Kinetics of MgO chlorination with HCl gas
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. INTRODUCTION
A common challenge in all variations of electrolytic magnesium production processes is that the magnesium chloride–loaded fused salt, which is used as the electrolyte, must be virtually free of oxygen or oxygen-containing compounds. The low oxygen content in the electrolyte is needed because any oxygen present in the electrolyte will either react with and consume the components of the electrolysis cells or will form stable compounds that interfere with the cell performance and severely lower productivity of the electrolysis cells. The oxygen-containing compounds that are found in magnesium electrolytes can either be formed during the dehydration of magnesium chloride hydrates in the industrial feed preparation process or, less likely, by hydrolysis of the electrolyte itself. These oxides are either in the form of magnesium oxide (MgO) or magnesium hydroxychlorides and have the general chemical formula Mgx(OH)yClz. If present, these oxides need to be chlorinated, and when the chlorinating agent is HCl gas, the chlorination reactions are as follows: MgO 2HCl MgCl2 H2O
[1]
MgOHCl HCl MgCl2 H2O
[2]
Knowledge of the rates of chlorination of these oxides is important to the industry for controlling of the oxide content in the molten salt electrolyte. Recent work by authors[1] found that under certain conditions, MgOHCl can be the dominant oxide species produced during the dehydration of MgCl26H2O and that fine MgO can be formed by the thermal decomposition of MgOHCl2; i.e., MgOHCl MgO HCl
[3]
K.W. NG, Research Associate, S. KASHANI-NEJAD, Postdoctoral Candidate, and R. HARRIS, Professor, are with the Department of Mining, Metals and Material Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted July 20, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
The kinetics of the chlorination of MgO have been studied by a number of researchers. Ishii et al.[3] studied the chlorination of MgO by Cl2 gas in the presence of carbon in the temperature range from 25 °C to 970 °C. They reported that chlorination of samples with different C:MgO mass ratios demonstrated exothermic differential thermal analysis (DTA) peaks at various temperatures ranging from 475 °C to 530 °C. Their final samples contained about 45 to 49 wt pct MgCl2. However, no significant DTA peaks were observed when a sample containing only MgO was used, and X-ray analysis of this sample showed that only a very small amount of MgCl2 was formed upon its heating in Cl2 gas at 970 °C. Ishii et al. described the kinetics of the chlorination using Jander’s equation and found the activation energies to be 182.4, 191.6, and 260 kJ/mole for C:MgO ratios of 0.5, 1, and 2, respectively. The mechanism of the carbochlorination was represented as MgO C Cl2 [MgCl2 # O*] C MgCl2 CO [4] Similar results were obtained by Ino et al.[4] in their studies of direct chlorination of Mg(OH)2 and MgO in both fixed and fluidized bed reactors. They reported that the rate of chlorination of MgO at 400 °C to 500 °C witho
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