Effect of Cooling Rate on Cold Compressive Strength of Carbon-Burdened Hematite Pellets

Abstract

The pelletization of iron ore fines involves mainly two steps, (i) green ball production (ii) induration of green pellets. The induration of green pellets is the thermal treatment of pellets wherein drying, preheating, firing and cooling stages occur. The carbonaceous materials are admixed in pellets mix to lower the energy requirement and induration temperature of hematite ore pellets. Some extent of iron oxides is always reduced to lower iron oxide during induration process due to the presence of the coke in pelletization mixture. For the better quality of pellets, reduced iron oxide in pellets should re-oxidized into hematite phase. The degree of pellets re-oxidation depends on induration conditions and furnace atmosphere on induration stand for the given type of iron ore. The firing and cooling of pellets can control the phase formation and quality of the pellets. The FeO or Fe3O4 content in final pellets should be as low as possible for good cold compressive strength of the pellets. The extent of formation of FeO (Fe3O4) in final pellets mainly depends on induration parameters and atmospheric condition of the induration furnace. Therefore, we have studied the effect of cooling rate on CCS of the carbon burned pellets. The CCS of furnace-cooled pellets (annealing) is more than that of air-cooling (normalizing) and water-cooling (quenching) pellets because of more recrystallization bond, diffusion bond and uniform distribution of phases in the final products of furnace cooled pellets. CCS of the furnace, air and water-cooled pellets at 1280 ◦C /10 min are 235, 140 and 35 kg/pellet, respectively. However, the only furnace cooled pellets displayed acceptable strength for blast furnace operation at 1280 ◦C /10 min.