PURPOSE: To enable effective recovery of waste gas heat energy, as of a steel mill, by the use of high-temperature exhaust gas with a specified O2 concentration in order to dry and grind solid fuel such as coal.

CONSTITUTION: The factory waste gas with a ten percent or less O2 concentration is led to the heat exchanger for heat recovery. Then, the waste gas now maintained at temperatures of 200W400°C is fed to the device which is designed to dry and grind solid reducing agents. Then, the waste gas now maintained at temperatures of 60W120°C is led to a dust collector to be made free of dust. This not only enables effective use of waste gas heat energy, but also provides ease of removal of dust from waste gas. Besides, this provides at low cost the hot blast required for drying and grinding solid reducing agents including coal.