Method and apparatus for high frequency screening of materials

Jan. 20, 1970 l. balamuth 3,490,584

METHOD AND APPARATUS FOR HIGH FREQUENCY SCREENING OF MATERIALS

Filed Aug. 31, 1965

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United States Patent Office

3,490,584

Patented Jan. 20, 1970

3,490,584 METHOD AND APPARATUS FOR HIGH FREQUENCY SCREENING OF MATERIALS Lewis Balamuth, New York, N.Y., assignor to Cavitron Corporation, a corporation of New York Filed Aug. 31, 1965, Ser. No. 484,066 Int. Cl. B07b 1/40 U.S. CI. 209—1 6 Claims

ABSTRACT OF THE DISCLOSURE

A method of and apparatus for screening a liquid suspension of particulate matter or an emulsion. A high frequency vibration is applied to the suspension or emulsion so as to cause cavitation within the suspension or emulsion. The direction of vibration is substantially perpendicular to the plane of the screen, and the area of the output surface of the high frequency vibratory means is generally coextensive with the screen. The flow of the particulate matter or emulsion is further enhanced by simultaneously vibrating the screen.

This invention relates to a method and apparatus for screening materials, and more particularly to an improved method and apparatus for processing a liquid suspension of particulate matter or an emulsion through a screening element in which high frequency vibratory energy is employed for enhancing the screening action.

In certain existing screening devices the screening elements are mounted in supporting structures and vibratory energy is imparted thereto by vibrating either the structures or imparting the vibratory energy directly to the screening element. In these devices, a screen is usually tightly drawn and firmly secured to the screening element and the screen may be vibrated by coupling the vibratory energy directly thereto. In the case of large screens a serious difficulty is encountered in that various gaps or dead zones are formed on the screening surface where no screening action occurs when the vibratory energy is imparted to the screen in any of the above mentioned ways. It is also wasteful from the aspect of energy-economy, to first induce vibration of either the screening element or supporting structure, in order to impart vibrations to the materials being processed.

Thus it has been proposed in the prior art, to effect vibration of the screen in various way at a high frequency and minute amplitude by the use of different mechanical means, for instance by unbalanced weights, or electromagnetic vibrators. Ordinarily, these screening operations are carried out by the transmitting of high frequency vibrations to the screen in such a manner as to induce vibrations in the wire composing the screen. It is clear that, in this case, the energy for effecting the screening operation must first be transmitted through the screen before it can be absorbed by the material being processed. By reason of the foregoing, this method has the disadvantages of requiring a plurality of transducers if a large screen is to be vibrated, and the obtaining of various areas in the screen wherein no energy is received.

The prior art also discloses the application of ultrasonic energy to sieving of suspensions in fluids. However, said prior art discloses nothing more than the insertion of an ultrasonic probe in a tank containing a fluid with suspensions. It is a further sophistication of this concept to which applicants invention is directed.

Accordingly, it is an object of this invention to provide improved methods and apparatus for screening materials speedily and conveniently through the use of high frequency vibrations.

Another object of this invention is that of providing improved methods and apparatus of screening, through the use of high frequency vibrations, and wherein said vibrations are imparted directly to the material being screened.

Another object of this invention is to provide improved screening methods and apparatus, through the use of high frequency cavitational energy and wherein said vibrations are imparted directly to the material and in a direction substantially perpendicular to the plane of a screening element.

Another object of this invention is to provide novel screening methods and apparatus wherein vibratory forces are applied simultaneously to the screening element and material to further enhance the flow of the material through the screen.

This method takes advantage of the effects of high frequency waves when introduced on suspended particles in a solution. When particles are suspended in a fluid or in a thixotropic mixture and irradiated with progressive high frequency waves, the pressure of radiation sets the particles in motion. The mobilities of the particles depend on their sizes and on the viscosity of the fluid, as well as the acoustic intensity. Although the actual phenomena which occurs may be complex in the presence of high frequency vibration, there are nevertheless some general principles which may be outlined, in particular there are three very great advantages in applying “high frequency cavitational energy” in accordance with this invention.

The first advantage of applying high frequency energy is the effect of reducing the viscosity of the material being screened. There are two known ways of increasing the internal kinetic energy of a solid or liquid body. One way is to add heat or to heat up the body (i.e., raise its temperature); the other way is to add acoustical or mechanical vibrational energy to the system. From the point of view of the individual molecule, it does not know whether its increased motion is due to heat or sound, only when the phase relations of the vibrations of different molecules are compared can it be ascertained which is which. For thermal motion, phase relationships are random, for sound, the motions have a definite space phase aspect as exemplified by the wave lengths of the various sonic or ultrasonic frequencies present. Thus it is not surprising that many phenomena requiring high temperatures for their occurrence, can be reproduced at low temperatures by using high frequency energy.

The second advantage of applying high frequency cavitational energy is that such energy imparts a type of pumping action to the material being screened. This pumping effect is related to the amplitude of vibration of the vibrator working face, the spacing between said face and the screening surface as well as the material composition and mesh size of the screen. The high frequency vibrator is designed so that its entire working face attains a plane wave front, that is, in phase vibrations of uniform amplitude, over the entire area of the working face or output surface of the vibrator. These high frequency in phase vibrations recurring in the order of 1,000 to 100,000 cycles per second act upon the material being processed to induce a pumping action in the column of material disposed between the screen surface and the vibrator output surface. This high frequency pumping effect occurs simultaneously with the above referred to change in the material viscosity to substantially increase the flow of the processed material through the screening element. The term high frequency vibrations or forces in the present invention is intended to include vibrations starting from approximately 1,000 cycles per second.

The third advantage of applying high frequency cavitational energy is that such energy insures a more uniform dispersion of the particles within the liquid or a more uni3,490,584 form dispersion of one liquid within another liquid. This “dispersive” effect of high frequency cavitational energy is what is relied upon for the success of ultrasonic cleaners.

In summary, the combination of these three advantages of applying high frequency cavitational energy in accordance with this invention gives rise to the unexpected results of this invention. This cavitation creates intense and localized disruption of the material extending from the surface of the screen to the vibrator output surface or working face.

Of particular importance is that the area of the output surface is substantially coextensive with the area of the screening element.

Of secondary importance is the feature that the direction of the vibrations is substantially perpendicular to the plane of the screening element. Finally, properly spacing the output surface or working face of the vibratory member from the screening surface and proper choice of frequency, are also important parameters.

It would seem that the high frequency effects observed with respect to the flow properties of the material are derived from reduction of surface friction, shear thinning of the material, particle orientations, surface-film rupture, and wetting phenomena. The role that each of the phenomena contribute to the increased flow rate has yet to be determined. As a result of these forces produced by intense cavitational action at the area of the vibrator output surface or working face, the flow rate has been increased on certain materials by as much as 200 per cent.

The method and apparatus of one embodiment of this invention embraces the use of a vibrator or transducer which is vibrated at a high frequency in the order of 1,000 to 100,000 cycles per second, and which is mounted to present its vibrator working face in spaced relationship to one surface of the screening element and may be so maintained by any suitable mounting arrangement. The close proximity to which the vibrating working face of the vibrator is positioned with respect to the adjacent surface of the filtering screen, produces cavitation of a limited column of the material being screened, covering an area directly under and immediately adjacent to the vibrating working face of the vibrator) The vibration applied to the material is also partially transmitted to the screen and the beneficial effects thereof are also incorporated into the present invention.

Means are additionally provided for both supplying the material being processed onto the screening element to maintain a layer thereof in between the working face and screening surface as well as maintaining relative movement between the vibratory working face and screen element along a path substantially normal thereof so that progressive areas of the screening surface are exposed through a sweeping movement to the cavitational action. As a result of this relative movement a fresh supply of the composition to be screened is continuously subjected to the high frequency forces.

In accordance with another aspect of the invention, it is possible to further enhance the above referred to method of high frequency screening by introducing a secondary component of motion substantially perpendicular to or parallel with the direction of high frequency vibration. This vibratory movement is introduced either to the frame supporting the screening element or to the latter directly and which in turn is transmitted to the material being processed or the interwoven fibers of the screen to further reduce the coefficient of friction between said screen and material. Thus, by combining vibratory motion which is imparted directly to the screening element in the approximate range of 20 to 1,000 cycles per second and high frequency vibrations in the range of 1,000 to 100,000 cycles per second, which is imparted to the material being screened, an efficient screening method is obtained which utilizes high and low frequency motion in one system.

For a better understanding of the present invention, reference should be had to the accompanying drawings, wherein like numerals of reference indicate similar parts throughout the several views and wherein:

FIG. 1 is an end elevational view, partly broken away and in section, of the high frequency screening apparatus embodying this invention;

FIG. 2 is a front elevational view, partly broken away and in section;

FIG. 3 is a fragmentary sectional view illustrating a modification of the high frequency screening apparatus of FIG. 1 and 2; and

FIG. 4 is a fragmentary sectional view similar to that of FIG 3 but illustrating a further embodiment of the invention.

Referring to the drawings in detail, and initially to FIGS. 1 and 2 thereof, it will be seen that a high frequency screening machine embodying the present invention and there generally identified by the reference numeral 10 may include an upwardly opening housing 11 having a partition 12 extending horizontally thereacross at the upper portion of said housing so as to provide a flange 13 for the support of the U-shaped overhead frame 14 which rests upon said flange. The housing further includes a bottom wall 15 thereby creating a compartment 16 for the positioning of the equipment which will be hereinafter described in detail.

A screening element 19 containing a screen 20 extending horizontally thereacross, which may be constructed of metallic fibres or other material, is disposed through an opening 22 in partition 12 and has a surface 21 adapted to have the material 18 deposited thereon. A vibrator assembly 30 is supported vertically above screen member 20 by a mounting plate 28 which is rigidly fixed thereto and is secured to the overhead frame 14 by bolts 29. The vibrator assembly 30 may be any one of a number of electro-mechanical types, such as, electro-dynamic, piezo-electric or magnetostrictive, however, for purposes of discussing the present invention, we have selected a vibrator assembly of the magnetostrictive type.

The vibrator assembly 30 is vertically disposed, watercooled magnetostrictive transducer, which is of the type disclosed in United States Letters Patent No. Re. 25,033 issued Aug. 29, 1961, to Lewis Balamuth and Arthur Kuris. The vibrator assembly 30 dagramatically shown, generally includes a transducer driver unit 33 and an acoustic impedance transformer 34. The transducer 33 may comprise a stock of laminations of magnetstrictive materials, for example, nickel, and a diagrammatically illustrated winding 35 adapted to carry a biased, high frequency alternating energizing current. The lower ends of the laminations making up the stack of the transducer 33 are fixedly secured, as by welding or soldering, to the upper end 34α of the transformer 34. The transformer 34 has an enlarged section 36 intermediate its ends in the general area of a nodal plane of motion, and this section 36 constitutes a flange seemed, as by bolts 38, to a casing or enclosure 39 for the transducer 33 and the upper portion of the transformer 34. An inlet hose 40 and an outlet hose 41 are connected to the enclosure or casing 39 for circulating a cooling fluid, preferably water, through the enclosure, to remove heat generated in the transducer 33 during operation of the device.

A biased, high frequency alternating current is supplied to winding 35 through conductors enclosed in a flexible conduit 42 extending from a suitable oscillation generator 37 which may be of the type disclosed at page 270 of “Ultrasonic Engineering” by Alan E. Crawford, published 1955 by Butterworths Scientific Publications, London. An oscillation generator of this type is effective to supply a biased alternating current to the winding 35 at a resonant frequency of the transducer 33 and is further effective to vary the frequency of the supplied biased, alternating current when the resonant frequency of the transducer is varied due to changes in temperature, or changes in the loading thereof.

3,490, ⁵

The lower or output end of transformer 34 is preferably provided with a depending threaded projection 43 which is coupled to a similar threaded projection 44 at the upper end of a vibration transmitting member 31 by means of an internally threaded coupling sleeve or nut 45. A thin disk 46 of copper or other deformable metal is preferably interposed between the smooth flat end surfaces of the projections 43 and 44 so that, when sleeve or nut 45 draws projections 43 and 44 axially toward each other, disk 46 ensures uninterrupted metallic ₁₀contact between transformer 34 and vibration transmitting member 31 over the substantial cross sectional area of projections 43 and 44, whereby, the transmission of vibrations from transformer 34 to vibration transmitting member 31 is enhanced. 15

When transducer 33 is operated, by electrical oscillations supplied from generator 37, compressional waves are generated in the stack of laminations thereof, the transformer 34 and transmission member 31, so as to cause vibrational movements in the vertical direction, 20 that is, along the longitudinal axis of the vibrator assembly 30. For the purposes of the present invention, such vibrations preferably have a frequency in the range between approximately 1,000 cycles per second and 100,000 cycles per second, but preferably from 10,000 to 25 30,000 cycles per second, and are of sizable amplitude, for example, in the range between approximately .0001 and .10 inch. In order to ensure that the maximum applitude of vibration in the vertical direction (substantially perpendicular to the plane of the screen) is obtained at 30 the output surface or working face 32 of transmission member 31, as indicated by the double headed arrow 47, thus ensuring the maximum transmission of working energy. The overall length of the stack of magnetostrictive laminations 33, the transformer 34, and the vibratory 35 member 31 is selected so that, at the frequency of the electrical oscillations supplied to winding 35 of the transducer, a loop of longitudinal motion of the generated compressional waves occurs at or near the output surface or working face 32 of transmission member 31. In 40 other words, the overall length of the transducer 33, transformer 34 and transmission member 31 is approximately equal to an integral number of one-half wavelengths of the sound waves generated in the particular materials they are comprised of. ^5

Thus the vibratory member 31 is held stationary and has acoustical or high frequency vibratory energy imparted thereto by the transmission of a wave motion through transformer 34, while its output surface or working face 32 is positioned in spaced relationship to the ⁵⁰upper surface 21 of screen 20.

In the preferred embodiment relative lateral movement is maintained between the vibratory member 31 and the screening element 19, as the latter is caused to rotate so as to subject different portions of the material to the high frequency cavitational forces. The screening element 19 consists of a rim or cylindrical section 23 which merges into a frusto-conical surface portion 24 and which terminates in a cylindrical neck portion 25. The screen 20 is disposed across the cylindrical section 23 and secured thereto by any suitable means.

The screening element 19 is mounted in a manner to permit rotation by any suitable means about its vertical axis. As shown in FIG. 1, it is rotated during operation _6gof the screening process by drive means consisting of an electric motor 50 mounted in compartment 16 on a shelf 51 and driving a variable speed drive 52 which in turn drives a bevel gear 53 meshing with a bevel gear 54 mounted on the neck portion 25 of screening element 19. 70 As the screening element 19 is rotated, the cylindrical portion 23 of said element is supported so that the output surface 32 of the vibratory member 31 is maintained in spaced relationship with the screen surface 21, for example, by means of a flexible sealing ring 26 engaging the 75

584 surface of rim 23 and being mounted in a counterbore 27 which is provided in the partition 12.

It will also be seen in FIG. 1 that the neck portion 25 terminates into a flexible coupling 55 that permits the screened material 18 to pass therethrough while a fluid tight seal is maintained. The screened material 18 then enters the pump 56 by means of pipe 57 which connects the coupling 55 to the latter and is pumped out of the screening machine 10 through pipe 58 to either a storage tank or for immediate use in the process in which the screened material 18 is to be utilized. Since the screened material flows continuously from the screening machine 10, the supply of material 18 to be screened must be continuously replenished as through a pipe 59 which is joined through pipe fitting 60 into an inlet pipe 61 which is joined to the latter. Thus, a fresh supply of screening material 18 is continuously supplied to the upper surface 21 of screen 20.

As is evident from the above disclosure, the method and apparatus of this invention can be advantageously and effectively employed in the screening of a wide range of materials with various viscosities and through a variety of screen mesh sizes.

In use the vibrating or energy transmitting output surface 32 of the member 31 is placed substantially parallel to the surface 21 of screen 20. A gap is maintained to permit the fresh supply of materials 18 to be screened to flow in between said vibrator surface 32 and the screen 20. In accordance with this invention, the material to be screened is caused to flow onto the screen 20 and a layer of said material 18 is maintained so as to be in contact with the output surface 32 at all times. Thus, since the vibrating face 32 is in direct contact with the material 18, alternate positive and negative pressure fronts are created which travel from the surface 32 through the material 18 and onto the screen 20. A major portion of the vibratory energy is consumed in the material being screened and the remainder imparted to the screen causing it to vibrate so that the passage of the material through the screening element is facilitated.

These variable pressure fronts emitted by the output surface 32 of member 31 cause, a cavitational effect to take place in the material 18 which acts upon the molecules of said material causing increased molecular movement. The effect of this movement is to increase the flowability of the material due to an increase in the. viscosity of the material. With the arrangement herein shown, it is seen that the column of the material 18 is subjected to intense cavitation in the area below the vibratory surface 32. In addition to the localized disruption of the material due to the cavitational forces, the in-phase vibrations act as a high frequency pump which forces the material through the maximum screen 20. To obtain the beneficial effects of the increased viscosity in the material and the high frequency pumping action, it has been found that the gap or spacing between the output surface 32 and screen surface 21 is important and should be controlled. It has been found that generally for a distance of between approximately .04 inch to .40 inch at 20,000 cycles per second and for screens ranging in size between 120 and 270 mesh, desirable results are obtained. It can be appreciated that the dimensional spacing will vary with the material, screen mesh size and flow rate required. Thus, for very viscous materials spacings less than .10 inch might be very desirable.

In a practical embodiment, a clay base material used in the making of paper was processed through a standard size No. 270 mesh screen. The vibratory working face was vibrated at 20,000 cycles per second and spaced .062 inch from the screen surface. The rate of material passage through the screen was increased approximately 100 percent as compared to when no vibration was used at all.

To present progressive areas of the material to the high frequency forces, the screening element 19 is caused to rotate, as previously described, by means of electric

3,490,584 motor 50 and the associated linkages. Alternate methods may be utilized and are within the scope of the present invention, for example, to obtain the desired relative motion between the. screening element 19 and the blade like vibratory member 31, a rotary transducer may be used instead. This eliminates the need to rotate the screening element since the rotary transducer is actually a combination of a standard type rotary motor similar to motor 50 and the high frequency motor 30. In this manner the driving mechanism for rotating the screening element can be eliminated.

In accordance with another aspect of the invention, and as illustrated in FIG. 3, the vibratory tool and the screening element are each maintained in a fixed location to each other. This design is utilized when the surface area of the vibrator working face is substantially the same as that of screening surface. It will be seen that the high frequency screening machine 10' is generally similar to the previously described machine 10 and differs substantially from the latter only with respect to the configuration of the vibratory member and the surface area it covers of the screen itself. The several parts of the machine 10' which correspond to parts of the machine 10 are identified by the same reference numerals, but with a prime appended to each of them.

A major expense in any high frequency application is the initial cost of both the high frequency generator which converts the 60 cycle current into the frequency desired, and the high frequency motor. In FIGS. 1 and 2, the vibratory member illustrated is of the type disclosed in U.S. Patent No. 3,113,225, issued Dec. 3, 1963, to Claus Kleesattel, et al., entitled Ultrasonic Vibration Generator, and assigned to the present assignee. By using a vibratory member of this design which has a blade like body, which is uniformly vibrated along its entire length, and maintaining relative movement between the member and screen, a large surface area may be subjected to high frequency recurring forces with a minimal power. If on the other hand, the screen is of a smaller diameter, a member having an output surface covering substantially the entire surface area of said screen may be utilized. Thus, it will be seen that, in machine 10', the vibratory member 31' has a working face 32' which substantially covers the entire surface area 21' of screen 20' and is supported in a plane substantially parallel thereto. The screening element 19', is supported by a resilient sealing ring 26' which is positioned in partition 12'. This type of arrangement avoids the necessity of rotating the vibratory member 31' relative to the screening element 19'. In all other respects, the process is essentially the same in that the vibratory motion indicated by arrow 47' is imparted directly to the material 18' being processed.

Referring now to FIG. 4, the high frequency screening machine 10 is generally similar to the previously described machine 10 and differs substantially from the latter only with respect to the fact that vibratory forces are also applied directly to the screening element 19 to further enhance the flow of the material 18 therethrough It has been found that by introducing a secondary component of motion either substantially perpendicular to or parallel with the screen surface 20, the coefficient of friction is reduced to permit an increased rate of flow of the material.

The forces may be applied by means of a vibrator 70, that is mounted horizontally to the housing 11, in any conventional manner, and a portion of which extends therethrough so that its vibrator element 71 engages the rim 23 of the screening element 19 and the latter is rotated in the previously described manner. The vibratory motion imparted to the screen 20 is preferably in the approximate frequency range of 10 to 1000 cycles per second but may be in the high sonic or ultrasonic frequencies as well. The flexible sealing ring 26 permits the pivotal movement of the screening element about its vertical axis. It is appreciated that for certain amplitudes of vibration, the mounting apparatus for the screening ele ment may be varied accordingly. High frequency vibrations are simultaneously applied directly to the material by the vibratory member 31 so that screening is obtained which utilizes high and low frequency motion in one system.

It has been found that for those applications in which the vibatory member 31 and screening element 19 are moved relative to each other, if the same are also angularly disposed to obtain a variable gap, a better flow of the material there-between results. The angular displacement may be obtained by providing a tapered surface 62 on the working face 32 of the vibratory member, which aids in the flow of the material 18 between said face and screen surface 21. The tapered surface 62 preferably extends in the direction of relative movement to permit a gradually decreasing spacing as the material is confined and subjected to the high frequency vibratory forces. The taper may be in the order of from approximately 5 to 30 degrees and extend the length of the vibratory member.

In a practical embodiment, a clay base material used in the making of paper was processed through a standard size No. 270 mesh screen. The vibratory working face was vibrated at 20,000 cycles per second and spaced .062 inch from the screen surface. The screening element was simultaneously vibrated at a frequency of 60 cycles per second, the flow rate increased by approximately 150 per cent as compared to when no vibration was used at all.

From the above, it will be apparent that the high frequency screening apparatus embodying this invention employs a high frequency vibratory member which may have a flat working face for introducing vibratory energy into the material being screened and said working face is maintained in spaced relationship to the screening element through which the material is to pass and a layer of the material is maintained between and in contact with a surface of the screening element and working face. Since the area of the working face of the vibratory member may be smaller than that of the screening surface, the two may be moved relative to each other to permit progressive portions of the material to be subjected to the high frequency vibratory forces which creates an intense and localized disruption of the material of such force to increase its flowability characteristics as a higher viscosity and a greater dispersion, and simultaneously act as a high frequency pump forcing the material through the screening element. Vibratory forces may also simultaneously be applied directly to the screening element to further enhance the flow characteristics of the material.