Abstract: A method is proposed for processing high-ash coal sludges by flotation of a slurry in the cells of flotation units, particularly for processing gas coal and open burning coal which are difficult to float, in which the coal slurry to be processed flows through the cells of the flotation unit pre-conditioned and controllably, particularly with control of the dwell time. In a preferred embodiment, the control of the dwell time occurs by a controlled distribution of the slurry to cells of the flotation unit which operate in parallel. For the purpose of controlling the dwell time of the slurry in a flotation unit, cells which are traversed in parallel are additionally connected or disconnected as a function of operating parameters, such as slurry density or solids content or solids distribution.