Formation and decomposition of C4F7 type calcium ferrites in superfluxed magnetite based pellets during oxidation and reduction

Abstract

In the current work the reactions of magnetite based pellets with large additions of calcite (3%CaO) during reduction have been investigated. This made it possible to use both X-ray diffraction (XRD) and scanning electron microscopy (SEM) to detect reaction phases that normally occur in very small amounts. The main binding phase in the pellets after oxidation was (CaO,MgO,FeO)₄(Fe₂O₃)₇, whereas the one commonly reported in the literature is (CaO)(Fe₂O₃)₂. During reduction at 500–700°C severe cracking occurred in these pellets, especially in the calcium ferrite phase. However, the decomposition of this phase began at 600°C, and therefore it is believed that the reason for the cracks is low strength of the phase itself, rather than weakness induced by reduction of the phase. Upon reduction of magnetite into wüstite at 800°C, the calcium began dissolving in the wüstite, and at 900°C porous calciowüstite had formed in the entire sample, except for some remaining magnetite left in the pellet cores.

Keywords:

• Reduction
• Iron ore pellets
• Additives
• Calcite
• Calcium ferrites
• C₄F₇
• C₃WF₇

Acknowledgements

The work presented in this paper has been done in remembrance of our former friend and colleague Anders Rutqvist who through XRD first identified the calcium ferrite in superfluxed magnetite pellets as C₄F₇.

Financing from HLRC (Hjalmar Lundbohm Research Centre) and CAMM (Centre for Advanced Mining and Metallurgy) is gratefully acknowledged. We would also like to thank LKAB for providing material, as well as valuable assistance and discussions about tests and results.