1. Introduction
Ore-based steelmaking generates a variety of residues including dusts, sludges, scales, and slags. Recycling of these residues within the process or via other applications is essential for the sustainable production of steel. Domestic environmental legislation [
1] as well as the cost of raw material and energy continue to drive efforts towards increased recycling. However, the recycling has to be economically feasible and possible from a process technical standpoint.
One of the residues that is in general recycled completely—with no landfill—is the dry blast furnace (BF) dust. The BF dust is collected in the gas-cleaning equipment treating the top-gas from the BF. In addition to the coarse and dry dust, a wet finer residue is normally collected by scrubbing the gas; namely, the BF sludge. The BF sludge is generally landfilled despite having a chemical composition dominated by iron and carbon. There are three principal reasons that this residue is not recycled: the zinc content, the fine particle size distribution and the water content. Generally, the zinc content is the limiting factor. Depending on how the BF is operated, the top-gas is more or less the main outlet of zinc from the furnace. Therefore, if the dust is recycled to the BF, the sludge needs to be pre-treated, removing zinc, before recycling in order to avoid its accumulation. The effects of zinc in the BF include increased reductant rates, reduced life of carbon-based refractories and scaffold formations, which may lead to a disturbed descent [
2]. Thus, avoiding the accumulation of zinc in the furnace is essential.
The removal of zinc from BF sludge and recycling of the low-zinc fraction via the sinter [
3,
4,
5,
6,
7] or cold-bonded pellets [
7] to the BF has been implemented in industrial scale. However, on-site recycling of the high-zinc fraction generated in the dezincing process has not been reported. A logical step in recycling of the high-zinc fraction is to consider the available on-site processes, e.g., the hot metal (HM) desulfurization (deS) plant and basic oxygen furnace (BOF).
The BOF has been acknowledged as an alternative route for recycling of off-gas dusts from the integrated steel plant [
8,
9,
10,
11,
12]. In one publication, recycling to the BOF by replacing the sinter used as coolant was stated to be limited in tonnage and was recognized as a partial solution for recycling of integrated steel plant dusts [
8]. Nonetheless, industrial-scale trials have shown that off-gas dust from the BOF as well as BF dust can successfully be recycled via cold-bonded agglomerates to the BOF in the amounts of 23 [
9] and 40 [
10] kg per ton HM (kg/tHM). In the former study, 23 kg/tHM was the maximum recycling rate used in the trials [
9]. In another study, cold-bonded briquettes were shown to be suitable to recycle all BOF sludge back to the BOF [
11]. In addition, hot briquetting has been employed to recycle the BOF dust back to the BOF in industrial practice [
12]. As the BOF has been recognized as a possible recycling route for in-plant residues, a logical succession is to study the potential of recycling the high-zinc fraction of upgraded BF sludge using this process.
Recycling to the BOF sets limitation on the agglomerates regarding their sulfur content, as recycling to this process is accompanied by sulfur pick-up in the crude steel [
10]. This might limit the recycling rate in this process depending on the quality requirement of the produced steel and the sulfur removal capacity of the BOF process. Tang et al. [
10] reported steel grades with sulfur requirements stricter than 0.008 wt.% were not eligible for recycling whereas steel grades allowing up to 0.015 wt.% sulfur could be used to recycle up to 40 kg/tHM. In order to meet the required quality of the steel, recycling prior to the deS of the HM should be considered as well. Prior to the current paper, research on this recycling route was yet to be reported.
In the present paper, smelting reduction of cold-bonded agglomerates in the form of briquettes and pellets was studied in technical-scale experiments aiming for recycling the high-zinc fraction of upgraded BF sludge in the HM deS plant. In addition, the potential for recycling the sludge was studied in industrial-scale trials by charging cold-bonded briquettes, without BF sludge, to the HM deS plant. In order to study the feasibility of improving the recycling capacity while maintaining the high quality of the final steel, the cold-bonded briquettes were charged to the BOF in industrial-scale trials as well. Furthermore, the sulfur pick-up of the crude steel was addressed by producing and characterizing cold-bonded briquettes produced with binders of low-sulfur contents in laboratory-scale.
Author Contributions
Conceptualization, B.B. and L.S.-Ö.; Methodology, A.A., A.K., M.A., and E.M.; Formal Analysis, A.A., M.A., and E.M.; Investigation, A.A., A.K., M.A., and E.M.; Writing—Original Draft Preparation, A.A.; Writing—Review & Editing, A.A., H.A., L.S.-Ö., and B.B.; Supervision, H.A., L.S.-Ö., and B.B.; Project Administration, H.A.; Funding Acquisition, L.S.-Ö. and B.B.
Funding
This research was funded by the Swedish Energy Agency and the research program Iron and Steel Industry Energy Use (JoSEn). The work was carried out within CAMM—Centre of Advanced Mining and Metallurgy at Luleå University of Technology.
Conflicts of Interest
The authors declare no conflict of interest.
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