| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 101 === MgO-C bricks are widely used in steelmaking industry, usually as linings in basic oxygen furnace and electric arc furnace. They are also the best materials for lining the slag-line in steel ladles. Because the MgO-C bricks are subjected to corrosion by steel melt and molten slag, they flake off from furnace and ladle; therefore, are one of the consumable materials. This thesis investigated the mineral reactions in the MgO-C brick + slag system and addressed the association of these mineral reactions with the corrosion of the MgO-C bricks. The outcome provides critical references for prolonging the lifetime of the steel ladle MgO-C bricks.
SEM and EDS analyses on a MgO-C brick removed from ladle after ~70 times of steel refining process showed three different mineral assemblage zones between the slag–brick transition. They are, from closest to slag toward brick, decarburization zone, slag-side reaction zone, and brick-side reaction zone. The decarburization zone is characterized by relatively large volume of pore space, which has been commonly interpreted as vacancies resulted from carbon removal from the MgO-C bricks during heating. The pore space was filled by periclase originated as crystals from MgO vapor and as relicts from brick degradation. The simple mineral assemblage in the decarburization zone contrasts to the sophisticated ones in the reaction zone.
The slag-side reaction zone is composed of gelhenite, spinel, melilite, merwinite, monticellite and glassy matrix. The majority (9 of 18) of the glass composition analyses resulted in sub-equal amounts of Al2O3 and CaO (~39%) with ~20% SiO2 and 〈 1% MgO. This composition can be explained as addition of SiO2 and MgO derived from bricks into the molten slag that crystallized C3A and C12A7. Other glass compositions are characterized by lower Al2O3 contents of 22–8%. The association of these glass compositions and coexisting minerals was justified by relevant phase diagrams in the Al2O3–CaO–SiO2–MgO system. The observed mineral assemblage is consistent with crystallization from a melt with composition similar to the high Al2O3 glass in a general sequence of gelhenite spinel + melilite spinel + melilite + merwinite melilite + merwinite + monticellite. Melts with compositions similar to the ~10%-Al2O3 glass were in equilibrium with monticellite-containing mineral assemblages. Element mapping showed systematical increases in Al2O3, MgO, and SiO2 and a decrease in CaO contents toward brick. Based on the fact that such a variation pattern reflects changes in mineral assemblages, this feature is explained by two models: (1) crystallization from melts whose compositions subjected to varying extents of contamination from bricks, and (2) fractional crystallization from a compositionally homogeneous melt.
The brick-side reaction zone is dominated by spinel, fosterite and monticellite. Fosterite formed as a product of interaction between periclase and SiO2 vapor derived from oxidation of Si metal in the bricks. The fosterite then reacted with the slag-derived or brick-derived CaO components to form monticellite.
The formation mechanisms of minerals in the reaction zone lead to three lines of connections to brick degradation. (1) Interaction between periclase in the brick and the FeO and MnO components in the slag formed (Mg, Fe, Mn)O, which subsequently dissolved into the slag for lower melting point. (2) Partial melting of the interstitial phases in the sintered periclase led to gradual disintegration of brick. (3) Dissolution of minerals formed in the reaction zone facilitated brick degradation. These should be considered for extending the lifetime of the steel ladle MgO-C brick.
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