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 WO₄^2-, which could be restrained by a certain amount of CO₃^2- 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