along the mixing chamber 14 and the delivery duct 14 u2032 slides a cleaning member 16 substantially provided by an extension of the rod of the piston 17 of an intermediate hydraulic control cylinder 18 which forms part of the mixing device 10 on the side opposite to the cleaning member 16 the piston rod 16 extends with a rear portion 16 u2032 which operates in a per se known way a limit switch or a sensor 19 screwed into a housing 20 to provide a control signal at the opening of the mixing chamber 14 still with reference to the example shown in fig3 it can be seen that each injection nozzle 15 communicates with a respective pipe fitting 23 a for connection to a feed pipe for a chemical component by means of a duct 21 a in the body 13 of the mixing device which extends rearward with a duct 21 b in the peripheral wall of the hydraulic cylinder 18 towards a manually operable on off valve member 22 seated in the support plate 11 to stop the flow of the component whenever the mixing device 10 must be removed and or replaced the valve members 22 are in the form of a cylindrical plug rotatingly supported by the body 11 of the valve unit and provided with a cross hole 24 to connect or respectively to close each pipe fitting 23 a in the direction of the feed duct 21 b and the respective injection nozzle each valve member 22 can be manually operated by inserting an appropriate tool for example by inserting the tip of a screwdriver in a slot through a hole 25 provided in a ring 26 outside of the body 11 for retaining all the on off valve members 22.