Extensive work is reported in literature on the reduction of iron oxides with carbonaceous reductants. Most of this work considered isothermal reaction of the material mixture, although as shown in some studies, isothermal reaction conditions are not often the norm because of sample size and heating arrangement in the experiment. In industrial processes, such as the rotary hearth type processes and the IFCON® process for iron ore reduction, the norm is non-isothermal reaction. Simulation of industrial processes should take non-isothermal reaction into account if the heat transfer effects within the process are to be investigated. To avoid the complications of coal volatiles in the experimental set-up, few studies were done with coal as reductant. The primary aim of the work presented here is to quantify radiation heat transfer to the surface of an iron ore and coal mixture heated uni-directionally from the sample surface to show the importance of heat transfer in the IFCON® process. Secondary aim of this work are to show the effects of layer thickness, coal volatiles, phase chemistry and particle size in this reaction system. The experimental set-up consists of a tube furnace modified to transport the sample into and out of the experimental tube furnace heating zone under a protected atmosphere, whilst the product gas is analysed throughout the experiment by quadropole mass spectrometer. The sample surface temperature, heating zone temperatures and material bed temperatures were measured throughout the experiment. A sample cutter-splitter was developed to divide the reacted sample into three horizontal segments for chemical analyses. The sample surface temperature and the heating zone temperatures were used as inputs to a radiation network calculation to quantify radiation heat transferred to the sample surface. The radiation network calculation was calibrated against heat-mass balance calculations for pre-reduced ore and graphite samples reacted at furnace temperatures of 1300, 1400 and 1500°C. The results show that radiative and conduction heat transfer control prevails for 16 mm to 40 mm material layers heated uni-directionally from the material layer surface. It is shown that coal volatiles contribute to reduction in the stagnant material layer. Also, smaller particle sizes result in increased reaction rates because of a decrease in the diffusion limited effects which were seen in reaction of the base size of coal and ore particles.
Thesis (PhD (Metallurgical Engineering))--University of Pretoria, 2009.