Comparative evaluation of energy and resource consumption for vacuum carbothermal reduction and Pidgeon process used in magnesium production

Yang Tian ,Lipeng Wang ,Bin Yang ,Yongnian Dai ,Baoqiang Xu ,Fei Wang ,Neng Xiong

a State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization,Kunming University of Science and Technology,Kunming,PR China

b National Engineering Laboratory for Vacuum Metallurgy,Kunming University of Science and Technology,Kunming,PR China

c Key Laboratory of Vacuum Metallurgy for Nonferrous Metal of Yunnan Province,Kunming University of Science and Technology,Kunming,PR China

d School of Metallurgy and Energy Engineering,Kunming University of Science and Technology,Kunming,PR China

Abstract With the fast development of the application of magnesium based alloys,the demand for primary magnesium is increasing dramatically all over the world.The Pidgeon process is the most widely used process for producing magnesium in China,but suffers from problems such as high energy,resource consumption and environmental pollution.While the process of vacuum carbothermal reduction to produce magnesium (VCTRM) has attracted more and more attention as its advantages,but it has not been well-practiced in industrial applications and there also is no comprehensive and quantitative analysis of this process.This study quantifie the fl ws of resource and energy for the Pidgeon process and the VCTRM process,then compared and analyzed these two processes with each other from three aspects.The VCTRM process results in 63.14% and 69.16% lower of non-renewable mineral resources and energy consumptions when compared to the Pidgeon process,respectively.Moreover,the low energy consumption (2.675 tce vs.8.681 tce) and material to magnesium ratio (2.953:1vs.6.429:1)of the VCTRM process,which lead to lower greenhouse gas (GHG) emissions (8.777 t vs.26.337 t) and solid waste generation(0.522 t vs.5.465 t) with a decrease of 66.67% and 90.45%,respectively.Results indicate that the VCTRM process is a more environmentally friendly process for magnesium production with high efficien y but low cost and low pollution,and it shows a good potential to be industrialized in the future after solving the bottleneck problem of the reverse reaction.

Keywords: Magnesium production;Vacuum carbothermal reduction process;Pidgeon process;Energy and resource consumption;Greenhouse gas emissions.

1.Introduction

Magnesium is one of the lightest metals used in structural alloys.Magnesium alloys are widely used in the fiel of aerospace,transportation and electronics.Especially,they have been used as lightweight materials in the vehicle industry to decrease weight of vehicles.This would reduce the emission of greenhouse gas (GHG) from vehicles,and result in contributing to an alleviation of the global warming issue[1,2].There has been a dramatic increase in applications and demand for magnesium based alloys all over the world.In recent years,the global primary magnesium demand is experiencing continued growth at a high rate [3].

At present,the world’s commercial methods of magnesium production are electrolysis and thermal process which is also known as the Pidgeon process [4].The Pidgeon process underwent a boom during the past three decades.Particularly in China which is the largest producer of magnesium in the world,over 80% of the global primary production of this metal in amount of 930,000 tonnes has been achieved in 2017[5].However,magnesium production is energy-intensive in China,resulting in a large amount of GHG emission,which is fi e times higher than steel production.This situation seriously restricts the development of the magnesium industry and may weaken the contribution to alleviate global warming by using Mg based alloys as lightweight parts in automobiles[6].

The magnesium industry of China has been making efforts in energy conservation and emission reduction along with the increase of magnesium production.Those efforts achieved remarkable results.The reports of annual magnesium industry show that the processes have been subject to considerable technological and equipment’s improvements during the past years,mainly including the development and the widespread application of the energy-saving equipment (e.g.new energysaving rotary kiln,regenerative reduction furnace,vertical reduction furnace,preheater,etc.) and the novel regenerative combustion technology with the benefi of high energy utilization efficien y [7-8].Consequently,the energy consumption for per tonne magnesium production in China has decreased from 11 to 12 tonnes of coal equivalent (tce) in 2000 to 8-9.5 tce in 2005 [9].This value was further reduced to 4.8-5.2 tce in 2010[10].From 2012 to 2015,the energy consumption has been even slightly declined and maintained at a level of 4.3-4.7 tce [11,12],and again dropped to around 4 tce by 2018[13].The large-scale and clustering of Chinese magnesium production enterprises and the research of new processes,such as the two-step and even one-step magnesium smelting process [14],will promote the magnesium production industry to become more energy-efficien and cleaner,realizing the target of a reduction of its average energy consumption to 3.5 tce by 2020,as reported inthe 13th Five-Year Plan (2016-2020)of the non-ferrous metal industry[3].

In order to further achieve a cleaner production of the magnesium industry,a better production magnesium process in line with future environmental protection requirements is necessary and becomes more and more urgent.Compared with the other magnesium production processes,the process of vacuum carbothermal reduction to produce magnesium (VCTRM process) attracts more attention recently,because of its advantages of high efficien y,low cost,low energy and resource consumption,and eco-friendly.The VCTRM process is considered as an ideal clean production process for magnesium,although it’s also facing some challenges.Among them,as reported by Geoffrey Brooks et al.[15] that the contamination of metallic magnesium by MgO via the reversion reaction of the magnesium vapor with CO,the biggest challenge faced is the reverse reaction in the condensation process.How to control and reduce the reverse reaction has become the key problem but bottleneck to achieve industrialization in the future.To solve this critical problem,Australia’s Commonwealth Scientifi and Industrial Research organisation(CSIRO)[16-17]has developed the MagsonicTMcarbothermal technology based on carbothermal reduction technology with supersonic quenching,and has obtained magnesium powder by this technology.However,this technology has not yet been commercialized.Kunming University of Science and Technology (KUST) has conducted systematic researches on the VCTRM process for nearly 20 years.These researches cover the thermodynamics of the carbothermal reduction reaction[18],the catalytic mechanism of CaF2[19],the laws of reverse reaction and the magnesium vapor condensation [20].Well-crystallized bulk magnesium has been obtained from the internal heat vacuum resistance furnace by controlling condensation process parameters,which avoided the phenomenon that condensed magnesium is prone to deflagratio or explosion [21-24].

However,before exerting any effort for the industrialization of the VCTRM process,it’s necessary to quantify an estimation on the consumption of material resources,energy fl w and environmental implications of the VCTRM process,because such data of the VCTRM process was seldom reported in the past.It would offer a good basis for the industrialization of the VCTRM process in the future and play a positive role in the promotion of a cleaner production of magnesium.In this study,the resource and energy consumption in the VCTRM process were firstl calculated and quantifie based on the latest data of the Pidgeon process,and then presented a comparison with the Pidgeon process in three aspects(i.e.the consumption of resources and comprehensive energy,waste generation) and a key parameter.These results provide a good benchmark for future improvement of these two processes and would hopefully promote the implementation of cleaner techniques for magnesium production.

2.Method

The Pidgeon process,was invented by Lloyd Montgomery Pidgeon in the 1940s,is currently the main commercial process for magnesium production.Fig.1 shows a schematic of this process fl w.Dolomite (CaCO3·MgCO3) as a raw material is calcined to form dolime (CaO·MgO) in rotary-kiln at 1323-1423K.Ferrosilicon (contains 75% silicon and 25%iron) as a reductant is produced by carbothermic reaction of quartzite and coke in arc furnace at 1873K.Dolime,ferrosilicon and fluorit (CaF2) are mixed and briquetted to form small bricks,and later fed into Ni-Cr stainless steel reactor.At the condition of 1433-1473K and 1-13.3Pa,a magnesium vapor is developed by the reaction (1).When the temperature is decreased to approx.723K,the magnesium vapor would then condense inside the condensers as crown magnesium.

Fig.1.Schematic representation of the working stages of two different technological processes of magnesium production (numbers show process stages).

The VCTRM process is based on carbothermal reduction that at temperatures higher than 1273K under vacuum condition.With the carbothermal reduction reaction,carbon can reduce different kinds of metal oxides (ZnO,MgO,SnO2and etc.) to the corresponding metals either in a liquid or a gaseous phase and by-product CO [25-27].In the VCTRM process,coking coke is preferred as a carbonaceous reductant in possible fi ed carbons [28,29].And it would react with the MgO via a solid-solid reaction type,and correspondingly produce metallic magnesium vapor and CO in an internal heat vacuum resistance furnace(like the Bolzano Process,have higher thermal efficien y than reduction furnace of the Pidgeon process.For detailed equipment information,see references [21-23]) at 1673K and 30-100Pa.Subsequently,magnesium vapor condenses at the lower temperature zone nearby the water-cooled condenser.As the reaction Eq.(2) in this process can’t take place under these technological conditions,with △GT ＞0 [23],the reduction process is postulated to be expressed by the reaction Eq.(3) [15]:

The reverse reaction of reaction Eq.(3) occurs during condensation resulting in a loose structure of the condensate.But the study found that under the proper condensation temperature and temperature gradient,increasing the magnesium vapor concentration or partial pressure to reduce the reverse reaction rate of crystalline magnesium [24].

The main differences between these two technological processes in reduction stage are the reduction equipment and raw materials used,which would be resulting from different reduction principles [16,21-25].Therefore,the calculation of energy and recourse consumption in the reduction process of carbothermal reaction is the focus of the estimation in this study,and the basis for the according comparison and discussion.

2.1.Scope of calculation and quantificatio

In this paper,the resource and energy consumption in each stage of the Pidgeon process and the VCTRM process were firstl calculated.The magnesium production process from raw materials to magnesium ingots was considered as a product system.The calculation of energy consumption covers production system and auxiliary production system.The energy consumption of production auxiliary system including the vacuum system,illuminating system,the mechanical equipment and the water treatment,which is counted into the“other” category in consolidated tables and assumed that it is consistent with that of the Pidgeon process.The product system is define as composed of four process stages as shown in Fig.1,including the reductant production,the calcination,the magnesium production by reduction,and the refinin &casting.The briquetting stage is a physical and mechanical process,and the energy consumed is also counted in the “other”category.Hence,this process wasn’t considered as a separate stage in the system.

2.2.Principles of calculation and quantificatio

In the process of calculation and quantification some principles as shown below are applied:

(1) The functional unit of the product is chosen to be 1 tonne of magnesium ingot produced;

(2) All of these resources related to the technology process consumed are measured in tonnes;

(3) In order to achieve the same fina product,energy consumption is compared to the same industry,namely comparability principle.The consumption of all energy materials includes the primary energy (coal,nature gas),the secondary energy (electricity) and the energy-consumed medium (industrial fresh water) in a technological process.They need to be converted into tonnes coal equivalent according the equivalent calorifi value or conversion coefficient as expressed by tonne of coal equivalent (tce).The standard coal define have a calorifi value of 2.9307×107kJ/t.

(4) The comprehensive energy consumption is a systematic measure of the energy consumption intensity for a technological process,and is calculated according to the formula (4):

whereEcompreis the comprehensive energy consumption for producing 1 t of magnesium ingot;Eiis the primary energy consumption inith process;Qiis the energy recovered from process waste heat and is calculated intoith process with a waste heat recovery device in the process;Ziis the secondary energy(e.g.coke gas) or energy-consumed medium(e.g.industrial fresh water) consumption inith process;∑Yiis the energy used for auxiliary production system.

2.3.Data sources

The main data of the Pidgeon process was obtained from the published data and Hui-ye magnesium company of Ningxia province[30],whose magnesium production in China has consistently ranked in the top three for many years.Some data were quoted fromChinese magnesium industry development report[7-11] and referred to the latest Chinese national standard GB/T 2589-2008 [31],GB/T 21347-2012 [32] and GB/T 21341-2017 [33].Then,the data of magnesium reduction stage in the VCTRM process mainly from laboratory and larger scale experiment data,the others stages was calculated based on the actual production data and parameters of the Pidgeon.This enables the realistic detailed data which truly reflec the VCTRM process as much as possible,rather than just theoretical values.

3.Results and discussion

Both the Pidgeon and the VCTRM processes belong to the type of thermal reduction method for magnesium production.There are many similarities in these two technological processes.In the VCTRM process,except some differences in the reduction stage,the other stages are almost the same as those ones in the Pidgeon process.This means if the reduction stage of the well-established Pidgeon process can be replaced and adjusted with new technologies,the commercial production of metallic magnesium by the VCTRM process would show a good chance of industrialization.

In this work,two scenarios (using dolomite and magnesite as raw material) have been considered for the VCTRM process,in which using dolomite as a scenario is a transition from the Pidgeon process to the VCTRM process using magnesite,to ensure their reliability and comparability.The results show that magnesite is an ideal raw material for the VCTRM process,and it has a greatly economic advantage and development trend.Hence,compared the Pidgeon process with the VCTRM process of using magnesite in following discussion.

The fina results show that the calculated results are largely in line with the annual magnesium industry reports andthe 13th Five-Year Planof China,reflectin the average level and trend of energy consumption of each technological process.However,it has to be noted that the calculated results dependent not only on the specifi technology and equipment employed but also on the producers who carried out the process.This dependency is influence by different energy and raw material supplies,the technical characteristics of each stage,and economic factors.

3.1.Resource and energy consumption of the Pidgeon process

According to the data stated above of Section 2.3 and statistical methods,the comprehensive energy consumption and resource consumption for per tonne magnesium ingot produced by the Pidgeon process can be calculated,as shown in Table 1.

Table 1 Resource and energy consumption for each process stage of the Pidgeon process.

Table 2 Results of resource consumption for per tonne magnesium product in the VCTRM process using dolomite.

3.2.Resource and energy consumption of the VCTRM process

In the reduction process of VCTRM,the materials include(i) MgO-containing materials,i.e.dolime and reactive magnesia obtained by calcining dolomite and magnesite,respectively.(ii) the reductant and (iii) the fluorite Except for the reductant,the other materials are consistent with the Pidgeon process.In this section,the VCTRM process using dolomite as the ore is taken as an example for the calculation.

3.2.1.Resource consumption

Dolime consumption:One tonne dolime is obtained from the calcination of 2.040 tonnes of dolomite,in which containing 40.03wt.% MgO and the rest is CaO.The magnesium reduction rate of the VCTRM process is more than 90% [36],so selecting 90%as the reduction ratio in the following calculation.Meanwhile,the incidence of reverse reaction at similar conditions during the condensation process is 3.42%-7.41%according to the reference [24],the average value of the reverse reaction rate is used as the calculated value of this work,and its value is 5.11%.Accounting for the losses in the collecting and the refinin the crown magnesium,approximately 96% of crown magnesium can be refine into magnesium ingots [35].Therefore,the mass of dolime required to produce each unit of magnesium ingot can be obtained by the following formula:

WheremMgingotis the mass of magnesium ingot,which is define as one tonne;γis rate of reverse reaction,5.11%.ZMgOandZMgare the molecular masses of magnesia and magnesium,which are 40.305g/mol and 24.305g/mol,respectively;ηis the magnesium reduction rate,90%;ωis the rate of refin ing,96%;wt.%(MgO)is the mass fraction of MgO in dolime,40.03%.

Reductant consumption:The carbon consumption should be calculated in the way that the molar ratio of carbon to MgO is 1:1 according to the reaction formula (3),and theamount of reductant can ensure the reaction fully proceeds.The content of fi ed carbon (dry basis) in coking coal is 73.52% [28].Then,the mass of reductant can be determined by the following equation with respect tomreducant:

wherenMgOis the number of mole;ZCis the molecular mass of carbon,which is 12g/mol;αis the amount of fi ed carbon(dry basis) in the coking coal,73.52%.

Fluorite consumption:The research and industrial production of the Pidgeon process indicated that the calcium fluorid as a catalyst,which is the main component of fluorite can greatly increase the rate of reaction [19].Fluorite consumption is 3% of the total mass of the dolime and the reductant,and the content of CaF2is more than 95%,so the fluorit is treated as a pure CaF2in following calculation.Therefore,Fluorite consumption can be calculated by the expression:

The dolime,the coking coal and the fluorit are blended in a certain proportion.The yielding mixture is then finel ground and pressed into pellets and in the last step charged into a reduction reactor.The corresponding results of estimation are shown in Table 2.In this scenario,the mass of the raw material to magnesium ratio is 6.048:1,which is a parameter in actual production process to measure resource consumption.

3.2.2.Energy consumption

In the reduction stage,the reaction (Equ.3) is carried out in a vacuum reactor,as we have demonstrated earlier[19,21-24].Theoretical calculation shows that,this reaction is carried out at 1473K when the system pressure is around 60Pa.However,in order to achieve the catalytic process of CaF2and a high reduction efficien y,setting the reaction proceed at 1673K and 30-100Pa.The total energy consumption (Q) of the reduction process system include the heat absorbed in the reduction reaction (Q1),the amount of heat needed to heat reduction products(Q2) and the amount of heat needed to heat reduction residues (Q3) [37],as shown in the following reaction:

The heat absorbed by reduction reaction,Q1:According to the reaction Eq.(1),the thermal effect of the reduction reaction,namely the heat absorbed in the reduction process,is calculated by Gibbs free energy function [38],as shown with formula (9)-(12).The thermodynamic parameters of the related substances are listed in Table 3.

Table 3 Thermodynamic parameters on substances related to carbothermal reduction [38,39].

whereis the heat of the reaction at temperature T K;νiis the stoichiometric number for the substancei,where the number of reactants is negative and the number of products is positive;is the enthalpy of the formation for substanceiat T K.

The gas does not do work under vacuum condition,which is the same situation as for the reaction enthalpy under the condition of the standard pressure.Therefore,the values ofis 615.10kJ/mol.That means generation one mole of magnesium vapor needs an energy of 615.10kJ at 1673K,and the heat required for a tonne magnesium ingot is represented asQ1.

The amount of heat needed to heat reduction products,Q2:In order to produce 1 t of magnesium ingot,1.098 t magnesium vapor need to obtained from the reduction process along with 1.265 t of CO.The amount of the heat of the CO and the magnesium vapor at 1673K is calculated as following:

The amount of heat needed to heat reduction residues,Q3:The reactants are not completely reacted due to the reduction rate was 90%,and they are heated to 1673K.The reduction residues contain the unreacted the MgO,the CaO,the coking coal and the CaF2,which can be obtained by the conservation of mass.Their needed energy is calculated with formula (15)and the components and the contents of the reduced slag and the calculated data are shown in Table 4.

Table 4 Calculated data of reaction residues in the VCTRM process using dolomite.

Theoretically,producing per tonne magnesium ingot need 1.098 t magnesium vapor and absorbed energy 3.646×107kJaccording to the formula (16).Moreover,taking the thermal efficien y of equipment in actual production into consideration,the thermal efficien y of the vacuum reduction furnace heated from inside by electric power was assumed to a value of approx.70% [21,40].The actual demand of the energy is 5.209×107kJ,which is equivalent to 1.777 tce.

The resource and energy consumption in the VCTRM process are calculated by using dolomite as the ore and shown in Table 5,and accordingly the results of the VCTRM process using magnesite is obtained,and the data related to the three processes are listed in Table 6.

By comparing Table 1 with Table 5,it indicates that dolomite is not the ideal raw material for the VCTRM process,in which CaO does not play any role but only wastes energy.Likewise,nickel laterite [41] and magnesite [42] can also as ore to attained magnesium by the VCTRM process,according to the reports.It is clear that,calcined magnesite(contains almost only MgO),is an ideal raw material in all of raw material for producing magnesium by VCTRM process.

3.3.Comparison of Pidgeon and VCTRM processes

Table 6 and Fig.2 show the calculated results,comparing the three aspects of resource,comprehensive energy consumption and waste generation.It has to be pointed out that even with the same VCTRM process,the selection of raw material in the calcination stage (Fig.1) plays also an important role on energy and resource consumption of magnesium production.The energy and resource consumption of the VCTRM process using magnesite as raw material is significantl lower than that of this process using dolomite.

Table 5 Resource and energy consumption for each process stage of the VCTRM process using dolomite.

Table 6 The resource and energy consumption and waste generated of the three Mg production processes.

3.3.1.Resource consumption

The VCTRM process consumed 5.656 t of non-renewable mineral resources,which is 9.689 t less than that used in the Pidgeon process with a decrease by 63.14%.Among them,the ore consumption decreased 6.010 t which is the most contributive item (the contribution rate is 62.03%) for reducing resource consumption.However,the Pidgeon process necessitates more additional substances to participate in the process (e.g.the CaO with SiO2form a stable compound(2CaO)·SiO2to promote the reaction,and even the Fe does not directly participate in the reaction),resulting in a higher resource consumption.

Fig.2.Comparison chart of three magnesium production process(in terms of resource and comprehensive energy consumption,waste generation and parameter).

For reductant,shown in Table 6,the resource consumption of the reductant (i.e.coking coal) used in the VCTRM process is only 23.75% (0.819 t/3.449 t) of the resource consumption to product ferrosilicon of the Pidgeon process.This is attributed to the fact that the coking coal can be almost used directly after the mining and the grinding without additional energy-consuming processes like ferrosilicon.This is the most distinct characteristics of the VCTRM process,and also is its advantage.Accordingly,in the VCTRM process,the consumption of the fluorit was decreased by 53.76%with the reduction of the demands for ore and reductant.

Moreover,the replacement of non-renewable coal by clean energy like natural gas led to reduce environmental damage and pollution caused by coal mining and transportation.Reducing resource consumption can fundamentally depress energy consumption and waste generation,in order to achieve energy conservation and emission reduction in the magnesium production industry.

3.3.2.Energy consumption

The production comprehensive energy consumption measured in standard coal equivalent has been one of the most significan indicators of the metal production industry and the national environmental protection departments of China.The current Pidgeon process of magnesium production is a high energy consuming industry,with a comprehensive energy consumption of 8.681 tce,of which the energy consumption of reductant production is 4.511 tce and account for 51.96%of the total energy consumption in the Pidgeon process.The remaining consumption (4.170 tce) is related to the other production processes and auxiliary systems,which is close to the data published by Chinese Magnesium Association (CMA).

The comprehensive energy consumption of per tonne magnesium ingot produced by the VCTRM process using magnesite is 2.675 tce,which is 6.006 t less than that of the Pidgeon process with a decrease by 69.16%.This is mainly attributed to the replacement of the ferrosilicon production process,thereby reducing energy consumption,accounting for 75.11% of all reduced energy consumption.In addition,in the calcination stage,the energy consumption of the VCTRM process has been significantl reduced (0.396 tce vs.1.529 tce of the Pidgeon process) as shown in Table 6,which is linked with (i) the less demand of ore (4.325 t of magnesite vs.10.335 t of dolomite) and (ii) lower calcination temperature (1023-1123K vs.1323-1423K of the Pidgeon process)as shown in Fig.1.Therefore,it can be concluded that in the VTRTM process,the use of carbonaceous reductant is significantl lower than the use of ferrosilicon of high energy consumption as reductant in the Pidgeon process at the same stage.

The thermal efficien y of equipment is also one of the most important factors affecting energy consumption.In the Pidgeon process,the effective energy consumption for heating materials in the reduction furnace accounts for only 9.85%of all energy input [30].While the internal resistance heating mode of the VCTRM process like the Bolzano Process has higher thermal efficien y than the external heating mode through fuel combustion in the VCTRM process [11,44].The higher thermal efficien y of the equipment,result in more products can be produced with the same energy consumption.Hence,it is so necessary to adopt waste heat utilization technology and improve fuel combustion efficien y to reduce energy consumption.

3.3.3.Waste generation

Most industrial fresh water is used as cooling water for reduction equipment.This kind of water can be recycled or used for other purposes after purification Therefore,it is assumed that no waste liquid is generated.

Solid waste is always associated with the metallurgical process,regardless of the kind of metal produced.The composition of the solid waste mainly depends on the composition of the material and varies with it,and the fluctuatio range is not large,because the smelter would control the proportion of ingredients within a reasonable range.In the Pidgeon process,5.465 t solid wastes,composed of 3.692 t (2CaO)·SiO2,unreacted MgO,CaO and ferrosilicon alloy,is an off-white powder with pH＞11.This kind of waste will seriously pollute the air,earth and water.Besides,its application as building materials was greatly limited and cannot be used well at present,due to the addition of MgO,the mechanical properties of cement have decreased.However,it has broad application prospects as compound fertilizer in agriculture [45,46].Meanwhile,0.079 t slags and 3.878 t fume (included CO) were produced in the production of ferrosilicon,which is a great burden on the environment.

Magnesium vapor is condensed,processed and made into magnesium ingots for commercial sale,or directly used in the production of magnesium alloys.Although CO of 1.201 t produced in the VCTRM process has not been utilized well up to now,it has a high application value.For example,it may be firs collected and used to fuel or chemical raw materials,even though this is still a matter of research.The collection and the application of the blast furnace gas in iron work are very worthy as a reference for this purpose.Moreover,the C and MgO as slag (add up to 0.121 t) in crown magnesium generated by the reverse reaction during the condensation process will inevitably increase the burden of the refinin stage to some extent.Taking the reduction rate,catalyst and other factors into consideration,0.401 t of solid residue (including unreacted MgO,carbonaceous reductant and added fluorite were remained in the reactor after end of the reaction.This only accounts for 9.55% of the solid waste of the Pidgeon process.

GHG emission is mainly concentrate on the calcination stage and the fuel combustion process for suppling energy.In this work,the energy materials in the whole production process are converted into standard coal according to different proportions,assuming that the electricity is supplied by thermal power.According to published data,per tonne of standard coal emits 2.4567 t of GHG after complete combustion [47].Hence,26.337 t CO2was emitted by the Pidgeon process,in which 21.327 t CO2was from the fuel combustion and accounted for 80.98%,and this proportion also is as high as 74.84% in the VCTRM process.Hence,the energy consumption is identifie as the key factor with the highest influenc on greenhouse gas emissions of magnesium production.The VCTRM process only emits 8.777 t greenhouse gasses,which is nearly 17.560 t less than that of the Pidgeon process.Although CO2emission from carbonate ore calcination is unavoidable,the use of alternative clean and renewable energy could decrease the CO2emission and further improve the overall sustainability of magnesium production.

Electricity consumption accounts for nearly 3/4 of the total energy consumption in the VCTRM process.If electricity could be provided by clean energy such as hydropower or wind power,it will dramatically reduce GHG emissions.The use of biomass charcoal (such as wood charcoal) as carbon source is also a promising attempt,which maintain the carbon circulating in green plants and the atmosphere,achieve zero carbon emission in the most ideal case [48].But further large-scale practice must ensure that it meets the requirements of the process for carbon source.

Overall,this study provides quantifie data and detailed evaluation on resource,energy fl w of the VCTRM process.Given the industrialization scale of the process,the energy consumption estimates may be revised up or down by 30-50% depending on the actual situation.Compared with the current Pidgeon process of China,the carbothermal method still have advantages.

4.Conclusions

By quantificatio the fl w of resource and energy for these two magnesium production technological processes,especially in combination with the current status of the magnesium industry of China,the environmental burden of resource,energy consumption and waste discharge were evaluated.Based on these results obtained,the following conclusions can be drawn:

(1) In the resource consumption,the VCTRM process consumes non-renewable mineral resources totally 5.656 t,which is 9.689 t less than those of the Pidgeon process.This value is decreased by 63.14%,caused by using magnesite as ore and coking coal as reductant.Reducing resource consumption can fundamentally depress energy consumption and waste generation.

(2) In the energy consumption,production 1 tonne of magnesium ingot by the VCTRM process consume 2.675 tce in total,which is 6.006 tce less than that of the Pidgeon process.Using coking coal instead of ferrosilicon can reduce energy consumption 4.511 tce.

(3) In the waste and the airborne emissions,5.465 t of solid waste,3.878 t of fume and 26.337 t of CO2are generated in the Pidgeon process,while only 0.522 t of solid wastes are remained with emission of 1.201 t CO and 8.777 t CO2in the VCTRM process.Energy consumption is identifie as the key factor on greenhouse gas emissions of magnesium production.

(4) The VCTRM process has dominating advantages in almost terms,which is most intuitively reflecte by the parameter of material to magnesium ratio,i.e.2.953 (VCTRM process) vs.6.429 (Pidgeon process).It means an increase of 54.07% in production efficien y.

It shows that the VCTRM process is a promising and environmentally friendly magnesium production process of low energy and resource consumption,and high efficien y compared with the current Pidgeon process of China.But there is still a long distance from industrialization.To minimize the incidence of reverse reaction is still a problem that the VCTRM process needs to overcome in the future.In addition,the collection and the application of by-product CO is also a meaningful matter of research.

Declaration of Competing Interest

None.

Acknowledgement

This research supported by the Yunnan Ten Thousand Talents Plan Industrial Technology Champion Project Foundation of China (No.YNWR-CYJS-2018-015),and this research also was supported by Basic Research Project of Yunnan Province(No.2019FB080).