Interaction between pellet properties and blast furnace operation

Abstract: The raw material properties of iron-bearing material and coke have a significant impact on the blast furnace (BF) process. The raw materials are designed and chosen to achieve smooth BF operation but the operational parameters can be simultaneously modified to meet properties of selected raw materials. As the pre-set process conditions change, e. g. due to an increase in pulverized coal injection rate (PCR), the in-furnace conditions for iron oxide reduction change, burden distribution pattern has to be adapted and demands on raw material strength will be affected, etc. In order to maintain stable operation, but also to obtain low amounts of material losses through the off-gas, it is important to understand fines generation and behaviour in the blast furnace. The present investigation focuses on how the pellet properties during reduction can affect the BF process as well as how pre-set process conditions will influence pellet properties. The performed tests involve full-scale, pilot and laboratory investigations. Raw material sampling of, among other things, pellets was carried out during a period of fluctuations in the hot metal Si content at the SSAB BF No. 3 at Luleå. Although differences in pellet low-temperature reduction disintegration and the high-temperature breakdown were observed, the reduction behaviours during laboratory blast furnace simulation tests were almost identical. Differences in the hot metal Si content in a production blast furnace were difficult to correlate to raw material properties, since the process conditions were changed in order to control the heat level of the blast furnace. Tests in the LKAB Experimental Blast Furnace (EBF) were carried out under different pre-set process conditions. Injection of high-volatile (HV) coal resulted in a higher reduction potential in the ascending gas due to a higher H2 content and an increased shaft temperature compared to operation with low-volatile (LV) coal. A higher pellet reduction degree was attained in pellets sampled with the upper shaft probe during operation with HV coal. The differences receded through the shaft and no differences in pellet reduction degree that can be correlated to the pre-set process conditions were observed in samples taken out with the lower shaft probe. However, differences in the Femet pellet texture were observed. The present investigations showed an increased carburization of Femet and an increase in the potassium content in pellets sampled with the lower shaft probe during injection with HV coal. Off-gas dust and fines generated in the EBF were characterized and attempts to relate the findings to furnace conditions have been made. Flue dust was mechanically formed and the predominant share was <0.5 mm. Fe-containing particles, dominating <0.063 mm fractions, were present as Fe2O3, showing that they originated from the top of the shaft. Coke particles were observed in all flue dust fractions but were commonly >0.075 mm. Fines of quartzite, limestone and BOF-slag were identified in flue dust when these materials were used as slag formers. For blast furnace operation with low amounts of flue dust, operation with low off-gas temperatures and velocities together with an even off-gas flow pattern is desirable. The properties of fine particles in terms of size and density are also important when outflow of mechanical dust, such as flue dust, is concerned. Material in the >0.5 mm size range did not leave the EBF with the off-gas but influenced in-furnace conditions. Chemically formed dust was precipitated from the ascending gas through the furnace and appeared as spherical sub-micron particles that ended up in the sludge. Laboratory blast furnace simulation reduction tests for hypothetical PCR indicated that an increase in hypothetical PCR was necessary to compensate for the decrease in reduction time between a slow and a fast temperature profile. The reduction time influenced the Femet texture in the pellet periphery. The initial reduction conditions, in terms of temperature level and reduction gas composition, will have a significant effect on the pellet texture up to a reduction degree of at least 60 percent. However, outlines of the original pellet texture can be preserved during reduction and recognized as far as to the formation of wustite. To learn more about the relationship between; the textures of reduced pellets, breakage properties and compression strength, an initial study when using the Pellet Multi Press equipment for compression strength tests of pellets taken from the EBF was conducted. Furthermore, the pellet pieces generated after compression tests were characterized. Occurrence of Femet in the pellet texture increased the compression strength, while less reduced and less sintered textures showed the reverse effect. So far, the results from CS-tests indicate that disintegration of pellets takes place at a reaction front, at transition between different texture types of an iron oxide or at the location of a visible surface crack.