Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution

Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution | 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 Research Article Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution SANDEEP KUMAR JENA, Baijayantimala Mohanty, Ranil Bahalia, Sapan Kumar Kandi This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6930760/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Nov, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted 6 You are reading this latest preprint version Abstract The generation of large quantities of red mud, or bauxite residue (BR), during the Bayer process of alumina production from bauxite poses a significant global environmental challenge. Approximately 1 to 2.5 tons of waste BR is produced for every ton of alumina manufactured, depending on the chemistry and mineralogy of the feed bauxite sample. Effectively managing, safely storing, and finding productive uses for this waste are three major concerns for global aluminum producers. This study investigates the effectiveness of a hydrochloric acid solution impregnated with calcium (Ca²⁺) and magnesium (Mg²⁺) ions for the settling and dewatering of BR slurry. Dolomite was used as the source of Ca²⁺and Mg²⁺ions. Various experimental parameters including acid concentration, agitation time, temperature, dolomite weight percentage, settling time, and solid weight percentage in the slurry were optimized to evaluate the dewatering process. Initial comparison experiments using 1.5-12N HCl and 1.5-12N HCl with Ca²⁺ and Mg²⁺ revealed the effectiveness of the acid solution enhanced by these divalent cations. The solid percentage in the slurry significantly influenced the settling efficiency; as the solid weight percentage increased from 2.5% to 15%, the settling efficiency gradually declined from 98% to 92%. However, when the solid weight percentage increased beyond this range, the settling value sharply dropped to 40%. The experiments maintained 5 minutes of slurry agitation followed by 2 minutes of settling using 1.5N HCl with 5g of dolomite. Further studies at higher solid concentrations indicated that either extended settling times or greater volumes of the impregnated acid solution would be necessary to achieve settling efficiencies greater than 90%. Additionally, the effects of dolomite amount and acid concentration were also considered during these experiments. The physicochemical characteristics of the BR and the processed product were analyzed using techniques such as Particle Size Analysis, ICP-OES, Zeta-Potential, XRD, FTIR, and FESEM-EDX studies to support the experimental findings. The incorporation of divalent cations like Ca²⁺ and Mg²⁺ into the low-concentration HCl solution significantly enhances the settling characteristics of BR particles. The possible settling mechanism is discussed based on experimental evidence, characterization results, and relevant literature. Bauxite Residue Dolomite Dewatering Characterization Separation Mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Red mud, also known as bauxite residue (BR), is a reddish-brown by-product generated during the Bayer process for alumina production. This hydrometallurgical method is used worldwide to produce alumina. The process involves treating bauxite at high temperatures of approximately 170–180°C, followed by filtration to separate the waste, which is known as red mud. In 2023, global alumina production reached around 140 million tons, resulting in approximately 177 million tons of red mud as waste (USGS 2024). On average, producing one ton of alumina generates about 1 to 2.5 tons of red mud waste, with the exact amount depending primarily on the quality of the bauxite used (Hendrick 2022; Ding et al. 2024 ). In India, this high-volume, low-toxicity waste is classified as industrial waste rather than hazardous waste, allowing for potential resource recovery and reuse (CPCB 2024). The management of large amounts of alkaline waste is a significant global issue. The disposal of this waste primarily depends on local topography, state regulations, and the availability of land (Patil and Thorat, 2022). Alumina refineries can choose between directly pumping red mud slurry to a disposal area or processing the slurry through filtration. The two main methods for disposing of and storing red mud waste are dry stacking and wet stacking. Wet stacking is more costly, as it involves storing red mud slurry in large ponds or dams, where the slurry settles and dries naturally. However, this method poses environmental risks, including the potential for leakage. An example of such a disaster occurred in 2010 at the Ajka Alumina Plant in Hungary (Ruyters et al., 2011 ). In contrast, dry stacking is more economical and environmentally friendly. This method involves dewatering the red mud slurry to create a material with a high solid content, making it a safer option compared to wet stacking. Recent advancements in the stockpiling of red mud have explored both wet and dry stacking methods. This mixed-stacking approach offers several advantages over either method used alone (Liu and Wu, 2012 ). Red mud waste is highly alkaline, with a pH value ranging from 10 to 12.5. Direct contact with soil significantly degrades its quality and contaminates groundwater due to the leaching of heavy metal ions present in the red mud. Despite these drawbacks, several techniques have been developed for the direct utilization of this waste. Landfilling is the most common application for red mud, which accounts for the largest portion of waste generated throughout the year. However, considering the environmental risks associated with direct disposal, significant improvements have been made to the current disposal system (Klauber et al. 2011 ; Liu and Naidu 2014 ).Various applications of red mud have been reported in the literature, including the production of ceramic tiles, paver blocks, bricks, and other construction materials (Silveira et al. 2021 ; Venkatesh 2021; Raj et al. 2024 ; Nayak et al. 2024 ). The chemical composition of red mud makes it suitable as a supplementary cementitious material, allowing it to replace cement and enhance the physical and chemical properties of geopolymer concrete. This beneficial characteristic stems from its high iron and alumina content, as well as its alkalinity (Zakira et al. 2023 ; Ai et al. 2021 ). The natural red colour of the waste, caused by its iron content, results in its significant use in dye and pigment industries (Raj et al. 2024 ).A considerable amount of literature is available regarding the use of red mud as an adsorbent for heavy metal ions. Its high surface area, composition, and fine particle size contribute to its adsorption capacity (Bhatnagar 2011; Samal 2021 ; Rajkovic 2025). Numerous studies in the literature highlight the excellent adsorption properties of red mud and modified red mud for removing toxic heavy metal ions in wastewater treatment (Taneez and Hurel, 2019 ; Bai et al., 2022 ; Sitorus et al., 2024 ). Additionally, significant attention has been directed towards the recovery of various metal values, such as iron, titanium, vanadium, germanium, gallium, and rare earth elements (REEs), by researchers worldwide (Gome et al., 2016; Lu et al., 2018 ; Sadangi et al., 2018 ; Rai et al., 2019 ; Jena et al., 2025; Habibi et al., 2023 ; Li et al., 2021 ; Panda et al., 2021 ; Pan et al., 2023 ). Red mud is a fine-grained waste that consists of particles ranging in size from 0.1 µm to 90 µm, with approximately 90% of the particles being smaller than 10 µm (Borra et al., 2015 ; Kumar and Ramakrishna, 2023 ). The dewatering behavior of red mud is significantly affected by the presence of these extremely fine particles, which possess a very high surface area. A larger surface area in red mud particles increases moisture absorption capacity, making dewatering more challenging. When discharged into dump areas, red mud slurry contains approximately 15%-40% solid content. Many efforts have been made to investigate the settling behavior of the slurry, with the aim of utilizing the solid residue for value addition. Mineral acids such as hydrochloric acid (HCl), nitric acid (HNO 3 ), and sulfuric acid (H 2 SO 4 ), as well as mild organic acids like citric and oxalic acids, are utilized for neutralizing and studying the settling behavior of waste (Smiciklas et al. 2014 ; Rai et al. 2017 ; Luo et al. 2017 ; Kang et al. 2023 ). Reddy and Rao ( 2018 ) investigated the settling characteristics of red mud particles under various aqueous environmental conditions. Their findings showed that solutions containing sodium chloride (NaCl) resulted in higher settlement compared to HCl solutions, likely due to the flocculation or aggregation of finer particles. Pickling waste liquor, which is produced in cold rolling and galvanizing mills, is highly acidic, with a pH of around 2. It can efficiently neutralize red mud slurry, leading to the settlement of solid particles (Rai et al. 2012 ). Additionally, bubbling carbon dioxide into red mud slurry effectively reduces the pH, neutralizing its alkaline properties (Sahu et al. 2010 ; Qi et al. 2021; Suman and Tripathy 2024). Various natural and commercial flocculants have also been reported, including polyacrylamide (PAM), polyaluminum ferric chloride, aluminum sulfate (alum), ferric chloride, ferric sulfate, chitosan, and starch-based flocculants (Trampus and Franka 2020; Archambo 2021 ; Cheng et al. 2022 ).Industrial products and wastes, such as coal dust, superphosphate, gypsum, and fly ash, are used as amendments that play a beneficial role in the neutralization and settling of red mud (Rai et al., 2017 ; Dong et al., 2023 ). Bio-neutralization and settling behavior of red mud slurry represent a new approach that is simple, cost-effective, and highly efficient for adding value to the waste (Li et al., 2024 ; Qin et al., 2023 ). The use of seawater, or other calcium and magnesium-rich brines, provides a very cost-effective and efficient method for neutralizing red mud slurry. However, a limitation of this method is the distance between the refinery and the coast (Kanan et al., 2021 ; Yang et al., 2025 ). This article discusses an improved and cost-effective method for dewatering red mud using a calcium and magnesium impregnated acid solution. Various operating parameters, such as acid concentration, solid weight percentage, and dolomite ratio at different acid concentrations, were varied to optimize the maximum settling values. The work is further supported by various physical and chemical characterizations of the sample. 2. Materials and method 2.1 Materials Bauxite Residue (BR) The red coloured lumpy BR sample was collected from western part of Odisha, India. It was made into powdered form with the help of a mortar-grinder (Retch model 200M). In order to remove the presence of any moisture in the sample, it was dried for over-night using a hot air oven maintained at 100 ± 5°C. The snapshots of the sample is provided in the process flowsheet (Fig. 1 ). Dolomite The dolomite samples used in this experiment were collected from the western part of Odisha state, India. The sample was processed by different size-reducing equipment followed by ball milling to < 100 microns size. Thereafter, the fine-sized dolomite was washed repeatedly with deionised water and dried in an oven overnight before any use. The analytical grade high purity inorganic acids and salts were purchased from CDH (India) Ltd., Mumbai. For instrumental analysis (ICP-OES and Flame Photometer), the required chemicals and standards were procured from Avantor Performance Materials India Limited, New Delhi and used as such. 2.2 Materials Characterization The major and minor constituents of the samples were determined by wet chemical (Bureau of Indian Standards, New Delhi) and instrumental analysis methods (ICP-OES - Perkin Elmer Optima 8300, flame photometer - Systronic 128 µC). Further, it was confirmed by XRF analysis. Rigaku X-ray diffractometer (Ultima IV) using Cu-Kα irradiation source was used to identify the presence of different mineral phases. The material’s surface textural analysis along with elemental composition was investigated using ZEISS (EVO) make SEM-EDX. Further, the FTIR spectra were recorded within the 400–4000cm-1 range by Perkin-Elmer GX- Spectrum instrument over a KBr disc pellet. The particle size distribution of the feed and settled/unsettled mass were measured in the Malvern Mastersizer-2000 instrument. 2.3 Dewatering Experiments: The calculated amount of dolomite was digested in an HCl solution of different concentrations for approximately 1 hour. Then filtered into a 100ml conical flask, after that the insoluble parts were rejected and the mother liquor (Ca-Mg impregnated acid solution or D-HCl) was preserved for different experimentations. Subsequently, to study the effect of different parameters, the desired amount (ml) of D-HCl was used to note the settling value. The schematic diagram for the dewatering study is provided in Fig. 1 . The fine ground red mud sample was preconditioned with deionised water for 30 minutes at 1000 RPM to get a homogenous slurry. Further, the slurry was agitated for 5 min after the addition of D-HCl solution and then allowed to settle. The settling time was maintained for 2 min, then slowly decanted to separate the settled and unsettled portions. The settled portion was dried in a hot air oven to calculate the settling value. Repetition of some selected experiments indicates the repeatability of the results within an error range of ± 0.2%. Settling Value (wt. %) Calculation : The following formula was used to calculate the settling value (in wt. %) of Bauxite residue. % S = (100 x W2) / W1 Where W1 = Total mass of solid in the slurry; W2 = Settled mass after a predetermined time. 3. Results and Discussion 3.1 Characterization Studies The chemical analysis of the red mud sample is presented in Table 1 . Iron (Fe 2 O 3 ) and alumina (Al 2 O 3 ) are two major constituents; contribute almost 65% of the total mass. The LoI of the sample is around 11% with sodium, titanium and silica in the range of 6–8%. The particle size analysis of the feed and settled bauxite residue is provided in Fig. 2 . The D90 of both the sample were measured as 76.4 µm and 98.8µm respectively, which indicates the agglomeration of fine particles in settled mass. XRD of the feed sample confirms the presence of goethite, hematite, gibbsite, calcite and sodalite mineral phases. The FTIR spectra of the raw sample are provided in Fig. 3 . The peak position at 465.2 cm -1 corresponds to the stretching vibrations of Fe-O bonds or bending vibrations of Si-O-Si bonds (quartz and hematite phases) (Ramirez et al. 2022 ; Feng et al. 2022 ). The peak at 989.4 cm -1 is assigned to the asymmetric stretching vibration of the Si–O–Si bond (Kan et al. 2022 ; Hai et al. 2024 ). The presence of organic hydrocarbons as impurities shows two stretching vibration peaks at 2855.3 cm -1 and 2922.4 cm -1 (Singh et al. 2019 ). Table 1 Chemical analysis of the feed red mud and Dolomite sample. Constituents Red Mud (NALCO) Dolomite Wt.% Wt.% Fe 2 O 3 49.67 1.23 SiO 2 7.5 4.64 Al 2 O 3 19.9 0.64 LoI 11.24 40.41 CaO 1.5 32.24 MgO 0.64 20.12 K 2 O 0.16 0.08 Na 2 O 6.42 0.12 TiO2 2.62 0.061 V 2 O 5 0.16 0.042 The SEM-EDX (Fig. 3 ) analysis indicates the appearance of very fine particles with disparate shapes and sizes. The appearance of aggregates in patches with relatively loose arrangements is observed in the settled mass. The settled mass seems to be of high porosity in nature due to the overlapping of particles with each other. The elemental analysis of the sample is provided below the SEM image, which shows no major elemental changes in the samples. However, the sodium content in the settled mass decreases to the tune of 10%, while the titanium and iron increase to some extent. 3.2 Dewatering Studies Effective dewatering of Red mud slurry using hydrochloric acid of different concentrations and its demerit is well documented in the literature (Kong et al. 2017 ; Yang et al. 2025 ). An effort has been made here using low-concentration HCl impregnated with divalent calcium and magnesium ions. For this purpose, we have selected to use the dolomite sample as a source of Ca 2+ / Mg 2+ ions. Few preliminary experiments were performed to evaluate the effectiveness of the impregnated acid solution. Red mud slurry (10 wt. %) was prepared by taking 20g dried red mud powder in 200 ml deionised water and agitating for 30min at 1000 RPM in order to generate a homogenized slurry sample. 2ml of each acid solution of concentration ranging between 1N to 12N were added individually to the slurry and agitated for 5min under the same conditions. At the end of time, it was allowed to settle for 2min followed by slow decantation of the slurry. Similarly, the same experiments were performed with dolomite (5g/100ml) added acid solutions to compare the settling value. The results are provided in Fig. 4 . Parameters influencing the Dewatering behaviour (a) Effect of acid concentration For HCl, the settling value rises slowly from 50 to 92 on differing the concentration from 1N to 6N. Further, an increase in acid concentration (up to 12N) has no effect on the settling value. The D-HCl concentration variation from 1 to 12 N, suggests an approximate settling value of 94% for 1.5N D-HCl and any higher change in concentration affects minutely the settling data. This experiment concludes that almost 30% higher settling value for the 1.5N impregnated HCl (D-HCl, 2ml) compared with the pure 1.5N HCl solution. Hence further experiments were conducted using 1.5N D-HCl to optimize the dewatering performance of red mud slurry. (b) Volume of Ca-Mg impregnated Acid The volume of acid plays a vital role in the dewatering of red mud slurry. The results are provided in Fig. 4 (b), performed by changing the modified acid (D-HCl) volume from 0.5ml to 5ml at 10% solid: liquid ratio. The slurry was agitated for 5 min at 800 rpm followed by the addition of D-HCl. The settling time was maintained for 2 min for each experiment. The sharp mounting of settling value (from 68 to 94wt. %) was observed when the acid volume gradually increased from 0.5ml to 2ml. However, a further increase in acid volume noticed minimal changes in the settling value. Therefore, all other experiments were executed using 2ml D-HCl acid for the settling study. (c) Effect of solid weight,% The Fig. 4 c shows the effect of solid wt. % on settling of red mud slurry. The experiments were conducted using 1.5N D-HCl (2ml) varying the solid wt. % from 2.5 to 25, whereas other parameters are the same as above. The settling value slowly decreases from 98 to 89 wt. % when the solid % rose from 2.5 to 15%. However, it is noticed that a further increase in solid % abruptly affects the settling value. For 20% solid in the slurry, the settling value comes down to approximately 50%. This may be due to an increase in repulsive forces between the particles, which hinders the settling rate or may be due to increased interactions and inter-particle collisions (Khelifi et al. 2013 ). Also, the alkali content increases due to an increase in solid wt. %, which favours the particles to disperse freely in the fluid system resulting in a lower settling value (Sousa et al. 2021 ; Zaidi 2021 ) (d) Effect of settling time (in min) The response of settling time on settling value was demonstrated by varying the settling time and solid wt. % simultaneously. The agitation speed and time were fixed at 800rpm and 5 min respectively. 2 ml of D-HCl (5g dolomite/100 ml HCl) was used in each of the experiments. The outcome of the study is presented in Fig. 4 d. Increasing the settling time for all the different solid wt. % used, it has been noticed that settling value increases gently and beyond 2 minutes of settling time no significant changes in settling value were noted. However, the solid wt. % considerably influence the settling value. As shown in Fig. 4 d, for 40% solid in the slurry, the settling value is marked to be around 30% even changing the time from 2 to 7.5min. For 1 min settling time, the value comes to be around 24%. Decreasing the solid % from 40 to 15%, the settling value moves up to 94% at 2 min. Further increasing the settling time has no impact. For 15% solid, the lowest settling value of 72% is noted at 1 min with a maximum of 94% at 2 min settling time. When solid% is high (above 15wt.%), then the addition of 2ml -1.5N D-HCl following the same experimental condition as discussed above shows inferior results. Therefore few experiments were performed altering the acid volume, concentration and dolomite wt. % in the acid solution with respect to different solid wt. %, to note the settling data. The results are provided in Fig. 5 . Increasing the acid volume (1.5N D-HCl) steadily increases the settling efficiency of 20–40% solid-containing slurry, whereas for slurry containing 15% solid minor changes are observed in comparison to previous results. For slurry containing 20% and 40% solid, the maximum settling efficiency was recorded as 85% and 56% respectively after the addition of 8ml D-HCl. Simultaneous variation of acid concentration and solid wt. % in the slurry reflects the increase of settling value with increasing the acid concentration, for all the ranges of solid %. However, using a high concentration of acid for settling of red mud slurry is not wise (Nie et al. 2020 ). The experiments (Fig. 5 c) show 80% and 27% (at 12N and 1.5N respectively) settling values for 40% solid mass in the slurry. Whereas, the settling value is more than 90% for 15% solid. Therefore the lowest acid concentration of 1.5N with 15% solid in the slurry is the best chosen condition for the settling study. Variation of dolomite weight in 1.5N HCl (100ml) also influences the settling data. The results are provided in Fig. 5 b. Addition of dolomite (1g to 5g) with the hydrochloric acid (1.5N) slowly increases the settling of red mud. However, further increment of dolomite in the acid solution has an insignificant effect. Therefore 5g of dolomite in 1.5N is the best condition to settle the red mud slurry containing 15 wt.% solid. The feed red mud and settled red mud sample images are provided in Fig .6, clearly showing the agglomeration of fined red mud particles. 3.3 Dewatering Mechanism Bauxite residue is generally composed of various metal oxides majorly iron and aluminium oxides, which contribute almost 60%-70% of the total solid mass. Apart from this titanium oxide also contributes 4 to 12% with a loss on ignition to the tune of 10%. The mineralogical and chemical composition of bauxite residue significantly varies depending on the bauxite characteristics used in the Bayer Process. The high alkalinity of bauxite residue slurry (pH 10–13) and the existence of fine particles cause the slurry to settle extremely slowly (Panda et al. 2017 ; Silva 2022 ). The particle size, shape, density and viscosity of the fluid through which it is settling decides the settling rate (Ville Roitto 2014 ). The bauxite residue particles are extremely fine-sized (average particle size ~ 10µm) and have a very high surface area. As a result, the particles experience a very slow settling rate due to frictional drag forces in the fluid. Further, the high pH favours the formation of hydroxides at a definite temperature and salinity (Li and Hynes, 2017). The plant-rejected red mud in general contains around ~ 70% crystalline and ~ 30% amorphous materials (Grafe et al. 2011 ). The presence of this ~ 30% amorphous material having a very high surface area with surface charging property prevents the slurry from settling easily. Also, in amorphous compounds, the intermolecular forces are weaker, as a result, the molecular mobility will be also very high. This results in better chemical and physical reactivity with the slurry fluid, making a negative impact on the settling value. XRD pattern presented in Fig. 3 (d) shows the presence of iron minerals such as hematite and goethite. Literature informs that any bauxite residue composed of hematite settles faster than the goethite iron minerals. High specific gravity and low surface area of hematite in comparison to goethite results in speedy settling of bauxite residue (Li and Rutherford 1996 ). Goethite in comparison with hematite have more affection towards aqueous solution. Depending on the solution pH, the iron minerals exist in different hydroxide forms. The formation of different hydroxide forms viz. Fe(OH) 2 + , Fe(OH) 3 + , and Fe(OH) 4 - majorly depend on the available OH - ion, solution temperature and salinity (Morgan and Lahav 2007 ). The formation of ferric and ferrous hydroxide in the aqueous solution is shown in the below equations. The ferrous hydroxide is more soluble under high alkaline conditions due to its lower lattice energy than the ferric hydroxide, therefore the ferrous form is slowly converted to its ferric hydroxide form. Further, the ferrous hydroxide under the influence of temperature (150°C-450°C) arranges into mesoporous iron hydroxide forms (FeO-OH) (Mahasti et al. 2019 ). The formation of mesoporous materials in the bauxite residue slurry opposes the finer particles to settle down. Also, the pores in the mesoporous material capture the very fine or tiny particles and suspend them in the bauxite residue Slurry. Therefore settling in bauxite residue slurry is very difficult unless the addition of any external reagents prevents the formation of this type of mesoporous materials. Similarly, gibbsite at high alkaline conditions leads to the development of a porous complex form with pore size of 2-50nm range via the formation of aluminate [Al(OH) 4 − ] ions (Li et al. 2011 ). Further, the high sodium ion content in the red mud slurry interacts with the alumino-silicate and aluminium oxide surface leading to the dispersion of particles (Duraisamy and Chaunsali 2025 ). In a highly alkaline medium, the Fe(OH) 3 and OH- interact with each other forming a complex cage structure, which possibly entraps the extremely fine-sized amorphous materials present in the slurry. Similarly, the aluminium and silicon complexes are formed as described in Fig. 7 . Further, the high viscosity due to the presence of alkali-bearing phases makes the complex Fe, Al and Si structure in suspension for a long time. However, the addition of Ca-impregnated acid solutions removes the sodium ion as NaCl and relates with the freely available OH- ions, as a result, the settling behaviour of the slurry increases. The addition of diluted HCl, to the red mud slurry can effectively disintegrate the mesoporous structure formed due to Fe and Al minerals, as a result, the destabilization of suspended materials is hindered. The high pH of red mud slurry is because of the presence of soluble Na-bearing compounds such as NaOH, NaCO 3 , NaAlO 2 and insoluble sodalite. The soluble sodium compounds can be easily leached out from the slurry in comparison to insoluble sodalite. However, treatment of HCl can effectively reduce the sodium content of the slurry via the production of NaCl salt (XRD, Fig. 8 ). As a result, the viscosity of the slurry reduces, which favours the settling behaviour of materials (Pejcinovic et al. 2007 ; Kashif et al. 2023 ). Fe-O-OH + 3HCl = FeCl 3 + 3H 2 O Al- O-OH + 3HCl = AlCl 3 + 3H 2 O Al(OH) 3 + 3HCl = AlCl 3 + 3H 2 O In highly alkaline red mud, the particles are negatively charged due to the presence of OH-ions. The iron, aluminium and other metal oxide surfaces interact with the hydroxyl ions to develop a negatively charged surface. The D-HCl used for settling contains a sufficient amount of positively charged calcium ions (Ca 2+ ), which plays a major role in the neutralization of the negatively charged surfaces. Therefore, the inter-particle gap decreases leading to agglomeration of particles. In addition, the calcium ions (D-HCl) act as a negotiator between smaller particles for the formation of larger particle masses (Liang et al. 2018 ; Archambo 2021 ). Similarly, the presence of a small amount of magnesium ions in the D-HCl also supports the formation of a larger agglomerated mass (Chen et al. 2023 ). Therefore, the synergistic effect of both the acid and calcium /magnesium ions effectively neutralizes the highly alkaline red mud slurry, leading to the settling of finer particles in a very short time. The zeta potential and XRD of the treated red mud agree with the experimental findings. At 15% solid, the red mud slurry zeta potentials were measured at different volumes of HCl and D-HCl. The raw red mud samples show their initial zeta-potential value of -48.94 mV with a pH of 11.2. Subsequent addition of different volumes of acid affects severely the zeta-potential (Fig. 9 ). At 2ml of HCl and D-HCl addition, the zeta potential sharply goes down to -15.5 mV and − 4.96 mV respectively. Particle agglomeration/precipitation of fine particles occurs only when the zeta potential lies in a range of + 3 to -5mV (Rao and Reddy, 2017 ). Therefore, a minimum of 2ml D-HCl addition could be possibly able to settle almost 92% of red mud particles. The settled red mud XRD pattern shows a tall peak of halite, which appears in the sample because of salt formation after the addition of D-HCl. The calcite and goethite peaks seem to completely disappear, however small intensity peaks of sodalite and boehmite are observed. This is due to the dissolution of calcite and the dehydroxylation of gibbsite phases in the presence of the added D-HCl ions (Filho et al. 2017). Conclusion The generation of large quantities of red mud during alumina production, along with its safe storage and disposal, presents a significant global challenge for alumina producers. Existing literature outlines several methods for the safe storage, disposal, and reuse of this waste to add value. While the direct use of mineral acids (HCl, HNO 3 , and H 2 SO 4 ) can effectively neutralize and settle red mud slurry, the present study investigates the superior effectiveness of Ca²⁺ and Mg²⁺ impregnated HCl solutions as a more cost-effective solution. Dolomite was dissolved in various concentrations of HCl solutions to supply the necessary divalent ions. Experimental parameters, including acid concentrations, dolomite weight percentage, agitation speed, settling time, solid weight percentage, and volume of D-HCl, were optimized to achieve maximum settling efficiency. At 15% solid content, using 2 ml of a 1.5N D-HCl solution, it was possible to settle approximately 92% of the red mud solid particles within a 2-minute timeframe. However, increasing the solid weight percentage resulted in a significant drop in settling efficiency to around 50%. XRD characterization of the settled red mud indicated the disappearance of calcite and goethite, accompanied by a strong presence of halite due to the formation of NaCl. The SEM images of the settled mass showed the appearance of aggregates in patches with relatively loose arrangements. The zeta potential (along with pH) values of the red mud samples in water, HCl, and D-HCl were -48.94 mV (pH 11.2), -15.5 mV (pH 8.04), and -4.96 mV (pH 8.1), respectively. These findings suggest that the addition of D-HCl significantly influences the surface charge of red mud particles. The highly alkaline environment allows iron and aluminium hydroxides to interact with OH⁻ ions, forming complex, cage-like structures. This likely traps various extremely fine amorphous particles, preventing them from settling freely. Treatment with synthetic divalent ion acid solutions can neutralize the surface charges of red mud particles and facilitate the eviction of sodium ions as NaCl. The effectiveness of the divalent cation-impregnated solution has been discussed based on experimental results and existing literature. Declarations Acknowledgements: The authors are thankful to the Director, CSIR-IMMT, Bhubaneswar for his kind permission to publish this paper. The authors also wish to gratefully acknowledge Dr S.P Das, Chief Scientist, CSIR-IMMT for the useful discussion during the experimentation. Also, thanks to Dr P. C. Beuria, Head, Mineral Processing Department, CSIR-IMMT, for granting permission to carry out all necessary laboratory experiments. Author contributions : All the authors contributed to the conception and design of the study. Sandeep Kumar Jena and Sapan Kumar Kandi performed the literature search, design of experiment and data analysis. Ms. Baijayantimala Mohanty and Ms. Ranil Bahalia performed laboratory experiments and analysis. The first draft of the manuscript was prepared by Sandeep Kumar Jena. All authors commented on the previous versions of the manuscript. All authors read and approved of the final manuscript. Availability of data and materials: Data will be made available at a reasonable request. Funding: No specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Ethical approval : Authors contribute to maintaining the integrity of the research and its presentation by following the rules of good scientific practice. Consent to participate: All authors agreed with the content and that all gave explicit consent to participate. Consent for publication: All authors agreed with the content and that all gave explicit consent to submit. Competing Interests: The authors declare no competing interests. Clinical Trial number: Not Applicable References Ai T, Zhong D, Zhang Y, Zong J, Yan X, Niu Y (2021) The effect of red mud content on the compressive strength of geopolymers under different curing systems. 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Ruyters S, Mertens J, Vassilieva E, Dehandschutter B, Poffijn A, Smolders E (2011) The Red mud accident in ajka (hungary): plant toxicity and trace metal bioavailability in red mud contaminated soil. Environ Sci Technol 45(4):1616-1622 Sadangi JK, Das SP, Tripathy A, Biswal SK (2018) Investigation into recovery of iron values from red mud dumps, Sep Sci Technol 53 (14): 2186-2191 Sahu RC, Patel RK, Ray BC (2010) Neutralization of red mud using CO2 sequestration cycle. J Hazard Matter 179:28–34 Samal S (2021) Utilization of red mud as a source for metal ions—a review. Materials 14: 2211.https://doi.org/10.3390/ma14092211 Silva RC (2022) Experimental characterization techniques for solid-liquid slurry flows in pipelines: A review. Processes 10:597. https://doi.org/10.3390/pr10030597 Silveira NCG, Martins MLF, Bezerra ACS, Araujo FGS (2021) Red Mud from the Aluminium Industry: Production,Characteristics, and Alternative Applications in Construction Materials—A Review. Sustainability 13:12741.https://doi.org/10.3390/su132212741 Singh S, Aswath MU, Biswas RD, Ranganath RV, Choudhary HK, Kumar K, Sahoo B (2019) Role of iron in the enhanced reactivity of pulverized Red mud: Analysis by Mössbauer spectroscopy and FTIR spectroscopy. Case Stud. Constr. Mater. 11: e00266 Sitorus B, Yoyo T, Setiawati S, Khairi S (2024) Improving Pb(II) Removal via ascorbic acid-enhanced red mud adsorption: kinetics and isotherm analysis. Trends Sci 21(6): 7534. https://doi.org/10.48048/tis.2024.7534 Smiciklas I, Smiljanic S, A. Peric-Grujic A, Sljivic-Ivanovic M, Mitric M, Antonovic D (2014) Effect of acid treatment on red mud properties with implications on Ni(II) sorption and stability. Chem Eng J 242: 27–35 Sousa TAT, Monte FP, Silva JVN, Lopes WS, Leite VD, Van Lier JB, Sousa JT (2021) Alkaline and acid solubilisation of waste activated sludge. Water Sci Technol 83:12. doi: 10.2166/wst.2021.179 Suman, Tripathy A (2024) CO2 capture using red mud & mechanically-activated red mud and its kinetics under ambient conditions. Chem Eng J 498:155609 Taneez M, Hurel C (2019) A review on the potential uses of red mud as amendment for pollution control in environmental media. Environ Sci Pollut Res 26:22106–22125 Trampus BC, Franca SCA (2020) Performances of two flocculants and their mixtures for red mud dewatering and disposal based on mineral paste production. J Clean Prod 257:120534 USGS Mineral Commodity Summaries 2024 report (2024). https://pubs.usgs.gov/ periodicals /mcs2024/mcs2024.pdf Yang J, Liu X, Cui K, Lyu J, Liu H, Qiu J (2025) Hazards and dealkalization technology of red mud —a critical review. Minerals 15:343. https://doi.org/10.3390/min15040343 Zaidi AA (2021) Characteristics of settling of dilute suspension of particles with different density at high Reynolds numbers. Particuology 56: 62–74 Zakira U, Zheng K, Xie N, Birgisson B (2023) Development of high-strength geopolymers from red mud and blast furnace slag. J Clean Prod 383:135439 Cite Share Download PDF Status: Published Journal Publication published 06 Nov, 2025 Read the published version in Environmental Science and Pollution Research → Version 1 posted Editorial decision: Major Revision 08 Aug, 2025 Reviewers agreed at journal 09 Jul, 2025 Reviewers invited by journal 25 Jun, 2025 Editor invited by journal 25 Jun, 2025 Editor assigned by journal 23 Jun, 2025 First submitted to journal 19 Jun, 2025 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-6930760","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":476284438,"identity":"c582d400-72ad-4020-8400-9d05c87f58d5","order_by":0,"name":"SANDEEP KUMAR 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2","display":"","copyAsset":false,"role":"figure","size":76293,"visible":true,"origin":"","legend":"\u003cp\u003eParticle size analysis of raw red mud and settled red mud.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/b2cdbfb047eb6dcf5e70a5ad.png"},{"id":85663207,"identity":"b960bcc5-088d-4fa4-9d8e-4cc0170d28ac","added_by":"auto","created_at":"2025-06-30 12:09:11","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":552427,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of feed Red mud \u003cstrong\u003e(a)\u003c/strong\u003e and settled Red mud \u003cstrong\u003e(b)\u003c/strong\u003e The table shows the elemental mapping of both the sample, \u003cstrong\u003e(c)\u003c/strong\u003eFTIR spectra and \u003cstrong\u003e(d)\u003c/strong\u003e XRD spectra of raw BR.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/1c2120a186152ef5b36ae77b.png"},{"id":85663210,"identity":"21232892-7192-4fd2-8d88-cfe9832480f8","added_by":"auto","created_at":"2025-06-30 12:09:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":127660,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of dewatering parameters \u003cstrong\u003e(a)\u003c/strong\u003e acid concentration \u003cstrong\u003e(b)\u003c/strong\u003e acid volume \u003cstrong\u003e(c)\u003c/strong\u003e solid wt. % and \u003cstrong\u003e(d)\u003c/strong\u003esettling time\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/462b6ed20ee6bafb9b375a00.png"},{"id":85663214,"identity":"188108c3-a964-49c3-b75c-cb64899f0d40","added_by":"auto","created_at":"2025-06-30 12:09:11","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":149778,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of dewatering parameters at high solid wt.% (a) acid volume(1ml to 8ml) (b) dolomite concentration(1g to 10g) in 1.5N HCl (c) acid concentration (1.5N to 12N).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/bd86071f8eab8a4ad2939bf5.png"},{"id":85664368,"identity":"bd9f1bfb-7735-4918-9252-35aa9a5cb080","added_by":"auto","created_at":"2025-06-30 12:25:11","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":375183,"visible":true,"origin":"","legend":"\u003cp\u003eAs received red mud (a) and the red mud after dewatering (b).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/b5453b32072242b4611898df.png"},{"id":85663208,"identity":"3bfccaa8-8ad2-4d46-bae1-2926dec63dfa","added_by":"auto","created_at":"2025-06-30 12:09:11","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":252140,"visible":true,"origin":"","legend":"\u003cp\u003eBehaviour of iron, alumino-silicate complexes in alkaline red mud on account of D-HCl addition.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/b3f27390fa307caa88704383.png"},{"id":85663744,"identity":"75d096a7-148d-4ad4-b7c8-e42403f356cb","added_by":"auto","created_at":"2025-06-30 12:17:11","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":68274,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of D-HCl treated red mud.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/1c93066a00773d7a7e0018d0.png"},{"id":85663747,"identity":"1d5c36fc-5722-4fe3-b1a3-02055da033c9","added_by":"auto","created_at":"2025-06-30 12:17:11","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":68549,"visible":true,"origin":"","legend":"\u003cp\u003eZeta Potential vs volume (ml) of HCl/D-HCl of red mud slurry.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/71f1c1df94eb1d1588dde36a.png"},{"id":95563986,"identity":"c51d50e8-5f8b-459d-b9d6-35df5c0217f2","added_by":"auto","created_at":"2025-11-10 16:06:05","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2556966,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6930760/v1/83cdc6ec-1545-40b0-8842-2c3d92a5c82e.pdf"}],"financialInterests":"","formattedTitle":"Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRed mud, also known as bauxite residue (BR), is a reddish-brown by-product generated during the Bayer process for alumina production. This hydrometallurgical method is used worldwide to produce alumina. The process involves treating bauxite at high temperatures of approximately 170\u0026ndash;180\u0026deg;C, followed by filtration to separate the waste, which is known as red mud. In 2023, global alumina production reached around 140\u0026nbsp;million tons, resulting in approximately 177\u0026nbsp;million tons of red mud as waste (USGS 2024). On average, producing one ton of alumina generates about 1 to 2.5 tons of red mud waste, with the exact amount depending primarily on the quality of the bauxite used (Hendrick 2022; Ding et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). In India, this high-volume, low-toxicity waste is classified as industrial waste rather than hazardous waste, allowing for potential resource recovery and reuse (CPCB 2024).\u003c/p\u003e \u003cp\u003eThe management of large amounts of alkaline waste is a significant global issue. The disposal of this waste primarily depends on local topography, state regulations, and the availability of land (Patil and Thorat, 2022). Alumina refineries can choose between directly pumping red mud slurry to a disposal area or processing the slurry through filtration. The two main methods for disposing of and storing red mud waste are dry stacking and wet stacking. Wet stacking is more costly, as it involves storing red mud slurry in large ponds or dams, where the slurry settles and dries naturally. However, this method poses environmental risks, including the potential for leakage. An example of such a disaster occurred in 2010 at the Ajka Alumina Plant in Hungary (Ruyters et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). In contrast, dry stacking is more economical and environmentally friendly. This method involves dewatering the red mud slurry to create a material with a high solid content, making it a safer option compared to wet stacking. Recent advancements in the stockpiling of red mud have explored both wet and dry stacking methods. This mixed-stacking approach offers several advantages over either method used alone (Liu and Wu, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRed mud waste is highly alkaline, with a pH value ranging from 10 to 12.5. Direct contact with soil significantly degrades its quality and contaminates groundwater due to the leaching of heavy metal ions present in the red mud. Despite these drawbacks, several techniques have been developed for the direct utilization of this waste. Landfilling is the most common application for red mud, which accounts for the largest portion of waste generated throughout the year. However, considering the environmental risks associated with direct disposal, significant improvements have been made to the current disposal system (Klauber et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Liu and Naidu \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).Various applications of red mud have been reported in the literature, including the production of ceramic tiles, paver blocks, bricks, and other construction materials (Silveira et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Venkatesh 2021; Raj et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Nayak et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The chemical composition of red mud makes it suitable as a supplementary cementitious material, allowing it to replace cement and enhance the physical and chemical properties of geopolymer concrete. This beneficial characteristic stems from its high iron and alumina content, as well as its alkalinity (Zakira et al. \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ai et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The natural red colour of the waste, caused by its iron content, results in its significant use in dye and pigment industries (Raj et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).A considerable amount of literature is available regarding the use of red mud as an adsorbent for heavy metal ions. Its high surface area, composition, and fine particle size contribute to its adsorption capacity (Bhatnagar 2011; Samal \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Rajkovic 2025).\u003c/p\u003e \u003cp\u003eNumerous studies in the literature highlight the excellent adsorption properties of red mud and modified red mud for removing toxic heavy metal ions in wastewater treatment (Taneez and Hurel, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Bai et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sitorus et al., \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Additionally, significant attention has been directed towards the recovery of various metal values, such as iron, titanium, vanadium, germanium, gallium, and rare earth elements (REEs), by researchers worldwide (Gome et al., 2016; Lu et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Sadangi et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Rai et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Jena et al., 2025; Habibi et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Panda et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Pan et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRed mud is a fine-grained waste that consists of particles ranging in size from 0.1 \u0026micro;m to 90 \u0026micro;m, with approximately 90% of the particles being smaller than 10 \u0026micro;m (Borra et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Kumar and Ramakrishna, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The dewatering behavior of red mud is significantly affected by the presence of these extremely fine particles, which possess a very high surface area. A larger surface area in red mud particles increases moisture absorption capacity, making dewatering more challenging. When discharged into dump areas, red mud slurry contains approximately 15%-40% solid content. Many efforts have been made to investigate the settling behavior of the slurry, with the aim of utilizing the solid residue for value addition.\u003c/p\u003e \u003cp\u003eMineral acids such as hydrochloric acid (HCl), nitric acid (HNO\u003csub\u003e3\u003c/sub\u003e), and sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), as well as mild organic acids like citric and oxalic acids, are utilized for neutralizing and studying the settling behavior of waste (Smiciklas et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Rai et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Luo et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Kang et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Reddy and Rao (\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) investigated the settling characteristics of red mud particles under various aqueous environmental conditions. Their findings showed that solutions containing sodium chloride (NaCl) resulted in higher settlement compared to HCl solutions, likely due to the flocculation or aggregation of finer particles.\u003c/p\u003e \u003cp\u003ePickling waste liquor, which is produced in cold rolling and galvanizing mills, is highly acidic, with a pH of around 2. It can efficiently neutralize red mud slurry, leading to the settlement of solid particles (Rai et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Additionally, bubbling carbon dioxide into red mud slurry effectively reduces the pH, neutralizing its alkaline properties (Sahu et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Qi et al. 2021; Suman and Tripathy 2024). Various natural and commercial flocculants have also been reported, including polyacrylamide (PAM), polyaluminum ferric chloride, aluminum sulfate (alum), ferric chloride, ferric sulfate, chitosan, and starch-based flocculants (Trampus and Franka 2020; Archambo \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Cheng et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).Industrial products and wastes, such as coal dust, superphosphate, gypsum, and fly ash, are used as amendments that play a beneficial role in the neutralization and settling of red mud (Rai et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Dong et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Bio-neutralization and settling behavior of red mud slurry represent a new approach that is simple, cost-effective, and highly efficient for adding value to the waste (Li et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Qin et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The use of seawater, or other calcium and magnesium-rich brines, provides a very cost-effective and efficient method for neutralizing red mud slurry. However, a limitation of this method is the distance between the refinery and the coast (Kanan et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThis article discusses an improved and cost-effective method for dewatering red mud using a calcium and magnesium impregnated acid solution. Various operating parameters, such as acid concentration, solid weight percentage, and dolomite ratio at different acid concentrations, were varied to optimize the maximum settling values. The work is further supported by various physical and chemical characterizations of the sample.\u003c/p\u003e"},{"header":"2. Materials and method","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003e \u003cstrong\u003eBauxite Residue (BR)\u003c/strong\u003e \u003cp\u003eThe red coloured lumpy BR sample was collected from western part of Odisha, India. It was made into powdered form with the help of a mortar-grinder (Retch model 200M). In order to remove the presence of any moisture in the sample, it was dried for over-night using a hot air oven maintained at 100\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C. The snapshots of the sample is provided in the process flowsheet (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eDolomite\u003c/strong\u003e \u003cp\u003eThe dolomite samples used in this experiment were collected from the western part of Odisha state, India. The sample was processed by different size-reducing equipment followed by ball milling to \u0026lt;\u0026thinsp;100 microns size. Thereafter, the fine-sized dolomite was washed repeatedly with deionised water and dried in an oven overnight before any use.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe analytical grade high purity inorganic acids and salts were purchased from CDH (India) Ltd., Mumbai. For instrumental analysis (ICP-OES and Flame Photometer), the required chemicals and standards were procured from Avantor Performance Materials India Limited, New Delhi and used as such.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Materials Characterization\u003c/h2\u003e \u003cp\u003eThe major and minor constituents of the samples were determined by wet chemical (Bureau of Indian Standards, New Delhi) and instrumental analysis methods (ICP-OES - Perkin Elmer Optima 8300, flame photometer - Systronic 128 \u0026micro;C). Further, it was confirmed by XRF analysis.\u003c/p\u003e \u003cp\u003eRigaku X-ray diffractometer (Ultima IV) using Cu-Kα irradiation source was used to identify the presence of different mineral phases. The material\u0026rsquo;s surface textural analysis along with elemental composition was investigated using ZEISS (EVO) make SEM-EDX. Further, the FTIR spectra were recorded within the 400\u0026ndash;4000cm-1 range by Perkin-Elmer GX- Spectrum instrument over a KBr disc pellet. The particle size distribution of the feed and settled/unsettled mass were measured in the Malvern Mastersizer-2000 instrument.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Dewatering Experiments:\u003c/h2\u003e \u003cp\u003eThe calculated amount of dolomite was digested in an HCl solution of different concentrations for approximately 1 hour. Then filtered into a 100ml conical flask, after that the insoluble parts were rejected and the mother liquor (Ca-Mg impregnated acid solution or D-HCl) was preserved for different experimentations. Subsequently, to study the effect of different parameters, the desired amount (ml) of D-HCl was used to note the settling value.\u003c/p\u003e \u003cp\u003eThe schematic diagram for the dewatering study is provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The fine ground red mud sample was preconditioned with deionised water for 30 minutes at 1000 RPM to get a homogenous slurry. Further, the slurry was agitated for 5 min after the addition of D-HCl solution and then allowed to settle. The settling time was maintained for 2 min, then slowly decanted to separate the settled and unsettled portions. The settled portion was dried in a hot air oven to calculate the settling value. Repetition of some selected experiments indicates the repeatability of the results within an error range of \u0026plusmn;\u0026thinsp;0.2%.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eSettling Value (wt. %) Calculation\u003c/b\u003e:\u003c/p\u003e \u003cp\u003eThe following formula was used to calculate the settling value (in wt. %) of Bauxite residue.\u003c/p\u003e \u003cp\u003e% S = (100 x W2) / W1\u003c/p\u003e \u003cp\u003eWhere W1\u0026thinsp;=\u0026thinsp;Total mass of solid in the slurry; W2\u0026thinsp;=\u0026thinsp;Settled mass after a predetermined time.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Characterization Studies\u003c/h2\u003e\n \u003cp\u003eThe chemical analysis of the red mud sample is presented in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Iron (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) and alumina (Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) are two major constituents; contribute almost 65% of the total mass. The LoI of the sample is around 11% with sodium, titanium and silica in the range of 6\u0026ndash;8%. The particle size analysis of the feed and settled bauxite residue is provided in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The D90 of both the sample were measured as 76.4 \u0026micro;m and 98.8\u0026micro;m respectively, which indicates the agglomeration of fine particles in settled mass.\u003c/p\u003e\n \u003cp\u003eXRD of the feed sample confirms the presence of goethite, hematite, gibbsite, calcite and sodalite mineral phases. The FTIR spectra of the raw sample are provided in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. The peak position at 465.2 cm\u003csup\u003e-1\u003c/sup\u003e corresponds to the stretching vibrations of Fe-O bonds or bending vibrations of Si-O-Si bonds (quartz and hematite phases) (Ramirez et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e; Feng et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e). The peak at 989.4 cm\u003csup\u003e-1\u003c/sup\u003e is assigned to the asymmetric stretching vibration of the Si\u0026ndash;O\u0026ndash;Si bond (Kan et al. \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e; Hai et al. \u003cspan class=\"CitationRef\"\u003e2024\u003c/span\u003e). The presence of organic hydrocarbons as impurities shows two stretching vibration peaks at 2855.3 cm\u003csup\u003e-1\u003c/sup\u003e and 2922.4 cm\u003csup\u003e-1\u003c/sup\u003e (Singh et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eChemical analysis of the feed red mud and Dolomite sample.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eConstituents\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRed Mud (NALCO)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDolomite\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWt.%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWt.%\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eFe\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e49.67\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.23\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eSiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.64\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eAl\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e19.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eLoI\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e11.24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e40.41\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eCaO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e32.24\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eMgO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e20.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eK\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.08\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eNa\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eTiO2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.061\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003eV\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003c/sub\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.16\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.042\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eThe SEM-EDX (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) analysis indicates the appearance of very fine particles with disparate shapes and sizes. The appearance of aggregates in patches with relatively loose arrangements is observed in the settled mass. The settled mass seems to be of high porosity in nature due to the overlapping of particles with each other. The elemental analysis of the sample is provided below the SEM image, which shows no major elemental changes in the samples. However, the sodium content in the settled mass decreases to the tune of 10%, while the titanium and iron increase to some extent.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 Dewatering Studies\u003c/h2\u003e\n \u003cp\u003eEffective dewatering of Red mud slurry using hydrochloric acid of different concentrations and its demerit is well documented in the literature (Kong et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e; Yang et al. \u003cspan class=\"CitationRef\"\u003e2025\u003c/span\u003e). An effort has been made here using low-concentration HCl impregnated with divalent calcium and magnesium ions. For this purpose, we have selected to use the dolomite sample as a source of Ca\u003csup\u003e2+\u003c/sup\u003e / Mg\u003csup\u003e2+\u003c/sup\u003e ions. Few preliminary experiments were performed to evaluate the effectiveness of the impregnated acid solution.\u003c/p\u003e\n \u003cp\u003eRed mud slurry (10 wt. %) was prepared by taking 20g dried red mud powder in 200 ml deionised water and agitating for 30min at 1000 RPM in order to generate a homogenized slurry sample. 2ml of each acid solution of concentration ranging between 1N to 12N were added individually to the slurry and agitated for 5min under the same conditions. At the end of time, it was allowed to settle for 2min followed by slow decantation of the slurry. Similarly, the same experiments were performed with dolomite (5g/100ml) added acid solutions to compare the settling value. The results are provided in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eParameters influencing the Dewatering behaviour\u003c/strong\u003e\u003c/p\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003e\u003cstrong\u003e(a) Effect of acid concentration\u003c/strong\u003e\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eFor HCl, the settling value rises slowly from 50 to 92 on differing the concentration from 1N to 6N. Further, an increase in acid concentration (up to 12N) has no effect on the settling value. The D-HCl concentration variation from 1 to 12 N, suggests an approximate settling value of 94% for 1.5N D-HCl and any higher change in concentration affects minutely the settling data. This experiment concludes that almost 30% higher settling value for the 1.5N impregnated HCl (D-HCl, 2ml) compared with the pure 1.5N HCl solution. Hence further experiments were conducted using 1.5N D-HCl to optimize the dewatering performance of red mud slurry.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(b) Volume of Ca-Mg impregnated Acid\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eThe volume of acid plays a vital role in the dewatering of red mud slurry. The results are provided in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e(b), performed by changing the modified acid (D-HCl) volume from 0.5ml to 5ml at 10% solid: liquid ratio. The slurry was agitated for 5 min at 800 rpm followed by the addition of D-HCl. The settling time was maintained for 2 min for each experiment. The sharp mounting of settling value (from 68 to 94wt. %) was observed when the acid volume gradually increased from 0.5ml to 2ml. However, a further increase in acid volume noticed minimal changes in the settling value. Therefore, all other experiments were executed using 2ml D-HCl acid for the settling study.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(c) Effect of solid weight,%\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eThe Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec shows the effect of solid wt. % on settling of red mud slurry. The experiments were conducted using 1.5N D-HCl (2ml) varying the solid wt. % from 2.5 to 25, whereas other parameters are the same as above. The settling value slowly decreases from 98 to 89 wt. % when the solid % rose from 2.5 to 15%. However, it is noticed that a further increase in solid % abruptly affects the settling value. For 20% solid in the slurry, the settling value comes down to approximately 50%. This may be due to an increase in repulsive forces between the particles, which hinders the settling rate or may be due to increased interactions and inter-particle collisions (Khelifi et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e). Also, the alkali content increases due to an increase in solid wt. %, which favours the particles to disperse freely in the fluid system resulting in a lower settling value (Sousa et al. \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zaidi \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e)\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(d) Effect of settling time (in min)\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eThe response of settling time on settling value was demonstrated by varying the settling time and solid wt. % simultaneously. The agitation speed and time were fixed at 800rpm and 5 min respectively. 2 ml of D-HCl (5g dolomite/100 ml HCl) was used in each of the experiments. The outcome of the study is presented in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed.\u003c/p\u003e\n \u003cp\u003eIncreasing the settling time for all the different solid wt. % used, it has been noticed that settling value increases gently and beyond 2 minutes of settling time no significant changes in settling value were noted. However, the solid wt. % considerably influence the settling value. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed, for 40% solid in the slurry, the settling value is marked to be around 30% even changing the time from 2 to 7.5min. For 1 min settling time, the value comes to be around 24%. Decreasing the solid % from 40 to 15%, the settling value moves up to 94% at 2 min. Further increasing the settling time has no impact. For 15% solid, the lowest settling value of 72% is noted at 1 min with a maximum of 94% at 2 min settling time.\u003c/p\u003e\n \u003cp\u003eWhen solid% is high (above 15wt.%), then the addition of 2ml -1.5N D-HCl following the same experimental condition as discussed above shows inferior results. Therefore few experiments were performed altering the acid volume, concentration and dolomite wt. % in the acid solution with respect to different solid wt. %, to note the settling data. The results are provided in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. Increasing the acid volume (1.5N D-HCl) steadily increases the settling efficiency of 20\u0026ndash;40% solid-containing slurry, whereas for slurry containing 15% solid minor changes are observed in comparison to previous results. For slurry containing 20% and 40% solid, the maximum settling efficiency was recorded as 85% and 56% respectively after the addition of 8ml D-HCl.\u003c/p\u003e\n \u003cp\u003eSimultaneous variation of acid concentration and solid wt. % in the slurry reflects the increase of settling value with increasing the acid concentration, for all the ranges of solid %. However, using a high concentration of acid for settling of red mud slurry is not wise (Nie et al. \u003cspan class=\"CitationRef\"\u003e2020\u003c/span\u003e). The experiments (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec) show 80% and 27% (at 12N and 1.5N respectively) settling values for 40% solid mass in the slurry. Whereas, the settling value is more than 90% for 15% solid. Therefore the lowest acid concentration of 1.5N with 15% solid in the slurry is the best chosen condition for the settling study. Variation of dolomite weight in 1.5N HCl (100ml) also influences the settling data. The results are provided in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb. Addition of dolomite (1g to 5g) with the hydrochloric acid (1.5N) slowly increases the settling of red mud. However, further increment of dolomite in the acid solution has an insignificant effect. Therefore 5g of dolomite in 1.5N is the best condition to settle the red mud slurry containing 15 wt.% solid. The feed red mud and settled red mud sample images are provided in Fig .6, clearly showing the agglomeration of fined red mud particles.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Dewatering Mechanism\u003c/h2\u003e\n \u003cp\u003eBauxite residue is generally composed of various metal oxides majorly iron and aluminium oxides, which contribute almost 60%-70% of the total solid mass. Apart from this titanium oxide also contributes 4 to 12% with a loss on ignition to the tune of 10%. The mineralogical and chemical composition of bauxite residue significantly varies depending on the bauxite characteristics used in the Bayer Process. The high alkalinity of bauxite residue slurry (pH 10\u0026ndash;13) and the existence of fine particles cause the slurry to settle extremely slowly (Panda et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e; Silva \u003cspan class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eThe particle size, shape, density and viscosity of the fluid through which it is settling decides the settling rate (Ville Roitto \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e). The bauxite residue particles are extremely fine-sized (average particle size\u0026thinsp;~\u0026thinsp;10\u0026micro;m) and have a very high surface area. As a result, the particles experience a very slow settling rate due to frictional drag forces in the fluid. Further, the high pH favours the formation of hydroxides at a definite temperature and salinity (Li and Hynes, 2017).\u003c/p\u003e\n \u003cp\u003eThe plant-rejected red mud in general contains around ~\u0026thinsp;70% crystalline and ~\u0026thinsp;30% amorphous materials (Grafe et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e). The presence of this\u0026thinsp;~\u0026thinsp;30% amorphous material having a very high surface area with surface charging property prevents the slurry from settling easily. Also, in amorphous compounds, the intermolecular forces are weaker, as a result, the molecular mobility will be also very high. This results in better chemical and physical reactivity with the slurry fluid, making a negative impact on the settling value.\u003c/p\u003e\n \u003cp\u003eXRD pattern presented in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(d) shows the presence of iron minerals such as hematite and goethite. Literature informs that any bauxite residue composed of hematite settles faster than the goethite iron minerals. High specific gravity and low surface area of hematite in comparison to goethite results in speedy settling of bauxite residue (Li and Rutherford \u003cspan class=\"CitationRef\"\u003e1996\u003c/span\u003e). Goethite in comparison with hematite have more affection towards aqueous solution. Depending on the solution pH, the iron minerals exist in different hydroxide forms. The formation of different hydroxide forms viz. Fe(OH)\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, Fe(OH)\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e, and Fe(OH)\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e majorly depend on the available OH\u003csup\u003e-\u003c/sup\u003e ion, solution temperature and salinity (Morgan and Lahav \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e). The formation of ferric and ferrous hydroxide in the aqueous solution is shown in the below equations. The ferrous hydroxide is more soluble under high alkaline conditions due to its lower lattice energy than the ferric hydroxide, therefore the ferrous form is slowly converted to its ferric hydroxide form. Further, the ferrous hydroxide under the influence of temperature (150\u0026deg;C-450\u0026deg;C) arranges into mesoporous iron hydroxide forms (FeO-OH) (Mahasti et al. \u003cspan class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n \u003cp\u003eThe formation of mesoporous materials in the bauxite residue slurry opposes the finer particles to settle down. Also, the pores in the mesoporous material capture the very fine or tiny particles and suspend them in the bauxite residue Slurry. Therefore settling in bauxite residue slurry is very difficult unless the addition of any external reagents prevents the formation of this type of mesoporous materials.\u003c/p\u003e\n \u003cp\u003eSimilarly, gibbsite at high alkaline conditions leads to the development of a porous complex form with pore size of 2-50nm range via the formation of aluminate [Al(OH)\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e] ions (Li et al. \u003cspan class=\"CitationRef\"\u003e2011\u003c/span\u003e). Further, the high sodium ion content in the red mud slurry interacts with the alumino-silicate and aluminium oxide surface leading to the dispersion of particles (Duraisamy and Chaunsali \u003cspan class=\"CitationRef\"\u003e2025\u003c/span\u003e). In a highly alkaline medium, the Fe(OH)\u003csub\u003e3\u003c/sub\u003e and OH- interact with each other forming a complex cage structure, which possibly entraps the extremely fine-sized amorphous materials present in the slurry. Similarly, the aluminium and silicon complexes are formed as described in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e. Further, the high viscosity due to the presence of alkali-bearing phases makes the complex Fe, Al and Si structure in suspension for a long time. However, the addition of Ca-impregnated acid solutions removes the sodium ion as NaCl and relates with the freely available OH- ions, as a result, the settling behaviour of the slurry increases.\u003c/p\u003e\n \u003cp\u003eThe addition of diluted HCl, to the red mud slurry can effectively disintegrate the mesoporous structure formed due to Fe and Al minerals, as a result, the destabilization of suspended materials is hindered. The high pH of red mud slurry is because of the presence of soluble Na-bearing compounds such as NaOH, NaCO\u003csub\u003e3\u003c/sub\u003e, NaAlO\u003csub\u003e2\u003c/sub\u003e and insoluble sodalite. The soluble sodium compounds can be easily leached out from the slurry in comparison to insoluble sodalite. However, treatment of HCl can effectively reduce the sodium content of the slurry via the production of NaCl salt (XRD, Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e). As a result, the viscosity of the slurry reduces, which favours the settling behaviour of materials (Pejcinovic et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e; Kashif et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eFe-O-OH\u0026thinsp;+\u0026thinsp;3HCl\u0026thinsp;=\u0026thinsp;FeCl\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003eAl- O-OH\u0026thinsp;+\u0026thinsp;3HCl\u0026thinsp;=\u0026thinsp;AlCl\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003eAl(OH)\u003csub\u003e3\u003c/sub\u003e + 3HCl\u0026thinsp;=\u0026thinsp;AlCl\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e\n \u003cp\u003eIn highly alkaline red mud, the particles are negatively charged due to the presence of OH-ions. The iron, aluminium and other metal oxide surfaces interact with the hydroxyl ions to develop a negatively charged surface. The D-HCl used for settling contains a sufficient amount of positively charged calcium ions (Ca\u003csup\u003e2+\u003c/sup\u003e), which plays a major role in the neutralization of the negatively charged surfaces. Therefore, the inter-particle gap decreases leading to agglomeration of particles. In addition, the calcium ions (D-HCl) act as a negotiator between smaller particles for the formation of larger particle masses (Liang et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e; Archambo \u003cspan class=\"CitationRef\"\u003e2021\u003c/span\u003e). Similarly, the presence of a small amount of magnesium ions in the D-HCl also supports the formation of a larger agglomerated mass (Chen et al. \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eTherefore, the synergistic effect of both the acid and calcium /magnesium ions effectively neutralizes the highly alkaline red mud slurry, leading to the settling of finer particles in a very short time. The zeta potential and XRD of the treated red mud agree with the experimental findings. At 15% solid, the red mud slurry zeta potentials were measured at different volumes of HCl and D-HCl. The raw red mud samples show their initial zeta-potential value of -48.94 mV with a pH of 11.2. Subsequent addition of different volumes of acid affects severely the zeta-potential (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e). At 2ml of HCl and D-HCl addition, the zeta potential sharply goes down to -15.5 mV and \u0026minus;\u0026thinsp;4.96 mV respectively. Particle agglomeration/precipitation of fine particles occurs only when the zeta potential lies in a range of +\u0026thinsp;3 to -5mV (Rao and Reddy, \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e). Therefore, a minimum of 2ml D-HCl addition could be possibly able to settle almost 92% of red mud particles.\u003c/p\u003e\n \u003cp\u003eThe settled red mud XRD pattern shows a tall peak of halite, which appears in the sample because of salt formation after the addition of D-HCl. The calcite and goethite peaks seem to completely disappear, however small intensity peaks of sodalite and boehmite are observed. This is due to the dissolution of calcite and the dehydroxylation of gibbsite phases in the presence of the added D-HCl ions (Filho et al. 2017).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe generation of large quantities of red mud during alumina production, along with its safe storage and disposal, presents a significant global challenge for alumina producers. Existing literature outlines several methods for the safe storage, disposal, and reuse of this waste to add value. While the direct use of mineral acids (HCl, HNO\u003csub\u003e3\u003c/sub\u003e, and H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) can effectively neutralize and settle red mud slurry, the present study investigates the superior effectiveness of Ca²⁺\u0026nbsp;and Mg²⁺\u0026nbsp;impregnated HCl solutions as a more cost-effective solution.\u003c/p\u003e\n\u003cp\u003eDolomite was dissolved in various concentrations of HCl solutions to supply the necessary divalent ions. Experimental parameters, including acid concentrations, dolomite weight percentage, agitation speed, settling time, solid weight percentage, and volume of D-HCl, were optimized to achieve maximum settling efficiency. At 15% solid content, using 2 ml of a 1.5N D-HCl solution, it was possible to settle approximately 92% of the red mud solid particles within a 2-minute timeframe. However, increasing the solid weight percentage resulted in a significant drop in settling efficiency to around 50%. XRD characterization of the settled red mud indicated the disappearance of calcite and goethite, accompanied by a strong presence of halite due to the formation of NaCl. The SEM images of the settled mass showed the appearance of aggregates in patches with relatively loose arrangements.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe zeta potential (along with pH) values of the red mud samples in water, HCl, and D-HCl were -48.94 mV (pH 11.2), -15.5 mV (pH 8.04), and -4.96 mV (pH 8.1), respectively. These findings suggest that the addition of D-HCl significantly influences the surface charge of red mud particles. The highly alkaline environment allows iron and aluminium hydroxides to interact with OH⁻ ions, forming complex, cage-like structures. This likely traps various extremely fine amorphous particles, preventing them from settling freely. Treatment with synthetic divalent ion acid solutions can neutralize the surface charges of red mud particles and facilitate the eviction of sodium ions as NaCl. The effectiveness of the divalent cation-impregnated solution has been discussed based on experimental results and existing literature.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are thankful to the Director, CSIR-IMMT, Bhubaneswar for his kind permission to publish this paper. The authors also wish to gratefully acknowledge Dr S.P Das, Chief Scientist, CSIR-IMMT for the useful discussion during the experimentation. Also, thanks to Dr P. C. Beuria, Head, Mineral Processing Department, CSIR-IMMT, for granting permission to carry out all necessary laboratory experiments.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e: All the authors contributed to the conception and design of the study. Sandeep Kumar Jena and Sapan Kumar Kandi performed the literature search, design of experiment and data analysis. Ms. Baijayantimala Mohanty and Ms. Ranil Bahalia performed laboratory experiments and analysis. The first draft of the manuscript was prepared by Sandeep Kumar Jena. All authors commented on the previous versions of the manuscript. All authors read and approved of the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials:\u003c/strong\u003e Data will be made available at a reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e No specific grant from any funding agency in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical approval\u003c/strong\u003e: Authors contribute to maintaining the integrity of the research and its presentation by following the rules of good scientific practice.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate:\u0026nbsp;\u003c/strong\u003e All authors agreed with the content and that all gave explicit consent to participate.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication:\u0026nbsp;\u003c/strong\u003e All authors agreed with the content and that all gave explicit consent to submit.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical Trial number:\u0026nbsp;\u003c/strong\u003eNot Applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAi T, Zhong D, Zhang Y, Zong J, Yan X, Niu Y (2021) The effect of red mud content on the compressive strength of geopolymers under different curing systems. 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J Clean Prod 257:120534\u003c/li\u003e\n \u003cli\u003eUSGS Mineral Commodity Summaries 2024 report (2024). https://pubs.usgs.gov/ periodicals /mcs2024/mcs2024.pdf\u003c/li\u003e\n \u003cli\u003eYang J, Liu X, Cui K, Lyu J, Liu H, Qiu J (2025) Hazards and dealkalization technology of red mud \u0026mdash;a critical review. Minerals 15:343. https://doi.org/10.3390/min15040343\u003c/li\u003e\n \u003cli\u003eZaidi AA (2021) Characteristics of settling of dilute suspension of particles with different density at high Reynolds numbers. Particuology 56: 62\u0026ndash;74\u003c/li\u003e\n \u003cli\u003eZakira U, Zheng K, Xie N, Birgisson B (2023) Development of high-strength geopolymers from red mud and blast furnace slag. J Clean Prod 383:135439\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Bauxite Residue, Dolomite, Dewatering, Characterization, Separation Mechanism","lastPublishedDoi":"10.21203/rs.3.rs-6930760/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6930760/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe generation of large quantities of red mud, or bauxite residue (BR), during the Bayer process of alumina production from bauxite poses a significant global environmental challenge. Approximately 1 to 2.5 tons of waste BR is produced for every ton of alumina manufactured, depending on the chemistry and mineralogy of the feed bauxite sample. Effectively managing, safely storing, and finding productive uses for this waste are three major concerns for global aluminum producers. This study investigates the effectiveness of a hydrochloric acid solution impregnated with calcium (Ca²⁺) and magnesium (Mg²⁺) ions for the settling and dewatering of BR slurry. Dolomite was used as the source of Ca²⁺and Mg²⁺ions. Various experimental parameters including acid concentration, agitation time, temperature, dolomite weight percentage, settling time, and solid weight percentage in the slurry were optimized to evaluate the dewatering process. Initial comparison experiments using 1.5-12N HCl and 1.5-12N HCl with Ca²⁺ and Mg²⁺ revealed the effectiveness of the acid solution enhanced by these divalent cations. The solid percentage in the slurry significantly influenced the settling efficiency; as the solid weight percentage increased from 2.5% to 15%, the settling efficiency gradually declined from 98% to 92%. However, when the solid weight percentage increased beyond this range, the settling value sharply dropped to 40%. The experiments maintained 5 minutes of slurry agitation followed by 2 minutes of settling using 1.5N HCl with 5g of dolomite. Further studies at higher solid concentrations indicated that either extended settling times or greater volumes of the impregnated acid solution would be necessary to achieve settling efficiencies greater than 90%. Additionally, the effects of dolomite amount and acid concentration were also considered during these experiments. The physicochemical characteristics of the BR and the processed product were analyzed using techniques such as Particle Size Analysis, ICP-OES, Zeta-Potential, XRD, FTIR, and FESEM-EDX studies to support the experimental findings. The incorporation of divalent cations like Ca²⁺ and Mg²⁺ into the low-concentration HCl solution significantly enhances the settling characteristics of BR particles. The possible settling mechanism is discussed based on experimental evidence, characterization results, and relevant literature.\u003c/p\u003e","manuscriptTitle":"Improved dewatering behaviour of Bauxite Residue using Calcium-Magnesium-impregnated acid solution","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-30 12:09:06","doi":"10.21203/rs.3.rs-6930760/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2025-08-08T14:34:12+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-07-09T09:45:03+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-25T11:36:35+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2025-06-25T10:14:10+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-23T04:19:50+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2025-06-20T02:48:27+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"77a70e08-2997-4047-8e9f-2c216467b563","owner":[],"postedDate":"June 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-11-10T16:00:08+00:00","versionOfRecord":{"articleIdentity":"rs-6930760","link":"https://doi.org/10.1007/s11356-025-37003-0","journal":{"identity":"environmental-science-and-pollution-research","isVorOnly":false,"title":"Environmental Science and Pollution Research"},"publishedOn":"2025-11-06 15:57:19","publishedOnDateReadable":"November 6th, 2025"},"versionCreatedAt":"2025-06-30 12:09:06","video":"","vorDoi":"10.1007/s11356-025-37003-0","vorDoiUrl":"https://doi.org/10.1007/s11356-025-37003-0","workflowStages":[]},"version":"v1","identity":"rs-6930760","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6930760","identity":"rs-6930760","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}