Abstract: An apparatus for removing SO.sub.2 from flue gas treats the hot flue gas with a sump liquid to quench the flue gas and saturate it with water while concentrating ammonium sulfate in the recycled liquid. The gas is then scrubbed in a packed scrubbing zone with a solution of ammonia water, untreated recycled ammonium sulfite and ammonium hydrogen sulfite, and ammonium sulfate solution from an oxidizer to which a partial stream of the scrubbing liquid is fed.

Abstract: SO.sub.2 containing flue gas is scrubbed with a scrubbing solution made up from ammonia water, untreated ammonium sulfate scrubbing solution recycled from the bottom of the scrubbing zone and ammonium sulfate solution formed by oxidizing a portion of the recovered ammonium sulfite scrubbing solution. The ammonium sulfite/ammonium sulfate mixing ratio is maintained at 15:1 to 3:1. The ammonium sulfite containing scrub liquid in combination with aqueous ammonia and an ammonium sulfate solution which has undergone oxidation can have an unusually high sulfur dioxide absorption capacity while minimizing the formation of aerosols.