The influence of CaC03 addition and clay binder selection on electricity efficiency and other operational aspects during pre-reduction of pelletised chromite
Van Zyl, P.G.
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Production of ferrochrome is an energy intensive process. Pelletised chromite pre-reduction is most likely the ferrochrome production process with the lowest specific electricity consumption, i.e. MWh/t ferrochrome produced. Higher chromite pre-reduction levels correspond to lower specific electricity corzsumptions. In this paper the effect of CaC03 addition and clay binder selection on pelletised chromite pre-reduction is presented. Results indicated that CaC03 addition significantly enhances the level of chromite pre-reduction achieved within a given time and temperature horizon. Hovvever, CaC03 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 CaC03 in the pelletised chromite pre-reduction process will therefore most probably result in the formation of excessively fine feed materials. In practise, this reduces the usefulness of this technique for submerged arc furnace f errochrome 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 C02 released from the CaC03 will result in additional carbon consumption, which is an additional negative associated with its use as an additive during pre-reduction of composite chromite pellets. Although the research work covered in this paper was not specifically aimed at obtaining mechanistic information, thermo-mechanical analysis indicated that especially iron pre-reduction rates were enhanced by CaC03 addition. It was also proved that clay binder selection not only ilifluence the physical strength of the pre-reduced pellets, but also the level of achievable pre-reduction. Additionally, increased addition of clay binder to the composite pellet was found to result in progressive decline in the degree of chromite pre-reduction achieved within a specified timetemperature reference frame