US3539116A - Vibratory grinding - Google Patents

Description

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FOREIGN PATENTS 907,049 6/1945 France............

[] Patented Nov. 10,1970

[32] Priority July 27, 1966 [33] Great Britain [31 33,766/66 259/72 .259/(Vib. Meeh.)

961,416 6/1964 GreatBritain..........::

, Bachmann (A.P.C.),'

Ser. v published May 4, 1943.

[54] VIBRATORY GRINDING 11 Claims, 4 Drawing Figs.

241/172, Primary ExaminerRobert C, Riordon 259/4, 259/108 Assistant Examiner-Donald G. Kelly [51] Int. 17/04, Attorney-lmirie, Smiley B02c17/16, B01f1l/00 Field 241/30, 21, l,46,46.02, 46.04, 46.17, 170, 171, 172, (Cursor-y), (Dispersion Digest); 259/ l 4-9 (Mechanic Vibrators Digest), 100, 108, 113 ABSTRACT: A quantity of grinding media and material to be References Cited processed is placed in a grinding chamber and subjected to UNlTED STATES PATENTS 2,350,534 6/1944 Rosinger vibratory grinding action by means of a pulsator extending into the mass, the chamber being maintained in a static position.

Patented Nov. 10, 1970 Sheet 1 Of 2 Inventor HENRY LQPODMORE A Home y;

Patented Nov. 10, 1970 3,539,116

Sheet & 012

' lnvenlor HENRY L. PanMoRE' Attorney;

VIBRATORY GRINDING BRIEF SUMMARY OF THE INVENTION A fixed grinding chamber has an electric motor mounted over it driving a shaft extending vertically downwards'and to the shaft a pulsator is connected, by means of a flexible coupling, so that the pulsator is rotated and by means of outof-balance weights carried by it imparts ahigh frequency low amplitude vibration to the mass of grinding media and material to be processed in the chamber.

Many types of vibration mills are now known in which grinding media is maintained ina state of high frequency vibration thereby effecting grinding of the charge material. In all known mills, the necessary energy is imparted to the media by applying a vibrational movement to the grinding chamber. For example, in one known application, as set forth in the specifications of our prior United Kingdom Pat. Nos. 813,654 and 862,535 the grinding chamber in the form of an annulus about a vertical axis, is supported on springs and vibrated by an electric motor driving out-of-balance weightsthe shaft of the motor being on the vertical axis with the weights disposed equally above and below the centre of the vibrating mass. Mills are also known in which the grinding media is stirred or caused to move at random by the action of rotating blades within the media and charge. Mills of the first type, in general, grind efficiently by producing high frequency impacts, while mills of the second type prove effective (by virtue of high shear forces) for dispersing powders in liquids.

The object of the present invention is to produce an improved form of vibration mill which will facilitate the production of both high frequency impact and shear forces.

According to the present invention a charge of material to be processed is fed into a grinding chamber containing grinding media, the chamber being maintained in a static position, and the charge and media are then subjected to a high frequency low-amplitude high frequency by the action of a pulsator extending into the material.

According to another aspect of the invention a vibration mill is provided with a grinding chamber which is maintained in a static position and a pulsator which extends into the chamber and is operated to set up a high frequency vibration of the material being processed.

In a preferred arrangement the pulsator is in the form of a single out-of-balance device located on the centre vertical axis of the grinding chamber and depending into the part of the chamber which will in use be occupied by the grinding media and charge of material.

Our experiments suggest that the best results will be achieved if the pulsator is capable of causing the entire mass of material in the chamber to vibrate through an amplitude of not less than one one-thousandth inch and not more than onehalf inch but these limits may be inapplicable in particular cases. The amplitude is governed mainly by the design of the pulsator and speed at which it is operated.

C onstructional forms of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a sectional elevation of a mill and associated equipment.

FIG. 2 is a section through a pulsator unit and FIG. 3 a plan thereof.

FIG. 4 is a plan of an alternative form of pulsator.

FIGS. 2, 3 and 4 are drawn to a larger scale than FIG. 1.

An electric motor is mounted centrally over a grinding chamber 11, the axis of the motor represented by its output shaft 10a and the axis of the chamber being vertical. The pulsator as shown in FIG. I is in the form of a circular hollow housing closed at the bottom and carried by a central shaft 16. This shaft is rapidly rotated by means of the motor 10 and universal joint coupling 19, and an out-of-balance weight causing the lower end of the housing 15 to vibrate through one cycle for each rotation so that in this case a 1,500 revolutions per minute motor will produce vibrations of 1,500 cycles per minute. The deflection is controlled by springs 17 mounted between plates 18 cam'ed by the shaft 16. This circular form of rotator produces a low starting torque, and wear on the pulsator is approximately uniform all round so that the working life is long. It is found advantageous to connect the motor shaft 10a to the pulsator shaft 16 through a flexible coupling. The work done isrelated to the revolutions per minute of the pulsator and the moment of inertia of the out-of-balance weight. Hence, by using a variable speed motor a wide range of grinding conditions can be produced.

In the preferred form of pulsator as shown in FIGS. 2 and 3 the housing 15 carries a tubular enclosure 20 on its inside wall, closed at the bottom by a welded plug 21. In the enclosure 20 weights 22 are inserted and the space above them is filled up by tubular distance pieces 23. The enclosure '20 is fixed in the top cover 24 of the housing 15 and its contained weights and distance pieces are held in place by a plug 25 having a square or similar upstanding boss 26 by means of which the plug can be screwed tightly into the enclosure 20. The shaft 16 fits into a hole in the cover 24 and is welded in position, a collar 27 being fitted round it.

In another form of pulsator, see FIG. 4, the housing 15 is of elliptical cross section, the enclosure 20 and weights 21 being omitted. The housing is rotated on the approximately vertical axis by the electric motor 10. The difference between the major and minor axes of the cross section of the pulsator determines the amplitude of the vibration applied to the grinding media which is in contact with the surface of the pulsator. The vibrational energy is then transmitted to adjacent pieces of media so causing a rapid succession of grinding impacts upon the charge material located in the interstices between the media. The rotary movement of the pulsator produces high shear forces in the media and between the surface of the pulsator and the adjacent media and hence such an arrangement will provide very effective fine grinding and in addition will disperse the products in the liquid present.

It will be understood that if the cross section of the pulsator is a simple ellipse, then for each rotation of the pulsator there will be two vibratory cycles so that if a 1,500 revolutions per minute motor is used, the vibrational frequency will be 3,000 cycles per minute.

Pulsator systems as described in either of the foregoing examples can maintain large quantities of media in a state of vibration. For many applications it is found that the most suitable shape for the grinding chamber is approximately cylindrical arranged symmetrically about the pulsator; the bottom of the pulsator being approximately 4 inches above the base of the grinding chamber. The chamber 11 may, however,

' as shown in FIG. 1 at 11a be frustoconical, wider at the base than at the top, and the bottom 1112 may be domed in shape, internally concave, to facilitate a central discharge. The housing 15 also preferably has a domed lower end 1511. Material to be processed is delivered into a feed and recirculation tank 36. Circulation is maintained by a pump 37 through pipes 38 controlled by valves 39, the chamber having a central discharge grid 40 at the base. The distance from the axis of the pulsator to the vertical wall of the grinding chamber is dependent upon the distance through the media that vibrational energy can effectively be transmitted and this in turn depends upon the type of grinding media, the nature of the charge material and the amplitude of the pulsator. In most cases, it is found that media 3 feet from the pulsator is maintained in active vibration and hence this will permit the use of a grinding chamber 6 feet in diameter. Normally the most satisfactory height-to-diameter ratio for the grinding chamber is approximately 3 :2;

The particular pulsators described tend to set up compound vibrations in the media and charge. Obviously as the pulsator moves outwards radially there will bean outward and inward movement of the material in a substantially horizontal plane. Since the lower end of the pulsator will have a greater displacement than the higher portion nearer the coupling, there will also be a vertical component in the movement of the material. The resultant compound vibration has been found to be of great value -in promoting efficient and expeditious processing of the material.

This system of grinding is equally effective for both dry and wet processing. For batch processing, it is merely necessary to charge the mill by way of the pipe 38 and after grinding for the requisite time to discharge through the grid 40 at the base, the apertures of the grid being large enough to pass the ground product but sufficiently small to retain the grinding media. The mill can also be operated continuously by introducing raw material at the top and removing the ground product at the base. Alternatively, for wet processing the slurry can be pumped in at the base and the ground product removed by overflow discharge; in this case a certain amount of classification is achieved particularly if the upper surface of the grinding media is a few inches below the surface of the slurry. Continuous dry grinding can also be effected by air sweeping the upper part of the grinding chamber.

The thin layer of liquid (in wet processes) or gas (in dry processes) carried round on the surface of the pulsator tends to produce a zone comparatively free from media and this reduces the frictional contact and pressure between the rotating pulsator and the media, the direct result being a considerable reduction in rate of wear on the pulsator.

The lining of the grinding chamber can be fabricated from a wide variety of materials according to the particular type of product. Manganese steel, stainless steel, or steel covered by rubber are suitable for most applications but at as the grinding chamber is completely static, it is possible to use virtually any type of lining including ceramic blocks. Similarly the pulsator can be fabricated from various types of steel, steel rubber covered or steel fitted with a ceramic sleeve. The latter, in conjunction with a ceramic lining, gives a mill admirably suited to the grinding of materials which are required to be free from all traces of metallic impurity. Such a mill is, for example, most effective for the processing of white paints.

The choice of grinding media will depend largely upon the particular process. In general, the composition of the grinding media should be such as to provide no disadvantages if introduced in very small quantity to the product. The media should have very high abrasion resistance and be capable of taking a high polish to give a smooth surface free from indentations. For most applications where both impact and shear forces are desirable the media should be spherical, or near spherical. The size of the media will vary according to the size of the material being processed and to the size of the mill but one-half inch or less diameter spheres are found suitable for most applications. Where a very large range of size reduction is required, it is advantageous to use media graded in size. Sand granules or glass ballotini may also be used.

I claim:

I. A vibratory grinding mill having a grinding chamber which is maintained in a static position, a bed of grinding media and material to be processed in said chamber, an elongate pulsator extending into the chamber so that its outer surface is in contact with said bed, and means for rotating said pulsator about an axis extending longitudinally thereof for simultaneously sweeping said outer surface of the pulsator past the material of the bed in contact therewith to produce shear forces in said bed and displacing the bed back and forth generally radially' of said pulsator to produce high frequency, low amplitude grinding impact movement of said bed.

2. A mill as claimed in claim 1 having the pulsator located upon a central vertical axis inthe grinding chamber.

3. A mill as claimed in claim 2 in which the pulsator comprises a central shaft, a housing attached to the shaft, an enclosure attached to the housing and off centre with respect to the shaft, and weights contained in the enclosure.

4. A mill as claimed in claim 2 in which the pulsator comprises a member of oval cross section attached to a central shaft.

5. A mill as claimed in claim 2 in which the pulsator comprises a housing attached to a central shaft and an out-ofbalance weight attached to the shaft within the housing.

6. A mill as claimed in claim 1 having an electric motor mounted centrally above the grinding chamber, a shaft driven by and depending vertically from the motor, and a flexible coupling including deflection control springs connecting the shaft with the pulsator.

7. A mill as claimed in claim 1 having a grinding chamber of frustoconical form, wider at the base, and having the base internally concave to facilitate central discharge therefrom, and a central grid in the said base for that purpose.

8. A vibratory grinding mill comprising, in combination:

a receptacle defining a vertical grinding chamber;

a vertically elongate pulsator having a longitudinal axis and projecting into said receptacle, said pulsator having an outer surface substantially smaller than said chamber and positioned therein to define a region around said outer surface between the outer surface of said pulsator and the inner surface of said receptacle for receiving grinding media;

a bed of grinding media within said region and being of a depth to bury a substantial vertical length of said pulsator therein, with said outer surface of the pulsator being in contact with said grinding media;

said pulsator adapted to displace a volume in said chamber in response to rotation of said pulsator about said axis which volume is greater than the volume of the buried portion of said pulsator and to this impart inward and outward grinding impact movement to said media; and

means for rapidly rotating said pulsator about said axis to produce high frequency low-amplitude grinding impact movement of said media and additionally to sweep said outer surface of the pulsator past the grinding media and thereby produce shear forces in said bed of media.

9. The vibratory grinding mill according to claim 8 wherein said pulsator is cylindrical, and including a flexible coupling connecting the upper end of said pulsator to said means whereby to allow the pulsator to cone therebelow.

10. The vibratory grinding mill according to claim 8 wherein said pulsator is of elliptical cross section.

11. The vibratory grinding mill according to claim 8 including a flexible coupling connecting the upper end of said pulsator to said means whereby to allow the pulsator to cone therebelow.

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