MECHANISM OF IRON-OXIDE REDUCTION AND HEAT-TRANSFER IN THE SMELTING REDUCTION PROCESS WITH A THICK LAYER OF SLAG

It is possible to stably operate the smelting reduction process of iron oxide with a thick layer of slag by controlling slag foaming with carbonaceous materials. In the present paper, the iron oxide reduction and the heat transfer in this process is investigated. (1 ) The overall rate constant of reduction, post combustion, heat efficiency and the state of the slag layer (temperature gradient and distribution of metal droplets) were measured by 5 t and 100 t scale experiments. (2) The amounts of reduction occurring at slag-metal bath, slag-metal droplets and slag-carbonaceous materials interfaces were estimated for the process under various conditions. Though the reduction at slag-metal bath interface is predominant in a small furnace, the amounts of reduction at each site are almost equal in a 100 t experiment. (3) Heat transfer by radiation and gas convection was calculated by using a mathematical model and was compared with actual data. This model was constructed by dividing the furnace into three regions depending on the modes of reaction and heat transfer and by taking the mass and heat balances throughout each region. When the gas temperature is as high as 2 170-degrees-C, the actual heat transfer can be explained mainly by radiation. But, under practical conditions (gas temperature less-than-or-equal-to 1 765-degrees-C), the contribution of radiation and gas convection on heat transfer is only 20-30%. Therefore, heat transfer by circulation of super-heated carbonaceous material was assumed. The assumption coincides with the dependence of apparent activation energy of reduction on an experimental scale. (4) The appropriate condition for operating the smelting reduction process with a thick layer of slag is the combination of a sufficient amount of carbonaceous material and moderate intensity of stirring.