US3477568A

US3477568A - Electrostatic separation of round and nonround particles

Info

Publication number: US3477568A
Application number: US591174A
Authority: US
Inventors: Robert W Madrid
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-11-01
Filing date: 1966-11-01
Publication date: 1969-11-11
Anticipated expiration: 1986-11-11

Description

Nov. 11, 1969 R. w. MADRID 3,477,568

ELECTROSTATIC SEPARATION OF ROUND AND NONROUND PARTICLES Filed Nov. 1, 1966 2 Sheets-Sheet 1 FIG. 1

INVENTOR. @PERfi W. MADRID BY 5 ATTORNEYS Nov. 11, 1969 R. w. MADRID 3,477,568

ELECTROSTATIC SEPARATION OF ROUND AND NONROUND PARTICLES Filed Nov. 1, 1966 I 2 Sheets-Sheet 2 INVENTOR. ROBERT w. MADRID agma C. p.21;

A T TORNEYS United States Patent 6 3,477,568 ELECTROSTATIC SEPARATION OF ROUND AND NONROUND PARTICLES Robert W. Madrid, Macedon, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Nov. 1, 1966, Ser. No. 591,174 Int. Cl. B03]: 7/08, 7/04 US. Cl. 209-127 16 Claims ABSTRACT OF THE DISCLOSURE This invention relates in general to a sorting system and more particularly to a novel system for separating spherical particles from nonspherical or otherwise imperfect, irregular particles including fine chips, filaments, powder and dust.

It is often desirable and necessary in certain commercial applications to separate, sort, or refine a batch of unrefined particles as to shape and more specifically to separate spherical particles, hereafter called rounds, in the batch from the nonrounds which may include irregular rounds, dumbbells i.e. two rounds fused together, aggregates of two or more rounds, angular shaped particles and fine chips, filaments, powder and dust. One such application is in the manufacture of developer materials in the process of xerography.

In the process of xerography, for example, as disclosed in Carlson Patent No. 2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charge finely divided material, such as an electroscopic powder, that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a patern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a transfer support surface for example paper to which it may be afiixed by any suitable means.

The cascade method of latent image development has found extensive commercial acceptance and generally consists of gravitationally flowing across the xerographic plate developer material consisting of a two component material consisting of an electroscopic marking powder termed toner and a granular material called carrier which consists preferably of spherical particles devoid of nonrounds including fines which may freely reside in the carrier mixture or adhere tenaciously to the surface of individual carrier beads.

Illustrative examples of such two component developer materials are disclosed in Walkup et al. Patent 2,638,416, Walkup Patent 2,618,551, Wise Patent 2,618,552 and Copley Patent 2,659,670. Generally carrier particles are much larger relative to the toner particles, toner particles 3,477,568 Patented Nov. 11, 1969 generally having an average particle diameter between about 1 and 30 microns whereas the carrier particles for example may have an average particle diameter from about 50 to 700 microns.

As indicated in the aforementioned patents, conventionally the carrier particles act as vehicles to carry toner to latent image areas and serve to triboelectrically charge toner so that toner may be pulled off of the carrier particles to image areas but not to background areas. Carrier beads also pick up toner particles which might tend to ad here to uncharged or background areas.

As pointed out in Walkup et al. Patent 2,638,416 it is extremely desirable to have carriers round or nearly mind as to facilitate their movement in gravitating, without sticking, over the xerographic plate for example in the cascade method of xerographic image development. Carrier bead sticking is to be avoided since carrier beads on the plate after development are carried on the plate through subsequent image transfer and plate cleaning steps. Beads on the plate during the image transfer step prevent intimate contact of the plate with the transfer support surface in the region of the bead thus causing incomplete image transfer in that region. Also pressure is often required during this transfer step and this pressure may cause carrier particles to scar, dent or otherwise degrade the relatively delicate surface of the xerographic plate. Pressu re may also misshape or fracture carrier particles thus rendering them unfit for the further reuse and necessitating periodic replenishment.

Also in automatic recyclable xerographic copying machines the plate is generally cleaned after the transfer step to ready the plate for a new imaging cycle. In the cleaningstep where any remaining toner is removed from the plate to ready it for reuse, especially where web belts in skidding contact with the plate are employed as the cleaning means, carrier particles on the plate by being ground against it, may cause serious damage to the plate which ordinarily should be very smooth to produce quality prints.

To require remedial measures such as tapping or air pressure to remove granular carrier beads from the plate after the development step would add greatly to the complexity of xerographic apparatus.

In addition it is found that fine chips, powder, filaments and dust which adhere to the surface of both round and irregular carrier beads may cause unsightly deposition in non-charged background areas of the plate and no round, nonround sorting system has been found which also satisfactorily removes these fines from the bead surface.

In order to secure nearly perfectly round carrier particles devoid of nonrounds including surface adhering fines, various sorting systems are available in the art which have not proved entirely satisfactory.

For example sorting systems relying on revolving discs and centrifugal forces to separate rounds from nonrounds are quick and simple but provide a very rough degree of sorting and often fail to remove fines which adhere tenaciously to the surface of larger particles in the batch. Other systems are available each of which is lacking in some fundamental aspect, for example, simplicity, cost, speed of operation or degree of refinement as to render it not completely satisfactory for inclusion in a commercial manufacturing process.

It is therefore an object of this invention to provide a sorting system to separate rounds from nonrounds which overcomes the above noted disadvantages and satisfies the above noted wants.

It is a further object of this invention to provide a sorting system of general usefulness and particularly use ful in providing round carrier particles for use in developers in the process of xerography.

It is a further object of this invention to provide a sorting system which is simple and inexpensive to construct and operate.

It is a further object of this invention to provide a sorting system readily adaptable for batch or continuous type operation.

It is a still further object of this invention to provide a round, nonround sorting system capable of a high degree of refinement for each cycle of operation.

It is a still further object of this invention to provide a round, nonround sorting system capable of simultaneously removing larger irregulars as well as fine chip, filament, powder and dust particles which otherwise might adhere tenaciously to the periphery of larger particles.

It is a still further object of this invention to provide a sorting system capable of separating rounds from nonrounds of the same material.

The foregoing objects and others are accomplished in accordance with this invention by causing unsorted particles to be advanced relative to a conveyor so that particles are electrostatically attracted towards the conveyor surface as they are advanced relative thereto whereby rounds are caused to advance at a faster rate in response to advancing forces and nonrounds being effected to a greater degree by frictional and electrostatic retarding forces are caused to move at a slower rate or actually stick to the conveyor surface, thereby effecting the separation of rounds from nonrounds.

In one aspect of the invention merely advancing and moving the unsorted particles relative to the conveyor surface may in and of itself be sufiicient to triboelectrically generate electrostatic charges on the particles or the surface of the conveyor or both to cause an electrostatic attraction between the conveyor and the particles to aid in sorting rounds from nonrounds. This type of contact electrification is called triboelectrification from the Greek tribein meaning to rub.

In another aspect of the invention the surface of the conveyor or the particles or both may be charged from an external charge source. This charging may be accomplished both prior to and during processing and if both the particles and the conveyor surface are charged the charges will usually be of opposite polarities to increase the electrostatic force of attraction between conveyor and particles. It will be understood that in this aspect of the invention there also may be triboelectrically generated charges in addition to charges placed from external charge sources.

In both aspects of this invention it is thought that sorting of rounds from nonrounds is accomplished by the ability of the rounds in response to advancing and moving forces, to overcome conveyor surface friction and electrostatic forces to advance along the conveyor for example to a round collection point while nonrounds including fines, under the combined greater influence of conveyor surface friction and electrostatic forces advance more slowly than rounds or are stopped altogether to remain on the conveyor surface where they may be conveyed for example to a nonround collection point to be removed by any suitable means. The system herein is found to be preferred for separating particles having an average diameter of less than about 3,000 microns and especially preferred for separating particles having an average diameter of less than about 700 microns and greater than about 50 microns.

Also, uniquely, in both aspects of the invention fine chips, filaments, powder and dust are removed from the surface of rounds to render them clean for commercial use.

The advantages of this novel sorting system will become apparent upon consideration of the following detailed disclosure of the invention especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an isometric view of an apparatus embodiment for carrying out the invention.

FIG. 2 is a partially schematic representation of another apparatus embodiment for carrying out the invention.

FIG. 3 is a partially schematic representation of yet another apparatus embodiment for carrying out the invention.

FIGS. 4A-4D are outline drawings of 17.5 X photographs of 4A a batch of unsorted glass carrie'r core particles the rounds of an average diameter of about 600 microns, 4B nonrounds from said batch deposited after 1 cycle according to an embodiment hereof; 4C nonrounds deposited after a second cycle according to an embodiment hereof and 4D sorted rounds after two processing cycles.

Referring now to FIG. 1 there is shown an apparatus embodiment for batch processing unsorted particle's according to the invention.

According to the first aspect of the invention unsorted particles 15 are stored in feeder 11, the rate of flow of particles onto the conveyor 16 in the form of a fiat bottomed chute being regulated by gate 13.

Conveyor 16 is secured to vibrator 14 comprising flexible leaf springs 17, reciprocating member 9, electromagnetic drive mechanism 19 connected electrically to electric controller 20 used to control the amplitude and frequency of the vibration, the controller in turn connected electrically to electrical power source 21. Vibrator 14 and feeder 11 are in turn secured to support frame 12.

The magnet in mechanism 19, energized by pulsating current, pulls the conveyor down and back toward mechanism 19, then the leaf springs 17 return it up and forward to its original position, the direction of travel of the conveyor determined by the angle of the leaf springs.

Although conveyor 16 is illustrated to be a fiat bottom type, which is preferred in order to provide conveyor contact with a maximum number of particles, various other shaped conveyors may be used including V-shaped, half round, tubular and radius bottom conveyors.

The particles are caused to advance down the tilted conveyor by gravitational forces and optionally by vibrational or other forces which for selected particle material and conveyor surface material combinations causes triboelectrically generated electrostatic charges to be formed on particles and the conveyor surface to cause an electrostatic attraction between same.

The rounds being to a lesser degree effected by frictional and electrostatic retarding forces between themselves and the conveyor surface gravitate toward round collection point 18 while nonrounds including fines are caused by the same retarding forces to remain on the surface of the conveyor thus effecting sorting of rounds from nonrounds.

Of course the tilt angle, amplitude and frequency of vibration, if vibrational motion is used, batch feed rate and other factors will depend on many factors including each other, and the size and density of the particles to be sorted, the position in the triboelectric series of the unsorted particles and the conveyor surface material, the relative softness or hardness of the particles and the conveyor surface materials and the degree of separation or sorting desired for each processing cycle.

It will be appreciated that the type of apparatus illustrated in FIG. 1 is preferred for sorting a single batch of particles since the sorting process should preferably be interrupted periodically to wipe nonrounds and fines from the surface of the conveyor. Of course a particular batch of particles may be reprocessed a number of times depending upon the degree of refinement which is desired. In addition with certain combinations of materials an undesirably large charge may be built upon on the conveyor surface after periods of continuous use to cause even rounds to stick to the conveyor. In these situations periodic discharging of the conveyor for example during 1cleaning will reduce the electric fields to a more desirable evel.

Vibration of the conveyor surface has been found to be especially advantageous for conveyor surfaces which are relatively soft for example the conveyor surface used in Example I, which appears below, as opposed to the relatively harder amorphous selenium conveyor surface utilized in Example V and for shorter length conveyors since vibration increases both the sliding and the rolling contact of the particles with the surface of the conveyor thus increasing triboelectric charging and triboelectric attraction between the particles and the conveyor surface for a given conveyor length. Vibration is also found to be advantageous in those cases where the particle material and the conveyor surface material are relatively far apart in the triboelectric series which when coupled with low tilt angles and relatively low density particle material and small particles may cause, because of the magnitude of Van Der Walls and electrostatic forces, a particles including rounds to become fixed to the conveyor surface unless vibratory motion is used to prevent such sticking. The characteristics of the vibratory motion used to sort a particular batch of material according to an embodiment of this invention may be widely varied to optimize sorting of rounds from nonrounds. For example the amplitude of vibration of the vibratory feeder used in Example I and illustrated in FIG. 1 may be varied from about 20 mils to about 55 mils. Also in any specific application, frequencies can be varied as desired as well as the angle of the line of amplitude.

It should be appreciated that if the angle of the line of amplitude is acute with respect to the upwardly extending portions of a tilted conveyor surface at least the larger nonrounds may be caused to actually move up the conveyor surface to a round collection point while at least the larger nonrounds move down to provide a self cleaning feature especially useful for sorting batches with a relatively low amount of fines. It should be appreciated that in all embodiments of the present invention the conveyor need not be tilted since vibrating motion and many other advancing forces may advance particles along for example a horizontal conveyor.

In the first aspect of the invention, wherein the unsorted particles themselves triboelectrically generate charges by rolling or skidding in contact with the conveyor, it seems that at least the surface material of the conveyor and of the particles must be selected in accordance with their tribolelectric properties so that when particles are advanced relative to the surface of the conveyor either the particles or the conveyor surface is charged positively if the other material is below it in the triboelectric series and negatively if the other material is above it in the triboelectric series to create sufficient triboelectric charging to cause an appreciable separation of rounds from nonrounds for each processing cycle.

For unsorted particles of a particular material or at least surface coated with a particular material, the material selected for the surface of the conveyor may be selected from a large group of materials that have been tested and which occupy recognized positions in the triboelectric series so that charges are generated upon advancement of the particles over such a surface. The magnitude of triboelectric charging and the electrostatic forces generated thereby when two materials are contacted is roughly correlated with the degree of separation of the two materials in the triboelectric series.

There is still much to be learned about the triboelectric effects and the relative position of the various materials in the triboelectric series but it is generally found to be preferred herein for both aspects of the invention, that at least the surface of either the conveyor or the particles to be sorted should have a bulk electrical resistivity greater than about ohm-centimeters.

All of the various published triboelectric series are not in complete agreement because of the many intangibles found to be a part of research done in this area.

But by consulting the various published series a reasonably accurate prediction can be made as to how a particular conveyor surface material will triboelectrically react with a particular batch of unsorted particles.

Three of the most famous of such series are given below. Table I appears in an article by Shaw and Leavy, E. W. L. Proc. of Royal Society, 138, p. 506 (1932) which gives a triboelectric series of a number of metals as compared with silica. It will be noted] that selenium lies at the very bottom of the series indicating that to sort silica or glass type particles according to this invention a selenium coated conveyor may probably be advantageously used in the first aspect of this invention to create appreciable triboelectric charging. I

Table II is one of the earliest of such series called the Smithsonian series and may be found published in Smithsonian Physical Tables, p. 375, 9th Rev. Ed. (1954).

Table III appears in an article by Hersh and Montgomery, Textile Research Laboratories, 28, p. 903 (1956).

Lucite is a class of acrylic resins from Du Pont, Dacron is a synethetic fiber made from the condensation of dimethyl terephthalate and ethylene glycol from Du Pont, Orlon is a synthetic fiber made principally from polyacrylonitrile from Du Pont, Dynel is a synthetic fiber made by copolymerization of 40% acrylonitrile and 60% vinyl chloride, Velon is a line of synthetic plastics and resins available from Firestone Plastics Co., and Teflon is a tetrafluoroethylene polymer from Du Pont.

Other series are known to those skilled in the art and may be used and compared with each. other to select a conveyor material which will most probably react triboelectrically with the particles to be sorted to be used in accordance with this invention to separate rounds fromnonrounds and to clean the surface of rounds of fine 1) Asbestos (sheet) (2) Rabbits fur (Hg) (3) Glass (combn. tubing) (4) Vitreous silica, opposums fur (5) Glass (fusn.)

(6) Mica (8) Glass (pol.), quartz (pol.), glazed porcelain (9) Glass (broken edge), ivory (l0) Calcite (11) Cats fur (12) Ca, Mg, Pb, fluorspar, borax (13) Silk (14) Al, Mn, Zn, Cd, Cr, felt, hand, wash-leather (15) Filter paper 7 Vulcanized fiber Cotton Magnalium K-alum, rock-salt, satin spar Woods, Fe Unglazed porcelain, salammoniac K-bichromate, parafiin, tinned Fe Cork, ebony (24) Amber (25) Slate, chrome-alum (26) Shellac, resin, sealing wax (27) Ebonite (28) Co, Ni, Sn, Cu, As, Bi, Sb, Ag, Pd, C, Te,

Eureka, straw, copper sulfate, brass (29) Para rubber, iron alum. (30) Guttapercha (31) Sulfur (32) Pt, Ag, Au (33) Celluloid (34) India rubber Negative TABLE III Positive Wool Nylon Viscose Cotton Silk Acetate Lucite Polyvinyl alcohol Dacron Orlon Dynel Velon Polyethylene Negative Because most glasses have a bulk electrical resistivity greater than about ohm-centimeters it is found that for example when glass beads are to be sorted the conveyor may be surface coated with an electrically conductive material including most metals as long as the electrically conductive layer is backed with an electrical insulating layer which in turn is suitably backed with an electrically conductive layer, as Well as any suitable electrically insulating material such as natural and synthetic resins, and the like; so long as the material is separated from the glass bead material in the triboelectric series.

On the other hand if the conveyor surface is of a material of bulk resistivity greater than about 10 ohm-centimeters particles of almost any material within the size ranges specified hereinabove may be sorted in accordance with this invention so long as the particle material or at least its surface coating is suitably separated from the conveyor surface material in the triboelectric series.

Any suitable film formable material of bulk resistivity greater than about 10 ohm-centimeters may be used as a conveyor surface coating. Typical such materials include polysulfone, polyethylene terephthalate polyester, acrylics for example alphamethyl styrene copolymer, cellulose nitrate, epoxy resins, phenolics, phenolformaldehyde, silicones, urethanes, urea-formaldehyde, cellulose acetate, polycarbonates, cellophane, polychlorotrifluoroethylene copolymers, polyvinyl-butyral, polymethyl methacrylate, polystyrene, polyethylene, fiuorohalocarbons, cellulose triacetate, cellulose acetate butyrate, polyurethane elastomer, cellulose propionate, ethyl cellulose, polypropylene, polyvinyl fluoride, vinyl-chloride-acetate copolymers, vinylidene chloride-vinyl chloride copolymer, tetrafiuoroethylene available under the trademark Teflon from E. I. du Pont de Nemours & Co., copolymers of hexafluoropropylene and polytetrafluoroethylene, polyvinyl chloride,

polyacrylonitrile, cellulose nitrate plasticized with camphor, hard rubber such as ebonite and chlorinated rubber, nylons or polyamides, polyvinyl alcohol, polyvinylidenefluoride, copolymers of chlorotrifiuoroethylene and vinylidine fluoride, casein, polyglycols, alkyds and others.

In order to prevent dynamic charge build up resulting from extended periods of use of a sorting system of a conveyor surface and particles relatively far apart in the triboelectric series, certain film formable materials which temporarily hold a charge are especially preferred for use as conveyor surface materials herein since they may be charged to sort according to the invention but are electrically leaky or conductive enough to continually discharge a part of the charge build up to prevent overcharging and resultant sticking of even rounds to the conveyor surface.

Preferred film formable materials which are electrically leaky for use herein include cellulose acetate, ethyl, cellulose, epoxy resins, phenolics, polyvinyl fluoride, polystyrenes and others.

It will be understood that various fillers, plasticizers and additives may also be added to the aforementioned film formable materials and to others to change their electrical properties and that other suitable leaky materials are available in the art and may be used herein. In fact it is generally found that any film formable material possessing a dielectric constant and a bulk electrical resistivity such that the product of these tWo values is between about 10 and 10 ohm-centimeters is leaky and satisfies that requirement for use as a preferred conveyor surface material herein.

In the second aspect of the invention, the conveyor surface or the particles or both may be charged from an outside charge source rather than relying solely on triboelectric charging. This aspect of the invention is found to be advantageous Where shorter conveyor lengths are to be used.

For maximum sorting effect the conveyor should be surface charged to a negative potential of the particles are of a material above the conveyor surface material in the triboelectric series and to a positive potential if the particles to be sorted are in a lower position in the triboelectrieseries. For example to sort glass beads with a conveyor surface of selenium which is below or negatively situated in the triboelectric series in relation to the glass beads, the conveyor surface should be charged negatively for maximum electrostatic attractive effect. If such a charge causes sticking of rounds to the conveyor, negative charging of the conveyor surface to a lower potential or posi tive charging probably to a low potential might be tried in order to lower the electrostatic attractive forces sufficiently to permit rounds to advance. Of course increasing the tilt angle of the conveyor and using vibratory motion are also useful in preventing sticking of rounds.

Photoconductive insulating materials such as amorphous selenium are advantageously used as conveyor surface materials in this as well as the first aspect of the invention because in the absence of actinic light such photoconductive insulating materials are electrically insulating thus will accept and retain a charge applied either from an outside charge source or generated triboelectrically for example, during the sorting process. Then, for example by exposing the conveyor surface illustrated in FIG. 1 to actinic light and, for example, by increasing the angle of the conveyor with the horizontal, nonrounds may be readily wiped from the conveyor surface, for example, by a vibratory or a brushing action, to prepare the apparatus for another batch to be sorted. Also this periodic exposure discharges dynamic charge build up.

Any suitable hotoconductive insulating material may be used as a conveyor surface material. Typical ones include amorphous selenium, alloys of sulfur, arsenic or tellurium with selenium, selenium doped with materials such as thallium, cadmium sulfide, cadmium selenide, etc., particulate photoconductive materials such as zinc sulfide,

zinc cadmium sulfide, French process zinc oxide, phthalocyanine, cadmium sulfide, cadmium selenide, zinc silicate, cadmium sulfoselenide, linear quinacridones, etc. dispersed in an insulating inorganic film forming binder such as a glass or an insulating organic film forming binder such as an epoxy resin, a silicone resin, an alkyd resin, a styrenebutadiene resin, a wax or the like. Other typical photoconductive insulating materials include: blends, copolymer, terpolymers, etc. of photoconductors and non-photoconductive materials which are either copolymerizable or miscible together to form solid solutions and organic photoconductive materials of this type include: anthracene, polyvinylanthracene, anthraquinone, oxidiazole derivatives such as 2,5-bis-(p-amino-phenyl-l), 1,3,4-oxidiazole; 2- phenylbenzoxazole; and charge transfer complexes made by complexing resins such as polyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates, etc., with Lewis acid such as phthalic anhydride; 2,4,7-trinintrofluorenone; metallic chlorides such as aluminum, zinc or ferric chlorides; 4,4-bis(dimethylamino) benzophenone; chloranil; picric acid; 1,3,5-trinitrobenzene; l-chloroanthraquinone; bromal; 4-nitrobenzaldehyde; 4-nitrophenol; acetic anhydride; maleic anhydride; boron trichloride; maleic acid; cinnamic acid, benzoic acid; tartaric acid; malonic acid and mixtures thereof.

The conveyor surface may be charged in a wide variety of Ways including vigorously rubbing the surface with a softened material such as a cotton or silk handkerchief or a soft brush or fur chosen to impart charge of the desired polarity, induction charging for example, as described in Walkup Patent 2,934,649, roll charging as described in Straugham, Mayer, Proc. Nat. Electronics Conf. 13, 959-,- 962 (1958), depositing charge from a corona discharge device and other techniques. Charging by corona discharge devices which generally can apply either positive or negative charge of varying potentials and which may be adapted for many applications are found to be preferred outside source applicators for use herein. For example corona discharge devices of the general description and generally operated as disclosed in Vyverberg Patent 2,836,725 and Walkup Patent 2,777,957 have been found to be excellent sources of corona useful in the charging of photoconductive insulating materials or other conveyor surfaces. It will be understood that not only may electrically insulating conveyor surfaces be charged in accordance with the invention but electrical conductors, if properly insulated may also accept and retain a charge.

It is preferred in this second aspect of the invention if higher electrostatic forces are being sought, that the conveyor surface material and the particle material be selected according to their bulk electrical resistivity and according to their relative placement in the triboelectric series as described herein for use in the first aspect of the invention since triboelectrieally generated charges would add to externally applied charges to increase the resultant electrostatic attraction. Of course for some materials and operating parameters such selection is not necessary since an increase of the electrostatic attraction between the particles and the conveyor surface by reason of triboelectrification would not be needed or desired. An advantage of the second aspect of the invention is that a conveyor surface electrically conductive or insulating identical to the unsorted particle material may be used to produce electrostatically generated forces between the conveyor surface and the particles according to the second aspect of the invention.

Although uniform charging of the conveyor surface 1s found to create electrostatic lines of force even in the middle of large charge areas, these lines of force may be strengthened for the same amount of charge potentlal by utilizing various techniques such as development electrodes and screening techniques for example as described 1n Dessauer and Clark, Xerography, Focal Press, pp. 274-279 1965 In ad dition to charging the conveyor surface from an outside charge source the unsorted particles may be charged prior to their being advanced relative to the conveyor surface by corona techniques or for example by utilizing a vibrating supply hopper illustratively of the type available from Syntron Co., Homer City, Pa., to cause particles held therein to acquire charge by rubbing against the sides of the feeder. The inside of the feeder may preferably be coated with a layer of material selectively chosen from the triboelectric series to charge the particles to a polarity opposite to the polarity of charge applied to the conveyor surface to enhance the eletcrostatic attraction between the conveyor surface and the particles. Such charging although not required may be desirable in certain systems. It will be understood in connection with the second aspect of the invention that charging from external charge sources may take place not only prior to the contact. of particles with the conveyor but during the advancement of particles relative thereto.

The versatility of the sorting system disclosed herein is illustrated by the various continuous or automatic apparatus embodiments for carrying out the invention two embodiments of which are illustrated in FIGS. 2 and 3.

Referring now to FIG. 2, and according to a first aspect of the invention, belt 22 carries unsorted particles 15 from a particle manufacturing operation to deposit the particles in feeder 26. Gate 28 allows a variable feed rate of particles 24 to conveyor 30 in the form of an endless belt. Conveyor 30 is held in tension by roller 32 and drive roller 34 driven by motor 36 through drive means not shown to advance conveyor 32 illustratively in a direction opposite to the direction in which rounds will tend to gravitate.

The factors of tilt angle, vibrational characteristics, if vibration is used, size and density of particles to be sorted, the relative positions in the triboelectric series of the conveyor surface and the particle material, the relative hardness of the conveyor surface and particle material and the degree of separation or sorting desired for each sorting cycle are all interrelated and depend on each other in a belt conveyor configuration in much the same way as in the apparatus embodiment illustrated in FIG. 1.

The rate of advancement of the conveyor belt may be very low for example one inch per second or lower which is generally sufficient to continuously remove nonrounds and present clean conveyor areas to the cascading particles to effect significant sorting, especially when the particles are fed at a preferred rate to create a monolayer i.e. a layer of particles one particle thick cascading down the conveyor. The conveyor belt may be advanced at much higher rates such as 5 or 10 inches per second or more especially when used in conjunction with greater tilt angles and vibration.

Of course a monolayer of cascading particles is not required and greater feed rates may be used with an increase in the number of layers of particles advancing along the conveyor but the degree of sorting will correspondingly decrease because of less particle contact with the conveyor surface and because the greater particle flow forces tend to Wash away nonrounds deposited on the conveyor surface, and more processing cycles may be needed depending on the degree of refinement desired.

As illustrated, with at least the surface of the conveyor and the particle surface material being selectively chosen as herein described, the particles flow from gate 28, become engaged in rolling and sliding contact with advancing conveyor 30 thereby triboelectrieally charging both the surface of the conveyor 30 and the particles. The rounds being less effected than nonrounds by retarding forces tend to gravitate toward collection point 18 while the advancement of nonrounds and fines toward collection point 18 is retarded more than for the rounds by both frictional forces and electrostatic forces. In fact the rate of advancement of nonrounds towards collection point 18 is stopped or at least slowed to a velocity less than the velocity of the moving conveyor thus causing nonrounds to be carried away from round collection point 18 and toward roller 32 and nonround collection point 38 whereat some of the larger nonrounds will be caused by gravitational and centrifugal forces to leave the surface of the conveyor. Nonrounds including fines which adhere tenaciously to the conveyor may be removed by rotating brush 40 powered by motor 42 through drive means not shown.

It will be understood that belt 30 may comprise a photoconductive insulator on a conductive substrate with sorting taking place in the absence of actinic light. Also for example an exposure station or other charge dissipating means may be positioned immediately subsequent to rotating brush 40 in the direction of belt advancement to at least partially discharge the belt to prevent the build up of charge to an unworkable level.

Referring now to FIG. 3 there is illustrated an exemplary automated apparatus embodiment for carrying out the second aspect of the invention where the conveyor surface or the particles or both are charged from an external source and not solely by triboelectric means. Unsorted particles may be fed to the conveyor in any convenient manner for example similar to that manner illustrated in FIG. 2. In FIG. 3 conveyor 44 illustratively comprises a surface coating of a photoconductive insulating material on a grounded electrically conductive substrate which may be a thin film of a metal, for example, aluminum. The photoconductive insulating surface of the conveyor is charged by corona charging device 52 illustratively shown to be emitting negative charge producing particles.

Preferably, conveyor 44 should be charged when the photoconductive insulating material is at its highest insulating value or when there is an absence of actinic radiation i.e. that electromagnetic radiation that would make the photoconductive insulating layer electrically conductive. To allow the charge to remain on the surface of the layer once deposited there, charging and the balance of particle sorting should preferably take place in the absence of that wavelength radiation or light to which the particular photoconductive material is sensitive. As particles are discharged from gate 28 and begin their gravitational descent towards round collection point 18, electrostatic forces generated between the conveyor surface and the particles together with frictional forces exert a selectively greater influence on nonrounds and fines thus slowing their rate of advancement to an average rate of advancement less than the rate of advancement of conveyor 44 or causing them to stick to the conveyor as is the case with most fines, thus causing nonrounds and fines to move towards roller 32 and nonround collection point 38. In the region of roller 32 light source 54 aids in discharging the charge on the photoconductive insulating material which will dissipate at least some of the electrostatic attraction that the conveyor holds for nonrounds and fines thus facilitating their removal. The larger nonrounds by the co-action of gravitational and centrifugal forces will be impelled into nonround collection station 38. Nonrounds including fines which adhere to the surface of the conveyor may be brushed off by a brush similar to brushing means 40 illustrated in FIG. 2 or by brushing means 56 illustrated in FIG. 3 which comprises a revolving brush 58, hood 60 and a vacuum flow line 62 connected to an externally positioned vacuum source 63. A further description of brushing removal means 56 may be found in Walkup et al. Patent 2,832,977.

Of course for the belt conveyors illustrated in FIGS. 2 and 3 vibratory motion may be imparted to the region of the conveyor surface between

rollers

32 and 34 upon which the sorting is taking place by positioning vibratory means beneath the conveyor in said region or for example by causing vibration of one or both of

rollers

32 and 34.

The following examples further specifically define the present invention with respect to the use of frictional and electrostatically generated retarding forces to sort rounds from nonrounds. The parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the refining system of this invention. Particles are fed in each example by releasing them onto the conveyor with a minimum of momentum before contacting the conveyor.

Example I A steel conveyor chute about 20 inches in length, about 5 inches wide with about 2 inch sides, shaped similar to the chute illustrated in FIG. 1 and available as vibratory feeder Model F0l0 from the Syntron Co., Homer City, Pa. is coated with a coating solution of about 50 parts vinyl chloride-vinyl acetate copolymer available as EXON 470 available from the Firestone Plastics Co., about 8 parts of Luxol Fast Blue Dye available from the E. I. du Pont de Nemours & Co., and about 200 parts methyl ethyl ketone, by placing a quantity of the coating solution at the end of the conveyor chute and tilting the conveyor to about 30 from the horizontal allowing the coating solution to run down the conveyor with any excess draining at the end. The conveyor chute is air dried and tilted at an angle of about 10 with the horizontal.

About a 10 1b. batch of unrefined glass beads having a density of about lbs. per cubic foot comprising rounds and nonrounds including fines, the rounds of an average diameter of about 600 microns is fed uniformly at a constant rate of about 2 /2 lbs. per minute onto and across the entire width of the conveyor at its elevated end and allowed to run down the chute while the chute is caused to vibrate at an amplitude of about 50 mils at an angle of about 30 degrees with the horizontal at a frequency of about 3,600 cycles/minute.

For purposes of illustration, FIG. 4A is a drawing of a 17.5 photograph of rounds and nonrounds comprising the unrefined batch of particles. Fines are not illustrated.

Rounds advance down the conveyor chute and are collected in a receiver at the end thereof. A quantity of nonrounds including fines comprising chips, filaments, powder and dust some of which is pulled by the conveyor surface from the surface of the rounds themselves are deposited on the coated conveyor chute. FIG. 4B is a drawing of a 17.5 photograph of nonrounds deposited on the surface of the conveyor chute after a first pass. After this first pass, the relatively more round material which passed over the chute is taken from the receiver and passed a second time over a cleaned conveyor chute in the same position and operating under the same conditions to further refine the particles. FIG. 4C is a drawing of a 17.5 photograph of nonrounds including dumbbells, angular particles and nonspherical curvilinear particles which remain behind on the conveyor chute after the second pass.

After the second pass a much refined batch of round materials is collected in the receiver at the lower end of the conveyor. FIG. 4D is a drawing of a 17.5 photograph of the refined material after two passes as described in this example.

Example II A coated conveyor as in Example I is tilted at an angle of about 30 degrees with the horizontal.

About a 10 lb. batch of the same type of particles as in Example I is fed uniformly at a constant rate of about 2 /2 pounds per minute across the entire width of the conveyor at its elevated end and allowed to run down the conveyor with no vibration of the conveyor.

With each processing cycle a significant amount of separation of rounds from nonrounds is found to occur.

Example III Example 11 is followed except that the conveyor chute is surface coated with a layer of a plastic, tetrafluoroethylene polymer available from E. I. du Pont de 13 Nemours & Co. under the trademark Teflon and is tilted at angle of about 20 degrees with the horizontal.

A significant amount of separation of round from nonrounds is found to occur for each processing cycle.

Example IV Example II is followed except that the conveyor surface is layered with about a mil layer of Mylar polyethylene terethalate polyester film available from the E. I. du Pont de Nemours & Co.

A significant amount of sorting of rounds from nonrounds is found to occur for each processing cycle.

Example V A removable bottom plate of a conveyor chute of the type used. in Example I, comprising about a 50 micron layer of amorphous selenium on a substrate of aluminum of about 60 microns in thickness is uniformly charged negatively by a corona discharge device available commercially as a part of the Model D Processor available from Xerox Corporation, to a substantially uniform surface potential of about 600 volts and removed from the Processor being careful to avoid exposure to room light and secured to the bottom of the conveyor chute. Working in infra red light is found to be satisfactory. The conveyor chute is placed at an angle of about 10 degrees with the horizontal.

About a 10 pound batch of the same type of unrefined material as in Example I, in the absence of room light, is fed uniformly at a constant rate of about 2 /2 pounds per minute across the entire width of the conveyor at its elevated end and allowed to gravitate down the nega-' tively charged selenium coated conveyor with no vibration of the conveyor.

A comparison of the refined material with the unrefined material before processng, indicates a significant increase of rounds and a decrease of nonrounds in the refined material.

The conveyor chute is cleaned of nonrounds including fines to ready the plate for another batch to be processed by exposing the plate to room light thereby at least partially discharging the plate, increasing the angle of the plate with the horizontal and brushing with a brush made up of New Zealand sheared and dyed rabbit fur.

Example VI A conveyor chute of the type used in Example I is coated with an organic photoconductive coating solution of the photoconductor 2,5-bis (p-aminophenyl)-1,3,4-oxadiazole available under the trademark TO 1920 from Kalle & Co. and the resinous binder material Vinylite VYNS, a copolymer of vinyl chloride and vinyl acetate available from Carbide and Carbon Chemicals Co in diethylketone in proportion of about 30 grams of the photoconductor and about 30 grams of the VYNS to about every 300 milliliters of diethylketone. The solution is applied to the conveyor using a gravure roller. The photoconductive solution air dries to a thickness of about 10 microns.

The bottom of the conveyor is then uniformly charged negatively by a corona discharge device to a substantially uniform surface potential of about -l50 volts in the absence of room light. The conveyor is then placed at an angle of about degrees with the horizontal.

A batch of particles is processed as in the last three paragraphs of Example V.

Example VII The bottom of a conveyor chute of the type used in Example I is first layered with about a 10 mil thick film of Mylar which is overcoated with about a 60 micron layer of copper completely electrically insulated from the rest of the conveyor by the Mylar layer. The conveyor is placed at an angle of about 10 degrees with the horizontal.

About a 10 lb. batch of Teflon heads the rounds of an average diameter of about 400 microns is fed uniformly at a constant rate across the width of the conveyor at its elevated end and allowed to run down the conveyor with no vibration of the conveyor at a rate to create about a monolayer of beads cascading down the conveyor.

Example VIII Example I is followed except that about a 10 lb. batch of copper beads the rounds of an average diameter of about 300 microns is fed uniformly at a constant rate across the width of the conveyor at its elevated end and allowed to run down the conveyor with the conveyor vibrating, the feed rate sufiicient to create about a monolayer of beads cascading down the conveyor.

Although specific components and proportions have been stated in the above description of exemplary preferred embodiments of the sorting system hereof, other suitable materials as specified herein may be used with similar results. In addition, other materials may be added to the mixtures of the materials specified herein or variations may be made in the various processing steps or in the apparatus embodiments to synergize, enhance or otherwise modify the properties of the sorting system hereof.

For example various methods are available to discharge the particles and the conveyor surface to facilitate removal of nonrounds from the conveyor surface after sorting including using a fine aqueous mist or ionized air to serve as a charge path to ground.

Also, although gravitational and vibrational forces are convenient for advancing particles relative to the conveyor other advancing forces are available and any means which causes advancement of particles relative to a conveyor surface may be suitable. For example, particles may be sorted according to this invention by lightly brushing particles with a charged or uncharged brush along on a conveyor which may be tilted or in a horizontal position. Also gas jets for example directing a stream of air may be used to advance particles.

It will be understood that various other changes in the details, material steps and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure, and such changes are intended to be included within the principle and scope of this invention.

What is claimed is:

1. A process for sorting an unsorted mixture of particles into rounds and nonrounds, wherein the rounds of said unsorted mixture have an average diameter less than about 3,000 microns, by imparting advancing forces to the unsorted particles tending to move the particles relative to a conveyor surface wherein said conveyor surface is at least surface layered With a first material and the particles are at least surface coated with a second material, the first and second materials being separated in the triboelectric series at least one of said first and second materials having a bulk electrical resistivity greater than about 10 ohm-cm; the process comprising, the steps of:

(a) surface charging said conveyor surface to a first polarity;

(b) charging at least some of the unsorted particles to a surface charge of a second polarity, opposite in sign to said first polarity, prior to the imparting of advancing forces to said particles; and

(c) imparting advancing forces to the unsorted particles tending to move the particles relative to said conveyor surface wherein at least some: of said particles are advanced in rolling and sliding contact with said 15 conveyor surface to additionally triboelectrically charge said particles to a second polarity and additionally triboelectrically charge the conveyor surface to a first polarity.

2. A process according to claim 1 wherein the first polarity charge is electrically positive or negative depending upon if the first material occupies a higher or lower position, respectively, relative to the second material in the triboelectric series and the second polarity charge is electrically opposite in sign to said first polarity charge.

3. A process according to claim 2 wherein said conveyor surface is surface layered with a photoconductor in electrical contact With an electrically conductive substrate.

4. A process according to claim 2 wherein said conveyor surface is surface layered with a film forming material wherein the product of the dielectric constant and the bulk electrical resistivity of said film forming material is between about and 10 ohm-cm.

5. A process according to claim 1 wherein the rounds of said unsorted mixture have an average diameter between about 50 and 700 microns.

6. A process according to claim 1 wherein said conveyor surface is surface layered with a photoconductor in electrical contact with an electrically conductive substrate.

7. A process according to claim 1 wherein said conveyor surface is surface layered with a film forming material wherein the product of the dielectric constant and the bulk electrical resistivity of said film forming material is between about 10 and 10 ohm-cm.

8. A process for sorting an unsorted mixture of particles into rounds and nonrounds by imparting advancing forces to the unsorted particles tending to move the particles relative to a conveyor surface wherein said conveyor surface is surface layered with a photoconductor in electrical contact with an electrically conductive substrate, the process comprising the steps of:

(a) imparting advancing forces to the unsorted particles tending to move the particles relative to said photoconductor conveyor surface;

(b) providing an electrostatic attractive force between the unsorted particles and the conveyor surface, thereby electrostatically attracting, at least during part of the advancing step, charged particles to the oppositely charged conveyor surface; and

(c) exposing said photoconductor to actinic light to at least partially discharge said photoconductor; whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces as compared to nonrounds; to cause sorting of rounds and nonrounds.

9. Apparatus for sorting an unsorted mixture of particles into rounds and nonrounds, wherein the rounds of said unsorted mixture have an average diameter less than about 3,000 microns, comprising in combination:

(a) a flexible, elongate conveyor surface comprising a photoconductor overlying an electrically conductive substrate, capable of accepting and retaining, at least temporarily, a uniform electrostatic charge;

(b) means for supporting and advancing said conveyor surface through a predetermined path;

(c) electrostatic charging means positioned adjacent said path adapted to uniformly charge said conveyor surface as portionsof said surface pass by;

(d) a particle feeding means next in the path of said conveyor surface, in the direction of its advancement positioned adjacent said path adapted to feed particles to said conveyor surface;

(e) a conveyor surface cleaning means next in the path of said conveyor surface positioned adjacent said surface and adapted to clean nonrounds including fine chips, filaments, powder and dust from said surface; whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater'retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces, as compared to nonrounds, to cause sorting of rounds from nonrounds.

10. Apparatus for sorting an unsorted mixture of particles into rounds and nonrounds, wherein the rounds of said unsorted mixture have an average diameter less than about 3,000 microns, wherein said particles are at least surface coated with a first material and wherein said conveyor surface is at least surface layered with a second material the first and second materials being separated in the triboelectric series at least one of said first and second materials having a bulk electrical resistivity greater than about 10 ohm-centimeters comprising in combination:

(a) a flexible elongate conveyor surface comprising a photoconductor overlying an electrically conductive substrate, capable of accepting and retaining, at least temporarily, a uniform electrostatic charge;

(c) electrostatic charging means positioned adjacent said path adapted to uniformly charge said conveyor surface as portions of said surface pass by;

(e) a conveyor surface cleaning means next in the path of said conveyor surface positioned adjacent said surface and adapted to clean nonrounds including fine chips, filaments, powder and dust from said surface; whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces, as compared to nonrounds, to cause sorting of rounds from nonrounds.

11. Apparatus according to claim 10 including in combination a conveyor surface charge dissipating means positioned adjacent said conveyor surface and between said particle feeding means and said conveyor surface cleaning means in the path of said conveyor surface taken in the direction of advancement thereof.

12. Apparatus according to claim 11 wherein said charge dissipating means is positioned between said particle feeding means and said conveyor surface cleaning means.

13. Apparatus according to claim 11 wherein said charge dissipating means comprises a source of actinic radiation for said photoconductor.

14. Apparatus for sorting an unsorted mixture of particles into rounds and nonrounds, wherein the rounds of said unsorted mixture have an average diameter less than about 3,000 microns, comprising in combination:

(a) a flexible, elongate conveyor surface comprising a film forming material wherein the product of the dielectric constant and the bulk electrical resistivity of said film forming material is between about 10 and 10 ohm-centimeters;

(0) electrostatic charging means positioned adjacent said path adapted to uniformly charge said conveyor surface as portions of said surface pass by;

(e) a conveyor surface cleaning means next in the path of said conveyor surface positioned adjacent said surface and adapted to clean nonrounds including fine chips, filaments, powder and dust from said surface;

whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces, as compared to nonrounds, to cause sorting of rounds from nonrounds. I;

15. Apparatus for sorting an unsorted mixture of particles into rounds and nonrounds, ,wherein the rounds of said unsorted mixture have an average diameter of less than about 3,000 microns, by imparting advancing forces to the unsorted particles tending to move the particles relative to a conveyor surface the, appaartus comprising in combination:

(a) a conveyor surface, surface layered with a photoconductor in electrical contact with an electrically conductive substrate;

(b) means for imparting advancing forces to the unsorted particles tending to move the particles relative to said photoconductive conveyor surface;

(c) means for providing an electrostatic attractive force between the unsorted particles and the conveyor surface, thereby electrostatically attracting, at least during part of the advancing step, charged particles to the oppositely charged conveyor. surface; and

(d) means to expose at least portions of said photoconductor to radiation to at least partially discharge electrostatic charge therefrom; whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces as compared to nonrounds; to cause sorting of rounds and nonrounds.

16. Apparatus for sorting an unsorted mixture of particles into rounds and nonrounds, wherein the rounds of said unsorted mixture have an average diameter of less than about 3,000 microns, by imparting advancing forces to the unsorted particles tending to move the particles relative to a conveyor surface the apparatus comprising in combination:

(a) a conveyor surface, surface layered with a film forming material wherein the product of the dielectric constant and the bulk electrical resistivity of said film forming material is between about 10' and 10 ohm-cm;

(b) means for imparting advancing forces to the unsorted particles tending to move the particles relative to said conveyor surface; and

(c) means for providing an electrostatic attractive force between the unsorted particles and the conveyor surface, thereby electrostatically attracting, at least during part of the advancing step, charged particles to the oppositely charged conveyor surface; whereby electrostatic and frictional forces between said conveyor surface and said particles exert a selectively greater retarding influence on nonrounds causing rounds to advance faster and further along the conveyor surface in response to advancing forces as compared to nonrounds; to cause sorting of rounds and nonrounds.

References Cited UNITED STATES PATENTS 714,649 11/1902 Sutton 209- 1,744,967 1/ 1930 Johnson 209-131 2,314,939 3/ 1943 Hewitt 209-127 3,059,772 10/1962 Le Baron 209-127 3,249,225 5/1966 Stuetzer 209-129 2,638,416 5/1953 Walkup 252-621 OTHER REFERENCES Ralston, Electrostatic Separation of Mixed Granular Solids, Elsevier Pub. Co., N.Y., 1961, (TP 156, E5R3), pages 30-36.

Fraas, Contact Potential in Electrostatic Separation, U.S.B.M. R1. 3667, 1942,, pages 49.

Fraas, Electrostatic Separation of Granular Materials, U.S.B.M. Bull. 603 (TN23U4), 1964,

pages

12 and 13.

FRANK W. LUTTER, Primary Examiner