Choose from the questions below to learn more about perfluorocarbons and primary aluminum production.
- What are perfluorocarbons (PFCs)?
- How do PFCs play a role in global climate change?
- How is primary aluminum produced?
- How are PFCs generated during the primary aluminum production process?
- How can PFC emissions be reduced?
- Are there other benefits to PFC reduction?
1. What are Perfluorocarbons (PFCs)?
Perfluorocarbons (PFCs) are carbon compounds where all available bond sites are attached to fluorine. These compounds have extremely stable molecular structures and unless they are struck by lightning or happen to be combusted in an incinerator, they are largely immune to the chemical processes that break down most pollutants. Not until the PFCs reach the mesosphere, about 60 kilometers above Earth, do very high-energy ultraviolet rays from the sun destroy them. This removal mechanism is extremely slow and as a result PFCs can accumulate in the atmosphere and remain there essentially forever, by any human timeframe. Primary aluminum production and semiconductor manufacture are the largest known human-related sources of two perfluorocarbons - tetrafluoromethane (CF4) and hexafluoroethane (C2F6).
2. 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 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 respectively of carbon dioxide emissions, when the warming is considered over a 100-year period (IPCC Fourth Assessment 2007).
3. How is primary aluminum produced?
Aluminum is produced in much the same way it has been produced 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.
4. 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 the above 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.
5. 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 like employee training, use of computer monitoring, and changes in feeding techniques are used to minimize anode effects without sacrificing competitiveness.
6. Are there other benefits to PFC reduction?
In addition to the environmental benefits, avoiding anode effects improves operational efficiency and adds to the bottom line. When an aluminum cell is on 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 a number of benefits associated with reducing anode effects:
- Decrease in power consumption;
- Improve aluminum production;
- Improve aluminum purity;
- Decrease in carbon consumption;
- Decrease in fluoride consumption;
- Decrease in labor costs; and
- Increase in production pot life.