The rate of MnO reduction from silicate slag melted in a graphite crucible as a reductant has been investigated by means of a thermo-balance. At small values of MnO activity a_MnO, a large portion of the produced molten Mn was evaporated, and the rate of Mn evaporation was nearly equal to that of MnO reduction. In particular, at the value of a_MnO less than 0.1, the reaction was of the first order in regard to a_MnO, and the activation energy was 375 kJ/mol. It was supposed that the reaction at the slag-graphite interface proceeded by the direct reduction, (MnO)+C(s)=Mn(l)+CO(g).
At large values of a_MnO, reduction was greatly accelerated by the presence of liquid Mn, because the following reactions occurred at the slag-metal interface:
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\ oindentThis finding was observed mainly in the MnO-SiO₂ binary slags.
It was found that the reaction rate was markedly dependent on the wetting of slag on the graphite crucible wall. In general, with lower a_MnO, the slag gave immersional wetting on the graphite, whereas with higher a_MnO, it gave adhesional wetting. In the latter, the reduction rate was by 1 or 2 orders of magnitude lower than that in the former. These results were explained by the assumption that the wettability of the slag on a graphite crucible was intimately related to the formation of SiC at the slag-graphite interface.
It might be inferred that the reduction rate was controlled by chemical reactions.