Energy and Exergy Analysis of the Primary Aluminum Production Processes: A Review on Current and Future Sustainability

Abstract

The common industrial practice for primary aluminum production consists of the Bayer process for the production of alumina followed by the Hall–He´roult process for the production of aluminum. Both processes were developed at the end of the 19th century and despite continuous optimization, their basic thermodynamic inefficiencies and environmental issues remain till today unchanged. As a result, primary aluminum production industry is the world’s larger industrial consumer of energy, is ranked among the most CO2 intensive industries, and is associated with the generation of enormous quantities of solid wastes. In this paper a detail energy and exergy analysis of the primary production of aluminum is presented and alternative sustainable processes are reviewed.

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Energy and Exergy Analysis of the Primary Aluminum Production
Processes: A Review on Current and Future Sustainability
Efthymios Balomenosa; Dimitrios Paniasa; Ioannis Paspaliarisa
a Laboratory of Metallurgy, School of Mining and Metallurgical Engineering, National Technical
University of Athens, Zografos Campus, Athens, Greece
Online publication date: 09 March 2011
To cite this Article Balomenos, Efthymios , Panias, Dimitrios and Paspaliaris, Ioannis(2011) 'Energy and Exergy Analysis
of the Primary Aluminum Production Processes: A Review on Current and Future Sustainability', Mineral Processing
and Extractive Metallurgy Review, 32: 2, 69 — 89
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ENERGY AND EXERGY ANALYSIS OF THE PRIMARY
ALUMINUM PRODUCTION PROCESSES: A REVIEW
ON CURRENT AND FUTURE SUSTAINABILITY
Efthymios Balomenos, Dimitrios Panias, and
Ioannis Paspaliaris
Laboratory of Metallurgy, School of Mining and Metallurgical Engineering,
National Technical University of Athens, Zografos Campus, Athens, Greece
The common industrial practice for primary aluminum production consists of the Bayer
process for the production of alumina followed by the Hall–He
´roult process for the
production of aluminum. Both processes were developed at the end of the 19th century
and despite continuous optimization, their basic thermodynamic inefficiencies and environ-
mental issues remain till today unchanged. As a result, primary aluminum production
industry is the world’s larger industrial consumer of energy, is ranked among the most
CO
2
intensive industries, and is associated with the generation of enormous quantities of
solid wastes. In this paper a detail energy and exergy analysis of the primary production
of aluminum is presented and alternative sustainable processes are reviewed.
Keywords:chemical exergy analysis, energy and exergy efficiency, primary aluminum production
OVERVIEW: PRIMARY ALUMINUM PRODUCTION TODAY
Aluminum represents 8% of the Earth’s crust, being the third (following
oxygen and silicon) most abundant element in our planet (Meyers 2004); however
its commercial production is relatively recent, dating a little more than a century.
Despite this, the world’s aluminum production exceeds today the production of all
the other nonferrous metals combined (Meyers 2004). Due to a unique combination
of properties, aluminum is a prevalent material for numerous applications.
Aluminum is a light-weight, durable, flexible, corrosion-resistant metal with high
electrical and thermal conductivity, which is used in a vast array of products in all
areas of modern life including transport, construction, food, medicine, packaging,
electronics, and electricity transmission. As a result, aluminum is the world’s second
most-used metal after steel and the aluminum production industry is the largest, in
volume of metal produced, in nonferrous metal industry. In 2006, the world’s pri-
mary aluminum production was approximately 34 million tonnes, while aluminum
recycling produced another 16 million tonnes (Barrientos 2009). This total of about
Address correspondence to Dimitrios Panias, National Technical University of Athens, Laboratory
of Metallurgy, Athens, Greece. E-mail: panias@metal.ntua.gr
Mineral Processing & Extractive Metall. Rev., 32: 69–89, 2011
Copyright #Taylor & Francis Group, LLC
ISSN: 0882-7508 print=1547-7401 online
DOI: 10.1080/08827508.2010.530721
69
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50 million tonnes in aluminum greatly exceeds the 17 million tonnes of copper, 8
million tonnes of lead, and 0.4 million tonnes of tin produced worldwide.
All of the primary aluminum production is conducted through one industrial
practice, which consists of two separate stages: the Bayer process for the production
of high grade alumina (Al
2
O
3
) from bauxite ore and the Hall–He
´roult process for the
electrolytic reduction of alumina to aluminum. The Bayer process was patented in
1888 by Karl Joseph Bayer, while the Hall–He
´roult process was developed nearly
simultaneously and completely independently in 1886 by Paul-Louis He
´roult and
Charles Martin Hall (Ju
¨rgen Buschow et al. 2001). Since then, both processes have
been extensively researched and many technological improvements have been made
upon them; however their basic scientific principles and environmental issues remain
unchanged (Choate and Green 2003).
Primary aluminum production today is ranked among the most energy and
CO
2
intensive industrial processes. Particularly, primary aluminum industry is the
world’s largest industrial consumer of energy, having an energy cost which accounts
for approximately 30% of its total production cost. Nearly all the electricity
consumed in primary aluminum production (approximately two-third of the total
energy input) is used in the Hall–He
´roult process. This process is also currently
responsible for the 2.5% of the world’s anthropogenetic CO
2
-equivalent emissions
(Steinfeld 1997). The Green-House Gas (GHG) emissions of the Hall–He
´roult
process include direct (from the reaction of oxygen with the carbon-based anodes)
and indirect (from the use of fossil fuels based electricity) CO
2
emissions, as well as
emissions of perfluorcarbons (PFCs), like CF
4
and C
2
F
6
, released due to process upset
(anode effect) in the cryolite bath. Both CF
4
and C
2
F
6
are powerful climate gases with
100 years Global Warming Potential (GWP) of 6500 and 9200, respectively.
Additionally, the primary aluminum production industry generates enormous
quantities of wastes, the most significant among them, being the insoluble bauxite
residue known as red mud. Approximately 1 kg of red mud (on a dry basis) is gen-
erated per kg of produced alumina (European Commission 2001; Mason et al. 2007;
Dimas, Gianopoulou, and Panias 2009) creating both environmental and economic
costs for its disposal. In 2006 the production of 71 million tonnes of alumina world-
wide (Barrientos 2009), generated practically an equal amount of red mud waste.
THE PRODUCTION PROCESS OF PRIMARY ALUMINUM: MASS AND
ENERGY ANALYSIS
The primary production of aluminum consists of the two fundamental
processes: the Bayer process and the Hall–Heroult process. The mass and energy
analysis of these two processes are presented in Tables 1–5 and Figure 1.
The Bayer Process
In the Bayer process, alumina is extracted from bauxite ore with sodium
hydroxide solution, under pressure (3.5 MPa) and elevated temperature (140–
300C), in digesters (digestion stage). The produced slurry contains dissolved sodium
aluminate and a solid residue called ‘‘red mud,’’ which is removed in thickeners
(clarification stage). The aluminate solution is then seeded with gibbsite (trihydrate
70 E. BALOMENOS ET AL.
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alumina) at 60C to accelerate the precipitation of gibbsite (precipitation stage).
Finally the precipitate is removed from the solution and is calcined at 1100Cto
produce powdered, high-grade metallurgical alumina (calcination stage).
According to literature (European Commission 2001; Choate and Green 2003;
Hudson et al. 2009) the Bayer process requires in total (including the energy for lime-
stone calcination) 24.61 MJ=kg Al of energy for heat, which is supplied through fuel
Table 1 Gibbsite extraction
1kgofAl
2
O
3
1kgofAl
Mass Exergy Mass Exergy
INPUT Bauxite input 2.65 kg 0.38 MJ 5.10 kg 0.74 MJ
NaOH 0.05 kg 0.09 MJ 0.09 kg 0.17 MJ
Limestone 0.03 kg 0.01 MJ 0.06 kg 0.01 MJ
Water 6.00 kg 0.94 MJ 11.57 kg 1.82 MJ
TOTAL INPUT (IN1) 8.72 kg 1.42 MJ 16.81 kg 2.73 MJ
UTILITIES Diesel Fuel 0.19 kg 8.64 MJ 0.36 kg 16.65 MJ
TOTAL UTILITIES (IN2) 0.19 kg 8.64 MJ 0.36 kg 16.65 MJ
PRODUCT Gibbsite 1.53 kg 0.28 MJ 2.95 kg 0.54 MJ
TOTAL PRODUCT (OUT1) 1.53 kg 0.28 MJ 2.95 kg 0.54 MJ
WASTE Red Mud 1.04 kg 0.17 MJ 2.00 kg 0.33 MJ
CO
2
from limestone calcination 0.01 kg 0.01 MJ 0.03 kg 0.01 MJ
Water 6.00 kg 0.94 MJ 11.57 kg 1.82 MJ
CO
2
from diesel 0.54 kg 0.24 MJ 1.04 kg 0.47 MJ
Steam from diesel 0.22 kg 0.14 MJ 0.43 kg 0.27 MJ
TOTAL WASTE (OUT2) 7.81 kg 1.50 MJ 15.06 kg 2.89 MJ
WASTE HEAT (IN1 þIN2-OUT1-OUT2) 8.27 MJ 15.95 MJ
Total Energy consumption (Utilities) 8.64 MJ 16.65 MJ
Total CO
2
emissions 0.55 kg 1.06 kg
Exergy Efficiency of process 2.79% 2.79%
Table 2 Gibbsite calcination (1100C)
1kg of Al
2
O
3
1kg of Al
Mass Exergy Mass Exergy
INPUT Gibbsite 1.53 kg 0.28 MJ 2.95 kg 0.54 MJ
TOTAL INPUT (IN1) 1.53 kg 0.28 MJ 2.95 kg 0.54 MJ
UTILITIES Diesel fuel 0.09 kg 4.13 MJ 0.17 kg 7.96 MJ
TOTAL UTILITIES (IN2) 0.09 kg 4.13 MJ 0.17 kg 7.96 MJ
PRODUCT Alumina 1.00 kg 0.42 MJ 1.93 kg 0.80 MJ
TOTAL PRODUCT (OUT1) 1.00 kg 0.42 MJ 1.93 kg 0.80 MJ
WASTE Steam from gibbsite 0.53 kg 0.33 MJ 1.02 kg 0.64 MJ
CO
2
from diesel 0.28 kg 0.13 MJ 0.54 kg 0.25 MJ
Steam from diesel 0.11 kg 0.07 MJ 0.21 kg 0.13 MJ
TOTAL WASTE (OUT2) 0.92 kg 0.53 MJ 1.77 kg 1.02 MJ
WASTE HEAT (IN1 þIN2-OUT1-OUT2) 3.47 MJ 6.68 MJ
Total Energy consumption (Utilities) 4.13 MJ 7.96 MJ
Total CO
2
emissions 0.28 kg 0.54 kg
Exergy Efficiency of process 9.45% 9.45%
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 71
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burning. In this study the used fuel is assumed to be diesel fuel which when burned
produces 45.90 MJ=kg of fuel (higher heating value [HHV, Bossel 2003]) and emits
2.86 kg of CO
2
=kg of fuel (U.S. Environmental Protection Agency 2009) and 1.19 kg
of H
2
O=kg of fuel (for simplicity in the calculations no other species are considered).
The Bayer process is responsible for the generation of a solid residue known as
red mud, which consists from metal oxides of Fe, Al, Ti, Si, K, Na, V, Ga, according
to the chemical composition of the initial bauxite ore (Table 8). Although red mud is
classified as a nonhazardous waste (Commission Decision 2000), its small particle
size (dust-like), high alkalinity, and large amounts (approximately 1 kg of red mud
is produced for each kg of alumina) makes its disposal a significant problem. Today,
red mud is disposed into sealed or unsealed artificial impoundments, leading to
important environmental issues (e.g., groundwater pH change, leakage, overflow,
air pollution by dust) and substantial land use (Gleich et al. 2006; Mason et al.
2007; Dimas et al. 2009).
Table 4 Hall–Heroult process
Powered from
hydroelectric plants
Powered from coal
burning plants
Mass Exergy Mass Exergy
INPUT Alumina 1.93 kg 0.80 MJ 1.93 kg 0.80 MJ
Carbon anodes 0.45 kg 15.37 MJ 0.45 kg 15.37 MJ
Na
3
AlF
6
0.03 kg 0.01 MJ 0.03 kg 0.01 MJ
AlF
3
0.04 kg 0.05 MJ 0.04 kg 0.05 MJ
TOTAL INPUT (IN1) 2.45 kg 16.23 MJ 2.45 kg 16.23 MJ
UTILITIES Electricity 56.24 MJ 170.27 MJ
Fuel for anode baking 0.06 kg 2.66 MJ 0.06 kg 2.66 MJ
TOTAL UTILITIES (IN2) 58.90 MJ 172.94 MJ
PRODUCT Aluminum 1.00kg 30.00 MJ 1.00 kg 30.00 MJ
TOTAL PRODUCT (OUT1) 1.00 kg 30.00 MJ 1.00 kg 30.00 MJ
WASTE CO
2
from electrolysis 1.53 kg 0.69 MJ 1.53 kg 0.69 MJ
CO
2
from anode baking 0.12 kg 0.05 MJ 0.12 kg 0.05 MJ
CF
4
(g) 2.99E-04kg 1.62E-03 MJ 2.99E-04 kg 1.62E-03 MJ
C
2
F
6
(g) 2.45E-05 kg 1.77E-04 MJ 2.45E-05 kg 1.77E-04 MJ
Spent Pot Lines (SPL) 0.02 kg 0.35 MJ 0.02 kg 0.35 MJ
CO
2
from electricity 0.00 kg 0.00 MJ 14.84 kg 6.68 MJ
TOTAL WASTE (OUT2) 1.67 kg 1.10 MJ 16.51 kg 7.78 MJ
WASTE HEAT (IN1 þIN2-OUT1-OUT2) 44.04MJ 151.39 MJ
Total energy consumption (utilities þcarbon anodes) 74.27 MJ 188.30 MJ
Total CO
2
–eq emissions 3.82 kg 18.66 kg
Exergy efficiency of process 39.93% 15.86%
Table 3 Total Bayer process
1kg of Al
2
O
3
1kg of Al
Total energy consumption 12.77 MJ 24.61 MJ
Total CO
2
emissions 0.83 kg 1.61 kg
Exergy efficiency of process 2.94% 2.94%
72 E. BALOMENOS ET AL.
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The Hall–He
´roult Process
In the Hall–He
´roult process, aluminum is produced by the electrolytic
reduction of high-grade alumina, which is dissolved in a molten bath consisting
mainly of cryolite (Na
3
AlF
6
), at a temperature of about 960C. Consumable carbon
anodes (0.45 kg for each kg of Al) and a potential of 4.6 V (Choate and Green 2003)
are employed in the electrolytic cell to produce molten aluminum, which is period-
ically withdrawn from the cathodes by vacuum siphon. The electrolytic cells
used in the process need to be periodically replaced, producing a carbon-based solid
waste (0.02 kg=kg Al) known as Spent Pot Lining (SPLs), which is classified as a
hazardous waste (Commission Decision 2000), due to its chemical content (12%
F

and 0.15% CN

).
Figure 1 Mass & energy balances in current industrial process of primary aluminum production.
Table 5 Primary production of 1 kg of aluminum
Powered from
hydroelectric plants
Powered from
coal burning plants
Total energy consumption 98.88 MJ 212.92 MJ
Total CO
2
-eq emissions 5.43 kg 20.27 kg
Exergy efficiency of process 30.17% 14.05%
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 73
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Approximately 56.24 MJ of electrical energy is consumed directly in the elec-
trolytic reduction of alumina for each kilogram of aluminum produced, assuming
hydroelectric energy utilization (which constitutes the best practice according to
IPPC directive [European Commission 2001]). Additionally, another 2.66 MJ=kg
Al of fuel energy are spent on carbon anode baking to produce the consumable car-
bon anodes. As these anodes are made of carbon, they are also in sense a fuel spent
in the process. Based on the HHV of coal (Bossel 2003), to produce 1 kg of Al
another 15.37 MJ of energy is consumed. In total 3.82 kg of CO
2
equivalent GHG
(Choate and Green 2003) are directly released for each kilogram of aluminum pro-
duced; 1.53 kg of CO
2
are released from anode consumption, 0.12 kg are released
during the anode baking and approximately 2.18 kg of CO
2
equivalent of hazardous
perfluorocarbons (PFCs) (Commission Decision 2000) are resulted from the process
upset known as anode effect.
These figures for the energy consumption of the Hall–He
´roult process refer to
hydroelectric energy utilization. This means that the electrical energy needed for the
electrolytic reduction is produced in hydroelectric dams at practically no additional
energy-resource cost (the energy cost for transporting the electrical current is
neglected [Choate and Green 2003]). However, most aluminum plants are not pow-
ered by hydroelectric electricity but from coal-based electricity. This means that
there is an extra energy-resource cost in the primary production of aluminum,
related to the consumption of coal. To produce 1 MJ of electrical energy approxi-
mately 3.03 MJ of heat produced from coal burning is required and approximately
0.26 kg CO
2
equivalent emissions are produced (U.S. Environmental Protection
Agency 2000). Therefore, in most cases, approximately 170.27 MJ of energy is con-
sumed to power the electrolytic reduction of alumina and an additional 14.84 kg of
CO
2
is indirectly released through coal burning for each kg of aluminum produced
by the Hall–He
´roult process. The Hall–He
´roult process is therefore, by design, the
most energy intensive stage in the primary production of aluminum, consuming
up to 188.30 MJ=kg Al (including the heating value of the consumable carbon
anodes), while its total (direct and indirect) CO
2
and CO
2
equivalent emissions are
up to 18.66 kg per kg of Al produced.
It is therefore obvious that a simple mass and energy balance is not enough to
fully describe the efficiency of resources utilization and the environmental impact of
an industrial process such as the production of primary aluminum. What is missing
is the second law analysis or exergy balance of the process, which as explained in
Appendix I, quantifies the cost of each material or utility resource used, in relation
to the theoretical minimum energy required to extract this resource from the
environment.
THE PRODUCTION PROCESS OF PRIMARY ALUMINUM: EXERGY
ANALYSIS
To perform an exergy analysis for the primary aluminum production, it is
necessary to calculate the chemical exergy values for all the compounds that are used
in the process. As shown in Table 6, chemical exergies of elements involved are taken
from literature (De Meester et al. 2006) and chemical exergies of species involved are
calculated according to the methodology presented in Appendix I and shown in
74 E. BALOMENOS ET AL.
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Tables 7 and 8. Fuel exergies are derived from their HHVs found in literature (Bossel
2003) Based on these data and assuming that all material flows entering and exiting
each production process are considered to be at the environmental state (T
0
¼25C,
P
0
¼1 atm), the detailed exergy analysis of the primary production of aluminum is
presented in Tables 1–5 and Figure 2.
The exergy analysis shows that the most exergy inefficient process is not the
high-energy consuming Hall–He
´roult process, which has an exergy efficiency of
39.93% (in the case of hydroelectric energy utilization) or 15.86% (in the case of
Table 7 Standard chemical exergy of species involved
Species MW (gr=mol) DG0
f(kJ=mol) e0
x(kJ=mol) e0
x(MJ=kg)
Al
2
O
3
101.96 1582.27 42.48 0.42
Al
2
O
3
3H
2
O 156.01 2310.12 28.59 0.18
Al
2
O
3
H
2
O (Boehmite) 119.98 1831.43 31.31 0.26
AlF
3
83.98 1431.11 95.74 1.14
CaCO
3
100.09 1128.08 19.67 0.20
CaOAl
2
O
3
2SiO
2
278.21 4002.17 71.71 0.26
CO
(g)
28.01 137.18 275.01 9.82
CO
2(g)
44.01 394.36 19.81 0.45
CF
4(g)
88.00 888.52 478.28 5.43
C
2
F
6(g)
138.01 1257.30 998.00 7.23
Fe
2
O
3
159.69 741.04 20.51 0.13
H
2
O
(g)
18.02 228.58 11.39 0.63
H
2
O
(l)
18.02 237.14 2.83 0.16
Na
3
AlF
6
209.94 3157.72 96.08 0.46
NaOH 40.00 379.65 76.83 1.92
NaAlSiO
4
142.05 1980.05 27.59 0.19
SiO
2
60.08 856.44 1.33 0.02
TiO
2
79.90 889.42 20.65 0.26
Calculation using exergy data from Table 6 and Equation (I-2) from Appendix I.
All Gibbs Free Energy of Formation are derived from the HSC 6.12 database.
Table 6 Standard chemical exergy of elements involved
Element e0
x(kJ=mol)MW (gr=mol) e0
x(MJ=kg)
Al 809.40 26.98 30.00
C 410.20 12.01 34.15
Ca 731.60 40.08 18.25
F
2
478.30 38.00 12.59
Fe 377.80 55.85 6.76
H
2
236.00 2.02 117.08
Na 336.50 22.99 14.64
O
2
3.97 32.00 0.12
Si 853.80 28.09 30.40
Ti 906.10 47.90 18.92
Based on De Meester B. et al. 2006.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 75
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coal-based electricity generation) but the less-energy intensive Bayer process, which
has an exergy efficiency of only 2.94%.
Exergy Analysis of the Bayer Process
In the Bayer process, as shown in Tables 1 and 2, for 1 kg of aluminum the
material input of the process (excluding fuel) embodies in total 2.73 MJ of chemical
exergy while the material output (excluding off-gases from fuel burning) embodies
3.60 MJ, of which 22% (0.80 MJ) in alumina, 9% (0.33 MJ) in the red mud waste,
0.3% (0.01 MJ) in CO
2
off-gases from limestone calcination, 18% (0.64 MJ) in water
vapors from gibbsite calcinations, and 50% (1.82 MJ) in waste water. In total, the
increase from input to output is 0.87 MJ, representing only 4% of the 24.61 MJ of
fuel exergy spent in the Bayer process.
To understand the reasons for this exergetic inefficiency, a closer look in the
process is needed. First of all there is an internal inefficiency in the chemical process
itself, as in process of gibbsite extraction (which includes limestone calcination,
bauxite digestion, liquor clarification, and gibbsite precipitation), the chemical
exergy embodied in the raw materials (2.73 MJ) is separated in gibbsite (0.54 MJ),
red mud solid waste (0.33 MJ), waste water (1.82 MJ), and CO
2
produced from lime-
stone calcination (0.01 MJ). In total the exergy embodied in the material output of
Table 8 Standard chemical exergy of species involved
Bauxite ore gr n
i
(mol) x
i
ex;i(kJ=mol) niex;i
Al
2
O
3
H
2
O (Boehmite) 100.00 0.83 0.11 25.79 21.50
Al
2
O
3
3H
2
O 500.00 3.20 0.42 26.41 84.64
Fe
2
O
3
200.00 1.25 0.16 16.00 20.04
SiO
2
90.00 1.50 0.19 2.74 4.10
TiO
2
60.00 0.75 0.10 14.88 11.17
CaOAl
2
O
3
2SiO
2
50.00 0.18 0.02 62.39 11.21
TOTAL 1000.00 7.72 1.00 144.47 kJ=kg of
Bauxite
0.14 MJ=kg of
Bauxite
Red Mud Waste gr n
i
(mol) x
i
ex;i(kJ=mol) niex;i
Fe
2
O
3
510.00 3.19 0.47 18.65 59.57
TiO
2
153.00 1.91 0.28 17.53 33.56
CaOAl
2
O
3
2SiO
2
209.27 0.75 0.11 66.27 49.85
NaAlSiO
4
127.73 0.90 0.13 22.59 20.31
TOTAL 1000.00 6.76 1.00 163.29 kJ=kg of
Red Mud
0.16 MJ=kg of
Red Mud
Calculations assuming an ideal mixture according to Equation (I-3) from Appendix I. Mineral species
concentrations reflect typical samples of Bauxite ore.
The red mud waste is assumed to be produced from the remaining species of the bauxite ore according
to the bauxite to red mud ratio given in Table 1. The lime addition is converted to anorthite
(CaO Al
2
O
3
2SiO
2
) while the sodium addition reacts with bauxite silica to form nepheline (NaAlSiO
4
,
0.5 (Na
2
OAl
2
O
3
2SiO
2
)).
76 E. BALOMENOS ET AL.
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this process is 2.70 MJ, meaning that not only does it not increase the exergetic value
of the feedstock but instead it results in a slight decrease (0.03 MJ). This inefficiency
is the result of a cyclic chemical process designed to separate gibbsite from bauxite
and of a series of spontaneous reactions resulting in the production of the red
mud waste.
The gibbsite extraction from the bauxite could be represented as
Al2O33H2O½bauxiteþ2NaOH½aq!2NaAlO2½aqþ4H2OðlÞ!Al2O33H2O½s
þ2NaOH½aq
If this process, is viewed as beginning with gibbsite embodied in bauxite ore
and resulting in solid gibbsite, then from a thermodynamic point of view no chemical
transformation takes place and all that is needed, is work to separate gibbsite from
the bauxite ore. If bauxite is assumed to be an ideal mixture of mineral species
(Table 8), the minimum work needed to extractgibbsite is 6.98 kJ=kg of Bauxite
or 0.04 MJ=kg of Al produced.
The hematite (Fe
2
O
3
), rutile (TiO
2
), and anorthite (CaO Al
2
O
3
SiO
2
) mineral
species of bauxite are assumed to be transferred without any chemical transform-
ation to the red mud waste, while bauxite silica (SiO
2
) is assumed to react both with
the added lime (CaO, produced form limestone calcination) producing more
Figure 2 Exergy analysis of the current industrial process for primary aluminum production,
with coal-based electricity (all input and output flows are calculated at 25C and atmospheric
pressure).
The minimum work required to separate the i-component of an ideal mixture is equal to the
opposite of the exergy of mixing of the i-component, W ¼–n
i
RT
0
ln(x
i
).
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 77
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anorthite as well as with the sodium aluminate liquor producing nepheline
1
(NaAlSiO
4
), according to
SiO2½bauxiteþNaAlO2½aq!NaAlSiO4½s
CaO½sþSiO2½bauxiteþAl2O3½bauxite!CaO Al2O3SiO2½s
As both reactions are exothermic, they occur spontaneously, consuming exergy
from the material input (and not from the fuel exergy). While the red mud waste and
the gibbsite have in total 0.13 MJ more chemical exergy than the original bauxite ore,
this increase is mainly caused by the ‘‘consumption’’ of the sodium hydroxide input
(0.17 MJ). This is why, in total the chemical exergy embodied in the material output
is less than the chemical exergy embodied in the material input and the process is
characterized by low exergy efficiency.
From the 16.65 MJ spent in utilities, 0.2% (0.04 MJ) is spent in gibbsite
extraction, 0.6% (0.10 MJ) is spent in limestone calcination (according to the
enthalpy of the reaction found in HSC 6.12 database), and 4.4% (0.74 MJ) of
the fuel exergy is embodied in the off-gases produced from fuel burning, leaving
the remaining 94.7% of fuel exergy to be theoretically unutilized. Obviously a por-
tion of this exergy is consumed due to the nonideality of the bauxite ore mixture
(which results in higher work needed for gibbsite extraction), while additional
exergy has to be spent in order to increase during precipitation the gibbsite crystal
size at levels that will satisfy the standards needed in the Hall–He
´roult process (a
final crystal size >44 mm is needed while crystal growth happens at rates from
9mm=h (Hudson et al. 2009) to 1 mm=24 h according to precipitation conditions).
Finally, fuel exergy is lost due to external exergy losses in the industrial site
(e.g., heat radiation to the environment due to imperfect insulation) and of course
due to the irreversibility of the process itself (internal exergy losses). Assuming that
the aluminum industry has taken all steps to minimize external exergy losses and
that the conditions are optimized in respect to production rates, no further increase
in the exergy efficiency of these three processes can be foreseen. What is clearly
needed is a novel chemical route that will allow for a higher degree of fuel exergy
to be embodied in the material output, meaning a chemical route that will result in
products with higher chemical exergy.
In the next stage of the Bayer process, for the production of 1 kg of aluminum
approximately 0.17 kg of diesel fuel (data from the aluminum S.A. plant in Greece)
with 7.96 MJ of chemical exergy is consumed to produce sufficient heat for the
calcination at 1100C of 2.95 kg of gibbsite, with chemical exergy of 0.54 MJ, to
1.93 kg of alumina, with chemical exergy of 0.80 MJ. This means that from the
7.96 MJ of exergy spent in the process only 0.26 MJ is embodied in the product of
the process (alumina), while the rest is discarded as CO
2
off gases, water vapors,
and waste heat.
1
Cancrinite (Na
6
Ca
2
[Al
6
Si
6
O
24
](CO
3
)
2
¼6 NaAlSiO
4
þ2 CaCO
3
) is a more accurate representation
of the sodium containing species in the red mud. However as no consistent themodynamic data for the
cancrinite formation were found, nepheline and anorthite were chosen instead.
78 E. BALOMENOS ET AL.
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To analyze this process further, the chemical reaction of the gibbsite calcination
Al2O33H2OþQ!Al2O3þ3H2O
for the production of 1.93 kg of alumina has a total energy requirement (enthalpy of
reaction, according to the HSC 6.12 database) 3.51 MJ, or 44% of the fuel exergy
spent. The chemical exergy of the produced alumina and water vapors are 0.91 MJ
higher than the exergy of the consumed gibbsite, accounting for another 11% of fuel
exergy. The burning of 0.17 kg of diesel fuel with atmospheric oxygen (zero chemical
exergy) produces CO
2
and H
2
O off-gasses, which in total embody 5% (0.38MJ) of the
initial fuel exergy. Therefore only 60% of the total fuel exergy is utilized in the process,
the remaining 40% of fuel exergy is lost. Considering the calciner as an ideal heat
engine, utilizing heat content (fuel burning) from a thermal reservoir of 1100C to pro-
duce work (chemical reaction) and reject heat at a thermal reservoir of 25C (environ-
ment), then its maximum efficiency would be 78%.
2
So at least 22% (1.73 MJ) of the
fuel exergy is lost as waste heat due the irreversibility of heat transfer between the
two thermal reservoirs (internal exergy loss). The remaining 18% (1.45MJ) is the heat
content in the hot off-gasses and alumina product as well as the external heat losses.
Bearing in mind that the diesel fuel consumption mentioned, includes the utilization
of the heat content of off-gasses in preheating, it is clear that little can be done to
improve the exergy efficiency of the calcination process under these conditions.
Theoretically, the only way to improve the exergy efficiency of this process would
be to conduct the calcination at a higher temperature thereby increasing the efficiency
of the heat transfer. However, this would not only result in higher fuel consumption
but it will also lead to the formation of a-Al
2
O
3
, an allotropic form of alumina
favored in higher temperatures, which is outside the metallurgical standards for
alumina currently required in the Hall–He
´roult process (<20% in a-Al
2
O
3
content
[Hudson et al. 2009]).
Exergy Analysis of the Hall–He
´roult Process
As mentioned earlier the exergy efficiency of the Hall–He
´roult process is not
as low as the Bayer process, as its final product, aluminum, embodies significant
chemical exergy (30 MJ=kg), representing a 3634% increase from the alumina
input (0.80 MJ). However this increase comes at a high exergy cost as when electricity
is produced from coal burning, from the 188.30MJ of exergy spent in utilities and
carbon anodes only 16% (29.19 MJ) are embodied in the aluminum product. The total
energy requirement of the reaction
2Al2O3þ3C !4Al þ3CO2
is 20.11 MJ=kg of Al (enthalpy of reaction, according to the HSC 6.12 database) repre-
senting only 11% of the utilities input. The aluminum is produced at a temperature of
960C and therefore embodies another 1.42 MJ=kg of exergy (heat calculation from
the HSC 6.12 database) or 1% of utilities input. Finally, the waste products of the
2
The maximum available work (exergy) of an ideal heat engine operating between two thermal
reservoirs T >T
o
is W
max
¼B¼Q
fuel
(TT
o
)=T.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 79
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process (CO
2
off-gases, PFCs, Spent Pot Lines waste from the electrolysis and CO
2
off-gases from coal-based electricity generation) embody only 4% (7.78MJ) of the
total ulitilies exergy. Therefore 69% (129.80 MJ) of the utilities exergy is not utilized
during the Hall–He
´roult process. This high exergy loss is mainly attributed to the exer-
getic cost of producing electricity from nonrenewable energy sources like coal. If the
electricity was produced entirely from renewable sources only 21% (15.76MJ) would
be lost during the process. These 15.76 MJ can be attributed both to electrical overpo-
tentials in the electrolyte bath as well as to the inherent volumetric inefficiency of the
electrolytic reduction process: As the reaction takes place in a practically two-
dimensional space between the electrodes (average anode–cathode distance is
4–5 cm while the surface of the carbon anode is typically 70125 cm (Ju
¨rgen Buschow
et al. 2001) lengthy installations are required and thus great heat losses are produced.
Therefore to reduce the exergy losses in the Hall–He
´roult process either renew-
able energy sources must be used to produce electricity or a different chemical route
must be followed.
FUTURE SUSTAINABILITY PROSPECTS
The energy and exergy analysis of the primary aluminum production process,
revealed that to produce 1 kg of aluminum utilizing coal-produced electricity a total
of 212.92 MJ is spent in utilities, 20.27 kg of CO
2
-eq gasses are released in the atmos-
phere and the exergy efficiency of the process is only 14.05%. Bearing in mind that
both, Bayer and Hall–He
´roult, have been continuously optimized for more than a
century, it is logical to conclude that significant energy and exergy improvements
cannot be achieved through further optimizations of these technologies. The exergy
analysis preformed in this study clearly indicates that radical changes are needed in
all processes, in order to achieve major breakthroughs in the energy and exergy
efficiency of the industry. A list of proven or achievable technologies that could
produce such changes, is presented in the following section.
The Boehmite Process
The Bohemite process, developed originally by the National Technical
University of Athens (NTUA) Laboratory of Metallurgy (Paspaliaris and Panias
1998; Panias, Paspaliaris, Amanatidis, and Schmidt 2001; Skoufadis, Panias, and
Paspaliaris 2003) and recently advanced by the Institute of Minerals and Materials
Technology (CSIR) of India (Dash et al. 2009), follows a similar flow sheet with
the Bayer process, but through the replacement of gibbsite seeding with boehmite
seeding and an optimization of the alumina to caustic ratio in the precipitation
solution, it can achieve the precipitation of monohydrate (boehmite) rather than tri-
hydrate (gibbsite) alumina under atmospheric pressure in temperatures lower than
90C. In accordance with the description given earlier for the first stages of the Bayer
process, the Bohemite process can be thought of as
Al2O33H2O½bauxiteþ2NaOH½aq!2NaAlO2½aqþ4H2OðlÞ!Al2O3H2O½s
þ2NaOH½aqþ2H2OðlÞ
80 E. BALOMENOS ET AL.
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The process is no longer a chemical cycle as the final product (boehmite) differs
from the initial alumina phase (gibbsite). Assuming similar energy and material
requirements with the four first stages of the Bayer process (Figure 3), the Boehmite
process produces a product with higher exergy value (2.28 kg of boehmite embody
0.59 MJ of chemical exergy) and thus the process is characterized by higher exergy
efficiency (n
ex
¼3.06% rather than n
ex
¼2.79% in the Bayer process).
More importantly, with the boehmite process the weight of the precipitate
(boehmite) needed to produce 1.93 kg of alumina after calcination is 2.28 kg, mean-
ing 23% less than the weight of the corresponding gibbsite produced from the Bayer
process. Therefore less fuel exergy will be required in the subsequent calcination
stage as less mass will have to be calcinated. Moreover, thermodynamically the cal-
cination reaction of boehmite can take place at 900C and requires 115 kJ=mole
Al
2
O
3
less enthalpy than the gibbsite calcinations (Panias et al. 2001). In total
boehmite calcination results in 60% fuel reduction (and thus a 60% CO
2
emissions
reductions from fuel burning) and a 9.45% point increase in the exergy efficiency
of the calcination step (n
ex
¼21%).
The entire alumina production process utilizing the Boehmite process, when
compared to the conventional Bayer process, can achieve 19% energy savings and
22% CO
2
emissions reduction and a 2% point increase in the exergy efficiency
(n
ex
¼5%). Additionally this technology stands to be economically viable as the mono-
hydrate alumina produced is a suitable precursor for the production of a variety of spe-
cialty=transitional aluminas, currently produced from expensive synthetical processes.
Red Mud Utilization
The red mud waste embodies 44% of the chemical exergy of the bauxite input
and is produced at a 2:1 mass ratio to primary aluminum. The relatively high exergy
Figure 3 Mass & energy balance of the Boehmite process.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 81
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embodied in the red mud is mainly due to its high content of Fe
2
O
3
(in this study
51% per weight), which could characterize red mud as an industrial feedstock rather
than an industrial waste. To this end, many attempts have been made to produce pig
iron from red mud, but till recently no economically viable solution has been found.
The extremely fine particles of red mud require agglomeration prior to feeding in
conventional reactors; their high alkaline nature is unsuitable for blast furnace
reductive smelting and the low, when compared to iron ores, content in iron oxides
makes the production of pig iron a cost ineffective process (Stivanakis et al. 2002;
Kumar et al. 2006). Valuable metals like Ti, Ga, and V contained in the red mud
can be extracted (Maitra 1994; Agatzini-Leonardou et al. 2008), but again with no
economical benefits (Stivanakis et al. 2002). Other proposed uses for red mud include
the production of construction or ceramic materials (Yang et al. 2008; Dimas et al.
2009), as well as utilizing red mud in wastewaters or polluted soils treatments
(Santona, Castaldi, and Melis 2006; Mason et al. 2007; Wang et al. 2008).
A patent-pending process has been recently developed by the Advanced Mineral
Recovery Technologies (AMRT, Ltd.) and the NTUA’s Laboratory of Metallurgy,
for the direct transformation of red mud into valuable products. The treatment is
based on the innovative AMRT-Electric Arc Furnace (AMRT-EAF), which has
the capability of processing finely sized materials, notably below 1 mm in particle size
(dust like), without any dusty material loss in the off-gas stream (AMRT 2009a). The
operation of the AMRT-EAF is controlled by a unique patented PLC control system
(AMRT 2009b), which, based on system-dependent programmable parameters pre-
dicted accurately by a proprietary thermodynamic model, can control impedance
of the electric arc produced by the electrodes through the control of the positioning
of the electrodes within the furnace influencing the energy input, the arc stability,
the solid charge melting pattern, and the electrode consumption.
Therefore this innovative EAF technology is ideal for processing the dust-like
red mud (mean particle size less than 500 nm) without any pretreatment or substan-
tial energy losses, thus providing the proposed process with a significant industrial
advantage.
Utilizing the AMRT-EAF the red mud can be directly transformed into pig iron
and viscous slag that can be converted into glassy fibers suitable for mineral wool
production, thereby converting all red mud into valuable products. The reductive
smelting of red mud has been achieved at 1500C, using carbon as a reducing agent
along with appropriate fluxes (e.g., SiO
2
) to regulate the composition of the generated
slag. From 1 kg of red mud approximately 0.36 kg of pig iron (chemical exergy
2.41 MJ, assuming the same chemical exergy value as metallic iron) and 0.68 kg of
a viscous slag (chemical exergy 0.10 MJ, assuming that no chemical transformation
takes place in the remaining red mud species) were produced, consuming 0.19 kg of
coke (chemical exergy 6.42 MJ) and 6.20 MJ of electricity and releasing 0.49 kg of
CO
2
. If this electricity is produced from coal burning then in total 18.79 MJ of coal
exergy is consumed and an additional 1.64 kg of CO
2
is released. The exergy efficiency
of this process is 20% when electricity from hydroelectric dams is used and 10% when
electricity from coal burning is used, while the process is solid and liquid waste-free.
In total the new process (Figure 4) for the complete bauxite exploitation (for
alumina, pig iron, and mineral wool production) will increase the exergy efficiency
from 3% in the conventional Bayer process to 14% with hydroelectric dam produced
82 E. BALOMENOS ET AL.
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electricity, or to 8% with coal-produced electricity, as the solid waste of the Bayer
process (red mud) with chemical exergy of 0.33 MJ=kgAl is replaced by pig iron
and mineral wool products, with total chemical exergy 5.04 MJ=kgAl. From an
economic perspective a solid waste with costly disposal, is replaced, in a single step
process, by two valuable by-products thereby significantly increasing the versatility
and profit margin of the industry.
High Temperature Carbothermic Reduction of Alumina
The only alternative to the electrolytic reduction of alumina is its direct chemi-
cal reduction. From the reducing agents considered suitable for industrial production
like C, CO, CH
4
,H
2
, Si only carbon has the thermodynamic capacity to reduce alu-
mina, as aluminum is one of the most reactive metals and one of the most efficient
reducing agent itself. Thus, the basic alternative to the Hall–He
´roult process is the
carbothermic reduction of alumina, that has been proposed by various researchers
in the last 50 years (Ju
¨rgen Buschow et al. 2001; Choate and Green 2003). This
process is theoretically described by the following chemical reaction occurring at
temperatures higher than 1900C:
Al2O3þ3C ¼2Al þ3CO
The industrial application of this process has not yet been achieved, as the
occurrence of side-reactions, resulting in the formation of undesired products, such
as the aluminum carbide Al
4
C
3
and the Al-oxycarbides Al
2
OC and Al
4
O
4
C, as well
as of aluminum and aluminum suboxide Al
2
O vapors (Ju
¨rgen Buschow et al. 2001;
Frank et al. 2009), which substantially reduce the aluminum yield. Aluminum yields
as high as 67% have been obtained by staging the reactions to produce aluminum car-
bides at 1930–2030C and then in a second step reducing the carbides with alumina to
Figure 4 Mass & energy balance of the Bayer process followed by the Red Mud process.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 83
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produce aluminum and carbon monoxide at 2030–2130C (Cohran 1976; Choate and
Green 2003). Scientific research is focused on increasing this yield through either,
alternative chemical routes or the development of specific reactors with better thermal
efficiency and advance vapor management (Choate and Green 2003; Halmann, Frei,
and Steinfeld 2007; Frank et al. 2009). More recently a high temperature route for
carbothermic reduction of alumina was proposed by Halmann et al. (2007) where
carbide formation is avoided at high temperatures (2100–2400C) and yields as high
as 90% can be achieved.
The main exergetic benefit from replacing the electrolytic reduction of alumina
with a carbothermic process, is that carbon will be used as a direct reducing agent
supplying the 47% (based on the figures given in literature [Halmann et al. 2007])
of the total energy for the process instead of the 8% that is currently being utilized
as consumable carbon anodes. The high exergetic cost of transforming carbon into
electricity to drive the redox reaction will be avoided, while the alumina reduction
will take place in a three-dimensional space thus avoiding heat losses due the volu-
metric ineffieciency of the Hall–He
´roult process. Additionally such a carbothermic
process would avoid entirely PFC emissions and SPL solid wastes.
The process heat needed to reach the high temperatures for the carbothermic
reduction can be provided either through electricity or in the future through con-
centrated solar heat (Murray 1999; Halmann et al. 2007). Even with the case of
coal-produced electrical heating for a process at 2100C at least a 16% savings in uti-
lities and 23% in CO
2
–eq. emissions and an increase of 3% points in the exergy
efficient of the process can be foreseen when compared to the Hall–He
´roult process
powered from coal-produced electricity (Figure 5). Energy and exergy savings can
increase further if the hot CO off-gases of the process are utilized, for example, meth-
anol production (Steinfeld 1997) (through a water-gas shift reaction to produce first
hydrogen gas) as well as if in the future concentrated solar heat is used to supply a
portion of the required process heat.
CONCLUSIONS
The primary aluminum production is characterized by low exergy yield in the
Bayer process and high energy and CO
2
intensity in the Hall–He
´roult process.
Figure 5 Mass & energy balance of the Carbothermic Reduction process at high temperatures. With
dashed lines, the alternative prospect of utilizing CO for methanol production process is also shown.
84 E. BALOMENOS ET AL.
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The reason for both these inefficiencies lies not in the methods of their industrial
realization but in their thermodynamic principles.
In the case of the Bayer process the cyclic chemical procedure used to separate
gibbsite from bauxite spends significant amounts of unembodied exergy while produc-
ing red mud waste practically in a 1:1 mass ratio with alumina. The Boehmite process
can achieve the precipitation of monohydrate alumina instead of trihydrate thereby
reducing energy consumption and increasing exergy efficiency in the subsequent cal-
cinations step. In addition, if the red mud waste is utilized as industrial feedstock for
pig iron and mineral wool production, the total exergy efficiency of the Bayer process
will increase significantly, while the solid wastes of the process will be eliminated.
In the case of the Hall–He
´roult process, the high energy and exergy cost is
related primarily to the cost of electricity generation. If the process is replaced by
a high temperature carbothermic reduction, less electrical energy (high exergy con-
tent produced from carbon burning) and more carbon (medium exergy content) will
be used directly for the reduction of alumina. Additionally, if in the future the
process heat is not supplied through electrical heating but through concentrated
solar power, the process can become truly sustainable.
These novel technologies presented here for the reformation of the primary
aluminum industry are proven in principle but obviously require substantial techno-
logical optimizations and large-scale industrial demonstrations before they can be
deemed ready for industrial application. What is therefore missing, are not the techno-
logical alternatives to the 100-year-old Bayer and Hall–He
´roult processes but the will
of primary aluminum industry to invest in high-risk high-impact novel production pro-
cesses, which can combine improved profit margins with environmental sustainability.
REFERENCES
Agatzini-Leonardou, S., Oustadakis, P., Tsakiridis, P. E., and Markopoulos, Ch., 2008,
‘‘Titanium leaching from red mud by diluted sulfuric acid at atmospheric pressure.’’
Journal of Hazardous Materials, 157, pp. 579–586.
AMRT, 2009a, Advanced Mineral Recovery Technologies Ltd., web page: http://amrt.co.uk/
index.html (accessed 10=12=2009).
AMRT, 2009b, Advanced Mineral Recovery Technologies Ltd., 2009, Control System for an
arc furnace, UK Patent, GB 2430276 B, 16=09=2009.
Barrientos, M., 2009, Index Mundi, http://indexmundi.com/ (accessed 10=12=2009).
Bossel, U., 2003, Well-to-Wheel Studies, Heating Values, and the energy conservation
Principle, European Fuel Cell Forum.
Brodyansky, V. M., et al., 1994, The Efficiency of Industrial Processes: Exergy Analysis and
Optimization, Amsterdam: Elsevier Science B.V.
Choate, W. T. and Green, J. A. S., 2003, U.S. Energy Requirements for Aluminum Production:
Historical Perspective, Theoretical Limits and New Opportunities, Washington, D.C.: U.S.
Department of Energy Efficiency and Renewable Energy.
Cohran, C., 1976, ‘‘Carbothermic Production of Aluminum.’’ United State Patent 3,971,65.
Commission Decision, 2000, 2000=532=EC, Official Journal of the European Communities,
http://eur-ex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2000:226:0003:0024:EN:
PDF (accessed 10=12=2009).
Dash, B., et al., 2009, ‘‘Precipitation of boehmite in sodium aluminate liquor.’’ Hydrometal-
lurgy, 95, pp. 297–301.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 85
Downloaded By: [Panias, Dimitrios] At: 11:21 9 March 2011

De Meester, B., et al., 2006, ‘‘An improved calculation of the exergy of natural resources for
exergetic life cycle assessment.’’ Environmental Science & Technology, 40, pp. 6844–6851.
Dimas, D., Gianopoulou, I. P., and Panias, D., 2009, ‘‘Utilization of alumina red mud for
synthesis of inorganic polymeric materials.’’ Mineral Processing and Extractive Metal-
lurgy Review, 30, pp. 1–29.
European Commission, 2001, Integrated Pollution Prevention and Control (IPPC), 2001,
‘‘Best Available Techniques Reference Document in the non ferrous metals industries’’.
Frank, W. B., et al., 2009, Aluminium, Ullmann’s Encyclopedia of Industrial Chemistry,
Weinheim: Wiley-VCH Verlag GmbH & Co.
Gleich, A., et al (editors.), 2006, Sustainable Metals Management, Chapter 4: Prospects for a
Sustainable Aluminium Industry, Dordrecht: Springer, pp. 97–111.
Halmann, M., Frei, A., and Steinfeld, A., 2007, ‘‘Carbothermal reduction of alumina:
Thermochemical equilibrium calculations and experimental investigation.’’ Energy, 32,
pp. 2420–2427.
Hudson, L. K., et al., 2009, Aluminium Oxide, Ullmann’s Encyclopedia of Industrial Chemistry,
Weinheim: Wiley-VCH Verlag GmbH & Co. KGa.
Ju
¨rgen Buschow, K. H., et al. (editors), 2001, Encyclopedia of Materials: Science and Technology,
Chapter: Aluminum Production and Refining, Oxford: Elsevier Science Ltd., pp. 132–140.
Kumar, S., et al., 2006, ‘‘Innovative methodologies for the utilisation of wastes from metallur-
gical and allied industries Resources.’’ Conservation and Recycling, 48, pp. 301–314.
Maitra, P. K., 1994, ‘‘Recovery of TiO
2
from red mud for abatement of pollution and for
conservation of land and mineral resources.’’ Light Metals, pp. 159–165.
Mason, L. G., et al. (editors), 2007, Focus on Hazardous Materials Research, Chapter 3:
Red Mud and Paper Mill Sludge Treatment and Reuse for the Recovery of Contaminated
Soils, Sediments and Waters: A focus on latest trends, New York: Nova Science
Publishers, Inc., pp. 77–142.
Meyers, R. A. (editor), 2004, Encyclopedia of Physical Science and Technology: Materials
Chapter: Aluminum, New York: Academic Press, pp. 495–518.
Murray, J. P., 1999, ‘‘Aluminum production using high-temperature solar process heat.’’ Solar
Energy, 66(2), pp. 133–142.
Panias, D., Paspaliaris, I., Amanatidis, A., and Schmidt, H. W., 2001, A. Hollnagel, ‘‘Boeh-
mite Process: An Alternative technology in alumina production.’’ Light Metals New
Orleans, USA, pp. 97–103.
Panias, D. and Paspaliaris, I., 2003, ‘‘Boehmite process—a new approach in alumina
production.’’ Erzmetall, 56(2), pp. 75–80.
Paspaliaris, I. and Panias, D., 1998, ‘‘Precipitation of boehmite—an innovative route in the
alumina production’’. Fortschritte in der Hydrometallurgie, Metallurgischen Seminar
des Fachausschusses fur Metallurgische Aus- und Weiterbildung der GDMB, Heft 82
der Schriftenreihe der GDMB Gesellshaft fur Bergbau, Metallurgie, Rohstoff- und
Umwelttechnik, Goslar, Germany, 18–20 November, pp. 111–127.
Santona, L., Castaldi, P., and Melis, P., 2006, ‘‘Evaluation of the interaction mechanisms
between red muds and heavy metals.’’ Journal of Hazardous Materials, B136,
pp. 324–329.
Skoufadis, C., Panias, D., and Paspaliaris, I., 2003, ‘‘Kinetics of boehmite precipitation from
supersaturated sodium aluminate solutions.’’ Hydrometallurgy, 68, pp. 57–68.
Steinfeld, A., 1997, ‘‘High-temperature solar thermochemistry for CO
2
mitigation in the
extractive metallurgical industry.’’ Energy, 22, pp. 311–6.
Stivanakis, V. M., et al., 2002, ‘‘On the utilization of red mud in the heavy clay industry in
Greece.’’ CIMTEC 2002, International Symposium on Modern Materials & Technolo-
gies, 10th International Ceramics Forum & 3rd Forum on New Materials.
Szargut, J., 1980, ‘‘International Progress in second law analysis.’’ Energy, 5, pp. 709–718.
86 E. BALOMENOS ET AL.
Downloaded By: [Panias, Dimitrios] At: 11:21 9 March 2011

U.S. Environmental Protection Agency, 2000, Carbon Dioxide Emissions from the
Generation of Electric Power in the United States, Washington DC: Department of
Energy and Environmental Protection Agency.
U.S. Environmental Protection Agency, 2009, Average Carbon Dioxide Emissions Resulting
from Gasoline and Diesel Fuel, http//www.epa.gov/otaq/greenhousegases.htm,
(accessed 10=12=2009)
Wang, S., et al. 2008, ‘‘Novel applications of red mud as coagulant, adsorbent and catalyst for
environmentally benign processes.’’ Chemosphere, 72, pp. 1621–1635.
Warner, N. A., 2008, ‘‘Conceptual Design for Lower-Energy Primary Aluminum.’’ Metallur-
gical and Materials Transactions B, 39B, pp. 247.
Yang, J., et al. 2008, ‘‘Preparation of glass-ceramics from red mud in the aluminium indus-
tries.’’ Ceramics International, 34, pp. 125–130.
APPENDIX I: EXERGY ANALYSIS OF A CHEMICAL PROCESS
In thermodynamics the exergy of a nonisolated system is the maximum work
that can be extracted from this system during a process that brings the system into
equilibrium with its surroundings. By choosing as a surrounding medium for all sys-
tems the environment of our planet, exergy, which is measured in energy units, can
become a universal measure of the quality of matter and energy (Brodyansky et al.
1994). In general, exergy can be defined as the resource consumed by dissipative struc-
tures to produce structure=information and remain in states far from thermodynamic
equilibrium with their environment or the resource consumed by decaying structures
as they proceed to thermodynamic equilibrium with their environment (Szargut 1980).
Exergy is divided according to its carrier flow into exergy of energy (heat and
radiation) and exergy of matter (chemical and thermomechanical) (Brodyansky et al.
1994). Other forms of energy like electrical, magnetic, mechanical, and even nuclear
are considered ordered forms of energy (meaning that ideally they can be converted
to other forms of energy without entropy related losses) and their exergy values are
the same as their energy values.
In general the chemical exergy (e
x
) of matter defines the maximum work
obtainable by the chemical interaction (reaction) of matter with its environment.
In cases of fuel, chemical exergy is measured by the heating value of the fuel
(Brodyansky et al. 1994). To produce 1 MJ of electrical exergy 3.03 MJ of coal exergy
has to be consumed (the HHV, of coal that can be equated with its chemical exergy,
is 34.1 MJ=kg of coal [Bossel 2003]).
For matter that is not fuel, the standard chemical exergy e0
x

can be calculated
from its theoretical reaction of formation at the environmental standard state (T
0
,P
0
)
aA þxX þyY âAaXxYyðI-1Þ
according to the relationship
e0
x;AaXxYy¼DG0
f;AaXxYyT0
ðÞþ
X
i
vie0
i;ðI-2Þ
where DG0
f;AaXxYyT0
ðÞis chemical free energy of formation of the substance, v
i
is the
stoichiometric coefficient, and e0
iis the standard chemical energies of element i.
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 87
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The standard chemical exergy of elements is related to reference substances found
more commonly in the environment and are given in literature (Brodyansky et al.
1994; Szargut 1980; De Meester et al. 2006).
The chemical exergy of solution containing nchemical species at the environ-
mental state (T
0
,P
0
)is
ex¼X
i
niex;i¼X
i
nie0
x;iþRT0X
i
niln cixiðI-3Þ
The thermomechanical exergy of matter ep;T

which is the exergy of matter
due only to its differences in pressure and temperature from the environment, is
ep;T¼hh0T0ss0
ðÞ¼DhT0DsðI-4Þ
where h, s are the specific enthalpy and entropy of matter at T, P and at T
0
,P
0
.
Finally the total exergy of a solution at conditions T, P different from the
environmental state is
e¼exþep;T:ðI-5Þ
With Equations (I-2–I-5), an exergy value can be assigned to any chemical
component or chemical solution of an industrial process. The chemical exergy quan-
tifies in energy units the quality of matter in relation to our environment (planet
Earth). The higher the chemical exergy value of a compound the further away this
compound is from the dead state (total thermodynamic equilibrium with the
environment, e
x
¼e
p
,T¼0). As seen in Table I-1 the chemical exergy of hematite,
which is the most common iron oxide, is very low, but as the oxidation numbers
of iron decrease the chemical exergy of the iron species (and its scarcity in the oxidiz-
ing environment of our planet) increases. The same is not true for the corresponding
Gibbs free energy of formation, which provides no information for the value of the
oxide in relation to the environment. A compound with high chemical exergy is
therefore either a valuable product or a potential hazardous waste (as it is far from
chemical equilibrium with the environment).
Table I-1. Exergetic classification of metal oxides
Chemical type Oxidation number of metal e0
x(kJ=mole) DG0
f(kJ=mole)
Fe
2
O
3
þ3 20.51 741.04
Fe
3
O
4
þ2,þ3 129.00 1012.34
FeO þ2 134.06 245.72
Fe 0 377.80 0.0
Al
2
O
3
3H
2
Oþ3 28.59 2310.12
Al
2
O
3
H
2
O (B) þ3 31.31 1831.43
Al
2
O
3
þ3 42.48 1582.27
Al 0 809.40 0.0
88 E. BALOMENOS ET AL.
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Any industrial process related to extractive metallurgy can be defined as a
process where matter with low chemical exergy (ore) is transformed into matter with
high chemical exergy (metal) with the consumption of exergy provided by utilities
and chemical agents (fuel for heating, reducing agents, etc.). The theoretical mini-
mum exergy that has to be consumed in an ideal transformation process (reversible
process) can be found by the difference of the standard chemical exergies of feed-
stock and product. As seen in Table I-1, to reversibly reduce of hematite to iron
the minimum exergy that has to be spent is 357.3 kJ=mole or 6.4 MJ=kg of iron while
to reduce gibbsite to aluminum the minimum exergy is 795.1 kJ=mole or 29.5 MJ=kg
of aluminum.
Moreover, chemical exergy allows the definition of a thermodynamic efficiency
coefficient for any process, based on a simple exergy balance
Etot
in ¼Etot
out þDtot;ðI-6Þ
where Etot
in ;Etot
out are the total exergy flows entering and exiting the system (a steady
state process is assumed), and D
tot
is the total exergy losses or exergy consumption or
waste heat due to irreversible processes inside the system (internal losses) and
between the system and the environment (external losses). According to the
Guy-Stolola equation and the second law of thermodynamics
Dtot ¼T0DStot 0;ðI-7Þ
where DS
tot
is the total increase in entropy in both system and environment.
By dividing the exergy outflow Etot
out into products and wastes (e.g., chemical
wastes) according to equation
Etot
out ¼Etot
product þEtot
waste:ðI-8Þ
The efficiency of the processes can be defined as
nex ¼Etot
product
Etot
in
;0nex 1:ðI-9Þ
ENERGY AND EXERGY ANALYSIS OF PRIMARY ALUMINUM PRODUCTION 89
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