US3012673A - Oscillating sieve - Google Patents

Description

Dec. 12, 1961 N. J. PORTER 3,012,673

OSCILLATING SIEVE Filed Sept. 8, 1959 OVERSIZE e FINES INVENTOR. N .J PORT ER A T TORNE rs tent 3,012,673 Patented Dec. 12, 1961 3,012,673 CILLATING 5IEVE Norman 5. Porter, Berger, Tex, assignor to Phillips Petroleum Company, a corporation oi Delaware Filed Sept. 8, 1959, Ser. No. 833,633 5 Claims. (Cl. 209-315) This invention relates to an improved oscillating sieve. In one of its aspects the invention relates to an oscillating sieve composed of a plurality of screens, down through which travel substantially all particles of desired and smaller sizes it is desired to separate from average particles, and in which sieve the top screen is equipped or coacts with a substantially helical or curved flexible deflector or strip to elfect quick removal of oversize/d par ticles, fed to the apparatus, while at the same time avoiding surging of such oversized particles into the reject bin prior to one revolution travel thereof. In one embodiment of the invention, the deflector strip is attached to the top surface of the top screen. in another embodiment of the invention, the strip is attached to the apparatus above the top screen. Presently, the former embodiment is preferred for reasons appearing herein. In another of its aspects the invention relates to a method of segregating particles of size greater than desired size from a mixture of particles which comprises feeding a mixture of particles into a separating zone, in said zone imparting to said particles an oscillatory motion in a substantially horizontal plane, causing said particles to roll and thus segregating smaller desired particles to the bottom of said zone, removing smaller desired particles through the bottom of said zone and from said plane, causing larger particles travelling and rolling in a widening substantially helical path in said zone to impinge upon a flexibly defined intermediate boundary in said zone, extending from the peripherjof said zone, at a point at the downstream side of the large particle discharge therefrom, helically to a point near the center of said zone in a direction opposite to the general path of travel of said larger particles, thus causing said larger particles to reach said discharge within approximately one revolution about the center of said zone.

The general combination of rn-ulti-layered oscillatory sieves or screens is well known. Even the placement of a convoluted or spiral partition adjacent a screen in such a mechanism is known.

It has been observed that'larger particles remaining on the top screen of a conventional sieve as here described undergo attrition resulting in increased fines to be reprocessed and increased fines unavoidably in the final product. This undue attrition also causes undue Wear of the screen.

The presently disclosed dew'ce and its method of operation are based upon a new concept of providing a flexible strip such as of rubber, which does not significantly dampen the oscillating motion and, therefore, does not cause screen fatigue. A flexible strip which has been used in actual plant practice with excellent results is made of a flexible material as found in regularly available V-belts. Indeed, V-belting has been used to provide the strip in question. This is essentially a rubber strip and has the characteristic of flexibility of the well known V-bel't. T his belt is made of reinforced rubber.

It is to be noted especially that it has been considered to use a flexible solid metal spiral deflector strip and that it appears that such a strip would cause significant fatigue and, therefore, accelerated failure of the screen. Also, there "would be caused an undue dampening of the oscillatory motion causing the system to be overloaded when operating at expected rates.

Therefore, the present invention includes within its scope only those deflector strips which are of such material, so-shaped, and s-o-attached to the screen or above the same that the kinetic energies involved in the motions of the strip, screen, and apparatus as a Whole will avoid an duo fatigue due to dampening action of the strip or strips. At present writing, the rubbery flexible material as in. a V-belting which has a, relatively speaking, low mass compared with that of the overall screen, is preferred.

An object of the present invention is to provide an improved oscillatory sieve. Another object of the invention is to provide a method of operating an oscillating sieve. A further object of the invention is to provide an improved -method of segregating particles acc-ordingto size to obtain a mass of desired particles from a mass containing particles of larger sizes. A still'f urther object of the'invention is to provide an improved internal construction in a plural screen sieve for segregation of oversized particles. A still further object is to avoid undue wear and tear on the top screen of such a sieve. A fu ther object of the invention is to avoid unnecessary attrition of oversized particles which results in undue amounts of fines to be reprocessed and unavoidably in fines in the product.

Other aspects, objects, and the several advantages of the invention are apparent from a study of this disclosure, the drawing, and the appended claims.

According to the present invention there is provided a method as described in which the oversized particles are, in effect, repeatedly bounced by the flexible intermediate boundary, during their helical path of travel, toward the periphery of the segregation zone and into the outlet from said zone in not more than approximately one revolution of said particles.

In accordance with the present invention the action has been accomplished by providing on the screen, at-

tached at several points along its length, a flexible strip extending helically from the downstream side of the large particle discharge orifice or outlet to a-poi nt near thev center of the screen essentially in the direction of oscillation of the apparatus.

In the drawings, FIGURE I is an elevation-a1 cross section of a conventional oscillating sieve. FIGURE II is a plan view of the sieve of FIGURE I generally showing the overall path of travel of the particles of all sizes in the sieve and FIGURE III is a plan view of the top screen of the sieve modified according to the. invention to provide'the flexible strip which, as will be noted, is

shaped substantially along the edge of a gradually increasing diameter.

Referring now to FIGURE I, a mixture of various sized particles obtained from an ammonium nitrate plant is fed to the apparatus by way of orifice 10 and onto screen H. The entire apparatus is oscillated and this results in a substantially ever widening circulatory particle movement as can be seen exemplifiedin FIGURE II, along the path of travel indicated by the dashed line and arrows. T he particles of ammonium nitrate rollon the screen with smaller particles Working their way to the bottom of the layer of particles on the screen andeven tually through the screen to screen 12. A similar action takes place on screen 12 with the smallest-particles or titles dropping to the bottom of the sieve shellat 13: on

each of the screens 11 and 12, particles which will not go through the screen mesh work to the periphery and ultimately are removed from the screen by way of provided draw-oil openings as shown at 14, 15 -and 16. The

operation and the details of construction of an oscillating sieve, as described in FIGURES I and II, are conventional and well known'in the art.

Referring now toFIGURE III, there is provided on top screen 11, flexible strip 18, in this instance, made of a V-belt having essentially a trapezoidal cross section and the following dimensions: bottom base /2 inch, top base A; inch, and height /8 inch. The length of the strip used in this embodiment is 40 inches. The diameter of the screen employed is 4 feet. The sieve into which the strip was actually installed, as here described, is known in the field as a Sweco sieve made by the Sweco Corporation. The sieve used oscillates at 1,200 revolutions per minute. The ammonium nitrate which is being screened by the apparatus here described passes through a 6 mesh screen as a top screen and a 16 mesh screen as a bottom screen. Thus, the ammonium nitrate product taken off at 16 passes through 6 mesh but will not pass 16 mesh to any substantial extent. As will be understood by one skilled in the art in possession of this disclosure, the oversized material existing at 14 and the fines exiting at 15 can be combined and returned to a remelt system for further processing into ammonium nitrate prills or particles.

The shape of Strip 18 is essentially as shown in the drawing. This shape can be varied, depending upon desired throughput, particle sizes, screen mesh, rate of oscillation, and other related factors as one skilled in the art in possession of this disclosure will understand. Essentially, however, it will be noted that the strip does not make a complete revolution on the screen and that it is in juxta position to the screen. It is within the scope of the invention, however, to attach the flexible deflector strip somewhat above the screen so as to allow for motion of any small particles which have not yet dropped through the screen, on the screen, under the strip to continue on the screen until these have been dropped therethrough or eliminated on a further revolution with the oversized material. It will be noted, however, that such a space is not provided in the operating unit at this time since all fines dropped to the product level screen very early in the first revolution around the screen. Thus, any particles being deflected on the upstream side are undesirable as they have failed to pass through the 6 mesh screen during one complete circuit. Nevertheless, if rates were increases to a point at which all desirable product or fines would not fall through the top screen in one circuit then the deflector strip spaced from the screen would desirably act to perform its primary service yet permit continued rotation on the screen of the fines until these have been segregated and dropped through the screen.

It is also within the scope of the present invention but as stated not now preferred, to suspend the flexible strip over the top screen so that it will essentially be in the same position relative to the screen as already described but would actually be attached to the apparatus at the top partition or otherwise. Thus, if the deflector was placed just at the point above the screen so the oversized particles being removed would not flow under it and at the same time so the strip would not contact the oscillating screen which would cause wear of the deflector and screen and interference with the oscillating balance of the unit, the strip could be attached elsewhere than on the screen.

The strip fastened directly to the screen, as earlier described, is preferred for reasons as will be understood by one skilled in the art having studied this disclosure; also because it requires no maintenance or adjustment and does not interfere in any way when the top screens are removed. When replacing the top screen care is taken to replace it with the attached strip located so as to deflect the oversized particles out to the periphery at the desired point, namely, so that the particles will pass through outlet 14.

The flexible strip of the invention is believed to be unique from other deflectors in that it is flexible and, therefore, disturbance of the screen power unit function is prevented.

In the operation being described, the deflector, accord- 4 ing to the invention, has considerably decreased remelt recycle rate and has resulted in improved product quality. Thus, in a dollar and cents appraisal, the installation of a deflector on each of several sieve units, as here have been described, has resulted not only in a savings but also in increased product quality.

By installing the strips according to the invention, chunks of nitrate can be immediately removed to the remelt chute so that grinding caused by continued circuits of such chunks around the screen has been eliminated. As one skilled in the art will understand, the chunks remaining on the screen an undue length of time were grinding and misshaping the prills causing increased fines to the remelt and inevitably to the product.

Example Eight tons per hour of ammonium nitrate prills are passed through a four foot diameter Sweco sieve modifled according to the invention with the V-belting as herein described. About ten percent coarse material is removed without undue attrition or production of additional fines. Thus, about ten percent fines in the product being screened is recovered as fines from the sieve with practically no significant increase in the percentage of said fines. Six mesh screen was used to remove the coarse particles. A 16 mesh screen was used to remove the product, fines passing through the latter screen.

The materials which can be used to replace the V-belting are those which are tough, yet flexible, and relatively soft as compared with metal.

The Sweco separator which has been modified according to the present invention is Model D-2D8 of the Southwestern Engineering Corporation of Los Angcles, California.

Reasonable variation and modification are possible within the scope of the foregoing disclosure, drawing and the appended claims to the invention the essence of which is that a flexible deflector strip has been placed on the top screen of a multi-scrccn oscillating sieve permitting the separation of oversized particles from desired and undersized particles more rapidly and with considerably reduced attrition of oversized particles, resulting in decreased remelt stream and decreased unavoidable fines in the product, as well as in increased overall capacity of the sieve without loss of product quality and that a method for segregating desired size particles from undesired size particles have been set forth substantially as described herein.

I claim:

1. In a multi-screen oscillatory sieve wherein there is provided a peripheral outlet from a top screen for the removal of oversized particles therefrom, a flexible strip attached to and integral with said screen extending from the downstream side of said outlet in a substantially helical manner toward and to a point near the center of said screen the strip being attached only to the screen, permitting the bulk of the strip, due to its flexibility, and particles of material being sieved which are up against the strip or contacted thereby as it moves, to take up the full motion imparted to it by the screen in a delayed manner.

2. A device according to claim 1 wherein the flexible strip is made essentially of a rubber-like material.

3. A device according to claim 1 wherein the flexible strip is made of a flexible V-belting essentially consisting of reinforced rubber.

4. In a multi-screen oscillatory sieve wherein there is provided a peripheral outlet from a top screen for the removal of oversized particles therefrom, a flexible strip extending from the downstream side of said discharge in a substantially helical direction toward and to a point near the center of said screen, said strip extending around the center of the screen less than one complete revolution and being attached to said screen for integral motion therewith at the points of attachment while per- 5 6 mitting the bulk of the strip, due to its flexibility, and References Cited in the file of this patent particles of material being sieved which are up against UNITED STATES PATENTS the strip or contacted thereby as it moves, to take up in v R h 1 t b a 641,986 McAnulty Jan. 23, 1900 g l manmt e ful mctwn Impar ed to 1t y 16 5 2,777,578 Miner Jan. 15, 1957 2,849,113 Boume Aug. 26, 1958 5. A sieve according to claim 4 wherein the flexible strip is made of a rubber.

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