Electrolytic production of magnesium: Effect of current density

Abstract

The effect of current density on current efficiency for the electrolytic production of magnesium metal and on the wetting of a steel cathode by magnesium has been studied in a proprietary sodium chloride-rich NaCl−MgCl2 melt at 710±10°C. With the exception of the smallest applied current density, current reversal chronopotentiograms at a small steel electrode in an argon blanketed melt showed a linear increase of current efficiency with increasing current density up to 0.7A cm−2. The current efficiency then remained constant around 94% over a current range of 0.7–1.2 A cm−2. Further increase in the current density, however, produced a decline in the current efficiency. Interestingly, a linear increase in the current efficiency with current density even up to 1.2 A cm−2, the highest applied current, was observed in a laboratory cell which simulated the production cell practice. The extent of the steel wetting by molten magnesium decreased with decrease in the cathodic current density, and the wetting was poor below 0.4 A cm−2. This study suggests that magnesium cells can be run at higher current densities than 0.3–0.5 A cm−2, the average operating current density in commercial cells, to maximize the current efficiency, the production rate and the space time yield. Other variables in production cells may make higher current density operation difficult in practice.