MgOHCl thermal decomposition kinetics
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I. INTRODUCTION
MAGNESIUM chloride hydrates produced from the dewatering of commercially pure aqueous magnesium chloride brines and containing varying numbers of waters of crystallization and varying amounts of magnesium oxide or magnesium hydroxychlorides are used as feed for many industrial electrolytic magnesium production processes.[1–4] In some operations, they are added directly to the fused salt electrolyte to increase the magnesium chloride content of the spent electrolyte returning from the electrolysis cells. In other operations, before being added to the electrolyte, they are first converted to commercially anhydrous magnesium chloride; in other words, a magnesium chloride material that is free of waters of crystallization but may contain magnesium oxide or magnesium hydroxychloride as impurities. In the former instances, final dehydration of these hydrates occurs upon their contact with the fused salt that is maintained at about 600 °C 50 °C, but it has been found that as the hydrates heat and become digested into the bath, hydrolysis of the magnesium chloride inevitably occurs and the products of hydrolysis enter the bath with negative consequences in the electrolysis cells unless corrective action is taken.[5–10] II.
LITERATURE REVIEW
Moldnehauer[11] was the first to report in 1906 that MgOHCl was formed during the dehydration of magnesium chloride hydrates. Kelley[12] in 1945 and Kassner[13] in 1958 studied the equilibrium formation of MgOHCl and determined its thermodynamic properties. Equilibrium thermal decomposition of magnesium hydroxychlorides was also studied by Kelly and Kassner along with many others.[14–19] In particular, it was found that under equilibrium conditions,
S. KASHANI-NEJAD, Postdoctoral Candidate, K.-W. NG, Research Associate, and R. HARRIS, Professor, are with the Department of Mining, Metals and Materials Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Contact e-mail: [email protected] Manuscript submitted January 16, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
pure, solid MgOHCl decomposes to HCl and MgO at a temperature above 533 °C (806 K).[12,20] MgOHCl(s) MgO(s) HCl(g)
[1]
Recent work by one of the authors has found that when magnesium chloride hydrates with nominally 2 moles of water per mole of magnesium are added to fused salt baths, per some industrial operations, a considerable amount of the MgOHCl that was produced during the hydrolysis that occurred during the digestion process reported to the electrolyte[21,22] was not decomposed per Reaction [1] neither prior to nor after its digestion, as was considered during the development and subsequent operations of such commercial operations. Galway and Laverty[23] studied the thermal decomposition of magnesium chloride dihydrate prepared by the heating of bischofite (magnesium chloride hexahydrate) at 400 °C for 1 hour following the known thermodynamics of the system. However, such a method of preparing magnesium chloride dihydrate would not likely produce a single product but rather a mi
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