GB2074895A - Agitator mill - Google Patents

Abstract

An agitator mill comprises a grinding vessel 2 defined by a cylindrical wall 17 within which an agitator including a cylindrical wall 15, 16 is mounted for rotation. The wall 15, 16 divides the grinding vessel into two coaxial chambers 11, 12. Within the inner chamber is a coaxially disposed central column 10 defined by two cylindrical walls 13, 14. Projections 25, 26 extend outwardly from the walls 14 and 16 respectively but the inner surface of the cylindrical wall of the agitator has no such projections. In use, material to be ground enters the inner chamber through the central column and is preground by the action of grinding bodies in the inner chamber which are brought into motion by the projections 25 and frictional engagement with the inner surface of the cylindrical wall of the agitator. The partially ground material then passes to the outer chamber 12 and is ground further by additional grinding bodies therein. <IMAGE>

Description

SPECIFICATION Agitator mill The present invention relates to agitator mills, and is concerned with that type of mill including a grinding vessel, an agitator which is rapidly rotatable within the grinding vessel, at least two chambers which are coaxially disposed inside the grinding vessel and separated from each other by a cylindrical wall forming part of the agitator and through which material to be ground can flow sucessively, a stationary central column within the inner chamber and projections which, in use, influence the movement of the material to be ground and of grinding bodies within the chambers.

Agitator mills of this type are referred to as quick-running multi-chamber mills. The speed of revolution of the agitator is normally in inverse relationship to the capacity of the grinding vessel.

In large capacity agitator mills the speed is about 300 r.p.m. and it goes up to about 3000 r.p.m. in smaller agitator mills.

French patent specification No. 2014753 discloses an agitator mill of the type referred to above which consists essentially of two axially engaging tools each comprising two concentric cylindrical walls and being open at one end and closed at the other. The central axes of the two tools are both vertical and are offset parallel to one another. The upper tool is stationary, while the lower one is rotatable about its own central axis which is offset with respect to the central axis of the upper tool. The latter is additionally adapted to perform circular oscillations about its central axis. To this end an agitator shaft carrying the lower tool and adapted to be driven in rotation by a motor is supported eccentrically in a bushing which in turn is supported concentrically with respect to the upper tool and adapted to be driven in rotation by a separate motor.The upper stationary tool includes a hollow central shaft through which the material to be ground flows from top to bottom and then from the bottom to the top in the radially inner chamber defined by the two tools.

The eccentric arrangement of the two tools with respect to each other has the effect that the chambers which are each defined by two circular cylindrical walls form tapered gaps in which the material to be ground, mixed with the grinding bodies, is entrained by projections disposed on the circular cylindrical walls in the direction towards the narrowest part of each chamber and is then crushed and, if desired, suspended in a liquid.

When the additional motor is switched on to cause the circular oscillating movement of the lower tool about the central axis of the upper tool, the narrowest place of the tapering gap in each grinding chamber constantly changes its location.

If, on the other hand, the lower tool merely rotates about its axis which is eccentric with respect to the central axis of the upper tool, the location of the narrowest place of each chamber does not alter.

At any rate, extremely great drive power is required to propel the material to be ground and the grinding bodies through the various cross sections of the chambers, and the cylindrical walls are subject to heavy wear. Moreover, the tools and the agitator shaft are subjected to considerable bending stress and flexural vibrations occur which cause eneven running of the entire agitator mill and problems in supporting the agitator shaft.

A similar agitator mill is disclosed in German Offenlegungschrift No. 1 6 07 531.

It is an object of the present invention to reduce the drive power required as well as the wear and flexural vibrations to a minimum in an agitator mill of the type referred to above without sacrificing the known advantages of a plurality of grinding chambers disposed inside the grinding vessel for the material to be ground to flow through successively.

According to the present invention an agitator mill comprises a grinding vessel, an agitator within the grinding vessel mounted for rotation about an axis of rotation, at least two substantially annular chambers which are coaxially disposed inside the grinding vessel and separated from one another by a cylindrical wall forming part of the agitator and through which, in use, material to be ground flows successively and a stationary central column within the inner chamber, the central column and the cylindrical wall of the agitator being arranged coaxially with the axis of rotation of the agitator and the inner chamber containing projections extending from the central column whilst the cylindrical wall of the agitator has no projections on its inner surface but projections on its outer surface and is disposed opposite the inner surface of a cylindrical wall of the grinding vessel.If the mill has only two chambers the outer chamber is defined by the cylindrical wall of the agitator and the outer cylindrical wall of the mill. The chamber communicate via one or more openings through which the material to be ground may pass but which may prevent the passage of the grinding bodies which, in use, are in the chambers.

The invention is based on the discovery that the projections alone which project from the central column and thus are also stationary, in combination with the friction transmitted by the smooth inner surface of the cylindrical wall of the agitator to the grinding bodies and material to be ground in the inner chamber which is defined by the stationary central column and by the cylindrical wall of the agitator are sufficient to activate the grinding bodies, distribute them substantially evenly throughout the annular cross section of this inner chamber, and also achieve a good grinding performance without requiring any additional eccentrically rotating or oscillating motion. The surfaces of the central column and agitator are subjected to little wear because ths grinding bodies and material to be ground need not be forced through a tapering or wedge-shaped gap.

In a preferred embodiment of the invention the projections on the central column extend to points immediately adjacent the inner surface of the cylindrical wall of the agitator and are shaped to perform a scraping action. Preferably each projection has a portion which extends in a direction substantially opposite to that in which, in use, the inner surface of the annular wall of the agitator moves and has a substantially straight edge adjacent the inner surface of the annular wall of the agitator. In this manner a layer of grinding bodies and material to be ground is prevented from forming at the inner surface of the annular wall of the agitator where it would constitute a deposit constantly rotating with the agitator and not being activated or only to a very small extent.

There may be a similarity between an agitator mill according to the invention and known agitator mill of the type referred to in that a substantially vertical agitator shaft is provided to drive the agitator about a substantially vertical axis of rotation and the central column affords an internal space through which the material to be ground is introduced into the inner chamber. In accordance with the invention, this arrangement is preferably such that the material to be ground flows upwardly through the central column and the agitator surrounds the central column from above in the manner of a bell.This permits the multichamber mill according to the invention to be designed like known single-chamber mills in that the grinding vessel can be suspended releasably on a stationary machine frame and lowered upon termination of the grinding process so as to be moved away, if desired, on a carriage of its own. A multi-chamber mill according to the invention of such design can be exchanged readily for a singlechamber mill of similar overall dimensions.

Furthermore, it is advantageous to dispose a check valve at the upper end of the central column. Thus the grinding bodies can be prevented from entering into the central column and perhaps blocking it.

The internal space is preferably defined by a supply pipe, which conveniently carries the check valve, composed of a plurality of pipe sections within the central column. The pipe sections are so dimensioned that the supply pipe and check valve it carries can be disassembled in sections even if the space available below the grinding vessel is much less than the total length of the supply pipe.

As discribed above in connection with known agitator mills, the individual chambers of the agitator mill according to the invention may contain grinding bodies of different sizes and be interconnected by grinding body separators. In this event it is convenient in accordance with the invention if the spacing between the projections on the central column is greater than that between the projections arranged in the or each outer chamber and if the radially inner chamber contains grinding bodies of larger size than those in the or each outer chamber. This makes it possible to effect particularly intensive pre-crushing of the material to be ground in the inner chamber.

Preferably the central column carries at least one bearing for the agitator radially within the cylindrical wall of the agitator. This is particularly preferred if the agitator has a horizontaliy disposed axis of rotation. Preferably two bearings of this kind are provided, again especially if the agitator mill is a horizontal one.

Preferably the central column is defined by two coaxial tubular walls between which a cooling space is defined, which space communicates with a coolant inlet and a coolant outlet. The cylindrical wall of the agitator and, if desired, the outer wall of the grinding chamber may also be constituted by two spaced walls defining a cooling space provided with an inlet and an outlet for a coolant.

Further features and details of the invention will be apparent from the following description of certain specific embodiment which is given by way of example only with reference to the accompanying diagrammatic drawings, in which Figure 1 is a side elevation of a vertical agitator mill and frame Figure 2 is an axial sectional view on an enlarged scale of the agitator mill shown in Figure 1; Figure 3 is a cross sectional view along the line Ill-Ill in Figure 2; Figure 4 is an axial section through a different construction of vertical agitator mill; and Figure 5 is an axial section through a horizontal agitator mill.

The agitator mill shown in Figure 1 includes a grinding vessel 2 of cylindrical shape removably secured to a stationary frame 1 and carrying a plurality of feet 3 each provided with a roller 4.

The grinding vessel 2 is adapted to be lowered from the operational position to stand on its rollers 4 so that it can be moved away.-The frame 1 is hollow and accommodates arnongst other things a drive motor (not shown) together with appropriate gearing to transmit rotational movement to an agitator which will be described in greater detail below and which rotates about an axis of rotation 5. Above the grinding vessel 2 there is a tubular nozzle 6 through which the ground material is removed when finished.

Many structural elements of the three agitator mills shown in Figures 2 and 3, Figure 4 and Figure 5 are the same or similar and these are designated by the same reference numerals and will be described below.

The grinding vessel 2 is partly filled with grinding bodies 7 and has a bottom wall 8, a top wall 9, and a central column 10 secured to the bottom wall 8. The grinding bodies 7 are distributed in two annular chambers 11 and 12 which are coaxial with the axis of rotation 5 and extend around the central column 10.

The central column 10 comprises two cylinders 13 and 14 which are disposed coaxially about the axis of rotation 5. The chamber 11 is defined by the cylinder 14 and another coaxial cylinder 1 5.

Similarly, the chamber 12 is defined by two spaced cylinders 1 6 and 1 7 coaxial with the axis of rotation 5. Another cylinder 1 8 which is also coaxial with the axis of rotation 5 is arranged radially outside the cylinder 1 7. In the embodiment shown in Figure 4 a cylindrical separating wall 19, which is also coaxial with the axis of rotation 5, is disposed between the cylinders 13 and 14. The cylinders 13 and 14 of the central column 10 are secured to the bottom wall 8 and define a cooling space 20.The cylinders 1 5 and 16 form a circular cylindrical wall forming part of the agitator which is rotatable about the axis of rotation 5 and define between them a cooling space 23 which, in the embodiment shown in Figure 4, is sub-divided into two cooling spaces 21 and 22 by a separating wall 1 9. The cylinders 17 and 1 8 define a further cooling space 24, and in the embodiment shown in Figures 2 and 3 and also that shown in Figure 4 are fixed to the bottom wall 8 and the top wall 9.

In the embodiment shown in Figure 5 the cylinders 1 7 and 1 8 are fixed similarly either directly or indirectly to the corresponding walls which comprise vertical end wall 8' and 9'. In each case the cylinders 1 7 and 1 8 form the stationary outer circular cylindrical wall of the grinding vessel.

Rows of projections 25 are disposed at regular angular spacings on the outer cylinder 14 of the central column 10. In the embodiment shown in Figure 3, for instance, four such rows extending parallel to the axis of rotation 5 are provided at an angular spacing of 900 A smaller number of rows may be sufficient, depending on the nature of the material to be ground and the size of the grinding bodies. For example, it is also possible to provide two diametrically opposed rows of projections 25 directed away from each other. As seen in Figure 2, the projections 25 of each row are preferably axially offset with respect to the projections of the or each adjacent row, and they may be constructed to effect a scraping action, as shown in the embodiment, of Figures 2 and 3.As seen in Figure 3, the projections have a portion which extends up to a point close to the cylinder 1 5 of the agitator at an angle opposed to the direction of rotation of the agitator which is indicated by an arrow.

It may also be sufficient for the projections 25 to be simple radially extending bars, as shown in Figures 4 and 5. In all cases, the inner cylinder 1 5 of the cylindrical wall 1 5, 16, of the agitator has no projections and is constructed as smooth as possible.

However, the outer cylinder 1 6 carries projections 26 in the form of radially outwardly protruding agitator bars fixed to its outer surface.

Similarly, projections 26' in the form of radially inwardly protruding counter bars are fixed to the inner surface of the inner cylinder 1 7. of the outer wall 17, of the grinding vessel.

The spacing between adjacent projections 25 measured in the axial direction is greater, as seen in Figure 2, than the corresponding spacing between both the projections 26 and the projections 26'. On the other hand, in the embodiments shown in Figures 4 and 5 the spacing between adjacent projections 25 is the same as that between adjacent projections 26 or 26'. As shown in Figure 2, the grinding bodies 7 in the radially inner chamber 11 are larger than those in the outer chamber 12. For instance, the grinding bodies in chamber 11 have a diameter of 2 mm, while those in chamber 12 have a diameter of only 1 mm.These different diameters and the resulting diffrences in mass, provided the specific mass of the grinding bodies 7 is the same, may be justified because, at a given speed of the agitator, the peripheral speed of the projections 26 in chamber 12 is greater than the peripheral speed of the cylinder 1 5 which partially defines the chamber 11 and causes the grinding bodies contained in it to be moved by friction.

The grinding bodies 7 in the chambers 11 and 12 are kept apart by a separator divice disposed between the two chambers. As may be seen in Figure 2, this consists of a stationary ring 27 coaxial with the axis of rotation 5 and fixed to the bottom wall 8 and a ring 29 likewise coaxial with the axis of rotation 5 and fixed to the lower end of the cylindrical wall 1 5, 1 6. The rings 27 and 29 define between them an annular gap 28. The inner surface of the ring 29 is aligned with the inner surface of the cylinder 15. Both rings 27 and 29 preferably are made of hard metal. The radial width of the annular gap 28 is smaller than the diameter ofthe grinding bodies enclosed in the first chamber 11. Thus, the partially ground material can flow through the annular gap 28, whereas the grinding bodies 7 are retained in the first chamber 11.

The agitator mill shown in Figure 4 does not include a comparable separator divice but permits the unobstructed passage of grinding bodies between the chambers 11 and 12. In the agitator mill shown in figure 5, however, a separating device for the grinding bodies is provided between the chambers 11 and 12. In this case it takes the form of a screen 32 rotating with the annular wall 15,16 of the agitator.

The exit of the ground material from the second chamber 12 of the agitator mill may occur by different routes as illustrated in Figures 2, 4 and 5.

In Figures 2 and 5, a separator device is provided which is constituted by a ring 34 rotating with the agitator and a stationary ring 36 which defines an annular gap 35 between them. This annular gap 35 is so narrow that it permits passage of the ground material only and not of the grinding bodies 7 from the chamber 12. In the embodiment of Figure 4, the separator device comprises an annular gap 38 formed between an agitator shaft 37 which extends through the top wall 9 of the grinding vessel 2 and a ring 39 set in the top wall 9.

In the case of the vertical agitator mills shown in Figures 2 and 4 the material to be ground is supplied from the bottom. The material to be ground passes through an inlet 41 in the bottom wall 8 of the grindcg vessel and thence through the interior 43 of the central column 10 to the top thereof so that it will enter from above into the top end of the first chamber 11. At this point a check valve 42 is positioned to prevent return flow of the material to be ground and the exit of grinding bodies. In the embodiment of Figure 2, a supply pipe 44 divided into a plurality of relatively short pipe sections is arranged within the interior 43 of the central column 10. By virtue of this division only a small space is required underneath the grinding vessel 2 for disassembly of the supply pipe 44 together with the check valve 42.

In the wall of the grinding vessel 2 an inlet 45 and an outlet 46 are formed in the outer cylinder 1 8 by way of which a coolant may circulate through the cooling space 24 between the cylinders 1 7 and 18. As may be seen in Figure 2, the cooling space 20 of the central column 10 can be connected to the coolant circuit by a supply line 47 and a discharge line 48. In the cooling space 20 a conduit 49 is disposed parallel to the axis 5 which extends over almost the entire height of the cooling space and ends in the discharge line 48.

Two different arrangements are shown in the drawings for cooling the agitator. In Figure 2 a conduit 50 connected to a supply pipe 51 is arranged parallel to the axis in the cooling space 23 between the cylinders 13 and 14 and extends nearly to the bottom of the space 23. The coolant thus enters the cooling space 23 at the lower end and leaves through a bore 52 at the upper end for discharge through an annular space 53 in the agitator shaft 37. The arrangement of the conduit 50 and bore 52, is only shown in one axial plane in Figure 2 but several may be provided, if required, in different axial planes.The separating wall 19 shown in Figure 4 replaces the conduit 50 shown in Figure 2 and affords apertures 54 in its lower portion for the passage of the coolant from one side of it to the other In the embodiment of Figure 2, the agitator mill is so designed that upon removal of the grinding vessel 2 from the top wall 9 the entire agitator can be uncoupled from a flange 56 of the agitator shaft 37 situated approximately at the level of the top wall 9 by ioosening screws 57. The screws 57 of which only one is shown, connect the flange 56 to a closure ring 59, 60, the lower part 59 of which is fixed to the cylinder 1 5 and the upper part 60 of which is fixed to the cylinder 1 6.

Together with the agitator shaft 37 the flange 56 is held at a constant level by a bearing (not shown) of the agitator shaft so that the annular gap 35 defined by the rings 34 and 36 for discharge of the ground material also remains unchanged. The bearing of the agitator shaft 37 is protected from the material to be ground by a sliding ring seal 58.

As shown in Figure 2, a tube 61 serves for filling grinding bodies into the first chamber 11.

This tube extends through the upper closure ring 59, 60 and is accessible through an opening in the top wall 9. An opening 62 is formed in the bottom wall 8 for emptying the chamber 11 of grinding bodies. The second chamber 12 is filled with grinding bodies through a tube 63 and emptied through an opening 64 in the bottom wall 8.

During operation of the two-chamber mill shown in Figure 2 both chambers 11 and 12 are partly filled with grinding bodies 7. The material to be ground is pumped in through the supply pipe 44 disposed in the central column 10 and enters into the radially inner chamber 11 through the check valve 42. In the inner chamber it is pretreated by the grinding bodies 7 which are activated by the projections 25 and thus also, at the same time cooled intensively by the coolant which circulates through the cooling spaces 20 and 23. The material being treated then flows through the annular gap 28 into the second radially outer chamber 12 where it is subjected to aftertreatment by the more closely spaced projections 26 and 26' and the smaller grinding bodies 7. The chamber 12, too, is intensively cooled from both the inside and outside surfaces.

The finished, ground material leaves the chamber 12 through the annular gap 35 and is discharged through the tubular nozzle 6 from a discharge chamber 65.

In the horizontal agitator mill of Figure 5, the agitator is supported at both ends on the central column 10. In this case the central column 10 is secured to the end walls 8' and 9' so that it extends through the entire agitator mill. It carries two bearings 67 and 68, of which the bearing 67 defines the left hand end of the grinding vessel 2 while the bearing 68 is disposed entirely within the grinding vessel. The bearing 67 is protected by a sliding ring seal 69, whilst the bearing 68 is protected on both sides by a sliding ring seal 70. A V-belt pulley 71 surrounded by a housing 72 is provided for driving the agitator. The central column 10 has a central section 73 of increased diameter to replace the cylinder 1 3 in its interior as shown in Figures 2 and 4.

The material to be ground enters through the inlet 41 into the first chamber 11 where it is pretreated and then leaves the chamber through the screen 32 and is subjected to aftertreatment in the second chamber 12 which it leaves through the annular gap 35 to enter into the discharge chamber 65.

Double ended support of the agitator, such as illustrated in Figure 5 in connection with a horizontal agitator mill is suitable also for a vertical agitator mill. All the agitator mills shown may be readily enlarged to constitute multichamber mills with more than two coaxial chambers, by simply arranging addittional chambers outside of chamber 12 and coaxially with it. It is possible to provide in all chambers only projections which extend outwardly from the wall defining the inner extent of the respective chamber.

Claims (9)

1. An agitator mill comprising a grinding vessel, an agitator within the grinding vessel mounted for rotation about an axis of rotation, at least two substantially annular chambers which are coaxially disposed inside the grinding vessel and separated from one another by a cylindrical wall forming part of the agitator and through which, in use, material to be ground flows successively and a stationary central column within the inner chamber, the central column and the cylindrical wall of the agitator being arranged coaxially with the axis of rotation of the agitator and the inner chamber containing projections extending from the central column whilst the cylindrical wall of the agitator has no projections on its inner surface but projections on its outer surface and is disposed opposite the inner surface of a cylindrical wall of the grinding vessel.

2. An agitator mill as claimed in Claim 1 in which the projections on the central column extend to points immediately adjacent the inner surface of the cylindrical wall of the agitator and are shaped to perform a scraping action.

3. An agitator mill as claimed in Claim 1 or Claim 2 including a substantially vertical agitator shaft mounted to rotate the agitator about a substantially vertical axis of rotation and the central column affords an internal space through which the material to be ground may be introduced into the inner chamber through the -lower end of the central column, whereby the material to be ground flows upwardly through the central column, and the agitator surrounds the central column from above in the manner of a bell.

surrounds the central column from above in the manner of a bell.

4. An agitator mill as claimed in Claim 3 in which the internal space is defined by a supply pipe composed of a plurality of pipe sections within the central column.

5. An agitator mill as claimed in Claim 3 or Claim 4 including a check valve arranged at the upper end of the central column.

6. An agitator mill as claimed in any one of the preceding claims in which the projections extending from the central column into the inner, chamber are spaced apart by a greater distance than those in the or each outer chamber, the inner chamber contains grinding bodies of larger size than those in the or each outer chamber and the inner chamber communicates with the adjacent chamber by means of grinding body separating means which prevent the passage of the iarger grinding bodies.

7. An agitator mill as claimed in Claim 1 in which the central column carries at least one bearing for the agitator within the cylindrical wall of the agitator.

8. An agitator mill as claimed in any one of the preceding claims in which the central column is defined by two coaxial tubular walls between which a cooling space is defined, which space communicates with a coolant inlet and a coolant outlet,

9. An agitator mill substantially as specifically herein described with reference to Figures 2 and 3, Figure 4 or Figure 5 of the accompanying drawings.