The Quantitative Effect of Blast Furnace Slag Composition and Temperature on the Kinetics of Potassium Evaporation
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TION
A. Context
INCREASED in-plant recycling and lower quality raw material in terms of alkali content drive the alkali load in the blast furnace (BF) to higher levels. Due to the detrimental effects of alkalis on the BF operation, these elements have to be controlled in order to achieve a smooth and efficient process.
ANTON ANDERSSON and BO BJO¨RKMAN are with the Minerals and Metallurgical Engineering, Lulea˚ University of Technology, 971 87 Lulea˚, Sweden. Contact e-mail: [email protected] HESHAM AHMED is with the Minerals and Metallurgical Engineering, Lulea˚ University of Technology and also with the Central Metallurgical Research and Development Institute, P.O. Box 87, 114 21 Helwan, Egypt. LENA SUNDQVIST O¨KVIST is with the Minerals and Metallurgical Engineering, Lulea˚ University of Technology, Lulea˚ and also with the Swerim AB, Box 812, 971 25 Lulea˚, Sweden. Manuscript submitted June 17, 2020; Accepted September 10, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The alkalis of interest for the BF process are sodium and potassium, both of which enter the BF via the primary raw materials as well as recycled in-plant residues. From material balances, the introduction of alkalis in the top-charged materials is mainly attributed to the ferrous burden.[1,2] However, Sukarovs et al.[3] characterized a range of coke samples, finding that the alkali content varied between 0.11 and 0.30 wt pct, which suggests the possibility of significant contributions from the coke ash as well. Previously published material balances recognized the contribution of coke, presenting that the coke ash may account for as much as 22 to 56 pct of the total alkali load in the BF.[2,4] The upper limit of allowed alkali load differs between different plants. Geerdes et al.[5] reported that typical figures of upper limits vary between 1.5 and 5 kg of alkalis per ton hot metal (tHM). However, significantly higher alkali loads have been reported in commercial operation, e.g., publications with figures varying between 6.5 and 14 kg alkalis/tHM exist.[1,2,6–9] In addition, Lu[10] reported on declining qualities regarding the chemical composition of fine iron ore products and stated that the trend for continued downshift in iron ore quality is expected
for the future. Similar observations have been reported, but specifically for the alkali content, in other publications as well.[2,6,11,12] This trend suggests that alkali loads may increase in the future. Another factor that drives the alkali load to higher levels is the increased recycling of in-plant residues. Both the domestic environmental legislation[13] and efforts on improving raw material efficiency motivate increased recycling within the integrated steel plant. As an example, Wedholm[14] reported that the amount of materials recycled via cold-bonded briquettes had increased fivefold going from 1993 to 2015. In terms of cold-bonded briquettes, the reported recycling rate employed in industrial scale is 100 kg/tHM.[15] The reported K2O and Na2O contents of cold-bonded briquettes are 0.16 to 0.
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