Magnesium production and casting processes utilize sulfur hexafluoride (SF6), a potent greenhouse gas (GHG) to prevent oxidation of molten magnesium during production and processing operations.
The EPA and the U.S. magnesium industry, with the support of the International Magnesium Association (IMA), worked together from 1999 to 2010 through the SF6 Emission Reduction Partnership for the Magnesium Industry. The Partnership’s goal was to better understand and reduce emissions of SF6 by identifying, evaluating and implementing cost-effective climate protection strategies and technologies including alternative cover gases. EPA continues to track facility specific emissions from the magnesium industry through the Greenhouse Gas Reporting Program Subpart T. A summary of this industry, emission sources and key resources developed during the Partnership are archived below.
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.
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. Given the high GWP and long atmospheric lifetime of SF6, 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.
- Secondary Magnesium Ingot Casting
- Magnesium Die Casting
- Alternatives to SF6/SO2 for Magnesium Melt Protection
- Resources for reporters to the Greenhouse Gas Reporting Program (GHGRP): Subpart T - Magnesium Production
- Emissions data and trends from the GHGRP metals sector
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.