Dissolution of scheelite under soft acidic conditions via the formation of polyoxotungstate: kinetics and mechanism supported by small-angle X-ray scattering (SAXS) to identify and quantify H3PW12O40

Abstract

The decomposition of scheelite in a synergistic H₂SO₄-H₃PO₄ mixture is an effective method for extracting tungsten from mining concentrates. The driving force behind the dissolution of scheelite in the H₂SO₄-H₃PO₄ mixture is the formation of a soluble Keggin-type polyoxotungstate, H₃PW₁₂O₄₀, which prevents the formation of very low solubility tungstic acid. In this work, a multiparametric study of scheelite dissolution kinetics was carried out in a synergistic acid mixture. In particular, the independent contributions of temperature, acid concentration, and W:P molar ratio on scheelite dissolution kinetics and H₃PW₁₂O₄₀ formation yield were evaluated. To this end, a method based on the use of small-angle X-ray scattering (SAXS) was developed to identify and quantify H₃PW₁₂O₄₀ under different operating conditions. The results provide a better understanding of the stability range of H₃PW₁₂O₄₀ in terms of H₂SO₄ concentration and stoichiometric W:P ratio. These findings led to the selection of optimized, soft-leaching conditions that ensure rapid dissolution of scheelite while avoiding surface passivation by the precipitation of secondary phases. A comparison with speciation calculations using thermodynamic data reported in the literature reveals an absence of a self-consistent thermodynamic dataset. Thus, measuring the concentration of H₃PW₁₂O₄₀ in the leachate was necessary to optimize the dissolution conditions. From this perpective, SAXS appears to be a suitable quantitative method.