During the operation of a basic oxygen furnace (BOF)，slag that contains a high (％MgO) is formed by the addition of dolomite to decrease the dissolution rate of the refractory. Nevertheless, the refractory has a limited lifespan because the dissolution of MgO from the dolomite and refractory occurs competitively. For the efficient protection of the refractory, MgO has to be supplied from the flux at a rate higher than the dissolution rate of the refractory. To clarify the factors that affect the dissolution rate of MgO, various MgO-containing solid oxides were melted in liquid slag. The dissolution rates of the oxides were evaluated by calculating the change in the MgO content of the liquid slag, and the following results were obtained : (1) The dissolution rate of sintered MgO in MgO-SiO2-B2O3 slag was much higher than that in FeO-CaO-SiO2 slag. A solid solution of MgO and FeO was formed at the interface of the FeO-CaO-SiO2 slag and sintered MgO. (2) The dissolution rate of a MgO-C brick was higher than those of the burnt dolomite and sintered MgO ; moreover, the dissolution rate of decarburization slag was much higher than those of them. These results indicate that the coating layer of the decarburization slag on the refractory easily melts; however, the addition of the decarburization slag is effective in increasing the MgO content of the slag.

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