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

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.