Method and apparatus for grinding material

Abstract: A sonic generator comprises a resonant bar (11), a housing (20) and magnetic excitation (13) units connected between the resonant bar (11) and the housing (20). The magnetic excitation units (13) are connected to the bar (11) at highly stressed points using a mounting connection (14) which is a sleeve having an inside diameter slightly greater than the outside diameter of the bar (11). A resilient elastomer (21) such as urethane is mounted between the sleeve (14) and the resonant member (11). The sonic generator may be used for grinding purposes and, in particular, may be used to grind material exceptionally fine with reduced power consumption. The bar (11) is substantially unrestrained so as to allow substantially free vibration.

Description: This invention relates to a method and apparatus for grinding material employing a sonic generator used for the transmission of energy into fluid mediums and, more particularly, to a sonic generator in which the resonant member is substantially unrestrained when vibrating. Sonic generators used to convert electrical energy into acoustic and kinetic energy for transmission to fluid mediums are known. Such devices are illustrated, for example, in U.K. Patent Specification 2,152,728 to Bodine and U.S. Patent 2,468,515 to Robinson. The sonic generators there disclosed, however, suffer disadvantages. For example, the apparatus taught by Bodine in the aforementioned U. K. patent utilizes resonant drives similar to the types described in Bodine's U.S. Patents 3,633,877, 3,684,037, 3,360,056 and 4,265,129. Such drives limit the upper range of frequencies such units can transmit to a fluid medium. Since the transmission of acoustic power between the resonant bar and the fluid is less efficient at lower frequencies, the frequency limitation lowers the acoustic efficiency of the Bodine apparatuses. Bodine also necessarily utilizes a coupling between his orbiting mass oscillators and his resonant bar which attempts to isolate the inertial forces of the vibrating bar from the oscillators. Without such a coupling, the magnitude of the forces generated are sufficient to cause relatively rapid failure of the orbiting mass oscillator drive motors, be they hydraulic or electric. The above-identified Robinson reference teaches restraining a resonating bar with steel bushings at respectively oppositely located end portions of the bar. such supports cause energy to be lost through the steel bushing support structure which energy would be better utilized in the fluid medium. This is so since Robinson's method does not allow for free and unrestrained vibration of the resonant member. Rather the resonant member is forced to assume a specific mode shape by the steel retaining bushings. Robinson's structure also imposes a very high stress concentration on the resonant member both at the support point and at the point of maximum bending stress. Such stress concentration can eventually cause unnecessary damage to the resonant member and/or premature failure. It also limits the mechanical stresses that can be sustained by the member without failure. Existing vibratory grinders generally consist of a rigid housing containing the grinding media such as steel balls mounted on a spring system for support and vibration isolation. The vibration is transmitted through a rotating unbalanced shaft that is rigidly attached to the body of the grinding unit or through an electric motor using an integral unbalanced weight mounted directly to the housing. The unbalanced shaft embodiment is typically driven by a standard electric motor through a cardan shaft to isolate the motor from the vibration or the eccentric weight which is integral to the electric motor If the latter, the electric motor vibrates with essentially the same intensity at the grinder itself. When inducing vibration with an eccentric weight, forces are introduced which must be transmitted from the rotating member through bearings to the body of the grinder. These forces increase with the square of the vibration frequency which practically limits the frequency of the vibration to 30 Hz in commercial scale applications because of the very large force required to vibrate the mass of the grinder unit as the frequency increases. As the rotational frequency increases, the roller bearing becomes limited in load carrying capacity. When existing vibratory designs are referred to as resonant grinders, it merely means that the vibration frequency corresponds to the natural frequency of the spring-mass system used to isolate the grinder. Resonant operation of such grinders will increase the energy efficiency in the system. However, it is difficult to strain the necessary vibration amplitude for the grinder with the required spring stiffness for the large grinding masses. From AU-A-419,984 a grinding method and a grinding apparatus according to the preamble of claim 4 are known. The grinding material is crushed between a fixed jaw and a movable jaw, the movable jaw being driven by longitudinal vibrations of the member. From DE-C-872 144 a method of grinding material and a grinding apparatus are known, wherein the material is crunched in a ball oscillating mill having a grinding chamber. It is supported by tension and pressure springs as resilient supporting means. It is the object of the present invention to provide an apparatus and a method that avoids the initially mentioned draw backs and which in particular provide an improved power transmission to the grinding material. This object is solved by a method according to claim 1 or an apparatus according to claim 4. Preferred developments of the invention are given in the corresponding subclaims. A specific embodiment of the invention will now be described, by way of example only, with the use of drawings in which: Figure 1A is a diagrammatic front view of a sonic generator used in a grinding apparatus of an embodiment of the invention; Figure 1B is a diagrammatic view of the second mode shape of the generator corresponding to Figure 1A; Figure 2 is an end view of the sonic generator of Figure 1; Figures 3A - 3D diagrammatically illustrate the resonant member in its various characteristic mode shapes; Figure 4 is an enlarged side view of the motor mounting area of the sonic generator according to the invention; Figure 5 is an end of the motor mounting area of Figure 4 taken along the plane V-V of Figure 4; Figure 6a is a side view illustrating the sonic generator in operating condition in an agitated tank; Figure 6b is a top view of the sonic generator at Figure 6a; Figure 7 is an end view of the electromagnetic excitation units and the armatures attached to the bar through the mounting means; Figure 8 is a partial side view of Figure 7; Figure 9A is a partial sectional side view of the resonant member of Figure 1 with grinding chambers attached forming one embodiment of the grinding apparatus of the invention; Figure 9B is an end view taken from the right of Figure 9A; Figure 9C is an end sectional view taken along IXB - IXB of Figure 9A; and Figure 10 is a diagrammatic view of the resonant member of Figure 9 at the first resonant frequency. Referring now to the drawings, a sonic generator is illustrated generally at 10 in Figure 1A. It comprises a resonant member such as a bar or tube 11 extending substantially the length of the generator 10 and being mounted, at one end, within a housing 20. Two variable frequency electromagnetic excitation units 13 (only one of which is illustrated in Figure 9A), each consisting of one electromagnet per phase are energized by a three phase AC power supply (not illustrated). The armature components are rigidly mounted using a bolted, clamped or welded connection. They can be directly mounted to the bar if located at a free end or, if located otherwise, to an excitation unit isolation sleeve 14. The sleeve 14 is made from a metal tube, preferably steel, concentrically located about the resonant bar and having an inside diameter 22 slightly greater than the outside diameter of the resonant bar 11 and having a length which is desirably at least as great as its diameter. The annular space between the inside diameter of the isolation sleeve 14 and the outside diameter of the resonant bar 11 is solidly filled with a resilient elastomeric compound 21 such as urethane. The urethane further solidly bonds the sleeve 14 to the bar 11. In operation, it will be assumed that the sonic generator 10 is to be mounted in a fluid medium such as illustrated in Figure 6, such that when the sonic generator 10 is operative, energy is transferred from the resonant bar 11 of the sonic generator 10 to the fluid medium 15. The mode shape of the resonant member 11 including the isolation sleeves 14 and the armatures 12 is known for the various natural frequencies of the system. For example and with reference to Figure 3, the undeflected resonant bar 11 is illustrated diagrammatically in Figure 3A and the three mode shapes for the three lowest natural frequencies of the member 11 are illustrated in Figures 3B, 3C and 3D, respectively. To excite the resonant member 11, the magnetic excitation units 13 are energized at the desired natural frequency of the resonant member 11 by a variable frequency three pase AC power supply (not shown). Each phase of the power supply energizes one of three phases of the magnetic excitation units 13. Each phase of the excitation units 13 is spaced at 120 degrees radially about the resonant member 11 as illustrated in Figure 2 such that the force vector produced by the excitation unit rotates at a constant rate about the longitudinal axis 16 of the resonant member 11 at the driven frequency. This causes a three dimensional, nutational vibration of the resonant member 11 which allows acoustic energy to propagate radially off of the resonant bar 11 in all directions. When the member 11 is excited at its natural frequency, an increased power transmission ability for a given excitation force is obtained. The resonant bar 11 is elastically mounted within the housing 20, as illustrated in Figure 1A, at the node points illustrated diagrammatically in Figure 1B where there is substantially zero vibration amplitude of the resonant bar 11. It is advantageous to mount the resonant bar 11 to the housing 20 at these points since, with such a mounting, relatively little power is lost through the nodal support positioner 23 and, since there is little or no movement of the resonant bar 11 at these node points when the resonant bar 11 is resonating, locating the resonant system, particularly the armatures 12, with respect to the electromagnetic excitation units 13 with the nodal support positioners 23 can be accomplished with good accuracy as illustrated in Figures 7 and 8. As seen in Figure 1B, four (4) points of maximum amplitude are illustrated. These points, shown at 24, 25, 30 and 31, are anti-nodes. Within the housing 20, the excitation unit armatures 12 are mounted to the resonant bar 11 through the isolation sleeves 14 and resilient elastomeric compound 21 at the highly stressed point or anti-node 30. A resilient mounting means 21 is not required at the anti-node 31 because the free end of the bar 11 is substantially non-stressed. The power transmission between both the excitation unit electromagnets 13 and the armatures 12, and the resonant bar 11 and the fluid medium 15 (Figure 6A) in which the sonic generator 10 is mounted, is most efficient at the anti-nodes, since power is transferred both to the fluid medium 15 from the resonant bar 11, and to the resonant member 11 from the electromagnetic excitation units 13, in direct proportion to the amplitude of vibration of the resonant bar 11. The use of the resilient elastomeric component and the sleeve 14 reduces stress concentrations applied to the resonant bar 11 caused by rigidly mounting, as by clamping or bolting or welding, the excitation unit armatures 12 directly to the bar. This is so because the free surface area of the elastomer 21 is quite small relative to the constrained surface area, which results in a layer which has very little compressibility. Thus, it will transmit substantially all of the force between the components of the resonant member 11, while at the same time the elastomer 21 allows the resonant bar 11 to freely flex in a substantially unrestrained configuration in its characteristic mode shape by absorbing the small relative deflections of the bar 11 caused by the flexure inherent in the characteristic mode shape. While the elastomeric substance 21 used between the resonant bar 11 and the sleeve 14 is described as being urethane, it is clear that many other substances could be used with the appropriate operating characteristics including, as described, a resilient elastomeric component. The sonic generator 10 has been described as being used in a fluid medium and it should be understood that such a fluid medium could be a liquid, a gas or a solid which has been fluidized by grinding to a finite particle size. Reference is now made to Figures 9 and 10 where the above described sonic generator 35 is utilized in a grinding environment. A resonant member 32 is supported on nodal locations 33, 34 by airbags 36 (Figure 9C) which can be inflated and placed at the nodal points 33, 34 of the resonant member 32, which nodal points have been calculated or have been found by simply resonating the resonant member 32 and observing the nodal locations. Super-Cushion Air Spring airbags manufactured by Goodyear have been found appropriate. The electromagnetic drive units 40, 41 are connected to the resonant member 32 preferably at the anti-nodes 42, 43 (Figure 9A) and are excited at the appropriate resonant frequency of the member 32. As previously described, the orientation of the excitation units causes a three dimensional, nutational vibration of the resonant member 32 which translates into a circular motion of the grinding chambers 44, 50 when viewed along the axis of the resonant member 32 which is important to grinding efficiency. The grinding chambers 44, 50 are mounted to the resonant member 32, preferably also at the anti-nodes 42, 43 by bolts 47. The grinding chambers 44, 50 and the drive units 40, 41 are ridigly connected to the resonant member 32 and for the purposes of determining the mode shape and resonance frequencies, become essentially a part of the resonant member 32. The grinding chambers 44, 50 are filled with grinding media 45 such as steel, ceramic, cast iron, etc. The shape of the media 45 is arbitrary although balls or cylinders are preferable. The containers 44, 50 are filled with steel balls of an appropriate size, for example and the material to be ground will be fluidized in order to proceed through the media 45 in the grinding chambers 44, 50. The material to be ground is fed into the grinding media 45 in the grinding chambers 44, 50 through ingress 52, and out the egress 51 of the grinding chambers 44, 50. Control of the material to be ground can be accomplished, for example, by pumping the fluidized material in to the grinding chambers 44, 50 at the desired rate. For grinding a typical gold bearing ore, it is contemplated that the resonant member 32 will be a steel bar having a diameter of approximately 13 inches and a length of approximately 120 inches. With such dimensions, a natural frequency of the resonant member 32 would be approximately 120 Hz. The electromagnetic drive motors 40, 41 will have a power rating of approximately 75 kW total and one will be mounted at each of the outer anti-nodes 42, 43. At the resonant frequency of 120 Hz, there will be three anti-nodes 43, 42, 46 and two nodes 33, 34 as seen in Figure 10, one anti-node 46 being located at the centre of the resonant member 32. It is contemplated that each of the grinding chambers 44, 50 would have a volume of approximately 15 litres with dimensions of approximately 20.3 cm (8 in.) diameter and about 45.7 cm (18 in.) length. Replaceable liners are also intended to be used for convenient replacement when the liners are worn. For gold, a grinding media such as steel, tungsten carbide, zirconia, alumina, etc. could be used. With the apparatus as contemplated and given above, there is intended to be size reduction from 1500 to 50 µm at a rate of 500 tonnes per day. It will be appreciated that these quantities are approximations only and that under actual operating conditions, they may change substantially.

Claims: A method of grinding material comprising supporting a bar-type or tube-type member (32) having at least one resonant frequency at nodal points (33, 34) such that said member (32) is substantially unrestrained, rigidly mounting at least one grinding chamber (44, 50) on said member (32) at a point (42, 43) other than said nodal points (33, 34), introducing grinding media (45) into said grinding chamber (44, 50), electromagnetically exciting said unrestrained member (32) at a resonant frequency of said member to vibrate in the transverse direction at said point (42, 43) other than said nodal points (33, 34), introducing material to be ground into said grinding chamber (44, 50) and removing said material after passing through said grinding media (45). A method as in claim 1 wherein said grinding chamber (44, 50) is located at an anti-node (42, 43) of said member (32). A method as in claim 1 or 2 wherein said member (32) is electromagnetically excited with an electromagnet located at an anti-node (46) of said member (32). Grinding apparatus comprising a bar-type or tube-type member (32) having a plurality of nodes (33, 34) and anti-nodes (42, 43, 46) at a resonant frequency, mounting means (36) located between a housing and said member (32) at at least two nodes (33, 34) of said member (32), electromagnetic drive means (40, 41) mounted to said member (32) at at least one anti-node (46), characterized by at least one grinding chamber (44, 50) mounted to said member (32) close to an anti-node (42, 43), and wherein the mounting means (36) are resilient and the member (32) is vibratable in the transverse direction at said anti-nodes (42, 43, 46). Apparatus as in claim 4 and further comprising means to allow material (45) to be ground ingress to and egress from said grinding chamber (44, 50). Apparatus as in claim 4 or 5 wherein said grinding chamber (44, 50) is integral with said member (32). Apparatus as in claim 4 or 5 wherein said grinding chamber (44, 50) is ridigly connected to said member (32). Apparatus as in one of claims 4 through 7 wherein said member (32) is free to vibrate in the natural mode shape of the member (32) without substantial constraint. Apparatus as in claim 8 wherein said vibration supplied to said grinding chamber (44, 50) is substantially circular. Apparatus as in one of claims 4 through 9 wherein said resilient mounting means includes air bags (36).

Inventors: BRDICKO JAN, JACKSON JAMES G, NYBERG CHRISTOPHER A, BRDICKO, JAN, JACKSON, JAMES G., NYBERG, CHRISTOPHER A.
Applicants: ARC SONICS INC, ARC SONICS INC.
Application year: 1989
Main classification: B06B3
IPC codes: B02C17, B06B3, A61B17