US3045822A

US3045822A - Magnetic separator

Info

Publication number: US3045822A
Application number: US678714A
Authority: US
Inventors: Patrick E Cavanagh
Original assignee: Research Cottrell Inc
Current assignee: Research Cottrell Inc
Priority date: 1957-08-16
Filing date: 1957-08-16
Publication date: 1962-07-24
Anticipated expiration: 1979-07-24

Description

pail

Filed Aug. 16, 1957 6 Sheets-Sheet 1 INVENTOR PATRICK E. CAVANAGH BY HM 7. W

ATTORNEY S July 24, 1962 P. E. CAVANAGH MAGNETIC SEPARATOR Filed Aug. 16, 1957 BY fla HmZ/A- 6 Sheets-Shee 2 'INVENTOR PATRICK E. CAVANAGH ATTORNEYS y 1952 P. E. CAVANAGH 3,045,822

MAGNETIC SEPARATOR Filed Aug. 16, 1957 6 SheetsSheet 3 I28 I28 h zlo INVENTOR h PATRICK E. CAVANAGH I ,9 M A. AM

ATTORNEYS July 24,

Filed Aug.

P. E. CAVANAGH 3,045,822

MAGNETIC SEPARATOR 6 Sheets-Sheet 4 INVENTOR PATRICK E. CAVANAGH BY HM TAM NMA- AW ATTORNEYS July 24, 1962 P. E. CAVANAGH 3,045,822

MAGNETIC SEPARATOR Filed Aug. 16, 1957 6 Sheets-Sheet 5 N o N 00 It I o I I l w 2 N v I N I r I v m I W Q 1 N I g m N m 5 w w w E I. a 8" i I ig g 5. co 9 T m .1 5 I In o o l I ---!1 o I w k Kw in" I I a I gun I?) I 5 g, ////i /i I I u 111 a w I g 9 o N 4 IO j q; w i w E #3 {g m o mvmon N PATRICK E.CAVANAGH ATTORNEY S July 24, 1962 P. E. CAVANAGH MAGNETIC SEPARATOR 6 Sheets-Sheet 6 Filed Aug. 16, 1957 INVENTOR PATRICK E. CAVANAGH ATfonNEYs United States Patent 3,045,822 MAGNETIC SEPARATOR Patrick E. Cavanagh, Oalrville, Ontario, Canada, assignor to Research-Cottrell, Inc., Bridgewater, N.J., a corporation of New Jersey Filed Aug. 16, 1957, Ser. No. 678,714 3 Claims. (Cl. 209-219) This invention relates to a new and highly improved apparatus for separating and concentrating magnetic susceptible materials.

It is a particular object of the present invention to provide apparatus for concentrating magnetic iron bearing low grade ores to provide a premium ore containing a substantial iron content.

A further object of this invention is to provide new and improved apparatus for separating magnetic susceptible particles in a dry state which eliminates the disadvantages which are inherent in the prior art wet magnetic processing equipment.

It is a further object of the present invention to provide apparatus for the beneficiation of low grade iron ore capable of treating large tonnages of ore at low overall cost in apparatus which is relatively inexpensive to manufacture, rugged and dependable in operation, substantially automatic in operation and which substantially eliminates an injury to the health of the operators.

It is a further object of the present invention to provide apparatus for concentrating low grade iron containing minerals having relatively high magnetic attractabilities such as magnetite, franklinite, ilmenite, and the like. These minerals have appreciably higher magnetic attractabilities than, for example, the low magnetic susceptible iron minerals such as hematite.

The apparatus of the present invention is particularly adaptable to the concentration of ground ore in a size range of from about to about +200 mesh, and the apparatus finds particularly utility in ore treating processes such as disclosed and claimed in copending United States patent application entitled System and Apparatus for Separating Magnetic Susceptible Particles Serial No. 678,- 468, filed August 16, 1957, now Patent No. 2,990,124 by P. E. Cavanagh and Carl W. Hedberg.

These and other objects and advantages of the present invention are provided by a magnetic separator which generally comprises a first cylindrical drum, means axially mounting said drum for rotation in a generally horizontal plane, means for rotating said drum about the axial mounting, a second cylindrical drum having a diameter less than the diameter of the said first drum, a plurality of permanent magnets secured in an axial array to the outer surface of said second cylindrical drum, means for eccentrically mounting said second cylindrical drum for rotation within said first cylindrical drum, means for rotating said second cylindrical drum in a direction opposite to the direction of rotation of said first cylindrical drum, means for directing particulate material including magnetic susceptible particles, to the outer surface of said first cylindrical drum at a point adjacent the zone of strongest magnetic field created by said permanent magnets, means for collecting magnetic susceptible particles adjacent the zone where the attractive force of the magnetic field is overcome by centrifugal force and means adjacent said collection zone for directing a stream of pressure fluid such as air in a direction opposite to the direction of rotation of said first cylindrical drum.

The principles of the present invention and other objects and advantages thereof will become more apparent to those skilled in the art from the following detailed description of the apparatus which will be more completely described with reference to the accompanying drawings wherein:

FIG. 1 is a vertical sectional view through an apparatus for concentrating magnetic susceptible particles constructed in accordance with the teachings of the present invention;

FIG. 2 is an elevational view of the magnetic separating apparatus shown in FIG. 1;

FIG. 3 is an elevational end view of the structure shown in FIGS. 1 and 2;

FIG. 4 is an enlarged fragmentary sectional view through one of the separator units comprising the present invention substantially on line 4-4 of FIG. 1.

FIG. 5 is an elevational view of the portion of the apparatus shown in FIG. 4;

FIG. 6 is an enlarged sectional view substantially on line 6-6 of FIG. 5;

FIG. 7 is an enlarged sectional fragmentary view of the feed mechanism comprising a portion of the present invention; and

FIG. 8 is a perspective view of a preferred form of permanent magnets employed on the magnetic separators shown in FIGS. 1 through 7.

In general, in the concentration of from about -10 to about +200 mesh ground material in the apparatus of the present invention, the ground material is fed into a hopper generally designated 14 where the particulate material is permitted to settle. The outer surface of a cylindrical, preferably stainless steel of the nonmagnetic types such as stainless steel numbers 304 or 316, drum generally designated 16forms the bottom of the feed hopper 14. The self-feeding hopper 14 does not depend upon the friction of the stainless steel cylindrical drum 16 to drag the material to be separated, generally designated 18, from the hopper 14. The magnetic field created by the inner magnetic drum generally designated 20 agitates the magnetite or other magnetic susceptible particles in the lower portion of the hopper 14 and brings the magnetic susceptible particles onto the surface of the outer cylindrical drum 16, thus transporting it automatically out of the hopper. Where the inner drum is rotated in the direction of the directional arrow in FIG. 7, the resulting magnetic field on the surface of the stainless steel outer drum will cause magnetic particles to travel individually over its surface in the opposite direction to the rotation of the inner drum. Gangue particles pass out of the hopper 14 on top of the layer of magnetic susceptible particles where the rotational speed of the outer drum 16 accelerate the gangue particles to a high velocity and they are thrown free of the outer drum 16 by centrifugal force. The magnetic susceptible particles tumbling over and over and travelling mainly as individual particles proceed around the surface of the drum 16 as it is rotated. The outer drum 16, as will be more fully described hereinafter, is mounted eccentrically to the inner magnetic drum 20 and the space between the drums is greatest at the zone designated B where the magnetic susceptible particles are discharged.

In this zone the speed of rotation of the drum and theelfective magnetic field on the outer drum surface are so selected that the attractive force of the magnetic field is overcome by centrifugal force and the magnetic susceptible particles are thrown free of the drum into the concentrate hopper 22 therebelow. In leaving the surface f the outer cylindrical drum 16, the magnetic susceptible particles pass through an opposing air stream which aids in the differential collection of the magnetic susceptible from the non-magnetic particles. The air stream also aids in freeing the magnetic particles from the outer surface of the drum. The entire assembly is maintained within a dustproof housing or shell 12 to reduce the possibilities of contamination of the surrounding air, thus preventing silicosis and other respiratory diseases to workers maintaining and operating the device of the present invention.

The Drum Structure The eccentric counter rotating

drums

16 and 20 and their associated components and structures will be more particularly described with reference to FIGS. 4, 5 and 6 of the accompanying drawings. The outer drum 16 comprises a cylindrical shell 24 preferably constructed of stainless steel. The stainless steel shell 24 is secured about the marginal edges thereof to drum heads generally designated 26 and 28. Each of the drum heads 26 and 28 includes an annular member 30 which may be constructed of stainless steel and a second annular member 32 which is preferably constructed of aluminum. To further reduce the flow of eddy currents established by the magnets on the inner rotating drum of the separator, a thin band 29 of non-electrical conductive plastic material is interspaced between the shell 24 and its supporting side flanges 26 and 28. The annular aluminum members 32 are secured by bolts 34 to the inner marginal portions of the outer annular members 30 and they are also secured to sleeves 36 and 36' at the respective ends of the drums. The sleeves are mounted concentric to the outer cylindrical shell 24. Sleeve 36' is rotatably mounted in a bearing block 38 secured by bolts 40 to a shell or housing 42 having dimensions Slightly greater than the diameter of the outer cylindrical drum 16.

The outer sleeve 36 is rotatably mounted in bearing block 44 which is similarly secured by bolts 46 to the other side of the housing 42. The sleeve 36 extends axially outwardly of the bearing block 44 and has secured to said outer end a pulley 48 about which is trained a driving belt 50, whereby movement of the belt causes the pulley 48 to rotate bringing about the rotation of the sleeve 36 and the entire drum assembly 16.

Eccentrically mounted within the outer drum 16 is inner drum 20. The inner drum generally comprises a stainless steel cylindrical shell 52 which is secured adja;

cent the marginal edges thereof to

drum heads

54 and 56 by bolts 58. Preferably the

drum heads

54 and 56 are constructed of, for example, aluminum or other non magnetic material.

Each of the

drum heads

54 and 56 is axially bored as at 60 and 62. The bore 62 is slightly larger than bore 60 and is adapted to receive a flanged sleeve 64 which abuts in bearing relationship the sleeve 36' while the inner portion of the sleeve 64 is keyed as at 66 to a shaft 68 which extends through the sleeve 64 and the bore 60 in drum head 54 to project outwardly from each end of the barrel through sleeves 36 and 36'. The extended ends of shaft 68 are rotatably supported in bearing blocks 70 secured by bolts 72 to plates 74. Each of the plates 74 as more clearly shown in FIG. 5 of the drawings is circular in plan and is provided with a plurality or arcuate slots 76 adjacent the periphery thereof. A bolt 78 is received in each of the arcuate slots 76 and threadedly received in a further portion 80 of the frame 42. When the bolts 78 are cinched, the bearing blocks 70 for the shaft 68 are rigidly fixed relative to the frame for the drums. However, by loosening the bolts, the plates 74, there attached bearing blocks 70, and the shaft 68 carrying the inner magnetic drum 20 may be shifted eccentrically to the axis of rotation of the outer drum whereby an optimum spacing between the magnets carried on the inner drum and the inner surface of the outer drum is obtainable.

Between the outer end of sleeve 36' and the inner end of bearing block 70 is provided a pulley 84 which is keyed to the shaft 68. An endless belt 86 is trained about the pulley 84 and about a motor-driven pulley, as to be more fully described hereinafter, for rotating the inner drum 20 opposite to and at a different speed from the outer drum 16.

Secured to the outer surface 52 of the inner drum 20 are a plurality of permanent magents generally designated 88. The arrangement and attaching means for the magnets 88 are more clearly shown in FIGS. 6, 7 and 8.

In the illustrated form of the present invention, each of the permanent magnets 88 comprises an elongated generally U-shaped

magnet having legs

90 and 92 which terminate in pole faces 94 and 96. The lower face 98 of each magnet is concave in configuration and conforms to drum deck substantially equal to the height of the legs,

90 and 92, is substantially equal to a spacing D between said legs whereby a substantially uniform sinusoidal mag-- netic field is established adjacent the surface of the outerdrum deck 24. Further, the magnets are so arranged that the polarity of adjacent pole faces are opposite.

For convenience in manufacture of the magnets 88, each axial strip comprises a pair of identical magnets joined at the center of the drum by a single attaching bolt 98 shown in dotted lines in FIG. 6. The attaching bolt 98 passes through arcuate openings 100 in each of the opposed ends of the magnets and similar arcuate bores 102 are provided in the other ends of the magnets and adapted to receive attaching bolts 104.

As more clearly shown in FIG. 6 of the drawings, the outer surface of the inner drum 52 is provided with three bands of non-magnetic material designated 106, 108 and 110, upon which the ends of magnets 88 lie and through which the

bolts

98 and 104 pass. The

bolts

98 and 104, however, only threadedly engage the drum deck 52.

In the illustrated form of the invention the magnets 88 are shown and described as being of the generally horseshoe type, however, other forms and shapes of permanent magnets may be employed on the outer surface of the inner drum without materially affecting the efficient operation of the separator. For example, the magnets may comprise bars, strips, rods or a plurality of small horseshoe-shaped magnets.

The Housing Structure The housing structure for the magnetic separators will be more particularly described with reference to FIGS. 1, 2 and 3. The housing or shell 12 is adapted to receiveeach of the eccentric drum mechanisms hereinbefore described and the housing may enclose one or more of such eccentric drum units and in the drawings the specific embodiment of the present invention discloses an arrangement wherein a single housing encloses two separator units generally designated 114 and 116. The housing 12 generally comprises a top portion 118 to which is secured the feed hopper 14,

sides

120 and 122 and ends 124 and 124. The lower end of the housing 12 is shaped to provide paired

hoppers

22 and 126. The

side walls

120 and 122 are provided with cutout portions so that each

magnetic drum unit

114 and 116 may he slipped into place as a fully assembled unit, which units are then provided with a cover plate 128, FIG. 3 of the drawings, to provide a substantially gas-tight envelope about the units but for openings for the passage of

belts

50 and 86.

The

side walls

120 and 122 and end walls 124 may be conveniently provided with access and inspection windows generally designated and 132, respectively, so that an operator may, without exposing himself to the suspended dust particles, view the operation of the machine.

Within the housing and carried by the

sidewalls

120 and 122 are a plurality of baflies arranged to maintain separate flow paths for tails and magnetic susceptible concentrates and to provide an internal hopper for the lowermost magnetic drum assembly where plural magnetic drum units are installed in a single housing.

Referring particularly to FIG. 1 of the drawings, the internal baflles are designated 134, 136, 138, and 142.

Bafiles

134 and 136, in cooperation with the inner surfaces of

side walls

120 and 122 of the housing, form a generally V-shaped trough having a restricted elongated open end 144 substantially identical in construction to the open end of hopper 14 which will be described in detail with reference to FIG. 7 of the drawings. Baffle 13-8, which slopes downwardly and outwardly within the housing, provides, with the side walls of the

housing

120 and 122, a passage 146 for middlings comprising relatively large particles and particles having relatively high magnetic attractabilities. The outer limits of the passage 146 is defined by bafiie 140 pivotally mounted within the housing by adjustable screw and nut assembly generally designated 148.

The upper portion of baffle 140 designated 150 is curved to provide a sloping restricted passage between the outer surface of the outer drum deck 24 of separator unit 114 through which fine and highly magnetic attractable particles pass. The particles passing between the upper end 150 of baflle 140 and the rotating deck 24 of the outer magnetic drum are conveyed by the battle 140 into a passage 152 defined by the baffle 140, the inner surface 124' of the end of the housing and the inner surfaces of the

side plates

120 and 122. In the form of the apparatus shown in the drawings, both the middlings passing through the passage 146 and the concentrates flowing in passage 152 are collected in the common hopper 22. However, where desired, the middlings and the concentrates from

passages

146 and 152 may be separately collected by extending the baffle 140 and providing an additional outlet from the hopper.

To complete the baffle arrangement for the upper magnetic separator 1'14, bafile 142, which forms an extension of baffle '136, is pivotally mounted adjacent the outer surface of the outer magnetic drum 16 by a screw and slotted opening arrangement generally designated 154. The bafile 142 with its adjustable mounting permits a relatively clean cut between the non-magnetic and the magnetic susceptible materials which pass between the uppermost edge of the baffle 142 and the outer surface 24 of the drum 16.

The lower magnetic separator unit 116 is provided with similar baffles, which are designated 156 and 158. Baffle 156 corresponds to adjustable baffie 154 in the upper unit and its lower end forms an extension of the sloping wall 160 of hopper 22. Bafile 158 corresponds to bafile 140 in the upper unit and is adjustably mounted as at 162. In the lowermost unit the non-magnetic particles pass into the hopper 126, the middlings pass into the hopper 22 through passage 164 formed by the sloping wall 160 and the baffle 158 while the highly magnetic susceptible particles pass into passage 166 formed between the baffle 158 and the other sloping wall 168 of the hopper 22.

In order to provide an airsweep system for the separator units, hereinbefore described, a conduit 170 is connected to a suitable blower not shown in the drawings. Plural oiftake conduits designated 172 and 174 for the upper and

lower separators

114 and 116, respectively, are provided in the other end wall 124. Air entering the inner chamber through conduit 170' is divided into two main branches as indicated by the directional arrows. One of the streams passes down into the lower portion of the housing '12, past the upper end of the bafiie 158 and between the upper surface thereof and the outer surface of the outer drum of unit 116, thence between the baffie 156 and the outer surface of the outer drum, thence to the outlet duct 174. The other portion of the airstream entering through the conduit 170 pass-es between the upper surface of the curved portion 150 of bafile 148 and the outer surface of drum unit 116, between the inner surface of baflle 142 and the outer surface of drum unit 116, and then out through duct 172. The airstreams sweeping through the housing between the baffles and drums purge the housing of fine particles of suspended particles and aid in insuring a relatively clean separation between the magnetic susceptible and nonmagnetic particles and further aid in the removal of the magneto susceptible particles from the outer surfaces of the outermost drums adjacent the discharge zone where the eccentric drums are spaced the greatest distance apart and the magnetic fields created by permanent magnets 38 are at a minimum. The fine dust suspended in the

airstream leaving conduits

172 and 174 may then be directed to conventional gas cleaning apparatus of the mechanical, electrostatic or combined mechanical and electrostatic types such as shown in copending applicatlon hereinabove referred to.

The Drive Mechanism The drive mechanism for the

plural drum units

114 and 116 will be more particularly described with reference to FIGS. 2 and 3 of the drawings. The uppermost magnetic drum unit 114 is driven from an electric motor generally designated 176 which is mounted on the top 118 of the housing 12. The motor 176 is of the type wherein the motor shaft 178 extends from both ends of the motor housing. One end of the shaft 178 is provided with a pulley 180 which pulley has trained thereover a V-belt 182 which is also trained over pulley 184 connected to shaft 186, the ends of which are journalled in bearings carried by bracket 188 secured to the outer wall of the casing 12. The shaft 188 also has secured thereto a pulley 190 which has trained thereover V-belt 50 which drives the inner drum 20 through pulley 48 and sleeve 36 as more clearly shown in FIG. 6 of the drawings.

The other portion of the shaft 178 extending from the other end of the motor 176 passes through a support bearing 194 and receives on the end of the shaft a pulley 196 differing in diameter from the pulley 180 mounted on the other end of said shaft. Pulley 196 is connected to pulley 193 by V-belt 200. Pulley 198 is secured to shaft 202 carried in bracket 188', secured to the opposite side of the housing 12. A further pulley 204 is secured to shaft 202 and is, in turn, connected to pulley 84 of the outermost magnetic drum 16 by V-belt 86 again as more clearly shown in FIG. 6 of the drawings. Through the use of the foregoing motor, pulley, and belt assemblies, it will be seen that a single motor is utilized to rotate the inner and outer drums in opposite directions at different speeds in a very economical and efficient manner.

The lower unit 116, of eccentric counter-rotating magnetic drums, is driven by a substantially identical system which generally comprises motor 204 having shaft 206 extending from either end thereof. This shaft 206 carries

pulleys

208 and 210 which, in turn, are connected to

pulleys

212 and 214, respectively, with the latter pulleys being connected to the corresponding inner and outer drum pulleys through conventional V-belts as hereinabove described with reference to counter-rotating drum unit 114.

Material Feed Hopper As hereinbefore described, the material to be separated in the magnetic separating units is conveyed from a suit able grinding apparatus and/or classifiers, as more fully described in said copending application filed on even date herewith, passes through conduit 215 into the V-shaped hopper 14. The V-shaped hopper 14 as more clearly shown in FIG. 7 of the drawings comprises a pair of sloping walls 216 and 213 which are connected by vertically extending V-shaped side plates 220. The lower end of the hopper 14 is provided with an elongated rectangular opening generally designated 222. The elongated rectangular opening 222 communicates with a smaller V-shaped outlet 224 having walls which slopes similar to sloping

walls

216 and 218 and a vertically extending lower portion 226. The lowermost end of the vertical extension 226 terminates adjacent the outer surface 24 of the outer magnetic drum 16. The clearance between the extended end of the outlet 226 and the surface of the drum 16 is maintained sufiicient to permit the passage of the largest particles fed to the hopper 14. Very satisfactory results have been obtained when the clearance is about three times the largest particle size.

The material to be separated is fed into the hopper 14 at a rate approximating the rate of discharge of material 7 from the hopper in order to constantly maintain a head of material in the hopper 14. As the outer drum 16 rotates in the direction of the directional arrow 228 and the inner drum 52 rotates in the direction of the directional arrow 230 carrying therewith the magnets 88, the magnetic fields created by the magnets cause the particles of material in the hopper 14 and the discharge outlet 226 to be in a constant state of flux whereby the magnetic particles are drawn from the outlet 226 onto the surface of the rotating drum 16 along with the nonmagnetic materials.

Operation It has been found that very satisfactory separation is brought about in

separators

114 and 116 when the diameter of the outermost drums is from about 6 to about 18 inches in diameter. It will be appreciated that the speed of the drums and their diameter and the strength of the magnetic fields created by the magnets 88 secured to the outer surface of the inner drum are all interdependent so that if the speed of rotation of the drums is increased or the diameter of the drums is decreased the field strength of the magnets must be increased. With separator units wherein the outer drum is about 12 inches in diameter very satisfactory results are obtained when the outer drum is rotated at about 200 r.p.m.; the inner drum is rotated at about 400 r.p.m. and the magnetic field created by the magnets is approximately 700 or more gaus at the surface of the outer drum 16 when the counter-rotating drums are at their point of closest spaced relationship with the magnetic field decreasing to a minimum of about 200 gauss at the surface of the outer drum. With this arrangement of field strength, separation of particles in the size range of from about to +150 mesh or from about 20 to about +200 mesh has been found to be very satisfactory with drum diameters of from about 6 to about 18 inches, outer drum speed of from about to about 200 r.p.m. and inner drum speeds of from about 100 to about 600 r.p.m.

Particularly good results are obtained from ground Oceanic iron ore having the following screen analysis:

Percent -20 to mesh 27 to 30 -65 to mesh 33 to 36 --l00 to mesh 30 to 33 From the foregoing description, it will be seen that the present invention provides a new and useful apparatus for concentrating low-grade magnetic susceptible iron ores, and while the invention has been described with specific reference to a particular form of apparatus and magnet arrangements, it will be appreciated that variou changes may be made therein and the number of separating stages in each of the separating units may be variously modified,

depending upon the particular type of ore being concentrated.

I claim:

1. A magnetic separator comprising a first drum having a non-magnetic cylindrical deck portion, means axially mounting said drum for rotation in a generally horizontal plane, means for rotating said drum about the axial mounting, a second cylindrical drum having a diameter less than the diameter of the said first drum, a plurality of permanent magnets secured in an axial array to the outer surface of said second cylindrical drum and extending across the cylindrical surface thereof, each of said magnets having a generally U-shape in cross-section and said magnets being spaced one from the other a distance substantially equal to the spacing between the U-shaped legs, means for eccentrically mounting said second cylindrical drum for rotation within said first cylindrical drum, means for rotating said second cylindrical drum in a direction opposite to the direction of rotation of said first cylindrical drum, means for directing particulate material including magnetic susceptible particles to the outer surface of said first cylindrical drum at a point adjacent the zone of strongest magnetic field created by said permanent magnets, said particle directing means including a substantially V-shaped hopper adapted to receive particulate material, a quadrangular opening at the lower end of the hopper and mean positioning said quadrangular opening vertically above the surface of said drum at a distance about three times the diameter of the largest particle fed to said hopper, means for collecting magnetic susceptible particles adjacent the zone where the attractive force of the magnetic field is overcome by centrifugal force and means adjacent said collection zone for directing a stream of pressure fluid in a direction opposite to the direction of rotation of said first cylindrical drum.

2. The invention defined in claim 1 wherein the diameter of the said first drum is from about 6 to about 18 inches and is rotated at from about 50 to about 200 r.p.m. and said second drum is rotated at from about 100 to about 600 r.p.m.

3. The invention defined in claim 2 wherein the magnetic field created by said magnets adjacent the hopper outlet is at least about 700 gauss.

References Cited in the file of this patent UNITED STATES PATENTS 478,551 Ball et al. July 12, 1892 1,529,970 Ullrich Mar. 17, 1925 FOREIGN PATENTS 191,492 Germany Nov. 9, 1907 751,415 Great Britain June 27, 1956