US2723029A - Ore beneficiation method - Google Patents

Description

'United States Patent Office 2,723,029 Patented Nov. 8, 1955 ORE BENEFICIATIN METHOD James E. Lawver, Lakeland, Fla., assignor to International Minerals & Chemical Corporation, 4a corporation of New York Application April 24, 1951,*Serial No. 222,689

14 Claims. (.C-l. 209-127.)

This invention relates to the beneiciation of multicomponent materials. More particularly, it relates to the electrostatic separation of multicomponent ore material. Still more particularly, it relates to a treatment of granular ore feeds'to render separations in the electrostatic iield more complete and effective.

When beneciating orcs in accordance with the electrostatic methods known heretofore, a number of deleterious effects appear which make `it diflcult to obtain purity of products obtainable by other methods such as flotation. After suliicient passes through an electrostatic field or fields, ore'particles apparently lose the vproper charge -or magnitude of charge, either becoming passive or inactive, or even exhibiting `random charges, sometimes even of the opposite polarity from the charge originally exhibited. Further, the drying of the ore apparently has a marked -eiiect upon `the ore particles. Indirect-ly heated ore, such as ore dried :in an electric oven, exhibits greater selectivity in response to `an electrostatic eld than ore dried by tumbling in a'rotary kiln, or 'equivalent device, in cont-act with combustion products. 'Indirect heating suffers from the disadvantage 'that low heating efficiency does not lend itself to economical drying of 'low value, low grade ore. Y

It is an object of Vthe present :invention to overcome disadvantagesand limitations of the processes heretofore known.

lt is a further object to provide a method wherein ore is maintained at Va high 'level of lsuscepfilzvi'lity to separation by electrostatic fields.

`It is a still vfurtherobject/of the invention to provide a `method whereinpart'icles which have lost their `char-ge are .restored `to a highly `chargeable state.

It is still another object to provide a method wherein ore particles are dried by 'large capacity V.direct heating and the deleterious teiects overcome/by an after treatment to restore the selectivity of charging and response to an electrostatic eld.

lt is a still further object to provide a method wherein particle surfaces are .cleaned and exposed whereby the particles retain high `selectivityrto charging.

It is a still further object to provide amethod which in a few 4passes through .electrostatic fields produces concentrates containing only a few percent of foreign matter.

It is a still further object to provide a method for the beneiiciation of phosphate ores to produce phosphatic material concentrates of 74% to`85% bone phosphate of lime (B. P. L.) content l,containing Aonly approximately 1% to 4% of insoluble matter.

lt is a still further object to provide a method `for the recovery of crystalline products from potash ores, such Vas .sylvinite, which have become .electrostatically inactive.

These and other objects will become apparent as the description proceeds.

This invention is based vupon the discovery that ore particles which have reached .an inactive stage, .as regards response to electrostatic fields, may be restored to electrostatic separationsusceptibility by exposure `of surfaces 4by impact action. Exposure of surface may be brought about either by contact between particles or .contact between particles `and mechanical elements, as in grinders, whereby old surfaces 'are cleansed and/ or new surfaces .are uncovered.

The impacting action referred to may be illustrated by reference to a type of grinding accomplishable in a hammer mill or impactor. In this type of grinding, material is put through the mill as rapidly as possible with a mill setting such that there is a maximum of Aaction at the surface of the particles and minimum particle disintegration. lIt is not intended by this statement to rule out particle disintegration by normal grinding since disintegration does expose new surface areas, but it has been found that such drastic reformation of particles as is accomplished' in grinding toliberation of components is not necessary in order to revive the response of the particles to an electrostatic field. The following sieve analysis of an elcctrostatically inactive phosphate ore before and after a reactivating treatment in a Ahammer `mill illustrates the small amount of reformation necessary.

Asive. Aielve n ysis a ysis Tyler Screen Size Before After Grinding Grinding The above analyses show that only about 4.1% of lines mesh) were produced by passage through the hammer mill. It further shows a general reduction in quantity from one mesh size to another. In other words, the vaction was one of uncovering clean surfaces rather than formation -of new surfaces by cleavage, an action which markedly alters sieve analyses and generally produces large quantities of fines.

The novel method of beneficiating ore comprises im pacting concentrates which exhibit appreciable reduction in response to an electrostatic eld and subjecting the impacted material to further electrostatic field concentration.

In the over-all process of effecting beneiciation of an ore, the method comprises inducing multicomponent dry ore of relatively uniform particle size selectively to accept an electrical charge, subjecting the charged ore to at least one electrostatic field separation, baring or exposing clean surfaces by predominantly surface attrition action of at least the concentrate fraction recovered in the separation, and subjecting the materials after such attrition action to additional separations in electrostatic elds.

In the preliminary .treatment of ores prior to any passage through electrostatic lields, the ore is treated so as .to have a reasonably uniform particle size. In the case of Florida phosphate pebble ore, the operation is simply washing to free the phosphate pebbles of clay and sizing to produce Va fraction in the particle size range of -14 mesh and +200 mesh. Florida pebbles of +14 mesh size are washed and sold without further beneiiciation. If further beneiiciation is required it is necessary to grind pebbles for liberation. On the other hand, phosphate ores from other regions and other nonconductor mineral ores, require grinding to the economical liberation stage for freeing Vor unlocking the ore components.

The material is next dried. This may be accomplished either by direct or indirect heating; for example,`by contact of hot gases with a bed of granular material, as in a rotary kiln, fluo-dryer, and the like. Dry ore, preferably in a hot condition, is then induced selectively to accept an electrical charge and the charged particles passed through one or more electrostatic fields to effect preliminary concentration.

Upon exhibiting a reduction in the response to separation in an electrostatic field of a tield gradient equivalent to that of the prior separation in the sequence, the particles are treated to again approach their initial activity or respense to strong electrostatic fields. This treatment, in the nature of a grinding action, is not to be confused with grinding for liberation. The above sieve analyses show that treatment does not make suflicient change in particle sizes to effect any appreciable liberation. The particles are treated to bring about exposure of clean selectively chargeable surfaces, following which the particles may again be subjected to electrostatic beneciation with or without reheating and recharging o the ore particle, depending upon the characteristics of the ore being beneciated.

More specifically, feed material, a granular product having a particle size in the range of approximately -14 mesh and approximately +200 mesh, determined by standard screens, is fed to the initial electrostatic separations. Charging of the particles preferably is carried out while particles are at an elevated temperature in the range of approximately 150 F. to approximately 350 F. One method of charging is by conveyance of particles to a feeder having a grounded contact chute surface of graphite, lead, zinc, aluminum, tin, and the like. The charged particles are then passed between the electrodes of one or more electrostatic separation units, preferably being fed as freefalling bodies moving in a path normally not in contact with the electrodes.

The strength of the electrostatic Alield which will eieetively alter the path of particle movement varies with the average particle size of the ore fed to the separator and the magnitude of charge on the particle. This may vary from 3,000 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 beneficiating of coarscr particles. ln all such discussion of eld 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 70,000 volts to 250,000 volts, preferably giving field gradients in initial separations of 3,000 volts to 10,000 volts per inch, and gradients of 5,000 volts to 15,000 volts per inch for final separations. 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 a rectilied radio frequency power supply.

When, upon passage through electrostatic fields of the above indicated strengths, the ore particles are rendered inactive with respect to the electrostatic field, i. e., fail to upgrade upon additional passes through an electrostatic field, clean surfaces on particles are exposed by passage through an agitator or impactor; such as, for example, a pebble mill, hammer mill, grinder, rod mill, an impactar, screen, or the like. Following the treatment to clean surfaces, the concentrate particles may or may not exhibit a restored response to electrostatic fields without being retreated to induce a selective recharging. lf the material is to be recharged, it may again be passed over a grounded Contact surface such as has been described above. The charged material then will be resubjected to electrostatic fields of the strength hereinbefore described.

The method will be more fully understood from the following description taken in conjunction with Figures 1 and 2, in which:

Figure l is a diagrammatic flow sheet showing one embodiment of the invention.

Figure 2 is a graph showing the B. P. L. content of ore fractions effected by electrostatic separations when orc concentrate has become unresponsive to an electrostatic eld and when the ore has been rejuvenated by the novel method.

Sized ore from a storage hopper 10 is conveyed to a drying station 11. Hot dry ore is then moved by suitable means to a charging unit 12, such as a grounded vibratory galvanized metal chute. Charged ere issuing from the chute passes between electrodes of one or more electrostatic fields in series indicated by separation station 13. The tail product 14 is generally a discard material but may be reworked by additional passes through a tail scavenging section to reduce the content of the desired product component.

The intermediate product 15 is delivered to an impactor 16; for example, a hammer mill. A reformed product may then be treated in a number of alternating ways7 two of which are illustrated. In the preferred method, the reformed intermediate product is fed directly between the electrodes of one or more electrostatic separation units indicated at station 17. In unit 17, three products are generally produced; a tail 18, which may or may not be subjected to additional electrostatic processing; a concentrate 19; and a middling 20, which is recycled to the impactor.

When greater disintegration of particles takes place, as where materials are prone to fracture, the material may be treated as shown by the dotted line alternative method. In this method, the material from the impactor 16 is delivered by line 26 to a screening station 21. Material 22, larger than some predetermined size which is separated, is recycled by a conveyor, indicated at 22, to the impactor station 16. If any appreciable quantity of fine material is produced, the fines 24 may be removed in an air sizer 23. The granular product of the air sizing or the product from the screening operation 21 are then conveyed with or without reheating and/ or recharging to electrostatic separation station 17 by means of conveyor 25 for treatment as above described. The invention is illustrated by the following example:

Example Florida pebble phosphate was washed and deslimed to produce a fraction having material of a particle size in the range of about -14 mesh and about +150 mesh. The granular material was dried in a duo-dryer through which combustion gases at a temperature of about 325 F. were passed. The original granular feed had a B. P. L. content of approximately 31.5%. The material, while hot, was charged by passage through the trough of a galvanized iron vibratory feeder which was grounded to the earth by an electrical conductor. The charged particles were then dropped as free-falling bodies through a series of four electrostatic fields having plate electrodes 10 feet in length. The total impressed dilerence of potential was maintained at approximately 90,000 volts, and the electrode plates were separated by approximately 9 inches.

At the fourth pass (stage 4) through electrostatic fields, an intermediate concentrate or intermediate product was produced which had a B. P. L. content of about 73.7% and about 8% insoluble material and which was the feed material fed to electrostatic separations hereinafter described to produce curves A and B of Figure 2. The curves of Figure 2 are plots of the B. P. L. content of a concentrate fraction and the B. P. L. content of the corresponding tail fraction produced by an electrostatic separation at an impressed diterence of potential of 90,000 volts and a field gradient of 10,000 volts per inch of distance separating electrodes. Curve A shows the results of passing a portion of the intermediate concentrate or product obtained from the fourth stage through additienal stages of separation without any ,intermediate treatment. *Lhis `curve shows, .for example, `that the .intermediate `concentra-te from stage 4 gave-a separation in stage 5 in which the fraction collected at the electrode normally attracting the phosphate concentrate had a B- P- L. .content of 73%; whereas, the vso-called tail fraction had a B. P. L. content `of 73.5%. VFrom this curve it can be seen that added stages of `electrostatic separation are no longer making eective separations and the concentrate has reached a B. P. L content where `no further upgrading is being accomplished.

A second portion of this intermediate .concentrate obtained from stage 4 was y given a surface grind or attrition treatment in a hammer mill .having no screen. The 150 :mesh material fthe fines :produced in this grind) was removed Lby screening. |lfihe -35 +150 mesh material, constituting '9,6% of the ,mater-i211 fed to thegrinder (4% fines produced), -was then reheated to approximately 350 `and'passed-through one stage `of electrostatic separation in the equipment .used for the ypreliminary separations above.

Results .obtained in this separation are shown in curve B of Figure 2. From this curve it can he seen that the B. P. L. of the concentrate feed can be improved to about the 7.6% to 80% B. L. lhigh `grade .phosphate level of commerce with about `1.5% Vto .about 2.2% in soluble matter; whereas, without the intermediate treatment, such products were unattainable.

For example, from the '73.7% B. P. L. surface ground feed, a concentrate was produced Yin a fifth stage of electrostatic separation having a B. P. L. content of 76.5% and a tail fraction 'having a B. P. L. content of 69%. At the sixth stage of electrostatic separation, afconcentrate was produced having a B. P. Lsrrntent tof `78.2% and a tail fraction having 70.8% B. P. L. The tails from these separation operations are high phosphate content materials of relatively small volume from which the phosphate is recovered by recycling the material to an appropriate intermediate processing stage.

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

l. A method of beneficiating multicomponent ore which comprises inducing dry ore of relatively uniform particle size and having the components thereof substantially completely liberated selectively to accept an electrical charge, subjecting the charged ore to at least one electrostatic field separation, exposing clean surfaces by predominantly surface attrition action on particles of at least the concentrate fraction recovered in the electrostatic separation, and subjecting the materials after such treatment to at least one additional electrostatic separation.

2. A method of beneficiating multicomponent ore which comprises inducing dry ore of relatively uniform particle size and having the components thereof substantially completely liberated selectively to accept an electrical charge, subjecting the charged ore to at least one electrostatic field separation from which is recovered a concentrate having substantially no response to further electrostatic field separations, exposing clean surfaces by predominantly surface attrition action on particles of at least the concentrate fraction recovered in the separation, inducing the clean concentrate particles selectively to accept an electrical charge, and subjecting the materials after such treatment to atleast one additional electrostatic separation.

3. A method of beneciating multicomponent ore which comprises inducing dry granular ore of relatively uniform particle size and having the components thereof substantially completely liberated selectively to accept an electrical charge, subjecting the charged ore as free-falling bodies to the attractive and repulsive forces of at least one electrostatic field, collecting at least a concentrate and a tail fraction, grinding lightly at least the concentrate fraction to expose .clean surfaces, and subjecting the materials after such treatment to at least one electrostatic separation effected by a relatively ,high .impressed .dierence .of potential.

. 4. A method of benelciating multicomponent ore whichcomprises ,inducing :dry granular `Vore of relatively uniform particle :size .and .having .the components thereof Substantially completely liberated selectively to accept an .electrical charge, subjecting the charged ore as freefalliing bodies .to the attractive 'and repulsive forces of at yleast one electrostatic lfield, collecting at least a concentrate and .a tail fraction, grinding lightly at least the concentrate fraction to expose clean surfaces, subjecting-the material-s .after such treatmentto at least one electrostatic .separation effected `by a relatively high impressed ,difference of potential, and collecting at least a concentrate vfraction of higher B. P. L. content than the impacted intermediate concentrate.

:5. A method-of beneciating phosphate ore which comprises .inducing phosphate ore -of .a particle size in the frange-between about 114 imesh and about 200 mesh, pass- ;ing the charged ore .through vat least one electrostatic field as free-falling bodies, collecting at least a concentrate and a Vtail fraction, the concentrate exhibiting substantially no .response Vto an Yelectrostatic field of at least va field gradient equivalent to that of the prior separations :in the sequence, subjecting at least the concentrate fraction to predominantly surface attrition action to expose `clean surfaces on particles, and subjecting the materials after such impact action to at least one electrostatic field separa-tion at a relatively high impressed difference of potential.

|`6. A method 'of `beneficiating phosphate ore which comprises inducing phosphate .ore of a particle size in the range between about 1,4 mesh and about 200 mesh selec- 'tively to accept an electrical charge, subjecting the charged ore as free-falling bodies to the attractive and repulsive forces of at least one electrostatic field having a field gradient in the range of about 3,000 volts per inch to about 10,000 volts per inch, collecting at least a concentrate and a tail fraction, at least the concentrate fraction of which exhibits substantially no response to additional passes through electrostatic fields, lightly grinding the concentrate fraction in a hammer mill, and subjecting the ground concentrate to at least one electrostatic field separation at a field gradient in the range between 3,000 volts per inch and about 15,000 volts per inch.

7. A method of beneficiating phosphate ore which comprises inducing phosphate ore of a particle size in the range of about 14 mesh to about 200 mesh selectively to accept an electrical charge, subjecting the charged orc to the attractive and repulsive forces of electrostatic fields until an intermediate concentrate is produced which eX- hibits substantially no beneficiation in subsequent electrostatic fields, lightly grinding the intermediate concentrate in a hammer mill, and subjecting the ground intermediate concentrate to at least one electrostatic field separation at a field gradient in the range of between about 3,000 and 15,000 volts per inch of distance separating electrodes and equal to the field gradients maintained in preparing the intermediate concentrate.

8. A method of beneficiating multicomponent ore concentrates which exhibit substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises grinding lightly the concentrate at least sufficiently to expose clean surfaces on concentrate particles, and subjecting the impacted material to further electrostatic field concentration.

9. A method of beneficiating ore concentrates which exhibit substantially no response to an electrostatic eld following subjection to at least one electrostatic field separation which comprises grinding lightly the concentrate at least sufficiently to expose clean surfaces on the concentrate particles, inducing the impacted concentrate selectively to accept an electrical charge, and subjecting the charged material to additional electrostatic field concentration.

10. A method of beneficiating ore concentrates which exhibit substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises abrading the concentrate particles at least sufiiciently to expose chargeable clean surfaces, inducing the abraded concentrate selectively to exhibit an electrical charge by exchange of electrons with a surface of low work function relative to said ore grounded to the earth by an electrical conductor, and subjecting the charged material to additional electrostatic field concentration.

11. A method of beneficiating an electrostatically prepared ore concentrate which exhibits substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises grinding lightly the concentrate particles at least sufciently to expose chargeable clean surfaces, and subjecting the charged particles to the attractive and repulsive forces of at least one electrostatic field having field gradients in the range between about 3,000 volts per inch and about 15,000 volts per inch of distance between electrodes.

12. A method of beneficiating an electrostatically prepared ore concentrate which exhibits substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises grinding lightly the concentrate particles at least suiciently to expose clean surfaces, passing the impacted particles as free-falling bodies through at least one electrostatic field bounded by electrodes having impressed thereon a difference of potential in the range between about 90,000 volts and about 250,000 volts, and collecting at least a tail and a concentrate fraction in the vicinity of the lower portions of the electrodes.

13. A method of beneficiating electrostatically prepared phosphate ore concentrates which exhibit substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises grinding lightly the concentrate particles at least sufficiently to expose chargeable clean surfaces, removing ne material produced from said impacted concentrate, and subjecting the charged particles to the attractive and repulsive forces of at least one electrostatic field having field gradients in the range of about 3,000 volts per inch to about 15,000 volts per inch of distance between electrodes.

14. A method of beneficiating electrostatically prepared Florida pebble phosphate ore concentrates of a particle size in the range between about 14 mesh and about 200 mesh which concentrates exhibit substantially no response to an electrostatic field following subjection to at least one electrostatic field separation which comprises lightly grinding the concentrate particles in a hammer mill and subjecting the ground particles to the attractive and repulsive forces of at least one electrostatic field having a field gradient in the range of about 3,000 volts to about 15,000 volts per inch of distance between electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,197,865 Johnson Apr. 23, 1940 FOREIGN PATENTS 542,988 Great Britain Feb. 5, 1942 OTHER REFERENCES Science, December 7, 1945, vol. 102, pages 573-576. Copy in Scientific Library.

Claims (1)

1. A METHOD OF BENEFICIATING MULTICOMPONENT ORE WHICH COMPRISES INDUCING DRY ORE OF RELATIVELY UNIFORM PARTICLE SIZE AND HAVING THE COMPONENTS THEREOF SUBSTANTIALLY COMPLETELY LIBERATED SELECTIVELY TO ACCEPT AN ELECTRICAL CHARGE, SUBJECTING THE CHARGED ORE TO AT LEAST ONE ELECTROSTATIC FIELD SEPERATION, EXPOSING CLEAN SURFACES BY PREDOMINANTLY SURFACE ATTRITION ACTION ON PARTICLES OF AT LEAST THE CONCENTRATED FRACTION RECOVERED IN THE ELECTROSTATIC SEPARATION, AND SUBJECTING THE MATERIALS AFTER SUCH TREATMENT TO AT LEAST ONE ADDITIONAL ELECTROSTATIC SEPARATION.

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