Monitoring O^2– 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 O^2- during electrolysis presents a significant challenge for the commercial application of CaCl₂-based technologies. Understanding the behavior of O^2- in the melt is crucial for optimizing the process. To address this, we developed an electromotive force (EMF) measuring system to monitor O^2- behavior during the Si electrodeposition in molten CaCl₂. We explored the thermodynamic properties of O^2- in molten CaCl₂-CaO (<i>X</i>_O^2- < 0.03) at 1123 K. The relationship between activity and molar fraction of O^2- was established as <i>a</i>_O^2- = <i>X</i>_O^2-^2.2, indicating strong interactions between O^2- and other ions in the melt. EMF measurements confirmed the accumulation of O^2- during the electrolysis in molten CaCl₂-CaO-SiO₂, as evidenced by the significant increase in <i>a</i>_O^2-. Notably, this accumulation becomes more pronounced with larger cathode areas, which leads to greater carbon contamination. Further investigation revealed that solid SiO₂ effectively reduces <i>a</i>_O^2- in molten CaCl₂-CaO to lower levels. By immersing solid SiO₂ in melt during electrolysis, the accumulation rate of O^2- 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 O^2- behavior and optimizing electrolysis processes in molten CaCl₂.