Influence of potassium on the nucleation and early growth of magnetite in the reduction of hematite with carbon monoxide-carbon dioxide

Abstract

The enhancement of the reduction rate of hematite to magnetite in the presence of potassium oxide is well known. On the other hand, the nucleation of magnetite and its first growth steps have recently been investigated. It was therefore of interest to study, on the same scale, the influence that potassium may have on these stages and to understand the reasons. A series of samples were therefore prepared with reaction times ranging from 0.5 to 45 min during the reduction of hematite single crystals (prepared by chemical vapour transport) at 600 ° C with CO-CO 2 (2:98). Also the morphology and growth rate of samples with and without potassium were compared with a scanning electron microscope. The observation of a strong influence of potassium on the number and growth rate of the nuclei, which are often anisotropic, is reported. The interpretation is based on the transient formation of KFe 11O 17 which, owing to its magnetoplumbite structure, can easily form an epitaxic layer on Fe 2O 3; a detailed mechanism of reduction and nuclei formation is suggested. The following stages in the reduction of KFe 11O 17 are also discussed with the help of a proposed iron-potassium-oxygen diagram.