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 anity for

8 water and is found in nature as a plurality of hydrates ,MgCl2 Á nH2O, n ˆ 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 sti€ness. 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 anity for water as evidenced by the large number of sium in the form of serpentine Mg3Si2O5 OH†4† [4]. 58 34 hydrates ,MgCl2 Á nH2O, n ˆ 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 e€ortUNCORRECTED 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† ˆ 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† ˆ 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 ˆ 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 di€erences 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† ˆ 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 ˆ 0:01, then the ratio of 186 MgOHCl MgCl2 PHCl=PH O necessary to chlorinate the impurity is greater 187 MgCl Á 2H2O s† ˆ 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. ,4). Fig. 2 plots the temperature dependence of this ride concerns the stability of the oxychloride, MgOHCl, 194 142 threshold PHCl. which can decompose according to the reaction 195 143 The ®nal stage in dehydrating magnesium chloride is MgOHCl ˆ MgO ‡ HCl 8† 144 to remove the last water of crystallization from the solid s† s† g† 145 monohydrate by the reaction: The decomposition temperaturePROOF of MgOHCl, i.e., the 197 temperature at which the pressure of HCl reaches 1 atm, 198 MgCl Á H2O s† ˆ MgCl ‡ H2O v† 5† 2 2 s† is calculated [5] to be 828 K. Above this temperature the 199 147 The hydrolysis reaction corresponding to Eq. ,5) is oxychloride formed during dehydration will be con- 200 MgCl Á H O ˆ MgOHCl ‡ HCl 6† verted entirely to MgO according to reaction ,8). Solu- 201 2 2 s† s† g† bilities of the various oxidation products have been 202 149 Fig. 2 shows variation in the partial pressure of HCl determined [8]. Hydroxychloride formation and its e€ect 203 150 with temperature for Eq. ,6). on melt chemistry is probably more complex than it 204 151 Reactions ,5) and ,6) can be combined to express the appears, as there many hydroxychloride species reported 205

152 hydrolysis of MgCl2 as in literature. To complicate further the issue, some of 206 MgCl ‡ H O ˆ MgOHCl ‡ HCl 7† these hydroxychlorides are reported to have hydrates. 207 2 s† 2 v† s† g† Due to the complexity of the system some authors refer 208 154 which allows a calculation of the minimum ratio of to the retained water as ``occluded'' water. Table 1 209 155 P =P to avert hydrolysis. As shown in Fig. 2, the shows the formulae of all the hydroxychlorides and their 210 HCl H2O 156 vapor pressure of HCl under these circumstances is hydrates for which powder di€raction ®les ,PDF) have 211

157 considerably higher than that of H2O. This suggests that been reported in the cards of the Joint Committee on 212 158 unless precautions are taken, the hydrolysis reaction ,7) Powder Di€raction Standards ,JCPDS). Although the 213 159 will proceed in preference to the dehydration reaction plethora of the proposed hydroxychlorides may cast 214 160 ,5). Alternatively, hydrolysis can be averted when the doubt on their existence, nevertheless, it testi®es to the 215

161 applied partial pressure of HCl exceeds that shown in complexity of the Mg±Cl2±O2±H2 system. 216 162 Fig. 2 for reaction ,7). In industrial practice, kinetics, which do not appear 217 163 In the light of the above reactions ,1)±,7), the dehy- to have been extensively studied in this system, play a 218

164 dration of MgCl2 Á 6H2O will produce anhydrous MgCl2 major role. Indeed, spray drying under optimum con- 219 165 as long as the ®nal step, reaction ,5), is performed under ditions can produce material with 1% H2O and 1% 220 166 a blanket of HCl at a partial pressure given in Fig. 2. It MgO, although normal practice gives 2±5% H2O and 2± 221 167 has been reported [6] that the entire dehydration se- 5% MgO. 222 168 quence starting from hexahydrate is conducted under To avoid some of the problems presented by the de- 223 169 HCl. The thermodynamic analysis presented in [5] hydration of hydrated magnesium chloride, other 224 170 shows that up to reaction ,4) it is not necessary to use chemistries for cell feed preparation have been investi- 225 171 HCl provided that no liquid phase is present. gated. Some of these, particularly those involving am- 226 172 In an attempt to improve the kinetics of the process, monium chloride salts, have been tested at the pilot 227 173 workers may be tempted to conduct dehydration at el- plant level. These include dehydration of double salts 228 174 evated temperaturesUNCORRECTED where either the hydrates or the and chlorination of magnesium oxide. 229 175 product MgCl2 are present as liquid. Under these cir- Magnesium chloride and potassium chloride form a 230 176 cumstances, the value of the partial pressure of HCl hydrated double salt called carnallite, 231

177 capable of preventing hydrolysis is substantially higher KCl Á MgCl2 Á 6H2O. Since KCl is also a major compo- 232 178 than that shown in Fig. 2. Unfortunately, the thermo- nent of the electrolyte, one option in cell feed prepara- 233

179 dynamic properties of multicomponent chloride melts tion is to produce anhydrous KCl Á MgCl2 directly from 234 180 containing dissolved oxycompounds have not been carnallite. Proponents of the use of carnallite have ar- 235 ARTICLE IN PRESS

4 G.J. Kipouros, D.R. Sadoway / Journal of Light Metals xxx 82001) xxx±xxx

236 gued that it is easier to dehydrate than MgCl2 Á nH2O. 413 K 237 Closer examination shows this not to be the case. NH4Cl Á MgCl2 Á 4H2O s† ! NH4Cl Á MgCl2 Á 2H2O s†

238 The dehydration of carnallite proceeds according to ‡2H2O v† 13† 239 the reactions

KCl Á MgCl2 Á 6H2O s† ˆ KCl Á MgCl2 Á 2H2O s† ‡ 4H2O v† 433 K NH4Cl Á MgCl2 Á 2H2O s† ! NH4Cl Á MgCl2 s† 9† ‡2H2O v† 14† 241 and

KCl Á MgCl Á 2H2O s† ˆ KCl Á MgCl ‡ 2H2O v† 2 2 s† NH Cl Á MgCl 509! K NH Cl ‡ MgCl 15† 10† 4 2 s† 4 v† 2 s† 243 accompanied to a certain extent by hydrolysis according Heating ammonium carnallite to 450 K will result in 269 244 to reaction ,7). Although it appears that dehydration of dehydration according to reactions ,12)±,14). In a sec- 270 245 carnallite is similar to that of magnesium chloride, there ond step, reaction ,15) is carried out by maintaining a 271 246 is a signi®cant di€erence between the two. Any hy- temperature of 573 K. The last step should be conducted 272 247 droxychloride generated during the dehydration of car- upon complete removal of the water of crystallization, 273 248 nallite forms a solid solution with it. Upon melting, the otherwise hydrolysis will occur via reaction ,11). The 274 249 hydroxychloride dissociates according to patent literature [9] describesPROOF several variations of this 275 ‡ À method of dehydration. Dehydration of ammonium 276 MgOHCl ˆ MgOH ‡ Cl 11† chloride in an ammonia atmosphere or in the presence 277 251 thereby generating cationic impurities, which can inter- of an organic phase is also described [10]. 278 252 fere with the electrochemical reduction of magnesium in Recently dehydration of ammonium double salts, 279 253 the electrolytic cell. To prevent hydrolysis a blanket of which has its merits, has been tried at the pilot plant 280 254 dry hydrogen chloride gas should be present. Thus, the scale. It seems that the higher energy consumption may 281 255 process is in practice as complicated as the dehydration o€set the advantages. Carnallite dehydration has been 282 256 of magnesium chloride and appears to o€er no advan- used on an industrial scale in Russia and recently in 283 257 tages over the latter. Furthermore, long-term cell feed- Israel. Ammonium carnallite dehydration has been 284 258 ing with dehydrated carnallite eventually requires either practiced at the laboratory scale by the authors of the 285

259 that pure MgCl2 make-up be added or that supporting present paper. The product anhydrous MgCl2 was then 286 260 electrolyte be withdrawn to keep its composition within sublimed in an inert atmosphere containing SOCl2 to 287 261 limits. give a highly puri®ed material for spectral studies 288 262 Another double salt is ammonium carnallite, [11,12]. 289

263 NH4Cl Á MgCl2 Á 6H2O. Dehydration occurs by the fol- Diculties in the dehydration of MgCl2 Á 6H2O led to 290 264 lowing steps: the development of processes for the direct chlorination 291

373 K of magnesium oxide. Magnesium oxide can be produced 292 NH4Cl Á MgCl2 Á 6H2O s† ! NH4Cl Á MgCl2 Á 4H2O s† by calcination ,1000°C) of either magnesite or mag- 293

‡2H2O v† 12† nesium hydroxide precipitated by mixing seawater with 294 dolime. 295

Table 1 PDF for reported hydroxides of magnesium and their hydrates

Formula Name File number Year

MgClOH Magnesium chloride hydroxide 24±726 1970

Mg2 OH†3Cl b-magnesium chloride hydroxide 12±410 1954 MgOHCl Magnesium chloride hydroxide 3±100 1944 Mg,OH)Cl Magnesium chloride hydroxide 11±328 1953

Mg2Cl OH†3 Magnesium chloride hydroxide 12±120 1958 Mg2Cl OH†3 Á 4H2O Magnesium chloride hydroxide hydrate 36±388 1982 Mg3Cl2 OH†4 Á 2H2O Magnesium chloride hydroxide hydrate 12±133 1958 Mg Cl OH† Á 5H O Magnesium chloride hydroxide hydrate 12±123 1958 10 2 UNCORRECTED18 2 Mg3Cl OH†5 Á 4H2O Magnesium chloride hydroxide hydrate 12±122 1958 Mg3 OH†5Cl Á 4H2O Magnesium chloride hydroxide hydrate 7±420 1949 Mg2 OH†3Cl Á 2H2O Magnesium chloride hydroxide hydrate 7±419 Mg3 OH†5Cl Á 3H2O Magnesium chloride hydroxide hydrate 7±416 Mg2 OH†3Cl Á 4H2O Magnesium chloride hydroxide hydrate 7±412 Mg10 OH†18Cl Á 5H2O Magnesium chloride hydroxide hydrate 7±409 Mg2 OH†3Cl Á 3H2O Magnesium chloride hydroxide hydrate 7±403 ARTICLE IN PRESS

G.J. Kipouros, D.R. Sadoway / Journal of Light Metals xxx 82001) xxx±xxx 5 296 The chlorination of magnesium oxide can be repre- tapped periodically into crucibles and fed to the elec- 347 297 sented by the reaction trolysis cells. 348 Newenvironmental restrictions have forced re-ex- 349 MgO ‡ Cl ˆ MgCl ‡ 1=2O 16† s† 2 g† 2 sorl† 2 g† amination of the carbochlorination process for two 350 main reasons. First, under certain conditions environ- 351 299 where MgCl2 is solid below987 K and molten above this 300 temperature. The thermodynamics of the process are mentally unacceptable polychlorinated hydrocarbons 352 301 explained in [5,8]. may be produced [15] and second, the chlorination 353 302 At 1300 K, which is the temperature at which in- furnaces are not easy to control. For improved eciency 354 303 dustrial chlorinators operate, the oxygen content of the it is necessary to optimize the charge with respect to 355 304 gas should be kept below1.4 mol% at 1 atm total particle size distribution, particle porosity, activity of 356 305 pressure. Failure to do so will result in oxidation of the reducing agent, i.e., grade of carbon, as well as with 357 respect to the concentrations of certain impurities. 358 306 MgCl2, i.e., reaction ,16) proceeds to the left. In indus- 307 trial practice, to achieve an acceptable rate of chlori- While the focus of this article has been the dehydra- 359 308 nation, the pressure of oxygen will be allowed to tion of various forms of MgCl2 Á nH2O, the production 360 309 increase above the equilibrium value. This necessitates of electrolytic-grade, anhydrous MgCl2, implies material 361 310 the use of a suitable reducing agent such as carbon to act free of impurities. Impurities not only contaminate the 362 311 as an oxygen sink. metal but also interfere with the operation of the elec- 363 312 Carbochlorination occurs by the reaction trolytic cells and a€ect theirPROOF productivity. This is due to 364 the fact that, even at trace concentration levels, impu- 365 MgO s† ‡ Cl2 g† ‡ C s† ˆ MgCl2 l† ‡ CO g† 17† rities can adversely change the physical properties of the 366 electrolyte. 367 314 The reaction products are very stable. Thus, when a Sources of MgCl can also contain oxides and sul- 368 315 stream of chlorine gas passes through a mixture of MgO 2 fates, as well as boron. Table 2 shows the maximum 369 316 and C maintained at a constant temperature of 1300 K permissible levels of commonly occurring impurities. 370 317 the gas phase will consist almost entirely of CO provided When cell feed is produced by dehydration of hy- 371 318 that equilibrium is achieved. On the other hand, since drated magnesium chloride concentrated from seawater 372 319 most industrial chlorination reactors have been designed or brines, sulfates are removed as calcium sulfate. It was 373 320 to consume a minimum amount of reagent ,carbon) and found [5] that the solubility of CaSO decreases as 374 321 do not operate isothermally, the gas phase tends to be a 4 MgCl content increases. For a brine containing 35% 375 322 mixture of CO, CO ,Cl and volatile chlorides. 2 2 2 MgCl the solubility of CaSO is 1.37 g/l. A similar trend 376 323 Carbon monoxide itself, however, is also a reducing 2 4 has also been observed for MgSO . At this stage of brine 377 324 agent initiating a strongly exothermic reaction: 4 concentration the sulfates are precipitated by the addi- 378

MgO s† ‡ Cl2 g† ‡ 2CO g† ˆ MgCl2 l† ‡ CO2 g† 18† tion of CaCl2. This treatment also results in precipita- 379 tion of other solids, such as clays and silica. In 380 326 Proper control of the temperature of the chlorination exceptional cases BaCl2 may be added to remove the last 381 327 furnace is essential for its successful operation. traces of sulfate. Following this, boron is removed by 382 328 In a patent [13] the carbochlorination of magnesium ion exchange [8] or solvent extraction [14]. In the sea- 383 329 carbonate ,instead of magnesium oxide) by chlorine and water process, after Mg OH†2 has been dissolved in HCl 384 330 carbon monoxide gases in a packed bed reactor to the excess calcium is precipitated along with magnesium 385 331 produce anhydrous molten magnesium chloride is de- by the addition of sulfuric acid. 386 332 scribed. The reaction is The residual sulfate is then removed by the intro- 387 duction of small amounts of barium carbonate or bar- 388 MgCO ‡ CO g† ‡ Cl2 g† ˆ MgCl ‡ 2CO2 g† 19† 3 s† 2 l† ium chloride. 389 334 Carbon dioxide is removed from the top of the reactor, 335 and anhydrous molten magnesium chloride is with- Table 2 336 drawn below the packed bed. The energy requirements Maximum permissible levels of impurities in electrolytic cell feed 337 are claimed to be low, and the kinetics favorable. The 338 one attempt to commercialize the process was unsuc- B 0.001% 339 cessful. MgO 0.2% 340 A detailedUNCORRECTED description of chlorination technology as SO4 0.05% Fe 0.005% 341 it applies to MgO in general has been given elsewhere Ca 0.2% a 342 [14]. Magnesium oxide mixed with carbon and magne- H2O 0.1% 343 sium chloride acting as a binder is formed into pellets, Tib 0.005% 344 which are dried and fed into the top of an electrically Mnb 0.1% 345 heated shaft furnace while chlorine enters at the bottom. a For anhydrous feed. b 346 In a continuous operation molten magnesium chloride is For MgCl2 recycled from Ti, Zr plants. ARTICLE IN PRESS

6 G.J. Kipouros, D.R. Sadoway / Journal of Light Metals xxx 82001) xxx±xxx 390 When cell feed is prepared by carbochlorination of designs must reduce internal cell resistance. Improved 433 391 magnesium oxide, impurities are removed by di€erent diaphragmless cells and bipolar-array electrode con®g- 434 392 reactions. For example, the other oxides present as im- urations are two possibilities that in the past have not 435 393 purities in MgO are also carbochlorinated by reactions been fully exploited due to materials limitations. How- 436 394 such as ever, the advent of a newgeneration of high perfor- 437 CaO ‡ C ‡ Cl ˆ CaCl ‡ CO 20† mance materials nowo€ers a newopportunity for 438 s† s† 2 g† 2 l† g† success. For example, some of the candidate materials 439 for inert anodes [16] and wettable cathodes [17] that 440 2Fe2O3 s† ‡ 3C s† ‡ 6Cl2 g† ˆ 4FeCl3 v† ‡ 3CO g† 21† have been researched by the aluminum industry may 441 ®nd utility in advanced magnesium cell designs. Last, 442 SiO2 s† ‡ 2C s† ‡ 2Cl2 g† ˆ SiCl4 v† ‡ 2CO2 g† 22† and most radically, is the prospect of totally avoiding 443

2Al2O3 s† ‡ 3C s† ‡ 6Cl2 g† ˆ 4AlCl3: v† ‡ 3CO2 g† 23† chloride metallurgy by direct electrolysis of MgO from a 444 molten oxide electrolyte to produce magnesium metal 445 399 These reactions waste chlorine and carbon. Further- and oxygen gas [18]. 446 400 more, not all the above reactions go to completion, and

401 the remaining oxides, particularly SiO2, MgO, and 402 Al O form a slag, the removal of which complicates the 2 3 Acknowledgements 447 403 operation. 404 Sulfates are also chlorinated according to the fol- PROOF The authors wish to acknowledge the Natural Science 448 405 lowing reactions: and Engineering Research Council ,NSERC) of Canada 449

MgSO4 s† ‡ C s† ‡ Cl2 g† ˆ MgCl2 l† ‡ SO2: g† ‡ CO2 g† ,GJK) and the MacVicar Foundation at the Massa- 450 chusetts Institute of Technology ,DRS) for partial ®- 451 24† nancial support. 452

CaSO4 s† ‡ C s† ‡ Cl2 g† ˆ CaCl2 l† ‡ SO2 g† ‡ CO2 g† 25† References 453 408 Finally, up to 50% of the moisture present can evaporate 409 upon introduction into the furnace, while the rest is [1] E.A. Brandes ,Ed.), Smithells Metals Reference Book, sixth ed., 454 410 chlorinated according to the reactions Butterworths, London, 1983. 455 [2] G.J. Kipouros, Bringing magnesium to automobiles, in: H. 456 2H2O v† ‡ 2Cl2 g† ‡ C s† ˆ 4HCl g† ‡ CO2 g† 26† Mostaghasi ,Ed.), Materials Solutions for Environmental Prob- 457 lems, The Metallurgical Society, Canadian Institute of Mining, 458 H2O v† ‡ Cl2 g† ‡ C s† ˆ 2HCl g† ‡ CO g† 27† Metallurgy, and Petroleum, Sudbury, Ont., Canada, August 17± 459 20, 1997, pp. 265±267. 460 [3] R.E. Brown, Magnesium industry growth in the 1990 period, in: 461 H.I. Kaplan, J.N. Hryn, B.B. Clow,Eds.), Magnesium Technol- 462 413 4. Implications for producing electrolytic cell feed ogy 2000, TMS, Warrendale, PA, 2000, pp. 3±12. 463 [4] K. Watson, P. Ficara, M. Charron, J. Peacey, E. Chin, G. Bishop, 464 The magnola demonstration plant: a valuable investment in 465 414 It is somewhat ironic that the least dense structural technology development and improvement, in: H.I. Kaplan, J.N. 466 415 metal, magnesium, which can reduce energy consump- Hryn, B.B. Clow,Eds.), Magnesium Technology 2000, TMS, 467 416 tion when used as a material for vehicular construction, Warrendale, PA, 2000, pp. 27±30. 468 417 is one of the most energy-intensive metals to produce [5] G.J. Kipouros, D.R. Sadoway, The chemistry and electrochem- 469 istry of magnesium production, in: G. Mamantov, C.B. Maman- 470 418 with presently available technologies. Thus e€orts to tov, J. Braunstein ,Eds.), Advances in Molten Salt Chemistry, vol. 471 419 reduce the cost of production must address the energy 6, Elsevier, Amsterdam, 1987, pp. 127±209. 472 420 requirements of extraction and bring them closer to the [6] P.M.D. Pinfold, Primary magnesium production at Becancour, 473 421 theoretical minima. An understanding of the scienti®c in: M. Sahoo, P. Pinfold ,Eds.), Extraction, Re®ning, and 474 422 principles of the relevant chemistry and electrochemistry Fabrication of Light Metals, Pergamon Press, Oxford, 1991, pp. 475 43±53. 476 423 serves to specify these theoretical limitations and to [7] P. Ficara, E. Chin, T. Walker, D. Laroche, E. Palumbo, C. Celic, 477 424 guide research and development in this regard. M. Avedesian, Magnola: a novel commercial process for the 478 425 In electrolytic processes, high-purity cell feed prepa- primary production of magnesium, in: M. Avedesian, R. Guil- 479 426 ration is energy-intensive.UNCORRECTED Newchemistries for energy- bault, D. Ksinsik ,Eds.), Light Metals 1996, The Metallurgy 480 427 ecient dehydration of MgCl may be found through Sociecty of CIM, Montreal, Canada, 1996, pp. 183±195. 481 2 [8] J.E. Vindstad, Hydrolysis of and oxide solubilities in melts related 482 428 better understanding the dehydration process. As for the to electrolytic magnesium production, Ph.D. Thesis 81, Norwe- 483 429 electrolysis operation itself, improvements in energy ef- gian Institute of Technology, Trondheim, February 1996, p. 114. 484 430 ®ciency will accrue from decreasing the cell voltage. This [9] G.J. Kipouros, Magnesium and other reactive metals: have they 485 431 can be accomplished through changes in cell design and matured?, J. Metall. 47 ,12) ,1995) 38±39. 486 432 the use and development of newmaterials. Newcell ARTICLE IN PRESS

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UNCORRECTED