US2065460A - Magnetic separation - Google Patents

Description

Dec. 22, 1936. F. R. JOHNSON 2,065,460

MAGNETIC SEPARATION Filed May 20, 1933 INVENTOR ATTORN EYS Patented Dec. 22, 1936 UNITED STATES PATENT OFFICE The Exolon Company, poration of Massachusetts ell, N. Y., a cor- Application May 20, 1983, Serial No. 671,995

6 Claims. (01. 209-214) This invention relates to magnetic separation, and relates more particularly to magnetic separation of the type in which a rapidly rotating attractor, commonly called a rotor, is magnetized by induction and in which the comminuted mixture to be separated, consisting of materials of varying degrees of magnetic susceptibility and hereinafter termed the grain stream, is fed upon the top of and over the rotor so as to be carried by the rotor through a magnetic air gap between the rotor and an adjacent magnetic pole, herein called the active or working pole.

The operation of magnetic separators of this type depends upon the fact that the non-magnetic or less magnetic particles of the grain stream, which are not attracted or but very slightly attracted by the rotor, are thrown from the rotor by centrifugal force while the more magnetic particles are still held to the rotor by the magnetic attraction existing therebetween. Since the non-magnetic or less magnetic particles and the more magnetic particles of the grain stream leave the rotor at different points, they travel in different trajectories, and hence may be separated by a suitable splitter or divider, for example, a knife edge, located between the two trajectories. The non-magnetic or less magnetic particles thus first thrown from the rotor are collected in front of the divider as a separate fraction. The more magnetic particles leave the rotor at later points in its rotation, and are collected in back of the divider either as a single fraction or as two or more fractions. Unless a contrary meaning definitely appears, the expression non-magnetic fraction will be used hereinafter to designate said first fraction and the particles composing the same will be termed "non-magnetic particles, although such fraction may in fact consist largely or wholly of particles possessing some magnetic permeability. Similarly, the more magnetic particles will usually be termed simply magnetic particles.

A non-magnetic particle will be projected from the rotor at a point depending upon the physical factors affecting the particle, such as the speed of rotation of the rotor, the friction between the particle and the surface of the rotor, and the size and density of the particle. The location of this leaving point may conveniently be defined by its angular position on the surface of the rotor measured with respect to the longitudinal axis of the rotor and the horizontal plane through that axis. In practice, for a given rotor speed, this angle will not be quite the same for all of the non-magnetic particles of the grain stream because of variations in the size and mass of the non-magnetic particles. The area on the surface of the rotor embraced between the angle at which the first non-magnetic particle leaves the rotor and the angle at which the last nonmagnetic particle leaves the rotor, is termed the leaving point area for the non-magnetic particles. In this specification, this area is referred to for the sake of convenience simply as the leaving point area. This area will vary in size and position on the rotor with the rotor speed and the character of the grain stream.

The magnetic particles of the grain stream will be held on the rotor longer than the nonmagnetic particles, due to the force exerted by the lines of force from the pole converging towards the rotor. The leaving point for the magnetic particles depends upon the strength of the magnetic force so exerted, that is to say, the stronger the magnetic force, the longer the magnetic particles will be retained on the rotor.

A magnetic separator of the foregoing type is shown in Johnson United States Patent No. 1,836,252, issued December 15, 1931, upon which this invention is an improvement.

The purpose of this invention is to attain greater efficiency of separation, economy of power consumption, and economy of construction of such a magnetic separator by the correct control of the flux between the pole and the rotor.

It has heretofore been recognized that from the standpoint of economy it is impractical to maintain a magnetic field of uniformly high strength over the entire working surface of the rotor. It was therefore realized that the magnetic field should have a high concentration of lines of force at the point where the most useful work could be done by the field. There have heretofore been two schools of thought with regard to the proper location of the active pole nose with respect to the rotor.

One school of thought believed that the highest concentration of the magnetic field should be directed on the rotor just below the leaving point area for the non-magnetic particles. This conclusion was based on the opinion that it was there that it was most necessary that the magnetic material should be strongly attracted and held to the rotor. The other school of thought believed that the highest concentration of the magnetic field should be at the leaving point area. This idea was based on the hypothesis that were it not for a strong magnetic field in that area, the magnetic particles would tend to leave the rotor at the same time as the non-magnetic particles.

The present invention contemplates a location of the active pole nose radically different from those Just described. According-to the present invention, the active pole nose is so shaped and positioned that a strong magnetic flux, and preferably the maximum density of magnetic fiux, is obtained near the top of the rotor in the area above the leaving point area for the non-magnetic particles. This area above the leaving point area is termed herein the area of pre-alignment of the magnetic material, or more briefly, the pre-alignment area. It has been discovered that by directing a strong magnetic flux on that pre-alignment area, the magnetic particles are drawn into immediate proximity to the magnetic portions of the rotor and become attached thereto during their passage through the pre-alignment area. By virtue of this immediacy of contact between the magnetic portions of the rotor and the magnetic particles, the magnetic force tending to hold the particles to the rotor is at a maximum for the particular flux density employed, since the magnitude of that force decreases very rapidly as the distance between the magnetic portions of the rotor and the particles is increased. As a result of this invention, not only do the magnetic particles which have alighted on the magnetic portions of the rotor tend to be attached more firmly to the magnetic portions, but also the magnetic particles which have alighted on the non-magnetic portions of the rotor have a greater tendency to be drawn to the edges of the magnetic portions, due to the greater strength of the converging magnetic lines of force which are available over that portion of the rotor surface wherein the particles to be separated are adjusting themselves to the moving rotor surface. The maximum number of magnetic particles thereby become attached to the magnetic portions of the rotor, giving a high degree of efficiency of separation. Even very feebly magnetic particles are more greatly influenced and are attracted to the rotor to such an extent as to permit their separation from still less magnetic or non-magnetic particles.

While this concept finds its greatest usefulness in the treatment of very weakly magnetic materials, the same principles apply in the treatment of somewhat more magnetic substances. However, the higher the magnetic permeability of the magnetic fraction of the grain stream, the less importance need be attached to the position of the active pole nose. The reason for this result is that with highly magnetic particles, an excess of magnetic fiux over-that necessary with a properly designed active pole may readily be supplied, whereas with very feebly magnetic particles the limit of available magnetic flux is reached without separation taking place unless the principles of this invention are followed.

This invention wil be more clearly understood by reference to the following description and to the accompanying drawing, which forms a part of this specification, and in which:

Figure 1 is a diagram illustrating schematically the construction of a closed circuit magnetic separator in accordance with this invention;

Figure 2 is a diagram illustrating schematically the magnetic field existing in the separator of Figure 1;

Figure 3 is a view similar to Figure 1 showing a modification; and

Figure 4 is a diagram illustrating schematically an open circuit magnetic separator in accordance with this invention and the magnetic field existing therein.

InFigureLarotor llisshownlocatedina magnetic circuit between a front or active pole nose II and a rear pole nose II. A chute I8 is arranged to deliver the grain stream upon the top of and over the rotor ll. As indicated by the arrow I, the rotor I. is arranged to revolve at the desired speed in a clockwise direction. The rotor is intended to be revolved at a speed sufficient to project the grain stream through the magnetic air gap ll formed between the active pole nose II and the rotor II, as distinguished from a mere revolving conveyor that rotates so slowly as simply to allow the material to slide off.

For the purpose of defining the angular positions of points on the surface of the rotor II, the rotor is assumed to be calibrated in degrees according to the usual algebraic scheme, as shown by the symbols 0, and 270.

The magnetic field existing in the separator of Figure 1 is illustrated in Figure 2. The broken line NN' represents the neutral line of the magnetic field, the flux entering the rotor on one side of this neutral line and leaving on the other side.

The known laws of mechanics tell us that a non-magnetic particle deposited on the rotor I. from the chute It will remain on the rotor until it reaches a certain point, indicated at a, at which point it leaves the rotor and becomes a freely falling body describing a substantially parabolic path A through the air. The angle alpha gives the angular position of the leaving point a. The particles deposited on the rotor attain a speed equal to or nearly equal to the surface speed of the rotor. Due to such variables as particle size and slippage in adjusting to the speed of the rotor, different non-magnetic particles will leave at slightly different leaving points. For the purpose of illustration, it is assumed that the point it represents the leaving point of the first particle of the non-magnetic fraction to leave the rotor, the point D the leaving point for the last particle of that fraction, and the parabolic trajectories A and B their respective paths. The angle beta indicates the angular position of point b, and the arc ab, which is measured by alpha minus beta, defines the leaving point area.

The length and position of the arc ab defining the leaving point area are dependent primarily on the size and character of the non-magnetic particles and on the rotor speed. For a given grain stream, the leaving point are tends to shorten and to assume a higher position on the rotor as the rotor speed increases, the change in position being more marked than the change in length. The length of the arc ab is largely determined by the uniformity of size and mass of the non-magnetic particles, and, with the grain streams usually employed, will ordinarily not exceed 10 or 12 degrees in length. Considering the variations of rotor speed and of grain size and character usually encountered in practice, it is approximately correct to say that the leaving point area will be found somewhere between 45 degrees and 65 degrees. That is to say, non-magnetic particles will not commence to leave the rotor until they reach the 65 degree position, and the last of such particles will be projected from the rotor by the time the 45 degree position is reached. It will be understood that one or the other of these limits may be exceeded in cases of very rapid r very slow rotor speeds.

Likewise for the sake of illustration, it is assumed that the point dis the leaving p i of the particle of the magnetic fraction to leave the rotor and the point e that of the last partlcle of that fraction. The parabolic trajectories of these particles are illustrated at D and E, respectively.

A divider or knife edge I6 is arranged between the paths described by the non-magnetic and the magnetic fractions, and serves to direct these two fractions into separate collecting compartments, not shown. In some cases, the fractions are not as clearly defined as illustrated, and it is advisable to use two dividers instead of one, whereby the falling stream may be separated into a non-magnetic fraction, an intermediate fraction consisting of both magnetic and non-magnetic particles, and a magnetic fraction. The use of two or more dividers is also advisable where the magnetic particles are of different kinds and assume trajectories such that they may be separated from one another as well as from the nonmagnetic particles.

The pre-alignment area to which reference has already been made and which is of vital importance to the present invention, extends from the upper terminus of the leaving point area to the point of delivery of the grain stream on the rotor, and may even include the lower end of the feed chute.

It will be noted that the face ll of the active pole nose ll instead of being circular in contour and concentric with the rotor I0, is shaped parabolically so as to give an increasing width of the air gap l5 from the leading edge l8 of the pole nose I l to the rear of the pole nose. This has the advantage of spacing the face ll sufficiently far from the rotor l0 so that the particles projected from the rotor will clear the pole nose even with high trajectories produced by high rotor speeds. It will be observed that the rounded tip IQ of the pole nose is located well up toward the top of the rotor. This design gives the maximum density of the magnetic field at the top of the rotor well above the leaving point area defined by the are ab, that is to say, the maximum density is directed upon the pre-alignment area, as shown in Figure 2.

In the practice of this invention, the active pole should preferably be so shaped and located that the upper boundary of the concentrated band of flux from the pole is at least ten degrees above the leaving point area. By so doing, the entire grain stream passes through an effective pre-alignment area of at least ten degrees in length, during which passage it is supported on the rotor and is subjected to a magnetic field of high density. Preferably also, the concentration of the magnetic flux on the effective prealignment area should be at least as high as that on any other portion of the rotor.

The general contour of the working pole nose I l shown in Figure l is preferred because it gives the maximum concentration of the magnetic flux in the pre-alignment area and because it is well adapted for use with a wide variety of rotor speeds and of grain size. However, it will be understood that the face I! may if desired be circular in contour and concentric with the rotor III. In such case, the width of the air gap should preferably be variable to accommodate the magnetic separator to differing conditions of rotor speed and grain size, unless the machine is intended only for a particular class of work. Furthermore, with this circular contour, the ma netic fiux will be substantially constant in intensity over the entire air gap. This will give the desired high concentration of magnetic flux in the pre-alignment area, but is somewhat wasteful of flux, since more fiuxis provided than is actually necessary at and below the leaving point area.

Figure 3 shows a' magnetic separator very similar to that of Figure 1, except that the tip IQ of the working pole nose II is located even higher up on the rotor and nearer to the base of the chute than is the case in Figure 1. The

'arrangement of Figure 3 is particularly useful with separating operations requiring high rotor speeds, as is indicated in this figure by the fact that the arc ab defining the leaving point area is located higher on the rotor than is the case in Figure 1. Also the pole nose of Figure 3 is particularly useful with very fcebly magnetic materials where extremely high flux densities are required. The pole nose of Figure 3 will not be useful with more strongly magnetic materials unless the flux density is sufficiently low to ob viate their tendency to jump to the tip IQ of the pole nose and clog the air gap.

The magnetic separator diagrammatically illustrated in Figure 4 is an open circuit separator embodying the principles of this invention. The single pole 40 is arranged sufficiently high on the rotor 4| so that a strong magnetic field is obtained near the top of the rotor in the prealignment area. The face 42 of the pole nose 40 is preferably parabolic as shown so as to provide an air gap of increasing width. In general, closed circuit magnetic separators are to be preferred to open circuit separators from the standpoint of economy of power consumption, but the underlying principles of this invention apply equally to both types of separators.

It will be understood by those skilled in this art that the rotors hereinabove referred to must be so constructed that the magnetic field will in general converge from the active or working pole nose toward the rotor. As is well known, this convergence of the magnetic field toward the rotor may be produced in a number of ways. For example, the rotor may be made of alternate discs of magnetic and non-magnetic materials. The magnetic fields illustrated in Figures 2 and 4 are of the character that would be obtained with rotors of the alternate disc type, since there the convergence of the magnetic field would appear in a longitudinal section of the magnetic field, and not in a cross section as illustrated. This invention is independent of the particular form of rotor employed, and the advantages thereof may be secured with any of the known types of rotors. The magnetic fields illustrated are therefore to be taken as illustrative and not as limitative to the invention.

The essential requirement is that the magnetic material of the rotor shall at all times present an irregular contour to the active pole nose. The valleys in this irregular contour may be and preferably are filled with non-magnetic material, so that the physical surface of the rotor is smooth. These valleys, whether or not filled with nonmagnetic material, will hereinafter be referred to as the non-magnetic portions of the rotor.

The surfaces of the rotors employed in magnetic separators of the type with which this invention is concerned therefore consist of alternate magnetic and non-magnetic portions. The magnetic lines of force from the active pole will converge toward the magnetic portions of the rotor, thus dividing the magnetic field in the air gap into a plurality of small fields each converg- CPI ing toward a magnetic portion of the rotor. This creates desirable strong attractive forces in the air gap toward the magnetic portions of the rotor. It will be evident, however, that the lines of force in converging toward the magnetic portions also form a like number of small magnetic fields over the non-magnetic portions of the rotor converging in the opposite direction toward the active pole. That is to say, the magnetic field in the air gap between the active pole and the rotor consists of many alternately arranged small fields of opposing directions, one over a magnetic portion converging toward the rotor, the next over a nonmagnetic portion converging toward the active pole, and so forth. This phenomenon may be referred to for convenience as double convergence, and is inherent in all separators of the type mentioned.

It will be understood that the small fields converging toward the rotor are very much stronger than the small fields converging toward the active pole. The general convergence of the field is therefore properly described as being toward the rotor. Nevertheless, the phenomenonof double convergence introduces complications in magnetic separation, and renders inaccurate the common conception of the rotor as an attractor exerting a uniform pull on the magnetic particles deposited on it.

The attractive force toward the active pole resulting from the convergence of the small field over a non-magnetic rotor portion will be termed herein a reverse force, since it is not in the desired direction. Because of these reverse forces, magnetic particles located in the small fields converging toward the active pole, that is to say, located on or above the non-magnetic portions of the rotor, are urged away from the rotor. It will be understood that these reverse forces are only effective on magnetic particles that are in the small fields converging toward the active pole, and do not influence those magnetic particles that are attached to the magnetic portions of the rotor or otherwise located in the small fields converging toward the rotor. In prior magnetic separators in which the magnetic field is concentrated on the leaving point area or below, it frequently happens that a considerable number of magnetic particles are found in the stream of nonmagnetic particles leaving the rotor, and are collected therewith. Such a condition is of course inimical to efllcient separation. This condition usually occurs because the magnetic particles originally deposited on or above the non-magnetic portions of the rotor are largely still in such position at the time of entering the strong magnetic field in the air gap. At such time, the magnetic particles so located are subjected to the reverse forces mentioned while these forces are assisted by centrifugal force and are only slightly, if at all, opposed by the force of gravity, since in the leaving point area and below the reverse forces are nearly horizontal. The reverse forces are of course exerted more strongly on magnetic particles in their fields that have left the surface of the rotor, since such particles are in a stronger part of the reverse field, which field increases in concentration as the active pole is approached.

The present invention overcomes this inherent difilculty of prior magnetic separators due to the unavoidable phenomenon of double convergence. This is done by directing a magnetic fiux of high density upon what is herein termed the prealignment area, that is, the area on the rotor above the leaving point area for the non-magnetic particles. In the preferred form of this invention, not only is a magnetic fiux of high density directed on the pre-alignment area, but the pole nose is so designed that the magnetic flux is greater on that area than on any other area of the rotor. By following these teachings, there is obtained not only a greater efiiciency of separation in treating mineral mixtures that have heretofore been subjected to magnetic separation, but magnetic separation has been rendered feasible for weakly magnetic materials of a character such that magnetic separation has heretofore not been considered possible, or if attempted, has failed to achieve practical results.

The unusual results achieved by the present invention may be explained in the following manner. The strong magnetic flux directed on the pre-alignment area, as above defined, alters the position of the magnetic particles that are likely to be afiected by the undesirable reverse forces and results in attaching these magnetic particles to the magnetic portions of the rotor before these magnetic particles enter the leaving point area, in which area the weak reverse forces mentioned would be assisted by the centrifugal force exerted upon the particles. In other words, the strong magnetic flux directed on the pre-alignment area not only tends to retain the magnetic particles that are in contact with the magnetic portions of the rotor, but alsb tends to pull the magnetic particles that are not in contact with the magnetic portions of the rotor into contact therewith while the magnetic particles are passing through that area. The natural shifting of the various particles of the grain stream at the top of the rotor in assuming the rotor speed assists in the prearrangement of the magnetic particles. This prearrangement requires considerable energy for the displacement of some of the non-magnetic particles, and can be accomplished near the top of the rotor because there the magnetic attraction is not opposed by the force of gravity on the magnetic particles, but is aided by the force of gravity. Furthermore, in the pre-alignment area at the top of the rotor, the force of gravity opposes the undesirable reverse forces.

As a result of the prearrangement of the magnetic particles in the pre-alignment area, the majority of the non-magnetic particles will be left either overlying the magnetic particles or located on the non-magnetic portions of the rotor. Consequently, the non-magnetic particles may be discharged from the rotor when passing through the leaving point area with little tendency to carry with them the magnetic particles. Furthermore, the magnetic particles being in direct contact with the magnetic portions of the rotor, a relatively small magnetic attraction is sufilcient to hold them on the rotor for a considerable time longer than the non-magnetic particles. A higher efilciency of separation and an increased effectiveness in treating very feebly magnetic materials are therefore obtained. Indeed, the advantages of prearrangement of the magnetic particles before entering the leaving point area are so marked that it is preferable to have the highest concentration of the magnetic fiux directed on the pre-alignment area. By so doing, adequate power is available in the pre-alignment area to displace the non-magnetic particles and arrange the magnetic particles in direct contact with the magnetic portions of the rotor, while at the same time the lesser flux densities obtained in the leaving point area and below are ample to retain even very feebly magnetic particles in contact with the rotor for a sufficient time, provided they have been properly prepared as described in the pre-alignment area.

It will be understood that the foregoing explanation oi the effectiveness of this invention is somewhat a matter of theory at the present time, precise observation of the operations taking place being exceedingly difiicult. This explanation is therefore not to be taken as limiting, although it is believed to be correct, and is substantiated by tests already made.

As examples of the separations of feebly and very feebly magnetic particles for which this invention is peculiarly useful, there may be mentioned the treatment of nelsonite ore and borate ore.

Nelsonite ore is an aggregate consisting of particles of ilmenite (an iron-bearing titanium oxide mineral containing about 46 per cent. TiOz), micas of various kinds, and apatite (a calcium phosphate mineral containing 41.53 per cent. P205), held together as an aggregate by a clay-like bond. This ore is found in Nelson County, Virginia, and is mined in an open pit. After crushing and processing to remove the clay-like bond, a mixture of ilmenite; micas and apatite is obtained in the form of sand-like grains. This mixture, called sands, is then ready for separation according to this invention. The ilmenite may be described as feebly magnetic, and the micas as very feebly magnetic, and they are separated from the sands in that order in successive stages, the non-removed material being the apatite, which is substantially non-magnetic.

In actual present-day commercial operation, eight stages of magnetic separation are employed in succession, the non-removed material passing over the rotor of each stage being fed to the succeeding stage. The first rotor is subjected to a low intensity field and consequently removes only tramp iron and highly magnetic iron-contaminated particles, together with a small amount of ilmenite. Higher intensity fields are used on the second and third rotors, and high quality ilmenite is removed in these stages. Still higher intensity fields are employed on the last five rotors, and the very feebly magnetic micas are removed in these stages. The efiiciency of separation is such that the commercial separation yields a recovery of about 85 per cent. of the ilmenite with a purity of 94.5 per cent., and a recovery of more than 80 per cent. of the apatite with a purity of nearly 99 per cent.

In the separation of the ilmenite from the sands, the very feebly magnetic micas are relatively non-magnetic as compared to the feebly magnetic ilmenite, and in the later removal of the micas, the difference in magnetic susceptibility between the very feebly magnetic micas and the substantially non-magnetic apatite is sufficient to permit a further separation by the practice of my invention.

Borate ore from Kern County, California, has been successfully separated by this invention, using raw borate ore just as it comes from the mine, except for preliminary crushing. The ore consists principally of shale and borate, the borate being present in two crystalline forms, known respectively as tinkal and razorite. The shale is very feebly magnetic, and the borate is substantially non-magnetic. The magnetic sepathe spirit of this invention.

ration according to this invention, conducted usually in four stages, the first of which is merely for the removal of tramp iron and other highly magnetic materials, results on a commercial scale in recovering a borate concentrate containing over 80 per cent. of the total borate originally present in the ore and sufficiently pure to be ready for marketing without further treatment other than a partial dehydration to remove some of the water of crystallization.

The position of the active pole in accordance with this invention that will give the best results in separating a given grain stream cannot be precisely defined, but must be determined in each case by experimentation. As pointed out above, the position of the leaving point area is determined by the ohysical characteristics of the grain stream and the surface velocity of the ro tor, and this invention requires that a strong magnetic flux be directed upon the rotor above the leaving point area in the area herein termed the pre-alignment area. The eflective prealignment area may be considered to be that portion of the rotor surface above the leaving point area on which a concentrated band of flux is directed. The precise position and extent of the effective pre-alignment area, the flux density thereon and the rotor speed for the most eificient separation of a given grain stream must be experimentally determined.

Many changes and modifications may be made by those skilled in the art without departing from This invention is intended to be limited, therefore, only by the prior art and the scope of the appended claims.

I claim:

1. The method of magnetically separating a grain stream composed of a mixture of feebly magnetic particles and relatively non-magnetic particles, which comprises rapidly moving the grain stream about a horizontal axis in a circular path commencing near the 90 positionthrough an air gap of .a magnetic field having local convergences directed toward said circular path, mechanically supporting the grain stream and progressing the same successively through a pre-alignment area in which it is adjusting its speed, and a leaving point area in which the non-magnetic particles are projected outwardly from said circular path in a falling trajectory by centrifugal force suificient to overcome the force of gravity, said leaving point area lying wholly within and occupying a part of a range extending from not greatly above the 65 position to substantially the 45 position, relating the flux concentration of said magnetic field to said circular path so as to direct the concentrated portion thereof from a point adjacent to the 90 position to a point materially below said leaving point area, and directing a high flux density on said prealignment area, whereby the feebly magnetic particles are so attracted in the direction of said convergences as to continue their circular motion through an arc of substantial extent below said leaving point area and are then projected outwardly from said circular path in a falling trajectory by centrifugal force suflicient to overcome the magnetic attraction.

2. The method of magnetically separating a grain stream composed of a mixture of feebly magnetic particles and relatively non-magnetic particles, which comprises rapidly moving the grain stream about a horizontal axis in a circular path commencing near the 90 position through an air gap of a magnetic field having local convergences directed toward said circular path, mechanically supporting the grain stream and progressing the same successively through a prealignment area in which it is adjusting its speed, and a leaving point area in which the non-magnetic particles are projected outwardly from said circular path in a falling trajectory by centrifugal force sufiicient to overcome the force of gravity, said leaving point area lying wholly within and occupying a part or a range extending from not greatly above the 65 position to substantially the 45 position, relating the flux concentration of said magnetic field to said circular path so as to direct the concentrated portion thereof from a point adjacent to the 90 position to a point materially below said leaving point area, and directing a high flux density on said pre-alignment area which is substantially as high as that on any other portion of said circular path, whereby the feebly magnetic particles are so attracted in the direction of said convergences as to continue their circular motion through an arc of substantial extent below said leaving point area and are then projected outwardly from said circular path in a falling trajectory by centrifugal force sufiicient to overcome the magnetic attraction.

3. The method of magnetically separating a grain stream composed of a mixture of feebly magnetic particles, and relatively non-magnetic particles, which comprises rapidly moving the grain stream about a horizontal axis in a circular path commencing near the 90 position through an air gap of a magnetic field having local convergences directed toward said circular path, mechanically supporting the grain stream and progressing the same successively through a prealignment area in which it is adjusting its speed, and a leaving point area in which the non-magnetic particles are projected outwardly from said circular path in a falling trajectory by centrifugal force sufiicient to overcome the force of gravity, said leaving point area lying wholly within and occupyin a part of a range extending from not greatly above the 65 position to substantially the 45 position, relating the fiux concentration of said magnetic field to said circular path so as to direct the concentrated portion thereof from a point adjacent to the 90 position to a point materially below said leaving point area, and directing the maximum flux density of said magnetic field on said pre-alignment area, whereby the feebly magnetic particles are so attracted in the direction of said convergences as to continue their circular motion through an arc of substantial extent below said leaving point area and are then projected outwardly from said circular path in a falling trajectory by centrifugal force suflicient to overcome the ma netic attraction.

4. In a magnetic separator adapted to separate a grain stream composed of a mixture of feebly magnetic particles and relatively non-magnetic particles, in combination, a pole, a horizontally disposed rotor having alternate magnetic and non-magnetic portions upon its surface and spaced from said pole to form a magnetic air gap, whereby a magnetic field is produced having local convergences directed toward the rotor, means for feeding the grain stream on said rotor near the 90 position, and means so constructed and arranged as to revolve said rotor at a speed such that the entire grain stream remains on the rotor during its passage through a pre-alignment area of substantial extent in which the grain stream is positively supported and is adjusting its speed to the speed of the rotor, and said rotor speed being such that the non-magnetic particles in said grain stream are projected outwardly from said rotor in a falling trajectory by centrifugal force sufilcient to overcome the force of gravity while passing through a leaving point area lying wholly within and occupying a part of a range extending from not greatly above the 65 position to substantially the 45 position, and said pole being so shaped and positioned as to produce a band of concentrated flux directed on the rotor from a point adjacent to the 90 position to a point materially below said leaving point area and as to direct a high flux density on said pro-alignment area, whereby the feebly magnetic particles are retained on the rotor by magnetic attraction in the direction of said convergences while passing through an arc of substantial extent below said leaving point area and are then projected out-- wardly from the rotor in a falling trajectory by centrifugal force suificient to overcome the magnetic attraction.

5. In a magnetic separator adapted to separate a grain stream composed of a mixture of feebly magnetic particles and relatively non-magnetic particles, in combination, a pole, a horizontally disposed rotor having alternate magnetic and non-magnetic portions upon its surface and spaced from said pole to form a magnetic air gap, whereby a magnetic field is produced having local convergences directed toward the rotor, means for feeding the grain stream on said rotor near the 90 position, and means so constructed and arranged as to revolve said rotor at a speed such that the entire grain stream remains on the rotor during its passage through a pre-alignment area of substantial extent in which the grain stream is positively supported and is adjusting its speed to the speed of the rotor, and said rotor speed being such that the non-magnetic particles in said grain stream are projected outwardly from said rotor in a falling trajectory by centrifugal force sufllcient to overcome the force of gravity while passing through a leaving point area lying wholly within and occupying a part of a range extending from not greatly above the 65 position to substantially the 45 position, and said pole being so shaped and positioned as to produce a band of concentrated flux directed on the rotor from a point adjacent to the 90 position to a point materially below said leaving point area and as to direct a high flux density on said prealignment area which is substantially as high as that on any other portion of said rotor, whereby the feebly magnetic particles are retained on the rotor by magnetic attraction in the direction of said convergences while passing through an arc of substantial extent below said leaving point area and are then projected outwardly from the rotor in a falling trajectory by centrifugal force sufllcient to overcome the magnetic attraction.

6. In a magnetic separator adapted to separate a grain stream composed of a mixture of i'eebly magnetic particles and relatively non-magnetic particles, in combination, a pole, a horizontally disposed rotor having alternate magnetic and non-magnetic portions upon its surface and spaced from said pole to form a magnetic air gap, whereby a magnetic fleld is produced having local convergences directed toward the rotor, means for feeding the grain stream on said rotor near the 90 position, and means so constructed and arranged as to revolve said rotor at a speed such that the entire grain stream remains on the rotor during its passage through a pro-alignment area of substantial extent in which the grain stream is positively supported and is adjusting its speed to the speed of the rotor, and said rotor speed being such that the non-magnetic particles in said grain stream are projected outwardly from said rotor in a tailing trajectory by centrifugal force suflicient to overcome the force of gravity while passing through a leaving point area lying wholly within and occupying a part of a range extending from'not greatly above the 65 position to substantially the 45 position, and said pole being so shaped and positioned as to produce a band of concentrated flux directed on the rotor from a point adjacent to the 90 position to a point materially below said leaving point area and as to direct the maximum flux density of said magnetic field on said pre-alignment area, whereby the feebly magnetic particles are retained on the rotor by magnetic attraction in the direction of said convergences while passing through an arc of substantial extent below said leaving point area and are then projected outwardly from the rotor in a falling trajectory by centrifugal force suflicient to overcome the magnetic attraction.

FRED R. JOHNSON.

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