MgOHCl thermal decomposition kinetics

Abstract

Experiments to determine the kinetics of the thermal decomposition of MgOHCl were performed. It was found that the decomposition of MgOHCl commenced at 649 K, and it directly converted into MgO and HCl without undergoing any intermediate step. Decomposition vs time data showed that the thermal decomposition of MgOHCl was a first-order process with respect to the amount of MgOHCl remaining, and the mass transfer of the product HCl gas away from the interface was likely the rate-limiting step. It was also found that the time required to completely decompose MgOHCl into MgO, more than 20 minutes, at the operating temperatures of electrolytic magnesium production processes, 600 °C±50 °C, was significantly longer than the time required, less than 1 minute, to digest the solid magnesium chloride containing feed material into the molten salt electrolyte in these processes. Such delay in the decomposition would mean that any MgOHCl produced during heating and digestion of the feed would not be decomposed by the heat of the electrolyte and thus the persistent MgOHCl would dissolve into the molten salt electrolyte with potentially severe negative consequences on electrolysis cell operation.