Abstract: Molten slag fresh from a smelting or refining furnace is ladled into a pan on a wheeled carriage and is thereby transported to a first heat recovery station, where water is heated by radiation from the slag while the latter is being agitated and granulated. On granulation the slag is conveyed as by a skip hoist to a second heat recovery station, where the residual heat of the slag is utilized to heat air by convection. The heated air is directed into a utility device such as a boiler, from which the cooled air is fed back to the second heat recovery station for reheating. Several embodiments are disclosed.