Refractory gunning - Millennium...
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AUTHORS: Christian Wolf and Kurt Wolf VELCO GmbH GUNNING PRINCIPLES T he gunning repair is a well-proven procedure by which refractory material can be applied quickly and cheaply. It is used for the hot repair of ladles and melting furnaces as well as relining or cold repair of the back lining. There are two basic methods: dry and wet gunning. With dry gunning the material is discharged from the machine with a maximum of 5% moisture and then fed to the gunning nozzle by an air stream where the required water, typically 5–10%, is added. With wet gunning the gunning material is moistened with water in a mixer and then pumped through a hose by means of an eccentric screw or a piston pump. At the end of the line the material is dispersed with compressed air and, if necessary, an additional liquid bonding/hardening agent can be added (shotcreting). Wet gunning has the problem that the machine or the gunning hose may get clogged by already moistened refractory material, particularly when not in continuous use; hence the process has to be carefully controlled. Also, the equipment requires more intensive cleaning and is not considered an efficient operation for applications less than 400kg. With dry gunning, blockages in the conveying hose can be blown free by compressed air only. For optimum gunning and refractory performance the gunning material is essentially the same as the original refractory (plus binder material), but with a size distribution of 4mm maximum. The applied thickness is typically in the range 10–30mm and a vessel can normally be used again after 3–5 minutes. Standard gunning materials are of magnesia, alumina or silica base. These materials differ in price so the optimum choice depends on many factors such as steel grade, slag composition and the vessel used. Consumption of gunned refractory is also extremely varied, ranging from 0.2kg/t steel for new vessels and up to 2kg/t steel for an old vessel or one in an aggressive environment. MIXING OF WATER AND MATERIAL Three essential requirements for a good gunning repair are optimal moistening, homogeneous mixing of the gunning material with water, and a high quality gunning machine that guarantees even conveying. With pre-moistening a share of the gunning water is added some metres away from the gunning nozzle and relies on the turbulence within the conveyance for premixing. This approach, however, is very susceptible to operational problems. For instance if too much water is added in advance this can lead to clogging, particularly with quick binding systems where hardening starts inside the hose. Too much water reduces refractory quality and hence refractory lifetime. Also, the water can dissociate in the liquid steel to hydrogen, which can be detrimental to some steel grades (micro-cracks). With standard mixing heads (for dry gunning) the water is jetted through radial borings and, in order to compensate for inadequate initial moistening, the operator often works with a surplus of water so that the dust is minimised. This, however, often leads to use of an incorrect water/ cement ratio, resulting in reduced refractory durability. Improvements have been achieved by use of pressure- increasing pumps so that the water jet has become sharper and more readily reaches the centre of the nozzle cross- section. Optimum nozzle-vessel distance is quite varied; between 40 and 1,000mm depending on application. TYPES OF GUNNING MACHINES Both rotor and pressure vessel gunning machines can be used for the dry gunning process. The ROTAMAT gunning Dry gunning, with or without the use of gunning robots, is a well-known, modern procedure to increase the life of refractory-lined vessels quickly, accurately and with low rebound. Additionally, with the new GUNMIX moistening system, it is now possible to dry gun low cement castables with low rebound, and for small repairs this is a cost-saving alternative to shotcreting. Refractory gunning a r Fig.1 Gunning machine Rotamat 32 f/l MILLENNIUM STEEL 2005 120
Transcript of Refractory gunning - Millennium...
AUTHORS: Christian Wolf and Kurt Wolf VELCO GmbH
GUNNING PRINCIPLES
The gunning repair is a well-proven procedure bywhich refractory material can be applied quickly and
cheaply. It is used for the hot repair of ladles and meltingfurnaces as well as relining or cold repair of the backlining. There are two basic methods: dry and wet gunning.With dry gunning the material is discharged from themachine with a maximum of 5% moisture and then fedto the gunning nozzle by an air stream where the requiredwater, typically 5–10%, is added. With wet gunning thegunning material is moistened with water in a mixer andthen pumped through a hose by means of an eccentricscrew or a piston pump. At the end of the line thematerial is dispersed with compressed air and, ifnecessary, an additional liquid bonding/hardening agentcan be added (shotcreting).
Wet gunning has the problem that the machine or thegunning hose may get clogged by already moistenedrefractory material, particularly when not in continuoususe; hence the process has to be carefully controlled. Also,the equipment requires more intensive cleaning and isnot considered an efficient operation for applications lessthan 400kg. With dry gunning, blockages in theconveying hose can be blown free by compressed air only.
For optimum gunning and refractory performance the
gunning material is essentially the same as the originalrefractory (plus binder material), but with a size distributionof 4mm maximum. The applied thickness is typically in therange 10–30mm and a vessel can normally be used againafter 3–5 minutes. Standard gunning materials are ofmagnesia, alumina or silica base. These materials differ inprice so the optimum choice depends on many factors suchas steel grade, slag composition and the vessel used.
Consumption of gunned refractory is also extremelyvaried, ranging from 0.2kg/t steel for new vessels and upto 2kg/t steel for an old vessel or one in an aggressiveenvironment.
MIXING OF WATER AND MATERIALThree essential requirements for a good gunning repair areoptimal moistening, homogeneous mixing of the gunningmaterial with water, and a high quality gunning machinethat guarantees even conveying. With pre-moistening ashare of the gunning water is added some metres awayfrom the gunning nozzle and relies on the turbulencewithin the conveyance for premixing. This approach,however, is very susceptible to operational problems. Forinstance if too much water is added in advance this canlead to clogging, particularly with quick binding systemswhere hardening starts inside the hose. Too much waterreduces refractory quality and hence refractory lifetime.Also, the water can dissociate in the liquid steel tohydrogen, which can be detrimental to some steel grades(micro-cracks).
With standard mixing heads (for dry gunning) the wateris jetted through radial borings and, in order to compensatefor inadequate initial moistening, the operator often workswith a surplus of water so that the dust is minimised. This,however, often leads to use of an incorrect water/cement ratio, resulting in reduced refractory durability.Improvements have been achieved by use of pressure-increasing pumps so that the water jet has become sharperand more readily reaches the centre of the nozzle cross-section. Optimum nozzle-vessel distance is quite varied;between 40 and 1,000mm depending on application.
TYPES OF GUNNING MACHINESBoth rotor and pressure vessel gunning machines can beused for the dry gunning process. The ROTAMAT gunning
Dry gunning, with or without the use of gunning robots, is a well-known, modern procedure toincrease the life of refractory-lined vessels quickly, accurately and with low rebound. Additionally,with the new GUNMIX moistening system, it is now possible to dry gun low cement castables withlow rebound, and for small repairs this is a cost-saving alternative to shotcreting.
Refractory gunning
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r Fig.1 GunningmachineRotamat 32 f/l
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machine (see Figure 1) has a continuously rotating rotor,similar to the cylinder of a revolver, which is tightenedbetween two rubber disks. On one side the rotorchambers are filled by the gunning material flowing in bygravity and on the other side it is blown out, forming acontinuous stream of gunning material and compressedair. Nozzle velocity is about 30m/sec, with a gunning rateof 1–100kg/min. The principle is shown in Figure 2.
This machine type is mobile and easy to use, however,the construction principle leads to wear between the rotorand the rubber sealing disk, and so for higher gunningcapacities pressure vessel machines with a volume of1,000–3,000 litres are often used, and are mostly filled bylarge capacity bags – so-called big bags.
VELCO has developed the EKU-type pressure vesselgunning machine with pneumatic dosing system andthe EKS-type with a mechanical dosing system, both fordry gunning (see Figure 3). Both can be manufacturedas mobile (with wheels) or as stationary (transport bycrane and fork lift). The dosing wheel of the EKS workshorizontally (see Figure 4) and has a service life ofseveral thousand tonnes of gunning material. This kindof construction is fundamentally different from theprinciple of the cellular wheel sluice, common to manyother designs, which works vertically and is subject to high wear. The dosing system guarantees even mass flow, even when the gunning material hasagglomerated, as can occur with humid compressed airor from vibrations on the furnace platform.
GUNNING MANIPULATORS FOR HOT REPAIRThe hot gunning repair of aggregates for vessels such asconverter, ladle, EAF and RH-snorkels, is a cheapalternative to renewal of the complete lining. Thisprocess increases the service life of the vesselconsiderably and reduces vessel reheating costs. Due tothe rapid repair, the number of circulating vessels, eg,ladles, can also potentially be reduced. The use ofgunning manipulators for hot repair is necessary becauseof the hot conditions which are unsuitable for humanoperators. These devices also allow a more preciselylocated repair and higher gunning capacities
r Fig.2 Rotorprinciple
r Fig.4 Dosing wheels
r Fig.3 Gunning machine EKS-K
(100–300kg/min), which considerably reduces therepair time and the loss of heat of the vessels. Reboundrates do tend to increase as injection rate increases.
Figure 5 shows the METOP gunning manipulator in usefor EAF repairs. The gunning nozzle is driven to therequested working position with the furnace roofremoved. The operator starts the gunning machine andcontrols the repair either by means of radio remotecontrol or by use of a pre-programmed gunning program.Figure 6 shows a manipulator for the repair of ladles. It is driven in front of the horizontal ladle and operated bya radio or cable remote control and it can it be linked toa laser scanner. Manipulators can also be used for therepair of RH-snorkels and, if the manipulator is equippedwith two gunning lances, simultaneous internal andexternal repair can be carried out.
THE GUNMIX SYSTEMGUNMIX is VELCO’s latest moistening system in whichwater is dispersed by a compressed air jet and thegunning material is then moistened by a water fog. Thisgreatly improves wettability of the refractory particles,especially of the ultrafine binders (less than 50μm). Theaddition of water is exactly adjusted and can be metered,allowing reproduction of an exact mixing ratio of gunning a
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material and water. This improved moistening systemconsiderably reduces the amount of dust and reboundand produces a consistently high quality of gunningconcrete. Rebound typically reduces from 20–30% to10–15%. The system can be used for both hot and coldrepair. Of particular note is that even special refractoriessuch as LCC (low cement castable), ULC (ultra low cementcastable) and NCC (no cement castable) can be workedwith less dust and less rebound by means of this system.
The ability to gun these special materials has enablednew fields of application to develop, such as dry gunningof ladle bottoms and blast furnace runners. Due to thebetter moistening by GUNMIX these materials can beprocessed by adding a small amount of liquid bindingagent water glass (sodium metasilicate), acid or water.Also, unlike conventional refractory installation bycasting, there is no need to use a template. Processingwith GUNMIX is also a cost-saving alternative to costlyprocedures such as shotcreting since it is worked withsimple machine technology, and less material is lostduring the adjustment and cleaning of the machine. TheGUNMIX system can be combined with rotor or pressurevessel gunning machines and used to upgrade olderVELCO gunning machines.
Examples of the system in use are shown in Figures 7–9.
CONCLUSIONS Dry gunning is a well-known, modern procedure toincrease the life of melting furnaces, ladles and so on.When combined with gunning robots, refractories can beapplied accurately, quickly and with low rebound. By usingthe new GUNMIX moistening system it is now possible todry gun LC castables with low rebound, and for smallrepairs this is a cost-saving alternative to shotcreting. MS
Christian Wolf is Managing Director and Kurt Wolf isSenior Manager, both at VELCO GmbH, Velbert,Germany
CONTACT: [email protected]
GUNMIX® is a registered trademark of Velco GmbH
r Fig.6 Ladle gunning manipulator
r Fig.7a Pig iron ladle before gunningFig.7b Pig iron ladle after gunning
r Fig.8 Repair of tundish back lining
r Fig.5 Gunning manipulator METOP for EAF
r Fig.9 Repairof torpedoladle
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