Surplus concrete or return concrete to be processed is poured into a high-capacity receiving bin and is transferred at a metered rate from said bin by means of a screw conveyor to a washing apparatus, in which fine components are separated from very fine components. The transfer at the metered rate is effected in dependence on the driving power of rotary motors of the rotary washing apparatus. When the driving power of the motors exceeds a predetermined value, then the motor of the screw conveyor is deenergized. When the driving power decreases below that value or below another value, the supply at the metered rate is resumed. The plant operates fully automatically and when the receiving bin has been emptied the plant is automatically cleaned and automatically shut down.