as can be inferred from fig1 an apparatus according to the invention has a stand 1 , in which an electric drive motor 2 is seated . the motor has a lower , first power take - off 3 and an upper , second power take - off 4 , which is driven by the motor 2 , by way of a gear 5 having an infinitely - variably - adjustable transmission . a grinding receptacle 6 is rotatably seated on the stand 1 by means of a lower bearing journal 7 in bearings 8 . the first power take - off 3 employs a belt drive 9 in driving the receptacle to rotate . a grinding roller 10 , which is mounted to a roller drive shaft 11 , is disposed in the grinding receptacle 6 . the drive shaft 11 is extremely sturdy , and is seated in two bearings 12 , 13 , which are spaced as far apart as possible in a bearing slide element 14 . in the upper region of the stand 1 , the bearing slide element 14 is seated in a guide 15 so as to be displaced with the grinding roller 10 in the direction 17 , which extends transversely to the essentially vertical , central longitudinal axis 16 of the drive shaft 11 . the drive shaft 11 , and therefore the grinding roller 10 , can be driven by the second power take - off 4 of the motor 2 by way of an articulated shaft 18 and a belt drive 19 , with the articulated shaft 18 being coupled to the second power take - off 4 by way of a sliding bearing 20 , so displacements of the bearing slide element 14 in the direction 17 can be compensated . the bearing slide element 14 can be moved into a desired position by hydraulically - actuatable adjusting drives 21 , e . g ., in the form of hydraulically - actuatable piston - cylinder drives , then secured or blocked in this position . the adjusting drives 21 are articulated to the stand 1 . they can also be actuated individually for reasons that will be explained below . the grinding receptacle 6 is approximately frustoconical , and is embodied to be rotationally symmetrical relative to an axis of rotation 22 extending through the bearing journal 7 . the receptacle has a bottom 23 and a frustoconical receptacle wall 24 , which widens toward the top . the grinding receptacle 6 has an upper opening 25 ; near this opening , on the outside of the receptacle wall 24 , is an annular , preferably cylindrical - ring - shaped , support surface 26 , against which support rollers 27 rest , the rollers being supported in turn in the stand 1 . a grinding stock supply line 29 leads into the interior 28 of the grinding receptacle 6 , namely in the open region between the bottom 23 and the grinding roller 10 . provided in the region of the opening 25 is a grinding stock discharge 30 , the discharge surrounding the opening 25 . the material to be ground is guided through the line 29 in the supply direction 31 , and is carried off in the discharge direction 32 . the inside surface of the receptacle wall 24 forms a polished , frustoconical , first grinding surface 33 . the surface of the grinding roller 10 also forms a polished , cylindrical , second grinding surface 34 . the axis of rotation 22 of the grinding receptacle 6 and the axis 16 of the grinding roller 10 form an angle a , which corresponds , entirely or at least essentially , to one - half of the opening angle of the frustoconical first grinding surface 33 . the two grinding surfaces 33 and 34 can also be slightly convex . also provided on the stand 1 is a sensor 35 , which can be used to detect the fullness level 36 of the material in the grinding receptacle 6 . the signals of this sensor 35 are transmitted to a central control 37 , which influences the drive motor 2 , the gear 5 , and thus the speeds of the grinding receptacle 6 and the grinding roller 10 . this central control 37 also actuates a grinding stock pump 38 . the control 37 is freely programmable . the first grinding surface 33 and the second grinding surface 34 define a grinding gap 39 , which extends essentially parallel to the axis 16 , as can be seen in fig1 and 2 . the overlap of the first grinding surface 33 and the second grinding surface 34 in the direction of the axis 16 , that is , the length b of the grinding gap 39 , extends essentially from the bottom 23 to the opening 25 of the grinding receptacle 6 . the diameter c of the grinding roller 10 is always smaller than the projection of the diameter d of the inside bottom surface 40 onto the diameter c . the following applies : c & lt ; d × cos a . the following preferably applies : the adjusting drives 21 have moved the bearing slide element 14 , with the drive shaft 11 and the grinding roller 10 , into a position in which the grinding gap 39 has a desired width and , if applicable , a desired extension , the grinding receptacle 6 is driven in the direction of rotation 41 , and the grinding roller 10 is driven in the direction of rotation 42 — in other words , as can be seen in fig3 in the same direction — so the material is conveyed through the grinding gap 39 in the same conveying direction 43 . as can be seen from fig1 and 2 , the fullness level 36 of the grinding stork is very low , so the grinding roller 10 only dips slightly into the material . when the grinding receptacle 6 and the grinding roller 10 are inoperative , therefore , the majority of the grinding gap 39 is located above the fullness level 36 of the material . upon rotary actuation , the frustoconical widening of the first grinding source 33 toward the opening 25 located at the top causes the grinding stock to be conveyed upward on helical paths 44 ; consequently , the material passes through the grinding gap 39 multiple times until it reaches the opening 25 , and thus the discharge 30 . for cleaning purposes , a closable outlet 45 is provided in the bottom 23 . because of the at least essentially frustoconical embodiment of the first grinding surface 33 and the at least essentially cylindrical embodiment of the second grinding surface 34 , relative speeds exist in the grinding gap 39 between the grinding surfaces 33 and 34 ; in theory , identical circumferential speeds of the grinding surfaces 33 and 34 can dominate at a single location . this is the case when a & gt ; 0 , that is , when the first grinding surface 33 is actually frustoconical and the second grinding surface 34 is cylindrical . in general , 0 & lt ; a ≦ 45 °. preferably , 10 °≦ a ≦ 30 °. if the first grinding surface 33 is also cylindrical , that is , a = 0 , which is entirely possible , the above - described conveying actions still take place , i . e ., the grinding stock moves upward toward the opening 25 in an approximately helical movement , relative to the first grinding surface 33 . generally , the relative speeds can be significantly influenced if the grinding receptacle 6 and the grinding roller 10 are driven at different , variable rpms . the angle a can be modifiable such that the width of the grinding gap 39 changes over its length b . for example , the arrangement can be such that the grinding gap 39 has its greatest width downwards , i . e ., where it begins in the vicinity of the bottom 23 , and the width continuously decreases toward the top . this can be achieved in that the grinding receptacle 6 is seated to pivot by a small angle in the region of its bearing 8 . in particular , this can be achieved simply by actuating the adjusting drives 21 slightly differently , which effects a corresponding change in the width of the grinding gap 39 over its length b . because such changes in the width of the grinding gap 39 over its length b lie within a range of thousandths of millimeters , this can be achieved by the corresponding different actuation of the adjusting drives 21 based on the unavoidable play present in the guide 15 . in addition , changes in the width of the grinding gap 39 with a frustoconical embodiment of the first grinding surface 33 can also be effected by the relative displacement of the grinding receptacle 6 and the grinding roller 10 in the direction of the axis 16 . the width of the grinding gap 39 lies in a range of 3 to 500 μm preferably in a range of 5 to 50 μm .