Thermodynamic study on acid decomposition of calcium molybdate and its application

According to the laws of simultaneous equilibria and mass conservation, the lgC-pH diagram of a CaMoO₄-H₂SO₄/HCl-H₂O system was established on the basis of the modynamic date at 298 K. Thermodynamic analysis was carried out to show the effect of technical conditions on acid decomposition of CaMoO₄. The results show that there are three stable areas of various solid phases of the CaMoO₄-H₂SO₄-H₂O system in the pH range of 0~7.0. The first one is H₂MoO_4(s) and CaSO_4(s) (0 < pH < 3.5), the second one is H₂MoO_4(s) CaMoO_4(s) and CaSO_4(s) (3.5 < pH < 4.5), and the third one is CaMoO_4(s) (4.5 < pH < 7.0). There are three stable areas of various solid phases of the CaMoO₄-HCl-H₂O system in the pH range of 0~7.0. The first one is H₂MoO_4(s) (0 < pH < 2.9), the second one is H₂MoO_4(s) and CaMoO_4(s) (2.9 < pH < 4.5), and the last one is CaMoO_4(s) (4.5 < pH < 7.0). Thermodynamics analysis results show that calcium molybdate is easily decomposed by acid; compared with the sulfuric acid leaching process, hydrochloric acid leaching calcium molybdate does not produce CaSO₄, which reduces leaching product covered by calcium molybdate. The experiment proves that compared with hydrochloric acid, calcium molybdate is a kind of better leaching agent, and the leaching of molybdenum reaches 98.56 % at 25 ℃ for 45 min with HCl concentration of 2.4 mol/L at a VL/WS ratio of 10 mL/g.