CAPTURE AND RECYCLE FOR EMISSION REDUCTION OF SULFUR HEXAFLUORIDE IN MAGNESIUM CASTING

A partnership between MagCorp and Air Liquide was made to develop and test the potential of reducing sulfur hexafluoride emissions from magnesium casting operations. Air Liquide has developed the Floxal SF 6 Recycle Process, which is capable of capturing dilute SF6 in the parts per million (ppm) range from casting system exhaust and recycling it as a concentrated feed gas in percentage levels. This process utilizes proprietary membrane technology to separate the SF6 from air in four stages: pretreatment, compression, separation, and recycling. In order to benchmark the process, the test program was conducted in three phases. Phase 1 was a pre-test visit, Phase 2 was an on-site analytical campaign to characterize the emissions of the casting process and provide a mass balance on the SF 6, and Phase 3 was a second analytical campaign to measure the performance of the Floxal SF 6 Recovery Unit while it operated. Phase 2 had the largest impact for the recovery system, because the levels of corrosive byproducts and particulate affected the design of the pretreatment. Also, the amount of vacuum pulled from the casting system had to be optimized with the makeup rate so it would not deplete the SF 6 atmosphere. Preliminary results show that the Floxal Recovery Unit, after one month of service, had a 90% recovery or better of SF6 supplied to the unit. The Floxal unit produced concentrations of SF6 in the permeate at approximately 34 ppm, while the product was approximately 0.4%. The machine, on average, increased the concentration of the diluted SF6 from the casting system 10 times.