Relationships Between Basic Oxygen Furnace Maintenance .
Relationships BetweenBasic Oxygen FurnaceMaintenance Strategies andSteelmaking ProductivityThis article is available online at www.aist.org for 30 days following publication.The increasing necessity to improve productivity in the basic oxygen furnace(BOF) shop led to the evaluation of differentBOF maintenance strategies. The principaltargets of most integrated steel mills are maximizing automation, BOF application speed,and production time by reducing downtimesand improving vessel availability. The standard North American and European BOFmaintenance strategies were examined, andthe financial implications of various aspects —including the refractory materials, missed production opportunities, and refractory installation time — were compared. The results indicated that considerable cost savings throughproductivity increases can be achieved byoptimizing vessel maintenance, and multipleapproaches to realize improvements in thisarea are described.IntroductionOver many years, the steel industries worldwide have evolved more or less independentlyfrom each other. Substantial differences intargets and philosophies between Europeanand North American steel producers haveled to production strategies that vary significantly. With the increasing globalization in thesteel industry, a discussion regarding optimization potentials was initiated, mainly due toglobal companies’ internal benchmarking.With respect to BOF operations, completelydifferent and almost noncomparable developments have taken place over the last 10–15years. This paper details a comparison of thestandard North American and European BOFmaintenance operations, including BOF lining and maintenance costs, time consumptionfor maintenance, and the influence of otherrelevant cost factors using a theoretical casestudy.Basic Oxygen FurnaceMaintenance StrategiesThe following methods are commonly usedfor BOF refractory ntenance strategies are examined. Considerablecost savings can be achieved by optimizing vesselmaintenance, and multiple approaches to realizeimprovements in this area are described. Zero maintenance.Slag splashing.Slag washing.Patching.Gunning.Super gunning.Zero Maintenance — The zero-maintenanceconverter, with campaign lives of up to 2,500heats, is very common in many European steelplants. Only the tapholes are changed, andsometimes the mouth area is gunned to avoidskull formation. In many cases, this practice isused when three vessels are available and twoof them have to produce a maximum numberof heats per day.Slag Splashing — This practice was developedapproximately 15 years ago in the UnitedStates. With increasing experience, up to60,000 heats with one vessel lining have beenrealized. After steel tapping, the remainingslag in the vessel is splashed with a high-pressure nitrogen jet onto different areas of thelining during a 2- to 5-minute period. ToAuthorsHans Jörg Junger (left), director of marketing — steel, North America, RHI CanadaInc., Steel Division, Burlington, Ont., Canada (hansjoerg.junger@rhicanada.ca);Christoph Jandl (center), project manager — marketing, steel, North America, andJürgen Cappel (right), director — Metallurgy Department, RHI AG, Steel Division,Vienna, Austria (christoph.jandl@rhi-ag.com, juergen.cappel@rhi-ag.com)November 2008 29
Table 1Standard BOF Operating Conditions in North Americaand EuropePerformance criterionNorth AmericaEuropeLining life 42–7Gunning consumption (kg/t)0.3–0.70.2–0.4Slag amount 0–25NoYesTap-to-tap (min)50–7040–60Heats per day18–2624–36Slag MgO (wt.%)[P] in hot metal (ppm)[C] x [O]Ar/N2 stirringavoid slag buildup in the bottom, the excessslag is then poured off before charging.1–7Slag Coating and Slag Washing — After tapping, the vessel is tilted for deslagging and asmall amount of liquid slag is retained in thevessel. This slag is enriched with doloma orraw dolomite to cool the slag and increase itsadhesive properties. Afterward, the vessel isrocked several times to cover the bottom, bot-Table 2Standard Operating Details for the North American andEuropean Plants Used for Model Cost CalculationsModel calculation detailsNorth Americansplashing modeEuropean liningand gunning .60OperationNo. of vessels in the plant (n)Tap weight (t/heat)No. of heats (n/day)Production days (days/year)Tap-to-tap time (min)Yearly production (t/year)LiningLining weight (t/lining)Lining price (US /lining)Lining life (heats/campaign)Lining costs (US /t crude steel)Installation time (hours/lining)Gunning and splashingGunning consumption (kg/t crude steel)Mix price (US /t refractory)Gunning costs (US /t crude steel)Machinery costs (US /t crude steel)Gunning speed (kg/min)Mix consumption (kg/gunning)Gunning time (min/heat)Average gunning interval (heats/gunning)Slag splashing (min/heat)30 Iron & Steel Technologytom joint, tapping pad, and scrap impact zonewith a thin slag layer.1Hot Patching — Self-flowing refractory mixeswith optimized solidification times enableprecise care of the scrap impact zone, tapping pad and bottom joint. While the repairis longer-lasting than the aforementioned slagcoating, it requires a longer period to achievemaximum durability. Therefore, hot patchingis usually planned in advance and performedduring a scheduled production interruption.Gunning — By gunning preworn areas withspecial gunning mixes, an extension of thevessel lining service life is targeted. Accurategunning leads to a uniform lining wear rateand maximizes utilization of all the installedrefractory materials.Super Gunning — Intensive gunning may costproduction time; however, by increasing thegunning speed, super gunning can replaceslag splashing as a maintenance method if itis carried out almost fully automatically andin short available breaks. Mix throughputs ofup to 400 kg/minute can be achieved, andprecise repairs of all areas are possible. A longmix lifetime, no influence on the purgingplug availability, and adapted lining conceptscan solve the requirement for maximum productivity at the lowest costs.8Comparison of North Americanand European Basic OxygenFurnace Operating ConditionsA comparison of the standard BOF operating conditions in North American andEuropean steel plants (Table 1) provided thebasis for an investigation of different maintenance methods (or different combinationsof methods) and operating philosophies withrespect to productivity and costs. While NorthAmerica targets the almost everlasting liningand thereby sacrifices productivity (typicallywith a 2/2 vessel operation), European steelproducers aim for a higher steel qualityduring primary crude steel production andmaximum productivity (typically with a 3/3vessel operation).An additional difference is the use of lowphosphorous iron ore in North America thatenables local steel plants to produce phosphorous-critical steel grades without additional argon gas purging and without highamounts of reactive slag. Furthermore, theexcessive slag splashing practice in NorthAmerica and high MgO content required inthe slag to counteract refractory corrosionalso differ from the European approach toBOF steelmaking.
Figure 2Figure 1Specific refractory costs/ton of liquid steel (CPT-R), comprising the lining, gunning and machinery costs/ton of liquid steelunder standard North American conditions.Refractory and MissedProduction Opportunity CostsIn the following cost calculations, a typicalNorth American BOF steel plant with two vessels was assumed. The standard plant operating conditions are summarized in Table2. For this model calculation, the BOF wasconsidered to be the bottleneck vessel in thesteel plant.A preliminary analysis of the calculatedspecific refractory costs/tonliquid steel (CPT-R),which includes lining, gunning and machinery, supported the North American operational philosophy (Figure 1). The minimumCPT-R was achieved after more than 20,000heats, and the negligible cost increases for thegunning required to extend the lining life didnot justify the installation costs and the necessary downtime to reline a BOF.In further calculations, non-utilized productivity was represented as the costs resultingfrom missed production opportunities/tonliquid steel (CPT-O), and thereby a proportion ofthe time required for unscheduled maintenance activities was expressed in the analyses.For the calculated example, it was assumedthat 50% of the maintenance was performedduring planned breaks, and the other 50%resulted in lost production time and wastherefore included as a cost in the CPT-O.The calculation results indicated that theNorth American BOF production philosophyresulted in a CPT-R below US 1.0/tonliquidsteel after approximately 10,000 heats (Figure2). However, since intensive maintenanceresulted in a very high CPT-O, the sum ofthe refractory and missed production opportunity costs/tonliquid steel (CPT-R-O) reachedonly a minimum of US 5.6/tonliquid steel afterabout 20,000 heats. However, once this mini-Total refractory and missed opportunity costs/ton of liquidsteel (CPT-O-R), comprising the specific refractory costs/tonof liquid steel (CPT-R) and the missed production opportunity costs/ton of liquid steel (CPT-O) for standard NorthAmerican BOF maintenance operations.mum CPT-R-O value was reached, due to theincreasing slag splashing intensity, the refractory cost savings no longer compensated forthe missed production opportunities and theCPT-R-O increased. These results demonstrated the potential for operational improvements, mainly realizable through productivityincreases.Subsequently, the CPT-R and CPT-O werecalculated for the model steel plant operating under the standard European conditionsdetailed in Table 2. The operational differences included that the maintenance wasperformed mainly during scheduled breaks(i.e., 80% of maintenance during scheduledstandstills, and therefore only 20% of themaintenance resulted in a loss of production time) and the gunning equipment wasmodernized (i.e., introduction of super gunning). In addition, top-quality gunning mixeswere introduced, and slag splashing as astandard practice after every heat was waived.A cheaper, balanced lining with a targetlifetime of 5,000 heats was installed, and theinstallation time for relining was reduced.The results of the model plant operatingunder the European conditions (Figure 3)were significantly different from the sameplant operating under the North Americanphilosophy of BOF maintenance.The CPT-R and CPT-R-O approached aminimum at a lining life of about 5,500 heats,which reflected realistic targets for EuropeanBOF steel plants. A direct comparison of theCPT-R values calculated for the European andNorth American BOF maintenance schedules over 30,000 heats demonstrated thatthe minimum CPT-R of US 1.45/tonliquid steelwith the European strategy was considerablyhigher than the US 0.69/ton determined forNovember 2008 31
Figure 3Figure 4Total refractory and missed opportunity costs/ton of liquidsteel (CPT-O-R), compirsinig the specific refractory costs/ton of liquid steel (CPT-R) and the mised productionopportunity costs/ton of liquid steel (CPT-O) for standardEuropean BOF maintenance operations.Comparison of the CPT-R values for BOF maintenance usingthe European and North American approaches.the North American approach (Figure 4).However, when the calculated CPT-O valueswere included in the analysis (Figure 5) andthe resulting CPT-R-O costs were compared,the European strategy reached a minimum ofUS 2.3/ton at 5,500 heats, while the NorthAmerican approach reached only a minimum of US 5.6/tonliquid steel at approximately20,000 heats. These potential savings of atleast US 3/tonliquid steel can be achieved evenwithout taking the metallurgical benefits intoaccount and are realizable through improvedmaintenance planning and a better utilizationof production time.In all the analyses performed, it was demonstrated that the cost of time utilized formaintenance impacts the CPT-R-O so significantly that a comprehensive examination ofFigure 5Comparison of the CPT-R-O values for BOF maintenanceusing the European and North American approaches.32 Iron & Steel Technologydifferent approaches to increase productivityis fully justified.Potentials to IncreaseProductivityUtilization of Breaks for Vessel Maintenance— Depending on the number of castersavailable and the bottleneck vessel in thesteel plant, optimization of the productionschedule is possible. The fewer vessels available, the smaller the amount of idle time, andtherefore the expedient use of these shortbreaks for efficient maintenance becomesmore important (Figure 6).Relining — Detailed planning and optimization of all possible aspects — such as specialFigure 6Distribution of idle and process time depending on the number of vessels in operation.
Figure 7brick packaging, labeling and special-shapedbricks — reduces the time required for relining. In addition, there are significant potentials to be realized through the effectivecooperation between steel plant management,refractory supplier and lining service, as illustrated in Figure 7. The investment in specialshapes adapted to the steel plant’s vessel andoptimized transport logistics of the materialinto the vessel also result in faster vessel linings (i.e., less than five days from the last tofirst heat).Slag Splashing — Although slag splashing hasbecome more common in Europe over the lastdecade, the frequency and procedures differfrom the North American practice. Whereasin North America slag splashing is carried outafter almost every heat using a highly MgOsaturated refining slag, typically in Europeansteel plants the splashing is performed afterevery second to 10th heat. In addition, theexcess slag is initially poured out, and then theremaining slag is conditioned by first addingdoloma lime or another MgO carrier.CONREP — High-Speed State-of-the-ArtGunning Robot — The newly developedCONREP high-speed BOF gunning system(Figure 8), which has already been successfully tested in two European steel plants, enablesLining speeds at various European and North American steel plants.highly automated and precise gunning repairsin all vessel areas. The specially developedmixes provide a high material throughput(i.e., up to 400 kg/minute) with perfect initial adherence and low rebound. Due to thehigher mix densities, longer lifetimes can beachieved. The CONREP machine can be combined with CONSCAN, a residual thicknesslaser measurement system, to enhance themaintenance accuracy.8Figure 8CONREP high-speed gunning system in operation.November 2008 33
Figure 9Repair precision of different maintenance methods.The repair precision of different maintenance methods is illustrated in Figure 9. Usingslag splashing, slag foaming, slag baking oreven hot repair methods, no precise measuresto counteract prewear in specific areas can beconducted. This results in a considerable lossof production time due to servicing areas inthe vessel that do not require repair. For thisreason, slag splashing needs to be performedin combination with gunning to achieve lininglifetimes of more than 6,000 heats. Therefore,the apparent benefit of rapid slag splashingrepair is offset by the enormous time requirement to perform both, since the older thevessel becomes the more gunning material isrequired and the higher the production timeloss.While it is not necessary to discontinue slagsplashing completely, it should not be part ofthe standard procedure at the end of everyheat. A more effective use of slag splashing orslag coating is to utilize idle times expedientlyand condition the slag only if required.Additional AspectsWith the increased freedom and flexibility ofBOF maintenance options and process strategies, other potentials can be realized: Lower lining price due to less complicated lining design demands. Ability to use purging plugs during theentire campaign. Modification of slag composition andamounts.As the lining life is adjusted to incorporateeconomical aspects, a balanced lining conceptcan be introduced with the advantage of bothreduced weight and a larger vessel volume. Inthe majority of cases, this will also improve theBOF operating conditions.34 Iron & Steel TechnologyTo achieve the lowest phosphorous content with an everlasting lining, iron ore withlow phosphorous impurities must be used inthe blast furnace due to the poor dephosphorization capability of the MgO-rich BOFslag necessary for efficient slag splashing.With a change in the slag composition (i.e., alower MgO content) and the introduction ofbottom gas purging, a higher phosphorouscontent in the hot metal is permissible. Thisprovides more flexibility to the blast furnaceburden and a significant benefit for theironmaking divisions, because the increasedmining yield will result in considerably loweriron ore prices. Studies have shown that,depending on the slag basicity, the MgOsaturation is only approximately 6 wt. % inthe final stage with higher phosphorous levels.9–10 However, a MgO content around thislevel is sufficient to produce a reactive liquidslag that is not excessively corrosive to therefractory material.ConclusionThe calculations presented in this paper demonstrate a paradigm shift from the BOF everlasting lining to lining lifetimes of 5,000–7,000heats is possible and enables steel operatorsto reduce costs. Together with changes inthe installation routine (e.g., logistics), liningconcept, and maintenance program, this newphilosophy is more efficient economically aswell as organizationally. As a result of shiftingthe necessary vessel equipment maintenanceto the scheduled relining periods, unplannedinterruptions caused by breakdowns are minimized. These achievements are possible onlyin an open partnership between steel plant,refractory supplier, and original equipmentmanufacturer. Thereby, individual strategiesthat address the targets as well as the limitations can be developed collaboratively forthe benefit of both the steel plant and therefractory supplier. Furthermore, while therealizable refractory cost reductions per tonof crude steel are only cents, by increasingproductivity and having a higher flexibilityregarding raw materials and steel grades, savings in the range of several dollars can beachieved.References1. Mills, K.C.; Su, Y.; Fox, A.B.; Li, Z.; Thackray,R.P.; and Tsai, H., “A Review of Slag Splashing,” ISIJInternational, 2005, 45, No. 5, pp. 619–633.2. Chigwedu, C., and Kempken, J., “Using SlagSplashing Technology for Low Gunning Rates and EvenLower Refractory Costs,” Stahl und Eisen, 2006, 126,No. 2, pp. 19–26.3. Russel, R.; Donaghy, N.; Meyer, E.; and Goodson,K., “Everlasting BOF Linings at LTV Steel?” 1st EOSCProceedings, Düsseldorf/Neuss, Germany, 21–23 June,1993, pp. 220–225.
4. Messina, C.J., “Slag Splashing in the BOF— Worldwide Status, Practices and Results,” Iron andSteel Engineer, 1996, 73, pp. 17–19.5. Goodson, K.; Donaghy, N.; and Russel, R.,“Furnace Refractory Maintenance and Slag Splashing,”Iron & Steelmaker, 1995, 22, June, pp. 31–34.6. Singh, K.; Dhali, S.; Sahay, J.; and Nagpal,O., “Slag Splashing Technology in BOFs,” MPTInternational, 2000, No. 5, pp. 110–111.7. Bagsarian, T., “Slag Splashing Reaches NewLevels,” New Steel, 1999, 15, No. 11, pp. 38–40.8. Pudack, A.; Lanzenberger, R.; Pungersek, R.; andWimmer, V., “CONSCAN and CONREP: An InnovativeBasic Oxygen Furnace High-Speed Gunning System,”RHI Bulletin, 2007, No. 1, pp. 14–19.9. Obst, K.; Schürmann, E.; Mahn, G.; Münchberg,W.; and Nolle, D., “On the Dissolution of MagnesiumOxide in the Slag During the LD Process,” Stahl undEisen, 1980, 100, No. 20, pp. 1194–1200.10. Schürmann, E., and Kolm, I., “Dissolution ofDolomite in Steel Plant Slags, Limitations in Solubilityand Conseq
Basic Oxygen Furnace Maintenance Strategies The following methods are commonly used for BOF refractory maintenance: Zero maintenance. Slag splashing. Slag washing. Patching. Gunning. Super gunning. Zero Maintenance — The zero-
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