US3209912A

US3209912A - Magnetic separator

Info

Publication number: US3209912A
Application number: US254223A
Authority: US
Inventors: Carroll D Sloan
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1963-01-28
Filing date: 1963-01-28
Publication date: 1965-10-05
Anticipated expiration: 1982-10-05
Also published as: GB998237A

Description

Oct. 5, 1965 c. D. SLOAN MAGNETIC SEPARATOR 2 Sheets- Sheet 1 Filed Jan. 28, 1963 INVENTOR.

CARROLL 0. SLOA/V ATTU Q/VEY.

Oct. 5, 1965 c. D. SLOAN MAGNETIC SEPARATOR 2 Sheets-Sheet 2 Filed Jan. 28, 1965 Will! INVENTOR United States Patent Office 2,209,912 MAGNETIC SEPARATOR Carroll D. Sloan, Franklin Township, Westmoreland County, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 28, 1963, Ser. No. 254,223 12 Claims. (Cl. 209-223) This invention relates to magnet assemblies particularly useful, by way of example, in magnetic separators for the resolution of magnetic ores into a desired fraction and a gangue fraction. It also concerns apparatus that includes such magnet assemblies.

There are presently available commercially a number of different type magnetic separators. In those devices, permanent magnets and pole pieces are arranged relative to a surface, designated the separation surface, to exert a magnetic influence along that surface whereby magnetic ores are attracted and held there and can thus be separated from remaining constituents, commonly designated as gangue. The magnetic conveyor or surface generally is rotated relative to the magnet arrangement and the separated materials are removed from the gangue to a place where they can be collected.

Analysis of the prior art magnet arrangements in those devices demonstates the existence of isgniflcant drawbacks by which about /2 to /3 of the magnetic flux is not utilized. Thus, in many such devices, substantial spaces are located between the permanent magnets and magnetic -flux exists in those spaces. Since the spaces are within the drum and between adjacent magnets, it is evident that this internal flux constitutes a leakage or a loss of flux compared to the total that should be available to effect magnetic separation. In still other designs, the spaces between magnets have been avoided by placing the pole pieces therein; that eliminates internal flux between the magnets. However, in that arrangement, magnetic flux passes between the pole pieces rearwardly of the magnets. Hence, while a portion of the flux extends externally of the conveyor surface and is useful, another and significant portion appears rearwardly of the magnet and pole assembly and thus is wholly removed from any area where it can exert a useful influence for the intended purposes. That arrangement, therefore, simply relocates internal flux leakage and does not eliminate it. Similar problems exist with regard to magnet chucks or assemblies used in conveyors, in sheet spreaders and other applications.

It is the primary object of the present invention to provide an arrangement of magnets and pole pieces particularly useful in magnetic apparatus and by which magnetic efiiciency, measured by the degree of application of magnetic flux to a location of useful work, approaches 100 per cent.

Another object is to provide magnetic separator, conveying and spreading apparatus including an assembly of permanent magnets and pole pieces in accordance with the foregoing object.

Other objects and advantages of the invention will be apparent from the following detailed description and drawings in which:

FIG. 1 is a diagrammatic illustration of a magnetic drum separator;

FIG. 2 is a perspective view, with parts broken away, showing a permanent magnet assembly for use in a drum separator, the assembly of magnets being arranged in accordance with this invention;

FIG. 3 is a sectional view taken along the line IIIIII of FIG. 4;

FIG. 4 is a sectional view taken along line IV1V of FIG. 3;

3,209,912 Patented Oct. 5, 1965 FIG. 5 is an end view of an assembly of permanent magnets and pole pieces showing a particular Way of joining those members to one another; and

FIG. 6 is a sectional view of an alternate embodiment of the invention.

The objects are attained in the present invention by an apparatus assembly including either a vpair or more of permanent magnets, or a pair or more of stacks of permanent magnets, that are arranged, magnetically, in parallel.

- Broadly, the magnetic assembly of this invention is a structure composed of soft magnetic pole pieces and permanent magnetic material, shaped, arranged and assembled to provide a first elongated pole face of one polarity, and a pair of elongated pole faces of the other polarity at either side of said first pole face and substantially parallel thereto. The three pole faces, which define the working face of the magnetic assembly, usually are parallel to a plane or curved surface of large radius.

More specifically, the pole pieces of the assembly of this invention take the form of a trough-shaped yoke having forwardly extending arms, in which the ends of the arms of the yoke provide a pair of pole faces, and a bar, having a pole face along one side thereof, constitutes the center pole. The yoke and bar are held in fixed spaced relation to each other and the space between them is occupied by oriented permanent magnet material; the permanent magnet material being positioned to converge one polarity upon the center pole piece. The yoke surrounds the rear surfaces of the magnets and the bar member is located within the space defined by the forward surfaces of the magnets. The magnets are magnetized in a direction that is perpendicular to the planes of the aforementioned surfaces.

In utilizing the basic assembly described, ordinarily a plurality of assemblies will be used depending upon the number of poles desired. For example, a five-pole arrangement may be obtained by employing two of the described assemblies. In this simple fashion, the efliciency achieved approaches percent, based on the location of the magnetic flux relative to the working surface, for this structural and magnetic arrangement eliminates material open spaces between permanent magnets and concentrates and conducts the magnetic flux into the area where it can be utilized to effect a desired object, for example a magnetic separation.

In a preferred embodiment, the of this invention takes a form which may be designated the V type. In this type, the yoke is V shaped in cross-section and the bar is triangular in cross-section with an apex thereof aligned with the apex of the yoke, and a pair of magnets or a pair of stacks of slab-shaped magnets are secured between the yoke and the bar, magnetically in parallel.

The invention will be further described in detail with reference to the utilization of V type magnetic assemblies in drum shape magnetic separators for resolving magnetic ores. However, it is to be understood that this is for purposes of illustration and is not to be construed as limiting the invention.

Referring now to the drawings, the magnetic separator shown comprises a supporting frame 10 on which is mounted the active separator unit indicated generally by the numeral 12. As will be apparent in the detailed description hereinafter, the separator unit structurally comprises a generally cylindrical drum 13 of non-magnetic material surrounding an assembly of magnets and pole pieces located along, but spaced from, a portion of the inside surface 14 (FIGS. 3 and 4) of the cylindrical wall 15 of the drum 13. The magnet arrangement is supported independently of the drum 13 so that the drum magnetic assembly can be rotated while the assembly of magnets remains statlonary.

Accordingly, the drum 13 is supported on the frame by a shaft 16. A motor 17 operatively connected to the drum 13 through a sprocket and chain arrangement '18 attached to the end walls 19 of the drum 13 is also supported on frame 10. Shaft 16 suitably supports the drum 13 by means of bearings 19a attached to the end walls 19.

Internally of frame 10, there is structure by which feed to be resolved is directed to the external surface of the cylindrical wall of the drum of the magnetic separator unit, and by which the fractions into which the feed is resolved can be separately removed. For this general purpose, a conduit 22 leads into an L-shaped chamber 24 at one side of the frame 10. The materials that are non-magnetic and thus not influenced by the magnet assembly pass downwardly along the L-shaped chamber 24 and are withdrawn through an outlet conduit 25 and are stored or otherwise disposed of as desired. The magnetic materials in the feed are attracted and held to the external surface of the sidewall 15 of the drum until they are carried beyond a point where they could fall into the path to the outlet conduit 25. Once they have been carried beyond this point, they are permitted to drop into a bin or outlet conduit 26 provided to receive them.

Within the drum 13 is the permanent magnet assembly, indicated by thenumeral 30 in FIG. 2. The arrangement of the permanent magnets and pole pieces in this embodiment of the invention is shown in FIGS. 2 and 3. This unit includes a pair of supporting

end plates

32 and 34 having means 35 associated with each by which the end plates can be supported in any desired fixed position on shaft 16. For example, means 35 can be collars rigidly attached to the end plates and having screw locks or other means (not shown) by which the fixed relationship to the shaft 16 is achieved. The supporting plates shown are shaped in the form of a segment of a circle and are made of non-magnetic material.

The permanent magnets and pole pieces preferably are maintained as close as possible to the inside surface 14 (FIG. 3) of the drum side wall consistent with rotation of the drum relative to those magnets. The magnet-s may be supported by a non-magnetic cylindrical segment 36 joined to and extending between

plates

32 and 34 at their peripheries. Permanent magnets, central pole pieces and field frames or yokes are then arranged on the inner surface of cylindrical segment 36. An arrangement of permanent magnets to operate, magnetically, in parallel in accordance with this invention is achieved by aligning at least two permanent magnets, or two stacks thereof, generally to an inverted V form. In the embodiment shown, each stack of magnets is com posed of four

slab magnets

41, 42, 43 and 44 of increasing width from bottom to top. The actual width of each magnet preferably is determined considering the width of the magnet with which it is associated in its companion stack so thatgaps, if any, are minimized. A pole piece 48 of a shape to be received in the V defined by the magnets is located therein. A soft steel V-shaped field frame 50 or yoke surrounds the outer surface of magnet assemblies of the companion stacks with the ends 51 and 52 of the field frame terminating adjacent the cylindrical support segment 36, and, therefore, adjacent the inside surface 14 of the drum sidewall 15.

In the embodiment shown in FIG. 2, there are two such V arrangement of magnets that, in the aggregate and in view of their arrangement, contribute five poles. Considering the pole pieces within the V to be south poles, the termini or ends of the V-shaped field frames or yokes all will be north poles. Where two Such ends come together, the two north poles operate as a single north pole. Accordingly, the double V contributes five poles though any single V would provide three poles. Additional unit V assemblies contribute twomore poles, and

accordingly, any odd number of poles desired can be easily provided. Where a 360 arc of magnet assemblies is employed an even number of poles is obtained, for the V that closes the circle contributes but one pole.

While the magnetic assembly of the invention has been described primarily in terms of a V configuration, other satisfactory arrangements may be used. One such alternate arrangement is shown in cross-section in FIG. 6 in which three contacting sets of stacked magnets are employed with their axes of magnetization (106, 107, 108) converging at the central pole. As shown, these axes are 60 apart. The yoke contacts the rear surfaces of the magnet stacks which are made up of the

individual magnets

101, 102, 103, 104. Pole piece 120 constitutes the central magnetic pole. The ends 109, 110 of the arms of the yoke provide a pair of poles of a first polarity and the central magnetic pole provides a magnetic pole of a second polarity.

It will be obvious that the basic magnetic assembly may have numerous sets of magnetic stacks, all with their axes of magnetization substantially convergent at the central pole. Further, the yoke and the magnets may also be arcuate in cross-section.

For a drum type magnetic separator of the type being discussed, a diameter of 30 to 36 inches frequently is used and the drum may be as much as 4 to 6 feet in length. The individual permanent magnets used Would be functionally of substantially similar length, though that length can be achieved by using a plurality of magnets each of a smaller length so that in the aggregate the 4 to 6 feet of magnets is attained. Of course, the pole pieces also extend the full length, and similarly, the full length required may be achieved by using a plurality of smaller pole pieces in contact with each other. In the embodiment shown where the magnet and pole assembly is along the lower surface of the separator unit, they can be held in place against the cylindrical segment 36 simply by strips of metal attached to the field frames or yokes, as by welding, and then anchored to cylindrical segment 36. Still further, it is possible to maintain the units in place by use of a conventional adhesive to join the tips, for example 51 and 52, of the field frames to the inside surface of cylindrical segment 36. Another way of accomplishing this is to provide collars 35 of a size and shape to bear directly on the field frames. Another advantageous arrangement, in which the cylindrical segment 36 can be eliminated, is evident in FIG. 5 in which a bolt 59 extends between the pole pieces through the magnets and is screwed tight, for example to collars 35 that extend to the field frame. There is some advantage in eliminating the cylindrical segment 36, for the magnet assemblies may then be located closer to the rotating drum 13, thus increasing efficiency. Other ways of securing the magnet asemblie in position will be apparent to those skilled in the art.

The materials used to produce the permanent magnets and pole pieces in accordance with this invention can vary widely, as is apparent to those skilled in the art. Preferably, they are made from ceramic permanent magnet high coercive force materials such as the barium ferrites, or strontium ferrites or other materials of like character, since the arrangement provides a large magnet area in relation'to the magnet length. However, the metallic permanent magnet materials such as Alnico, and the like also can be used. Similarly, the pole pieces can be made of any soft magnetic material desired. Particularly suitable are iron and the common soft steel, non-permanent magnetic materials.

material essentially converge one polarity thereof toward the central pole member.

As has already been indicated, the pairs of magnets, or pairs of stacks of magnets are magnetically coupled in parallel. Consequently, the south pole of each magnet (or stack) is adjacent one pole piece, while the north pole is adjacent the other pole piece. In the embodiment shown, the north pole of each magnet stack is at the face which is in contact with the field frame while the south pole is at the face in contact with the central pole piece. While magnetic parallelism is a requirement for the pair of magnets, or pair of stacks, it should be evident to the artisan that magnets in any given stack are in series arrangement; accordingly, the face of magnet 41 against central pole piece 48 is a south pole; its opposite face is a north pole; the face of magnet 42 contacting magnet 41 is a south pole; and so on.

In consequence of the just described arrangement of pole pieces and magnets, there is no substantial internal flux leakage since gaps do not exist between cooperating magnets. Moreover, there is little flux leakage rearwardly of the magnet arrangement since the flux behind the magnets is concentrated in the field frames and is conducted by those frames to their tips. Hence, the

flux is principally located in front of the magnets in an are between the pole pieces and thus substantially all of it, for practical purposes, is available to effect the desired separation.

Operation of a magnetic separator as just described follows that of prior art devices. Thus, an ore that is to be resolved is comminuted to a relatively small size, for example, from fines ranging up to particles of A1 to /2 inch or more. For wet operation, the finely divided ore, is dispersed in a carrier fluid, normally water, and fed to the separator through the inlet conduit. With the conveyor or drum surface rotating, the feed passes into contact with it and the magnetic material is attracted to and held on the drum surface. The remaining materials pass forwardly to the first outlet conduit. The drum surface and its magnet assembly are of such size that the magnetic force is maintained on the particles on the surface until they are carried beyond the area in which they would pass outwardly with the tailings. Once they are beyond that point, the end of the magnetic chuck or separator arrangement is reached and the particles fall away from the drum surface under the influence of gravity and the failure, due to remoteness, of sufiicient magnetic force to retain them. The falling magnetic particles are directed to a separate outlet conduit provided for them. The alternate polarities of the magnetic poles to which the particles are subjected as the drum surface passes relative to the magnetic drum arrangement, tends to vibrate these particles on the surface, reorientating them and permitting entrapped non-magnetic material to fall away. It is believed that the use of magnets, in the assembly, having sharp corners contributes to the walking or reorientation phenomenon.

In the resolution of materials with a separator as in this invention by what can be termed a dry process, the feed usually is arranged to pass over the rotating surface under the influence of gravity. Normally for dry separations, more magnetic assemblies are used, often covering on the order of 50 percent or more of the surface of the separating drum. In such practice, the feed may be introduced into contact with the separating drum near its .top and the feed inlet, commonly a simple funnel arrangement to which a vibrator may be attached if necessary or desirable, is supported from the frame above the drum. The tailings fall free of the drum at its side to an outlet provided for them, while the magnetic particles are held to the surface beyond that point and are separately removed.

Another arrangement, especially suited to dry separations or conveying applications, utilizes a drum having magnet assemblies along the entire inner surface of the '6 sidewall ofthe drum. In such instance, the drum sidewall, which is driven, may provide all or part of the motive power for the conveyor.

Magnet assemblies as described are useful for many other industrial applications. For example, they can be used in conjunction with conveyors by which cans may be held to a conveyor and moved as desired. Further, they are useful as spreaders of metal sheets arranged in stacks.

From the foregoing discussion and description, it is apparent that the present invention is a significant advance in magnet assemblies. The V shape of the preferred configuration contributes a mechanical advance of strength and a structural advantage of mounting simplicity in addition to the magnetic benefits already noted. These advantages can be attained even with variations of the details of the invention. Thus, it will be evident that sizes, number of elements and the like can be changed. For conveying applications, the magnet assembly can be arranged, relative to a conveyor, so that articles being moved are subjected to the same pole at all times rather than to poles of alternating polarity as is advantageous in magnetic separations. Furthermore, the face of the magnet assembly can be made flat or even concave, rather than convex as shown, for particular applications. Tips can be added at the ends of the field frame or on the central pole piece to aid in directing the flux as desired. In all events, the materials of construction for different parts of apparatus with which the magnet assembly is used should be chosen such that no materially adverse influence on the utilization of essentially all the available flux exists.

Having illustrated and described the invention in detail for purposes of exemplification, it should be understood that the invention may be practiced otherwise and is not to be limited by the details expressed.

I claim as my invention:

1. A highly eflicient magnetic assembly having three magnetic poles at a working face thereof, comprising a unitary trough-shaped yoke composed substantially entirely of permeable soft magnetic material having two ends extending to the working face of the magnetic assembly, a central pole member of permeable soft magnetic material disposed within the trough-shaped yoke in fixed spaced relation thereto to provide an end terminating at the working face at a point intermediate the two ends of the yoke, a body of permanent magnet material substantially completely filling the space within the yoke from the working face of the assembly, the arrangement and directions of strong magnetization of the permanent magnet material being such as to essentially converge one polarity thereof toward the said central pole member and the other polarity being directed to the trough-shaped yoke, whereby the end of the central pole member has the one polarity and the ends of the yoke have the other polarity.

2. The magnetic assembly of claim I wherein the body of permanent magnetic material consists of a plurality of stacks of oriented ceramic permanent magnets in slab form.

3. In a magnetic separator comprising a conveyor having a material receiving surface for conveying materials to be separated and a permanent magnet assembly having three poles at a working face thereof to attract magnetic material to be separated towards said surface, the conveyor being movable relative to the permanent magnet assembly, the improvement comprising forming said assembly of a unitary trough-shaped yoke composed substantially entirely of permeable soft magnetic material having two ends extending to the working face of the magnetic assembly in close proximity to said surface, a central pole member of permeable soft magnetic material disposed within said yoke and attached thereto to provide an end terminating at the working face in close proximity to said surface, a body of oriented permanent completely filling the space within the yoke from the working face of the magnetic assembly, the arrangement and direction of magnetization of the oriented permanent magnet material being such as to essentially converge one polarity thereof toward the central pole member and the other polarity being directed to the troughshaped yoke, whereby the end of the central pole member has the one polarity and the ends of the yoke have the other polarity.

4. In a magnetic separator in accordance with claim 3, at least two such assemblies of permanent magnets arranged with an end of the yoke of one assembly proximate one end of the yoke of the adjacent assembly, the working faces of said assemblies being closely adjacent the surface of said conveyor.

5. In a magnetic separator, a drum having a cylindrical sidewall composed ofa non-magnetic material and mounted for rotation on a shaft, a permanent magnet assembly having three poles at a working face thereof adapted to attract magnetic material to be separated towards the external surface of the side wall of the drum and mounted within the drum, said working face being positioned in close proximity to the internal surface of said sidewall, the magnetic assembly comprising a unitary trough-shaped yoke composed substantially entirely of permeable soft magnetic material having two ends eX- tending to the working face thereof, a central pole member of permeable soft magnetic material disposed within said yoke and fixed in spaced relation thereto to provide an end terminating at the working face, a body of oriented permanent magnetic material substantially completely filling the space within the yoke from the working face of the magnetic assembly, the arrangement and direction of magnetization of the oriented permanent magnet material being such as to essentially converge one polarity thereof toward the central pole member and the other vpolarity being directed to the trough-shaped yoke, whereby the end of the central pole member has the one polarity and the ends of the yoke have the other polarity.

6. In a magnetic separator in accordance with claim 5, at least two such permanent magnet assemblies adjacent one another, arranged with an end of the yoke of one assembly proximate one end of the yoke of the adjacent assembly. I

7. An assembly of magnets for use in separating procedures and conveying applications comprising'a generally V shaped yoke composed of a soft magnetic material, at least a pair of permanent magnets arranged in a V form fitted within said yoke and a magnetic pole piece within the frame of magnets within said yoke, said ,magnets being magnetized in a direction perpendicular to the portion of the yoke with which it is associated, and said magnets being arrangd therein with their polarities magnetically coupled in parallel.

8. In a magnetic separator comprising a movable conveyor for conveying material to be separated and a permanent magnet assembly located adjacent the conveyor to attract magnetic material to be separated towards its surface for movement therewith, the improvement comprising a magnet assembly comprising at least a pair of permanent magnets arranged into a V shape and magnetically coupled in parallel, a first magnetic pole piece within that V shape of magnets and extending to adjacent the surface of the conveyor, and a second magnetic pole piece comprising a field frame surrounding the outer surface of the V shaped magnet pair assembly with the ends of the field frame in close proximity to the surface of the conveyor, whereby essentially all magnetic flux is concentrated along the conveyor.

9. In a magnetic separator comprising a, conveyor having a material receiving surface OI conveying materials to be separated and a permanent magnet assembly to attract magnetic material to be separated towards said surface, the conveyor being movable relative to the permanent magnet assembly, the improvement comprising forming said assembly with at least a pair of permanent 'magnets angularly disposed with respect to each other and arranged magnetically in parallel about a soft magnetic central pole piece to confer a first polarity on said central pole piece,.a unitary trough-shaped field frame formed of soft magnetic material surrounding and in mating relationship with the pair of magnets and having its ends extending to adjacent the plane of the central pole piece, the ends of the frame providing a pair of poles of a second polarity, each of said magnets being polarized in a direction that passes through the central pole pieces and is at about a 45 angle to the surface of said conveyor.

10. In a magnetic separator in accordance with claim 9, two such assemblies of permanent magnets and pole pieces arranged with an end of the field frame of one assembly proximate one end of the field frame of the other assembly, all of said poles being adjacent the surface of the conveyor.

11. In a magnetic separator, a drum having a cylindrical sidewall composed of a non-magnetic material and mounted for rotation on a shaft, a permanent magnet assembly adapted to attract magnetic material to be separated towards the external surface of the sidewall of the drum and mounted within the drum, the magnetic assembly comprising at least a pair of permanent magnets arranged generally in a V shape and magnetically coupled in parallel and having the ends of the V adjacent the internal surface of the sidewall of the drum, a first soft rnagnetic pole piece within the V defined by the pair of permanent magnets, a second soft magnetic pole piece comprising a field frame surrounding the outer surfaces of the pair of magnets, the first-mentioned pole piece and the ends of the field frame extending to adjacent the internal surface of the sidewall of the drum.

12. A permanent magnet assembly comprising a pair of stacks of oriented ceramic permanent magnet slabs,

each slab being magnetically oriented in a direction perpendicular to its largest surface and the slabs of each stack arranged so that they are magnetically in series, the

stacks arranged generally in a V-shape in cross section with the base slab of each stack in contact with one arm of a confining integral V-shaped soft magnetic field frame vwhereby the direction of magnetic orientation of each base slab is perpendicular to the frame arm surface with which it is in contact, the ends of the field frame arms providing .pole faces of one polarity, the two magnetic stacks when thus assembled within the arms of the field frame defining a space which is V-shaped in cross-section in which there is positioned a bar of soft magnetic material constituting a center pole piece having surfaces in contact with the top slab, respectively, of each stack, the .pair of stacks being magnetically in parallel whereby the center pole piece has a second polarity, the pole faces of the field frame and the pole face of the central pole piece lying -in a plane of relatively large radius of curvature.

I References Cited by the Examiner UNITED STATES PATENTS 2,461,008 2/49 Stea-rns 2o9 223 2,992,736, 7/61 Buus 209 223 2,992,738 7/61 Maynard 209 223 FOREIGN PATENTS 433,616 8/35 Great Britain.

HARRY B. THORNTON, Primary Examiner.

HERBERT L. MARTIN, Examiner,

Claims

3. IN A MAGNETIC SEPARATOR COMPRISING A CONVEYOR HAVING A MATERIAL RECEIVING SURFACE FOR CONVEYING MATERIALS TO BE SEPARATED AND A PERMANENT MAGNET ASSEMBLY HAVING THREE POLES AT A WORKING FACE THEREOF TO ATTRACT MAGNETIC MATERIAL TO BE SEPARATED TOWARDS SAID SURFACE, THE CONVEYOR BEING MOVABLE RELATIVE TO THE PERMANENT MAGNET ASSEMBLY, THE IMPROVEMENT COMPRISING FORMING SAID ASSEMBLY OF A UNITARY TROUGH-SHAPED YOKE COMPOSED SUBSTANTIALLY ENTIRELY OF PERMEABLE SOFT MAGNETIC MATERIAL HAVING TWO ENDS EXTENDING TO THE WORKING FACE OF THE MAGNETIC ASSEMBLY IN CLOSE PROXIMITY TO SAID SURFACE, A CENTRAL POLE MEMBER OF PERMEABLE SOFT MAGNETIC MATERIAL DISPOSED WITHIN SAID YOKE AND ATTACHED THERETO TO PROVIDE AN END TERMINATING AT THE WORKING FACE IN CLOSE PROXIMITY TO SAID SURFACE, A BODY OF ORIENTED PERMANENT MAGNETIC MATERIAL CONSISTING OF AT LEAST TWO STACKS OF CERAMIC PERMANENT MAGNETS IN SLAB FORM SUBSTANTIALLY COMPLETE FILLING THE SPACE WITHIN THE YOKE FROM THE WORKING FACE OF THE MAGNETIC ASSEMBLY, THE ARRANGEMENT AND DIRECTION OF MAGNETIZATION OF THE ORIENTED PERMANENT MAGNET MATERIAL BEING SUCH AS TO ESSENTIALLY CONVERGE ONE POLARITY THEREOF TOWARD THE CENTRAL POLE MEMBER AND THE OTHER POLARITY BEING DIRECTED TO THE TROUGHSHAPED YOKE, WHEREBY THE END OF THE CENTRAL POLE MEMBER HAS THE ONE POLARITY AND THE ENDS OF THE YOKE HAVE THE OTHER POLARITY.