The softening and melting of blast furnace burden materials.

The conditions prevailing in the cohesive zone of the blast furnace have been determined from published data and applied in a study of burden materials on an experimental scale. The mechanism of softening and melting of burden materials has been evaluated and it was found that two opposing reactions are important, namely the liquid slag formation reaction and the gaseous reduction reaction. At high temperatures the permeability of burden materials was found to be strongly dependent upon the degree of reduction and the pertaining temperature. A concept, summarised by Temperature-Reduction-Isobar Diagrams, was developed to portray this relationship, each burden material having its own characteristic diagram which can be used to predict the permeability of materials at elevated temperatures.The influence of gaseous sulphur and alkali vapour in the reducing gas upon the softening and melting behaviour of acid pellets has been evaluated. Sulphur is detrimental because of the formation of a low melting point Fe-S-O eutectic phase which hinders gaseous reduction and lowers the melt-down temperature. Alkali vapours increased the reduction rate of acid pellets, creating a rise in the melt-down temperature. Although the pellets absorbed alkali from the gas phase it had no discernable influence upon the liquid slag formation process. A joint sulphur/alkali study showed that the detrimental influence of sulphur was capable of eliminating the beneficial effect of alkali vapour. The information accrued from the experimental studies has been used to explain the structure of the cohesive zone found in dissected blast furnaces and to suggest methods to improve the performance of existing furnaces.