Sulphur hexafluoride (SF6) is used in metal casting processes in magnesium foundries as well as in aluminium foundries. The function of SF6 in both processes is different:
Magnesium is a highly reactive metal. As temperatures of up to 800 °C can occur during the casting process, the surface of the melt has therefore to be protected against ignition, oxidation and the formation of nitrides. In the early years of industrial magnesium production only salt fluxes or powdered sulphur were known as protective cover for this purpose. In order to prevent contamination of the cast products, the salt fluxes were replaced by protective gas mixtures. Sulphur dioxide (SO2) was the first cover gas used in magnesium foundries to protect the molten metal, and plays an important role still today. Due to its toxic and corrosive properties (especially humid SO2 enhances corrosion of steel equipment) it was replaced by SF6 in most foundries.
More recently the use of alternative cover gases with lower GWPs has been developed, leading to use restrictions in the EU for SF6 first implemented under the F-Gas Regulation 842/2006. Now, through Regulation 517/2014, the use of sulphur hexafluoride in magnesium die-casting and in the recycling of magnesium die-casting alloys is totally prohibited from 1 January 2018.
For further information see Magnesium smelting: bans on use of sulphur hexafluoride.
As well as SO2 and dilute SO2 mixtures, HFC-134a is being used as a cover gas. A study has shown that in die casting, on average 79% of the HFC-134a is destroyed over the melt during processing, reducing emissions of HFC-134a, and further reducing the environmental impact when compared to SF6. For further information see the Öko-Recherche 2009 report for the European Commission.
Porosity caused by the hydrogen content of the aluminium melt is a fundamental problem in the production of aluminium cast components, as it leads to a reduction in strength. Such an effect can only be avoided during the aluminium alloy smelting. During the smelting process also oxides and solid impurities should be removed. This can be achieved by injecting chlorine (pure or in mixture with inert gas) or an SF6/inert gas mixture. The Öko-Recherche 2009 report for the European Commission reports that in aluminium smelting, SF6 gas bubbles are finely dispersed in the melt by impeller technology. SF6 is largely decomposed in the hot metal and broken into sulphur and fluorine compounds. As a consequence, only 1.5% of the SF6 consumption is considered to be global warming emissions.
©Shutterstock via Cefic