Study on the law and mechanism by which oxalic acid dissolves tungstic acid.

The traditional alkali autoclaving tungsten smelting process has several disadvantages, such as high production costs, high water consumption and prominent "three wastes" problem, all of which seriously hinder the sustainable development of the tungsten smelting industry. We found that tungstic acid (H₂WO₄) can be dissolved in oxalic acid and that H₂WO₄ can be easily precipitated from a solution at relatively high temperatures. Thus, traditional ammonium para-tungstate (APT) can potentially be replaced by H₂WO₄ as the smelting intermediate, which is expected to overcome the problems associated with the alkali autoclaving process, and solve the problem of ammonia/ammonium pollution at the source. To provide theoretical guidance for the development of novel tungsten smelting processes, the law and mechanism of H₂WO₄ dissolution in oxalic acid are urgently needed. These results indicate that when H₂WO₄ is dissolved in an oxalic acid solution, H₂C₂O₄ and H₂WO₄ can ionize C₂O^2-₄ and WO^2-₄, respectively. Then, some of the oxygen atoms bonded with W in WO^2-₄ are replaced by free C₂O^2-₄ to form [WO₃(C₂O₄)·H₂O]^2-. H₂[WO₃(C₂O₄)·H₂O] is subsequently formed by combining [WO₃(C₂O₄)·H₂O]^2- with H⁺ in solution. Under the conditions of an oxalic acid concentration of 0.05-0.4 mol/L, a reaction temperature of 25-65 ℃ and a liquid-solid ratio of 2:1-10:1 mL/g, the dissolution efficiency of H₂WO₄ in oxalic acid increases with increasing oxalic acid concentration or liquid-solid ratio; a low temperature (25 ℃-35 ℃) is more conducive to the dissolution of H₂WO₄ than a high temperature (45 ℃-65 ℃).