US3021082A - Vibratory ball mill system - Google Patents

Description

Feb. 13, 1962 R. M. E. SULLIVAN 3,021,032

VIBRATORY BALL MILL SYSTEM Filed Jan. 21, 1960 2 i 22 t I W22 3 l l 1 g t I' :70 I i I l 22 I i [2 I j l I l INVENTOR RALPH MAJOR EDWARD SULLIVAN 3,2l,82 Patented Feb. 13, 1%62 fice 3,021,082 VIBRATORY BALL MlLL SYSTEM Ralph Major Edward Sullivan, llayswater, London, England, assignor to National Research Development Corporation, London, England, a British corporation Filed Jan. 21, 1966, Ser. No. 3,759 Claims priority, application Great Britain Jan. 22, 1959 5 Claims. (Cl. 241-175) This invention relates to the generation of vibrations in loads, and aims at providing a method of and means for vibrating a load in the most eflicient manner. In particular, the invention relates to vibratory systems in which the frequency and mode of vibration of a load is imposed by an external exciter. A common example of such systems is the vibratory ball mill, to which particular reference will be made hereinafter.

Investigations have been made into the relative effectiveness of linear and circular or orbital forms of vibration in ball mill grinding, and it has been observed that the vibratory energy reappearing as heat in the contents of such a mill is much greater when circular or orbital excitation is applied than when either horizontal or vertical rectilinear excitation is applied. In this connection it must be noted that the vibratory displacement of a ball mill normally takes place in one plane, i.e. it is two-dimensional, its components being normally horizontal and vertical linear displacements.

Normally, a large vibrating ball mill is operated at a frequency in excess of the resonant frequency of the mill. It has been found, however, that by varying the form of the vibration at constant frequency, a large variation in rate of grinding is produced. Such variation in form can only be obtained by segregating the components of displacement in two mutually inclined directions preferably at right angles, and adjusting either in relation to the other.

The present invention is essentially a compound suspension in which each of two or more displacements in mutually inclined directions is controlled by a primary resilient suspension anchored to a foundation through a secondary resilient suspension. The primary resilient suspension may consist of respective individual resilient units each having one degree of freedom of deflection only, viz. that of the relevant component direction. A source of excitation is advantageously applied to the load directly or at the point of its attachment to the compound system.

In the design of vibratory systems generally, a well known parameter of the design to ensure effective isolation of the vibratory system from its surroundings is the resonant frequency of the loaded suspension. As a general rule, if the excitation frequency is approximately four times the resonant frequency, little or no vibration is transmitted through the suspension anchorages to the foundations of the system. This principle has also been assumed to have the further advantages that the spring stresses are low and their anchorages light.

It has now been found, however, that in vibratory ball mills a much increased rate of grinding can be obtained by the use of high frequency excitation with low amplitudes of displacement of the load. This kind of excitation produces fluidisation of at least the grinding balls, i.e. a state of random agitation extending throughout the mass with substantially uniform amplitude so as to give an apparent reduction in density throughout the mass. Furthermore, in such a modified system, power loss is considerably reduced, so that a lower power input can be tolerated for the increased grinding rate. Where excitation is provided by rotary eccentric masses, these can be much lighter, with a corresponding reduction in the size of bearings.

In an externally excited system for vibrating a load the present invention provides a primary resilient suspension for the load which is designed so as totransmit a major pro-portion of the total input excitation force. This means that the primary suspension is tuned at least approximately to resonance when under load and, although the damping effects of a granular or pulverulent charge in a ball mill are complex and difiicult to estimate, in practice the resonance curve normally exhibits a sharp peak. For all practical purposes, therefore an excitation frequency of the order of 0.9 times the resonant frequency is adequate in a ball mill operated in accordance with the present invention.

A resilient suspension of. high rate imposes on its anchorages a relatively high static loading at high frequency, so that the system will normally be required to be designed with a view to isolating the high frequency excitation transmitted by the suspension from surrounding buildings. Known techniques may be adopted for this purpose, as will be understood, but as the frequency of excitation increases in order to increase the grinding rate, the static loading on the anchorages of a resonant suspension also increases. Consequently, heavier foundations are required, and the known techniques for isolating them from their surroundings become much more diflicult and costly to apply. Although experiments have shown that the size of a mill, and hence its weight, can be reduced almost in proportion to the increase of resonant frequency excitation, and that this offsets to a significant degree the above-mentioned increase in static loading on the anchorages of the resonant suspension, the relative stiffness of a resonant or near-resonant suspension compared with that of a conventional suspension still imposes very large forces on its anchorages at high frequencies of excitation, and the problem of isolating heavy foundations is still formidable.

The present invention reduces this problem to a substantial extent by providing a secondary suspension, between the primary suspension and the foundation, which has a different rating from that of the primary suspension. Preferably, this secondary suspension has a resonant frequency much lower than that of the primary suspension, so that it can absorb the major proportion of the loading on the primary suspension anchorages. Hence, a much lighter foundation becomes possible, and final isolation thereof from a surrounding building becomes a much simpler problem.

The present invention also aims at further reducing the problem of foundation size and isolation by interconnecting a plurality of grinding vessels or containers by means of an inter-vessel suspension arranged as a selfbalancing or substantially self-balancing system having a minimum external resultant force, and mounting the as sembly on a relatively light foundation by means of a suspension having a substantially different resonant frequency.

Preferably, the vessels or containers are located at the corners of a polygon in a closed loop and each container is connected to its neighbour on either side by a suspension designed so as to transmit the major proportion of the input excitation force. Where four vessels are interconnected in the form of a square, each vessel is excited in anti-phase with its immediate neighbours.

As an alternative to a rotating out-of-balance system, the vibration may also be induced electro-magnetically by a suitable arrangement of electro-magnets between the individual grinding vessels, the coils of the magnets being energised from a suitable A.C. source.

When operating a vibration ball mill at or near natural frequency of the system, changes in the damping effects of the loose charge in a container, due to the cushioning action of the increasing fineness of the charge, cause relatively large changes in amplitude of vibration. In order to control these amplitude changes, the present invention also provides for some form of continuous amplitude monitoring device. If the out-of-balance force is generated by means of a rotating eccentric mass system, the output signal from the detector unit when the vibration amplitude rises above a predetermined level may be used to vary the excitation frequency in the sense for reducing the amplitude of vibration. If the vibration is excited electro-magnetically, the signal may be used to vary the input voltage to the coil of the electro-magnet.

The detector unit may be a photo-electric device, the light beam of which is arranged to be obturated to a greater or less extent as the vibration amplitude rises above a predetermined level or alternatively, a variable inductance or capacitance detecting device may be preferred.

A practical embodiment of the present invention'will now be described by way of illustration only with reference to the accompanying drawings in which:

FIGURE 1 is an end view, partly in section, of a fourvessel mill, and

FIGURE 2 is a plan view of FIGURE 1.

High grinding rates are obtainable with greatly reduced bearing stresses and consequent simplification of bearing design and improved efficiency due to lower heat losses. Both capital and running costs per unit weight of ground material can thus be substantially reduced.

FIGURES 1 and 2 of the drawings show a vibrating ball mill having four cylindrical grinding vessel-s or containers 1, 2, 3, 4 whose axes are parallel and equi-angularly spaced at the corners of an imaginary cube. Each pair of adjacent vessels 1 and 2, 2 and 3, etc. are coupled together by a respective primary suspension unit 5, 6, 7, 8 each of which is tuned to resonance or substantially to resonance at a relatively high excitation frequency. Each such inter-vessel primary suspension unit is shown as comprising a pair of similar leaf springs 9, '10 clamped together by a rigid spacer 11 at their mid-points and anchored at their ends by rigid blocks 12 to the respective vessel.

Through each vessel 1 4 passes a shaft 13 carrying small out-of-balance weights 14 at either end of the vessel. The shafts 13 are coupled through universal joints 15 to respective outputs 16 from a common gear box 17, and an electric motor 18 drives a common input 19. The gear box 17 drives each pair of weights 14 in anti-phase with the corresponding pair associated with an adjacent vessel, each pair of shafts 13 being contrarotated.

The entire assembly of vessels or containers 1 4 .and high natural frequency primary suspension units 8 is supported by a low natural frequency secondary suspension system consisting of helical springs 20 on a common foundation or bed 21, each spring 20 having an anchorage 22 fixed on or integral with a corresponding vessel 1 4.

When the vessels ll 4 are loaded and the motor 18 is run up to speed, the small out-of-balance weights 14 induce high frequency vibrations at or near the resonant or natural frequency of the primary suspension system Since the vessel assembly is symmetrically arranged, a high proportion of the excitation forces is balanced out in the primary suspension system 9, It 11, each vessel being, in effect, suspended from another mass constituted by the neighbouring vessels. Any resultant unbalanced force is absorbed by the low natural frequency secondary suspension springs 20 which thus serve to isolate the foundation or bed 21 from the vibrations. Thus it is possible to excite the vessels 1 4 at high frequencies with the high efficiencies resulting from the use of high natural frequency suspensions 5 8 without the need for very heavy and expensive foundations and their insulation from surrounding structures.

An alternative form of excitation mechanism is a polyphase electromagnetic system in which four or more electromagnets (not shown) are mounted on a ring yoke around the vessel assembly and use the vessels or containers 1 4 as armatures. Alternatively, armatures may be fixed to, or constituted by extensions of, the spacer blocks 1 If space permits, the magnets may be mounted within the ring of vessels 1 4'. Such a field system could be excited from the public mains supply, a frequency of 50 cycles per second giving a vibration rate of 3000 cycles per minute.

So long as the vessels or containers 1 4- are arranged symmetrically and each is excited in anti-phase with its immediate neighbours, the machine is very largely self-balancing with respect to resultant external vibrations, so that there is no theoretical limit to the number of vessels used. Clearly, however, practical considerations of complication and multiplicity of components requiring fairly close dynamic matching, together with considerations of accessibility and space occupied, will limit the number adopted in any one design of ball mill according to the invention.

FIGURES l and 2 also show a schematic arrangement of vibration amplitude detector which can be used in conjunction with a conventional form of electrical motor speed controller to keep the vibration amplitude below a predetermined maximum value. The detector consists of a light source 50 and photocell 51 mounted on opposite sides of the bed structure 21, and an apertured shutter 52 mounted on the vessel 4 with its aperture on the axis of the light beam from the source 50. So long as the amplitude of vibration of the vessel 4 remains within the predetermined limit, the photocell output is constant. Excessive amplitude of vibration, however, causes partial or total obscuration of the light falling on the photocell 51 during part of each half cycle of vibration of the vessel, with a corresponding change in output from the photocell which can operate an indicator or a motor speed controller.

The vessels 1 4 may be replaced by any other .piece of apparatus or equipment which it is desired to subject to oscillation. For example, the vibratory system may be used for classifying a mixture of granular mate rials having different particle sizes. Each vessel 1 4 would then be replaced by the tray or plate on which the granular material is placed for classification. Alternatively, bodies of liquid or liquid/solid mixtures may be vibrated to emulsify them or to promote intimate admixture where one constituent is insoluble in the liquid phase. In these applications of the invention, it may be necessary or preferable to induce the vibrations in the horizontal plane.

Other forms of excitation can be used if desired-for example, electromagnetic. In such a system the vessel assembly 1, 2, 3, 4 is arranged as a four-pole armature symmetrically located between the poles of four fixed electro-magnets respectively whose axes are equiangularly spaced. The magnet system is energised from a Z-phase supply in known manner. Such an armature will be subjected, when the fixed field system is energised from the supply, to radial forces which appear to rotate in the same manner as the rotating eccentric weight 7. An alternative excitation system is a pneumatic or hydraulic system.

I claim:

1. A vibration ball mill comprising four containers symmetrically arranged at equal spacings and adapted to carry grinding elements and a charge of material to be ground; a primary suspension unit interconnecting each pair of adjacent containers and tuned at least approximately to resonance at the frequency which induces a condition of fiuidization of the mass of grinding elements in each container, said primary suspension unit consisting of leaf springs coupled in series and interconnecting each pair of adjacent containers; an exciter for exciting each container in at least two mutually inclined directions, the excitation of each pair of adjacent containers being in anti-phase along one common direction; and a secondary suspension system having a lower natural frequency of vibration than that of the primary suspension.

2. A vibration ball mill comprising four cylindrical containers whose axes are parallel and equiangularly spaced around the circumference of an imaginary cylinder; 2. primary suspension unit interconnecting each pair of adjacent containers and consisting of a pair of leaf springs coupled in series and tuned approximately to resonance at a relatively high frequency; an eccentric exciting mass journalled on the axis of each container; means for driving each exciting mass in phase opposition to each adjacent mass; and a secondary suspension system supporting said primary suspension system and having a natural frequency lower than that of said primary system.

3. A vibrating load system comprising four symmetrically disposed load carriers; a high frequency leaf spring suspension interconnecting each pair of adjacent carriers for displacement in mutually inclined directions; an exciter for vibrating said carriers simultaneously in said directions at approximately the resonant frequency of said primary suspension; a secondary suspension supporting said carriers and said primary suspension and having a natural frequency remote from said resonant frequency, and a carrier vibration amplitude detector comprising an element mounted on a carrier and a fixed coacting element, and means for indicating relative displacements of said elements.

4. A vibration ball mill comprising a group of four symmetrically disposed vessels for containing a charge to be ground and a quantity of grinding elements; a high frequency primary leaf spring suspension interconnecting each pair of adjacent vessels for displacement along mutually inclined directions and tuned at least approximately to resonance at the desired high vibration frequency; means for exciting adjacent vessels in phase opposition; a secondary resilient suspension supporting said vessels and the primary suspension and having a natural frequency substantially lower than said vibration frequency; and vibration amplitude control means comprising a displacement detector associated with a vessel, and means operated by said detector for controlling the input energy to said exciter.

5. Apparatus according to claim 4 wherein the vibration amplitude detector comprises an apertured shutter mounted on the vessel, a fixed photocell, and a fixed light source arranged to irradiate said photocell through the aperture in said shutter.

References Cited in the file of this patent UNITED STATES PATENTS 2,760,729 Mittag et al. Aug. 28, 1956 FOREIGN PATENTS 633,699 Germany July 16, 1936 936,003 Germany Dec. 1, 1955

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