CA1080667A - Methods and apparatus for separating particles using a magnetic barrier - Google Patents

Abstract

METHODS AND APPARATUS FOR SEPARATING
PARTICLES USING A MAGNETIC BARRIER
Abstract of the Disclosure A flowable mixture of particles is separated in accordance with the magnetic susceptibilities of the particles by feeding the mixture into a magnetic field in such a manner that the mixture is urged by a non-magnetic force, e.g., gravity, towards the locus at which the magnetic energy gradient H.theta.H/.theta.X of the field is at a maximum. The magnetic energy gradient defines a magnetic barrier along the locus of its maximum magnitude which exerts a magnetic force on the particles in opposition to the non-magnetic feeding force.
Particles having a magnetic susceptibility lower than that value at which the force exerted by the magnetic barrier balances the non-magnetic feeding force pass through the barrier, whereas particles of greater magnetic susceptibility are prevented from crossing from one side of the barrier to the other side and may therefore be recovered separately from the less susceptible particles. Continuous separations may be carried out by giving the mixture a velocity component lengthwise of the barrier such that particles deflected by the barrier will move therealong to a collection point. The magnitude of the maximum magnetic energy gradient may be varied along the length of the barrier or plural barriers of different strengths may be provided in succession to provide for progressive separation of the mixture into fractions of different magnetic susceptibilities. Naturally non-magnetic, but conductive, particles may be separated by employing superimposed a.c. and d.c. magnetic fields, the d.c.
field establishing the magnetic barrier and the a.c. field inducing magnetism in the conductive particles. Also, particles may be suspended in a fluid and separated along the magnetic barrier in accordance with the relative values of the magnetic susceptibility and density of the particles and fluid, respectively.

Description

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BACKGROUND OF THE INVENTION
Field of the Invention ~ he present invention relates in general to magnetic separation and, in particular, to improved methods and apparatus for making continuous magnetic separations of flowable mixtures - of particles of different magnetic susceptibilities.
The Prior Art Heretofore magnetic separations have fallen generally into two classes, those which employ a katadynamic magnetic field, i.e., a field in which the magnetic force exerted on a paramagnetic particle-(as used herein including ferromagnetic and ferrimagnetic particles) increases in the direction of increasing magnetic field strength, and those which employ an;
isodynamic magnetic field, i.e., a field in which the force exerted on a paramagnetic particle is substan-tially constant throughout the working area of the field. The prior art also discloses an anadynamic magnetic field, i.e., a field in which the magnetic -_ I
~' ' .

:

-2-10~0~ti'7 forcc ~Y.crted on a paramagnetic particle decreascs in the direction of increasing field strength. Uses of the anadynamic field for magnetic separation have not been broadly developed.
Typically, katadynamic magnetic separators make separations by attracting particles towards the magnetic poles or by otnerwise deflecting the particles from their original path of travel through the separator. They have the advantage that many pole piece configurations may~be used, thereby allowing flexibility in the design and arrangement of the pole pieces to suit particular applicationsj, and of allo~ing control of the magnetic force exerted on the particles to be separated by adjustment of the spacing between the pole pieces Qr the applied magnetic field intensity, or both. Separators of this type, however, have either brought the particles into contact with the poles, thus presenting a cleaning problem among others, or have required the use of moving elements, such as belts, discs, or the like, pr a moving ~luid stream to intercept particles attracted or deflected by the field and carry them out of the ~Jor~ing area of the field for collection. In addition, prior katadynamic separators have largely been limited to the separatlon ol ferro-magnetic and paramagnetic particles. In the main, they have not been successfully applied to the~separation of diamagnetic materials.
The ~irs~ magnetic separator to employ an isodynamic working field was disclosed by S. G. Frantz and patented by him in U. S. Patent No. 2,056,426, issucd on Octobcr 6, 1936 and assigned to the assignee of ~he present application. Such separators are manufactured ~y the S. G. Frantz Company, Inc., r ~ 9 - iO80 . , - .
' .
Trenton, New Jersey, under the trademark ISO~YNAMIC. The Frantz ISODYNAMIC magnetic separator affords advantages relative to the katadynamic magnetic separator in that it permits precise separations according to magnetie suseeptibilities of both paramagnetie and diamagnetic materials. It also enables separa-tions to be made continuously and without bringing the particles into contact with the pole pieces or without requiring the use of mechanieal elements or a moving fluid stream to intereept and remove the par.ieles from the magnetie fi-eld. Unlike the katadynamie separator, powever, the ISODYNA~IC separator re-quires speeially shaped pole pieees to establish the isodynamic field. The eonfiguration and separation of the pole pieces, onee determined, eannot readily be varied to allow adjustment of th~ magnetie force exerted on the partisles without loss of the isodynamie eharaeter of the field.
The present invention is direeted to the provision of improved magnetie separation methods and apparatus whieh eombine in a unique way the advantages and eapabilities of kata-dynamic, isodynamie, and anadynamic magnetic fields.

SUMMARY OF THE INVENTION

~ here is provided, in acco~dance with the invention, a method, and an apparatus for praetieing sueh method, for separating a flowabie mixture of partieles according to the m~gnetie susceptibilities of the particles in whieh a magnetic field is established having a loeus at which the magnetie energy gradient Ha~aX is a ma~imum and in whieh the mixture to be separated is fed into the magnetie ficld in sueh a way 108Vtj~'7 as to be urged by non-magnetic forces towards the locus of maximum gradient. Particles within the mixture having a magnetic susceptibility greater than that value at which the magnetic force exerted by the maximum energy gradient on the particles balances the non-magnetic force urging the particles towards the locus of maximum energy gradient are prevented by the magnetic force from crossing from the side of the locus from which they were`fed to the other side, while particles of lower magnetic susceptibility cross the locus. The locus of the maximum magnetic energy gradient thus defines a magnetic barrier along which more magnetically susceptible particles may bè separated from less susceptible particles. Any suitable non-magnetic force may be used to urge the mixture towards the barrier, including, for example, gravitational, centrifugal, fluid viscous force, frictional force and the like.
In accordance with the invention, the magnetic field is established using 2 pair of spaced-apart pole pieces having opposing faces that are shaped in cross section to form a katadynamic field on one side of the locus of maximum nagnetic energy gradient and an anadynamic field on the other side of the lo_us. An isodynamic field exists at the locus of maximum magnetic energy g adient. This field combination allows both paramagnetic and diamagnetic particles to be separated. h'hen paramagnetic particles are to be prevented from crossing the magnetic barrier, the mixture is fed between the pole pieces ~rom the anadynamic field side of the locus of maximum - gra~ient. Tl~e paramagne~ic particles will thereEore move - towards the barrier in opposition to the magnetic force exerted 10~

on them by the anadynamic field. When the barrier is to be used to prev~nt diamagnetic particles from crossing thereover, the feed direction is reversed and the particles are fed between the pole pieces from the katadynamic field side of the locus of maximum gradient. Progress of the diamagnetic particles through the barrier will therefore be opposed ~y the magnetic force exerted on them by the katadynamic field. The strength of the barrier is defined by the energy gradient of the iso-dynamic field which is the locus of the maximum energy gradient.
According to a further feature of the invention, the opposing pole faces are preferably symmetrical in cross section to provide a region in the vicinity of the plane of symmetry of the pole faces in which the magnetic energy gradient in the direction normal to the plane of symmetry is small in comparison to the maximum energy gradient defining the magnetic barrier. By feeding the mixture between the pole pieces generally along the plane of symmetry of the pole faces, the magnetic force tending to attract the particles towards either pole face is minimized. This facilitates separation of the mixture according to magnetic susceptibility and without bringing the particles into contact with the poles. If desired, the pole faces may be coated, e.g., with a corrosion resistane~ material, to confine q ~ Ç-particle flow to the vicinity of the plane of symmetry.
In a preferred embodiment of the invention, the pole pieces are elongated and are shaped in cross section to establish a magnetic field therebetween in which the direction of the magnetic energy gradient HaH/aX is transverse to the lengthwise direction of the pole pieces and is at a maximum at a locus in .

10~0~;~7 the vicinity of tlie line of closest approach of the opposing pole faces. The mixture is fed betwcen the pole faces so as to have velocity components in the direction of the locus of maximum gradient, i.e., the magnetic barrier, and lengthwise of the locus, so that the particles deflected by the barrier move lengthwise thereof to a downstream point where they may be collected separately from the particles which cross the barrier. Preferably, provision is made for control of the angle of approach of the particles towards the barrier to per-mit adjustment of the magnitude of the velocity componentwhich must be opposed by the magnetic force of the barrier.
This arrang~.nent enables continuous separ~tions to be effected by providing for continuous movement of the particles throug~
the magnetic field and for continuous collection of the mag-netic and non-magnetic fractions. In a convenient arrangement, flow of the mixture through the magnetic field is confined within an elongate flow channel positioned between the pole pieces in generally parallel alignment therewith. The pole pieces and the flow channel are inclined to the horizontal both in the transverse direction and in the lengthwise direc.ion, with the result that the mixture is urged by gravity along a path of travel which crosses the locus of maximum gradient.
Particles deflected by the barrier, however, are diverted rom such flow path and flow through the channel along the loc-ls of the barrier. Separate particle outlets from the channel permit - separate collection of the magnetic and non-magnetic fractions.
Des1rabl~, the flow channel is loca~ed generally along the plane of symmetry o the pole faces so as to minimize the lO~

magnetic force on the particles in the direction normal to the plane of the flow channel.
As another feature of the invention, provision may be ma~e for varying the magnitude of the maximum magnetic energy gradient along the length of its locus. This may be-done in several ways, including varying the spacing between the opposing pole faces along the line of closest approach therebetween, varying the intensity of the magnetic field, and varying the shape of the pole faces which create the kata-dynamic, isodynamic and anadynamic fields. In one sucharrangement, the barrier height may be maintained at the maximum value and substantially uniform over an upstream region of the magnetic field and then caused to decrease in a downstream region. The particles which do not cross the barrier move lengthwise along the barrier in the upstream field region but are released to pass through the barrier, for collection purposes, upon reaching the downstream region.
In another embodiment, a plurality of pairs of pole pieces may be arranged in side by side relation to establish a succession of magnetic barriers. By feeding the mixture through these barriers in succession, particles of different magnetic susceptibilities may be deflected and subsequently collected, along each barrier.
In accordance with one broad aspect, the invention relates to apparatus for separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising: means for establishing a nonuniform magnetiC field consisting essentially of, in a direction transverse to the direction of the field and in contiguous sequence, one or more series of field regions, each said series consisting of a katadynamic field region, an isodynamic field ~-~ region, and an anadynamic field region, the dimension of said ~~

lO~V~'7 each isodynamic field region in said transverse direction being small relative to the dimension in said transverse direction of either the adjoining anadynamic field region or the adjoining katadynamic field region in said series, the magnetic energy gradient -~ of said magnetic field in said transverse direction being at a maximum at the locus of at least one of said isodynamic field regions; means for feeding a mixture of particles into the magnetic field on one side of the locus of said maximum transverse gradient in such a manner that the particles are urged by non-magnetic force in said transverse direction towards said locus of maximum transverse gradient;
said field establishing means and said mixture feeding means being arranged such that the magnitude of the transversely-acting magnetic force exerted by said maximum transverse gradient, in the direction opposite to the transverse direction of movement of the particles towards said locus, on those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected is greater than the transversely-acting nonmagnetic force urging said particles towards said locus, whereby said particles of equal or greater susceptibility are prevented by the magnetic force acting thereon from passing from said one side of the locus of maximum transverse gradient to the other : side while par~icles having a magnetic susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by the transversely-acting nonmagnetic force; and means for collecting the particles prevented from passing from said one side of the locus of maximum transverse gradient to the other side separately from the particles which cross said locus.

~ In accordance with another aspect, the invention ~ - relates to apparatus for separating a flowable mixture of .--8a~

10t~0~;'7 particles in accordance with the magnetic susceptibilities of the particles, comprising: means, including a pair of spaced-apart elongate pole pieces having opposing faces, for establishing a non-uniform magnetic field, said opposing pole faces being shaped in cross section transversely of the elongate direction of the pole pieces such that said magnetic field consists primarily of in said transverse direction, in contiguous se~uence, a katadynamic field region, an isodynamic field region, and an anadynamic field region, the dimension of the isodynamic field region in said transverse direction being small relative to the dimension in said transverse direction of either the anadynamic field region or the katadynamic field region whereby the magnetic energy gradient HaH of the field in said transverse direction is at a maximum at the locus of said isodynamic field region and decreases therefrom on either side thereof; means for feeding a mixture of particles between the pole pieces on one side of the locus of said maximum transverse gradient in a manner such that the particles are urged by non-magnetic force generally towards and lengthwise of the locus of maximum transverse gradient; said field establishing means and said mixture feeding means being arranged such that those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected are retained on said one side of the locus of maximum transverse - gradient by the transversely-acting magnetic force exerted .. thereon by said ma~imùm transverse gradient in the direction opposite to the transverse direction of movement of the ~ particles towards such locus and are urged lengthwise along .-. 30 said one side of such locus by a lengthwise-acting component of said non-magnetic force, while particles having a magnetic _ -8b t 10~0~'7 susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by a transversely-acting component of the non-magnetic force; and means for collecting the particles retained on said one side of the locus of maximum transverse gradient separately from the particles which cross said locus.
In accordance with a further aspect, the invention relates to a method of separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising: establishing a nonuniform magnetic field having in a direction transverse to the direction of the field, in contiguous sequence, a katadynamic field region, an isodynamic field region, and an anadynamic field region, whereby the magnetic energy gradient H aH/ ax in said transverse direction is at a maximum at the locus of said isodynamic field region and decreases therefrom on either side thereof;
feeding a mixture of particles into the magnetic field on one side of the locus of maximum transverse gradient in a manner such that the particles are urged by non-magnetic force in said transverse direction towards the locus of maximum transverse gradient, the magnitude of the maximum transverse gradient being such that particles having a magnetic susceptibility to the field equal to or greater than a selected susceptibility at which a separation is to be effected are prevented by the transverse magnetic force acting thereon from passing from said one side of the locus of maximum transverse gradient to the other side while particles having a magnetic susceptibility to the field less than said selected susceptibility are urged across the locus of maximum transverse gradient by the non-magnetic force; and collecting the particles which do not cross said locus of maximum _ ~ .. , -8c ~

~0~0~'7 transverse gradient separately from the particles which cross said locus.
In accordance with yet another aspect, the invention relates to apparatus for separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising: means, including a pair of spaced-apart elongate pole pieces having opposing faces, for establishing a nonuniform magnetic field, said opposing pole faces being shaped in cross section transversely of the elongate direction of the pole pieces so as to produce therebetween, in contiguous transverse sequence, a katadynamic field region, an isodynamic field region, and an anadynamic field region, whereby the magnetic energy gradient HaH in said transverse direction is at a maximum at the locus of said isodynamic field region and decreases therefrom on either side thereof; an elongate flow channel located between the pole pieces in general parallel alignment therewith and with the locus of maximum transverse gradient lying within the cross section of the channel; means for supporting the pole pieces and the flow channel such that the flow channel is inclined to . the horizontal both in the lengthwise direction and in the transverse direction; means for feeding a mixture of particles .~ into an upstream region of the flow channel on the uppermost side of the locus of maximum transverse gradient, whereby the particles are urged by gravity generally towards and lengthwise of said locus; said field establishing means and said supporting means being arranged such that those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected are retained on said uppermost side of the locus of maximum transverse gradient by the transversely-acting magnetic force ., ~
~ ~ .... -8d- ~
~ ~ -;

. - ' ' '' ',. , ' '`~ ` `
-iOt~O~i~'7 exerted thereon by said maximum transverse gradient and are urged lengthwise along said uppermost side of such locus by a lengthwise-acting gravitational component, while particles having a magnetic susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by a transversely-acting gravitational component; and means for collecting the particles retained on said uppermost side of the locus of maximum transverse gradient separately from the particles which cross said locus.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be made to the following detailed description of exemplary embodiments thereof, taken in conjunction with the accompanying drawings, in which:

-~ 20 ~ ~ -8e-~ 10~0~i'7 7 Figure 1 is a representation of a magnetic circuit ~ J-useful in establishing a magnetic circui~ in accordance with ~., "7 ~ r7the invention;

~ igures 2a and 2b are enlarged schematic views of S the pole pieces of the magnetic circuit of Figure l;

Figures 3a, 3b and 3c are graphical representations of the variations over the cross section of the pole pieces of Figures 2a and 2b of certain parameters of the magnetic field established between the pole faces;

lQ Figure 4a is a schematic view of the pole pieces . of Figures 2a and 2b as arranged to deflect paramagnetic . particles along the magnetic barrier established therebetween against the force of a gravity particle feed;

Figure 4b is a graph of the magnetic forces acting on the particles fed between the pole pieces of Figure 4a;

Figure 5a is a schematic view of the pole pieces of Figures 2a and 2b as arranged to deflect diamagnetic particles along the magnetic barrier established therebetween against the force of a gravity particle feed;

Figure 5b is a graph of the magnetic forces acting on the particles fed between the pole pieces of Figure 5a;

Figures 6a, 6b and 6c illustrate various ways in which the strength of the magnetic barrier may be varied;

Figure 7 is a pictorial representation of a magnetic barrier created in accordance w~th the invention in which the ' ' r 108~ 7 strcngth o~ the barrier is shown as height at the locus of maximum energy gradient;

Figure 8a is a pictorial representation of a mag-netic barrier having a substantially uniform strength in an upstream region of the magnetic field and a decreasing strength in a downstream region of the field;

Figure 8b is a cross-sectional view of pole pieces ` - useful in producing the magnetic barrier of Figure 8a;

Figure 9a is a schematic view of a flow channel - 10 and magnetic barrier arrangement for ~eparating paramagnetic particles in accordance with the invention;

Figure 9b is a diagram of the magnetic and gravita-tional forces acting on particles in the separator arrangement -. - of Figure 9a;

; 15 Figure lOa is a schematic view of a flow channel and magnetic barrier arrangement for separating diamagnetic . particles in accordance with the invention;

Figure lOb is a diagram of the forces acting on particles in the separator arrangement of Figure lOa;

Figure 11 is a schematic view of a flow channel and magnetic ~arrier arrangement in which the barrier strength , progressively decreases in the do~nstream direction;

-~ Figure 12a is a schematic view of a flow channcl ~ and magnetic barrier arrangement in which a succession of -.

10~0~7 magnetic barriers of progrcssively increasing stren~ths are provided; and Figure 12b is a schematic view of a magnetic circuit useful in generating the magnetic barriers depicted in Figure S 12a, with parts broken away for clarity cf illustration.

DETAILED DESCRIPTION

For convenience, exemplary embodiments of the inven-tion are described herein by reference to the separation of dry flowable mixtures. It will be understood that the invention is likewise applicable to the separation of particles suspended in a liquid carrier. For example, water, solvent, colloidal suspensions of magnetic particles or other magnetic fluids such as SOiUti~llS of par~l~agrletic substances mdy be used.
- Fig. 1 depicts in schematic form the magnetic circuit of a separator, including the opposed pole pieces 10a and 10~, a U-shaped core 12 and a winding 14. The winding 14 is adapted to be connected to a conventional source (not shown~ of d.c.
current. Alternatively, the core and winding and also the poles if so desired could be replaced by a permanent magnet or - 20 magnets. In this case the field strength may be varied by ad-justing the reluctance of the magnetic circuit. As shown in enlarged detail in Figs. 2a and 2b, the pole pieces 10a and 10b are elongated in the direction of axis y-y' and have opposing faces 16a and 16b which are symmetrical in cross section relative to a plane of symmetry x-x'. For the purpose of establishing a reference system useful in descrihing the .. . .

iO~0~'7 invention hereinafter, the point along axis y-y' coinciding with the remote end o~ pole pieces lOa and lOb has been dcsig-nated in Fig. 2a as the zero point and the point therealong which coincides with the near end of the pole pieces has been designed Yl Fig. 2b depicts the pole pieces of Fig. 2a looking down the y-y' axis. Since, as noted, the opposing pole faces 16a and 16b are symmetrical,relative to the x-x' plane, the magnetic energy gradient in the direction normal to the x-x' plane, and hence the net magnetic force exerted on particles, such as indicated at 18 and 20 in Fig. 2b, lying on or near the x-x' plane, is much smaller than .he magnetic energy gradient and the resulting net magnetic force acting on the particles in the direction of the x-x' plane. Consequently, there will be little tendency for the particles 18 and 20 to move in the direction of either pole piece lOa or pole piece lOb. The present invention allows full advantage to be taken of this characteristic, as described in more detail hereinafter, by permitting introduction of the particles into the magnetic field generallv along the plane of sym~etry x-x' of the opposing pole pieces. In this connection, each of the pole faces 16a and 16b may be covered with a corrosion resistant coating 17 which is of a thickness such that particle flow between the pole pieces i~ confined to the vicinity of the plane of symmetry x-x'.
Where such a corrosion resistant coating is provided, all of the parts of the separator whlch come into contact with the flowable mi~ture may similarl~ be coated. The coating may comprise any suitable corrosion-resistant,-non-magnetic material, e.g. a -. plastic such as polytetrafluorethylene, polyethylene, polyvinyl chloride, etc., or a ceramic such as glass, etc.

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The pole faccs 16a and 16b are also prcfcra~ly shaped in cross section to definc over adjacent regions ther~of a katadynamic magnetic field and an anadynamic mag- -netic field separated by an isodynamic field. Various pole face configurations may be used for this purpose. For simplicity, the pole faces 16a and 16b are shown in Fig. 2b as being defined by a quarter circle of radius r which merges at its ends into tangential surfaces. As depicted in Fig. 2b, the plane passing through the inner ends of the quarter circle portions of faces 16a and 16b passes through the y-y' axis, whereby the Yl-O line along the pole faces defines their line of closest appxoacn. The distance of such closest approach, hereinafter referred to as the gap between the pole pieces, is designated in Fig. 2b as 2d.
Fig. 3a depicts the variation of the square of the magnetic field strength H2 and Fig. 3b depicts the gradient - of the square of the field strength HaH/aX, defined herein as the ~.agnetic energy gradient, produced by the ~ole faces 16a and 16b along the plane of symmetry x-x'. As shown in Fig. 3a, H2 increases toward the right until the line Yl-O is reached, at which point it becomes uniform owing to the parallel planar nature of the pole faces to the right of the Yl-O line. The dashed line curve of Fig. 3b indicates that the gradient HaHfaX increases toward the right until the point of inflection of the H2 curve (Fig. 3a) and thereafter decreases to zero at the Yl-O line. To the right of the Yl-O line the energy gradient ~IaH/aX is zero because the field is uniform over that that region. ~n isodynamic field at the locus of maximum ~ . " .

10~ i7 energy gradient llall/aX de~ines the strength o~ the magnetic barrier ~hich e~tends leng~hwise of the pole pieces in the vicinity of the line of their closcst approach Yl-0. The locus of the maximum magnetic energy gradient, which defines the position of the barrier, is identified in Figs. 3a and 3b, and in later views, by the designation XB. For convenience of illustration herein, the dashed-line gradient curve of Fig.
3b is approximated by two straight lines which connect the peak or maximum gradient value with the zero points on either side of the locus XB.
The magnetic forces exerted on a small paramagnetic particle by an applied field (assuming a single dimensional case) is given by:
FM = KvHaH/aX = 1/2 ~va~i /aX (1) where FM is the masnetic force (dynesi, K is the volume susceptibiliiy, v is the volume of the particle (cm3), is the magnetic field intensit~r (Oe.) aH/~X is the field gradient along the x-x' plane ~Oe./cm), and /4~HaH~aX is the magnetic energy grad~ent (Oe.2/cm).
By convention paramagnetic particles are considered to have positive K values and diamagnetic materials are con-sidered to have negaiive R values. The magnetic forces exerted on a paramagnetic particle l~ and a diamagnetic particle 20 located between the pole pieces 10a and 10b (Fig.
2b) may therefore be represented in the manner shown in Fig. 3c.
The maximum maynetic force for eith~r particle occurs at the . iO~Oti~'7 .
locus XB of the m~ximum value of the gr~dient }S~H/aX. For the paramagnetic p~rticle 18, this force, indicated by the curve 22 in Fig. 3c, increases in the direction of increasing field streng.h H2 and is considered to be a positive force.
S The force exerted on diamagne~ic particle 20, indicated by curve 24 in Fig. 3c, on the other hand, decreases in the direction of increasing field strength H2 and is considered to be a negative force. Accordingly, the magnetic force +FM
acting on paramagnetic particle 18 is toward the right in Fig.
3c while the magnetic force -FM acting on diamagn-etic~particle 20 is toward the left. In accoraance with the aforementioned definitions, therefore, it may be seen from Fig. 3c that the magnetic field established between the pole pieces lOa and lOb is katadynamic over the region to the left of the XB of the nlaximum gradient ~aH/aX, anadynamic over ihe region to the right of ~B~ and isodynamic at XB. It will also be appreciated that the magnetic energy gradient H~H/aX,by virtue of the - force which it-exerts on particles coming ~ithin the field, - defines a magnetic barrier along the locus XB against the movement thereacross of particles which are subject to a non-magnetic force sufficient to overcome the opposing magnetic - force exerted by the barrier. Since the maximum magnetic force is exerted along the locus of maximum gradient, the magnitude or value of the maxi~lum gradient is herein describea and represented in the drawings as the barrier. It should be .. .
understood that the barrier does not have physical height but consists of a region in the vicinity of the x-x' plane where the ma~nctic force exerted in the direction of increasing .
.,- ~ ' ., .
.

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magnetic ~i~ld on par~magnetic particles and in thc rcvcrsc direction on diamagnetic par~icles reachcs a maximum.
~he manner in which the forcc illustrated in Fig.
3c may be used to separate particles according to their mag-netic susceptibilities may be further understood from considera-. tion of Figs. 4a and 4b and Figs. 5a and 5b. In Fig. 4a, the pole pieces lOa and lOb have been tilted such that the plane of symmetry x-x' is inclined at an angle ~ to the horizontal.
If a group of five particles Pl, P2, P3, 4 5 decreasing susceptibilities Kl>K2>K3>K4>0 >K5, respectively, is introduced between t~hë pole pieces from the right along a surface 21, lying in or near the x-x' plane, all of the particles will be ~rged downward along the surface by a gravitational force FG = mg Sin ~, where m is the mass of the particle, g is the acceleration due to gravity and ~ is the angle of tilt from the horizontal. Particles Pl to P4 will be opposed in such movement by a magnetic force +FM proportional to their susceptibilities Kl to K4, and the particle P5 w-ll experience additional downward force -FM. If the magnitude of the grav-itational force FG is superimposed on a plot of the magnetic forces aciing on the respective particles, as has been done in Fig. 4b, it may be seen that those particles having a suscep-tibility K greater than the susceptibility at which the upwardly acting magnetic force F~ balances the downwardly acting gravitational force FG will be retained on the right hand side of the barrier locus XB, but that particles of lesser suscep-- tibility ox of a neqative susceptibility will cross the barrier locus XB and con~inue downward alon~ the surfacc 21.

~ , . .

lO~V~

The critc~ion de~crmining ~JIIether a particle be deflected by thc magnetic barrier or whether it will penetrate the barrier may be expressed as:

FNM = KoV (H~H/aX)max (2) where FNM is the applied non-magnetic force (FG in Fig. 4a) tending to urge the particles in the direction of the barrier;
Ko is that-vaiue of magne~ic susceptibility at which, for a constant magnetic energy gradient HaHf3X, the magnetic force acting on a particle will just balance the applied non-magnetic force; and (H~H/aX)m is the maximum value of the magnetic energy gradient HaH/ ax .
For a constant applied non-magnetic force FNM and constant maximum barrier strength (HaH/aX)ma , particles with susceptibilities greater than Ko will be deflected by the barrier whiie particles of lower susceptibilities will penetrate the barrier. As will be apparent, this criterion affords a basis for separating particles according to their magnetic susceptibilities.
Turning to Fig. ~b, it may be seen that the particle P3, which has a susceptibility K3 equal to Ko for ~he barrier of Fig. 4a, will undergo a magnetic force F 3 which just balallces the gravitational force ~G and that the ~articlc P3 will therefore be prevented from crossing from one side of the barrier to the other side at a balance point X3 which coincides with the locus XB of the barrier. The particles Pl and P2, having stiil higher magnetic susceptibilitic-s-~Ki and K2, will be subject to the stronger magnetic forces F 1 .

io80~

and F 2' respectively, and will be deflected at balance points Xl and X2. The less stron~ly paramagnetic particle P4, however, which has a susceptibility K4 lower than Ko~ will experience a magnetic force Fp4 which is insufficient to balance the grav-S itational force FG, with the result that particle P4 will crossthe barrier locus XB. The diamagnetic particle P5, having a negative susceptibility K5, will be subject to a negative magnetic force Fp5 and will of course pass through the barrier.
Diamagnetic particles approaching the barrier in Fig. 4a from the right will always pass through the barrier.
Figs. 5a and 5b depict a separator arrangement for deflecting diamagnetic particles on the magnetic barrier. In this arrangement, the pole pieces lOa and lOb are tilted in the opposite direction from that of Fig. 4a, such that the x-x' J
lS plane is inclined to the left at an angle ~, and the particles"~
~ q7' Pl to P5 are fed along the surface 21 from the left hand side of the barrier locus XB. The paramagnetic particles Pl to P4 will be urged downward to the right by both the magnetic forces Fpl to Fp4, see Fig. 5a, and the gravitational force FG.
They will therefore cross the barrier locus XB without resis-tance. The diamagnetic particle P5, by contrast, will be opposed by the upwardly acting magnetic force Fp5, ana, assuming ` its susceptibility K5 is more diamagnetic than Ko for the ~arrier, will be deflected by the barrier at the balanc~ point X5, as shown in Fig. 5b.
It has been assumed in the above discussion of Figs.
4a and 5a that the barrier strength, i.e., the maximum value of the magnetic gradient HaH/aX, was held constant. This ;, ~

10~0~;~;;7 condition, with a constant applied. non-magnctic force FNM, gives a separation into two fractions, one having susceptibili-ties greater than Ko and the other having susceptibilities less than Ko~ I~, however, the barrier strength is varied S while maintaining the applied non-magnetic force constant, it is possible to vary the value of Ko at which the barrier will be effective to deflect particles and thereby obtain multiple fractions of different susceptibilities.
Three ways in which the barrier s'rength ~ay be varied are illustrated in Figs 6a, 6b and 6c, namely, by varying the strength of the applied field, by varying the spacing or gap between the pole pieces, and by varying the cross sectional shape of the opposing pole faces. Fig. 6a depicts three barrier strengths correspon~ing to three different field strengths Hl, H2 and H3, where the gap and the pole face shape are unchanged.
The strength of the barrier HaH/aX increases with increasing field strength, but changes in the strength of the barrier do not significantly change its locus.
In Fig. 6b, the shape of the pole pieces and the - 20 applied field strength H have been ~ept unchanged and the barrier strength ~aried b~ changing the gap 2d between the pole pieces.
The barriers corresponding to three gaps dl, d2 and d3 are represented, with dl being the smallest gap and d3 the largest ~ap. It may also be seen from Fig. 6b that the locus Xl, X2 and ~3 of the respective barriers shifts to the right, i.e., toward the line of closest approach of the pole pieces, with decreasing g~p si~e.

.

' ' ' -19-, 1080~

Fig. 6c del~icts thc condition where field intensity and gap size are kept constant and the shape of the pole faces is varied, as' for exctmple, by varying the radius r of the quarter circle portion of the faces. In Fig. 6c, rl repre ents the smallest radius and r3 the largest radius, from which it will be apparent that the strength of the barrier increases as the radius decreases. Here again, the locus Xl, X2 and X3 of the respective barriers ~hifts toward the line of closest approach as the radius of curvature of the pole faces is decreased.
With reference to Fig. 7, the magnetic barrier established in accordan`ce with the invention, and specifically the barrier established by the pole piece arrangement of Figs.
2a and 2b, may be visualized as a virtual planar member of finite extent, having a locus XB along the x-x' axis, 2 strength (HaH/aX) max. shown as the height,and a length parallel to the y-y' axis from O to Yl- The pole pieces are not shown in Fig.
7 for clarity. Since the gap between the pole pieces lOa and lOb in the arrangement of Figs. 2a and 2b is uniform over the full length of the pieces and since the cross sectional shape of the pole faces 16a and 16b are likewise uniformly shaped over the full length or the pole pieces, the barrier established between the pole pieces will be of a uniform strength over the full length of the pole pieces.
Fig. 8a shows a barrier in which the strength is at a maximum and uniform over the lengthwise region O-Y2 and gradually decreases over the lengthwise region Y2-yl. Such a barrier configuration is useful in ma~ing continuous separations of a flowable mixture in which the mixture is given velocity _~o_ ``. ' 9 0 10~0~'7 components both toward the barrier and along its lcltgth, e., in the direction of the y-y' axis. Since the direction of the magnetic force generated by the barrier is perpendicular to the barrier, the particles will encounter no resistance to lengthwise movement along thc barrier. Hence the particles will be guided by the barrier to a downstream point where they may be collected. This point is depicted in Fig. 8a at Y2, do~mstream of which the b~rrier strength falls off. As the particles prevented from crossing the barrier pass the point Y2 they will begin to pass through the barrier as the barrier strength decreases. They may thereupon be intercepted by one or more dividers 26 and guided to a suita~le collecting receptacle (not shown).
- Pole pieces lOa and lOb useful in establishing the barrier configuration of Fig. 8a are shown in lengthwise cross section in Fig. 8b. As there illustrated, the decrease in barrier strength over the downstream region Y2-yl is achieved by progressively incxeasing the gap 2d in the direction from point Y2 to point Yl- It will be understood that the vriation in barrier strength over the region Y2-Yl may also be accom-plished by varying the shape of the opposing pole faces.
The mixture of particles may be urged towards and along the magnetic barrier by any suitable non-magnetic force, including, for example, gravitational forc~l centrifugal force, fluid viscous force, frictional force and the li~e. Various representative separator arrangements for separatin~ p~rticles according to their r.lagiletic susccp~i~ilities by mea..s of opposing magnetic force 'o gravitational force are illustrated in Fi~s.
9a, lOa, 11 ~nd 12a. For clarity of illustr~tion of the .

1080f~

magnetic barri~rs and thc flow o~ particlcs rel~tive thercto, the polc picces havc been omittcd ~rom thcse views.
Fig. 9a pr~sents a separator arrangement suitable for separating paramagnetic particles. It includes an elon-S gated non-magnetizable flow channel 28 of generally U-shaped cross section positioned between the pole pieces (not shown) in generally parallel alignment therewith. Preferably, the channel is located such ~hat its transverse plane is in or near the plane o~ symmetry x-x' of the pole pieces.
As shown in Fig. 9a, the channel 28 has a transverse slope to the left at an~angle 9 and a lengthwise or forward slope at an angle ~ . The angle ~ is measured in the plane normal to the transverse plane of the channel 28. The magnetic barrier 30 extends lengthwise of the channel 28 and is o a uniform strength over substantially the full length thereol.
The forces acting in the plane of the channel on a paramagnetic particle moving downward therealong, such as the particle 32 in Fig. 9a, are shown in Fig. 9b. A gravitational force component FG acts along the x-x' plane towards the left 20 in Figs. 9a and 9b. For the separator arrangement of Fig. ga, this component is given by: -FGX = mgsin ~ (3) where g is the gravitational acceleration constant, and m is the mass of the particle.
-` The transverse gravitational component FGX is opposed by the magnetic force FM which acts at ri~ht angles ~22-lO~

to the barrier 30 and, for paramagnetic particles, in the opposite direction from FGX. As discussed above in connection with Figs. 4a and 5a, those particles, such as particles 34 in Fig. 9a, having magnetic susceptibilities greater than the Ko value of barrier 30 will be deflected by the barrier 30 on the righthand side of the barrier locus XB, , while more weakly paramagnetic particles and diamagnetic particles, such as are indicated at 36 in Fig. 9a, will cross the barrier 30 under the influence of the gravitational force component FGX. By virtue of the forward slope ~ of the channel 28, the particles, referring here again to particle 32 in Fig. 9a by way of example, will also have a lengthwise gravitational force component FGy acting along the y-y' plane. FGy will be of a magnitude given by:
FGy = mg cos ~ sin ~ (4) Since the magnetic barrier 30 does not impede particle movement in the y-y' direction, the particles will flow lengthwise of flow channel 28 generally along two separate paths, one for the particles 34 which do not pass the barrier and the other for the particles 36 which pass through the barrier. The two distinct groups of particles may therefore be readily recovered at the downstream end of the flow channel 28. To that end, a guide member 38 may be provided on the downstream side of the barrier 30. Another divider 39 may be provided on the upstream side of the barrier 30 at the upstream end of the channel 28 to facilitate proper particle feed into the channel.
As may be appreciated from Figs. 9a, 9b, lOa and lOb - the magnetic force produced by a magnetic barrier opposes _~.~

, 10~

the componcn~ of the non-magnetic forcc acting perpendicular to the barrier. For a const~nt magnitude non-magnetic force, the magnitude of this component is controlled by the angle between the direction of the resultant non-magnetic force and the magnetic barrier, i.e., the angle of approach of the par-ticles towards the barrier. As this angle becomes smaller the component of the non-magnetic force which the magnetic force - opposes becomes smaller and less susceptible magnetic particles will be deflected along the barrier. The susceptibility Ko at which particles will bé prevented from crossing the barrier may therefore also be controlled by appropriate selection of the angle of approach of the particles towards the barrier.
Fig. lOa depicts a separator arrangement similar to that of Fig. 9a but arranged such that the barrier 40 thereof will deflect diamagnetic particles. Whereas in Fig. 9a the flow channel 28 was inclined so as to have a transverse slope to the left, in Fig. lOa the flow channel 42 is inclined by an angle ~ to have a transverse slope to the right. The chute 42 is also inclined in the forward direction at an angle ~ , as measured in a planc normal to the plane of the channel. The !
particle mixture is fed to channel 42 on the lefthand side of barrier 40, for which purpose a divider 44 may be provided.
The forces acting on diamagnetic particles flowing along channel 42, such as particle 45, are represented in Fig. lOb.
Those diamagnetic particles 46 having a diamagnetic susceptibility greater than that value at which the magnetic force F~, acting normal to the barrier 40 and to the left in Fig. lOa, balances the rightwardly acting transverse .

1080~7 gravitational ~orce FGX will be prcvented from passing the barrier 40, whereas less susceptible diamagnetic particles and paramagnetic particles, indicated at 48 in Fig. lOa, will eross to the righthand side of the barrier. A guide member S 50 at the downstream end of channel ~2 provides for separate eollection of the two fractions 46 and 48 of particles. In this instance, the guide member 50 is located on the righthand side of the barrier 40.
Fig. 11 illustrates a separator arrangement generally similar to that of Fig. lOa except that the strength of the magnetic barrier 52 progressively decreases from the upstream end to the downstream end of flow channel 54. The diamagnetic particles fed to the channel 54 will therefore progressively eross over the barrier-52, in the course of their downstream movement therealong, in accordance with their magnetic suscep-tibilities. This arrangement, therefore, permits a number of fraetions of different magnetic susceptibilities to be obtained.
For instance, the most weakly susceptible particles and any paramagnetic particles in the mixture, such as the particles 56 in Fig. 11, will cross the barrier 52 in a comparatively up-stream region of the channel 54 and may be guided from the ehannel 54 along a separate path formed by the guide members 5 and 60.
Particles having sufficient diamagnetic susceptibility 2~ to be initially retained by the barrier 52 but which are not suf~iciently susceptible to be retained as the barrier strength decreases along the length of the channel 54 will cross the barrier at that position along its length at which the magne.ic ~'' , '" ' .

0~'7 force on the particle a~ t~e barrier has decrc.lsed and no longer exceeds ~hc gravitational opposincJ force. Thus, in Fig. 11, the particles 62 wllich are collectcd between the guide members 60 and 64 ~ill be undcrstood to be more diamag-netic than particles 56 collected between the more upstreamguide members 58 and 60, and the particles 66 collected be~ween the guide members 64 and 68 will be more diamagnetic than particles 62. Those particles, indicated at 70 in Fig.
11, having a sufficiently high diamagnetic susceptibility to be prevented from crossing the barrier 52 over substantially the full ler.gth of the channel 54 will be collected to the left Qf the guide member 68. In the embodiment of Fig. 11, therefore, four separate fractions may be obtained from the particle mixture by use of a single pair of pole pieces.
A separator arrangement in which a plurality of pairs of pole pieces, and hence a plurality of magnetic barriers, may be used to produce multiple fractions of different magnetic susceptibilities is depicted in Fig. 12a. In this arrangement, a flow channel 72 of expanded width and the associated pole pieces ~not shown in Fig. 12a) are inclined to the horizontal in the transverse and lengthwise directions in the same ~.anner as has been explained in connection with Fig. lOa. Three magnetic barriers 74a, 74b and 74c are located in side-by-side parallel relation across the transverse extent of the flow channel. Each barrier has a configuration similar to that - depicted in Fig. 8a but is of a different strength, with the barrier strength progressively incrcasing from barrier 74a to barrier 74c. The particle mixture is fed to the channel 72 ~080~;7 adjaccnt the upper lethand corner thcreof so that thc par-ticles not retaincd by barrier 7qa will flow through the downstream barriers in succession as they progress along the flow channel. Diamagnetic particles having diamagnetic sus-ceptibilities greater than the X0 of barrier 74a will be - deflected by that barrier and be guided therealong lengthwise of the channel 72 until collected by the guïde member 76 down-stream of point Y2. Less diamagnetic particles will pass through barrier 74a and, by virtue of the gravitational force componeni acting trans~ersely of channel 72, approach barrier 74b. Since barrier 74b has a greater strength than 74a, it will deflect particles of lower diamagnetic susceptibility than would barrier 74a. Accordingly, another fraction of particles ~ay be collected downstream of point Y2 by use of a guide member 78. Particles of still less diamagnetic suscep-tibility cross barrier 74b and are urged towards barrier 74c.
- Since barrier 74c has a higher strength than barrier 74b, it will deflect still less diamagnetically susceptible particles, which particles may be separately collected along a guide mem-ber 80. ~ny particle~ in the mixture having diamagnetic sus-ceptibilities lower than the Ko of barrier 74c or any paramagne-tic particles in the mixture will continue on across the flow channel 72 and may be collected along the righthand side of the channel. The arrangement of Fig. 12a thus provides in a continuous manner four separate fractions of particles of different magnetic susceptibilities.
A magnetic circuit sui';able fGr crcating the barriers 74a, 74~ and 7~c of Fig. 12a is shown in schcmatic form in .

,~ , .

1080~;~'7 .

Fig. 12b. It includes thr~e pairs 82a, 82b and 82c of op~osed pole picces, a U-shap~d core 84 and a windinq 86. The pole pieces have the cross sectional shapc shown in Figs. 2a and 2b and are spaced along the legs o~ the core 8~ in side-by-side parallel relation. The flow channel 72 is located betwecn the pole pieces such that the plane of the channel is in the region of the plane of symmetry x-x' of the pole faces. By employing progressively smaller gaps between pole piece pair 82a, pair 82b and pair 82c, the strengths of the associaled barriers 74a, 74b and 74c, respectively, may be progressively increased in the manner illustrated in Fig. 12a. As noted in connection with Fig. 6c, like variation in barrier strength can be achieved by using three different radii of curvature for the-opposing pole faces of the three pole pairs.
A number of barriers can be provided in the gap of one magnetic circuit. The pole piece pairs producing the barriers may be stacked in tiers with a number of pairs in each tier. The particles can be fed so that the flow stream is divided into a number of parts at every tier and each part o.^
the flow stream passes through one or morQ barriers successively, and the separated particles can be collecteæ in such a manner - that magnetic particles are discharged at one exit from the separator and non-magnetic particles are discharged at another exit. Such an arrangement provides means for continuous pro-cessing of a substantial volume of material.
The magnetic circuits of Figs. 1 and 12b are useful for separating materials b~sed on their natural magnetic properties. However, the invention may also be employed t:o 1080~'7 make separations on the basis of induced magnetic susceptibili-ties. Accordingly, the ci~-cuit of Fig. 1 may be modi~ied to - include a second winding 88 adapted to be connected to a suit-able source (not shown) of a.c. current. Superimposed a.c.
and d.c. maynetic fields will thereby be created. The a.c.
field will generate eddy currents in conductive particles passing through it which in turn will produce an induced magnetic field within the particles. The d.c. magnetic field will, as dis-cussed above, establish the magnetic barrler. The barrier will act to deflect particles in accordance with their induced sus-ceptibilities or, more preciseiy, according to the conductivity and shape of such particles.
Where separations are to be made in a liquid carrier, ~ the same analysis used in the dry separation can be used, except the magnetic susceptibility term K in equation (1) becomes (R - KL), where KL is the magnetic susceptibility of the liquid.
The magnetic force exerted on a particle by an applied field w~ld then become:
M (K XL3 vHaaH (5) and it can be used to oppose a non-magnetic force, for e~ample, the gravitational force.
If the same apparatus shown in Fig. 9a is used, then the non-magnetic force opposing the magnetic force along the plane x-x' becomes:

FNN = tdp - aL) vqsin ~ (6) . . ' - . .

-r~
.. . .

.
.

10~ '7 where dp is the density of the particle; and dL is the density of the liquid carrier.
When FM (equation (5)) exceeds FNM (equation (6)), the particles are deflected by the barrier, and FM = (IC - KL) v(Hdx) max. (7) and exceeds FNM = (dp - dL) vgsin ~ (8) If K iS much larger than KL then K-KL = K
(approximately) and KV (H dH) max. exceeds (dp - dL) Vgsin ~.
The separation is made according to the value Ofd Kd -If K is much smaller than KL then K - KL = -KL
(approximately). In this case the particle acts as though it is diamagnetic having a susceptibility -KL. Separation requires reversal of direction of the magnetic force and occurs when KL V (HaX) max. exceeds (dp - dL) vgsin ~. Separation in this instance is made according to the value of d Ld However, since dL and KL are both constants, the separation is made solely on the basis of the particle density dp. In a case where neither K nor KL may be neglected the separation is made according to the value of ~ .
If the liquid is moving the analysis will be somewhat different but the separation will be in accordance with the same particle properties. When a liquid is used, whether moving or stationary, the flow channel preferably is fully enclosed rather than open as shown in Figs. 9a and lOa.
Additional 10~0~

apparatus and methods which may be used to feed a moving liquid carrier through a magnetic field is disclosed in U.S. Patent 4,102,780 of July 25, 1978 for METHOD AND APPARATUS FOR MAGNETIC
SEPARATION OF PARTICLES IN A FLUID CARRIER.
In the description of methods and apparatus for separating particles in accordance with the magnetic susceptibilities of the particles the word "particles" should be understood to include pieces of the material of any size and the words "magnetic susceptibilities" should be interpreted in the following manner.
Since magnetic separation of particles is based upon opposing a magnetic force to a non-magnetic force, which non-magnetic force may be gravitational, centrifugal, fluid viscous force, frictional force and the like, the separation depends not only upon the force exerted on the particle by the magnetic field but also upon the non-magnetic force which is determined by other physical properties of the particle.
For example, when the opposing force is gravitational in a low density medium such as air, the separation is in accordance with the particle susceptibility K divided by its density dp or dp. When the particle is immersed in a liquid the K
separation is in accordance with dp-dL, the dL is the density of the liquid. When the particle is immersed in a magnetic KL
- liquid, the separation is in accordance with dp ~ , where ~ and j-KL are the susceptibilities of the particle and the '.

~ 30 W
~ !

_31- !

10~0~'7 liquid. In the case where R is much larger than Kp, so that Kp may be neglected and KL and dL are constants, separation is in accordance with the density of the particle dp and K
is not involved.
In cases of other external forces opposing the magnetic force~similar analyses are required to determine the nature of the separation.
The apparatus shown in Figs. 11, 12a and 12b is arranged for the separation of particles according to their diamagnetic susceptibilities. The paxticle flow is from left to right into a barrier having a katadynamic field on the lefthand side and an anadynamic field on the righthand side as shown in Fig. 3c. If the pole pieces producing the field are rearranged so as to produce an anadynamic field on the lefthand side and a katadynamic field on the righthand side, the appara-tus of Figs. 11, 12a and 12b may be used for separating particles according to their paramagnetic properties.

Example 1 A sample containing paramagnetic particles was pre-pared from a larger sample of river delta mud having a minimum particle size of 106 microns. The larger sample was first passed in conventional fashion through a Model Ll, Frantz ISODYNAMIC magnetic separator with the chute arranged to have a -transverse slope to the left at an angle ~ of 15 and a forward slope at an angle ~ of 25. With a field strength of 18,500 gauss in the gap between the pole pieces, a non-magnetic fraction of 18.75 grams was obtained. The 18.75 gram sample 10~0~;~7 was determined by microscopic inspection to include many pre-dominately black particles, which were known to be very weakly paramagnetic, and many light grey particles.
A magnetic barrier was established in accordance with the invention by using the outside or non-isodynamic por-tion of the Model Ll pole pieces. This portion of the pole pieces has the form of a quarter circle of 3/8ths inch radius which merges at its ends into tangential plane surfaces in the manner illustrated in Fig. 2b. The gap between the pole pieces, corresponding to 2d in Fig. 2b, was 5/32nds of an inch over the upper half (12.7 cm) of the pole pieces and gradually increased from the 5/32nd inch value over the lower half of the pole pieces. The barrier generated had the configuration ~ ~ "~, n~t3~ 7~r_-shown in Fig. 8a and had a maximum ~ t (H~H/aX) of 38.525 x 10 lS Oe. /cm over the upper half of the Ll pole pieces, corresponding to region O-Y2 in Fig. 8a. The flow channel structure and orientation employed was similar to that depicted in Fig. 9a.
The sample was fed to the upper righthand end of the flow channel on the upstream side of the barrier.
Five successive separations were performed, using the full 18.75 gram sample in the first run and only the non-` magnetic fraction from the preceding run for each successive run. The following results were obtained:
. Table I

25 Side Forward Gap Field Magnetic Non-Magnetic Slope Slope Strength Fraction Fraction Total (~o)(~o) (gauss) (g) (g) g 13,800 2.7 16.05 18.75 18,500 4.1 11.95 16.05 19,500 .6 11.35 ll.g5 6 25 19,500 3.2 8.15 11.35 4 25 19,500 1.6 6.55 8.15 1080~7 Each separation was repeated and found to he clo~.cly reproducihle. The ~inal non-magnetic fraction (6.55 grams) was ~ery light grey in color and found to contain almost no predominately black particles when examined by microscope.

Example 2 A C-shaped core for a magnetic circuit of the type illustrated in Fig. 1 was constructed by cutting a 1/8th inch gap in one leg of a square loop of steel having a 1.27 cm x 1.27 cm cross section. The inner portions of the core surfaces bordering the gap were cut away so as to form opposing pole faces which tapered outwardly to a sh~rp edge at the outer surface of the core. The gap between the sharp edges of the pole faces was approximately l/8th inch. The core was wound with a 1000 turn winding, which winding was connected to a half wa~e rectified a.c. current. This current can be viewed as eguivalent to an average d.c. current with a superimposed a.c.
current. A single a.c. ammeter was used to measure the current supplied to the coil. The magnetic circuit was arranged wi~h the plane of symmetry of the pole faces in the horizontal plane. A mixture composed of beach sand particles and copper disks was placed on a flat plastic sheet. The plastic sheet was then introduced by hand between the pole faces generally in the plane symmetry of the faces and moving from the outside of the core toward the inside of the core in a direction per-pendicular to the lengthwise extent of the pole faces. Witha current of 1.25 amperes applied to the ~Jinding, the copper disks were blocked by the barrier established between the 1080~
sharp opposing edges of the polc faces, whercas the ~and particles moved with the plastic sheet through the barrier.

E~mple 3 A mixture of silicon carbide par~icles and natural diamond particles ranging in size from 90 microns to 75 microns was separated using the Model Ll Frantz ISODYNAMIC magnetic separator, having the same pole face configuration, size, and separation as described in Example 1, with the flow channel and pole pieces oriented hn the manner shown in Fig. 10a. The flow channel had a transverse slope to the right at an angle of 3 and a forward slope at an angl~ of 15. The current to the magnetizing coils of the separator was adjusted until a field strength of 18,500 gauss was established in the gap between the pole pieces. At that field strength, a diamagnetic fraction consisting essentially of natural diamond particles was obtained along the barrier, and a separate fraction contain-ing substantially all of the silicon carbide particles was cbllected on the downstream side of the barrier, thereby indicating that the diamagnetic diamond particles had been blocked and retained by the barrier while the silicon carbide particles passed through the barrier. The diamond particles were predominately of a light grey color and the silicon carbide particles of a darker color, which readily permitted their identification by visual inspection.
With no magnetic field applied, all of the particles of the samplcs tested in E~amples 1, 2 and 3 above passed through the locus at which the barrier had been established.
.. . . .
' ~5~

1080~'7 ~ lthough ~hc invention h?s been described and illustrated herein with reference to specific embodimen~s thereo, it will be understood that thc invention is not limited to such specific embodiments but is subject to varia-tion and modification without departing from the inventiveconcepts embodied therein. For example, whereas all of the embodiments described herein have embodied magnetic barriers which are straight in the lengthwise direction, the pole pieces may be constructed, if desired, to provide a locus XB
of maximum magnetic energy gradient which lies along any line - on a simple surface. All such variations and modifications, therefore, are intended to be included within the scope of the appended claims.

. .

~, .
:
.~ . '' . ' .

.
.',. - . .
.

Claims (47)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising:
means for establishing a nonuniform magnetic field consisting essentially of, in a direction transverse to the direction of the field and in contiguous sequence, one or more series of field regions, each said series consisting of a katadynamic field region, an isodynamic field region, and an anadynamic field region, the dimension of said each isodynamic field region in said transverse direction being small relative to the dimension in said transverse direction of either the adjoining anadynamic field region or the adjoining katadynamic field region in said series, the magnetic energy gradient of said magnetic field in said transverse direction being at a maximum at the locus of at least one of said isodynamic field regions;
means for feeding a mixture of particles into the magnetic field on one side of the locus of said maximum transverse gradient in such a manner that the particles are urged by non-magnetic force in said transverse direction towards said locus of maximum transverse gradient;
said field establishing means and said mixture feeding means being arranged such that the magnitude of the transversely-acting magnetic force exerted by said maximum transverse gradient, in the direction opposite to the transverse direction of movement of the particles towards said locus, on those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected is greater than the transversely-acting nonmagnetic force urging said particles towards said locus, whereby said particles of equal or greater susceptibility are prevented by the magnetic force acting thereon from passing from said one side of the locus of maximum transverse gradient to the other side while particles having a magnetic susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by the transversely-acting nonmagnetic force; and means for collecting the particles prevented from passing from said one side of the locus of maximum transverse gradient to the other side separately from the particles which cross said locus.

2. The apparatus of claim 1 in which said transversely-acting non-magnetic force is a component of a non-magnetic force acting in a direction oblique to said transverse direction.

3. The apparatus of claim 1 wherein the field establishing means includes a pair of spaced-apart pole pieces, the opposing faces of said pole pieces being substantially symmetrical in a plane transverse to the direction of the field; and the mixture feeding including means for feeding the mixture between the pole pieces generally along the plane of symmetry of said opposing faces.

4. The apparatus of claim 1 wherein the particles to be prevented from crossing from said one side of the locus of maximum transverse gradient are paramagnetic; and the mixture feeding means includes means for feeding the mixture into the magnetic field on the side of the locus of maximum transverse gradient on which said anadynamic field is established.

5. The apparatus of claim 1 wherein the particles to be prevented from crossing from said one side of the locus of maximum transverse gradient are diamagnetic; and the mixture feeding means includes means for feeding the mixture into the magnetic field on the side of the locus of maximum transverse gradient on which said katadynamic field is established.

6. The apparatus of claim 1 wherein the field establishing means includes means for establishing super-imposed d.c. and a.c. magnetic fields, said d.c. field being effective to create said maximum transverse gradient and said a.c. magnetic field being effective to cause eddy currents in conductive particles in the mixture to induce values of magnetic susceptibility greater than said selected susceptibility, whereby the induced conductive particles are prevented by magnetic force from passing from said one side of the locus of maximum transverse gradient to the other side.

7. The apparatus of claim 1 wherein the field establishing means includes means for varying the magnitude of the maximum transverse gradient over at least a portion of the length of said locus.

8. Apparatus for separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising:
means, including a pair of spaced-apart elongate pole pieces having opposing faces, for establishing a nonuniform magnetic field, said opposing pole faces being shaped, in a plane perpendicular to the elongate direction of the pole pieces, such that the magnetic field consists primarily of, in a direction transversely of said elongate direction, in contiguous sequence, one or more series of field regions, each of said series consisting of a katadynamic field region, an isodynamic field region, and an anadynamic field region, the dimension of said each isodynamic field region in said transverse direction being small relative to the dimension in said transverse direction of either the adjoining anadynamic field region or the adjoining katadynamic field region in said region, the magnetic energy gradient of the field in said transverse direction being at a maximum at the locus of at least one of said isodynamic field regions;
means for feeding a mixture of particles between the pole pieces on one side of the locus of said maximum transverse gradient in a manner such that the particles are urged by non-magnetic force towards and lengthwise of the locus of maximum transverse gradient;
said field establishing means and said mixture feeding means being arranged such that those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected are retained on said one side of the locus of maximum transverse gradient by the transversely-acting magnetic force exerted thereon by said maximum transverse gradient, in the direction opposite to the transverse direction of movement of the particles towards such locus, and are urged lengthwise along said one side of such locus by a lengthwise-acting component of said nonmagnetic force, while particles having a magnetic susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by a transversely-acting component of the nonmagnetic force; and means for collecting the particles retained on said one side of the locus of maximum transverse gradient separately from the particles which cross said locus.

9. The apparatus of claim 8 wherein the opposing faces of said pole pieces are substantially symmetrical in said transverse cross section; and the mixture feeding includes means for feeding the mixture between the pole pieces generally along the plane of symmetry of said opposing faces.

10. The apparatus of claim 8 wherein the mixture feeding means includes means for varying the angle of approach at which the particles are urged by said non-magnetic force towards the locus of maximum transverse gradient.

11. The apparatus of claim 8 wherein the mixture feeding means includes an elongated flow channel; means for supporting the flow channel between the pole pieces in general parallel alignment therewith and with the locus of maximum transverse gradient lying within the cross section of the channel; and means for introducing the mixture into an upstream region of the flow channel on said one side of the locus of maximum transverse gradient.

12. The apparatus of claim 11 wherein the opposing faces of said pole pieces are substantially symmetrical in said transverse cross section; and said flow channel lies generally along the plane of symmetry of said opposing faces.

13. The apparatus of claim 11 wherein the particle collecting means includes a divider located in said flow channel downstream of the particle feeding region for guiding the particles deflected on said one side of the locus of maximum transverse gradient from the flow channel separately from the particles which cross said locus.

14. The apparatus of claim 11 wherein: the flow channel supporting means includes means for supporting the pole pieces and the flow channel such that the flow channel is inclined to the horizontal both in the lengthwise direction and in the transverse direction; and said one side of the locus of maximum transverse gradient is the uppermost side, whereby the mixture is urged by gravity generally towards and lengthwise of said locus.

15. The apparatus of claim 8 wherein the opposing pole faces are shaped to provide a maximum transverse gradient which varies in magnitude over at least a portion of the length of the pole pieces.

16. The apparatus of claim 15 wherein the pole faces are shaped over an upstream lengthwise region of said pole pieces to provide a substantially uniform maximum transverse gradient magnitude over said upstream region and are shaped over a down-stream lengthwise region to provide a maximum transverse gradient magnitude which decreases over said downstream region, whereby the particles deflected by the maximum transverse gradient move lengthwise of said upstream region on said one side of the locus of maximum transverse gradient and are released to cross said locus after moving downstream of said region.

17. The apparatus of claim 15 wherein: the pole faces are shaped to progressively vary the magnitude of said maximum transverse gradient from an upstream point of maximum magnitude to a downstream point of minimum magnitude; and the mixture feeding means feeds the mixture between the pole pieces adjacent said point of maximum magnitude, whereby the particles progressively cross the locus of maximum transverse gradient according to their magnetic susceptibilities in the course of downstream movement along said locus.

18. The apparatus of claim 17 wherein the particle collecting means includes means for collecting the particles which cross the locus of maximum transverse gradient in the course of downstream movement therealong into one or more separate fractions of different magnetic susceptibilities.

19. The apparatus of claim 8 wherein the spacing between the opposing pole faces varies over at least a portion of the length of the pole pieces to provide a maximum transverse gradient which varies in magnitude over at least said portion of the length of the pole pieces.

20. The apparatus of claim 8 wherein the field establishing means includes means for varying the field strength and the maximum transverse gradient established between the pole pieces.

21. The apparatus of claim 9 wherein the field establishing means includes a plurality of said pairs of elongate pole pieces arranged in side-by-side relation and means for establishing said magnetic field between each pair of pole pieces, thereby to establish a locus between each pair of pole pieces at which the local transverse magnetic energy gradient H.theta.H/.theta.X is a maximum, the magnitude of each of said maximum transverse gradients being different;
the mixture feeding means includes means for feeding the particles between the pole pieces of said pairs of side-by-side pole pieces in succession, thereby to pass the particles through a succession of maximum transverse magnetic energy gradients; and the particle collecting means includes means for separately collecting the particles deflected by each maximum transverse gradient.

22. The apparatus of claim 21 wherein the means for establishing said magnetic field beween each pair of pole pieces includes means for progressively increasing the magnitude of the local maximum transverse gradient from pole pair to pole pair in said succession, thereby to provide for the retention between succeeding pole pairs of particles of progressively lower magnetic susceptibility.

23. The apparatus of claim 8 wherein: the particles to be deflected on said one side of the locus of maximum transverse gradient are paramagnetic; and the mixture feeding means includes means for feeding the mixture between the pole pieces on the side of the locus of maximum transverse gradient on which said anadynamic field is established.

24. The apparatus of claim 8 wherein: the particles to be deflected on said one side of the locus of maximum transverse gradient are diamagnetic; and the mixture feeding means includes means for feeding the mixture between the pole pieces on the side of the locus of maximum transverse gradient on which said katadynamic field is established.

25. The apparatus of claim 8 wherein the field establishing means includes means for establishing super-imposed d.c. and a.c. magnetic fields, said d.c. magnetic field being effective to create said maximum transverse gradient and said a.c. magnetic field being effective to cause eddy currents in conductive particles in the mixture to induce values of magnetic susceptibility greater than said selected susceptibility, whereby the magnetized conductive particles are deflected by magnetic force on said one side of the locus of maximum transverse gradient.

26. The apparatus of claim 8 wherein the opposing faces of said pole pieces are substantially symmetrical in said transverse cross section; and the pole faces are coated with a corrosion resistant material of a thickness to confine the flow of the mixture to within the vicinity of the plane of symmetry of said pole faces.

27. A method of separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising:
establishing a nonuniform magnetic field having in a direction transverse to the direction of the field, in contiguous sequence, a katadynamic field region, an isodynamic field region, and an anadynamic field region, whereby the magnetic energy gradient H.theta.H/.theta.X in said transverse direction is at a maximum at the locus of said isodynamic field region and decreases therefrom on either side thereof;
feeding a mixture of particles into the magnetic field on one side of the locus of maximum transverse gradient in a manner such that the particles are urged by non-magnetic force in said transverse direction towards the locus of maximum transverse gradient, the magnitude of the maximum transverse gradient being such that particles having a magnetic susceptibility to the field equal to or greater than a selected susceptibility at which a separation is to be effected are prevented by the transverse magnetic force acting thereon from passing from said one side of the locus of maximum transverse gradient to the other side while particles having a magnetic susceptibility to the field less than said selected susceptibility are urged across the locus of maximum transverse gradient by the non-magnetic force; and collecting the particles which do not cross said locus of maximum transverse gradient separately from the particles which cross said locus.

28. The method of claim 27 wherein the magnetic field is established such that in a transversely extending region thereof the magnitude of the magnetic energy gradient in the direction of the field is small relative to the magnitude of said maximum transverse gradient; and the mixture is fed into the magnetic field within said region where the magnetic energy gradient in the field direction is relatively small.

29. The method of claim 27 wherein the particles to be prevented from crossing from said one side of the locus of maximum transverse gradient to the other side are paramagnetic;
and said one side of the locus of maximum transverse gradient on which the mixture is fed is the side on which said anadynamic field is established.

30. The method of claim 27 wherein the particles to be prevented from crossing from said one side of the locus of maximum transverse gradient to the other side are diamagnetic;
and said one side of the locus of maximum transverse gradient on which the mixture is fed is the side on which said katadynamic field is established.

31. The method of claim 27 wherein the mixture is fed into the magnetic field in a manner such that the particles are urged by said non-magnetic force both towards and lengthwise of the locus of maximum transverse gradient, whereby particles deflected on said one side of said locus move therealong lengthwise of said locus.

32. The method of claim 31 further comprising varying the magnitude of the maximum transverse gradient over at least a portion of the length of said locus.

33. The method of claim 32 wherein the magnitude of the maximum transverse gradient is at a maximum value and sub-stantially uniform over an upstream lengthwise region of said locus and decreases from said maximum value downstream thereof, whereby the particles deflected by the maximum transverse gradient move lengthwise of said upstream region on said one side of the locus of maximum transverse gradient and are released to cross said locus according to their magnetic susceptibilities after moving downstream of said region.

34. The method of claim 32 wherein said variation in the magnitude of the maximum transverse gradient along said locus is progressive from an upstream point of maximum magnitude to a downstream point of minimum magnitude; and the mixture is fed into the magnetic field adjacent said point of maximum magnitude, whereby the particles progressively cross the locus of maximum transverse gradient according to their magnetic susceptibilities in the course of downstream movement along said locus.

35. The method of claim 34 wherein the particles which cross the locus of maximum transverse gradient in the course of downstream movement therealong are collected into a plurality of fractions of different magnetic susceptibilities.

36. The method of claim 27 further comprising establishing a plurality of said magnetic fields in side-by-side relation, each of said fields having a locus at which the transverse magnetic energy gradient H.theta.H/.theta.X thereof is at a maximum, the magnitude of each said maximum transverse gradient being different, thereby to provide a succession of maximum transverse gradients;
feeding the mixture through said succession of maximum transverse gradients; and separately collecting the particles deflected by each maximum transverse gradient.

37. The method of claim 36 wherein the magnitude of the maximum transverse gradient progressively increases from field to field in said succession, thereby to provide for the deflection by succeeding maximum transverse gradients of particles of progressively lower magnetic susceptibility.

38. The method of claim 27 wherein the field establishing step comprises establishing superimposed a.c. and d.c. magnetic fields, said d.c. field being effective to create said maximum transverse gradient and said a.c. field being effective to cause eddy currents in conductive particles in the mixture to induce values of magnetic susceptibility greater than said selected susceptibility, whereby the magnetized conductive particles will be deflected on said one side of the locus of maximum transverse gradient.

39. The method of claim 27 wherein the mixture feeding step includes dispersing the mixture in a fluid and feeding the fluid-liquid dispersion into the magnetic field, whereby the particles are separated along the locus of maximum transverse gradient substantially in accordance with the relative value of where KP is the magnetic susceptibility of the particles, KF is the magnetic susceptibility of the fluid, and dP is the density of the particles, and dF is the density of the fluid.

40. The method of claim 39 wherein the magnetic susceptibility of the fluid is very much greater than that of any particle contained in the mixture, whereby the particles are separated substantially in accordance with the relative value of dP.

41. The apparatus of claim 1 wherein: the field establishing means includes means for defining within the magnetic field a transversely extending region in which the magnitude of the magnetic energy gradient in the direction of the field is small relative to the magnitude of said maximum transverse gradient; and the mixture feeding means includes means for feeding the mixture into the magnetic field generally within said region where the magnetic energy gradient in the field direction is relatively small.

42. The method of claim 46 wherein said magnetic field consists essentially of, in said transverse direction, one or more series of field regions, each of said series consisting of katadynamic, isodynamic and anadynamic field regions, the dimension of each isodynamic field region in said transverse direction being small relative to the dimension in said transverse direction of either the adjoining anadynamic field region or the adjoining katadynamic field region in said series.

43. Apparatus for separating a flowable mixture of particles in accordance with the magnetic susceptibilities of the particles, comprising:
means, including a pair of spaced-apart elongate pole pieces having opposing faces, for establishing a nonuniform magnetic field, said opposing pole faces being shaped, in a plane transverse to the elongate direction of the pole pieces, so as to produce therebetween, in a direction transverse to said elongate direction and in contiguous transverse sequence, a katadynamic field region, an isodynamic field region, and an anadynamic field region, whereby the magnetic energy gradient in said transverse direction is at a maximum at the locus of said isodynamic field region and decreases therefrom on either side thereof;
an elongate flow channel located between the pole pieces in general parallel alignment therewith and with the locus of maximum transverse gradient lying within the cross section of the channel;
means for supporting the pole pieces and the flow channel such that the flow channel is inclined to the horizontal both in the lengthwise direction and in the transverse direction;

means for feeding a mixture of particles into an upstream region of the flow channel on the uppermost side of the locus of maximum transverse gradient, whereby the particles are urged by gravity towards and lengthwise of said locus;
said field establishing means and said supporting means being arranged such that those particles having a magnetic susceptibility equal to or greater than a selected susceptibility at which a separation is to be effected are retained on said uppermost side of the locus of maximum transverse gradient by the transversely-acting magnetic force exerted thereon by said maximum transverse gradient and are urged lengthwise along said uppermost side of such locus by a lengthwise-acting gravitational component, while particles having a magnetic susceptibility less than said selected susceptibility are urged across the locus of maximum transverse gradient by a tranvsersely-acting gravitational component; and means for collecting the particles retained on said uppermost side of the locus of maximum transverse gradient separately from the particles which cross said locus.

44. The apparatus of claim 43 wherein the opposing faces of said pole pieces are substantially symmetrical in said transverse plane; and said supporting means support said flow channel generally in the plane of symmetry of said opposing faces.

45. The apparatus of claim 43 wherein the pole faces are shaped over an upstream lengthwise region of said pole pieces to provide a substantially uniform maximum transverse gradient magnitude over said upstream region and are shaped over a downstream lengthwise region to provide a maximum transverse gradient magnitude which decreases over said downstream region, whereby the particles retained on said uppermost side by the maximum transverse gradient move lengthwise of said upstream region on said uppermost side of the locus of maximum transverse gradient and are released to cross said locus after moving downstream of said region.

46. The apparatus of claim 43 further comprising:
a plurality of said pairs of elongate pole pieces arranged in side-by-side relation;
means for establishing said magnetic field between each pair of pole pieces, thereby to establish a locus trans-versely of each pair of pole pieces at which the local tranvserse magnetic energy gradient is a maximum, the magnitude of each local maximum transverse gradient being different; and wherein said elongate flow channel extends between the pole pieces of all of said pole-piece pairs in general parallel alignment with the poles thereof and with the loci of all of said local maximum transverse gradients lying within the cross section of said flow channel;
said supporting means supports all of said pole piece pairs and said flow channel such that the flow channel is inclined to the horizontal both in the lengthwise direction and in the transverse direction;
said mixture feeding means includes means for feeding the particles into an upstream region of the flow channel on the uppermost side of the uppermost of said loci of maximum transverse gradients, whereby the particles are urged by gravity through said loci of maximum transverse gradients in succession; and the particle collecting means includes means for separately collecting the particles deflected by each maximum transverse gradient.

47. The apparatus of claim 46 wherein the magnitudes of the maximum transverse gradients progressively increase from pole pair to pole pair in said succession, thereby to provide for the retention between succeeding pole pairs of particles of progressively lower magnetic susceptibility.