Design principles for spray-roasted iron oxides for the manufacturing of ferrites

A range of high purity iron oxides are prepared by varying basic operation parameters of an industrial spray roasting process. These iron oxides are investigated in relation to their morphology and subsequently evaluated as raw materials for MnZn-ferrite preparation. It appears that the most important morphological parameters for determining the reactivity (defined as firing shrinkage at equal compaction density) of the high purity iron oxide, and consequently the final density and magnetic properties of the ferrite specimens, are the primary particle size and the number of primary particles per aggregate. As found, the specific surface area of the iron oxide is of no predictive value for the behavior of the iron oxide in a MnZn-ferrite manufacturing process. A small primary particle size is important for a high reactivity; however, when particles are packed together in large aggregates, they are not available for the prefiring reactions. As a result, reactive sintering takes place leading to high porosity and bad microstructure. As found by the characterization methods employed in this article, the optimum iron oxides for MnZn-ferrite preparation should have a primary particle size between 0.45 and 0.55 μm with an aggregate size below 1.60 μm.