PURPOSE: To fix noxious substance contained in heavy oil ash as unnoxious matter in slag, by charging the heavy oil ash to a blast furnace, and decomposing it in a high-temp. reducing atmosphere.

CONSTITUTION: Heavy oil ash captured by an electric dust precipitator or the like after the combustion of heavy oil is charged to a blast furnace for manufacturing pig iron or the like through its tuyere or throat. A carbon part and hydrocarbon in the charged heavy oil ash are burnt in a high-temp. reducing atmosphere, while ammonium sulfate (NH4)2SO4 is decomposed into NH4 and S in the same atmosphere. Said NH4 is further reductively decomposed into N2 and H2, and the S part is formed into polysulfide with alkali metals from CaS, etc. and fixed in slag. On the other hand, valuable metals such as Ni and V are reduced into a metallic state or lower oxides and dissolved in pig iron. In addition, trace elements such as Pb are made unnoxious by fixing them in blast furnace or steel-making slag.