Activity coefficients of NiO and CoO in CaO–Al_2O_3–SiO_2 slag and their application to the recycling of Ni–Co–Fe-based end-of-life superalloys via remelting

To design optimal pyrometallurgical processes for nickel and cobalt recycling, and more particularly for the end-of-life process of Ni–Co–Fe-based end-of-life(EoL) superalloys, knowledge of their activity coefficients in slags is essential. In this study, the activity coefficients of NiO and CoO in CaO–Al_2O_3–SiO_2 slag, a candidate slag used for the EoL superalloy remelting process, were measured using gas/slag/metal equilibrium experiments. These activity coefficients were then used to consider the recycling efficiency of nickel and cobalt by remelting EoL superalloys using CaO–Al_2O_3–SiO_2 slag. The activity coefficients of NiO and CoO in CaO–Al_2O_3–SiO_2 slag both show a positive deviation from Raoult's law, with values that vary from 1 to 5 depending on the change in basicity. The activity coefficients of NiO and CoO peak in the slag with a composition near B =(%CaO)/(%SiO_2) = 1, where B is the basicity. We observed that controlling the slag composition at approximately B = 1 effectively reduces the cobalt and nickel oxidation losses and promotes the oxidation removal of iron during the remelting process of EoL superalloys.