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The performance of a blast furnace is judged by its productivity, coke rate and the quality of hot metal produced. In the case of Indian blast furnaces, these performance indices are distinctly inferior to the corresponding figures for good blast furnace operation elsewhere in the world. Some of these important indices are shown in Table 1.
It is apparent from the data presented in Table 1 that the high ash in coke, as well as the high coke rate, are unique features of Indian blast furnace operation. These factors, coupled with a high Al203/Si02 ratio in the iron ore, not only increase the slag volume and the alumina content in the slag, but also produce highly limey and viscous bosh slags which hinder the smooth movement of the burden inside the furnace, thereby adversely affecting productivity, coke rate and hot metal quality.
The main features of the bosh and hearth regimes of the Indian blast furnaces can be analysed bearing the following factors in mind:
(a) The basicity and the fluidity of the bosh slag vis-a-vis the flush slag.
(b) Alumina content of the slag.
(c) Generation of SiO vapour from Si02 released from the coke ash at the tuyere level.
(d) RAFT conditions.
(e) Softening and melting behaviour of iron ores/agglomerates.
The basicity and fluidity of bosh slag vis-a-vis flush slag
To arrive at the desired slag quality, it is necessary to consider the fact that there are two distinct types of slags within a blast furnace - the bosh slag (the slag made above the tuyeres) and the flush slag (the slag below the tuyeres), the latter being formed by the dissolution in the bosh slag of the coke-ash released at the tuyeres. Since all the flux is charged from the top of the furnace, the bosh slag contained the entire flux constituents including that required for fluxing the coke ash, the bulk of which, almost 70-80%, is released only at the tuyere level when the coke burns. Thus, the bosh slag basicity is always higher than the flush slag basicity since the coke ash contains mainly silica and alumina (55-60% and 25-28%, respectively). This gap between the bosh slag and the flush slag basicity increases with the following :
(a) Increase in...