Model of Draining of the Blast Furnace Hearth with an Impermeable Zone
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DUCTION
THE hearth is an important part of the blast furnace process, because the main product, hot metal, assumes its final temperature and composition in this region.[1] The drainage of the hearth is also crucial for the operation of the furnace as it dictates the rate at which the liquids can be extracted,[2] so it may limit the production rate. Important factors affecting the draining process are the hearth coke particle size and permeability, and the state (sitting, floating) of the coke bed (often referred to as deadman) in the hearth, the volume and viscosity of the slag, and the taphole length, diameter and erosion rate during tapping. Furthermore, the draining patterns are known to affect the erosion of the hearth lining, which is a factor that often limits the campaign length of the furnace. In order to guarantee a smooth drainage of the hearth, an understanding of the present state of operation of the region should be gained. However, due to the hostile environment the main variables characterizing the state of the hearth cannot be measured directly, so one has to rely on indirect measurements.[3–5] The outflow rates of iron and slag can be estimated on the basis of weighing torpedoes continuously, or by measuring the iron and slag level in ladles and pots where the liquids are tapped. For gaining information about the liquid levels in the hearth, devices can be installed at
HENRIK SAXE´N, Professor, is with the Thermal and Flow Engineering Laboratory, A˚bo Akademi University, Biskopsgatan 8, 20500 A˚bo, Finland. Contact e-mail: hsaxen@abo.fi Manuscript submitted December 22, 2013. Article published online August 23, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
the outer part of the hearth boundary. Such measurements include electromotive forces measured at the furnace shell[6,7] and strain-gauge information from the region between the carbon bricks and the steel shell.[7] However, these measurements are always corrupted by noise and drift and are difficult to calibrate accurately. Despite being inaccurate, such measurements in large blast furnace have revealed that the operation of different tapholes deviates considerably from each other under alternating tapping. The existence of low-permeability zones in the blast furnace hearth has been described by many authors.[8–10] Nouchi et al.[11] studied experimentally the hearth drainage in a small-scale model and analyzed the effect of different permeability of the hearth core on the drainage behavior. Nishioka et al.[12] developed a CFD model and extensively studied the effect of coke-bed permeability, coke diameter, slag viscosity, etc. on the main drainage characteristics. Iida et al.[13] studied the operation of a blast furnace with a mathematical model that considered the pressure loss in the coke bed in front of the taphole and in the taphole itself, and extended the model to a hearth in a state where the slag flow from the two operating tapholes deviated strongly from each other,[10] based on the observations by other investigators, including Nouchi et al.[14
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