Alkali Control in the Blast Furnace – Influence of Modified Ash Composition and Charging Practice

The enrichment of alkali in the blast furnace has been proven to be a catalyst of coke gasification and is thus a key parameter in the degradation of coke. Alkali also directly destroys the carbon structure, increases the risk of scaffold formation, increases the load and attacks the refractory. It is thus important to decrease the recirculation of alkali in the blast furnace and the gasification of coke to ensure sufficient strength of the coke. The aim of the present study was to examine possible ways of alkali control in the blast furnace. This was done by investigating if coke with a modified ash composition contributed to a higher capacity of binding alkali in stable phases, which can be drained via the slag. This would decrease the recirculation of alkali in the blast furnace and prohibit coke degradation. Two campaigns were studied to determine the distribution of alkali in the shaft when the charging differed, this to improve the understanding of alkali control in the blast furnace with respect to the charging practice. Three different test cokes were produced in pilot scale with a mineral addition of kaolin, silica or bauxite. The test cokes were together with base coke used as a reference, charged in baskets to LKAB’s Experimental Blast Furnace (EBF) at the end of a campaign. When the campaign was finished the EBF was quenched with nitrogen and the charged baskets were excavated. The influence of alkali on coke with a modified ash composition was examined with XRF, XRD, SEM-EDS and TGA. This was done in order to confirm any difference between the test cokes and the base coke in terms of chemical composition, phases in the coke ash, degree of graphitisation and reactivity. The results showed that the base coke in most cases had collected more alkali compared to the test coke with a mineral addition of kaolin and silica. For the test coke with addition of bauxite the alkali content was higher in three out of four samples compared with the corresponding base coke. Unreacted grains with bauxite were detected, which indicates that bauxite was completely or partly inactive in the capturing of alkali. All aluminosilicates detected in the coke samples contained alkali, which indicates that aluminosilicates contributes in the capturing of alkali in the EBF. The main mineral phases containing potassium in the coke were kalsilite, leucite and other aluminosilicates with varying alkali content. The carbon conversion and thus the reactivity increased with the alkali content in both the test coke and the base coke. The reactivity of the test coke was thus not decreased due to the mineral addition. No indications of an increased capacity of capturing alkali in stable phases could be seen in the test cokes, this could be due to the low amount of minerals added. The uptake of alkali in the different coke types was dependent of the horizontal and vertical position in the EBF, and thus the conditions the baskets had been exposed to and the distribution of alkali within the EBF. It was concluded that the charging had an impact on the alkali distribution in the EBF. During campaign 31 the alkali content was more evenly distributed over the horizontal section in the upper part of the furnace, in the lower shaft the alkali content increased towards the centre. During campaign 32 the alkali content was increasing towards the walls in shaft of the EBF. The content of alkali in the lower shaft was higher during campaign 32 compared with campaign 31