PURPOSE: To obtain steam and lumped bodies having high density and strength by transmitting the heat retained in the melt of blast furnace slag of a high temp. to a boiler by means of the lumped melt of the blast furnace slag which is solidified by heat exchange.

CONSTITUTION: A melt 1 of blast furnace slag and lumped bodies 2 of blast furnace slag of a low temp. are supplied to a rotary cylindrical vessel 10. Both are subjected to heat exchange in the furnace 10 whereby the lumped bodies having a uniform high temp. are obtained. The lumped bodies are supplied to a double cylindrical rotary furnace 20, where the bodies are broughtout into contact in counter current with powder and granules 3 of blast furnace slag at a low temp., whereby said bodies are heat exchanged to produce the lumped bodies 2 at a low temp. Said lumped bodies are circulated and the rest are removed to the outside of the system. The powder and granules 4 heated to a high temp. are introduced into the circulating air flow and the heat transmittion tubes 51AWC of a boiler 50 are heated by the resulted solid-gas mixture, whereby steam 8 is generated. Part of the powder and granules 3 cooled to a low temp. are supplied to the furnace 20 and the rest are removed to the outside of the system.