The invention relates to a processing unit (1, 51) for a liquid washing medium contaminated with sulphur oxides and/or nitrogen oxides, comprising an evaporation stage (2, 2) for concentrating the active components of the washing medium by means of an evaporator (3) and/or by means of a heat exchanger (53), and comprising a collecting tank (7) connected to the evaporator (3) and/or to the heat exchanger (53). The invention is characterized in that the collecting tank (7) is configured as a crystallizer (9) for removing sulfur oxides from the washing medium by crystallization of a sulphate, in particular of potassium sulphate. The invention further relates to a separating device (61) for carbon dioxide having a corresponding processing unit (1, 51), and to a method for processing a washing medium contaminated with sulphur oxides and/or nitrogen oxides, wherein a corresponding processing unit (1, 51) is used.
