No abstract

The heat hardening by oxidation is a process commonly used in iron ore pelletization process. The green pellets are fired in induration machine using Corex gas at JSW Steel Limited Pellet Plant. The firing temperature of induration machine strongly influences the physical and metallurgical properties of the pellet. Due to absence of exothermic reaction and poor roasting property of hematite pellet, the energy consumption of hematite pellet production is at higher side and requires higher roasting temperature. In present pelletization process, carbon burdened method is found to be more favorable technique in practice to enhance the induration of hematite pellets. Coke breeze is added in the pellet mix at JSW pellet plants to get the inherent fuel value of a hematite green ball equal to that of a magnetite pelletizing feed. The firing temperature (from corex gas) of the induration machine and carbon addition in the pellet mix is interrelated and decides physical and metallurgical properties of the pellets. At JSW Steel Pellet Plant the carbon addition varies from 0.90 to 1.50% and firing temperature varies from 1 230 to 1 320°C. Fluctuations in physical and metallurgical properties were observed due to deviation in carbon addition and firing temperature. Optimization of external firing temperature and coke breeze addition in the green pellet mix is necessary to get the desired properties of the pellet for iron making units. Basket trials were carried out at pellet plant induration machine by varying the external firing temperature from 1 220 to 1 330°C and coke breeze addition from 0.7 to 1.4%. At firing temperature of 1 220, 1 250, 1 280, 1 310 and 1 330°C the optimum carbon addition 1.30, 1.20, 1.10, 0.90 and 0.70% achieved the optimum physical and metallurgical properties of the pellet for iron making units respectively.

During induration in a straight grate machine, the green pellets pass through four different thermal treatments, namely drying, preheating, heating and cooling. The pellet bed is fired with downdraught firing leading to thermal gradients through the bed. Corex sludge, which is used as fuel in the pellet mix, supplies the necessary energy for uniform heating of the pellet. The physicochemical conditions, e.g. the temperature and oxygen partial pressure mainly depend on the amount of fuel incorporated in the pellet mix. As a result the percentage and the distribution of various phases in the pellets vary, leading to deviation in quality. To study the distribution of phases and their impact on cold crushing strength at different carbon levels (1?20 and 1?35%), pellets from different layers of the induration bed in an industrial straight grate were characterised. It was observed that the strength of the pellets varied from 142 to 268 kg/pellet and 128 to 245 kg/pellet across bed, with carbon 1?20 and 1?35% respectively. It was found that middle layer pellets had higher strength compared to top and bottom layers. It was observed that amount of hematite, magnetite, porosity and the pore size plays a significant role on the pellet strength. Pellets with 1?20% carbon showed better physical and microstructural properties across the pellet bed compared to pellets with 1?35% carbon.