Effect of KF additions in NA3ALF6-AL2O3 electrolytes on expansion of cathode blocks

Abstract

Effects of K and Na on expansion of cathode carbon block were studied through an improved instrument for measurement of expansion rate. For a 30% graphitic cathode block, based on the results, in KCl-NaCl electrolysis system K and Na are reacting with graphite and forming graphite intercalation compounds (GICs) during penetration from surface to inside of the cathode block, and K plays more important role in expansion and penetration than Na. The results show that for a NaF-AlF3-Al2O3 system, containing 7% Al2O3 and with CR = 6.5, during electrolysis at 1000°C the expansion rate of the 30% graphitic cathode block is increased from 0.47% to 0.82% with 0 to 7% KF addition to the electrolytes. It is also shown that the expansion due to K and Na penetration in cathode becomes lower with increasing content of graphite in cathode material, and that expansion of fully-graphitized cathode is very low.