Formation of Metallic Iron Shell and Its Mutual Cohesion during the Reduction of Iron Ore Pellets

An investigation has been carried out on the behavior of pellets during the reduction in the lower part of blast furnace. Pellets samples taken from an industrial blast furnace were large and very hard masses, whose section showed a zonal structure; a spherical envelope of metallic iron was formed in the external part of each pellet and unreduced core remains in the central part.When pellets become such a large mass with unreduced ore in the blast furnace, this core must be reduced by direct reduction in the liquid state after melting of this mass. In consequence, the blast furnace performance becomes undesirable, with an increase of coke rate, for example.We have studied therefore, in the first step, on the properties of pellets samples taken from the blast furnace and, in the second step, on the condition on metallic iron shell formation and its mutual cohesion in order to obtain the fundamental data. The results obtained are as follows:(1) The chemical analysis and microscopic investigation into the pellets taken from the blast furnace shows that the shell consists of much metallic iron, a little wustite and slag. Its melting point is 1560°C. On the other hand, the core consists of much wustite, and small amounts of metallic iron and slag. Its melting point is 1360°C.(2) When a pellet prereduced to wustite is reduced by reducing gas containing 30%CO+70%N2 at 1200°-1250°C during 30-60 minutes, a dense metallic iron shell of about 1.5mm in thickness is formed.(3) Strong mutual cohesion takes place among pellet particles, by heating pellets having metallic iron shell in the atmosphere of N2 at 1250°-1300°C during 30-60 minutes under a lord of 1.0kg/cmcm2.