WO2000012215A1

WO2000012215A1 - Improved drive mechanism for centrifugal grinding mills

Info

Publication number: WO2000012215A1
Application number: PCT/AU1999/000695
Authority: WO
Inventors: Lawrence Roos; William Richards Rayner
Original assignee: Hicom International Pty. Ltd.
Priority date: 1998-08-31
Filing date: 1999-08-27
Publication date: 2000-03-09
Also published as: AUPP556298A0

Abstract

This invention relates generally to a centrifugal grinding mill comprising a relatively stationary axis or vertical axis or vertical axis of revolution (1), a longitudinal axis of symmetry or nutating axis (2) intersecting the vertical axis (1) at a point of nutation symmetry (3), a grinding chamber (4) symmetrical about the nutating axis (2) connecting with the feed passage at its upper end and having discharge opening (6) located in its periphery, a support member or frame member (7) being adapted to support the mill and transmit forces and moments generated by the mill to suitable foundations, constraint means for determining the form of nutating motion of the grinding chamber (4), and drive means located below the grinding chamber (4) comprising a drive shaft (14) adapted to be driven at its lower end by an electric motor (15) or other power transmission device through a flexible drive coupling (16), the drive shaft (14) having an eccentric stub shaft (19) mounted at its upper end, the stub shaft axis (20) held coincident with the nutating axis (2) by engagement of the stub shaft (19) with a grinding chamber bearing (21) mounted at a lower extremity of the grinding chamber (4).

Description

IMPROVED DRIVE MECHANISM FOR CENTRIFUGAL GRINDING MILLS

FIELD OF THE INVENTION

This invention relates to centrifugal grinding mills of the kind which perform the size reduction of solid particles by the action of loose grinding media which are driven with a gyrating and tumbling motion. The invention further relates to a bearing seal arrangement and a flexible drive coupling being adapted for use in a grinding mill.

BACKGROUND TO THE INVENTION

Centrifugal grinding mills have been widely applied for the comminution of solid particles, in particular for the size reduction of mineral ores. A feature of centrifugal grinding mills is that the centrifugal acceleration imparted to the mill contents as a consequence of their gyrating motion has the effect of greatly enhancing the rate of comminution compared with conventional tumbling mills, which are limited by gravitational acceleration.

A particular type of centrifugal grinding mill to which this invention applies is that in which the grinding chamber axis is inclined to and intersects the axis of rotation of the chamber, which is characteristic of nutating motion. For the purpose of this specification nutating motion of a machine element relative to a fixed frame is defined as the motion of the element, an axis of which intersects with and traces out a conical surface about a stationary axis of the fixed frame. In the general case, the nutating element has a net rotational motion about its axis, relative to the fixed frame. A special case of nutating motion is one in which the nutating element has a net irrotational motion.

Figure 1 shows a cross section through a known centrifugal grinding mill of this type, as described in Australian Patent No. 568949. Figure 1 shows a grinding chamber 104, having an axis of symmetry 102, which rotates about a fixed axis 101, and intersects therewith at a point of nutation symmetry 103. The chamber is constrained to perform nutating motion by the engagement of complementary annular bearing surface pairs 109 and 111, and 108 an 110, which together form a spherical bearing symmetrical about the point of nutation symmetry 103, and which limit the amplitude of nutating motion. Feed material, in the form of coarse granular material, enters the grinding chamber from the feed passage 105, and discharges from the opposite extremity of the grinding chamber, via openings 106, as a fine granular product. The grinding chamber is driven with a nutating motion by multiple oil pressurised pistons 159 which sequentially engage a radial flange element 163, attached to the grinding chamber, and which surrounds the point of nutation symmetry 103. In another embodiment of the machine shown in Figure 1, the grinding chamber 104 is driven by multiple push- rods, mounted above, and acting upon the flange element 163. In that embodiment the push rods transmit both tension and compression drive forces to the flange element, compared with only compression forces in Figure 1.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a centrifugal grinding mill comprising: a grinding chamber having a longitudinal axis of symmetry, the grinding chamber being constrained and adapted to permit nutating motion of the axis of symmetry about a relatively stationary axis of the grinding mill, said axes intersecting at a point of nutation symmetry; a support member on which the grinding chamber is supported; and driving means operatively coupled to the grinding chamber at an opposite axial extremity of said chamber relative to the point of nutation symmetry, the driving means being designed to drive the grinding chamber to effect the nutating motion.

Advantageously, the application of drive input to the grinding chamber from a point below the chamber, and remote from the point of nutation symmetry, yields a more favourable mechanical advantage for application of drive force compared with examples from the prior art featuring power input from above the grinding chamber, adjacent the point of nutation symmetry. In the preferred embodiment of the present invention, the amplitude of motion of the drive point is relatively large, and the drive force is correspondingly reduced compared with prior art examples. This results in a lighter weight construction of the drive mechanism, with consequent reduced cost.

In a preferred embodiment of the invention constraint means comprising annular bearing surfaces associated with the grinding chamber engage complementary opposing bearing surfaces on the support member, said annular bearing surfaces comprising a spherical bearing symmetrical about the point of nutation symmetry, and adapted to limit the amplitude of nutating motion.

In another embodiment of the invention, rotation of the grinding chamber about its axis of symmetry is prevented by a torque restraining device connecting the grinding chamber to the support member.

In a preferred embodiment of the invention, the driving means comprises a drive shaft having an axis substantially co-linear with the stationary axis of nutating motion, mounted in bearings for rotation, and driven in rotation from an end remote from the grinding chamber by a power transmission unit connected therewith. The drive shaft is provided with a cantilevered eccentric stub shaft mounted at the end adjacent the grinding chamber, the stub shaft having an axis substantially co-linear with the axis of symmetry of the grinding chamber. The eccentric stub shaft engages the grinding chamber through an intermediate bearing element adapted to permit relative rotational motion about the chamber axis of the two engaging members .

In one embodiment, the eccentric stub shaft bearing element is resiliently mounted with respect to the grinding chamber.

In another embodiment of the invention, the eccentric stub shaft bearing element is resiliently mounted with respect to the stub shaft . The provision of a resilient coupling member interposed between the eccentric stub shaft and the grinding chamber isolates the drive means from transfer of inertial reaction forces and moments from the grinding chamber to the support means. All inertial reactions from the grinding chamber are transferred to the support means through the constraint means. The drive means transfers only torsional moment to the grinding chamber associated with power consumed by the nutating components.

In a further embodiment, the eccentric stub shaft bearing element is mounted in a support frame which is provided with independent means to adjust eccentricity of the bearing element relative to the axis of symmetry of the grinding chamber.

In a preferred embodiment of the invention, the eccentric stub shaft bearing and adjacent drive shaft upper bearing are each provided with sealing means to prevent contamination of bearings, and internal components of the drive means, by material discharging from the grinding chamber. Sealing means include first seal elements mounted adjacent the bearing elements, and adapted to contain lubricant within the bearing cavities, fluid purged second seal elements, and third seal elements mounted in serial arrangement between the bearing elements and the external extremities of the drive means. Fluid is supplied to first annular cavities, defined by the adjacent mounting of the first and second seal elements, at controlled pressures of sufficient magnitude to overcome interfacial forces acting between rotating and stationary faces of the second seal elements, and permit them to separate and discharge controlled flowrates of fluid to second annular cavities between rotating and stationary members of the third seal elements.

According to another aspect of the present invention there is provided a bearing seal arrangement comprising: a first seal being axially spaced from a bearing in which a shaft is rotatably mounted, the first seal being adapted to sealably contain lubricant in a sealed bearing cavity disposed about the bearing; a second seal located axially adjacent the first seal and being configured to define a seal cavity adapted to receive a pressurised fluid; and a third seal disposed radially about and surrounding the second seal, said seals being mounted in series and together being adapted to prevent the ingress of contaminant material to the sealed bearing cavity.

Preferably the first seal is a stationary radial lip seal being designed to seal against the shaft to prevent the egress of the lubricant from the sealed bearing cavity.

Preferably the second or the third seal is an axial end face seal having a stationary seal component and a rotating seal component being adapted to fix to the shaft, said seal components have respective sealing faces which are urged to contact with one another by elastic forces within said second or third seals, the pressurised fluid causes said sealing faces to break contact with one another whereby said fluid is purged from the seal cavity at a controlled flowrate.

Preferably the second or the third seal is a bushing or a labyrinth type interstitial seal having a rotating seal component disposed in close proximity to a stationary component at least part of which is of a complementary shape to the rotating seal component.

Advantageously, this embodiment of the bearing seal arrangement provides effective sealing means to prevent contamination of bearings, and other internal components of the drive means, by discharge products issuing from the grinding chamber, thus contributing towards long operating life between maintenance overhauls .

According to a further aspect of the present invention there is provided a flexible drive coupling being designed to couple an eccentric stub shaft to a grinding chamber having a longitudinal axis of symmetry and being constrained within a grinding mill to permit nutating motion of the axis of symmetry about a relatively stationary axis of the grinding mill, said flexible drive coupling being characterised by its relatively low radial stiffness in a first plane intersecting both the axis of symmetry and the stationary axis and relatively high radial stiffness in a second plane being orthogonal to said first plane, whereby in operation minimal inertial forces and moments from the grinding chamber are transmitted to the stub shaft whilst maintaining a substantially complete transfer of torsional moments about the relatively stationary axis from the stub shaft to the grinding chamber.

Preferably, the flexible drive coupling comprises multiple parallel laminae of rigid material separated by and attached to layers of a resiliently flexible material and together being orientated with their planes substantially parallel to the first plane.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to achieve a better understanding of the nature of the present invention a preferred embodiment of a centrifugal grinding mill, bearing seal arrangement and flexible drive coupling will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a sectional view of a prior art centrifugal grinding mill; Figure 2 is a sectional view of a centrifugal grinding mill according to one embodiment of the invention;

Figure 3 is an enlarged sectional view of a drive shaft and eccentric stub shaft together with a grinding chamber of the grinding mill of Figure 2; Figure 4 is an enlarged sectional view showing details of sealing means provided for an upper drive shaft bearing and eccentric stub shaft bearing mounted at the base of the grinding chamber of the grinding mill of Figure 2;

Figure 5 is an enlarged sectional view in a plane normal to the plane of Figure 4 showing details of a flexible drive coupling of the grinding mill of Figure 2; and

Figure 6 is an enlarged sectional view of constraint means of the grinding mill of Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As best shown in Figure 2 there is one embodiment of a centrifugal grinding mill comprising: a relatively stationary axis or vertical axis of revolution 1, a longitudinal axis of symmetry or nutating axis 2 intersecting the vertical axis 1 at a point of nutation symmetry 3, a grinding chamber 4 symmetrical about the nutating axis 2 connecting with a feed passage 5 at its upper end and having discharge openings 6 located in its periphery, a support member or frame member 7 being adapted to support the mill and transmit forces and moments generated by the mill to suitable foundations, constraint means for determining the form of nutating motion of the grinding chamber 4 comprising annular nutating bearing surfaces 8 and 9 rolling on corresponding fixed bearing surfaces 10 and 11 and nutating and fixed part spherical surfaces 12 and 13 respectively having centres coincident with the point of nutation symmetry 3, and drive means located below the grinding chamber 4 comprising a drive shaft 14 adapted to be driven at its lower end by an electric motor 15 or other power transmission device through a flexible drive coupling 16, the drive shaft 14 supported at its upper and lower ends by drive shaft bearings 17 and 18 respectively which are mounted in a base portion 46 of the frame member 7, the drive shaft 14 having an eccentric stub shaft 19 mounted at its upper end, with the stub shaft axis 20 held coincident with the nutating axis 2 by engagement of the stub shaft 19 with a grinding chamber bearing 21 mounted at a lower extremity of the grinding chamber 4.

In this example the feed passage 5 intersects the inner surface of the grinding chamber 4, and its upward extension, at a plane which defines the upper boundary of the grinding chamber 4, located below the point of nutation symmetry 3. The upper drive shaft bearing 17 and grinding chamber bearing 21 are provided with sealing means, as shown in Figure 4, to prevent entry of contaminants to the bearings. A first seal in the form of radial lip seals 22 and 23 are mounted on the upper and lower end respectively of the drive shaft bearing 17, defining a sealed bearing cavity, to limit leakage of lubricant therefrom. A second seal which in this embodiment is an axial end face seal 24 is mounted immediately above the radial lip seal 22, and fixed to the shaft 14, having rotating and stationary sealing faces 25 and 26 respectively which are held in contact by elastic forces within the seal. The drive shaft bearing 17 is further protected from contaminant entry by a third seal in the form of a bushing type interstitial seal 27, which surrounds the bearing housing 28 and the seal 24, and is formed by the close proximity to the housing periphery of the cylindrical inside surface of a rotating member 29 fixed to the drive shaft 14.

The grinding chamber bearing 21, mounted at the lower extremity of the grinding chamber 4, is protected against contaminant entry by first, second and third seals 30, 32 and 35, mounted in series at its lower end. The bearing cavity is closed at its upper end, and sealed against loss of lubricant by the first seal in the form of a radial lip seal 30 mounted adjacent the lower end of the grinding chamber bearing 21, and sealing against a rotating member 31 fixed to the inner race of the chamber bearing 21. The second seal which in this embodiment is an axial end face seal 32 is mounted immediately below the radial lip seal 30, and fixed to a rotating member 31, having rotating and non-rotating sealing faces 34 and 33 respectively, which are held in contact by elastic forces within the end face seal 32. The chamber bearing 21 is further protected from contaminant entry by the third seal in the form of a labyrinth type interstitial seal 35, located at the lower extremity of a nutating housing member 36. The labyrinth seal 35 is formed by the close proximity of annular downward projections from a nutating member 37, fixed to the nutating housing member 36, with corresponding annular projections on a rotating and nutating member 38. Although the sealing arrangement of the embodiment depicted in Figure 4 shows specific types of seals, comprising radial lip seals, axial end face seals, and interstitial seals, alternative constructions may include other seal types, or these same seal types may be used in different order.

The eccentric stub shaft 19 of this example of the grinding mill engages with the lower extremity of the grinding chamber 4 through a flexible drive coupling 39, which attaches at its outer extremity, comprising the rotating member 31, to an inner race of the chamber bearing 21, and at its inner extremity to the stub shaft 19, as shown in Figure 5. Inner and outer elements, 40 and 41, of the flexible drive coupling 39, have plane opposing surfaces which are separated by equi-spaced multiple plane laminae 42 aligned parallel with the plane faces of the inner and outer elements 40 and 41, which are all connected by an elastomer 43 which is moulded in place. The outer elements 41 have part cylindrical outer surfaces spaced apart from an inner surface of the rotating member 31, to which it is bonded by the elastomer 43. The elastomer 43 bonds the members and elements 31, 40 and 41, and 42 together to form the flexible drive coupling element 39, which permits a small amount of angular, radial and axial mis-alignment between the stub shaft 19, and the inner race of the chamber bearing 21.

In order to facilitate a further understanding of the invention, the use and operation of the centrifugal grinding mill described above will now be outlined.

In operation of the centrifugal grinding mill the input drive shaft 14 is driven in rotational motion by the drive motor 15, which is converted to nutating motion of the grinding chamber 4 by its interaction with the eccentric stub shaft 19, and the constraining means, comprising opposing bearing surfaces 8 and 10, and 9 and 11, disposed about the point of nutation symmetry 3. Solid feed particles 44 are fed into feed passage 5, where they move by gravity to the grinding chamber 4. The feed particles interact with loose solid particles of grinding media 45, which are driven with a gyrating and tumbling action by the motion of the grinding chamber 4, causing the feed particles to break down to finer size fractions. Fine size fractions of feed particles 44 discharge from grinding chamber 4 through the peripheral discharge openings 6.

The embodiment shown in Figure 2 is a wet grinding mill, in which liquid, usually water, is also introduced into the grinding chamber 4 with the solid feed particles 44. Fine product material discharges from the openings 6 in the form of a slurry, and drains to a central sump 46 surrounding the drive shaft 14, from where it flows to a discharge pipe 47. In other embodiments of the invention, the discharge openings 6 may be in other forms, typically elongated slots symmetrical about radial planes through the grinding chamber axis 2, and displaced toward the lower region of the chamber 4. Other embodiments may also include centrifugal grinding mills adapted for operating with a dry environment within the grinding chamber 4. In this case gas, usually air, is fed to the grinding chamber 4 together with feed particles 44, and discharge product issues from the discharge openings 6 as a suspension of fine particles in gas.

As shown in Figure 6, the grinding chamber 4, and attached members, are located and constrained to perform the required nutating motion about the vertical axis 1 by continuous rolling contact at opposing bearing surface pairs 8 and 10, and 9 and 11 of the constraining means. All inertial reactions from the assembly of nutating members, as a consequence of their nutating motion, are transmitted to the support frame 7 via the stationary bearing surfaces 10 and 11. The driving means, below the grinding chamber 4, are isolated from the inertial reactions of the chamber by the resilience of the flexible drive coupling 39, which connects the driving means and the grinding chamber 4. The flexible drive coupling 39 is characterised by its low radial stiffness in the plane of Figure 4, containing the fixed and nutating axes 1 and 2, ensuring negligible transmission of inertial forces and moments from grinding chamber 4 to the drive shaft 14. The coupling 39 has high radial stiffness normal to the plane of Figure 4, and parallel to the plane of Figure 5, as a consequence of its multiple parallel laminae construction, to enable transfer of torsional moment from the rotating drive shaft 14 to the nutating grinding chamber 4.

In addition to providing energy transfer from the power input unit 15 to the grinding chamber 4, an important function of the drive mechanism, shown in Figure 3, is to prevent the ingress of discharge products to the bearings, and other drive components. It is a feature of the type of centrifugal grinding mill shown in Figure 2 that the regions surrounding and below the grinding chamber 4 are permeated by slurry discharging from the grinding chamber 4. The slurry discharges with high velocity, impacting on, and rebounding as a dispersion of slurry particles, from the internal surface of the frame member 7, creating a zone of intensely turbulent high velocity particles of slurry. The maintenance of long term continuous operation of the grinding mill requires that the drive mechanism, located within this region of high velocity slurry particles, has effective sealing means to exclude all slurry contamination from the bearing cavities .

The drive shaft and chamber bearings 17 and 21 are protected from slurry contamination by three separate sealing units arranged in series. Each bearing operates in a sealed chamber fully immersed in lubricant. The sealed bearing chamber for the drive shaft bearing 17 is delineated by the first radial lip seals 22 and 23, whereas for the chamber bearing 21 a first single lip seal 30 provides complete sealing of the bearing chamber. The radial lip seals of the bearings 17 and 21 which in this example comprise the first stage of sealing, are further protected from contaminant entry by axial end face seal elements 24 and 32 respectively, comprising the second stage of sealing. Pressurised fluid, normally air, is fed to the internal cavities 44 and 45 of seals 24 and 32 respectively through distribution ports in the rotating shaft system. The pressure of fluid in the cavities is controlled to exert pressure forces in excess of the elastic forces acting within the seal, causing rotating sealing surfaces 25 and 34 to break contact with non-rotating surfaces 26 and 33 respectively, resulting in controlled flowrates of fluid to discharge from the seal cavities, and flow into seals 27 and 35. In normal operation the axial end face seals 24 and 32 operate with rotating and non-rotating surfaces 25 and 34, and 26 and 33 respectively, separated and lubricated by the pressurised fluid fed to seal cavities 44 and 45. This prevents abrasive wear of the adjacent seal surfaces.

When the mill is not operating, and the fluid flow to seal cavities 44 and 45 is discontinued, the adjacent seal surfaces 25 and 26, and 33 and 34, are held in close contact by elastic forces acting within the seals 24 and 32. This provides effective sealing against any contamination of seal cavities 44 and 45 in the event of the drive means becoming submerged in slurry caused by blockage of discharge pipe 47. The interstitial seals 27 and 35 in this embodiment comprise the third stage of sealing against contaminant entry to the bearings 17 and 21. These provide protection against entry of high velocity particles of slurry by the extended length of the gap between the adjacent rotating and stationary surfaces. In addition, fluid discharging from the axial end face seals 24 and 32 flows through the interstitial gaps of the interstitial seals 27 and 35, and prevents entry of fluid containing entrained water or fine slurry particles from penetrating the third seal stage. The three stage seal elements described above also protect the bearings 17 and 21 against contamination resulting from full submergence in slurry or liquid, in the event of blockage of the discharge pipe 47. In such event, the second stage axial end face seals 24 and 32, and third stage interstitial seals 27 and 35, would prevent any entry of contaminants to seal cavities 44 and 45, providing effective protection from contamination to bearings 17 and 21. Although the embodiment shown in Figures 4 and 5 indicate gas as the fluid commonly used to feed to seal cavities 44 and 45, other embodiments may be envisaged in which liquid is used as the fluid in lieu of gas.

As shown in Figure 6, the embodiments shown in the attached figures describe a centrifugal grinding mill in which the grinding chamber 4 is restrained from rotation about the vertical axis 1 by intermeshing bevel gears 46 and 47 disposed about the point of nutation symmetry 3. The movable bevel gear 47 is fixed to grinding chamber 4 which has nutating motion, and engages with the stationary gear 46, to transfer torsional moment from the grinding chamber 4 to the stationary frame 7. The centrifugal grinding mill described herein is not limited to mills having grinding chambers restrained from rotation, and may also be applied to alternative embodiments in which the grinding chamber is free to rotate about the nutating axis 2. In such alternative embodiments, it is common for the rotational speed of the grinding chamber 4 about its axis to be a small proportion, typically about two percent, of the nutating speed of the mill.

It is an important requirement of this embodiment of the invention that an axis 20 of the eccentric stub shaft 19 is co- linear with the nutating axis 2 within a small error tolerance. Small errors of co-linearity are absorbed by flexing of the elastomer 43 in the resilient drive coupling 39. However excessive errors will result in failure of the elastomer by fatigue. Errors in co-linearity of the axis result from accumulation of manufacturing tolerances of component parts to the mill, and cannot be avoided. In another embodiment (not shown) errors in co-linearity of the nutating and stub shaft axes 2 and 20 may be reduced by provision of bolt elements 48 of special design, incorporating and adjustable eccentricity between the shank and thread of the bolt. This enables eccentricity between axes 2 and 20 to be readily adjusted by angular displacement of bolts 48.

In the embodiment shown in the attached figures, lubrication of the bearings 17, 18, and 21 is provided by lubricant which is continuously recirculated through interconnecting passages in the rotating shaft elements, including 14 and 19. The recirculating lubricant provides cooling to remove excessive heat generated in bearings, and also removes contamination resulting from bearing wear and entry of any particles past the three stages of sealing. Lubricant discharging from the bearings is subsequently filtered to remove contaminants, and cooled by heat exchange equipment, if required, prior to recirculating to the bearings.

Now that a preferred embodiment of the various aspects of the present invention has been described in some detail it will be apparent to those skilled in the art that the centrifugal grinding mill, bearing seal arrangement, and flexible drive coupling have at least the following advantages over the admitted prior art: (1) driving of the grinding chamber remote from the point of nutation symmetry reduces the magnitude of drive forces applied to the grinding chamber, and axially separates the zones of drive application and feed entry to the chamber;

(2) the three stage sealing arrangement inhibits the ingress of contaminant materials to bearings which in this example are located within a highly contaminated environment in the grinding mill; and

(3) the flexible drive coupling which in this example is disposed between the grinding chamber and the stub shaft has low transmissibility of inertial forces in the plane of the fixed and nutating axes whilst exhibiting relatively high radial stiffness in an orthogonal plane so as to promote efficient transfer of torsional moment about the fixed axis from the stub shaft to the grinding chamber.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the specific construction of the driving means may vary from that described provided it engages the grinding chamber at an opposite axial extremity of said chamber relative to the point of nutation symmetry and effects nutation of the grinding chamber as described. The specific configuration of the seal arrangement and/or the flexible drive coupling may also vary provided they function in sealing a bearing or providing a resilient coupling as broadly disclosed in the specification. All such variations and modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A centrifugal grinding mill comprising: a grinding chamber having a longitudinal axis of symmetry, the grinding chamber being constrained and adapted to permit nutating motion of the axis of symmetry about a relatively stationary axis of the grinding mill, said axes intersecting at a point of nutation symmetry; a support member on which the grinding chamber is supported; and driving means operatively coupled to the grinding chamber at an opposite axial extremity of said chamber relative to the point of nutation symmetry, the driving means being designed to drive the grinding chamber to effect the nutating motion.

2. A centrifugal grinding mill as defined in claim 1 further comprising constraining means for determining the form of the nutation motion, the constraining means being connected to the support member and disposed about the point of nutation symmetry.

3. A centrifugal grinding mill as defined in claim 2 wherein the constraining means comprises annular bearing surfaces associated with the grinding chamber which engage complementary opposing bearing surfaces on the support member.

4. A centrifugal grinding mill as defined in claim 3 wherein said annular bearing surfaces comprise a spherical bearing element being symmetrical about the point of nutation symmetry and adapted to limit amplitude of the nutating motion.

5. A centrifugal grinding mill as defined in any one of the preceding claims also comprising a feed passage in communication with and directed towards the grinding chamber.

6. A centrifugal grinding mill as defined in claim 5 wherein the relatively stationary axis is substantially vertical and the feed passage is downwardly directed to the grinding chamber.

7. A centrifugal grinding mill as defined in any one of the preceding claims wherein the grinding chamber is provided with discharge openings located in a periphery wall of the chamber remote from the axis of symmetry.

8. A centrifugal grinding mill as defined in claim 7 wherein the discharge openings are generally circular in shape.

9. A centrifugal grinding mill as defined in claim 7 wherein the discharge openings are each in the form of elongated slots located in the periphery wall remote from the point of nutation symmetry.

10. A centrifugal grinding mill as defined in any one of the preceding claims wherein rotation of the grinding chamber about the axis of symmetry is prevented by a torque restraining device connecting the grinding chamber to the support member.

11. A centrifugal grinding mill as defined in any one of the preceding claims wherein said driving means comprises a drive shaft having an axis substantially co-linear with the stationary axis said drive shaft being driven in rotation from an end remote from the grinding chamber and having an eccentric stub shaft mounted at an opposite end adjacent the grinding chamber and adapted to engage therewith to effect the nutation motion, said stub shaft having an axis substantially co-linear with the axis of symmetry of the grinding chamber.

12. A centrifugal grinding mill as defined in claim 11 wherein the stub shaft engages the grinding chamber via a bearing element .

13. A centrifugal grinding mill as defined in claim 12 wherein the bearing element is resiliently mounted with respect to the grinding chamber.

14. A centrifugal grinding mill as defined in claim 12 wherein the bearing element is resiliently mounted with respect to the crank shaft.

15. A centrifugal grinding mill as defined in claim 13 or 14 herein the bearing element is mounted in a support frame which is provided with independent means to adjust eccentricity of the bearing element relative to the axis of symmetry.

16. A centrifugal grinding mill as defined in any one of claims 11 to 15 wherein an opposite end of the drive shaft adjacent the grinding chamber is mounted in a drive shaft bearing.

17. A centrifugal grinding mill as defined in claim 16 wherein the drive shaft bearing is provided with drive shaft bearing sealing means being designed to prevent the ingress of material discharged from the grinding chamber.

18. A centrifugal grinding mill as defined in any one of claims 12 to 15 wherein the bearing element is provided with bearing element sealing means being designed to prevent the ingress of contaminant material discharged from the grinding chamber.

19. A centrifugal grinding mill as defined in claim 17 or 18 wherein the sealing means includes a labyrinth sealing device.

20. A centrifugal grinding mill as defined in claim 19 wherein the sealing means includes a gas purged sealing device superposed between the labyrinth seal and the drive shaft bearing or the bearing elements.

21. A centrifugal grinding mill as defined in claim 20 wherein the gas purged sealing device comprises an annular cavity bounded at one end by a radial lip seal being configured to prevent leakage of lubricant from an adjacent bearing cavity and bounded at an opposite end by an axial end face seal.

22. A centrifugal grinding mill as defined in claim 21 wherein gas is fed to the annular cavity at a controlled pressure which is of sufficient magnitude to overcome interfac^'ial forces acting between faces of the axial end face seal and causing them to separate and discharge controlled flowrates of said gas.

23. A centrifugal grinding mill as defined in any one of the preceding claims wherein the grinding chamber is of a substantially circular cross section.

24. A bearing seal arrangement comprising: a first seal being axially spaced from a bearing in which a shaft is rotatably mounted, the first seal being adapted to sealably contain lubricant in a sealed bearing cavity disposed about the bearing; a second seal located axially adjacent the first seal and being configured to define a seal cavity adapted to receive a pressurised fluid; and a third seal disposed radially about and surrounding the second seal, said seals being mounted in series and together being adapted to prevent the ingress of contaminant material to the sealed bearing cavity.

25. A bearing seal arrangement as defined in claim 24 wherein the first seal is a stationary radial lip seal being designed to seal against the shaft to prevent the egress of the lubricant from the sealed bearing cavity.

26. A bearing seal arrangement as defined in claim 24 or 25 wherein the second or the third seal is an axial end face seal having a stationary seal component and a rotating seal component being adapted to fix to the shaft, said seal components have respective sealing faces which are urged to contact with one another by elastic forces within said second or third seals, the pressurised fluid causes said sealing faces to break contact with one another whereby said fluid is purged from the seal cavity at a controlled flowrate.

27. A bearing seal arrangement as defined in claim 24 or 25 wherein the second or the third seal is a bushing or a labyrinth type interstitial seal having a rotating seal component disposed in close proximity to a stationary component at least part of which is of a complementary shape to the rotating seal component .

28. A flexible drive coupling being designed to couple an eccentric stub shaft to a grinding chamber having a longitudinal axis of symmetry and being constrained within a grinding mill to permit nutating motion of the axis of symmetry about a relatively stationary axis of the grinding mill, said flexible drive coupling being characterised by its relatively low radial stiffness in a first plane intersecting both the axis of symmetry and the stationary axis and relatively high radial stiffness in a second plane being orthogonal to said first plane, whereby in operation minimal inertial forces and moments from the grinding chamber are transmitted to the stub shaft whilst maintaining a substantially complete transfer of torsional moments about the relatively stationary axis from the stub shaft to the grinding chamber.

29. A flexible drive coupling as defined in claim 28 comprising multiple parallel laminae of rigid material separated by and attached to layers of a resiliently flexible material and together being orientated with their planes substantially parallel to the first plane.