US3898156A

US3898156A - Hyperbolic magnet poles for sink-float separators

Info

Publication number: US3898156A
Application number: US454373A
Authority: US
Inventors: Robert Kaiser; Leon Mir
Original assignee: Avco Corp
Current assignee: Avco Corp
Priority date: 1974-03-25
Filing date: 1974-03-25
Publication date: 1975-08-05
Anticipated expiration: 1992-08-05
Also published as: DE2509959A1; BE827098A; NL7503359A; GB1504968A; IT1034590B; CH595140A5; FR2265458A1; JPS50130061A; FR2265458B3

Abstract

A hyperbolic magnet with pole which produces a constant magnetic gradient along an axis, usually a vertical axis, in a sink-float separator. Magnetic pieces are secured at the lateral edges of the poles to modify the magnetic field to prevent material being separated from being pinned to the outer walls of the separator and a mirror plate is adjustably separated from the end of the poles for tuning the magnetic field.

Description

United States Patent Kaiser et al. Aug. 5, 1975 [5 1 HYPERBOLIC MAGNET POLES FOR 2.768.746 10/1956 COlbUlTl 209/223 R SINK FLOAT SEPARATORS 3,289,836 12/1966 Weston 209/222 X 3.483.969 12/1969 Rosensweig 209/1 [7 In n r Robert Kaiser. m g Leon 3.733.465 1/1974 Reimers et a1 209/1 Mir, Brookline, both of Mass.

[73] Assignee: Avco Corporation, Cincinnati, Ohio [22] Filed: Mar. 25, 1974 [21] App]. No: 454,373

[52] US. Cl 209/1; 209/1725 [51] Int. Cl. B03B 5/00 {58] Field of Search 209/1, 172.5, 223 R, 232, 209/1118; 310/10 [56] References Cited UNITED STATES PATENTS 1,956,760 5/1934 Forrer 209/223 R 2,088,364 7/1937 Ellis et a1. 209/232 X 2,154,010 4/1939 Queneau 209/232 X Primary ExaminerFrank W. Lutter Assistant Examiner-Ralph J. Hill Attorney, Agent, or Firm-Charles M. Hogan, Esq.; Abraham Ogman, Esq.

[5 7] ABSTRACT A hyperbolic magnet with pole which produces a constant magnetic gradient along an axis, usually a vertical axis, in a sink-float separator. Magnetic pieces are secured at the lateral edges of the poles to modify the magnetic field to prevent material being separated from being pinned to the outer walls of the separator and a mirror plate is adjustably separated from the end of the poles for tuning the magnetic field.

8 Claims, 7 Drawing Figures PATENTEU WE SHEET PATENTEU AUG 51975 SHEET HYPERBOLIC MAGNET POLES FOR SINK-FLOAT SEPARATORS BACKGROUND OF THE INVENTION l. Field of the Invention The present invention relates generally to sink-float separators and more particulary to a new magnetic pole design for sink-float separators.

2. Description of the Prior Art In the sink-float separators, it has been found common to use magnetic fields to cause the separation of objects of different densities in a ferrofluid. The magnetic field produced in the prior art has not been suffcient to provide accurate separation of objects whose densities are very close. Similarly, the magnetic fields of the prior art have not been sufficiently adjustable so as to fine tune the system to guarantee a constant apparent density in a ferrofluid, hereinafter called magnetic density.

A problem that arises in sink-float tank separators of the prior art is that the magnetic field is such that it results in the pinning of the materials which are to be separated against the outer walls of the separator, thereby jamming the free-flow in the vertical direction of the objects to be separated.

Also, prior art devices which produce acceptable magnetic fields use large amounts of electric power.

SUMMARY OF THE INVENTION The present invention is a unique design of a magnet with hyperbolic poles and a mirror plate for sink-float separators to solve the problems of the prior art devices. The rectangular hyperbolic magnetic pole pieces and the mirror plate are designed to produce a constant vertical magnetic gradient in the magnet gap. The pole pieces are straight-line approximations of a portion of a hyperbola, said portions asymptotes being parallel to the direction of separation or gravity. Magnetic pieces, or shims, are secured to the lateral edges of the poles to locally modify the magnetic field to prevent the material being separated from being pinned to the outer walls of the separator by the force produced by the magnetic field of the poles. The magnetic plate is adjustably separated from the ends of the poles to fine tune the magnetic field produced by the hyperbolic poles to provide a constant magnetic density throughout the ferrofluid.

OBJECTS OF THE INVENTION An object of the present invention is to provide a magnetic pole design with the lower power requirements than the prior art devices.

Another object of the present invention is to provide a magnetic pole designed for use in the float-sink separator which provides a uniform vertical magnetic field gradient.

A further object of the present invention is to provide a magnetic pole design which is fine-tunable so as to achieve a uniform magnetic density across in a ferrofluid volume.

Still another object of the present invention is the modification of the magnetic field so as to prevent objects being separated from being pinned against the outer walls of the separator.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a hyperbolic curve from which the preferred embodiment of the magnetic poles were designed;

FIG. 2 is a front view of a float-sink separator with the preferred hyperbolic poles;

FIG. 3 is a front view of the gap defined by the hyperbolic poles and mirror plate of FIG. 2;

FIG. 4 is a view of the hyperbolic poles and mirror plate of FIG. 2 taken along Line 44 of FIG. 3;

FIG. 5 is a top view of the preferred embodiment shown in FIG. 3, showing a modification of the shims;

FIG. 6 is a graph of the magnetic field H along the Z axis; and

FIG. 7 is a graph of magnetic parameters as a function of vertical height before and after fine tuning using the adjustments of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The non-magnetic sink-float separator of the present invention is based on an unusual property of ferrofluids the ability to float a non-magnetic or weakly magnetic object of far greater density than the ferrofluid itself when the ferrofluid is placed in a suitable magnetic field.

Ferrofluids are very stable colloids of small (about A) magnetic particles. generally magnetite, suspended in a base liquid and stabilized by surface active agents. For sink-float separation, the base liquid could be, for example, a high flash point kerosene. The suspended particles do not settle out or agglomerate under the action of gravity or applied magnetic fields. A ferrofluid placed in a non-uniform magnetic field experiences a force directed along the magnetic field gradient. A consequence of this is that a non-magnetic object immersed in a ferrofluid experiences a force in the opposite sense and is expelled to the region of mini mum magnetic field.

In a ferrofluid separator, a body of ferrofluid is held between the poles of an electromagnet which generates a magnetic field with a constant gradient; directed downward, in the direction of gravity. Consequently, a non-magnetic object immersed in the ferrofluid pool experiences a magnetic force in the upward direction. By regulating the strength of the magnetic field gradiem and the strength of the ferrofluid, this magnetic force can be made larger or smaller than the force of gravity on the non-magnetic object. When the magnetic force is larger than the force of gravity, the object will float even though its density is larger than the density of the ferrofluid. When the magnetic force is less than the force of gravity, the object sinks.

According to the theory of ferrofluid levitation, in a suitable magnetic field with a gradient parallel to gravity, a ferrofluid can be viewed as a liquid that has a controllable apparent density:

M H p a p F 4 m where p a apparent density of the ferrofluid', g/cm p F physical density of ferrofluid; g/cn't M(H) magnetic dipole moment of ferrofluid. emu;

a function of the field strength, H

1 vertical gradient of magnetic field; oersted/cm 3 acceleration of gravity; cm/sec" The apparent density can be changed by changing which is done by changing the current to the coil of the electromagnet. With existing fcrrofluids and state of the art electromagnet design, it is possible to obtain apparent densities that range from about I g/cm to over 25 g/cm". This concept is more fully discussed in US. Pat. Nos. 3,483,968; 3,483,969 and 3,488.531, which are commonly assigned to the assignee hereof and are incorporated herein by reference.

In order to obtain accurate separations, it is necessary for the apparent density of the ferrofluid to be constant (within the accuracy desired) throughout the ferrofluid volume. Otherwise, the less dense objects might fall to the bottom in a region of an abnormally low apparent density and the more dense objects float in a region of abnormally high apparent density. This is accomplished by ensuring that the field throughout the separator is high enough to bring the ferrofluid magnetic dipole moment M close to its saturation value and that the vertical magnetic field gradientv" is essentially constant.

An additional constraint on the desired magnetic field is that the horizontal gradients (V x and z) be very small, or that they be outwardly directed. This constraint is required to prevent the levitated objects from moving toward the bounding walls of the separator as they rise or fall. Were this to occur, the objects would be pinned against these walls and jamming would result. By designing the magnet to provide outwardly pointing gradients, the particles move away from the walls to the center of the ferrofluid pool.

Although it is possible to achieve such a design by the use of Helmholtz coils, a design based on properly shaped iron pole pieces is more practical because of its very much lower power requirements. The basic shape of such pole pieces is a rectangular hyperbolic surface extending indefinitely in the +2 and z direction. A mirror plane plate is provided above the pole pieces to create a virtual image of the pole pieces. The pole is a straight line approximation of the solid segment of the hyperbola shown in FIG. 1.

For this configuration, the value of the vertical component of the magnetic field gradient is given by:

G, the value of the gradient along the y axis is controlled by the distance between the poles and the magnetizing current. By limiting the ferrofluid to region near the y axis, where the value of .r/y is small, the value of 8 H/lS y is kept as constant as the separation to be made dictates.

in translating this theoretical design to the hardware stage, the upper half of the hyperbolic surface (not shown) is replaced by a steel plate coincident with the x axis, as shown in FIG. 1. This plate forces the field lines to be perpendicular to the x axis, and is therefore functionally equivalent below the x axis to the upper hyperbolic surface. Three deviations to the shape of the lower hyperbolic surface are also introduce:

1. Since the gradient drops sharply for the large values of .r. the region between the x axis and the portion of the curve near this axis is not used for separation. The hyperbolic surface is therefore terminated at some finite value of .r.

2. The hyperbolic surface is also terminated at a finite value ofy, because for large values of y. the interpole distance becomes impractically small.

3. Likewise one cannot extend the poles indefinitely along the z axis.

The above cut-offs from the ideal hyperbolic shape necessitate corrections to the shape of the pole piece in order to achieve the two basic goals of the design given previously. The pole pieces shown in FIGS. 2, 3 and 4 were designed to include the necessary correction and achieve the aforementioned goals.

The preferred embodiment as shown in FIGS. 2, 3 and 4 includes the separator 10, having included therein two

hyperbolic segments

12 and 14. Each of the

hyperbolic pole pieces

12 and 14 may be made up of a plurality of plane segments which approximate a portion of the hyperbolic surface as shown in FIG. 1. Similarly, the pole pieces may be cast as a unitary piece and the face machined to form the plane segments.

The

hyperbolic pole pieces

12 and 14 are supported and secured to a yoke 16. Lying perpendicular to the y'axis on support blocks 18 is the mirror plate 20 and mirror plate support 22 discussed above. Though the mirror plate support block 18 is shown as blocks, they may be replaced by any device which will adjustably mount the mirror plate relative to the

pole pieces

12 and 14 so as to fine tune the system. As will be discussed later, a system may be fine tuned so as to find the perfect distance from the mirror plate to the

pole pieces

12 and 14.

Secured to both ends of the

magnetic pole pieces

12 and 14 are a plurality of magnetic pieces or shims 24. Magnetic pieces 24 are used to provide a magnetic field distribution as shown in FIG. 6. The shims increase the magnetic field at the marginal edges of the pole pieces to generate the hill-like perturbations 25 at the edge of the pole pieces. The perturbation is provided to assist the material being separated to flow towards the center of the magnetic field. This action prevents the force produced by the magnetic field from pinning the mate rial against the container wall.

The magnetic field gradients found in the perturbations are generally in the direction shown by

arrows

27 and 29. Materials to be separated which gravitates to a boundary 31 of the working volume, encounters the magnetic field gradient depicted by arrow 27. This material is forced back away from the boundary 31 by the outwardly directed lateral gradient 27 since, as discussed previously, the material tends to move toward regions of minimum magnetic field.

There is no necessity to modify the magnetic field along the .r axis as the pole pieces are designed to provide the dish-like configuration along the x axis as shown for the z axis of FIG. 6. If modification of the magnetic field along the x axis is desired, as is provided along the z axis, the modification of the shims to provide such a field modification is shown in FIG. 5 as 24' as being tapered in two planes. As can be seen clearly in FIG. 3, the shims 24 are rectangular and have a generally tapered configuration in one plane which may vary between /z to 7, dependent upon the size and structure of the magnetic pole pieces.

The working volume" marked in FIG. 3 is the region where the vertical component of the gradient should be constant within This region is obtained by having the ideal hyperbolic shape of the poles approximated by the straight line cords that intersect on the hyperbola and the mirror plate. The tapered shims along the edges of the poles are for the purpose of modifying the magnetic field and controlling the horizontal gradient.

In summary, without the shims, an inwardly directed gradient is produced by the pole piece. This gradient, along the z axis, exerts a force that will push the materials in the ferrofluid towards the front and rear walls of the separator. The shims modify the field to produce the field as shown in FIG. 6. An outwardly pointed 2 component of the magnetic field gradient prevents the particles which are being separated from being pinned to the containers outer wall. The objects to be separated move along the y axis which, as can be seen in FIG. 1, is one of the asymptotes of the hyperbola, of which the magnetic poles are sections.

As shown in FIG. 2, two

coils

26 and 28 are wrapped around the yoke 16 behind

pole pieces

12 and 14, respectively. It is to be understood that the magnetic field is of sufficient strength to support the ferrofluid therebetween. A container 30, shown in FIG. 2, is included to restrict the ferrofluid to the working volume and provide the front and rear walls of the separation. When the top and bottom of box 30 are deleted, there is easy horizontal access to the pool of ferrofluid. Since magnetic forces also retain the ferrofluid in the gap of the magnet, conveyors or other means of introducing the feed or removing the separated products can be introduced directly into the ferrofluid pool without fluid leakage or sealing problems from front or back.

It is generally well known to use a magnetic mirror plate to generate a virtual mirror image of the field distribution. The mirror plate of the present invention is used, however, to fine tune the present system. As illustrated in FIG. 7, the mirror plate 20 is located in an optimum position to provide a constant gradient (Curve 33), and the magnetic moment M, the field intensity H, and the density p depicted as

Curves

31, 32 and 37, respectively. As depicted in Curve 37, the density p is slightly less at the top of the magnetic pole than it is at the bottom.

To effectively separate objects having very close densities, it is imperative that the density p be constant. To achieve this, the magnetic plate 20 may be varied to fine tune locally the system so as to achieve a constant p. The height of the mirror plate 20 is determined by the height of the support blocks 18. When the mirror plate is lowered slightly, there is a large change in the field intensity H, a slight decrease in the magnetic moment M, and a relatively large increase in the vertical gradient in the vicinity of the mirror plate 20. If the mirror plate is lowered the proper amount, will increase in the vicinity of the mirror plate to provide a constant p throughout the working volume. The

6 parameters H. M. Wand p. as a function of height Y. after the adjustment. are represented as H. M. and p and are depicted by curves 35. 36. 34 and 38. respectively.

To summarize briefly, the hyperbolic magnetic design as depicted in FIGS. 2, 3 and 4 provide a constant p. The mirror plate 20 is adjustable to increase in the vicinity of the mirror plate to a greater extent than M is decreased, so that a constant p can be realized.

The magnetic field between the poles, a nominal 4" separator, were mapped by means of a Hall probe after energizing coils 26 using 10 amps. The results of such mapping is shown in Table I attached. It should be noted that the coordinate system x, y, and z differs as shown in FIGS. 2, 3 and 4 from the coordinate system shown in FIG. I because the x, z axes are offset along the y axis.

It is apparent that along the axis of the system (x' O, z 0), the gradient is very constant from y' 0.8 inches to y' 3.0 inches. Near the comers of the working volume (x 1.0, z 1.0), the gradient near y 3.0 is about 8% lower than the axis value. At intermediate points, the deviations are correspondingly lower. This pole shape therefore meets the first objective of the design, a nearly constant vertical gradient over a substantial portion of the interpole volume.

The data in Table I shows that the .i" component of the gradient points away from the center. towards the poles of the magnet. Within the controlled working volume, the z Component of the gradient is very small, less than 10% of the value of the y component. Near the inner portion, it points to the axis but near the outer portion, it points away from the axis of the interpole volume. Thus, the second objective of the design is also met by this pole geometry.

Scaling up this design can be carried out directly without any change in pole shape, while still maintaining the desired field characteristics. It is, of course, necessary to provide the appropriate increases in the ampere turns of magnetizing current, in order to achieve the same gradient values.

The present design can separate any two non-ferrous materials whose density differs by 5 to 10% or more. Apparent specific gravity of the ferrofluid can bc varied, from less than 1 g/cm to over 20 g/cm by a 5 mph: change in electric current flow through the coils of an energizing electromagnet. This range include from magnesium and aluminum (around 2-3 g/cm); through zinc, tin, brass, copper and lead (7-1 1 g/cm); up to gold and platinum 1922 g/cm). Some magnetic materials may also be separated as long as they are less magnetic than the ferrofluid used.

The separation of non-magnetic metals was carried out in a ferrofluid pool contained between the poles of this magnet. In all cases, the apparent ferrofluid density required to float an object was equal, within experimental error, to the objects known density, as the following table demonstrates:

TABLE I FIELD MAPPING OF HY PERBOLIC POLES Position of Probe Position of Probe Position of Probe Position of Probe Position of Probe x'=0.0", z'=0.0" x=0.8, z'=0.0 x'=-.8", z'=l .0 x'=l.0, z'=.0 x=l.0", z'=l.0 v I H ,1 H H H (inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss) TABLE I- Continued FIELD MAPPING OF HYPERBOLIC POLES Position of Probe Position of Probe x'=0.0", z'=0.0" x=0.8. z'=().0 x'=-.8". z=1.0 x'=l.0 z'=.0 x'=l.0", z=l.() H H H H (inches) (k. gauss) (inches) (k. gauss) (inches) (k. gauss) (inches) (1:. gauss) (Inches) (k. gauss) Apparent Ferrofluid it is possible to accurately separate materials differing Dens'w Densny in density by as little as the separation being es- Brass 8.3 8.4 sentially independent of the size or shape of the ob- Copper 8.9 8.7 jects Aluminum 2.7 2.8 zinc 7 7 Although the invention has been described and Illus- Titanium 4.5 4.6

The separation of mixtures of brass and copper occurred when the apparent ferrofluid density was adjusted to values intermediate between 8.4 and 8.7. This was obtained with ferrofluids varying in saturationmagnetization from 100 gauss to 240 gauss.

The especially designed hyperbolic magnetic poles for use in a sink-float separator achieves the desired design requirements and utilizes low power requirements.- The design provides a constant vertical magnetic gradient and includes the adjustment of the mirror plate as defined in the system to provide a constant density along the vertical axis. Shim pieces are provided at the edges of the magnetic poles to modify the magnetic field so as to prevent the objects which are being separated from being pinned against the outer walls of the container.

The pole configuration of the present invention can be used in the process described in the previously mentioned patents. The solid mixture to be separated is introduced into the pool of ferrofluids suspended between the poles by magnetization. Objects less dense than the apparent density of the ferrofluid float to the top, while those more dense sink to the bottom of the ferrofluid pool, resulting in a physical separation. An upper and lower conveyor will remove the more dense and less dense materials. respectively, from the top and bottom of the pool. Since magnetic forces also retain the ferrofluid in the gap of the magnet, conveyors can be introduced directly into the pool without fluid leakage or sealing problems.

The separation can be carried out on a batch or continuous basis. Mixtures of three or more components can be separated by making two or more passes through the separator during which the apparent density of the ferrofluid is adjusted to produce one pure component per pass. Such mixtures could also be separated by a sequence of separators operating at appropriate apparent density levels, with partly separated mixtures being conveyed from one separator to the next until all separations are achieved. By such means,

trated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the invention being limited only by the terms of the appended claims. What is claimed is: l. [n a ferrofluid separator having a magnet including a pair of spaced pole pieces defining an air gap containing a magnetic field and a pool of ferrofluid disposed in said air gap and magnetic field, the improvement comprising:

pole pieces that are a' mirror image of each other with respect to an axis, and each of said pole pieces is a segment of a hypobolic surface; and

a mirror plate means disposed above and spaced from said pole pieces and air gap for creating a virtual image of said pole pieces.

2. The separator as in claim 1 wherein each pole is formed by a plurality of pole pieces. each being a straight line approximation of a portion of said hyperbolic segment.

3. The separator as in claim 2 including means secured to said poles for preventing the materials to be separated from being pinned to the lateral walls of the ferrofluid pool.

4. The separator as in claim 3 wherein said means includes magnetic pieces secured to the lateral edges of said poles.

5. The separator as in claim 4 wherein said magnetic pieces are tapered shims with a taper in one plane.

6. The separator as in claim 4 wherein said magnetic pieces are tapered shims with a taper in two planes.

7. The separator as in claim 1 to include means for maintaining a constant apparent density along said axis which includes means for adjusting the magnetic field intensity adjacent to said mirror plate.

8. The separator as in claim 7 wherein said maintaining means includes said magentic plate, whose plane is perpendicular to said axis and said maintaining means being adjustable along said axis for varying the apparent density distribution of the magnetic field.

1. I II UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 898,156 DATED g t 5, 1975 INVENT()R(S) 1 Robert Kaiser, Leon Mir It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown betow:

Column 3, line 6, please insert-- before "Which.

Column 3, line 68, please change "introduce" to "introduced".

Column 7, line 50, please change t0 Signed and Scaled this twenty-first Day Of October 1975 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (umnu'ssz'uner uj'larenrs and Trademarks

Claims

1. IN A FERROFLUID SEPARATOR HAVING A MAGNET INCLUDING A PAIR OF SPACED POLE PIECES DEFINING AN AIR GAP CONTAINING A MAGNETIC FIELD AND A POOL OF FERROFLUID DISPOSED IN SAID AIR GAP AND MAGNETIC FIELD, THE IMPROVEMENT COMPRISING: POLE PIECES THAT ARE A MIRROR IMAGE OF EACH OTHER WITH RESPECT TO AN AXIS, AND EACH OF SAID POLE PIECES IS SEGMENT OF A HYPOBOLIC SURFACE, AND A MIRROR PLATE MEANS DISPOSED ABOVE AND SPACED FROM SAID POLE PIECES AND AIR GAP FOR CREATING A VIRTUAL IMAGE OF SAID POLE PIECES.

2. The separator as in claim 1 wherein each pole is formed by a plurality of pole pieces, each being a straight line approximation of a portion of said hyperbolic segment.