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AUTHORS: Rolf Gerling, Bernd Stallmann and Dieter Blissenbach Stahlwerk Bous GmbH, Minteq International GmbH andFerrotron Technologies GmbH

Continuous changes in the economic environmentand increasing competitive pressures require steel

producers to introduce innovative measures to reducecosts. Stahlwerk Bous/Saar GmbH, part of the steelsection in Georgsmarienhütte Holding, has successfullypursued this strategy over many years. More recently,working with Minteq International GmbH, a new conceptof automatic arc furnace refractory maintenance wasdeveloped to eliminate the disadvantages inherent inintermittent refractory maintenance.

DESCRIPTION OF THE STEEL PLANTStahlwerk Bous GmbH is located on the banks of the riverSaar in south west Germany. It was built in 1961 andproduces more than 260,000tpa of round, polygonal,rectangular, square, cylindrical and conical uphill castingots per year for seamless tube mills and forgings, with weights from 1.4 to 63 tonnes. The quality rangecomprises more than 800 different grades of aluminiumkilled, low to high carbon, manganese, chromium andmolybdenum-alloyed steels, together with desulphurised,vacuum-degassed and calcium treated grades.

The 70t AC EAF has a shell diameter of 5,180mm and an electrode pitch circle diameter of 1,220mm and20in electrodes, and is equipped with the following:

` 45 MVA transformer` Water cooled wall and roof, TW 2000 with heat

recovery` Nose tapping` Four bottom stirring plugs` Three wall burners with post combustion` Lance manipulator with one carbon and two oxygen

lances` Current conducting aluminium electrode arms` Electrode spray cooling` Waste gas spray cooling` Furnace enclosure` Process computer

` Alloy hopper` Neural network for optimal control of energy input.

REFRACTORY MAINTENANCE METHODSRefractory maintenance was a major cause of downtimeat Bous, both for furnace re-lining and refractorymaintenance by gunning and fettling. Classic gunningmaintenance practice involves very hard manual labour tomanipulate the lance in front of the furnace and requirestwo operatives; and for material application a rotor orpressure chamber gunning machine was required whichhad a maximum conveying capacity of 80kg/min. Thismeant that gunning, for example, one tonne of refractorymaterial to maintain the furnace slag line would takeapproximately 12.5 minutes. In addition, due to thevarying distances and angles necessary to apply thematerial to the slag line it was not possible to applymaterial uniformly at the optimum angle, resulting inrebound levels of up to 30%. Hence, in order toeffectively apply 1t of material to the slag line, 1.3t hadto be gunned. This typically increased the maintenancetime by 3.75 minutes, giving a total time for slag linemaintenance of 16.25 minutes per heat. Depending onthe refractory condition of the furnace, maintenancetimes of up to double this figure were possible.

If refractory wear became too severe the furnace had tobe taken out of service for repair and every week a brickrepair was carried out which would take around 8 hours,including the time to breakout the worn bricks. Thisrequired a gang of bricklayers. Bous decided to changethis practice with the aim of decreasing total refractorycosts and increasing furnace productivity. The aim was toreduce both the number of and the time taken forgunning repairs and to extend the production periodbetween brick repairs to two weeks.

Together with Minteq International GmbH an automaticEAF refractory maintenance system was installed inJanuary 2003. The concept was designed with a view torepairing EAF refractory linings rapidly and efficiently. Thesystem, which includes equipment, service and materialsto increase furnace steel production and reduce refractorycosts has now been optimised and has significantlyreduced the maintenance time and refractoryconsumption.

A new concept of automatic arc furnace refractory maintenance to meet the requirements ofmodern steel production at Stahlwerk Bous was installed in 2003. It has eliminated the financialand operational disadvantages inherent in intermittent maintenance, reduced refractoryconsumption and costs, and increased plant availability.

Automatic EAF refractory maintenance

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and smoother movement and a new gunning head andwater mixing system. To enable the gunning head to locatethe correct required position in the furnace, a co-ordinatingsystem was required which necessitated the design of acompletely new software system.

Due to the very small dog house the MINSCAN had tobe split into three separate components: the manipulator,the silos and hoppers, and the hydraulic system. Themanipulator is positioned in the dog house on a 2.3mhigh platform to ensure that there was no interferencewith other aspects of the furnace operation. This area wasalso the most frequent entry point to the dog house totake samples and temperature measurements during theprocess. The silos and hoppers are located on a platformoutside of the dog house. The hydraulic system with thetank, motors and cooling are positioned outside the doghouse on the ground.

The gunning head can perform a continuous 360°rotational and simultaneous vertical movement from thefurnace centre to the upper edge of the furnace water-cooling panels.

Incorporated inside the gunning head is a newlydeveloped eccentric jet mixing nozzle ‘MINJET’, designed

FULLY AUTOMATIC MAINTENANCE SYSTEMThe system comprises the following three components:

` A LaCam® laser scanner to measure the residualrefractory thickness of the furnace (see Figure 1)

` An enhanced MINSCAN™ robotic maintenancesystem to repair the refractory lining in differentareas of the EAF (see Figure 2)

` A SCANTROL™ interface module, linking the above-mentioned components, to evaluate themeasurement data and control the roboticmaintenance unit.

LaCam laser scanner The laser-based profile measuringsystem has been developed for non-contact measurementof refractory linings in metallurgical reaction vessels andtransport ladles. The scanning unit is installed in a ruggedcylindrical steel housing. Its measuring principle is basedon accurate range-finding by means of a pulsedsemiconductor diode laser with electro-optical rangingcapability and a biaxial beam scanning mechanism. The3D images are generated by performing a number ofindependent laser ranging measurements in different yetwell-defined angular directions. These ranging data,together with associated angles, form the basis of the 3Dimages. A three-dimensional framework of the furnaceinner surface is created with, typically, 200,000 rangingpoints being scanned within 20 seconds. The built-inindustrial PC evaluates the residual refractory thickness bycomparing it with a previous reference measurement.

LaCam manipulator At Bous the laser manipulator, isinstalled 9m above the furnace outside the dog house.The manipulator essentially comprises an arm with twoguides and a cooled box to accommodate the laserscanner. A powerful geared motor enables themanipulator to traverse at high speed, so as to minimisethe overall time taken for the measuring procedure.Additional cooling systems permit unlimitedmeasurement, even at extremely high furnacetemperatures. When a laser measurement is to be donethe manipulator drives into the dog house and makes twomeasurements in different positions, taking only 35seconds (see Figures 3a and 3b).

MINSCAN application equipment Many improvementshad to be made to the original MINSCAN to meet therequirements of fully automatic gunning based on LaCammeasurements. In addition, the small amount of availablespace and the small furnace diameter meant that theclassic MINSCAN had to be re-designed with a much faster

q Fig.1 LaCam laserr Fig.2 MINSCAN robotic maintenance

r Fig.3a & b The MINSCAN robotic maintenance unit

a

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to thoroughly wet the material at high speeds whilepreventing clogging and pipe drip. The new coolingtechnique ensures that the maintenance operation can operate continuously without any temperaturerestrictions. This equipment guarantees the application ofgunning and fettling material in a precise, efficient safeand rapid manner.

Scantrol This is an interface between laser andmanipulator which transforms the measurement datafrom the laser scanner in such a way that this informationis evaluated, and then a maintenance strategy is derivedto control the robotic maintenance unit.

REFRACTORY PRODUCTSThe MINSCAN system can work with gunning and drybottom material at very high rates under both hot andcold conditions. These materials are speciallyengineered for improved flowability, wetability andplasticity, and the unique particle sizing and binderpackage allows outstanding adhesion to the furnacesubstrate, thus improving on-wall density andminimising rebound. As a result, material durability isincreased which, in turn, reduces maintenanceoperations and increases furnace availability. Thechemical composition of the M-Frit KK, M-Frit and MgObottom construction and repair materials has beendeveloped to ensure an optimum balance betweenfusion behaviour and adhesive strength.

Quantum gunning material, which is used to maintainand repair the slag zone, has been successfullyredesigned for fully automatic gunning using theMINJET, resulting in a much higher application rate. Theuse of pure MgO and the synthetic additives contributeto resistance against hot erosion and slag attack, leadingto extremely high material durability. The binder systemdoes not contain any phosphate, ensuring that thephosphorus content in the steel is not affected.

EVALUATIONFigure 4 shows a flowchart for the process. The operatorat the EAF initiates the measuring procedure. When theexact position of the furnace has been determinedautomatically from the laser measurement by means of a3D matrix, the working-lining measuring points arefiltered out and transformed into a co-ordinate systemfor the furnace. The calculation of the residual brickthickness takes place on the basis of a comparison profileof the permanent lining. The individual measuring pointsin partial, high-resolution fields are defined in terms ofcylinder co-ordinates and distributed uniformly over thevessel area potentially requiring repair, and are thencombined. The system determines the co-ordinates with

minimum residual brick thickness in relation to three-dimensionally depicted sectors. Threshold values that theoperator defines for the permissible residual brickthicknesses, per sector, serve as the basis for deriving thematrix of the areas requiring repairs areas in which theresidual brick thickness is less than the appropriatethreshold value.

The operator sets the optimisation sequence (duration,material consumption, degree of restoration) and startsthe calculation of the optimised maintenance procedureso that the system carries out the maintenanceautomatically, as follows:

` Special matrix formulae combine the fragmented,high-resolution structures of the fields requiringrepair into three-dimensionally coherent, compactstructures

` The sizes and sequences of the rectangular areasrequiring repair as well as the type of repair materialsand application rate (applied thickness) aredetermined by means of strategies designed tooptimise the time taken, material consumption anddegree of restoration, and by taking into account thephysical properties of the mixes used for the repair(application from bottom to top, setting duration,maximum thickness of application)

` The manipulator co-ordinates for the areas requiringrepair are transmitted in the form of a telegram tothe PLC unit of the MINSCAN system

` The MINSCAN system carries out its maintenanceroutine fully automatically, ie, the right product is‘expertly’ applied at the exact location in therequired layer thickness. These parameters areintegrated in the preventative maintenance

r Fig.4 The maintenance process flow chart

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2002 2003 Consumption Without SCANTROL With SCANTROL Additional % Less %

Rental equipment % 0 100 100Slag conditioning % 0 100 100Gunning material kg/tls 3.14 3.45 9.9Bank/bottom hot repair kg/tls 2.79 1.86 - -33.3Bank/bottom cold repair kg/tls 1.68 1.01 - -39.9Bricks kg/tls 3.37 1.86 - -44.8Maintenance time min/heat 3.81 3.1 - -18.6Refractory repair shifts number/yr - -22 -Additional shifts number/yr - -22 -Specific net savings ¤/tls 1.39Total net savings ¤/yr 321,271

Table 1 Summary of savings and costs

r Fig.5a & b Operator console and display (a) Wall plot (b) Bottom plot

programme, thereby harmonising consumption and operating efficiency.

VISUALISATIONA monitor in the control pulpit is used for thevisualisation of the measured residual refractorythicknesses and the parameters for the refractorymaintenance process. The measured residual refractorythicknesses (wall and bottom) are shown in the left-hand half of the display (see Figures 5a and 5b). Themaintained areas or the thicknesses of the refractory(wall, bottom) after a pre-calculated, automaticmaintenance process are seen on the right-hand side.

The system permits a simple menu-driven dialoguethat enables the operator to read off the exact amountsof materials (Quantum for the slag zone and M-Frit forbanks and bottom), and the total maintenance time(including manipulator movements).

If the operator concurs with the maintenanceprocedure, he can initiate it directly at the push of a button. If the maintenance time or materialquantity appears excessive, for example, because ofproduction influences, he is able to alter the time orquantity. At the same time the residual maintainedrefractory areas and thicknesses are recalculated and re-displayed on the right of the screen. Once he has given his approval, the repair and maintenanceprocess is then carried out automatically. The operator thus has the possibility at any time to adjustthe maintenance process to the situation at thefurnace.

The exact measurements of the residual thickness inthe whole furnace by the LaCam system provide theoperator with exact information about the state of therefractory. The measurement is carried out at leastthree times per day and profiles of the refractory wearin the EAF are being determined and evaluated online. a

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RESULTSFigures 7 and 8 illustrate some average values ofsignificant consumptions in 2002 before installingSCANTROL and in 2003 after installation. With the newsystem it is possible to effectively maintain the refractoryin areas of the furnace that could not be reached using ahand lance. These areas include behind the furnace door,the door pillars and areas to the left and right of thedoor. Additionally, a larger proportion of the banks arenow maintained using gunning compared to thesituation before the introduction of the SCANTROL whenfettling maintenance would have been used. These dataare summarised in Table 1.

Associated costs are: ` Operating lease of LaCam, SCANTROL, MINSCAN` Increased consumption of gunning material` Cost of the new bottom/bank hot repair material` Additional consumption of regenerated MgO

The benefits are: ` Decreased bottom/bank cold repair material` Reduced brick consumption` Less refractory maintenance time` Doubling of the production period of the furnace

between brick relines ` Additional production shifts

The resulting specific net-savings are 1.39¤/t liquid steel,equivalent to 321,271¤/year.

CONCLUSIONSThe automatic EAF refractory maintenance system wasinstalled in the Bous steel plant in January 2003. After ashort period to optimise the operation to meet thespecialised requirements of Bous, the equipment hasoperated effectively with excellent availability. Thefunctionality of the SCANTROL system providing laserdriven measurement of refractory thickness, visualrepresentation of scanned results and automatic materialapplication, has significantly enhanced productivity,working conditions and decision-making capabilities ofsteel plant operators. The overall effect at Bous hasincluded:

` Reduction in total refractory consumption` Reduction in refractory maintenance time` The ability to effectively maintain all areas

of the furnace` Doubling the available production period

between brick relines ` The requirement for additional shifts has

been reduced

The measurements made during a furnace campaignare evaluated and depicted in Figures 5a and 5b. Thesefigures indicate the thickness of the slag zone, banksand bottom of the EAF. The various colours symbolisethe diverse residual thicknesses. The black horizontalline indicates the slag zone, and the water-cooledpanel are represented in white. It can be seen in thatthe main problem zones in the furnace are behind thedoor, the phase 2 and 3 electrodes in the slag zoneand the banks.

Accelerated refractory wear in the various sectors ofthe EAF is identified directly and with the aid of thesoftware it is possible to superimpose the differentfurnace refractory states horizontally and vertically atdifferent angles and levels as shown in Figures 6a and6b, thereby generating a refractory wear profile as afunction of time. The software immediately calculatesthe refractory wear. The original lining (thick black line)and the permeable blocks in bottom are also shown.

r Fig.6a & b Vertical cross-sections at different vessel positions

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` Improved operational safety ` Elimination of hand lance manual

operation MS

Rolf Gerling is Steel Plant Manager at Stahlwerk BousGmbH, Bous, GermanyBernd Stallmann is Sales Manager, Germany, MINTEQInternational GmbH, Duisburg, GermanyDieter Blissenbach is Project Manager at FerrotronTechnologies GmbH, Moers, Germany

CONTACT: marketing.europe@mineralstech.com

MINSCAN™, SCANTROL™, M-Frit KK, M-Frit andQuantum are products and/or trademarks of Minteq.LaCam® is a registered trademark of FerrotronTechnologies GmbH

r Fig.7 Refractory maintenance time before and after SCANTROL

r Fig.8 Brick consumption before and after SCANTROL

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