to each other around the periphery of inner wall 39 by half the distance between two plates 43 or 44 in one cylinder section 40 or 41 shovel like plates 43 as viewed from the outside to the inside are pitched in the rotational direction 45 of the mixing drum with respect to the corresponding radius so that a pocket 46 is formed between plate 43 and 44 and the associated section of inner wall 39 of mixing drum 1 as the drum moves in rotational direction 45 a specific quantity of wood chips is scooped up from the wood chip bed 47 in the lower part of mixing drum 1 into such a pocket 46 these wood chips in pocket 46 flow over the inner edge 48 of plate 43 44 as the drum rotates in the form of a relatively thin parabolic layer 49 50 back into wood chip bed 47 as may be seen from fig2 a wood chip layer 49 from the beginning of emptying of a pocket 46 former by a plate 43 until complete emptying migrates transversely through the mixing drum 1 so that between this layer 49 and the part of inner wall 39 turned toward it on the left in fig2 a space 51 is produced which enlarges as the pocket empties in which space a certain negative pressure arises as it steadily increases in size as may be seen from fig3 because of the staggered arrangement of plates 43 44 in adjacent cylinder sections 40 41 or 41 42 the layers 49 50 which form are also staggered with respect with one another so that the negative pressure developing in the individual spaces 51 behind layers 49 50.