Eco-friendly and economical preparation of permanent ferrite magnet with ultrapure magnetite concentrate: Focus on the action mechanism of silicon and aluminum impurities

Abstract

In this study, ultrapure magnetite concentrate (UMC) was used to replace high-quality iron oxide red (IOR) for permanent ferrite magnet (PFM) preparation. UMC needs to be pre-oxidized to eliminate FeO which easily generates nonmagnetic phases in PFM. The magnetic powder and PFM products prepared with UMC and IOR had close magnetic properties, but the calcination temperature was lower and calcination time was shorter with UMC. Si and Al impurities in UMC could not only form eutectic compounds but also dissolve into the ferrite grain by substituting Fe. Those eutectic compounds benefited uniformity of grains by inhibiting grain overgrowth but decreased the purity of PFM because they were non-magnetic substances. Mn impurity in IOR entered into the ferrite grain by replacing Fe instead of participating in the formation of eutectic mixtures, while Cl impurity was evaporated during calcination, causing equipment erosion and environment pollution. UMC has the advantages of low raw material cost, good product property and non-pollution, and thus it has a broad application prospect in PFM preparation.