Extraction Of Valuable Metals From Lepidolite

The lepidolite from Jiangxi Province (China)has a reserve of1.1million tons Li2O and accounts for30%of China’s domestic proven reserves, but little of ithas been utilized. The present methods of lithium extraction from lepidolite worldwide were summarized in this paper. Comprehensive extraction and utilization of lithium, potassium, rubidium and cesium from the lepidolite of Jiangxi Province were introduced. Chlorination roasting, sulfation roasting and autoclaving method were adopted. The effect and mechanism ofmineral phase reconstruction on the extraction behavior of Li, K, Rb, Cs from lepidolite with different methods were discussed in detail and conclusions were made as follows.Based on the analysis of thermodynamics, the experimental conditions of chlorination roasting-water leaching were studied. The conditional experiments indicated that the optimum mass ratio of lepidolite/CaCl2/NaCl is1:0.6:0.4during the roasting process. The extraction efficiencies of Li, K, Rb and Cs are92.86%,88.49%,94.05%and93.06%, respectively after roasting for0.5h at880℃. Li, K, Rb and Cs are selectively leached by chlorination roasting and water leaching at atmospheric pressure, and impurities extraction can be suppressed.The reaction mechanism of lepidolite and solid chloridizing agent was investigated in details. The results indicated that base-exchange reaction occurs between CaCl2and lepidolite under argon atmosphere. CaCl2is decomposed intohCl under vapor atmosphere andhCl will react with lepidolite under vapor atmosphere. Because CaCl2is decomposed into Cl2under oxygen atmosphere, lepidolite will react with CaCl2and Cl2.Sulfation roasting followed by water leaching process was used to selectively extract Li, K, Rb and Cs from lepidolite. Various operational parameters including roasting temperature, the amount of additives, and solid/liquid ratio in the leaching process were optimized. The extraction efficiencies of Li, K, Rb and Cs could reach91.61%,44.37%,29.33%and23.21%with a mass ratio of lepidolite/Na2SO4/K2SO4/CaO as1:0.5:0.1:0.1by roasting at850℃for0.5h. XRD analysis showed that the original aluminosilicate will decompose to NaSi3AlO8, KAlSi2O6and CaAl2Si2O8during sulfation roasting. The phases of CaF2and Ca4Si2O7F2are observed due to the addition of CaO.Salt roasting with Na2SO4+CaCl2followed by water leaching was used to extract alkali metals from lepidolite. The experiments indicated that the optinum mass ratio of lepidolite/Na2SO4/CaCl2during roasting is1:0.5:0.3. The extraction efficiencies of Li, Rb and Cs are over90%after0.5h at880℃. The recovery efficiency of Li is essentially constant at830-930℃. The flexible roasting condition is easily maintained for industrial application. After the residue was leached in water, the aqueous solution was cooled to-5℃for2h for92.1%of the sulphate and3.9%of the chloride to be crystallized as Na2SO4·10H2O and NaCl, respectively. By evaporation and precipitation with Na2CO3, Li2CO3crystal with a purity of over99.5%was produced and a solution was obtained from which Rb and Cs could be recovered.The lepidolite was pre-roasted athigh temperature with water steam atmosphere for defluorination. The defluorination percentage is61.43%at860℃when the duration of defluorination is40min. The XRD result indicated that as the defluorination time increases to40min, the diffraction peaks of lepidolite almost disappear and the main phases are aluminum silicate (LiAl(SiO3)2) and leucite (KAlSi2O6). The structure of the originalmineral is destroyed and transformed to aluminum silicate (LiAl(SiO3)2) and leucite (KAlSi2O6) during the defluorination process.The defluorinated lepidolite washandled by Na2SO4and Ca(OH)2in the autoclaving leaching. The results indicated that the optimum operating parameters with the extraction efficiencies of Li, K, Rb and Cs as91.98%,93.06%,70.18%and61.02%were established as follows: defluorination temperature860℃, defluorination time30min (the defluorination percentage of lepidolite is42.3%), leaching temperature150℃, leaching time60min, defluorinated lepidolite/Na2SO4/Ca(OH)2/H2O1:1:0.7:7. The results of the leaching kinetics of defluorinated lepidolite show that the dissolution rates of Li, K, Rb, Cs accord with unreacted shrinking core models for solid film control. And the activation energies for the leaching of Li, K, Rb, Cs are 20.179kJ·mol-1,19.61J·mol-1,45.274kJ·mol-1and36.338kJ·mol-1respectively.The defluorinated lepidolite was leached in a lime-milk autoclave. Various parameters including the defluorination percentage of lepidolite, milling time, temperature, time, lime-to-defluorinated lepidolite ratio, and liquid-to-solid ratio in the leaching process were optimized. The extraction efficiencies of Li, K, Rb and Cs can reach98.9%,68.67%,47.43%and43.54%under the optimal conditions.Aim at the chloride and sulphate leaching solution, the phase equilibria of ternary reciprocal system Li+, Na+, K+//Cl--H2O and Li+, Na+, K+//SO42--H2O were studied by isothermal method. The results indicated that lithium can be concentrated to20-30g·L-1in Li+, Na+, K+//Cl--H2O system. But the concentration of lithium was limited to8.6g·L-1due to the precipitation of LiKSO4fromhigh concentration of sulphate solutions in Li+, Na+, K+//SO42--H2O system. In order to overcome this problem, the purified solution was cooled to-5℃for2h so that92.1%of the sulphate and3.9%of the chloride ions were crystallized as Na2SO4·10H2O and NaCl, respectively. The sulphate-free solution was thermally evaporated and the concentration of lithium was increased to20-24g-L-1when5mol·L-1Na2CO3solution was added to precipitate～86%of the lithium as lithium carbonate.