The author gives a thermodynamic analysis and schematic description of halogen-based demagging processes which promise reduced atmospheric emissions for secondary aluminum producers. The ″demagging″ process (reduction of the magnesium content in aluminum casting alloys) has recently become a major problem for secondary smelters because of the resulting effluents containing noxious halogens. Metallurgical considerations indicate that the process can be made 100% efficient, thus eliminating this source of air pollution.