The Phase Stabilities of Magnesium Hydroxychlorides
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AGNESIUM hydroxychlorides are hydrated double salts between MgO and MgCl2 that form when MgO is added to MgCl2 brines or when water hydrolyzes MgCl2 at temperatures of a few hundred degrees Celsius. Several hydroxychloride species exist, all with the general stoichiometry xMgOÆyMgCl2ÆzH2O. When MgO is added to water or dilute salt solutions, the MgO undergoes a simple hydration reaction to form Mg(OH)2. However, when MgO is added to MgCl2 solutions that are more concentrated than approximately 10 wt pct MgCl2, a magnesium hydroxychloride precipitates out. As the equilibrium solubility of oxides in MgCl2 solutions is quite low, with only a small amount of MgO, the solution will always be saturated in an oxide phase. If magnesia is present in excess, then a paste forms that sets to a hard mass and becomes Sorel cement.[1] However, when smaller amounts of MgO are added to MgCl2 solutions, loose crystals of hydroxychloride are obtained and can be filtered out.
JAN DE BAKKER, Metallurgist, is with BBA, Inc., 630, boul. Rene-Levesque O, Suite 2500, Montreal, QC H3B 1S6, Canada. Contact e-mail: [email protected] JOSHUA LAMARRE was formerly an Undergraduate Student with the Department of Chemical Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada. JOHN PEACEY, Professor, and BOYD DAVIS, Associate Professor, are with the Department of Mining Engineering, Queen’s University. Manuscript submitted November 6, 2011. Article published online May 17, 2012. 758—VOLUME 43B, AUGUST 2012
The hydroxychloride precipitate can have one of several different stoichiometries. Several phases have been identified, named according to their MgO:MgCl2 ratio. These include the ‘‘3-form’’ 3MgOÆMgCl2Æ11H2O, the ‘‘2-form’’ 2MgOÆMgCl2Æ6H2O, the ‘‘5-form’’ 5MgOÆMgCl2Æ13H2O, and the ‘‘9-form’’ 9MgOÆMgCl2Æ14H2O. In addition, Mg(OH)2 frequently precipitates as a metastable phase. Previous studies of magnesium hydroxychlorides in the literature have focused on their role as the key components of Sorel or oxychloride cements. The current investigation, however, was motivated by the use of magnesium hydroxychlorides in novel flowsheets for the extraction of nickel from laterite ores.[2,3] The flowsheet envisioned leaching laterites with HCl; after several subsequent steps, this would leave a concentrated magnesium chloride solution. By adding MgO, therefore, the chloride units are ‘‘captured’’ in a magnesium hydroxychloride. The hydroxychloride can subsequently be thermally decomposed into HCl gas and MgO.[4] In this fashion, hydrogen chloride is recovered from an MgCl2 brine without requiring the solution to undergo the extensive evaporation of water. Another motivation for this research was a desire to use hydroxychloride precipitation and decomposition as a means of treating carnallite ‘‘mother liquor.’’ Mother liquor is the waste solution left over from aqueous processing of carnallite (KClÆMgCl2Æ6H2O); it has an approximate composition of 3.40 molal MgCl2 (approximately 22.6 wt pct), 0.73 molal KCl, 0.77 molal NaCl, and 0.12 molal MgSO4. The
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