Geology Reference
In-Depth Information
energy intensity was 120 GJ/t of alumina in 2006, with a range from 112 GJ to 145
GJ/t, depending on exact location. The smelting process consumes around 70% and
the alumina production around 18% of the total energy used per kg of commercial
primary aluminium. According to Norgate et al. (2007) in a cradle to gate LCA
study undertaken for various common metals, the GWP, of primary aluminium for
electricity generated by coal was as high as 22.4 kg CO
2
e=kg Al, a figure second
only to titanium. In the same report, 211 GJ/t was stated as the gross energy
requirement. Norgate et al. (2007) also mention that the new cell design referred to
“vertical electrode cell” could save as much energy as 25-30% over the Hall-Heroult
cell conventional process, thus reducing the typical value of 15 to 11 kWh/kg Al.
The ideal case of using hydropower instead of coal to decrease its GWP further is
subject to local availability. An intermediate case for such reductions is the natural
gas combined cycle, which produces electricity more e
ciently than conventional
power plants.
Additional information on energy requirements for aluminium production is that
of the Bureau of International Recycling (Grimes et al., 2008), who state a 47 GJ for
the Hall-Heroult process, a figure which drops to 2.4 GJ in secondary production.
Likewise, the carbon footprint for primary production is 3.8 t CO
2
and only 0.29
t CO
2
for secondary. Finally, Barkas (2009) reports that the energy required to
concentrate one tonne of ore in the mine through the Bayer process is 23 GJ/t of
aluminium, with an additional 7.5 GJ/t used in the transportation. The smelting
process consumes around 69 GJ/t.
The main environmental issues of primary aluminium production are perflu-
orocarbons (PFCs) and hydrogen fluoride waste in the form of gases, as well as
aluminium and sodium fluorides and unspent cryolite, as particulate material. All
are very toxic. The GWP of PFCs also present a problem as they are 6,500 to
9,200 times greater than that of carbon dioxide
4
. Emissions, however, have reduced
sharply over the last decade with new plants producing as low as 0.5 kg of fluorides
per tonne of aluminium. The Soderberg process, meanwhile, produces significant
emissions of polycyclic aromatic hydrocarbons whilst baking the pitch which will
go on to form the electrodes.
Indirect CO
2
emissions are also very important. Indeed electricity stemming
from either coal or natural gas, combined with the anode production and electrolysis
emits significant amounts of carbon dioxide, carbon monoxide, sulphur, and nitrous
oxides. The same emission scheme applies for the calcining process of alumina
(IPPC, 2009; Kawatra, 2011).
4
See U.S. Environmental Protection Agency. Voluntary Aluminum Industrial Partnership
(VAIP): Preventing, Responding to, and Mitigating the Impact of Anode Effects to Reduce
PFC Emissions http : ==www:aluminum:org=AM=CM=ContentDisplay:cfm?ContentFileID =
59256&FusePreview = Y es. Accessed Nov. 2011.
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