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Dissection Investigation of Blast Furnace Hearth—Kokura No. 2 Blast Furnace (2nd Campaign)
(Kaoru NAKANO, Shusaku KOMATSU and Akinobu OGAWA)
(Corporate Research and Development Laboratories, Japan.)
(January 22, 2009)
Dissection investigation of blast furnace hearth was made at Kokura No. 2 Blast Furnace (2nd Campaign). Before blow-out, tracer response test was carried out in order to estimate the molten iron flow in hearth, and the measured data indicated that the depth of “effective” flow region of molten iron was extremely shallow. According to the result of the dissection investigation, the deadman was floating in hearth, and deadman coke was considerably degraded. Therefore, the poor permeability of the deadman is supposed to cause a downsize in the flow space of molten iron, which coincides with the prediction through the tracer response test. The numerical method to estimate the boundary shape of the deadman was developed by means of evaluating the stress field of the deadman in hearth. The calculated result was in reasonable agreement with the observed shape of deadman. In addition, based on the data obtained by analyses of the boring samples, the thermal equilibrium erosion shape of hearth refractory was evaluated by numerical simulation, and a good agreement to the observed shape was found.
After the movement of large-sizing the inner volume and downsizing the number of furnaces, which occurred around the 1980’s in Japan, the furnace life extension has become one of the most important issues in blast furnace operation, since a huge investment is required to reline or rebuild a furnace. With the exception of scheduled blow-out based on a prior campaign plan, the furnace life is judged on the capability to keep the demand of production within ordinary maintenance activities. Recently, the technology for repairing inside the furnace, especially for the area above tuyere level, has developed significantly, such as replacing stave coolers and gunning castable refractory by means of the emptying operation technique. And these technologies have contributed to elongate the campaign life of blast furnaces up to over 10 years. However, as for the damage of the hearth area, which is occupied with molten iron, slag and coke, there is no effective way to empty and repair during a campaign. Therefore, the furnace life is supposed to be finally controlled by the damage of the hearth area.
The damage of the hearth area means the erosion of hearth refractory. Through dissection investigations of blast furnaces, which were carried out in the ’80s and the ’90s, the features of hearth erosion were found to be various, such as “bowl-shaped” erosion profiles and “elephant foot shaped” ones.
Therefore, the molten iron flow brings heat into the hearth, and then hearth refractory is exposed to high temperature iron and is eroded through thermo-chemical solution such as carbon dissolution into molten iron or melting. This process is supposed to be affected by the inner hearth state, such as permeability distribution, as well as refractory property and layout. As for the inner state of the hearth, some investigations report that deadman coke was observed floating in hearth metal and various materials such as Ti bear and kish graphite were found in the hearth bottom. On the other hand, many analyses have been made on heat load of hearth in operation, and then, it was found that the heat load fluctuates for the duration of operation and the distributions of heat load in hearth, as well as the fluctuation behavior, are different among furnaces. Through analysis about transition of the heat load based on the fundamental experiment and the result of dissection investigation of Mizushima No. 4 blast furnace, Sawa state that the poor permeability area exists in heath deadman and its distribution affects the heat load through the molten iron flow and the drainage condition.
Meanwhile, many researches by use of numerical simulation have also been made on the relationship among inner hearth condition and heat load as well as molten iron flow, and discussed the effect of the lower boundary level and permeability distribution of deadman in hearth. Though, these works didn’t go beyond the “case study” level, since it was too difficult to estimate and specify the packing condition of the hearth as a calculation condition, theoretically.
Recently, Nouchi applied a numerical simulation based on the discrete element method (DEM) to investigate the deadman behavior in the hearth. In addition to estimating the lower boundary shape of deadman, in other words, the coke free space in the hearth, they discussed the replacing mechanism of the deadman, such as the flow pattern and the less permeability layer formation, with reference to the dissection investigation of Mizushima No. 4 blast furnace.
As a matter of course, the inner hearth condition of the real furnaces can be observed only through the dissection after blow-out. Then, taking the complexity and variety of hearth phenomena mentioned above into account, therefore, the knowledge about the inner hearth phenomena is still not sufficient to understand and control.
In this paper, the results of the dissection of Kokura No.2 Blast furnace hearth, which was carried out from the viewpoint of clarifying the inner state as well as the refractory erosion, are described. And, by use of numerical analysis, the inner hearth phenomena are discussed with relation to the heat load and refractory erosion of blast furnace hearth.
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