Vessel relining and maintenance strategies play a major

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1 Maintenance matrices: overview of common refractory maintenance methods for BOFs and EAFs When dealing with refractory maintenance, steel workers face a range of possibilities and approaches. Often discussed controversially and influenced by suppliers promoting their system, it can be difficult to choose the right method. Therefore, RHI has compiled two matrices summarising BOF and EAF maintenance approaches. The rating of each method in relation to the application area, investment costs and operator stress is designed to support and justify selection of the most appropriate solution for a specific maintenance task. Authors: Gregor Lammer, Christoph Jandl and Karl-Michael Zettl RHI AG Vessel relining and maintenance strategies play a major role in determining steel plant efficiency. Usually the decisions when choosing the maintenance method and related equipment depend on the general plant strategy, budget and orders, production process bottlenecks, and any space restrictions on-site. Typically, steel producers look closely at maintenance to optimise one or more of the following topics: ` Reduction of specific brick consumption ` Targeted maintenance of premature wear areas ` Decreasing likelihood of breakouts ` Increasing lining lifetime ` Predictable lining lifetimes Following on from RHI s philosophy to consult with steel producers based on extensive global experience and know-how, the maintenance matrices provide an overview of commonly used maintenance methods and their suitability for different pre-wear areas in the vessels. The maintenance matrices provide a fundamental tool to determine the best customer-specific solution. REFRACTORY MAINTENANCE Often, refractory maintenance costs are rated as a minor economic factor when only the direct refractory costs are evaluated. However, the maintenance method contributes far more to the total cost of ownership estimate if the following issues are also considered in the analysis: r Fig 1 Manual gunning in a BOF plant ` Manpower and equipment costs for installation ` Vessel downtime due to maintenance ` Inventory costs ` Steel plant logistics a 61

2 Method Description Performance Gunning Targeted spraying of wet and chemically bonded refractory mixes kg/min to repair worn areas or preventively maintain a furnace damaged by mechanical, thermal, and chemical attack during its lifetime. Fettling Applying dry, non-chemically bonded mixes to repair Up to 800kg/min the wear of hearth and banks. r Table 1 Definition of gunning and fettling Thereby, the impact of the maintenance method becomes more evident. Minimising human influence during refractory application or reducing the physical stress on operators can also be decisive criteria, for example in the case of manual gunning (see Figure 1). r Fig 2 RHI shooter BOF MAINTENANCE In converter steel plants various techniques are used for refractory lining maintenance, including standard gunning using a hand lance or gunning (eg, Shooter (see Figure 2)), the use of self-flowing mixes, as well as the application of conditioned converter slag by, for example, splashing, washing and foaming. In summary, maintenance methods can be assigned to two main categories: Slag maintenance and maintenance using refractory products. EAF MAINTENANCE Steelmakers maintenance philosophy in EAF steel plants varies between zero and excessive maintenance, always depending on the conditions on-site (eg, production process bottlenecks, automation level, changeable vessel, available maintenance equipment, target productivity and produced steel grades). There are two main products used for EAF maintenance: gunning mixes and fettling mixes (see Table 1). The ANKER TWIN machine (see Figure 3) provides the efficient application of both. r Fig 3 ANKERTWIN machine MAINTENANCE MATRICES In the following maintenance matrices (Tables 2 and 3), the vessels are subdivided into the multiple common wear areas (see also Figures 4 and 5). Each maintenance method focuses on the technical rating for a specific vessel region. COMMON BOF MAINTENANCE METHODS Typical BOF maintenance methods in use include: Slag splashing After steel tapping, the remaining slag in the vessel is splashed with a high-pressure nitrogen jet on to different areas of the lining for 2 to 5 minutes. Slag coating and slag washing After tapping, the vessel is tilted for deslagging and a small amount of liquid slag is retained in the vessel. This slag is enriched with 62

3 doloma or to cool the slag and increase its adhesive properties. Subsequently, the vessel is rocked several times to apply a thin slag layer. Hot patching Self-flowing refractory mixes enable precise care of the scrap impact zone, tapping pad and bottom joint. While the repair is longer lasting than the aforementioned slag coating, it requires a longer period to achieve maximum durability. Manual and gunning By gunning preworn areas with special gunning mixes, an extension of the vessel lining service life is targeted. Automated gunning By increasing the gunning speed, super gunning requires an automated gunning process to handle mix throughputs of up to 400kg/min. A long mix lifetime, no influence on the gas purging plug availability, and adapted lining concepts can solve the requirement for maximum productivity at the lowest costs. r Fig 4 Common wear areas in a BOF Slag conditioning Adding fluxes containing MgO to increase the (MgO)-content closer to the saturation point helps to minimise dissolution of <MgO> and reduce refractory wear. Slag foaming After tapping the heat, carbon is added to the remaining slag causing it to foam and cover the walls. This method is similar to slag splashing but without additional investment and N 2 blowing. EQUIPMENT FOR BOF MAINTENANCE The following RHI equipment is available to maintain BOF vessels: ` ANKERJET (Type A): A pressure vessel machine used to feed the gunning system with the necessary mixes ` Shooter: A exclusively designed for the application of gunning material onto refractory bricks in furnaces ` CONREP: An advanced, completely automated gunning system designed for high precision gunning, with a laser interface and minimised human influence for maximum mix performance r Fig 5 Common wear areas in an EAF COMMON EAF MAINTENANCE METHODS The EAF maintenance methods currently in use and the RHI equipment available for efficient gunning and fettling mix application are summarised below: Maintenance by shovelling Fettling material is manually thrown into the vessel using a shovel from outside the vessel. a 63

4 Method Machinery Physical RHI s system solution investment stress on Machinery Refractory cost operator material Modified None None Dololime, raw (MgO) Maintenance using refractory products Slag maintenance saturated slag Slag High None Dololime, raw splashing Slag None None Dololime, raw washing Slag None None Dololime, raw foaming r Table 2 BOF maintenance matrix Lip ring Upper cone Charge pad, carbon Hand Low High ANKERJET gunning hand lance ANKERJET Manipulator Intermediate Intermediate ANKERJET ANKER GUN gunning Shooter ANKERJET Automated High Low ANKERJET ANKERJET gunning CONREP ANKERREP Hot repair None Low No equipment ANKERREP mix required Patching None Low No equipment with bricks required Appropriate (+), good (++), ideal (+++), and the specific area cannot be maintained with the method (blank) Bottom joint Bottom with plugs Bottom without plugs Tap pad Taphole replacement Trunnion Upper cone Hot repair with fettling mix in big bag Fettling material contained in a big bag is dropped into the vessel using a crane. Hand gunning light and ANKERJET Type M The gunning mix is fed by a small, mobile compressed-air system. The application is performed using a hand lance from outside the vessel. Dry spin ANKERROTO (dry) The fettling mix is stored in a silo and applied by a spin wheel below the silo. The system is crane operated. Wet spin ANKERROTO (wet) Similar to dry spin, but this method uses gunning mix. Hand gunning advanced ANKERJET Type A The gunning mix is fed by a pressure vessel based fixed system. The application is performed using a hand lance from outside the vessel. Fettling ANKERFRIT The fettling mix is stored in a silo and applied using a 360 rotatable chute. The system is crane operated and very high fettling mix throughputs are possible. Gunning basic ANKERSINGLE The gunning mix is fed by a pressure vessel based fixed system. The application is performed using a remote controlled from the centre of the vessel. The system is crane operated and provides targeted maintenance of all wall areas. Combined fettling and gunning ANKERTWIN This system combines a basic fettling 64

5 Method Application Machinery Physical Fettling material Gunning material RHI s system solution temperature investment stress on cost operator Hot Cold Hearth Bank Cooling Wall Hot Door Slag Equipment Refractory panels spots zone material Maintenance o None High (*) ANKERFRIT, by shovelling Hot repair o None Middle ++ + ANKERFRIT, using fettling mix in big bag Hand o o Very low High , ANKERJET, gunning ANKERJET light TYPE M Dry spin o Low Low ++ ANKERROTO ANKERFRIT, (dry) Wet spin o o Low Low ANKERROTO ANKERJET, (wet) Hand o o Low High ANKERJET ANKERJET gunning TYPE A advanced Fettling o Low Low ANKERFRIT ANKERFRIT, Gunning o o Intermediate Low ANKERSINGLE ANKERJET, basic Combined o o Intermediate Low ANKERTWIN ANKERFRIT, fettling and, gunning ANKERJET, Gunning o o High Low Third party ANKERJET, equipment (door access) Gunning o o High Low TERMINATOR S ANKERJET, advanced Automated o o High Low TERMINATOR XL ANKERJET, gunning robot Appropriate (+), good (++), ideal (+++), option (o), using fettling mix (*), and the specific area cannot be maintained with the method (blank) r Table 3 EAF maintenance matrix and gunning. The system is crane operated and remote controlled. Gunning (door access) The mix application is performed using a remote controlled through the door. Gunning advanced TERMINATOR S The gunning or fettling mix is fed by a pressure vessel based fixed system and high feeding rates are possible. The application is performed using a remote controlled boom from the centre of the vessel. Automated gunning robot TERMINATOR XL This is the fully automated version of the TERMINATOR and includes a residual thickness laser measurement system. CONCLUSIONS It is a well-established fact that successful, efficient and effective BOF maintenance has to be a combination of slag and refractory maintenance, whilst for EAFs a combination of fettling and gunning mix application is appropriate. The maintenance matrix approach incorporates more transparency and fact based decision making into selecting the appropriate refractory maintenance methods tailored to the customer s specific requirements. MS Gregor Lammer, Christoph Jandl and Karl-Michael Zettl are with RHI AG, Steel Division, Vienna, Austria. Contact: Gregor Lammer, gregor.lammer@rhi-ag.com 65