Thermodynamic Effect of NaCl and KCl Addition on the Equilibrium Between Molten ZnCl₂-Based Salt and Liquid Zn–Cd Alloy

Purification of molten salt is essential for producing highly pure metal from melts by chemical reactions and electrolysis. Cementation is a technique used for refining molten ZnCl₂-based salt. In this study, we investigated the thermodynamics involved in the reduction of CdCl₂ in molten ZnCl₂-based salt with Zn metal to optimize the separation conditions for Cd; this is difficult as both Cd and Zn have similar electrochemical potentials. We experimentally investigated the thermodynamic equilibrium between the molten ZnCl₂-based salt and liquid Zn–Cd alloy at 723 K (450 °C). The equilibrium concentration of CdCl₂ in 40 mol pct ZnCl₂-NaCl-KCl coexisting with Zn-Cd alloy changed as the ratio of NaCl and KCl was altered. The activity coefficient of CdCl₂ in the system at 723 K (450 °C) was obtained with the calculated activity coefficient of ZnCl₂ using several thermodynamic solution models. A two-step cementation process was used to demonstrate the possibility of lowering the concentration of Cd. The Cd content in molten salt was lowered to ~ 10 mass ppm by the two-step reaction, and the feasibility of multi-step cementation was demonstrated.