China Nonferrous Metallurgy

Gas bubbles in electrolytic cells present a dual effect: they diminish the effective electrical conductivity, thereby increasing cell voltage and energy consumption, while concurrently enhancing mass transfer through the promotion of electrolyte convection. Current research on anode bubble behavior in molten salt electrolysis predominantly focuses on aluminum production, with limited investigation into chloride-based molten salts. This study utilized a Qiu-style transparent electrolytic cell to investigate the evolution of bubbles at a graphite anode during the electrolysis of alkali/alkaline earth metal chloride molten salts. Bubble growth and detachment processes in various chloride salt systems were recorded from side-view using a video camera, with image analysis providing parameters such as bubble diameter and bubbles layer thickness. It was found that bubble behavior during the electrolysis of NaCl, 55wt.%LiCl-45wt.%KCl, and 5 wt.%MgCl-53wt.%NaCl-42wt.%KCl molten salts was similar. In these systems, large bubbles periodically detached from the anode surface, exhibiting diameters of approximately 50mm and bubble layer thicknesses ranging from 3.7 to 4mm. Conversely, the bubble behavior in the KCl molten salt system exhibited marked differences. Bubbles detached at a significantly faster rate, with diameters around 8mm and a layer thickness of 2mm, and failed to coalesce into large bubbles covering the anode surface. Additionally, an anode effect was observed in NaCl molten salts when the current density exceeded 0.7A·cm^-2. Following prolonged electrolysis, the NaCl, 5wt.%MgCl₂-53wt.%NaCl-42wt.%KCl, and KCl systems developed a reddish-brown coloration. This phenomenon is attributed to the dissolution or dispersion of electrolytically generated chlorine gas (Cl2) in the molten electrolyte.