Thermodynamic analysis on reaction behaviors of silicates in (NH4)2WO4-(NH4)2CO3-NH3-H2O system
(School of Metallurgy and Environment, Central South University, Changsha 410083, China)
Abstract: The reaction behaviors of silicate species in (NH4)2WO4-(NH4)2CO3-NH3-H2O system are crucial to developing a green manufacture technique for ammonium paratungstate. In order to efficiently remove silicon from the system, the reaction behaviors of silicate species were systematically investigated by thermodynamic analysis. The thermodynamic analysis shows that silicate in the tungstate clinker partly decomposes in the leaching process, with 150-160 mg/L silicon thermodynamically at 25 °C. The dissolved silicon can be removed by magnesium salts via forming insoluble MgSiO3. The low carbonate and high ammonia concentrations in the system are beneficial to the removal of silicon, with silicon concentration reaching 8-10 mg/L thermodynamically, whereas MgSiO3 precipitation is hardly formed when the concentration of total carbonate is more than 1.5 mol/L. The reaction behaviors of calcium and magnesium were also studied in the system. The results in the verification experiments consist with the theoretical calculation.
Key words: ammonium paratungstate; silicon removal; magnesium salt precipitation; solution purification