in the flake compartment 2 39 in the lower zone 2 34 miscella compartment two connection pieces 5 7 are provided the positions of these connections being of special importance for the operation of the novel apparatus connection piece 5 is connected to a pipe duct 17 see fig1 by which returned extraction solution miscella or also additional solvent l is brought into the miscella compartment 2 34 connection piece 7 connects the mixing zone or the miscella compartment 2 34 with a return duct 21 leading to a tank 16 fig1 in the interior of the mixing zone said connection piece 7 opens into an adjustable or controllable overflow 23 the position of which at any one time determines the level 12 39 12 34 and ensures that the space between the screw base 12 and the line 12 39 is constantly filled with extraction mass suspension the mixing screw 1 is in the form of a double screw 1 1 39 with intermeshing screw threads 3a the screw pitch 3 is of varying magnitude in zone 3b i e the zone where the feeding and equalizing vessel is mounted on the screw in this zone the pitch increases in the direction towards the discharge end 14 of the screw either by groups that means that the screw turns are arranged.