Thermodynamics of Formation of Al₆Mn Inter-Metallic Compound for Mn Removal from Molten AlMg Alloy

To reduce CO₂ emission, recycling Al scrap into wrought alloy is desired. However, impurity elements are inevitably contained in the Al scrap recovered from the society, and most of them are difficult to be removed by the current pyro-metallurgical processes. Mn is one of the major alloying elements of Al alloys and steels and is contained in Al scraps. Therefore, a removal method of Mn from Al is necessary. Mn easily forms an inter-metallic compound with Al; on the other hand, it is immiscible with Mg in the liquid state, which suggests that the repulsive interaction between Mg and Mn in molten Al enhances the precipitation of intermetallic compounds such as Al₆Mn. This study proposed Mn removal from molten AlMg alloy through precipitation of Al₆Mn inter-metallic compound. Molten AlMg alloy was equilibrated with Al₆Mn, and the reducing limit of Mn concentration was thermodynamically discussed. Mn concentration becomes lower at higher Mg content of molten Al and lower temperature. The activity coefficient of Mn in molten AlMg alloy was increased with the addition of Mg, and the following equation was obtained as a function of temperature and molar fraction of Mg: 873 ln £^o_Mnðl;in _AlMg alloy_Þ ¼ _T ð-0:459x_Mg² þ 17:94x_Mg - 8:80Þ It was found that Al₆Mn will precipitate due to the repulsive interaction of Mn and Mg when the Mg content of Al is increased. The thermodynamic analysis showed the possibility of reducing the Mn content of Al to 0.0030 mass% at 733 K and 34 mass%Mg by the present removal process.