Formation of a Network Structure in the Gaseous Reduction of Magnetite Doped with Alumina

Abstract

Reduction of un-doped magnetite is developed topochemically with the formation of a dense iron shell. However, the reduction of alumina-doped magnetite to wustite proceeds with the formation of a network-like structure which consists of criss-crossed horizontal and vertical plates of wustite. Reduction of magnetite includes the conversion of Fe3+ to Fe2+ and the movement of iron cations from the tetrahedral sites on the {400} and {220} planes of magnetite to the octahedral sites on the {200} planes of wustite. Alumina has a negligibly small solubility in wustite. In the reduction of magnetite doped with Al2O3, rejected Al3+ cations from wustite diffuse to the magnetite–hercynite solid solution. Enrichment of the Fe3O4–FeAl2O4 solution with alumina in the vicinity of the reduction interface restricts the growth of {220} planes of wustite and nucleation of {220} planes adjusted to the existing planes, preventing the merging of wustite plates during the reduction process. Reduction of magnetite from the magnetite–hercynite solid solution practically stops when the Al3+ content at the interface approaches the solubility limit. Wustite in the separated plates is reduced further to iron.