Structural investigations on iron-ore sinter phases – crystallography meets applied mineralogy

Sinter plants are an integral part for the preparation of iron ores for the blast furnace process. In a series of complex reactions, semi-molten, centimeter-sized aggregates are formed containing ore particles cemented by a matrix of Ca-rich ferrites that contain some silica and alumina. For these compounds, the acronym SFCA (Silico-Ferrites of Calcium and Aluminum) was coined. The SFCA phases with general composition A14+6nO20+8n (A: Ca, Mg, Fe2+, Fe3+, Al3+, Si4+) act as the binder that keeps the sinters intact in order to withstand the loads in a blast furnace.

Depending on the value of n in the chemical formula, four different SFCA-phases can be distinguished forming a polysomatic series. They can be described with a modular approach involving the stacking sequence of “P” and “S” modules that can be imagined as being cut from the well-known pyroxene and spinel structure-types. The representatives with n = 0 (A14O20) are related to the sapphirine supergroup of minerals.

Even though the SFCAs are critical for the production of iron-ore sinters, it is surprising that there are still many open questions regarding their composition, atomic structure, thermochemistry and stability. For any target-oriented improvement and optimization of ore-sintering as well as thermodynamic modeling of the sinters, a much more detailed understanding of these fundamental solid-state properties of the SFCAs is essential. The present contribution provides - for the first time - a detailed crystallographic analysis on the impact of chemical variations on these compounds that are of relevance for the field of applied mineralogy.