Furnace refractory erosion - Mintek

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Si 3 N 3.4Fine intergrowth ofphases that include—• Fe, Si, Co alloy• Cu, Fe sulfideSiC0 500 µmFigure 9. Near the Hot-Face of SiC-Si 3 N 4Refractory: Si-Enriched Metal.Micrograph of the backscattered-electron image.Graphite in RefractoriesOxygen fugacities in CRFS also played a role in the performances of the two refractories that contained carbon.The two were installed for their thermal conductivities, the spinel-carbon castable in the sidewall in contact withthe slag bath. After the campaign, it was found to be covered by a plate of metal; the plate took the unevenform of liquid iron running down the sidewall. The refractory behind the hot-face was marked by the absenceof graphite flakes; slag had penetrated the pores. There can be little doubt that graphite in the refractoryreduced some of the considerable FeO in the slag. Liquid metal precipitated against the hot-face, while the lossof carbon opened up pores to the infiltration of slag. The formation of metal could have had a beneficialconsequence, however: as the metal formed against the hot-face, it could have shielded the refractory from thecorrosive effects of the slag bath. On the whole, the erosion of this refractory was not noticeably worse than itsnon-carbon counterpart.2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS 374
-5BA-10SiO 2SiC-15FeOFe 0log p O 2CO 2CO + C-20SiO 2Si metal-25-30900 1100 1300 1500 1700Temperature (°C)Figure 10. Oxygen Potential Diagram: RelativeStabilities of Different Buffers.(Constructed from thermochemical data. 15 ) Thetwo shaded discs mark desirable operatingconditions—A For the smelting of LBFS.B For the smelting of CRFS.At these conditions, the reduction of Fe 2+ tometallic iron should be minimal.The magnesia-carbon bricks were never meant to come into contact with either molten slag or liquidmetal; they were shielded by an inner course of refractories. But the latter suffered excessive erosion, whichexposed some surface of the magnesia-carbon bricks to the bath. Erosion was severe.Spinel Refractories in the FreeboardIn several of the campaigns, investigators favoured spinel refractories in the sidewall of the freeboard. Brickswere preferred to castables; only in one of the earlier campaigns did a furnace run with a castable lining. Itspalled severely. In casting about for an explanation, we ruled out stress caused by a possible volume change infiring. The change in refractory volume was negligible.Firing—or rather the lack of it—seemed have been the critical factor in spalling. The manufacturer of thespinel castable stipulated a programme of drying followed by firing. All that the castable in the furnace got wasa partial curing at 400°C (higher temperatures would have damaged the cooling panels behind the refractorylining). Laboratory tests on pressed pellets of the castable demonstrated that firing at 1400°C (as stipulated)gave the refractory a strength that was wholly lacking in the material cured at 400°C. The lining, therefore, wasinadequately prepared for the rigour of a smelting campaign.That said, however, a brick of fused-spinel from the freeboard of the second LBFS campaign showedspalling (Figure 11). Although slag temperatures in the bath were much cooler (cf. figures 1b and 1c), the2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS 375
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 and 8: MgO and Al 2 O 3 , it will dissolve
Page 9 and 10: MgO(Mg,Fe 2+ )(Cr,Al,Fe 3+ ) 2O 4ho
Page 11 and 12: of slag into the refractory is mini
Page 13: • An alloy phase containing high
Page 17 and 18: with a freeze lining, there were ti

Furnace refractory erosion - Mintek

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