PURPOSE: To obtain a white ash free form a ferrous component contained in coal ash and at the same time, facilitate the treatment of coal ash by allowing the coal ash discharged due to the burning of coal to bring into contact with hydrogen chloride gas or a chloride gas at least a specified temperature.

CONSTITUTION: A white ash is obtained by removing a ferrous component contained in coal ash by allowing the coal ash discharged due to burning of coal to bring into contact with hydrogen chloride gas or a chloride gas at least 700°C. Alternatively, the coal ash, previously mixed with carbon, may be brought into contact with chlorine gas. In addition, the coal ash after coming into contact with hydrogen chloride gas is exposed to air or oxygen so that the carbon content is removed by burning. Thus the unburnt carbon and ferrous component contained in the coal ash are removed to obtain a finely white ash easily instead of the normally produced coal ash.