Use of SO_2 as protection gas for Mg-Recycling

摘要

Different protection gases are used to protect liquid magnesium metal. Most of them are containing fluoride except sulfur dioxide. Mixing with carrier gases is possible in different concentration of the protection gas, depending on produced alloys and equipment used. The most used protection gas today is SF_6, unfortunately this gas has a very high global warming potential and the magnesium industry is one of the largest emitter. Due to the high GWP in future production and use will be restricted or forbidden. Alternative cover gases are offered, also containing fluoride but with lower GWP compared to SF_6. Some of them have been tested at different companies with all kind of magnesium alloys. During reaction with the magnesium melt other gases can be generated with a global warming potential. The generation of HF is possible, too. SO_2 has no GWP but it is corrosive and toxic, so precautions have to be taken regarding the working environment. SO_2 is mainly generated during coal and oil/fuel combustion (in million tons/a in Germany), the SO_2 amount emitted during magnesium production is compared to that very small. Hydro Magnesiumgesellschaft (HMG) in Bottrop, Germany uses SO_2 as protection gas with air as carrier gas in the recycling and anode production. SF_6 was substituted by SO_2 already in the early '90th. First only SO_2 bottles located close to each casting station were used bringing pure SO_2 directly to the liquid metal surface. End of the '90th a central mixing station was installed to reduce the amount of total SO_2 using a mixture of air and SO_2. Since January 2004 a new mixing station is in place giving the possibility to adjust different outlets in flow rates and SO_2 concentration for different alloys. Additionally an online monitoring system is installed to measure SO_2 concentrations at the work places with visual indicators to comply with German law requirements. Technical data and some cost figures are shown.