The EPA and the U.S. magnesium industry, with the support of the International Magnesium Association (IMA), worked together to better understand and reduce emissions of sulfur hexafluoride (SF6), a potent greenhouse gas (GHG), from magnesium production and casting processes, with the goal of eliminating SF6 emissions by identifying, evaluating and implementing cost-effective climate protection strategies and technologies including alternative cover gases.
SF6 is widely used by producers, casters and recycling companies in the magnesium industry. A cover gas of dilute SF6 in dry air and/or carbon dioxide (CO2) protects molten metal from oxidation and potentially violent burning. Without protection, molten magnesium will oxidize in the presence of air and form magnesium oxide (MgO) deposits that greatly reduce the quality and strength of the final product. In contrast, an effective cover gas, such as SF6, modifies and stabilizes the MgO surface film to form a protective layer that prevents further oxidation.
Studies conducted to characterize the reaction byproducts of SF6 and molten magnesium determined that most of the SF6 introduced to the molten metal surface is emitted to the atmosphere and that only a small portion reacts or decomposes. With a global warming potential (GWP) 22,800 times as strong as carbon dioxide (CO2) and a 3,200-year atmospheric lifetime, reducing SF6 emissions yields significant environmental benefits.
Studies conducted by the magnesium industry identified technically proven alternative cover gas options for eliminating the use of SF6. EPA's partners in the magnesium industry made significant progress in improving their operational efficiencies and environmental performance by deploying alternative cover gas technologies and optimizing SF6 cover gas concentrations, flow rates and delivery mechanisms, as well as identifying and repairing leaks in SF6 gas distribution systems. These activities and technological innovations provided both economic and environmental benefits.
- Characterizing of Cover Gas and Byproduct Emissions from Secondary Magnesium Ingot Casting at Advanced Magnesium Alloys Corporation (AMACOR)
- Characterization of Cover Gas and Byproduct Emissions from Secondary Magnesium Ingot Casting
- Characterization of Emissions and Occupational Exposure Associated with Five Melt Protection Technologies for Magnesium Die Casting
- Characterization of Cover Gas Emissions from U.S. Magnesium Die Casting
- Measured SF6 Emissions From Magnesium Die Casting Operations
- Alternatives to SF6/SO2 for Magnesium Melt Protection
- Building a Bridge for Climate Protection: U.S. EPA and the Magnesium Industry
- Update On EPA’s Magnesium Industry Partnership For Climate Protection
- What is SF6 and why is EPA concerned?
SF6 is a highly potent GHG. Over a 100-year period, SF6 is 22,800 times more effective at trapping infrared radiation than an equivalent amount of CO2. SF6 is also a very stable chemical, with an atmospheric lifetime of 3,200 years.
- 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 cover gas 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.
- What are the potential abatement strategies available to the magnesium industry?
There are several pollution prevention options available to reduce and eliminate SF6 emissions from the magnesium industry. The International Magnesium Association (IMA) and EPA have studied alternate cover gas compounds. Initial studies 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) Novec™612. AMCover™ (HFC-134a blend) and 3M’s Novec™612 (FK) are commercially available cover gas alternatives for SF6 that have been tested and, in some instances, adopted by facilities in the United States.
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, developed a magnesium ingot casting machine that uses two processes (a 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.
- Will SF6 emissions from the magnesium industry grow in the future?
Estimated emissions of SF6 by the U.S. magnesium industry declined from nearly 6 MMTCO2e (million metric tons of carbon dioxide equivalent) in 1999 to roughly 1.3 MMTCO2e in 2010. Despite industry contraction within the U.S., the decrease in total U.S. emissions was enabled by transitions to alternative cover gas options and optimized cover gas usage. Growing demand for lightweight magnesium automotive parts and components of hand-held electronic devices is expected to support strong future growth in the magnesium casting sector both in the U.S. and internationally.
When considering business-as-usual (BAU) practices, overall U.S. industry emissions reported are estimated to have been cumulatively reduced by 5.7 MMTCO2e since 1999. With more environmentally friendly cover gas technologies such as 3M’s Novec™ 612 (FK) and AMCover™ (HFC-134a blend) available in the marketplace, it is unlikely the industry’s SF6 emissions will grow significantly in the future.
- Will companies in the magnesium industry continue to report data on GHG emissions to EPA?
Under subpart T of EPA’s Greenhouse Gas Reporting Program (40 CFR Part 98), owners or operators of facilities that contain magnesium production processes are required to annually report emissions from use of cover or carrier gases as well as for all other source categories located at the facility for which methods are defined in the rule. Facilities covered by subpart T are those that have emissions equal to or greater than 25,000 MTCO2e per year in combined emissions from stationary fuel combustion and all other source categories identified by the rule. Covered facilities were required to begin monitoring GHG emissions on January 1, 2011 in accordance with the methods specified in the rule. The first report was due to EPA by September 28, 2012; subsequent reports are due annually thereafter by March 31. Reporters report their data to EPA electronically through EPA’s Electronic Greenhouse Gas Reporting Tool (e-GGRT).
- 5th Annual Global Magnesium Industry Climate Protection Workshop
May 2009, San Francisco, CA
- 4th Annual Global Magnesium Industry Climate Protection Workshop
May 2008, Warsaw, Poland
- 4th International Conference on SF6 and the Environment
November 2006, San Antonio, TX
- 3rd International Conference on SF6 and the Environment
December 2004, Scottsdale, Arizona
- 2nd International Conference on SF6 and the Environment
November 2002, San Diego, CA
- 1st International Conference on SF6 and the Environment
November 2000, San Diego, CA