US3385435A

US3385435A - Electrostatic separation apparatus

Info

Publication number: US3385435A
Application number: US432369A
Authority: US
Inventors: James E Lawver; John H Sholine; John F French
Original assignee: International Minerals and Chemical Corp
Current assignee: International Minerals and Chemical Corp
Priority date: 1965-02-12
Filing date: 1965-02-12
Publication date: 1968-05-28
Anticipated expiration: 1985-05-28

Description

y 8, 1968 J. E. LAWVER ETAL 3,385,435

ELECTROSTATIC SEPARATION APPARATUS Filed Feb. 12, 1965 3 SheetsSheet l INVENTORS James 5. Lawyer; Jlm H. .Sfiazziw, John F. Frezw/v May 28, 1968 J. E. LAWVER ETAL 3 ELECTROSTATIC SEPARATION APPARATUS 5 Sheets-Sheet 2 Filed Feb. 12, 1965 mvsmons' James E. Lawyer; John HS/zol Jim,

z'rze, F. Franc/u May 28, 1968 J. E- LAWVER ETAL 3,385,435

ELECTROSTATIC SEPARATION APPARATUS 3 Sheets-Sheet 3 Filed Feb. 12, 1965 Ill? IIII llllmlllllllllfrllllll|l||||l|i|||lLLllllllilllll II I I'll; ||||||Il||l|||lllll-llllllllllllllllllllillllllllllll'lll|| INVENTORS June: 5 Lawyer; fa/zit H. S'lwlv'rze JZ/m F3 Franc/v United States Patent ELECTROSTATIC SEPARATION APPARATUS James E. Lawver, Edina, Minn., John H. Sholine, Lakeland, Fla., and John F. French, Atlanta, Ga., assignors to International Minerals & Chemical Corporation, a

corporation of New York Filed Feb. 12, 1965, Ser. No. 432,369 20 Claims. (Cl. 209-129) ABSTRACT OF THE DISCLOSURE An electrostatic separation apparatus including at least two electrodes mounted in upstanding parallel relationship to define at least one zone for free fall electrostatic separation with the electrostatic field being provided by a source of unidirectional electricity connected to adjacent electrodes. A plurality of separating zones is provided with a minimum of space by mounting at least three elec trodes to define separation zones between adjacent electrodes and the source of unidirectional electricity is connected to the electrodes so as to have alternate electrodes of like polarity and adjacent electrodes of opposite polarity. Each of the electrodes is characterized as 'having a rectangular surface facing the separation zone between it and the adjacent electrode with the perimeter of the rec tangular surface being curled away from the separation zone which it faces. Disposed above each of the separation zones is a feeder having a pair of horizontal rolls providing surfaces which are yieldably biased toward each other, and means for rotating the rolls in a direction to withdraw material from a supply hopper and introduce the same into the respective separation zones at a minimal vertical velocity.

This invention relates generally to the beneficiation of minerals and more particularly to apparatus for electrostatic separation of mineral material particles. Specifically, the invention is directed to apparatus for electrostatic separation of non-conductive particulate materials,

It has long been recognized that mineral beneficiation can be obtained by passing finely divided mineral material through an electrostatic field. The art has recognized several types of apparatus that may be employed for elec trostatic separation of mineral particles. One type of apparatus obtains electrostatic separation by reason of the differences in surface conductivity and capacitance of the particles of the comminuted mineral, This type of apparatus normally feeds the particulate material in a single layer over a revolving cylindrical electrode and the particles are charged by what is known as inductive conductance wherein the particles become charged by contact with the conductive surface of the cylindrical electrode in the presence of an external electrical field. The conductive particles become charged by inductive conductance much more rapidly than the non-conductive particles. Thus by controlling the time and the field structure in relation to the surface resistance and capacitance of the particles it is possible to control the particle trajectories in the electrical field and thereby to eifect a separation.

Another type of electrostatic separation apparatus is known as the mobile ion type of separator wherein the particulate material is delivered in a single layer over a grounded revolving cylindrical electrode which has spaced therefrom an oppositely charged second electrode. As the material particles pass between these electrodes they are in the normal operation charged by bombardment with ions and/or electrons. The most conductive particles rapidly share their charge with the grounded electrode and fall from the cylindrical electrode into a 3,385,435 Patented May 28, 1968 collecting compartment under the influence of gravity. The less conductive particles remain attracted to the revolving cylindrical electrode until removed and allowed to fall into a different collecting compartment.

The above described types of electrostatic separation apparatus are extensively used and are suitable for separations where conductive and non-conductive particles or where particles of different degrees of conductivity are involved. Different problems are present in attempting electrostatic separation of particulate materials that are essentially non-conductive in nature and the above-mentioned types of equipment are not satisfactory.

In the apparatus of the instant invention, electrostatic separation of essentially non-conductive particulate materials may be carried out. The apparatus embodies a free fall separation zone and the electrostatic separation action is attained by charging the non-conductive particles through the medium of contact electrification. This term is considered as generically embracing particle charging which results either by particle to particle contact within the particulate material or by contact of particles with a separate member such as a grounded metal surface, It has been recognized that particles of dissimilar non-conductive materials can exhibit differential electrification or charging upon being subjected to contact electrification operations such as agitation. A fuller discusson of this phenomenon and reference to additional sources considering the matter may be found in Lawver Patent 2,805,- 769 issued Sept. 10, 1957.

It is a principal object of the present invention to provide improved apparatus for electrostatic separation of non-conductive materials in particulate form.

It is a further object of this invention to provide electrostatic separation apparatus wherein high capacity operation for an apparatus of only limited size is obtained employing a free fall electrostatic separation zone following particle to particle contact electrification of the particles of the material being separated.

Another object of the invention is to provide electrostatic separation apparatus of relatively high capacity that is low in initial cost having a minimum overall height and which may be constructed to provide increased capacity as desired by increasing the number of separating zone cells by the mere addition of an electrode to form each additional separating zone cell.

It is also an object of the instant invention to provide an improved electrostatic separation apparatus wherein the charged particles are introduced into the separating zone at a minimal vertical velocity such that the electrostatic field in the zone can most efiectively perform separation of the particles.

Yet another object of the invention is to provide an electrostatic separation apparatus wherein the particulate material is introduced at a minimal entrance velocity and in a vertical direction so that the separating action carried out in the electrostatic field will not be afiected by the presence of lateral movement of the particles as introduced into the separating zone.

Generally described, the present invention embodies an electrostatic separation apparatus wherein a series of spaced planar electrodes are mounted in parallel upstanding relation to one another to define between adjacent electrodes a zone for free fall electrostatic separation and where the electrostatic field in each zone is obtained by alternate electrodes being of like polarity and adjacent electrodes being of opposite polarity. A feeder is disposed above the separation zone having rolls providing faces which are yieldably biased toward each other, these rolls being rotated in a direction to withdraw material from a supply hopper and introduce the particulate material into the zone in a vertical direction but at a minimal vertical velocity. Individual material receiving compartments are disposed beneath the electrodes to receive the separated products resulting from the electrostatic separation of the material falling between the electrodes.

The above and other objects and novel features of the instant invention will be readily apparent from the following description which is given in connection with the accompanying drawings. It is to be expressly understood that the drawings are for the purposes of illustration and are not intended to define the limits of the invention but rather to merely illustrate a preferred embodiment and structural apparatus incorporating the features of the instant invention. In the accompanying drawings forming a part of this specification and wherein like reference numerals are employed to designate like parts:

FIGURE 1 is a vertical sectional view of the electrostatic separation apparatus with parts within the housing shown in elevation;

FIGURE 2 is a plan view taken on line 22 of FIG- URE 1;

FIGURE 3 is a detailed sectional view of a material feeder in relation to the outlet of the material supply hopper and inlet to an electrostatic separating zone.

FIGURE 4 is an elevational view of one electrode with parts thereof cut away and its mounting bars shown in section;

FIGURE 5 is a sectional view taken on line 5--5 of FIGURE 4;

FIGURE 6 is a plan view showing the material collecting compartments and taken on line 66 of FIGURE 1; and

FIGURE 7 is a partial sectional view through the material collecting compartments of FIGURE 6.

Referring to FIGURE 1, the apparatus has a housing 10 provided with suitable supporting legs 11. Enclosed within the walls of the housing 10 there are provided frame members 12 which serve to support bars 13 that extend across the interior of the housing and support

electrodes

14, 15, and 16 by being fastened to the edges thereof as shown at 17. The bars 13 are made of a suitable electrical insulating material so that their position adjacent the electrostatic separating zones between the electrodes and their connection to the electrodes will not interfere with the electrostatic separation operation.

The bars 13, as connected to the respective electrodes, serve to mount these electrodes in parallel upstanding relation to one another such that the three electrodes are equally spaced and a free fall electrostatic separation zone A is formed between

electrodes

14 and 15 and a similar electrostatic free fall separating zone B is formed between

electrodes

15 and 16. It will be understood that although three electrodes forming a two cell apparatus have been illustrated in the embodiment of the drawings, the capacity of the apparatus can be increased by forming additional cells simply by the addition for each added cell of one additional electrode laterally spaced from and parallel to one of the existing outside electrodes. Thus, more free fall electrostatic separation zones up to the desired number of zones for the required apparatus capacity may easily and inexpensively be built into the apparatus.

To estabish the electrostatic field in the zone between adjacent electrodes, an appropriate source of uni-directional electricity (not shown) is connected to the electrodes to have alternate electrodes of like polarity and adjacent electrodes of opposite polarity. In other words, the leads for

electrodes

14 and 16 will be connected to one side of the source to render these two electrodes of the same polarity and electrode 15 will be connected to the other side of the source to be of a polarity opposite the polarity of the

electrodes

14 and 16. Where more than three electrodes are incorporated into the apparatus, the connections to the source will be made to have alternate electrodes of like polarity and adjacent electrodes of opposite polarity.

The novel construction of the

individual electrodes

14, 15, and 16 is illustrated in FIGURES 4 and 5. Each electrode consists of two

rectangular sheets

25 and 26. The entire perimeter of each of these sheets is curled as shown at 27 on FIGURE 5 with the four edges 28 of the sheet facing normal to the plane of the sheet.

The

sheets

25 and 26 are retained in spaced relation by

tubular frame members

29, 30, 31 and 32. These members are positioned between the sheets to extend along the respective perimeters thereof, i.e., along the respective four edges of the sheets which are rounded so as to embrace the outwardly facing exterior of the frame members. These rounded or curled portions of the sheets where they overlie the frame members are attached thereto by suitable screws 35.

The above described electrode construction, which is used for each of the three electrodes assembled in the apparatus as shown in FIGURE 1, provides a rectangular planar surface for each electrode with a peripheral portion surrounding this surface curving away from the planar surface. It will be noted, as shown in FIGURE 4, that the corners of the longitudinal electrode face are also rounded and curved away from the planar surface. This rounded or curled perimeter avoids the presence of sharp corners at the perimeter of the electrode which might detract from or interfere with the desired electrostatic field to be set up between the adjacent electrodes. The components assembled in forming an electrode may conveniently be formed from aluminum with the tubular frame members being of aluminum as well as the two sheets which are attached thereto and which form the oppositely facing planar surfaces of the electrode.

Referring again to FIGURE 1 on the drawings, the upper end of housing 10 is closed by a sheet 40 of electrical insulating material such as lucite. This sheet has a slot formed therein centrally disposed above each of the electrostatic separation zones A and B. The slot is shown at 41 for zone A and at 42 for zone B. As shown more clearly in FIGURE 2, the

slots

41 and 42 extend parallel to the planes of the

electrodes

14, 15 and 16 and are elongated in a direction such that a fiat wide stream of particulate material can be introduced therethrough into the respective separating zones. To achieve the high capacity to which the compact electrostatic separator of this invention is capable it is preferred that the width of each flat wide stream of material be substantially equal to the width of

electrodes

14, 15 and 16.

A hopper 45 is mounted above the housing 10 disposed to receive and retain a supply of particulate material that is to be separated. In the embodiment illustrated the hopper has two downwardly extending

outlets

46 and 47 terminating above the

slots

41 and 42 respectively. The relationship of these two

outlets

46 and 47 of hopper 45 to the

slots

41 and 42 and the feeders described hereinafter is shown by the

outlets

46 and 47 being illustrated in phantom on FIGURE 2.

Each

outlet

46 and 47 has a slide gate 50 disposed therein and which may be individually moved into stop flow of material from hopper 45 through the outlets or withdrawn to the position shown in FIGURE 1 to permit flow of material to the material feeders.

Between outlet 46 and slot 41 as well as between outlet 47 and slot 42 there is disposed a feeder which acts to control the introduction of particulate material into the separating zone. The construction of this feeder is of particular importance in that it is designed to introduce the particulate material at a controlled minimal vertical velocity and also introduce the material in a vertical direction without any movement in a lateral direction toward one or the other of the two electrodes between which the separating zone is formed.

It is to be noted as important to the invention herein that each feeder should be positioned as close as possible to the top of the electrodes. This close relationship of the feeder to the top of electrodes between which it is to supply the flat wide stream of particulate material precludes dispersion of the particle due to air currents occurring before the particles become properly exposed to the electrostatic field existing between the electrodes. However, each feeder must be sufficiently spaced and insulated from the electrodes so as to avoid arcing between the electrodes and feeder or undesirable building of electrostatic charge.

The two feeders above the

slots

41 and 42 are similar in construction and accordingly description of only one of these feeders need be given. Each feeder includes a pair of

rolls

55 and 56. As shown more clearly in FIG- URE 3, each of these rolls is formed with a core 57 enclosed within a layer 58 of resilient material such as rubber. The rubber covering on the rolls provides an advantage in that it tends to avoid interference of the rolls with the electrostatic field which is present immediately below the rolls and the insulating sheet 40. Also particularly where the shafts of the rolls are fixedly mounted, a rubber layer on one or both of the rolls will provide surfaces which are yieldably biased toward each other and thus as the rolls are rotated the particles can be fed between the rolls, the resilient covering layer yielding to permit the particles to pass therebetween.

In the embodiment as illustrated, the shafts of the

rolls

55 and 56 have their opposite ends mounted in

bearings

60 and 61, respectively. These bearings are supported in parallel slide tracks 62 and the bearings are biased toward each other by springs 63. Thus even in the absence of a resilient covering layer on one or both of the

rolls

55 and 56 the rolls may be mounted in a manner such that the bearings are urged toward each other by springs such as 63. With this construction, as the rolls rotate the particles may pass therebetween and the rolls yield by compressing the springs 63 as the

bearings

60 and 61 slide in tracks 62.

As shown in FIGURE 2 the shafts for

rolls

55 and 56 of both feeders are connected through a suitable drive 70 to a motor 71. Motor 71, when energized, drives the rolls of the two feeders in the direction shown by the arrows on FIGURES 2 and 3. With the rolls pressed together, the speed of motor 71 may be varied as desired to obtain the requisite speed of rotation of the feed rolls and obtain the appropriate rate of feed of material into the separating zone with minimal vertical velocity. If the rolls are slightly spaced, as on the order of V to inch for two-inch diameter rolls, the rolls function as a choke feed control. In either case, feed stoppage by bridging due to oversize or trash particles in the feed stream is eliminated by yielding of the rolls for their surfaces to move apart.

Downwardly tapering walls 74 at the lower end of housing 10 beneath the

electrodes

14, and 16 support a series of individual material receiving compartments generally designated by the numeral 75. The arrangement of these compartments is shown in plan view of FIGURE 6.

Each compartment is in the form of an elongate-d narrow upwardly open chamber. A plurality of these compartments are grouped beneath each of the electrostatic separating zones A and B. As illustrated, there are five compartments 76 centrally disposed beneath zone A and similarly, five compartments 77 centrally disposed beneath zone B. A wider compartment 78 is mounted directly beneath the plane of each of the

electrodes

14, 15, and 16. As shown in FIGURE 7, each compartment tapers downwardly to an outlet which has connected thereto a flexible material withdrawal tube 80.

By having a group of narrow material receiving

com partments

76 and 77 disposed beneath the respective separating zones the material is collected in what may be closely similar product collections as between the products in adjacent narrow compartments. The degree of differentiation between the product collections in the compartments is in relation to the separating effect achieved by the materials passage through the electrostatic separating zone. The separated products collected in these groups of compartments may thereafter be combined at the operators discretion by withdrawing the product from two or more of the compartments through the flexible tubes to a single collection point and then conveying the combined products from the apparatus. In practice, products colaccommodate the divider beneath the electrodes.

The manner of utilization of the apparatus should be apparent from the description of such apparatus as given hereinabove. Electrostatic fields are established in the zones A and B by connection of the

electrodes

14, 15 and 16 to a source of unidirectional electricity such that electrodes '14 and 16 are of the same polarity but of opposite polarity to that of electrode 15.

The hopper 45 is charged with a supply of material in particulate form which is to be subjected to electrostatic separation. Normally an average mean particle size of 35 to 48 mesh is preferred. However a particle range of 10 to mesh or finer can effectively be separated in the apparatus. These particles carry a charge so that effective separation of the material may be carried out in the electrostatic separation zones A and B. Particle charging can be accomplished by particle to particle contact, or the like, in any of a variety of charging devices.

Motor 71 would then be energized to rotate the rolls of the feeders at a speed such that the requisite rate of material feed into the apparatus will occur with the material being introduced vertically at a minimal vertical velocity, in a thin flat stream having a width substantially equal to the width of

electrodes

14, 15, and 16. With slide gates 50 open the material will then pass from hopper 45 through the feeders and

slots

41 and 42 into the electrostatic field in zones A and B.

The material moving in a free falling path through zones A and B will essentially be made up of non-conductive particles, although a small proportion of conductive particles would not interfere with the separating operation. However, such small proportion of conductive particles will not separate with any degree of predictability since in passing through the electrostatic field the conductive particles will repeatedly assume a charge and discharge rapidly. A substantial portion of conductive particles in the material could interfere with maintenance of the field between the electrodes or with the environment necessary to get electrostatic separation.

The apparatus as described can achieve a high capacity or rate of electrostatic separation of material. Capacities as high s thirty tons or more per hour in a separator such as illustrated and having outside dimensions of roughly 5' x 5' x 5' can be expected. The individual electrode may be about 18" wide and 60" long with around 13" space between adjacent electrodes. Actually, the electrode length can vary between 2 /2 and 7' while, in order to achieve the desired high separation capacities the electrode width should not be less than 18" and desirably not less than about 36". Indeed the electrode can have a width several times the length thereof. The design of the electrodes permits high field strength between the electrodes with a differential of 60 kilovolts per inch of spacing between the electrodes being feasible.

It is to be understood that the form of the invention herewith shown and described is to be taken only as a preferred embodiment of the invention and that various changes and modifications in the arrangement of parts and interconnection of components may be resorted to without departing from the spirit or scope of the appended claims.

We claim:

1. Electrostatic separation apparatus comprising at least three planar electrodes mounted in parallel upstanding relation to one another to define between adjacent electrodes a zone for free fall electrostatic separation of particulate material, each electrode having a width of at least 18" and including two spaced rectangular metal sheets with the entire perimeter of each sheet curled so that the edge of each sheet faces normal to the plane of the sheet and means fastening the spaced sheets of each electrode together,

means for connecting a source of unidirectional electricity to said electrodes to have alternate electrodes of like polarity and adjacent electrodes of opposite polarity,

feeding means disposed above each said zone to introduce charged particulate material in a free fall path into such zone,

and individual material receiving compartments disposed beneath said electrodes to receive the particulate material as electrostatically separated in falling between said electrodes.

2. Electrostatic separation apparatus as recited in claim 1 wherein said material receiving compartments are elongated and narrow with the upper ends opening upwardly and a plurality of compartments grouped beneath each said zone with a flexible material withdrawal tube leading from the bottom of each compartment so that the material collected in selected ones of these cells can be combined as desired in operation of the apparatus.

3. Electrostatic separation apparatus as recited in claim 2 wherein said grouped plurality of compartments includes a wider elongated compartment disposed directly beneath each electrode and a series of thin elongated compartments occupying the space between the wider elongated compartments.

4. Electrostatic separation apparatus as recited in claim 1 wherein said electrodes are between 2% and 7 in length.

5. Electrostatic separation apparatus as recited in claim 1 wherein each of said electrodes has a length of at least 60".

6. Electrostatic separation apparatus comprising a pair of electrodes each defining a planar surface and a third electrode defining planar surfaces on opposite faces thereof, said pair of electrodes being spaced from each other with the planar surfaces of such electrodes facing each other and with said third electrode positioned between said pair of electrodes to define a zone for free fall electrostatic separation on each side of said third electrode, each electrode having the surface facing the zone between it and the adjacent electrode formed from a rectangular metal sheet having the entire perimeter curled so that the edge of such sheet faces normal to the plane of the sheet, each of said electrodes having a width of at least 18",

means for connecting a source of unidirectional electricity to said electrodes to have said pair of electrodes of like polarity and said third electrode of a polarity opposite thereto,

feeding means disposed above each free fall zone to introduce charged particulate material to be separated in a free fall path into the zone, said feeding means introducing said material in a thin flat stream having a width substantially equal to the width of said electrodes,

:and individual material receiving compartments disposed beneath said electrodes to receive the particulate material as electrostatically separated in falling between said electrodes.

7. Electrostatic separation apparatus comprising a pair of electrodes each defining a planar surface, and a third electrode defining planar surfaces on the opposite faces thereof, said pair of electrodes being spaced from each other with the planar surfaces of such electrodes facing each other and with said third electrode positioned equidistantly between said pair of electrodes to define a zone for free fall electrostatic separation on each side of said third electrode, each of said electrodes providing a rectangular surface facing the zone between it and the adjacent electrode and having the perimeter of the rectangular surface curled away from the zone which it faces, the width of each of said electrodes being at least 1 means for connecting a source of unidirectional electricity to said electrodes to have said pair of electrodes of like polarity and said third electrode of a polarity opposite thereto,

feeding means disposed above each free fall zone to introduce charged particulate material to be separated in a free fall path into the zone,

8. Electrostatic separation apparatus comprising at least three planar electrodes mounted in parallel upstanding relation to one another to define between adjacent electrodes a zone for free fall electrostatic separation of particulate material, each of said electrodes including two rectangular metal sheets with the entire perimeter of each sheet curled so that the edge of each sheet faces normal to the plane of the sheet and tubular frame members positioned between said sheets along the four edges of said sheets with the curled perimeter embracing said tubular frame members, the width of each of said electrodes being at least 18",

feeding means disposed above each said zone to introduce charged particulate material in a free fall path into such zone, said feeding means introducing said material in a thin flat stream having a width substantially equal to the width of said electrodes,

9. Electrostatic separation apparatus comprising electrodes mounted in upstanding parallel relation to define therebetween a zone for free fall electrostatic separation of particulate material, each electrode having a width of at least 18" and including two spaced rectangular metal sheets with a limited portion of the perimeter of each sheet curled away from the plane of the sheet so that the edge of each sheet faces normal to the plane of the sheet,

tubular frame members positioned between said two sheets of each electrode and extending along the four edges thereof with the curled perimeter portion of these sheets embracing and attached to said tubular frame members,

means for connecting a source of unidirectional electricity to said electrodes to establish an electrostatic field therebetween,

feeding means disposed above said zone to introduce charged particulate material to be separated in a free fall path into the zone,

10. Electrostatic separation apparatus as recited in claim 9 wherein said electrodes are between 2 /2 and 7 in length.

11. Electrostatic separation apparatus as recited in claim 9 wherein each of said electrodes has a length of 60".

12. Electrostatic separation apparatus comprising at least three planar electrodes each including two spaced rectangular metal sheets with the entire perimeter of each sheet curled so that the edge of each sheet faces normal to the plane of the sheet and means fastening the spaced sheets of each electrode together, each of said electrodes having a width of at least 18",

a housing having spaced electrically insulating bars extending thereacross and connected to the edges of said electrodes to retain the electrodes in parallel upstanding relation to one another whereby there is defined between adjacent electrodes a zone for free fall electrostatic separation of particulate material,

a sheet of electrical insulating material covering the top of said housing, said sheet having a slot therein opening above each said zone for the introduction of material to be separated in such zone,

feeding means disposed above said sheet overlying each said slot in said sheet to introduce charged particulate material to be separated in a free fall path into such zone, said feeding means introducing said material in a thin flat stream having a width substantially equal to the width of said electrodes,

and a plurality of elongated narrow upwardly open compartments grouped at the lower end of said housing beneath each said zone to receive the particulate material as electrostatically separated in falling between said electrodes.

13. Electrostatic separation apparatus as recited in claim 12 wherein said grouped plurality of compartments include a wider elongated compartment disposed directly beneath each electrode and a series of thin elongated compartments occupying the space between the wider elongated compartments, and each compartment has a flexible material withdrawal tube leading from the bottom thereof.

14. Electrostatic separation apparatus comprising electrodes mounted in upstanding parallel relation to define therebetween :a zone for free fall electrostatic separation of particulate material, each electrode including two spaced rectangular metal sheets with a limited portion of the perimeter of each sheet curled away from the plane of the sheet so that the edge of each sheet faces normal to the plane of the sheet,

hopper to contain a supply of particulate material, said hopper being mounted above said electrodes and having an outlet opening into said zone to introduce particulate material into said zone,

a feeder associated with said outlet having a pair of horizontal parallel rolls providing surfaces which are yieldably biased toward each other and means connected to rotate said rolls in a direction to withdraw material from said outlet and introduce the charged material into the zone at a minimal vertical velocity, and in a thin flat stream having a width substantially equal to the width of said electrodes,

15. Electrostatic separation apparatus comprising at least three planar electrodes mounted in parallel upstanding relation to one another to define between adjacent electrodes a zone for free fall electrostatic separation of particulate material, each of said electrodes including two rectangular metal sheets with the perimeter of each sheet curled so that the edge of each sheet faces normal to the plane of the sheet and tubular frame members positioned between said sheets with the curled perimeter embracing and attached to the said tubular frame members,

means for connnecting a source of unidirectional electricity to said electrodes to have alternate electrodes of like polarity and adjacent electrodes of opposite polarity,

a hopper for a supply of particulate material mounted above said electrodes, said hopper having an outlet opening into each zone between adjacent electrodes to introduce particulate material into such zone,

a feeder associated with each said outlet opening having a pair of horizontal parallel rolls providing surfaces which are yieldably biased toward each other and means for driving said rolls to withdraw material from the outlet and introduce the charged material into the zone at a minimal vertical velocity, and in a thin flat stream having a width substantially equal to the width of said electrodes,

16. Electrostatic separation apparatus comprising a pair of upstanding spaced electrodes defining therebetween a zone for free fall electrostatic separation of particulate material,

a hopper to contain a supply of particulate material, said hopper being mounted above said electrodes and having an outlet opening into said zone to introduce particulate material into said zone,

a feeder associated with said outlet having a pair of horizontal parallel rolls with surfaces which are constructed of a resilient material capable of avoiding interference of said rolls with said electrostatic field and means connected to rotate said rolls in a direction to withdraw material from said outlet and introduce the charged material into the zone at a minimal vertical velocity, and in a thin fiat stream having a width substantially equal to the width of said electrodes,

17. Electrostatic separation apparatus as recited in claim 16 wherein the rotational mounting for said rolls includes means yieldably biasing the roll surfaces toward engagement with each other.

18. Electrostatic separation apparatus comprising a pair of upstanding spaced plate electrodes defining therebetween a zone for free fall electrostatic separation of particulate material, each electrode having the surface facing the zone between it and the adjacent electrode formed from a rectangular metal sheet having the perimeter curled so that the edge of the sheet faces normal to the plane of the sheet,

a sheet of electrical insulating material overlying the upper ends of said electrodes, said sheet having a slot therein centered above said zone for the introduction of material into such zone,

a hopper for material to be separated mounted above said sheet and having an outlet positioned above the slot in said sheet,

a pair of horizontal parallel rolls rotatably mounted between said outlet and said slot and providing surfaces which are yieldably biased toward each other,

means for rotating said rolls to withdraw material from said outlet and introduce the charged material through said slot into said zone at a minimal vertical velocity, and in a thin flat stream having a width substantially equal to the width of said electrodes,

and individual material receiving compartments dis posed beneath said electrodes to receive the particulate material as electrostatically separated in falling between said electrodes.

19. Electrostatic separation apparatus as recited in claim 18 wherein at least one of said rolls is provided with a layer of resilient material capable of avoiding interference of said rolls with said electrostatic field to form the References Cited UNITED STATES PATENTS 409,186 8/1889 Friend 209-46 2,168,681 8/1939 OBrien 209127 2,782,923 2/1957 Cook 209-127 3,058,589 10/1962 Carpenter 209-129 FOREIGN PATENTS 750,728 1/1945 Germany.

836,481 4/ 1952 Germany.

FRANK W. LUTTER, Primary Examiner.