An Anatomy Of Furnace Refractory Erosion: Evidence From

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Paper presented at 58th Electric Furnace Conference, Orlando, Florida, November 13th, 2000.In 58th Electric Furnace Conference Proceedings: 361–78. Warrendale, PA.: Iron & Steel Society. 2000.An Anatomy of Furnace Refractory Erosion:Evidence from a Pilot-Scale FacilityPaul den HoedMINTEKPrivate Bag X30152125 RandburgSouth AfricaTelephone 27-11-709-4735Telefax [email protected]: DC-arc, furnace, reverberatory, blast, copper, lead, slag, nickel laterite, chrome, magnesia,alumina, spinel, silicon carbide, graphite, refractory, corrosion, erosion, phase chemistryINTRODUCTIONPlasma-arc technology—and DC transferred-arc technology, in particular—has its adherents.1, 2 MINTEK is oneof them. Since the late 1970s, it has sought to apply this technology to the recovery of valuable metals fromcertain ores and from furnace slags and dusts. Commercial furnaces are now in place for the production offerrochromium from chromite and for the smelting of ilmenite. Beginning with small-scale DC-arc furnaces, ithas demonstrated the application in four other areas:2–7 The recovery of copper, nickel and cobalt from converter slagsThe recovery of nickel from nickel lateriteThe fuming of lead and zinc from lead blast-furnace slag (LBFS)The removal of zinc and lead from electric-arc furnace (EAF) dusts collected during steelmakingThen, several years ago, the scale of work leapt with the commissioning of a 5.6 MVA (1–3 MW) facility. Thefurnaces, although larger (about 2.5 m in diameter), follow earlier designs—a cylindrical shell of water-cooledpanels; a conical roof, through which the graphite cathode passes; and the facility to feed charge close to thearc. In most applications, an alumina castable protects the roof and an MgO rammable the hearth. Therefractories of the sidewall, being the focus in this matter, vary according to the demands and concerns of eachcampaign. The right choice of refractory has always been integral to the success of a campaign. These days,the assessment of their performance provides invaluable pointers in choosing refractories for large-scale,industrial furnaces—it is for the purpose of designing these furnaces, after all, that campaigns on a pilot scaleare conducted.The campaigns themselves have been good vehicles for testing refractories under particular conditions.One can cite two reasons:1. Conditions in the small, pilot-scale furnace are sometimes more severe than those likely to be encounteredin a industrial-scale furnace. The dimensions are such that, more than once, high temperatures have2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS361

prevented a freeze-lining from forming; and in at least one configuration, flaring from the arc impinged ona section of sidewall in the freeboard.2. Severity notwithstanding, of all the tests one can devise, a campaign in a pilot-scale furnace best simulatesthe conditions that will prevail in an industrial furnace. Heat transfer profiles are similar, and bothcorrosive and erosive forces are at play: The hot-face is at the refractory-slag interface and temperature drops across the refractory. This standsin contrast to the cup test, in which a crucible of the refractory, or a cavity drilled into a brick of thematerial, is filled with slag and heated in a furnace. This configuration forces temperature, whenconditions have stabilized, to be uniform throughout the slag and refractory. Continuous feeding and tapping keep the composition of slag in the bath constant. This maintains thechemical potentials driving corrosion. In the cup test, by contrast, the ratio of slag to refractory is low,with the effect that chemical potentials equalize when slag reacts with the refractory. The spindle test,in which a rotating rod of refractory is immersed in a bath of molten slag, would circumvent this flawof the cup test if the bath were large in comparison with the immersed refractory. Turbulence in the slag bath creates an erosive environment, one that a refractory must withstand.These advantages, however, cannot offset the fact that tests in a pilot-scale furnace fail to give full and precisecontrol over conditions at the slag-refractory interface. In a post-mortem examination, one is consequentlyunable to distinguish between, let alone measure, the interactive processes between slag and refractory—thevery processes researchers consider to constitute corrosion and erosion. (This is a concern being addressed by agroup at CSIRO Minerals, Australia. It has developed a gravimetric technique for providing “directinformation” on the dynamic processes of wetting, penetration, dissolution and erosion of refractories by moltenslags.8) We can, nonetheless, rank the performances of different refractories from similar campaigns; and,drawing on phase-chemical theory, we can interpret the clues offered by post-mortem examinations to identifythe causes of erosion in a particular refractory. The details may not all be there, but an account of the broadermechanisms is.A number of different refractories were used in several recent campaigns run in the 5.6 MVA, DC-arcfurnace at MINTEK. This paper describes aspects of their corrosion and erosion. It offers explanations for whathappened to them, and it draws some lessons regarding the choice of refractories in certain applications.MATERIALS AND CONDITIONSThe campaigns involved the smelting of siliceous materials at conditions designed to minimize the reduction ofiron from the slag in order to concentrate certain valuable metals. These metals can be recovered from anynumber of sources; this paper considers three: Nickel laterites Lead blast-furnace slags (LBFS) Copper reverberatory-furnace slags (CRFS)The smelting of these materials produced slags of different composition (Table I). Comparing just these slags,one might highlight their relative qualities: A slag rich in magnesia and silica A slag rich in calcia and iron oxide A slag rich in calcia and silica. The alkali levels in this slag were also unusually highThe refractories were both shaped and unshaped (tables II and III). 9 Except for the silicon carbide bricks,they were all of the oxide variety. Two of the refractories—one a magnesia brick, the other a spinel castable—contained graphite. (They were chosen for their high thermal conductivity; although graphite does inhibit slagpenetration, which increases resistance to spalling.10) Only sub-sets of the refractories were used in eachcampaign (Table IV). Several of them—the magnesia, magnesia-chrome, spinel, and silicon carbiderefractories—were used in three or more campaigns.2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS362

Table I. Average Compositions of Slags Tapped from the Furnace in 7 Campaigns(mass per cent)Ex Nickel LateriteCaOMgOFeOAl2O3Cr2O3SiO2ZnOK2O Na2OEx Lead Blast-Furnace SlagEx Copper Reverberatory-Furnace Slag12121230.3321621.247. . . . . . .0.3351351.345. . . . . . .202.53950.3254. . . .2353640.1224. . . .204179.50.246. . . .4193.51790.146. . . .41342090.149. . . .4Principal Phases in the Cooled Slag(Mg,Fe)2SiO4 O6 (pyroxene I)(Mg,Fe)SiO3 (pyroxene)(Fe,Mg)O (magnesiowüstite)CaFe0.7(Si,Al)2.3O6 (pyroxene II)Ca(Fe,Mg)SiO4 (kirschteinite)KAlSi2O6 (leucite)* Akermanite, which formed in slag 2 with cooling. It was the dominant phase.Table II. Compositions of Shaped Refractories: Chemical †(mass per agnesiaChromeFusedSpinelSiC-Si3N4SiC-SiO2960.3. . . . .0.30.8879. . . . .0.52.3597.520101.62872. . . . .0.10.1. . . . .0.3. . . . .0.30.5. . . . .0.7. . . . .0.78.5. . . . . . .97. . . . . . . . . . . . . . . . . . . . . .7523. . . . . . . . . . . . . . . .90. . . .9. . . .Principal Phases (Approximate)MgO (periclase)Mg(Cr,Fe,Al)2O4 (chromite)MgAl2O4 (spinel)SiCSi3N4SiO2 (cristobalite)C (graphite).96. . . . . . .74. . . .11. . . . . . . . . .15.4849. . . . . . . . . . .Physical Properties †Bulk Density ( –3)Apparent Porosity (%)Thermal Cond. (W.m–1.K–1)*2.87182.80103.23162.94174.1 (1000 C)4.1 (1000 C)2.6 (1000 C)3.0 (1200 C)2.65172.551816.3 (1480 C) 15.7 (1480 C)† From the manufacturers’ data sheets.* Thermal conductivity of the refractory at the temperature reported in brackets.Along with compositional differences in their slags, the campaigns differed in other respects. They didnot all run for the same duration. Nine days was the norm, but two of the campaigns ran for much longer2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS363

periods (Table IV). Temperatures were also different (Figure 1). On the assumption that the temperature oftapped slag reflects the temperature within the furnace, we can see that the second campaign in the smelting oflead blast-furnace slag maintained relatively low temperatures ( 1400 C); the second campaign in the smeltingof nickel laterite, the hottest temperatures ( 1700 C). The difference relates to the higher liquidus of the MgOFeO-SiO2 slag from nickel laterites compared with that of the CaO-FeO-SiO2 slag from LBFS.11, 12Table III. Compositions of Unshaped Refractories: Chemical(mass per 00.541.5593. . . . .—0.11.82276. . . . . . . . .0.10.1. . . . . . .8710. . . . . . . . . .ü. . . . . . . . . . . .8510. . . . . . . . . . . . .ü2.5Principal Phases (Approximate)MgO (periclase)(Mg,Fe)(Cr,Fe,Al)2O4 (chromite)MgAl2O4 (spinel)Al2O3 (corundum)(Al,Cr)2O3 (sesquioxide)CaMgSiO4 (monticellite)Mg2SiO4 (olivine)CAx (calcium aluminates)C (graphite).3055. . . . . . .96. . . . . . . . . . . . . .üü. . . . . . . . . . . . . . . . . . .1875. . . . . . . . . .ü. . . .Physical PropertiesBulk Density ( –3) successes with certain refractories and a willingness to try new ones played a part in the choices ofrefractories made for the different campaigns. Physical factors were also considered. Any choice, however,should not fail to take cognizance of an important phase-chemical principle, that of the compatibility betweenslag and refractory. With thought given to it, the following precautions could be sounded: The slags, which are rich in FeO, will tend to oxidize a silicon carbide refractory and acce lerate its erosion.Only a freeze lining will prevent this reaction. The choice of silicon carbide in three of the campaigns(Table IV) was prompted by a need for high thermal conductivities in order to establish a freeze lining. The slags will tend to oxidize the graphite in carbon-composite refractories, which will affect the wetting ofthe refractory and, therefore, slag penetration. Only a freeze lining will prevent this from happening. The slags, which contain little Al 2O3, will tend to dissolve alumina refractories. Avoid these refractoriesunless a freeze lining is guaranteed. LBFS, which has relatively little silica, will dissolve silicate phases in those refractories that contain them.Choose refractories with little or no SiO2.The same phase-chemical principle, on the other hand, enables one to recommend that magnesia refractories beused in the smelting of nickel laterite, because the slag is rich in MgO.2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS364

650175016501750Temperature ( 0Temperature ( 5016501750Temperature ( C)Figure 1. Temperatures of Tapped Slags(normalized variations in temperature).a. Smelting of nickel laterite.b. Smelting of lead blast-furnace slag.c. Smelting of Cu reverberatory-furnace slag.The numbers refer to campaigns (see Table I).2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS365

Table IV. Combinations of Slags and Refractories: Shaped RefractoriesEx Nickel elSiC Nitride bondedSiC Silicate bonded.12ü. .ü. . . . . . . .ü. . . . . . .Ex Lead Blast-Furnace Slag1. . . . .ü. . . . . . .2.Ex Copper Reverberatory-Furnace Slag1. . . . . . .üüüü. . .ü. . .ü. . . . .23ü. . . .üüüüü. . . . . . .üüü.Unshaped inelSpinel-carbon.Campaign Duration (days).9ü. . . . .10. . .ü. . . . . . .9.18. . . . . . .üüü9.25.9REACTIONS, RESISTANCE AND FAILURESIn all seven campaigns, corrosion was the cause of failure in many of the refractories lining the sidewall of thefurnace. It manifested itself in two ways:1. As a dissolution reaction at the hot-face. The driving force in this process is the lower activity of therefractory-oxide component—i.e., MgO, Al2O3 or Cr2O3—in the slag. (A similar imbalance drives FeOinto the refractory.) In a closed system, the dissolution process would continue until the slag reachedsaturation. In practice, however, because the slag composition is held constant, the point of saturation isnever reached and dissolution continues until the entire refractory is consumed.2. As a loss of refractoriness behind the hot-face. Here, slag penetrates the refractory. The introduction ofCaO, FeO and SiO2 lowers the solidus temperature of the refractory to well below the prevailingtemperature. The consequence is a turning of part of the refractory to liquid. This weakens the refractory,making it susceptible to any turbulence in the slag or metal bath. As these currents impinge on the lining,so the refractory succumbs to erosion.Several local factors would determine which of these mechanisms prevailed at any point in the furnace or in anyrefractory. Structural characteristics (i.e., the porosity and grain-size distribution of a refractory) and interfacialproperties (i.e., the surface tension between a given slag and refractory, which influences wetting) determine theextent to which a slag will penetrate a refractory. On the other hand, high temperatures in the furnace and sharpgradients in the refractory lining would tend to favour reactions at the hot-face over those behind it. Withoutour having measured the physical properties directly, we can only infer their likely effects from a post-mortemexamination of the refractories in the light of generally understood principles, or remain silent.MgAl2O4 (Spinel) in Contact with CRFSWe can represent this combination by compositions within the system CaO-MgO-Al2O3-SiO2. (The systemaccounts for the principal species in the slag at and behind the hot-face. We can ignore FeO on the grounds thatFe2 will diffuse into the grains of spinel, which accommodates it in solid solution. This, indeed, is whathappened.) Phase relations at liquidus temperatures in the system have been published for planes of constantAl2O3.11, 12 The composition of CRFS can be represented on the diagram cutting the 10% Al 2O3 plane of thesystem (10% approximates the alumina content of the slag—Table I). It lies over the pyroxene primary-phasefield. At 1550–1600 C, therefore, the slag is not in equilibrium with MgAl 2O4 (spinel); being unsaturated with2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS366

MgO and Al2O3, it will dissolve the refractory until it is in equilibrium with MgAl2O4. The dissolution processcan be tracked through the system across planes of increasing Al2O3: at 15% Al2O3, the spinel primary-phasefield has appeared and begun expanding; at 25% Al2O3, the slag composition has begun to move over the spinelfield; between 30 and 35% Al2O3, it moves across the 1550–1600 C isotherms. Only when the pointrepresenting the slag composition falls within the spinel primary-phase field and converges with the isothermrepresenting the bath temperature does further dissolution of spinel from the refractory cease. The point ofsaturation is reached when the Al2O3 fraction in the slag has risen to between 30 and 35%; in the process, theslag will have consumed 45–65% of its mass in MgAl2O4 (spinel). Without a freeze lining, therefore, one canexpect CRFS to do considerable damage to spinel refractories.That the bricks of fused spinel sustained severe erosion is a clear indication that a freeze lining was notmaintained at the base of the furnace (Figure 2a). Further up the sidewall, cooling panels held downtemperatures in the lining sufficiently for corrosion and erosion to have been minimal (Figure 2b). Themicroscopic evidence points to a dissolution of MgAl 2O4 (spinel) at the hot-face as the mechanism of erosion: As Figure 3 strikingly shows, the slag-refractory interface ‘slices through’ MgAl2O4 (spinel) grains at theeroded face of the fused-spinel brick; the interface is sharp and smooth over the full surface of the hot-face.No loose grains of spinel lay in the layer of slag adhering to the hot-face, a sign that the refractory had notfirst been weakened by corrosion and then washed by currents into the bath. Slag penetrates the matrix in both brick and castable. Within the pores, its composition is enriched inAl2O3. Analyses of the slag phase trace a sharp increase in the level of Al2O3 from a point at the hot-face( 10%) to one just behind it ( 25%). There was no evidence that the reaction of slag with MgAl 2O4(spinel) in the pores of the refractory contributed to erosion.ab010 cm010 cmFigure 2. Corrosion/Erosion of SpinelRefractories in Contact with CRFS.a. Brick of fused spinel from base of furnace.b. Castable against cooling panel.2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS367

slaghotfaceMgAl2O40300 µmFigure 3. Refractory-Slag Interactions at theHotface of a Spinel Brick.Micrograph of the backscattered-electron image.Magnesia-Chrome in Contact with LBFSIn theory, one can represent this combination by compositions in the system CaO-MgO-FeO-Cr2O3-SiO2. Inpractice, however, given the constraints of presenting phase diagrams in two dimensions and omissions in thecorpus of published diagrams, such a representation is no easy task. The composition of the slag mapsconveniently onto the phase diagram for the system CaO-iron oxide-SiO2 in contact with metallic iron.11, 12Introducing MgO (periclase) and Mg(Cr,Fe,Al)2O4 (chromite)—phases making up the refractory—complicatesmatters immeasurably. A simpler tack is desirable. The marked difference in compositions of the slag andrefractory suggests that the two might be incompatible. Whereas the refractory contains about 60% MgO(Table II), the slag contains no more than 5% MgO (Table I). The considerable disparity in these numbersmake it very likely indeed that LBFS, in contact with a magnesia-chrome refractory, is unsaturated with MgO.(Despite similar differences in Cr2O3, the chrome solubility in such FeO-rich slag—by implication, fairlyoxidizing—would be low.) The evidence of microscopy and energy dispersive spectrometry (EDS) supportsthis conclusion: The hot-face defines a sharp boundary between the refractory and the slag of the bath (Figure 4). Wewould interpret this feature as erosion by dissolution. Not only is the cooled slag adjacent to the hot-face enriched in Mg2 , but, where it has been left relativelyundisturbed, a spinel rich in magnesia and chrome has crystallized from the molten slag. This phase and itschrome-magnesia-rich composition indicate that the slag in this area, shielded from turbulence in the slagbath, had reached saturation, the outcome of a dissolution process.Not surprisingly, without a freeze-lining to protect them, the magnesia-chrome bricks that contained the slagbath were severely eroded in the shorter campaign and entirely consumed within 18 days (Table IV). Erosionwas just as severe in magnesia-chrome refractories lining the lower sections of the freeboard.2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS368

MgO(Mg,Fe2 )(Cr,Al,Fe3 )2O4hotfaceslag01.5 mmFe2O3Figure 4. Refractory-Slag Interactions at theHotface of a Magnesia-Chrome Brick.Microgr

The hot-face is at the refractory-slag interface and temperature drops across the refractory. This stands in contrast to the cup test, in which a crucible of the refractory, or a cavity drilled into a brick of the material, is filled with slag and heated in a furn

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