8th International Congress on Science and Technology of Ironmaking - ICSTI 2018

In the blast furnace (BF) process, the presence of alkalis (Na, K) is known to have negative influences on the process due to the catalytic effect on coke gasification, destruction of carbon structure by intercalation of alkali, scaffold formation and refractory attack. Alkalis are introduced with the raw materials and due to alkali circulation; alkalis are picked-up on the way to the higher temperature zone in the BF. In the combustion zone, a significant part of alkalis are reduced and vaporized. The formed alkali gas follows the furnace gas to the colder regions in the shaft. Here alkali is condensed on the burden or furnace walls and oxidized to more stable compounds. The rather low temperature and less reducing gas in upper part of the shaft are not favorable for alkali gas output via the top gas. Moreover, the solubility of alkalis in slag is dependent on the slag chemistry and amount while higher temperatures in the raceway region enhance the reduction and vaporization of alkalis which will prevent alkali dissolution in slag. The nature of alkali compounds in the BF combined with the character of the process causes alkalis to recirculate and accumulate in the BF. Within this study, the distribution of alkali at different depths in the shaft and for different charging practices has been studied by examination of excavated coke samples from LKABs Experimental Blast Furnace. A possible way of controlling the alkali load in blast furnace have been investigated by studying if alkali can be bound in stable phases in a modified coke ash and coke coating and thereafter drained out from the BF via the slag.