Softening and melting characteristics of self-fluxed pellets with and without the addition of BOF-slag to the pellet bed

Abstract

When using 100% self-fluxed pellets in the blast furnace burden, top charged fluxes, especially the BOF-slag, may cause irregularities in operation. The major reason has been theoretically attributed to the problematic slag formation in the furnace. As the melting of pellets is the first step of the slag formation process, the softening and melting properties of LKAB's self-fluxed pellets with and without addition of fluxes are studied experimentally. The results show that the softening and melting properties of the two types of LKAB's self-fluxed pellets are quite suitable for blast furnace operation. Contrarily, the melting-down characteristics of BOF-slag are variable and harmful to the slag formation under a reducing atmosphere. Adding 5% BOF-slag to the self-fluxed pellets can considerably worsen the softening and melting properties of pellets. It can either increase the softening and melting temperature interval substantially or cause the precipitation of solid phases, mainly di-calcium silicates, in the slag. It is quite likely that the resulting slag will therefore become very viscous, even entirely blocking the melting down of the pellets up to a temperature 1 520 °C.