PFCs
In 1991, Dr. Dean Abrahamson of the University of Minnesota discovered that
primary aluminum smelters produce two extremely potent greenhouse gases:
tetrafluoromethane and hexafluoroethane.
With atmospheric lifetimes are at least 10,000 years, these are some of the
longest-lived atmospheric pollutants. The gases have global warming
potentials that are 6,500 to 9,200 times higher than carbon dioxide. The
emission of one kilogram of tetrafluoromethane would have the same climate
change effect, over the subsequent 100 years, as 6.5 metric tons of carbon
dioxide. (“Greenhouse worries for the aluminum industry,” Energy, Economics
and Climate Change, Jan. 1992; Harnisch et al.)
The MIT scientists suggested that “in view of the PFCs’ atmospheric
stability and their large specific radiative forcing relative to most other
greenhouse gases, increased scientific focus on these compounds is
warranted.” (Harnisch et al.)
These two gases are the unintentional byproduct of using fluorine in the
electrolytic reduction of alumina, formed during the “anode effect,” when
the electrolyte is depleted.
Harnisch, et al, estimate that each of ton of aluminum production created
about 0.48 kilograms of tetrafluoromethane in 1995. Technological
improvements brought this level down 30 percent from emission rates in the
1980s. Still, the primary aluminum industry generated about 9,400 tons of
this PFC in 1995, or about 90 percent of all tetrafluoromethane emissions
worldwide. This is the global warming equivalent of 59.22 million metric
tons of carbon dioxide.
The MIT study further estimates that in 1992, aluminum smelters emitted
about 1,300 tons of hexafluoroethane, or the greenhouse gas equivalent of
11.96 million tons of CO-2. Primary aluminum production accounted for about
65 percent of this chemical’s total emissions; plasma etching in the
semiconductor industry accounted for the balance of hexafluoroethane
emissions..
Proportionally, smelters employing Vertical Stud Soderberg and Sidework
Prebaked technology account for most of the industry’s PFC emissions. The
MIT study estimated that in 1995, these two technologies generated 65
percent of emissions even though they accounted for only 30 percent of
production. (Harnisch et al)
The International Aluminium Institute uses a grouping of technologies.
First generation plants built between 1940 and 1955 emit between 12-15
kilograms of PFCs per ton of metal produced; those built between 1955-75
emit 2-6 kilograms per ton; and “third generation plants” (1975-today),
emit from 0.3 to 1 kilogram per ton. (Interantional Aluminium Institute,
“Smelter emissions,” on its website, world-aluminium.org)
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