Optimization of Crystallization Process for Preparation of Lithium Phosphate in Alkaline Solution System

The medium-temperature or low-temperature alkaline leaching method of spodumene ore is an environmentally friendly laboratory technology being explored. The low concentration of lithium and high concentration of impurity ions in the leaching solution make it impossible to prepare lithium carbonate. Herein, the preparation of low-solubility and high-value lithium phosphate (\rmL\rmi_\rm3\rmP\rmO_\rm4) is proposed. The solubility of \rmL\rmi_\rm3\rmP\rmO_\rm4 in alkaline solution system was determined. The effect of process conditions on the yield and particle size of \rmL\rmi_\rm3\rmP\rmO_\rm4 was investigated, as well as the influences of impurity including K, Si, Al on the purity, morphology and size of \rmL\rmi_\rm3\rmP\rmO_\rm4. The mechanism indicates that the synergy of ion-selective-adsorption on the crystal surface and ion-intercalation in the crystal lattice are responsible for the effects. Central composite design (CCD), a kind of response surface methodology, was employed to explore the effects of reaction temperature, lithium concentration, dosage of sodium phosphate and feeding rate. The optimal conditions were 80 ℃, 1.60 g/L of Li⁺ concentration, 130% of Na₃PO₄ dosage and 10 mL/min of feed speed, respectively. Under the conditions, the optimal yield and the average particle size were predicted to be 97.37% and 18.57 μm, respectively, while the corresponding experimental results were 96.85% and 18.70 μm, respectively. The errors between the predicted and experimental values were small.