Preparation of Anhydrous Magnesium Chloride: Solid-Liquid Phase Diagram for the System MgCl 2 -NH 3 -C 2 H 4 [OH] 2 at 323 K Mark I. Pownceby,* David H. Jenkins, Roman Ruzbacky, and Sophia Saunders CSIRO Process Science and Engineering, Bayview Avenue Clayton, Victoria 3168 Australia ABSTRACT: The ammoniated magnesium chloride hexammoniate compound (HEX) is the key precursor phase required for the production of anhydrous magnesium chloride by the Australian Magnesium (AM) process. It is produced by direct ammoniation of MgCl 2 -saturated ethylene glycol solutions at 323 K. To determine the conditions required to form HEX, the C 2 H 4 [OH] 2 -rich part of the MgCl 2 -NH 3 -C 2 H 4 [OH] 2 system was investigated at 323 ± 0.5 K. Seven phase regions were determined. These were: NH 3 (g)+Liq T , HEX+Liq T +NH 3 (g), HEX +Liq T , HEX+T+Liq T , T+Liq T , MgCl 2 ·3EG+T+Liq T , and Liq T . The symbol T represents a ternary compound of composition MgCl 2 ·2NH 3 ·2C 2 H 4 [OH] 2 , and Liq T represents a ternary liquid phase. To produce only hexammoniate in the AM process, bulk ammonia levels need to be maintained at levels of greater than about (11 to 13) % (w/w) NH 3 . At lower ammonia levels, the formation of T- phase is promoted, resulting in coprecipitation of HEX and T-phase. ■ INTRODUCTION Electrolytic magnesium production methods typically use magnesium chloride as the feedstock. 1 Magnesium chloride is however highly hygroscopic, and this presents a significant technical challenge to producers, as the feedstock for electrolysis is required to be anhydrous. Hydrous magnesium chloride cannot simply be dehydrated by drying or calcining due to decomposition to the hydroxychloride, MgOHCl. 2,3 During electrolysis the hydroxychloride dissociates, to produce hydrogen chloride gas and various oxycompounds of magnesium. Any residual hydrate water of chloride can therefore cause corrosion problems during electrolysis resulting in high maintenance costs, sludge formation, anode con- sumption, and low current efficiency. 2,4 As a consequence of the difficulty in producing pure, anhydrous magnesium chloride by simple dehydration or calcination methods, a number of alternative approaches have been proposed. Most involve the formation of a solution of hydrated magnesium chloride in a polar organic solvent (e.g., methanol or ethanol), removal of water from the solution, formation of a magnesium chloride complex by reaction with a precipitating agent, and then calcination of the magnesium chloride complex. A number of variations of this general approach have been proposed (Table 1) with a common feature being the use of ammonia as the precipitating agent. Differences in the processes lie in the use of different solvents to form the magnesium chloride solution. The current study describes experiments conducted at 323 K to determine solid-liquid phase relations in the MgCl 2 -NH 3 - C 2 H 4 [OH] 2 system. This system is the basis for the production of anhydrous magnesium chloride via the Australian Magnesium (AM) process 9,12,13 in which ethylene glycol (C 2 H 4 [OH] 2 ) is used as the organic solvent and ammonia (NH 3 ) as the precipitating agent. Experimental results were used to identify the conditions required for the formation of pure magnesium chloride hexammoniate (MgCl 2 ·6NH 3 ), the precursor phase used in the production of anhydrous magnesium chloride. ■ BACKGROUND In the AM process, a (30 to 32) % (w/w) purified MgCl 2 brine is mixed with sufficient amounts of ethylene glycol to give a (10 to 15) % (w/w) MgCl 2 solution (anhydrous basis). The water is removed by simple distillation according to: → + MgCl (in H O) MgCl (in C H [OH] ) H O(g) 2 2 2 2 4 2 2 (1) resulting in a (10 to 20) % (w/w) anhydrous magnesium chloride in ethylene glycol solution. To separate the MgCl 2 from the ethylene glycol, gaseous anhydrous ammonia is bubbled through the solution. The ammonia dissolves, initially saturating the ethylene glycol, then forming a MgCl 2 ·6NH 3 hexammoniate complex that precipitates: Received: July 9, 2012 Accepted: September 20, 2012 Published: October 2, 2012 Table 1. Previous Systems Proposed to Produce Anhydrous MgCl 2 system/solvent reference MgCl 2 -NH 3 -CH 3 OH (methanol) 5 a ;6 MgCl 2 -NH 3 -C 2 H 5 OH (ethanol) 5 a MgCl 2 -NH 3 -C 2 H 4 [OH] 2 (ethylene glycol) 7-9; this study MgCl 2 -NH 3 -C 5 H 11 OH (isoamyl alcohol) 10, 11 a Sivilotti et al. experiments in ref 5 also contained NH 4 Cl and H 2 O. Article pubs.acs.org/jced Published 2012 by the American Chemical Society 2855 dx.doi.org/10.1021/je300764z | J. Chem. Eng. Data 2012, 57, 2855-2862