Furnace refractory erosion - Mintek

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slaghotfaceMgAl 2O 40 300 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-Cr 2 O 3 -SiO 2 . 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 maps11, 12conveniently onto the phase diagram for the system CaO-iron oxide-SiO 2 in contact with metallic iron.Introducing MgO (periclase) and Mg(Cr,Fe,Al) 2 O 4 (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 Cr 2 O 3 , 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 Mg 2+ , 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 PROCEEDINGS 368
MgO(Mg,Fe 2+ )(Cr,Al,Fe 3+ ) 2O 4hotfaceslag0 1.5 mmFe 2O 3Figure 4. Refractory-Slag Interactions at theHotface of a Magnesia-Chrome Brick.Micrograph of the backscattered-electron image.Magnesia, Mag-Chrome and Ni Laterite SlagsConsidering the high level of MgO in the slag (Table I), one would be prudent in lining the furnace with amagnesia refractory. The evidence seems to bear out the validity of this line of reasoning. Not that erosion didnot occur; rather, bricks of magnesia in the samples we collected sustained less erosion over the hot-face incontact with the slag bath than those of magnesia-chrome (cf. figures 5 and 6. The localized erosion coincidentwith the metal-slag interface—grooves marked X—is a manifestation of the Marangoni effect 13, 14 ). We canexplain the difference with reference to the appropriate phase diagrams. The combination of a magnesiarefractory in contact with a slag produced in the smelting of nickel laterite can be represented by the systemMgO-SiO 2 . As in a previously discussed combination, we can ignore FeO on the grounds that Fe 2+ diffusesinto MgO (periclase), which accommodates it in solid solution. We found phases of the slag in close proximityto grains of MgO to contain far less FeO than the bulk slag; the grains themselves had become (Mg,Fe)O(magnesiowüstite). Looking at the phase diagram for the system MgO-SiO 2 , one can see that pointsrepresenting the slag—compositions between 55 and 60% SiO 2 at temperatures between 1600 and 1700°C—lie11, 12within the liquid field close to the Mg 2 SiO 4 (olivine) liquidus. As Mg 2 SiO 4 (olivine) co-exists with MgO(periclase) at these temperatures, we can conclude that the slag is just short of being saturated with MgO. Thedriving force in dissolution—corresponding to the displacement between the compositions of the slag andliquidus on the phase diagram—would be relatively small.For a magnesia-chrome refractory, by contrast, this driving force is much greater. Here, though, theexercise is complicated by more components in the system to be considered. There is as yet no complete phasediagram for system MgO-FeO-Cr 2 O 3 -Al 2 O 3 -SiO 2 in equilibrium with metallic iron. The phase diagrams tofour sub-systems, however, give some indication of what one can expect to find in the larger system. The four11, 12are—• MgO-Cr 2 O 3 -SiO 2 in equilibrium with air• MgO-Cr 2 O 3 -SiO 2 in equilibrium with metallic chromium• MgO-Al 2 O 3 -SiO 2• FeO-Al 2 O 3 -SiO 2 in equilibrium with metallic iron2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS 369
Page 1 and 2: Paper presented at 58th Electric Fu
Page 3 and 4: Table I. Average Compositions of Sl
Page 5 and 6: 1.81.5a2Frequency1.20.90.60.310.012
Page 7: MgO and Al 2 O 3 , it will dissolve
Page 11 and 12: of slag into the refractory is mini
Page 13 and 14: • An alloy phase containing high
Page 15 and 16: -5BA-10SiO 2SiC-15FeOFe 0log p O 2C
Page 17 and 18: with a freeze lining, there were ti

Furnace refractory erosion - Mintek

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