Article pubs.acs.org/jced
Preparation of Anhydrous Magnesium Chloride: Solid−Liquid Phase Diagram for the System MgCl2−NH3−C2H4[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 MgCl2-saturated ethylene glycol solutions at 323 K. To determine the conditions required to form HEX, the C2H4[OH]2-rich part of the MgCl2−NH3−C2H4[OH]2 system was investigated at 323 ± 0.5 K. Seven phase regions were determined. These were: NH3(g)+LiqT, HEX+LiqT+NH3(g), HEX +LiqT, HEX+T+LiqT, T+LiqT, MgCl2·3EG+T+LiqT, and LiqT. The symbol T represents a ternary compound of composition MgCl2·2NH3·2C2H4[OH]2, and LiqT 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) NH3. At lower ammonia levels, the formation of Tphase is promoted, resulting in coprecipitation of HEX and T-phase.
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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 consumption, 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 MgCl2−NH3− C2H4[OH]2 system. This system is the basis for the production of anhydrous magnesium chloride via the Australian Magnesium (AM) process9,12,13 in which ethylene glycol (C2H4[OH]2) is used as the organic solvent and ammonia (NH3) as the precipitating agent. Experimental results were Published 2012 by the American Chemical Society
Table 1. Previous Systems Proposed to Produce Anhydrous MgCl2 system/solvent MgCl2−NH3−CH3OH (methanol) MgCl2−NH3−C2H5OH (ethanol) MgCl2−NH3−C2H4[OH]2 (ethylene glycol) MgCl2−NH3−C5H11OH (isoamyl alcohol) a
reference a
5 ;6 5a 7−9; this study 10, 11
Sivilotti et al. experiments in ref 5 also contained NH4Cl and H2O.
used to identify the conditions required for the formation of pure magnesium chloride hexammoniate (MgCl2·6NH3), the precursor phase used in the production of anhydrous magnesium chloride.
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BACKGROUND In the AM process, a (30 to 32) % (w/w) purified MgCl2 brine is mixed with sufficient amounts of ethylene glycol to give a (10 to 15) % (w/w) MgCl2 solution (anhydrous basis). The water is removed by simple distillation according to: MgCl2(in H 2O) → MgCl2(in C2H4[OH]2 ) + H 2O(g) (1)
resulting in a (10 to 20) % (w/w) anhydrous magnesium chloride in ethylene glycol solution. To separate the MgCl2 from the ethylene glycol, gaseous anhydrous ammonia is bubbled through the solution. The ammonia dissolves, initially saturating the ethylene glycol, then forming a MgCl2·6NH3 hexammoniate complex that precipitates: Received: July 9, 2012 Accepted: September 20, 2012 Published: October 2, 2012 2855
dx.doi.org/10.1021/je300764z | J. Chem. Eng. Data 2012, 57, 2855−2862
Journal of Chemical & Engineering Data
Article
Water. Moisture determinations of the liquors and solid phases was carried out using a Metrohm 701 Karl Fischer (KF) Titrino apparatus using a stable hydranal hydrazine solution of iodine, sulfur dioxide, and imidazole dissolved in alcohol. For the water content determinations, 3 g of benzoic acid was added to the KF cell as a buffer for the high pH caused by the ammonia content of the crystals. All titrations were carried out in duplicate and gave a consistent relative standard deviation (2σ) of ± 2 %. Magnesium, Chloride, and Ethylene Glycol. All recovered liquid and solid phases were dissolved in 10 mL of 10 % nitric acid and made up to a 100 mL total volume with distilled water. Magnesium was determined by titration against a standard 0.05 M ethylenediaminetetraacetic acid (EDTA) solution, while chloride was measured potentiometrically using a Metrohm 672 Titroprocessor. A relative standard deviation of < 1.8 % was obtained for the magnesium determinations, while for the chloride analysis the relative standard deviation was also of the order of < (1.8 to 2.0) % Ethylene glycol was determined by the difference. Solid Phases. Characterization of all solid phases was by Xray diffraction (XRD). In experiments where only a small amount of solid material was produced, the wet solid was recovered from the bottom of the reaction vessel and smeared onto a quartz zero-background plate. In experiments where a larger amount of solid material was produced, the solid was collected by either centrifuging or filtering of the sample. The recovered solids were loaded into aluminum sample holders and covered with a clear plastic film of low density polyethylene to avoid reaction with water vapor in the atmosphere and/or loss of ammonia. X-ray diffraction patterns were collected using a Phillips PW1710 powder diffractometer fitted with a diffracted beam graphite monochromator and employing Cu Kα radiation. The diffractometer was operated at 40 kV and 40 mA, and samples were analyzed over the range 5° to 65° 2θ, using a step size of 0.04° 2θ and counting times of 0.25 s per step. Total scan times were of the order of (10 to 12) min per sample. Method. Solid−liquid and liquid−gas equilibration experiments were initially performed along the MgCl2−C2H4[OH]2 and C2H4[OH]2−NH3 binaries to determine the maximum solubility of MgCl2 and NH3 in C2H4[OH]2 as well as the composition of any solid compounds present at saturation. To determine the solubility of NH3 in C2H4[OH]2, ammonia was sparged at a flow rate of 1.9 L·min−1 into ∼100 mL of ethylene glycol contained in a 250 mL Schott bottle that was heated to 323 ± 0.5 K by partial immersion in an agitated ethylene glycol bath. The ammonia sparger consisted of a 3 mm diameter glass tube inserted through a rubber stopper which was also fitted with a plastic outlet port to prevent pressure build up from excess ammonia. The stopper also prevented any moisture pick up from the atmosphere. Ammoniation was continued for up to 30 min before sampling of the liquid for analysis. For solubility experiments along the MgCl2−C2H4[OH]2 binary, excess MgCl2 was added to C2H4[OH]2 in a 250 mL Schott bottle. The temperature of reaction was kept constant at 323 ± 0.5 K by partial immersion in an agitated water bath. Based upon previous work in similar systems,15 agitation for 48 h was sufficient to ensure equilibrium among all solid and liquid phases was reached. The mixture was then allowed to settle in an oven at 323 ± 0.5 K for at least a further 24 h to ensure separation between solid and liquid phases. The solid and liquid
MgCl2 + C2H4[OH]2 + x NH3 → MgCl2·6NH3(s) + C2H4[OH]2 + (x − 6)NH3 (2)
The hexammoniate solid is separated, washed with ammoniated methanol, and then deammoniated to produce anhydrous MgCl2 according to: MgCl2·6NH3 → MgCl2 + 6NH3(g)
(3)
Decomposition to anhydrous magnesium chloride is accomplished in two stages. In the first, the hexammoniate is decomposed to the diammoniate (MgCl2·2NH3) at 493 K, and this is subsequently decomposed to anhydrous MgCl2 at 723 K. The ethylene glycol and ammonia are recovered and recycled back to the process. Besides the end member compounds, key phases known to exist in the MgCl2−NH3−C2H4[OH]2 system include: three crystalline ammoniated compounds (magnesium chloride hexammoniate, MgCl2·6NH3; magnesium chloride tetrammoniate, MgCl2·4NH3; and magnesium chloride diammoniate; MgCl2·2NH31,14) and two magnesium chloride glycollate compounds (magnesium chloride triglycollate, MgCl2·3C2H4[OH]2;9,15 and magnesium chloride biglycollate biammoniate, MgCl2·2NH3·2C2H4[OH]29,14). The generation of glycollate compounds in the AM process is to be avoided as dehydration results in the formation of magnesium hydroxychloride, MgOHCl.14 If glycollate compounds are formed, they will require additional processing to separate them from the hexammoniate. Knowledge of the fundamental solid−liquid phase relationships operating within this complex system is therefore vital to understanding, controlling, and optimizing the precursor hexammoniate phase necessary for the production of pure anhydrous magnesium chloride by the AM process.
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EXPERIMENTAL SECTION Reagents. The anhydrous magnesium chloride contained > 99.7 % MgCl2 (BDH Laboratory Reagent grade). The ammonia gas was type 179R supplied by BOC gases and contained
