PURPOSE: To save electric power by utilizing the Joule heat produced by the electric resistance of melted substances, by inserting an electrode into melted substances to a specified depth when combustion ashes and dust are melt treated.

CONSTITUTION: In order to change heavy metals and sulfides contained in city refuse combustion ashes and dust into completely harmless artificial minerals and metals, thery are melted by a three-phase Heroult type fixed electric furnace, capacity 300kw, for example. At this time, an electrode is buried into the melted substance to a depth of 10W300mm and electricity is supplied to it. Melting is operated effectively by utilizing the Joule heat due to the electric resistance of the melted substance. By this, electricity can be saved, and at the same time, the melted substance is not heated to an extremely high temperature because of the heat generation by electric resistance, so that the lifetime of the electric furnace can be lengthened.