Abstract: A process for treating a flue gas containing sulfur dioxide and fly ash is disclosed in which an aqueous absorbent liquid is sprayed into the glue gas stream, and the resulting gas is fed to a vertical, open-ended pipe to form a jet annular stream, so that portions of the SO.sub.2 and the fly ash contained in the flue gas are transferred to the liquid phase. The jet annular stream is then sparged into the aqueous absorbent held in a well-mixed vessel to form a jet bubbling layer in a shallow, upper region of the aqueous absorbent, so that a greater part of the SO.sub.2 and the fly ash are removed from the gas. Air is introduced into a deep lower region of said aqueous absorbent to provide streams of fine oxygen-containing gas bubbles throughout the whole vessel, so that the absorbed SO.sub.2 and other sulfites are oxidized to form coarse gypsum crystals while COD of the absorbent is reduced.