Leaching of WO3 from Sulfuric Acid Converted Product of Scheelite in NH3·H2O–(NH4)2C2O4 Solution

Due to the existence of large auxiliary material consumption, huge wastewater discharge, and high production cost in present tungsten extractive metallurgy practice, a novel technology featuring sulfuric acid conversion-ammonium salts leaching was proposed. Based on the complete conversion of tungsten minerals in sulfuric acid solution, this paper studied the leaching of WO3 from sulfuric acid converted product of scheelite in NH3·H2O–(NH4)2C2O4 solution. The effect of leaching conditions on WO3 leaching efficiency and solid phase transformation was systemically investigated. The WO3 leaching efficiency was > 98% under optimized conditions of 1 mol/L (NH4)2C2O4, 3 mol/L NH3·H2O, 350 rpm, 5 min, and 25 °C. The formed flaky CaC2O4·H2O densely covered on the surface of banding or rodlike shaped CaSO4, which prevented the further transformation of CaSO4. The morphology of leaching residue was more irregular for converted product of scheelite concentrate than that for synthetic scheelite. Minor secondary reaction between CaSO4 and (NH4)2WO4 might occur with increased (NH4)2WO4 concentration, which could be restrained by the existence of (NH4)2C2O4 in solution due to the larger Ksp value of CaC2O4·H2O than CaWO4. The leaching process could be explained by acid–base neutralization of H2WO4 and phase transformation of CaSO4 in NH3·H2O–(NH4)2C2O4 solution.