thickener pump 6 and a back flow stop 7 the second section 3 branches off downstream of said thickener pump and said backflow stop and merges into the positive displacement chamber 8 of a piston cylinder unit 9 the third section 4 extends between the branching off position and the mixer head 5 the housing 10 of this unit 9 having a second hydraulic chamber 11 wherein a piston 12 is guided a hydraulic pipe 15 coming from a hydraulic fluid supply container 13 and containing a high pressure metering pump 14 merges into this chamber 11 a return pipe 16 branches from this hydraulic pipe 15 between said high pressure metering pump 14 and said hydraulic chamber 11 containing a throttle valve 17 leading into the housing 18 of the mixer head 5 and ending at housing passage 19 the rod 20 of said control element 26 consisting of a hydraulic piston 21 driven in a hydraulic chamber 24 having feed pipes and delivery pipes 22 23 and a control piston 25 movable within it the control piston 25 movable in the mixing chamber bore filling said bore in the position illustrated and thereby sealing the injection openings 27 28 a second section 29 of said return pipe leads from the housing 18 from the rear of the housing 18 in the drawing back into the supply container 13 the control element 26.