Enhancement of Thiourea Leaching Performance of Refractory Silver Tailings by Mechanical Activation

Enhancement of Thiourea Leaching Performance of Refractory Silver Tailings by Mechanical Activation | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Enhancement of Thiourea Leaching Performance of Refractory Silver Tailings by Mechanical Activation Fatih Apaydin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9194073/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract Refractory silver tailings are both an environmental liability and a potential secondary resource for precious metals. However, strong mineralogical locking of silver-bearing phases within barite and aluminosilicate matrices severely limits silver recovery under conventional leaching conditions. In this study, mechanical activation was investigated as a pretreatment method to enhance the thiourea leaching performance of refractory silver tailings from the Kütahya Eti Silver Plant (Turkey). Mechanical activation was carried out in a planetary ball mill for 0, 30, 60, and 90 min. The resulting changes in particle size, specific surface area, and structure were evaluated by particle size analysis, BET, XRD, and SEM. Mechanical activation caused marked particle fragmentation, increased surface area, and partial amorphization, indicating enhanced structural disorder and improved accessibility of silver-bearing phases. Thiourea leaching experiments were performed in acidic solution (pH 1.5) with Fe³⁺ as the oxidizing agent. Mechanical activation significantly improved silver dissolution, increasing recovery from about 68% for the non-activated sample to about 96% for the sample activated for 90 min at 50°C. Kinetic analysis under conditions minimizing external mass-transfer resistance showed that apparent rate constants increased with activation time, while the apparent activation energy decreased from 46.1 to 38.9 kJ·mol⁻¹. Post-leaching SEM and BET results revealed agglomerated and porous particle structures, indicating that diffusion through evolving porous particle assemblies contributes to the observed kinetic behavior. This work directly links mechanochemically induced structural evolution with apparent leaching kinetics in industrial refractory silver tailings and highlights mechanically assisted thiourea leaching as a promising cyanide-free alternative for refractory silver-bearing wastes. Physical sciences/Chemistry Physical sciences/Engineering Earth and environmental sciences/Environmental sciences Physical sciences/Materials science Mechanical Activation Thiourea Leaching Refractory silver tailings Hydrometallurg Leaching kinetics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1. Introduction Silver is a strategically important metal widely used in electronics, photonics, catalysis, and advanced energy technologies. The growing demand for silver, together with the progressive depletion of high-grade primary ores and increasingly stringent environmental regulations, has intensified interest in alternative resources for silver production. In this context, mine tailings generated during mineral beneficiation and metallurgical processing have attracted renewed attention as both an environmental liability and a potential secondary resource for valuable metals. Mine tailings are finely ground solid residues produced during ore processing and are commonly stored in large-scale tailings facilities. Due to their large volumes, fine particle size, and residual chemical reactivity, tailings pose significant long-term environmental risks, including metal leaching, acid generation, dust dispersion, and land degradation [ 1 ], [ 2 ], [ 3 ]. At the same time, advances in processing technologies and rising metal prices have demonstrated that tailings may contain economically relevant concentrations of precious and base metals that were not recoverable using historical technologies or were discarded due to unfavorable economics at the time of operation (Adiansyah et al., 2015; Jamieson et al., 2015; Blengini et al., 2019). Reprocessing of tailings therefore offers a dual benefit by mitigating environmental impacts while contributing to resource efficiency and circular economy objectives. The feasibility of metal recovery from tailings strongly depends on their physicochemical characteristics, particularly mineralogical locking, particle size distribution, surface properties, and structural disorder, which collectively control reagent accessibility and dissolution behavior. In hydrometallurgical processing, silver-bearing materials are classified as refractory when economically acceptable metal recoveries cannot be achieved under conventional leaching conditions due to mineralogical encapsulation, surface passivation, or diffusion limitations. In such systems, silver-bearing phases are commonly embedded within chemically inert gangue minerals, restricting direct contact between the lixiviant and the valuable phases and resulting in low dissolution rates and high reagent consumption [ 7 ], [ 8 ]. Previous investigations on tailings from the Kütahya Eti Silver Plant have consistently reported limited silver recovery in the absence of intensive pretreatment, primarily due to the strong association of silver with barite and aluminosilicate matrices (Celep, 2011; Celep et al., 2011; Dinçer Hayrünnisa, 1997). Accordingly, the material investigated in the present study is classified as refractory. Although mine tailings are often characterized by relatively fine particle sizes, this does not necessarily imply sufficient mineral liberation. Conventional grinding techniques are frequently inadequate for liberating finely disseminated silver-bearing phases from inert gangue matrices, particularly in refractory systems. Mechanical activation, a high-energy grinding approach, has emerged as an effective pretreatment method for such materials. In addition to particle size reduction, mechanical activation induces lattice distortion, defect generation, formation of new reactive surfaces, and partial amorphization, thereby enhancing mineral liberation and reagent accessibility [ 12 ], [ 13 ], [ 14 ]. These mechanochemical effects have been shown to significantly improve the leaching behavior of refractory mineral systems by reducing diffusion barriers and increasing surface reactivity. Growing environmental concerns associated with cyanide use have further stimulated research into alternative, more environmentally benign lixiviants for precious metal recovery. Among these, thiourea (CS(NH₂)₂) has received considerable attention due to its high selectivity toward silver and gold, its effectiveness under acidic conditions, and its relatively rapid dissolution kinetics compared to cyanide systems (Calla-Choque and Lapidus, 2021, 2020; Tian et al., 2024). In acidic thiourea systems, silver dissolution proceeds through complexation reactions, forming stable silver–thiourea complexes, as illustrated by Eq. (1) [ 18 ] Ag (s) + 2CS(NH₂)₂ (aq) + Fe³⁺ (aq) → [Ag(CS(NH₂)₂)₂]⁺ (aq) + Fe²⁺ (aq) (1) The presence of an oxidizing agent such as Fe³⁺ plays a critical role by facilitating electron transfer and sustaining silver oxidation through a reversible Fe³⁺/Fe²⁺ redox cycle, as represented by Eq. (2) (Jing-ying et al., 2012; Calla-Choque and Lapidus, 2020): 2Fe²⁺ + ½O₂ + 2H⁺ → 2Fe³⁺ + H₂O (2) Although thiourea leaching offers important advantages, its effectiveness is sensitive to operating conditions. Elevated temperatures, excessive oxidant concentrations, and highly acidic environments may accelerate thiourea degradation, leading to increased reagent consumption and formation of undesirable by-products [ 19 ], [ 20 ]. Pretreatment strategies that enable efficient leaching at moderate temperatures and shorter reaction times are therefore essential to improve process viability. Recent studies have shown that the combined application of mechanical activation and thiourea leaching can significantly enhance silver recovery from refractory materials by promoting structural disorder and improving diffusion pathways [ 21 ], [ 22 ]. However, most previous investigations have focused on laboratory-prepared ores or electronic waste and have often addressed either structural modification or leaching performance in isolation. Systematic studies linking mechanical activation–induced structural changes to leaching behavior and apparent kinetic response in industrial refractory silver tailings remain limited. Within this framework, the present study investigates the effect of mechanical activation on the thiourea leaching behavior of refractory silver tailings from the Kütahya Eti Silver Plant. Mechanical activation was systematically applied for 0, 30, 60, and 90 min to elucidate the progressive evolution of structural disorder and its influence on silver dissolution behavior, rather than limiting the analysis to non-activated and fully activated states. By integrating particle size analysis, BET surface area measurements, microstructural characterization (SEM/XRD), leaching performance evaluation, and kinetic analysis, this study establishes a physically consistent relationship between mechanically induced structural modifications and silver dissolution behavior. The applied kinetic expressions are interpreted as phenomenological descriptors, supported by microstructural observations and activation energy analysis, rather than as idealized mechanistic models, thereby providing a realistic and comprehensive framework for understanding thiourea leaching of mechanically activated refractory silver tailings. 2. Materials and Methods Figure 1 schematically illustrates the overall experimental workflow adopted in this study. The experimental procedure consisted of sequential stages including material characterization, mechanical activation, thiourea leaching, and kinetic evaluation. The mineralogical composition, surface morphology, and silver content of the samples were determined using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX), and atomic absorption spectroscopy (AAS), respectively. Mechanical activation was carried out by planetary ball milling for different durations (0, 30, 60, and 90 min), resulting in significant structural modifications such as particle size reduction, defect generation, and partial amorphization. Subsequently, the mechanically activated samples were subjected to thiourea leaching in the presence of Fe³⁺ as an oxidizing agent under controlled experimental conditions at temperatures ranging from 25 to 75°C. Silver dissolution was monitored as a function of time, and leaching kinetics were evaluated using heterogeneous kinetic expressions in combination with Arrhenius analysis to assess apparent rate trends and activation energies. 2.1 Materials The refractory silver-bearing material used in this study was obtained from the tailings disposal area of the Kütahya Eti Silver Plant (Turkey). These tailings originate from historical beneficiation and metallurgical processing operations and have previously been reported to exhibit low silver solubility under conventional leaching conditions due to mineralogical encapsulation and diffusion limitations [ 9 ], [ 10 ], [ 11 ], [ 23 ], [ 24 ]. Figure 3 presents the particle size distribution (PSD) curves of the as-received tailings and mechanically activated samples. Although tailings from beneficiation processes are generally considered fine-grained, the as-received material exhibits a median particle size (d₅₀) of approximately 28.3 µm, indicating that a significant fraction of silver-bearing phases remains locked within gangue minerals. Therefore, further size reduction is required not merely to decrease particle size, but to enhance mineral liberation, increase surface defect density, and improve reagent accessibility during leaching. Prior to mechanical activation and leaching experiments, the tailings were homogenized and dried at ambient conditions. Representative subsamples were prepared using coning and quartering to ensure compositional consistency throughout the experimental program. All leaching experiments were performed using analytical-grade reagents, including thiourea (≥ 99.0%), sulfuric acid (≥ 95.0%), and ferric sulfate (≥ 97.0%). Deionized water was used in the preparation of all solutions to eliminate the influence of extraneous ions. 2.2 Mechanical Activation 2.2.1 Equipment and operating conditions Mechanical activation of the silver-bearing tailings was performed using a planetary ball mill (Fritsch Pulverisette 5) equipped with hardened steel vials and 10 mm diameter steel balls. Dry milling experiments were conducted at a rotational speed of 400 rpm, while maintaining a constant ball-to-powder mass ratio of 10:1. Mechanical activation was applied for different durations (0, 30, 60, and 90 min) to systematically investigate its influence on the structural and surface properties of the material. 2.2.2 Characterization of mechanically activated samples Particle size distributions of the mechanically activated samples were measured by laser diffraction using a Mastersizer 2000 analyzer (Malvern Instruments). X-ray diffraction (XRD) patterns of both raw and mechanically activated samples were collected using a PANalytical Empyrean diffractometer with Cu Kα radiation (λ = 1.5406 Å), operated at 45 kV and 40 mA. Data were acquired over a 2θ range of 10°–80° with a step size of 0.02° and a scanning rate of 2°·min⁻¹. Phase identification and semi-quantitative analyses were carried out using HighScore Plus software (Malvern Panalytical) with reference to the ICDD PDF-4 + database. The specific surface area of the samples before and after leaching was determined by N₂ adsorption–desorption measurements at − 196°C using a Quantachrome® ASiQwin™ surface area and porosity analyzer. Prior to analysis, the samples were degassed under vacuum at 120°C for 12 h to remove physically adsorbed moisture and gases. Specific surface area values were calculated using the Brunauer–Emmett–Teller (BET) method within the relative pressure (P/P₀) range of 0.05–0.30. Morphological and surface features of the samples were examined using scanning electron microscopy (SEM, Supra 40 VP, Zeiss). Fourier-transform infrared (FTIR) spectra of non-activated and mechanically activated samples were recorded using a PerkinElmer Spectrum 100 spectrometer over the wavenumber range of 4000–400 cm⁻¹. The degree of amorphization induced by mechanical activation (0, 30, 60, and 90 min) was quantified based on XRD data following the method proposed by Baláž, (2003). XRD patterns used for this calculation were collected over a 2θ range of 10°–80° at a scanning rate of 10°·min⁻¹. The amorphization degree (A) was calculated using Eq. ( 3 ), considering the average intensities of the three most intense diffraction peaks: $$\:\begin{array}{cccc}&\:A=\left(1−\frac{{B}_{0}{I}_{x}}{{B}_{x}{I}_{0}}\right)\times\:100&\:&\:\end{array}$$ 3 where \(\:{I}_{0}\) and \(\:{B}_{0}\) represent the integral intensity and background intensity of the non-activated sample, respectively, while \(\:{I}_{x}\) and \(\:{B}_{x}\) correspond to the mechanically activated samples. 2.3 Thiourea Leaching 2.3.1 Leaching Procedure Thiourea leaching experiments were conducted under atmospheric pressure using a thermostatically controlled magnetic stirrer equipped with a heating plate. Leaching solutions were prepared with deionized water containing 0.5 M thiourea, and the pH was adjusted to 1.5 using sulfuric acid. For each experiment, 5 g of ore sample was contacted with 100 mL of leaching solution in a 250 mL glass reactor, followed by the addition of 20 mL of 0.01 M ferric sulfate (Fe₂(SO₄)₃) as an oxidizing agent. Ferric sulfate was selected due to its well-established effectiveness in thiourea-based silver leaching systems and its ability to sustain a reversible Fe³⁺/Fe²⁺ redox cycle, thereby promoting continuous silver oxidation under acidic conditions. Previous studies have demonstrated that Fe³⁺ acts as an efficient electron acceptor, enhancing silver dissolution kinetics while remaining compatible with low-pH thiourea media [ 18 ], [ 20 ], [ 25 ]. It is acknowledged that ferric ions may form coordination complexes with thiourea, such as FeSO₄(CS(NH₂)₂)⁺, which can increase thiourea consumption under unfavorable conditions [ 8 ], [ 26 ], [ 27 ]. To mitigate this effect, the ferric ion concentration was deliberately limited to 0.01 M, the solution pH was maintained at 1.5, and temperatures above 50°C were excluded from kinetic evaluation. These conditions are consistent with literature reports indicating that controlled Fe³⁺ addition can enhance silver dissolution without inducing excessive thiourea degradation [ 15 ], [ 28 ]. Although alternative oxidants such as hydrogen peroxide and manganese dioxide have been reported, these reagents often exhibit rapid and less controllable oxidation behavior, which may accelerate thiourea decomposition and complicate kinetic interpretation [ 29 ], [ 30 ]. In contrast, the Fe³⁺/Fe²⁺ redox couple provides improved process stability, cost-effectiveness, and operational control. Under the selected experimental conditions, no abnormal loss of leaching efficiency or evidence of thiourea instability was observed. Leaching experiments were performed at temperatures of 25, 30, 40, 50, and 75°C for leaching times of 15, 30, 60, 120, and 240 min. To assess hydrodynamic effects, stirring speeds of 300, 450, and 600 rpm were investigated. Visual inspection confirmed complete suspension of solid particles at 450 and 600 rpm, whereas partial settling was occasionally observed at 300 rpm, particularly at longer leaching times. Accordingly, a stirring speed of 600 rpm was selected for kinetic modeling and activation energy analysis to minimize external mass-transfer resistance. To prevent solvent loss during prolonged experiments and at elevated temperatures, all leaching tests were conducted in covered glass reactors. The solution volume was periodically monitored, and no measurable evaporation-related losses were detected, ensuring constant liquid-to-solid ratios and stable reagent concentrations throughout the leaching process. 2.3.2 Analytical methods After completion of the leaching experiments, the slurry was filtered to separate the solid residue from the leachate. Silver concentrations in the filtrate were determined by atomic absorption spectroscopy (AAS) using a PerkinElmer Analyst 700 instrument. Each sample was analyzed in triplicate to ensure analytical accuracy, and the reported values represent the average of three measurements. The leaching efficiency (X, %) was calculated according to Eq. ( 4 ): $$\:\begin{array}{cccc}&\:X\left(\text{%}\right)=\frac{{C}_{t}\cdot\:V}{{m}_{Ag}}\times\:100&\:&\:\end{array}$$ 4 where \(\:{C}_{t}\) is the silver concentration in the leachate at time \(\:t\) , V is the solution volume, and \(\:{m}_{Ag}\) is the total silver content in the initial sample. 2.3.3 Kinetic modeling and Arrhenius analysis The leaching kinetics of silver were evaluated using heterogeneous fluid–solid reaction expressions commonly applied in hydrometallurgical studies to describe conversion–time behavior. In the present work, these kinetic expressions are treated as limiting case and phenomenological descriptors, rather than as strict mechanistic models, in recognition of the non-ideal particle geometry, evolving particle size distribution, and agglomeration effects associated with mechanically activated fine materials. Within this framework, kinetic expressions derived from the shrinking-core concept were examined to represent three classical limiting cases: (i) surface chemical reaction control, (ii) film diffusion control, and (iii) product-layer (ash-layer) diffusion control. In addition, the Avrami model was included as an empirical reference to assess the robustness of regression behavior in systems exhibiting structural evolution and non-uniform reaction geometries. Similar comparative approaches have been widely adopted in leaching studies involving fine, porous, or mechanically activated particles [ 31 ], [ 32 ], [ 33 ], [ 34 ]. The kinetic expressions employed in this study are given as follows: Surface chemical reaction control: \(\:1-(1-X{)}^{1/3}=kt\) (5) Product-layer (ash-layer) diffusion control: \(\:1-\frac{2}{3}X-(1-X{)}^{2/3}=kt\) (6) Film diffusion control: \(\:X=kt\) (7) Avrami model: \(\:\text{l}\text{n}[-\text{l}\text{n}(1-X\left)\right]=\text{l}\text{n}k+n\text{l}\text{n}t\) (8) where X is the fractional silver extraction at time t , k is the apparent rate constant, and n is the Avrami exponent. It is emphasized that classical shrinking-core formulations are rigorously derived for systems involving particles of constant size and idealized geometries. However, in mechanically activated systems, particle size continuously evolves, agglomeration may occur during wet processing, and porous structures develop, which limits the strict applicability of ideal SCM assumptions. Under such conditions, kinetic models are commonly applied as practical descriptors of observable rate behavior, and deviations from linearity are expected [ 31 ], [ 33 ], [ 34 ]. The temperature dependence of the apparent rate constants was evaluated using the Arrhenius equation: $$\:k=A\text{e}\text{x}\text{p}(-{E}_{a}/RT)$$ 9 or in linearized form: $$\:\text{l}\text{n}k=\text{l}\text{n}A-\frac{{E}_{a}}{RT}$$ 10 where Eₐ is the apparent activation energy (kJ·mol⁻¹), R is the universal gas constant, and T is the absolute temperature (K). All kinetic expressions and their mathematical forms were verified against standard formulations reported in authoritative chemical reaction engineering and hydrometallurgical literature, including Levenspiel (2013) and recent critical analyses of kinetic model applicability in leaching systems involving fine and mechanically activated particles [ 31 ], [ 33 ], [ 34 ]. 3. Results and Discussions 3.1 Chemical Composition and Mineralogical Characteristics of the Refractory Silver Tailings The silver-bearing tailings investigated in this study were collected from the tailings disposal area of the Kütahya Eti Silver Plant and exhibit a complex chemical and mineralogical composition characteristic of refractory silver materials. As summarized in Table 2 , the tailings are dominated by SiO₂, Al₂O₃, Fe₂O₃, CaO, and Ba-bearing phases, with an average silver content of approximately 125 ppm. This composition closely agrees with previous investigations conducted on tailings from the same processing facility [ 8 ], [ 9 ], [ 11 ], [ 23 ], confirming the representativeness of the material used in the present study. Table 2 Chemical analysis of silver sample Component (%) Element (ppm) SiO 2 49.43 Ag 125 Al 2 O 3 10.90 Ba 9.73% Fe 2 O 3 4.97 Cu 178.32 CaO 5.21 Ni 26.45 MgO 1.10 Sb 881.19 SO 3 1.76 As 2 O 3 1.16 PbO 1.53 ZnO 1.75 LOI 10.36 As shown in Table 2 , the high SiO₂ and Ba contents confirm the dominance of chemically inert gangue phases, which strongly contribute to the refractory behavior of the investigated tailings. In hydrometallurgical processing, silver-bearing materials are classified as refractory when economically acceptable recoveries cannot be achieved under conventional leaching conditions due to mineralogical encapsulation, surface passivation, or diffusion-related limitations. The chemical and mineralogical features observed for the investigated tailings are fully consistent with this definition. X-ray diffraction analysis of the raw tailings (Fig. 2 ) indicates that quartz (SiO₂) and barite (BaSO₄) are the dominant crystalline phases, accompanied by dolomite [CaMg(CO₃)₂], calcium carbonate (CaCO₃), hematite (Fe₂O₃), muscovite, and microcline. This mineral assemblage reflects a silicate- and sulfate-rich gangue matrix that is highly resistant to chemical attack. No discrete silver-bearing mineral phases (e.g., argentite or native silver) were detected by XRD, which is consistent with the low silver concentration and its finely disseminated nature. The dominance of quartz and barite phases observed in Fig. 2 explains the limited reactivity of the raw tailings under conventional leaching conditions. SEM–EDS analyses (Figs. 4 – 11 ) further support these findings, confirming the predominance of Si-, Al-, Ba-, Ca-, and Fe-bearing phases. Due to its trace-level concentration and dispersed occurrence, silver could not be identified as a distinct phase by SEM–EDS. Previous studies on tailings from the same site have demonstrated that silver is structurally associated with barite and aluminosilicate matrices rather than occurring as liberated silver minerals [ 9 ], [ 11 ], [ 23 ]. Such encapsulation is a defining characteristic of refractory silver materials and explains the poor leaching performance observed under conventional processing conditions. The persistence of chemically inert gangue minerals and the absence of discrete silver phases provide a clear justification for the use of mechanical activation as a pretreatment strategy in this study. Mechanical activation is expected to enhance microstructural liberation by inducing lattice distortion, microcrack formation, and partial amorphization, thereby disrupting diffusion barriers imposed by the gangue matrix and improving silver accessibility during subsequent thiourea leaching. In addition to silver, the chemical analysis reveals appreciable concentrations of iron and copper (Table 2 ). Iron plays a dual role in thiourea leaching systems: Fe³⁺ species can promote silver dissolution through redox cycling, whereas excessive iron may increase thiourea consumption via complex formation. In the present work, potential adverse effects were mitigated by controlling ferric ion concentration, solution pH, and temperature, ensuring stable leaching conditions. Copper is known to form stable complexes with thiourea and may undergo parallel dissolution. However, under the selected experimental conditions, copper dissolution remained limited and did not dominate the leaching behavior. Accordingly, kinetic analyses in this study focus exclusively on silver dissolution, with the obtained apparent kinetic parameters reflecting the combined influence of mineralogical association and transport-related constraints rather than the independent behavior of accompanying metals. The presence of alkaline earth oxides and carbonate-bearing phases, including CaO, MgO, and dolomite, is also noteworthy. These phases can influence thiourea-based leaching systems by buffering acidic solutions and increasing reagent consumption through side reactions (Celep et al., 2015; John and House, 2006; Liu et al., 2019). In this study, such effects were minimized by maintaining the leaching pH at 1.5 and carefully controlling oxidant dosage. Nevertheless, the persistence of carbonate and silicate phases contributes to diffusion-related limitations by forming chemically inert matrices around silver-bearing species. Similar behavior has been widely reported for refractory tailings and mechanically activated systems [ 37 ], [ 38 ], [ 39 ]. Overall, the chemical and mineralogical characteristics of the investigated tailings confirm their refractory nature and provide a consistent framework for interpreting the effects of mechanical activation on silver liberation, leaching performance, and apparent kinetic behavior. 3.2 Effect of Mechanical Activation on Particle Properties Leaching efficiency in refractory silver systems is strongly governed by the accessibility of the lixiviant to reactive mineral surfaces. In thiourea-based leaching, dissolution performance is jointly controlled by particle size, surface reactivity, and transport pathways. Mechanical activation enhances these factors by inducing particle fragmentation, lattice distortion, defect generation, and partial loss of crystalline order, thereby increasing surface reactivity and reagent accessibility [ 12 ], [ 40 ], [ 41 ]. In the present study, mechanical activation times of 0, 30, 60, and 90 min were selected to represent successive stages of mechanochemical transformation and to identify an optimal activation window. The effects of activation were evaluated through particle size distribution, BET specific surface area, degree of amorphization (Fig. 3 ), and microstructural evolution observed by SEM (Fig. 4 ). The results indicate three distinct activation regimes. During the initial stage (0–30 min), rapid particle fragmentation occurred, with the median particle size decreasing from approximately 27 µm to about 5 µm, accompanied by an increase in amorphization to ~ 17%. This behavior reflects intensive defect generation, and microstructural disorder induced by high-energy milling and marks the onset of surface activation effects that begin to influence leaching behavior [ 42 ], [ 43 ]. At 60 min of activation, the system reached a highly activated structural state, characterized by a maximum specific surface area (~ 2.3 m²·g⁻¹) and an amorphization degree of approximately 27%. SEM and XRD analyses (Figs. 4 and 5 ) reveal the disappearance of relatively unstable carbonate phases, such as dolomite and CaCO₃, together with the development of amorphous regions within the mineral matrix. Such transformations are commonly associated with enhanced surface reactivity and improved leaching response in mechanically activated mineral systems [ 39 ]. Prolonged milling beyond 60 min resulted in diminishing structural gains. At 90 min, the amorphization degree approached a plateau (~ 31%), while a slight decrease in BET surface area was observed. This reduction is attributed to particle agglomeration and partial re-welding effects induced by extended mechanical activation. SEM micrographs of the 90 min activated sample (Fig. 4 d) clearly show clustered particle assemblies and reduced dispersion compared to samples activated for shorter durations, supporting the agglomeration-based interpretation. Similar agglomeration-driven surface area reductions at prolonged milling times have been widely reported in mechanochemically activated systems (Baláž, 2000; Liu et al., 2025). Despite the onset of agglomeration, the 90 min activation condition was selected for subsequent leaching and kinetic evaluations, as it provided the highest enhancement in silver dissolution under the investigated conditions. This choice reflects a balance between the benefits of extensive structural activation and the emergence of agglomeration-related limitations. Although mine tailings generated from beneficiation processes are often characterized by relatively fine particle sizes, this does not necessarily imply sufficient mineral liberation. As shown in Fig. 3 , the as-received silver tailings exhibit a median particle size of approximately 28.3 µm, indicating that a substantial fraction of silver-bearing phases remains encapsulated within the gangue matrix. Mechanical activation shifts the particle size distribution toward the ultrafine range, with median sizes decreasing to below 10 µm after 60–90 min of milling. This additional size reduction is therefore essential for enhancing mineral liberation, increasing surface defect density, and improving reagent accessibility, all of which directly contribute to improved leaching performance. 3.3 Microstructural and Phase Changes Mechanical activation induces lattice distortion, defect generation, and partial loss of crystalline order, which collectively enhances reagent accessibility by creating new reactive sites and disrupting diffusion barriers. SEM observations (Fig. 4 ) demonstrate that increasing activation time progressively modifies particle morphology, resulting in reduced particle size, increased surface roughness, and a more homogeneous particle distribution. These microstructural changes are consistent with enhanced surface activation and improved leaching response. SEM–EDS analyses were conducted to complement XRD results and to clarify the microstructural association between silver-bearing phases and the surrounding gangue minerals. The analyses indicate that silver is not present as a discrete, liberated mineral phase but occurs as a finely dispersed component occluded within SiO₂-rich and barite-dominated matrices. This microstructural configuration confirms the mineralogical locking inferred from XRD analysis and explains the limited silver accessibility observed under conventional leaching conditions. Encapsulation of silver-bearing phases within chemically inert gangue minerals such as quartz and barite imposes diffusion constraints and restricts direct lixiviant–mineral contact. This occlusion is a defining characteristic of refractory tailings and plays a dominant role in controlling leaching efficiency. Mechanical activation partially disrupts this locked microstructure by inducing lattice defects, microcracks, and localized disorder, thereby improving reagent penetration and facilitating silver dissolution. The evolution of particle morphology with activation time is clearly illustrated in Fig. 4 . The untreated sample (Fig. 4 a) exhibits irregular particle shapes and a broad size distribution. After 30 min of activation (Fig. 4 b), pronounced particle fragmentation and surface defect formation are observed, reflecting repeated fracture and plastic deformation during milling. At 60 min (Fig. 4 c), particles appear smaller and more uniformly distributed, corresponding to a highly activated structural state. Prolonged activation to 90 min (Fig. 4 d) results in the formation of agglomerated particle clusters, attributed to partial re-welding and interparticle adhesion under sustained mechanical stress [ 14 ], [ 18 ], [ 46 ]. XRD patterns of the mechanically activated samples (Fig. 5 ) reveal progressive phase transformations with increasing milling time. Unstable carbonate phases, particularly dolomite and CaCO₃, gradually disappear with prolonged activation, whereas quartz and barite remain largely unaffected, reflecting their chemical and structural inertness. Peak broadening and intensity reduction of the remaining phases confirm increasing amorphization, consistent with the BET and particle size evolution discussed in Section 3.2 and with previous reports on mechanically activated mineral systems [ 41 ], [ 47 ]. FTIR spectra (Fig. 6 ) further corroborates these structural modifications. In the non-activated sample, absorption bands at approximately 1093, 798, and 459 cm⁻¹ are assigned to Si–O–Si, sulfate (S–O), and Al–O vibrational modes, respectively. With increasing activation time, the progressive reduction in band intensity and peak broadening indicate increasing structural disorder and amorphous domain formation [ 48 ]. After 90 min of activation, the appearance of bands in the 1500–2000 cm⁻¹ region is associated with Mg–CO vibrations related to dolomite transformation, while carbonate bands near 1450–1425 cm⁻¹ and ~ 875 cm⁻¹ weaken markedly, confirming CaCO₃ decomposition [ 49 ]. In contrast, Ba–SO₄ vibrational bands in the 610–1100 cm⁻¹ range remain largely unchanged, confirming the inert behavior of barite during mechanical activation (Theng, 2024). SEM–EDS analyses of solid residues obtained after thiourea leaching under kinetic evaluation conditions (50°C, S/L = 1/10, 600 rpm) provide further insight into the dissolution mechanism. Post-leaching residues exhibit pronounced particle agglomeration, irregular morphologies, and porous surface structures. Elemental mapping reveals heterogeneous surface compositions dominated by residual Si- and Ba-rich regions corresponding to quartz and barite phases, which persist after leaching and contribute to the development of diffusion-related barriers as leaching progresses. Overall, the combined SEM, EDS, XRD, and FTIR results establish a clear link between microstructural evolution, phase composition, and leaching behavior. Silver dissolution proceeds within evolving porous and agglomerated particle assemblies rather than through idealized, uniformly shrinking particles, highlighting the importance of transport limitations in the later stages of leaching and providing a microstructural basis for the kinetic interpretation presented in Section 3.5 . 3.4 Silver Leaching Performance and Effects of Operating Parameters This section examines the influence of key operating parameters on silver dissolution in thiourea media to establish robust experimental conditions for subsequent kinetic analysis. Mechanical activation time, temperature, solid-to-liquid (S/L) ratio, and stirring speed were systematically evaluated to clarify their individual and combined effects on leaching performance and to identify conditions that minimize external mass-transfer limitations. Based on these results, kinetic modeling was performed under optimized hydrodynamic and reagent-availability conditions. 3.4.1. Effect of Mechanical Activation Time The effect of mechanical activation time on silver dissolution was evaluated at 50°C, S/L = 1/10, and 600 rpm (Fig. 7 ). Mechanical activation significantly enhanced both the dissolution rate and the final silver recovery. Under identical leaching conditions, the non-activated sample reached approximately 68% recovery after 240 min, whereas the sample activated for 90 min achieved about 96% recovery. Increasing the activation time from 0 to 60 min resulted in a pronounced acceleration of dissolution kinetics, particularly during the early leaching stage (≤ 60 min). This enhancement is attributed to particle size reduction, increased surface defect density, and higher surface reactivity, which collectively improve lixiviant accessibility to silver-bearing phases [ 50 ]. Extending the activation time to 90 min further increased silver recovery; however, the incremental gain diminished, indicating the onset of saturation effects. At prolonged milling durations, particle agglomeration and surface reorganization may partially counteract continued defect generation, as reported for other mechanically activated mineral systems [ 39 ], [ 47 ], [ 51 ]. These trends are consistent with previous studies demonstrating that mechanical activation enhances leaching by inducing structural disorder or partial amorphization, increasing the density of reactive surface sites, and shortening effective diffusion pathways within the ore matrix [ 18 ], [ 21 ], [ 27 ], [ 39 ]. Overall, mechanical activation predominantly accelerates early-stage dissolution, while the gradual reduction in dissolution rate at longer times suggests an increasing contribution of transport-related limitations. On this basis, an activation time of 90 min was selected for subsequent leaching performance evaluation and kinetic analysis, as it provided the highest overall recovery despite diminishing incremental improvements. 3.4.2. Effect of Temperature The influence of temperature on silver dissolution was investigated over the range of 25–75°C at S/L = 1/10 and 600 rpm (Fig. 8 ). Increasing the temperature from 25°C to 50°C resulted in a marked enhancement of dissolution rates and final recoveries for both non-activated and mechanically activated samples, indicating thermally activated leaching behavior. At all investigated temperatures, the mechanically activated sample consistently exhibited higher dissolution rates and recoveries than the non-activated material. A decline in silver recovery was observed at 75°C, despite the thermally activated nature of the process. This behavior is attributed to the thermal instability of thiourea in acidic media at elevated temperatures, which leads to reagent degradation and reduced effective lixiviant concentration [ 52 ], [ 53 ], [ 54 ]. To avoid reagent-controlled artifacts and ensure reliable kinetic interpretation, 50°C was selected as the optimal temperature for kinetic modeling and Arrhenius analysis. 3.4.3. Effect of Solid-to-Liquid Ratio The effect of the solid-to-liquid ratio on silver dissolution was evaluated at 50°C and 600 rpm (Fig. 9 ). Lower S/L ratios (i.e., higher solution volumes) resulted in higher dissolution rates and final recoveries for both sample types. This behavior reflects improved reagent availability, reduced local depletion of thiourea and oxidizing species near particle surfaces, and enhanced mass transport within the slurry. The mechanically activated sample consistently outperformed the non-activated material across the investigated S/L ratios, achieving approximately 96% recovery at an S/L ratio of 1/10. Reduced dissolution efficiency at higher solid loadings is consistent with mass-transfer limitations and reagent consumption effects widely reported in precious metal leaching systems [ 21 ], [ 50 ]. Accordingly, an S/L ratio of 1/10 was selected for kinetic evaluation to ensure sufficient reagent availability and stable leaching conditions. 3.4.4. Effect of Stirring Speed The influence of stirring speed on silver dissolution was investigated at 50°C and an S/L ratio of 1/10 (Fig. 10 ). Increasing the agitation speed from 300 to 600 rpm led to higher dissolution rates and final recoveries for both samples. This enhancement is attributed to a reduction in boundary-layer thickness and improved transport of thiourea and oxidant species to the particle surfaces [ 31 ], [ 55 ]. At a stirring speed of 600 rpm, dissolution curves approached a plateau, indicating that external mass-transfer resistance was substantially minimized and that intrinsic reaction steps and intra-particle transport processes became more influential. Consequently, a stirring speed of 600 rpm was selected for kinetic modeling and activation energy analysis to ensure that derived kinetic parameters reflect intrinsic and particle-scale phenomena rather than external hydrodynamic limitations. 3.5 Kinetic Modeling and Activation Energy Analysis Kinetic analysis was performed to quantify the effect of mechanical activation on silver dissolution in thiourea media and to interpret the conversion–time behavior under conditions of minimized external mass-transfer resistance. For mechanically activated fine solids, apparent kinetic parameters commonly reflect the combined contributions of interfacial reaction, intra-aggregate transport, and evolving microstructural features rather than a single well-defined rate-controlling step. Accordingly, the present analysis is framed within a phenomenological interpretation using limiting-case kinetic expressions [ 12 ], [ 31 ], [ 32 ]. This approach is consistent with recent hydrometallurgical studies in which ideal shrinking-core assumptions—such as constant particle size, regular geometry, and uniform product-layer growth—are not strictly satisfied due to broad particle-size distributions and particle agglomeration during leaching [ 16 ], [ 33 ], [ 34 ], [ 38 ], [ 39 ]. To suppress film-diffusion effects, kinetic tests were conducted at 600 rpm and S/L = 1/10, based on the agitation study in Section 3.4 showing improved dissolution with increasing stirring speed. Because thiourea is susceptible to degradation in acidic media at elevated temperatures, experiments at 75°C were excluded from kinetic modeling; consequently, kinetic evaluation and Arrhenius analysis were restricted to 25–50°C [ 52 ], [ 53 ], [ 54 ]. Conversion–time data were analyzed using the integral-form expressions introduced in Section 2.3.3 (Equations (5)–( 10 ), including shrinking-core-based limiting cases (surface reaction, film diffusion, and product-layer/ash-layer diffusion) and the Avrami formulation as an empirical comparator [ 31 ], [ 32 ]. These expressions were applied for comparative description of the data rather than as mechanistic proof, and regression quality was evaluated by linear fitting (R²). The resulting R² values for all mechanical activation durations (0, 30, 60, and 90 min) samples are summarized in Table 3 . Overall, diffusion-type limiting-case expressions provided the most consistent statistical description across the investigated temperatures, whereas film-diffusion fits were weaker, in agreement with the high agitation intensity used to minimize external mass-transfer resistance. Table 3 Comparison of correlation coefficients (R²) obtained from different kinetic expressions applied to silver leaching data for non-activated and mechanically activated samples (0, 30, 60, and 90 min) at different temperatures Mechanical activation duration (min) T (°C) Surface chemical reaction Product-layer diffusion Film diffusion Avrami 0 25 0.860 0.908 0.846 0.866 30 0.770 0.839 0.744 0.684 40 0.868 0.927 0.831 0.779 50 0.766 0.838 0.706 0.729 30 25 0.882 0.934 0.857 0.907 30 0.813 0.873 0.782 0.809 40 0.821 0.878 0.789 0.831 50 0.836 0.905 0.801 0.876 60 25 0.902 0.952 0.881 0.929 30 0.865 0.914 0.842 0.893 40 0.842 0.899 0.815 0.861 50 0.879 0.931 0.854 0.912 90 25 0.823 0.887 0.774 0.719 30 0.829 0.882 0.777 0.747 40 0.788 0.847 0.692 0.833 50 0.886 0.903 0.724 0.952 Importantly, model preference was not based on R² alone. The regression trends were interpreted together with characterization evidence. Post-leaching SEM images (Fig. 11 ) show pronounced agglomeration, irregular morphologies, and porous surface structures, indicating that dissolution does not proceed via an ideal retreating interface expected for classical shrinking-core geometry. Consistently, BET measurements show a decrease in specific surface area after leaching for the mechanically activated sample, implying surface collapse/agglomeration and/or partial passivation during wet processing. In addition, XRD/FTIR results confirm the persistence of chemically inert phases (e.g., quartz and barite), indicating that diffusion-related resistance associated with refractory gangue is not eliminated by activation. Collectively, these observations support interpreting the leaching kinetics as diffusion-influenced transport through evolving porous/agglomerated particle assemblies, rather than diffusion through a uniform product layer on a single shrinking particle. Apparent rate constants obtained from the most consistent expression were used for Arrhenius evaluation (Section 2.3.3 , Equations ( 9 )–( 10 ). The ln k versus 1/T plots (Fig. 12 ) were linear over 25–50°C for both samples, indicating Arrhenius-type behavior within this interval. The apparent activation energies decreased progressively from 46.1 kJ·mol⁻¹ (0 min) to 42.99, 40.13, and 38.9 kJ·mol⁻¹ for samples mechanically activated for 30, 60, and 90 min, respectively, indicating a gradual reduction in the energetic barrier with increasing mechanical activation. This decrease is consistent with mechanochemical defect generation and lattice distortion that reduce the energetic barrier and modify transport pathways, while diffusion-related contributions remain significant due to agglomeration and inert-phase persistence [ 18 ], [ 38 ]. The magnitude of Ea falls within ranges commonly reported for thiourea leaching systems where diffusion and reaction contributions coexist [ 52 ], [ 53 ], [ 54 ]. Overall, the combined regression results (Table 3 ), Arrhenius analysis (Fig. 12 ), and SEM/BET/XRD–FTIR evidence (Fig. 11 ) demonstrate that mechanical activation increases apparent dissolution rates and lowers the apparent activation energy under the investigated conditions. Accordingly, kinetic expressions are used here as physically consistent, limiting-case descriptors that link conversion–time behavior with observed microstructural evolution, without claiming a definitive single rate-controlling mechanism. 4. Conclusion This study demonstrates that mechanical activation is an effective pretreatment for enhancing thiourea leaching of refractory silver tailings characterized by strong mineralogical locking of silver-bearing phases within inert gangue matrices. Mechanical activation led to pronounced particle size reduction, defect generation, and partial amorphization, which collectively improved reagent accessibility and silver dissolution under acidic thiourea leaching conditions. Under optimized conditions (90 min mechanical activation, 50°C, S/L = 1/10, 600 rpm), silver recovery increased from approximately 68% for the non-activated material to about 96% for the mechanically activated sample. Kinetic evaluation conducted under minimized external mass-transfer resistance revealed a systematic enhancement of the apparent rate constants with increasing mechanical activation time, accompanied by a progressive decrease in apparent activation energy from 46.1 kJ·mol⁻¹ for the non-activated sample to 42.99, 40.13, and 38.9 kJ·mol⁻¹ for samples mechanically activated for 30, 60, and 90 min, respectively. This gradual reduction in activation energy reflects the mechanically induced structural disorder and improved accessibility of reactive surfaces, and the resulting values are consistent with diffusion-influenced thiourea leaching systems reported in the literature. Microstructural and surface analyses provide essential context for interpreting kinetic behavior. Post-leaching SEM observations revealed agglomerated and porous particle assemblies rather than ideal shrinking particles, while BET measurements indicated a reduction in specific surface area after leaching, suggesting surface collapse and/or passivation effects. Together with the persistence of inert mineral phases identified by XRD, these findings indicate that diffusion-related transport through evolving porous and agglomerated structures contributes significantly to the observed leaching behavior. Accordingly, the kinetic expressions employed in this study are interpreted as phenomenological, limiting-case descriptors rather than definitive mechanistic models. The combined use of SEM, BET, particle size analysis, and apparent activation energy trends provides a physically consistent framework for understanding silver dissolution from mechanically activated refractory tailings without invoking idealized particle-scale assumptions. From a practical perspective, the results indicate that mechanical activation enables high silver recoveries at moderate temperatures and relatively short leaching times, which may reduce energy demand and partially offset the higher reagent costs typically associated with thiourea leaching. Although a detailed techno-economic assessment was beyond the scope of this work, the findings highlight the potential of mechanically assisted thiourea leaching as a viable cyanide-free alternative for processing refractory silver tailings where conventional cyanidation is ineffective or environmentally constrained. Declarations Conflict of Interest The author(s) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The datasets used and/or analysed during the current study available from the corresponding author on reasonable request. Funding: The authors received no financial support for this research. Author Contribution The article was written by a single author. Acknowledgements The author(s) would like to thank the Kütahya Eti Silver Plant for providing the tailings samples used in this study. 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J. Sustainable Metall. 8 (2), 597–612. 10.1007/S40831-022-00499-8/TABLES/1 (Jun. 2022). Habashi, F. A Generalized Kinetic Model for Hydrometallurgical Processes Chemical Product and Process Modeling A Generalized Kinetic Model for Hydrometallurgical Processes, (2007). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 18 May, 2026 Reviewers agreed at journal 10 May, 2026 Reviews received at journal 05 May, 2026 Reviewers agreed at journal 22 Apr, 2026 Reviewers invited by journal 22 Apr, 2026 Editor assigned by journal 21 Apr, 2026 Submission checks completed at journal 27 Mar, 2026 First submitted to journal 27 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9194073","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":627720277,"identity":"bddff35c-c600-415d-92a0-62ff3a3eaa0f","order_by":0,"name":"Fatih Apaydin","email":"data:image/png;base64,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","orcid":"","institution":"Bilecik University","correspondingAuthor":true,"prefix":"","firstName":"Fatih","middleName":"","lastName":"Apaydin","suffix":""}],"badges":[],"createdAt":"2026-03-22 23:53:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9194073/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9194073/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108192671,"identity":"d9b2f7da-b2b5-4bf2-b1f4-f45e966c5661","added_by":"auto","created_at":"2026-04-30 10:11:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":327713,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental workflow\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/169603613f91fcdbc45784c3.png"},{"id":108491450,"identity":"8a893e77-00ce-439b-96a9-21d074f14ca5","added_by":"auto","created_at":"2026-05-05 09:53:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":47063,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of the waste silver tailings recorded using Cu Kα radiation (λ = 1.5406 Å) at 45 kV and 40 mA, over a 2θ range of 10–80°, with a step size of 0.02° and a scan rate of 2°·min⁻¹.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/986a888722999f2e87816fe2.png"},{"id":108192683,"identity":"64405d16-b5b6-4c61-a8ea-a8aee3d9b9dc","added_by":"auto","created_at":"2026-04-30 10:11:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":90311,"visible":true,"origin":"","legend":"\u003cp\u003eParticle size distribution curves of the as-received refractory silver tailings and the mechanically activated samples, illustrating the pronounced shift toward finer particle sizes induced by mechanical activation (d₅₀ of raw sample ≈ 28.3 µm). (a: particle size, b: surface area and amorphization)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/543abdc3db87c69dffea5c47.png"},{"id":108192684,"identity":"4f1d1605-f552-48b9-a85b-cb929141b58f","added_by":"auto","created_at":"2026-04-30 10:11:42","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":518205,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of mechanical activation on the microstructure of waste silver ore (a: 0 min, b: 30 min; c: 60 min; d: 90 min).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/7a2774b5c2408741c17ee056.png"},{"id":108491440,"identity":"e6a0eb5e-08dc-4565-aaa8-048ed3904fde","added_by":"auto","created_at":"2026-05-05 09:53:55","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":143086,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of mechanically activated silver tailings (0, 30, 60, and 90 min) obtained using Cu Kα radiation (λ = 1.5406 Å) at 45 kV and 40 mA, scanned over a 2θ range of 10–80° with a step size of 0.02° and a scan rate of 2°·min⁻¹.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/aa288239095064f3c0588b3d.png"},{"id":108192686,"identity":"ba156910-8680-4def-86a7-29c835f772a1","added_by":"auto","created_at":"2026-04-30 10:11:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":110400,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR Analysis of Mechanically Activated Waste Silver Ore Samples\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/7df064a3ad30788853f169df.png"},{"id":108491108,"identity":"fb3995f3-0d86-407a-8a1e-8b9513c0ed30","added_by":"auto","created_at":"2026-05-05 09:52:19","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":9993,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of mechanical activation time on silver dissolution at 50 °C (S/L = 1/10, 600 rpm).\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/cd0e923382f2eb2f57e02ed1.png"},{"id":108192692,"identity":"17fe8819-5d13-4733-bb6c-a9cba92b43b6","added_by":"auto","created_at":"2026-04-30 10:11:44","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":17352,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of temperature (25–75 °C) on silver dissolution for non-activated and 90 min activated samples (S/L = 1/10, 600 rpm).\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/b7ea19beeb866277f6be3fdc.png"},{"id":108192728,"identity":"c16c8e28-1981-4fd1-8129-2aa9e1fde875","added_by":"auto","created_at":"2026-04-30 10:11:49","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":17477,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of solid-to-liquid ratio on silver dissolution for non-activated and 90 min mechanically activated samples at 50 °C (600 rpm).\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/46a64069c0c7abdab28d2f11.png"},{"id":108192691,"identity":"66386cfe-b1f4-4c76-bb41-fd5d98d8c4bd","added_by":"auto","created_at":"2026-04-30 10:11:44","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":15889,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of stirring speed (300–600 rpm) on silver dissolution at 50 °C (S/L = 1/10).\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/466a645093d17929fbb04f2d.png"},{"id":108192666,"identity":"410efe6d-fef0-4c37-a490-6125d462336e","added_by":"auto","created_at":"2026-04-30 10:11:36","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":520018,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSEM–EDS micrographs of silver tailings subjected to thiourea leaching under optimized conditions (50 °C, S/L = 1/10, 600 rpm): \u003c/em\u003e\u003cem\u003e\u003cstrong\u003e(a)\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e non-activated sample before leaching, \u003c/em\u003e\u003cem\u003e\u003cstrong\u003e(b)\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e mechanically activated sample (90 min) before leaching, \u003c/em\u003e\u003cem\u003e\u003cstrong\u003e(c)\u003c/strong\u003e\u003c/em\u003e\u003cem\u003enon-activated sample after leaching, and \u003c/em\u003e\u003cem\u003e\u003cstrong\u003e(d)\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e mechanically activated sample after leaching (90 min).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/c4c07d8e367a2cc05705b5b9.png"},{"id":108491057,"identity":"3c11bb0c-1f1e-44aa-bd0b-eb0b88beeb80","added_by":"auto","created_at":"2026-05-05 09:51:40","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":47674,"visible":true,"origin":"","legend":"\u003cp\u003eArrhenius plots of the apparent rate constants derived from the diffusion-controlled shrinking core model for silver leaching in thiourea solution at different mechanical activation times (0, 30, 60, and 90 min).\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/a229bd923457d0715d336995.png"},{"id":109081143,"identity":"77ea832b-3f38-4b3b-a04d-8deb3e1d01b5","added_by":"auto","created_at":"2026-05-12 12:01:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2192761,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9194073/v1/38646567-120c-427a-a0c7-a334e659efe7.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhancement of Thiourea Leaching Performance of Refractory Silver Tailings by Mechanical Activation","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSilver is a strategically important metal widely used in electronics, photonics, catalysis, and advanced energy technologies. The growing demand for silver, together with the progressive depletion of high-grade primary ores and increasingly stringent environmental regulations, has intensified interest in alternative resources for silver production. In this context, mine tailings generated during mineral beneficiation and metallurgical processing have attracted renewed attention as both an environmental liability and a potential secondary resource for valuable metals.\u003c/p\u003e \u003cp\u003eMine tailings are finely ground solid residues produced during ore processing and are commonly stored in large-scale tailings facilities. Due to their large volumes, fine particle size, and residual chemical reactivity, tailings pose significant long-term environmental risks, including metal leaching, acid generation, dust dispersion, and land degradation [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. At the same time, advances in processing technologies and rising metal prices have demonstrated that tailings may contain economically relevant concentrations of precious and base metals that were not recoverable using historical technologies or were discarded due to unfavorable economics at the time of operation (Adiansyah et al., 2015; Jamieson et al., 2015; Blengini et al., 2019). Reprocessing of tailings therefore offers a dual benefit by mitigating environmental impacts while contributing to resource efficiency and circular economy objectives.\u003c/p\u003e \u003cp\u003eThe feasibility of metal recovery from tailings strongly depends on their physicochemical characteristics, particularly mineralogical locking, particle size distribution, surface properties, and structural disorder, which collectively control reagent accessibility and dissolution behavior. In hydrometallurgical processing, silver-bearing materials are classified as \u003cem\u003erefractory\u003c/em\u003e when economically acceptable metal recoveries cannot be achieved under conventional leaching conditions due to mineralogical encapsulation, surface passivation, or diffusion limitations. In such systems, silver-bearing phases are commonly embedded within chemically inert gangue minerals, restricting direct contact between the lixiviant and the valuable phases and resulting in low dissolution rates and high reagent consumption [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePrevious investigations on tailings from the K\u0026uuml;tahya Eti Silver Plant have consistently reported limited silver recovery in the absence of intensive pretreatment, primarily due to the strong association of silver with barite and aluminosilicate matrices (Celep, 2011; Celep et al., 2011; Din\u0026ccedil;er Hayr\u0026uuml;nnisa, 1997). Accordingly, the material investigated in the present study is classified as refractory. Although mine tailings are often characterized by relatively fine particle sizes, this does not necessarily imply sufficient mineral liberation. Conventional grinding techniques are frequently inadequate for liberating finely disseminated silver-bearing phases from inert gangue matrices, particularly in refractory systems.\u003c/p\u003e \u003cp\u003eMechanical activation, a high-energy grinding approach, has emerged as an effective pretreatment method for such materials. In addition to particle size reduction, mechanical activation induces lattice distortion, defect generation, formation of new reactive surfaces, and partial amorphization, thereby enhancing mineral liberation and reagent accessibility [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These mechanochemical effects have been shown to significantly improve the leaching behavior of refractory mineral systems by reducing diffusion barriers and increasing surface reactivity.\u003c/p\u003e \u003cp\u003eGrowing environmental concerns associated with cyanide use have further stimulated research into alternative, more environmentally benign lixiviants for precious metal recovery. Among these, thiourea (CS(NH₂)₂) has received considerable attention due to its high selectivity toward silver and gold, its effectiveness under acidic conditions, and its relatively rapid dissolution kinetics compared to cyanide systems (Calla-Choque and Lapidus, 2021, 2020; Tian et al., 2024). In acidic thiourea systems, silver dissolution proceeds through complexation reactions, forming stable silver\u0026ndash;thiourea complexes, as illustrated by Eq.\u0026nbsp;(1) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eAg (s) + 2CS(NH₂)₂ (aq)\u0026thinsp;+\u0026thinsp;Fe\u0026sup3;⁺ (aq) \u0026rarr; [Ag(CS(NH₂)₂)₂]⁺ (aq)\u0026thinsp;+\u0026thinsp;Fe\u0026sup2;⁺ (aq) (1)\u003c/p\u003e \u003cp\u003eThe presence of an oxidizing agent such as Fe\u0026sup3;⁺ plays a critical role by facilitating electron transfer and sustaining silver oxidation through a reversible Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox cycle, as represented by Eq.\u0026nbsp;(2) (Jing-ying et al., 2012; Calla-Choque and Lapidus, 2020):\u003c/p\u003e\n\u003ch3\u003e2Fe²⁺ + ½O₂ + 2H⁺ → 2Fe³⁺ + H₂O (2)\u003c/h3\u003e\n\u003cp\u003eAlthough thiourea leaching offers important advantages, its effectiveness is sensitive to operating conditions. Elevated temperatures, excessive oxidant concentrations, and highly acidic environments may accelerate thiourea degradation, leading to increased reagent consumption and formation of undesirable by-products [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Pretreatment strategies that enable efficient leaching at moderate temperatures and shorter reaction times are therefore essential to improve process viability.\u003c/p\u003e \u003cp\u003eRecent studies have shown that the combined application of mechanical activation and thiourea leaching can significantly enhance silver recovery from refractory materials by promoting structural disorder and improving diffusion pathways [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, most previous investigations have focused on laboratory-prepared ores or electronic waste and have often addressed either structural modification or leaching performance in isolation. Systematic studies linking mechanical activation\u0026ndash;induced structural changes to leaching behavior and apparent kinetic response in \u003cem\u003eindustrial refractory silver tailings\u003c/em\u003e remain limited.\u003c/p\u003e \u003cp\u003eWithin this framework, the present study investigates the effect of mechanical activation on the thiourea leaching behavior of refractory silver tailings from the K\u0026uuml;tahya Eti Silver Plant. Mechanical activation was systematically applied for 0, 30, 60, and 90 min to elucidate the progressive evolution of structural disorder and its influence on silver dissolution behavior, rather than limiting the analysis to non-activated and fully activated states. By integrating particle size analysis, BET surface area measurements, microstructural characterization (SEM/XRD), leaching performance evaluation, and kinetic analysis, this study establishes a physically consistent relationship between mechanically induced structural modifications and silver dissolution behavior. The applied kinetic expressions are interpreted as phenomenological descriptors, supported by microstructural observations and activation energy analysis, rather than as idealized mechanistic models, thereby providing a realistic and comprehensive framework for understanding thiourea leaching of mechanically activated refractory silver tailings.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e schematically illustrates the overall experimental workflow adopted in this study. The experimental procedure consisted of sequential stages including material characterization, mechanical activation, thiourea leaching, and kinetic evaluation. The mineralogical composition, surface morphology, and silver content of the samples were determined using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM\u0026ndash;EDX), and atomic absorption spectroscopy (AAS), respectively. Mechanical activation was carried out by planetary ball milling for different durations (0, 30, 60, and 90 min), resulting in significant structural modifications such as particle size reduction, defect generation, and partial amorphization. Subsequently, the mechanically activated samples were subjected to thiourea leaching in the presence of Fe\u0026sup3;⁺ as an oxidizing agent under controlled experimental conditions at temperatures ranging from 25 to 75\u0026deg;C. Silver dissolution was monitored as a function of time, and leaching kinetics were evaluated using heterogeneous kinetic expressions in combination with Arrhenius analysis to assess apparent rate trends and activation energies.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eThe refractory silver-bearing material used in this study was obtained from the tailings disposal area of the K\u0026uuml;tahya Eti Silver Plant (Turkey). These tailings originate from historical beneficiation and metallurgical processing operations and have previously been reported to exhibit low silver solubility under conventional leaching conditions due to mineralogical encapsulation and diffusion limitations [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents the particle size distribution (PSD) curves of the as-received tailings and mechanically activated samples. Although tailings from beneficiation processes are generally considered fine-grained, the as-received material exhibits a median particle size (d₅₀) of approximately 28.3 \u0026micro;m, indicating that a significant fraction of silver-bearing phases remains locked within gangue minerals. Therefore, further size reduction is required not merely to decrease particle size, but to enhance mineral liberation, increase surface defect density, and improve reagent accessibility during leaching.\u003c/p\u003e \u003cp\u003ePrior to mechanical activation and leaching experiments, the tailings were homogenized and dried at ambient conditions. Representative subsamples were prepared using coning and quartering to ensure compositional consistency throughout the experimental program. All leaching experiments were performed using analytical-grade reagents, including thiourea (\u0026ge;\u0026thinsp;99.0%), sulfuric acid (\u0026ge;\u0026thinsp;95.0%), and ferric sulfate (\u0026ge;\u0026thinsp;97.0%). Deionized water was used in the preparation of all solutions to eliminate the influence of extraneous ions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Mechanical Activation\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Equipment and operating conditions\u003c/h2\u003e \u003cp\u003eMechanical activation of the silver-bearing tailings was performed using a planetary ball mill (Fritsch Pulverisette 5) equipped with hardened steel vials and 10 mm diameter steel balls. Dry milling experiments were conducted at a rotational speed of 400 rpm, while maintaining a constant ball-to-powder mass ratio of 10:1. Mechanical activation was applied for different durations (0, 30, 60, and 90 min) to systematically investigate its influence on the structural and surface properties of the material.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 Characterization of mechanically activated samples\u003c/h2\u003e \u003cp\u003eParticle size distributions of the mechanically activated samples were measured by laser diffraction using a Mastersizer 2000 analyzer (Malvern Instruments). X-ray diffraction (XRD) patterns of both raw and mechanically activated samples were collected using a PANalytical Empyrean diffractometer with Cu Kα radiation (λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;), operated at 45 kV and 40 mA. Data were acquired over a 2θ range of 10\u0026deg;\u0026ndash;80\u0026deg; with a step size of 0.02\u0026deg; and a scanning rate of 2\u0026deg;\u0026middot;min⁻\u0026sup1;. Phase identification and semi-quantitative analyses were carried out using HighScore Plus software (Malvern Panalytical) with reference to the ICDD PDF-4\u0026thinsp;+\u0026thinsp;database.\u003c/p\u003e \u003cp\u003eThe specific surface area of the samples before and after leaching was determined by N₂ adsorption\u0026ndash;desorption measurements at \u0026minus;\u0026thinsp;196\u0026deg;C using a Quantachrome\u0026reg; ASiQwin\u0026trade; surface area and porosity analyzer. Prior to analysis, the samples were degassed under vacuum at 120\u0026deg;C for 12 h to remove physically adsorbed moisture and gases. Specific surface area values were calculated using the Brunauer\u0026ndash;Emmett\u0026ndash;Teller (BET) method within the relative pressure (P/P₀) range of 0.05\u0026ndash;0.30.\u003c/p\u003e \u003cp\u003eMorphological and surface features of the samples were examined using scanning electron microscopy (SEM, Supra 40 VP, Zeiss). Fourier-transform infrared (FTIR) spectra of non-activated and mechanically activated samples were recorded using a PerkinElmer Spectrum 100 spectrometer over the wavenumber range of 4000\u0026ndash;400 cm⁻\u0026sup1;.\u003c/p\u003e \u003cp\u003eThe degree of amorphization induced by mechanical activation (0, 30, 60, and 90 min) was quantified based on XRD data following the method proposed by Bal\u0026aacute;ž, (2003). XRD patterns used for this calculation were collected over a 2θ range of 10\u0026deg;\u0026ndash;80\u0026deg; at a scanning rate of 10\u0026deg;\u0026middot;min⁻\u0026sup1;. The amorphization degree (A) was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e3\u003c/span\u003e), considering the average intensities of the three most intense diffraction peaks:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{cccc}\u0026amp;\\:A=\\left(1\u0026minus;\\frac{{B}_{0}{I}_{x}}{{B}_{x}{I}_{0}}\\right)\\times\\:100\u0026amp;\\:\u0026amp;\\:\\end{array}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{I}_{0}\\)\u003c/span\u003e\u003c/span\u003eand \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{B}_{0}\\)\u003c/span\u003e\u003c/span\u003erepresent the integral intensity and background intensity of the non-activated sample, respectively, while \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{I}_{x}\\)\u003c/span\u003e\u003c/span\u003eand \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{B}_{x}\\)\u003c/span\u003e\u003c/span\u003ecorrespond to the mechanically activated samples.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Thiourea Leaching\u003c/h2\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Leaching Procedure\u003c/h2\u003e \u003cp\u003eThiourea leaching experiments were conducted under atmospheric pressure using a thermostatically controlled magnetic stirrer equipped with a heating plate. Leaching solutions were prepared with deionized water containing 0.5 M thiourea, and the pH was adjusted to 1.5 using sulfuric acid. For each experiment, 5 g of ore sample was contacted with 100 mL of leaching solution in a 250 mL glass reactor, followed by the addition of 20 mL of 0.01 M ferric sulfate (Fe₂(SO₄)₃) as an oxidizing agent.\u003c/p\u003e \u003cp\u003eFerric sulfate was selected due to its well-established effectiveness in thiourea-based silver leaching systems and its ability to sustain a reversible Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox cycle, thereby promoting continuous silver oxidation under acidic conditions. Previous studies have demonstrated that Fe\u0026sup3;⁺ acts as an efficient electron acceptor, enhancing silver dissolution kinetics while remaining compatible with low-pH thiourea media [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIt is acknowledged that ferric ions may form coordination complexes with thiourea, such as FeSO₄(CS(NH₂)₂)⁺, which can increase thiourea consumption under unfavorable conditions [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. To mitigate this effect, the ferric ion concentration was deliberately limited to 0.01 M, the solution pH was maintained at 1.5, and temperatures above 50\u0026deg;C were excluded from kinetic evaluation. These conditions are consistent with literature reports indicating that controlled Fe\u0026sup3;⁺ addition can enhance silver dissolution without inducing excessive thiourea degradation [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAlthough alternative oxidants such as hydrogen peroxide and manganese dioxide have been reported, these reagents often exhibit rapid and less controllable oxidation behavior, which may accelerate thiourea decomposition and complicate kinetic interpretation [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. In contrast, the Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox couple provides improved process stability, cost-effectiveness, and operational control. Under the selected experimental conditions, no abnormal loss of leaching efficiency or evidence of thiourea instability was observed.\u003c/p\u003e \u003cp\u003eLeaching experiments were performed at temperatures of 25, 30, 40, 50, and 75\u0026deg;C for leaching times of 15, 30, 60, 120, and 240 min. To assess hydrodynamic effects, stirring speeds of 300, 450, and 600 rpm were investigated. Visual inspection confirmed complete suspension of solid particles at 450 and 600 rpm, whereas partial settling was occasionally observed at 300 rpm, particularly at longer leaching times. Accordingly, a stirring speed of 600 rpm was selected for kinetic modeling and activation energy analysis to minimize external mass-transfer resistance.\u003c/p\u003e \u003cp\u003eTo prevent solvent loss during prolonged experiments and at elevated temperatures, all leaching tests were conducted in covered glass reactors. The solution volume was periodically monitored, and no measurable evaporation-related losses were detected, ensuring constant liquid-to-solid ratios and stable reagent concentrations throughout the leaching process.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Analytical methods\u003c/h2\u003e \u003cp\u003eAfter completion of the leaching experiments, the slurry was filtered to separate the solid residue from the leachate. Silver concentrations in the filtrate were determined by atomic absorption spectroscopy (AAS) using a PerkinElmer Analyst 700 instrument. Each sample was analyzed in triplicate to ensure analytical accuracy, and the reported values represent the average of three measurements. The leaching efficiency (X, %) was calculated according to Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e4\u003c/span\u003e):\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{cccc}\u0026amp;\\:X\\left(\\text{%}\\right)=\\frac{{C}_{t}\\cdot\\:V}{{m}_{Ag}}\\times\\:100\u0026amp;\\:\u0026amp;\\:\\end{array}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{t}\\)\u003c/span\u003e\u003c/span\u003eis the silver concentration in the leachate at time \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e, \u003cem\u003eV\u003c/em\u003e is the solution volume, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{m}_{Ag}\\)\u003c/span\u003e\u003c/span\u003eis the total silver content in the initial sample.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e2.3.3 Kinetic modeling and Arrhenius analysis\u003c/h2\u003e \u003cp\u003eThe leaching kinetics of silver were evaluated using heterogeneous fluid\u0026ndash;solid reaction expressions commonly applied in hydrometallurgical studies to describe conversion\u0026ndash;time behavior. In the present work, these kinetic expressions are treated as limiting case and phenomenological descriptors, rather than as strict mechanistic models, in recognition of the non-ideal particle geometry, evolving particle size distribution, and agglomeration effects associated with mechanically activated fine materials.\u003c/p\u003e \u003cp\u003eWithin this framework, kinetic expressions derived from the shrinking-core concept were examined to represent three classical limiting cases: (i) surface chemical reaction control, (ii) film diffusion control, and (iii) product-layer (ash-layer) diffusion control. In addition, the Avrami model was included as an empirical reference to assess the robustness of regression behavior in systems exhibiting structural evolution and non-uniform reaction geometries. Similar comparative approaches have been widely adopted in leaching studies involving fine, porous, or mechanically activated particles [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e], [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe kinetic expressions employed in this study are given as follows:\u003c/p\u003e \u003cp\u003eSurface chemical reaction control: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:1-(1-X{)}^{1/3}=kt\\)\u003c/span\u003e\u003c/span\u003e\u0026emsp; (5)\u003c/p\u003e \u003cp\u003eProduct-layer (ash-layer) diffusion control: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:1-\\frac{2}{3}X-(1-X{)}^{2/3}=kt\\)\u003c/span\u003e\u003c/span\u003e\u0026emsp; (6)\u003c/p\u003e \u003cp\u003eFilm diffusion control: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:X=kt\\)\u003c/span\u003e\u003c/span\u003e\u0026emsp; (7)\u003c/p\u003e \u003cp\u003eAvrami model: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{l}\\text{n}[-\\text{l}\\text{n}(1-X\\left)\\right]=\\text{l}\\text{n}k+n\\text{l}\\text{n}t\\)\u003c/span\u003e\u003c/span\u003e\u0026emsp; (8)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eX\u003c/em\u003e is the fractional silver extraction at time \u003cem\u003et\u003c/em\u003e, \u003cem\u003ek\u003c/em\u003e is the apparent rate constant, and \u003cem\u003en\u003c/em\u003e is the Avrami exponent.\u003c/p\u003e \u003cp\u003eIt is emphasized that classical shrinking-core formulations are rigorously derived for systems involving particles of constant size and idealized geometries. However, in mechanically activated systems, particle size continuously evolves, agglomeration may occur during wet processing, and porous structures develop, which limits the strict applicability of ideal SCM assumptions. Under such conditions, kinetic models are commonly applied as practical descriptors of observable rate behavior, and deviations from linearity are expected [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe temperature dependence of the apparent rate constants was evaluated using the Arrhenius equation:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:k=A\\text{e}\\text{x}\\text{p}(-{E}_{a}/RT)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e9\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eor in linearized form:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{l}\\text{n}k=\\text{l}\\text{n}A-\\frac{{E}_{a}}{RT}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e10\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eEₐ\u003c/em\u003e is the apparent activation energy (kJ\u0026middot;mol⁻\u0026sup1;), \u003cem\u003eR\u003c/em\u003e is the universal gas constant, and \u003cem\u003eT\u003c/em\u003e is the absolute temperature (K).\u003c/p\u003e \u003cp\u003eAll kinetic expressions and their mathematical forms were verified against standard formulations reported in authoritative chemical reaction engineering and hydrometallurgical literature, including Levenspiel (2013) and recent critical analyses of kinetic model applicability in leaching systems involving fine and mechanically activated particles [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results and Discussions","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Chemical Composition and Mineralogical Characteristics of the Refractory Silver Tailings\u003c/h2\u003e \u003cp\u003eThe silver-bearing tailings investigated in this study were collected from the tailings disposal area of the K\u0026uuml;tahya Eti Silver Plant and exhibit a complex chemical and mineralogical composition characteristic of refractory silver materials. As summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the tailings are dominated by SiO₂, Al₂O₃, Fe₂O₃, CaO, and Ba-bearing phases, with an average silver content of approximately 125 ppm. This composition closely agrees with previous investigations conducted on tailings from the same processing facility [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], confirming the representativeness of the material used in the present study.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical analysis of silver sample\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComponent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eElement\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(ppm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e49.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e125\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eBa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.73%\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCu\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e178.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e26.45\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMgO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e881.19\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAs\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePbO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZnO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLOI\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAs shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the high SiO₂ and Ba contents confirm the dominance of chemically inert gangue phases, which strongly contribute to the refractory behavior of the investigated tailings. In hydrometallurgical processing, silver-bearing materials are classified as refractory when economically acceptable recoveries cannot be achieved under conventional leaching conditions due to mineralogical encapsulation, surface passivation, or diffusion-related limitations. The chemical and mineralogical features observed for the investigated tailings are fully consistent with this definition.\u003c/p\u003e \u003cp\u003eX-ray diffraction analysis of the raw tailings (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) indicates that quartz (SiO₂) and barite (BaSO₄) are the dominant crystalline phases, accompanied by dolomite [CaMg(CO₃)₂], calcium carbonate (CaCO₃), hematite (Fe₂O₃), muscovite, and microcline. This mineral assemblage reflects a silicate- and sulfate-rich gangue matrix that is highly resistant to chemical attack. No discrete silver-bearing mineral phases (e.g., argentite or native silver) were detected by XRD, which is consistent with the low silver concentration and its finely disseminated nature. The dominance of quartz and barite phases observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e explains the limited reactivity of the raw tailings under conventional leaching conditions.\u003c/p\u003e \u003cp\u003eSEM\u0026ndash;EDS analyses (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e) further support these findings, confirming the predominance of Si-, Al-, Ba-, Ca-, and Fe-bearing phases. Due to its trace-level concentration and dispersed occurrence, silver could not be identified as a distinct phase by SEM\u0026ndash;EDS. Previous studies on tailings from the same site have demonstrated that silver is structurally associated with barite and aluminosilicate matrices rather than occurring as liberated silver minerals [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Such encapsulation is a defining characteristic of refractory silver materials and explains the poor leaching performance observed under conventional processing conditions.\u003c/p\u003e \u003cp\u003eThe persistence of chemically inert gangue minerals and the absence of discrete silver phases provide a clear justification for the use of mechanical activation as a pretreatment strategy in this study. Mechanical activation is expected to enhance microstructural liberation by inducing lattice distortion, microcrack formation, and partial amorphization, thereby disrupting diffusion barriers imposed by the gangue matrix and improving silver accessibility during subsequent thiourea leaching.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition to silver, the chemical analysis reveals appreciable concentrations of iron and copper (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Iron plays a dual role in thiourea leaching systems: Fe\u0026sup3;⁺ species can promote silver dissolution through redox cycling, whereas excessive iron may increase thiourea consumption via complex formation. In the present work, potential adverse effects were mitigated by controlling ferric ion concentration, solution pH, and temperature, ensuring stable leaching conditions.\u003c/p\u003e \u003cp\u003eCopper is known to form stable complexes with thiourea and may undergo parallel dissolution. However, under the selected experimental conditions, copper dissolution remained limited and did not dominate the leaching behavior. Accordingly, kinetic analyses in this study focus exclusively on silver dissolution, with the obtained apparent kinetic parameters reflecting the combined influence of mineralogical association and transport-related constraints rather than the independent behavior of accompanying metals.\u003c/p\u003e \u003cp\u003eThe presence of alkaline earth oxides and carbonate-bearing phases, including CaO, MgO, and dolomite, is also noteworthy. These phases can influence thiourea-based leaching systems by buffering acidic solutions and increasing reagent consumption through side reactions (Celep et al., 2015; John and House, 2006; Liu et al., 2019). In this study, such effects were minimized by maintaining the leaching pH at 1.5 and carefully controlling oxidant dosage. Nevertheless, the persistence of carbonate and silicate phases contributes to diffusion-related limitations by forming chemically inert matrices around silver-bearing species. Similar behavior has been widely reported for refractory tailings and mechanically activated systems [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOverall, the chemical and mineralogical characteristics of the investigated tailings confirm their refractory nature and provide a consistent framework for interpreting the effects of mechanical activation on silver liberation, leaching performance, and apparent kinetic behavior.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Effect of Mechanical Activation on Particle Properties\u003c/h2\u003e \u003cp\u003eLeaching efficiency in refractory silver systems is strongly governed by the accessibility of the lixiviant to reactive mineral surfaces. In thiourea-based leaching, dissolution performance is jointly controlled by particle size, surface reactivity, and transport pathways. Mechanical activation enhances these factors by inducing particle fragmentation, lattice distortion, defect generation, and partial loss of crystalline order, thereby increasing surface reactivity and reagent accessibility [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e], [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the present study, mechanical activation times of 0, 30, 60, and 90 min were selected to represent successive stages of mechanochemical transformation and to identify an optimal activation window. The effects of activation were evaluated through particle size distribution, BET specific surface area, degree of amorphization (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), and microstructural evolution observed by SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe results indicate three distinct activation regimes. During the initial stage (0\u0026ndash;30 min), rapid particle fragmentation occurred, with the median particle size decreasing from approximately 27 \u0026micro;m to about 5 \u0026micro;m, accompanied by an increase in amorphization to ~\u0026thinsp;17%. This behavior reflects intensive defect generation, and microstructural disorder induced by high-energy milling and marks the onset of surface activation effects that begin to influence leaching behavior [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e], [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt 60 min of activation, the system reached a highly activated structural state, characterized by a maximum specific surface area (~\u0026thinsp;2.3 m\u0026sup2;\u0026middot;g⁻\u0026sup1;) and an amorphization degree of approximately 27%. SEM and XRD analyses (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) reveal the disappearance of relatively unstable carbonate phases, such as dolomite and CaCO₃, together with the development of amorphous regions within the mineral matrix. Such transformations are commonly associated with enhanced surface reactivity and improved leaching response in mechanically activated mineral systems [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eProlonged milling beyond 60 min resulted in diminishing structural gains. At 90 min, the amorphization degree approached a plateau (~\u0026thinsp;31%), while a slight decrease in BET surface area was observed. This reduction is attributed to particle agglomeration and partial re-welding effects induced by extended mechanical activation. SEM micrographs of the 90 min activated sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) clearly show clustered particle assemblies and reduced dispersion compared to samples activated for shorter durations, supporting the agglomeration-based interpretation. Similar agglomeration-driven surface area reductions at prolonged milling times have been widely reported in mechanochemically activated systems (Bal\u0026aacute;ž, 2000; Liu et al., 2025).\u003c/p\u003e \u003cp\u003eDespite the onset of agglomeration, the 90 min activation condition was selected for subsequent leaching and kinetic evaluations, as it provided the highest enhancement in silver dissolution under the investigated conditions. This choice reflects a balance between the benefits of extensive structural activation and the emergence of agglomeration-related limitations.\u003c/p\u003e \u003cp\u003eAlthough mine tailings generated from beneficiation processes are often characterized by relatively fine particle sizes, this does not necessarily imply sufficient mineral liberation. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the as-received silver tailings exhibit a median particle size of approximately 28.3 \u0026micro;m, indicating that a substantial fraction of silver-bearing phases remains encapsulated within the gangue matrix. Mechanical activation shifts the particle size distribution toward the ultrafine range, with median sizes decreasing to below 10 \u0026micro;m after 60\u0026ndash;90 min of milling. This additional size reduction is therefore essential for enhancing mineral liberation, increasing surface defect density, and improving reagent accessibility, all of which directly contribute to improved leaching performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Microstructural and Phase Changes\u003c/h2\u003e \u003cp\u003eMechanical activation induces lattice distortion, defect generation, and partial loss of crystalline order, which collectively enhances reagent accessibility by creating new reactive sites and disrupting diffusion barriers. SEM observations (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) demonstrate that increasing activation time progressively modifies particle morphology, resulting in reduced particle size, increased surface roughness, and a more homogeneous particle distribution. These microstructural changes are consistent with enhanced surface activation and improved leaching response.\u003c/p\u003e \u003cp\u003eSEM\u0026ndash;EDS analyses were conducted to complement XRD results and to clarify the microstructural association between silver-bearing phases and the surrounding gangue minerals. The analyses indicate that silver is not present as a discrete, liberated mineral phase but occurs as a finely dispersed component occluded within SiO₂-rich and barite-dominated matrices. This microstructural configuration confirms the mineralogical locking inferred from XRD analysis and explains the limited silver accessibility observed under conventional leaching conditions.\u003c/p\u003e \u003cp\u003eEncapsulation of silver-bearing phases within chemically inert gangue minerals such as quartz and barite imposes diffusion constraints and restricts direct lixiviant\u0026ndash;mineral contact. This occlusion is a defining characteristic of refractory tailings and plays a dominant role in controlling leaching efficiency. Mechanical activation partially disrupts this locked microstructure by inducing lattice defects, microcracks, and localized disorder, thereby improving reagent penetration and facilitating silver dissolution.\u003c/p\u003e \u003cp\u003eThe evolution of particle morphology with activation time is clearly illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The untreated sample (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) exhibits irregular particle shapes and a broad size distribution. After 30 min of activation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), pronounced particle fragmentation and surface defect formation are observed, reflecting repeated fracture and plastic deformation during milling. At 60 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec), particles appear smaller and more uniformly distributed, corresponding to a highly activated structural state. Prolonged activation to 90 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed) results in the formation of agglomerated particle clusters, attributed to partial re-welding and interparticle adhesion under sustained mechanical stress [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eXRD patterns of the mechanically activated samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) reveal progressive phase transformations with increasing milling time. Unstable carbonate phases, particularly dolomite and CaCO₃, gradually disappear with prolonged activation, whereas quartz and barite remain largely unaffected, reflecting their chemical and structural inertness. Peak broadening and intensity reduction of the remaining phases confirm increasing amorphization, consistent with the BET and particle size evolution discussed in Section \u003cspan refid=\"Sec14\" class=\"InternalRef\"\u003e3.2\u003c/span\u003e and with previous reports on mechanically activated mineral systems [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFTIR spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) further corroborates these structural modifications. In the non-activated sample, absorption bands at approximately 1093, 798, and 459 cm⁻\u0026sup1; are assigned to Si\u0026ndash;O\u0026ndash;Si, sulfate (S\u0026ndash;O), and Al\u0026ndash;O vibrational modes, respectively. With increasing activation time, the progressive reduction in band intensity and peak broadening indicate increasing structural disorder and amorphous domain formation [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. After 90 min of activation, the appearance of bands in the 1500\u0026ndash;2000 cm⁻\u0026sup1; region is associated with Mg\u0026ndash;CO vibrations related to dolomite transformation, while carbonate bands near 1450\u0026ndash;1425 cm⁻\u0026sup1; and ~\u0026thinsp;875 cm⁻\u0026sup1; weaken markedly, confirming CaCO₃ decomposition [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. In contrast, Ba\u0026ndash;SO₄ vibrational bands in the 610\u0026ndash;1100 cm⁻\u0026sup1; range remain largely unchanged, confirming the inert behavior of barite during mechanical activation (Theng, 2024).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSEM\u0026ndash;EDS analyses of solid residues obtained after thiourea leaching under kinetic evaluation conditions (50\u0026deg;C, S/L\u0026thinsp;=\u0026thinsp;1/10, 600 rpm) provide further insight into the dissolution mechanism. Post-leaching residues exhibit pronounced particle agglomeration, irregular morphologies, and porous surface structures. Elemental mapping reveals heterogeneous surface compositions dominated by residual Si- and Ba-rich regions corresponding to quartz and barite phases, which persist after leaching and contribute to the development of diffusion-related barriers as leaching progresses.\u003c/p\u003e \u003cp\u003eOverall, the combined SEM, EDS, XRD, and FTIR results establish a clear link between microstructural evolution, phase composition, and leaching behavior. Silver dissolution proceeds within evolving porous and agglomerated particle assemblies rather than through idealized, uniformly shrinking particles, highlighting the importance of transport limitations in the later stages of leaching and providing a microstructural basis for the kinetic interpretation presented in Section \u003cspan refid=\"Sec21\" class=\"InternalRef\"\u003e3.5\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Silver Leaching Performance and Effects of Operating Parameters\u003c/h2\u003e \u003cp\u003eThis section examines the influence of key operating parameters on silver dissolution in thiourea media to establish robust experimental conditions for subsequent kinetic analysis. Mechanical activation time, temperature, solid-to-liquid (S/L) ratio, and stirring speed were systematically evaluated to clarify their individual and combined effects on leaching performance and to identify conditions that minimize external mass-transfer limitations. Based on these results, kinetic modeling was performed under optimized hydrodynamic and reagent-availability conditions.\u003c/p\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1. Effect of Mechanical Activation Time\u003c/h2\u003e \u003cp\u003eThe effect of mechanical activation time on silver dissolution was evaluated at 50\u0026deg;C, S/L\u0026thinsp;=\u0026thinsp;1/10, and 600 rpm (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). Mechanical activation significantly enhanced both the dissolution rate and the final silver recovery. Under identical leaching conditions, the non-activated sample reached approximately 68% recovery after 240 min, whereas the sample activated for 90 min achieved about 96% recovery.\u003c/p\u003e \u003cp\u003eIncreasing the activation time from 0 to 60 min resulted in a pronounced acceleration of dissolution kinetics, particularly during the early leaching stage (\u0026le;\u0026thinsp;60 min). This enhancement is attributed to particle size reduction, increased surface defect density, and higher surface reactivity, which collectively improve lixiviant accessibility to silver-bearing phases [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. Extending the activation time to 90 min further increased silver recovery; however, the incremental gain diminished, indicating the onset of saturation effects. At prolonged milling durations, particle agglomeration and surface reorganization may partially counteract continued defect generation, as reported for other mechanically activated mineral systems [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e], [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e], [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThese trends are consistent with previous studies demonstrating that mechanical activation enhances leaching by inducing structural disorder or partial amorphization, increasing the density of reactive surface sites, and shortening effective diffusion pathways within the ore matrix [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Overall, mechanical activation predominantly accelerates early-stage dissolution, while the gradual reduction in dissolution rate at longer times suggests an increasing contribution of transport-related limitations. On this basis, an activation time of 90 min was selected for subsequent leaching performance evaluation and kinetic analysis, as it provided the highest overall recovery despite diminishing incremental improvements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2. Effect of Temperature\u003c/h2\u003e \u003cp\u003eThe influence of temperature on silver dissolution was investigated over the range of 25\u0026ndash;75\u0026deg;C at S/L\u0026thinsp;=\u0026thinsp;1/10 and 600 rpm (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Increasing the temperature from 25\u0026deg;C to 50\u0026deg;C resulted in a marked enhancement of dissolution rates and final recoveries for both non-activated and mechanically activated samples, indicating thermally activated leaching behavior. At all investigated temperatures, the mechanically activated sample consistently exhibited higher dissolution rates and recoveries than the non-activated material.\u003c/p\u003e \u003cp\u003eA decline in silver recovery was observed at 75\u0026deg;C, despite the thermally activated nature of the process. This behavior is attributed to the thermal instability of thiourea in acidic media at elevated temperatures, which leads to reagent degradation and reduced effective lixiviant concentration [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. To avoid reagent-controlled artifacts and ensure reliable kinetic interpretation, 50\u0026deg;C was selected as the optimal temperature for kinetic modeling and Arrhenius analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.4.3. Effect of Solid-to-Liquid Ratio\u003c/h2\u003e \u003cp\u003eThe effect of the solid-to-liquid ratio on silver dissolution was evaluated at 50\u0026deg;C and 600 rpm (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). Lower S/L ratios (i.e., higher solution volumes) resulted in higher dissolution rates and final recoveries for both sample types. This behavior reflects improved reagent availability, reduced local depletion of thiourea and oxidizing species near particle surfaces, and enhanced mass transport within the slurry.\u003c/p\u003e \u003cp\u003eThe mechanically activated sample consistently outperformed the non-activated material across the investigated S/L ratios, achieving approximately 96% recovery at an S/L ratio of 1/10. Reduced dissolution efficiency at higher solid loadings is consistent with mass-transfer limitations and reagent consumption effects widely reported in precious metal leaching systems [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. Accordingly, an S/L ratio of 1/10 was selected for kinetic evaluation to ensure sufficient reagent availability and stable leaching conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003e3.4.4. Effect of Stirring Speed\u003c/h2\u003e \u003cp\u003eThe influence of stirring speed on silver dissolution was investigated at 50\u0026deg;C and an S/L ratio of 1/10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). Increasing the agitation speed from 300 to 600 rpm led to higher dissolution rates and final recoveries for both samples. This enhancement is attributed to a reduction in boundary-layer thickness and improved transport of thiourea and oxidant species to the particle surfaces [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAt a stirring speed of 600 rpm, dissolution curves approached a plateau, indicating that external mass-transfer resistance was substantially minimized and that intrinsic reaction steps and intra-particle transport processes became more influential. Consequently, a stirring speed of 600 rpm was selected for kinetic modeling and activation energy analysis to ensure that derived kinetic parameters reflect intrinsic and particle-scale phenomena rather than external hydrodynamic limitations.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Kinetic Modeling and Activation Energy Analysis\u003c/h2\u003e \u003cp\u003eKinetic analysis was performed to quantify the effect of mechanical activation on silver dissolution in thiourea media and to interpret the conversion\u0026ndash;time behavior under conditions of minimized external mass-transfer resistance. For mechanically activated fine solids, apparent kinetic parameters commonly reflect the combined contributions of interfacial reaction, intra-aggregate transport, and evolving microstructural features rather than a single well-defined rate-controlling step. Accordingly, the present analysis is framed within a phenomenological interpretation using limiting-case kinetic expressions [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. This approach is consistent with recent hydrometallurgical studies in which ideal shrinking-core assumptions\u0026mdash;such as constant particle size, regular geometry, and uniform product-layer growth\u0026mdash;are not strictly satisfied due to broad particle-size distributions and particle agglomeration during leaching [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo suppress film-diffusion effects, kinetic tests were conducted at 600 rpm and S/L\u0026thinsp;=\u0026thinsp;1/10, based on the agitation study in Section \u003cspan refid=\"Sec16\" class=\"InternalRef\"\u003e3.4\u003c/span\u003e showing improved dissolution with increasing stirring speed. Because thiourea is susceptible to degradation in acidic media at elevated temperatures, experiments at 75\u0026deg;C were excluded from kinetic modeling; consequently, kinetic evaluation and Arrhenius analysis were restricted to 25\u0026ndash;50\u0026deg;C [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eConversion\u0026ndash;time data were analyzed using the integral-form expressions introduced in Section \u003cspan refid=\"Sec11\" class=\"InternalRef\"\u003e2.3.3\u003c/span\u003e (Equations (5)\u0026ndash;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e10\u003c/span\u003e), including shrinking-core-based limiting cases (surface reaction, film diffusion, and product-layer/ash-layer diffusion) and the Avrami formulation as an empirical comparator [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. These expressions were applied for comparative description of the data rather than as mechanistic proof, and regression quality was evaluated by linear fitting (R\u0026sup2;). The resulting R\u0026sup2; values for all mechanical activation durations (0, 30, 60, and 90 min) samples are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Overall, diffusion-type limiting-case expressions provided the most consistent statistical description across the investigated temperatures, whereas film-diffusion fits were weaker, in agreement with the high agitation intensity used to minimize external mass-transfer resistance.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of correlation coefficients (R\u0026sup2;) obtained from different kinetic expressions applied to silver leaching data for non-activated and mechanically activated samples (0, 30, 60, and 90 min) at different temperatures\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMechanical activation duration (min)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSurface chemical reaction\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eProduct-layer diffusion\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFilm diffusion\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAvrami\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.860\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.908\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.846\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.866\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.770\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.839\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.744\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.684\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.868\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.927\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.831\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.779\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.766\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.838\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.706\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.729\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.882\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.934\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.857\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.907\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.813\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.873\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.782\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.809\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.821\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.878\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.789\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.831\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.836\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.905\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.801\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.876\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.902\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.952\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.881\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.929\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.865\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.914\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.842\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.893\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.842\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.899\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.815\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.861\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.879\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.931\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.854\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.912\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.823\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.887\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.774\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.719\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.829\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.882\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.777\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.747\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.788\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.847\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.692\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.833\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.886\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003e0.903\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.724\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.952\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eImportantly, model preference was not based on R\u0026sup2; alone. The regression trends were interpreted together with characterization evidence. Post-leaching SEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e) show pronounced agglomeration, irregular morphologies, and porous surface structures, indicating that dissolution does not proceed via an ideal retreating interface expected for classical shrinking-core geometry. Consistently, BET measurements show a decrease in specific surface area after leaching for the mechanically activated sample, implying surface collapse/agglomeration and/or partial passivation during wet processing. In addition, XRD/FTIR results confirm the persistence of chemically inert phases (e.g., quartz and barite), indicating that diffusion-related resistance associated with refractory gangue is not eliminated by activation. Collectively, these observations support interpreting the leaching kinetics as diffusion-influenced transport through evolving porous/agglomerated particle assemblies, rather than diffusion through a uniform product layer on a single shrinking particle.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eApparent rate constants obtained from the most consistent expression were used for Arrhenius evaluation (Section \u003cspan refid=\"Sec11\" class=\"InternalRef\"\u003e2.3.3\u003c/span\u003e, Equations (\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e9\u003c/span\u003e)\u0026ndash;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e10\u003c/span\u003e). The ln k versus 1/T plots (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e) were linear over 25\u0026ndash;50\u0026deg;C for both samples, indicating Arrhenius-type behavior within this interval. The apparent activation energies decreased progressively from 46.1 kJ\u0026middot;mol⁻\u0026sup1; (0 min) to 42.99, 40.13, and 38.9 kJ\u0026middot;mol⁻\u0026sup1; for samples mechanically activated for 30, 60, and 90 min, respectively, indicating a gradual reduction in the energetic barrier with increasing mechanical activation. This decrease is consistent with mechanochemical defect generation and lattice distortion that reduce the energetic barrier and modify transport pathways, while diffusion-related contributions remain significant due to agglomeration and inert-phase persistence [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The magnitude of Ea falls within ranges commonly reported for thiourea leaching systems where diffusion and reaction contributions coexist [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, the combined regression results (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e3\u003c/span\u003e), Arrhenius analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e), and SEM/BET/XRD\u0026ndash;FTIR evidence (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e) demonstrate that mechanical activation increases apparent dissolution rates and lowers the apparent activation energy under the investigated conditions. Accordingly, kinetic expressions are used here as physically consistent, limiting-case descriptors that link conversion\u0026ndash;time behavior with observed microstructural evolution, without claiming a definitive single rate-controlling mechanism.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study demonstrates that mechanical activation is an effective pretreatment for enhancing thiourea leaching of refractory silver tailings characterized by strong mineralogical locking of silver-bearing phases within inert gangue matrices. Mechanical activation led to pronounced particle size reduction, defect generation, and partial amorphization, which collectively improved reagent accessibility and silver dissolution under acidic thiourea leaching conditions.\u003c/p\u003e \u003cp\u003eUnder optimized conditions (90 min mechanical activation, 50\u0026deg;C, S/L\u0026thinsp;=\u0026thinsp;1/10, 600 rpm), silver recovery increased from approximately 68% for the non-activated material to about 96% for the mechanically activated sample. Kinetic evaluation conducted under minimized external mass-transfer resistance revealed a systematic enhancement of the apparent rate constants with increasing mechanical activation time, accompanied by a progressive decrease in apparent activation energy from 46.1 kJ\u0026middot;mol⁻\u0026sup1; for the non-activated sample to 42.99, 40.13, and 38.9 kJ\u0026middot;mol⁻\u0026sup1; for samples mechanically activated for 30, 60, and 90 min, respectively. This gradual reduction in activation energy reflects the mechanically induced structural disorder and improved accessibility of reactive surfaces, and the resulting values are consistent with diffusion-influenced thiourea leaching systems reported in the literature.\u003c/p\u003e \u003cp\u003eMicrostructural and surface analyses provide essential context for interpreting kinetic behavior. Post-leaching SEM observations revealed agglomerated and porous particle assemblies rather than ideal shrinking particles, while BET measurements indicated a reduction in specific surface area after leaching, suggesting surface collapse and/or passivation effects. Together with the persistence of inert mineral phases identified by XRD, these findings indicate that diffusion-related transport through evolving porous and agglomerated structures contributes significantly to the observed leaching behavior.\u003c/p\u003e \u003cp\u003eAccordingly, the kinetic expressions employed in this study are interpreted as phenomenological, limiting-case descriptors rather than definitive mechanistic models. The combined use of SEM, BET, particle size analysis, and apparent activation energy trends provides a physically consistent framework for understanding silver dissolution from mechanically activated refractory tailings without invoking idealized particle-scale assumptions.\u003c/p\u003e \u003cp\u003eFrom a practical perspective, the results indicate that mechanical activation enables high silver recoveries at moderate temperatures and relatively short leaching times, which may reduce energy demand and partially offset the higher reagent costs typically associated with thiourea leaching. Although a detailed techno-economic assessment was beyond the scope of this work, the findings highlight the potential of mechanically assisted thiourea leaching as a viable cyanide-free alternative for processing refractory silver tailings where conventional cyanidation is ineffective or environmentally constrained.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe author(s) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003cp\u003eThe datasets used and/or analysed during the current study available from the corresponding author on reasonable request.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e \u003cp\u003eThe authors received no financial support for this research.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eThe article was written by a single author.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThe author(s) would like to thank the K\u0026uuml;tahya Eti Silver Plant for providing the tailings samples used in this study. 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A Generalized Kinetic Model for Hydrometallurgical Processes Chemical Product and Process Modeling A Generalized Kinetic Model for Hydrometallurgical Processes, (2007).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Mechanical Activation, Thiourea Leaching, Refractory silver tailings, Hydrometallurg, Leaching kinetics","lastPublishedDoi":"10.21203/rs.3.rs-9194073/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9194073/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRefractory silver tailings are both an environmental liability and a potential secondary resource for precious metals. However, strong mineralogical locking of silver-bearing phases within barite and aluminosilicate matrices severely limits silver recovery under conventional leaching conditions. In this study, mechanical activation was investigated as a pretreatment method to enhance the thiourea leaching performance of refractory silver tailings from the K\u0026uuml;tahya Eti Silver Plant (Turkey).\u003c/p\u003e \u003cp\u003eMechanical activation was carried out in a planetary ball mill for 0, 30, 60, and 90 min. The resulting changes in particle size, specific surface area, and structure were evaluated by particle size analysis, BET, XRD, and SEM. Mechanical activation caused marked particle fragmentation, increased surface area, and partial amorphization, indicating enhanced structural disorder and improved accessibility of silver-bearing phases.\u003c/p\u003e \u003cp\u003eThiourea leaching experiments were performed in acidic solution (pH 1.5) with Fe\u0026sup3;⁺ as the oxidizing agent. Mechanical activation significantly improved silver dissolution, increasing recovery from about 68% for the non-activated sample to about 96% for the sample activated for 90 min at 50\u0026deg;C.\u003c/p\u003e \u003cp\u003eKinetic analysis under conditions minimizing external mass-transfer resistance showed that apparent rate constants increased with activation time, while the apparent activation energy decreased from 46.1 to 38.9 kJ\u0026middot;mol⁻\u0026sup1;. Post-leaching SEM and BET results revealed agglomerated and porous particle structures, indicating that diffusion through evolving porous particle assemblies contributes to the observed kinetic behavior.\u003c/p\u003e \u003cp\u003eThis work directly links mechanochemically induced structural evolution with apparent leaching kinetics in industrial refractory silver tailings and highlights mechanically assisted thiourea leaching as a promising cyanide-free alternative for refractory silver-bearing wastes.\u003c/p\u003e","manuscriptTitle":"Enhancement of Thiourea Leaching Performance of Refractory Silver Tailings by Mechanical Activation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-30 10:11:03","doi":"10.21203/rs.3.rs-9194073/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-05-18T20:11:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"73163736738197842320655591925126738353","date":"2026-05-11T02:04:27+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-05T22:05:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"209808542767103680308860499608073284023","date":"2026-04-22T04:38:54+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-22T04:13:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-21T08:28:36+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-27T16:30:31+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-03-27T16:23:38+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ca846da0-7882-4de4-9aa7-aab46d47e42c","owner":[],"postedDate":"April 30th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-18T20:11:39+00:00","index":32,"fulltext":""},{"type":"reviewerAgreed","content":"73163736738197842320655591925126738353","date":"2026-05-11T02:04:27+00:00","index":29,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-05T22:05:00+00:00","index":22,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":66812146,"name":"Physical sciences/Chemistry"},{"id":66812147,"name":"Physical sciences/Engineering"},{"id":66812148,"name":"Earth and environmental sciences/Environmental sciences"},{"id":66812149,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-04-30T10:11:09+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-30 10:11:03","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9194073","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9194073","identity":"rs-9194073","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}