Abstract: Molten slag discharged from a metallurgical furnace and flowing down from a slag trough is atomized by air jet into a primary heat-exchanger of rotary cylindrical type and hot slag granules thus obtained and air are caused to pass in a co-current therethrough so as to exchange the heat of the slag to the air flow to obtain a primary hot blast, then the hot slag granules discharged from the primary heat-exchanger are supplied to a secondary heat-exchanger connected to the primary heat-exchanger where the hot slag granules are brought into contact with a counter flow of cold air supplied from outside the system, or with a counter flow of a mixture of the above cold air and a part of the primary hot blast exhausted from the primary heat exchanger to effect heat-exchange and to recover a secondary hot blast.