Molten slag in a vessel or vessels is placed below heat absorbing tubes through which water is passed and converted into steam by the heat radiated by the slag of high temperature, the heat thus being recovered by utilizing the steam for heating purposes, or for driving a turbine or the like. A cover is placed over the tubes and the vessel or vessels to increase the heat transfer efficiency. Any cooled slag layer formed on the surface of the molten slag in the vessel is removed by skimming to promote the heat transfer and, after the temperature of the slag in the vessel has lowered to about 1,200 C, the slag is agitated to further promote the heat transfer. Alternatively, the slag in the vessel or vessels is agitated from the beginning until its temperature has lowered to about 700 C.
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