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ARTICLE IN PRESS Journal of Light Metals xxx ,2001) xxx±xxx www.elsevier.com/locate/ligandmet 2 A thermochemical analysis of the production of anhydrous MgCl2 a b, 3 Georges J. Kipouros , Donald R. Sadoway * 4 a Department of Mining and Metallurgical Engineering, Dalhousie University, 1360 Barrington Street, Halifax, Nova Scotia, Canada B3J 2X4 5 b Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA 6 Abstract 7 The electrolytic production of magnesium requires high-purity, anhydrous magnesium chloride which has a high anity for 8 water and is found in nature as a plurality of hydrates ,MgCl2 Á nH2O, n 1, 2, 4, 6, 8, 12). Their dehydration is nontrivial and can 9 be accompanied by hydrolysis leading to the production of undesirable oxycompounds of magnesium.PROOF Through an analysis of the 10 relevant thermochemistry this paper indicates howto prevent hydrolysis and make electrolytic-grade, anhydrous MgCl 2. Ó 2001 11 Published by Elsevier Science Ltd. 12 Keywords: Magnesium; Hydrolysis; Dehydration; Thermochemistry 13 1. Introduction 2. Background 38 14 With a density of 1:74 g=cm3 magnesium is 4.5 times A cubic kilometer of seawater contains approxi- 39 15 lighter than steel and 1.6 times lighter than aluminum mately a million tones of magnesium, more than has 40 16 [1]. The mechanical properties of magnesium castings ever been produced in one year by all the magnesium 41 17 make them competitive with aluminum castings. When plants in the world. Furthermore, seawater contains 42 18 hot worked, magnesium is the easiest to deep draw of only 3.7% of the total magnesium present in the earth's 43 19 the common metals and requires the least energy to crust. Clearly, magnesium resources are ubiquitous and 44 20 machine. It has superior damping capacity owing to its virtually inexhaustible. 45 21 high speci®c stiness. Due to its reactivity, magnesium is never found in 46 22 The expectation of increased utilization of magne- nature in its free state but rather in compounds: as 47 23 sium in automotive applications has stimulated renewed chlorides MgCl2 Á 12H2O in seawater ,0.25±0.55%), in 48 24 commercial interest in this metal [2]. In fact, at this underground brines ,up to 20%), and in salt deposits 49 25 moment, the combined tonnage of announced green®eld such as carnallite MgCl2 Á KCl Á 6H2O,andas car- 50 26 projects exceeds total global production capacity [3]. By bonates in the ores dolomite MgCO3 Á CaCO3 and 51 27 far the dominant extraction technology is molten salt magnesite MgCO3. Less important are the surface 52 28 electrolysis of magnesium chloride to produce liquid minerals, kieserite MgSO4 Á H2O, kainite MgSO4Á 53 29 magnesium and chlorine gas. While electrolytic reduc- KCl Á 3H2O and langbeinite 2MgSO4 Á K2SO4. Fi- 54 30 tion appears straightforward, the process cannot be nally, magnesium can be found in certain waste streams 55 31 sustained without the availability of high-purity feed, in concentrations high enough to warrant commercial 56 32 i.e., anhydrous magnesium chloride. Owing to its high interest. For example, asbestos tailings contain magne- 57 33 anity for water as evidenced by the large number of sium in the form of serpentine Mg3Si2O5 OH4 [4]. 58 34 hydrates ,MgCl2 Á nH2O, n 1, 2, 4, 6, 8, 12), the pro- In the electrolytic production of magnesium, mag- 59 35 duction of anhydrous magnesium chloride is not trivial. nesium chloride is dissolved in a solvent consisting of a 60 36 In an eortUNCORRECTED to help workers in the ®eld avoid the pitfalls, multicomponent solution of alkali and alkaline-earth 61 37 the present paper presents the relevant thermochemistry. chlorides. The action of electric current drives faradaic 62 processes that produce liquid magnesium and chlorine 63 gas through the decomposition of magnesium chloride. 64 The temperature of operation is typically between 700°C 65 and 750°C. Magnesium droplets form on the steel 66 * Corresponding author. Fax: +1-617-253-5418. cathode, and chlorine bubbles form on the carbon an- 67 E-mail address: [email protected] ,D.R. Sadoway). 1471-5317/01/$ - see front matter Ó 2001 Published by Elsevier Science Ltd. PII: S 1 4 7 1 - 5 3 1 7 , 0 1 ) 0 0 004-9 ARTICLE IN PRESS 2 G.J. Kipouros, D.R. Sadoway / Journal of Light Metals xxx 82001) xxx±xxx 68 ode. Both cell products rise to the surface of the elec- 69 trolyte where they are collected. As these reactions 70 proceed the electrolyte is depleted of magnesium chlo- 71 ride, which must be periodically added to the melt. 72 While the stoichiometry of the magnesium chloride sets 73 the molar ratio of cell products at 2:1 chlorine:magne- 74 sium, on a volumetric basis the ratio is 5800:1! One 75 might argue that the electrolysis of magnesium chloride 76 produces chlorine, and magnesium is the by-product. 77 Magnesium metal was ®rst produced electrolytically 78 by Faraday in 1833. Modern electrolytic cell technology 79 emerged in Germany in the late 19th century. Today 80 there are several variants 1 but all can be traced back to 81 the I.G. Farben cell which uses anhydrous MgCl2 as 82 feed. In the 1930s, Dowdeveloped a cell designed to 83 accept partially hydrated MgCl2 as feed. Since 1998, all 84 electrolytic magnesium is produced by anhydrous tech- 85 nology. Fig. 1. Phase diagram of the magnesium chloride ± water system ,schematic). PROOF 86 Although it is not the only harmful impurity present 87 in MgCl2, water is an unwanted component of the cell 88 feed for many reasons: form in the Mg±Cl2±O2±H2 system. In the event of hy- 115 89 1. At electrolysis temperatures hydrolysis occurs, i.e., drolysis, one can expect to ®nd magnesium bound not 116 only to chlorine, but also to oxygen and hydrogen, e.g., 117 90 the water of hydration in MgCl2 Á nH2O does not re- 91 main intact, but instead dissociates. This generates MgOHCl ,see Eq. ,4)). 118 92 highly reactive hydrogen and oxygen species which At room temperature the most fully hydrated form of 119 magnesium chloride is the hexahydrate, MgCl Á 6H O. 120 93 attack MgCl2 to produce hydrogen chloride gas and 2 2 94 various oxycompounds of magnesium. Hydrogen Its dehydration proceeds by the following reaction: 121 95 chloride attacks the equipment and renders the chlo- MgCl Á 6H2O s MgCl Á 4H2O s 2H2O v 1 96 rine produced at the anode unmarketable. Some oxy- 2 2 97 compounds are insoluble in the electrolyte and cause over the temperature range 298±390 K, where the solid 123 98 sludging. hexahydrate is stable. 124 99 2. Soluble oxycompounds can participate in parasitic The dehydration of MgCl2 Á 4H2O occurs by con- 125 100 electrochemical reactions on the carbon anode con- version to the dihydrate by the reaction: 126 101 suming it in the process. This attendant dimensional MgCl2 Á 4H2O s MgCl2 Á 2H2O s 2H2O v 2 102 instability means suboptimal cell operation, e.g., un- Dehydration of the solid dihydrate by conversion to the 128 103 desirable changes in joule heating rates and electro- monohydrate occurs by the reaction: 129 104 lyte circulation patterns. 105 3. Soluble oxycompounds can react with nascent mag- 106 nesium droplets forming a surface ®lm that prevents 107 their coalescence. 108 3. The thermochemistry of the hydrates of MgCl2 109 Magnesium chloride is very hygroscopic. As shown in 110 Fig. 1, water can become chemically bound to form a 111 series of hydrate compounds ,MgCl2 Á nH2O, n 1, 2, 4, 112 6, 8, 12) [5]. It should be noted that MgCl2 and H2O, the 113 two terminalUNCORRECTED components of the phase diagram depicted 114 in Fig. 1, are only two of the stable compounds that 1 The dierences between the I.G. Farben cell and the VAMI cell can be attributed to the Russian practice of producing anhydrous feed from carnallite. VAMI is the acronym for the All-Union Aluminum Fig. 2. Vapor pressures of H2O and HCl over the hydrates of mag- and Magnesium Institute. nesium chloride. ARTICLE IN PRESS G.J. Kipouros, D.R. Sadoway / Journal of Light Metals xxx 82001) xxx±xxx 3 measured. Thus, absent these data, accurate determi- 181 MgCl Á H2O s MgCl Á H2O s H2O v 3 2 2 nation of the P =P ratio is not possible. Just the 182 HCl H2O 131 over the temperature range where the dihydrate is sta- same, it is easy to appreciate that if dehydration of the 183 132 ble. monohydrate occurs under conditions that yield molten 184 133 It should be noted that dehydration of the dihydrate MgCl2 containing a small amount of dissolved MgOHCl 185 134 can also proceed by hydrolysis: such that a =a 0:01, then the ratio of 186 MgOHCl MgCl2 PHCl=PH O necessary to chlorinate the impurity is greater 187 MgCl Á 2H2O s MgOHCl HCl g H2O v 4 2 2 s by a factor of 100 compared to the situation in which 188 136 Reaction ,4) is inhibited by the greater vapor pressure of there is no liquid phase. Thus, dehydration at higher 189 137 water generated by reaction ,3), which occurs simulta- temperatures requires attendant changes in process 190 138 neously. Hydrolysis can be prevented by the use of HCl chemistry. By way of example, Magnola employs a so- 191 139 according to Le Chatelier's Principle provided that the called superchlorinator [7]. 192 140 partial pressure of HCl exceeds the value calculated by A ®nal note on the dehydration of magnesium chlo- 193 141 Eq.