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Improvements in Yield in an All-DRI-Fed EAF from Minimization of Feo Generation During Melting As Well As Post-Reduction of Feo from Residual Slag

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Improvements in Yield in an All-DRI-Fed EAF from Minimization of Feo Generation During Melting As Well As Post-Reduction of Feo from Residual Slag 36 Technical Article Improvements in Yield in an All-DRI-Fed EAF From Minimization of FeO Generation During Melting as Well as Post-Reduction of FeO From Residual Slag In recent years, ArcelorMittal Mexico has focused on improving iron yield in the continuing challenging economic environment. Previous work highlighted recycling and briquetting direct reduced iron fines. This improved yield more than 2%. In that work, it was noted that additional yield savings were possible by a tighter control of ppm oxygen at tap to avoid excess generation of FeO. This also applies to generation of FeO during the heat. By delaying the initiation of lancing, the final slag %FeO can be reduced and yet meet aim carbon. Lower slag %FeO not only improves yield, directly lowering costs, but additionally leads to cost savings from using less aluminum after tap. ver the past several years, the fracturing of the DRI pellet in the Oauthors have reported signifi- first place, upstream of the EAF, cant progress on improving yield focusing particularly on the DRI in all-direct-reduced-iron (DRI)-fed reduction units (Midrex and HYL). electric arc furnaces (EAFs).1–3 A Generally at issue is a compro- significant breakthrough was real- mise between pellet strength dur- izing that subsized DRI pellets and ing reduction and pellet porosity fines were short-circuiting the fur- needed for good reducibility that is nace by floating out with the slag largely determined upstream by ore over the breast rather than get- concentrate selection and the pellet ting ingested into the bath. This is production process. These phenom- Authors attributed to Stokes’ Law, whereby ena are complex, suggesting they the falling (terminal) velocity of the can be optimized. However, it was Rubén Lule González (top row, left) steelmaking and continuous casting pellet decreases in proportion to decided to not compromise metal- process engineering superintendent, the square of its diameter, so that lization in any trade-off between ArcelorMittal Lázaro Cárdenas Flat Carbon, Lázaro Cárdenas, Mich., Mexico the smaller the pellet, the greater compressive strength and porosity. [email protected] its descent velocity through the slag Instead, a recycling practice was Francisco López Acosta (top row, right) on feeding. While this velocity never, developed, although it does entail operaiton steelmaking superintendent, by itself, can reach zero, in the fur- additional costs with regard to bri- ArcelorMittal Lázaro Cárdenas Flat nace the falling DRI encounters an quetting. However, in this case, the Carbon, Lázaro Cárdenas, Mich., Mexico upward force from the evolution tremendous improvement in yield Michael Lowry (middle row, left) of CO from decarburization and more than offsets these costs. engineer, ArcelorMittal USA Research Laboratories, East Chicago, Ind., USA reduction of carbon and FeO in the In previous papers, the authors [email protected] pellet. This phenomena effectively elucidated the key factors affect- David Kundrat (middle row, right) suspends the smaller DRI pellets ing yield. Top of the list is slag vol- adjunct professor, University of and fines in the slag. ume because of the contained FeO Cincinnati, Cincinnati, Ohio, USA This has led to a successful strat- dictated thermodynamically by the Allen Wyatt (bottom row, left) egy based on screening pellets and carbon level in the metal at tap. In senior manager, customer projects, fines in the feed below 0.25 inch this case, there are unduly high slag SGL Group, The Carbon Company, AIST.ORG I and recycling them as briquettes. As volumes due to reliance on cheap Charlotte, N.C., USA [email protected] a result, yield has jumped some 2%, but high gangue ores to produce the Jackson Kuntze an enormous improvement for gen- DRI as mandated by management. regional technical manager, SGL Canada eral improvement in yields. Thus, aiming for lower slag volume Inc., Etobicoke, Ont., Canada Most recently, the authors is out of scope, as it is important to [email protected] explored factors related to maintain a sufficient basicity and Hagen Fuchs (bottom row, right) IRON & STEEL TECHNOLOGY I global director, commercial projects, SGL Group, The Carbon Company, 2018 Wiesbaden, Germany [email protected] JAN This article is available online at AIST.org for 30 days following publication. 37 adequate MgO levels to avoid erosion of the furnace furnace at the start of the subsequent heat. Since refractories, etc. the equilibrium value is about 20% FeO, the gap at From a thermodynamic point of view, low yields are the start of the heat is some 6% FeO, or a maximum the result of an imbalance between oxygen and car- potential yield differential of about 0.4%. bon in the system. In fact, there is also a kinetic aspect The slag volume then increases during the heat as that plays a significant role as well. This is because the DRI and fluxes are added. If the DRI is particularly EAF is not nearly well mixed as, say, the basic oxygen high in acidic gangue (Al2O3 and SiO2), the slag vol- furnace (BOF). As a result there can be global and ume will reach as high as 300 to 400 kg/ton, or as local departures from equilibrium. Since the slag and high as 66,000 to 88,000 kg/heat. At the end of the metal are not well mixed, the slag, for example, can heat, the slag typically reaches about 35% FeO. This be excessively oxidizing relative to the metal, yet the represents a widening of the gap (with respect to the metal tap remains at the aim ppm oxygen. In consid- equilibrium value) to about 15% FeO. Taking a slag eration of this phenomenon, the focus will be on the volume of 350 kg/ton, this represents a maximum lancing practice and any role it might have in generat- differential in yield of more than 4% relative to equi- ing excess levels of FeO. librium conditions! The difference in potential yields The present paper explores not only the benefits of from the beginning of the heat to the end of the heat a tighter control of the lancing practice near the end represents a substantial deterioration of yield during of the heat to avoid overoxidizing the slag, but also melting. This depends on not only increasing slag any benefits of delaying initiation of lancing early in %FeO but also the very large slag volume generated the heat. These two practices can lead to lowering slag during the heat. It is clear that, while there is a signifi- FeO levels for an essentially fixed slag volume without cant improvement in yield from the pre-melting stage an adverse effect on ability to achieve the desired ppm practice, the bigger potential payback is improvement oxygen and power-on-time. from preventing increases in slag %FeO during melt- Additionally, the benefits are also reported of a ing in the first place. In this case, this is especially further reduction of FeO from the residual slag in glaring, given the unduly high slag volume. the system by making use of the time gap in between heats (where, for example, the slag pot is changed out) and the relatively large hot heel. Role of Kinetics and Excessive FeO Levels During Together, the practices of recycling DRI of a sub- EAF Melting par pellet size, more optimized lancing practice and further reduction of FeO in the residual slag after tap In a previous paper, the authors revealed a key have resulted in a combined substantial improvement mechanism for the production of excessive FeO in the in furnace yield. all-DRI-fed EAF, namely the role of the rate of mass transfer of carbon in the bath during decarburization. This includes not just the mass transfer coefficient, Yield Loss Occurring During Melting but also the driving force due to carbon dissolved in the bath relative to the equilibrium level. Essentially, As stated earlier, it is important that the carbon and when the mass transfer of carbon is limited compared oxygen flows in the furnace be in balance. It can be to the lancing rate, the incoming oxygen finds a home shown from thermodynamic calculations of the FeO- oxidizing iron instead. This phenomenon can be O-C equilibrium for a given slag basicity and tem- particularly important near the end of the heat, when perature that the slag has an FeO composition that is the driving force for mass transfer of carbon can be at dictated by the aim ppm O if the system is in complete its lowest when carbon levels are at their lowest point equilibrium. For example, for the ratio CaO/SiO2 in the heat. Here it is easy to overblow the heat and in the slag taken at 1.5, at 1,600°C and for an aim of produce excessive FeO by this mechanism. 1,300 ppm O, the slag FeO is about 20%. But, this same mechanism can be a factor early JAN It is of interest for this paper to examine the actual in the heat as well, now possibly from a slower rate 2018 slag %FeO relative to this level during the progress of dissolution and transfer of carbon due to lower I of the heat. This calculation shows the gap from the temperatures. Early in the heat, bath temperatures IRON & STEEL TECHNOLOGY equilibrium value to widen during melting, thus a are at their lowest, so perhaps access of the bath to deterioration of yield had the gap remained constant. carbon locked in the DRI can be limited, resulting At the start of the heat, the residual slag in the hot in a lower driving force for diffusion of carbon. Also, heel is about 26% FeO, which is a gap of some 6% the mass transfer coefficient may be lower as well due from the equilibrium value as a result of the practice to less mixing if the DRI is not reacting as vigorously I to add coke in the pre-melting stage.
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