Thermodynamic and experimental evaluation of the sustainable recycling of magnesium alloy scrap by vacuum distillation based on vapor-liquid equilibrium

Abstract

Magnesium (Mg) alloys are widely used lightweight structural materials for automobiles and help reduce carbon emissions. However, their use increases the production of Mg alloy scrap, which is recycled at a much lower rate than aluminum, and its greater complexity poses challenges to existing recycling processes. Although vacuum distillation can be used to recycle Mg alloy scrap, this requires optimizing and maximizing metal recirculation, but there has been no thermodynamic analysis of this process. In this study, the feasibility and controllability of separating inclusions and 23 metal impurities were evaluated, and their distribution and removal limits were quantified. Thermodynamic analyses and experimental results showed that inclusions and impurity metals of separation coefficient lgβ_i ≤ -5, including Cu, Fe, Co, and Ni below 0.001 ppm, could be removed from the matrix. All Zn entered the recycled Mg, while impurities with -1 < lgβ_i < -5 such as Li, Ca, and Mn severely affected the purity of the recycled Mg during the later stage of distillation. Therefore, an optimization strategy for vacuum distillation recycling: lower temperatures and higher system pressures for Zn separation in the early stage, and the early termination of the recovery process in the later stage or a continuous supply of raw melt can also prevent contamination during recycling. The alloying elements Al and Zn in Mg alloy scrap can be further recovered and purified by vacuum distillation when economically feasible, to maximize the recycling of metal resources.