Reduction kinetics of iron ore concentrate particles relevant to a novel green ironmaking process

A novel green ironmaking technology is under development at the University of Utah. This technology produces iron directly from fine magnetite concentrates by the suspension reduction by gaseous reductants. The direct use of concentrates allows bypassing the problematic pelletization/sintering and cokemaking steps in conventional blast furnace ironmaking. Hydrogen is the main reducing agent for high reactivity and for reduced carbon dioxide release during ironmaking. Feasibility tests indicated that high reduction degrees above 90% could be achieved within several seconds by this method. The purpose of this research was to determine comprehensive kinetics of the suspension reduction reaction of magnetite concentrate particles by hydrogen. Experiments were carried out in the temperature range of 1150 - 1400? with other experimental variables being hydrogen partial pressure and particle size. The nucleation-and-growth kinetics expression was found to describe the reduction rate in this study. In order to obtain a complete rate expression, a series of experiments were conducted to determine the functional dependence of the rate on partial pressures of hydrogen and water vapor, particle size, and then activation energy value. The residence time was varied by changing the total gas flowrate. It was determined that the reduction rate has 1/2-order dependence with respect to hydrogen partial pressure. The effect of particle size factor changed with temperature. The reduction rate increased with particle size at 1150? and 1200?, but the effect of particle size was negligible when reduction temperature was above 1300?. The activation energy was 464 kJ/mol, indicating that this reaction has a greater dependence on temperature than most reactions. Finally, a complete reaction rate expression was formulated. Hydrogen is best as a reductant and/or fuel from the environmental and reduction kinetics points of view, but it is currently expensive. Instead, syngas which is mainly composed of H2 and CO from the reforming of natural gas or coal gasification has been used as a reducing gas mixture for the majority of direct reduction processes. CO was added to replace H2 and N2 in the reducing gas mixture to examine the effect of CO addition on the reduction rate on the base of the experimental conditions examined in the kinetics measurement. It was determined that the pure hydrogen reduction was much faster than the syngas reduction. When nitrogen was replaced by CO, the reduction rate increased significantly.