J. Cent. South Univ. (2020) 27: 27-36

DOI: https://doi.org/10.1007/s11771-020-4275-4

Kinetics of leaching lithium from lepidolite using mixture of hydrofluoric and sulfuric acid

WANG Hai-dong(王海东)¹, ZHOU An-an(周安安)¹, GUO Hui(郭慧)^{1, 2},LU Meng-hua(吕梦华)¹, YU Hai-zhao(余海钊)¹

1. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China;

2. School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: The fluorine-based chemical method shows great potential in leaching lithium (Li) from lepidolite. Leaching kinetics of Li in a mixture of sulfuric acid and hydrofluoric acid, which is a typical lixivant for the fluorine-based chemical method, was carried out under crucial factors such as different HF/ore ratios (1:1-3:1 g/mL) and leaching temperatures (50-85 °C). The kinetics data fit well with the developed shrinking-core model, indicating that the leaching rate of Li was controlled by the chemical reaction and inner diffusion at the beginning of leaching (0-30 min) as a calculated apparent activation energy (E_a) of 20.62 kJ/mol. The inner diffusion became the rate-limiting step as the leaching continues (60-180 min). Moreover, effects of HF/ore ratio and leaching temperature on selective leaching behavior of Li, Al and Si were discussed. 90% of fluorine mainly existed as HF/F^- in leaching solution, which can provide theoretical guidance for further removal or recovery of F.

Key words: lepidolite; lithium extraction; fluorine-based chemical method; hydrofluoric acid; selective leaching; leaching kinetics

Cite this article as: WANG Hai-dong, ZHOU An-an, GUO Hui, LU Meng-hua, YU Hai-zhao. Kinetics of leaching lithium from lepidolite using mixture of hydrofluoric and sulfuric acid [J]. Journal of Central South University, 2020, 27(1): 27-36. DOI: https://doi.org/10.1007/s11771-020-4275-4.

1 Introduction

Lithium (Li) is projected to experience an increasing demand in the coming years due to its incredible application of lithium on primary and secondary batteries in electrical vehicles, portable devices and other energy storage materials. Currently, lithium is mainly extracted from lithium brines. However, about 70% of the global lithium brines are distributed in Argentina, Bolivia and Chile [1-4]. To diversify the lithium resource, the Li-bearing minerals, including spodumene, zinnwaldite and lepidolite have received increasing attention as important alternative Li resources due to its relatively wide distributions and shorter production cycle [4-10]. Different methods have been proposed aiming to extract lithium from lepidolite and listed in Table 1. The sulfuric acid method has been currently considered as the most efficient processes. However, the low lithium grade (~3% Li₂O in concentrates) resulted in a much more complex process to economically extract Li than that from spodumene, which has been realized commercial production. Moreover, the lepidolite sampled from Yichun, Jiangxi Province, China is obtained as the flotation tailings of Ta and Nb. Therefore, more efficient methods need to be proposed to enhance Li leaching from lepidolite.

Here, a fluorine-based chemical method has been proposed considering that 2%-4% fluorine (F) contained in lepidolite. The dissolution reaction of lepidolite in hydrofluoric acid and sulfuric acid (HF/H₂SO₄) occurred as reactions 1-3 [8, 11, 12].

Table 1 Different methods to extract lithium from lepidolite

More detailed discussion on dissolution behavior has been presented in our previous work [8]. More importantly, the added H⁺ can influence the complex equilibrium between Al³⁺ and F^- as reaction 2 [11]. Finally, the proposed fluorine-based chemical method converts Al, Li and K into sulfates as reaction 3 to maximize the leaching efficiency of Li and efficient consumption of HF.

(n: 0-6; m: 1-6)           (1)

             (2)

                     (3)

Figure 1 shows the schematic process to extract Li from lepidolite using HF/H₂SO₄. 98% of Li was efficiently leached under investigated optimal conditions: ore/HF/H₂SO₄ratio of 1: 2: 3.5 (g/mL/mL) at 85°C for 3h [8]. The relatively selective leaching of Li over Si was received due to the lepidolite preferentially dissolved over quartz, which is important for the downstream purification and separation.

As for acidizing in the petrochemical industry, the dissolving of sandstones in HF/HCl system (called mud acid) was considered that the surficial adsorption of HF molecules was the rate-limiting step [22-24]. In this work, the leaching kinetics of Li from lepidolite has been discussed in the mixture of sulfuric acid and hydrofluoric acid (H₂SO₄/HF). The compositions of lepidolite are significantly different from sandstones (which are mainly carbonate minerals), and the H₂SO₄ is less volatile than HCl, resulting in a different leaching behavior. Our previous work indicated that the HF/ore ratio showed significant effects on the leaching efficiency and the compositions of generated insoluble residues. Here, effects of different HF/ore ratio and leaching temperature on leaching kinetics of Li were carried out to further understand selective leaching of Li.

Figure 1 Schematic diagram of extract Li from lepidolite using H₂SO₄/HF as lixiviant

2 Experimental

2.1 Materials and reagents

The lepidolite sampled from Yichun was accompanied with albite (NaAlSi₃O₈) and some quartz (SiO₂). About 3.2% Li₂O was contained in the concentrate. Other major elements analysis was determined and listed in Table 2. The ore sample was ground by planetary ball mill for 30 min for leaching experiments.

Table 2 Chemical analysis of lepidolite ore sample (mass fraction, %)

Except for the analytical grade hydrofluoric acid (40 wt%), 1:1 (V/V) diluted sulfuric acid was added to convert Li, Al and K into sulfates because the concentrated H₂SO₄ (98 wt%) presents more oxidative and too viscous for H⁺ transportation.

2.2 Procedures and methods

The leaching system was typically set up in a 100 mL Teflon crucible reactor with an oil bath as heating system. The ore sample, pre-wetted with certain amount of deionized water, was then mixed with 1:1 H₂SO₄ under the optimal conditions. The hydrofluoric acid was immediately added once the desired temperature was reached (50-85°C). Then the slurry kept stirred continuously at 150 r/min for different leaching time. The resulted slurry was separated and analyzed to obtain series of kinetics data. The kinetics data were valued as averaged by repeating three times. The leaching efficiency of Li (L) and the insoluble residues ratio (R) were introduced to evaluate the efficiency of leaching process. The retention percentage of F (F_rent,l) in the leaching solution was also introduced.

Elemental analysis along with some characterization such as XRD (X-ray diffraction), FTIR (Fourier-transform infrared spectroscopy) and NMR (Nuclear magnetic resonance) were employed to determine the compositions of leaching solution and insoluble residues. Detailed instrument information has presented in our previous work [8].

                   (4)

                          (5)

       (6)

where Q_L and Q_F are mass concentrations of Li and F in leaching solution, respectively, g/L; V and V_HF are volumes of leaching solution and the added HF, respectively, L; m_ore is mass of lepidolite ore sample, g; c_HF is concentration of the HF added, mol/L; w_Li,ore and w_F,ore are mass fractions of Li and F in the lepidolite, respectively, %.

3 Results and discussion

3.1 Investigation of leaching kinetics

Effects of different leaching conditions have been discussed in our previous work [8], such as HF/ore ratio (1:1-3.5:1 mL/g), H₂SO₄/ore ratio (2:1-4:1 mL/g), leaching temperature (50-100°C) and leaching time (2-4 h). The results showed that the added amount of HF (HF/ore) and leaching temperature are the crucial factor that significantly influenced on the leaching process. The introduced H⁺ was employed to convert insoluble residues into soluble sulfates. However, the added H₂SO₄ shows slight effects on leaching efficiency of Li under the investigated range, which can be verified by XRD analyses of insoluble residues in our previous work [8]. The leaching solution is strongly acidic under the investigated conditions with a negative pH value without specific meaning (pH -0.5). However, the pH is very important for Al removal during separation and purification, which will be discussed in detailed in our future works.

The leaching kinetics of Li in Figure 2 was investigated under different leaching temperatures (50-85°C) with an optimal ore/HF/H₂SO₄ ratio of 1: 2: 3.5 (g/mL/mL) and stirred at 150 r/min. The distribution curve of particle size was showed in Figure 3(a). Moreover, effect of HF/ore ratio and leaching temperature on leaching of Li, Al and Si were also investigated, aiming to further understand the selective leaching behavior of lepidolite.

Figure 2 Leaching kinetics of lithium from lepidolite in HF/H₂SO₄ under different leaching temperatures

The shrinking-core model, which is commonly applied in leaching system or hydrometallurgy fields, is employed to analyze the kinetics data with the ore particles treated as spherical particles. Our previous work indicated that insoluble products like amorphous SiO₂ and AlF₃ were generated on the surface of minerals [8]. The insoluble fluorides can be further dissolved by the introduced H⁺. Different equations were employed to analyze the kinetics data according to different rate-limiting steps theories [25-31].

Chemical reaction (Eq. (7)):

                          (7)

Inner diffusion (Eq. (8)):

                  (8)

Mixed controlling process (Eq. (9)):

             (9)

However, the fitting results showed that none of these three equations could fit well with the whole leaching process. The leaching process was then separated into two periods due to that the L reached 85% for 30 min. The results in Figure 3(b) showed that the chemical reaction and inner diffusion were the limiting step of leaching Li at the beginning of leaching (0-30 min) with apparent energy (E_a) calculated as 20.62 kJ/mol (Figure 3(c)). While as for the leaching (60-180 min) continues, the inner diffusion became as the rate-limiting step that the Li⁺ diffused through solid product layers with E_a calculated as 19.37 kJ/mol (Figure 3(d)).

The leaching rate of Li during 0-30 min, which was controlled by the surface chemical reaction, could be attributed to the protonation attack of HF and H⁺. With the reaction proceeding, the insoluble products resulted in the inner diffusion leaching as rate-limiting step. This kinetics of leaching Li from lepidolite in HF/H₂SO₄ was different from that of α-spodumene [30], which has a higher E_a due to the lower chemical reactivity of α-spodumene.

3.2 Leaching of Li, Al and Si

The HF/ore and leaching temperature show obvious effect on leaching efficiency. Therefore, their effects on the leaching of Li, Al and Si were carried out to further understand the selective leaching behavior of lepidolite.

3.2.1 Effect of HF/ore

Figure 4 indicates that the HF/ore obviously influenced the leaching process that the leaching efficiency of Li, Al and Si increased dramatically with the increasing of HF/ore ratio, especially for the L.

3.2.2 Effect of leaching temperature

Figure 5 indicates that the leaching temperature affects the leaching of Li more obviously than that of Al and Si, due to that Al and Si formed insoluble products like AlF₃ and SiO₂ while Li mainly existed as soluble Li₂SO₄.

Figure 3 Kinetics investigation of leaching Li from lepidolite:

3.3 F retention in leaching system

3.3.1 Elemental analysis of leaching solution

The elemental analysis of leaching solution in Table 3 shows that 98% of Li can be leached along with more than 90% of Al and other metal elements (except K, which remained as insoluble K₂SiF₆) under the optimal leaching conditions (ore/HF/ H₂SO₄ ratio of 1: 2: 3.5 g/mL/mL at 85 °C for 3 h). However, considerable F was contained in leaching solution, which should be removed or recovered with subsequent process. Here, effects of different HF/ore ratio and leaching temperature on F retention were discussed. XRD and FTIR were employed to further determine the existing forms of fluorine in insoluble residues. As for leaching solution, chemical environment of typical element ²⁷Al and ¹⁹F was analyzed by NMR to reveal the existing form of F, which is important for further removal or recovery of F.

3.3.2 Effect on F retention in leaching solution

The results in Figure 6 showed that the F retention reached above 90% under the investigated

conditions, which is important for efficient using of F. The investigation also showed that the higher F retention was achieved at lower HF/ore due to the efficient conversion from fluorides into sulfates by H⁺. The higher leaching temperature is benefit for converting fluorides into sulfates. Considering that the crucial effect of HF/ore on leaching, the leaching is conducted with the HF/ore ratio of 2:1 (mL/g) at 85 °C.

3.3.3 ²⁷Al and ¹⁹F NMR analysis of leaching solution

Considering that the Al³⁺ can easily form coordination with F^-, the existing form of F in the liquid phase is important for subsequent separation and purification. Therefore, typical elements of ²⁷Al and ¹⁹F were investigated using NMR to determine the existing form of F.

Figure 4 Effect of HF/ore on leaching of Li, Al and Si:

Based on our previous NMR analysis of liquid phase compositions of spodumene dissolved in HF/H₂SO₄ [30], it can be concluded that the existing forms of Al and F in the liquid phase were determined by the complexation equilibrium between Al and F. The Al and F can exist as M₃[AlF₆] or [AlF_n]^3-n, n=1, 2, 3, …, 6 [32-34] or even complexing with other metal elements. Thus,the NMR analysis of existing form of F is important for understand the dissolution behavior.

Figure 5 Effect of leaching temperature on difference among leaching of Li, Al and Si:

More importantly, the introduced H⁺ affects the complexing equilibrium between Al and F. ²⁷Al NMR analysis in Figure 7 of the leaching solution of lepidolite shows that Al mainly exists in two forms: four-coordinated aluminium (Al^IV, ~80×10^-6) and six-coordinated aluminium (Al^VI, ~0). The slight shifting is mainly due to the exchange among different ligands. Because F^- as ligand can exchange rapidly with different ligands such as H₃O⁺ since F as the most negative non-metallic element. Combined with the analysis of ¹⁹F NMR signal in Figure 8, the signal near -129.00×10^-6 may belong to F^-/HF and SiF₆^2- chemical shift, and the chemical shift around -150.41×10^-6 belongs to the resonance signal formed by four-coordinate complexation between Al^IV and F, while the -156×10^-6 can be attributed to the six-coordination complexation between Al^VI and F [32-34]. The NMR analysis results showed that the Al^IV-F in 15 min was decreased compared with 180 min, indicating insoluble Al-F fluoride was gradually dissolved with leaching proceeding.

Table 3 Elemental analysis of leaching solution and insoluble residues under optimal conditions

Figure 6 Effect of HF/ore and leaching temperature on F retention in the liquid phase

The ²⁷Al and ¹⁹F NMR analysis of the leaching solution showed that Al^VI and F respectively existed as Al³⁺ and F^-/HF, which can be confirmed with the subsequent analysis of corresponding residues.

Figure 7 ²⁷Al NMR spectra of leaching solution of kinetics at 85 °C

Figure 8 ¹⁹F NMR spectra of leaching solution of kinetics at 85 °C

3.3.4 XRD analysis of insoluble residues

The XRD analysis in Figure 9 indicated that the characteristic peaks of albite and lepidolite were gradually decreased. Characteristic peaks of generated K₂SiF₆ (PDF#07-0217) were obviously detected. However, the quartz remained during the leaching process, resulting in an incongruent leaching of Li over Si, which is important for selective leaching of Li from lepidolite. However, no coordinated products between Al and F or even with Li were detected by XRD. Then, FTIR was carried out to further determine the composition of insoluble residues.

3.3.5 FTIR analysis of insoluble residues

The FTIR analysis in Figure 10 of lepidolite ore sample and insoluble residues showed that the M-O (M: Li, Al, K … metal elements) stretching vibration absorption peaks at 482.42 cm^-1 in the near infrared region <500 cm^-1 was obviously decreased during the leaching, indicating that the metal elements Li, K, Na and Al were efficiently leached. The absorption peaks of 694.81 cm^-1 occurred due to the stretching vibration of Al-F and the stretching vibration absorption peaks of Si—F—Si at 771.03 cm^-1 were detected [35]. The peaks at 1071.81 cm^-1 belonging to the stretching vibration peak of Si—O always remained during leaching process, indicating that a relatively selectively leaching of Li, Al over Si resulted in this fluorine-based acid treatment using HF/H₂SO₄.

Figure 9 XRD analyses of insoluble residues obtained from kinetics at 85°C:

Figure 10 FTIR analyses of insoluble residues obtained from kinetics at 85°C

Combined with the analysis of the corresponding leaching solution, the F in solid phase was insoluble fluorides Al-F and K₂SiF₆. These insoluble residues formed solid layers which affected the leaching rate of Li, which somehow resulted in this selective and incongruent leaching behavior of lepidolite.

4 Conclusions

1) The leaching kinetics of Li indicated that the chemical reaction and inner diffusion were rate-limiting step at the early stage of the leaching (0-30 min) at 50-85 °C, with an apparent energy E_a of 20.62 kJ/mol. The inner diffusion through solid layers became the rate-limiting step with leaching continues.

2) The HF/ore ratio and leaching temperature had an obvious effect on selective leaching Li over Al and Si.

3) More than 90% of fluorine was remained in leaching solution as HF/F^-, which is important for further utilization of F. The F in the solid phase mainly existed as insoluble AlF₃·3H₂O and K₂SiF₆.

Acknowledgments

The authors also thank for the Changsha Research Institute of Mining and Metallurgy for elemental analyses.

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(Edited by HE Yun-bin)

中文导读

锂云母混酸HF/H₂SO₄浸出锂的动力学研究

摘要：锂云母混酸HF/H₂SO₄浸出动力学研究表明：50~85 °C下锂浸出速率在浸出前期(0~30 min)由表面化学反应以及内扩散共同控制，表观活化能E_a为20.62 kJ/mol；浸出后期(60~180 min)则主要由产物内扩散控制。氢氟酸添加量相比浸出温度对Li、Al和Si的浸出速率影响更显著。浸出温度对Li浸出效率影响比对Al和Si的影响更明显。F元素在固相不溶渣中的存在形式则主要为Al-F不溶氟化物和K₂SiF₆。此外，液相中F元素在所研究的氢氟酸添加量及浸出温度下均可保持较高保留率(>90%)，为F元素的高效利用及后续回收利用提供保障。

关键词：锂云母；矿物提锂；氟化学法；氢氟酸；选择性浸出；动力学

Foundation item: Project(51474237) supported by the National Natural Science Foundation of China

Received date: 2019-04-08; Accepted date: 2019-12-20

Corresponding author: GUO Hui, PhD; Tel: +86-731-88879622; E-mail: hnhguo89@126.com; ORCID: 0000-0002-3554-7212