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Analysis of Drainage Rate Variation of Molten Iron and Slag from Blast Furnace during Tapping
Masakazu IIDA, Kazuhiro OGURA and Tetsui HAKONE
(Materials Development Department., Japan.)
( January 22, 2008)
Despite its importance in practical blast furnace (BF) operation, the dominant factors to control drainage rate or tapping time have not been well studied. In most cases, short tapping time has been attributed to rapid tap hole diameter enlargement. On the other hand, the experiential tendency about positive correlation between furnace hearth bottom temperature and drainage rate has been widely recognized.
In order to examine the dominant factors to control the liquid drainage rate or tapping time at BF, a simulative calculation model is introduced, where the liquid drainage path consists of coke particles packed layer (coke filter) and tapping hole and the overall drainage rate is determined as one of smaller fluid rate in coke filter or tapping hole. For calculating the fluid rate in coke filter, a hypothesis that liquid iron and slag in coke filter is driven toward the tap hole entry point consuming the coke particles, whose extent depends on molten iron C saturation degree and FeO fraction in molten slag, was introduced.
The calculation results present good matches with the observed tapping operation. This result can be explained by the two influences of low permeability zone formation or elimination at furnace hearth. Considering the two influences of low permeability zone formation, (1) to lower hearth bottom temperature and (2) to induce low C saturation of pig iron due to short traveling time in liquid pool to tap hole entry point, the simulation result conforms to the above mentioned experiential tendency.
The drainage of molten iron and slag out of blast furnace (hereinafter referred to as BF), “tapping” in the operation field term, is carried out usually from two tap holes alternately. Tapping starts by opening the tap hole which has been plugged with tap hole mix in the previous operation. The drainage or tapping rate, whose initial value is smaller than iron and slag production rate in BF, increases in accordance with the progress of tapping. Once the tapping rate exceeds iron and slag production rate, the slag level in BF is gradually lowered. When the slag level in the furnace reaches close to tap hole entry position, the tapping is ended with tap hole mix plugging, followed by the tapping start at the other tap hole. The time duration is called as tapping time.
Lengthy tapping time is appreciated for heavy duty alleviation in tapping operation as well as reduction in tap hole mix consumption. From this standpoint, highly durable tap hole mix with smaller erosion speed is required. Even using a certain tap hole mix with constant refractories compositions and the similar erosion properties, the tapping time broadly varies occasionally in the practical operation, for example, from 120 to 240 min. Such a large fluctuation in tapping time is unexplainable only with the refractory wear rate of tap hole mix, because the refractory properties influencing on its wear rate, such as chemical erosion, mechanical abrasion, thermal shock peeling, have been improved so as to establish constant and stable performance, and the variations in operational factors influencing on refractory wear rate, such as hot metal temperature, slag ratio, metal and/or slag compositions, are not large enough to give irrelevant impacts on tap hole mix wear.
This outlook is authentically supported by the investigation of Nishioka. They calculated the drainage rate variation during tapping taking account of the pressure drop of fluids through a constant length tap hole with actually measured tap hole diameter change and compared to the observed one, and concluded that it was not possible to describe the drainage rate variation only by tap hole diameter enlargement. They also suggested that a conformity between observed and calculate drainage rate variation could be achieved only when assuming that tap hole length is shortened from 4m to 1 m, which is apparently unrealistic and denied by the reports of Yamanaka and Ando on the BF tap hole boring studies.
Whilst, an apparent relation between BF hearth bottom temperature and tapping time is widely recognized, that is, the lower hearth bottom temperature, the shorter tapping time. The change in hearth bottom temperature is explained with formation and disappearance of low permeability zone in the hearth, whose influences on tap hole refractory wear is thought to be quite small or null.
In this report, introducing a model in which the drainagepath of fluid out of BF consists of coke filter layer and tap hole tube, the authors discuss the extents of influences of refractory and operation factors on tapping time variation.
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