Efficient recovery of tungsten from scheelite concentrates using a sulfur-phosphorus mixed acid leaching system

In recent years, a new process for leaching scheelite concentrates with sulfur-phosphorus mixed acid has been successfully industrialized. This process suffers from the elimination of the retarding effect of calcium sulfate, the selective extraction of tungsten from the H₃PW₁₂O₄₀-H₂SO₄-H₃PO₄ mixture solution, and the removal of introduced phosphorus. Therefore, in this paper, we investigated the inhibition measures of a calcium sulfate-blocking membrane, the selective extraction of tungsten by ion exchange, and the removal of phosphorus by the magnesium ammonium salt method. The results showed that the addition of calcium sulfate whisker seeds and a higher H₃PO₄ concentration inhibited spontaneous diffuse nucleation, promoted crystal growth, and avoided the formation of a dense blocking film. About 98 wt.% of the scheelite concentrate was digested by H₂SO₄-H₃PO₄. The selective adsorption of PW₁₂O₄₀³⁻ was realized using D301 resin, and the extraction efficiency of tungsten exceeded 99.9%. After desorption with ammonia water, PW₁₂O₄₀³⁻ was depolymerized into WO₄²⁻ and PO₄³⁻. 99.9 wt.% of phosphorus was removed in the form of MgNH₄PO₄ by the magnesium ammonium salt method, and the residual phosphorus concentration in the solution was lower than 10 mg·L⁻¹. After purification, the highest-grade ammonium paratungstate product was prepared by evaporative crystallization.