Leaching of WO3 from sulfuric acid converted product of scheelite in NH3·H2O-NH4HCO3 solution
(School of Metallurgy and Environment, Central South University, Changsha 410083, China)
Abstract: The leaching of sulfuric acid converted product of scheelite in NH3·H2O-NH4HCO3 solution was systemically studied to improve sulfuric acid conversion-ammonium salts leaching technology route for ammonium paratungstate production. The results showed that the WO3 leaching efficiency was about 99% under optimal conditions of 350 r/min, liquid-to-solid ratio of 3 mL/g, 1 mol/L NH4HCO3, 4 mol/L NH3·H2O, 25 °C, and 15 min. During the leaching, CaSO4 almost had no change and was still in a banding or rod-like shape in short leaching time, while conglobate CaCO3 was gradually formed on the CaSO4 surface. A secondary reaction might occur between CaSO4 and WO42-, which could be restrained by a certain amount of CO32- in the solution. There was no CaCO3 phase determined by XRD in leaching residue of converted product for scheelite concentrate under optimal conditions, which was different from that for synthetic scheelite. The leaching process could be explained by neutralization reaction of H2WO4 and solid transformation of CaSO4 in NH3·H2O-NH4HCO3 solution.
Key words: tungstic acid; calcium sulfate; leaching mechanism; calcium carbonate; ammonium tungstate