Abstract:
An agitator mill with a grinding receptacle comprises a grinding chamber defined by an inner wall thereof and a rotor which is located therein and driven in high-speed actuation. Mounted on the rotor are agitator elements which stand out radially and to which are allocated counterpart elements mounted on the inner wall. A grinding-stock inlet is provided on an end of the grinding chamber and a grinding-stock outlet is provided on the other end of the grinding chamber, comprising an auxiliary-grinding-body separating device for the auxiliary grinding bodies in the grinding chamber to be held back. The distance of agitator elements and counterpart elements which adjoin each other decreases from the grinding-stock inlet towards the auxiliary-grinding-body separating device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an agitator mill comprising a grinding receptacle, which has a cylindrical wall, a bottom, and a cover; a grinding chamber formed in the grinding receptacle; an agitator unit, which has a driving shaft, which is disposed substantially outside the grinding chamber and driven in rotation, and a rotor, which is located in the grinding chamber and connected to the driving shaft; agitator elements mounted on the rotor, which are disposed in radial planes radially of the central longitudinal axis, and which project into the grinding chamber radially of the central longitudinal axis towards the wall of the grinding receptacle; counterpart elements mounted on the wall of the grinding receptacle, which are disposed in radial planes radially of the central longitudinal axis, and which project towards the rotor into the grinding chamber; a grinding-stock inlet, which leads through the bottom into the grinding chamber; an auxiliary-grinding-body separating device disposed in the vicinity of the cover and which is disposed upstream of a grinding-stock outlet; and a filling of auxiliary grinding bodies, with a diameter a, in the grinding chamber. 
     2. Background Art 
     Agitator mills of the generic type pose the fundamental problem that the auxiliary grinding bodies are entrained in the grinding-stock flow direction and accumulate in front of the separating device, which may lead to impairment of the grinding and dispersing process and, in the extreme, to the agitator mill being blocked. 
     For avoidance of the mentioned effect, U.S. Pat. No. 4,848,676 teaches, in the case of an agitator mill of the generic type, to provide a device for detection of the distribution of the auxiliary grinding bodies in the grinding chamber, which comprises a measuring point for detection of the pressure drop in the grinding chamber as compared to atmospheric, this measuring point sensing the grinding-stock pressure directly before the grinding chamber, with transgression of a given pressure drop constituting a measure for a concentration of auxiliary grinding bodies at the grinding-stock inlet and in front of the separating device, respectively. Furthermore, a device is provided for the regular distribution of the auxiliary grinding bodies in the grinding chamber, the effect of which resides in a reduction of the mass flow when the auxiliary grinding bodies concentrate in front of the separating device. This design has proved very successful, but it requires some implementation in terms of measuring and regulation. 
     DE 32 45 825 A1 teaches to provide a device in an agitator mill, which selectively exercises, at least substantially only on the auxiliary grinding bodies, a force in the opposite direction of the flow of grinding stock. This is to prevent the auxiliary grinding bodies from migrating before the separating device. For detection of auxiliary grinding bodies that arrive before the separating device, a pressure sensor is provided, by means of which the pressure of the auxiliary grinding bodies is sensed in this area. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody an agitator mill of the generic type in such a way that accumulation of the auxiliary grinding bodies before the separating device is at least largely precluded by simple means. 
     According to the invention, this object is attained by the features which consist in that the distance from each other of agitator elements and adjacent counterpart elements in the direction of the central longitudinal axis decreases from a greatest distance c in the vicinity of the grinding-stock inlet to a smallest distance b in the vicinity of the auxiliary-grinding-body separating device. Reducing the distance of agitator elements from neighboring counterpart elements towards the separating device helps obtain a higher local shear intensity and thus an increased power density as the distance decreases. As the auxiliary grinding bodies tend to avoid the status of increased power density, they migrate to the area of lower power density, i.e. to the area where the agitator elements have greater distances from the neighboring counterpart elements. This effect is of special importance when a very tough i.e., very viscous, grinding stock is ground. The grinding i.e., the comminution, of the grinding stock particles, increases the surface thereof—related to their mass. This increases the toughness of the grinding stock in the flow direction. The growing toughness leads to increasingly higher entrainment forces in the flow direction which would give rise to an accumulation of auxiliary grinding bodies before the separating device. This effect is opposed by the described measures. The comminution effect intensifies as the grinding process progresses during the flow of the grinding stock through the grinding chamber. 
     Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagrammatic illustration of a side view of an agitator mill; and 
     FIG. 2 is a longitudinal section through the grinding receptacle of the agitator mill. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The agitator mill seen in FIG. 1 customarily comprises a stand  1  which supports itself on the ground  2  and to which is fixable a cylindrical grinding receptacle  3  of vertical arrangement. An electric driving motor  4  lodges in the stand  1 ; it is equipped with a V-belt pulley  5 , by which a V-belt pulley  8 , which is non-rotatably joined to a driving shaft  7 , is drivable in high-speed rotation. 
     As seen in particular in FIG. 2, the grinding receptacle  3  comprises a cylindrical inner wall  10  which envelops a grinding chamber  9  and which is enveloped by a substantially cylindrical outer jacket  11 . Between them, the inner cylinder  10  and the outer jacket  11  define a cooling chamber  12  which is connected to a coolant inlet  13  and a coolant outlet  14 . Downwards the grinding chamber  9  is finished by a circular bottom plate  15  which is mounted on the grinding receptacle  3  by means of screws  16  (roughly outlined). 
     The grinding receptacle  3  has an upper annular flange  17 , by means of which it is mounted on the underside of a carrying housing  18  via screws  19  (also roughly outlined); this carrying housing  18  is mounted on the stand  1  of the agitator mill. The grinding chamber  9  is closed by means of a cover  20  which is secured by means of screws  21  (also roughly outlined). 
     The carrying housing  18  comprises a central bearing and sealing housing  22  which is disposed coaxially of the vertical central longitudinal axis  23  of the grinding receptacle  3 . The driving shaft  7 , which equally extends coaxially of the axis  23  and on which is mounted an agitator unit  24 , passes through this bearing and sealing housing  22 . 
     The agitator unit  24  has a cylindrical rotor  25  which comprises a cylindrical outer wall  26  and a inner wall  27  which is disposed coaxially thereof; between them, the walls  26  and  27  define an annular cylindrical cooling jacket  28 . On its lower side that faces away from the driving shaft  7 , the rotor  25  is closed by a rotor bottom  29  which also finishes the cooling jacket  28  downwardly. On its upper side that is adjacent to the shaft  7 , the rotor  25  is closed by a rotor cover  30 , on which also the shaft  7  is mounted. 
     Through a coolant supply pipe  31 , coolant is supplied to the cooling jacket  28 . Coolant discharge takes place through an annular coolant discharge channel  32  which is formed between the shaft  7  and the supply pipe  31 . For the supply and discharge of coolant, a rotating joint  33  is attached to the upper end of the driving shaft  7 . 
     In the vicinity of the axis  23 , the rotor bottom  29  is bulged concavely into the rotor  25 . This is where a grinding stock inlet  34 —coaxially of the axis  23 —opens into the lower grinding stock supply area  35 , substantially in the shape of an annular disk, of the grinding chamber  9 . Between the inner wall  10  and the outer wall  26 , the grinding chamber  9  comprises an annular cylindrical grinding area  36  in which the grinding process takes place substantially. A grinding stock outlet  37  leads from this grinding area  36  through the cover  20  of the grinding receptacle  3 . Disposed between the grinding area  36  and the grinding stock outlet  37  is an auxiliary-grinding-body separating device  38 , which is a so-called annular separating gap. It comprises a stationary annular disk  39  which is mounted on the cover  20  and an annular disk  40  which is mounted on the rotor cover  30 , rotating together with the rotor  25 , these two annular disks  39 ,  40  defining, between them, a separating gap  41  which connects the grinding area  36  with the grinding stock outlet  37  and which, in a manner known per se, has a width that corresponds maximally to half the diameter a of the auxiliary grinding bodies  42  available in the grinding chamber  9 . An auxiliary-grinding-body feed opening  44  is formed in the cover  20  and can be closed by means of a plug  43 . An auxiliary-grinding-body evacuation opening  46  is formed in the bottom plate  15  and can also be closed by a plug  45 . As for the ratio that the diameter D of the inner wall  10  bears to the diameter d of the rotor, d≧0.5 D applies. 
     The outer wall  26  of the rotor  25  is equipped with annular cylindrical, pin or rod-shaped agitator elements  47  which project therefrom radially towards the inner wall  10 . Several agitator elements  47  are disposed on the periphery of the rotor  25  in a radial plane relative to the axis  23  i.e., in a horizontal plane; in the direction of the axis  23 , agitator elements  47  are disposed in several radial planes. Mounted on the inner wall  10  of the grinding receptacle  3  are counterpart elements  48  of the same kind as the agitator elements  47 , which likewise project towards the rotor  25  and are disposed in radial planes i.e., horizontal planes, with the counterpart elements  48  of a radial plane being centered between the neighboring agitator elements  47  in the neighboring radial planes. The rod-shaped agitator elements  47  and the counterpart elements  48  overlap considerably in the radial direction. Fundamentally, the agitator elements and the counterpart elements may have any shape available in practice. 
     As seen in FIG. 2, the axial distance of the agitator elements  47  one the one hand and thus also that of the counterpart elements  48  on the other hand decreases in the grinding area  36  in the flow direction  49  i.e., from the grinding-stock supply area  35  towards the auxiliary-grinding-body separating device  38 . The smallest distance b in the flow direction  49  and in the direction of the axis  23 , respectively, of the last agitator elements  47  disposed in a horizontal radial plane before the separating device  38 , from the last adjacent counterpart elements  48  is not smaller than three times the diameter a of the auxiliary grinding bodies  42  used. Consequently b≧3a applies. The greatest distance c of the lowermost agitator elements  47 , which adjoin the grinding-stock supply area  35 , from the equally lowermost i.e., neighboring, counterpart elements  48  should be at least 1.5 times greater than b. In this regard c≧1.5b applies. c≦3b should apply to a practically oriented ratio. For clarification, the distances are exaggerated in FIG.  2 . As regards the diameter a of the auxiliary grinding bodies  42 , 0.1 mm≦a≦6 mm applies. In particular in the case of an annular separating gap  38 , 0.5 mm≦a applies preferably. In particular within this range of diameter, the auxiliary grinding bodies  42  of a filling of a grinding chamber  9  have substantially the same diameter. 
     The decrease in distance from c to b can be continuous—as seen in FIG.  2 . However, the annular cylindrical grinding area  36  may also be divided into three subsequent zones by the agitator elements  47  and the counterpart elements  48  having the same distance c from each other in the lowermost section that adjoins the grinding-stock supply area  35 . In the third section that adjoins the separating device  38 , they also have the same distance b which corresponds to the shortest or smallest distance. In the middle section, they have a mean distance that ranges between the greatest distance c and the smallest distance b. As seen in FIG. 2, the described distances b and c are illustrated as the clear distance of an agitator element  47  from an axially adjacent counterpart element  48 . 
     High-speed actuation of the agitator unit  24  takes place by means of the driving motor  4  so that the auxiliary grinding bodies  42  are subject to intense acceleration pulses from the agitator elements  47 . The auxiliary grinding bodies are braked by the counterpart elements  48  so that intense motions take place of the auxiliary grinding bodies  42  relative to each other and relative to the individual grinding-stock particles. This results in intense grinding and dispersing effects. Reducing the distance of the agitator elements  47  from the counterpart elements  48  towards the separating device  38  i.e., towards the grinding-stock outlet  37 , helps obtain higher local shear intensity, and thus higher power density, as the distance decreases from c to b. The auxiliary grinding bodies  42  tend to avoid the status of higher power density, consequently they migrate to the area of lower power density i.e., to the area of greater distances c of the agitator elements  47  from the adjacent counterpart elements  48 . This effect is of special importance when very tough i.e., very viscous, grinding stock is ground. The grinding i.e., the comminution, of the grinding-stock particles causes their surface to grow, related to their mass. As a result, the toughness of the grinding stock increases in the flow direction  49  from the grinding-stock supply area  35  to the grinding-stock outlet  37 . Owing to the growing viscosity, increasingly higher entrainment forces occur in the flow direction  49 , the result of which would actually be an accumulation of auxiliary grinding bodies  42  before the separating device  38 . This effect is opposed by the described effects that are produced by the modification of the distance of the agitator elements  47  from the counterpart elements  48 ; this effect is compensated.