Effect of Alumina on Reduction Softening and Melting Behaviour of Iron Ore Pellets

Abstract

Aim of the blast furnace ironmaking process is the extraction of iron from iron ore, and prepare the raw iron (hot metal) for steel making. The iron oxides (ore) are charged into the furnace in the form of sinters and pellets containing the iron ore. Iron ore interacts with reducing gases and coke to form molten iron which in combination with slag are collected in the hearth of the blast furnace and tapped separately. Since blast furnace is a counter-current reactor, the permeability is of prime importance for its efficiency. When temperatures reach 1100 °C to 1200 °C the ore layers start to soften and gradually melt, and this region is called the cohesive zone. The cohesive zone consists of a series of approximate ring-shaped, doughnut-like masses of semi-molten, pasty ferruginous burden material. Here the loss of permeability is caused by melt onset and deformation of the solid phases due to the pressure of the burden. The shape and position of the cohesive zone affect the efficiency of the process and it is advantageous to have the cohesive zone located at as high temperatures as possible and its thickness as small as possible. This is governed by the properties of the pellets and sinter which in turn depends on the nature of ore or concentrate, associated gangue, type and amount of fluxes added, and their subsequent treatment. With the depletion of high-quality iron ore resources, the consumption of iron ores with high alumina (Al2O3) content is increasing. Therefore, it is important to investigate the influence of alumina on the reduction, softening, and melting behaviour of iron-bearing materials like sinters and pellets, hence its effect on the cohesive zone. This thesis explores the effect of alumina on the pellet’s behaviour in the cohesive zone, for which Reduction, Softening, and Melting apparatus (RSM) is used as a blast furnace simulator. With RSM, the pellet samples with similar basicity but varying alumina content are subjected to conditions similar to the blast furnace, and their reduction, softening, and melting behaviour are compared. It is observed that with an increase in alumina content the cohesive zone shifts to a lower temperature and its thickness increases, therefore the resistance offered to the gas flow increases. The alumina plays a multifaceted role, where it affects reduction degree, degree of carburization, the viscosity of primary slag, and finally wettability of slags with coke and iron. Alumina causes reduction retardation in heterogeneous conditions, due to the formation of low melting interfaces. This on one hand does not influence the total reduction degree but on the other increases the rate of homogenization of the pellet microstructure. Alumina induces reduction retardation in homogenous condition, by increasing the rate of melt formation due to an increase in the fraction of low melting phases. This causes an increase in the rate of softening and early melting. Degree of reduction is not the only factor that influences softening and melting behaviour, as a pellet sample with lower reduction degree and lower alumina showcase better softening melting properties than the pellet sample with a higher reduction degree and higher alumina content. Alumina acts as network former in the chemistries considered, increasing the viscosity of primary slag, hence increasing the resistance to gas flow. Alumina increases the proportion of high melting phases in the FeO lean slag. The possible effects of alumina on carburization and wettability are explored with the help of literature, one possibility is that alumina decreases the wettability of slag with iron shell, hence increases the degree of carburization causing early melting of iron shells. Simultaneously, the possible effects of basicity are also explored using literature; it seems with an increase in basicity the negative effects of alumina lessen.