Why is CaCO3 not used as an additive in the pelletised chromite pre-reduction process?

Abstract

The production of ferrochrome is an energy intensive process. At present, the pelletised chromite pre-reduction process is most likely the ferrochrome production process with the lowest specific energy consumption, i.e. MW h/t ferrochrome produced. Higher chromite pre-reduction levels correspond to lower specific energy consumptions. It was previously proven that various compounds could enhance the level of chromite pre-reduction. In this paper the effect of CaCO3 addition on pelletised chromite pre-reduction is presented. CaCO3 (as limestone) is already used as a flux in some FeCr production processes, hence its use does not constitute the addition of an extra raw material. Results indicated that CaCO3 addition enhance the level of chromite pre-reduction achieved significantly, which could results in substantial specific energy consumption improvements. However, CaCO3 addition caused severe decreases in both compressive and abrasion strengths of pre-reduced pellets, which is unlikely to be negated by mitigation measures. The addition of CaCO3 in the pelletised chromite pre-reduction process is therefore likely to result in the formation of excessively fine feed materials. In practise, this reduces the usefulness of this technique for submerged arc furnace ferrochrome production significantly, since excessive fines in the feed material are likely to result in increased operational instabilities, equipment damage and safety risks. TGA and thermochemical calculations also indicated that CO2 released from the CaCO3 will result in in situ carbon and energy consumption, which is an additional negative associated with its use as an additive during pre-reduction of composite chromite pellets. Although this paper was not specifically aimed at obtaining mechanistic information, thermo-mechanical analysis indicated that especially iron pre-reduction rates were enhanced by CaCO3 addition.