Investigating the effects of the mechanical activation process on the microwave reduction of magnetite ore concentrates with tea plant wastes

This study aimed to produce pig iron from domestic iron ores with an alternative method to blast furnace and rotary hearth furnace processes. For this purpose, magnetite ores with 49.87% Fe grade and magnetite ore with hematite obtained from the firm Hun Madencilik located in the region of Elazığ-Keban-Yukarı Çakmak in Turkey were concentrated by using low intensity dry and wet magnetic separators. Tea plant wastes that were used as a reducing agent and initially contained 49.15% C and 0.39% S were carbonized at different temperatures in the range of 400-900 oC for different times in the range of 30-2160 min, and the optimal result in terms of carbon, sulphur and calorific value was obtained in the sample processed at 800 oC for 1440 min. For the product that was obtained in these conditions, the carbon content was 94.68%, sulphur content was 0.03%, and calorific value was 8823 kcal/kg. The concentrate with -45 µm of particle size and carbonized tea plant wastes were blended at basicity ratios of 0.8, 1, 1.2 and 1.4 with 1, 2 and 4 times the stochiometric ratio was achieved, as a bonding agent, 10% of the mixture that was used (concentrate + flux + reducing agent) from the 30% molasses solution, gel was added by ratios of 0.5, 1, 1.5, 3, 4, 8 and 10% into this solution, and composite pellets were obtained. These pellets were dried at temperatures of 100, 150, 200 and 250 oC for 30, 60, 90 and 120 min, and it was observed that the composite pellets produced by adding 3% gel and drying at 200 oC for 2 h provided the pellet strength value of 289 N/pellet and 20-24% porosity. The pellets that were dried were subjected to reduction for 5, 10, 15 and 20 min of times in a microwave oven under vacuum where oxygen was fed with the speed of 2 l/h. In the pellets whose reduction rates were calculated by XRD, XRF and SEM/EDX analyses, 3 different products that are known in the literature as DRI, TDRI and iron nuggets were obtained. The product that is named iron nugget was obtained as a result of the reduction processes that were carried out for 20 min in the series with the basicity ratios of 1.2 and stoichiometric ratio of 2. The iron nuggets were characterized, and they were observed to correspond to the region that is known as white cast iron in the iron-carbon phase diagram. The analysis of this product in a metal spectrometer device determined that it contained 96.6% Fe, 2.4% C, 0.42% Si, 0.04% Mn, 0.14% S and 0.06% P. Moreover, as a result of the microstructure examinations that were carried out, it was observed that the product that was obtained as iron nuggets presented similar characteristics to those of white cast iron. In the most suitable basicity ratio and stoichiometry levels, concentrates with different particle sizes were pelleted under the same conditions, and it was determined that their porosity values varied in the range of 28-32%, while their pellet strength values were 230 N/pellet on average. As a result of the microwave reduction experiments that were conducted by oxygen feeding and vacuuming, the TDRI form of the product was obtained in pellets that were produced by powders with -212 +150 µm particle sizes, but metal-slag separation was not complete. The blend with the particle size of -45 µm on which the best results were obtained was subjected to mechanical activation for 5, 10, 15, 30 and 45 min, and the strength value of the composite pellets that were produced from the activated powders was 320 N/pellet on average, while their porosity values varied in the range of 14-19%. In the reduction processes of the pellets that were produced from mechanically activated powders by feeding oxygen, almost all of especially the pellets that were subjected to reduction process in a microwave oven for 5 and 10 min after ground for 30 and 45 min turned into hematite. At the further stages of reduction, this hematite phase turned into wustite and magnetite phases. As a result of the experiments, it was understood that mechanical activation processes for this ore did not contribute to microwave reduction, but as the time of mechanical activation increased, reduction became more difficult. In the last section, the product that was obtained in this study was compared to blast furnace processes, rotary hearth furnace processes and other sponge iron production processes by conducting a feasibility analysis. As a result of the analysis, a product on par with the quality of white cast iron was obtained, and it was proven that this method may be an alternative to other sponge iron and pig iron production processes in terms of economy, process duration and energy efficiency.