Fundamental Research on a New Process to Remove Al3+ as Potassium Alum during Lithium Extraction from Lepidolite

Abstract

Sulfuric acid method can effectively extract lithium from lepidolite. However, purification problems make the method still tough before being widely applied. Especially, a considerable amount of Al3+ is contained in leaching solution, which needs to be removed before Li2CO3 precipitation. Based on the characteristic that K+, Rb+, and Cs+ can form corresponding alums with Al3+, an alum crystallization method was introduced and applied in our research. The feasibility of this method was also verified by phase equilibrium investigations in Li2SO4-K2SO4-Al2(SO4)3-H2O and its subternary system, which were typical subsystems of leaching solution. The results showed that Al2(SO4)3 can form potassium alum (KAl(SO4)2·12H2O) with K2SO4 easily, while no alum or double salt was found between Li2SO4 and Al2(SO4)3. Although Li2SO4 can generate KLiSO4 with K2SO4, the crystallization of KAl(SO4)2·12H2O still has priority over KLiSO4 when K2SO4 concentration is relatively low. Therefore, aluminum probably can be removed by regulating the concentration of leaching solution located in the crystallization field of KAl(SO4)2·12H2O. Besides, the effects of controlling factors specifically temperature, pH, and molar ratio of K+/Al3+ (m (K+/Al3+)) on alum crystallization were investigated. The results indicated that lower temperature and the addition of K2SO4 can accelerate the removal of Al2(SO4)3 obviously. The alum crystallization method turned out to be effective such that about 78 pct of Al2(SO4)3 can be removed mainly as potassium alum (KAl(SO4)2·12H2O) at 278 K (5 °C) with m (K+/Al3+) at 0.9 and pH of 2. A slight loss of lithium was caused by entrainment during alum crystallization. This research can be a promising exploration to purify leaching solution of lepidolite or other aluminosilicates.