Patent Publication Number: US-7216822-B2

Title: Agitator mill

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an agitator mill for treating free-flowing grinding stock, comprising a grinding receptacle which defines a substantially closed grinding chamber by means of an inner wall; and an agitator which is rotarily drivably disposed therein and which is cup-shaped in relation to a common central longitudinal axis, having an annular cylindrical rotor which has a closed wall; and an interior stator which is disposed within the rotor and fixedly joined to the grinding receptacle; wherein an annular cylindrical exterior grinding chamber in the form of an annular gap is formed between the inner wall of the grinding receptacle and an outer wall of the rotor, the exterior grinding chamber having a radial gap width; and an annular cylindrical interior grinding chamber in the form of an annular gap is formed between an inner wall of the rotor and an outer casing of the interior stator, the interior grinding chamber being arranged coaxially within the exterior grinding chamber and connected thereto via a deflection chamber and having a radial gap width h; wherein the exterior grinding chamber, the deflection chamber and the interior grinding chamber constitute the grinding chamber which is partially filled with auxiliary grinding bodies; wherein a grinding-stock supply area, which is disposed upstream of the exterior grinding chamber and opens into it in the direction of flow of the grinding stock, and a separator device, which is disposed downstream of the interior grinding chamber in the direction of flow, are disposed approximately on the same side of the grinding receptacle for the grinding stock to pass through; wherein auxiliary-grinding-body return conduits are provided in the agitator for returning the auxiliary grinding bodies from the vicinity of the separator device into the grinding-stock supply area, the return conduits connecting the end of the interior grinding chamber to the beginning of the exterior grinding chamber; and wherein the inner wall of the grinding receptacle and the outer wall and the inner wall of the rotor are free of interruptions, and the inner wall of the grinding receptacle and the outer wall of the rotor are smooth and free of agitator implements. 
   2. Background Art 
   In an agitator mill of the generic type known from U.S. Pat. No. 5,950,943 the interior grinding chamber as well as the exterior grinding chamber are smooth-walled without any interruptions and free from agitator elements. The gap width i.e., the radial extension of the exterior grinding chamber, distinctly exceeds that of the interior grinding chamber. This is meant to accomplish that grinding and dispersing the free-flowing, slurried grinding stock takes place predominantly by shearing effects in such a way that the local intensity of strain on the grinding stock is substantially constant throughout the entire grinding length of path. The smooth-walled design of the cylindrical boundary walls of the exterior grinding chamber and the interior grinding chamber produces a flow in which the auxiliary grinding bodies are moved relative to each other in layers. The shearing gradient and thus the local intensity of strain is constant over the respective grinding-chamber height in the exterior grinding chamber on the one hand and in the interior grinding chamber on the other. With the gap width of the interior grinding chamber being smaller than the gap width of the exterior grinding chamber, the shearing gradient can be made equal in the exterior grinding chamber and in the interior grinding chamber; it is then virtually constant throughout the grinding chamber. Problems have turned out to be posed by the fact that start-up of the agitator mill is difficult in the case of a high auxiliary-grinding-body fill factor. Because of its start-up problems the agitator mill is operated at a reduced auxiliary-grinding-body fill, which again leads to unfavorably rough distribution in grinding-stock particle size. Since this reduction of auxiliary grinding bodies reduces the amount of auxiliary grinding bodies that return through the auxiliary-grinding-body return conduits, there is an increase in the risk of so-called grinding-stock shooting flow i.e., grinding stock that has been supplied to the grinding-stock supply chamber for being ground or dispersed may short-circuit through the auxiliary-grinding-body return conduits towards the separator device. 
   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 start-up of the agitator mill is facilitated and fine distribution of grinding-stock particle size is obtained. 
   According to the invention, this object is attained by g&lt;h applying to the radial gap width g of the exterior grinding chamber in relation to the radial gap width h of the interior grinding chamber. The measures according to the invention help to ensure that, when the agitator mill is switched off, the auxiliary grinding bodies that deposit downwards do not stick together with the adjacent walls in particular in the interior grinding chamber. Upon start-up of the agitator mill, the auxiliary grinding bodies can therefore be set moving easily. The measures according to the invention further ensure that there is no accumulation of the auxiliary grinding bodies in the exterior grinding chamber in front of the interior grinding chamber, because the gap width of the interior grinding chamber exceeds that of the exterior grinding chamber. Grinding by shearing takes place in the exterior grinding chamber. With the auxiliary grinding bodies tending to escape from increased shearing action, they flow into the interior grinding chamber through the deflection chamber which expands towards the interior grinding chamber. Owing to the described effects, the agitator mill can be run at a high fill ratio of auxiliary grinding bodies i.e, the fill of auxiliary grinding bodies need not be reduced. This leads to especially intensive grinding while avoiding grinding-stock shooting flows, because sufficient quantities of auxiliary grinding bodies are returned through the auxiliary-grinding-body return conduits. 
   The effects which the invention aims at are influenced particularly favorably by the feature wherein Fa≦Fb, and preferably 1.2 Fa≦Fb≦7 Fa, applies to the cross-sectional area Fa of the exterior grinding chamber in relation to the cross-sectional area Fb of the interior grinding chamber. This is still supported by the development wherein g≧3 i applies to the gap width g of the exterior grinding chamber in relation to the diameter i of the biggest auxiliary grinding bodies in the grinding chamber; wherein i≦3.0 mm, and preferably i≦1.5 mm, applies to the diameter i of the auxiliary grinding bodies; and wherein g≦9.0 mm, and preferably g≦5.0 mm applies to the gap width of the exterior grinding chamber. 
   This effect of loosening up the grinding stock in the interior grinding chamber and thus facilitated flow of the mixture of grinding stock and auxiliary grinding bodies is supported by the elevations which are attached at least to the interior stator and which may be designed as implements, in particular implements in the form of pegs. Thorough swirling of the auxiliary grinding bodies takes place by the elevations or implements attached to the interior stator, which again means intensive strain on the grinding stock. This intensive swirling effect also counteracts any boundary layer at rest to form on the grinding-chamber boundary walls, improving the cooling of the grinding stock. 
   The development according to which the elevations are disposed helically on the interior stator and the inner wall of the rotor is smooth, free of agitator implements prevents auxiliary bodies from depositing on the inner wall of the rotor; due to the helical arrangement of the implements on the outer casing of the interior stator, the inner wall of the rotor is entirely wiped and thus kept free from deposits. 
   The further development according to which the interior grinding chamber is followed by a discharge conduit in the shape of a truncated cone which is directed towards the grinding-stock/auxiliary-grinding-body separator device ensures that a certain accumulation effect is exercised on the interior grinding chamber, increasing the dispersing and grinding intensity. This effect can be attained in particular by a further development according to which the discharge conduit is defined by a face, neighbouring the separator device, of the interior stator and a dam-up device. A locally increased auxiliary-grinding-body concentration in the upper end portion can be achieved by such a dam-up device, which again leads to an especially intensive grinding or dispersing effect and thus to very closely distributed grinding-stock particle size. Being a separate component, such a separately incorporated dam-up device can be suited to any concrete application. The gap width of the discharge conduit can be constant in the direction towards the separator device or it may grow. 
   Fundamentally it is of special advantage when the interior stator is provided with a wearing protection in the vicinity of the discharge conduit, which is particularly advantageous when the gap width of the discharge conduit does not grow towards the separator device i.e., radially inwards, and, consequently, when the cross section of flow is reduced, accompanied with corresponding acceleration of the grinding-stock/auxiliary-grinding-body flow. 
   The further development, namely of the auxiliary-grinding-body return conduits being formed in an independent auxiliary-grinding-body return module, and in particular of the auxiliary-grinding-body return conduits being open towards a front of the return module, enables the size of the auxiliary-grinding-body return conduits to be adapted to the aims of grinding and dispersing in a simple way. Providing these return conduits in an auxiliary-grinding-body return module enables them to be incorporated laterally into the module, which is particularly simple in terms of implementation. This design also ensures the auxiliary-grinding-body return conduits to be provided with any desired contours by simple manufacturing steps. This simple fabrication also ensures the cross sections of flow of the auxiliary-grinding-body return channels to be optimized in their course from the inside out, with optimal ranges of the relationship of widths of the inlets and outlets consisting in that the return conduits have an inlet of a width c and an outlet of a width d, wherein d&gt;c, and preferably d≧1.5 c, applies to the width c of the inlet in relation to the width d of the outlet. With the height of the auxiliary-grinding-body return conduits being kept comparatively small in the direction of the central longitudinal axis, the risk of auxiliary grinding bodies shooting flow can be reduced without excellent separation of the auxiliary grinding bodies from the grinding stock being affected. In this regard, optimal marginal conditions reside in that the auxiliary-grinding-body return conduits have a height e and the grinding-stock/auxiliary-grinding-body separator device has a height f—each in the direction of the central longitudinal axis; and in that e≦f, and preferably e&lt;0.5 f, applies to the height e in relation to the height f. Those optimal conditions are further improved by the design wherein the return module, in vicinity to the separator device, is provided with wipers which pass continuously without interruption into the return conduits, and wherein the wipers extend along the height f of the auxiliary-grinding-body separator device. 
   Of course, the design specified above can also be employed by advantage in agitator mills of the generic type which are not embodied for g&lt;h applying to the radial gap width g of the exterior grinding chamber in relation to the radial gap width h of the interior grinding chamber. 
   Further features and advantages of the invention will become apparent from the ensuing description of exemplary embodiments, 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; 
       FIG. 2  is a longitudinal sectional view of a first embodiment of a grinding receptacle of the agitator mill; 
       FIG. 3  is a cross-sectional view of the grinding receptacle on the line III—III of  FIG. 2 ; 
       FIG. 4  is a longitudinal side view of an interior stator of the agitator mill; 
       FIG. 5  is a perspective view of an auxiliary-grinding-body return module of the agitator mill according to  FIGS. 2 to 4 ; 
       FIG. 6  is a longitudinal sectional view of a second embodiment of a grinding receptacle of the agitator mill; 
       FIG. 7  is a perspective view of the auxiliary-grinding-body return module of the agitator according to  FIG. 6 ; 
       FIG. 8  is a longitudinal sectional view of a third embodiment of a grinding receptacle of the agitator mill; 
       FIG. 9  is a longitudinal sectional view of a fourth embodiment of a grinding receptacle of the agitator mill; 
       FIG. 10  is a longitudinal sectional view of a fifth embodiment of a grinding receptacle of the agitator mill; 
       FIG. 11  is a longitudinal sectional view of a sixth embodiment of a grinding receptacle of the agitator mill; 
       FIG. 12  is a side view of an auxiliary-grinding-body return module of the agitator mill according to  FIG. 11 ; and 
       FIG. 13  is a view from below of the auxiliary-grinding-body return module according to  FIG. 12 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The agitator mill seen in  FIG. 1  conventionally comprises a stand  1  to which to attach a cylindrical grinding receptacle  2 . An electric drive motor  3  is housed in the stand  1  and is provided with a V-belt pulley  4  by means of which a V-belt pulley  7 , fixed against rotation on a shaft  6 , is rotarily drivable. 
   As shown in particular in  FIGS. 2 and 3 , the grinding receptacle  2  comprises a cylindrical inner wall  9  which surrounds a grinding chamber  8  and is surrounded by a substantially cylindrical outer casing  10 . The inner wall  9  and the outer casing  10  define between each other a cooling chamber  11 . The bottom closure of the grinding chamber  8  is formed by a circular bottom plate  12  which is fastened by means of screws  13  to the grinding receptacle  22 . 
   The grinding receptacle  2  has an upper annular flange  14  by means of which is it fixed by screws  16  to the underside of a support housing  15  that is mounted on the stand  1  of the agitator mill. The grinding chamber  8  is closed by a lid  17 . The support housing  15  has a central bearing and sealing housing  18  which is disposed coaxially with the central longitudinal axis  19  of the grinding receptacle  2 . The bearing and sealing housing  18  is penetrated by the shaft  6  which also extends coaxially with the axis  19  and on which is provided an agitator  20 . A grinding-stock supply line  21  opens into the area, adjacent to the grinding chamber  8 , of the bearing and sealing housing  18 . 
   An approximately cup-shaped cylindrical interior stator  22  is fixed to the circular bottom plate  12  and projects into the grinding chamber  8 ; it is comprised of a cylindrical outer casing  23  which is coaxial with the axis  19  and defines the grinding chamber  8 ; and of a cylindrical inner casing  24  which is also coaxial with the axis  19 . Between themselves they define a cooling chamber  25 . The cooling chamber  25  is connected with a cooling chamber  26  in the bottom  12 , to which cooling water is supplied via a cooling-water supply connector  27  and discharged via a cooling-water discharge connector  28 . Cooling water is supplied to the cooling chamber  11  of the grinding receptacle  2  via a cooling-water supply connector  29  and discharged via a cooling-water discharge connector  30 . 
   Disposed on the upper annular face  31 , located above the grinding chamber  8 , of the interior stator  22  is a grinding-stock/auxiliary-grinding body separator device  32  which is connected with a grinding-stock discharge line  33 . Between the separator device  32  and the discharge line  33  provision is made for a grinding-stock collection funnel  34 . In the vicinity of the bottom plate  12 , the discharge line  33  is provided with a handle  35  which, by means of screws  36 , is detachably joined to the bottom plate  12  and, respectively, to the interior stator  22  that is fixedly connected thereto. The separator device  32  is sealed towards the annular face  31  of the interior stator  22  by means of a seal  37  and, together with the discharge line  33  and the collection funnel  34 , can be pulled downwards out of the interior stator  22  once the screws  36  have been loosened. The separator device  32  can be removed from the grinding chamber  8  without the auxiliary grinding bodies  38  in the grinding chamber  8  having to be removed therefrom, because, with the agitator  20  not being driven, the level to which the grinding chamber  8  is filled with these auxiliary grinding bodies  38  does not extend to the face  31 . 
   The basic structure of the agitator  20  is cup-shaped i.e., it has a substantially annular cylindrical rotor  39 . The rotor  39  has a cylindrical outer wall  40  and a cylindrical inner wall  41  which is disposed coaxially there-with and coaxially with the axis  19 . The outer wall  40  and the inner wall  41  are smooth, forming closed surfaces and consequently not exhibiting any interruptions. A cooling chamber  42  is formed between the outer wall  40  and the inner wall  41  of the rotor  39 . 
   The top end of the agitator  20  is provided with a lid-type closing member  43 , with a closing plate  44  being fixed to the underside thereof that is turned towards the rotor  39 . The closing member  43  and the closing plate  44  are mounted on the shaft  6 . 
   An auxiliary-grinding-body return module  45  is disposed between the rotor  39  and the closing plate  44  of the agitator  20 . The rotor  39 , the return module  45  and the closing plate  44  are detachably united by means of tie rods  46 . The supply and discharge of cooling water to the cooling chamber  42  takes place via cooling-water conduits  47 ,  48  formed in the shaft  6  and in the return module  45 . 
   An exterior grinding chamber  8   a  is formed by the smooth design of the inner wall  9  of the grinding receptacle  2 , which does not possess any implements, and the equally smooth design of the outer wall  40  of the rotor  39 . The smooth-walled design, also free of implements, of the inner wall  41  of the rotor  39  and the outer casing  23  of the interior stator  22  define an interior grinding chamber  8   b . Elevations in the form of peg-style implements  49  that are mounted on the outer casing  23  of the interior stator  22  extend into this interior grinding chamber  8   b ; as seen in particular in  FIG. 4 , they are arranged helically along the circumference and length of the outer casing  23 . As seen in particular in  FIG. 4 , implements  49  which adjoin in the peripheral direction of the interior stator  22  overlap in the direction of the central longitudinal axis  19  so that, upon rotation of the rotor  39 , the inner wall  41  thereof will be wiped entirely by the implements  49 . 
   As seen above, the grinding chamber  8  is divided into a cylindrical exterior grinding chamber  8   a  on the one hand and a cylindrical interior grinding chamber  8   b  on the other, these chambers being interconnected in vicinity to the bottom plate  12  by a deflection chamber  50  which expands steadily from the outside inwards. 
   As seen in  FIGS. 2 and 4 , the cylindrical separator device  32  is comprised of a stack of annular disks  51 , between each of which a separating gap  52  has been left, the width of which is less than the diameter of the smallest auxiliary grinding bodies  38  used; however, the width may also exceed it, separation of the auxiliary grinding bodies  38  taking place before the separator device  32  has been reached. The stack of annular disks  51  is closed off frontally i.e., on the side turned towards the closing plate  44 , by a closing plate  53 . The separator device  32  is disposed within the return module  45 . 
   As seen in  FIGS. 2 and 5 , the auxiliary-grinding-body return module  45  is provided with auxiliary-grinding-body return conduits  54 . Their respective inlet  55  directly adjoins the separator device  32 . Their respective outlet  56  discharges into an annular cylindrical grinding-stock supply area  57  which is formed between the return module  45  and the inner wall  9  of the grinding receptacle  2 . The return conduits  54  have their minimum width c at the inlet  55  and their maximum width d at the outlet  56 , with the widths c and d being respectively measured in the peripheral direction. From the inlet  55  towards the outlet  56 , the return conduits  54  are curved counter to the direction of rotation  58  of the agitator  20 , namely convexly from the inside outwards. As for the width c in relation to the width d, d&gt;c applies, and preferably d≧1.5 c. 
   In the embodiment according to  FIGS. 2 to 5 , the return conduits  54  extend in the direction of the axis  19  nearly along the total height of the return module  45 , their axial height e exceeding the axial height f of the separator device  32 . In this embodiment, the return conduits  54 , apart from extending across the separator device  32  in the direction of the axis  19 , also reach across a discharge conduit  59  leading from the top end of the interior grinding chamber  8   b  obliquely upwards and inwards to the separator device  32  i.e., tapering in the shape of a truncated cone in the direction towards the closing plate  44 . In this embodiment, the return conduits  54  are open also towards the discharge conduit  59  as seen in  FIG. 2 . Consequently, the discharge conduit  59  is not spatially defined upwards. Rather, it is open in the direction of the central longitudinal axis  19  towards the interior grinding chamber  8   b , leaking auxiliary grinding bodies  38  while the grinding stock flows through the discharge conduit  59  in the direction towards the separator device  32 . 
   The grinding stock flows through the grinding chamber  8  in accordance with the arrows of flow direction  60 , passing from the grinding-stock supply line  21  through a grinding-stock supply chamber  61  between the closing member  43  of the agitator  20  on the one hand and the lid  17  and the adjacent area of the inner wall  9  on the other hand, through the grinding-stock supply area  57 , through the exterior grinding chamber  8   a  downwards, radially inwards through the steadily expanding deflection chamber  50  and from there through the interior grinding chamber  8   b  upwards to the discharge conduit  59  and from there to the separator device  32 . On its way through the exterior grinding chamber  8   a , the deflection chamber  50  and the interior grinding chamber  8   b , the grinding stock is being ground with the agitator  20  being rotarily driven in cooperation with the auxiliary grinding bodies  38 . The grinding stock leaves the interior grinding chamber  8   b  via the separator device  32 , from where it flows off through the grinding-stock discharge line  33 . 
   As seen in particular from  FIG. 2 , the radial gap width g of the exterior grinding chamber  8   a  is distinctly less than the radial gap width h of the interior grinding chamber  8   b . The relationship of the gap widths g and h to each other is such that the cross-sectional area Fb of the interior grinding chamber  8   b  equals or exceeds the cross-sectional area Fa of the exterior grinding chamber  8   a . The exterior grinding chamber  8   a  as well as the interior grinding chamber  8   b  are designed as grinding gaps. As for the gap width g of the exterior grinding chamber  8   a  in relation to the diameter i of the biggest auxiliary grinding bodies  38  in the agitator mill, the following applies:
     g≧3 i,   with i≦3.0 mm, and preferably i≦1.5 mm,
 
applying to the diameter i.
   

   As for the gap width g of the exterior grinding chamber  8   a ,
     g≦9.0 mm, and preferably g≦5.0 mm,
 
applies absolutely.
   

   As for the cross-sectional area Fa of the exterior grinding chamber  8   a  in relation to the cross-sectional area Fb of the interior grinding chamber  8   b : Fa≦Fb applies, and preferably 1.2 Fa≦Fb≦7 Fa. 
   The embodiment of  FIGS. 6 and 7  differs from that of  FIGS. 2 to 5  substantially in that, in addition to an auxiliary-grinding-body return module  45 ′, a dam-up device  62  is provided as part of the agitator  20 ′ between the closing plate  44  and the rotor  39 . The discharge conduit  59 ′ is defined between the face  31  of the interior stator  22  and this dam-up device  62  so that, by variation of the embodiment of  FIGS. 2 to 5 , it is defined not only at its underside by the face  31 , but also at its top side by the dam-up device  62 . Other than in the embodiment of  FIGS. 2 to 5 , the interior grinding chamber  8   b  does not discharge by its top end directly into the return conduits  54 ′, but the mixture of grinding stock and auxiliary grinding bodies is forcibly deviated by the dam-up device  62  in a direction obliquely upwards and inwards towards the separator device  32 ′. The gap width j of the discharge conduit  59 ′ is constant in this embodiment. 
   In as much as parts are identical with those of the embodiment according to  FIGS. 2 to 5 , the same reference numerals are used. Functionally identical and constructionally similar parts have the same reference numerals with a prime added. The same applies to further embodiments with a correspondingly higher number of primes. The height e′ of the return conduits  54 ′ is clearly inferior to the height e in the embodiment of  FIGS. 2 to 5 . Furthermore the height e′ is clearly inferior to the axial height f′ of the separator device  32 ′. This is a simple way of ensuring that the height e′ of the return conduits  54 ′ can be adapted to reduced grinding-stock throughputs and that the risk of grinding-stock-particle shooting flow can additionally be reduced, in particular in the case of little grinding-stock throughput or a low speed of the agitator  10 . It applies:
     e′≦f′ and in particular   e′≦0.8 f′ and especially   e′≦0.5 f′.   

   Furthermore, the separator device  32 ′ does not extend across the entire area above the face  31 . Rather, a closed annular section is provided as a wearing protection  63  between the face  31  and the separator device  32 ′; the wearing protection  63  and the separator device  32 ′ are one piece. The discharge conduit  59 ′ ends ahead of, or at, the wearing protection  63  so that any auxiliary grinding bodies  38 , leaking from the discharge conduit  59 ′ and being deflected into a motion parallel to the axis  19 , do not hit the separator device  32 ′. 
   The embodiment according to  FIG. 8  differs from that of  FIGS. 6 and 7  only in that the auxiliary-grinding-body return conduits  54 ″ have a minimum height e″ required for trouble-free operation at inferior grinding-stock throughputs. In this case too the auxiliary-grinding-body return module  45 ″ adjoins the dam-up device  62 , with the return conduits  54 ″, at their top side, being defined by the closing plate  44  in this embodiment as well as in the two embodiments mentioned above. However the axial height k is the same in the return modules  45 ′ and  45 ″. 
   As for the minimal axial height e″ of the return conduits  54 ″ the following applies: e″≧3 i, and at least e″≧4 mm. 
   The embodiment according to  FIG. 9  corresponds to that of  FIG. 6  with the difference residing in that no wearing protection  63  is provided and that the discharge conduits  59 ′″ expand towards the auxiliary-grinding-body separator device  32  i.e., the gap width j′″ of the discharge conduit  59 ′″ grows inwards to such an extent that the total cross-sectional area of this conduit  59 ′″ does not decrease in the direction towards the separator device  32  so that no acceleration of the flow of grinding stock and auxiliary grinding bodies takes place in the discharge conduit  59 ′″ towards the separator device  32 . For this reason, the separator device  32  can extend as far as to the face  31 , because the auxiliary grinding bodies  38  do not hit the separator device  32 . 
   The embodiment according to  FIG. 10  substantially corresponds to that of  FIG. 9 , with the auxiliary-grinding-body return module  45 ″″ not leading as far as to the separator device  32 . The inlets  55 ″″ of the auxiliary-grinding-body return conduits  54 ″″ have a clear radial distance from the separator device  32 . In this annular chamber  64 , provision is made for several wipers  65  which are mounted on the closing plate  44  and rotate together with the agitator  20 ″″. 
   The embodiment according to  FIGS. 11 to 13  comprises an auxiliary-grinding-body return module  45 ′″″ which, towards the dam-up device  62 , bears against an intermediate ring  66 . The module  45 ′″″ is open downwards towards the grinding chamber  8  i.e., towards a front  67 . The axial height e′″″ is constant from the respective inlet  55 ′″″ to the outlet  56 ′″″ and distinctly less that the height f′ of the separator device  32 ′. The wipers  65 ′″″ directly adjoin the return conduits  54 ′″″ so that there is a continuous transition from these wipers  65 ′″″ into the return conduits  54 ′″″, as shown in particular in  FIG. 13 . This leads to optimal flow conditions. As seen in  FIG. 11 , the wipers  65 ′″″ extend in the direction of the axis  19  approximately along the height f′ of the separator device  32 ′.