GB2047118A - Mill for grinding solids - Google Patents

Abstract

A mill for grinding solids, has milling surfaces 14, 27 which are covered with grooves comparable with the size of the particles to be ground and extending over substantially all of the milling surfaces transversely to the direction of relative movement (rotation) of the surfaces. The milling surfaces have means 23 for maintaining predeterminable spacing and pressure between them, and are preferably annular. The operation of the mill grips the particles in the grooves so that, effectively, the grooved surfaces carry a layer of particles with them and milling takes place in the mass of particles (i.e. by particle to particle abrasion) rather than at the milling surfaces themselves. <IMAGE>

Description

SPECIFICATION Mill for grinding solids The invention relates to a mill for grinding solids, and more particularly to a mill for grinding solids wherein the attrition of the solid particles is carried out in a manner which improves the efficiency of the grinding process and the uniformity and/or predictability of the size of the milled particles.

It is already known to mill solids by subjecting them to pressure between two relatively moving surfaces, most commonly rotating surfaces. The procedures and machinery so far known, however, have the disadvantage that they are not sufficiently precise or predictable in operation to enable the operator to achieve a desired particle size in the milled product without first starting up the mill and making adjustments until the desired results are obtained.

Also many forms of mill presently in use tend to be inefficient, as they grind only a small proportion of the particles present in the mill.

We have now found that the disadvantages may be overcome by constructing the mill with two milling surfaces covered with grooves which are made to dimensions which assist the milling surfaces to grip the particles which are to be ground, and thereby also promote attrition between the particles thus gripped, either with each other or with particles which are not adjacent to the milling surfaces, as relative movement between the milling surfaces takes place. We have also discovered that this form of mill can be operated so as to subject the particles being milled to stress and strain rates which are calculable, and reproducible.

Thus according to our invention we provide amill comprising:- (a) two milling members having milling surfaces adapted so that, by movement of one or both, a relative motion between the two can be maintained, (b) grooves in both the said milling surfaces (i) which are in directions transverse to the direction of relative motion, (ii) are of dimensions comparable with the size of particles to be ground and (iii) cover substantially the whole of said surfaces so as to leave no significant flat surfaces between the grooves.

(c) means for causing relative motion between the milling surfaces by motion of one or both of the milling members.

(d) means for forcing one milling surface towards the other so as to maintain a predetermined space between the said surfaces or a predetermined pressure on particles placed within the space for milling.

The space may be determined in terms of specific measurements, or more conveniently, in terms of a specified quantity of material to be ground.

The grooves in the milling surfaces may lie perpendicular to the line of relative motion of the surfaces, i.e. they may be radially disposed on the annuli, radiating from the geometric centres of the annuli. This radial disposition is not essential, and the grooves may if desired be disposed at an angle to the radii of the annuli. The radial disposition, however, is usually easier to construct.

The dimensions of the grooves should be such as to retain a layer of the particles to be ground over the milling surface. This achieved by making their dimensions such that: (a) The depth is in the range 0.1 to 1 times the mean diameter of the largest particles to be ground, and(b) the width is in the range 0.1 to 1 times the mean diameter ofthe largest particles to be ground.

The cross-section of the grooves is most conveniently a simple "V" form, principally because this is easy to fabricate and is less likely to retain the particles (and fragments abraded from them) too firmly. A variety of other cross-sections for the grooves may be used, however, as the requirement is for the groove to grip the particles sufficiently firmly to carry them along with the millling surface but not to grip them so firmly that the particles cannot be dislodged from them. Thus, examples of alternative cross-sections include curved (e.g. semicircular), trapezoidal (e.g. wedge-shaped) or combinations of these. It is preferred that lip of the grooves is not narrower than the body of the groove within the milling surface.

The coverage of the milling surfaces should be sufficient to leave no substantial area of the milling surfaces flat, because flat areas contribute relatively little to the milling action. The milling surfaces may be of any convenient form which permits the desired relative motion. For example, they may be in the form of belts or spinning plates, but we much prefer them to be annular surfaces. When using annular surfaces, the desired coverage of the milling surfaces with grooves of such dimensions that the edges of the grooves at the outer edge of the grooved milling surface are separated by a distance which is less than the width of the grooves. As a general guide, we find that the best control of the strain rate in the particles is achieved when the annulus is narrow in relation to the radius of the annulus itself.

The annular milling surfaces may be made of any material which is of appropriate hardness, usually a hardness greater than that of the material to be ground. In most cases the material will be a metal, for example mild steel or stainless steel. The choice is thus made on the basis of durability in service.

The grooves therein may be made by means well known in the art, for example by casting, machining or the like.

The annular surfaces are usually carried on rotating carrier members, which may be made of mate rials which are appropriate for the fabrication, operation and any necessary balancing of the moving parts. Usually these too are of metal, but the metal for these may be less hard, for example a light metal (e.g. aluminium or an aluminium alloy).

The relative motion of the milling surfaces may be achieved by mounting the two milling members so that their annular milling surfaces have a common geometric centre and then one is adapted to be rotatable (preferably in a suitable bearing) relative to the other. The use of a common geometric centre is most advantageous, but is not absolutely necessary provided the desired relative motion of the two surfaces is achieved.

We find it more convenient to maintain the upper milling member stationary and rotate the lower one, but the opposite arrangement may be used if desired.

To make a complete mill the two milling members are mounted, with their annular milling surfaces coaxial and together with such shafts and bearings as are required to ensure their desired relative rotation, inside a casting which also carries the means for forcing the milling surfaces together at predetermined pressure.

The means for forcing the milling surfaces together are most conveniently hydraulic or pneumatic devices, as the pressure exerted by these is most readily controllable. Alternative means, for example springs, may be used if desired. Preferably these means are used in a multiple assembly, so as to achieve more evenly controlled pressures over the whole a.rea of the milling surfaces.

In operation at constant volume the means for maintaining the desired spacing between the milling surfaces may be any conveniently placed stops or bearing rings on one or other of the milling members or on the members on which they are supported or rotated. These stops or rings should be placed so that they do not engage the grooved annular milling surfaces. In operation at constant pressure the predetermined pressure between the milling faces during milling may be achieved by applying pressure-exerting devices to one of the milling surfaces. The precise pressure to be exerted in any particular case depends upon the particular particles being ground (i.e. their physical properties) and the size to which they are to be ground.

In operation one of the milling surfaces is rotated, relative to the other, by means of any appropriate motor or its equivalent and the particles to be milled are fed into the space between the surfaces. The particles are held within the grooves of the surfaces and contained therein so that they become substantially static at the milling surface and the attrition is then achieved by these trapped particles and those in neighbouring layers or zones being abraded by their relative motion instead of being rolled or ground between the milling surfaces themselves, as occurs in conventional milling devices.This has the advantages of reducing the wear on the milling surfaces themselves and also controlling more closely the actual attrition of the particles, as this now occurs under conditions in which the sizes of the particles, their relative motions, and the pressures between particles are all more specifically determinable and the results can therefore be calculated and predicted with much more certainty when the relative "trial and error" conditions in conventional mills. This can help to eliminate the need or lengthy and expensive trials to determine the efficiency of the mill and the size of particles it can produce from a given feed.

The milling operation does not require a great depth of particles between the milling surfaces, and indeed great depths are to be avoided as wasteful, since the stresses and strain rates are then not so well controlled. The preferred spacing ofthe milling surfaces is in the range 1 to 25 times the mean particle diameter of the material to be milled, and preferable 5 to 15 times said diameter.

As the mill operates, the particles are pressed and rubbed against each other, and the fragments thus produced tend to accumulate in the lower regions of the particle bed, and even in the grooves of the lower face. Consequently, it is sometimes possible to improve the efficiency of the mill by providing it with means by which it can be inverted periodically. This allows fresh particles to be ground, and this segregation (and hence unequal exposure) of the particles to be minimised.

The present invention represents a valuable advance in the art of milling solids, partly because it provides a means for enabling the milling process to be studied but especially because it enables a mill to be designed specificallyforthe grinding of particulate materials and the degree of grinding to be controlled and predicted more reliably. This greatly reduces the wasteful uncertainty of mills in which the milled output may vary considerably from time to time as different feeds are used.

An especially useful application for the milling device of the present invention is in the study of the milling process and of the properties of materials when subjected to milling, and in the gaining of knowledge systematically and rationally whether in this particular new form of mill or in the older prior art devices). The operation and control of the mill may utilise conventional motors, speed regulators, measuring devices, sensors and the like, as may be appropriate for the measurement or study of the particular materials and/or parameters of interest to the user. Examples include arrangements for regulating the speed of the rotation of the milling surfaces (e.g. maintaining this constant) or for counting the number of revolutions of the annulus or the time of milling.If desired, the equipment may include controls which give the mill a suitably pre-determined operating cycle, to facilitate repetition oftests.

The versatility of the mill may also be increased by making the grooved milling surfaces on members which can be interchangeable, so that a grooved surface having grooving appropriate to one material can be readily replaced by another having grooving appropriate to a different one.

The invention is illustrated but not limited by the accompanying drawings in which Figure 1 represents a vertical sectional view through an apparatus according to one embodiment of the present invention, and Figures 2 and 3 represent horizontal sectional views, downwardly and upwardly respectively, along the line A-A of Figure 1. All the said drawings are schematic and not to scale.

The numerals used to indicate the parts in the drawings represent the same items in all drawings.

The apparatus represented comprises two supporting frame members or brackets (1) and (2) carrying bearings (3) and (4) respectively which fit within surrounding rims (5) and (6) respectively, attached to the opposing outer sides of a frame (7).

This permits the whole frame to be rotated about a horizontal axis (by means not shown).

The said frame (7) may be of closed or open structure and carries in its lower (as depicted in the drawing) face (8) a bearing (9) through which passes a vertical shaft (10). On this shaft (10) are carried a heavy central spindle (11) within the frame and an electric motor (12) below and outside the frame.

The heavy central spindle (11) is thus rotatable by the electric motor (12), within the frame (7). This spindle (11) carries a disc (13) on which is cut an annular band or track (14) of closely spaced radial grooves. This track (14) is bounded by two cylindrical walls (15) and (16), disposed coaxially with each other and with the spindle (11), so as to define an annular channel (17) with smooth sides and a transversely grooved bottom face.

The upper face of the frame (20) carries multiple pneumatic cylinders (21) within which, by application of pneumatic pressure through inlet pipes (22), move pneumatic plungers (23). These plungers (23) are secured to a locking ring (24), which is substantially parallel to the disc (13) on spindle (11), bearing on its lower face a cylindrical member (25) whch carries at its lower extremity, an annular ring (26).

This annular ring (26) is made to dimensions which allow it to fit exactly within the annular channel (17) without engaging with its side walls (15) and (16), and carries on its lower face (27) a band of closely spaced radial grooves. These grooves are substantially the same as those on the annular track (14) on the lower disc (13).

In operation, the material to be milled is fed into the annular channel (17), and the annular ring (26) is lowered into the chanel (17) so that it is then confined between the two grooved bands (14) and (27) within the retaining walls (15) and (16). The quantity of material fed in is required so as to achieve the desired depth of solid particles within the said channel.

Limitation of the movement of the faces towards each other is achieved by a collar (28) inside the loading ring, so that the two faces cannot come into direct contact with each other.

The disc (13) is then rotated by the electric motor (12) while the other disc (24) and the annulus (26) attached to it is impelled towards it by pneumatic pressure fed in through inlets (22) and acting on the plungers (23). The speed of rotation and the pressure between the opposing faces are set to predetermined magnitude, and milling is carried on. This occurs by the particles being engaged in the grooves of the two grooved tracks and thus being held to form two relatively moving bands of particles separated by an intermediate layer of particles which thus abrade each other and are broken down.

If it is desired to avoid particle size segregation, the whole frame (7) (together with its attendant discs, pneumatic cylinders, etc.) is inverted by rotation about the bearings (3) and (4) so as to invert the layer of abraded and unabraded particles. Then, the milling is continued.

This operation may be continued and repeated as often as desired. Finally the ground material may be discharged by reverting the frame to its original orientation, releasing the pneumatic pressure, and separating the discs (13) and (24).

In operation, the mill may be run in any comment orientation. Usually it is most convenientfor the milling surfaces to be substantially horizontal, but this is not essential. Similarly, the mill may be partly or completely inverted, usually through about 180 degrees but optionally more or less if desired.

The pneumatic plungers may be replaced if desired by other pressure devices, for example hydraulic plungers. It is preferred to use those which can be used to measure or predetermine the pressure they exert.

Claims (8)

1. A mill comprising two milling members having milling surfaces adapted so that relative motion between the two can be maintained, and means for causing such relative motion by movement of one or both of the milling members, wherein the milling surfaces are substantially completely covered with grooves which are in directions transverse to the direction of relative motion and are of dimensions comparable with the size of the particles to be ground, and means are provided for maintaining a predetermined space between the said surfaces or a predetermined pressure on material placed within the space for milling.

2. A mill as claimed in Claim 1 wherein the dimensions of the grooves are of such dimensions that (a) their depth is in the range 0.1 to 1 times the mean diameter of the largest particle to be ground, and (b) their width is in the range 0.1 to 1 times the mean diameter of the largest particle to be ground.

3. A mill as claimed in Claim 1 or Claim 2 wherein the milling surfaces are annular in form.

4. A mill as claimed in Claim 3 wherein the grooves are radially disposed on the annular surfaces.

5. A mill as claimed in any one of Claims 1 to 4 wherein the spacing between the milling surfaces is in the range 1 to 25 times the mean particle diameter of the material to be milled.

6. A mill as claimed in Claim 5 wherein the spacing between the milling surfaces is in the range 5 to 15 times the mean particle diameter of the material to be milled.

7. A mill as claimed in any one of Claims 1 to 6 wherein the mill is provided with means for inverting it.

8. Method of operation of a mill as claimed in any one of Claims 1 to 7, adapted for purposes of study and control of the milling process.