Washing and grinding water can be cleaned in a clarification unit that uses the “Dortmund design principle”. Water is fed into the inner pipe and due to restraint guidance and the effect of gravity the fines are accelerated down­wards and clear water upwards, in an outer pipe. A thickener in the lower part of the tower keeps the fines from going hard. The sludge can be pumped away and offers the possibility of being returned to the batching process or further processed through a filter press.