Reductant characterisation and selection in ferrochromium production

Abstract

A variety of carbonaceous materials are being used for the production of ferrochromium. Reductant materials include carbons such as coal, coke and char. Due to complexity and heterogeneity of carbonaceous reducing materials and a wide range of process requirements, the selection of the most appropriate reducing agent will depend on a number of considerations such as availability of raw material and associated costs, product and process requirements. Matching the correct carbon reductant for each specific process and product requirements has become a vital function of the ferrochrome industry. This selection process however, has become increasingly difficult. In many cases, the substitution of new a reductant, despite being similar in all relevant chemical specification characteristics to previous material, has resulted in different performance properties relative to that expected. An investigation prior to the usage of the substituting reductant in the submerged arc furnace was done in order to obtain required performance properties. The investigation concerned the search for a more reliable model in characterizing the reductant according to their reactivity performance, and using the model parameters to choose the appropriate reductant. The model chosen for this investigation was the Arrhenius model. Sintered chromite ore was reduced with the reductant under investigation in a Thermogravimetric analyzer TGA) at different temperatures. The resultant data was then fitted into the Arrhenius model to obtain the indication of the reactivity of the reductant. The TGA tests suggest that the Vietnamese anthracite has the highest reactivity since its activation energies are the lowest and requires less energy to induce the reductant reaction. The tests also suggest that the rate controlling mechanism is the diffusion of species to the reactive site since the effective diffusion constant is of the order 10-10 to 3 10-11, which is far less than the chemical rate constant of the order 10-6 A generalized rate model developed to describe the reduction of chromite shows that at given particle size and up to a reduction of 40%, the rate of reduction is controlled mainly by interfacial-area chemical reaction, and after which the rate is dominated by diffusion.