Kinetics of sodium hydroxide leaching of scheelite

Abstract

It has been reported, historically, that scheelite (CaWO 4) cannot be decomposed using sodium hydroxide under commercial conditions. However, a mechanochemical leaching process using caustic soda at elevated temperature + pressure for the treatment of scheelite has been developed and put into practice successfully in China since the late 1980s. This technology can treat not only high-calcium-content wolframite, but also scheelite–wolframite concentrate blends and even scheelite concentrates directly. Recently, a thermodynamic analysis was made to explain the digestion of scheelite by NaOH, but the kinetics of this leaching process is still not well understood. In this study, the influence of variables, such as temperature, NaOH concentration and mineral particle size on NaOH leaching of scheelite was studied. The experimental data agreed quite well with the shrinking core model for leaching. The leaching rate was controlled by a surface chemical reaction, with an apparent activation energy of 64.6 kJ/mol and a reaction order of 1.74. The kinetics equation of the process was established as: 1 − ( 1 − α ) 1 3 = 23.67 × 10 − 3 [ N a O H ] 1.74 r 0 − 1 e ( − 64600 8.314 T ) t