In blast furnace ironmaking one major challenge is to control and measure the alkalis circulating and accumulating in the blast furnace (BF). Alkali enter the BF with the primary raw material and will form a cycle where it is first reduced to metal at the lower parts forming gas. Alkali then follows the gas flow up where it oxidizes and solidies as the oxide form has a higher melting and volatilization temperature. Condensation then occurs on burden material and in their pores and by that it is following the burden downwards. The circular nature of the reactions leads to a build-up of alkali in the form of potassium in the BF that is hard to control or measure. Condensation of alkali compounds can also occur on the BF walls functioning like a glue to which particles attach, forming scaffolds that can rapidly increase and disturb the burden descent. The increased alkali catalyzes gasication of coke with CO2 that increasescoke consumption and leads to disintegration of coke. A common method today to control alkali is by varying the basicity in the BF. As lower basicity increases the amount alkali removed through slag while at the same time reducing the amount of sulfur that can be removed with the slag. This project was divided into two parts. The first part was a continuation of a previous study performed at Swerea MEFOS. Where to control the effect of alkali on coke gasication a method was tested using coke ash modication to inhibit the catalyzing properties of alkali bound on coke. The method has previously shown that alkalis are bound in the desired form but the added amount was not sufficient for inhibition of all picked-up alkalis. In this study, additional trials with higher additions of kaolin was performed. 2 wt% kaolin was added to the coal blend for producing coke that was then added to LKAB's experimental blast furnace (EBF) as basket samples in the end of a campaign. The excavated samples were analyzed using XRF, XRD, SEM-EDS and TGA to find if the alkali was bound in aluminum silicates in the coke ash, if the addition was sufficient for binding all alkalis and if the catalytic effect in coke gasication had been achieved. The second part was a novel approach with a statistical process analysis using SIMCA to connect top gas composition of SSAB Oxelösund's BF No. 4 to alkali content using process data. The approach investigated the correlation between NH3(g) and HCN(g) in the top gas to alkali content. Expanding on the possibility to measure alkali content quickly for the operators using top gas measurements. Top gas composition was measured using a mass spectrometer (MS) and where complimented with process and tap data provided by SSAB. Data was analyzed using the multivariate analysis tool SIMCA 15 to find possible correlations. Results from the first part showed that the alkali that was found was present as alkali aluminum silicates independent of kaolin addition after the EBF. As temperature along gas composition was the main factors behind alkali uptake in coke. Main differences in alkali uptake and development of coke properties in the BF was linked to the temperature and gas composition profile during tests campaigns compared. Results from TGA showed that the reaction rate of coke with CO2 increases with increasing K2O and that start of reaction was lower with increasing alkali. The results from the second approach did not find a correlation between HCN(g) and K2O in slag. Positive correlation could be seen between HCN(g) and increased SiO2 in slag and that H2O(g) would affect HCN(g) negatively.

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.