The effect of replacing fine ore by coarse ore on sintering reactions was investigated using an infrared furnace on laboratory scale and sinter pots on pilot plant scale. Five sinter mixes were prepared by changing the percentage coarse ore from 0% to 100% in 25% increments. Coarse ore fraction, sintering temperature, holding time and oxygen partial pressure were selected as sintering parameters, and two-level factorial design was used for identification of parameters that significantly influence the formation of sinter phases. Experimental results showed that the coarse ore fraction has a higher effect on the sintering process compared to those of other parameters. The experiment design also enabled to set these parameters to their optimum values. The porosity of compacted pellets was measured using a helium pycnometer. The replacement of fine ore by coarse ore resulted in a decrease in porosity (increase in packing density) of compacted pellets. The particles are closer to each other in pellets consisting of more coarse particles than fine particles. Laboratory experiments were performed at 1300°C in air, using a high heating rate (15°C/s). The holding time was set to 2.5 minutes. X-ray diffraction (XRD), reflected light microscopy (RLM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) were used to characterize sintering reactions and sinter phases. XRD analysis revealed that sintered pellets consisted of hematite, SFCA, SFCA-I and calcium silicate. The proportions of SFCA slightly increased when the fraction of coarse ore varied from 0% to 25%, but decreased with a further increase in percentage coarse ore. At 25% coarse ore fraction, the porosity of the compacted pellets decreased, resulting in an increase in packing density and sintering rate. More hematite reacted, resulting in the formation of high amounts of SFCA. Above 25% coarse ore fraction, the amount of hematite increased, and the concentrations of columnar SFCA decreased despite a further decrease in porosity. This was attributed to the decrease in reaction surface area for coarse ore, and the short reaction time, which limited the extent of reaction of the coarse particles. The variation of SFCA-I and calcium silicate was not significant under laboratory conditions. Reflected light microscopy and SEM analysis easily identified two major sinter phases: hematite and SFCA. A clear distinction between the different types of SFCA could not be made using EDS analysis. Sinter pot tests were carried out in order to examine the effect of coarse ore fraction on physical and metallurgical properties of sinters. The tumbler and reduction disintegration indexes increased with increasing coarse ore fraction in the sinter bed. This was presumably due to the increase in amounts of hematite and decrease in surface area for reaction. Consequently, the reducibility of sinter decreased as the percentage coarse ore increased. This study has concluded that the presence of 25% coarse ore in the sinter mix led to enhance sintering reactions. The amounts of SFCA increased, and sinter quality was improved. It is recommended that in future work, sintering reactions should further be investigated by also measuring the permeability of the sinter bed and the reaction surface area of solid particles. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Materials Science and Metallurgical Engineering / unrestricted