11 and of the lower part 12 of the digester through the material advancing in the spaces between adjacent blades 8 securing thereby a uniform treatment of this material in the course of its passage through the digester arrows 16 are showing the directions of this passage fig4 shows in top view side view and cross section an exemplary embodiment of a blade 8 of the blade conveyor 6 the blade 8 is made either of sheet material or of cast material and has advantageously in cross section a slightly cranked shape a number of openings 17 are provided over the whole surface of the blade 8 the diameter of these openings being generally between 7 and 12 mm and their distance between 30 to 60 mm in order to prevent clogging of these openings 17 it is advantageous to make these openings not cylindrical but as shown in fig5 conical or as shown in fig6 as double cones narrowing towards the center of thickness of the blade the size of these openings depends of course on the size of chips which are treated and their diameter should be smaller than the minimum diameter of chips this arrangement operates satisfactory as long as the treated material does not disintegrate in the course of its passage through the digester for instance in the case of an aqueous initial hydrolysis of wood chips if however the treated material is partly disintegrated or contains small particles this arrangement would fail as the particles of material would pass together with the boiling liquid through these openings and the boiling conditions would be.