Preparation of Anhydrous Magnesium Chloride from Magnesia

Jul 13, 2012 - ABSTRACT: High-purity anhydrous magnesium chloride was prepared by using magnesia and ammonium chloride as reactants, alumina as ...
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Preparation of Anhydrous Magnesium Chloride from Magnesia Zhimin Zhang,†,‡ Xuchen Lu,*,†,§ Suping Yang,†,‡ and Feng Pan†,‡ †

State Key Laboratory of Multi-phase Complex systems, Institution of Process Engineering, Chinese Academy of Science, Haidian District, Beijing 100190, P.R. China ‡ Graduate University of Chinese Academy of Science, Beijing 100049, P.R. China § United Research Center for Resource and Materials, Wuhai 016000, P.R. China ABSTRACT: High-purity anhydrous magnesium chloride was prepared by using magnesia and ammonium chloride as reactants, alumina as covering agent. The process was characterized by X-ray diffraction and thermogravimetric and differential scanning calorimetry analysis. The reaction mechanism involved in the process was proposed and discussed. In addition, the effects of molar ratio of ammonium chloride to magnesia, thickness of the covering agent (alumina), reaction temperature, and reaction time on the purity of anhydrous magnesium chloride were investigated. The content of magnesia in anhydrous magnesium chloride was achieved 0.014% under the optimum conditions: ammonium chloride and magnesia with the molar ratio of NH4Cl:MgO = 5:1 were mixed evenly. Alumina with the thickness of 1.1 cm was used as a covering agent. The mixture was maintained at 450 °C for 1.5 h for the complete reaction, and then calcined at 700 °C for 0.5 h to obtain well-sintered anhydrous magnesium chloride.

1. INTRODUCTION Magnesium and its alloys are attractive materials for the transportation industry because of their low density and good mechanical properties.1−3 The two principal magnesium production technologies are electrolysis of molten magnesium chloride (electrochemical method) and thermal reduction of magnesium oxide (thermal reduction method).4,5 The electrochemical method is energy-intensive and environment-friendly, whereas the thermal reduction method suffers from high energy consumption and low productivity. Currently, however, the majority of magnesium in the world is produced by the thermal reduction method because of its low costs and simplicity of the process.6 High production costs of preparation of anhydrous magnesium chloride have limited the electrolytic magnesium production.7,8 Various economical methods have been proposed for preparing anhydrous magnesium chloride. For example, Zhou et al.9 prepared anhydrous magnesium chloride using dehydrated ammonium carnallite (NH4Cl·MgCl2·nH2O, 1 ≥ n ≥ 0.5) and ammonium chloride. The dehydrated ammonium carnallite reacted with ammonium chloride to form ammonation ammonium carnallite (NH4Cl·MgCl2·mNH3, 1 ≥ m ≥ 0.5). Ammonation ammonium carnallite was calcined at elevated temperature and anhydrous magnesium chloride was obtained. His results revealed that the content of magnesia in anhydrous magnesium chloride could achieve 0.087% when the mass ratio of dehydrated ammonium carnallite to ammonium chloride was 1:4. The recovery of plenty of ammonium chloride can cause big problems in this process. Madorsky10 and Suzukawa et al.11 produced anhydrous magnesium chloride using ammonium carnallite as starting material and hydrogen chloride-containing gas or ammonia as protective gas. The complexity of the operation is an obvious disadvantage. Several researchers have studied the preparation of anhydrous magnesium chloride from magnesia. The conventional carbochlorination method provides a way of preparing © 2012 American Chemical Society

anhydrous magnesium chloride by using magnesia, carbon or carbon monoxide, and chlorine as starting materials.12 The degree of utilization of chlorine is low in the process. Sharma13 reported that anhydrous magnesium chloride could be prepared by using magnesia reacted with neodymium chloride in a magnesium chloride−neodymium chloride molten salt at elevated temperature. The consumption of neodymium chloride increases the costs of the process and the end product is magnesium−neodymium alloy instead of pure magnesium. Peacey and others14−16 proposed a method of producing anhydrous magnesium chloride molten salt by chloridizing magnesia-containing feed at high temperature. The low degree of utilization of chlorine is an obvious disadvantage. Ou17 proposed that magnesia can react with ammonium chloride in ethylene glycol to form anhydrous magnesium chloride. The considerable consumption of organic solvent has limited the large-scale industrial applications. In this paper, high-purity anhydrous magnesium chloride was prepared by using magnesia and ammonium chloride as reactants and alumina as covering agent. The process was investigated by X-ray diffraction and thermogravimetric and differential scanning calorimetry (TG-DSC) analysis. The mechanism involved in the process was proposed. The factors affecting the content of magnesia in anhydrous magnesium chloride were studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade magnesia (98.0 wt % magnesia, 1.5 wt % carbonate; Sinopharm Chem. Reagent Co. Ltd., China) and analytical-grade ammonium chloride (99.5 wt Received: Revised: Accepted: Published: 9713

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2.4.3. EDTA Titration. According to Kashani-Nejad et al.,18 the only oxide-containing compound in the sample was magnesia under the experimental conditions. The sample obtained was dissolved in distillated water to separate insoluble magnesia from soluble anhydrous magnesium chloride. The precipitate was filtered and washed with distillated water to eliminate Mg2+. Then the precipitate was dissolved in 0.2 N sulfuric acid and the solution was titrated with EDTA to determine the content of magnesia.

%, Xilong Chem. Co. Ltd., China) were used for the preparation of anhydrous magnesium chloride. Analyticalgrade alumina (99.4 wt %; Sinopharm Chem. Co. Ltd., China) was used as a covering agent in the process. 2.2. Preparation of Anhydrous Magnesium Chloride. Magnesia and ammonium chloride were used for the preparation of anhydrous magnesium chloride. Alumina was employed as a covering agent. Magnesia and ammonium chloride with an appropriate molar ratio were mixed evenly by grinding and then putting them into a 50 mL corundum crucible. Alumina with a given thickness was put on top of the mixture, and then the lid was put on. The crucible was placed in a muffle furnace and maintained at a certain temperature for complete reaction. After the reaction, the mixture was calcined at 700 °C for 30 min to drive off excessive ammonium chloride. Then well-sintered anhydrous magnesium chloride was obtained. 2.3. Single Factor Test. The main affecting factors that influence the purity of the anhydrous magnesium chloride are molar ratio of ammonium chloride to magnesia, thickness of alumina, reaction temperature, and reaction time. To find the effects of the molar ratio of ammonium chloride to magnesia on the purity of anhydrous magnesium chloride, the molar ratio of ammonium chloride to magnesia was varied from 2 to 6 while other parameters were fixed. To find the effects of the thickness of alumina on the purity of anhydrous magnesium chloride, the thickness of alumina was varied from 0 to 2 cm while other factors were fixed. To find the effects of reaction temperature and reaction time on the purity of anhydrous magnesium chloride, the reaction temperature was varied from 300 to 500 °C and the reaction time was varied from 0.5 to 2.5 h while other conditions were fixed. 2.4. Characterization. 2.4.1. XRD Analysis. Solid powder diffraction patterns were collected by using an X-ray diffractometer (X’Pert MPD Pro, PANalytical, The Netherlands). The XRD analysis was carried out in ambient atmosphere at room temperature. To determine the products obtained at different temperatures and the effects of alumina on the products, two experiments were carried out for the analysis. In the first experiment, a 17.6 g mixture of magnesia and ammonium chloride (the molar ratio of NH4Cl:MgO was 3:1 for the complete reaction) was charged into a 50 mL corundum crucible. Alumina with the thickness of 1.9 cm was put on top of the mixture, and then the lid was put on. The crucible was placed into a muffle furnace and maintained at 200, 300, 400, 500, and 700 °C for 0.5 h, respectively. The products at different temperatures were taken and stored in a vacuum drybox until XRD analysis was carried out. The operation of the second experiment was the same as the first one except that alumina was not used as the covering agent. 2.4.2. TG-DSC Analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were carried out by a simultaneous TGA/DSC apparatus (TGA/ DSC 1, Mettler-Toledo). Two samples were prepared for the analysis: the first sample was ammonium chloride, and a 20.22 mg sample was used for the thermal analysis; the second sample was the mixture of ammonium chloride and magnesia (the molar ratio of NH4Cl:MgO was 3:1), and a 8.15 mg mixture was used for the thermal analysis. The samples were respectively put in alumina crucibles and heated from room temperature to 750 °C with the nitrogen flow rate of 40 mL·min−1 and the heating rate of 10 °C/min.

3. RESULTS AND DISCUSSION 3.1. Single Factor Test Analysis. The effects of molar ratio of ammonium chloride to magnesia, thickness of alumina, reaction temperature, and reaction time on the purity of anhydrous magnesium chloride were investigated. To obtain well-sintered anhydrous magnesium chloride and avoid volatilization of anhydrous magnesium chloride at high temperature, 700 °C and 0.5 h were chosen as the calcination temperature and calcination time in the experiment. 3.1.1. Effect of Molar Ratio of NH4Cl/MgO. The effect of the molar ratio of ammonium chloride to magnesia on the purity of anhydrous magnesium chloride was investigated with the thickness of alumina being 1.3 cm, the reaction temperature 410 °C, and the reaction time 1.5 h (Figure 1). With increasing

Figure 1. Effect of molar ratio of NH4Cl to MgO on the purity of anhydrous magnesium chloride.

nNH4Cl/nMgO (nNH4Cl was the molar number of NH4Cl; nMgO was the molar number of MgO) from 2 to 6, the content of magnesia in anhydrous magnesium chloride decreased gradually. When the molar ratio was 5:1, the mass fraction of magnesia in anhydrous magnesium chloride was 0.026%, which was far below the industrial demand 0.5%. The result reveals that when nNH4Cl/nMgO exceeds a certain value, the reaction between ammonium chloride and magnesia can proceed thoroughly with high-purity anhydrous magnesium chloride as final product. 3.1.2. Effect of Thickness of Alumina. Alumina with average particle diameter of 6.8 μm and surface area of 299 m2/kg was chosen to be the covering agent in this experiment. It neither sinters nor reacts with anhydrous magnesium chloride under the experimental temperature. The effect of the thickness of alumina as the covering agent on the purity of anhydrous 9714

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when the reaction temperature exceeded 450 °C, the content of magnesia in anhydrous magnesium chloride increased sharply. When the reaction temperature is below 350 °C, it is impossible to make complete reaction between ammonium chloride and magnesia during the experimental time. However, when the reaction temperature exceeds 450 °C, ammonium chloride decomposes quickly without reacting with magnesia. Therefore, the optimum reaction temperature range for the experiment is 400−450 °C. 3.1.4. Effect of Reaction Time. The effect of reaction time on the purity of anhydrous magnesium chloride was investigated with nNH4Cl/nMgO being 5:1, the thickness of alumina 1.1 cm, and the reaction temperature 450 °C (Figure 4). With an increase in reaction time from 0.5 to 2.5 h, the

magnesium chloride was investigated with nNH4Cl/nMgO being 5:1, the reaction temperature 410 °C, and the reaction time 1.5 h (Figure 2). With increasing the thickness of alumina from 0

Figure 2. Effect of thickness of alumina on the purity of anhydrous magnesium chloride.

to 1.1 cm, the content of magnesia decreased until a minimum of 0.025% was obtained. Further increase in the thickness of alumina was followed by a slight increase in the content of magnesia. Alumina as covering agent can efficiently suppress the emission of ammonium chloride to make a complete reaction with magnesia. In addition, alumina can isolate the unsintered anhydrous magnesium chloride from an ambient atmosphere to avoid hydrolysis. However, too much alumina on top of the reactants will handicap the emission of water vapor, which results in high magnesia content in anhydrous magnesium chloride. 3.1.3. Effect of Reaction Temperature. The effect of reaction temperature on the purity of anhydrous magnesium chloride was investigated with nNH4Cl/nMgO being 5:1, the thickness of alumina 1.1 cm, and the reaction time 1.5 h (Figure 3). The content of magnesia was reduced gradually as the reaction temperature increased from 300 to 450 °C. However,

Figure 4. Effect of reaction time on the purity of anhydrous magnesium chloride.

content of magnesia in anhydrous magnesium chloride sharply decreased. The magnesia content remained constant with a further increase in reaction time from 1.5 to 2.5 h. Therefore, 1.5 h was selected for the reaction time in this experiment. In summary, high-purity anhydrous magnesium chloride was prepared by using ammonium chloride and magnesia as reactants and alumina as the covering agent. The magnesia content in anhydrous magnesium chloride could achieve 0.014%, which is far lower than the industrial demand of 0.5%. The optimum conditions for the preparation of anhydrous magnesium chloride can be concluded as follows: ammonium chloride and magnesia with the molar ratio of NH4Cl:MgO = 5:1 were mixed evenly. Alumina with the thickness of 1.1 cm was used as the covering agent. The mixture was maintained at 450 °C for 1.5 h for the complete reaction and then calcined at 700 °C for 0.5 h to obtain well-sintered anhydrous magnesium chloride. 3.2. XRD Analysis. The XRD spectra of the mixture of magnesia and ammonium chloride (the molar ratio of NH4Cl:MgO was 3:1) with alumina (the thickness of alumina was 1.9 cm) as covering agent were recorded at various temperatures (Figure 5). It can be observed that the products at 200 °C were still MgO and NH4Cl. However, when the temperature was increased to 300 °C, magnesia peaks disappeared and new diffraction peaks were detected which did not match with any standard data. To determine the new diffraction peaks, about 2 g of product was taken and stored in

Figure 3. Effect of reaction temperature on the purity of anhydrous magnesium chloride. 9715

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in Figure 7. Compared with Figure 5, anhydrous MgCl2 was observed at 400 °C, which indicated that alumina as covering

Figure 5. XRD patterns of the mixture with alumina as covering agent: (a) MgO; (b) NH4Cl; (c) NH4Cl·MgCl2·nH2O; (d) MgCl2·4H2O; (e) MgCl2. Figure 7. XRD patterns of the mixture without alumina as the covering agent: (a) MgO; (b) NH4Cl; (c) NH4Cl·MgCl2·nH2O; (d) MgCl2·4H2O; (e) MgCl2.

ambient atmosphere (the average ambient humidity was 65%) for 18 h and then the XRD analysis was carried out. The XRD spectra obtained matched well with the characteristic peaks of ammonium carnallite (NH4Cl·MgCl2·6H2O) (Figure 6), which

agent could efficiently suppress the emission of ammonium chloride. In the temperature range of 200−400 °C, the content of magnesia in the sample decreased with the increase of temperature. However, when the temperature was increased to over 400 °C, the content of magnesia in the sample increased with the increase of temperature. The result indicates that the unsintered anhydrous MgCl2 could easily react with water vapor in ambient atmosphere to form MgO. Therefore, alumina as covering agent can also isolate the unsintered anhydrous MgCl2 from ambient atmosphere to guarantee the formation of high-purity anhydrous MgCl2. 3.3. TG-DSC Analysis. The TG-DSC curves of the mixture of MgO and NH4Cl (the molar ratio of NH4Cl:MgO was 3:1) are shown in Figure 8. The comparison between the DSC curve of NH4Cl and the DSC curve of the mixture (MgO and NH4Cl) are shown in Figure 9. As can be seen from Figure 8, four distinctive endothermic peaks, each of them accompanied by a weight loss, were observed. The first endothermic peak (187.52−209.16 °C) on the DSC curve corresponded to the

Figure 6. XRD patterns of the product at 300 °C stored for 18 h in ambient atmosphere and the unknown matter.

implied that the new diffraction peaks obtained at 300 °C were ascribed to NH4Cl·MgCl2·nH2O (0 ≤ n < 6). When the temperature was increased to 400 °C, well-crystallized NH4Cl·MgCl2·nH2O could still be observed, which indicated that alumina as covering agent could efficiently suppress the emission of ammonium chloride. The products at 500 °C were MgCl2 and MgCl2·4H2O. The formation of MgCl2·4H2O was due to the strong hygroscopy of the unsintered anhydrous magnesium chloride obtained at 500 °C. The characteristic peaks of MgCl2·H2O and MgCl2·2H2O were not detected because their content was lower than 5 wt %. When the temperature was increased to 700 °C, high-purity anhydrous MgCl2 with good crystallinity was formed. To investigate the effects of alumina as covering agent on the products, the same operation, except that alumina was not used as the covering agent, was performed. XRD spectra were shown

Figure 8. TG-DSC curve of the mixture of MgO and NH4Cl. 9716

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product. Alumina as covering agent guarantees that high-purity anhydrous magnesium chloride can be obtained. The content of magnesia in anhydrous magnesium chloride was achieved 0.014% under the optimum conditions: MgO:NH4Cl molar ratio of 1:5, thickness of alumina of 1.1 cm, reaction temperature of 450 °C, reaction time of 1.5 h, calcination temperature of 700 °C, and calcination time of 0.5 h.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding author's address: No. 1 Bei-er-tiao, Zhongguan-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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Figure 9. DSC curves of NH4Cl and the mixture of NH4Cl and MgO.

REFERENCES

(1) Busk, R. S. Magnesium products design; Marcel Dekker: New York, 1987. (2) Ignat, S.; Sallamand, P.; Grevey, D.; Lambertin, M. Magnesium alloys (WE43 and ZE41) characterization for laser application. Appl. Surf. Sci. 2004, 233, 382. (3) Ruden, T. J.; Albright, D. L. High ductility magnesium alloys in automotive applications. Adv. Mater. Process 1999, 145, 28. (4) Eom, H. C.; Park, H.; Yoon, H. Preparation of anhydrous magnesium chloride from ammonium magnesium chloride hexahydrate. Adv. Powder Technol. 2010, 21, 125. (5) Froes, F. H.; Eliezer, D.; Aghion, E. The science, technology, and applications of magnesium. JOM 1998, 50, 30. (6) Brooks, G.; Trang, S.; Witt, P.; Khan, M.; Nagle, M. The carbothermic route to magnesium. JOM 2006, 58, 51. (7) Cathro, K. J.; Deutscher, R. L.; Sharma, R. A. Electrowinning magnesium from its 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) Zhou, N. B.; Chen, B. Z.; He, X. K.; Li, Y. B. Preparation and characteristic research of anhydrous magnesium chloride with dehydrated ammonium carnallite. J. Cent. South Univ. Technol. 2006, 13, 373. (10) Madorsky, S. L. Process for producing electrolytic magnesium. U.S. Patent 2165284, 1939. (11) Suzukawa, Y.; Kobayshi, W.; Uehori, K.; Ohtaka, S.; Yoshida, K. Process of manufacturing anhydrous magnesium chloride. U.S. Patent 3798314, 1974. (12) Strelets, Kh. L. Electrolytic production of magnesium; Israel Program for Scientific Translation: Israel, 1977. (13) Sharma, R. A. Method for producing magnesium metal from magnesium oxide. U.S. Patent 5279716, 1994. (14) Toomey, R. D. Removing oxide contaminants from magnesium chloride by chlorinating melt in the presence of carbon and iron. U.S. Patent 3953574, 1976. (15) Peacey, J. G. Production of anhydrous magnesium chloride. U.S. Patent 4981674, 1991. (16) Shackleton, C. E. E.; Shackleton, E. A. B.; Wickens, A. J.; Turner, J. H. W. Preparation of anhydrous magnesium chloride. U.S. Patent 4269816, 1981. (17) Ou, T. J. Leaching kinetics of calcined magnesite from glycol solution dissolved with ammonium chloride. J. Process. Eng. 2007, 7, 928. (18) Kashani-Nejad, S.; Ng, K. W.; Harris, R. MgOHCl thermal decomposition kinetics. Metall. Mater. Trans. B 2005, 36, 153. (19) 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. 1965, 60, 1095.

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phase transformation of NH4Cl. The second endothermic peak (251.99−315.38 °C) was ascribed to the formation of NH4Cl·MgCl2·nH2O (6 > n ≥ 0), which was consistent with the XRD analysis. The third endothermic peak (323.53−353.60 °C) was assigned to the decomposition of NH4Cl, which could be confirmed by Figure 9. The first three steps in Figure 8 (187.52−353.60 °C) were ascribed to the formation of anhydrous MgCl2. The measured weight loss of these steps was 68.41%, which was higher than the theoretical value 52.62% owing to the fact that ammonium chloride was emitted quickly without reacting with magnesia. Therefore, the existence of covering agent, which can suppress the emission of ammonium chloride, was necessary. The fourth endothermic peak (705− 713 °C) was attributed to the melting process of anhydrous MgCl2 at its melting point 712 °C. The weight loss of the last step (350−700 °C) should be attributed to the oxidation of anhydrous MgCl2 to MgO.20 Therefore, the existence of covering agent, which can isolate the unsintered anhydrous MgCl2 from ambient atmosphere, was necessary. 3.4. Reaction Mechanism. According to the above XRD and TG-DSC experimental results, the possible reaction mechanism involved in the process can be concluded. The reaction between MgO and NH4Cl occurs in the temperature range of 200−300 °C with NH4Cl·MgCl2·nH2O (6 > n ≥ 0) as product. The crystallized water is gradually released from NH4Cl·MgCl2·nH2O at elevated temperature. When temperature reaches 300−400 °C, NH4Cl is released in the form of HCl and NH3 from NH4Cl·MgCl2·nH2O with anhydrous MgCl2 as final product. Alumina as covering agent has two functions in the process: first, it can effectively suppress the emission of NH4Cl to make sure complete reaction between MgO and NH4Cl occurred. Second, it can isolate the unsintered anhydrous magnesium chloride from the atmosphere to avoid hydrolysis.

4. CONCLUSIONS In summary, we prepared high-purity anhydrous magnesium chloride using magnesia and ammonium chloride as reactants, alumina as covering agent. Magnesia reacts with ammonium chloride at 200−300 °C with NH4Cl·MgCl2·nH2O (6 > n ≥ 0) as product. Crystallized water and ammonium chloride are gradually released from NH4Cl·MgCl2·nH2O at elevated temperature with anhydrous magnesium chloride as the final 9717

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(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, 412, 149.

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