Abstract: The dew point of the exhaust gas is firstly reduced to a value in the range of 60.degree.-65.degree. C. Thereafter the exhaust gas flows through a washing and condensing apparatus. A part of the prepurified condensate is recirculated in a loop for carrying out the latter-mentioned washing. Thereafter the gas flows through a wet-deep bed filter apparatus. The sprinkler apparatus belonging thereto is fed again by a partly purified liquid. A drying chamber follows the wet-deep bed filter apparatus. The gas is then finally purified in an electro-filter plant. The condensate in the washing and condensing apparatus, to which also the washing liquid stemming from the sprinkler apparatus of the wet-deep-filter apparatus is added, is fed to a flotation plant. This condensate-liquid mixture has an increased temperature such that a better working in the flotation plant is obtained.