U.S. patent number 3,627,678 [Application Number 04/854,895] was granted by the patent office on 1971-12-14 for magnetic separator and magnetic separation method.
This patent grant is currently assigned to Magnetic Engineering Associates, Inc.. Invention is credited to Laszlo Miklos Lontai, Peter Grant Marston, John Joseph Nolan.
United States Patent |
3,627,678 |
Marston , et al. |
December 14, 1971 |
MAGNETIC SEPARATOR AND MAGNETIC SEPARATION METHOD
Abstract
A magnetic separation method and magnetic separator is disclosed
including an enclosure within an electromagnetic coil surrounded by
a ferromagnetic return frame including a first portion adjacent one
side of the coil and covering the area enclosed by the coil and a
second portion adjacent the other side of the coil and covering the
area enclosed by the coil, and inlet and outlet means in the return
frame for introducing and removing fluid from the enclosure.
Inventors: |
Marston; Peter Grant (Glouster,
MA), Nolan; John Joseph (Randolph, MA), Lontai; Laszlo
Miklos (South Bend, IN) |
Assignee: |
Magnetic Engineering Associates,
Inc. (Cambridge, MA)
|
Family
ID: |
25319809 |
Appl.
No.: |
04/854,895 |
Filed: |
September 3, 1969 |
Current U.S.
Class: |
210/695; 210/222;
209/214; 210/427 |
Current CPC
Class: |
B03C
1/025 (20130101); B01D 35/06 (20130101) |
Current International
Class: |
B03C
1/025 (20060101); B01D 35/06 (20060101); B03C
1/02 (20060101); B01d 035/06 () |
Field of
Search: |
;210/42,222,223,436,456
;209/212,223R,213,221,224 ;335/297,296,216 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Scientific American, Vol. 212, No. 4, April, 1965 page 72.
|
Primary Examiner: Friedman; Reuben
Assistant Examiner: Granger; T. A.
Claims
What is claimed is:
1. A magnetic separator including an electromagnetic coil, a
ferromagnetic return frame proximate to and covering said coil, an
enclosure within said coil, a magnetic matrix within said
enclosure, said ferromagnetic return frame including a first
low-reluctance magnetic pole section arranged transverse to the
direction of fluid flow and adjacent and covering a first end of
said enclosure for concentrating at said first end the magnetic
field produced by said coil and a second low-reluctance magnetic
pole section arranged transverse to the direction of fluid flow and
adjacent and covering the second end of said enclosure for
concentrating at said second end the magnetic field produced by
said coil to produce a concentrated magnetic field uniform and
parallel to the direction of fluid flow between said pole sections
in said magnetic matrix, inlet means including at least one input
channel in said first pole section to provide fluid flow through
said first pole section into said magnetic matrix parallel to the
direction of the magnetic field in said magnetic matrix and outlet
means including at least one outlet channel in said second pole
section to provide fluid flow through said second pole section from
said magnetic matrix.
2. The separator of claim 1 in which said return frame includes a
shell extending closely adjacent about the external periphery of
said coil and interconnecting said first and second pole
sections.
3. The separator of claim 1 in which said inlet means includes a
first recess that increases in cross section area towards said
enclosure and a first ferromagnetic plug in said first recess
spaced from the surface of said first recess to form a first
peripheral channel and increase the ferromagnetic mass of said
first pole section.
4. The separator of claim 3 in which said outlet means includes a
second recess that increases in cross section area towards said
enclosure and a second ferromagnetic plug in said second recess
spaced from the surface of said second recess to form a second
peripheral channel and increase the ferromagnetic mass of said
second pole section.
5. The separator of claim 4 in which approximately one-half of the
cross-sectional area of said matrix is within the area encompassed
by said peripheral channels.
6. The separator of claim 1 including a flow system comprising
means for providing raw material to be processed to said inlet
means, means for delivering less-magnetic particles from said
outlet means to a first output channel, means for providing a wash
fluid to said enclosure to rinse out middlings, means for
delivering said middlings to a second output channel, means for
providing a wash fluid to said enclosure to flush out the
more-magnetic particles, and means for delivering said
more-magnetic particles to a third output channel.
7. The separator of claim 6 further including control means for
sequencing operations of said flow system.
8. The separator of claim 1 in which said coil includes a plurality
of individual segments each having a separate input and output
terminal accessible external to said coil.
9. The separator of claim 7 in which each said segment is made of a
conductor having a longitudinal cooling conduit.
10. The separator of claim 8 in which said input and output
terminals include electrical connection means and fluid connection
means for injecting coolant into and receiving coolant from said
longitudinal cooling conduits.
11. The separator of claim 1 in which said coil includes a
plurality of regions subject to different magnetic field strengths,
each of said regions containing conductors of a different
material.
12. The separator of claim 11 in which said coil is a
superconducting coil.
13. The separator of claim 12 in which one of said materials is a
niobium tin alloy, and another of said materials is a niobium
titanium alloy.
14. The separator of claim 1 in which said coil is a
superconducting coil.
15. The separator of claim 1 in which said coil is a cryogenic
coil.
16. The separator of claim 14 further including refrigeration
apparatus surrounding said coil including a vacuum vessel, a
coolant vessel with said vacuum vessel, and a boil-off outlet pipe
making said coolant vessel accessible to an external source of
refrigerant.
17. The separator of claim 16 in which said refrigeration apparatus
further includes a radiation shield between said vacuum vessel and
coolant vessel.
18. The separator of claim 17 in which said refrigeration apparatus
further includes cooling coils, a portion of which are proximate
said boil-off outlet pipe and a portion of which are proximate said
radiation shield for conducting heat from said shield to said
boil-off outlet pipe to maintain said shield at a temperature
intermediate said ambient temperature and the temperature of said
superconducting coil.
19. The separator of claim 14 in which said superconductors are
thermally stabilized by intimate thermal and electrical contact
with a high conductivity normal conductor, the composite thereof
being suitably cooled.
20. The separator of claim 19 in which said superconductors are
positionally transposed along the direction of the current
flow.
21. A magnetic separator comprising:
a superconducting coil;
a ferromagnetic return frame proximate said coil including a first
portion adjacent one side of said coil and covering the area
enclosed by said coil and a second portion adjacent the other side
of said coil and covering the area enclosed by said coil;
an enclosure within said coil and return frame and within the
magnetic field produced by said coil;
inlet and outlet means in said return frame for introducing and
removing, respectively, fluid from said enclosure;
refrigeration apparatus surrounding said coil including a vacuum
vessel, a coolant vessel with said vacuum vessel;
a boil-off outlet pipe making said coolant vessel accessible to an
external source of refrigerant;
a radiation shield between said vacuum vessel and coolant vessel;
and
cooling coils, a portion of which are proximate said boil-off
outlet pipe and a portion of which are proximate said radiation
shield for conducting heat from said shield to said boil-off outlet
pipe to maintain said shield at a temperature intermediate said
ambient temperature and the temperature of said superconducting
coil.
22. A method of magnetically separating materials of different
magnetic susceptibility comprising: energizing an electromagnetic
coil to produce a magnetic field; directing the magnetic field
through the center of said coil through an enclosure within the
coil and a magnetic matrix within the enclosure; concentrating the
magnetic field in first and second low reluctance magnetic pole
sections arranged transverse to the direction of fluid flow through
said matrix and adjacent and covering the ends of said enclosure to
produce a uniform magnetic field parallel to the direction of fluid
flow through said matrix; and submitting a slurry including the
materials to be separated through said magnetic matrix parallel to
the concentrated magnetic field therein through inlet means
including at least one input channel in said first pole section and
removing the slurry from said matrix through outlet means including
at least one outlet channel in said second pole section.
23. The method of claim 22 in which said magnetic field is further
directed through a ferromagnetic shell extending closely about the
external periphery of said coil and interconnecting said first and
second pole sections.
24. The method of claim 22 in which said inlet mens includes a
first recess that increases in cross section area toward said
enclosure and a ferromagnetic plug in said first recess spaced from
the surface of said first recess to provide a first peripheral
channel.
25. The method of claim 24 in which said by outlet means includes a
second recess that increases in cross sectional area toward said
enclosure and a second ferromagnetic plug in said second recess
spaced from the surface of said second recess to provide a second
peripheral channel.
26. The method of claim 25 in which approximately half of the
slurry is distributed through the enclosure within the area defined
by the peripheral channels.
27. The method of claim 22 further including providing raw material
to be processed to said inlet means delivering less-magnetic
particles from said outlet means to a first outlet channel
providing a wash fluid to said enclosure to rinse out middlings
delivering said middlings to a second output channel providing wash
fluid to said enclosure to wash out more-magnetic particles and
delivering said more-magnetic particles to a third output
channel.
28. The method of claim 22 in which said electromagnetic coil is a
superconducting coil.
29. The method of claim 22 in which said electromagnetic coil is a
cryogenic coil.
Description
BACKGROUND OF INVENTION
The invention relates to a magnetic separation method and a
magnetic separator, and more particularly to the separating of
materials of different magnet susceptibility.
One method of magnetic separation involves passing a fluid
containing the higher susceptibility material and lower
susceptibility material to be separated through a canister
containing a matrix of ferromagnetic material such as steel balls,
steel wool or tacks subject to a magnetic field. The higher
susceptibility materials adhere to the magnetic collection sites
and the low susceptibility materials pass through the canister.
Periodically the flow of fluid to be processed may be halted and a
flushing operation initiated simultaneously with a deenergization
of the magnetic field to remove the higher susceptibility material
from the canister. Interest in separating small particles such as
colloidal or subcolloidal particles, and in separating materials
having low magnetic susceptibility, including diamagnetic and
paramagnetic substances is increasing. With this increase comes the
demand for magnetic separators having intense magnetic fields, and
maximum effective utilization thereof to make magnetic separation
techniques technically and economically efficient for separation of
materials of minute size and low magnetic susceptibility in large
flow volume processes.
SUMMARY OF INVENTION
It is therefore an object of this invention to provide a magnetic
separation method and a magnetic separator capable of quickly,
efficiently, and effectively separating particles of differing
magnetic properties with particle sizes as small as
subcolloidal.
It is a further object of this invention to provide a magnetic
separator having a high-intensity magnetic field in its separation
chamber to provide therein high magnetic field gradients at a
multiplicity of collection sites.
It is a further object of this invention to provide a magnetic
separation method and a magnetic separator utilizing cryogenic or
superconducting electromagnet coils.
It is a further object of this invention to provide a magnetic
separator having a highly efficient magnetic circuit and uniform
flow characteristics in the separation chamber.
The invention may be accomplished by a magnetic separator including
an electromagnetic coil positioned within a recess in a
ferromagnetic return frame. Centered within the coil is an
enclosure containing a magnetic matrix of ferromagnetic materials
such as steel balls, wool, or tacks. Fluid to be processed flows
into the enclosure through an inlet means and out through an outlet
means. With the coil energized, a high intensity, axial magnetic
field establishes high field gradients at a multiplicity of
collection sites within the matrix at which the high susceptibility
materials collect, while lower susceptibility or nonmagnetic
materials pass through the enclosure. Maximum utilization of the
available ampere turns is aided by maximizing the amount of
ferromagnetic material in the magnetic circuit contributing to the
magnetic field in the matrix. For example, the return frame
includes a first portion covering the area enclosed by the coil and
a second portion adjacent the other side of the coil and covering
the area enclosed by the coil. The return frame may include a third
portion extending about the external periphery of the coil between
the first and second portions of the frame.
In preferred embodiments the inlet means may include an enlarged
section whose cross section area increases toward the enclosure and
a ferromagnetic plug spaced from the surface of the enlarged
section to provide a peripheral channel between the section and the
plug. The outlet means may also be similarly constructed and the
area within the peripheral channel in the enclosure is
approximately the same as the area of the enclosure outside the
channel whereby more uniform axial flow to and through the
enclosure is effected.
DISCLOSURE OF PREFERRED EMBODIMENT
Other objects, features, and advantages will occur from the
following description of a preferred embodiment and the
accompanying drawings, in which:
FIG. 1 is a cross-sectional, diagrammatic view of a cylindrically
symmetrical magnetic circuit configuration according to this
invention with a magnet matrix having a great number of collection
sites.
FIG. 2 is a diagram of one collection site in the magnetic matrix
shown in FIG. 1 and the field gradient thereat.
FIG. 3 is a schematic diagram of the flow system of a magnetic
separator using the magnet circuit of FIG. 1 according to this
invention.
FIG. 4 is a timing chart for the flow system of FIG. 3.
FIG. 5 is a sectional diagram of a conventional coil usable in the
magnetic circuit of FIG. 1.
FIG. 6 is a sectional diagram of a cryogenic or a superconducting
coil with refrigeration chamber usable in the magnetic circuit of
FIG. 1.
FIG. 7 is a schematic sectional diagram of a coil showing
regionalization of the conductors.
FIG. 8 is a sectional diagram of the arrangement of the
superconductors and normal conductors in the conductors of the
outer region of the coil of FIG. 7.
FIG. 9 is a sectional diagram of the arrangement of the
superconductors and normal conductors in the conductors of the
inner region of the coil of FIG. 7.
The magnetic circuit configuration according to this invention may
include a cylindrically symmetrical soft iron return frame 10
having a coil 12 located in a central recess 14. The chamber 16
formed centrally of coil 12 in recess 14 may be lined with a
canister 18 which extends through inlet 20 and outlet 22 to
external connections. Inlet 20 and outlet 22 are provided with
widened inner ports 24, 26 having conical shaped surfaces 28, 30.
Located within ports 24, 26 are conical iron plugs 32, 34 attached
to surfaces 28, 30 by spacers 36, 38, respectively, which establish
peripheral channels 40, 42 between those plugs and surfaces 28, 30.
The fluid to be processed flows in inlet 20 through matrix 52 in
canister 18 and out outlet 22. Plugs 32, 34 and surfaces 28, 30 are
shown having conical shapes, but this is not necessary: they may
have shapes resembling pyramids, hemispheres, cylinders or they may
be asymmetrical and each of different shapes; further, their
surfaces may be irregular. The return frame 10 and coil 12 may also
deviate from symmetry, cylindrical or otherwise.
Preferably, channels 40, 42 are uniform and enclose an area of
cross-sectional flow through chamber 16 between points 44, 46, and
48, 50, respectively, that is equal to approximately one half the
total cross-sectional flow area of chamber 16. The volume of
chamber 16 fed by channel 40 and emptied by channel 42 is evenly
divided by the peripheral channels 40, 42 whereby more uniform
axial flow is achieved.
The more intense magnetic field throughout chamber 16 and the more
uniform flow produced by the arrangement of FIG. 1 enables the use
of a more dense or finer matrix 52 in chamber 16 whereby more
effective utilization is made of the magnetic field volume provided
in chamber 16. The magnetic utilization factor, i.e. the ratio of
ampere turns or magnetomotive force present at the chamber to the
total ampere turns provided, is very high with the structure of
FIG. 1. The low leakage flux and low return path magnetomotive
force drop enabled by the magnetic circuit of this invention has
realized a magnetic utilization factor greater than 0.9: more than
90 percent of the magnetomotive force generated by the coil appears
across the magnetized volume at the canister. The uniform axial
flow contributes to the effective use of a very dense matrix and a
fine matrix provides a very high number of collection sites for
magnetic particles. Further, the high-intensity axial magnetic
field available throughout chamber 16 produces high field gradients
at those sites to attract magnetic particles.
Such sites as discussed supra are depicted in FIG. 2 where
collection sites 60, 62 are shown as north N and south S magnetic
poles, respectively, in a field 64 which is concentrated in their
vicinity to provide a high field gradient. A body placed in a
magnetic field can become magnetized whereby a magnetic dipole
moment is induced in it: magnetic poles are induced at the ends of
the body aligned with the magnetizing field. The magnitude of the
dipole moment is a function of the magnetic properties and geometry
of the body, and also of the intensity of the applied magnetic
field. In a uniform field the force on each pole is the same and
there is no net force on the body. In a field gradient the force
exerted on the pole at the higher field is greater than that
exerted on the other pole and there is a net force on the body.
Such bodies as would be present in the fluid flow through chamber
16 are shown adhered to sites 60, 62; particles 66, and moving to
those sites, particles 68. Lower susceptibility particles 70, those
of such low susceptibility that they are nearly unaffected by field
64, move freely past sites 60, 62. For the smallest and most weakly
magnetic particles viscous drag will limit their motion through the
fluid to a collection site. Such particles will be retained in the
matrix if they impinge directly on a collection site. The magnetic
separator of this invention is particularly well suited for
operation in this mode because of its high magnetic and hydraulic
efficiency which permit uniform flow distribution throughout a
great multiplicity of small, high-gradient collection sites in
large volumes of intense magnetic field.
The magnetic circuit configuration of this invention, such as
embodied in the device of FIG. 1 contributes to the
high-efficiency, high-intensity magnetic field in chamber 16
containing canister 18 and matrix 52. One section 10' of return
frame 10 adjacent one side of coil 12 covers the coil 12 and the
area enclosed by it, the other section 10" adjacent the other side
of the coil 12 covers the coil 12 and the area enclosed thereby. In
this manner the field applied at the matrix 52 is optimized both as
to uniformity and high intensity. A third section 10'" extending
about the outer periphery of coil 12 may also be used to increase
the utilization of the available ampere turns and reduce leakage
flux which is a consideration both as a safety factor and to
further improve efficiency. In FIG. 1 section 10'" is shown
coextensive with section 10' and 10" but the relationship may as
well reversed, i.e. sections 10' and 10" may extend beyond coil 12
to the outer surface of section 10'".
The arrangement of FIG. 1 may be used in a flow system such as
depicted in FIG. 3 including a feed tank 80, two pumps 82, 84, four
directional valves 86, 88, 90, 92, with four drive units 94, 96,
98, 100, a throttling valve 102, a magnet power supply 104, and a
timing control 108 for passing raw feed slurry to be separated
through canister 18 to separate the higher susceptibility particles
or fraction(s) from the lower susceptibility fraction(s). The two
outputs are, as a convenience, referred to respectively as the
magnetics and the nonmagnetics. In addition, there may be a third
output, referred to as the middlings, whose magnetic susceptibility
is between that of the higher and lower susceptibility particles.
The lower susceptibility particles pass through the matrix, the
higher susceptibility particles adhere to the collection sites and
the middlings are loosely attached to the sites. A flushing of the
matrix with the field on produces middlings, while flushing the
canister with the field off, produces the high-susceptibility
particles. Any one or more of these three separate outputs may be a
"product" desired for a particular purpose. If coil 12 is a
conventional water cooled coil its cooling may be performed by the
system.
In the first part of the operation cycle, or feed period, the
nonmagnetics may be produced at pipe 106. The coil 12 is energized
by the power supply 104. Raw feed is moved by pump 82 from tank 80
up pipe 110 to chamber 112 between pistons 114, 116 of valve 86.
From there the feed is pumped through pipe 118 to canister 18. The
speed of pump 82 is adjusted to obtain a flow-velocity in the
canister that is neither so great that the magnetic particles 66
are stripped from collection sites 60, 62, nor so low that settling
of the slurry occurs. Out of canister 18 the processed slurry
enters chamber 120 between pistons 122, 124 of valve 88 and out
pipe 106. During this part of the cycle, water from pipe 126 is
drawn by pump 84 and delivered through chamber 128 between pistons
130, 132 of valve 90 through pipe 134 to cool coil 12 and then
exits from pipe 136. And water from pipe 126 also reaches chamber
138, beyond piston 132 of valve 90, through throttling valve 102.
Pipe 150 connects chamber 138 to chamber 148, beyond piston 124 of
valve 88. Since chamber 148 communicates only with pipe 150 during
this period, there is no flow of rinse water to the canister
18.
In the second part of the cycle, or rinse period, timing control
108 operates units 94, 96 to change the state of valves 86, 88. As
a result, in valve 86, chamber 112 now connects pipe 110 to pipe
140; and chamber 142, between pistons 116 and 144, connects pipe
118 to pipe 146. In valve 88 chamber 120 communicates only with
pipe 106 while chamber 148 connects pipe 150 to canister 18. Feed
now flows from tank 80 through pump 82, pipe 110, chamber 112, and
pipe 140 back to tank 80. In this way the fluid is kept moving to
prevent settling. Water flows through valve 102, chamber 138, pipe
150, and chamber 148 to canister 18 where it back flushes middlings
into pipe 118. The coil 12 is still energized and excessive flow
velocity in the canister 18 is prevented by the throttling valve
102. The magnetics are thus retained by the matrix. From pipe 118
the middlings flow through chamber 142 of valve 86, pipe 146,
chamber 152, between pistons 154 and 156 of valve 92, and out pipe
158. Cooling water continues to flow through coil 12 as
previously.
In the third part of the cycle, or flush period, timing control 108
operates units 98 and 100 and coil 12 is deenergized. As a result,
in valve 92 chamber 152 now connects pipe 146 to pipe 160 and in
valve 90 chamber 128 connects the output of pump 84 to line 150.
The matrix in canister 18, no longer subject to the magnetic field
of coil 12, is now back flushed by water under high pressure from
pump 84 through chamber 128 of valve 90, through pipe 150 and
chamber 148 of valve 88. The freed magnetic particles are now
driven through pipe 118, chamber 142 of valve 86, pipe 146, chamber
152 of valve 92 and out pipe 160. No cooling water is supplied to
deenergized coil 12 and pump 82 continues to recirculate the feed
in tank 80. At the end of this part of the cycle the first part of
the cycle or feed period begins again.
The relationship of the feed, rinse, and flush periods of the
operation cycle is shown in conjunction with the coil energization
time in FIG. 4. Typically in a 20-minute cycle, the field is on the
first 17 minutes, line 162, the feed period lasts for the first 15
minutes, line 164, the rinse period occupies the next 2 minutes,
line 166, and the flush period occupies the last 3 minutes, line
168.
Coil 12 may be a conventional water cooled coil 12', FIG. 5, having
a toroidal body 170 with a hollow center 172 for receiving a
canister. The body is formed of a plurality of double-wound layers
174, 176, 178, 180 of hollow conductor 182 having longitudinal
channels 184 therein for receiving coolant. Each such double layer
begins with an inlet member 186 to be connected to a source of
electrical energy and to a source of coolant to pass through
channel 184 of conductors 182, and is wound inwardly in the first
layer 188 to the I.D. then lapped over and wound outwardly in the
second layer 190 to the O.D. finally terminating in outlet member
192. Successive double layers are generally connected in series to
a single source of electrical energy and hydraulically in parallel
to a single source of coolant, but numerous alternate hookups are
possible. Insulation 194 is provided between conductors.
Coil 12 may also be a cryogenic or a superconducting coil 12", FIG.
6. In systems, such as shown in FIG. 3, wherein cyclical operation
of the magnet is contemplated, cryogenic magnetics may be preferred
to superconductor types because of their greater efficiency in such
operations. In the context of this patent a cryogenic coil is one
which utilizes conductor materials which exhibit a large reduction
in resistivity when cooled to low temperatures. A notable example
of such a material would be aluminum of 99.9999 percent purity
which when cooled from room temperature to 4.2.degree. Kelvin
exhibits a reduction in resistance of 10,000 to 1. Thus, the
electric power required to operate a device utilizing such a
"cryogenic conductor" would also be reduced by a factor of 10,000
to 1. Other high-purity materials may be used. Operating
temperatures may be as high as 100.degree. Kelvin. Coil 12" may
include conductors 200 of high-purity aluminum, niobium tin alloy,
or niobium titanium alloy separated by insulation 202 and
maintained at 4.2.degree. K, or other "cryogenic" temperature, in a
refrigeration unit 204. Unit 204 may include an insulating support
206 for coil 12" mounted in helium vessel 208 which receives liquid
helium or other "cryogenic" coolant through neck 210 integral with
vessel 208. Vents 209 may be provided in support 206 to permit flow
of coolant beneath the coil 12". Suspended from neck 210 is vacuum
vessel 212; a radiation shield 214 may be positioned between
vessels 208, 212. Electrical connection to coil 12" is made through
helium boiloff cooled and properly insulated leads to reduce
parasitic heat transfer. Helium boiloff is recovered from neck 210
which may be provided with cooling coils 216 containing a suitable
coolant for utilizing the still quite low temperature of the helium
boiloff to reduce the temperature of shield 214 with lower cooling
coils 218 to further reduce heat transfer. The same double wound
layer construction used in FIG. 5 may be used to construct the
superconducting magnet of FIG. 6.
The cryogenic or superconducting magnet represents an extremely
significant improvement in the performance and also in the
processing economy of the magnetic separation device.
Superconductors are materials which when cooled to near absolute
zero exhibit a transition from a normal resistive state to a
superconducting state characterized by zero resistivity. This means
that there is zero or very little power generated by a current
flowing in a superconductor. Thus, superconducting windings can
maintain a magnetic field for an indefinite period of time without
requiring any power. It is this fact which allows the production of
very large volumes of very high magnetic fields and field gradients
in working magnetic separation systems having low operating
costs.
The systems described utilize superconductors whose normal to
superconducting transition temperature is below 20.degree. Kelvin
and therefore for convenience are operated in a bath of liquid
helium at approximately 4.2.degree. Kelvin. Significant improvement
may be obtained in some separation processes by operating at field
strengths in excess of 15 Testla. The device described is capable
of separating magnetic particles of subcolloidal size having
magnetic susceptibility of less than 10.sup.-.sup.5 cg.
Coil 12'" may consist of two regions 230, 232, FIG. 7. The most
economic utilization of materials can be achieved by proper
selection of superconducting alloys, stabilizing material, current
density and mechanical design for each region. In general, the
outer region 230, conductors 234, use copper stabilized niobium
titanium alloys and the inner 232 (higher field) region, conductors
236, use niobium tin alloys stabilized with either copper or
high-purity aluminum. Stabilization may be achieved in the
conductors 234 of outer region 230 by placing low resistivity
normal conductors 201 in intimate electrical and thermal contact
with the superconductors 203, FIG. 8, and may be achieved in the
conductors 236 of inner region 232 by placing low resistivity
normal conductors 201 in intimate electrical and thermal contact
with superconductors 203, FIG. 9. Thus, if a normal region is
established in the superconductor 203 the current simply transfers
into the stabilizing conductors 201 and "shunts" around the normal
region. The cross-sectional area and heat-transfer surface of the
high-conductivity normal conductors 201 is selected so that the
composite conductor temperature does not exceed the superconductor
transition temperature under the above condition and the normal
region reverts to the superconducting state. The superconductors
203 FIG. 8 and 9, may be twisted or otherwise positionally
transposed along the direction of the current flow to reduce
parasitic heating associated with time changing magnetic fields and
a phenomena generally referred to as "flux jumping". The
regionalization technique described in connection with FIG. 7 in
relation to superconducting coils is also beneficial to use with
cryogenic and conventional coils.
Other embodiments will occur to those skilled in the art and are
within the following claims:
* * * * *