The EPA and the aluminum industry worked together to improve aluminum production efficiency while reducing perfluorocarbon (PFC) emissions. PFCs are potent greenhouse gases characterized by strong infrared radiation absorption and relative inertness in the atmosphere. Primary aluminum production is a major source of global PFC emissions, and the EPA promoted the development and adoption of cost-effective PFC emission reduction opportunities.
Aluminum is produced using the Hall-Heroult process, which involves running an electric current between a carbon anode and a cathode through a high-temperature bath of cryolite and aluminum fluoride. Alumina (Al2O3) is fed into the bath at pre-determined intervals. When the current passes through the bath, the alumina is reduced to aluminum, which can then be removed or “tapped” from the bottom of the bath. The anodes, made of baked carbon, are immersed in the bath to complete the electric path.
PFCs are emitted during “anode effects” that occur when the alumina ore content of the electrolytic process bath falls below critical levels optimal for the production of aluminum. During an anode effect, two PFCs - Tetrafluoromethane (CF4) and Hexafluoroethane (C2F6) - are produced. The magnitude of PFC emissions for a given level of aluminum production depends on the frequency and duration of the anode effects. CF4 and C2F6 have global warming potentials (GWP) of 6,500 and 9,200 times as strong as carbon dioxide (CO2) and atmospheric lifetimes over 10,000 years. Therefore, reducing PFC emissions yields significant environmental benefits.
Companies in the aluminum industry have worked to reduce PFC emissions by minimizing the number and duration of anode effects. Technological and operational changes, such as employee training, use of computer monitoring and changes in alumina feeding techniques, have been used. The industry has succeeded in implementing cost-effective actions by optimizing the production process and through operation and management practices.
- The Aluminum AssociationExit
- International Aluminum AssociationExit
- Australian Aluminum CouncilExit
- Aluminum Association of CanadaExit
- European Aluminum AssociationExit
- The Minerals, Metals, and Materials SocietyExit
- USGS Minerals Information: Aluminum
- U.S. Department of Energy Industries for the Future: Aluminum
- Protocol for Measurement of CF4 and C2F6 from Primary Aluminum Production
- Factors Affecting PFC Emissions from Commercial Aluminum Reduction Cells
- PFC Emissions and Reductions: The Domestic and International Perspective
- International Efforts to Reduce Perfluorocarbon Emissions from Primary Aluminum Production
- Perfluorocarbon (PFC) Generation at Primary Aluminum Smelters
- Aluminum Industry Technology RoadmapExit
- What are Perfluorocarbons (PFCs)?
Perfluorocarbons (PFCs) are carbon compounds in which all available bond sites are attached to fluorine. These compounds have extremely stable molecular structures and, unless they are struck by lightning or are combusted in an incinerator, they are largely immune to the chemical processes that break down most pollutants. PFCs are only destroyed by very high-energy ultraviolet rays in the mesosphere, about 60 kilometers above Earth. This removal mechanism is extremely slow, and as a result, PFCs can accumulate in the atmosphere and remain there essentially forever. Primary aluminum production and semiconductor manufacture are the largest known human-related sources of two perfluorocarbons - tetrafluoromethane (CF4) and hexafluoroethane (C2F6).
- How do PFCs play a role in global climate change?
PFCs are potent greenhouse gases that are very stable in the atmosphere. PFCs are removed very slowly from the atmosphere due to their long atmospheric lifetimes. The estimated atmospheric lifetimes for CF4 and C2F6 are 50,000 and 10,000 years respectively. The global warming potential (GWP) of these compounds, a measure that combines expected atmospheric lifetime and infrared absorbing capacity and which provides a common unit for comparing the relative impacts of gases on global warming, is amongst the highest. One tonne of CF4 and C2F6 emissions is equivalent to approximately 7,390 and 12,200 tonnes of carbon dioxide (CO2) emissions, respectively, when the warming is considered over a 100-year period.
- How is primary aluminum produced?
Aluminum is produced in much the same way for the last century, using the Hall-Heroult process. This process involves running an electric current between a carbon anode and a cathode, through a high-temperature bath of cryolite and aluminum fluoride. Alumina (Al2O3) is fed into the bath at pre-determined intervals. When the current passes through the bath, the alumina is reduced to aluminum, which can then be removed or “tapped” from the bottom of the bath. Aluminum production is carried out in a semi-batch manner in large electrolytic cells called pots with a direct electric current input. The anodes, made of baked carbon, are immersed in the bath to complete the electric path.
- How are PFCs generated during the primary aluminum production process?
When the alumina ore content of the electrolytic bath falls below critical levels optimal for chemical reaction to take place, rapid voltage increases occur, termed “anode effects.” During an anode effect, carbon from the anode and fluorine from the dissociated molten cryolite bath combine, producing CF4 and C2F6. These gases are emitted from the exhaust ducting system or other pathways from the cell (e.g., the hood of the cell). The magnitude of PFC emissions for a given level of aluminum production depends on the frequency and duration of anode effects.
- How can PFC emissions be reduced?
PFCs are emitted during “anode effects” that occur when the alumina ore content of the electrolytic bath falls below critical levels optimal for the production of aluminum. Although PFC emissions cannot be eliminated completely with current technology, emissions can be reduced by lowering the frequency and duration of anode effects. Technological and operational changes such as employee training, use of computer monitoring and changes in feeding techniques are used to minimize anode effects without sacrificing competitiveness.
- Are there other benefits to PFC reduction?
In addition to the environmental benefits, avoiding anode effects improves operational efficiency and adds to productivity. When an aluminum cell is on an anode effect, no aluminum is being produced even though process inputs such as energy and alumina are being consumed. A study conducted by the VAIP Partnership found the following benefits associated with reducing anode effects:
- Decreased power consumption
- Improved aluminum production
- Improved aluminum purity
- Decreased carbon consumption
- Decreased fluoride consumption
- Decreased labor costs
- Increased production pot life