10 11 12 13 and 14 the foam which arises in the flotation plant 6 is fed via a pipeline 15 to a centrifuge 16 in this centrifuge water which is to be recycled is obtained and this water is fed via a pipeline 17 back to the flotation plant 6 thickened sediment from centrifuge 16 is collected in a container 18 the hardening of the water is carried out in a mixing container 19 which is supplied with the water to be hardened via a pipeline 20 in the container 19 the water reacts with calcium carbonate and carbon dioxide the calcium carbonate passes from a silo 21 via a pipe 37 into the mixing container 19 the carbon dioxide passes from a tank 22 via a treatment device 23 and a blower 24 through the pipeline 36 to the mixing container 19 the foam which forms in the mixing container 19 is fed to a cyclone 25 in which the carbon dioxide is degasified out of the liquid and returned to the treatment device 23 in particular the closure of the carbon dioxide cycle is of economic significance the liquid phase passes into a flotation apparatus 26 for the separation of dirt and suspended particles the dirt concentrated in the foam is expelled as.