Abstract: The alkalinity of the solids, slag and ash, which occur in a combustion plant such as an incinerator is utilized for the flue gas scrubber, while at the same time the storability and the alkaline flue gas washing stage are improved. For this purpose, the scrubbing liquor is removed from the tank of a flue gas scrubber for SO.sub.2, so that it can be used for the separate scrubbing of slag and ash in, for example, stirrer vessels. Subsequently, the slag is separated in a sieve according to particle size, and the fine portion, together with the ash, is supplied to a dewatering stage, from which they pass to a stirrer vessel, in order to be re-washed there with scrubbing liquor from tank emanating from a flue gas scrubber for HCl. In a subsequent washing stage, the solids are freed of chlorides. The scrubbing liquor from the dewatering stage is returned to the tank, while the scrubbing liquor from the settling tank is returned to another tank.