Analysis of the behavior of alkalis in a blast furnace

Abstract

The average monthly alkali balances for a 3200-m 3 blast furnace are used to evaluate the effect of the furnace’s intake of alkalis on its performance indices. A method is proposed for evaluating the amount of alkalis removed from a furnace with the sludge and water from the gas-cleaning system. The effect of the main parameters of the smelting operation on the rate of removal of alkalis from the furnace is examined, and it is shown that all of the alkalis which enter the furnace can be removed if it is operated with a well-developed central gas flow and low-basicity slags. The alkalis that enter a blast furnace with the charge materials have an adverse effect on the furnace’s performance, decreasing the hot strength of the coke, weakening the refractory lining, and facilitating the formation of slag crusts in the furnace, scaffolding, and burning of the tuyeres. All these developments ultimately increase fuel consumption and reduce the productivity of the furnace. The amount of excess coke consumed due to the entrance of alkalis into the furnace may be as high as 33 kg for each additional kilogram of these compounds [1]. All this points to the need to limit the intake of alkalis by blast furnaces as much as possible ‐ to no more than 2.5‐3.0 kg/ton pig iron. Many years of experience and the results of studies [1‐5] show that most alkalis are removed from the furnace with the slag. The amount removed in this manner increases with a decrease in the slag’s basicity and temperature. It should also be mentioned that the quantity of alkalis leaving a blast furnace with the slag decreases when the furnace is operated with an underloaded periphery [3]. In this article, we examine the effect of different operating parameters of a blast furnace on the amount of alkalis removed from it. Alkali Balances. A preliminary analysis of the average monthly alkali balances for blast furnace No. 6 at the Novolipetsk Metallurgical Combine (NLMK) during a 12-month period (Table 1) showed that the imbalance reaches 20‐30% (rel.). This might be related to the fact that the furnace is not equipped to monitor the amount of alkalis that leave the furnace in the top gas ‐ this amount can reach 8‐20% [3, 5, 6]. Most of the alkalis are dissolved in the water of the gas-cleaning system. To refine the data on alkali balance, we statistically analyzed the complete alkali balances on four blast furnaces during the period 1989‐1991. We also used data from the publications [3, 5, 6]. Based on this information, we obtained an empirical relation linking the quantity of alkalis dissolved in the water of the gas-cleaning system and the quantity removed with the top dust: R 2 O gcw = 11.842·R 2 O td ; R 2 = 0.6911,