Study on leaching behaviour of tungstates in acid solution containing phosphoric acid

Abstract

For decades, methods for leaching scheelite based on the generation of soluble phosphotungstic heteropoly acids have been continuously studied. These methods can be performed under normal pressure and can achieve a high leaching ratio when treating scheelite, but their most obvious disadvantage is that the leaching speed is very slow when treating wolframite, which has limited their application in the leaching of wolframite-containing ores. Considering the difference between the crystal forms of these two minerals, the relationship between the crystal structure of MeWO4-type tungstate and the leaching rate in acid solution containing phosphoric acid was systematically studied. The results showed that the leaching rates of all tetragonal tungstates were significantly higher than those of monoclinic and triclinic tungstates. Even in H3PO4-H2SO4 solution, sulfate precipitates were produced when leaching tetragonal tungstates, but their leaching rates were still significantly higher than those of other crystalline tungstates. In addition, the tungsten leaching rate was roughly proportional to the volume occupied by the MeWO4 unit, and NiWO4, with the closest-packed atomic arrangement, had the lowest leaching rate. It was found that the evolution of the tungstate crystal structure parameters was gradual and regular with the gradual change in the CX (cилoвыe xapaктepиcтики, obtained by the ratio of a cation's n-th ionization potential to its orbital radius), although these divalent cations are in different regions of the periodic table. Furthermore, leaching in the acid solution containing phosphoric acid yielded a better relationship between the change in the tungsten leaching rate and the change in the cation's CX in the same group or period of the periodic table.