A novel pathway for the preparation of Mg metal from magnesia

Zhimin Zhang,Xuchen Lu,Yan Yan

State Key Laboratory of Multi-phase Complex systems,Institute of Process Engineering,Chinese Academy of Science,Haidian District,Beijing 100190,PR China

Abstract High-purity anhydrous magnesium chloride was prepared from magnesia and ammonium chloride.The chlorination process was analyzed and then the critical stages affecting the purity of anhydrous magnesium chloride were pinpointed.The effect of sample dimension on the above critical stages was investigated respectively.The purity guarantee mechanism of anhydrous magnesium chloride was proposed.After that,magnesium metal was obtained via electrolyzing the anhydrous magnesium chloride-containing molten salt.The new process for the continuous production of magnesium metal from magnesia was proposed and discussed.The incomplete chlorination reaction and the hydrolysis of anhydrous magnesium chloride are the two critical stages affecting the purity of the anhydrous magnesium chloride.The dimension of the sample can influence reaction process and reaction mechanism,and thus the problems of incomplete chlorination reaction and hydrolysis can be solved together.The magnesia content in anhydrous magnesium chloride was below 0.1wt.% when the ratio of height to diameter of the sample was over 2.43.The content of impurities in the magnesium metal obtained met the specifications of the product Mg9980.The current efficiency was(94.7±1.8)% and the electricity consumption was(9107±97)kWh/t.

Keywords:Magnesium;Electrolysis;Anhydrous magnesium chloride;Hydrolysis.?Corresponding author.

1.Introduction

Magnesium metal and its alloys have many special properties,such as low density,high specific strength,remarkable anti-electromagnetic interference capability and extraordinary damping ability[1,2].Recently,the research and development of magnesium metal and its alloys has been greatly promoted by the lightweight demand in the automobile industry[3–5].Automobile manufacturers worldwide plan to increase the magnesium content of automobile from 5 to 6kg to 45–160kg[6].From the perspective of magnesium resources,magnesium is the eighth most common element in the earth’s crust and it is also extractable from brine and seawater.Because of the substantial magnesium resources and the attractive magnesium products,magnesium metal is regarded as a potentially ideal substitute for aluminum metalbut the global annual production of magnesium metal is only 1.8% that of aluminum metal[7].Therefore,the commercial potential for the production of magnesium metal is enormous.At present,magnesium metal is industrially manufactured using the thermal reduction method(i.e.the Pidgeon process)and the electrochemical reduction method[8].The thermal reduction method is based on the chemical reduction between calcined dolomite(CaO·MgO)and ferrosilicon(Si-Fe)at high temperature(1100–1250°C)and high vacuum(1.33–13.3Pa)[9].Compared with the electrochemical reduction method,the thermal reduction method has the drawbacks of severe environmental pollution(37–47kg CO2eq/kg Mg ignot)and high energy consumption(354.5MJeq/kg Mg ignot)[10-12].Nevertheless,more than 80% of the global magnesium metal is produced in China every year via the thermal reduction method because of its relatively low production costs[13].The production costs of high-purity anhydrous magnesium chloride account for almost 50% of the overall costs of electrolytic magnesium production,which greatly hinders the industrial development of the electrochemical reduction method[14,15].The main impurities in anhydrousmagnesium chloride are magnesia(MgO)and magnesium hydroxychloride(MgOHCl),and usually they are formed via the hydrolysis reaction of dehydrated magnesium chloride(MgCl2·nH2O,6＞n≥0)at elevated temperature.Those impurities seriously disturb the following electrolysis process and significantly decrease current efficiency.Therefore,the content of magnesia in anhydrous magnesium chloride(i.e.w(MgO)/w(MgCl2))should be lower than 0.5wt.%and 0.1wt.% when common electrolytic cell and the most advanced multipolar cell are being used respectively[16].

To utilize the large magnesium resources reserved in seawater and brine,some researchers attempted to prepare highpurity anhydrous magnesium chloride from magnesium chloride hexahydrate(MgCl2·6H2O)[17–21].The emphasis of the above research was placed on the inhibition of the hydrolysis reaction occurred in dehydration process.Meanwhile,to avoid the nerve-wrecking hydrolysis problem of dehydrated magnesium chloride,some researchers shifted the emphasis away and magnesia(or magnesite)was used for the preparation of high-purity anhydrous magnesium chloride.Shackleton et al.[22,23]prepared magnesia by calcining magnesite at 700–800°C and then the magnesia obtained was chloridized at approximately 1100°C using chlorine gas as chlorinating agent and carbon(or carbon monoxide)as reducing agent.After that,liquid anhydrous magnesium chloride was obtained(the melting point of anhydrous magnesium chloride is 714°C).The low utilization ratio of chlorine gas and the great hydrolytic tendency of liquid anhydrous magnesium chloride are serious drawbacks.Ou et al.[24–26]chloridized magnesia(obtained by calcining magnesite)at 130–150°C in glycol solvent by using ammonium chloride as chlorinating agent.The water produced in the chlorination process was removed by distillation and then solid-state magnesium chloride hexammoniate(MgCl2·6NH3)was obtained by introducing ammonia gas into the organic solvent.After that,anhydrous magnesium chloride was obtained by the thermal decomposition of magnesium chloride hexammoniate.However,much glycol solvent is adhered to the solid-state magnesium chloride hexammoniate and thus a great deal of anhydrous organic solvent is needed to wash off the attached glycol solvent.Sharma et al.[27–30]found that magnesia could react with neodymium chloride(NdCl3)in NdCl3-containing molten salt with anhydrous magnesium chloride and neodymium oxychloride(NdOCl)as products.However,magnesium-neodymium alloy instead of magnesium metal is produced and high-purity graphite anode is consumed in the electrolytic process,which severely limit the large-scale application of this method.The advantages and disadvantages of the traditional preparation methods of anhydrous magnesium chloride are shown in Table 1.

Table 1The advantages and disadvantages of the traditional preparation methods of anhydrous magnesium chloride.

Table 2Chemical analysis and phase structure of the products obtained at different temperatures.

Inspired by Shackleton’s and Ou’s methods,our research group has prepared high-purity anhydrous magnesium chloride from magnesia using solid-state ammonium chloride as chlorinating agent,and thus the disadvantages caused by gas chlorinating agent and organic solvent were overcome.Our method for the preparation of magnesium metal from magnesia(or magnesite)and its development are shown in Fig.1.We found that magnesia and ammonium chloride reacted at 200–300°C with the complex salt NH4Cl·MgCl2·nH2O(6＞n≥0)as product.Anhydrous magnesium chloride(w(MgO)/w(MgCl2)=0.014wt.%)was obtained via the thermal decomposition of NH4Cl·MgCl2·nH2O by using alumina powder as a covering agent[31].The covering agent used in the experiment could effectively isolate the unsintered anhydrous magnesium chloride from the oxygen-containing gas,so the hydrolysis was effectively inhibited.However,the covering agent can be easily brought into the anhydrous magnesium chloride below,which causes great inconvenience in the electrolytic magnesium process.To avoid the aforementioned problem,high-purity K3NaMgCl6(w(MgO)/w(MgCl2)=0.02wt.%)instead of anhydrous magnesium chloride was directly synthesized and then magnesium metal was obtained by electrolyzing K3NaMgCl6[32].Compared with anhydrous magnesium chloride,K3NaMgCl6had a lower tendency to hydrolyze at elevated temperature and thus covering agent was not used.However,it is difficult to achieve continuous electrolytic magnesium production because of the accumulation of potassium chloride and sodium chloride in electrolyzer.

In this study,the chlorination process of magnesia using solid-state ammonium chloride as chlorinating agent was investigated,and then the decisive factors affecting the purity of anhydrous magnesium chloride were pinpointed.By investigating the effect of sample dimension on the above decisive factors,the strategy for the preparation of high-purity anhydrous magnesium chloride was proposed and discussed.After that,magnesium metal was obtained by electrolyzing the anhydrous magnesium chloride-containing molten salt.Then a new process for the continuous production of magnesium metal from magnesia was proposed and its advantages for large-scale application were discussed.

2.Experimental

2.1.Raw materials

The main chemicals used in this research,including magnesia(mean particle size 26μm,purity≥98.0wt.%,1.5wt.%carbonate),anhydrous magnesium chloride(mean particle size 73μm,purity≥99.0wt.%,w(MgOHCl)=0.12wt.%),ammonium chloride(mean particle size 84μm,purity≥99.5wt.%),potassium chloride(purity≥99.5wt.%),sodium chloride(purity≥99.5wt.%)and anhydrous calcium chloride(purity≥96.0wt.%),were all analytical grade and obtained from Xilong Chem.Co.Ltd.(China).

2.2.Analysis of chlorination process

To pinpoint the critical stages affecting the purity of anhydrous magnesium chloride,the chlorination process of magnesia was investigated using ammonium chloride as a chlorinating agent.To guarantee the complete reaction,magnesia and ammonium chloride with the molar ratio ofn(NH4Cl):n(MgO)being 3:1 were well mixed and then the mixture was equallycharged into six corundum crucibles(φ35mm×50mm)with the same sample height of 38mm.The crucibles were placed in a muffle furnace and then the furnace was heated up continuously from room temperature to 700°C(the heating rate was 5°C/min)with holding at 200°C,300°C,400°C,500°C,600°C and 700°C for 1.0 h,respectively.The products obtained at different temperatures were taken out and sealed in vacuum bags before further analysis was carried out.The relative humidity of the ambient atmosphere ranged from 87%to 94% in the heating process.

Fig.1.Our method for the preparation of Mg metal from magnesia(magnesite)and its development.

2.3.Preparation of high-purity anhydrous MgCl2

In this section,samples of the same mass were placed in corundum crucibles with different inner diameters respectively,and thus the samples with different ratios of height to diameter(i.e.H/D,similarly hereinafter)were acquired.The relative humidity of the ambient air ranged from 54% to 68%in all the experiments.After the experiments,the products obtained were stored in vacuum bags before further analysis was conducted.

Firstly,to investigateH/Dof the sample on the chlorination process,two experiments were carried out.In the first experiment,the mixture of magnesia and ammonium chloride with the molar ratio ofn(NH4Cl):n(MgO)being 3:1 was used.The samples with differentH/Dwere kept at 600°C for different time.In the second experiment,the mixture of magnesia and ammonium chloride with the molar ratio ofn(NH4Cl):n(MgO)being 4:1 was used,magnesia of the same mass with the thickness ranged from of 3mm to 10mm was put on top of the mixture(n(NH4Cl)/n(MgO)total=3:1).The schematic diagram of the sample is shown in Fig.2.The samples were placed in a muffle furnace and kept at 700°C for 1h and then the surface of the sample was analyzed.

Fig.2.The schematic diagram of the sample used in the experiment.

Secondly,to investigateH/Dof the sample on the hydrolysis process,two experiments were carried out.In the first experiment,anhydrous magnesium chloride with the height of 45mm was charged in a corundum crucible(φ35mm×50mm)and then it was maintained at 700°C for 1.0 h.In the second experiment,samples of anhydrous magnesium chloride with differentH/Dwere heated up from room temperature to 700°C with the heating rate of 5°C/min and then maintained at that temperature for 1.0 h.

Thirdly,to investigate the effect ofH/Dof the sample on the purity of the obtained anhydrous magnesium chloride,the samples with differentH/Dwere heated up from room temperature to 700°C with the heating rate of 5°C/min and then maintained at that temperature for 1.0 hour to ensure the complete removal of ammonium chloride.

2.4.Electrolysis of anhydrous mgcl2-containing molten salt

The anhydrous magnesium chloride obtained(w(MgO)/w(MgCl2)=0.06wt.%)was used to prepare the anhydrous magnesium chloride-containing molten salt.Magnesium metal was prepared by galanostatic electrolysis using quaternary molten salt MgCl2(18wt.%)-NaCl(38wt.%)-KCl(29wt.%)-CaCl2(15wt.%)as electrolyte[33].Graphite crucible(φ65mm×140mm)was used as electrolytic cell.Molybdenum rod(d=6mm)and spectrally pure graphite rod(d=6mm)were used as cathode and anode respectively.The experiment was carried out under an argon atmosphere and the electrolytic conditions were as follows:electrolytic temperature 700°C,electrolytic time 4 h,interelectrode distance 4cm,and cathode current density 0.42Acm-2[16,23].Liquid magnesium drop with spherical shape was formed and gathered around the cathode.As the electrolysis process went on,spherical liquid magnesium gradually grew bigger and floated on the surface of the molten salt.

2.5.Characterization

The phase structure of the products was analyzed by using an X-ray diffractometer(X’Pert MPD Pro,PANalytical,The Netherlands)with Cu-Kαradiation(λ=1.5408?A)at 40kV and 40mA.The content of magnesium ion and the content of chloride ion in the products were determined by titration,and then the content of ammonium chloride was deduced based on charge balance.The content of magnesia in the products was determined by ethylene diamine tetraacetic acide(EDTA)titration[31].The detailed process for determining current efficiency and electricity consumption can be found elsewhere[34].According to the government standard GB/T 13748.20-2009,the content of Fe,Si,Ni,Cu,Al and Mn in the magnesium metal obtained was measured by using an inductively coupled plasma optical emission spectrometer(ICP-OES,iCAP 6300,Thermo Scientific).According to the government standard GB/T 13748.17-2005,the content of K and Na in the magnesium metal obtained was measured by using an atomic absorption spectroscopy(AAS,ice3000,Thermo Fisher).

3.Results and discussion

3.1.Analysis of chlorination process

Fig.3.The content variations of MgO and NH4Cl with heating temperature.

To pinpoint the critical stages affecting the purity of anhydrous magnesium chloride,the chlorination process of the mixture of magnesia and ammonium chloride was investigated.The chemical analysis and the phase structure of the products obtained at different temperatures are shown in Table 2.According to the previous study[31,35],the reaction mechanism of magnesia and ammonium chloride could be concluded as follows:The solid-solid reaction between magnesia and ammonium chloride occurred at 200–300°C with the complex salt NH4Cl·MgCl2·nH2O(6＞n≥0)as product,which explained the decrease of magnesia content at this period.With increasing the heating temperature to 400°C,NH4Cl·MgCl2·nH2O gradually decomposed and then anhydrous magnesium chloride was obtained.The molar ratio of Mg2+to Cl-in the product obtained at 400°C was close to 1:2,which proved the thermal decomposition of the complex salt and the formation of anhydrous magnesium chloride.The content variations of magnesia and ammonium chloride in the chlorination products with reaction temperature are shown in Fig.3.According to the trend of the broken lines,the chlorination process could be divided into two parts:room temperature-400°C(the first part)and 400–700°C(the second part).In the first part,the content of magnesia and the content of ammonium chloride both decreased sharply,which was due to their solid-solid reaction and the thermal decomposition of ammonium chloride.In the second part,the content of magnesia increased while the content of ammonium chloride decreased,which was ascribed to the hydrolysis of the newly-formed anhydrous magnesium chloride in the absence of ammonium chloride[36,37].Based on the above analysis,when preparing anhydrous magnesium chloride from magnesia,the incomplete chlorination reaction and the hydrolysisoccurring at elevated temperature are the two main reasons affecting the purity of the anhydrous magnesium chloride obtained.

Table 3The content of impurities of the magnesium metal obtained,wt.%.

Fig.4.The effect of H/D of the mixture of magnesia and ammonium chloride on the decomposition and emission of ammonium chloride(a),XRD patterns of the upper magnesia(b),and chemical analysis of the upper magnesia(c).

3.2.Preparation of high-purity anhydrous magnesium chloride

The complete chlorination reaction(200–400°C)and the suppression of the hydrolysis reaction(400–700°C)were proved to be crucial when preparing high-purity anhydrous magnesium chloride from magnesia.In the previous study,we found that the dimension of the sample could influence mass transfer and reaction process when preparing anhydrous magnesium chloride from magnesium chloride hexahydrate[36].To verify if the above result could be applied in this work,the effect of the ratio of height to diameter of the sample(i.e.H/D)on chlorination process and hydrolysis process were investigated respectively.

3.2.1.Effect ofH/Don chlorination process

The effect ofH/Dof the mixture of magnesia and ammonium chloride on the decomposition and emission of ammonium chloride was investigated(Fig.4(a)).With the increase ofH/Dof the mixture,the content of the remaining ammonium chloride in the product increased in the heating process,which indicated thatH/Dof the mixture could be used to control the decomposition and emission rate of ammonium chloride.By decreasing the decomposition and emission rate of ammonium chloride,the contact time between magnesia and ammonium chloride increases,which is beneficial to the complete chlorination reaction.

Hydrogen chloride gas and ammonia gas were produced by the thermal decomposition of ammonium chloride at 337.8°C[38].The mixture of magnesia and ammonium chloride formed a fixed-bed structure in crucible with hydrogen chloride gas and ammonia gas flowing through at elevated temperature.According to Kozeny equation(Eq.(1)),the volume flow rate of gas flowing through fixed-bed reactor can be expressed as Eq.(2)[39]:

wherea,ε,△pf,μandqvare specific surface area of particles,fixed-bed voidage,fixed-bed pressure drop,gas viscosity and volume flow rate of gas respectively.DandL(i.e.H)donate diameter and height of the sample respectively.In this experiment,with the increase ofH/Dof the mixture of magnesia and ammonium chloride,the volume flow rates of hydrogen chloride gas and ammonia gas decreased,and thus the decomposition and emission rate of ammonium chloride was slower.

The effect ofH/Dof the mixture of magnesia and ammonium chloride on the chlorination product of the uppermagnesia was investigated(Fig.4(b)and 4(c)).With the increase ofH/Dof the mixture below from 0.23 to 3.59,the intensity of the characteristic peaks of anhydrous magnesium chloride increased significantly.The chemical analysis of the upper products showed that the content of anhydrous magnesium chloride increased from 3.46wt.% to 96.11wt.% when increasingH/Dof the mixture below from 0.23 to 3.59.The above results illustrated the following three facts:(1).Magnesia could react with hydrogen chloride gas at elevated temperature with anhydrous magnesium chloride as product,which agreed well with Lamy’s work[40,41].(2).Two chlorination reaction mechanisms were involved in the heating process of the mixture of magnesia and ammonium chloride,including solid-solid reaction(between magnesia and ammonium chloride)and gas-solid reaction(between hydrogen chloride gas and magnesia).(3).By increasingH/Dof the mixture of magnesia and ammonium chloride,the chlorination mechanism at work changed from only solid-solid reaction to solid-solid reaction and gas-solid reaction.When only the solid-solid reaction is involved in the heating process,the not well-mixed reactants can easily lead to the incomplete chlorination reaction.However,this problem can be effectively solved by adopting two chlorination mechanisms-the solid-solid reaction and the gas-solid reaction.

Fig.5.The effect of H/D of anhydrous magnesium chloride on XRD patterns of hydrolysate(a)and content of hydrolysate(b).

3.2.2.Effect ofH/Don hydrolysis process

The sample of anhydrous magnesium chloride was maintained at 700°C for 1.0 h,and then the upper part,the middle part and the bottom part of the sample were characterized(Fig.5(a)).The intensity of the characteristic peaks of magnesia decreased with the increase of sample height.The above results proved the following facts:(1).The hydrolysis of anhydrous magnesium chloride occured on the surface of the sample and thus the content of hydrolysate could be directly controlled via decreasing the diameter of the sampleD(assuming the sample is cylindrical).(2).Magnesia formed in the hydrolysis process could be served as a covering agent and thus it could effectively protect the anhydrous magnesium chloride below from further hydrolyzing.Therefore,the hydrolysis of anhydrous magnesium chloride could be effectively inhibited by increasing the height of the sampleH.By coupling the above results,it can be concluded that the hydrolysis of anhydrous magnesium chloride is inhibited significantly by increasingH/Dof the sample.

Fig.6.The effect of the H/D of the mixture of magnesia and ammonium chloride on the content of magnesia in anhydrous magnesium chloride.

To further confirm the above result,the effect ofH/Dof anhydrous magnesium chloride on the content of magnesia(i.e.w(MgO)/w(MgCl2))was studied(Fig.5(b)).When increasingH/Dof the sample from 0.086 to 2.86,the content of magnesia in anhydrous magnesium chloride decreased sharply from 18.21wt.% to 0.08wt.%.The above result indicated that the hydrolysis of anhydrous magnesium chloride at elevated temperature could be effectively inhibited by increasingH/Dof the sample.It can be easily deduced that the reaction atmosphere also has a marked effect on inhibiting the hydrolysis reaction when using magnesia and ammonium chloride as raw materials.Ammonia gas,hydrogen chloride gas and ammonium chloride gas produced in the heating process effectively dilute the partial pressure of oxygen and water vapor,which inhibits the hydrolysis reaction.The increase inH/Dreduces the emission rate and the decomposition rate of ammonium chloride and thus the protection of the chlorination product is more lasting.

3.2.3.Effect ofH/Don the content of magnesia

As can be seen from the above results,the dimension of the sample influences mass transfer of reactants,chlorination reaction process and hydrolysis reaction process.By increasingH/Dof the mixture of magnesia and ammonium chloride,the complete chlorination reaction(200–400°C)can be achieved and the hydrolysis of anhydrous magnesium chloride obtained(400–700°C)can be inhibited.

To verify the above conclusion in the whole heating process,the effect ofH/Dof the mixture of magnesia and ammonium chloride on the content of magnesia was investigated(Fig.6).The content of magnesia in anhydrous magnesium chloride decreased sharply from 6.74wt.% to 0.06wt.% when increasingH/Dof the sample from 0.18 to 4.75.The above result also implied that the content of magnesia in anhydrous magnesium chloride was lower than 0.1wt.% when increasingH/Dof the mixture to above 2.43.Therefore,the anhydrous magnesium chloride obtainedcould be used as the raw material for multipolar cell and then the electrolytic energy consumption would decrease dramatically[42,43].The anhydrous magnesium chloride obtained(w(MgO)/w(MgCl2)=0.06wt.%)was pure white with the mean particle size of 0.24mm(Fig.7(a)).The characteristic peaks of the anhydrous magnesium chloride obtained agreed well with the standard ones(Fig.7(b)).

Fig.7.The actual picture(a)and the XRD patterns(b)of the anhydrous magnesium chloride obtained.

Fig.8.The purity guarantee mechanism involved in the heating process.

3.2.4.Purity guarantee mechanism

According to the above analysis,the purity guarantee mechanism of anhydrous magnesium chloride is concluded(Fig.8).The critical stages affecting the purity of anhydrous magnesium chloride are the chlorination process occurred at 200–400°C and the hydrolysis process occurred at 400–700°C.When increasingH/Dof the mixture of magnesia and ammonium chloride,there are two beneficial effects for the chlorination process:(1).The decomposition and emission rate of ammonium chloride is slower.Therefore,the reaction time between magnesia and ammonium chloride is prolonged and also the protection time of anhydrous magnesium chloride is prolonged.(2).The reaction mechanism changes from only solid-solid reaction to solid-solid reaction and gas-solid reaction,which solves the incomplete reaction issue brought by the unevenly mixed raw materials.In addition,by increasingH/Dof the mixture,the diffusion resistance of the oxygen-containing gas increases and the reaction area decreases,which effectively inhibit the hydrolysis reaction.Therefore,by controlling the diffusion of reactants and the reaction process,high-purity anhydrous magnesium chloride is obtained.

3.3.Preparation of magnesium metal

The actual picture and the XRD patterns of the magnesium metal obtained are shown in Fig.9.Because of the protection of molten salt,the magnesium metal obtained possessed a gleaming metallic color.The XRD patterns of the magnesium metal obtained were in consistent with the standard ones.The current efficiency was(94.7±1.8)% and the electricity consumption was(9107±97)kWh/t.Compared with the electrolytic parameters when using the multipolar cell(current efficiency being 85% and electricity consumption being 10,000kWh/t[44]),the current efficiency was higher and electricity consumption was lower.The content of impurities of the magnesium metal obtained is shown in Table.3.According to the government standard(GB/T 3499-2011),thecontent of impurities met the specifications of the product Mg9980.

Fig.9.The actual picture(a)and the XRD patterns(b)of the magnesium metal obtained.

Fig.10.Flow chart of the NCPE process for magnesium production(a)and the schematic diagram of the integrated equipment for both chlorination and electrolysis(b).

3.4.New process design

The NH4Cl assisted chlorination,protection and electrolysis process(i.e.the NCPE process)can be used for continuous magnesium production and the flow chart of this process is shown in Fig.10(a).Magnesia(or calcined magnesite)and ammonium chloride with the molar ratio ofn(NH4Cl):n(MgO)≥3[31]are mixed and used as raw materials.The mixture with a certainH/Dreacts at 200–300°C with the complex salt NH4Cl·MgCl2·nH2O(6＞n≥0)as product,and the ammonia gas produced is absorbed.The complex salt completely decomposes at above 400°C and then anhydrous magnesium chloride is obtained.The excess ammonium chloride is recycled and reused.Anhydrous magnesium chloride obtained is used for the electrolytic magnesium production.The fixed bed reactor for the preparation of anhydrous magnesium chloride should be placed on top of electrolyzer to form the integrated equipment for both chlorination and electrolysis,and then the continuous production of magnesium metal can be achieved(Fig.10(b)).To continuously protect the reaction product,the continuous production of anhydrous magnesium chloride and magnesium metal is crucial.

The NCPE process proposed is suitable for large-scale application and the reasons are as follows:(1).H/Dof the fixed bed reactor used in large-scale production can be very high and thus high-purity anhydrous magnesium chloride can be easily obtained.(2).Ammonia gas and hydrogen chloride gas are continuously produced and thus the chlorination product is continuously protected.However,ammonium chloride released at high temperature deposits and accumulates in the low temperature section of the pipes.Therefore,the pipes can easily be blocked in the preparation process of anhydrous magnesium chloride and the feasible solution needs to be further investigated.

4.Conclusions

(1)when preparing anhydrous magnesium chloride from magnesia,the incomplete chlorination reaction(200–400°C)and the hydrolysis of anhydrous magnesium chloride(400–700°C)are the two main reasons affecting the purity of the anhydrous magnesium chloride.

(2)H/Dof the mixture of magnesia and ammonium chloride influences diffusion of reactants and reaction pro-cess,and thus the complete chlorination reaction is achieved and the hydrolysis reaction is effectively inhibited.The content of magnesia in anhydrous magnesium chloride obtained was below 0.1wt.% when theH/Dof the mixture was over 2.43,and the content of magnesia further deceased with further increasingH/Dof the mixture.

(3)Magnesium metal was prepared via electrochemical method using the high-purity anhydrous magnesium chloride obtained as raw material.The current efficiency was(94.7±1.8)% and the electricity consumption was(9107±97)kWh/t.The content of impurities in the magnesium metal obtained met the requirements of the product Mg9980.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This project was supported by the National Natural Science Foundation of China(51501178),and Autonomous Research Fund of State Key Laboratory of Multiphase Complex Systems(MPCS-2019-A-10).