When slag solidified on a container, in a molten metal, and on the surface of a molten metal, residuals in the container and deposit therein are preheated, heated and melted during the refining and casting of a metal, fine powder of a thermosoftening waste plastic alone or fine powder of a mixture of such a plastic and a thermosetting waste plastic is used as a combustion and heating promoting agent. This enables the heating effect to be improved, and the natural gas expenses to be reduced greatly.
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