Résumé
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.