Monitoring O2– Behavior during Electrolysis in Molten CaCl2

Molten CaCl2 is a viable solvent and electrolyte for extracting target elements from their oxides through electrolysis. However, the accumulation of O2– during electrolysis presents a significant challenge for the commercial application of CaCl2-based technologies. Understanding the behavior of O2– in the melt is crucial for optimizing the process. To address this, we developed an electromotive force (EMF) measuring system to monitor O2– behavior during the Si electrodeposition in molten CaCl2. We explored the thermodynamic properties of O2– in molten CaCl2–CaO (XO2– < 0.03) at 1123 K. The relationship between activity and molar fraction of O2– was established as aO2– = XO2–2.2, indicating strong interactions between O2– and other ions in the melt. EMF measurements confirmed the accumulation of O2– during the electrolysis in molten CaCl2–CaO–SiO2, as evidenced by the significant increase in aO2–. Notably, this accumulation becomes more pronounced with larger cathode areas, which leads to greater carbon contamination. Further investigation revealed that solid SiO2 effectively reduces aO2– in molten CaCl2–CaO to lower levels. By immersing solid SiO2 in melt during electrolysis, the accumulation rate of O2– is slowed, thereby inhibiting carbon contamination and improving current efficiency. These findings provide valuable insights into the use of the EMF measuring system as an effective tool for revealing O2– behavior and optimizing electrolysis processes in molten CaCl2.