- What is sulfur hexafluoride (SF6) and why is EPA concerned?
- How is SF6 used in the magnesium industry?
- What are the potential abatement strategies available to the magnesium industry?
- What is the goal of this partnership?
- How do I know this partnership will work for my company?
- Will SF6 emissions from the magnesium industry grow in the future?
1. What is sulfur hexafluoride (SF6) and why is EPA concerned?
SF6 is a highly potent greenhouse gas. Over a 100-year period, SF6 is 23,900 times more effective at trapping infrared radiation than an equivalent amount of carbon dioxide (CO2). SF6 is also a very stable chemical, with an atmospheric lifetime of 3,200 years. Consequently, it will accumulate in the atmosphere and its effect on the climate will be felt by many future generations. This characteristic is highlighted by recent measurements that have identified an increase in atmospheric SF6 concentrations of approximately 7% per year since 1978 (Maiss and Brenninkmeijer, 1998). For more information on Climate Change proceed to EPA’s Climate Change Web site.
2. How is SF6 used in the magnesium industry?
SF6 is a non-hazardous, inert gas that is used to prevent the oxidation of molten magnesium during production and processing operations. Approximately 0.2% to 0.5% by volume of SF6 is used in gaseous mixtures containing air and/or CO2. The mixture is supplied to the molten magnesium surface, where SF6, in particular the fluorine component, forms a protective film of fluorine-containing magnesium oxides (MgO). Without an effective cover gas, molten magnesium oxidizes with atmospheric oxygen, producing a lower quality product and a potentially violent fire. Any SF6 that is not consumed in the process is typically emitted directly to the atmosphere.
The use of SF6 mixtures has been the predominant method for protecting molten magnesium surfaces for several decades. It replaced the use of more toxic and corrosive compounds, such as solid salt fluxes and sulfur dioxide (SO2), as the primary cover gas mechanism.
3. What are the potential abatement strategies available to the magnesium industry?
There are several pollution prevention options available to reduce and potentially eliminate SF6 emissions from the magnesium industry. The International Magnesium Association (IMA) and EPA are currently studying alternate cover gas compounds. The initial study identified compounds such as boron trifluoride (BF3), sulphuryl fluoride (SO2F2), 1,1,1,2-tetrafluoroethane (HFC-134a), hydrofluoroethers (HFE 7100 and HFE 7200), and a fluoroketone (FK). While these compounds are still under evaluation, some companies, primarily in Europe, have switched back to SO2 as a short-term SF6 mitigation strategy. For those companies still using SF6, research is also being conducted into advanced recycle/capture techniques. These devices collect the process exhaust gases, and filter the SF6 component for reuse on site. On another level, research has also been conducted into innovative equipment designs. Brochot, a casting equipment manufacturer, has developed a new magnesium ingot casting machine. The machine reportedly uses two innovative processes (a new casting wheel design, and patented inert cover gas containing a mixture of xenon, argon, and CO2) to reduce the impact of melt surface oxidation on casting productivity and operational costs.
4. What is the goal of this partnership?
The goal is to reduce emissions of SF6 from magnesium production and processing through a voluntary collaborative approach. In February 2003, EPA’s partner companies and the IMA committed to eliminate SF6 emissions by year-end 2010.
5. How do I know this partnership will work for my company?
The partnership is extremely flexible, and provides Partners with broad latitude to pursue the goal of reducing SF6 emissions. For example, the partnership asks Partners to develop a strategy to reduce SF6 usage rates. It is up to the Partner to determine the best approach. This may include improving the efficiency of the cover gas system, such as tightening furnace enclosure hoods, and improving controls for the distribution of low concentration SF6 gas to the molten magnesium, etc.
6. Will SF6 emissions from the magnesium industry grow in the future?
EPA’s partners and the IMA have voluntarily committed to eliminate SF6 emissions by December 31, 2010. With promising, more environmentally friendly cover gas technologies such as 3M’s Novec™ 612 (fluorinated ketone) and AMCover™ (HFC-134a blend) arriving in the marketplace, it is unlikely the industry’s SF6 emissions will grow in the future. It will be important to share information with China’s rapidly growing magnesium industry to assure they have access to and implement the most environmentally friendly production and casting technologies.