US3031079A

US3031079A - Electrostatic separation

Info

Publication number: US3031079A
Application number: US822655A
Authority: US
Inventors: Charles G Boss
Original assignee: Quaker Oats Co
Current assignee: Quaker Oats Co
Priority date: 1959-06-24
Filing date: 1959-06-24
Publication date: 1962-04-24
Anticipated expiration: 1979-04-24

Description

April 24, 1962 c. G. BOSS 3,031,079

ELECTROSTATIC SEPARATION Filed June 24, 1959 3 Sheets-Sheet 1 i i i I I i I I I 23 24 25 (DUMP) (RECYCLED) (DESI RED CONCENTRATE) F I G. l

INVENTOR. CHARLES G. BOSS ATTO RN EYS April 24, 1962 Filed June 24, 1959 v c. G. BOSS ELECTROSTATIC SEPARATION OR 20 MESH 3 Sheets-Sheet 2 D RYING (APPRO X- 230' F) ELECTROSTATIC SEPERATOR um'r (S EE FIGJ) MIDDLING CONCENTRATE SCREENING FINAL OR PRODUCT GONCENTRATE +29 ROLL rmuus a CRUSH'NG DUMP 20 ELECTROSTATIC um'r (s55 FIG-l) FIG. 2;; 25 conczm'nnz 78a N I I -3s mesa -2o+2a mzsn 1 Y as musmznc MAGNETIC MAGNETIC SEPERATOR SEPERATOR SEPERATOR TA ILING TAILING a TAILING a INVENTOR. DUMP L DUMP VCHARLES e. BOSS FINA L OONCENTRATRATE 4 f/ ia ATTORNEYS April 24, 1962 Filed June 24, 1959 OR 20 MESH C. G. BOSS ELECTROSTATiC SEPARATION 3 Sheets-Sheet 55 FIG. 3

CRUSHING (SET FOR 20 MESH) 2O MESH DR YIN G (APPROX. 230 F) ELECTROSTATIC SEPERATOR um'r (SEE FIG. I) i A MIDDLING 4(' CONCEVTRATE 23 24 25 MAGNETIC 5 -MIDDLING V TAILING e- DUMP FINAL OR PRODUCT CON CENTRATE I INVENTOR. CHARLES G. BOSS 1 ATTORNEYS United States Patent 9 3,031,07 9 ELECTROSTATIC SEPARATION Charles G. Boss, Milwaukee, Wis., assignor to The Quaker Oats Company, Chicago, 111., a corporation of New Jersey Filed June 24, 1959, Ser. No. 822,655 Claims. (Cl. 209-128) The present invention relates to improvements in electrostatic separation. More particularly, the invention relates to improved apparatus and process concepts for the concentration or beneficiation of materials having inherent properties which lend themselves to the use of electrostatic separation principles. Specifically, the invention relates to concepts particularly suited for the dry concentration of iron-bearing ores, such as specular hematite from Canada.

Adequate supply sources for iron ore are a matter of grave concern to a steel industry. It is known, for example, that certain areas in Northern Quebec and Western Labrador contain multi-billion ton deposits of low grade ore. In the metallurgical sense, low grade ore deposits that can be concentrated or beneficiated can be tailored to meet blast furnace requirements. In the economic sense, pre-treated high grade ore should be made available at or near the mine site, at a reasonable processing cost (for example, not to exceed approximately $35 per ton of annual capacity to produce one ton of concentrate), and should be suitable for pelletizing treatment and stockpiling for shipment to the mills when transportation is available. The present invention is considered to afford important and significant advantages in both the metallurgical and economic respects. I

It is a general object of the invention to provide improvements in the application of electrostatic separation principles to the art of concentration and beneficiation. More specifically, it is an object to provide improved apparatus and process concepts for dry (as distinguished from wet flotation techniques) concentration or beneficiation of materials, such as specular hematite.

Specific objects of the invention as well as the advantages thereof will be apparent to those skilled in this art in view of the following detailed description taken in conjunction with the attached drawings.

In the drawings:

FIG. 1 is a somewhat schematic sectional side view of an improved electrostatic separator unit according to the invention;

FIG. 2 is a flow sheet of an improved process according to the invention including the steps of crushing, scalping, drying, electrostatic treatment in the unit of FIG. 1, and screening;

FIG. 2a is a flow sheet of steps which are optional or supplementary to the process of FIG. 2 and which include, after electrostatic treatment, multiple-screening and magnetic separation treatment;

FIG. 3 is a flow sheet of a further modified process according to the invention including the steps of crushing, scalping, drying, electrostatic separation and magnetic separation.

In general, an improved electrostatic separation unit according to the invention is indicated at 20. Each unit 20 has a housing with a feed chamber in the upper portion and a series of discharge compartments, defined by transverse divider plates and the housing walls, in the lower portion. A large diameter grounded rotor extends transversely of the housing beneath the feed chamber and in vertical alignment with a divider plate between two of the discharge compartments. Arranged radially of the grounded rotor, within an arc of about or not more than 90 extending from beneath the feed chamber in the diice rection of rotation of the rotor, are a sparkless electrode charged with a negative polarity at a potential sufiicient to pin all the material being fed over the rotor to the surface thereof, followed by a smooth surface low intensity electrode charged with the same polarity at the same potential as the sparkless electrode to selectively repel or pull the iron bearing material particles from the pinned material on the rotor, and finally, a booster electrode also charged with the same polarity at the same potential as the sparkless electrode to keep the repelled or pulled iron bearing material particles moving in a direction away from the grounded rotor. The iron-bearing material particles are preferably collected in the discharge compartments as concentrate and middling fractions. The non-iron-bearing particles or waste are collected as a tailing fraction.

Referring to FIG. 1, a separator unit 20 includes a preferably rectangular or box-like outer casing or housing 21 having an inlet 22 and a series of outlets; preferably three as indicated at 23, 24 and 25. The inlet 22 receives mined ore which has been crushed, scalped and dried as described below, or otherwise made ready for electrostatic separation.

The ore passes through inlet 22 into a hopper or feed chamber having sloping bottom plates 26 and a feed adjusting slideplate 27. Because the materials being separated are very abrasive, it is preferred that the feed chamber have located therein several diverting elements which are readily replaceable when worn. As shown, an inverted C-channel section 28 extends transversely of the feed chamber above the outlet. Material impinging upon section 28 will defiect onto a similar section 29, extending transversely of the feed chamber adjacent the slideplate 27, or onto a section 3% located alongside the outlet.

Below the feed chamber outlet is a spreader channel 31 which distributes the material to be separated, as a thin layer of particles, on the upper surface of a large diameter and rotating roll 35. The rotor or roll 35 has a diameter of at least 6 inches, although a rotor having a diameter of at least 12 inches or somewhat larger than 12 inches could be used. In practice, a diameter of 12 inches has been found to be satisfactory. The rotor is electrically grounded and is suitably mounted by shaft and bearing means (not shown) for rotation by suitable drive means (also, not shown) at a speed, as determined by the mesh size of the ore particles being concentrated.

As shown in FIG. 1, the rotor 35 is located so that its axis of rotation is above and preferably vertically aligned with a divider or separator plate 36 which extends transversely of the separator housing to define a tailing compartment (37) and one wall of a middlings compartment (38). A similar plate 39 defines the other wall of middlings compartment 38 and a product or concentrate compartment (40). It will be noted that the

discharge outlets

23, 24 and 25, communicate with the lower ends of

compartments

37, 38 and 40, respectively.

To maintain the surface of rotor 35 in operable condition, it is preferred that a conventional felt wiper 41 be located adjacent the spreader channel 31. Also, it is preferred that a revolving rotor brush 42 be located in contact with the surface or roll 35 to dislodge pinned or clinging waste particles into the tailing compartment 37.

Arranged in succession radially of rotor 35, within an arc of about and not more than extending from the spreader channel 31 and below the feed chamber outlet in the direction of rotation of rotor 35 (as shown by the arrows in FIG. 1) are three

electrodes

45, 59 and 55 described in detail below.

Electrode 45 is of a type known to this art as a sparkless electrode. The sparkless electrode 45 preferably is of the type and construction as disclosed in US. Patent No. 2,860,276, issued November 11, 1958. The sparkless electrode 45 includes a hollow dielectric member 46, a bus element in the interior of the member 46, a series or comb of spaced pointed emitting elements or needles 48 extending through the wall of member 46 toward the rotor 35, and a liquid current conducting medium, sealed in the member 46, having substantial electrical resistance and contacting the bus and emission elements 48. For further details as are required to more fully understand the nature of electrode 45, reference is made to Patent No. 2,860,276.

The sparkless electrode 45 is carried by suitably insulated brackets attached to housing 21. The electrode is located radially of the rotor 35 and preferably at an angle of about 45 from the vertical plane of the rotor. The electrode is positioned so that the gap between the point of each needle element 48 and the surface of rotor 35 is about one and one 'half (1%") inches.

The bus of sparkless electrode 45 is connected to a conventional power source (not shown) supplying negative polarity current at a potential sufficient to pin the thin layer of material particles from the feed chamber onto the surface of grounded roll 35. The current potential may be in the order of 15,000 volts but selection of the actual operating voltage is deemed within the knowledge and abilities of those skilled in this art; once they have had the benefit of the full disclosure herein. In any event, the electrostatic field created by the current passing from the needles 48 to the rotor 35 should energize the material particles so that they will electrically adhere to the surface of the grounded rotor.

The second electrode 50 is of a type known to this art as a low intensity electrode. The low intensity electrode 50 is preferably of the type identified by the numeral 39, in U.S. Patent No. 2,398,792, issued April 23, 1946. For purposes of the present invention, the electrode 50 has a smooth cylindrical surface 51 of copper or other suitable electrode material and a diameter not less than one (1) inch and not more than about two (2) inches. The electrode may be stationary or rotating. For further details as are required to more fully understand the nature of electrode 50, reference is made to Patent No. 2,398,792.

The low intensity electrode 50 is carried by suitably insulated brackets attached to housing 21. The electrode is located radially of the rotor 35 preferably about below electrode 45 or about 65 from the vertical plane of the rotor. The electrode is positioned so that the gap between the surface 51 and the surface of rotor 35 is about two (2) inches.

The electrode 50 is electrically connected to a conventional power source, preferably common to the source for the sparkless electrode 45 and on the same side thereof (not shown), supplying negative polarity current at substantially the same potential as the sparkless electrode 45. It has been found that the current passing from the surface 51 to the grounded rotor 35 will cause iron bearing material particles pinned thereon to be preferentially or selectively repelled or pulled away and become free falling.

The third electrode 55 is of a type known to this art as a booster electrode. The booster electrode 55 is preferably of the type designated an auxiliary electrode and identified by the numeral 27 in U.S. Patent No. 2,357,658, issued September 5, 1944. For purposes of the present invention, the electrode 55 has a smooth cylindrical surface 56 of copper or other suitable electrode material and a diameter of about one (1) inch. The electrode is preferably stationary. For further details as are required to more fully understand the nature of electrode 55, reference is made to Patent No. 2,357,658.

The booster electrode 55 is carried by suitably insulated brackets attached to housing 21. The electrode is located radially of the rotor -35 preferably about 15 below electrode 50 or about 80 from the vertical plane of the rotor. The electrode is positioned so that the gap between the surface 56 and the surface of rotor is about two and five-eighths (2%) inches.

The electrode 55 is electrically connected to a cotrli ventional power source, also preferably common to t c) source for the sparkless electrode and the low inten; sity electrode and on the same side th fb (t shown), supplying negative polarity current at 2111i tially the same potential as the sparkless electrode g low intensity electrode 50. It has been found that current passing from the surface 56 to the grounded rOJ 35 will keep the iron-bearing material part1cles, pref erentially pulled by electrode 50, moving 1n a dlI'CC'tlOfl away from the grounded rotor.

The iron-bearing material particles are preferably drvided into two portions. A divider means or sphtter 59 carried atop plate 39 is adjustable withln a limlted range (e.g. 10 to 15) to direct predominantly iron-bearing particles into the concentrate compartment 40. A second divider or splitter means 60 carried atop plate 36 and located directly beneath the rotor 35, is similarly adplstable to direct material particles having lesser iron content into the middling compartment 38. The waste particles: containing predominant amounts of impurities such as silica, remain pinned to the grounded rotor 35 and at: scraped or otherwise directed, as by the rotor brush 4 or wiper 41 into the tailings compartment 37. I

The concentrate particles in compartment 40, the m ddling particles in compartment 38 and the waste partlcles' in compartment 37, are passed through

outlets

25, 24, and 23, respectively, for post-separator utilization or treatment in manners such as described below.

FIG. 2 is a flow sheet illustrating the use of the novel electrostatic separator unit 20, in combination with. other operations, for the concentration or beneficmuon of iron bearing ore such as specular hematite from a region of Quebec. I I

The concentration process begins with mmed ore. the mined ore is then subjected to a crushing operation, indicated generally at 75. Crushing may 1nvolve or require the use of first coarse and then fine crushmgt equipment. Coarse crushing may be accomplished by jaw crushers and selected size and capacity gyratory crushers. Fine crushing may be accomplished by cone crushers. Further details of the crushing operation are considered within the knowledge and abilities of those skilled in this art.

The ore from crushing operation should be at least as fine as 20 mesh. In practice, with the Quebec Or 1t was found that approximately 96% by weight of the fin crushed ore was 20 mesh or smaller. The plus 20 mesh portion (about 4% by weight) can be removed either by screens associated with the crushing equipment or by conventional scalping equipment, indicated at 75a, and could be either recirculated through the grinders or dumped as waste.

After crushing, the ore is brought to a surface-dry condition by a drying operation indicated generally at 76. The drying operation 76 is to be distinguished from the prolonged roasting operations sometimes practiced in the ore beneficiation art. The purpose of operation 76 is to reduce the ambient surface moisture on the fine crushed particles to an inconsequential amount; for example, less than l% by weight. That is, the ore i to b heated until it is dry to the touch and free flowing. It need not be heated so that it is bone dry. In tests conducted with the Quebec ore, it has been found that agitating the fine crushed ore and heating at atemperature of about 230 F. is sufiicient. This type of drying may be accomplished at or near the mine site and crushing Operation, as y gas or oil-fired cylindrical, rotary, or drum type equipment.

After drying, the free flowing minus 20 mesh ore isintroduced into an electrostatic unit, preferably a unit: 20 as described above. By way of example, actual re-- sults of a test conducted using samples of Quebec ore are set forth below. After drying (operation 76), the ore analyzed for 37.4% Fe. Assuming the dry ore to be 100% by weight, after treatment in an electrostatic unit (12 diam. large rotor 35 rotated at 125 r.p.m.; current potential of about 15,000 volts each on

electrodes

45, 50 and 55), a single pass of dry ore separated as follows:

Concentrate, 50.0% by weight, 66.7% Fe; Middling, 8.0% by weight, 32.1% Fe; Tailing, 42.0% by weight, 3.6% Fe.

After separation, the concentrate portion (e.g. 66.7% Fe) may be even further beneficiated. The middling portion (e.g. 32.1% Fe, 20 mesh or smaller) from outlet 24 is preferably recirculated through the separator unit unit 20. It has been found that further beneficiation of the concentrate portion from outlet 25 may be obtained by post-separator treatment including a screening operation indicated generally at 78.

The screening operation 78 is carried out, using conventional screening equipment, within the knowledge and abilities of those skilled in this art to produce a final concentrate of ore particles having a size of at least minus 28 mesh, and preferably smaller. In actual test, the minus 28 mesh final or product concentrate was 43.4% by weight of the head feed to the separator unit 20 and analyzed for 68.5% Fe.

After post-separator screening, the minus 20 plus 28 mesh particles may be subjected to further reduction in particle size by an attrition type crushing operation indicated generally at 79. The crushing operation 79 may be accomplished by a set of conventional secondary crushing rolls. Other attrition types of crushing equipment could also be employed. The portion from crushing operation 79 is then recirculated through the separator unit 20. In an actual test, the recirculated portion was 6.6% of the head feed to the separator unit 20 and analyzed for 54.7% Fe.

As shown in FIG. 2 and described above, a typical iron-bearing ore such as specular hematite from Quebec may be concentrated or beneficiated, from an initial Fe analysis of 37.4% to an analysis of 66.7%, solely by use of the novel separator unit 20. This is a result not readily obtainable by any known process or equipment. It has also been shown that post-separator treatment employing the screening operation 78 will produce a minus 28 mesh fraction having an Fe analysis of 68.5%.

The invention contemplates the use of still other postseparator operations. It has been found that conventional multiple screening equipment, employed after'a separator unit 20 operated as described above, followed by treatment with conventional magnetic separator equipment, will ultimately produce 'a final or product concentrate comprising more than one-half of the head feed to the separator unit 20 and having an analysis of at least 66.5% Fe.

FIG. 2a is a flow sheet illustrating these other postseparator operations. Concentrate from outlet 25 of the electrostatic separation unit 20 is subjected to a screening operation 78a and is carried out with conventional multiple shaker screening equipment having an upper screen 80 with a 28 mesh size and lower screen 81 with a 35 mesh size. In actual test, the three portions from screening operation 78a weighed and analyzed as follows:

Minus 20 plus 28 mesh portion, 6.6% by weight, 54.7%

Minus 28 plus 35 mesh portion, 11.15% by weight,

Minus 35 mesh portion, 32.25% by weight, 69.3% Fe.

The three intermediate concentrate ore portions from the screening operation 78a may be further beneficiated by a magnetic separator operation indicated generally at 85. The magnetic separation may be carried out using conventional high intensity magnetic separator units within the knowledge and abilities of those skilled in this art; preferably units of the induced rotor (IR) type in which the magnetic poles remain stationary while the rotor revolves. As the ore portions pass through the units, the magnetic particles are deflected and strategically located divider means or splitters accomplish a separation into magnetic-concentrate portion and non-magnetic tailing portions.

In actual test, the three portions from the screening operation 78a after separation in individual magnetic separator units 85 of a three roll induced rotor type, with a gap and a current supply inducing from 16,000 to 24,000 gausses, weighed and analyzed as follows:

v Minus 20 plus 28 mesh portion:

Concentrate, 5.28% by weight, 67.3% Fe, Tailing 1.32% by weight, 4.4% Fe; Minus 28 plus 35 mesh portion:

Concentrate, 10.59% by weight, 69.2% Fe, Tailing, 0.56% by weight, 12.0% Fe; Minus 35 mesh portion:

Concentrate, 32.09% by weight, 69.5% Fe, Tailing, 0.16% by Weight, 28.7% Fe.

It will be observed that beneficiation according to FIG. 2a produces an ore comprising almost 50% of the head feed to the separator unit 20 and having a median Fe analysis in excess of 69%. This result is of great significance to the art.

Still other variations from the flows of FIG. 2 or 2a could be practiced. For example, if recirculation of the middling portion (8.0% by weight, 32.1% Fe) from outlet 24 of the separator unit 20' were not desired, the screening operation 78 or 7811 could be employed. In actual test, using the

multiple screens

80 and 81, the resulting three portions weighed and analyzed as follows:

Minus 20 plus 28 mesh, 1.14% by Weight, 5.7% Fe; Minus 28 plus 35 mesh, 1.94% by weight, 8.9% Fe; Minus 35 mesh, 4.92% by weight, 47.3% Fe.

Whether such a small and low analysis yield would have commercial significance depends upon the economies of the particular beneficiation project under consideration. In beneficiation projects seeking extreme of purity, it is apparent that recirculation rather than screening of the middling portion from separator unit 20 would be preferred. However, the 47.3% portion could be mixed with the other much higher analysis portions if desired.

FIG. 3 is a flow sheet illustrating the use of the electrostatic separator unit 20, in combination with the crushing and scalping operations 75 and 75a, and the drying operation 76, as shown in FIG. 2 and described above. After the separator unit 20', operated as described above, a single magnetic separator unit is employed rather than the

screening operation

78 or 78a to obtain a final or product concentrate.

In actual tests, using Quebec ore, the head feed to the magnetic separator unit 85 was 50% by weight of the head feed to the electrostatic separator unit 20 and analyzed for 66.7% Fe. After a magnetic separation operation 85 (as described above) the two portions weighed and analyzed as follows:

Concentrate, 47.96% by weight, 69.2% Fe; Middling, 2.04% by weight, 8.39% Fe.

The middling portion from the separator unit 85 may either be recirculated through the separator unit 20 or dumped as waste.

The flow of FIG. 3 has also provided excellent results for other iron-bearing ores such as specular hematite with relatively high amounts of SiO as an impurity. In actual test, the electrostatic unit 20 (operated as described above), produced a final or product concentrate comprising 43.4% by weight of the head feed and analyzing for 65.43% Fe and 6.12% SiOg. After a magnetic separa- 'tion operation 85 (as described above), the two portions weighed and analyzed as follows:

Concentrate, 40.86% by weight, 68.38% Fe, 1.93% SiO Middling, 2.54% by weight, 17.98% Fe, 73.48% SiO A high SiO ore may also be beneficiated with excellent results according to the flows of FIGS. 2 and 2a and the operations described above in connection therewith. In actual test, such an ore processed according to FIG. 2 was separated by an electrostatic separator unit operated as described above, and weighed and analyzed as follows:

Concentrate, 43.4% by weight, 65.43% Fe, 6.12% SiO Middling, 9.4% by weight, (analysis not available); Tailing 56.6% by weight, 5.52% Fe.

The concentrate portion (65.43% Fe, 6.12 SiO from outlet was subjected to screening operation 78, using a 28 mesh screen. The minus 28 mesh final or product concentrate was 34.1% by weight of the head feed to the separator unit 20 and analyzed for 68.33% Fe and 2.11% SiO In actual test, a high SiO ore processed according to FIG. 211, using the combination of screening operation 78a and magnetic separator units 85, was also beneficiated with results having great significance to the art. The concentrate from outlet 25 of the electrostatic separator unit 20 (43.4% by weight, 65.43% Fe, 6.12% SiO was subjected to screening operation 78a using an upper screen 80 (28 mesh) and a lower screen 81 mesh). The three portions from screening operation 78a weighed and analyzed as follows:

Minus 20 plus 28 mesh portion: 9.3% by weight, 54.79%

Fe, 20.81% SiO Minus 28 plus 35 mesh portion: 12.8% by weight, 67.05%

Fe, 3.95% SiO Minus 35 mesh portion: 21.34% by weight, 69.10% Fe,

1.01% SiO After the screening operation 78a, each portion was passed through a magnetic separator unit 85 (operating as described above). After a magnetic separation operation, the three portions weighed and analyzed as follows:

Minus 20 plus 28 mesh portion:

Concentrate, 7.33% by Weight, 65.81% Fe, 5.05%

SiO Tailing, 1.97% by weight, 13.80% Fe, 79.46% SiO Minus 28 plus 35 mesh portion:

Concentrate, 12.39% by weight, 68.45% Fe, 1.96%

SiO Tailing, 0.41% by weight, 24.57% Fe, 64.15% SiO Minus 35 mesh portion:

Concentrate, 21.14% by weight, 69.23% Fe, 0.84%

SiO Tailing 0.16% by weight, 52.55% Fe, 23.83% SiO As used above, the numerical values of mesh size refer to US. standard wire mesh screens.

Conclusion As clearly illustrated by the product Weights and analyses as set forth above, the novel electrostatic separator unit 20 according to the invention is of great value to iron-bearing ore beneficiation projects. It is also apparent that the concepts of construction and operation involved therein could be used for the electrostatic treatment, beneficiation or concentration of other materials known to the art as being capable of separation by the electrostatic techniques.

It is also apparent that the concepts of material treatment and procession of operations disclosed herein may be employed with significant results. The conditioning operations of crushing, scalping and drying, the electrostatic separation treatment per se, and the post-separation operations of screening or magnetic separation, or both,

as preferred, represent novel processes or methods for the beneficiation of materials such as iron-bearing ores, particularly specular hematite with or without high amount of SiO; as an impurity.

What is claimed is:

1. In a gravity feed electrostatic separator unit having a housing, said housing having a feed chamber in the upper portion and a series of discharge compartments, defined by transverse divider plates and the housing walls, in the lower portion, a large diameter electrically grounded rotor extending transversely of said housing beneath said feed chamber and in vertical alignment with a divider plate between two of said discharge compartments, means for rotating said rotor, a sparkless electrode, a low intensity electrode, and a booster electrode, said sparkless, low intensity and booster electrodes being arranged successively radially of said rotor within an arc of not more than extending from beneath said feed chamber in the direction of rotation of said rotor, each of said electrodes being electrically connected to a power source supplying negative polarity current at substantially the same potential, the potential of said current being sufiicient to pin all material from said feed chamber passing through the electrostatic field of said sparkless electrode to the surface r of said rotor.

2. In a gravity feed electrostatic separator unit having a housing, said housing having a feed chamber in the upper portion and a series of discharge compartments, defined by transverse divider plates and the housing walls, in the lower portion, a large diameter electrically grounded rotor extending transversely of said housing beneath said feed chamber and in vertical alignment With a divider plate between two of said discharge compartments, means for rotating said rotor, a sparkless electrode located at an angle of about 45 from the vertical plane of said rotor, a low intensity electrode located below said sparkless electrode, and a booster electrode located below said low intensity electrode, said sparkless, low intensity and booster electrodes being arranged successively radially of said rotor within an arc of not more than 90 extending from beneath said feed chamber in the direction of rotation of said rotor, each of said electrodes being electrically connected to a power source supplying negative polarity current at substantially the same potential, the potential of said current being sufficient to pin all material from said feed chamber passing through the electrostatic field of said sparkless electrode to the surface of said rotor.

3. In a gravity feed electrostatic separator unit having a housing, said housing having a feed chamber in the upper portion and a series of discharge compartments, defined by transverse divider plates and the housing walls, in the lower portion, a large diameter electrically grounded rotor extending transversely of said housing beneath said feed chamber and in vertical alignment with a divider plate between two of said discharge compartments, means for rotating said rotor, a sparkless electrode located at an angle of about 45 from the vertical plane of said rotor, a low intensity electrode located at an angle of about 65 from the vertical plane of said rotor, and a booster electrode located below said low intensity electrode, said sparkless, low intensity and booster electrodes being arranged successively radially of said rotor within an arc of not more than 90 extending from beneath said r feed chamber in the direction of rotation of said rotor,

each of said electrodes being electrically connected to a power source supplying negative polarity current at substantially the same potential, the potential of said current being sufficient to pin all material from said feed chamber passing through the electrostatic field of said sparkless electrode to the surface of said rotor.

4. In a gravity feed electrostatic separator unit having a housing, said housing having a feed chamber in the upper portion and a series of discharge compartments, defined by transverse divider plates and the housing walls,

in the lower portion, a large diameter electrically grounded rotor extending transversely of said housing beneath said feed chamber and in vertical alignment with a divider plate between two of said discharge compartments, means for rotating said rotor, a sparkless electrode located at an angle of about 45 from the vertical plane of said rotor, a low intensity electrode located at an angle of about 65 from the vertical plane of said rotor, and a booster electrode located at an angle of about 80 from the vertical plane of said rotor, each of said electrodes being electrically connected to a power source supplying negative polarity current at substantially the same potential, the potential of said current being sufficient to pin all material from said feed chamber passing through the electrostatic field of said sparkless electrode to the surface of said rotor.

5. In a process of concentrating an ore containing particles of specular hematite of 20 mesh size and smaller, the steps of subjecting said ore particles to the electrostatic fields created between an electrically grounded rotor and a series of three electrodes positioned radially of said rotor, the first of said electrodes having a series of spaced emitting elements extending toward said rotor at an angle of about 45 from the vertical plane thereof, the second of said electrodes having a smooth surface and being located below said first electrode at an angle of about 65" from the vertical plane of said rotor, the third of said electrodes having a smooth surface and being located References Cited in the file of this patent UNITED STATES PATENTS 2,187,637 Sutton Jan. 16, 1940 2,314,939 Hewitt Mar. 30, 1943 2,357,658 Johnson Sept. 5, 1944 FOREIGN PATENTS 650,025 Great Britain Feb. 14, 1951 OTHER REFERENCES Mine and Quarry Engineering, July 1941, page 198.

Engineering and Mining Journal, vol. 159; No. 1; January 1958; pages 88-89.

American Institute of Mining and Metallurgical Engineers, TP No. 2257; 1947 (pages 1 and 2).

Industrial and Engineering Chemistry, vol. 32, N0. 5, May 1940, pages 600-604.