US3817461A

US3817461A - Agitator mill

Info

Publication number: US3817461A
Application number: US00296526A
Authority: US
Inventors: R Forster; A Geissel; R Lehnert
Original assignee: Erich Netzsch GmbH and Co Holding KG
Current assignee: Erich Netzsch GmbH and Co Holding KG
Priority date: 1971-10-14
Filing date: 1972-10-11
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18
Also published as: IT972191B; NL7213826A; DE2151246C3; AT329953B; CH543307A; DE2151246A1; BE790105A; ATA877172A; FR2157432A5; GB1404605A; DE2151246B2

Abstract

An agitator mill for grinding and/or dispersion of flowable material subjected to grinding comprising an enclosed grinding container in which a charge of grinding particles may be disposed, an outlet chamber communicating with said container and spaced therefrom by a separator which allows passage of only ground material. The said mill also including an agitator extending through the outlet chamber, the separator and into the grinding container. The said agitator in operation being driven by a rotary drive equipped with stirring elements and sealed with respect to the outlet chamber at the point where it passes through said chamber by hydrodynamic sealing disc means secured on said agitator.

Description

United States Patent [191 Geissel et al.

[111 3,817,461 June 18, 1974 1 AGITATOR MILL [75] Inventors: August Geissel; Richard Lehnert,

both of Selb; Rudolf Forster, Weissenstadt, all of Germany [73] Assignee: Gebruder Netzsch, Maschinenfabrik Selb/Bayern, Germany [22] Filed: Oct. 11, .1972

21 Appl. No: 296,526

[30] Foreign Application Priority Data Oct. 14, 1971 Germany 2151246 [52] US. Cl 241/73, 241/84, 241/98,

241/171, 241/184 [51] Int. Cl. .L B02c 13/00 [58] Field of Search 241/73, 74, 83, 84, 91,

[56] References Cited UNITED STATES PATENTS 3,720,379 3/1973 Szegvari 241/73 X Primary Examin'erGerald A. Dost Assistant Examiner-DeWalden W. Jones Attorney, Agent, or FirmKarl W. Flocks ABSTRACT An agitator mill for grinding and/or dispersion of flowable material subjected to grinding comprising an enclosed grinding container in which a charge of grinding particles may be disposed, an outlet chamber communicatingwith said container and spaced therefrom by aseparator which allows passage of only ground material. The said mill also including an agitator extending through the outlet chamber, the separator and into the grinding container. The said agitator in operation being driven by a rotary drive equipped with stirring elements and sealed with respect to the outlet chamber at the point where it passes through said chamber by hydrodynamic sealing disc means secured on said agitator.

19 Claims, 10 Drawing Figures PATENTEDJUN 18 I974 3Q8NLdl sum. 1 m 4 minimum m4 3.817161 sum 3 0f 4 1 AGITATOR MILL The invention relates to an agitator mill comprising a container for a material to be ground and particles for grinding it, an adjacent outlet chamber separated from the container by a separator which only passes the ground material, agitating means for'stirring the contents of the container, mounted on a shaft passing through the separator and out through an opening in an opposed wall of the outlet chamber. The invention is concerned with the sealing of the agitator shaft in relation to the outlet chamber and the grinding particle separator.

Flowable material to be ground may, for example, be powdered substances, granulates, agglomerates, 'plastics substances and/or liquids in which relatively large particles may be contained. lnparticular, the material to be ground may be a paste prepared from solid particles contained in a liquid medium. Flowable material has to be processed inter alia in the foodstuffs industry, chemicals industry and in the dyestuffs and paints industry, for which purpose agitator millsof the type described are particularly suitable and are to an ever increasing extent replacing roller stands.

In the case of such agitator mills,which preferably operate continuously, agitating means in-the grinding container stir a mixture of material to be ground and a charge of solid grinding particles, which term is used hereinto include spherical or' approximately spherical bodies made from steel, glass, porcelain or similar materials generally with dimensions of 0.5 to 3 or 5 mm. The energy transmitted to the contents of the grinding container through the agitating means, which are mounted on an agitator shaft rotating at high speed,

' produces gaps in the contents of the container, which is usually highly viscous. In the gaps fonned between the individual grinding particles and between the latter and the wall of the grinding container and between the particles and the agitator shaft, there occur differential speeds with high gradients and as a result shearing forces, sometimes combined with mechanical compression or friction forces, that cause comminution or dispersion of the particles to be ground;

The material to be ground is in most cases passed into thegrinding container from below by an infinitely variable feed pump and, after passing through the grinding zone, it is drawn off at the upper end of the grinding container through a grinding particle separating device which retains the grinding particles in the container and only allows passage of the ground material into an outlet chamber, on the top of the grinding container. The adequately ground or dispersed material flows through a lateral outlet from the outlet chamber into a receiving vessel or the like.

The separating device takes the form of a plane or slightly conical sieve, a perforated wall or a so-called friction gap, which is an annular gap between coaxial parts of the grinding container and the agitator shaft.

To an increasing extent, agitator mills are also used for comparatively highly. viscous media, e.g. printing ink, offset inks, letterpress inks, filling compositions, color concentrates and the like. In consequence, the agitator shaft which, in the case of agitator mills with vertical cylindrical and mostly very slim grinding containers, is usually introduced upwardly into the latter, must be sealed with respect to an upper closure of the container, e.g. with respect to the cover of an outlet chamber. The outlet chamber is regarded as the space provided above the separating device which constitutes the upper closure of the grinding container.

The outlet chamber must be separated from the grinding zone (in the grinding container) to prevent the ingress of grinding particles and at its upper end it must be so sealed in relation to the agitator shaft, that no processed material can emerge.

A problem in the case of the hitherto known agitator mills which are closed in this way is the sealing of the agitator shaft which extends up through the outlet chamber from the grinding container. This shaft which is generally freely mounted only above the grinding container cannot be maintained free from vibrations, particularly as it has to rotate in a mixture of grinding particles and material being ground, such a mixture not being completely homogeneous. Also the occurrence of resonance phenomena or the like leads to radial oscillatory movements of the agitator shaft. These oscillations can give rise to difficulties in the sealing of the agitator shaft by means of a compression gland. Difficulties, however, also arise due to solvents attacking the materials of the gland packing or other elastic seal, for example, lipped seal.

An object of the invention is to improve the sealing of the agitator shaft at the point where it passes through the cover of the outlet chamber, and preferably also where it passes through the separating device, in such a way that problems of wear are alleviated, the seal being easily constructed with a few parts in order to facilitate servicing and cleaning of the outlet chamber.

According to the present invention an. agitator mil] comprises a container for the material to be ground and particles for grinding it, an adjacent outlet chamber separated from the container by a separator which only passes the ground material, agitating means for stirring the contents of the container, mounted on a shaft passing through the separator and out through an opening in an opposed wall of the outlet chamber, and, for sealing the said opening, a hydrodynamic seal comprising a rotary body mounted coaxially with the shaft to rotate with it and having a sealing surface separated from the inner surface of the said wall of the discharge chamber by a small clearance.

The invention is based on the fact that a material which it is difficult to seal, such as a flowable very finely comminuted material which is present at the sealing locations, and which isat least at dynamic pressure, can be sealed off with a hydrodynamic seal, in other words a seal which dispenses with parts which are in frictional contact with one another. Since the sealing body, like all the other parts of the mill, may, in the region of ground material emergence, be constructed from metal, the problem of solvent attack which is present with other soft-elastic seals, does not arise.

In its simplest form the sealing body is therefore in the form of a disc having a plane sealing surface facing a plane surface of the wall of the outlet chamber. This represents a considerable simplification of the construction.

Conveniently the shaft is mounted substantially vertically and passes out through the top wall of the outlet chamber which is situated above the container, the container being of circular section coaxial with the shaft. Where the outlet chamber is cylindrical, the sealing disc preferably extends to a point close to its peripheral wall. This has the effect of increasing the length of the narrow gap in the direction of possible leakage. This length may also be increased by providing a labyrinth seal in the opening in the wall of the outlet chamber where the shaft passes through it, for example, by providing a sealing disc with a hub projecting through the opening and having a flange overlying the outer face of the wall. Again, if the materials are not too viscous, the sealing action may be increased by providing the surface of the sealing plate facing the wall of the outlet chamber with a spiral groove or a spiral web which carries the material to be ground outwards, or with annular ribs coaxial with the shaft and projecting into grooves in the wall of the outlet chamber or vice versa.

The possibility of employing various features of the sealing device depends on how large the clearance must be, and this in turn depends on bending oscillations of the agitator shaft which may occur. If the agitator shaft is freely mounted only above the grinding container, in a stand in the agitator mill, so that considerable bending oscillations may occur, then the clearance must be suitably large to ensure that even if slightly tilted the sealing disc will not come into contact with the cover or the wall of the outlet chamber. Conveniently the sealing disc and the cover of the outlet chamber may be divided so that they can be removed from the shaft to give access to the outlet chamber. Moreover, preferably the sealing disc is adjustably mounted on the agitator shaft so that the clearance can be adjusted.

In one form of the invention a top wall of the outlet chamber has on it a cup surrounding the agitator shaft to receive a sealing fluid. The cup may be closed by a cover sealed in relation to the shaft so as to be capable of holding the sealing fluid under pressure. Thus, when the mill is stationary, it may be desirable for the cup to be filled with a liquid or with an inert gas in order to avoid evaporation of solvent in the ground material.

In certain conditions it is found that by correct dimensioning of the clearance between the rotating sealing disc and the wall of the outlet chamber, it is possible to reach a state of equilibrium between the delivery pressure of the pump feeding the material into the grinding container and the pressure due to centrifugal force exerted by the sealin g disc on the ground material so that liquid in the cup outside the outlet chamber is not drawn in or otherwise affected.

To relieve the pressure on the sealing clearance, particularly when there is a super atmospheric pressure in the outlet chamber, the sealing body may carry pump vanes arranged to relieve the pressure on the sealing clearance by directing medium away from it. Such vanes may be adjustable and/or replaceably secured to the sealing body so that their pumping action can be adapted to particular circumstances such as the output or drive load, rate of delivery and the viscosity of the ground material. Preferably the pump vanes are inclined obliquely to their direction of movement and to avoid excessive stressing of the ground material the vanes may have apertures in them.

A seal is also required between the agitator shaft and the separator for the grinding particles. Thus, the shaft and separator may be provided with a separating structure comprising a sleeve mounted on the shaft with radial clearance relatively to the opening in the separator, and flanges projecting radially beyond the margin of the said opening and spaced from it by a distance smaller than the radius of the smallest grinding particles employed.

Preferably the upper and lower flanges of the separator unit are axially adjustable relatively to the shaft. Such an arrangement relies upon the axial clearances to prevent the passage of the grinding particles and hence the radial clearance may be somewhat greater to allow for bending oscillations and radial deflections of the agitator shaft. The lower face of the lower separating ring may taper conically downwards so as to give improved deflection of the grinding particles, particularly if the separating device is in the form of a slightly conical sieve having an inclined angle of between l40 and 160, preferably about 150. Such a slightly conical separating sieve having its outer edge situated higher than its middle has the advantage that upon completion of the grinding process drying out can be prevented. Non-drying suspensions of ground material can remain, since they cannot flow out through the outlet for the ground material which is at a higher level, or additional solvent may be added from above in order. to keep the apertures in the separating sieve clear.

In addition the conical shape, in conjunction with the conical form of the lower part of the separating ring and the agitator shaft, ensures that the grinding particles stirred by the agitator circulate in vertical planes and do not build up as they may if the sieve lies in a horizontal plane.

The invention may be put into practice in various ways, but one specific embodiment and certain modifications will be described by way of example with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a sectional elevation of the upper part of an agitator mill;

FIG. 2A shows an axial sectional view of a second embodiment of the gasket;

FIG. 2B shows a fragmentary side elevational view of the gasket of FIG. 2A;

FIG. 2C shows a partial top plan view of the gasket of FIG. 2A;

FIG. 3 is a bottom plan view of the sealing disc employed in FIG. 1;

FIG. 4 is a sectional elevation of a further modified form of sealing disc;

FIG. 5 is a plan view of a further modification of the sealing disc;

FIG. 6 is a vertical section through the sealing disc illustrated in FIG. 5 and adjacent parts of the agitator mill;

FIG. 7 is a plan view of another modification of the sealing disc; and

FIG. 8 is a vertical section through the sealing disc shown in FIG. 7 and adjacent parts of the agitator mill which has, also, in part, been modified.

The agitator mill of which the upper part is shown in FIG. 1 has a double-walled grinding container of which the outer shell 1 and the inner shell 2 are of coaxial vertical circular cylindrical form. The grinding space 3 enclosed by the inner shell 2 is bounded at the top by a separating sieve 4 which serves to retain the grinding particles 30. The separating sieve 4 is of slightly conical form and is clamped at its outer edge between the upper end face of the grinding container 1, 2 and the cylindrical outer wall 5 of an outlet chamber 6 provided above the separating sieve 4; the outer wall 5 has an outwardly projecting flange 7 which is bolted to the upper end face of the grinding container. At the top, the outlet chamber is closed by a cover 8 bolted to its outer wall 5. The material to be ground, as it passes,

through the separating sieve 4, can emerge through a lateral outlet 9 to which a downwardly directed elbow 11 is connected by a nut 10.

The charge of grinding particles 30 is agitated by agitating means 12 on an agitator shaft 13 which projects vertically from above coaxially through the outlet chamber 6 and the separating sieve 4, to stir the grinding particles 30 and the material to be ground. The agitator shaft 13, at the point where it passes through the cover 8 of the outlet chamber 6, carries a sealing disc 14 made from metal, which is axially adjustable in such a way as to leave a thin sealing clearance between its upper side and the underside of the cover 8. The sealing disc 14 extends radially to a point close to the peripheral wall 5 of the discharge member 6, from which it is also spaced by a narrow clearance.

The sealing disc 14.has a hub 15 extending through the opening in the cover 8 and formed with a recess 15a into which the margin of the opening projects with slight clearance on the upper side and underside. On its upper face the cover 8 is formed with an upstanding cylindrical sleeve 17 forming a cup 16 which can be filled with asealing fluid, e.g. a fluid to prevent evaporation of solvent from the ground material. The top of the cup 16 can be closed by a cover (not shown) if a pressurized gas, for-example, an inert gas, is to be introduced into the sealing gap from above.

On its underside the sealing disc 14 carries pump vanes or blades 18 to relieve the pressure on the sealing gap. The construction of these is best seen in FIGS. 3 and 4. In order that they may produce an impulse downwardly, and preferably towards the outlet on the ground materiaL'these pump vanes l8are preferably to be located obliquely with respect to the plane of the sealing disc 14 and/or with respect to the radial direction, as indicated by dash-dotted lines in the lefthand half of FIG. 3. For this purpose, each pump vane 18 has a base formed with slots 19 and 20 approximately at right angles to oneanother, by which screws 21 secure it to the underside of the sealing disc 14. In order to reduce the stressing of the ground material, some or all of the pump vanes 18 may have apertures 22 as shown in FIG. 4.

Instead of providing the pump vanes 18 on the underside of the sealing disc 14, it is possible also on the outer edge thereof to provide downwardly conveying radial blades 23, which are oblique to the axis as well as to the radius, as can be seen from the partial side view in FIG. 2B and the plan view in FIG. 2C.

At the point where it passes through the separating sieve 4, the agitator shaft 13 carries a separating device 24, consisting of a lower separating ring 24 and an upper separating ring 25. The separating rings 24 and 25 project on the upper and lower sides of the separating sieve 4 radially'beyond its inner periphery with a clearancewhich is smaller than the radius (or smaller clearance between this sleeve and the margin of the opening in the'sieve may be greater than the lower axial clearance between the separating sieve 4 and the lower separating ring 24, so that the agitator shaft 13 has adequate radial clearance; After it has been adjusted with respect to the separating sieve 4, the lower separating ring 24 is secured inposition by an upper lock nut 27. The lower face of the lower separating ring 24 is conical in order not to interfere with circulation of the contents of the grinding container.

It will be obvious to those 'skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown in the drawings and described in the specification.

We claim:

1. An agitator mill comprising a container for a material to be ground and particles for grinding the material, an outlet chamber spaced apart from said container by a separator and communicating with said container through said separator which allows passage of only ground material, agitating means for stirring the contents of the container, the agitating means being mounted on a shaft passing through said separator and through said outlet chamber and out through an opening in an opposed wall of said outlet chamber, and a hydrodynamic seal in the form of a rotary body mounted coaxially with the shaft in said outlet chamber to rotate with the shaft in sealing relationship with said opening and having a sealing surface separated from the inner surface of the said opposed wall of the outlet chamber by a small clearance.

2. A mill as claimed in claim 1 in which the sealing body is in the form of a disc having a plane sealing surface facing a plane surface of said opposed wall of the outlet chamber.

3. A mill as claimed in claim 2 in which the shaft is mounted substantially vertically and passes out through the top wall of the outlet chamber which is situated above the container, the container being of circular section coaxial with the shaft.

4. A mill as claimed in claim 2 in which the outlet chamber is cylindrical and the sealing disc extends to a point close to its peripheral wall.

5.A mill as claimed in claim 2 in which the surface of the sealing disc facing the wall of the outlet chamber is provided with a spiral groove or a spiral web which carries outwards the ground material.

6. A mill as claimed in claim 2 in which the surface of the sealing disc facing the wall of the outlet chamber is provided with annular ribs coaxial with the shaft and projecting into grooves in the wall of the outlet chamber, or vice versa.

7. A mill as claimed in claim 3 including a labyrinth seal in the opening in the wall of the outlet chamber where the shaft passes through it.

8. A mill as claimed in claim 2 in which the sealing disc and wall of the outlet chamber are divided so that they can be removed from the shaft to give access to the outlet chamber.

9. A mill as claimed in claim 2 in which the sealing disc is vertically adjustable on the agitator shaft.

10. A mill as claimed in claim 3 in which the top wall of the outlet chamber has on it a cup surrounding the agitator shaft to receive a sealing fluid.

11. A mill as claimed in claim 10 in which the cup is closed by a cover sealed in relation to the shaft to be capable of holding the sealing fluid under pressure.

12. A mill as claimed in claim 2 in which the sealing body carries pump vanes arranged to relieve the pressure on the sealing clearance by directing medium away from it.

13. A mill as claimed in claim 12 in which the pump vanes are adjustable and/or replaceably secured to the sealing body.

14. A mill as claimed in claim 12 in which the vanes have apertures in them.

15. A mill as claimed in claim 12 in which the pump vanes are inclined obliquely to their direction of movement.

16. A mill as claimed in claim 12 in which the shaft and separator are provided with a separating structure comprising a sleeve mounted on the shaft with radial clearance relatively to the opening in the separator, and flanges projecting radially beyond the margin of the said opening and spaced from it by a distance smaller than the radius of the smallest grinding particles employed.

17. A mill as claimed in claim 16 in which the upper and lower flanges of the separating unit are axially adjustable relatively to the shaft.

18. A mill as claimed in claim 16 in which the separator comprises a conical sieve having an inclined angle of between 140 and 160, preferably about 150.

19. A mill as claimed in claim 16 in which the face of the lower separating ring tapers conically downwards.

Claims

5. A mill as claimed in claim 2 in which the surface of the sealing disc facing the wall of the outlet chamber is provided with a spiral groove or a spiral web which carries outwards the ground material.

14. A mill as claimed in claim 12 in which the vanes have apertures in them.

18. A mill as claimed in claim 16 in which the separator comprises a conical sieve having an inclined angle of between 140* and 160*, preferably about 150*.