Synthesis of sodium silicate from the acid leached calcined kaolinitic clay residue of aluminum sulfate industry

Synthesis of sodium silicate from the acid leached calcined kaolinitic clay residue of aluminum sulfate industry | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Synthesis of sodium silicate from the acid leached calcined kaolinitic clay residue of aluminum sulfate industry nabil Alsagheer, Gamal Aboulfotouh This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3860849/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Acid leached calcined kaolinitic clay (ALCKC) is a residue of aluminum sulfate coagulant manufacturing process; it is a solid acidic waste that is harmful to environment. Silica dissolution process was systematically studied, including the thermodynamic analysis and the influence of aluminum content on the leaching of amorphous silica. Simulation studies have shown that aluminum combine with silicon to form silica–alumina gel, and zeolite, thereby preventing the leaching of silica. Maximizing the removal of aluminum, iron and titanium can effectively improve the leaching of silica in the subsequent process, and corresponding element removal rates are 45%, 41% and 15%, respectively. The removed aluminum is reused to prepare PAC. The silica extraction rate reached 88% at a conditions of (NaOH; 20%, NaOH to ALCKC; (v/ w) 5, 75°C, 2h), and sodium silicate modulus ( n SiO 2 : n Na 2 O) is 1.1. The results indicated that a large amount of silica was existed in amorphous form. Precipitated silica was obtained by acidifying sodium silicate solution at a pH 7.0 using sulphuric acid. The prepared sodium silicate (1.1) was used for further synthesis of sodium silicate with n SiO 2 : n Na 2 O = more than 3 Scientific community and society/Scientific community/Research data/Databases Scientific community and society/Scientific community/Research data Acid leached kaolin residue silica extraction amorphous silica sodium silicate Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1 Introduction Aluminum sulphate (AS) is the most widely used inorganic coagulant for water treatment because of its satisfactory performance for water clarification, easy to be used and relatively cheap. There are well known technologies for preparing AS. In general, an acceptable national synthesis method involves a one-step method where the aluminum hydroxide and sulphuric acid were used as raw materials for preparation of liquid AS. Other two-step method is used where kaolinitic clay is calcined to metakaolin and sulphuric acid is added for extraction of alumina and formation of aluminum sulphate. However, this method results in the production of large quantities of acid residues, known as “Acid leached kaolinitic clay (ALCKC). The annual output of AS from kaolin is about 0.25 Million tons. Approximately 320 kg of ALCKC are produced per ton liquid AS produced. ALCKC is growing with increasing the development of industrial production. ALCKC contains traces of aluminum sulphate. This feature will lead to high processing costs and even difficult to handle. For a long time, those solid wastes were disposed by stacking in many enterprises. A considerable amount of waste has not only restricted the development of the AS industry, but also caused serious environmental problems. However, from another perspective, ALCKC is also a potential raw material for some industries such as zeolite adsorbent, geopolymer, pozzolanic cement, valuable metal recovery, and building materials. Sodium silicate can be prepared by this by-product (its aqueous solution is commonly known as water glass) is an important chemical product and also the main raw material for other silica-containing products [1]. Sodium silicate is widely used as anti-corrosion materials, binders, refractory materials, white carbon black, acid-resistant cement, impregnates, fixing agents and molecular sieve catalysts, and other fields, covering almost all aspects of human life [2]. Therefore sodium silicate is the most extensively used industrial raw material after acids and bases [3]. However, the current industrial production method of sodium silicate requires a huge energy input, which is, fusing sodium carbonate and high quality quartz sand at temperatures 1,300°C or 1,600°C [4]. So, the synthesis of sodium silicate and separation silica from rich silica residue (such as slag, industrial solid waste) is being intensively investigated. The recovery of silica from different wastes such as fly ashes, and rice husk ash has been reported [5–7]. It was reported that the waste of corn stalks are used as raw materials, roasted at 700°C, and then leached with sodium hydroxide to prepare liquid sodium silicate [8]. The municipal waste incineration bottom ash was studied to prepare sodium silicate and meso-porous silica at low temperature [9]. The above mentioned research works have achieved the alkali leaching silica extraction from silica-rich slag, but the modulus and concentration of liquid sodium silicate are less than the commercial standard, which is unsuitable for some applications. There is a report on the hydrothermal preparation of high modulus liquid sodium silicate (3.0–3.8) using precipitated silica from silica sand, which produced in the titanium dioxide pigment manufacturing process [10]. However, there were no reports on the preparation of high modulus sodium silicate from ALCKC. In addition, from an economic and environmental point of view, if silica can be extracted from ALCKC and synthesized high modulus sodium silicate at low temperature, it will greatly promote the application of ALCKC. Based on the previous work, the aim of the present study was to extract precipitated silica from ALCKC and synthesize high-modulus sodium silicate at low temperature. First, the acid leaching pretreatment of ALCKC was carried out to reduce the influence of other elements on the extraction of silica. Usually, most reports only focus on the extraction of valuable metals from waste and rarely care about the reuse of leachate (pre-treat solution). The obtained acid leaching residue (ALCKC) is a potential raw material for preparing silicon-containing products. The silica dissolution from ALCKC was studied by varying the NaOH-to-ALCKC ratio, leaching time, temperature, and sodium hydroxide concentration to optimize the conditions and understand dissolution process. The reaction thermodynamics are studied. The sodium silicate solution gained by the alkaline leaching process is used to prepare precipitated silicon and synthesize high-modulus liquid sodium silicate at normal pressure. The followed process has performed at low temperature, low with simple equipment, and zero liquid waste. The process minimizes the residue of alum industry and at the same time reduces the residue of ALCKC. 2 Materials and methods 2.1 Material and analysis ALCKC material used was collected from the kaolin manufacturing process of AS in Abu Zaabal Province Egypt. The samples washed carefully to remove aluminum sulphate. The collected ALCKC has approximately 20 wt % moisture content. Hydrochloric acid (37%), sodium hydroxide pellets, and concentrated sulfuric acid (98%) were obtained from the local producers. All the chemicals used in this study were of general purpose reagents (GPR) and were applied without purification. Table 1 presents the chemical composition of ALCKC, indicating that silica and aluminum are the main components of the solid waste. In addition, ALCKC contains a series of potentially leachable elements: Ti and Fe. The aluminum sulphate content is the main reason for ALCKC becoming acidic waste, and its content is as high as 2.0% as SO 4 − 2 . Table 1 Chemical composition of acid leached calcined kaolinitic clay Compound Amount (weight %) O 45.3 Si 42.2 Si (amorphous) 31 Al 4.15 Fe 0.21 Ti 1.44 Mg 0.036 Ca 0.043 Na 0.008 K 0.009 S 0.72 P 0.004 Sr 0.008 Cl 0.06 L.O.I 4.8 2.2 Experimental procedure An acid treatment of ALCKC was performed to extract the residual aluminum, iron and titanium, to reduce their effect on the extraction of silica. ALCKC leaching experiment was carried out in a 2.5 L flask. First, ALCKC was mixed with hydrochloric acid solution (8M) to form a mixture with an acid – ALCKC ratio of 5 (v/w). After stirring the mixture for 3h at 85°C, it was filtered to obtain leachate and acid-leached ALCKC respectively. The ALCKC was washed with tap water and dried at 110°C for 6 h. The washing liquid and leachate can be used to prepare PAC, so that no waste liquid was discharged. 2.3 Alkaline leaching The silica extraction from the ALCKC was investigated by varying the mass ratio of ALCKC to NaOH (1: 2, 3, 4, 5, and 6), leaching time (0.5, 1, 2, 3, and 5 h), temperature (25°C, 45°C, 60°C, 75°C, and 90°C), and sodium hydroxide concentration (5, 10, 15, 20, and 25%) to determine the optimum conditions for leaching. The optimum conditions for the procedure were done as follows: 100 g of ALCKC and 20 NaOH solutions with a NaOH to ALCKC ratio of 5 are mixed in 1,000 ml flask, and then heated at 75°C for 2 h. 2.4 Preparation of silica powder Silica powder was prepared using the method described in U.S. Patents 8287822B2; 2012. [ 11 ]. 100 mL of water was added into 1L precipitation vessel and heated to 85°C. The pH was initially adjusted to be between 8.2 to 8.5 with keeping the temperature constant, little sodium silicate solution was added. Then, a certain amount of sodium silicate is continuously added to the above aqueous solution at a rate of 20 mL min − 1 and a sufficient quantity of 25% sulfuric acid solution to ensure that the pH was held constant. The solution was allowed to settle for 30 min. the precipitated silica was washed with deionized water several times and dried. 2.5 High modulus sodium silicate preparation According to the theoretical SiO 2 /Na 2 O ratio as a dissolved silica and sodium hydroxide (4/1) were prepared under alkaline leaching conditions to produce high-modulus liquid under temperature range of 85–200°C. The reaction was carried out for 3h and then the solution was filtered. Clear liquid was obtained with high modulus sodium silicate. 2.6 Characterization methods The mineralogical composition of the ALCKC was determined with XRD; Shimadzu ZU, Japan). X-ray powder diffraction patterns were obtained using a Rigaku D/max-TTR III X-ray diffractometer, at 40 kV and 250 mA, and using Cu K α filtered radiation ( λ = 0.1542 nm). The samples were subjected to full- element analysis using XRF-1800 wavelength dispersive X-ray fluorescence spectrometer (XRF; Test equipment comes from Shimadzu Corporation, Japan). The concentrations of Al and Ti were digested by hydrochloric acid (8M) followed by inductively coupled plasma emission spectrometry (ICAP7400 Radial, Thermo Fisher Scientific, USA) analysis. FT-IR spectra were recorded in the region 4,000– 400 cm − 1 in a WQF-200 model FTIR spectrometer made by Beijing Optical Instrument Factory, using the KBr pellet technique (about 1 mg of sample and 300 mg of KBr were used in the preparation of the pellets). 2.7 Determination of sodium silicate modulus The determination of sodium oxide and silica content in sodium silicate was carried out according to CS standard GB/T 4209 − 2008. The modulus of sodium silicate is calculated as the molar ratio (Mod) of silica to sodium oxide (SiO 2 /Na 2 O), and calculated according to Eq. 1: Sodium silicate modulus (SiO 2 /Na 2 O), = ( w 1 / w 2 ) × 1.03 (1) where w 1 and w 2 are the mass fraction of silica and sodium oxide in the water glass, respectively, and 1.03 is the relative molecular mass ratio. 3 Results and discussion 3.1 ALCKC XRD The XRD pattern of ALCKC is shown in Fig. 1 . It contains minerals such as quartz and anatase. There is a large amount of amorphous silica in the ALCKC. Before the production of aluminum sulphate from kaolin, the kaolin has been calcined at 700°C for 90 min. to form activated aluminum silicate that react with sulphuric acid to yield aluminum sulphate solution and amorphous silica (ALCKC) [ 12 ]. The dissolution of calcined kaolinitic clay and the formation of aluminum sulphate and the precipitation of silica are carried out according to the equations: (2) and (3): Al 2 O 3 ⋅2SiO 2 ⋅ 3H 2 O + 6H 2 SO 4 → 2Al 2 (SO 4 ) 3 (Alum) + 2H 2 SiO 3 + 3H 2 O (2) H 2 SiO 3 → SiO 2 (Amorphous) + H 2 O. (3) 3.2 ALCKC grain size distribution The grain size-distribution of the ALCKC by laser granulometry was plotted in Fig. 2 which shows that the diameter of ALCKC in cumulative 90% of ~ 78 µm and 10% of ~ 4.9 µm with an average diameter of ~ 9.6. BET analysis revealed a high fineness for ALCKC (20 m 2 /g). Thermogravimetry and differential scanning calorimetry of ALCKC (TG/DSC) The thermal behavior of the starting ALCKC is presented in the Fig. 3 . The main changes revealed by TG and DSC analysis are as follows: The DSC line indicates the removal of absorption water or free water as demonstrated in the small band at 186.6°C (water absorbed in pores and on the surface = 1.73%). The thermal degradation occurs between 186.6 and 550°C is associated with the presence of silicate and the chemical dewatering within the structure. The remaining weight loss at between 550 and 1000°C can be attributed to more bound water which results from silanol or aluminol. The results show that approximately 6.93% measured over the temperature range of ambient to 988°C. 3.3 Acid leaching and aluminum recovery ALCKC is an acidic solid waste containing traces of aluminum sulphate (AS). The presence of AS causes that the grains of ALCKC to stick to each other, and it is difficult for ALCKC to dump or to reuse without perfect washing, and a large amount of washing is generated. The ALCKC produced in this process is a low-priced raw material for preparing commercial-grade sodium silicate. ALCKC was leached with hydrochloric acid solution (8M) at 85°C for 3h. Figure 4 show that when the HCl/ALCKC was 1:1, the removal rates of aluminum, iron, and titanium were: 44.6%, 38%, and 7.5%, respectively. The HCl/ALCKC ratio was continued to 6, and the extraction rates were: Al: 45%, Fe: 41.2% and Ti: 15% respectively with slight change except for titanium. It was noticed that the trend of titanium extraction first increases up to 15% and then decreases to 13.8 at higher HCl ratio. This is because after the titanium is dissolved from the titanate mineral, it is precipitated again in the form of rutile mineral [ 13 ]. The result shows that the removal effect of aluminum and iron are better than that of titanium. The reason is that titanium mainly exists in ALCKC in the form of anatase (TiO 2 ). It is difficult to achieve titanium leaching at low temperature. To minimize the effect of other aluminum and titanium on the subsequent silica extraction, ALCKC was leached with excess hydrochloric acid solution (8M). Therefore, the best HCl/ALCKC ratio was set as 5. The chemical composition of the ALCKC is presented in Table 2 . Compared with Table 1 , the solute ions after acid treatment can be easily washed with tap water. View of thermodynamic, silicate mineral such as ALCKC react with hydrochloric acid at room temperature. However, the solubility of minerals is significantly affected by grain size and particle density. The extract solution is used to prepare poly aluminum chloride to reuse of the extract as indicated in Eq. 5 [ 14 ]: Table 2 Elemental composition of ALCKC after treatment with HCl Compound Amount (weight %) O 45.8 Si 45.3 Si (amorphous) 34.2 Al 6.7 Fe 0.05 Ti 1.8 Mg < 0.01 Ca < 0.01 Na ND K ND S ND P ND Sr ND Cl ND L.O.I 0.2 PAC was determined according to the drinking water standard [ 14 ]. The content of Al 2 O 3 in the liquid PAC was 11.5%, the basicity was 80%, and the heavy metal content complies with Egyptian standards (in Table 3 ). It can be seen from Table 3 that the quality of PAC can reach the drinking water standard of Egyptian standards. This process achieves efficient leaching of aluminum in the residual aluminum and reuse of leaching solution. 3.4 Effect of aluminum content on the alkaline leaching on the silica extraction Presence of aluminum affects the silica extraction [ 15 ]. The hindering effect of aluminum on the extraction of silica was investigated. Before the start of the ALCKC silica extraction experiment, sodium aluminate simulation experiments were used to study the effect of aluminum on the silica extraction process. 1000 mL of sodium hydroxide (20%) was used to dissolve 73.3 g of pure amorphous silica (the amount of this amorphous silica is equivalent to the total amount of silica in 100 g of ALCKC). The effect of alkaline solution dissolving different sodium aluminate alone on silica extraction was tested. Figure 5 shows that aluminum affect the dissolution of silica, and the dissolution rate of silica decreases with the addition of sodium aluminate. First, the silicate mineral reacts chemically with the alkali during the reaction, and then the silica enters the solution in the form of SiO 2− . The silicate ion reacts with sodium aluminate and then silicon precipitates out in the form of hydrated sodium aluminosilicates [16]. The precipitated particulates coat the surface of the ALCKC and prevent the contact of silica with the alkali. The dissolution of amorphous silica and the formation of silicon residue in the silica extraction take place according to the equations 4–6: SiO 2 (amorphous) + 2NaOH → Na 2 [H 2 SiO 4 ], (4) Al 2 O 3 .2SiO 2 .H 2 O + 2OH− → 2H 4 SiO 4 + 2Al(OH) 4 − (5) x Na 2 [H 2 SiO 4 ] + 2NaAl(OH) 4 → Na 2 O⋅Al 2 O 3 ⋅ x SiO 2 ⋅2H 2 O + 2 x NaOH (6) XRD of sodium aluminum silicate residue shows that different hydrated sodium aluminum silicates are coexisting in the filter residue (Fig. 6 ). The acid leached calcined kaolin dissolves in alkaline media, giving rise to silica [SiO 2 (OH) 2 ] 2− and [SiO(OH) 3 ] − as well as aluminum [Al(OH) 4 ] − monomers. These monomers can inter-react to yield aluminosilicates that precipitates in the form of a Na 2 O–Al 2 O 3 –SiO 2 –H 2 O gel [ 9 , 17 ]. Therefore, the hydrochloric acid pretreatment is necessary to effectively remove aluminum in the ALCKC. 3.5 The influence of NaOH–ALCKC ratio on silica extraction Sodium silicate was prepared using ALCKC with NaOH solution (20%) at 75°C for 2h. The effect of NaOH to ALCKC ratio was tested to determine the maximum extraction of silica, and the results are provided in Fig. 7 . About 65% of the available silica was dissolved during the first NaOH–ALCKC ratio of 2. The dissolution of silica increased to 88% when the liquid– solid ratio was reached to 5. Moreover, when the ratio was increased to 6 under the same conditions, a minor increase for the extraction rate occurred. The ratio SiO 2 /Na 2 O of liquid sodium silicate showed the opposite result, decreasing from 2.83 to 0.95 and this is normal. As the NaOH ratio increases, the dissolved silica in the solution increased slowly. Small amount of titanium dissolved into the alkaline leaching solution without significant effect on silica extraction, compared with aluminum. The aluminum dissolution rate is relatively high, which is 8.31% when the NaOH/ALCKC ratio is 6 as shown in Table 3 and Fig. 8 . Aluminum and silicon are present in ALCKC, in a stratified structure. Table 3: Effect of NaOH–ALCKC ratios on the dissolution of aluminum and titanium Parameter NaOH/ALCKC 2 3 4 5 6 Concentration (%) Aluminum 3.12 5.2 5.35 7.48 8.31 Titanium 0.15 0.17 0.19 0.2 0.2 The alkaline leaching of ALCKC is shown in equations: 7–11[ 18 ]. The corresponding thermodynamic calculation results are presented in Table 4 . Thermodynamic data may show that TiO 2 , and quartz can react with sodium hydroxide at room temperature. However, as the Gibbs free energy of the reaction is very low, it can be considered that there is no reaction takes places at room temperature for titanium oxide and quartz. Total dissolution rate of aluminum is about 7.48% when the NaOH /ALCKC ratio is 5. The dissolved aluminum resulted from un-calcined kaolinite. This means that less sodium silicate is obtained from decomposition of kaolinitic residue in ALCKC and there was no considerable effect of titanium. The majority of silicon dissolved in the sodium hydroxide solution comes from the amorphous silica in ALCKC. Al(OH) 3 (s) + NaOH(aq) = Al(OH) 4 − (aq) + Na + (aq) (7) Al 2 O 3 ⋅ 2SiO 2 ⋅ 3H 2 O (s) + 6NaOH (aq) = 2NaAlO 2(aq) + 2Na 2 SiO 3(aq) + 5H 2 O (aq) , (8) TiO 2(s) + 2H 2 O (aq) = Ti(OH) 4(aq) , (9) SiO 2 (quartz) + 2NaOH (aq) = Na 2 SiO 3(aq) + H 2 O (aq) , (10) nSiO 2 (amorphous) + 2NaOH(aq) = nSiO.Na2O ⋅ H 2 O (11) Table 4 Thermodynamic data for the reaction process taken place in sodium hydroxide leaching process) Equation Thermodynamic data Enthalpy of formation ∆H f 0, 25°C Entropy S 0 , 25°C Free energy of formation ∆ G f 0 , 25°C Free energy of formation ∆ G f 0 , 75°C Hθ kJ mol − 1 J mol − 1 kJ mol K − 1 kJ mol K − 1 10 10.5 52 168.04 -5.25 11 -1980 385.7 -1887.5 -2072.2 As shown in Figs. 9 the residue remains after silica extraction processes with varying NaOH to ALCKC (5/1, 6/1), the amplitude of the peak that locate in 2θ from 18° to 30° decreased [ 19 ]. This means minimizing of amorphous silica concentration after digestion with alkali and converting of another form. With the increase in the NaOH–ALCKC ratio, the diffraction peaks of other mineral phases are relatively strengthened in alkali leaching residue. This is caused by the dissolution of the soluble silica coating the surface of the mineral. From the cost benefit point of view, the ratio 5:1 is the optimum one for perfect reaction of silica extraction. And the SiO 2 /Na 2 O ration is 1.1 and the corresponding extracted Silica is 88%. 3.6 Effect of reaction time on silica extraction The effect of time on silica extraction using NaOH: ALCKC ratio of 5:1 and at constant temperature and ambient pressure was tested (Fig. 10 ). The results show that the solubility of silica in ALCKC is high and the extraction rate of the amorphous phase silica reached 57% in a relatively low period of time (30 min.). The extraction reached 87.6 after 3h. It can be deduced that amorphous silica precipitates remain in the ALCKC after extraction of aluminum by sulphuric acid in the process of producing aluminum sulphate from calcined kaolin. Most of the silica in the ALCKC is not coated with other minerals, so that the silica is easily leached by sodium hydroxide. 3.7 Effect of reaction temperature on silica extraction The reaction temperature is an important factor that affects the silica extraction. Figure 11shows that as the reaction temperature increase the rate of extraction increases. It was noticed that silica extraction rate increases from 50–88% when leaching temperature is increased from 25°C to 100°C, taking in account that the modulus of liquid sodium silicate was between 1.0 and 1.15. The leachability of silica was achieved at relatively lower temperature (70–100°C), this refers to the high solubility of amorphous silicate of ALCKC. 3.8 Effect of sodium hydroxide concentration on silica extraction The silica extraction was investigated under the several NaOH concentrations (Fig. 12 ). When the concentration of sodium hydroxide increases from 5 to 25%, the silica extraction rate of in ALCKC increases significantly from 35.3% up to 88%. Table 5 present the chemical composition of ALCKC after vigorous leaching. From table 6, it can be concluded that the silica in the ALCKC mainly exists in two forms of amorphous silicate and crystalline quartz. The amorphous was extracted with a percentage of 88%, therefore the quantity of solid was decreased by the alkaline leaching. It was known that the strength of covalent bond Si-O in the crystalline silica is much higher than the bond in the amorphous phase. Therefore the crystalline bond Si-O requires 460 kJ/mole to distract this bond and high pressure for destruction and formation of sodium silicate and it is difficult to rise up the SiO 2 /Na 2 O ratio [ 10 ]. It was reported that amorphous silica as the main component, such as silica fume is used to prepare sodium silicate applying high pressure hydrothermal reaction. Table 5 Element composition of the final residue of ALCKC after alkaline leaching Compound Amount (weight %) O 41.4 Si 17.3 Si (amorphous) 4 Al 10.3 Fe 0.05 Ti 4.2 Mg < 0.01 Ca < 0.01 Na ND K ND S ND P ND Sr ND Cl ND L.O.I 0.2 3.9 Synthesis of amorphous silica The soluble species is in the form of Si(OH) 4 [ 10 ]. As the solution is alkaline (pH 9), so precipitation of silica takes place by acidification of the alkaline solution (resulted using NaOH solution (20%) at 85°C for 3h). Sulfuric acid (30%) was used. Figure 13 illustrates the effect of pH of precipitation of silica. The recovery rate of silica has achieved more than 96% at pH value of 7 [ 25 ]. The silica has a higher recovery rate of 98.62%, with low sulfuric acid adding, for neutral liquid sodium sulfate solution. It was found that liquid sodium silicate can be neutralized by using H 2 SO 4 to form pure amorphous silica. The neutralization process of the sodium silicate solution resulting in mono and poly nucleation silicon and their polymerization create polysilicic acid in a colloidal form that upon drying, silicon precipitate is formed. The rate of polymerization of silicic acid affects the aggregation and the grain size of silica. The cross-linking reactions and physical aggregations causing anionic charged silica “sols and finally formation of gels [ 26 ]. The XRD pattern of amorphous silica obtained under neutral conditions is shown in Fig. 14 . Because amorphous silica shows broad diffraction peaks (2 θ from 15° to 30°) [ 10 ]. The IR spectrum for the prepared silica is shown in Fig. 15 . The characteristics bonds of silica appeared at 793, and 475 cm − 1 , these bands for symmetric and asymmetric stretching vibrations of Si–O bond, respectively. Another absorption peak emerged at 1,090 cm − 1 for the bending vibration of the Si–O–Si bond [ 23 ]. The absorption peaks at 1,640 and 3486 cm − 1 are the absorption peaks of water, surface adsorption water, and structured water. The 1,640 band is the bending vibration peak of H–O–H, and it is relevant by capillary water and surface adsorbed water. The latter is the asymmetric stretching vibration peak of O–H, which is related to structured water [ 27 , 28 ]. IR spectrum shows that the formed silica has small water content without any absorption peak of cross-linking reactions and physical aggregations causing anionic silica. 3.10 Preparation of high modulus liquid sodium silicate Amorphous silica and alkaline leaching solution were mixed according to the theoretical ratio or SiO 2/ Na 2 O of 4 to prepare high modulus liquid sodium silicate. The process was continued to 3h at different temperature, starting from 60°C up to 200°C, the digested solution was filtered to get the sodium silicate solution. As illustrated in Fig. 16 , the amorphous silica is easily soluble in alkaline medium. Liquid sodium silicate with a modulus of 3 can be prepared at 80°C under normal pressure. As the temperature increases the modulus increases. At 200°C the SiO 2 to Na 2 O become 3.55. There are three important principle in the amorphous silicon solubility - pH diagram: first the precipitation range of amorphous silicon; second: the stable silicon poly-anions; and third: the thermodynamic prevailing monomeric Si species [Si(OH) 4 , SiO(OH) − , and SiO 2 (OH) 2− ]. The pH value of the solution is 12, and this pH confirms the formation of stable polymeric structure of sodium silicate [ 26 ]. 4 Conclusions Acid leached kaolin of alum industry (environmentally harmful acid solid waste) can be used for synthesis of silica and of high-modulus sodium silicate. The leachate solution resulted from digestion of the acid leached kaolin was reused to prepare high basicity PAC. The silica extraction rate from ALCKC reached 88% and the SiO 2 :nNa 2 O of the prepared sodium silicate was 1.1 with NaOH–ALCKC ratio of 5:1 at the 20% NaOH at 75°C for 2 h. The recovery rate of amorphous silica from alkaline leachate reached 98.62% using sulfuric acid at pH of 7. 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Titanium metallurgy. 2nd ed. China: Metallurgical Industry Press; 2007. Brahmi D, Merabet D, Belkacemi H, Mostefaoui TA, Ouakli NA. Preparation of amorphous silica gel from Algerian siliceous by- product of kaolin and its physico chemical properties. Ceram Int. 2014;40(7):10499–10503. doi: 10.1016/ j.ceramint.2014.03.021 Li FT, Jiang JQ, Wu SJ, Zhang BR. Preparation and performance of a high purity poly-aluminum chloride. Chem Eng J. 2010;156:64–9. doi: 10.1016/j.cej.2009.09.034 Peng H, Peters S, Vaughan J. Leaching kinetics of thermally- activated, high silica bauxite. Light Met. 2019;11–7. doi: 10.1007/978-3-030-05864-7-2 Strachan DM, Croak TL. Compositional effects on long-term dissolution of borosilicate glass. J Non-Cryst Solids. 2000;272(1):22–33. doi: 10.1016/S0022-3093(00)00154-X Wang H, Feng QM, Liu K. The dissolution behavior and mechanism of kaolinite in alkali-acid leaching process. Appl Clay Sci. 2016;132–133:273–80. doi: 10.1016/ j.clay.2016.06.013 John DA. Lang’s handbook of chemistry. Beijing: Science Press; 2003. ISBN: 7-03-010409-9. Costa JAS, Paranhos CM. Systematic evaluation of amorphous silica production from rice husk ashes. J Clean Prod. 2018;192:688–97. doi: 10.1016/j.jclepro.2018.05.028 Zhang S, Liu Y. Molecular-level mechanisms of quartz dissolution under neutral and alkaline conditions in the presence of electrolytes. Geochem J. 2014;48(2):189–205. doi: 10.2343/ geochemj.2.0298 Gorrepati EA, Wongthahan P, Raha S. Silica precipitation in acidic solutions: mechanism, pH effect, and salt effect. Langmuir. 2010;26(13):10467–74. doi: 10.1021/la904685x Stumm W, Huper H, Champlin RL. Formulation of polysilicates as determined by coagulation effects. Enviorn Sci Technol. 1967;1(3):221–7. doi: 10.1021/es60003a004 Owoeye SS, Jegede FI, Borisade SG. Preparation and characterization of nano-sized silica xerogel particles using sodium silicate solution extracted from waste container glasses. Mater Chem Phys. 2020;248:122915. doi: 10.1016/j.matchemphys.2020.122915 Lai S, Yue L, Zhao X. Preparation of silica powder with high whiteness from palygorskite. Appl Clay Sci. 2010;50(3):432–7. doi: 10.1016/j.clay.2010.08.019 Additional Declarations There is NO Competing Interest. Supplementary Files highlights13012024.docx Dataset 3 Cite Share Download PDF Status: Posted Version 1 posted 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. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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-3860849","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":267229244,"identity":"f81efc8c-95aa-4298-8b6d-f963cd735cac","order_by":0,"name":"nabil Alsagheer","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA00lEQVRIiWNgGAWjYFACHoYDDxgY5PhB7IQCYrUkMDAYSzaAtBgQqYUBqCVxwwEQhxgt5uxnDx5IbLuTuPn86sQPDwwY5PnFDuDXYtmTl3Ag4cwz42033m6WADrMcObsBPxaDA7kGBxIqDgsu+3G2Q0gLQkGtwlpOf8GqMXgMOPmGWc3/yBOyw2ILYob+Hu3EWnLjXcQv0jc4N1mkWAgQYRfzuce/vCx7Y4cf//ZzTd/VNjI80sT0AIFBxgYJMAqJYhSDtXCf4Bo1aNgFIyCUTDCAAAowU/mG0fvnAAAAABJRU5ErkJggg==","orcid":"","institution":"Aluminum Sulfate Co., Egypt","correspondingAuthor":true,"prefix":"","firstName":"nabil","middleName":"","lastName":"Alsagheer","suffix":""},{"id":267229245,"identity":"76f0f85a-c74b-4ad3-988a-d7dfb2c70ec4","order_by":1,"name":"Gamal Aboulfotouh","email":"","orcid":"","institution":"Aluminum sulphate corporation of egypt","correspondingAuthor":false,"prefix":"","firstName":"Gamal","middleName":"","lastName":"Aboulfotouh","suffix":""}],"badges":[],"createdAt":"2024-01-13 17:45:24","currentVersionCode":1,"declarations":{"humanSubjects":false,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":false,"humanSubjectConsent":false,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-3860849/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3860849/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":49694491,"identity":"2a37c0a5-4924-4a36-bc6f-8f836532a464","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":78676,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of ALCKC\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/251606da7a52e58f042d4833.png"},{"id":49694492,"identity":"5dbe8cc1-b58b-4feb-8c5e-f4234c02c3b5","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":34173,"visible":true,"origin":"","legend":"\u003cp\u003eThe grain size-distribution of the ALCKC by laser granulometry\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/7dbc8ecbbbd10c653bd018a5.png"},{"id":49992675,"identity":"ece3b550-c270-4eed-ae58-ab31ff31999e","added_by":"auto","created_at":"2024-01-22 18:58:23","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":70833,"visible":true,"origin":"","legend":"\u003cp\u003e\u003ca href=\"https://www.eag.com/techniques/phys-chem/thermogravimetry-differential-thermal-analysis-tg-dta/\"\u003eThermogravimetry and differential scanning calorimetry of ALCKC (TG/DSC)\u003c/a\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/41ca8ec461895fe44ced2879.png"},{"id":49696217,"identity":"0653b9d4-eade-4b22-b400-73d500944b81","added_by":"auto","created_at":"2024-01-16 15:06:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":18536,"visible":true,"origin":"","legend":"\u003cp\u003eRemoval of harmful elements by acid leaching pretreatment; (HCl concentration:\u003c/p\u003e\n\u003cp\u003e8M, time: 3 h, temperature: 85°C).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/252344d8ad1d3cf79928fc6d.png"},{"id":49695239,"identity":"806f342e-93e7-4cde-9f26-633a59107301","added_by":"auto","created_at":"2024-01-16 14:50:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":12753,"visible":true,"origin":"","legend":"\u003cp\u003ethe effect of alkaline dissolving different aluminum on silica extraction.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/5a674766a7d5de9cb549a355.png"},{"id":49696219,"identity":"64fb394f-d032-4d38-80b2-dbe731b0928f","added_by":"auto","created_at":"2024-01-16 15:06:43","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":86976,"visible":true,"origin":"","legend":"\u003cp\u003ealuminum-silicon residue\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/99ba680ecef16c51b7fa0865.png"},{"id":49695738,"identity":"9845ab03-7d11-4a60-b545-68c752d2816b","added_by":"auto","created_at":"2024-01-16 14:58:43","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":21212,"visible":true,"origin":"","legend":"\u003cp\u003ethe effect of NaOH/ALCKC ratio on sodium silicate Modulus and silica extraction rate (NaOH concentration: 20%, time: 2h, temperature: 75°C).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/e7fd1ffe4ce1eabf1c8f7b00.png"},{"id":49694870,"identity":"8455d2ae-076d-4e8e-83f5-0bed1e6cc18c","added_by":"auto","created_at":"2024-01-16 14:42:43","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":14907,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of NaOH–ALCKC ratios on the dissolution of \u0026nbsp;aluminum and titanium\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/f0dc02b8cd3170a766cb0f03.png"},{"id":49694498,"identity":"9a3ace5f-390d-4de3-b310-191c6b91cb4f","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":100465,"visible":true,"origin":"","legend":"\u003cp\u003eXRD diffractogram mineral phases present in the ALCKC recovered after the extraction experiments with NaOH/ALCKC (5/1 and 6/1) for the duration of 2 h at 75°C\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/ef5f0ef96940ba0c7d40de5e.png"},{"id":49694875,"identity":"1102fbb2-31a0-4384-961e-5cbc6802855d","added_by":"auto","created_at":"2024-01-16 14:42:43","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":14140,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of time on silica extraction using NaOH: ALCKC ratio of 5:1 and at constant temperature and ambient pressure\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/ccb0d8d6f37d5c521cbeaddd.png"},{"id":49695742,"identity":"5881a985-5226-46e6-b29e-f06dd9d75837","added_by":"auto","created_at":"2024-01-16 14:58:43","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":17198,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of reaction temperature on silica extraction (NaOH concentration: (20%) temperature: 75°C).\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/add0eb8822ef6372b0f47480.png"},{"id":49695741,"identity":"c74a46f2-6b81-4756-9add-e4c8e941ea44","added_by":"auto","created_at":"2024-01-16 14:58:43","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":14863,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of sodium hydroxide concentrations on silica extraction (20%), time 2 h, temperature: 75°C).\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/ad543dd45e79303114b23601.png"},{"id":49694505,"identity":"c66656ed-bdba-496a-a1cf-8b1f59e9902d","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":12784,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of pH of precipitation of silica.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/c50be6ab217f0ea288a12744.png"},{"id":49694880,"identity":"79df0331-b1ce-439e-bf27-fdee91fe922f","added_by":"auto","created_at":"2024-01-16 14:42:43","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":44419,"visible":true,"origin":"","legend":"\u003cp\u003eThe XRD pattern of amorphous silica obtained under neutral conditions\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/02f877fb811fd35e4db7b1c0.png"},{"id":49694507,"identity":"ee7eca97-695b-4f73-a52d-e2f03ccdb271","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":97524,"visible":true,"origin":"","legend":"\u003cp\u003eThe IR spectrums for the prepared silica\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/b5bb45c0251b18e0a09338ec.png"},{"id":49694504,"identity":"3891a0fe-20de-4bf8-b255-497750bd8f6b","added_by":"auto","created_at":"2024-01-16 14:34:43","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":14091,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of reaction temperature on the preparation of high-modulus sodium silicate (reaction duration 3h).\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/79b5a6fb660e17ad95446528.png"},{"id":49992779,"identity":"c3feeb48-e86d-463e-bfa8-0f94f119ca65","added_by":"auto","created_at":"2024-01-22 18:58:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1024121,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/3658be70-2a6d-4a20-bda8-e25a49d795f6.pdf"},{"id":49695243,"identity":"a8b97bf1-84f3-4bd8-9a5c-d7b8b340a914","added_by":"auto","created_at":"2024-01-16 14:50:43","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":16597,"visible":true,"origin":"","legend":"Dataset 3","description":"","filename":"highlights13012024.docx","url":"https://assets-eu.researchsquare.com/files/rs-3860849/v1/0e1a4affdbac79c91688eee5.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eSynthesis of sodium silicate from the acid leached calcined kaolinitic clay residue of aluminum sulfate industry\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eAluminum sulphate (AS) is the most widely used inorganic coagulant for water treatment because of its satisfactory performance for water clarification, easy to be used and relatively cheap. \u0026nbsp;There are well known technologies for preparing AS. In general, an acceptable national synthesis method involves a one-step method where the aluminum hydroxide and sulphuric acid were used as raw materials for preparation of liquid AS. Other two-step method is used where kaolinitic clay is calcined to metakaolin and sulphuric acid is added for extraction of alumina and formation of aluminum sulphate. However, this method results in the production of large quantities of acid residues, known as “Acid leached kaolinitic clay (ALCKC).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe annual output of AS from kaolin is about 0.25 Million tons. Approximately 320 kg of ALCKC are produced per ton liquid AS produced. ALCKC is growing with increasing the development of industrial production. ALCKC contains traces of aluminum sulphate. This feature will lead to high processing costs and even difficult to handle. For a long time, those solid wastes were disposed by stacking in many enterprises. A considerable amount of waste has not only restricted the development of the AS industry, but also caused serious environmental problems. However, from another perspective, ALCKC is also a potential raw material for some industries such as zeolite adsorbent, geopolymer, pozzolanic cement, valuable metal recovery, and building materials. \u0026nbsp;Sodium silicate can be prepared by this by-product (its aqueous solution is commonly known as water glass) is an important chemical product and also the main raw material for other silica-containing products [1].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;Sodium silicate is widely used as anti-corrosion materials, binders, refractory materials, white carbon black, acid-resistant cement, impregnates, fixing agents and molecular sieve catalysts, and other fields, covering almost \u0026nbsp; all \u0026nbsp; aspects \u0026nbsp; \u0026nbsp;of \u0026nbsp; human \u0026nbsp; life \u0026nbsp; \u0026nbsp;[2]. \u0026nbsp; Therefore sodium silicate is the most extensively used industrial raw material after acids and bases [3]. However, the current industrial production method of sodium silicate requires a huge energy input, which is, fusing sodium carbonate and high quality quartz sand at temperatures 1,300°C or 1,600°C [4]. So, the synthesis of sodium silicate and separation silica from rich silica residue (such as slag, industrial solid waste) is being intensively investigated.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;The recovery of silica from different wastes such as fly ashes, and rice husk ash has been reported [5–7]. \u0026nbsp; It was reported that the waste of corn stalks are used as raw materials, roasted at 700°C, and then leached with sodium hydroxide to prepare liquid sodium silicate [8]. The municipal waste incineration bottom ash was studied to prepare sodium silicate and meso-porous silica at low temperature [9]. The above mentioned research works have achieved the alkali leaching silica extraction from silica-rich slag, but the modulus and concentration of liquid sodium silicate are less than the commercial standard, which is unsuitable for some applications.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;There is a report on the hydrothermal preparation of high modulus liquid sodium silicate (3.0–3.8) using precipitated silica from silica sand, which produced in the titanium dioxide pigment manufacturing process [10]. However, there were no reports on the preparation of high modulus sodium silicate from ALCKC. In addition, from an economic and environmental point of view, if silica can be extracted from ALCKC and synthesized high modulus sodium silicate at low temperature, it will greatly promote the application of ALCKC.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;Based on the previous work, the aim of the present study was to extract precipitated silica from ALCKC and synthesize high-modulus sodium silicate at low temperature. First, the acid leaching pretreatment of ALCKC was carried out to reduce the influence of other elements on the extraction of silica. Usually, most reports only focus on the extraction of valuable metals from waste and rarely care about the reuse of leachate (pre-treat solution). The obtained acid leaching residue (ALCKC) is a potential raw material for preparing silicon-containing products. The silica dissolution from ALCKC was studied by varying the NaOH-to-ALCKC ratio, leaching time, temperature, and sodium hydroxide concentration to optimize the conditions and understand dissolution process. The reaction thermodynamics are studied. The sodium silicate solution gained by the alkaline leaching process is used to prepare precipitated silicon and synthesize high-modulus liquid sodium silicate at normal pressure. The followed process has performed at low temperature, low with simple equipment, and zero liquid waste. The process minimizes the residue of alum industry and at the same time reduces the residue of ALCKC.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Material and analysis\u003c/h2\u003e \u003cp\u003eALCKC material used was collected from the kaolin manufacturing process of AS in Abu Zaabal Province Egypt. The samples washed carefully to remove aluminum sulphate. The collected ALCKC has approximately 20 wt % moisture content. Hydrochloric acid (37%), sodium hydroxide pellets, and concentrated sulfuric acid (98%) were obtained from the local producers. All the chemicals used in this study were of general purpose reagents (GPR) and were applied without purification.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the chemical composition of ALCKC, indicating that silica and aluminum are the main components of the solid waste. In addition, ALCKC contains a series of potentially leachable elements: Ti and Fe. The aluminum sulphate content is the main reason for ALCKC becoming acidic waste, and its content is as high as 2.0% as SO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eChemical composition of acid leached calcined kaolinitic clay\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCompound\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAmount (weight %)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e45.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e42.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi (amorphous)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e31\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTi\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMg\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.036\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.043\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.009\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.72\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.004\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSr\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eL.O.I\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Experimental procedure\u003c/h2\u003e \u003cp\u003eAn acid treatment of ALCKC was performed to extract the residual aluminum, iron and titanium, to reduce their effect on the extraction of silica. ALCKC leaching experiment was carried out in a 2.5 L flask. First, ALCKC was mixed with hydrochloric acid solution (8M) to form a mixture with an acid \u0026ndash; ALCKC ratio of 5 (v/w). After stirring the mixture for 3h at 85\u0026deg;C, it was filtered to obtain leachate and acid-leached ALCKC respectively. The ALCKC was washed with tap water and dried at 110\u0026deg;C for 6 h. The washing liquid and leachate can be used to prepare PAC, so that no waste liquid was discharged.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Alkaline leaching\u003c/h2\u003e \u003cp\u003eThe silica extraction from the ALCKC was investigated by varying the mass ratio of ALCKC to NaOH (1: 2, 3, 4, 5, and 6), leaching time (0.5, 1, 2, 3, and 5 h), temperature (25\u0026deg;C, 45\u0026deg;C, 60\u0026deg;C, 75\u0026deg;C, and 90\u0026deg;C), and sodium hydroxide concentration (5, 10, 15, 20, and 25%) to determine the optimum conditions for leaching. The optimum conditions for the procedure were done as follows: 100 g of ALCKC and 20 NaOH solutions with a NaOH to ALCKC ratio of 5 are mixed in 1,000 ml flask, and then heated at 75\u0026deg;C for 2 h.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Preparation of silica powder\u003c/h2\u003e \u003cp\u003eSilica powder was prepared using the method described in U.S. Patents 8287822B2; 2012. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. 100 mL of water was added into 1L precipitation vessel and heated to 85\u0026deg;C. The pH was initially adjusted to be between 8.2 to 8.5 with keeping the temperature constant, little sodium silicate solution was added. Then, a certain amount of sodium silicate is continuously added to the above aqueous solution at a rate of 20 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a sufficient quantity of 25% sulfuric acid solution to ensure that the pH was held constant. The solution was allowed to settle for 30 min. the precipitated silica was washed with deionized water several times and dried.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 High modulus sodium silicate preparation\u003c/h2\u003e \u003cp\u003eAccording to the theoretical SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO ratio as a dissolved silica and sodium hydroxide (4/1) were prepared under alkaline leaching conditions to produce high-modulus liquid under temperature range of 85\u0026ndash;200\u0026deg;C. The reaction was carried out for 3h and then the solution was filtered. Clear liquid was obtained with high modulus sodium silicate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Characterization methods\u003c/h2\u003e \u003cp\u003eThe mineralogical composition of the ALCKC was determined with XRD; Shimadzu ZU, Japan). X-ray powder diffraction patterns were obtained using a Rigaku D/max-TTR III X-ray diffractometer, at 40 kV and 250 mA, and using Cu K\u003cem\u003eα\u003c/em\u003e filtered radiation (\u003cem\u003eλ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.1542 nm). The samples were subjected to full- element analysis using XRF-1800 wavelength dispersive X-ray fluorescence spectrometer (XRF; Test equipment comes from Shimadzu Corporation, Japan). The concentrations of Al and Ti were digested by hydrochloric acid (8M) followed by inductively coupled plasma emission spectrometry (ICAP7400 Radial, Thermo Fisher Scientific, USA) analysis. FT-IR spectra were recorded in the region 4,000\u0026ndash; 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in a WQF-200 model FTIR spectrometer made by Beijing Optical Instrument Factory, using the KBr pellet technique (about 1 mg of sample and 300 mg of KBr were used in the preparation of the pellets).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7 Determination of sodium silicate modulus\u003c/h2\u003e \u003cp\u003eThe determination of sodium oxide and silica content in sodium silicate was carried out according to CS standard GB/T 4209\u0026thinsp;\u0026minus;\u0026thinsp;2008. The modulus of sodium silicate is calculated as the molar ratio (Mod) of silica to sodium oxide (SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO), and calculated according to Eq.\u0026nbsp;1:\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eSodium silicate modulus (SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO), = (\u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e/\u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e) \u0026times; 1.03 (1)\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003ew\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e and \u003cem\u003ew\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e are the mass fraction of silica and sodium oxide in the water glass, respectively, and 1.03 is the relative molecular mass ratio.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 ALCKC XRD\u003c/h2\u003e\n \u003cp\u003eThe XRD pattern of ALCKC is shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. It contains minerals such as quartz and anatase. There is a large amount of amorphous silica in the ALCKC. Before the production of aluminum sulphate from kaolin, the kaolin has been calcined at 700\u0026deg;C for 90 min. to form activated aluminum silicate that react with sulphuric acid to yield aluminum sulphate solution and amorphous silica (ALCKC) [\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e]. The dissolution of calcined kaolinitic clay and the formation of aluminum sulphate and the precipitation of silica are carried out according to the equations: (2) and (3):\u003c/p\u003e\n \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026sdot;2SiO\u003csub\u003e2\u003c/sub\u003e\u0026sdot; 3H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;6H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e \u0026rarr; 2Al\u003csub\u003e2\u003c/sub\u003e(SO\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e (Alum)\u0026thinsp;+\u0026thinsp;2H\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;3H\u003csub\u003e2\u003c/sub\u003eO (2)\u003c/p\u003e\n \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e \u0026rarr; SiO\u003csub\u003e2\u003c/sub\u003e (Amorphous)\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO. (3)\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 ALCKC grain size distribution\u003c/h2\u003e\n \u003cp\u003eThe grain size-distribution of the ALCKC by laser granulometry was plotted in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e which shows that the diameter of ALCKC in cumulative 90% of ~\u0026thinsp;78 \u0026micro;m and 10% of ~\u0026thinsp;4.9 \u0026micro;m with an average diameter of ~\u0026thinsp;9.6. BET analysis revealed a high fineness for ALCKC (20 m\u003csup\u003e2\u003c/sup\u003e/g).\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eThermogravimetry and differential scanning calorimetry of ALCKC (TG/DSC)\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eThe thermal behavior of the starting ALCKC is presented in the Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. The main changes revealed by TG and DSC analysis are as follows: The DSC line indicates the removal of absorption water or free water as demonstrated in the small band at 186.6\u0026deg;C (water absorbed in pores and on the surface\u0026thinsp;=\u0026thinsp;1.73%). The thermal degradation occurs between 186.6 and 550\u0026deg;C is associated with the presence of silicate and the chemical dewatering within the structure. The remaining weight loss at between 550 and 1000\u0026deg;C can be attributed to more bound water which results from silanol or aluminol. The results show that approximately 6.93% measured over the temperature range of ambient to 988\u0026deg;C.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Acid leaching and aluminum recovery\u003c/h2\u003e\n \u003cp\u003eALCKC is an acidic solid waste containing traces of aluminum sulphate (AS). The presence of AS causes that the grains of ALCKC to stick to each other, and it is difficult for ALCKC to dump or to reuse without perfect washing, and a large amount of washing is generated. The ALCKC produced in this process is a low-priced raw material for preparing commercial-grade sodium silicate. ALCKC was leached with hydrochloric acid solution (8M) at 85\u0026deg;C for 3h.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e show that when the HCl/ALCKC was 1:1, the removal rates of aluminum, iron, and titanium were: 44.6%, 38%, and 7.5%, respectively. The HCl/ALCKC ratio was continued to 6, and the extraction rates were: Al: 45%, Fe: 41.2% and Ti: 15% respectively with slight change except for titanium. It was noticed that the trend of titanium extraction first increases up to 15% and then decreases to 13.8 at higher HCl ratio. This is because after the titanium is dissolved from the titanate mineral, it is precipitated again in the form of rutile mineral [\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e]. The result shows that the removal effect of aluminum and iron are better than that of titanium. The reason is that titanium mainly exists in ALCKC in the form of anatase (TiO\u003csub\u003e2\u003c/sub\u003e). It is difficult to achieve titanium leaching at low temperature.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eTo minimize the effect of other aluminum and titanium on the subsequent silica extraction, ALCKC was leached with excess hydrochloric acid solution (8M). Therefore, the best HCl/ALCKC ratio was set as 5. The chemical composition of the ALCKC is presented in Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. Compared with Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, the solute ions after acid treatment can be easily washed with tap water. View of thermodynamic, silicate mineral such as ALCKC react with hydrochloric acid at room temperature. However, the solubility of minerals is significantly affected by grain size and particle density. The extract solution is used to prepare poly aluminum chloride to reuse of the extract as indicated in Eq.\u0026nbsp;5 [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]:\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eElemental composition of ALCKC after treatment with HCl\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"2\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCompound\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAmount (weight %)\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\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e45.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e45.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSi (amorphous)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e34.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.7\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eMg\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt; 0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eK\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eL.O.I\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2\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\u003ePAC was determined according to the drinking water standard [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e]. The content of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in the liquid PAC was 11.5%, the basicity was 80%, and the heavy metal content complies with Egyptian standards (in Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). It can be seen from Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e that the quality of PAC can reach the drinking water standard of Egyptian standards. This process achieves efficient leaching of aluminum in the residual aluminum and reuse of leaching solution.\u003c/p\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 Effect of aluminum content on the alkaline leaching on the silica extraction\u003c/h2\u003e\n \u003cp\u003ePresence of aluminum affects the silica extraction [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e]. The hindering effect of aluminum on the extraction of silica was investigated. Before the start of the ALCKC silica extraction experiment, sodium aluminate simulation experiments were used to study the effect of aluminum on the silica extraction process. 1000 mL of sodium hydroxide (20%) was used to dissolve 73.3 g of pure amorphous silica (the amount of this amorphous silica is equivalent to the total amount of silica in 100 g of ALCKC). The effect of alkaline solution dissolving different sodium aluminate alone on silica extraction was tested.\u003c/p\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;Figure 5 shows that aluminum affect the dissolution of silica, and the dissolution rate of silica decreases with the addition of sodium aluminate. First, the silicate mineral reacts chemically with the alkali during the reaction, and then the silica enters the solution in the form of SiO\u003csup\u003e2\u0026minus;\u003c/sup\u003e. The silicate ion reacts with sodium aluminate and then silicon precipitates out in the form of hydrated sodium aluminosilicates [16]. The precipitated particulates coat the surface of the ALCKC and prevent the contact of silica with the alkali.\u003c/p\u003e\n \u003cp\u003eThe dissolution of amorphous silica and the formation of silicon residue in the silica extraction take place according to the equations 4\u0026ndash;6:\u003c/p\u003e\n \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e (amorphous)\u0026thinsp;+\u0026thinsp;2NaOH \u0026rarr; Na\u003csub\u003e2\u003c/sub\u003e[H\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e], (4)\u003c/p\u003e\n \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.2SiO\u003csub\u003e2\u003c/sub\u003e.H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;2OH\u0026minus; \u0026rarr; 2H\u003csub\u003e4\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2Al(OH)\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (5)\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ex\u003c/em\u003eNa\u003csub\u003e2\u003c/sub\u003e[H\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e]\u0026thinsp;+\u0026thinsp;2NaAl(OH)\u003csub\u003e4\u003c/sub\u003e \u0026rarr; Na\u003csub\u003e2\u003c/sub\u003eO\u0026sdot;Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026sdot;\u003cem\u003ex\u003c/em\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u0026sdot;2H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;2\u003cem\u003ex\u003c/em\u003eNaOH (6)\u003c/p\u003e\n \u003cp\u003eXRD of sodium aluminum silicate residue shows that different hydrated sodium aluminum silicates are coexisting in the filter residue (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e). The acid leached calcined kaolin dissolves in alkaline media, giving rise to silica [SiO\u003csub\u003e2\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e]\u003csup\u003e2\u0026minus;\u003c/sup\u003e and [SiO(OH)\u003csub\u003e3\u003c/sub\u003e]\u0026thinsp;\u0026minus;\u0026thinsp;as well as aluminum [Al(OH)\u003csub\u003e4\u003c/sub\u003e]\u0026thinsp;\u0026minus;\u0026thinsp;monomers. These monomers can inter-react to yield aluminosilicates that precipitates in the form of a Na\u003csub\u003e2\u003c/sub\u003eO\u0026ndash;Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026ndash;SiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;H\u003csub\u003e2\u003c/sub\u003eO gel [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]. Therefore, the hydrochloric acid pretreatment is necessary to effectively remove aluminum in the ALCKC.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 The influence of NaOH\u0026ndash;ALCKC ratio on silica extraction\u003c/h2\u003e\n \u003cp\u003eSodium silicate was prepared using ALCKC with NaOH solution (20%) at 75\u0026deg;C for 2h. The effect of NaOH to ALCKC ratio was tested to determine the maximum extraction of silica, and the results are provided in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e. About 65% of the available silica was dissolved during the first NaOH\u0026ndash;ALCKC ratio of 2. The dissolution of silica increased to 88% when the liquid\u0026ndash; solid ratio was reached to 5. Moreover, when the ratio was increased to 6 under the same conditions, a minor increase for the extraction rate occurred. The ratio SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO of liquid sodium silicate showed the opposite result, decreasing from 2.83 to 0.95 and this is normal. As the NaOH ratio increases, the dissolved silica in the solution increased slowly.\u003c/p\u003e\n \u003cp\u003eSmall amount of titanium dissolved into the alkaline leaching solution without significant effect on silica extraction, compared with aluminum. The aluminum dissolution rate is relatively high, which is 8.31% when the NaOH/ALCKC ratio is 6 as shown in Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. Aluminum and silicon are present in ALCKC, in a stratified structure.\u003c/p\u003e\n \u003cp\u003eTable 3: Effect of NaOH\u0026ndash;ALCKC ratios on the dissolution of \u0026nbsp;aluminum and titanium\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" align=\"left\" width=\"476\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"24.159663865546218%\" rowspan=\"3\" valign=\"top\"\u003e\n \u003cp\u003eParameter\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"75.84033613445378%\" colspan=\"5\" valign=\"top\"\u003e\n \u003cp\u003eNaOH/ALCKC\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"20.775623268698062%\" valign=\"top\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.94459833795014%\" valign=\"top\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"21.606648199445985%\" valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.94459833795014%\" valign=\"top\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.72853185595568%\" valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"100%\" colspan=\"5\" valign=\"top\"\u003e\n \u003cp\u003eConcentration (%)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"24.159663865546218%\" valign=\"top\"\u003e\n \u003cp\u003eAluminum\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.756302521008404%\" valign=\"top\"\u003e\n \u003cp\u003e3.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.126050420168067%\" valign=\"top\"\u003e\n \u003cp\u003e5.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.386554621848738%\" valign=\"top\"\u003e\n \u003cp\u003e5.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.126050420168067%\" valign=\"top\"\u003e\n \u003cp\u003e7.48\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.445378151260504%\" valign=\"top\"\u003e\n \u003cp\u003e8.31\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"24.159663865546218%\" valign=\"top\"\u003e\n \u003cp\u003eTitanium \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.756302521008404%\" valign=\"top\"\u003e\n \u003cp\u003e0.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.126050420168067%\" valign=\"top\"\u003e\n \u003cp\u003e0.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.386554621848738%\" valign=\"top\"\u003e\n \u003cp\u003e0.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"15.126050420168067%\" valign=\"top\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.445378151260504%\" valign=\"top\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003cdiv align=\"char\" class=\"colspec\"\u003e\u003cbr\u003e\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eThe alkaline leaching of ALCKC is shown in equations: 7\u0026ndash;11[\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. The corresponding thermodynamic calculation results are presented in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. Thermodynamic data may show that TiO\u003csub\u003e2\u003c/sub\u003e, and quartz can react with sodium hydroxide at room temperature. However, as the Gibbs free energy of the reaction is very low, it can be considered that there is no reaction takes places at room temperature for titanium oxide and quartz. Total dissolution rate of aluminum is about 7.48% when the NaOH /ALCKC ratio is 5. The dissolved aluminum resulted from un-calcined kaolinite. This means that less sodium silicate is obtained from decomposition of kaolinitic residue in ALCKC and there was no considerable effect of titanium. The majority of silicon dissolved in the sodium hydroxide solution comes from the amorphous silica in ALCKC.\u003c/p\u003e\n \u003cp\u003eAl(OH)\u003csub\u003e3\u003c/sub\u003e(s)\u0026thinsp;+\u0026thinsp;NaOH(aq)\u0026thinsp;=\u0026thinsp;Al(OH)\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(aq)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;Na\u003csup\u003e+\u003c/sup\u003e \u003csub\u003e(aq)\u003c/sub\u003e (7)\u003c/p\u003e\n \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e \u0026sdot; 2SiO\u003csub\u003e2\u003c/sub\u003e \u0026sdot; 3H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e(s)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;6NaOH\u003csub\u003e(aq)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2NaAlO\u003csub\u003e2(aq)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3(aq)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;5H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e(aq)\u003c/sub\u003e, (8)\u003c/p\u003e\n \u003cp\u003eTiO\u003csub\u003e2(s)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e(aq)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Ti(OH)\u003csub\u003e4(aq)\u003c/sub\u003e, (9)\u003c/p\u003e\n \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e(quartz)\u0026thinsp;+\u0026thinsp;2NaOH\u003csub\u003e(aq)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;Na\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3(aq)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e(aq)\u003c/sub\u003e, (10)\u003c/p\u003e\n \u003cp\u003enSiO\u003csub\u003e2\u003c/sub\u003e(amorphous)\u0026thinsp;+\u0026thinsp;2NaOH(aq)\u0026thinsp;=\u0026thinsp;nSiO.Na2O \u0026sdot; H\u003csub\u003e2\u003c/sub\u003eO (11)\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eThermodynamic data for the reaction process taken place in sodium hydroxide leaching process)\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eEquation\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"4\"\u003e\n \u003cp\u003eThermodynamic data\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eEnthalpy of formation\u003c/p\u003e\n \u003cp\u003e∆H\u003csub\u003ef\u003c/sub\u003e 0, 25\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eEntropy\u003c/p\u003e\n \u003cp\u003eS\u003csup\u003e0\u003c/sup\u003e, 25\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFree energy of formation\u003c/p\u003e\n \u003cp\u003e∆ G\u003csub\u003ef\u003c/sub\u003e \u003csup\u003e0\u003c/sup\u003e, 25\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFree energy of formation\u003c/p\u003e\n \u003cp\u003e∆ G\u003csub\u003ef\u003c/sub\u003e \u003csup\u003e0\u003c/sup\u003e, 75\u0026deg;C\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\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eH\u0026theta; kJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eJ mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ekJ mol K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ekJ mol K\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e168.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-5.25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1980\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e385.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-1887.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-2072.2\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\u003eAs shown in Figs. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e the residue remains after silica extraction processes with varying NaOH to ALCKC (5/1, 6/1), the amplitude of the peak that locate in 2\u0026theta; from 18\u0026deg; to 30\u0026deg; decreased [\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e]. This means minimizing of amorphous silica concentration after digestion with alkali and converting of another form. With the increase in the NaOH\u0026ndash;ALCKC ratio, the diffraction peaks of other mineral phases are relatively strengthened in alkali leaching residue. This is caused by the dissolution of the soluble silica coating the surface of the mineral. From the cost benefit point of view, the ratio 5:1 is the optimum one for perfect reaction of silica extraction. And the SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO ration is 1.1 and the corresponding extracted Silica is 88%.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6 Effect of reaction time on silica extraction\u003c/h2\u003e\n \u003cp\u003eThe effect of time on silica extraction using NaOH: ALCKC ratio of 5:1 and at constant temperature and ambient pressure was tested (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e). The results show that the solubility of silica in ALCKC is high and the extraction rate of the amorphous phase silica reached 57% in a relatively low period of time (30 min.). The extraction reached 87.6 after 3h. It can be deduced that amorphous silica precipitates remain in the ALCKC after extraction of aluminum by sulphuric acid in the process of producing aluminum sulphate from calcined kaolin. Most of the silica in the ALCKC is not coated with other minerals, so that the silica is easily leached by sodium hydroxide.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7 Effect of reaction temperature on silica extraction\u003c/h2\u003e\n \u003cp\u003eThe reaction temperature is an important factor that affects the silica extraction. Figure 11shows that as the reaction temperature increase the rate of extraction increases. It was noticed that silica extraction rate increases from 50\u0026ndash;88% when leaching temperature is increased from 25\u0026deg;C to 100\u0026deg;C, taking in account that the modulus of liquid sodium silicate was between 1.0 and 1.15. The leachability of silica was achieved at relatively lower temperature (70\u0026ndash;100\u0026deg;C), this refers to the high solubility of amorphous silicate of ALCKC.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\n \u003ch2\u003e3.8 Effect of sodium hydroxide concentration on silica extraction\u003c/h2\u003e\n \u003cdiv class=\"BlockQuote\"\u003e\n \u003cp\u003eThe silica extraction was investigated under the several NaOH concentrations (Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e). When the concentration of sodium hydroxide increases from 5 to 25%, the silica extraction rate of in ALCKC increases significantly from 35.3% up to 88%. Table \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e present the chemical composition of ALCKC after vigorous leaching. From table 6, it can be concluded that the silica in the ALCKC mainly exists in two forms of amorphous silicate and crystalline quartz. The amorphous was extracted with a percentage of 88%, therefore the quantity of solid was decreased by the alkaline leaching.\u003c/p\u003e\n \u003c/div\u003e\n \u003cp\u003eIt was known that the strength of covalent bond Si-O in the crystalline silica is much higher than the bond in the amorphous phase. Therefore the crystalline bond Si-O requires 460 kJ/mole to distract this bond and high pressure for destruction and formation of sodium silicate and it is difficult to rise up the SiO\u003csub\u003e2\u003c/sub\u003e/Na\u003csub\u003e2\u003c/sub\u003eO ratio [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e]. It was reported that amorphous silica as the main component, such as silica fume is used to prepare sodium silicate applying high pressure hydrothermal reaction.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab5\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eElement composition of the final residue of ALCKC after alkaline leaching\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"2\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCompound\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAmount (weight %)\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\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e41.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e17.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSi (amorphous)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eFe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTi\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eMg\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt;\u0026thinsp;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026lt; 0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eK\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSr\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eCl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eND\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eL.O.I\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\n \u003ch2\u003e3.9 Synthesis of amorphous silica\u003c/h2\u003e\n \u003cp\u003eThe soluble species is in the form of Si(OH)\u003csub\u003e4\u003c/sub\u003e [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e]. As the solution is alkaline (pH 9), so precipitation of silica takes place by acidification of the alkaline solution (resulted using NaOH solution (20%) at 85\u0026deg;C for 3h). Sulfuric acid (30%) was used. Figure \u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e illustrates the effect of pH of precipitation of silica. The recovery rate of silica has achieved more than 96% at pH value of 7 [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]. The silica has a higher recovery rate of 98.62%, with low sulfuric acid adding, for neutral liquid sodium sulfate solution.\u003c/p\u003e\n \u003cp\u003eIt was found that liquid sodium silicate can be neutralized by using H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e to form pure amorphous silica. The neutralization process of the sodium silicate solution resulting in mono and poly nucleation silicon and their polymerization create polysilicic acid in a colloidal form that upon drying, silicon precipitate is formed. The rate of polymerization of silicic acid affects the aggregation and the grain size of silica. The cross-linking reactions and physical aggregations causing anionic charged silica \u0026ldquo;sols and finally formation of gels [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]. The XRD pattern of amorphous silica obtained under neutral conditions is shown in Fig. \u003cspan class=\"InternalRef\"\u003e14\u003c/span\u003e. Because amorphous silica shows broad diffraction peaks (2\u003cem\u003e\u0026theta;\u003c/em\u003e from 15\u0026deg; to 30\u0026deg;) [\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eThe IR spectrum for the prepared silica is shown in Fig. \u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e. The characteristics bonds of silica appeared at 793, and 475 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, these bands for symmetric and asymmetric stretching vibrations of Si\u0026ndash;O bond, respectively. Another absorption peak emerged at 1,090 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the bending vibration of the Si\u0026ndash;O\u0026ndash;Si bond [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. The absorption peaks at 1,640 and 3486 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are the absorption peaks of water, surface adsorption water, and structured water. The 1,640 band is the bending vibration peak of H\u0026ndash;O\u0026ndash;H, and it is relevant by capillary water and surface adsorbed water. The latter is the asymmetric stretching vibration peak of O\u0026ndash;H, which is related to structured water [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]. IR spectrum shows that the formed silica has small water content without any absorption peak of cross-linking reactions and physical aggregations causing anionic silica.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\n \u003ch2\u003e3.10 Preparation of high modulus liquid sodium silicate\u003c/h2\u003e\n \u003cp\u003eAmorphous silica and alkaline leaching solution were mixed according to the theoretical ratio or SiO\u003csub\u003e2/\u003c/sub\u003eNa\u003csub\u003e2\u003c/sub\u003eO of 4 to prepare high modulus liquid sodium silicate. The process was continued to 3h at different temperature, starting from 60\u0026deg;C up to 200\u0026deg;C, the digested solution was filtered to get the sodium silicate solution. As illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e16\u003c/span\u003e, the amorphous silica is easily soluble in alkaline medium. Liquid sodium silicate with a modulus of 3 can be prepared at 80\u0026deg;C under normal pressure. As the temperature increases the modulus increases. At 200\u0026deg;C the SiO\u003csub\u003e2\u003c/sub\u003e to Na\u003csub\u003e2\u003c/sub\u003eO become 3.55.\u003c/p\u003e\n \u003cp\u003eThere are three important principle in the amorphous silicon solubility - pH diagram: first the precipitation range of amorphous silicon; second: the stable silicon poly-anions; and third: the thermodynamic prevailing monomeric Si species [Si(OH)\u003csub\u003e4\u003c/sub\u003e, SiO(OH)\u003csup\u003e\u0026minus;\u003c/sup\u003e, and SiO\u003csub\u003e2\u003c/sub\u003e(OH)\u003csup\u003e2\u0026minus;\u003c/sup\u003e]. The pH value of the solution is 12, and this pH confirms the formation of stable polymeric structure of sodium silicate [\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusions","content":"\u003cul class=\"decimal_type\"\u003e\n \u003cli\u003eAcid leached kaolin of alum industry (environmentally harmful acid solid waste) can be used for synthesis of silica and of high-modulus sodium silicate.\u003c/li\u003e\n \u003cli\u003eThe leachate solution resulted from digestion of the acid leached kaolin was reused to prepare high basicity PAC.\u003c/li\u003e\n \u003cli\u003eThe silica extraction rate from ALCKC reached 88% and the SiO\u003csub\u003e2\u003c/sub\u003e:nNa\u003csub\u003e2\u003c/sub\u003eO of the prepared sodium silicate was 1.1 with NaOH\u0026ndash;ALCKC ratio of 5:1 at the 20% NaOH at 75\u0026deg;C for 2 h.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eThe recovery rate of amorphous silica from alkaline leachate reached 98.62% using sulfuric acid at pH of 7. Then liquid sodium silicate in combination with amorphous silica is used to prepare liquid sodium silicate with a modulus of 3 at 80\u0026deg;C for 3h.\u0026nbsp;\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding information:\u003c/strong\u003e This work was supported by aluminum Sulphate Corporation of Egypt.\u003c/p\u003e\n\u003cp\u003eAuthor contributions:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNabil A. ABDULLAH\u003c/strong\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003ecompleted the experiment and paper writing; paper revision; data analysis and discussion.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest:\u003c/strong\u003e The author declares that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNabil A. ABDULLAH\u003c/strong\u003e completed the experiment and paper writing; paper revision; data analysis and discussion.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eNematollahi B, Sanjayan J, Shaik FUA. Synthesis of heat and ambient cured one-part geopolymer mixes with different grades of sodium silicate. 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Molecular-level mechanisms of quartz dissolution under neutral and alkaline conditions in the presence of electrolytes. Geochem J. 2014;48(2):189\u0026ndash;205. \u003cu\u003edoi: 10.2343/ geochemj.2.0298\u003c/u\u003e\u003c/li\u003e\n \u003cli\u003eGorrepati EA, Wongthahan P, Raha S. Silica precipitation in acidic solutions: mechanism, pH effect, and salt effect. Langmuir. 2010;26(13):10467\u0026ndash;74.\u0026nbsp;\u003cu\u003edoi: 10.1021/la904685x\u003c/u\u003e\u003c/li\u003e\n \u003cli\u003eStumm W, Huper H, Champlin RL. Formulation of polysilicates as determined by coagulation effects. Enviorn Sci Technol. 1967;1(3):221\u0026ndash;7. \u003cu\u003edoi: 10.1021/es60003a004\u003c/u\u003e\u003c/li\u003e\n \u003cli\u003eOwoeye SS, Jegede FI, Borisade SG. Preparation and characterization of nano-sized silica xerogel particles using sodium silicate solution extracted from waste container glasses. Mater Chem Phys. 2020;248:122915. \u003cu\u003edoi: 10.1016/j.matchemphys.2020.122915\u003c/u\u003e\u003c/li\u003e\n \u003cli\u003eLai S, Yue L, Zhao X. Preparation of silica powder with high whiteness from palygorskite. Appl Clay Sci. 2010;50(3):432\u0026ndash;7. \u003cu\u003edoi: 10.1016/j.clay.2010.08.019\u003c/u\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Acid leached kaolin residue, silica extraction, amorphous silica, sodium silicate","lastPublishedDoi":"10.21203/rs.3.rs-3860849/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3860849/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAcid leached calcined kaolinitic clay (ALCKC) is a residue of aluminum sulfate coagulant manufacturing process; it is a solid acidic waste that is harmful to environment. Silica dissolution process was systematically studied, including the thermodynamic analysis and the influence of aluminum content on the leaching of amorphous silica. Simulation studies have shown that aluminum combine with silicon to form silica–alumina gel, and zeolite, thereby preventing the leaching of silica. Maximizing the removal of aluminum, iron and titanium can effectively improve the leaching of silica in the subsequent process, and corresponding element removal rates are 45%, 41% and 15%, respectively. The removed aluminum is reused to prepare PAC. The silica extraction rate reached 88% at a conditions of (NaOH; 20%, NaOH to ALCKC; (v/ w) 5, 75°C, 2h), and sodium silicate modulus (\u003cem\u003en\u003c/em\u003eSiO\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003en\u003c/em\u003eNa\u003csub\u003e2\u003c/sub\u003eO) is 1.1. The results indicated that a large amount of silica was existed in amorphous form. Precipitated silica was obtained by acidifying sodium silicate solution at a pH 7.0 using sulphuric acid. The prepared sodium silicate (1.1) was used for further synthesis of sodium silicate with \u003cem\u003en\u003c/em\u003eSiO\u003csub\u003e2\u003c/sub\u003e:\u003cem\u003en\u003c/em\u003eNa\u003csub\u003e2\u003c/sub\u003eO = more than 3\u003c/p\u003e","manuscriptTitle":"Synthesis of sodium silicate from the acid leached calcined kaolinitic clay residue of aluminum sulfate industry","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-16 14:34:38","doi":"10.21203/rs.3.rs-3860849/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"91c3c561-4225-4ce3-a1c9-eb9a81e9fa7e","owner":[],"postedDate":"January 16th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":28173097,"name":"Scientific community and society/Scientific community/Research data/Databases"},{"id":28173098,"name":"Scientific community and society/Scientific community/Research data"}],"tags":[],"updatedAt":"2024-01-16T14:34:41+00:00","versionOfRecord":[],"versionCreatedAt":"2024-01-16 14:34:38","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3860849","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3860849","identity":"rs-3860849","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}