Leaching of lepidolite and recovery of lithium hydroxide from purified alkaline pressure leach liquor by phosphate precipitation and lime addition

This study investigated the pressure leaching of lithium and other valuable metals from lepidolite using NaOH and Ca(OH)₂. The findings showed that NaOH concentration, temperature, and stirring rate are the most significant process parameters. The addition of Ca(OH)₂ facilitates the leaching of lithium and minimizes the concentration of silicate and fluoride in the leach liquor by forming solid phases identified as Ca₃Al₂Si₃O₁₂, CaF₂ and NaCaHSiO₄, with a loss of about 7% NaOH. The leaching efficiencies after 2 h were 94% Li, 98% K, 96% Rb, and 90% Cs under the most suitable conditions (250 °C, 320 g/L NaOH, liquid-to-solid ratio of 10, 300 rpm stirring speed, lime addition of 0.3 g/g). Precipitation of lithium as Li₃PO₄ by adding phosphoric acid and subsequent conversion of Li₃PO₄ to LiOH using Ca(OH)₂ recovered 83% of lithium as LiOH allowing recycling the rest along with residual remnant NaOH for leaching.