In 1999, EPA and the U.S. magnesium industry, with the support of the International Magnesium Association (IMA), launched a voluntary Partnership to better understand and reduce emissions of sulfur hexafluoride (SF6), a very potent greenhouse gas (GHG). EPA’s Partnership with the magnesium industry promoted technically feasible and economically attractive actions aimed at minimizing SF6 emissions and reducing the threat of global climate change.
SF6 has been widely used by the magnesium industry for more than 25 years. Magnesium producers, casters, and recycling companies commonly use a cover gas of dilute SF6 in dry air and/or carbon dioxide (CO2) to protect the 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 (see Molten Magnesium images).
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. For emission estimation purposes per the IPCC Guidelines, it is assumed that all SF6 used is emitted to the atmosphere. With a global warming potential (GWP) 23,900 times as strong as CO2 and a 3,200-year atmospheric lifetime, reducing SF6 emissions will have significant environmental benefits for many future generations. In 2010, the U.S. magnesium industry’s SF6 emissions were equal to 1.3 million metric tons of CO2, a 76% improvement since the partnership began in 1999.
In February 2003, EPA’s partners and the International Magnesium Association established a goal to eliminate SF6 emissions by year-end 2010. In pursuit of this technically aggressive commitment, the industry evaluated and implemented alternative cover gases such as HFC-134a, 3M’s fluorinated ketone, Novec™ 612, and possibly sulfur dioxide (SO2 is not a greenhouse gas). Table 1 shows the atmospheric lifetimes and GWPs of the chemicals that are GHGs.
Table 1: Atmospheric Lifetimes and Global Warming Potentials (100 year)*
|Compound||Atmospheric Lifetime (years)||Global Warming Potential (100 Year)|
|3M™ Novec™ 612
* IPCC 2007
*** Carbon dioxide's lifetime is poorly defined because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.
Despite a highly competitive global market for magnesium production and processing and recent declines in U.S. casting activity (USGS 2011), 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 directly contributed to the Partnership’s achievements and provided both economic and environmental benefits. More detailed information on F-GHGs is available at EPA's Emissions Homepage.