The solid state reduction of chromite.
High carbon ferrochromium serves as the main chromium source for almost all chromium containing steel alloys. The traditional method for the production of high carbon ferrochromium via the reduction of chromite using coke in electric arc furnaces, draws its considerable energy requirement from electrical power. The escalation in cost of electric power in South Africa has motivated research into alternative, fuel fired, reduction processes. One such process involves the partial solid state reduction of chromite in coal fired rotary kilns at temperatures between 1200 and 140rrc, prior to electric smelting. such processes are currently operated on a commercial scale and result in considerable savings in electrical energy, despite slow reduction kinetics and low reaction extents. A large amount of research conducted in the past, . aimed at establishing the fundamentals of the reduction process, has not provided satisfactory answers to questions regarding the mechanism of reduction. It was therefore necessary to conduct further test work on the process to establish the mechanism and factors limiting the rate and extent of reduction. Thermodynamic analysis of the reaction system indicates that at temperatures above 1050C reduction of the ore will proceed, and should reach an extent of approximately 90% reduction at 1200C. Complete reduction should be achievable at approximately 125ifc. However experimental results indicate the persistence of a stable magnesiochromite spinel under normal reducing conditions even at 140ifc. This limits the degree of chromium reduction to approximately 65%. Kinetic data from thermobalance studies and electron microscope examination of the reduction product showed independent reduction of iron and chromium. The rate of iron reduction was found to be relatively rapid and to go to completion, compared to that of chromium where the formation of a relatively inert picrochromite- spinel solid solution (MgO(Cr,AI)203) at the surface of the grain liinited the rate and extent of reduction to approximately 65% in the case of LG6 chromite. These findings suggested that the only way in which the kinetics of the process might be improved was through the addition of a component capable of disrupting the spinel layer at the surface of the chromite grain. In this study, fluoride containing mixtures such as CaF2 - NaF and fluorspar- feldspar- silica were successfully used to accelerate the reaction. Such mixtures are commercially interesting and highly effective even at low additions (4- 10%) . The mechanism whereby such mixtures operate was shown to involve the dissolution of all the spinel components in the liquid flux phase. Following dissolution, rapid i i recrystallization of spinel (Mg . Al2 ~) occurs , simultaneous to the transport of Fe 2+ and cr 3+ ions through the liquid to a site where reduction can take place. The main effect of this is to increase the rate and extent of chromium reduction to the point where virtual total reduction can be achieved in less than 90 min at temperatures as low as 1200C. Although the reduction kinetics in the presence of such solvent flux phases are still largely limited by the rate of solid state diffusion, the disruption of the surface enables faster overall diffusion rates to be achieved. Ultimately as the particle size and separation between oxide and reductant is increased, the rate of dissolution and transport through the flux phase become rate limiting.