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.