US2738875A - Method and apparatus for electrostatic separation - Google Patents

Description

March 20, 1956 1. M. LE BARON 2,738,875

METHOD AND APPARATUS FOR ELECTROSTATIC SEPARATION Filed March 30, 1951 j gj ,6

k. f U/ 'w'" W j 4 q 7 c m NT AT dgz dl Wl/M ATTORNEY United States Patent O F METHOD AND APPARATUS FOR ELECTROSTATIC SEPARATION Ira Le Baron, Lakeland, Fla., assignor to International Minerals & Chemical Corporation, a corporation of New York Application March 30, 1951, Serial No. 218,472 11 Claims. (Cl. 209 -127 This invention relates to electrostatic separation of material particles. More particularly, it relates to a method and apparatus for effecting electrostatic separations. Still more particularly, it relates to electrodes of improved design for electrostatic separation apparatus.

One of the simplest methods for separation of nonco'nductor materials is to pass the composite material or ore through one or more electrostatic fields formed between essentially paired and oppositely charged plate electrodes. In the past, these electrodes have consisted of smooth surfaced, straight, flat, or curved plates.

Plate electrodes have, however, been a source of numerous difficulties. A primary difficulty has been the tendency of particles to ride down the face of theelectrode and to take on an opposite charge by contact with the electrode. When the polarity of the particle charge is reversed, the particles are repelled and travel towards the oppositely charged electrode. versed polarity thereupon mix with repelled particles originally of opposite charge reducing the eifectiveness of the separation. build up material on theface thereof whichdoes not reverse the charge but which does interfere with the electrostatic field. Further, straight electrodes must be divergent from parallel vertical planes in order to give the stream of falling particles room to spread out according to their relative charges and intensity of charge. This divergence weakens the electrostatic field and therefore the efficiency of separation effected between the lower portions of the divergent electrodes falls off rapidly.

It is an object of the instant invention to overcome the disadvantages and shortcomings of apparatus heretofore utilized.

It is a further object of the invention to provide an electrostatic method of beneficiating ores wherein charged particles are moved in an unobstructed path thus permitting the movement of charged particles of opposite polarity a greater distance in opposite directions.

It is a still further object of the invention to provide an electrostatic method of beneficiating ore wherein negatively and positively charged particles are moved such distances in opposite directions relative to one another than cleaner separations may be effected.

It is a further object to provide electrodeswhich are maintainable equidistantly spaced over their entire length.

It is a further object to provide electrodes which produce an elongated field free of impediments to particle travel.

It is a further object to provide electrodes which constrict the particle travel path into elongated relatively narrow channels.

It is a still further object to provide electrodes. such that the material fed betwen the electrodes is moved laterally a greater distance and thereby purer end products are produced.

It is a further object to provide electrodes which have Additionally, the electrodes tend to.

2,738,875 Patented Mar. 20, 1956 trodes is substantially parallel to the direction of material movement in response to the electrostatic field.

These and other objects will be apparent to those skilled in the art as the description proceeds. I

It has now been discovered that, provided the path is unobstructed, an electrostatic field of high flux density in the area of introduction of a multicomponent ore feed will give the particles an initial thrust and cause the particles to pass into an area having a field of substantially zero flux density, bounded by electrode plates of identical polarity and magnitude of charge, thereby allowing 7 ticles a thrust in the direction of elongated relatively Repelled particlesof resubstantially reduced to zero. The particle is permitted; by such a field, to travel in its original direction-with narrow channels, bounded by electrodes of like polarity and magnitude of charge on both the negative and positive electrode sides of the field, and collecting a plurality of fractions, one of which on each side of the field is enriched as to one component of'the feed.

In the preferred embodiment of the invention, an electrostatic field of high concentration is built up in the vertical plane in line with the feed inlet. This field progressively decreases in strength as it is constricted into the portion of the channels between companion electrodes of the same polarity closest to the feed.

That portion of the material which moves through the channels between electrodes of like polarity apparently has been given sulficient initial energy to travel through an area having a field gradient which has been out adding energy to the electrical field. Material contacting the electrodes tends to travel along the surfaceof the electrode toward the weak end of the electrostatic field before dropping in collection hoppers.

In this .method of beneficiating ore, material is sub-- divided until reduced to suitable particle size, the oreis then dried .andthe dry ore of relatively uniform particle size is induced to accept an electric charge. Charged materialsubsequently is passed through an electrostatic field set upbetween convex electrodes of opposite polarity and is collected as concentrate fractions in the vicinity of the lower portions of said electrodes or conductors.

In the operation of this method, the feed material, generally a raw ore, is sized if it is of the nature of Florida phosphate pebble or is crushed sufficientlyto unlock or liberate its constitutents, and is screened by the usual procedures to produce a granular product having a previously determined or selected particle size; for example, in the range of approximately 14 mesh and +200 mesh lobtained by the use of standard screens.

a configurationsuch that the body portion of the elec- Comminuted ore is dried and then induced to accept an electric charge. The material is preferably charged in the absence of an electric field by causing the particles to contact a conducting or donor surface connected to a source of free electrons as by being grounded through an electrical conductor. One method of accomplishing this result is to convey warm particles having a temperature in the range of approximately F. to approximately 500 F. to a feeder having a grounded contact chute surface of iron, lead, zinc, aluminum, copper, tin, and the like. When the particles flow in a thin stream over the chute surface, which is connected by an electrical conductor to the earth, the individual particles take on a 3 charge either negative or positive according to the nature of the material.

The charged particles are preferably fed as free-falling bodies between the oppositely charged electrodes of one or more electrostatic separating units, i. e., ina path normally not in contact with said electrodes. The strength of the electrostatic field which will effectively alter the path of particle movement varies with the average particle size of the ore fed to the separator. This voltage may vary from 5,000 volts per inch of distance between electrodes in separating material of relatively fine particle size, to 15,000 volts per inch of distance separating electrodes for beneficiationof relatively coarser particles. In all such discussion of field strength, it must be borne in mind that corona discharges which ionize air are to be avoided. In general, it is preferred to operate with a total impressed difference in potential of voltage in the range of 50,000 volts to 250,000 volts. This voltage should be maintained at a high direct voltage potential substantially free of alternating current components, i. e., filtered D. C. current should be low in the so-called A. C. ripple. A steady supply of D. C. voltage may also be obtained without expensive filtering apparatus by the use of such equipment as radio frequency power supply.

The invention will be more fully understood by reference to the drawings in which:

Figure 1 is a top plan view of a set of electrodes shaped and arranged in accordance with the instant invention.

Figure 2 is a side elevational view of the electrodes of Figure 1.

Figure 3 is a top plan view of a multiple electrode arrangement having some electrodes common to two electrode separation paths or channels.

Referring to Figures 1 and 2, the numeral indicates an electrode having a convexly curved face or shoulder portion 11 and an elongated extension or body portion 12 of lesser or even zero curvature. These electrodes must be operated in clusters of four, or the equivalent, with units of opposite polarity arranged in a spaced, shoulder-to-shoulder, endwise alignment. Companion negatively charged electrodes are indicated by numeral 13 and companion positively charged electrodes are indicated by numeral 14. Intermediate the electrode cluster is an ore feed distributor mechanism 15.

The curvature of the electrodes is subject to marked variation. Units may be formed, for example, as a segment of a circle, the body portion formed as a straight line tangent to the circle at the end point of the segment; or, the unit may be a segment of a parabola terminating at the point of greatest width; or, the configuration may I be a segment of an ellipse from approximately the intersection with the longest axis to approximately the intersection with the shortest axis, hyperbolic in shape, or equivalent construction.

Material is delivered to distributor 15 through grounded chute 16 which is preferably of the vibratory type. Adjacent the bottoms of electrodes are collection hoppers 17, 18, and 19. Hoppers 17 and 18, and hoppers 18 and 19, have common walls 20 and 21, respectively, provided with pivot arms 22 and 23, respectively, adapted to be secured at any desired angle for altering the cut-point of material to be collected in the various hoppers.

In Figure 3 is shown an arrangement of a multiplicity of parallel channeled portions of the electrostatic field as set up between electrodes wherein each electrode, except those at the extremities of each group, serve as electrodes for two individual channels. Electrodes at the extremities of a group of like polarity are the type shown in Figure l and are indicated as 24 for negative electrodes and 25 for positive electrodes. Intermediate the end negative electrodes 24 are negative electrodes 26a and 2617, each consisting of parailel walls connecting semicircular ends. Intermediate positive electrodes 25 are positive electrodes 27a and 27b of construction similar to negative electrodes 26a and 26b. Material is distributed to an area intermediate the electrodes by distributors 28, 29, and 30, respectively. Material passing through the electrostatic field is directed outwardly and downwardly. Positively charged material passes between electrodes 24 and 26a, electrodes 26a and 26b, and electrodes 26b and 24, while negatively charged material passes between electrodes 25 and 27a, 27a and 27b, and 27b and 25. The separated material is collected in hoppers 31, 32, and 33.

Operation of the equipment illustrated in Figures 1 and 2 is shown by the following example.

Example Florida pebble phosphate ore was ground to a particle size in the range of approximtaely l4 mesh to approximately +200 mesh. It had a B. P. L. content of approximately 25%. The ore was heated to dryness in an electric oven maintaining a temperature of about 300 F. The warm material was charged by passage through a zinc-coated trough grounded to the earth by an electrical conductor. Two separate portions of ore were electrostatically tested, and one portion was fed to the distributor of the apparatus of the instant invention at a rate of one ton per hour per linear foot of electrode. The ore travelled as free-falling particles between electrodes of the instant invention, 10 feet long, about 4 feet in width, and having a convex curvature shoulder of 30 inch radius of curvature arranged as shown in Figures 1 and 2. Electrodes of like polarity were spaced 3 inches apart adjacent the' shoulder and 1 inch apart at the outer end, and electrodes of opposite polarity were spaced 3 inches apart in shoulder-to-shoulder arrangement. Material was collected in hoppers approximately as shown in Figure 2. The second portion of ore was passed between plate electrodes 10 feet long and 18 inches wide in face-to-face arrangement at approximately the same rate of feed. These electrodes were spaced 9 inches apart. The potential difference between electrodes in both types of apparatus was about 90,000 volts. In each instance the material was given one pass between electrodes and results were as follows:

Plate electrodes give slightly higher than 5% B. P. L. in the tail product when producing a concentrate having slightly more than 40% B. P. L. in the concentrate. Electrodes of the configuration described, on the other hand, have approximately 2.5% B. P. L. in the tail product when producing the 40% B. P. L. concentrate. When utilizing plate electrodes to produce concentrates of greater than 50% B. P. L. content, the B. P. L. content of tail fractions rapidly rises to the 10% to 15% B. P. L. level; whereas, when utilizing the electrodes of the instant invention, concentrates of about B. P. L. content are obtained before there is any appreciable increase in B. I. L. content of the tail product. In addition, the 13. P. L. content of the concentrate may be pushed almost to before the B. P. L. content of the tail product rises to the 10% to 15% level.

Having thus fully described and illustrated the character of the invention, what is desired to be secured and claimed by Letters Patent is:

1. An electrostatic separation machine comprising a source of unidirectional electricity, at least a first pair of electrodes electrically connected to be of like polarity, at least asecond pair of electrodes electrically connected to be of the same polarity, said polarity being opposite to that of said'first pair of electrodes, each of said electrodes in cross sectional configuration being formed with a straight body portion and a shoulder formed as a segment of a circle, the various pairs of electrodes being arranged with straight body portions substantially parallel and with shoulders diverging, said pairs of electrodes of opposite polarity being arranged with shoulders adjoining one another and with straight body portions in substantially straight line alignment, means to feed ore particles between said electrodes of opposite polarity, and collection hoppers for receiving separated material.

2. An electrostatic separation machine comprising a source of unidirectional electricity, a plurality of spaced electrodes connected to said source of electricity, said electrodes extending vertically parallel to the free fall path of feed material and being in cross sectional configuration formed with a convex curvature shoulder and an elongated straight body portion, said electrodes being arranged in spaced pairs with convex curvature shoulders adjoining one another and body portions substantially parallel, said pairs of electrodes being arranged in parallel as groups having one polarity, said groups of electrodes of opposite polarity being positioned with electrode shoulders adjacent and with the straight body portions in substantially straight line alignment, and collection hoppers for separated material.

3. For use in an electrostatic machine, the unit comprising a plurality of spaced electrodes, said electrodes extending vertically parallel to the free fall path of feed material and in cross sectional configuration being formed with a convex curvature shoulder and an elongated straight body portion, said electrodes being arranged in spaced pairs with body portions substantially parallel, and convex curvature shoulders adjoining one another and diverging, said pairs of electrodes being arranged in parallel groups having one polarity, groups of pairs of electrodes of opposite polarity being positioned with electrode shoulders adjacent and with the straight body portions in substantially straight line alignment.

4. Electrostatic separation apparatus comprising a first pair of horizontally spaced electrodes facing each other, a second pair of similarly horizontally spaced electrodes facing each other, the electrodes of said second pair being horizontally spaced from and positioned opposite the corresponding electrodes of the first pair, means for feeding particles to be separated to fall freely in a zone substantially equidistant from all said electrodes, electrical means for applying an electrical charge of like polarity to the electrodes of the first pair and for applying an electrical charge of like polarity to electrodes of the second pair with the charges on the electrodes of the second pair being opposite to the charges on the electrodes of the first pair, and collection means for receiving separated particles.

5. A method of electrostatic separation of oppositely electrically charged particles which comprises dropping said particles to fall freely into an electrostatic field having an elongated zone of uniform positive potential and an oppositely extending elongated zone of uniform negative potential in alignment with the first mentioned zone, the particles being introduced between the two zones and in substantial alignment with the two zones, thus giving 5 the positively charged particles a horizontal thrust in a direction to impart to such positively charged particles a movement having a horizontal component through said elongated zone of uniform negative potential, and giving the negatively charged particles a horizontal thrust in a 10 direction to impart to such negatively charged particles a movement having a horizontal component through said elongated zone of uniform positive potential, and collecting the separated particles falling through at least one of said zones.

6. A process as in claim 5 wherein the particles to be separated are particles of phosphate ore of -14 to ,{200 mesh.

7. A process as in claim 5 wherein the difierence of potential between the zone of positive potential and the zone of negative potential ranges between about 50,000

volts and about 250,000 volts.

8. A process as in claim 7 wherein the particles to be separated are particles of phosphate ore of l4 to +200 mesh.

9. A process as in claim 5 wherein the oppositely electrically charged particles are particles of a finely divided multicomponent ore.

10. A process as in claim 5 wherein the oppositely electrically charged particles are particles of comminuted ore.

11. A process as in claim 10 wherein the difference of potential between the zone of positive potential and the zone of negative potential ranges between about 50,000

volts and about 250,000 volts.

References Cited in the file of this patent UNITED STATES PATENTS 1,335,758 Schmidt Apr. 6, 1920 2,247,596 Boer July 1, 1941 2,536,693 Okolicsanyi Jan. 2, 1951 OTHER REFERENCES Bureau of Mines Report of Investigations 3667, dated November 1942, pages 2, 3, and Figure 1.

Bureau of Mines RI 3667, dated November 1942, page 11. (Copies in Scientific Library.)

Claims (2)

1. AN ELECTROSTATIC SEPARATION MACHINE COMPRISING A SOURCE OF UNIDIRECTIONAL ELECTRICITY, AT LEAST A FIRST PAIR OF ELECTRODES ELECTRICALLY CONNECTED TO BE OF LIKE POLARITY AT LEAST A SECOND PAIR OF ELECTRODES ELECTRICALLY CONNECTED TO BE OF THE SAME POLARITY, SAID POLARITY BEING OPPOSITE TO THAT OF SAID FIRST PAIR OF ELECTRODES, EACH OF SAID ELECTRODES IN CROSS SECTIONAL CONFIGURATION BEING FORMED WITH A STRAIGHT BODY PORTION AND A SHOULDER FORMED AS A SEGMENT OF A CIRCLE, THE VARIOUS PAIRS OF ELECTRODES BEING ARRAGNED WITH STRAIGHT BODY PORTIONS SUBSTANTIALLY PARALLEL AND WITH SHOULDERS DIVERGING, SAID PAIRS OF ELECTRODES OF OPPOSITE POLARITY BEING ARRANGED WITH SHOULDERS ADJOINING ONE ANOTHER AND WITH STRAIGHT BODY PORTIONS IN SUBSTANTIALLY STRAIGHT LINE ALIGNMENT, MEANS TO FEED ORE PARTICLES BETWEEN SAID ELECTRODES OF OPPOSITE POLARITY, AND COLLECTION HOPPERS FOR RECEIVING SEPARATED MATERIAL.

5. A METHOD OF ELECTROSTATIC SEPARATION OF OPPOSITELY ELECTRICALLY CHARGED PARTICLES WHICH COMPRISES DROPPING ING AN ELONGATED ZONE OF UNIFORM POSITIVE POTENTIAL AND AN OPPOSITELY EXTENDING ELONGATED ZONE OF UNIFORM NEGATIVE POTENTIAL IN ALIGNMENT WITH THE FIRST MENTIONED ZONE, THE PARTICLES BEING INTRODUCED BETWEEN THE TWO ZONES AND IN SUBSTANTIAL ALIGNMENT WITH THE TWO ZONES THUS GIVING THE POSITIVELY CHARGED PARTICLES A HORIZONTAL THRUST IN A DIRECTION TO IMPART TO SUCH POSITIVELY CHARGED PARTICLES A MOVEMENT HAVING A HORIZONTAL COMPONENT THROUGH SAID ELONGATED ZONE OF UNIFORM NEGATIVE POTENIAL, AND GIVING THE NEGATIVELY CHARGED PARTICLES A HORIZONTAL THRUST IN A DIRECTION TO IMPART TO SUCH NEGATIVELY CHARGED PARTICLES A MOVEMENT HAVING A HORIZONTAL COMPONENT THROUGH SAID ELONGATED ZONE OF UNIFORM POSITIVE POTENTIAL, AND COLLECTING THE SEPARATED PARTICLES FALLING THROUGH AT LEAST ONE OF SAID ZONES.

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