On hearth phenomena and hot metal carbon content in blast furnace

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Doctoral thesis (monograph)
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80, [3]
Helsinki University of Technology publications in materials science and metallurgy, 151
The objective of this work was to investigate the hearth operation and the factors influencing the total quality of hot metal, especially its carbon and sulphur contents. Regarding the hot metal quality there are many sources of disturbances starting from raw materials, continuing through the blast furnace process and ending the hot metal casting. The carbon content of hot metal is an important part of the total quality of hot metal. Nearly 80 % of the total reaction energy in the BOF process comes from the oxidation of carbon in hot metal. Therefore, a high and stable carbon content of hot metal is most beneficial. Sulphur in hot metal is known to retard the carbonisation of hot metal. The sulphur content in hot metal is controlled by the sulphur load in the coke and other fuels as well as the slag basicity and low level of unreduced iron oxides. Both carbonisation and desulphurisation are strongly dependent on the hearth operation. To secure a proper hearth operation the hearth coke, the deadman should neither be clogged nor get inactive. Good hearth operation prolongs the campaign life of the hearth lining improving thus the total economy. In this work the factors influencing the hot metal carbon content have been investigated. The carbonisation of solid iron starts in the shaft by CO gas and also by coke carbon when the metal melts down. Hot metal does not get saturated with carbon. The difference between the saturation and actual carbon contents was observed to be around 0.7 % in average and it increases with increasing sulphur content. The carbon content of hot metal can be maintained on a high level by keeping the sulphur content of hot metal below 0.05 % which can be attained with a sufficient slag basicity, e.g. CaO / SiO = 1.2. A progressive vicious circle in blast furnace operation was observed. Problems may start with some disturbance in the hearth operation gradually resulting in low permeability or total clogging of the deadman. The raceways become shorter which leads to a hanging cohesive zone. A hanging cohesive zone transfers insufficiently reduced iron oxides into the slag. Desulphurisation of hot metal weakens due to a higher FeO content in the slag. A high sulphur content in hot metal promotes the precipitation of calcium sulphide. Calcium sulphide together with other compounds with high melting point may clog the hearth further. There are several factors which strengthen vicious circle like a low heat level, an overfilled hearth, too acid slag etc. Different factors influencing the hearth phenomena and start up of the vicious circle including CaS precipitation were observed and analysed in this work. Early warning symptoms of a clogged hearth and methods to identify an inactive deadman were collected. Measures to cure an inactive deadman were developed and practised as well.
blast furnace, hearth, deadman, salamander, hot metal, sulphur, carbon
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