Effect of Unstable Tapping of Smelting Products on the Performances of the Evraz Ntmk Blast Furnaces

Timely and full tapping of liquid smelting products is an important condition for forcing the blast-furnace process. Unstable tapping of iron and slag leads to fluctuations of the liquid level in the hearth, changes in the charging rate, and fluctuations in the time the burden stays in the blast furnace. This, in turn, affects the thermal state and the composition of the liquid products. The ratio of tapped slag weight to the hot metal weight is determined by the physical properties of slag and the length and diameter of the taphole. An analysis of the operation of the EVRAZ NTMK blast furnaces revealed substantial fluctuations of this ratio. When the volume of tapped slag is 340 to 360 kg per ton of pig iron, the slag/metal weight ratio is between 200 and 850 kg per ton of pig iron. The objective is to analyze the causes of unstable tapping and the effect of the instability on the furnace performances using the laws of fluid mechanics. It is assumed that the melt flow in the taphole is stratified. The effect of the slag properties and the dimensions of the taphole on the velocity of the melt and, hence, on the slag/metal weight ratio in the taphole is studied. Since the velocity of the melt depends on its physical properties, a mathematical model was developed to assess the slag/metal weight ratio from the viscosity of the slag and the dimensions of the taphole. The results of modeling are validated against the industrial data acquired by the EVRAZ NTMK. It was established that an increase in the tapped slag/metal weight ratio leads to worse performance and lower vanadium recovery from pig iron.