Thermodynamics of (Ca(OH) 2 -)NaOH-Na 2 WO 4 -H 2 O and (Ca(OH) 2 -)NaOH-Na 3 PO 4 -H 2 O systems and their application

The caustic soda autoclaving process for digesting scheelite (CaWO 4 ) is a unique and widely used tungsten (W) metallurgy technology in China. However, some experimental phenomena still need a proper theoretical explanation. In this work, some operational details and the theoretical basis of the technology are reported. During the digestion, some impurities, such as P, As, and Si are also digested. The P impurity was taken as an example. Based on the analysis of thermodynamic equilibrium phase diagram analysis, low levels of P impurities in the leaching solution were obtained. However, during the next dilution of the residue, P was supplemented as an inhibitor to reduce W loss. The inhibition of the reverse reaction mechanism is explained. The washed Na 2 WO 4 solution containing excess NaOH can be reused in the evaporation–crystallization process; meanwhile, the supplemented P should be removed by adding Ca(OH) 2 . A theoretical analysis of the thermodynamic equilibrium phase diagram shows that there is an area where Ca 5 (PO 4 ) 3 OH is stable, but CaWO 4 is not. The experimental results show that over 90% of P can be removed by adding the theoretical Ca(OH) 2 amount at the beginning of the evaporation-crystallization step. The W loss ratio can be controlled below 0.2%.