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ORIGINAL ARTICL E
The Aluminum Smelting Process and Innovative
Alternative Technologies
Halvor Kvande, PhD and Per Arne Drabløs, MD
Objective: The industrial aluminum production process is addressed. The
purpose is to give a short but comprehensive description of the electrolysis
cell technology, the raw materials used, and the health and safety relevance
of the process. Methods: This article is based on a study of the extensive
chemical and medical literature on primary aluminum production. Results:
At present, there are two main technological challenges for the process—
to reduce energy consumption and to mitigate greenhouse gas emissions. A
future step may be carbon dioxide gas capture and sequestration related to the
electric power generation from fossil sources. Conclusions: Workers’ health
and safety have now become an integrated part of the aluminum business.
Work-related injuries and illnesses are preventable, and the ultimate goal to
eliminate accidents with lost-time injuries may hopefully be approached in
the future.
Industrial production of primary aluminum is carried out by the
Hall–H´
eroult process, named after its inventors, who indepen-
dently of each other, in 1886, developed and patented an electrolytic
process in which aluminum oxide (or alumina, Al2O3) is dissolved
in an electrolyte consisting mainly of molten cryolite (Na3AlF6)and
aluminum fluoride (AlF3). In modern aluminum electrolysis cells,
several prebaked carbon anodes are dipped into the electrolyte, and
oxide ions from the dissolved alumina are discharged electrolytically
onto the anodes as an intermediate product. Nevertheless, the oxide
immediately reacts further with the carbon anodes and gradually con-
sumes them by formation of gaseous carbon dioxide (CO2). Below
the electrolyte, there is a pool of molten aluminum, which is the cath-
ode in the cell. Fresh aluminum is formed from aluminum-containing
anions that are reduced at the electrolyte–aluminum interface.
AN ALUMINUM PRODUCTION PLANT (SMELTER)
The buildings where the electrolysis cells are located (the
potrooms) are huge. They can be more than 1 km long, in some cases
about 50 m wide, and perhaps 20 m high. In a potroom, between 100
and 400 electrolysis cells are arranged in series, with the cathode
of one cell electrically connected to the anode of the next cell, to
form a cell line (which in the industry jargon is called a potline).
Series connection allows the use of high-voltage rectifiers; and for
modern potlines, the maximum voltage now may be well above 1500
V. Although the current of the potlines are kept constant, the cells
have individual voltage adjustments to satisfy special technological
requirements, such as heat balance, cell-operating conditions, and
the age and condition of the cathode. Figure 1 shows a modern
potline.
Modern potlines now typically have amperages from about
300 kA and up to about 600 kA, which are the largest cells in
operation at present. These cells are placed side by side as shown in
From the Norwegian University of Science and Technology (Dr Kvande), Trond-
heim, Norway. Both of the authors are now retired.
This is an open-access article distributed under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 3.0 License, where it is permis-
sible to download and share the work provided it is properly cited. The work
cannot be changed in any way or used commercially.
The authors declare no conflicts of interest.
Address correspondence to: Halvor Kvande, PhD, Bestumaasen 4 A, NO-0281
Oslo, Norway (halvor.kvande@gmail.com).
Copyright C2014 by American College of Occupational and Environmental
Medicine
DOI: 10.1097/JOM.0000000000000062
Figure 1, to reduce the adverse magnetic effects of the high electrical
current and also to reduce the heat loss from the cells. Older cells,
which can have amperages less than 200 kA, are often placed end to
end, but not always.
Day and night, each of these cells produces this valuable metal
in large amounts, maybe 100 kg or more every hour. Added together,
the aluminum production in the plant can be huge, and the largest
aluminum plants in the world now report an annual production close
to or even more than 1 million metric tons.
The process is still far from fully automated. Cranes are moved
back and forth for transportation and changing of anodes and for
removal of aluminum from the cells. Large vehicles transport the
metal out of the potline building. They bring the metal to the cast
house for further treatment and casting of aluminum products.
The upper part of the cell is called the cell superstructure.
Aluminum hoods are there to facilitate collection of the anode gases
and fluoride vapors from the electrolyte, and these are sent to the
fume-treatment plant. Large vertical aluminum bars (called anode
risers) conduct the current from the negative cathode of the neighbor
cell to the positive anode of the present cell.
A layer of alumina plus solid electrolyte cover the top of the
anodes. There should preferably be open holes in the crust along the
center line between the two rows of anodes, where the alumina is
added automatically to the electrolyte. Underneath the crust, there
is a 15- to 20-cm deep layer of electrolyte, and with 10 to 20 cm of
molten aluminum beneath. These two melts have different densities,
and as such, they do not mix with each other. Alumina is dissolved
in the electrolyte and is electrolyzed at the cathode to form molten
aluminum.
There is a high ambient temperature in the potrooms, due
to the heat emitted from the cells. Ambient temperature in potlines
is poor if there is no designed natural ventilation system, as may
be the case for Søderberg potlines. In hot regions, heat exposure
is a serious problem in the potrooms, and extensive programs for
information, acclimatization, and preventive measures are set up.
It is, therefore, important to have strict rules for fluid intake, rest
areas, and measures to be taken when the operators show signs of
heat stress and heat exhaustion. Some individuals are more at risk
than others; for example, high body mass index is a well-known risk
factor for reduced tolerance to heat exposure.
The high electric current flowing through each cell creates
strong static magnetic fields, and because these cannot be felt by
the human body, the fields can cause damage to watches and credit
cards. People will not be allowed to enter the potroom if they have a
pacemaker, as these also can be affected.
In these huge systems, there are lots of joints that used to
be packed with asbestos material, usually chrysotile. Previously,
asbestos was also used to cover metal that had leaked from the cells.
Asbestos is a known carcinogen; however, a study supported by
the Norwegian Cancer Institute could not find any asbestos-related
lung cancer among former and present operators in the Norwegian
aluminum industry.1
THE ALUMINUM PRODUCTION PROCESS—FROM
ART TO SCIENCE
Throughout the years since its invention in 1886, the industrial
aluminum production has developed from art to science. Steadily
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
JOEM rVolume 56, Number 5S, May 2014 S23
Kvande and Drabløs JOEM rVolume 56, Number 5S, May 2014
FIGURE 1. A modern potline with high-amperage side-by-
side prebake cells.
increased understanding of the process has been achieved as a result
of extensive research and development work, particularly in the latter
half of the twentieth century, both in aluminum plants and in several
universities and academic institutions. During monitoring and inter-
vention of the process, the cell operators are constantly faced with
decision-making situations. Theoretical and practical training of the
operators and their supervisors and superintendents give them the
skills and knowledge needed to improve steadily cell operation and
work practices.
The overall electrochemical reaction for industrial production
of molten aluminum may be written as follows:
1/2Al2O3(dissolved) +3
/4C(s) =Al(l) +3
/4CO2(g) [1]
This reaction is simple and shows that the two main raw materials are
alumina and carbon and that there are two chemical products, molten
aluminum, which we want, and gaseous CO2, which we really do
not want.
The amounts of raw materials used in the process are illus-
trated in Fig. 2. Alumina is consumed according to the stoichiometric
ratio predicted from Equation 1. The consumption of alumina the-
oretically amounts to 1.89 kg per kilogram of aluminum produced.
Nevertheless, in practice, the real value for the specific alumina con-
sumption in the industry is a little higher, typically 1.93 kg, because
the alumina supplied is not 100% pure. It always contains small
amounts of impurity oxides like Na2O, CaO, Fe2O3,andSiO
2.Fur-
thermore, from the aforementioned chemical equation, we see that
FIGURE 2. The amounts of raw materials needed to produce
1 kg of aluminum.
we produce three-fourth moles of CO2per mole of aluminum. One-
half mole of alumina should then theoretically react with 0.33 kg of
carbon and produce 1 kg of aluminum and 1.22 kg of CO2.Never-
theless, because of other reactions of carbon with both oxygen and
CO2, between 0.40 and 0.45 kg carbon are consumed per kilogram of
aluminum in practice. This is called the net anode consumption, and
this in turn produces about 1.5 kg of CO2per kilogram of aluminum.
Alumina must be added regularly to the electrolyte to keep
the normal electrolytic production going on continuously. Older alu-
minum electrolysis cell designs had large and infrequent additions
of alumina, while modern cells are equipped with so-called point
feeders. Alumina is then supplied automatically from an overhead
bin or hopper, which is built into the superstructure of the cell. Two
to six volumetric feeders successively add about 1 kg of alumina to
the electrolyte every minute or so. These small additions increase
the ability for the alumina powder to dissolve, mix, and disperse
rapidly in the electrolyte. The average alumina concentration in the
electrolyte is usually kept within the narrow range of 2 to 4 wt%
alumina. Higher concentrations may lead to the formation of exces-
sive amounts of undissolved alumina, which in the industry is called
sludge. Because of its higher density, the sludge is collected at the
bottom of the molten metal. Sludge has no useful purpose in the
cell, and it is unwanted, mainly because it contributes to increase the
electrical resistance in the cell and thereby the cell voltage.
On the contrary, low alumina concentrations in the electrolyte
can give a dramatic change in the anode process, which leads to a so-
called anode effect. An anode effect causes a very high cell voltage,
perhaps up to 30 to 40 V instead of the normal 4.0 to 4.5 V, by
forming an electrically insulating layer of gas underneath the anodes.
The anode gas composition then changes abruptly from almost pure
CO2(g) to mainly CO (g) and also some gaseous perfluorocarbon
compounds, CF4(g) and smaller amounts of C2F6(g). These are
greenhouse gases with high global-warming potential and extremely
long atmospheric life times (of the order of 10,000 years).
The formation of these gases can be lowered by reducing the
anode effect frequency (the number of anode effects per cell per
day) and the anode effect duration (given in minutes). All aluminum
producers have now made significant progress in reducing their emis-
sions of perfluorocarbon gases. Most modern prebake cells can now
be controlled to operate for more than 1 week and even for several
months without an anode effect.
Before leaving the topic of anode effects, it should be men-
tioned that 70% to 80% of the anode gas evolved is then CO (g).
In some cases, termination of anode effects may require manual
intervention, and the operators may then breathe in this poisonous
gas. Nevertheless, even if this effect has probably not been studied
in detail, the concentration of CO (g) in the working atmosphere in
potlines may be so low that it is not harmful to humans.
In addition to being the raw material for production of alu-
minum, alumina also acts as a thermal insulator when it is placed
on top of the self-formed solid crust above the electrolyte, thereby
reducing heat losses. Alumina is also used for covering the top of
the anodes, which conserves heat and minimizes air burning of the
carbon anodes. More frequently, a mixture of alumina powder and
crushed pieces of solid electrolyte is used.
The third major role fulfilled by alumina is a very important
one. Alumina is used to capture fluoride emissions from the cells by
anode gas cleaning, by use of the so-called dry scrubbing method.
Alumina powder adsorbs the hydrogen fluoride (HF) gas evolved,
and it also entraps fluoride condensates, mainly particulate sodium
tetrafluoroaluminate (NaAlF4). The resulting alumina is called sec-
ondary alumina and is then used as feed material to the cells. The
cleaned exhaust gas, containing CO2and smaller amounts of perflu-
orocarbon gases, is discharged to the atmosphere.
Figure 3 shows a flow sheet of the industrial aluminum pro-
duction process. The processes made before the metal is sent to the
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
S24 C2014 American College of Occupational and Environmental Medicine
JOEM rVolume 56, Number 5S, May 2014 Aluminum Smelting Process and Alternative Technologies
+ + .
.
.
.
Alumina
Alumina
Gas scrubber
Sheet ingot
Extrusion ingot
Electrical
power
Anode
(carbon)
Silo
Steel shell
Cathode
(carbon in base and sides)
Liquid
aluminum
Primary foundry
alloys
Furnace
Wire rod
FIGURE 3. Flow sheet of the aluminum production process.
cast house are called upstream processes, while the processes in the
cast house to make extrusion ingots, sheet ingots, primary foundry
alloys, and/or wire rods are called downstream processes. A much
more detailed description of the electrolysis process can be found
in several textbooks, for example, in Grjotheim and Kvande2and
Thonstad et al.3
RAW MATERIALS USED IN THE ALUMINUM
PRODUCTION PROCESS
Bauxite Mining
Aluminum is the most abundant metallic element (8 wt%) in
the earth’s crust. It is found in nature in a wide variety of minerals
combined with oxygen, silicon, and other metals. Because all the
aluminum compounds are very stable chemically, aluminum is never
found as a metal in nature.
It is mainly bauxite that is used as raw material for the alu-
minum industry. Bauxite contains typically between 40 and 60 wt%
alumina, with smaller amounts of iron, silicon, and titanium com-
pounds, as well as many other trace impurities.2
One of these impurities is beryllium, the concentrations of
which vary from less than 1 parts per million to several parts per
million in different bauxite mines. Because beryllium is toxic to
humans and can be found to some extent in the potroom atmosphere
(usually in concentrations less than 100 ng/m3), some concerns have
been raised. Nevertheless, studies in various aluminum plants have
found a very low incidence of sensitization against beryllium among
the potroom workers.4,5
Alumina
In alumina refineries, bauxite is processed into pure alumina.
The Bayer process extracts alumina by caustic digestion of crushed
bauxite at high temperature and high pressure in an autoclave, fol-
lowed by clarification, precipitation, washing, and finally calcination
to produce pure anhydrous alumina. This is a white powder that looks
like ordinary table salt. Alumina has a high melting point, more than
2050◦C, and is chemically a very stable compound. This is why so
much energy is required to produce aluminum from alumina.
Electric Power
A large amount of electrical energy is, therefore, required to
reduce alumina to aluminum. The most modern aluminum smelters
need close to 13 kWh to produce 1 kg of aluminum, while the world
average value for the direct current energy consumption now may be
closeto14kWh/kgAl.
2
Data for the mix of power sources for aluminum production
in 2009 show the following percentages, when including Chinese
aluminum production*:
Coal 51%
Hydro 39%
Natural gas 8%
Nuclear 2%
Whereas hydropower traditionally has been the dominant electricity
source for aluminum production, we see that coal now accounts for
more than 50% of the world production.
Energy typically counts for roughly 30% of the aluminum
production cost,†and its price is, therefore, highly significant for
the economy of the process. Energy consumption of aluminum pro-
duction has decreased in recent years by means of technological
improvements of the process. Nevertheless, with the global demand
for electric energy increasing steadily, energy savings in all parts of
the production process will be a very important task for aluminum
producers in the coming years. New aluminum plants will be built
only in areas with available and cheap electric power.
Prebaked Carbon Anodes
Today, all aluminum smelters use carbon anodes in their elec-
trolysis cells. Carbon is a reasonably good electrical conductor, and
more importantly, it is able to withstand the action of the corrosive
fluoride-containing molten electrolyte at about 960◦C. Furthermore,
carbon is an active part of the electrochemical reaction, and thereby,
it contributes to reduce the cell voltage by 1.0 V. As such, electri-
cal energy is saved by burning carbon. On the basis of Equation 1,
we may consider carbon as a raw material in aluminum production,
because carbon is consumed by the anode reaction.
A typical prebaked anode is made from a mixture of petroleum
coke, coal tar pitch, and butts. An anode butt is the rest of the
used anode removed from the cell during anode changing. The butts
content in the new anodes can vary, but normally it is between 15%
and 25%.2
The main constituent of prebaked carbon anodes is calcined
petroleum coke. When crude oil is refined, there is a residue of
*Data reported to the International Aluminium Institute in 2009.
†Source: CRU, based on average weighted global aluminum production.
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
C2014 American College of Occupational and Environmental Medicine S25
Kvande and Drabløs JOEM rVolume 56, Number 5S, May 2014
about 30% from the distillation unit. This residue is treated at about
450◦C and 4 to 5 bar pressure to form what we call green coke. The
process is known as delayed coking. This implies that coking is used
to upgrade waste products from oil refineries that would otherwise
have to be sold as low-value fuels.
The coke residue from petroleum refining is quite pure, and
therefore, it has been the major source of carbon for anodes. This
coke requires calcining at about 1200◦C to remove volatile con-
stituents and increase its density, strength, and porosity before it
is blended into the anode mix. The product, now called calcined
petroleum coke, is then ready to be shipped to the anode production
plant in the aluminum smelter.
In addition, the carbon anodes contain 13 to 16 wt% coal tar
pitch to be used as a binder material, thus binding the coke and butts
particles together in the anode.2The pitch is distilled from the coal
tar produced when coke for the iron and steel industry is made from
coking of bituminous coal. Coal tar pitch is a complex hydrocarbon
mixture consisting of thousands of compounds, of which only a few
hundred have been identified chemically. Liquid pitch can be kept at
about 200◦C and transported by ship to the aluminum smelters.
In the anode production process, the petroleum coke and the
recycled anode material (butts) are crushed and sieved into fractions,
which are then blended to obtain an optimum particle size compo-
sition. This blend is mixed with sufficient coal tar pitch (usually
between 13 and 16 wt%) to allow molding into green anode blocks
by pressing or by vibrating. Before these green anodes can be used
in the electrolysis cells, they have to be prebaked in a special an-
ode baking furnace at about 1150 to 1200◦C, causing the pitch to
carbonize and forming strong and dense anode blocks.
To provide electrical contact and physical support, an alu-
minum or copper rod with an iron yoke and from one to six iron
stubs are attached to the anode. The stubs are placed into cavities on
the top of the carbon anode and are attached by applying molten cast
iron around the stubs. The purpose of the cast iron is to make a good
mechanical and electrical connection between the carbon anode and
the stubs. This process is called anode rodding.
The Søderberg Anode
There are two basic anode designs presently in use. Prebaked
anodes are the dominating type now. The other main anode type
is the Søderberg anode, invented by the Norwegian engineer Carl
Wilhelm Søderberg (1876 to 1955). The Søderberg anode can be
characterized as a monolithic, continuous, and self-baking anode.
This type of anode is also made from a mix of petroleum coke and
coal tar pitch, but here the mix typically contains between 25 and 28
wt% pitch,2which is about twice the pitch content used for making
prebaked anodes. Small briquettes of Søderberg anode paste are
then made, and these are added regularly to the top of the Søderberg
anode.
Although the anode paste passes slowly downward through a
rectangular steel casing, it is baked into an electrically conducting
solid composite by pyrolysis of the pitch from the waste heat gen-
erated in the electrolyte and in the anode itself. The baked portion
of the anode extends past the steel casing and into the molten elec-
trolyte. The briquettes added on the top replace the part of the anode
that is being consumed at the working surface on the bottom.
Electric current usually enters the Søderberg anode through
vertical spikes or studs, although in some older Søderberg cells, side-
entry horizontal studs are used. These spikes are pulled and reset to a
higher level when they approach the lower anode surface. Søderberg
anodes have an electrical resistivity that is about 30% higher than
that of prebaked anodes. Søderberg anodes suffer from resulting
lower efficiency and great difficulty in collecting and disposing of
anode baking fumes, especially polycyclic aromatic hydrocarbons
(PAHs). These hydrocarbons are mainly volatiles from the pitch
used in the anode paste, but the PAH emissions can also depend
on the conditions of the anode top. Polycyclic aromatic hydrocar-
bons consist of many different organic compounds, which have been
shown to be carcinogenic. Benzo(a)pyrene is considered as the most
dangerous compound here. Its concentration is, therefore, measured
regularly both in the working atmosphere and in the air outside of
the Søderberg potroom.
In Søderberg plants, epidemiological studies have found an
increased incidence of bladder cancer, which is considered to be
caused by PAH exposures. Some studies have also found an increase
in lung cancer among Søderberg potroom workers. This is also be-
lieved to be caused by PAH exposures.1,6
The trend now is that Søderberg cells are gradually being
replaced by prebaked anode cells, even though the former save the
capital cost, labor, and energy required to manufacture the latter.
Particularly in the recent 5 to 10 years, many Søderberg potlines have
been shut down, because they cannot cope with the new stringent
emissions limit values to air for total fluorides, gaseous hydrogen
fluoride, particulate fluorides, and dust. Nevertheless, there are still
several Søderberg plants in operation in Russia, Europe, Brazil, and
the United States.
ELECTROLYTE MATERIALS
The four main functions of the electrolyte are as follows:
rTo be the solvent for alumina to enable its electrolytic decompo-
sition, forming molten aluminum and CO2
rTo pass electricity from the anode to the cathode
rTo provide a physical separation between the cathodically pro-
duced aluminum metal and the anodically evolved CO2gas
rTo provide a heat-generating resistor that allows the cell to be
self-heating
Cryolite usually comprises 75 to 80 wt% of the molten elec-
trolyte, which typically also contains excess aluminum fluoride (9%
to 12%), calcium fluoride (4% to 7%), and alumina (2% to 4%).
These three additives lower the melting point of the electrolyte, as
well as the cell operating temperature, and they increase the effi-
ciency of the process.
Cryolite
The mineral cryolite is a double fluoride of sodium and alu-
minum and has a stoichiometric composition very close to the for-
mula Na3AlF6and a melting point of about 1011◦C. It has been found
in substantial quantities only in Greenland. Cryolite was mined ex-
tensively there in the early twentieth century, but the mine is now
essentially exhausted. Cryolite, thus, has to be made synthetically
now. It can be produced by reacting hydrofluoric acid with an alka-
line sodium aluminate solution according to the overall reaction:
6HF(g) +2 NaOH +NaAlO2=Na3AlF6+4H
2O(g) [2]
Aluminum Fluoride
Aluminum fluoride, AlF3, may comprise as much as 9 to
12 wt% of the electrolyte, when it is recorded in excess of the
amount represented by the cryolite composition. Aluminum fluoride
is consumed during normal operation by three major mechanisms.
First and foremost, aluminum fluoride reacts with sodium oxide that
is always added as an impurity material in alumina. This amount has
to be replaced, and it requires addition of about 20 kg of aluminum
fluoride per metric ton of aluminum produced to keep the AlF3
concentration in the electrolyte constant.
The second consumption mechanism is that aluminum fluo-
ride can be depleted by hydrolysis due to moisture in different forms
Copyright © 2014 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
S26 C2014 American College of Occupational and Environmental Medicine
JOEM rVolume 56, Number 5S, May 2014 Aluminum Smelting Process and Alternative Technologies
in the cell:
2AlF
3+3H
2O(g) =Al2O3+6HF(g) [3]
Gaseous hydrogen fluoride is extremely hazardous. Fortunately, fume
capture and gas scrubbing efficiencies have been improved strongly
in aluminum smelters, and very little HF (g) is emitted now to the
potroom and the surrounding atmosphere.
Finally, losses of aluminum fluoride by vaporization from the
electrolyte are appreciable. The most volatile species evolved from
the electrolyte is sodium tetrafluoroaluminate vapor, NaAlF4(g). It
has a partial pressure of 400 to 600 Pa over the operating electrolyte,
depending on its composition and temperature. Fortunately, more
than 98% of the fluorides, including HF (g), are collected by the gas
cleaning process in the fume-treatment plant and are returned to the
cell together with the secondary alumina.
Exposures to dust and fluorides in the prebake potrooms are
typically over short periods with extremely high exposures for cer-
tain tasks, followed by longer periods with very low exposures. HF
(g) may reach high concentrations, up to 100 parts per million,
during short episodes during certain job procedures. These peak ex-
posures are considered to be a risk factor in causing occupational
asthma. Occupational asthma among smelter workers has been ex-
tensively reported in several epidemiological studies. Recent reports
from Australia and Norway, however, have shown a considerable
decrease in the incidence of occupational asthma among potroom
workers.7,8 Recent methods of simultaneous exposure measures and
video surveillance when the operators carry out their jobs visualize
the exposure during work performance and help to mitigate exposure
through work practice changes.9
Calcium fluoride is seldom added intentionally to the elec-
trolyte. Because of the small amount of calcium oxide impurity in
the alumina (typically only about 0.035 wt%), it attains a stable
steady-state concentration of calcium fluoride of 4 to 7 wt% in the
melt. At this level, a minor amount of calcium is codeposited into
the aluminum, while some is emitted as a calcium compound, maybe
CaCO3vapor, in the off-gas at a rate equal to its rate of introduction
with the alumina.
Finally, a few words are needed about safety when working
with molten cryolite. Many reactive substances can result in danger
by contact with electrolyte and metal, and moisture is the most
hazardous. The molten electrolyte can give splashes and must be
treated with respect and awareness. The electrolyte and also the
metal have a temperature of about 950◦C. In addition, the fluoride-
containing electrolyte is corrosive. Remember that electrolyte burns
must be cooled immediately with temperate water for at least an
hour.
THE CATHODE AND CATHODE MATERIALS
In the cell, the electrolyte and the molten aluminum are con-
tained in a preformed carbon lining that has refractory and thermally
insulating materials inside a steel shell. Graphitic or semigraphi-
tized materials are now used extensively as prebaked carbon cathode
blocks. The other materials used are silicon carbide (SiC) sidewall
bricks and carbonaceous ramming paste. Several steel current collec-
tor bars are embedded in the carbon cathode and conduct the electric
current away from the cell. Figure 4 shows a schematic drawing of
an aluminum electrolysis cell.
Insulation bricks are used to insulate the cathode thermally.
These bricks are porous and vulnerable to penetration of electrolyte
components through the cathode blocks. The insulation materials are
protected by using refractory bricks, and sometimes a special barrier
material made of steel, glass, or other materials can be added. Refrac-
tory bricks also have some insulating effect, so that the temperature
in the insulation materials does not become too high.
FIGURE 4. Schematic drawing of an aluminum electrolysis
cell.
It should be noted here that the word cathode is used in the
aluminum industry to describe the whole container of electrolyte and
metal. Nevertheless, the real acting cathode from an electrochemical
point of view is the top surface of the molten aluminum pool. Thus,
the aluminum atoms are formed from aluminum-containing ions that
are reduced at the electrolyte–aluminum interface.
Cells are now typically 9 to 18 m long, 3 to 5 m wide, and 1 to
1.5 m deep. The depth of the operating cell cavity is relatively low,
however, only 0.4 to 0.5 m. Although carbon is the material known
to withstand best the combined corrosive action of molten fluorides
and molten aluminum, even carbon would have a very limited life
time in contact with the electrolyte at the sides of the cell if it was
not protected by a layer of frozen electrolyte. Now silicon carbide is
used as sidewall material, but this material is also corroded by the
electrolyte and needs to be protected.
TECHNOLOGICAL KEY OPERATIONAL PARAMETERS
FOR THE ALUMINUM SMELTING PROCESS
Current efficiency (CE) is a very important technological pa-
rameter used to describe the performance of the process. One may
simply say that current efficiency is the part of the current that is
used to produce aluminum. According to Faraday’s first law, 1 kAh of
electric current should theoretically produce 0.335 kg of aluminum,
but only 90% to 96% of this amount can be obtained in indus-
trial cells. Loss in metal production is typical for all electrolytic
processes and is, therefore, very difficult to avoid completely. The
principal loss mechanism in aluminum electrolysis is recombination
of the anodic and cathodic products, the so-called “back reaction,”
where aluminum back-reacts with CO2to form alumina and carbon
monoxide.
To account for these losses and to measure the electrochem-
ical efficiency of the process, the concept of current efficiency has
been introduced in the industry as the ratio between measured and
theoretical production rates:
CE =p/po×100% [4]
Here, pis the measured production rate (kg/h), and pois the theoret-
ical production rate (kg/h), calculated from Faraday’s first law.
In addition to the “back reaction,” there are several other
mechanisms accounting for additional small losses in current effi-
ciency. A new cell lining will absorb sodium. Fortunately, the cell
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C2014 American College of Occupational and Environmental Medicine S27
Kvande and Drabløs JOEM rVolume 56, Number 5S, May 2014
lining becomes saturated early in the cell’s life, but until this occurs,
current efficiency will be low. When a metal dissolves in a molten
salt, it usually imparts electronic conductivity to the melt, thereby
lowering the current efficiency. This is because the electrons “steal”
current without producing any metal. Some investigators have found
a small electronic conductivity for cryolite-based melts in contact
with molten aluminum in laboratory cells, while others have not; so
more studies are needed here. In any case, this contribution is small
compared to the “back reaction.”
Energy consumption (EC) is given in kWh/kg Al and can be
calculated by the following equation:
EC =2.98 ×CE/Vo l t a g e [ 5 ]
Here Voltage is the operational cell voltage given in volts (V), and
CE is current efficiency given as a fraction (and not in percent here).
Energy consumption is the best technological parameter in aluminum
production, because it also includes current efficiency.
Energy efficiency is defined as the part of the electrical energy
(amperage multiplied by voltage) that is used to produce aluminum.
Typical values are only between 45% and 50%, even in modern cells.
The rest of the energy produces heat, which is lost to the surround-
ings. One important task for the industry in the future is to reduce
the energy consumption and thereby increase energy efficiency.
CELL OPERATION
The following operational procedures have to be performed
regularly, although at different time intervals in the potlines:
rAlumina feeding
rAnode change and anode covering
rMetal tapping
rAddition of aluminum fluoride
rRack raising
Alumina feeding is nowadays automated through the use of
point feeders, and therefore, anode changing is now the most labor-
intensive manual routine operation. Prebaked anodes must be re-
placed at regular intervals when they have reacted down to about one
fourth of their original size. This occurs after 25 to 30 days. Because
each cell may have between 16 and 40 prebaked anodes, this means
that one anode in average has to be changed approximately each
day in every cell. Modern potlines are equipped with sophisticated
overhead cranes that allow the operator to sit in an air-conditioned
cabin and perform the anode-changing operation by manipulating
robotic arms. Alternatively, the use of anode-changing vehicles is
also common in many plants.
Anode changing causes the largest operating disturbance in
cells with prebaked anodes. When a new, cold anode is inserted,
weighing about 1 ton, a layer of solid electrolyte quickly freezes on
the underside of the anode, and it can take up to 24 hours to melt
this layer completely. This reduces the temperature of the electrolyte
locally, as the new anode draws very little current during this remelt-
ing process. The solid electrolyte layer is a poor electrical conductor.
It also disturbs the anodic current distribution in the cell. Several
aluminum plants are now changing two anodes simultaneously, and
this introduces an even larger thermal and electrical disturbance in
the cell.
There will unavoidably be some dust present in the air inside
the potrooms. This potroom dust consists both of alumina and fluo-
rides from the electrolyte and gives exposure of fine particulates to
operators. The presence and compositional nature of these airborne
particles have been discussed recently by Wong et al.10
Particularly during anode change, significant densities of
nanoparticles with a median particle size smaller than 20 nm can
be recorded in the vicinity of the cells. They are possibly produced
when the molten mass is exposed to the colder environment. The
surface of these particles is large, and HF, SO2, Be, and other parti-
cles on the surface represent potential health hazards of which there
is limited knowledge at present. After being released into the air, the
nanoparticle mode is subject to ageing, leading to a shift in the size
distribution toward larger particles.11
Individual aerosol particles of aluminum oxide/cryolite with
a high cryolite content immediately become surrounded by a surface
water film when exposed to relative high humidity (such as in the
upper respiratory tract). Because gaseous HF and SO2are highly
soluble in water, the aerosol particles may act as carrier for these
gases into the lower respiratory tract.12
Anode covering is usually done about 4 hours after anode
changing. Because the anodes are hot, we need a method of protecting
them from air oxidation (air burn) and heat loss. The anode cover
material must not introduce any metal contamination, and hence a
mixture of alumina and recycled electrolyte is used. The composition
of the anode cover can play an important part by reacting with the
fume and fusing the under surface of the crust. Poor anode cover
practice can result in air burn of the anodes.
The spent anodes, the butts, are cleaned outside the cell in
a separate butts-cleaning station. First the adhering electrolyte and
alumina are removed and are recycled to the cells. The cleaned
butts are then crushed and reused as a carbon raw material in the
manufacture of new prebaked anodes.
Removal of molten aluminum from the cells is called tapping,
and this is also a labor-intensive routine manual operation. The spout
of a vacuum ladle, or crucible, is dipped into the metal pad in the
cell, and the metal is then siphoned out and into the crucible by
the suction from an air ejector system. The molten metal is then
weighed and transported to the cast house. Although overhead cranes
are usually used to assist the manual tapping work practices, also
specially constructed motorized vehicles can be used. But otherwise,
these tapping procedures are identical.
Addition of aluminum fluoride is performed automatically in
modern cells. One or more silos for aluminum fluoride are built
into the superstructure of the cell, and the addition is done through
the hole in the crust made by the point feeder breakers for alumina
addition.
The last manual routine operation in the aforementioned list
is called rack raising or anode beam raising. Because the anodes
are consumed, the anode beam, which is holding all the anodes
in position, has to be gradually lowered downward into the cell to
maintain a constant anode–cathode distance. The cathode, which
is the metal surface, is kept approximately at the same position by
regular tapping. Finally, the position of the anode beam becomes so
low that it reaches a stopping device that can be electronic or physical.
The beam then has to be raised by use of a special anode beam–
raising machine carried by an overhead crane. All anodes are first
connected electrically to this machine and are held in their correct
positions in the electrolyte, while the anode clamps are loosened and
the anodes are electrically disconnected from the anode beam. The
beam is then raised to its upper position, the anodes are refastened
to the anode beam in their correct position again, and the machine is
finally removed from the top of the cell superstructure. This operation
is carried out every 2 to 3 weeks for each cell, and it then raises the
anode beam by about 20 cm.
If potline operators are asked what they think is the most risky
or hazardous work they do on the cells, the answer will probably be
beam raising. The reason for this is that sparks can occur through
electric arcing, if the electrical contact with the anodes is not satisfac-
tory. The possible loss of electrical continuity during beam raising
if an anode effect occurs on the cell is the underlying reason for
operators’ concern here. A potline open circuit, with explosive con-
sequences, can and has indeed occurred, with a high risk of fatality
for those in the proximity.
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S28 C2014 American College of Occupational and Environmental Medicine
JOEM rVolume 56, Number 5S, May 2014 Aluminum Smelting Process and Alternative Technologies
The extensive work to run the potrooms, as well as continuous
maintenance, causes much noise at times, and the operators usually
have to wear devices for noise protection. In spite of this, hearing
loss is a risk that has to be managed by the industry.
CELL PREHEATING AND START-UP
Before a new cell can be started, it has to be preheated. There
are two main preheating methods that are used now. One is based
on electrical resistance preheating with a thin bed of small coke
or graphite particles (2 to 5 mm usually) between the anodes and
the cathode. The other main method is flame preheating, where gas
burners are used. Resistance preheating is the oldest method here,
but both methods are now frequently in use in the aluminum industry.
The objective of preheating is to heat the cell materials as
close as possible to the operating temperature of a normal cell,
which really means 960◦C for the cathode surface and the underside
of the anodes. This provides a careful transition from the cold cell
to the operating temperature, and it contributes to avoid thermal
shock of the cathode materials when the molten electrolyte is added.
The higher the preheat temperature, the easier is the cell start-up.
Nevertheless, this target is difficult to reach in practice, because
unwanted hot spots on the cathode surface may then be hard to
avoid. The target for the average cathode surface temperature at the
end of the preheating is, therefore, usually around 900◦C.
The actual start-up is done by adding molten electrolyte to the
cell and raising the anodes carefully when the electric current is cut
in. Then the electrolysis process starts. The temperature preferably
should be kept less than 1000◦C in the first few hours and then
lowered gradually to increase the aluminum production efficiency.
During the start-up and early operation of the cell, many of the
hoods are removed so that the operators will be able to observe the
movement of the molten electrolyte. In this period, there is formation
of fluoride vapors and also HF (g), which can cause asthma-like
symptoms for the operators when this gas in inhaled. The use of
proper respiratory protection is, therefore, very important during
this type of work, and certainly also in other work where people are
exposed to fluoride vapors from the electrolyte.
MAGNETOHYDRODYNAMICS
The large electric currents used in modern aluminum electrol-
ysis cells (300 to 600 kA) generate strong magnetic fields, both out-
side and within the cell. These magnetic fields interact with the high
electric currents and exert so-called Lorentz forces. These forces are
strong enough to produce movement of liquid conductors. Molten
aluminum, which in itself is nonmagnetic, is influenced by these
strong magnetic fields. The reason is that molten aluminum then
acts as a movable current conductor.
The magnetic movement of the metal may give rise to high
metal velocities, metal height variations, and metal instabilities. To
minimize the adverse consequences of these effects, it is desirable
to compensate for the magnetic forces by a special arrangement of
the interconnecting electrical current conductor system.
Calculation of the magnetic fields and electric current flow
patterns is complicated. Nevertheless, for many years now, at least
four decades, powerful computer programs have been designed
to make these calculations and describe the fluid-dynamic conse-
quences. These calculations have been refined to the point where
cells with amperages up to 600 kA have been designed and such
cells are now in operation.
Even if these static magnetic fields usually range from 5 to 15
mT in the potrooms, there is no indication that that they cause any
serious health effects.13, 14 Nevertheless, during maintenance work
in the rectifiers where the static fields may be even higher, there are
some operators who experience magnetophosphenes (visual blurring
or light flashes) and some operators who have dental fillings with
mercury, describe metal taste in the mouth.
SAFETY IN ALUMINUM PRODUCTION PLANTS
Safety is number 1 priority for all aluminum producers. This
area has received considerable attention in the last decades and
rightly so. Workers’ health and safety have become an integrated
part of the aluminum business. Protection of the employees is criti-
cal, because there are numerous possible exposures to the employees
in this industry. Nothing is more important than to send the em-
ployees home safely at the end of the working day. Good working
environment is crucial, and good housekeeping is a prerequisite here.
In principle, all accidents at work can be avoided. The expres-
sion “Accidents don’t happen; they are caused” is a good philosophy.
Thus, work-related injuries and also illnesses are preventable. The
ultimate goal to eliminate accidents with lost-time injuries will hope-
fully be approached in the future.
ALUMINUM IN THE HUMAN BODY
Aluminum is a metal that is all around us. People wear it, cook
in it, and eat and drink it. Much food is packaged in aluminum foil,
and beverage cans are usually made from aluminum. Daily intake
of aluminum may range from 10 to 100 mg, the majority being
through oral routes. Nevertheless, most of this will be excreted. Still
the amount of aluminum in the human body ranges between 50 and
150 mg, with an average value of about 65 mg. About half of the
aluminum in the human body is stored in the bones, and about one
quarter in the lungs.
Measurements of aluminum in serum of industrial workers
have always been difficult because so much sampling equipment
contains aluminum, and thus, contamination of the sample may eas-
ily occur. Healthy subjects usually have less than 10 μg/L aluminum
in serum. Postshift serum aluminum will increase to some extent
in potroom workers. With normal renal function, aluminum is read-
ily excreted in the urine. So far, there are no consistent findings
of an increased incidence of either aluminum-induced diseases or
neurological disorders among potroom workers.
INNOVATIVE ALTERNATIVE ALUMINUM
PRODUCTION TECHNOLOGIES
Electrolysis Cells With Inert Anodes
The concept of inert anodes for aluminum electrolysis is by
no means a new idea. It was suggested first by Charles Martin Hall
already in his famous patent from 1886. Hall tried to use copper
anodes, but he soon found out that they did not work in practice.
Copper dissolved quickly in the electrolyte, and he had to give up this
idea. Carbon anodes have since then been the only possible practical
solution for the anode material in industrial alumina reduction cells.
Nevertheless, in 2000, Alcoa announced that it was working
hard with inert anodes, and 2 years later, Alcoa’s chief executive offi-
cer Alain Belda stated that “the science is proved, so we have an inert
anode, but we have not proved the commercial aspects.” Of course,
this led to highly increased interest, curiosity, and activities, and it
inspired several aluminum companies and research institutions to
start work to try to find an inert anode material. Much of this work is
surely unpublished and will probably remain so for proprietary rea-
sons. Still, the open literature now offers a vast amount of individual
publications and patents on inert anode materials. A comprehensive
literature review of inert anodes was given by Galasiu et al15 in 2007.
So what is actually meant by an inert anode? The word inert
means chemically nonreactive, and a completely inert anode will,
therefore, not react chemically or electrochemically in the electrol-
ysis process. This means that it would ideally not be consumed by
the anode reaction. An inert anode has been given many names,
like dimensionally stable anode, nonconsumable anode, and passive
anode.
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C2014 American College of Occupational and Environmental Medicine S29
Kvande and Drabløs JOEM rVolume 56, Number 5S, May 2014
With inert anodes in the electrolysis cell, the total cell reaction
will be very simple:
Al2O3(dissolved) =2Al(l)+3/2O
2(g) [6]
We see immediately that this reaction is different from Equa-
tion 1. Here, oxygen is formed at the anode, which environmentally
is a highly favorable gas, compared with CO2.
The dream would of course be to have anodes that lasted as
long as the cell life, which now may be up to 5 years or longer.
Anode changes would then not be necessary after the cell has been
started. Nevertheless, it is a chemical fact that all materials have a
finite solubility in the very corrosive cryolitic melts at about 960◦C,
so a totally inert anode will probably never be found for use in
these electrolytes. Thus, what we really are looking for is a slowly
consumable anode. But how slow a consumption rate can we tolerate?
The potential inert anode material must have low solubility
and low reactivity in the electrolyte and also show good chemical re-
sistance against the anodically produced hot oxygen gas. In addition,
the anode material should be physically stable at the operating tem-
perature, mechanically robust and resistant to thermal shock. There
will be extreme requirements for keeping the wear rates of these
anodes low. A wear rate of the order of 10 mm/y may perhaps be
sufficient, but lower values would certainly be beneficial.
There are two main challenges in the development of inert
anode materials. In addition to the requirement that the anode ma-
terial should survive sufficiently long in the electrolyte, the metal
produced must be of adequate purity. The impurity metal content in
the aluminum can indeed be very significant for the customers, and
the need for making pure aluminum will become more stringent in
coming years. The corrosion products, caused by the dissolution of
the anode material into the electrolyte, predominantly will end up in
the metal phase and thereby contaminate the aluminum produced.
Hence, the anode corrosion should be low enough to give impu-
rity contents corresponding to the present specifications for smelter
grade aluminum.
There are three principal potential advantages in favor of de-
veloping a new cell technology with inert anodes:
1. Cost reduction. All costs directly associated with the consumable
carbon anode will then be eliminated, including the capital saving
and raw materials costs by eliminating the need for the carbon
anode fabrication, baking, and also the anode rodding plant. These
cost savings may be significant. It has been indicated that there
might be 25% to 30% lower capital costs for a new potline with
inert anode cell technology.3
2. Environmental friendliness. Inert anodes would eliminate all
greenhouse gas generation and emissions from the electrolysis
cells. Smelters would no longer generate CO2, carbon monoxide,
or perfluorocarbon gases (CF4and C2F6), because carbon would
no longer be used as anode material. Carbon residues (butts) will
of course disappear. In addition, the fluoride and dust emissions
during anode change will also be eliminated.
3. Improved occupational health issues. Inert anodes would reduce
the work practices associated with the present prebaked carbon
anode change. The frequency of anode changes will certainly be
drastically reduced with inert anodes. Working conditions in the
potrooms would also be improved by avoiding all anode effects.
What types of materials are the most promising for inert an-
odes? Two main paths have emerged so far:
1. The cermet conducting electrodes, which is used by Alcoa. The
word cermet means a combination of ceramics and metals and
consists of a mixture of oxides and metals, NiFe2O4+NiO +Cu
+Ag
2. The so-called “metal anodes” were previously developed by
Moltech. These were metal alloys made of Ni +Fe +Cu
Both these groups have reported large-scale trials retrofitting
the conventional cell design. In reality, these two types of the elec-
trodes become extremely similar at the anode–electrolyte interface,
because this electroactive surface necessarily has to be an oxide,
irrespectively of what materials the anode substrate is made of.16
Present Conclusion on Inert Anodes
Several companies and research institutions have studied inert
anode materials actively in recent years. It is no doubt that substantial
progress has been made in inert anode development during the last
decade, particularly regarding the two main challenges: anode wear
and metal purity in inert anode cells.
UC Rusal has now started rig testing of a small 3 kA cell with
inert anodes. On success of the rig tests, the company plans to begin
production tests on its inert anode cells in 2015 at the Krasnoyarsk
aluminum smelter.17
Operation of inert anode cells will certainly be very chal-
lenging. The commercial aspects of inert anodes have not yet been
proven. At present, a number of engineering problems remain to be
solved. It is presently impossible to say when, or even if, this may be
a proven technology. In any case, there will probably go several years
before the issues mentioned previously will be solved satisfactorily.
Maybe, cell retrofitting will not be the preferred development path
in the future, and it is possible that a completely new cell design will
be necessary.15
Carbothermic Production of Aluminum
History
The idea of carbothermic reduction of alumina to aluminum
is also an old dream. Aluminum–copper alloys with about 15% alu-
minum were produced industrially by this method already in 1886,18
the same year as the present industrial process was invented. In the
1920s, Al–Si alloys with 40% to 60% aluminum were produced
in Germany, and about 10,000 tons of these alloys were produced
annually up to 1945.
The first attempt to produce pure aluminum by carbothermic
reduction of alumina was made around 1955. In France, Pechiney
worked on the process from 1955 to 1967, but terminated the program
for technical reasons. Reynolds worked on an electric arc furnace to
produce aluminum from 1971 to 1984. Alcan acquired information
from Pechiney and continued their research, but stopped in the early
1980s. Alcoa tried to develop the process to produce Al–Si alloys
from 1977 to 1982.
Nevertheless, in 1998, Alcoa started the carbothermic pro-
duction project again together with Elkem R&D in Norway. Elkem
had a long-time experience with modern silicon furnace technology
and came up with the idea to use their experience to design a new
type of tailor-made high-temperature electric reduction reactor for
carbothermic production of aluminum. Alcoa had a strong funda-
mental understanding and a long-time experience with carbothermic
production of aluminum from the work in the 1960s, 1970s, and
1980s. Together Alcoa and Elkem then agreed to try this again, and
the work is still going on, as we will see later.
But what is really meant by carbothermic aluminum produc-
tion? As the name says, the carbothermic method is to use carbon
and heat to reduce alumina to aluminum, according to the overall
reaction:
1/2Al2O3(s) +3/2C(s)+heat =Al(l) +3/2CO(g) [7]
The reaction proceeds close to and higher than 2000◦Cand
produces CO as the primary gas. From Equation 7, it is easily seen
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S30 C2014 American College of Occupational and Environmental Medicine
JOEM rVolume 56, Number 5S, May 2014 Aluminum Smelting Process and Alternative Technologies
that the carbothermic process is different from the present electrolytic
process. No electrolysis is involved here, but electrical energy is of
course required. In the carbothermic process, alternating current is
used to heat up the raw materials alumina and carbon.
The Three Main Steps in the Carbothermic Process
The reduction of alumina to aluminum must take place in three
stages (see Kvande et al,18 Bruno,19 and Johansen et al20), which are
characterized by three different phase combinations:
Al2O3+3C=Al4O4C+2CO(g) [8]
Al4O4C+6C =Al4C3+4CO(g) [9]
Al4O4C+Al4C3=8Al(l) +4CO(g) [10]
Nevertheless, reaction equations with pure condensed solid phases
will give only an approximate description of the chemical system
involved here. Therefore, these reactions do not give a correct de-
scription of the reactions that will actually occur. Equations 8 and 9
will really give production of a molten slag, which contains a molten
mixture of alumina and aluminum carbide.
The molten aluminum phase will contain some dissolved car-
bon, and therefore, it can be considered chemically as an Al–C
alloy. Equation 10, therefore, actually gives production of a molten
aluminum–carbon–(carbide) alloy rather than pure aluminum. This
molten alloy will float on top of the molten slag. An additional re-
action step required would then be the production of pure aluminum
(refining) from the alloy containing aluminum–carbon–(carbide).
The two most difficult steps here are the production of the
aluminum–carbon alloy and the subsequent refining of the alloy. In
addition, a gas scrubber is needed for collection of all the aluminum-
containing gases that will evaporate from the furnace at these high
temperatures. This is also an engineering challenge. A simplified
flowchart of the process is shown in Figure 5.
Greenhouse Gas Emissions From Carbothermic Alu-
minum Production
The carbothermic reaction produces CO as the primary gas,
and the gaseous by-product is, therefore, different from the present
industrial process. Equation 7 shows that theoretically 1.5 mol of
CO are formed per mole of aluminum, which on a mass basis means
1.56 kg of CO per kilogram of aluminum produced.
In the atmosphere, CO generally has a lifetime of several
months before it converts to CO2by natural atmospheric processes.
Still it is obvious, from both health and environmental reasons, that
poisonous CO gas cannot be emitted directly into the atmosphere
from the carbothermic aluminum production. The gas will have to
Alloy Al – C
Pure Al
Vapor recovery reactor
1) Slag pr oductio n reacto r 2) Alloy production reactor
3) Aluminum recovery
Al
4
C
3
(Al
2
O
3
)
(C)
Al
4
C
3
CO(g)
(Al
2
O
3
)
C
Al
2
O
3
C
CO(g)
Al
2
O(g)
Al(g)
CO(g)
Al
2
O(g)
Al(g) Slag
Al
4
C
3
-Al
2
O
3
FIGURE 5. Flowchart of the aluminum carbothermic
technology–advanced reactor process concept of Alcoa and
Elkem.20,21
be burnt to CO2, and this reaction can be written as follows:
3/2CO(g) +3/4O
2(g) =3/2CO
2(g) [11]
This means that 1.56 kg of CO will form 2.44 kg CO2per kilogram
of aluminum. This value does not include any CO2resulting from
electrode consumption during carbothermic reduction, but this is
expected to be small here. Thus, the theoretical production of CO2is
then increased by about 60% in the carbothermic process, compared
with the present process.
In practice, this means that the entire amount of CO produced
has to be captured. In a recent article by White et al,21 it is reported
that the CO generated from the process is more than 90% pure and
it can then potentially be collected and used as a chemical. The CO
gas can, for example, be used industrially as raw material for several
different chemical products. According to these authors,21 this can
include use as a reductant for removing Fe2O3from bauxite, or as
a reductant in direct reduced iron processes. If CO is used as fuel,
it would produce CO2, which then will have to be stored by carbon
capture and sequestration.
The conclusion is then that the carbothermic process itself
will increase the specific greenhouse gas emissions by about 60%.
Nevertheless, the process promises to reduce energy consumption
from 13 to 11 kWh/kg Al. For hydroelectric power, this does not
matter much for the overall greenhouse gas emissions. Nevertheless,
if the energy used to produce aluminum is coal-fired power and if the
energy consumption can be reduced from 13 to 11 kWh/kg Al, this
in itself can contribute to reduce the overall greenhouse gas emis-
sions by about 20%. Even if this reduction may be considered to be
significant, it can be concluded that unless all the CO (g) is captured
and used industrially, the carbothermic process is not the solution to
minimize the carbon footprint from aluminum production.
CONCLUDING REMARKS
The outlook of the primary aluminum industry may be sum-
marized as follows: This is now a mature industry, which presently
(2013) suffers severely from low aluminum prices and a very chal-
lenging market situation.
Technologically, the present aluminum production process can
be a close-to-zero greenhouse gas producer. The first step, which is
actually ongoing, is to focus on lower specific energy consumption,
and also to eliminate the occurrence of anode effects. Furthermore,
it is possible to reduce the inherent production of CO2by reducing
the net carbon anode consumption, although this reduction can only
be perhaps 10% or even less with the existing carbon anode tech-
nology. Here, an inert anode, if such a material can be found and
developed for use in industrial aluminum production, would repre-
sent a remarkable technological breakthrough, because then oxygen
is formed at the anodes instead of CO2. On the contrary, another
alternative process, carbothermic production of aluminum, would
increase the CO2emissions if the produced CO is not captured and
stored.
A natural step to save energy in the present electrolysis process
would be to recover energy from the main heat loss sources of the
cells, the cathode sidewalls and the anode gas exhaust systems. A
future step may be CO2gas capture and sequestration related to
the electric power generation. Finally, collection and cleaning of the
CO2from the electrolysis process itself may perhaps be a technical
possible scenario in the future.
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