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 ratelimiting 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 Ϯ 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.

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From this result, it was concluded that crystalline MgOHCl was successfully produced by the described method.Further work was carried out to determine the purity of MgOHCl produced. The sample was dissolved in 0.1 N HCl (aq). The solution was then titrated against 0.1 N NaOH (aq) for determination of acid consumption. From the volume of acid consumed, the amount of MgOHCl in the sample was determined according to the following reaction:

The kinetics of chlorination of MgO particles with HCl gas in the temperature range from 450 °C to 650°C were measured and analyzed in terms of the shrinking core model. The MgO particles were produced by the thermal decomposition of MgOHCl manufactured in-house and were found to have a d 50 of 510 nanometers. Over 90 pct of the MgO was converted to MgCl 2 within 30 minutes at 650°C. Analysis of the data indicated that the chlorination process was initially controlled by the kinetics of the chemical reaction between MgO and HCl, but as the thickness of the MgCl 2 product layer increased as the chlorination progressed, the rate of chlorination became controlled by the diffusion of HCl through the MgCl 2 ash layer surrounding a MgO core in the particles. A mathematical model that predicted the shrinkage of the MgO core with time was found to be in good agreement with the measurements over the range of temperatures studied. The ash layer thickness for the onset of the diffusion control regime was found to increase linearly with temperature. The apparent rate constant for chlorination in the initial reactioncontrolled regime was well described by the Arrhenius equation: k (m/s) ϭ 0.0787 exp (Ϫ7596/T).

Chlorination of MgOHCl particles with HCl gas was examined and complete conversion to MgCl 2 was observed within 60 min in the temperature range 325-525uC. X-ray diffraction analysis of samples treated for varying times revealed that MgOHCl was directly converted into MgCl 2 without first decomposing into MgO. Such behaviour was observed even at temperatures significantly higher than the equilibrium decomposition temperature of MgOHCl under ambient atmosphere. The results also showed that the rate of chlorination of MgOHCl was independent of temperature.