Removal of Nickel Impurities from Recycled Mg-Al Alloys by Precipitation of Intermetallic Compounds

The primary production of pure magnesium and its alloys is highly energy intensive and expensive.
Thus, their recycling is necessary considering energy saving and metal content. Magnesium postconsumer scrap often is contaminated with copper, nickel and iron resulting from aluminium alloy adherences, nickel platings and steel. Especially nickel is detrimental to the corrosion properties of magnesium alloys. To produce secondary magnesium alloys nickel impurities have to be removed to obtain appropriate corrosion resistance. In this case no traditional metallurgical refining process is applicable because nickel is nobler than magnesium. It has been found that the refining can be done by a combined addition of aluminium and zirconium to remove nickel by formation of high melting intermetallic compounds and hence their precipitation. The aim of the investigation is to determine and verify the amounts of zirconium and aluminium which are necessary to reach the low nickel content of 10 ppm (high purity quality). However, the mutual solubilities of nickel and zirconium in aluminium containing magnesium alloys are unknown. Therefore, the solubility isotherms of this system at 720 °C were experimentally investigated by adding nickel to magnesium melts with different aluminium contents (5 and 8 mass.-%). Subsequently aluminium and zirconium have been added to remove the nickel. At the near equilibrium state samples have been taken from the melt and analysed for the components. Furthermore, the formed intermetallic aluminium-nickelzirconium compounds have to be determined for identification by chemical analysis and EDX. Besides the solubility isotherms for the system are calculated using a thermodynamic model.