2 are in communication with separate collection chambers in such a case the closing device 4 is mounted on the rotary shaft 310 in external position on the drum 1 the support 44 of the cup spring 43 is a collar mounted on the shaft 310 the spring 43 supports the ferromagnetic element 41 whereon the plug 40 is mounted to open and close the outlet of the heavy phase u 2 the electromagnetic coil 42 is mounted on a support 50 fixed to the fixed structure 51 of the machine the plug 40 is normally open fig8 when the coil 42 is electrically powered when the coil 42 is not electrically powered the ferromagnetic element 41 is pushed together with the plug 40 towards the outlet holes of the heavy phase u 2 because of the spring 43 fig1 and 11 disclose an improved decanter 400 provided with outlet holes of the light phase u 1 on the flange 319 connected to the rotary shaft 310 if any the outlet of the heavy phase u 2 is obtained by means of a radial pipe in association with an obturating disk 480 inside the drum disposed immediately upstream said pipe in the outlet flow said decanter 400 is provided with a service opening u 4 situated in the end flange of the drum in peripheral position with respect to the outlet of the light phase u.