Article pubs.acs.org/IECR
Synthesis and Electrolysis of K3NaMgCl6 Zhimin Zhang,† Xuchen Lu,*,†,‡ Tizhuang Wang,† Yan Yan,† and Shiwei Chen†,§ †
State Key Laboratory of Multi-phase Complex Systems, Institution of Process Engineering, Chinese Academy of Science, Haidian District, Beijing 100190, PR China ‡ United Research Center for Resource and Materials, Wuhai 016000, PR China § Graduate University of Chinese Academy of Science, Beijing 100049, PR China S Supporting Information *
ABSTRACT: High-purity K3NaMgCl6 was synthesized from magnesia.The factors affecting the purity of K3NaMgCl6 were investigated. The hygroscopic property of K3NaMgCl6 was studied. The preparation process of K3NaMgCl6 was investigated by X-ray diffraction analysis and differential scanning calorimetry analysis, and the reaction mechanism involved was determined. Then, magnesium metal was prepared by an electrochemical method using K3NaMgCl6 as raw material. The electrolytic parameters were measured, and the electrochemical behavior of magnesium ion in K3NaMgCl6 molten salt was investigated. Magnesia content in K3NaMgCl6 achieved 0.02 wt % under the optimum conditions. K3NaMgCl6 had a lower hygroscopy at room temperature and had a lower tendency to hydrolyze at high temperature. The purity of the obtained magnesium metal was 99.4 wt %, and the current efficiency in the electrolysis process was 94.8%.
1. INTRODUCTION Magnesium and its alloys have been widely used in transportation industries, military industries and electronic industries because of their many advantages, such as low density, high specific strength, good machinability, good damping ability, and high energetic particle penetration resistance.1−5 Compared with the thermal reduction method, the electrochemical method is very promising for the preparation of magnesium metal because of its low pollution and low energy consumption.6 However, the high production cost of highpurity anhydrous magnesium chloride has limited electrolytic magnesium production.7,8 The bonding force between magnesium chloride and crystal water in magnesium chloride hexahydrate is strong, resulting in the hydrolysis in the heating process with magnesia and magnesium hydroxychloride as products. Therefore, the production cost of high-purity anhydrous magnesium chloride is high.9−12 To overcome the hydrolysis of magnesium chloride hexahydrate at elevated temperature, many researchers prepared high-purity anhydrous magnesium chloride from magnesia via various methods. These methods can be classified as rare earth chloride chlorination, 7,13−15 MgCl 2 −NH 3 decomposition,16,17 and the carbochlorination process.18,19 However, each of these methods has certain disadvantages. For the rare earth chloride chlorination method, the consumption of a great amount of neodymium chloride increases the preparation costs of anhydrous magnesium chloride. In addition, it is difficult to separate anhydrous magnesium chloride from neodymium oxichloride. For the organic solvent distillation method, much organic solvent is attached to magnesium chloride hexammoniate when taken out, and washing off the organic solvent consumes a large quantity of anhydrous organic solvent. For the carbochlorination process, the high reaction temperature and the consumption of a great deal of chlorine gas increase the © 2015 American Chemical Society
production costs. In addition, we found that adding anhydrous magnesium chloride in the electrolytic magnesium process could cause the following problems: (1) The melting point of anhydrous magnesium chloride is 714 °C, which makes it hard for anhydrous magnesium chloride to melt and diffuse in the electrolyte (the electrolytic temperature is about 700 °C). (2) Anhydrous magnesium chloride has high hygroscopic property at room temperature, which makes it very difficult to store and transport anhydrous magnesium chloride. (3) Anhydrous magnesium chloride reacts with oxygen and water vapor in ambient atmosphere above 450 °C,20 resulting in high magnesia content.
2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade magnesia (98.0 wt % magnesia, 1.5 wt % carbonate; Sinopharm Chem. Reagent Co. Ltd., China), analytical grade ammonium chloride (99.5 wt %; Xilong Chem. Co. Ltd., China), analytical grade sodium chloride (99.5 wt %; Xilong Chem. Co. Ltd., China), and analytical grade potassium chloride (99.5 wt %; Xilong Chem. Co. Ltd., China) were used in the experiments. 2.2. Synthesis of High-Purity K3NaMgCl6. Magnesia, ammonium chloride, sodium chloride, and potassium chloride with appropriate molar ratio were mixed evenly by grinding. The mixture was charged into a 50 mL corundum crucible and then maintained at a certain temperature for the complete reaction. After that, the mixture was calcined at 700 °C for 0.5 h to drive off the excessive ammonium chloride and to obtain the molten salt which has homogeneous chemical components. Received: Revised: Accepted: Published: 1433
November 13, 2014 December 30, 2014 January 1, 2015 January 1, 2015 DOI: 10.1021/ie504494n Ind. Eng. Chem. Res. 2015, 54, 1433−1438
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Industrial & Engineering Chemistry Research
potassium chloride with the molar ratio n(MgO):n(NH4Cl):n(NaCl):n(KCl) = 1:3:1:3. The samples were heated from room temperature to 750 °C with the heating rate of 10 °C· min−1 and the nitrogen flow rate of 40 mL·min−1. 2.6.3. Purity Analysis of K3NaMgCl6. The only impurity in K3NaMgCl6 was MgO under our experimental conditions.24,25 To compare with the purity of anhydrous MgCl2, the purity of K3NaMgCl6 was characterized by w(MgO):w(MgCl2) in the sample. Ethylene diamine tetraacetic acide (EDTA) titration was employed to determine the value of w(MgO):w(MgCl2). The titration process has been previously described, and the titration error was less than 5%.21 2.6.4. Purity Analysis of the Magnesium Metal. The obtained magnesium metal was precisely measured, and then it was dissolved in hydrochloric acid (6 mol·L−1). After that, the purity of magnesium was measured by EDTA titration. 2.6.5. Analysis of Electrolytic Magnesium Process. The equations used for calculating the current efficiency and the electricity consumption can be found elsewhere.11,21
The molten salt obtained was placed in ambient atmosphere to cool, and then the compact solid K3NaMgCl6 was obtained. 2.3. Hygroscopic Property of K3NaMgCl6. To determine whether K3NaMgCl6 can be stored and transported at room temperature, the comparison between the hygroscopic property of K3NaMgCl6 and anhydrous MgCl2 at room temperature was investigated. Long et al.20 reported that anhydrous MgCl2 could react with oxygen and water vapor in ambient atmosphere above 450 °C, which resulted in high magnesia content in anhydrous magnesium chloride. To determine the stability of K3NaMgCl6 at elevated temperature, the comparison between the hygroscopic property of K3NaMgCl6 and anhydrous MgCl2 at 450 °C was investigated. Anhydrous MgCl2 was prepared according to the previously reported method.21 Anhydrous MgCl2 (w(MgO):w(MgCl2) = 0.014 wt %) and K3NaMgCl6 (w(MgO):w(MgCl2) = 0.02 wt %) were both ground, and then the samples obtained with the particle size of 50−100 μm were used in our experiment. After that, anhydrous MgCl2 and K3NaMgCl6 with the same mole number were placed separately in corundum crucibles with the same sample thickness. Then the corundum crucibles were maintained at room temperature and 450 °C for different times. The hygroscopic property at room temperature can be reflected by the weight increase of the sample. The hygroscopic property at 450 °C can be reflected by w(MgO):w(MgCl2) in the sample.24 2.4. Preparation of Magnesium Metal from K3NaMgCl6. Magnesium metal was prepared by the electrochemical method using K3NaMgCl6 as raw material. Graphite crucible (Ø 65 × 140 mm2) was used as the electrolysis cell. Steel rod (d = 6 mm) and spectrally pure graphite rod (d = 6 mm) were used as cathode and anode, respectively. The electrolysis process was carried out with electrolytic temperature, 700 °C; electrolytic time, 4 h; interelectrode distance, 4 cm; and cathodic current density, 0.42 A·cm−2.19,21 2.5. Electrochemical Behavior of Magnesium Ion in K3NaMgCl6 Molten Salt. The electrolysis process was potentiostatic. Cyclic voltammetry was employed to study the electrochemical behavior of magnesium ion in K3NaMgCl6 molten salt. A silver wire (d = 0.4 mm) dipped into a solution of AgCl (1 wt %) in K3NaMgCl6 melt was used as a reference electrode (Ag/AgCl couple). The working electrode was a platinum wire (d = 0.5 mm; 99.99% purity) and the counter electrode was a spectrally pure graphite (d = 6 mm). The lower ends of the platinum electrode and the graphite electrode were polished by using SiC papers, and then they were cleaned in ethanol by using ultrasonic cleaning. The active electrode surface was determined by measuring the immersion depth of the electrode in the melt. 2.6. Characterization. 2.6.1. X-ray Diffraction (XRD) Analysis. Solid powder diffraction patterns were collected by using an X-ray diffractometer (X’Pert MPD Pro, PANalytical, The Netherlands) operating with a Cu anode at 40 kV and 40 mA in the range of 2θ value between 5° and 90° with a speed of 26.5 deg·min−1. 2.6.2. Differential Scanning Calorimetry (DSC) Analysis. The differential scanning calorimetry analysis were carried out by a simultaneous thermogravimetric analysis (TGA)−DSC apparatus (TGA/DSC 1, Mettler-Toledo). Two samples were used for the DSC analysis. The first sample was 8.2 mg mixture of magnesia and ammonium chloride with the molar ratio of n(MgO):n(NH4Cl) = 1:3. The second sample was a 14.4 mg mixture of magnesia, ammonium chloride, sodium chloride, and
3. RESULTS AND DISCUSSION 3.1. Synthesis of High-Purity K3NaMgCl6. 3.1.1. Effect of n(NH4Cl):n(MgO). The effect of the molar ratio of NH4Cl to MgO on the purity of K3NaMgCl6 was investigated with the molar ratio of n(MgO):n(NaCl):n(KCl) = 1:1:3; reaction temperature, 400 °C; and reaction time, 1.0 h (Figure 1). When
Figure 1. Effect of the molar ratio of NH4Cl to MgO on the purity of K3NaMgCl6.
increasing n(NH4Cl):n(MgO) (n(NH4Cl) was the molar number of NH4Cl, n(MgO) was the molar number of MgO) from 2.0 to 3.0, the content of MgO in K3NaMgCl6 decreased from 0.88 to 0.02 wt %. MgO content remained constant with the further increase of n(NH4Cl):n(MgO). We previously prepared high-purity anhydrous MgCl2 by using MgO and NH4Cl as raw materials. The result revealed that the content of MgO in anhydrous MgCl2 could reach 0.014 wt % when n(NH4Cl):n(MgO) = 5:1.21 Therefore, much less NH4Cl was used in the preparation process of high-purity K3NaMgCl6, which indicated that KCl and NaCl played an important role in the preparation process of high-purity K3NaMgCl6. 1434
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Industrial & Engineering Chemistry Research 3.1.2. Effect of n(KCl+NaCl):n(MgO). The effect of the amount of KCl and NaCl on the purity of K3NaMgCl6 was investigated with n(KCl):n(NaCl) being 3:1, n(NH4Cl):n(MgO) 3:1, reaction temperature 400 °C, and reaction time 1.0 h (Figure 2). When n(KCl+NaCl):n(MgO) increased from
Figure 3. Effect of reaction temperature on the purity of K3NaMgCl6.
Figure 2. Effect of the molar ratio of KCl and NaCl to MgO on the purity of K3NaMgCl6.
0 to 4 (n(KCl+NaCl) was the sum of the molar number of KCl and NaCl), the content of MgO decreased sharply. When n(KCl+NaCl):n(MgO) was increased further, the content of MgO remained constant. When KCl and NaCl were not used, the product was anhydrous MgCl2.21 Anhydrous MgCl2 could react with oxygen and water vapor above 450 °C with magnesia as product, which resulted in the high MgO content.22 The result indicated that K3NaMgCl6 had a lower tendency to react with oxygen and water vapor in ambient atmosphere at elevated temperature, which led to high-purity K3NaMgCl6. 3.1.3. Effect of Reaction Temperature. The effect of reaction temperature on the purity of K3NaMgCl6 was investigated with n(KCl):n(NaCl) being 3:1, n(KCl +NaCl):n(MgO) 4:1, n(NH4Cl):n(MgO) 3:1, and reaction time 1.0 h (Figure 3). When reaction temperature increased from 350 to 550 °C, the content of MgO varied from 0.020 to 0.033 wt % with no obvious regularity. When KCl and NaCl were not used, a covering agent had to be used to suppress the emission and decomposition of NH4Cl at high temperature.21 By observation, we found that the mixture of KCl, NaCl, MgO, and NH4Cl was compact in the heating process because of the sintering. Therefore, the emission and the decomposition of NH4Cl were effectively suppressed. Therefore, the complete reaction between NH4Cl and MgO would occur at elevated temperature. 3.1.4. Effect of Reaction Time. The effect of reaction time on the purity of K 3 NaMgCl 6 was investigated with n(KCl):n(NaCl) being 3:1, n(KCl+NaCl):n(MgO) 4:1, n(NH4Cl):n(MgO) 3:1, and reaction temperature 400 °C (Figure 4). When reaction time increased from 0.2 to 1.0 h, the MgO content decreased sharply. When reaction time was increased further, MgO content remained constant. Therefore, 1.0 h was selected as the reaction time in our experiment. To summarize, high-purity K3NaMgCl6 was synthesized by using MgO, KCl, NaCl, and NH4Cl as raw materials. The
Figure 4. Effect of reaction time on the purity of K3NaMgCl6.
content of MgO in K3NaMgCl6 achieved 0.02 wt %, which is far lower than the 0.5 wt % demanded by industry. The optimum conditions for the preparation of high-purity K3NaMgCl6 were as follows: MgO, NH4Cl, KCl, and NaCl with the molar ratio of n(MgO):n(NH4Cl):n(KCl):n(NaCl) = 1:3:3:1 were mixed evenly. The aforementioned mixture was maintained at 350− 550 °C for 1.0 h to make the complete reaction. After that, the mixture was calcined at 700 °C for 0.5 h and then cooled in air to obtain compact K3NaMgCl6. 3.2. Hygroscopic Property of K3NaMgCl6. The comparison of the hygroscopic property between anhydrous MgCl2 and K3NaMgCl6 at room temperature and 450 °C was investigated (Figure 5 and Figure 6). As can be seen from Figure 5, K3NaMgCl6 had hygroscopy lower than anhydrous MgCl2 at room temperature, which made it easier to store and transport. As can be seen from Figure 6, K3NaMgCl6 had a tendency to hydrolyze that was lower than that of anhydrous MgCl2 at 450 °C, which guaranteed the purity of K3NaMgCl6 at elevated temperature. 3.3. Study of the Preparation Process of K3NaMgCl6. 3.3.1. XRD Analysis. The XRD patterns of the mixture of magnesia, ammonium chloride, sodium chloride, and potassium 1435
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Figure 5. Comparison of hygroscopy between K3NaMgCl6 and anhydrous MgCl2 at room temperature.
Figure 7. XRD patterns of the mixture at different temperatures. a, NaCl; b, KCl; c, NH4Cl·MgCl2·nH2O; d, K3NaMgCl6.
3.3.2. DSC Analysis. The comparison of the DSC curves between the mixture of MgO and NH4Cl and the mixture of KCl, NaCl, MgO, and NH4Cl are shown in Figure 8. As can be
Figure 6. Comparison of hygroscopy between K3NaMgCl6 and anhydrous MgCl2 at 450 °C.
chloride at different temperatures are shown in Figure 7. Distinctive XRD patterns of NH4Cl·MgCl2·nH2O (0 ≤ n < 6) were observed at 300 °C, which has been confirmed previously.21 When temperature increased to 400 °C, new diffraction peaks were detected, which matched well with the characteristic peaks of K3NaMgCl6 reported by Fink et al.23 Therefore, the reaction between NaCl, KCl, and NH4Cl· MgCl2·nH2O could occur in the following way:
Figure 8. Comparison of the DSC curves.
seen from Figure 8, three endothermic peaks (187−209 °C, 236−315 °C, and 323−343 °C) were detected at both curves. Two endothermic peaks (397−407 °C and 461−473 °C) could be observed at the DSC curves of the mixture of KCl, NaCl, MgO, and NH4Cl. The endothermic peak between 187 and 209 °C corresponded to the phase transformation of NH4Cl.26 The endothermic peak between 236 and 315 °C was ascribed to the formation of NH4Cl·MgCl2·nH2O (0 ≤ n < 6), which was consistent with the results of the XRD analysis.21 The endothermic peak between 323 and 343 °C was assigned to the decomposition of NH4Cl, which could be confirmed by the decomposition temperature of NH4Cl. The endothermic peak between 397 and 407 °C was ascribed to the formation of K3NaMgCl6, which agreed well with the XRD analysis. The endothermic peak between 461 and 473 °C corresponded to
3KCl + NaCl + NH4Cl ·MgCl2·nH 2O = K3NaMgCl6 + NH3 + HCl + nH 2O
(1)
When temperature increased over 500 °C, only the characteristic peaks of K3NaMgCl6 could be observed. MgCl2 existed in the form of the complex salt NH4Cl·MgCl2·nH2O and K3NaMgCl6 in the heating process. NH4Cl·MgCl2·nH2O had a weak tendency to hydrolyze at low temperature,22 and K3NaMgCl6 had a great stability at high temperature, which guaranteed the purity of the obtained K3NaMgCl6. 1436
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Industrial & Engineering Chemistry Research the melting point of K3NaMgCl6, which matched well with the observation results. 3.3.3. Reaction Mechanism. According to the above experimental results, the reaction mechanism involved in the process can be determined. MgO reacts with NH4Cl below 300 °C, forming NH4Cl·MgCl2·nH2O (0 ≤ n < 6). The force bonding between crystal water and magnesium chloride in NH4Cl·MgCl2·nH2O is weak, which is beneficial to the dehydration process and the purity of the products at lower temperature.22 Even if hydrolysis occurs, NH4Cl and the HCl gas decomposed by NH4Cl can convert the hydrolysate (MgO and MgOHCl) to anhydrous MgCl2.21 The reaction among NH4Cl·MgCl2·nH2O, KCl, and NaCl at about 400 °C could be expressed as eq 1, forming the complex salt K3NaMgCl6. K3NaMgCl6 is stable above 450 °C, which guarantees the purity of the product at elevated temperature. Therefore, the complex salt NH4Cl·MgCl2·nH2O and K3NaMgCl6 were formed in the heating process, which can effectively avoid the hydrolysis of MgCl 2 from hydrolyzing and guarantees high-purity K3NaMgCl6 can be obtained. 3.4. Preparation of Magnesium Metal from K3NaMgCl6. 3.4.1. Electrolytic Parameters. Magnesium metal was prepared by electrochemical method using K3NaMgCl6 as raw material. Magnesium metal obtained was accumulated around the cathode, at the surface of the molten salt with a spherical shape. The XRD patterns of the magnesium metal obtained are shown in Figure 9. The purity
Figure 10. Voltammogram on platinum electrode (S = 0.5 cm2) in K3NaMgCl6 molten salt at 953 K (scan rate, 0.01 V·s−1).
metal. The cathodic peak B was due to Na−Mg alloy formation, which has been confirmed by Martinez et al.28 Upon a further scan of the platinum cathode at the negative potential region, cathodic peak A was detected, which was due to the deposition of sodium. In the reverse scan direction, anodic peaks of C′ and D′ corresponded to the dissolution of magnesium from magnesium metal and the dissolution of magnesium from magnesium−platinum alloy, respectively. According to the above analysis, the electrochemical reduction of magnesium ion took place in one reaction step with the transfer of two electrons. Cyclic voltammograms were recorded on a platinum electrode at different scan rates in K3NaMgCl6 molten salt at 953 K (Figure 11). As can be seen from Figure 11, the cathodic peak potential shifted cathodically with the increase of the potential scan rate, which revealed that the reduction of magnesium ion on the platinum electrode was irreversible.
Figure 9. XRD patterns of the magnesium metal obtained.
of the magnesium metal obtained was 99.4 wt %, and the current efficiency in the electrolysis process was 94.8%. The electricity consumption in the electrolysis process was 8819 kW·h, which is lower than that of the most advanced multipolar magnesium electrolytic cell. 3.4.2. Electrochemical Behavior Analysis. To study the electrochemical behavior of magnesium ion in K3NaMgCl6 molten salt, cyclic voltammetry was carried out on a platinum electrode at 953 K (Figure 10). The cathodic signal D at around −0.85 V (vs Ag/AgCl) was detected. According to the phase diagram of the Mg−Pt system,27 cathodic signal D corresponded to the formation of an Mg−Pt alloy. Considering the fact that the deposition potential of Mg ion is more positive than that of K and Na ions, the cathodic peak at around −1.36 V (cathodic peak C) represented the deposition of magnesium
Figure 11. Cyclic voltammogram at various potential scan rates for platinum electrode in the K3NaMgCl6 molten salt at 953 K (S = 0.5 cm2). 1437
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(7) Cathro, K. J.; Deutscher, R. L.; Sharma, R. A. Electrowinning magnesium from its oxide in a melt containing neodymium chloride. J. Appl. Electrochem. 1997, 27, 404. (8) Ratvik, A. P.; Laher, T. M.; Mamantov, G.; Roberts, S. S.; Josefowicz, J. Y. Studies of composite anodes for the production of magnesium and aluminum. J. Electrochem. Sol. 1987, 134, 321. (9) Kipouros, G. J.; Sadoway, D. R. A thermochemical analysis of the production of anhydrous MgCl2. JOM 2001, 1, 111. (10) Eom, H. C.; Park, H.; Yoon, H. S. Preparation of anhydrous magnesium chloride from ammonium magnesium chloride hexahydrate. Adv. Powder Technol. 2010, 21, 125. (11) Xu, R. Y. The Production Technology of Magnesium Metal; Central South Press: Changsha, China, 2003. (12) Pansy, J. G.; Kennedy, M. W.; Woelsker, T. P. The preparation of anhydrous magnesium chloride-containing molten salt from dehydrated magnesium chloride and the preparation of magnesium metal. U.S. Patent 1146757, 1997. (13) Sharma, R. A. A new electrolytic magnesium production process. JOM 1996, 6, 39. (14) Mediaas, H.; Tkatcheva, O.; Dracopoulos, V.; Papatheodorou, G. N.; Kipouros, G. J.; Ostvold, T. Solubilities and raman spectra of NdOCl in some chloride melts of interest for the electrowinning of magnesium from its oxide. Metall. Mater. Trans. B 2005, 36, 153. (15) Sharma, R. A. Method for producing magnesium metal from magnesium oxide. U.S. Patent 5279716, 1994. (16) Ou, T. J.; Lu, X. C.; Liang, X. F.; Yao, S. Y. Leaching kinetics of calcined magnesite from glycol solution dissolved with ammonium chloride. J. Process. Eng. 2007, 7, 928. (17) Ketil, A.; Ragnar, H. E.; Ralf, S. Process for producing anhydrous MgCl2. U.S. Patent 6042794, 1998. (18) Strelets, K. L. Electrolytic Production of Magnesium; Israel Program for Scientific Translation: Israel, 1977. (19) Zhang, Y. J. Electrolytic Metallurgy of Magnesium; Central South University Press: Changsha, China, 2006. (20) Long, G. M.; Ma, P. H.; Wu, Z. M.; Li, M. Z.; Chu, M. X. Investigation of thermal decomposition of MgCl2 hexammoniate and MgCl2 biglycollate biammoniate by DTA−TG, XRD and chemical analysis. Thermochim. Acta 2004, 142, 149. (21) Zhang, Z. M.; Lu, X. C.; Yang, S. P.; Pan, F. Preparation of anhydrous magnesium chloride from magnesia. Ind. Eng. Chem. Res. 2012, 51, 9713. (22) Zhang, Z. M.; Lu, X. C.; Wang, T. Z.; Yan, Y.; Pan, F. The dehydration of MgCl2·6H2O in MgCl2·6H2O-KCl-NH4Cl system. J. Anal. Appl. Pyrolysis 2014, 110, 248. (23) Fink, H.; Seifert, H. J. Quaternary compounds in the system KCl/NaCl/MgCl2. Thermochim. Acta 1984, 72, 195. (24) Kashani-Nejad, S.; Ng, K. W.; Harris, R. MgOHCl thermal decomposition kinetics. Metall. Mater. Trans. B 2005, 36, 153. (25) Chen, J. Z. Study of the system of MgO-MgCl2, Ph.D. Thesis, East China University of Science and Technology, Shanghai, 2006. (26) Schultz, R. D.; Dekker, A. D. The effect of physical adsorption on the absolute decomposition rates of crystalline ammonium chloride and cupric sulfate trihydrate. J. Phys. Chem. 1956, 60, 1095. (27) Guo, Q. W. Binary Alloy Phase Diagram; Chemical Industry Press: Beijing, 2008. (28) Martinez, A. M.; Borresen, B.; Haarberg, G. M.; Castrillejo, Y.; Tunold, R. Electrodeposition of magnesium from CaCl2-NaCl-KClMgCl2. J. Electrochem. Soc. 2004, 151, C508.
4. CONCLUSIONS High-purity K3NaMgCl6 was synthesized from magnesia. After that, magnesium metal was prepared by an electrochemical method using K3NaMgCl6 as raw material. (1) Magnesia content in K3NaMgCl6 achieved 0.02 wt % under the optimum conditions: n(NH4Cl):n(MgO) molar ratio of 3:1, n(KCl+NaCl):n(MgO) molar ratio of 4:1, reaction temperature of 350−550 °C, and reaction time of 1.0 h. (2) Compared with anhydrous MgCl2, K3NaMgCl6 had lower hygroscopy at room temperature and had a lower tendency to hydrolyze at high temperature. (3) Magnesium metal was prepared by an electrochemical method using K3NaMgCl6 as raw material. The purity of the obtained magnesium metal was 99.4 wt %, and the current efficiency in the electrolysis process was 94.8%. The electricity consumption in the electrolysis process was 8819 kW·h. (4) The reduction of magnesium ion on platinum electrode was irreversible. The electrochemical reduction of magnesium ion took place in one reaction step with the transfer of two electrons.
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ASSOCIATED CONTENT
S Supporting Information *
Schematic diagram of electrochemical process and photograph of magnesium metal obtained on the surface of the solidified molten salt. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*No. 1 Bei-er-tiao, Zhong-guan-cun, Haidian District, Beijing 100190, China. E-mail:
[email protected]. Tel.: +86 10 82544889. Fax: +86 10 62561822. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (U1407136, 21406247). REFERENCES
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