a research on energy-saving and environmental impacts of primary magnesium and magnesium alloy...
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A Research on Energy-saving and Environmental Impacts of Primary
Magnesium and Magnesium Alloy Production in China
Feng Gaoa, Zuoren Nieb, Zhihong Wangc,
Xianzheng Gongd and Tieyong Zuoe
College of Materials Science and Engineering, Beijing University of Technology,
Beijing 100124, China
[email protected], [email protected], [email protected], [email protected], [email protected]
Keyword: magnesium alloy; energy consumption; global warming potential; acidification potential;
life cycle assessment.
Abstract. China is the largest primary magnesium producer in the world, because of nearly 80% of
the global market share. In the present paper, an approach of life cycle assessment (LCA) was
applied to build an inventory of air emissions and to analyze the environmental impact of the global
warming potential (GWP) and the acidification potential (AP) related to the production of AZ91D
magnesium alloy. A summary of environmental impacts of primary magnesium and primary
aluminum production with various studies was made to show the influence of uncertainties on the
impacts. The results showed that the cumulative GWP and the acidification potential (AP) of
AZ91D Mg-alloy are 33.4 t CO2 eq/t ingot and 139 kg SO2 eq/t ingot, with the range of 29.5-36.3 t
CO2 eq/t ingot and 104-152 kg SO2 eq/t ingot, respectively. The GWP and AP of primary
magnesium account for 90% and 77% of the cumulative environmental impact of AZ91D Mg-alloy.
Under the grand background of advancing the development strategy of energy-saving and
emission-reducing, China magnesium smelting and manufacture industry has made rapid progress
in the structure optimization, energy efficiency improvement, and environment protection. The
calculated data show that the improvement measures, e.g. reduction of dolomite consumption and
energy consumption, in Chinese Pidgeon process led to 23% decrease of the GWP for the primary
magnesium production in 2009 compared with 2005. The global warming reduction potential for 1
ton AZ91D alloy ingots produced in China was estimated of substituting HFC-134a for SF6 as a
cover gas.
Introduction
Magnesium is of many attractive characteristics such as low density and high-specific strength and
stiffness. The requirements of weight loss and energy saving in automotive, aerospace, and
communication industries make a tremendous opportunity for the development and application of
magnesium alloy. Since the late 1990s, when the Pidgeon process was widely used in China, the
global production and technical structure have been changed by the rapid growth of China
magnesium industry. Magnesium alloy die casting parts have been the main field accounting for
nearly one third of total China magnesium consumption since 2006 [1], which are driven largely by
the growth in automotive applications.
Materials Science Forum Vol. 685 (2011) pp 152-160Online available since 2011/Jun/07 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.685.152
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On one hand the Pidgeon process does offer a lower cost compared with the electrolysis process,
but on the other hand, China has to face the impacts on intensive resources and energy consumption
and environmental burden, especially the greenhouse gas (GHG) emissions. The environmental
problems in China magnesium and its alloy production have caused the extensive concern in the
field of research, since the users and the manufacturers related to the magnesium output of China
account for nearly 80% of the global market share. China Magnesium Association combined with
several large production enterprises established the energy consumption limitation standard of
magnesium smelting products, and positively advocated and promoted the development and
application of energy-saving and consumption-reducing technologies in recent years [2,3].
Life Cycle Assessment (LCA) [4,5], as an effective technique for environment management and
assessment, has been used to assess the energy requirements and environmental impacts of a
number of magnesium products and magnesium production technologies. In the paper, an approach
of life cycle assessment (LCA) was applied to analyze the environmental impacts of the global
warming potential (GWP) and the acidification potential (AP) related to the magnesium alloy
production, which provide recent achievements in the development of China magnesium industry.
Magnesium and Magnesium Alloy Production Technology
Primary Magnesium Production Using Pidgeon Process. The main process of primary
magnesium production regarding the way of dolomite calcination, batch pelletizing, reduction,
refinement and ingot casting consists of four steps (Fig. 1). Materials consumption usually includes
dolomite, ferrosilicon and calcium fluoride. The dolomite ore which is mined and transported to a
magnesium plant is calcined in rotary or vertical furnaces at about 1200℃. The calcining process
yields dolime, as given by the following reaction:
( )( ) ( )( ) ( )gssCOMgOCaOheatMgCOCaCO 233 2+⋅→+⋅
. (1)
The dolime mixed with the ferrosilicon containing above 75% of silicon as reduction agent and
the fluorite containing around 95% of CaF2 as catalyst after calculating and measuring ingredients is
ground. Then put these three kinds of materials into the reduction pots after being compressed into
balls by pelleter and heat to 1200℃, and the magnesium vapor appears after reduction reaction.
Magnesium vapor sublimates crystal magnesium in the condenser in the front part of reduction pots.
The reaction describing the reduction process is as follows:
( ) ( )( ) ( ) ( )( ) ( )ssgss FeSiOCaOMgFeSiMgOCaO +⋅+→+⋅ 2222 ）（. (2)
The last step of the Pidgeon process is the refining, where the crystal magnesium containing
amounts of impurities are melted and treated with purifying agents. The surface of melted
magnesium needs to be blanketed with an appropriate flux or cover gas preventing oxidation. In this
process, the melted magnesium is highly combustible, thus serious safety problems are caused. The
molten magnesium is then transferred from the melting furnace and poured into ingot moulds to
produce magnesium ingots.
Materials Science Forum Vol. 685 153
Fig. 1 Typical process flowchart of Chinese primary magnesium and Mg-alloy production
Magnesium Alloy Production Technologies. Magnesium in its molten state burns in contact
with air or moist air. This rapid oxidative burning must be controlled in order to ensure safety in
production and magnesium alloy quality. For several decades the alkali halides-containing flux or
fluorine-bearing gas atmosphere has been used to inhibit melt surface oxidation in magnesium
production. In China the covering flux protection process (Fig. 2) and gas protection process (Fig.
3) are widespread methods for the magnesium alloy production. The technical flow of Mg alloy
production includes alloy elements proportioning, melting, alloying, refining, ladle analysis, and
casting. The producer gas and electricity are the main energy consumed in Chinese magnesium
alloy production.
Fig. 2 The covering flux protection process Fig. 3 The gas protection process
154 Energy, Environment and Biological Materials
For the covering flux protection method, primary magnesium and master alloys after
pretreatment were melted in a coal gas reverberatory furnace or a resistance crucible furnace at
about 730℃ under the protection of flux (a mixtures of chloride and fluoride). Refining flux, an
amount equivalent to 1.5-2.0% of charging mass, was added in the molten magnesium in order to
remove impurity. An inert gas such as argon or nitrogen was required to degas and supplementary
refining. The inhibitor (SF6 or SO2)/air cover gas mixtures were used to prevent from burning in the
process of standing and casting. The difference between the covering flux protection process and
gas protection process is that shielding for prevention of burning in the latter is free from covering
flux and uses SF6 or SO2/air cover gas mixtures for the whole process. When the gas concentration
of the SF6 is 0.10 to 0.40vol.%, a protective effect against burning can be obtained.
Methodology
System Boundary. Process-oriented life cycle analysis method was used to assess the energy
consumption and gaseous emissions of Chinese primary magnesium and AZ91D alloy (Al
8.3-9.7%, Zn 0.35-1.0%, Mn 0.15-0.5%) production process. The system boundary was from the
dolomite mining to magnesium alloy ingot produced (Fig. 1). An amount of flux, and aluminum as
alloying element added, and ferrosilicon produced, and their impacts on the LCA have been taken
into consideration. Although this flowchart includes dolomite mining or transport, their contribution
is not likely to change the results significantly [6]. Similarly, the impacts of slag waste and alloying
elements such as zinc and manganese have not been included in these calculations.
LCIA Methods. Life Cycle Impact Assessment (LCIA) [7] using science-based characterization
factors can provide a more meaningful basis to make comparisons between the product or process
and its potential environmental impacts. Midpoint impact assessment models [8] reflecting the
relative potency within the cause-effect chain were used to calculate the impacts resulted from
environmental release. The indicators, such as the Global Warming Potential (GWP) and the
Acidification Potential (AP), of impact categories associated with the gaseous emissions were
calculated. The gases responsible of rain acidification are SO2, NOx, HCl and HF, while the
greenhouse gases (GHG) contributed to GWP are CO2, CH4 and SF6. An accumulative model was
used for a comparison of the environmental impacts in the three main processes, i.e. primary
magnesium production, primary aluminum production, and alloy smelting. An uncertainty analysis
was conducted to estimate values for the lower and upper bounds of magnesium alloy process
parameters, as given in the tables below.
Data Collection. The input data of materials and energy consumption were based on
investigation to China magnesium factories. An upper bound, average and lower bound data of
Pidgeon process were summarized in Table 1.
For the magnesium alloys production, AZ91D that has a number of industrial applications was
chose to conduct a case study. The input data of 1t AZ91D ingot produced via the covering flux
protection process and gas protection process were collected in Table 2. Some of representative
characteristics of the Mg-alloy processes were described.
Materials Science Forum Vol. 685 155
Table 1 Materials Input inventory of 1 ton primary magnesium
Inputs Unit Normal Range
Dolomite t 10.5 10~11
Ferrosilicon t 1.08 1.05~1.12
Fluorite kg 180 170~210
Flux kg 200 170~220
Coal t 7.84 7.00~8.70
Sulfur kg 6 4~8
Electricity kWh 1200 1000~1400
Table 2 Materials input inventory of two methods for producing 1 ton AZ91D ingot
Inputs Unit covering flux protection process gas protection process
Primary magnesium kg 1070 1050
Primary aluminum kg 114 112
Zinc kg 11.2 11.0
Manganese kg 5.10 5.00
Flux kg 180 50.0
Electricity kWh 400 1150
Producer gas m3 650 0
Greenhouse gases emissions were calculated according to the methods recommended by
Intergovernmental Panel on Climate Change (IPCC) [9], while carbon emission coefficients and
carbon oxidation coefficients of fuels were chosen from the measured value based on the situation
of China [10]. The main components of flux widely used in China include 38-46%MgCl2,
32-40%KCl, 5-8%BaCl2, and 3-5%CaF2. The emission of acid gases such as HCl and HF from flux
pyrolyzed was taken into account. The concentration of cover gas mixtures we investigated was
0.2-0.3% SF6＋25%CO2＋75%air by volume fraction. Under the existing IPCC Good Practice
Guidance, SF6 emission from magnesium melt protection are assumed to be 100 percent of the
amount of SF6 utilized [9].The emission factors for electricity production were representative of the
data from Chinese grid [11]. Aluminum production processes were obtained from research reports,
including reference [12] and literatures provided by International Aluminum Institute (IAI) [13] and
European Aluminum Association [14].
Results and Discussion
Energy Consumption. The comprehensive energy consumption of 1 ton primary magnesium,
considering the system boundary from cradle to gate, decreases from 360GJ/t Mg to 297GJ/t Mg
between the year 2005 and 2009. With considerable improvements such as better production
management and equipment control, and especially coal gasification process and exhaust gas heat
recovery in reduction process, during the 2005 to 2009 period, unit product energy consumption of
Pidgeon process, excluding the ferrosilicon production, is 158-192GJ/t Mg in 2009, decreased by
about 27% than that in 2005. For the alloying process, the energy consumption of the covering flux
protection process and gas protection process are 8.1 GJ/t AZ91D ingot and 13.6GJ/t AZ91D ingot,
respectively.
156 Energy, Environment and Biological Materials
Comparison of Accumulative Environmental Impact. The accumulative impact of AZ91D
ingot from cradle to gate needs to take the environmental burden of primary magnesium, primary
aluminum, and alloying process into consideration. Differences in environmental impacts (GWP
and AP) between various studies for primary magnesium and primary aluminum production
indicate variation in time series, regions, technologies utilized, and assumptions in input data. These
values were compared with published data in Table 3 in order to estimate the influence of
uncertainties on these impacts. The time shown in Table 3 represents the reported period of these
interpreted data rather than the issue of these references.
Table 3 Environmental impact categories (GWP and AP) of primary Mg and Al production,
per tonne of primary metal
Country (process, time) GWP
(103 kg CO2 eq.)
AP
(kg SO2 eq.)
Primary magnesium China (Pidgeon process, 2009) 28.0 (25.6~30.0) 101 (86.4~108)
China (Pidgeon process, 2005) [6] 36.6 (34.1~41.9) 252 (217~293)
China (Pidgeon process, 2001) [15] 42.1 (37~47) N
Australia (Electrolysis, 2003) [16] 24.5 98.5
Australia (Electrolysis, 2003) [17] 24.3 (20.4-26.4) N
Primary aluminum China (2006) [12] 15.4 155
Europe (2005) [14] 9.68 43.9
World (2005) [13] 9.81 24.7 N = Not available
The results showed that the normal GWP value of primary magnesium production we calculated
in 2009 decreased by nearly 33% than that in Ramakrishnan S. et al [15]. The GWP value of
advanced level of Chinese Pidgeon process has been close to that of the electrolysis process, and
AP value could be even lower. The evident decline of the environmental burden was the result of
energy-saving measures in Chinese Pidgeon process. But the GWP and AP impacts of magnesium
are still 2-4 times higher than that of aluminum in Europe and world average level.
The accumulative global warming impact and acidification impact of AZ91D ingot were
illustrated in Fig.4 and Fig5. Considering the China average value of 15.4 kgCO2 eq/kg Al ingot for
the GWP and 155 kg SO2 eq/ kg Al ingot for the AP of aluminum ingots (Table 3), it may be seen
that the nominal values of process parameters yield a GWP impact of 33.4 t CO2 eq/t AZ91D ingot,
and a AP impact of 139 kg SO2 eq/t AZ91D ingot. The influence of uncertainties of the every
process on the impact is also shown. An uncertain range of global warming impact and acidification
impact of AZ91D magnesium alloy are 29.5-36.3 t CO2 eq/ t AZ91D ingot and 104-152 kg SO2 eq/
t AZ91D ingot, respectively. Since materials input varies with time and the technology level, the
range of GWP and AP of the primary magnesium production in 2009 are 26-30 t CO2 eq/t Mg ingot
and 86-108 kg SO2 eq/t Mg ingot, taking an average of 90% in the accumulative GWP of AZ91D
ingot, and 77% in the accumulative AP of that. According to the LCI of primary magnesium
between 2005 and 2009, it is calculated that 11% decrease of dolomite consumption and 29%
decrease of coal consumption in the Pidgeon process lead to 23% decrease of global warming
impact. It is identified that the environmental performances of primary magnesium process have
decisive influence on the accumulative environmental impacts of magnesium alloy.
Materials Science Forum Vol. 685 157
0
5000
10000
15000
20000
25000
30000
35000
GWP
kgCO2eq
primary
magnesium
primary
aluminum
alloy
production
Fig. 4 Global warming impact of producing 1t AZ91D magnesium alloy
0
20
40
60
80
100
120
140
AP
kgSO2eq
primary
magnesium
primary
aluminum
alloy
production
Fig.5 Acidification impact of producing 1t AZ91D magnesium alloy
Due to the environmental parameters of global aluminum ingot production varying in regions, if
Chinese data are used, the upper bounds for aluminum in 1 ton AZ91D alloy are 1760 kg CO2 eq
and 18 kg SO2 eq, respectively.
The contribution of GWP and AP for alloying process, accounting for 6% and 10%, respectively,
are lesser in the accumulative environmental impacts.
The impacts of SF6. Except for the emissions from the energy consumption, Sulfur hexafluoride
(SF6), used in magnesium smelting and die casting to prevent molten magnesium from rapid
oxidative burning, is an extremely powerful greenhouse gas, with a 100-year global warming
potential (GWP) estimated at 23,900 times that of carbon dioxide (CO2) [18]. The GWP
contribution of SF6 in covering flux process and gas protection process was 43% and 63%,
respectively, although the amount of SF6 is far less than CO2 and CH4. Due to a high global
warming potential of sulfur hexafluoride (SF6), Chinese magnesium enterprises have voluntarily
committed to phase out the use of SF6. There are some measures by improving the sealing
conditions of the furnace, and optimizing concentrations and flow rates of the cover gas to cut down
the SF6 emission. Meanwhile, the global magnesium producers are also seeking substitutes for SF6,
such as HFC-134a (1,1,1,2-tetrafluoroethane), fluorinated ketone liquid-to-gas, and dilute SO2
[19,20]. If HFC-134a were used, instead of SF6, for alloying process and ingots casting, the value
for GWP was estimated to be 4-8% decrease for 1 ton AZ91D alloy ingots produced in China.
158 Energy, Environment and Biological Materials
Conclusion
Since China is the largest producer of world primary magnesium and the Pidgeon process is widely
used in Chinese magnesium industry, it is important to investigate the environmental performances
of this magnesium production process in order to respond to the worldwide growing demand for
magnesium stimulated by cleaner automotive applications. The LCA results showed that the
environmental performances of primary magnesium process had decisive influence on the
accumulative environmental impacts of magnesium alloy. And the energy saving is the first priority
of improving the environmental performances of primary magnesium. With the considerable
improvements adopted in Chinese magnesium production process, a substantial reduction in energy
consumption, GHG emissions and acidic gas emissions has been achieved. And it is likely that the
environmental performances of Chinese magnesium could be further promoted based on more
energy-efficient and environmentally friendly technologies.
Acknowledgment
This work was carried out under the support from the National Basic Research Program of China
(973 Program) (No. 2007CB613706), National High Technology Research and Development
Program (863 Program) (No. 2007AA03Z432), Beijing Natural Science Foundation (No. 2081001),
National Natural Science Foundation of China (No. 50525413), and Scientific Research Initiative
Foundation of Beijing University of Technology (No. X0009011200902).
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160 Energy, Environment and Biological Materials
Energy, Environment and Biological Materials 10.4028/www.scientific.net/MSF.685 A Research on Energy-Saving and Environmental Impacts of Primary Magnesium and Magnesium
Alloy Production in China 10.4028/www.scientific.net/MSF.685.152