Lithium extraction and hydroxysodalite zeolite synthesis by

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Lithium extraction and hydroxysodalite zeolite synthesis by hydrothermal conversion of #-spodumene Peng Xing, Chengyan Wang, Lei Zeng, Baozhong Ma, Ling Wang, Yongqiang Chen, and Cheng Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00923 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Sustainable Chemistry & Engineering

Lithium extraction and hydroxysodalite zeolite synthesis by hydrothermal conversion of αspodumene Peng Xing, Chengyan Wang,* Lei Zeng, Baozhong Ma,* Ling Wang, Yongqiang Chen, and Cheng Yang School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China. Mailing address: 30 Xueyuan Road, Haidian District, Beijing 100083, China Corresponding Authors: [email protected] (C. Wang) and [email protected] (B. Ma).

ABSTRACT: The continuously increasing demand for lithium has made it one of the strategic metals, rendering its exploitation of critical importance. Natural α-spodumene is still the primary resource of lithium extraction. The traditional process for the treatment of α-spodumene generates immense quantities of waste residue and needs a high-temperature heat treatment, leading to high energy consumption. In addition to lithium, α-spodumene is rich in aluminum and silicon, and thus it is a potential raw material for zeolite synthesis. Herein, a novel process was developed for the clean and efficient extraction of lithium from α-spodumene coupled with the synthesis of hydroxysodalite zeolite. By hydrothermal alkaline treatment, α-spodumene was

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converted into hydroxysodalite; the lithium in α-spodumene was released into the solution, and subsequently recovered by precipitation with Na2CO3. A lithium extraction efficiency of 95.8% was obtained under the optimum conditions: temperature 250 °C, NaOH concentration 600 g/L, liquid/solid ratio 5:1, stirring speed 500 rpm, and reaction time 2 h. In addition, the influences of various factors on the composition and textural properties of the product were analyzed using XRD, SEM, TG, N2 adsorption/desorption, and FTIR.

KEYWORDS: α-Spodumene, Lithium, Extraction, Hydroxysodalite, Synthesis INTRODUCTION Lithium and its compounds have been widely applied in heat-resistant glass and ceramics, lithium grease lubricants, flux additive for aluminum production, and lithium ion batteries (LIBs).1 As the most promising rechargeable battery, LIB has become the largest consumer of lithium.2 The unprecedented growth in electric vehicles has dramatically increased the demand for lithium. A shortage of lithium carbonate, one of the most important raw materials for LIB production, is projected to occur around 2020.3 The high economic importance of lithium has made it one of the strategic metals. In recent years, much attention and research has been devoted to the extraction of lithium from brine.

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In comparison, there are few literature reports on the extraction of lithium from α-

spodumene. Natural α-spodumene is a lithium aluminum inosilicate mineral, which is the important mineral source for lithium extraction and generally has a high content of Li2O (~5%). Since the middle of the twentieth century, α-spodumene has been processed by a traditional method which involves a high-temperature heat treatment (~1100 °C) to transform the inert αspodumene into active β-spodumene, followed by sulfuric acid curing at around 250 °C and


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water leaching.9-12 Although some direct leaching processes have been reported,13-16 heat treatment is still a necessary step. The phase transformation of α-spodumene involves high energy consumption. In addition, the processing of 1 t of α-spodumene normally generates approximately 0.8 t of leaching residue using the traditional process. The treatment of immense quantities of this lithium leaching residue has become an increasingly serious problem. These have severely affected the sustainable exploitation of α-spodumene. Zeolites are microporous, aluminosilicate minerals commonly used in desiccation, catalysis, and waste water treatment.17-19 Industrially important zeolites are synthetically produced by a hydrothermal reaction of alumina, silicon, and sodium hydroxide. The searching for low-cost and easily available raw materials for the synthesis of zeolites has been a constant research focus.20,21 Natural raw materials have economic advantages over synthetic chemicals.22 Hydroxysodalite (Na8[AlSiO4]6(OH)2∙nH2O) is a mineral of the zeolite group, made of a cubic array of β-cages and exhibits a structure similar to that of sodalite.22,23 Hydroxysodalite is hydrophilic and has numerous micro and meso channels, which enables it to be used for the separation of small molecules from gas or liquid mixtures, hydrogen storage, and catalysis;24,25 it has also been used to modify mortar.26 So far, much work has focused on the synthesis of hydroxysodalite using low-cost resources containing alumina and silicon, such as kaolinite and coal fly ash.27-32 In addition, Kato et al. synthesized hydroxysodalite using the alkaline waste solution of glass polishing powder.33 Esaifan et al. synthesized hydroxysodalite using the basalt powder.24 In fact, in addition to lithium, α-spodumene is rich in aluminum and silicon, and thus it is a potential raw material for zeolite synthesis. However, there is no report on the conversion of α-spodumene to zeolite by hydrothermal treatment.


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In this work, a novel process was proposed for the clean and efficient extraction of lithium from α-spodumene coupled with the synthesis of hydroxysodalite zeolite. By hydrothermal alkaline treatment α-spodumene was converted to hydroxysodalite while the lithium in αspodumene was released into the solution and then recovered as Li2CO3 by adding Na2CO3. The process has the advantages of lower energy consumption and no gas emission. Additionally, the synthesized hydroxysodalite zeolite is a value-added product. The influences of various factors on the lithium extraction and the composition and textural properties of the product were investigated in detail. EXPERIMENTAL SECTION Materials. The α-spodumene ore was crushed and ground to a particle size below 0.074 mm. The chemical composition of the α-spodumene ore is shown in Table 1. Sodium hydroxide, cetyltrimethyl ammonium bromide (CTAB), calcium oxide, and sodium carbonate were of analytically pure grade. Table 1. Chemical Composition of the α-Spodumene Ore

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Apparatuses and Procedure. The hydrothermal alkaline treatment of α-spodumene was performed in a vertical autoclave. The α-spodumene ore (80 g) was first mixed with NaOH solution at a certain liquid-solid ratio in a liner. The slurry was agitated with electric-motor driven impellers. The temperature was controlled by regulating the heating and the cooling water. After the hydrothermal alkaline treatment was over, the slurry was filtered to separate the lithium solution and solid product. The solid product was washed with deionized water and oven-dried. The desilication was also carried out in the autoclave and the stirring rate was fixed


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at 500 rpm. The precipitation of lithium was performed in beaker with mechanical stirring at 500 rpm in a water bath. Analysis Methods. Lithium in α-spodumene ore and alkaline treatment products was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 7000DV, PerkinElmer). The silicon in ore and solution was analyzed by silicon molybdenum blue spectrophotometry. X-ray diffraction (XRD) measurements were carried out using a Rigaku Ultima IV diffractometer (Cu, Kα). The surface morphology and composition of the αspodumene and the products of alkaline treatment were analyzed using field-emission scanning electron microscope (FESEM; JSM-7001F) and energy dispersive X-ray spectroscope (EDS; BRUKER XFlash5010). Thermogravimetric (TG) measurement was performed using a NETZSCH STA 409C analyzer under a nitrogen atmosphere at a heating rate of 10 °C/min. The functional groups of the products of alkaline treatment were analyzed using an iN10MX FTIR spectrometer. The textural characterization of the products was obtained from low-temperature N2 adsorption/desorption at -196 °C on an ASAP 2020 Micromeritics instrument with an equilibrium time of 10 min. The specific surface area was estimated by the Brunauer-EmmetTeller (BET) method. The micropore volume was determined by the t-plot method. Total pore volume was estimated by the single point adsorption at P/P0 = 0.98. The pore size distribution was calculated by the Berrett-Joyner-Halenda (BJH) method. RESULTS AND DISCUSSION Characterization of the α-Spodumene Ore. The XRD pattern (Figure 1) and SEM−EDS analysis (Figure S1, Supporting Information) of the α-spodumene ore indicate that α-spodumene (LiAlSi2O6) is the major phase, with quartz (SiO2) and muscovite (KAl2Si3AlO10(OH)2) as minor


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phases. The cuboid shape α-spodumene shows a smooth surface with a size of ~20 μm in width and ~30 μm in length (Figure S1). 

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Figure 1. XRD pattern of the α-spodumene ore. Hydrothermal Conversion of α-Spodumene and Lithium Extraction. Effect of Temperature on Lithium Extraction and Product Composition. Hydrothermal treatment is an effective method for mineral dissociation and conversion.34-36 Temperature is an important factor in hydrothermal treatment. Figure 2a shows the lithium extraction vs. temperature curve under the conditions of 600 g/L NaOH concentration, 5:1 liquid/solid ratio, 2 h reaction time, and 500 rpm stirring rate. It was observed that the lithium extraction increased with the increase of temperature. The extraction ratio of lithium reached a plateau at 250 °C. Thus the optimum temperature was selected to be 250 °C. As shown in Figure 3, the diffraction peak of α-spodumene remained unchanged after hydrothermal treatment at 150 °C, which explains the low extraction efficiency of lithium. A new phase (hydroxysodalite, Na4Al3Si3O12(OH)) peak appeared at 200 °C. The peak of αspodumene decreased whereas the diffraction peak of hydroxysodalite markedly increased with the increase of temperature. When the temperature was increased to 250 °C, hydroxysodalite


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became the main phase. Meanwhile, the maximum extraction yield of lithium was obtained. A small amount of an impurity, zeolite (1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O) was derived from the reaction of the calcium, aluminum, and silicon in the ore with the alkaline solution. Figure 3 indicates that the conversion of α-spodumene in the alkaline solution can occur at above 200 °C and that increasing temperature can accelerate the conversion. The following reaction is predicted to take place during the hydrothermal conversion of α-spodumene: 6LiAlSi2O6 + 14NaOH → 2Na4Al3Si3O12(OH) + 3Li2SiO3 + 3Na2SiO3 + 6H2O 100

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Figure 3. XRD patterns of the α-spodumene ore converted at various temperatures. Effect of NaOH Concentration on Lithium Extraction and Product Composition. According to eq 1, NaOH concentration is also a key factor for extraction. Therefore, the variations in lithium extraction with NaOH concentration were investigated. The temperature, liquid/solid ratio, reaction time, and stirring rate were maintained at 250 °C, 5:1, 2 h, and 500 rpm, respectively. As shown in Figure 2b, the effect of the NaOH concentration on the extraction of lithium was significant. When the NaOH concentration was 600 g/L, the extraction of lithium reached a plateau and hence the optimum NaOH concentration was selected to be 600 g/L. As shown in Figure 4, the peak of α-spodumene decreased, while a new phase (Li2SiO3) peak appeared and the faujasite (Na14Al12Si13O51·6H2O) became the dominant phase in product after the hydrothermal treatment at NaOH concentration of 300 g/L. Interestingly, when the NaOH concentration was increased to 400 g/L, hydroxysodalite replaced faujasite as the dominant phase. Accordingly, the transformation of faujasite into hydroxysodalite strongly depends on the NaOH concentration. This result is consistent with the transformation of zeolite A into hydroxysodalite.37 The intensities of the α-spodumene and Li2SiO3 peaks decreased with increasing NaOH concentration, which accounts for the increase in lithium extraction.  -Spodumene  Li2SiO3 (PDF#29-0828)

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Figure 4. XRD patterns of the α-spodumene ore converted at various NaOH concentrations. Effect of Liquid/Solid Ratio on Lithium Extraction. The experiments were performed at different liquid/solid ratios (3:1–6:1) while the temperature, NaOH concentration, stirring rate, and reaction time were set at 250 °C, 600 g/L, 500 rpm, and 2 h, respectively. Figure 2c shows that the extraction efficiency of lithium initially increased with the increase of liquid/solid ratio. Increasing liquid/solid ratio promoted the mass transfer between the ore and the NaOH solution and increased the amount of NaOH, which was favorable to the conversion of α-spodumene. Thus, based on Figure 2c, the optimum liquid/solid ratio was selected to be 5:1. Figures 2a−2c indicate that the conditions for the conversion of α-spodumene into hydroxysodalite are more stringent than those for kaolinite.27,38,39 Effect of Stirring Rate on Lithium Extraction. Increasing stirring rate can also promote mass transfer. Hence, the effect of agitation on lithium extraction was investigated at different stirring speeds that ranged from 200 to 700 rpm. The temperature, NaOH concentration, liquid/solid ratio, and reaction time were maintained at 250 °C, 600 g/L, 5:1, and 2 h, respectively. As shown in Figure 2d, 500 rpm was sufficient for Li extraction. Effect of Reaction Time on Lithium Extraction and Composition and Textural Properties of Product. Numerous literatures have shown that the reaction time is an important factor affecting the synthesis of zeolites. In the case of the synthesis of hydroxysodalite, it usually takes several or many hours.23,29,30,40 Thus, the influence of reaction time on the Li extraction and the composition and textural properties of the product was investigated for durations from 0.5 to 5 h, under the constant conditions of 250 °C, 600 g/L NaOH concentration, 5:1 liquid/solid ratio, and 500 rpm stirring rate. It was observed that the hydrothermal conversion of α-spodumene was fast, with a Li extraction efficiency of above 70% within 0.5 h (Figure 2e), and a large amount of


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hydroxysodalite was found in the hydrothermal treatment product (Figures 5 and 6). The peaks of α-spodumene decreased with time and finally disappeared after 2 h (Figure 5). This observation explains the results in Figure 2e. As shown in Figure 6a, the hydroxysodalite shows a regular cubic structure and the particle size of which is about 20 μm. The surface of hydroxysodalite became rough and its particle size increased with increasing reaction time (Figure 6a, b, and d). Figure 7 shows the N2 adsorption/desorption isotherms for the products obtained under different hydrothermal conditions. According to the IUPAC classification, all the products display the typical reversible curve of Type II isotherm, which represents unrestricted monolayer-multilayer adsorption.41 This finding is consistent with the results obtained by Esaifan et al.24 Pore size distributions are shown in Figure 8, which indicates that the synthesized products present a porous structure. With the reaction time increasing from 0.5 to 5 h, the average pore diameter decreased from 5.92 to 4.82 nm. The textural properties of various products are listed in Table S1 (Supporting Information). The BET surface area and pore volume of the synthesized products increased with the reaction duration, which accounts for the much higher N2 adsorption capacity of the products obtained at 2 and 5 h (Figure 7). The Li extraction reached equilibrium within 2 h. The further time extension did not significantly improve the extraction or change the composition of the product. However, the intensity of the diffraction peak of hydroxysodalite did increase with time, indicating the increase in crystallinity of the product. Previous studies on the synthesis of hydroxysodalite from kaolin by hydrothermal alkaline treatment have shown that prior to the crystallization of hydroxysodalite, intermediate metastable phases, faujasite and zeolite A were generally formed at the initial stage of the reaction.32,42 Nevertheless, for the hydrothermal conversion of α-


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spodumene to hydroxysodalite, the formation of faujasite was mainly related to the NaOH concentration rather than to the reaction time, which implies that hydroxysodalite was directly formed from the precursor and the aluminosilicate gel.29 Figure 9 shows the FTIR spectra of the products synthesized at different reaction times. The peaks at 3543 and 3418 cm−1 are attributed to the –OH vibration of structural water molecules in hydroxysodalite. The peaks at 1650 and 1533 cm−1 correspond to the OH deformation of water. The pronounced wide absorption band at 885 cm−1 corresponds to the asymmetric stretching vibration of T–O–T (T = Si, Al). The absorption band at 727 and 661 cm−1 correspond to the symmetric stretching of T–O–T. The absorption band at 488 cm−1 is the characteristic of the bending vibration of O–T–O. The characteristics of the FTIR spectra of the products are consistent with those of hydroxysodalite.22,40,44 The weakening of the –OH and T–O–T vibrations and the disappearance of the O–T–O vibration were observed when the reaction time was increased from 2 to 5 h.  -Spodumene

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Figure 6. SEM images and EDX mapping of the hydroxysodalite produced from α-spodumene at different reaction time and dosage of CTAB: (a) 0.5 h; (b) 2 h; (c) 2 h with 3 g/L CTAB; (d) 5 h; (e) 5 h with 3 g/L CTAB (constant conditions: 250 °C, 600 g/L NaOH concentration, 5:1 liquid/solid ratio, and 500 rpm stirring rate).


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Figure 8. Pore size distribution for the products synthesized under different hydrothermal conditions. An integrated experiment was performed under the established optimized conditions (250 °C, 600 g/L NaOH concentration, 5:1 liquid/solid ratio, 2 h reaction time, and 500 rpm stirring rate) and a 95.8% Li extraction yield was obtained. The thermo-gravimetric analysis data of the hydroxysodalite zeolite synthesized at the optimized conditions revealed a 5.5% mass loss from room temperature to 1000 °C (Figure S2,


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Supporting Information). The mass loss from room temperature to 150 °C was due to the removal of adsorbed water while the larger mass loss from 400 to 700 °C was due to the dehydroxylation.22,43 Effect of the CTAB Surfactant on Lithium Extraction and Composition and Textural Properties of Product. Some literatures have been reported the promotion of cationic surfactants on minerals dissociation and crystal formation.45,46 Hence, a common cationic surfactant, CTAB was used in the present work to study its effects on lithium extraction and the composition and properties of the product. The dosage of CTAB was 3 g/L. Other parameters of hydrothermal reaction included a temperature of 250 °C, a NaOH concentration of 600 g/L, a liquid/solid ratio of 5:1, a stirring rate of 500 rpm, and a time of 2 h. It was observed that CTAB had no obvious effect on the lithium extraction. Interestingly, a small amount of a new phase, potassium aluminum oxide silicate (K0.85Al0.85Si0.15O2), was found in the product synthesized in the presence of CTAB (Figure S3, Supporting Information). The BET surface area of product increased with the addition of CTAB in the hydrothermal synthesis. This finding is in agreement with previous research results.47-49 However, CTAB had a small effect on the pore volume, which may be related to its relatively low concentration. Additionally, adding CTAB was conductive to obtaining near-spherical particles of a uniform size (Figure 6e). As shown in Figure 9, the addition of CTAB had little effect on the FTIR spectra of the products; only the weakening of the T–O–T vibration at 727 cm−1 was observed.


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488

661

885

5 h with CTAB

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3543

Transmission (%)

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3418


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2h

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3000

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Wavenumber (cm )

Figure 9. FTIR spectra of the products synthesized under different hydrothermal conditions. Desilication. The silicon in α-spodumene was partially leached in the hydrothermal treatment. The dissolved silica crystallizes easily, thereby causing the decrease in purity of lithium carbonate. To avoid it, the liquor containing lithium was further purified via desilication using inexpensively available calcium oxide to remove the silica. The desilication product was calcium silicate, which has many applications.50 In order to facilitate desilication, an equal volume of deionized water was added to dilute the liquor. After dilution, the concentration of SiO2 in solution was 36.4 g/L. Based on our previous research on desilication,51 the mass ratio of CaO to SiO2 and the time for desilication were set at 1.2 and 1 h, respectively. Only the effect of the reaction temperature on desilication was investigated in the present work. Figure S4 (Supporting Information) shows the SiO2 precipitation vs. desilication temperature curve. The extent of desilication was enhanced with increasing temperature, because the elevated temperature was conducive to the nucleation and growth of calcium silicate. As shown in Figure S4, a temperature of 95 °C was sufficient for desilication and a SiO2 precipitation of 96.2% was obtained.


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Lithium Precipitation. In order to enrich the Li in solution to facilitate precipitation, the solution after desilication was concentrated by evaporation, replenished with NaOH and then returned to the hydrothermal alkaline treatment step to leach raw ore. A continuous loop of desilication, evaporation concentration, and hydrothermal alkaline treatment was carried out and a concentrated lithium solution (Li 10.6 g/L) was obtained. Based on the widely used method of preparing lithium carbonate in industry,52 a stoichiometric amount of saturated Na2CO3 solution was added to 200 mL solution. The temperature and time for precipitation were 95 °C and 1 h, respectively. A lithium precipitation of 67.1% was achieved under these conditions. Although the direct precipitation rate of lithium is relatively low it would not affect the overall recovery rate since the solution is recycled. The XRD pattern of the obtained lithium carbonate (Figure S5, Supporting Information) agrees well with the standard pattern. CONCLUSIONS Lithium extraction and hydroxysodalite zeolite synthesis were successfully achieved by the hydrothermal conversion of α-spodumene. The optimum reaction temperature, NaOH concentration, liquid/solid ratio, reaction time, and stirring rate for Li extraction were determined to be 250 °C, 600 g/L, 5:1, 2 h, and 500 rpm, respectively. A 95.8% Li extraction yield was obtained under these conditions. After desilication, the lithium in solution was recovered by precipitation with Na2CO3. The conversion of α-spodumene occurred in the alkaline solution at 200 °C and was basically completed at 250 °C. At the low NaOH concentration of 300 g/L, the main product of the hydrothermal conversion was faujasite, which was transformed into hydroxysodalite with the further increase of the NaOH concentration. The hydrothermal conversion of α-spodumene was fast, as a large concentration of hydroxysodalite was present after only 0.5 h. The synthesized


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products presented a porous structure. Their BET surface area, pore volume, and average pore diameter increased with the duration of the treatment. Additionally, the FTIR spectra of the synthesized products indicates that the weakening of the –OH and T–O–T vibrations and the disappearance of the O–T–O vibration took place when the reaction time was increased from 2 to 5 h. The addition of CTAB had no obvious effect on the lithium extraction but increased the BET surface area of the products. Furthermore, adding CTAB was conductive to obtaining nearspherical particles with a uniform size. ASSOCIATED CONTENT Supporting Information Additional results as shown in Figures S1−S5, and Table S1 (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel.: +86-10-62332271. *E-mail: [email protected]. ORCID Chengyan Wang: 0000-0003-3982-6208 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS


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Synopsis The lithium extraction and hydroxysodalite zeolite synthesis have been successfully achieved by hydrothermal conversion of α-spodumene.


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Lithium extraction and hydroxysodalite zeolite synthesis by

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