Abstract
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 (H(2)WO(4)) can be dissolved in oxalic acid and that H(2)WO(4) can be easily precipitated from a solution at relatively high temperatures. Thus, traditional ammonium para-tungstate (APT) can potentially be replaced by H(2)WO(4) 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 H(2)WO(4) dissolution in oxalic acid are urgently needed. These results indicate that when H(2)WO(4) is dissolved in an oxalic acid solution, H(2)C(2)O(4) and H(2)WO(4) can ionize C(2)O(2-)(4) and WO(2-)(4), respectively. Then, some of the oxygen atoms bonded with W in WO(2-)(4) are replaced by free C(2)O(2-)(4) to form [WO(3)(C(2)O(4))·H(2)O](2-). H(2)[WO(3)(C(2)O(4))·H(2)O] is subsequently formed by combining [WO(3)(C(2)O(4))·H(2)O](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 H(2)WO(4) 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 H(2)WO(4) than a high temperature (45 ℃-65 ℃).