Abstract: SO.sub.2 is absorbed from hot flue gas by spray drying a Ca(OH).sub.2 -containing suspension in the flue gas. Fly ash is left in the flue gas which is to be treated in the spray absorption process, and the powder which is produced by the spray absorption process and which consequently contains the fly ash and partly reacted Ca(OH).sub.2 is partially recycled. Operation is controlled to obtain a temperature of the flue gas after the treatment which is 8.degree.-20.degree. C. above the saturation temperature of the flue gas at this stage. The process leads to optimum use of the Ca(OH).sub.2 used as absorbent and of the neutralization power inherent in the fly ash. Problems due to sedimentation of the absorbant before its atomization are avoided.

Abstract: Mercury is removed from a stream of hot waste gases by atomizing an aqueous liquid into the hot waste gases in the presence of fly ash suspended in the gas stream. Subsequently the fly ash is separated from the gas stream. A substantial part of the mercury originally present in the gas stream is absorbed or adsorbed by the fly ash so that the stream of the waste gas can safely be discharged to the atmosphere.