Phase Relations in the CaO-Nd2O3-Al2O3 System in Application for Rare Earth Recycling

Abstract

Phase relations in the CaO-Al2O3-Nd2O3 system were investigated in the temperature range between 1330°C and 1600°C using equilibration technique followed by characterization using x-ray diffraction and scanning electron microscopy combined with energy-dispersive x-ray spectroscopy (SEM/EDX). Melting relations were investigated using differential thermal analysis followed by microstructure study using SEM/EDX. It was found that CaNdAlO4 (τ1) compound melted congruently at 1808°C, while CaNdAl3O7 (τ2) compound melted incongruently at 1766°C. The sample in the stability fields adjacent to lowest eutectic in the CaO-Al2O3 system (CaAl2O4 + Ca3Al2O6) and τ2 as well as samples in the stability field τ2 + CaAl2O4 + CaAl4O7 and τ1 + τ2 + Ca3Al2O6 were completely melted at 1600°C. The Ca12Al14O33 phase considered as not stable in anhydrous atmosphere was found instead of Ca3Al2O6 phase after heat treatment at 1330°C. After melting Ca12Al14O33 formed glass. The obtained results are important for selection of conditions for process of rare earth recycling from waste permanent NdFeB magnets.