The Investigation On Reaction Mechanism Of Fluorine Chemical Method To Extract Lithium From Lepidolite And Separation Technology To Recover Aluminum In The Leaching Solution

The fluorine chemical method was proposed for the first time by our group that using the easily available fluosilicic acid to extract lithium from lepidolite. This research mainly revolved in reaction mechanism of the fluorine chemical method and the separation technology of the leaching solution to recover aluminum.In order to reveal the mechanism of lithium extraction, the lepidolite was characterized before and after reaction. By the SEM, XRD, XPS, ICP means, the structure and composition changement were analyzed and determined qualitatively and quantitatively. Based on the analysis, the possible mechanism of the reaction was assumed. In order to further verify the mechanism presumption, the thermodynamic calculation was carried out under the 298 K. The results showed that the reaction could be happened spontaneously under the normal temperature, which indicated that the presumption of the reaction mechanism was correct to a certain extent. The fluorine chemical reaction process was concluded that the crystal structure of lepidolite was completely destroyed by HF which was produced by fluosilicic acid decomposition. Then, the metal oxide reacted with sulfuric acid and recovered as soluble sulphate. Part of the oxide also can react with HF or fluosilicic acid to generate intermediate products. The fluorine chemical method we proposed is expected to be comparable with extraction from brine and ease the tension between uneven distribution and explosive demand on lithium in the future.The leaching solution of lepidolite contains a large amount of aluminum after lithium extraction, whether aluminum can be removed or recovered effectively will definitely affect the cost of lithium extraction. The comprehensive analysis was made on the merits and shortcomings of the traditional methods:alkali, acid and solvent extraction methods. Then a novel and effective way was proposed:recovered aluminum by alum formation. The phase equilibria in the ternary systems Li2SO4-Al2(SO4)3-H2O, K2SO4-Al2(SO4)3-H2O and the quaternary system Li2SO4-Al2(SO4)3-K2SO4-H2O at 298K were investigated with isothermal equilibrium method. Based on the solubility data, the phase diagrams were plotted. The crystallized fields were also determined. The phase diagrams showed that K2SO4 and A12(SO4)3 can form potassium alum (KAl(SO4)2·12H2O) in large range of concentration, while Li2SO4 and Al2(SO4)3 seemed not form double salts. Our investigation indicated that Li+ and Al3+ cannot form alum, but K+ and Al3+ can form potassium alum easily. Therefore, the aluminum can be recovered by crystallization to form alum.Since the NH4+ can form alum easily with Al3+ and could be easily removed, the aluminum recovery was enhanced by forming alum with (NH4)2SO4 as an additive. Eventually, above 80% aluminum was recovered effectively as high value co-products.The phase equilibrium of the Li2SO4-(NH4)2SO4-K2SO4-H2O quaternary system at 298K was investigated. The results showed that solid solution was generated to derease the recovery of K2SO4. In practice, the alum formation method should be carried out as twice separated process in order to recover Al2(SO4)3 and K2SO4. With the twice alum formation method, the recovery rate of Al2(SO4)3 and K2SO4 could reach more than 80%,90%, respectively.Additionally, the separation technology of the leaching solution was also discussed. The suitable temperature to form alum was 298K. The pH should be kept between 0 and 1. Less than 0.1% lithium was lost due to the entrainment of alum. The research can provide guidance for aluminum recovery, which will have theoretical and practical significance in lithium extraction from lepidolite with the fluorine chemical method.