Strength of Direct Reduced Iron Following Gas-Based Reduction and Carburization
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UCTION
DIRECT Reduced Iron (DRI), also known as sponge iron, is highly porous, much less dense than iron ore and sintered steel, and is used as a raw material in electric furnace steelmaking.[1] DRI is expected to sustain compressive impacts during handling, stockpiling, and shipping, which tend to break down DRI into fines. Fines are undesirable, requiring special handling (briquetting, for example) and potentially causing loss of material. The loss of DRI mostly occurs by cracking; the compression strength measured according to ISO 4700[2] is generally used to benchmark the physical properties of industrial pellets. The strength of pellets changes with each step of the reduction and carburization process. In this study, an underlying assumption is reaction steps that lower the compressive strength of pellets would cause more fines formation during handling. In order to improve DRI strength and reduce the loss of materials, it is necessary to identify factors that play a significant role in structural changes in DRI. Gas-based DR processes such as MIDREX and HYL accounted for about 80 pct of world DRI production in 2018.[3] Features of the gas-based processes related to DRI structure development are as follows. In both
GEONU KIM and PETRUS CHRISTIAAN PISTORIUS are with the Center for Iron and Steelmaking Research (CISR), Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted May 28, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
processes, high-temperature reducing gases flow upward in the opposite direction to iron ore that is loaded at the top of shaft furnaces. The processes differ in the inlet gas compositions,[4–6] the operating pressures,[6] and their use of reformers.[7] The H2/CO ratio in the bustle gas of Midrex process (1.5 to 1.6) is lower than that of HYL process (> 4);[4–6] Midrex operating pressure is slightly above 1 bar, whereas the HYL shaft is typically operated at higher pressure, about 5 to 8 bar. Unreduced pellets typically have a higher compression strength than gas-reduced pellets.[8] This implies that reduction (and possibly carburization) during DRI production involve a drop in strength. H2(g) and CO(g) are the major reductants in gas-based DR processes. There have been many reports that the reduction of iron oxides in H2 (g) and/or CO(g) gives rise to swelling of pellets with the generation of profuse cracks and higher porosity, which severely decreases the compression strength.[9–12] Tsujihata et al. showed that the crushing strength of pre-reduced pellets falls dramatically even when the degree of reduction is only about 20 pct.[13] Huang et al. reported that the most of strength loss of iron oxides occurs when reduced only for 1 minute in H2(g) and CO(g) mixtures.[11] The higher reduction rate by H2(g) causes more severe disintegration of iron ores with larger internal stress.[10] The rate of reduction in gas mixtures containing H2(g) and CO(g) tends to increase with the rise in the reacti
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