Assessing environmental impact of magnesium production using Pidgeon process in China

Available online at www.sciencedirect.com●CIENCE(Transactions of骂P@onmer.Nonferrous MetalsSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 18(2008) 749- 754Presswww.csu.edu. .cn/ysxb/Assessing environmental impact of magnesium production usingPidgeon process in ChinaGAO Feng(高峰), NIE Zuo-ren(聂祚仁), WANG Zhi-hong(王志宏),GONG Xian- zheng(龚先政), ZUO Tie-yong(左铁铺)College of Materials Science and Engineering, Beijing University of Technology, Bejig 10022, ChinaReceived 4 June 2007; accepted 29 August 2007Abstract: Based on the practice of magnesium production in China, a quantitative evaluation of the environment impact was carriedout according to the theory and famework of life cycle asessment(LCA) study. The major gasecous pollutants including CO2, SO2,NO, CH。, HF and particulates were calculated. The accumulative environmental performances of dfferent energy use strategies andthe characterization results, including abiotic depletion potential(ADP), global warming potentia(GWP), acidification potential(AP)and human-toxicity potential(HTP) were compared. The results show that the direct emission of fuel combustion in the process is themajor contributor to the pollutants emission of magnesium production. Global warming potential and acidification potential make themain contribution to the accumulative environmental impact. The different fuel use strategies in the practice of magnesiumproduction cause much different impacts on the environmental performance. The accumulative environmental impact of coal burneddirectly is the highest, and that of producer-gas comes to the next, while that of coke-oven gas is the lowest.Key words: magnesium production, Pidgeon process, life cycle asessmentmagnetic shielding, so these outstanding features would1 Introductionmake them possess both significant application value andbroad application prospects.Since 1990s, primary magnesium has had a rapidMagnesium production by the Pidgeon process hasincrease in its production in China and become the fifththe advantages of short technical process, low investmentdomestic major nonferrous metal afer aluminum, copper, input, quick completion of workshop and low productionlead and zinc[1]. By 2005, the production ability ofBut the process consumes large number of naturalprimary magnesium reached 816 000 t, and the produc-resources and leads to relatively severe environmenttion of primary magnesium reached 467 600 t; mean-pollution. The emission of air pollutants resulting fromwhile, its export volume stayed at 353 100 t accountingthe fuel combustion process in particular has alreadyfor 70% of the global output[2]. China is the largestattracted much attention from the local government andprimary magnesium producer in the world, and theenterprises.Pidgeon process invented in 1940s in Canada is anEvaluating sustainable development of magnesiumimportant technique to produce primary magnesium inproduction requires methods and tools to measure anChina. China is among the global countries that arecompare the environmental impacts. Life cyclerichest in magnesium, whose major raw materials of theassessment(LCA) is a systematic methodology that canmagnesium industry come ftom the resources ofbe used for such purpose to identify and quantify themagnesite, dolomite, lake brines and seawater[3].potential environmental impacts associated with aCompared with other structural materials, magnesiummaterial, process, application or disposal during its entireand magnesium alloys boast a number of advantageslife span, At present. the intermational LCA research on[4- 6], such as low density, high specific strength, goodboth中国煤化I lagnesium and thethermoformability, and high performance of electro-magnYHCNMH(-tial stage, and whatFoundaton item: Projec(50525413) supported by the National Natursl Science Foundatioa of China; Projec(2006BAE04B09 6) supported by NationalCorresponding suthor: NIE Zuo-ren; Tel: +86-10-67391536; E-mail: zmic@bjut.edu.cn750GAO Feng, et al/Trans. Nonferrous Met Soc. China 18(2008)environmental impacts caused by the extensive use of theof 2 000[9]. The proportion of MgO in dolomite ore ismagnesium products is not yet clear. A cradle-to-gate lifeusually at around 20%. Mining of dolomite ore is notcycle study was conducted using averaged data forincluded in this system. The environmental issue of oremagnesium production in China to calculate the globalfrom mining is considered in more detail later.warming impact of Chinese magnesium ingots[7].The form of energy use included in LCA study is .Followed the ISO1 4040 standard[8] (Environmentalthe gross energy consumption, which is the cumulativeManagement- Life Cycle Assessment- Principles anamount of primary energy consumed in all stages ofFramework)，this work carries out environmentalmagnesium production life cycle. The threeassessment based on the domestic practices ofrepresentative practices of energy use in magnesiummagnesium production, compares the characterizationproduction process are selected, which are coal, producerresults and the accumulative environmental impacts ofgas and coke-oven gas(COG). China enjoys rich anddifferent energy use scenarios and discusses the adoptionwidespread coal resources with relatively low priceof“clean energy" and technological improverments thatcompared with petroleum and natural gas, so coal is thereduce these impacts.major fuel of magnesium production. The coal and gasco-firing and combustion is adopted in the process of2 Life cycle assessmentmagnesium production because the lower calorific valueof producer gas and the limitations on the gas supply2.1 Goal and scope definitionconditions for COG make them unable to totally replaceA goal and scope definition, such as a stagecoal powder at present.generally associated with the main issues of goal, scope,A functional unit of 1 t magnesium ingot is set infunctional unit, system boundaries, is the first phase of athis paper. The primary objetives of this study are:life cycle assessment. In this section, the purpose of1) Display quantitatively the environmental emissionsapplying the LCA is to investigate the environmentaldirectly (during combustion of the fossil fuels andimpacts of magnesium production.processing) and indirectly (e.g. in the generation ofThe Pidgeon process can be subdivided in four mainelectric power) and compare the environmental impactssubsystems: dolomite calcinations, briquette production,of energy use in production practices; 2) llustrate thereduction of dolime, and refinement and ingot casting, asaccumulative environmental performances of magnesiumshown schematically in Fig.1. The system boundaryproduction with the three scenarios and provide a(Fig.1) includes the Pidgeon process and the auxiliarysuggestion about energy use strategy.subsystem (the thermal power plant supplies energy tothe processes). Dolomite is the major raw material ofmagnesium production. The domestic dolomite reservesTo assess the most accurate environmental impactsrequired by magnesium production are very rich, andassociated to China magnesium production, we mainlymineral resources spread all over the country. Theconsider data from China Magnesium Association. Theensured reserves stayed at above 230X 10。t by the endinventory of main resources and energy consumption ofthe three scenarios for the Pidgeon process are listed inTable 1. The gross energy consumption of the threeDolomite Ipractices calculated are 261, 247 and 171 GJ/,Calcinationrespectively.DolimeTable 1 Inventory of main resources and energy consumptionBatching, + F errosiliconEnergygrinding,Calciumfor ltMg of three scenariospelletizinghfluorideInputScenario 1 Scenario 2 Scenario 3BriquetteDolomite/(l0*kgt7)1.501.051.00Ferrosilicon/(I0kg.t门)1.20-ReductionFluorite/(10rkg:th)2.481.81.74Refinement andCoal/(10kgt7)11.90 .3.362.28ingot casting中国煤化工3.14x10* 0System boundaryPrimarymagnesium ingotYHCNMHG 06.42x10Electric power/Fig.1 System boundary' of magnesium production using1.10(10kWh:r)Pidgcon processGAO Feng, et al/Trans. Nonferrous Met. Soc. China 18(2008)751In order to present the results of the pollutants2.3 Life cycle impact assessmentemission legibly, the indirect emission from theAccording to the ISO 14044 standard[16], theelectricity generation and direct emission which includesimpact assessment method consists of three steps:the fuels combustion and processing are edited in thecharacterization, normalization and weighting. Theinventory. Life cycle emissions inventory of airproblem-oriented approach, which was developed by thepollutants of three scenarios for magnesium production isInstitute of Environmental Sciences(CML) of Leidenlisted in Table 2. The major waste gases produced in theUniversity, is used to calculate thefollowingprocess include CO2， SO2, CH4， NOx, HF andenvironmental impact of the case study: abiotic depletionparticulates.potential(ADP)，globalwarmingpotential(GWP),acidification potential(AP) and human toxicity potentialconsumed in magnesium production process also(HTP). .influences the environmental impact of that process. TheThe characterization factors of GWP, AP and HTPemission factors of electric power plants in China arefor the emissions are chosen from Ref.[17] For theobtained from Ref.[10].characterization factors of ADP, the model recommendedThe estimation of CO2 emissions from fuelby Ref.[18] is used to calculate the depletion potential ofcombustion in this study is based on energy consumptionminerals extraction. The “antimony" is chosen as aand emision factors by fuel type11-12]. SO2 and NO,reference. Based on the ensured reserves and extractionemission estimates depend on the energy consumptionrate of coal, dolomite, fluorite and antimony in China[9],and emission factors from the corresponding referenceshe characterization factors (antimony eq.) of coal,[13-14]. Particulates emission from the direct emissiondolomite, and fluorite are calculated to be 4.81x10-,is mainly based on the operation conditions, combustion2.64x10~3 and 1.35x 10-2 kgkg, respectively. Accordingequipment and technology, dust catcher and fuelsto Ref:[19], to produce 1 t frerosilicon needs the materialsconsumption[15]. HF emission estimate from theof 1 820 kg silica and 220 kg scrap iron. Calculated byprocessing is based on the fluorite consumption, thethe same model, the characterization factors (antimonycontent of CaF2 and emission factor[15].eq.) of silica and scrap iron are 1.18x10 4 and 5.20x10*The inventory results indicate that CO2 emissionkg/kg. The characterized results of ADP, GWP, AP andfrom Scenario 2 is increased by 14.7% compared withHTP for the three scenarios are listed in Table 3.Scenario 1. The producer gas used as the main fuels ofThe normalization factors (World, mid 1995) formagnesium production will not reduce the CO2 emission.GWP, AP and HTP are chosen from Ref.[17], while theBut the reduction purpose of SO2, NO, and particulatesnormalized value for depletion of abiotic resources isemission is evident. The overall pollutants emission frombased on China resources situation in 2002, which is 1.24Scenario 3 will be reduced clearly compared withX 10'4 kg antimony eq.[20]. The normalization resultsScenario 1 and Scenario 2. Especially, particulates fromfor three scenarios are listed in Table 4.the direct emission of Scenario 1 will decrease by 49.5%In the final stage, the normalized results multipliedand 65.4% compared with Scenario 2 and Scenario 3.by a weighting factor represent the relative importance ofTable 2 Air pollutants inventory of three scenarios (kg/t )ScenarioEmissionItemCO2SO2NO,CH4HFParticulatesIndirect emissionElectric power1.07*10'9.936.4620.2Fuels combustion2.04x10*20.789.475.9Scenario 1 Direct emissionProcessing7.02x10360.034.411Total2.85*10*90.695.92.60071.18x1010.97.112.8622.22.34x10*19.555.1-92.6Scenario 2 Direct emission4.91x1025.11.802.95x10* .90.462.2171.07x10' .中国煤化工二Fucls combustion1.59x10*13.262.8Scenario 3 Direct emissionMHCNMHG4.68x10324.12.17x10483.129.784.8GAO Feng, et al/Trans. Noferrous Met. Soc. China 18(2008)Table 3 Characterization results for Pidgeon processScenario 2 is higher than that of Scenario 1. It is shownImpact category Scenario 1Scenario 2Scenario 3that adopting producer gas as major fuel for magnesiumproduction cannot reduce the emission of greenhouseADP(Sb eq,)/4.32x10'3.04x10'2.90x10'gases. While GWP of Scenario 3, which uses COG as(kgt)major fuel for magncsium production, is decreased byGWP(CO2 eq.)V2.85*10*2.96*10*2.17x10*20% compared with that of Scenario 1. It can effectively(kgt")reduce global warming potential, but magnesium plantAP(SO2 cq.,V2.13x1021.74x1021.42x102needs to be constructed near the coke plant, and theHTP(1,4-DCBchoice of the location is restricted.eq,)(kg:th)3.52x10'2.54x10'2.38x103Fig.2 ilustrates that from Scenario 1-3, GWP offuel combustion respectively accounts for 72%, 79% andTable 4 Normalization results for three scenarios73% in their accumulative global warming potential.Therefore, fuels reduction and efficiency improvementImpactScenario 1/yr Scenario 2/yr Scenario 3/yrre the basic points of controlling greenhouse gascategoryADE3.48X10-13 2.45X10-13 2.34X 10-13emission. The reduction of greenhouse gases in dolomitecalcination is also significant, but this seems to beGWP7.38X10-10 7.66X10~10 5.62X 10-10AP7.11x10-10 5.82x10-10 4.76x 10~10dificult for the primary magnesium producer to use thePidgeon process because of the composition of dolomite.HTP7.07X10-11 5.10x10-11 4.77X 10~11the total environmental impact. It enables an overall3.2 Acidification potential(AP)The gases that contribute to acidification are mainlycomparison of the three scenarios. The analytic hierarchySO2 and NOx. SO2 is mainly generated from theprocess(AHP)，a matrix-based approach measuringdispersed sulphur oxidized on molten magnesium surfaceimpact priorities in a hierarchical structure[21- -22], isat high temperature (about 700 C) in the process ofused to determine the weight factors. The consistencytesting shows that the results have a very highrefinement and ingot casting. HF from magnesiumconsistency. The calculated weights of ADP, GWP, APproduction by reduction is another type of gasand HTP are 0.088, 0.482, 0.272 and 0.158, respectively.contributed to acidification. The total accumulativeThe final single results of Scenario 1-3 were 5.60x10~10,reductions of 18% and 33% are obtained for Scenario 25.36*10~10 and 4.08x 10 l0yr, respectively.and 3, respectively.The AP resulting fom the SO2 and NO2 emission is3 Interpretationdecreased with the reduction of direct consumption ofcoal (Fig.2). The processes of gasification and coking of3.1 Global warming potential(GWP)coal would help to reduce SO2 and NO, emission[12,23].In this study, CO2 is the major gas to cause globalo the contribution of AP in the process of fuelswarming impact. Global warming potential(GWP) ofcombustion is decreased. While the contribution in the120昌GWPScenario IScenario 3|100 0 HTP公8060-。4020JzE中国煤化工Electricpower combustionFuels Processing Electric Fuels.YHCNMHGPoessingpower combustiFig.2 Contribution of GWP, AP and HTP in different processesGAO Feng, et al/Trans. Nonferrous Met. Soc. China 18(2008)53processing is increased because the quantity of sulpburenvironmental performance. By considering theused changes a ltte.limitation of the COG supply condition and the locationof magnesium plants, producer gas, in areas where coke3.3 Human toxicity potential(HTP)production is not concentrated, can be used as the majorThe emissions contributing to this impact categoryfuel for primary magnesium production.are mainly SO2, NO, HF, and particulates. For threescenarios, HF from reduction in the processing, taking an4 Conclusionsaverage 93% in the cumulative human toxicity potential(Fig.2), poses to a large environmental threat to human1) According to the LCA procedure, thehealth. The total cumulative reductions of 28% and 33%environmental impact assessment was carried out on theare achieved for Scenario 2 and 3, respectively.practice of magnesium production with Pidgeon processin current China. The emissions inventory shows that the3.4 Abiotic depletion potential(ADP)combustion of fuels is the main contributor to theThe depletion of abiotic resources includes the coal,pollutant emissions in the life cycle of magnesiumdolomite and fluorite that are all directly used in theproduction.Pidgeon process. The characterization factor of coal is2) Adopting producer gas as major fuel forcomparatively small because of the abundant reserves, somagnesium production cannot reduce the emission ofits impact on the cumulative consumption of resourcesgreenbouse gases. Thcharacterization results indicatecan be neglected. ADP of Scenario 2 and 3 are droppedthat abiotic depletion potential(ADP)， acidificationby 30% and 33% respectively compared with that ofpotential (AP) and human-toxicity potential(HTP) areScenario 1. This is mostly due to the reduction ofdecreased cumulatively from Scenario 1 to 3, with thedolomite of Scenario 2 and 3.exception of global warming potential(GWP). The finalsingle scores show that the accumulative environmental3.5 Final single resultsperformance of Scenario 3 is the best compared withThe final single results indicate that thScenario 1 and 2.accumulative environmental performance of Scenario 33) The producer gas (Scenario 2) is an altemativeto produce primary magnesium is the lowest, and that offuel for the magnesium production rather than the coalScenario 2 comes to the next, which is 31% higher thanburmed directly (Scenario 1) in the areas where high costthat of Scenario 3. Scenario 1 shows the highestof coke oven gas is produced. The utilization of“clean”accumulative environmental load which is 37% higherenergy and reduction of greenhouse gases and acidicthan that of Scenario 3. Fig.3 ilustrates that the GWPgases emission are the main goal of the technologicaland AP are the major impacts of the magnesiumimprovements and cleaner production of the magnesiumproduction. 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