Co-injection of basic fluxes or BF flue dust with PC into a BF charged with 100% pellets : effects on slag formation and coal combustion

Abstract: Based on 100% pellets operation at BF No. 3 at SSAB Tunnplåt in Luleå a new pellet with CaO/SiO2=1 was developed during early nineties. The pellet showed good results in metallurgical laboratory test but caused slag formation problems in the bosh. A high basicity slag was formed during interaction with basic fluxes and its melting point was increased when the slag was finally reduced. By injection of basic fluxes, the slag formation problems in the bosh can be avoided. Without a sinter plant, dusts have to be recycled in a cold bonded briquette. Injection of some of BF flue dust would improve the properties of the briquette and may be beneficial for the BF process. Based on top charging or tuyere-injection of basic fluxes and the injection of BF flue dust studies have been done. The studies carried out will clarify some phenomena of slag formation and the effect of co- injection on coal combustion in the blast furnace, including the effect of: 1) basic fluxes on slag formation in the blast furnace, if they are top- charged; 2) basic fluxes on slag formation in the blast furnace, if they are tuyere injected; 3) chemical composition and metallurgical properties of pellet and fluxes on slag formation; 4) reduction conditions on (reduction temperature, reducing gas composition) on bosh slag formation; 5 co- injection of BOF slag or BF flue dust on coal combustion efficiency and BF performance. From the results, it can be concluded that the interaction between pellets and fluxes starts when softening and melting starts in the cohesive layer. By choosing fluxes of a high melting point the dissolution of them in the bosh slag can be delayed and the excessive basicity of the bosh slag can be limited. A basic flux with a low melting point will easily dissolve into the bosh slag causing its basicity to increase, which is beneficial to S refining when acid pellets are used. The formation of a bosh slag of excessive basicity (which causes BF operation disturbances, because its high melting point, increases further during reduction) can be avoided by tuyere injection of basic fluxes with the coal. The positive effect is greatest when a fluxed pellet of basicity B21 and with a high Fe content is used as ferrous burden. In this case, the slag amount can be significantly decreased. BOF slag has suitable high-temperature properties for use as a basic flux in combination with e.g., olivine pellets. It has a low softening and melting temperature, does not shrink, is slowly reduced in the BF shaft and its properties are almost unaffected by the partial reduction occurring in the shaft of the BF. BOF slag absorbs a low content of alkalies, which causes volume increase at elevated temperatures, when the basicity is still high. The results indicate that co-injection of BOF slag or BF flue dust with PC has positive effects on the BF operation. The injection of BOF slag decreases the Si content in HM by an increased basicity of the slag formed during combustion leading to a decreased activity of SiO2 in the slag and by an increased FeO content in the tuyere slag that will be reduced by SiFe in the metal at the tuyere level, if the SiFe is initially high. The consumption of reducing agents can be reduced because of a decreased Si content in HM and a decreased slag amount. The permeability is improved by prohibiting the formation of a shell at the raceway end because of improved melting properties of tuyere slag caused by an increased basicity and FeO content, consumption of coal and coke fines by FeO and improved radial transport of tuyere slag. The injection of BOF slag improves the bosh slag because an excessive basicity caused by top-charged fluxes added for neutralisation of coal and coke ashes and non-uniform slag formation caused by uneven distribution of top-charged fluxes are avoided. The productivity increases because of the possibility to decrease the slag amount. The injection of BOF slag can be done without negative effects on combustion efficiency by using very finely ground BOF slag and because of improved total consumption of coal and coke fines by direct reduction in the coke bed. The injection of BF flue dust decreases the Si content in HM by an increased FeO content of the tuyere slag that will be reduced by SiFe in the metal at the tuyere level, if the SiFe is initially high. The consumption of coal and coke is decreased because of a high of C content in BF flue dust and the decreased Si content in HM. The permeability is improved by prohibiting the formation of a shell at the raceway end because of consumption of coal and coke fines by FeO and improved melting properties of tuyere slag promoting the radial transport of the tuyere slag. The injection of BF flue dust can be done without negative effects on the total consumption of coal and coke fines by direct reduction in the coke bed. Injected BF flue dust supplies oxygen for combustion, but because of endothermic reactions as reduction of hematite and calcination of limestone occurring in the BF flue dust at the same temperatures as release and disintegration of VM, it will have a negative effect on the combustion of HV coal. SSAB Luleå and SSAB Oxelösund have started up projects aiming to make tuyere- injection of BF flue dust part of the standard BF operation.