Recovery of Lithium from Lepidolite by Sulfuric Acid and Separation of Al/Li by Nanoﬁltration

Article Recovery of Lithium from Lepidolite by Sulfuric Acid and Separation of Al/Li by Nanoﬁltration

Lin Gao 1,2,3, Huaiyou Wang 1,2,*, Jinli Li 1,2 and Min Wang 1,2,*

1 Key Laboratory of Comprehensive and Highly Eﬃcient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China; [email protected] (L.G.); [email protected] (J.L.) 2 Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, China 3 University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (H.W.); [email protected] (M.W.)

Received: 2 September 2020; Accepted: 30 October 2020; Published: 4 November 2020

Abstract: The recovery and leaching kinetics of lithium from lepidolite by sulfuric acid method were investigated in this study, and a new method of nanofiltration to separate Al/Li from lepidolite leaching solution was coupled. The results indicated the optimal conditions about leaching lithium from 1 lepidolite: leaching at 433 K for 4 h with the agitation rate of 120 r min− , sulfuric acid concentration of 60 wt%, liquid-solid mass ratio of 2.5:1, under which the Li yield could reach at 97%. The kinetics observations revealed that the leaching process was controlled by the hybrid control of solid product layer diffusion and the chemical reaction, and dominated by chemical reaction step, which improved the conclusion of single-step control in the previous literature. A successful attempt was made to couple nanofiltration separation with sulfuric acid extraction of lithium, and DK membrane was used to separate Al/Li from lepidolite leaching solution. DK membrane has shown excellent retention of Al3+ and Ca2+ and also can effectively permeate Li+, which may bring a new inspiration for lithium extraction from lepidolite in the future.

Keywords: sulfuric acid method; lithium extraction; kinetics; nanoﬁltration; separation of aluminum and lithium

1. Introduction Lithium, as a critical element of supporting new energy strategy, has been widely used in lithium high-energy batteries, nuclear power generation, glass ceramics, and grease [1–5] due to its unique physical and chemical properties. The utilization of lithium in batteries products has been increased to 65% in 2019 [6]. It is foreseeable that the security of lithium supply has become a top priority for technology companies in the world. Salt lake brines have been the main resource for the production of lithium compounds since the late 1990s because of cost effective [7,8], while the development of lithium ores should be improved under the urgent demand for lithium products. As a lithium-bearing ore with abundant reserves, lepidolite (KLi1.5Al1.5[AlSi3O10](OH,F)2) can alleviate the urgent demand for lithium to a certain extent. Several efforts have been developed to extract lithium from lepidolite, such as chlorination roasting method, sulfate roasting method, limestone roasting method, and sulfuric acid method [9–11]. The principle of all these methods is a displacement reaction via roasting with different additives (chloride, sulfate, or limestone) or digestion with concentrated sulfuric acid. In terms of roasting process, lepidolite ores are roasted with additives at high temperature to extract lithium from the stable aluminosilicate structure of lepidolite, and then the roasted product is leached to obtain a lithium bearing solution. Various additives such as lime–milk, NaCl + CaCl2, Na2SO4 + K2SO4 + CaO, Na2SO4 + CaCl2 were investigated by Yan.

Minerals 2020, 10, 0981; doi:10.3390/min10110981 www.mdpi.com/journal/minerals Minerals 2020, 10, 0981 2 of 17 et al. to extract lithium from lepidolite [12–14]. The extraction rate of lithium was greater than 90% and all experiments need to be carried out at a high temperature (>800 ◦C). Luong proposed two additives for extracting lithium extraction from lepidolite concentrates by roasting with Na2SO4, FeSO 7H O + CaO, respectively. Li SO and LiKSO were generated during roasting at 850–1000 C, 4· 2 2 4 4 ◦ and the yield of lithium is between 85% and 93% [11,15]. Although the yield of lithium obtained by roasting process is considerable, the disadvantages of excessive energy-consumption, harsh operating conditions, and stringent environmental restrictions make them unsuitable for lithium extraction from low-grade lithium ores such as lepidolite. Since R. B. Ellestad proposed that lithium can be extracted from spodumene by the sulfuric acid method in 1950 [16], Lajoie-Leroux and other researchers have studied the factors that affect the extraction rate of lithium during the extraction of spodumene [10,17,18]. Afterwards, the higher extraction rate of lithium and low energy consumption of the sulfuric acid method have attracted many researchers to apply it to the extraction of lithium from lepidolite. Vieceli has proposed a sulphuric acid digestion process for extracting lithium from lepidolite ores with mechanical activation pre-treatment. Lepidolite is reacted with concentrated sulfuric acid (98%) at 130 ◦C for 15 min, about 80% of Li yield was reached [19]. Zhang studied the reaction between lepidolite concentrate particles and 85% H2SO4 at 200 C, and the yield of Li was 97.1%. Acid insoluble sulfates KAl(SO ) , Al(SO )OH 5H O and ◦ 4 2 4 · 2 Li2SO4, were generated during the process [20]. Compared with the conventional roasting process, the lower reaction temperature required by sulfuric acid method can save a large amount of energy, and has been widely used in lithium ore extraction. However, the lepidolite leaching solution contains a large amount of aluminum. The removal 3+ of aluminum is more difficult because there are various forms of aluminum ions like Al , AlO2−, 2+ + Al(OH)3, Al(OH) , Al(OH)2 in the aqueous solution. Generally, the removal of aluminum from liquids need to add acid, base, or extractant, as shown in Table1[ 21], which will have a certain impact on subsequent lithium recovery and cannot achieve the flexible operation at minimum cost. The traditional impurity removal method of lepidolite leaching solution is chemical precipitation method, which requires the addition of alkaline substances, followed by stepwise precipitation to remove ions other than lithium, thereby obtaining a lithium-rich solution with less impurity ions [22]. Therefore, an effective method of separating Al/Li in the lepidolite leaching solution is critical to reduce the cost of lithium extraction.

Table 1. Traditional methods of aluminum removal.

Methods Principle 3+ Alkali Al + 4OH− = AlO2− + 2H2O Acid 2Al3+ + 3(COOH) 2H O = Al (C O ) 2H O + 6H+ 2· 2 2 2 4 3· 2 ↓ Extractant Selectively forming a complex with aluminum

Nanofiltration (NF) is a membrane separation technology in which nanofiltration membranes have a typical pore size (1 nm) and fixed charged groups on the surface of membrane. The mass transfer process of NF relies on the combination of steric hindrance and charge effects, which can effectively separate monovalent and multivalent ions. NF has been widely used in wastewater treatment and purification [23–25], as well as the extraction of lithium in salt lake brine. According to our previous study [26], the separation performance of four commercial NF membranes (DK, DL, NF270, and Duracid NF) were evaluated in simulated lepidolite leaching solution. The results showed that DK membrane exhibited extremely high Li/Al separation efficiency, which can be attributed to its pore size, smooth membrane surface, and appropriate Zeta potential. In this study, the recovery of lithium from lepidolite by sulfuric acid method was first investigated, and the leaching process kinetics was studied to provide theoretical instruction for expanding the development of lepidolite resources in Yichun, China. Then, a new method of NF (DK membrane) was Minerals 2020, 10, x FOR PEER REVIEW 3 of 17

Minerals 2020, 10, 0981 3 of 17

coupled to separate Al/Li from lepidolite leaching solution, as shown in Figure1. The related results of Mineralsthis work 2020 can, 10, bex FOR considered PEER REVIEW as a successful attempt to extract lithium simply and eﬀectively. 3 of 17

Figure 1. Roadmap difference between this study and traditional research.

2. Materials and Methods

2.1. Leaching Process of Lepidolite

2.1.1. Materials FigureFigure 1. 1. RoadmapRoadmap difference difference between between this this study study and and traditional traditional research. research. 2. MaterialsThe ground and lepidolite Methods concentrate powder used in this study was provided by Jiangxi Tengda Technology2. Materials Co.,and Ltd.,Methods Nanchang, China, and the fraction presents about 55.65 wt% particles below 1062.1. μ Leachingm. X-Ray Process Diffraction of Lepidolite (XRD) patterns of the lepidolite concentrate were determined by X’pert Pro2.1. Leaching(PANalytical Process B.V, of LepidoliteAlmelo, The Netherlands) using Cu-Kα (λ = 1.5405 Å) radiation source. The scans2.1.1. were Materials conducted with a speed of 2θ = 5° min−1 in the 2θ scan range from 5° to 80°, as shown in 2.1.1. Materials FigureThe 2. XRD ground pattern lepidolite has showed concentrate that main powder phases used present in this in study the concentrate was provided powder by Jiangxi are lepidolite, Tengda albite,TechnologyThe and ground quartz. Co., Ltd.,lepidolite Applying Nanchang, concentrate the Rietveld China, powder andRefineme the used fractionnt, in the this presentsmain study minerals was about provided present 55.65 wt% byin theJiangxi particles concentrate Tengda below canTechnology106 beµm. quantified: X-Ray Co., Di Ltd., ff56.5%raction Nanchang, lepidolite, (XRD) patternsChina, 29.0% andalbite, of the the lepidoliteand frac 14.5%tion concentratepresents quartz. about were 55.65 determined wt% particles by X’pert below Pro 106(PANalytical μThem. X-Raymorphology B.V, Diffraction Almelo, of lepidolite The(XRD) Netherlands) patterns concentrate of using the were lepidolite Cu-K observedα (λ concentrate= 1.5405 by a scanning Å) were radiation determined electron source. microscope Theby X’pert scans 1 (SEM,Prowere (PANalytical conductedSU8010, Hitachi, withB.V, Almelo, aTokyo, speed Japan)The of 2 θNetherlands) =with5◦ minthe accelerating− inusing the Cu-K 2θ voltagescanα (λ range =of 1.5405 2.0 from kV Å) (coated 5 ◦radiationto 80 with◦, assource. Pt). shown And The its in mainscansFigure chemicalwere2. XRD conducted patterncomposition haswith showed is a listedspeed thatin of Table 2 mainθ = 5°2. phases Themin −solutions1 presentin the 2 θused in scan the in concentraterange the experimental from powder5° to 80°, procedure are as lepidolite, shown were in preparedFigurealbite, and2. withXRD quartz. deionizedpattern Applying has water. showed the Conc Rietveld thatentrated main Refinement, sulfuric phases acidpresent the (95–98%) main in the minerals ofconcentrate the analytical present powder in grade the concentrate are was lepidolite, supplied can byalbite,be Sinopharm quantified: and quartz. Group 56.5% Applying Chemical lepidolite, the Reagen 29.0% Rietveldt albite,Co., Refineme Ltd. and (Shanghai, 14.5%nt, the quartz. China).main minerals present in the concentrate can be quantified: 56.5% lepidolite, 29.0% albite, and 14.5% quartz. The morphology of lepidolite concentrate were observed by a scanning electron microscope (SEM, SU8010, Hitachi, Tokyo, Japan) with the accelerating voltage of 2.0 kV (coated with Pt). And its main chemical composition is listed in Table 2.□ The solutions used□ Lepidolite in the experimental procedure were prepared with deionized water. Concentrated sulfuric acid (95–98%)∇ Albite of the analytical grade was supplied by Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).● Quratz □ □

∇ □ Intensity

□ Lepidolite □ □ □ □∇ □ ∇∇∇ □ □□ ● □ □ ∇ □● ∇ □ □●∇□ ● ∇ □□ ∇ ∇ □ □∇ ∇□ Albite●□ □ □□ ● Quratz □ 0 102030405060708090□ ∇ 2 theta/ (°) □ Intensity FigureFigure 2. 2. XRDXRD pattern pattern of of lepidolite lepidolite concentrate concentrate powder. powder.

□ The morphology of lepidolite concentrate□ □∇ were observed by a scanning electron microscope ∇∇ □ □ □□ ● ∇ ∇ □ □●∇ □ ● □ □● ∇ □ □ ∇ □ ∇□●□ □ □ (SEM, SU8010, Hitachi, Tokyo, Japan) with the accelerating□ ∇ voltage□ ∇ of□ 2.0 kV (coated with Pt). And its 0 102030405060708090 2 theta/ (°) Figure 2. XRD pattern of lepidolite concentrate powder.

Minerals 2020, 10, 0981 4 of 17 main chemical composition is listed in Table2. The solutions used in the experimental procedure were prepared with deionized water. Concentrated sulfuric acid (95–98%) of the analytical grade was supplied by Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).

Table 2. Main chemical composition of lepidolite concentrate powder (mass fraction, %).

Li Al Cs Mg Ca Na K Rb Si F H2O 1.68 7.55 0.13 0.26 1.04 2.29 5.07 0.70 20.37 0.74 13.37

2.1.2. Experimental Methods and Procedures Single factor optimization experiment on the factors affecting the lithium extraction such as leaching temperature, mass fraction of sulphuric acid, liquid-solid mass ratio, leaching time, and agitate rate was studied first. The experimental scheme on these factors is shown in Table3. A three-factor three-level orthogonal experiment was designed to explore the optimal conditions for the leaching of lepidolite by sulfuric acid method, on the basis of single factor optimization experiment. Moreover, the leaching solution under the optimal conditions was used as the feed for subsequent nanofiltration separation.

Table 3. Experimental conditions for leaching lepidolite with sulfuric acid.

Mass Fraction Liquid-Solid Agitation Rate Temperature (K) Time (h) 1 of H2SO4 (%) Mass Ratio (r min− ) 393 413 Temperature (K) 433 70 2.5:1 3 120 453 473 50 60 Mass fraction of 433 70 2.5:1 3 120 H SO (%) 2 4 80 90 1.5:1 2:1 Liquid-solid 433 60 2.5:1 3 120 Mass Ratio 3:1 3.5:1 1 2 Time (h) 433 60 2.5:1 3 120 4 5 80 100 Agitation Rate 433 60 2.5:1 4 120 (r min 1) − 140 160

Each experiment was conducted with 20 g lepidolite and implemented in a 200 mL high-temperature and high-pressure reactor with an inner chamber made of PTFE (CLF-200mL, Shanghai Yushen Instrument Co., Td, Shanghai, China). The temperature and agitation rate was controlled by a coupled external electric heating jacket and a timing magnetic stirrer. After the reaction is completed and the reactor has been cooled down to room temperature, the reaction liquid was separated by vacuum ﬁltration and the ﬁlter residue was washed 3 times with deionized water to obtain leaching solution. The residue was dried at 353 K for 12 h. The mean average value of Li yield was used after completing each experiment 3 times to ensure accuracy. Minerals 2020, 10, 0981 5 of 17

The reaction kinetics behavior of lepidolite leaching process will be further researched by conducted leaching experiments about roasting temperature-roasting time and sulfuric acid concentration-roasting time. The corresponding experimental design, results, and corresponding discussion are detailed in the following Section 3.3.

2.1.3. Analytical Methods The pH of the feed were measured by S210 pH meter (Mettler-Toledo Instruments Co., Ltd. Shanghai, China). Inductively coupled plasma-optical emission spectrometry (ICP-OES) (ICAP 6500 DUO, Thermo Fisher, Waltham, MA, USA) was used to measure the concentrations of + 3+ + + 2+ 2 Li , Al ,K , Na , Ca in the feed solution and penetrate solution. The concentration of SO4 − was determined by the barium sulfate gravimetric method, and the content of Cl− is determined by the mercury nitrate titration method.

2.1.4. Calculation The yield of lithium was calculated by Equation (1).

c V X% = i i 100%. (1) 1000 ωm × where ci is the concentration of lithium in the leaching solution, (g/L); Vi is the total volume of the leaching solution (mL); ω is the content of lithium in the lepidolite (mass fraction, %), and m is the initial mass of lepidolite concentrate powder (g).

2.2. Nanoﬁltration Separation

2.2.1. Materials The lepidolite leaching solution obtained under optimized leaching conditions was introduced into Cl− to prepare nanofiltration feed solution. Firstly, the dilution (1 L) of every 20 g lepidolite leaching solution was chosen and the influence of pH on nanofiltration separation was explored subsequently. Due to the excessive amount of unreacted sulfuric acid in the solution, the divalent alkali Ca(OH)2 solution (pH = 12.5), which can be retained by nanofiltration membrane, was used to adjust the pH to reduce the impact of competition on the subsequent separation. The volume of Ca(OH)2 solution required to adjust the acidity of the lepidolite leaching solution to the corresponding pH (1.4, 2.2) can 6 be calculated according to the solubility product of Ca(OH)2 (Ksp = 4.7 10− ). Secondly, nanofiltration 2 × membranes exhibit high retention for SO4 −, which may great influence the penetration of cations. Therefore, a monovalent anion Cl− was introduced into the solution this experiment, and the required 2 amount of CaCl2 could be calculated based on the molar ratio of SO4 −:Cl− = 0.6:0.4 according to our previous study to achieve the best Al/Li separation efficiency [26]. The supernatant and precipitate were separated, and the precipitate was washed several times. Then, the obtained lotion was mixed with the supernatant to reduce the entrainment loss of the precipitate. Finally, every 20 g of lepidolite sulfuric acid extract must be re-concentrated to 1 L as the feed solution for nanofiltration.

2.2.2. Experimental Methods and Procedures Filtrations were carried out in a complete recycling mode by circulating the permeate and retentate to the circling tank. To ensure that there was no residual water in the device, the device rinsed thoroughly with the feed solution. And, at the end of the experiment, the membrane unit was rinsed 3 times with deionized water to maintain the cleanliness of the membrane. Retention experiments were all conducted at a constant operating temperature of 293.15 K, pressure of 3.4 MPa, and ﬂow rate 1 of 3.5 L min− . Concentrate and permeate solutions were sampled after the equilibration of separation for 10 min. Minerals 2020, 10, 0981 6 of 17

2.2.3. Separation Equipment The laboratory-scale nanofiltration device (DSP-1812W-S) used in this study was provided by Hangzhou Donan Memtec Co., Ltd., Hangzhou, China. DK-1812 membrane was located in a stainless steelMinerals circular 2020, 10 unit,, x FOR the PEER feed REVIEW entered the center and flowed radially outward under external pressure6 of 17 (Figure3). The circulating tank was equipped with a water jacket, which can be used to control the temperaturetemperature of of the the feed feed between between 293.15 293.15 ±0.5 0.5 K. K. The The interrelatedinterrelated operating operating pressures pressures and and flows flows could could ± bebe adjusted adjusted by by manual manual valves. valves. ConcentrateConcentrateJ cJc andand permeatepermeate streamstreamJ pJp refersrefers toto thethestream stream has has not not passedpassed through through and and has has passed passed through through the membrane, the membra respectively,ne, respectively, which canwhich be removedcan be removed or recycled or torecycled the feed to tank. the feed Sampling tank. Sampling can be done can either be done in the either feed in tank the orfeed the tank permeate or the stream.permeate stream.

FigureFigure 3. 3. SeparationSeparation device of the the nanofiltration. nanofiltration. 1. 1. Circ Circulatingulating tank; tank; 2. Drain 2. Drain valve; valve; 3. Pipeline 3. Pipeline filter; filter;4. Pump; 4. Pump; 5. Frequency 5. Frequency converter; converter; 6. Safety 6. Safety relief relief valve; valve; 7. 7.Pressure Pressure gauge; gauge; 8. 8.Membrane Membrane unit; unit; 9. 9.Pressure Pressure gauge; gauge; 10. 10. Pressure Pressure regulating regulating valve; valve; 11. 11. Concentrate Concentrate flow flow meter. meter.

2.2.4.2.2.4. Calculation Calculation IonsIons retention retention rate, rate, separation separation factor factor of of lithium-aluminum, lithium-aluminum, and and flux flux are are three three essential essential aspects aspects to evaluateto evaluate membranes membranes separation separation performance. performance. RetentionRetention rate,rate, R, which which is is the the main main indicator indicator for for evaluating evaluating separation separation ability ability and andions ions permeability. permeability. ! C R = 1 P 100% (2) C C−P F × R =×  1- 100% (2) where Cp and CF are the concentration of ionsC ofF the permeate and feed solution (g/L), respectively. Separation factor, SF, means the concentration ratio of Li+ and Al3+ in the permeate and where Cp and CF are the concentration of ions of the permeate and feed solution (g/L), respectively. feed solution. + 3+ Separation factor, SF, means the concentrationC + /C ratio3+ of Li and Al in the permeate and feed Li Al P solution. SF = (3) C + /C 3+ ()Li Al F CC+3+/ When SF > 1, lithium could preferentially passLi through Al P the membrane than aluminum. SF = (3) Permeate flux, J, refers to the volume of permeate()CC/ permeated through the effective membrane area Li+3+ Al per unit time, and reflects the ability of the membrane to handleF a certain concentration of solution: When SF > 1, lithium could preferentially pass through the membrane than aluminum. V Permeate flux, J, refers to the volume Jof= permeate permeated through the effective membrane(4) t S 3600 area per unit time, and reflects the ability of the· · membrane to handle a certain concentration of wheresolution:V is the volume of the permeate, L; S is the effective area of the diaphragm, m2; and t is the time taken for sampling, h. V J = (4) t·S·3600 Where V is the volume of the permeate, L; S is the effective area of the diaphragm, m2; and t is the time taken for sampling, h.

Minerals 2020, 10, 0981x FOR PEER REVIEW 7 of 17

3. MineralsResults 2020 and, 10 Discussion, x FOR PEER REVIEW 7 of 17 3. Results and Discussion 3. Results and Discussion 3.1. Morphology and Composition of the Leaching Residue 3.1. Morphology and Composition of the Leaching Residue The Scanning Electron Microscopy (SEM) image of the concentrate powder and leaching residue after theThe reaction Scanning (Figure Electron 44)) showedMicroscopyshowed thatthat (SEM) layeredlayered image structurestructure of the concentrate ofof lepidolitelepidolite powder isis destroyeddestroyed and leaching andandlithium residuelithium isis successfullyafter the reaction transferred (Figure from 4) showed aluminosilicate that layered phase. structure In addition of lepidolite to the generationis destroyed of and soluble lithium sulfates is in successfullythe reaction, transferred the XRD spectrumfrom aluminosilicate (Figure 5) showsphase. Inthat addition there isto a the large generation amount of of soluble KAl(SO sulfates4)2 in the in the reaction, the XRD spectrum (Figure5) shows that there is a large amount of KAl(SO 4)2 in the residue.in the reaction, The chemicalchemical the XRD reactionreaction spectrum betweenbetween (Figure lepidolite lepidolite 5) shows and an thatd sulfuric sulfuric there acidis acid a large during during amount the the leaching ofleaching KAl(SO process process4)2 in canthe can be expressedberesidue. expressed The as follows:as chemical follows: reaction between lepidolite and sulfuric acid during the leaching process can be expressed as follows: 4KLi1.5Al1.5AlSi3O10OH2 + 20H2SO4→K2SO4 + 3Li2SO4 + 4Al2SO43 + 2KAlSO42 + 12H4SiO4 (5) 4K(Li 1.5Al1.5)[AlSi 3O10](OH) + 20H2SO4→ K2SO4 + 3Li2SO4 + 4Al 2(SO4) +2KAl(SO4) + 12H4SiO4 (5) 4K Li1.5Al1.5 AlSi3O10 OH 22 + 20H2SO4 →K2SO4 + 3Li2SO4 + 4Al2 SO4 3 + 2KAl3 SO4 2 + 12H2 4SiO4 (5)

Figure 4. SEM diagram of (a) lepidolite concentrate powder and (b) leaching residue leached after 4 h. FigureFigure 4. 4.SEM SEM diagramdiagram of ( aa)) lepidolite lepidolite concentrate concentrate powder powder and and (b) leaching (b) leaching residue residue leached leached after 4 afterh. 4 h.

♦ ♦♦ ————leachedleached ——unleached——unleached ♦

♦ ∇ ♦ ∇ ∇ ♦ ● ∇ ♦ ♦ ● ∇ ● ♦ ♦ ♦ ∇ ● ∇ ♦ ♦ ● ♦♦ ♦ ♦ ♦ ∇ ● ♦♦ ♦ □ ♦ ♦KAl(SO4)2 KAl(SO4)2

□ ∇ Albite □ □ ∇ Albite Intensity □ Intensity ∇ □ ● Quratz ∇ □ ● Quratz □ Lepidolite □ Lepidolite □ □ □∇ □ □ ∇ ∇∇ □ □ □ ● □ ● □ □● ∇□ □ □ □● □ ∇□ □ □∇ □∇□□ □ ∇□ ∇ ● ∇ □ ∇ ●□□ □ □ ∇ □●∇∇∇ □□ □● □ ● □ ∇ □ ∇□ ∇ ∇ □ □∇ ∇□●□□ □ □

0 102030405060708090 0 102030405060708090 2 theta/ (°) 2 theta/ (°) Figure 5. XRD patterns of lepidolite concentrate powder and leaching residue leached after 4 h. Figure 5. XRDXRD patterns of lepidolite concentrate powder and leaching residue leached after 4 h. 3.2. Leaching Process Optimization 3.2. Leaching Process Optimization 3.2.1. Effect of Leaching Temperature 3.2.1. Eﬀect of Leaching Temperature 3.2.1. Effect of Leaching Temperature As shown in Figure 6, the Li yield in the range of 393–473 K first increased from 73% to 89%, and As shown in Figure6, the Li yield in the range of 393–473 K ﬁrst increased from 73% to 89%, thenAs decreased shown in Figureto 77% 6,with the increasingLi yield in thetemperature. range of 393–473 That is Kbecause first increased the increase from in 73% temperature to 89%, and and then decreased to 77% with increasing temperature. That is because the increase in temperature thenwithin decreased a certain to range 77% willwith enhance increasing the collision temperature. probability That ofis molecules,because the which increase is conducive in temperature to the within a certain range will enhance the collision probability of molecules, which is conducive to the withinfull reactions a certain of range material will andenhance improves the collision the leachi probabilityng of lithium. of molecules, The Li yield which decreased is conducive when theto the full reactions of material and improves the leaching of lithium. The Li yield decreased when the fulltemperature reactions ofexceeds material 433 Kand because improves the boiling the leachi pointng of ofsulfuric lithium. acid The has Libeen yield exceeded, decreased and thewhen acid the temperature exceeds 433 K because the boiling point of sulfuric acid has been exceeded, and the acid

Minerals 2020, 10, 0981 8 of 17 Minerals 2020, 10, x FOR PEER REVIEW 8 of 17 Minerals 2020, 10, x FOR PEER REVIEW 8 of 17 temperature exceeds 433 K because the boiling point of sulfuric acid has been exceeded, and the acid gas will further corrode the device and bring about potential safety hazards. Therefore, the leaching gas will furtherfurther corrodecorrode thethe devicedevice andand bringbring aboutabout potentialpotential safetysafety hazards.hazards. Therefore, thethe leachingleaching temperature is kept constant at 433 K for the subsequent experiments. temperaturetemperature isis keptkept constantconstant atat 433433 KK forfor thethe subsequentsubsequent experiments.experiments.

100 100

90 90

80 80

70 70 Li yield (%)

Li yield (%) 60 60

50 50 380 400 420 440 460 480 380 400 420 440 460 480 Temperature (K) Temperature (K) Figure 6. Effect of leaching temperature on the extraction of lithium. Figure 6. EffectEﬀect of leaching temperature on thethe extractionextraction ofof lithium.lithium.

3.2.2. Effect Effect of Sulfuric Acid Mass Fraction 3.2.2. Effect of Sulfuric Acid Mass Fraction The eeffectffect ofof sulfuricsulfuric acidacid mass fraction on the Li yield was investigated in the range of 50 wt% The effect of sulfuric acid mass fraction on the Li yield was investigated in the range of 50 wt% toto 9090 wt%wt% asas shownshown inin FigureFigure7 .7. TheThe LiLi yieldyield reachedreached aa maximummaximum ofof 93%93% when when the the sulfuric sulfuric acid acid to 90 wt% as shown in Figure 7. The Li yield reached a maximum of 93% when the sulfuric acid concentration was was 60 60 wt%, wt%, and and then then drops drops with with the theincrease increase of the of concentration the concentration to the to lowest the lowest value concentration was 60 wt%, and then drops with the increase of the concentration to the lowest value valueof 52%. of This 52%. is This because is because the concentration the concentration of ionized of ionized H+ plays H+ playsa decisive a decisive role in role the in extraction, the extraction, and of 52%. This is because the concentration of ionized H+ plays a decisive role in the extraction, and anddissociated dissociated H+ isH more+ is moredifficult diffi tocult be ionized to be ionized in high in concentration high concentration sulfuric sulfuric acid. Furthermore, acid. Furthermore, higher dissociated H+ is more difficult to be ionized in high concentration sulfuric acid. Furthermore, higher higherconcentration concentration of sulfuric of sulfuric acid has acid poor has fluidity, poor fluidity, which is which not conducive is not conducive to the occurrence to the occurrence of reaction of concentration of sulfuric acid has poor fluidity, which is not conducive to the occurrence of reaction reaction[27]. Therefore, [27]. Therefore, the appr theopriate appropriate sulfuric acid sulfuric concentration acid concentration was 60 wt%. was 60 wt%. [27]. Therefore, the appropriate sulfuric acid concentration was 60 wt%.

100 100

90 90

80 80

70 70 Li yield (%)

Li yield (%) 60 60

50 50 50 60 70 80 90 50 60 70 80 90 Mass fraction (%) Mass fraction (%)

Figure 7. Effect of sulfuric acid mass fraction on the extraction of lithium. Figure 7. Effect of sulfuric acid mass fraction on the extraction of lithium. Figure 7. Effect of sulfuric acid mass fraction on the extraction of lithium. 3.2.3. Effect of Liquid-Solid Mass Ratio 3.2.3. Effect of Liquid-Solid Mass Ratio 3.2.3.The Effect effect of Liquid-Solid of liquid-solid Mass mass Ratio ratio on the Li yield was investigated in the concentration range of The effect of liquid-solid mass ratio on the Li yield was investigated in the concentration range 1.5:1 toThe 3.5:1 effect as shownof liquid-solid in Figure mass8. The ratio results on the show Li thatyield the was Li investigated yield increased in the from concentration 78% to 96% whenrange of 1.5:1 to 3.5:1 as shown in Figure 8. The results show that the Li yield increased from 78% to 96% theof 1.5:1 liquid-solid to 3.5:1 massas shown ratio in varies Figure from 8. The 1.5:1 results to 3.5:1. show that the Li yield increased from 78% to 96% when the liquid-solid mass ratio varies from 1.5: 1 to 3.5: 1. when the liquid-solid mass ratio varies from 1.5: 1 to 3.5: 1.

Minerals 2020, 10, x FOR PEER REVIEW 9 of 17 Minerals 2020, 10, 0981 9 of 17 Minerals 2020, 10, x FOR PEER REVIEW 9 of 17

100 100 90 90 80 80 70 70 Li yield (%) 60 Li yield (%) 60 50 50 1.5 2.0 2.5 3.0 3.5 1.5 2.0 2.5 3.0 3.5 Ratio Ratio Figure 8. Effect of liquid-solid mass ratio on the extraction of lithium. FigureFigure 8.8. EEffectffect ofof liquid-solidliquid-solid massmass ratioratio onon thethe extractionextraction ofof lithium.lithium. An increasing dosage of sulfuric acid is beneficial to lithium leaching, because if the amount of An increasing dosage of sulfuric acid is beneficial to lithium leaching, because if the amount sulfuricAn acidincreasing is insufficient, dosage of there sulfuric is notacid enough is beneficial sulfuric to lithium acid in leaching, the closed because vessel if tothe reduce amount the of of sulfuric acid is insufficient, there is not enough sulfuric acid in the closed vessel to reduce the condensationsulfuric acid effectis insufficient, between thethere reacted is not ore enough powder sulfurics. When acid the inliquid-solid the closed mass vessel ratio to was reduce greater the condensation effect between the reacted ore powders. When the liquid-solid mass ratio was greater thancondensation 2.5:1, a further effect betweenincrease thehas reacted no conspicuous ore powder influences. When on the the liquid-solid Li yield, butmass the ratio cost was is greatlygreater than 2.5:1, a further increase has no conspicuous influence on the Li yield, but the cost is greatly increasedthan 2.5:1, anda further the unreactedincrease has acid no willconspicuous also ad verselyinfluence affect on the the Li subsequentyield, but the separation cost is greatly and increased and the unreacted acid will also adversely affect the subsequent separation and purification. purification.increased and Therefore, the unreacted the liquid–solid acid will mass also ratio ad wasversely determined affect theto be subsequent 2.5:1. separation and Therefore,purification. the Therefore, liquid–solid the mass liquid–solid ratio was mass determined ratio was to determined be 2.5:1. to be 2.5:1. 3.2.4. Effect of Leaching Time 3.2.4. Effect of Leaching Time 3.2.4. Effect of Leaching Time The effect of leaching time in the range from 1 h to 5 h on lithium extraction was examined as The effect of leaching time in the range from 1 h to 5 h on lithium extraction was examined as shownThe in effectFigure of 9. leaching When the time leaching in the time range increase from 1d hfrom to 5 1 h h on to lithium5 h, the Liextraction yield increased was examined from 73% as shown in Figure9. When the leaching time increased from 1 h to 5 h, the Li yield increased from 73% toshown 98%. inLeaching Figure 9.time When has thea significantly leaching time increased increase effed fromct on 1 lithium h to 5 h, extraction the Li yield in theincreased first four from hours, 73% to 98%. Leaching time has a significantly increased effect on lithium extraction in the first four hours, butto 98%. the further Leaching increase time has of thea significantly leaching time increased (over 4 effeh) hasct on little lithium effect extraction on improving in the the first extraction four hours, of but the further increase of the leaching time (over 4 h) has little effect on improving the extraction lithium.but the further increase of the leaching time (over 4 h) has little effect on improving the extraction of of lithium. lithium.

100 100 90 90 80 80 70 70 Li yield (%)Li 60 Li yield (%)Li 60 50 50 12345 12345Time (h)

Time (h)

Figure 9. EffectEffect of leaching time on the extraction of lithium. Figure 9. Effect of leaching time on the extraction of lithium. 3.2.5. Effect of Agitation Rate 3.2.5. Effect of Agitation Rate 1 1 3.2.5.The Effect effect of Agitation of agitation Rate rate in the range from 80 r min− to 160 r min− on lithium extraction The effect of agitation rate in the range from 80 r min−1 to 160 r min−1 on lithium extraction was was studied as shown in Figure 10. The observation results indicated that the agitation rate has a studiedThe as effect shown of inagitation Figure 10. rate The in observationthe range from results 80 r indicated min−1 to 160that r the min agitation−1 on lithium rate has extraction a relatively was relatively small effect on lithium extraction, which also shows that the leaching process of lepidolite is smallstudied effect as shown on lithium in Figure extraction, 10. The observationwhich also resultsshows indicatedthat the leaching that the agitationprocess ofrate lepidolite has a relatively is not not controlled by external diffusion. In addition, a small agitation rate will make the materials easy to small effect on lithium extraction, which also shows that the leaching process of lepidolite is not controlled by external diffusion. In addition, a small agitation rate will make the materials1 easy to agglomerate during the leaching process, so the appropriate agitation rate was 120 r min− . agglomeratecontrolled by during external the diffusion. leaching process,In addition, so the a appropriatesmall agitation agitation rate will rate make was 120the rmaterials min−1. easy to agglomerate during the leaching process, so the appropriate agitation rate was 120 r min−1.

Minerals 2020, 10, 0981 10 of 17 Minerals 2020, 10, x FOR PEER REVIEW 10 of 17

100

Li yield (%) 60

50 80 100 120 140 160 -1 Agitation rate (r min ) Figure 10. EEffectﬀect of agitation rate on thethe extractionextraction ofof lithium.lithium. 3.2.6. Orthogonal Experiment 3.2.6. Orthogonal Experiment In order to explore the optimized conditions for the leaching of lepidolite by the sulfuric acid In order to explore the optimized conditions for the leaching of lepidolite by the sulfuric acid method, on the basis of single factor optimization experiment, the main factors of roasting temperature, method, on the basis of single factor optimization experiment, the main factors of roasting sulfuric acid mass fraction, and liquid-solid mass ratio were selected for orthogonal experiment temperature, sulfuric acid mass fraction, and liquid-solid mass ratio were selected for orthogonal investigation. A three-factor three-level orthogonal experiment table was designed as shown in Table4. experiment investigation. A three-factor three-level orthogonal experiment table was designed as The orthogonal experiment design and results were shown in Table5. shown in Table 4. The orthogonal experiment design and results were shown in Table 5.

Table 4. The design of orthogonal experiment. Table 4. The design of orthogonal experiment. A B C A B C Sulfuric Acid Mass Fraction (wt %) Liquid-solid Mass Ratio Roasting Temperature (K) Sulfuric Acid Mass Fraction (wt %) Liquid-solid Mass Ratio Roasting Temperature (K) 1 1 5050 2:1 2:1 413 413 2 2 6060 2.5:1 2.5:1 433 433 3 3 7070 3:1 3:1 453 453

Table 5.5. TheThe resultresult ofof orthogonalorthogonal experiment.experiment.

No. No. A A B B C C Li Yield Li (%) Yield (%) 1 1 5050 2:12:1 413413 70 70 2 2 5050 2.5:12.5:1 433433 86 86 3 3 5050 3:13:1 453453 84 84 4 4 6060 2:12:1 453453 86 86 5 5 6060 2.5:12.5:1 413413 90 90 6 6 6060 3:13:1 433433 99 99 7 7 7070 2:12:1 433433 87 87 8 8 7070 2.5:12.5:1 453453 85 85 9 70 3:1 413 83 9 70 3:1 413 83 K1 K1 240 240 243243 243243 K2 K2 275 275 261261 272272 K 255 266 255 K3 3 255 266 255 ⎯ K1 K1 80 80 8181 81 81 ⎯K2 91.7 87 90.7 Σ K2 91.7 87 90.7 Σ = 770.1 = 770.1 ⎯ K3 K3 85 85 88.788.7 85 85 Rj 11.7 7.7 9.7 Rj 11.7 7.7 9.7 Optimal level A2 B3 C2 OrderOptimal level A2 B A3 C B C2 Order A C B Through the range analysis, it was concluded that the influence of the three factors on the Li yield was as follows: roasting temperature > sulfuric acid mass fraction > liquid-solid mass ratio. And the optimal conditions of the leaching experiment were: leaching at 433 K for 4 h with the agitation

Minerals 2020, 10, x FOR PEER REVIEW 11 of 17

rate of 120 r min−1, sulfuric acid concentration of 60 wt%, liquid-solid mass ratio of 3:1. However, when the liquid-solid mass ratio increases from 2.5:1 to 3:1, the Li yield was only increased by 1.08%, which greatly increased the reaction cost and excessive sulfuric acid would also adversely affect the subsequent separation and purification. Therefore, 2.5:1 should be selected as the optimal liquid– solid mass ratio, and the composition of the lepidolite leaching solution under this condition is shown in Table 6.

Minerals 2020, 10, 0981 11 of 17 Table 6. The mass fraction (%) of each element in lepidolite powder and leaching solution.

Li Al Mg Ca Na K Si Through the range analysis, it was concluded that the influence of the three factors on the Li Lepidolite 1.68 7.55 0.26 1.04 2.29 5.07 20.37 yield was as follows: roasting temperature > sulfuric acid mass fraction > liquid-solid mass ratio. Solution 1.64 5.37 0.005 0.07 0.31 0.66 0.006 And the optimal conditions of the leaching experiment were: leaching at 433 K for 4 h with the 1 agitation3.3. Kinetic rate Analysis of 120 r min− , sulfuric acid concentration of 60 wt%, liquid-solid mass ratio of 3:1. However, when the liquid-solid mass ratio increases from 2.5:1 to 3:1, the Li yield was only increased by 1.08%,In whichthis study, greatly lepidolite increased concentrate the reaction particle cost ands could excessive be considered sulfuric acid as would non-porous also adversely spherical affparticles.ect the subsequent The reaction separation proceeds and on purification.the surface of Therefore, the particles 2.5:1 at should the beginning, be selected and as a thesolid optimal product liquid–solidlayer is formed mass ratio,on the and surface the composition of the particles of the by lepidolite the generated leaching aluminum solution sulfate under this and condition orthosilicate. is shownSulfuric in Table acid6 then. needs to pass through the product layer before continuing to react at the interface between the unreacted solid phase and the solid product layer [28]. The reaction process is shown in Figure Table11. 6. The mass fraction (%) of each element in lepidolite powder and leaching solution. The reaction of lepidolite and sulfuric acid can be interpreted by the shrinking core model (SCM) Li Al Mg Ca Na K Si with constant particle size of the non-catalytic liquid-solid reaction [29], the dynamic equation as Lepidolite 1.68 7.55 0.26 1.04 2.29 5.07 20.37 shown in Equation (6). Solution 1.64 5.37 0.005 0.07 0.31 0.66 0.006 1 2 2 ρ rL r 2 t = 1 - 1 - x3 + L 1 - x - 1 - x3 (6) bMc k 6D 3 3.3. Kinetic Analysis 0 c where t is the reaction time; b is the molar ratio of reactants; ρ is the density of solid particles; x is the Li Inyield; this study,M is the lepidolite relative concentrate molar mass particles of lepidolite could be concentrate considered aspowder; non-porous c is the spherical concentration particles. of The reaction proceeds on the surface of the particles at the beginning, and a solid product layer is sulfuric acid; rL is the initial radius of the lepidolite particles; k0 is the Arrhenius constant; Dc is the is formed on the surface of the particles by the generated aluminum sulfate and orthosilicate. Sulfuric acid the diffusion coefficient in porous product layer. The parameters rL, Dc, ρ, etc., which are not easy to then needs to pass through the product layer before continuing to react at the interface between the measure, can be expressed by the equation relationship with the reaction rate constant k (kC, kD) which unreactedis easier solidto be phasemeasured and theby the solid experiment. product layer [28]. The reaction process is shown in Figure 11.

FigureFigure 11. 11.Diagram Diagram of theof the reaction reaction process process between between lepidolite lepidolite concentrate concentrate powder powder and sulfuric and sulfuric acid (a acid), the(a product), the product layer; (layer;b), the (b reaction), the reaction interface; interface; and (c ),and the (c unreacted), the unreacted core of core the of lepidolite the lepidolite particles). particles).

TheAssuming reaction ofD lepidolitec >> k0, it andcan sulfuricbe considered acid can that beinterpreted the reaction by process the shrinking is only core controlled model (SCM) by the withchemical constant reaction, particle then size the of reaction the non-catalytic kinetic function liquid-solid of the reactionreaction [control29], the process dynamic can equation be simplified as shownand expressed in Equation by (6). YC as follows.  1  2 ρ r YC =1 1 - 1 r- x3 = kC2t 2  t =  L 1 (1 x) 3 + L 1 x (1 x) 3  (6)   (7) bMc k0 − − k0bMc6Dc − 3 − −  kC = rLρ where t is the reaction time; b is the molar ratio of reactants; ρ is the density of solid particles; x is the Li yield; M is the relative molar mass of lepidolite concentrate powder; c is the concentration of sulfuric acid; rL is the initial radius of the lepidolite particles; k0 is the Arrhenius constant; Dc is the is the diffusion coefficient in porous product layer. The parameters rL, Dc, ρ, etc., which are not easy to measure, can be expressed by the equation relationship with the reaction rate constant k (kC, kD) which is easier to be measured by the experiment. Minerals 2020, 10, x FOR PEER REVIEW 12 of 17 Minerals 2020, 10, 0981 12 of 17 On the contrary, if the reaction is controlled by the diffusion process in the solid product layer, i.e., k0 >> Dc, then the reaction kinetic function of diffusion control process can be simplified and Assuming Dc >> k0, it can be considered that the reaction process is only controlled by the chemicalexpressed reaction, by YD as then follows. the reaction [30]: kinetic function of the reaction control process can be simplified and expressed by YC as follows. 2 2 Y = 1 - x - 1 - x3 = k t D 3 D 1 (8) 3 YC = 1 6D(1cbMcx) = kCt k k= bMc− − (7) k =D 0 2 C rLρ rLρ

OnIn order the contrary, to obtain if the the controlling reaction is controlledstep and conduc by thet dikineticffusion analysis process in in this the work, solid productthe Li yield layer, at different temperatures and different leaching times were studied. A series of experiments with i.e., k0 >> Dc, then the reaction kinetic function of diffusion control process can be simplified and temperatures of 413 K, 423 K and 433 K were carried out under the conditions of fixed sulfuric acid expressed by YD as follows. [30]: mass fraction of 60 wt%, acid ore mass ratio of 2.5:1, agitation rate of 120 r min−1. 2 The lithium yield at different leaching times2 of 413 3K, 423 K and 433 K were substituted into YD = 1 3 x (1 x) = kDt Equations (7) and (8), respectively, to calculate6D−bMc the− values− of YC and YD. Then, use YC; YD as the Y-axis,(8) k = c D r2 ρ and leaching time t as the X-axis to get YC-t;L YD-t, as shown in Figure 12. The linear equation is fitted by linear regression using the least squares method, the slope of the straight line is the reaction rate In order to obtain the controlling step and conduct kinetic analysis in this work, the Li yield constant k at different temperatures. kc, kd and the correlation coefficient R2 at different temperatures at different temperatures and different leaching times were studied. A series of experiments with are shown in Table 7, the experimental results are shown in Figure 12. temperatures of 413 K, 423 K and 433 K were carried out under the conditions of fixed sulfuric acid 1 mass fraction of 60 wt%, acid ore mass ratio of 2.5:1, agitation rate of 120 r min− . Table 7. Reaction rate constant kC; kD and correlation coefficient R2. The lithium yield at different leaching times of 413 K, 423 K and 433 K were substituted into Equations (7) and (8), respectively, to calculate the valuesYC of Y and YYD. Then, use Y ; Y as the Y-axis, Temperature (K) C D C D −1 2 −1 2 and leaching time t as the X-axis to get YC-t; YkCD (h-t, as) shownR ink FigureD (h ) 12.R The linear equation is fitted by linear regression using the least413 squares method, 0.0657 the0.996 slope 0.0320 of the straight0.996 line is the reaction rate 2 constant k at different temperatures.423 kc, kd and 0.0873 the correlation 0.982 0.0422 coefficient 0.979R at different temperatures are shown in Table7, the experimental433 results0.104 are shown 0.979 in Figure0.0487 12 . 0.970

Figure 12. a Y –t; b Y –t Figure 12. Relationship between reactionreaction kinetickinetic functionfunction ((a)) YCC–t; ((b)) YDD –t atat different different temperatures. 2 Table 7. Reaction rate constant kC; kD and correlation coefficient R . The experimental results show that both YC and YD have a linear relationship with the leaching time t. Although the correlation coefficient (R2) YbetweenC Yc and t is greaterYD than 0.979, the correlation Temperature (K) coefficient between YD and t is also greater than1 0.970. This2 result is hard1 to judge2 whether the reaction kC (h− ) R kD (h− ) R control step of lepidolite 413and sulfuric acid 0.0657 is product 0.996 layer diffusion 0.0320 or chemical 0.996 reaction. Therefore, the apparent423 activation energy 0.0873 (Ea) of 0.982 the reaction 0.0422 needs to be 0.979 further studied to draw the rate-determining steps.433 Activation energy 0.104 is a co 0.979gent evidence 0.0487 to find the 0.970 rate-determining step of a reaction. If the apparent activation energy is less than 13 kJ mol−1, the rate-determining step is diffusion of the solid product layer, while the rate-determining step is the chemical reaction control The experimental results show that both YC and YD have a linear relationship with the leaching −1 timewhent. the Although apparent the activation correlation energy coeffi cientis greater (R2) betweenthan 40 kJYc mol and t[31].is greater The Arrhenius than 0.979, equation the correlation shows that the apparent reaction rate constant k is a function of temperature, and the equation between the coefficient between YD and t is also greater than 0.970. This result is hard to judge whether the reaction controlk, T, and step Ea is of shown lepidolite in Equation and sulfuric (6) [32]: acid is product layer diffusion or chemical reaction.

k = Aexp[- Ea/(RT)]; ln k = ln A - Ea/RT (9)

Minerals 2020, 10, 0981 13 of 17

Therefore, the apparent activation energy (Ea) of the reaction needs to be further studied to draw the rate-determining steps. Activation energy is a cogent evidence to ﬁnd the rate-determining step 1 of a reaction. If the apparent activation energy is less than 13 kJ mol− , the rate-determining step is diﬀusion of the solid product layer, while the rate-determining step is the chemical reaction control 1 when the apparent activation energy is greater than 40 kJ mol− [31]. The Arrhenius equation shows that the apparent reaction rate constant k is a function of temperature, and the equation between the k, T, and Ea is shown in Equation (6) [32]:

k = Aexp[ E /(RT)]; lnk = lnA Ea/(RT) (9) Minerals 2020, 10, x FOR PEER REVIEW − a − 13 of 17 wherewhere AA refersrefers toto pre-exponentialpre-exponential factors;factors; EEaa isis thethe reactionreaction activationactivation energy;energy; T isis thethe reactionreaction temperature;temperature; RR isis thethe molarmolar gasgas constant.constant. 1 Figure 13 was obtained by plotting ln k and ln k vs. T −1, with a slope of Ea/R and an intercept Figure 13 was obtained by plotting ln CkC and ln DkD vs. T− , with a slope of −−Ea/R and an intercept ofof lnln AA,, TheThe EEaa of the chemical reaction controlling step can be calculated from thethe slopeslope ofof thethe lineline asas 1 1 EEa(C)a(C) == 34.1434.14 kJ kJ mol mol−1, ,and and the the EEaa ofof the the product product layer didiffusionﬀusion controllingcontrolling step step is isE Ea(D)a(D) == 31.39 kJkJ molmol−−1..

1 1 FigureFigure 13.13. RelationshipRelationship betweenbetween reactionreaction raterate constantconstant ( a()a)k Ck–TC–T−−1;; ((bb)) kkDD–T–T−−1 of lepidolitelepidolite duringduring leachingleaching process.process.

The good linear relationship between Y (R2 > 0.979) and Y (R2 > 0.970) vs. leaching time t, The good linear relationship between YC (R2 > 0.979) and YD D(R2 > 0.970) vs. leaching time t, as as well as the value of E between 13 and 40 kJ mol 1, all indicated that reaction between lepidolite well as the value of Ea betweena 13 and 40 kJ mol−1, all− indicated that reaction between lepidolite and andsulphuric sulphuric acid acidwas wascontrolled controlled by the by hybrid the hybrid behavior behavior of diffusion of diffusion through through the insoluble the insoluble layer layer and andchemical chemical reaction. reaction. This is This different is di fffromerent the from conclu thesion conclusion of single of step single control step in control previous in literature previous literature [33,34]. The better correlation coefficients of Y (R2 > 0.979) and k (R2 = 0.969) indicate that [33,34]. The better correlation coefficients of YC (R2 >C 0.979) and kC (R2 C= 0.969) indicate that the thechemical chemical reaction reaction step step plays plays a major a major role role in in deter determiningmining the the leaching leaching proc process,ess, and and the activationactivation energy of this leaching reaction can be approximated as 34.14 kJ mol 1 which is equal to E . The value energy of this leaching reaction can be approximated as 34.14 kJ− mol−1 which is equala(C) to Ea(C). The of k calculated from the straight line in Figure 13a is 1389.209. value0 of k0 calculated from the straight line in Figure 13a is 1389.209. TheThe determinationdetermination of the reactionreaction orderorder nn requiresrequires furtherfurther studystudy ofof thethe relationshiprelationship betweenbetween LiLi yieldyield andand sulfuric sulfuric acid acid concentration. concentration. 20 20 g lepidoliteg lepidolite powder powder was was weighed weighed and and mixed mixed with with sulfuric sulfuric acid ofacid 40, of 50, 40, 60 50, wt%. 60 wt%. All experiments All experiments were conductedwere conducted at a series at a of series different of different leaching leaching times of times 1, 2, 3, of 4 and1, 2, 1 53, h, 4 andand the5 h, liquid-solid and the liquid-solid mass ratio mass and the ratio agitation and the rate agitation were fixed rate atwere 2.5:1, fixed 120 rat min 2.5:1,− , respectively.120 r min−1, respectively.The leaching kinetic equation of lepidolite can be approximated by the chemical reaction process kineticThe equation leachingY Ckinetic, according equation to the of lepidolite research of can this be experiment: approximated by the chemical reaction process kinetic equation YC, according to the research of this experiment: 1 n Y = 1 (1 x) 3 = k0exp[ Ea/(RT)]c t/rL (10) C − − 1 − n (10) YC = 1 - 1 - x3 = 𝑘exp-Ea/RTc t/𝑟 where k0 is Arrhenius constant, n is the reaction order of sulfuric acid concentration. where k0 is Arrhenius constant, n is the reaction order of sulfuric acid concentration. A similar fitting plot of YC and t was obtained at initial different sulphuric acid mass fractions c andA shown similar in fitting Figure plot 14a, ofk YandC and the t Rwas2 (correlation obtained at coe initialfficient) different are shown sulphuric in Table acid8 mass. The fractions apparent c 2 rateand constantshown in (k )Figure and a 14a, plot ofk and ln k thewith R ln (correlationc were calculated coefficient) from theare slopesshown of in the Table straight 8. The lines apparent shown rate constant (k) and a plot of ln k with ln c were calculated from the slopes of the straight lines shown in Figure 14b, and the apparent reaction order n = 1.711 relative to the sulfuric acid concentration is determined by the slope.

Minerals 2020, 10, 0981 14 of 17 in Figure 14b, and the apparent reaction order n = 1.711 relative to the sulfuric acid concentration is determinedMinerals 2020, 10 by, x the FOR slope. PEER REVIEW 14 of 17

FigureFigure 14.14. ReactionReaction kinetickinetic functionfunction ((aa)) YY atat didifferentﬀerent HH22SOSO44 mass fraction; and (b) thethe relationshiprelationship betweenbetween lnln kk andand lnln cc forfor lithiumlithium leachingleaching process.process.

Table 8. Reaction rate constant k and correlation coeﬃcient R2. Table 8. Reaction rate constant k and correlation coefficient R2. 1 2 Mass fraction of H2SO4 (wt%) k (h ) R Mass fraction of H2SO4 (wt%) k (h−−1) R2 4040 0.0411 0.0411 0.977 0.977 50 0.0675 0.993 50 0.0675 0.993 60 0.1039 0.979 60 0.1039 0.979

ThroughThrough thethe aboveabove analysis,analysis, thethe obtainedobtained ArrheniusArrhenius constantconstant kk00 == 1389.209 and thethe apparentapparent reactionreaction orderorder nn == 1.711 were substituted into the chemical reaction control kinetics equation, and the kinetickinetic equationequation forfor thethe leachingleaching ofof lepidolitelepidolite byby sulfuricsulfuric acidacid isis asas follows:follows:

1 13 1.711 Y = 1 (1 x) = 1389.209exp[ 34140(RT)]1.711c t/rL (11)(11) Y = −1 - 1-−x3 = 1389.209exp-34140− RTc t/𝑟 3.4. Nanofiltration Separation Process of Actual Leaching Solution 3.4. Nanofiltration Separation Process of Actual Leaching Solution The pH of lepidolite leaching solution (1 L dilution) was measured to be 0.5 under the optimal leachingThe conditions.pH of lepidolite The Ca(OH)leaching2 solutionsolution (1 (pH L di=lution)12.5) was was used measured to adjust to be the 0.5 pH under of the the lepidolite optimal leachingleaching solutionconditions. to 1.4 The and Ca(OH) 2.2 respectively,2 solution and(pH CaCl= 12.5)2 was was added used toto theadjust mother-liquid the pH of tothe ensure lepidolite that 2 theleaching molar solution ratio of to the 1.4 raw and material 2.2 respectively, solution and was CaCl SO4 2− was:Cl− added= 0.6:0.4. to the The mother-liquid pH, total salinity, to ensure and that the concentrationthe molar ratio of of ions the in raw the nanofiltrationmaterial solution feed was solution SO42− are:Cl− shown= 0.6:0.4. in TableThe pH,9. total salinity, and the concentration of ions in the nanofiltration feed solution are shown in Table 9. Table 9. pH and composition of raw material liquid separated by nanofiltration.

Table 9. pH and composition of raw material liquid1 separated 2 by nanofiltration. 3 pH 0.51 2 1.43 2.2 Ca(OH)2 solutionpH added (L)0.5 - 1.4 13.12.2 14.7 CaClCa(OH)2 added2 solution (g/L) added (L) 13.3 - 13.1 9.4 14.7 8.9 TotalCaCl salinity2 added (g/L) (g/L) 13.3 46.6 9.4 38.9 8.9 37.0 2 SOTotal4 − (gsalinity/L) (g/L) 46.633.1 38.9 24.0 37.0 22.8 Cl (g/L) 7.9 5.6 5.3 − SO42− (g/L) 33.1 24.0 22.8 2+ Ca (gCl/−L) (g/L) 7.91.1 5.6 2.5 5.3 2.7 Li+ (g/L) 0.3 0.3 0.3 Ca2+ (g/L) 1.1 2.5 2.7 Al3+ (g/L) 1.1 1.0 0.9 Li+ (g/L) 0.3 0.3 0.3 K+ (g/L) 0.1 0.1 0.1 3+ Na+ (gAl/L) (g/L) 1.1 0.06 1.0 0.06 0.9 0.05 K+ (g/L) 0.1 0.1 0.1 Na+ (g/L) 0.06 0.06 0.05

Nanofiltration separation experiments of the actual lepidolite leaching solution were carried out at a fixed operating pressure of 3.4 MPa and a temperature of 293.15 K ± 0.5 K. The results about retention of ions, separation factor of Al/Li and flux of the DK membrane are shown in Figure 15. The retention rate of ions in descending order were: R(Al3+) > R(Ca2+) > R(K+) > R(Na+) > R(Li+), which was

Minerals 2020, 10, 0981 15 of 17

Nanofiltration separation experiments of the actual lepidolite leaching solution were carried out at a fixed operating pressure of 3.4 MPa and a temperature of 293.15 K 0.5 K. The results about ± retentionMinerals 2020 of, 10 ions,, x FOR separation PEER REVIEW factor of Al/Li and flux of the DK membrane are shown in Figure15 of 15 17. The retention rate of ions in descending order were: R(Al3+) > R(Ca2+) > R(K+) > R(Na+) > R(Li+), whichconsistent was with consistent the ions with retention the ions of the retention simulated of the leaching simulated solution leaching in our solution previous in study our previous[26]. The studyretention [26]. rate The of retention DK membrane rate of for DK Al membrane3+ was stable for above Al3+ was 96%, stable and the above retention 96%,and ratethe for retentionLi+ was stable rate forbetween Li+ was 42–52%, stable betweenwhich has 42–52%, shown which an excellent has shown separation an excellent performance separation for performance Al3+/Li+. In addition, for Al3+/ LiDK+. Inmembrane addition, also DK membraneshown a high also retention shown aof high over retention 94% for ofCa over2+. 94% for Ca2+.

FigureFigure 15.15. EEffectsﬀects of pHpH onon ((aa)) retentionretention rate,rate, ((bb)) separationseparation factorfactor andand ﬂux.flux.

AsAs shownshown inin Figure Figure 15 15,, the the pH pH of of feed feed solution solution has has little little effect effect on theon retentionthe retention of multivalent of multivalent ions, 3+ 2+ whichions, which was because was because the larger the hydration larger hydr radiusation and radius higher and valence higher of Alvalenceand of Ca Al3+have and determined Ca2+ have thedetermined steric and the dielectric steric and exclusion dielectric resistance exclusion that resi enterstance into thethat permeate enter into through the permeate the membrane through were the + + + relativelymembrane large. were However, relatively the large. pH hasHowever, a greater the impact pH has on a thegreater retention impact of Kon ,the Na retention, and Li of, and K+, theirNa+, retentionand Li+, and all decreasedtheir retention to varying all decreased degrees asto varying the pH increases. degrees as The the corresponding pH increases. reasonThe corresponding was that the + increasereason was of pHthat will the reduceincrease the of contentpH will ofreduce H in the the content solution, of H and+ in there the solution, were more and negative there were charges more onnegative the membrane charges on surface the membrane accordingly, surface therefore, accordin moregly, cations therefore, were attractedmore cations by electrostatic were attracted action, by resultingelectrostatic in aaction, decrease resulting in the monovalent in a decrease cations in the retention. monovalent This changecations inretention. separation This performance change in 3+ + ofseparation ions caused performance by the pH of variance ions caused would by the lead pH to va theriance change would in the lead separation to the change efficiency in the of separation Al /Li correspondingly,efficiency of Al3+ and/Li+ thecorrespondingly, lower the pH ofand the the feed lower solution, the thepH betterof the the feed separation solution, performance the better the of 3+ + Alseparation/Li of performance the DK membrane. of Al3+/Li The+ of change the DK of membrane. pH has a small The change impact of on pH the has ion a retention,small impact but on has the a greaterion retention, impact but on thehas membranea greater impact flux and on separationthe membra factor.ne flux As and the separation pH of the factor. feed solution As the increases,pH of the thefeed membrane solution increases, flux gradually the membrane decreases. flux When graduall the pHy isdecreases. 0.5, the salinity When andthe pH the is membrane 0.5, the salinity flux of and the solutionthe membrane were the flux highest. of the The solution reason were was thatthe high the excessiveest. The reason unreacted was sulfuric that the acid excessive and large unreacted amount + ofsulfuric H in acid the solution and large has amount changed of H the+ in micro-structure the solution has of changed the membrane the micro-structure and resulted inof thethe increasemembrane on theand membrane resulted in flux. the increase on the membrane flux. TheThe experimentalexperimental resultsresults ofof nanofiltrationnanofiltration separationseparation havehave revealedrevealed thatthat DKDK membranemembrane hashas 3+ 2+ + excellentexcellent retentionretention of of Al Al3+ and Ca2+ andand also also can can effectively effectively permeate LiLi+,, which which means means it is completely 3+ + feasiblefeasible toto applyapply DKDK membranemembrane toto thethe separationseparation ofof AlAl3+/Li/Li+ inin the the lepidolite lepidolite leaching leaching solution. solution. 4. Conclusions 4. Conclusions The extraction of lithium from lepidolite via sulfuric acid method and the corresponding leaching The extraction of lithium from lepidolite via sulfuric acid method and the corresponding kinetics of lithium were systematically investigated in this study. The results revealed the recovery rate leaching kinetics of lithium were systematically investigated in this study. The results revealed the lithium from lepidolite could reach 97% under the optimal conditions and the reaction was dominated recovery rate lithium from lepidolite could reach 97% under the optimal conditions and the1 reaction controlled by chemical reaction step. The apparent activation energy (Ea) is 34.14 kJ mol− , and the was dominated controlled by chemical reaction step. The apparent activation energy (Ea) is 34.14 kJ reaction series n is 1.711. mol−1, and the reaction series n is 1.711. In the separation of actual lepidolite leaching solution with different pH values, nanofiltration In the separation of actual lepidolite leaching solution with different pH values, nanofiltration has exhibited an excellent separation efficiency of Al/Li, and the lower pH of the leaching solution, has exhibited an excellent separation efficiency of Al/Li, and the lower pH of the leaching solution, the better separation effect of the DK membrane. However, strong acidity of the solution may cause changes in the membrane structure and shorten its service life. It is necessary to modify the DK membrane in the future to obtain a membrane with superior acid resistance. Overall, it is a successful attempt to couple nanofiltration separation of Al/Li with sulfuric acid extraction of lithium, and may provide a feasible method for extracting lithium from lepidolite leaching solution.

Minerals 2020, 10, 0981 16 of 17 the better separation eﬀect of the DK membrane. However, strong acidity of the solution may cause changes in the membrane structure and shorten its service life. It is necessary to modify the DK membrane in the future to obtain a membrane with superior acid resistance. Overall, it is a successful attempt to couple nanoﬁltration separation of Al/Li with sulfuric acid extraction of lithium, and may provide a feasible method for extracting lithium from lepidolite leaching solution.

Author Contributions: Data curation, formal analysis and writing-original draft, L.G.; methodology, J.L. and H.W.; writing-review and editing, L.G., J.L. and H.W.; supervision, H.W. and M.W.; funding acquisition, M.W.; resources, J.L. and M.W. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China, NSFC-Qinghai Province (grant numbers U1507202, U1707603); Major science and technology projects, Qinghai Province (grant number 2019-GX-A7); and Major research and development project of China (grant number 2018YFC0604804). Acknowledgments: We are grateful for the financial support given by the National Natural Science Foundation of China, Chinese Academy of Sciences and Qinghai Province of China. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Tarascon, J.M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359–367. [CrossRef][PubMed] 2. Batistoni, P.; Angelone, M.; Carconi, P.; Fischer, U.; Fleischer, K.; Kondo, K.; Klix, A.; Kodeli, I.; Leichtle, D.; Petrizzi, L.; et al. Neutronics experiments on HCPB and HCLL TBM mock-ups in preparation of nuclear measurements in ITER. Fusion Eng. Des. 2010, 85, 1675–1680. [CrossRef] 3. Xu, W.; Birbilis, N.; Sha, G.; Wang, Y.; Daniels, J.E.; Xiao, Y.; Ferry, M. A high-specific-strength and corrosion-resistant magnesium alloy. Nat. Mater. 2015, 14, 1229–1235. [CrossRef][PubMed] 4. Rioja, R.J.; Liu, J. The Evolution of Al-Li Base Products for Aerospace and Space Applications. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 2012, 43, 3325–3337. [CrossRef] 5. De Laurentis, N.; Cann, P.M.; Lugt, P.M.; Kadiric, A. The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts. Tribol. Lett. 2017, 65, 128. [CrossRef] 6. Bernhardt, D.; Reilly, I.J. Mineral Commodity Summaries 2019; US Geological Survey: Reston, VA, USA, 2019. 7. Meshram, P.; Pandey, B.D.; Mankhand, T.R. Extraction of lithium from primary and secondary sources by pre-treatment, leaching and separation: A comprehensive review. Hydrometallurgy 2014, 150, 192–208. [CrossRef] 8. Choubey, P.K.; Kim, M.; Srivastava, R.R.; Lee, J.; Lee, J. Advance review on the exploitation of the prominent energy-storage element: Lithium. Part I: From mineral and brine resources. Miner. Eng. 2016, 89, 119–137. [CrossRef] 9. Barbosa, L.I.; Gonzalez, J.A.; Ruiz, M.D. Extraction of lithium from beta-spodumene using chlorination roasting with calcium chloride. Thermochim. Acta 2015, 605, 63–67. [CrossRef] 10. Guo, H.; Kuang, G.; Wang, H.; Yu, H.; Zhao, X. Investigation of Enhanced Leaching of Lithium from α-Spodumene Using Hydrofluoric and Sulfuric Acid. Minerals 2017, 7, 205. [CrossRef] 11. Luong, V.T.; Kang, D.J.; An, J.W.; Dao, D.A.; Kim, M.J.; Tran, T. Iron sulphate roasting for extraction of lithium from lepidolite. Hydrometallurgy 2014, 141, 8–16. [CrossRef] 12. Yan, Q.; Li, X.; Wang, Z.; Wu, X.; Guo, H.; Hu, Q.; Peng, W.; Wang, J. Extraction of valuable metals from lepidolite. Hydrometallurgy 2012, 117–118, 116–118. [CrossRef] 13. Yan, Q.; Li, X.; Yin, Z.; Wang, Z.; Guo, H.; Peng, W.; Hu, Q. A novel process for extracting lithium from lepidolite. Hydrometallurgy 2012, 121–124. [CrossRef] 14. Yan, Q.-X.; Li, X.-H.; Wang, Z.-X.; Wang, J.-X.; Guo, H.-J.; Hu, Q.-Y.; Peng, W.-J.; Wu, X.-F. Extraction of lithium from lepidolite using chlorination roasting–water leaching process. Trans. Nonferr. Metals Soc. of China 2012, 22, 1753–1759. [CrossRef] 15. Luong, V.T.; Dong, J.K.; An, J.W.; Kim, M.J.; Tran, T. Factors affecting the extraction of lithium from lepidolite. Hydrometallurgy 2013, 134–135, 54–61. [CrossRef] 16. Ellestad, R.B.; Milne, L.K. Method of Extracting Lithium Values from Spodumene Ores. U.S. Patent 2516109, 25 July 1950. Minerals 2020, 10, 0981 17 of 17

17. Lajoie-Leroux, F.; Dessemond, C.; Soucy, G.; Laroche, N.; Magnan, J.-F. Impact of the impurities on lithium extraction from β-spodumene in the sulfuric acid process. Miner. Eng. 2018, 129, 1–8. [CrossRef] 18. Tian, Q.; Chen, B.; Chen, Y.; Ma, L.; Shi, X. Roasting and leaching behavior of spodumene in sulphuric acid process. Chin. J. Rare Metals 2011, 35, 118–123. 19. Vieceli, N.; Nogueira, C.A.; Pereira, M.F.C.; Durão, F.O.; Margarido, F. Recovery of lithium carbonate by acid digestion and hydrometallurgical processing from mechanically activated lepidolite. Hydrometallurgy 2017, 175, 1–10. [CrossRef] 20. Zhang, X.; Tan, X.; Li, C.; Yi, Y.; Liu, W.; Zhang, L. Energy-efficient and simultaneous extraction of lithium, rubidium and cesium from lepidolite concentrate via sulfuric acid baking and water leaching. Hydrometallurgy 2019, 185, 244–249. [CrossRef] 21. Jia, J.; Zhang, Y.; Sheng, W.U.; Liao, C.; Yan, C.; Liu, J.; Deng, G.; Rui, X. Distribution and Separation of Aluminum in the Rare Earth Solvent Extraction Separation Processes(I). Chin. Rare Earths 2001, 22, 10–13. 22. Zhang, Y.; Peng, Q. Method for Extracting Lithium Carbonate and Removing Aluminum from Lepidolite. CN Patent 10222577, 26 October 2011. 23. Bracken, L.; Bruggen, B.V.D.; Vandecasteele, C. Regeneration of brewery waste water using nanofiltration. Water Res. 2004, 38, 3075–3082. [CrossRef] 24. Zhi, W.; Liu, G.C.; Fan, Z.F.; Yang, X.T.; Wang, J.X.; Wang, S.C. Experimental study on treatment of electroplating wastewater by nanofiltration. J. Membr. Sci. 2007, 305, 185–195. 25. Mohammad, A.W.; Teow, Y.H.; Ang, W.L.; Chung, Y.T.; Oatley-Radcliffe, D.L.; Hilal, N. Nanofiltration membranes review: Recent advances and future prospects. Desalination 2015, 356, 226–254. [CrossRef] 26. Gao, L.; Wang, H.; Zhang, Y.; Wang, M. Nanofiltration Membrane Characterization and Application: Extracting Lithium in Lepidolite Leaching Solution. Membranes 2020, 10, 178. [CrossRef] 27. Zhang, L.; Zhang, Y.; Zhang, X.; Tan, X. Extraction of Li, Rb, Cs from lepidolite by sulfuric acid and water leaching. Nonferr. Metals (Extr. Metall.) 2019, 04, 39–42. 28. Chi, R.; Zhu, G.; Tian, J. Leaching kinetics of rare earth from black weathering mud with hydrochloric acid. Trans. Nonferr. Metals Soc. China 2000, 10, 531–533. 29. Chen, J.; Yang, S.; Ke, J. The Studies and Progress of Metallurgy; China Metallurgy Industry Press: Beijing, China, 1998. 30. Tian, J.; Yin, J.; Chi, R.; Rao, G.; Jiang, M.; Ouyang, K. Kinetics on leaching rare earth from the weathered crust elution-deposited rare earth ores with ammonium sulfate solution. Hydrometallurgy 2010, 101, 166–170. 31. Abdel-Aal, E.A. Kinetics of Sulfuric Acid Leaching of Low-Grade Zinc Silicate Ore. Hydrometallurgy 2000, 55, 247–254. [CrossRef] 32. Zhu, B. Chemical Reaction Engineering; Chemical Industry Press: Beijing, China, 2012. 33. Liu, K.; Chen, Q.; Yin, Z.; Hu, H.; Ding, Z. Kinetics of leaching of a Chinese laterite containing maghemite and magnetite in sulfuric acid solutions. Hydrometallurgy 2012, 125–126, 125–136. [CrossRef] 34. Liu, J.; Yin, Z.; Li, X.; Hu, Q.; Liu, W. Recovery of valuable metals from lepidolite by atmosphere leaching and kinetics on dissolution of lithium. Trans. Nonferr. Metals Soc. China 2019, 29, 641–649. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).