U.S. patent number 3,676,337 [Application Number 05/053,497] was granted by the patent office on 1972-07-11 for process for magnetic separation.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Henry H. Kolm.
United States Patent |
3,676,337 |
Kolm |
July 11, 1972 |
PROCESS FOR MAGNETIC SEPARATION
Abstract
Process for separating colloidal and sub-colloidal ceramic
magnetic components and paramagnetic components from a slurry (or
other carrier) by passing the slurry through a column containing a
magnetic material, as a magnetic grade stainless steel wool. The
steel wool is subjected to a d-c magnetic field sufficient in
magnitude to effect magnetization to saturation and above and
provides a large number of regions of very high magnetic field and
magnetic field gradient along the paths of travel of the slurry to
attract and retain the magnetic components. It has been found that
in order to provide removal of such components on an industrial
scale at the high throughput rates required, a background field in
the wool of at least about 12,000 gauss is required to overcome the
forces of turbulence or the like in the slurry.
Inventors: |
Kolm; Henry H. (Wayland,
MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
21984694 |
Appl.
No.: |
05/053,497 |
Filed: |
July 9, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
761048 |
Sep 20, 1968 |
3567026 |
Mar 2, 1971 |
|
|
Current U.S.
Class: |
210/695 |
Current CPC
Class: |
G01N
27/72 (20130101); B03C 1/0337 (20130101); B03C
1/00 (20130101); B03C 1/032 (20130101); B03C
1/034 (20130101); B03C 1/027 (20130101); B03C
1/0335 (20130101); G01N 27/74 (20130101); B03C
1/0332 (20130101); B03C 2201/18 (20130101) |
Current International
Class: |
B03C
1/00 (20060101); B03C 1/033 (20060101); G01N
27/72 (20060101); B03C 1/02 (20060101); G01N
27/74 (20060101); C02b 001/00 () |
Field of
Search: |
;210/222,223,65,37,38,42
;209/212,213,224,223,232 ;335/216 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
polgreen, G. R., New Applications of Modern Magnets, MacDonald
& Co., London, 1966, p. 50-60, S.L.QC757P6 .
H. H. Kolm and A. J. Freeman, Intense Magnetic Fields, Scientific
American, p. 66, Vol. 212, No. 4, Apr. 1965.
|
Primary Examiner: Friedman; Reuben
Assistant Examiner: Granger; T. A.
Parent Case Text
This application is a continuation-in-part of application Ser. No.
761,048, filed Sept. 20, 1968, now U.S. Pat. No. 3,567,026, granted
Mar. 2, 1971.
Claims
What is claimed is:
1. A process for separating colloidal paramagnetic components from
a fluid carrier, that comprises, passing the carrier and components
through a substantial volume of magnetic stainless steel wool
material that contains a plurality of paths therethrough along
which the carrier and components can travel, the wool being placed
within the central opening of a coil which when energized is
adapted to provide a substantially uniform background magnetic
field in the volume occupied by the wool material substantially
parallel to the fluid flow and of an average strength of at least
12,000 gauss in the space through which the carrier flows,
energizing the coil to provide the 12,000 gauss d-c background
magnetic field, said 12,000 gauss field being substantially
stronger than required to magnetize the magnetic wool material
throughout said volume to saturation and to magnetize paramagnetic
components in the carrier, the magnetic flux thereby induced in the
saturated strands of said wool leaving the wool at a large number
of regions and passing into the space adjacent the strands along
said paths to provide a large number of regions of very high
magnetic field gradient within said volume, the background field in
said space and the filed gradient in said space being sufficiently
high in relation to the size of the components to attract and
retain said components.
2. A process as claimed in claim 1 that comprises, removing the
background field and applying a low intensity a-c magnetic field to
the material, gradually reducing the a-c field to zero, and
flushing said components from the wool.
3. A process as claimed in claim 1 and including the further steps
of compressing the wool to a density sufficiently high to provide a
large collection area per unit volume for the retention of said
components and adjusting the background field in said volume to
separate said components according to size and/or magnetic
susceptibility by the imposition of the background field and the
magnetic field gradient upon each component.
4. A process as claimed in claim 1 and including the further step
of adjusting the background field in said volume to separate said
components according to size and/or magnetic susceptibility by the
imposition of the background field and magnetic field gradient upon
each component.
5. A process for separating colloidal paramagnetic components from
a fluid carrier, that comprises, passing the carrier and components
through a substantial volume of ferromagnetic corrosion resistant
wool material that contains a plurality of paths therethrough along
which the carrier and components can travel, the wool being placed
within the central opening of a coil which when energized is
adapted to provide a substantially uniform background magnetic
field in the volume occupied by the wool material substantially
parallel to the fluid flow and of an average strength at least
adequate to magnetize the wool to saturation and above, energizing
the coil to provide said background magnetic field, said background
field being substantially stronger than required to magnetize the
magnetic wool material throughout said volume to saturation and
strong enough to magnetize paramagnetic components in the carrier,
the magnetic field thereby induced in the saturated strands of said
wool leaving the wool at a large number of regions and passing into
the space adjacent the strands along said paths to provide a large
number of regions of very high magnetic background field and
magnetic field gradient within said volume, the background field in
said space and the field gradient in said space being sufficiently
high in relation to the size of the components to attract and
retain said components.
6. A process for separating colloidal paramagnetic components from
a fluid carrier, that comprises, passing the carrier and components
through a substantial volume of ferromagnetic corrosion resistant
wool material that contains a plurality of paths therethrough along
which the carrier and components can travel, establishing a
substantially uniform background field in said volume,
substantially parallel to the fluid flow, substantially stronger
than required to magnetize the magnetic wool material throughout
said volume to saturation and strong enough to magnetize
paramagnetic components in the carrier, the magnetic flux thereby
induced in the saturated strands of said wool leaving the wool at a
large number of regions and passing into the space adjacent the
strands along said paths to provide a large number of regions of
very high magnetic background field and magnetic field gradient
within said volume, the background field in said space and the
field gradient in said space being sufficiently high in relation to
the size of the components to attract and retain said components.
Description
This invention was made in the course of work performed under a
contract with the Air Force Office of Scientific Research.
The present invention relates to separators adapted to remove
magnetic and conductive non-magnetic components from a slurry, the
non-magnetic components being removed by novel eddy current
means.
It is frequently desirable to separate magnetic components from a
slurry of predominantly non-magnetic particles suspended in water
or some other vehicle. The problem arises, for example, in ore
dressing where the magnetic component may be an impurity or a
usable mineral, and in the food industry where the magnetic
component may be harmful steel chips introduced during a processing
operation. To perform this separating function a number of "wet
separators" are in common use. All of these rely on the technique
of magnetizing the magnetic component by means of an applied
magnetic field and simultaneously subjecting the magnetized
particles to a divergent (fringing) magnetic field, that is, a
magnetic gradient. The magnetic force experienced by each particle
is directly proportional to three quantities: the induced
magnetization of the particle, the size of the particle and the
magnetic gradient. The effectiveness of a magnetic separator is
directly dependent upon this magnetic force since this is the force
which holds the magnetic particles against the competing forces of
viscosity, turbulence and gravitation. Very little can be done to
enhance the first of these quantities, the induced magnetization.
If the substance being separated is ferromagnetic, its
magnetization saturates in a moderate magnetic field and does not
increase at higher field intensities. If the substance is
paramagnetic, its magnetization increases linearly with the applied
field intensity but is usually too weak in fields which can be
applied in practice to be useful for separation although, as later
discussed herein, the advent of practical superconducting magnets
makes paramagnetic substances susceptible to magnetic separation on
an industrial scale. The second quantity, particle size, is
dictated by the intimacy of admixture of the magnetic component;
and this usually (at least on the applications of most interest)
requires that the parent substance be ground to colloidal size
before magnetic separation can be effected. The third quantity,
magnetic field gradient, therefore, is the crucial variable in the
process; and its enhancement is of primary concern to designers. It
is necessary not only to achieve the strongest possible field
gradient, but also to produce it over a surface area large enough
to collect a reasonable quantity of magnetic material before the
capacity of the separator is exhausted. To achieve this aim,
designers of the prior art use serrated steel plates, steel balls,
wire mesh screens and ribbon mesh screens, usually arranged in
multiple layers bridging the gap of an electromagnet. The types of
separators just described are adequate only for separating magnetic
particles of coarse size present in small quantities, and even
under such favorable conditions their collecting area is so small
that they require elaborate mechanical devices to move new
collecting areas into the magnet continuously while the saturated
area is being washed. The gradients produced in these known
fabricated structures are so weak that effective separation
requires flow rates so slow and retention times so long as to be
unrealistic for industrial application in the majority of cases. It
has been proposed, also, to provide loosely packed steel wool and
iron filings as the magnetic filtration material, the material
being placed between poles of permanent magnets, which rarely
provide fields above 500 gauss, and electromagnets designed to
achieve magnetic fields below 5,000 gauss. These latter separators
fail to provide the high field gradients hereinafter discussed,
and, furthermore, are not useful in connection with corrosive
materials of great commercial interest as, for example, kaolin
slurries, but appear to be useful for removing highly magnetizable
particles from non-corrosive petroleum slurries.
Accordingly, an object of the present invention is to provide a
process employing a corrosion resistant ferro-magnetic wool or the
like adapted to remove magnetic components from a slurry or the
like more completely and efficiently than is possible with the
before-mentioned separators.
A further object is to provide separator apparatus which is adapted
to remove colloidal and sub-colloidal ferro-magnetic components
from a slurry or the like but which can be used to remove colloidal
and sub-colloidal ceramic magnetic components and paramagnetic
components as well, and on an industrial scale.
In separators of the present invention, the ferro-magnetic wool is
placed in a high d-c magnetic field which is reduced to zero at
regular intervals to permit removal of trapped components from the
wool by flushing. It has been found, however, that the wool retains
some residual magnetization, and for this and other reasons a
sizable amount of the components are not flushed from the
separator. A further object of the invention is to provide means,
and particularly eddy current means, adapted to vibrate the wool to
shake loose retained components during the intervals of
flushing.
Although the eddy current concept is particularly useful for the
purposes discussed in the previous paragraph, it will be apparent
in the discussion to follow that it has other novel utility as
well. A still further object is, therefore, to provide eddy current
apparatus of more general utility, including apparatus adapted to
remove conductive particles, magnetic or non-magnetic, from a
slurry.
Other and still further objects will be evident in the
specification to follow and will be particularly delineated in the
appended claims.
By way of summary, the objects of the invention are attained,
generally, in a magnetic separator adapted to remove magnetic
components from a slurry or the like, that comprises, a magnetic
material comprising randomly oriented corrosion resistant fibers
adapted to provide a plurality of paths therethrough to effect
intimate contact between the slurry and the fibers. Inlet means is
provided to introduce the slurry to the column and outlet means to
remove the slurry therefrom. Magnetizing means is provided to
effect magnetization of the magnetic material to saturation and
above. The fibers are compressed to a density sufficiently high to
provide a multiplicity of regions of very high magnetic field
gradient within the space occupied by the material to attract and
retain the magnetic components at said regions, but not so high
that passage of the slurry therethrough is affected
appreciably.
The invention will now be described with reference to the
accompanying drawings in which:
FIG. 1 is a schematic representation of an embodiment of the
present invention showing two columns for receiving a slurry
containing magnetic components which are removed from the slurry
and retained within the columns;
FIG. 2 is an isometric view, partially cutaway, of one column, and
shows a magnetic material comprising randomly oriented corrosion
resistant fibers adapted to separate the magnetic components from
the slurry;
FIG. 3 is an isometric view showing a plurality of such columns
disposed between magnetic pole pieces with a central magnetic core
and coil adapted to magnetize the pole pieces;
FIG. 4 is a three dimensional view, on an enlarged scale, of two of
the fibers shown in FIG. 2;
FIG. 5 is a view taken upon the line 5--5 in FIG. 4 looking in the
direction of the arrows;
FIG. 6A is a schematic representation showing a column similar to
the columns of FIG. 1 with means for magnetizing and demagnetizing
the magnetic material within the columns and having, also, means
for applying magnetic pulses to the material to effect vibration
thereof, there being shown, as well, also schematically, an eddy
current means for removing conductive materials, magnetic or
non-magnetic, from a slurry;
FIG. 6B is a graph showing a sinusoidal wave to provide a
slow-rising field for the eddy current means and a square wave to
provide a fast-rising field therefor; and
FIG. 6C is a graph showing induced magnetic fields in magnetic
stainless steel wool and in bulk magnetic stainless steel of the
same grade as a function of background field and average field
gradient and peak field gradient, also as a function of background
field.
Referring now to FIG. 1 a magnetic separator 1 is shown for
removing magnetic components, as steel particles or the like, from
a slurry containing water or some other vehicle as the fluid
carrier. The separator is useful to remove such magnetic components
which may appear in the slurry as a foreign matter, or, in some
instances, the magnetic components may be separated out for their
own use. The separator 1 illustrated consists of two non-magnetic
casing tubes or columns designated 2 and 3, each of which contains
a corrosion resistant magnetic material 4, as shown in FIG. 2. As
explained hereinafter, there are times when two or more such
columns may be used; but, for purposes of explanation at this
point, it is necessary to refer to only one; and for that purpose
the explanation now to be made will be made with reference to the
column designated 2. The column 2 may be a single column, as that
shown in FIG. 2, or it may consist of the plurality of the
individual columns in FIG. 3 which may be connected in series or
parallel to provide a composite column 2.
A preferred magnetic material 4 is a fine steel wool of magnetic
grade stainless steel (other corrosion resistant ferromagnetic
wools as nickel or others may be used) adapted to provide a
plurality of paths therethrough to effect intimate contact between
the slurry and the magnetic material, the slurry being introduced
to the column through an inlet 5 and being removed therefrom
through an outlet 5'. The steel wool is compressed to a density
which gives it the desired specific magnetic reluctance, and, as
later discussed herein, enough magnetic flux leaks through the
voids between individual randomly oriented steel wool strands to
create a volume having a large number of regions of high magnetic
gradient. The column 2 is subjected to a magnetic field by placing
it within a coil 6, or the magnetizing means may be a yoke
comprising a core 8 coupled at the ends thereof to a pair of pole
plates 9 and 10. The fibrous material 4 is compressed to a density
sufficiently high to provide a multiplicity of regions of very high
magnetic field gradient within the column to attract and retain the
magnetic components.
The present invention is, thus, adapted to provide a large number
of regions of high magnetic background and high magnetic field
gradient, as more particularly explained hereinafter, along the
paths of travel of the slurry to attract and retain the magnetic
components. To affect the slurry, the regions mentioned must exist
in the space through which the slurry passes in its movement
through the column 2. If, for example, the reluctance of the
magnetic material 4 in the column is nearly equal to the bulk
reluctance thereof, the magnetic flux will be confined to the
interior of the material 4; and little or no flux (i.e.
environmental or background field) will be found in the path of
travel of the slurry. On the other hand, as the amount of magnetic
material approaches zero, the reluctance of the column will
approach the reluctance of air, resulting in low flux density in
the column 2 and effectively no regions of high magnetic field
gradient. Thus, if the reluctance is too low, there is, in effect,
a magnetic short circuit; and if it is too high, there is an open
circuit. Magnetic steel wool is particularly suited for present
purposes because the individual strands of the wool have
directional and other discontinuities that cause magnetic flux to
leave a strand and pass through the adjacent space. Thus, as the
slurry passes axially through the column 2, the paths of travel
provide intimate contact between the slurry and the magnetic
material 4 at a multiplicity of regions of very high magnetic field
gradient.
As previously mentioned, the vehicle or carrier fluid of the slurry
can be water. Although other carrier fluids are of interest, water
slurries are commercially of great importance; and it is necessary,
when removing magnetic components, as here, from water or other
corrosive slurries, that the material 4 used be corrosion
resistant. Corrosion is particularly harmful in separators of the
present type because, in addition to destroying the complete fibers
of the wool, it has the immediate effect of removing the serrated
edges of the wool upon which the effectiveness of the present
devices to a considerable extent depends, as later discussed in
greater detail with particular reference to FIGS. 4 and 5. To
prevent the harmful effects of corrosion in the present device, a
magnetic grade stainless steel wool is used as the material 4.
The magnetics of the present invention will now be explained with
particular reference to FIGS. 4 and 5, where two filaments or
fibers of the material 4 are shown at 15 and 16. Assuming a d-c
magnetic field in the vertical or y direction shown, a magnetic
flux will exist, as represented, for example, by the lines
designated 24. The flux lines will enter the fiber 15 to remain
there for various distances depending, in part, upon the contour of
the fiber in an x-y or y-z plane. Thus, the flux 24 will leave the
fiber 15 (and also the fiber 16) to create regions 18, 19 and 19',
of very high magnetic field gradient within the volume of the
material 4. The flux lines in the vicinity of the region 18 are
numbered 20, 21 and 22. The lines 20 and 21 pass into the fiber 15,
out into the region 18 and thence into the fiber 16 whereas the
flux line 22 passes directly from the fiber 15 to the fiber 16 at
the point of contact therebetween, designated 17. When fibers are
used for the material 4, most of the flux passes through the region
18 as the point contact at 17 is a low reluctance path, and the
fiber becomes saturated in the vicinity of the point as the cross
dimensions thereof decrease. Furthermore, a large percentage of the
contacts between fibers are of the point type shown in FIG. 4 so
there is little tendency for a magnetic short circuit to exist.
Also a large percentage of the volume occupied by the fibers
consists of the regions such as 18, so there is, in a given volume,
a considerable collecting area per unit volume available for
retention of the magnetic components. Furthermore, the serrated
profile of the fibers offers a large number of valleys, as 25,
which also provide collection regions of high field gradient for
the components. It should be noted, in this connection, that the
magnetic components as they accumulate tend to short circuit the
gaps, such as 18, thereby to reduce its effectiveness; thus, the
increase in the percentage of the volume occupied by such regions
is of more than negligible importance, and the detrimental effect
of corrosion is evident.
The importance of a high field gradient can best be explained with
reference to FIG. 5, where induced north and south poles within the
fibers 15 and 16 are designated N.sub.1, S.sub.1 and N.sub.2,
S.sub.2, respectively, and the induced poles in a magnetic
component 14 therebetween are designated N.sub.3, S.sub.3. The
poles N.sub.3, S.sub.3 have equal magnitude; so if the values of
S.sub.1 and N.sub.2 are also equal and if the particle 14 is
located half way between S.sub.1 and N.sub.2, no movement will
occur. Any change in the position toward either S.sub.1 or N.sub.2
will result in an unbalanced force upon the particle and movement
toward the closest pole, assuming no other forces. But other forces
as that due to slurry movement through the column 2, turbulence or
the like tend to sweep the particle 14 away from either S.sub.1 or
N.sub.2. Only if the gradient is high and the forces due to S.sub.1
and N.sub.2 change very rapidly with changes in spatial position
toward or away from either, will attraction be effective to remove
and retain the components, In the present device, as mentioned,
very high magnetic gradients are present.
The importance of the reluctance of the steel wool in the column 2
also becomes clearer after the explanation in the previous two
paragraphs. As mentioned, when the fibers are saturated, the flux
leaves the fibers at places of reduced cross dimensions; that is,
at places where the reluctance in the fibers becomes greater than
the reluctance of the adjacent air space, and the flux leaves also
at places where the fibers are oriented to cross the field. And it
is the flux lines (i.e., environmental or background magnetic
field) in the adjacent air space which affect the magnetic
components. But when the field strength within the volume occupied
by the material 4 is low due, for example, to the high reluctance
path in prior art devices resulting from the loosely packed wool,
there is a tendency for the flux lines to pass through the fibers
rather than into the space adjacent. Therefore, three unfavorable
conditions exist in separators with loosely packed steel wool:
first, the field in the column is less for a given exciting field
as might be effected between the pole plates 9 and 10; second, an
inordinate amount of the magnetic flux in the column passes along
through the fibers; and third, the distance between fibers will, on
the average, be greater. The level of magnetic field gradient that
exists in an operating separator is lessened due to the third
condition discussed, and the number of regions where a field
gradient exists is lessened due to the first and second
conditions.
For the foregoing reasons the present device has many more
collection points or stations for the magnetic components in a
given volume than are present in prior devices, the regions of such
points have a greater magnetic gradient than prior devices, and
magnetic short circuits which become more pronounced as magnetic
components are collected do not build as rapidly as they do in
prior devices. Since the capacity of the present apparatus to
retain the magnetic components far exceeds the capacity of prior
devices, it can be used to filter magnetic components from a far
greater quantity of slurry without flushing than previously known
apparatus and do so in a far more efficient manner. Furthermore,
colloidal and sub-colloidal particles are removed with facility by
apparatus embodying the present inventive concept.
As magnetic components are collected by the wool in increasing
quantity, they gradually short circuit the collection regions
forming paths for magnetic flux which bridge the magnetic gap. The
paths are periodically broken during demagnetization, flushing and
the vibration induced in the manner later discussed. However, to
reduce the effect of such paths and other short circuits that
occur, perforated equipotential plates 11 (made of magnetic-type
stainless steel, for example) in FIG. 2, having perforations 11',
are provided at intervals substantially at right angles to the
direction of magnetic flux flow. The plates 11 are sufficiently
massive to ensure uniform distribution of magnetic flux over the
entire cross section of the separator to mitigate the bridging
effect.
Turning again to FIG. 1, the apparatus illustrated, as mentioned,
has two columns, the coil 6 being adapted to create an axial
magnetic field in the column 2 and a further coil 13 being adapted
to create an axial magnetic field in the column 3, the fields being
thus parallel to the direction of slurry flow between inlets 5 and
outlets 5'. The tow columns shown allow cycling of the separator.
Thus, a switch 7 may be closed to energize the coil 6 from a
variable voltage d-c source 30 while simultaneously removing a
voltage from the coil 13, and electrically energizable valves 12
and 12' can be appropriately electrically interconnected to
introduce slurry at the input 5 and remove it at 5'. At the same
time a switch 32 is closed to introduce a voltage from a variable
voltage a-c source 31 to the coil 13 to remove any residual
magnetism from the circuit. Clear water introduced at 33 and
removed at 33' flushes magnetic components from the column 3. The
switch 7 connected to the coil 6 can then be opened and the d-c
source 30 connected to the coil 13, and the column 2 can be
flushed; a switch 34 and variable voltage a-c source 35 serve the
same purposes as the elements 32 and 31 before mentioned.
Appropriate electrical interconnection to execute the foregoing
operations is, of course, provided, and the a-c sources 31 and 35
are controllable so that the current output of each can be made to
decrease gradually to effect more complete demagnetization.
The described apparatus can be used with magnetic fields within the
column of say 12,000 gauss, but fields of 20,000 gauss can also be
used; and the coils 6 and 13 can be superconductive to provide
fields of 40,000 to 100,000 gauss and above for removing
paramagnetic impurities, for example. Ferromagnetic impurities are
removed or separated at the lower fields, but even ferromagnetics
are removed more effectively at the higher fields.
The strands of the steel wool have been called fibers and
filaments. In fact, commercial stainless steel wool fibers, as
shown in FIGS. 4 and 5, are flat ribbons with sharp, serrated edges
having a thickness in the various grades about one-tenth the value
of the typical predominant width dimensions given below:
GRADE WIDTH
__________________________________________________________________________
Very fine 0.002 inches Fine 0.004 inches Medium 0.006 inches Coarse
0.010 inches
__________________________________________________________________________
It should be noted, however, that the cross dimensions of the
fibers in general are non-uniform and that the dimensions given are
merely typical of the predominant fiber sizes. The grade used will
depend upon the size of the impurities to be extracted, but the
fine grade has been found to be best for colloidal and
sub-colloidal magnetic components although clogging of the
filtration material must be considered.
As mentioned, the steel wool 4 is demagnetized during the flushing
cycle; and this demagnetization may be accomplished by a relatively
low intensity alternating magnetic field, the field being gradually
reduced from some predetermined value to zero. The demagnetizing
field can be applied using the same coil as applies the magnetizing
field, as is shown schematically in FIG. 1. In FIG. 6A the
demagnetizing field is provided by a separate coil 27 energized by
the a-c source 35, the coil 27 being shown in FIG. 2 wound inside
the coil 6. It is pointed out at this juncture that the coils 6 and
27 and later discussed coils 28 and 29 can be interleaved and
arranged in various configurations to provide the desired field
values. Thus, in FIG. 2, the coils designated 6 and 27 can include,
also, coils to perform functions of the coils 28 and 29 discussed
in the following paragraph. On the other hand, a single coil, as 6,
in appropriate circumstances and proper switching, can be connected
to perform the functions of the coils 6, 27, 28 and 29.
Work done in connection with separators employing the present
invention has shown that complete demagnetization of the material 4
and collected components to allow flushing is not practically
possible. Also, the magnetic components become secured at various
regions in the material 4 in a manner that renders them difficult
to dislodge. It is necessary, then, to furnish some means for
vibrating the steel wool during the flushing cycle. A most
effective vibration action can be supplied by the eddy current
device now to be discussed. A sinusoidally alternating magnetic
field 40 in FIG. 6B having a skin depth that is greater than the
cross dimensions of the stainless steel fibers is supplied by the
coil 28 which is energized by an a-c current supply 36. The
frequency of the current supply 36 is typically in the lower sonic
range. A square wave alternating magnetic field 41 having an
amplitude that is, preferably, lower than the sinusoidal wave 40
but which is .pi./2 radians out of time phase therewith provides,
in combination with the field represented by the sinusoid 40, a
vibratory action far greater than might be furnished, for example,
by a single eddy current field as might be provided by a current
having a frequency in the upper sonic range (i.e. 18,000 - 20,000
cycles) and connected to either of the coils 28 and 29 during the
flushing cycle, although a single eddy current field may be used in
particular apparatus. The square wave is supplied by the coil 29
which is energized by an a-c current source 43 to furnish a square
wave 41 at the same frequency as the frequency of the sinusoidal
wave 40; but, whereas the sinusoid is slow rising and in the low
frequency used (i.e. in the lower sonic range) has a skin depth
that is large compared to the characteristic dimensions of the
fibers, the square wave is abruptly alternating or faster rising
and has a skin depth that is less than the characteristic
dimensions of the fibers. The slow rising field 40 acts as a pulsed
background field, the fast rising field 41 being applied to furnish
a rate of change that is maximal when the background field 40 is
also maximal thereby effecting a maximum vibratory force upon the
steel wool. In some instances, it may be expedient to mix a small
amount of copper wool in with the steel wool, the higher
conductivity of the copper wool acting to increase the intensity of
vibration within the column 2.
The previously discussed apparatus is adapted to remove
magnetizable components from a slurry, but is not adapted to remove
non-magnetic components. The lower schematic representation in FIG.
6A comprising a non-conductive tank 37 connected in series with the
column 2 by a pipe 42 is adapted to remove conductive components
whether magnetic or non-magnetic. The slow rising field 40, as
before, is furnished by a coil 38 which is energized by a current
source 36; and the fast rising field 41 is furnished by a coil 39
which is energized by the current source 43. Conductive components
42' within the slurry are propelled to travel in the direction of
the arrow designated A to pass from the tank 37 through an outlet
44; the usable slurry passes through an outlet 45. The force on any
conductive particle 42' will be to the left or right depending on
whether the wave 40 leads or lags the wave 41 in time and on the
relative orientation of the coils 38 and 39. Unlike devices with no
background field or a continuous background field in which the time
varying field must be asymmetric to provide a unidirection net
force, the present device, though having symmetric fields, produces
a unidirectional force. And the force is maximized because the
instant at which the rate of change is maximal coincides with the
instant at which the field is maximal. Thus, the induced eddy
currents reach their maximum at a time when the magnetic field is
maximum, which optimizes use of the potentially available force.
The use of high intensity pulsed fields, since the background field
is needed for only a brief instant of time, represents a
substantial saving of power over a continuous background field. In
addition, as explained before, the period or rise time of the
varying field is systematically matched to the object to be acted
on, in such a way as to achieve the proper skin depth or
penetration depth of the induced eddy currents. The eddy current
apparatus discussed here represents an improvement over the prior
art in that it uses two separately controlled pulsed or
periodically varying magnetic fields, one having a substantially
fast rise-time to induce eddy currents of appropriate skin depth in
the particular object or particle to be acted on, and the other
having a substantially slow rise-time to allow a background field
to penetrate into the object to be acted on. The fast rising pulse
has a skin depth equal to or smaller than the characteristic
dimensions of the object to be acted on in the direction of field
penetration while the slow rising pulse has a skin depth
substantially greater than the dimension of the object to be acted
on in the direction of field penetration. The two pulsed or
periodic magnetic fields are timed in such a way as to have an
expedient time phase relationship with respect to each other, here
shown to be .pi.2 radians, lead or lag so that the abrupt rise
coincides with the maximum or minimum of the sine wave and an
expedient spatial relationship to one another in order to produce
maximum force in the desired direction.
The discussion in this and the three following paragraphs covers in
detail some aspects of the invention discussed more broadly
previously herein. Mention has been made of the importance of
magnetizing the steel wool strands to saturation, and the 12,000
gauss and above average background field is sufficient to saturate
the steel wool as well as to provide a background field in the
space through which the carrier flows to magnetize ceramic magnetic
components and paramagnetic components in the carrier. It has been
found for present purposes that a background field of at least
about seven thousand gauss (see curve 81 in FIG. 6C) is needed to
saturate stainless steel wool of the type herein used (this will
depend to some extent on fiber size and shape) and is to be
compared with a field of about two thousand gauss background field
necessary to saturate the bulk material, as shown in the curve
labeled 83. The large background field strength needed to saturate
above that to be expected on the basis of bulk sample figures is
due to demagnetization effects in the wool. Furthermore, average
magnetic field gradients, as shown at 80, and peak magnetic field
gradients, as shown at 82, increase dramatically above the 7,000
gauss level, until about 12,000 gauss where substantial saturation
of the complete fiber occurs.
In FIG. 6C the abscissa represent the average background field (H)
in the region occupied by the wool. The curve 81 is the
magnetization curve (M) of the stainless steel strands, the knee of
the curve being at about the seven thousand gauss level mentioned
before, and the curve labeled 83 is the magnetization curve for
bulk stainless of the same type. The curve designated 80 is the
average field gradient at the regions within the wool at which the
field leaves the strands; the average gradient curve increases
rapidly above saturation due to saturation of frazzles of the steel
wool, which saturation continues beyond the field level at which
the major portion of the strand has been saturated. These frazzles
(or serrated edges 23 in FIG. 2) provide, when saturated, regions
such as the valleys 25 within which there occurs high background
fields -- the background fields in the valleys 25 often equal the
(M) fields of the curve 81 -- and high peak gradients are found at
the regions adjacent the ends of the serrations, as represented by
the curve labeled 82. Field gradient of the order of 1,300
kilogauss per centimeter are conservatively estimated to be needed
to remove the colloidal and sub-colloidal ceramic magnetic and
paramagnetic components in slurries moving at the thirty-five feet
per minute flow rate herein contemplated. The high average field
gradients are found at about 12,000 gauss background field, as
shown. Above about 12,000 gauss background field the increases in
retention forces is due mostly to the increases in the background
or inducing field in the space adjacent the fibers through which
the carrier flows and resultant increase in magnetization of the
materials which are only slightly magnetic and increase in
magnetization on substantially a straight-line basis. It should be
noted, however, that above saturation of the fibers, the peak field
gradient curve increases dramatically; and this is due to
saturation of the frazzles 23 since it is the gradients in the
region of the frazzles 23 and the neighboring valleys 25 which are
represented by the curve 82. The induced magnetization within a
particular particle of a certain susceptibility, as before
discussed, is determined by the strength of the magnetic field in
the region occupied by the particle, that is, in the space through
which the slurry passes in its movement through the column. The
strength of the magnetic field in the volume occupied by the
fibers, in turn, is determined by the inducing field and, to some
extent, by the reluctance of the magnetic material in the volume
within the center of the coil 6. Thus, by providing proper material
density, along with the high magnetic field that appears within the
center of the coil 2 (as best shown in FIG. 2), a condition of high
magnetic field exists substantially uniformly throughout the volume
of the separator, thereby supplying higher fields (and higher field
gradients) than heretofore available throughout the volume occupied
by the steel wool in such separator apparatus. For this reason, a
separator made in accordance with the present teaching is capable
of exerting greater forces on magnetic components than heretofore
possible throughout the active separating region; furthermore, the
forces thus exerted are substantially uniform throughout (i.e., the
field magnitude and gradient at the center axis region of the steel
wool in FIG. 2 is about equal to the field and gradient at the
circumference thereof, immediately adjacent the coil), thereby
rendering a greater portion of the steel wool available to effect
removal of contaminants.
It is pointed out above that most of the particles of commercial
interest are small, colloidal or sub-colloidal (that is, particles
small enough for brownian responses due to molecular forces to
overcome gravity forces), and this smallness (the particles are
little more than a stain in a kaolin slurry) results in a lower
induced field (N.sub.s,S.sub.3 in the drawings) for any given
inducing field intensity in the slurry, but has the further and
more detrimental effect of requiring very high field gradients in
order that there be any effective removal. In addition, the
components of interest here are only slightly magnetic. For
example, the iron oxide, titanium oxide and other impurites in
certain kaolin slurries have a magnetic susceptibility of 8.0
.times. 10.sup.-emu/gm at 12,900 gauss (as compared to iron which
has a susceptibility of 220 emu/gm) only four times higher than
that of the clay from which said impurities are separated.
Furthermore, separation on the tons per hour scale contemplated by
industry dictates passage of the slurry or other fluid carrier
through the separator at fluid velocities of the order of 35 feet
per minute. Thus, the forces, which tend to prevent separation and
also to dislodge separated components, are of considerable
magnitude. The background field and field gradient in the valleys
25 for this particular reason serve a most useful purpose in the
present apparatus; and it is only above saturation of the fibers,
as before explained, that high magnetic background fields and high
field gradient of the order of magnitude required really come into
existence.
Previous mention has been made about the particular importance of
the need for a high magnetic field gradient in separators for the
removal of small particles. Referring to FIG. 5, it will be noted
that an attractive force between S.sub.1 and N.sub.3, for example,
is countered by a force in the opposite direction between S.sub.1
and S.sub.3 leaving a difference .DELTA.f to provide movement
toward S.sub.1. The difference .DELTA.f is proportional to the
field gradient between the N.sub.3 and the S.sub.3 portions of the
particle, i.e., the change in field intensity between N.sub.3 and
S.sub.3. If the particle is quite small in cross dimensions, as
colloidal and sub-colloidal particles are, then only the existence
of a high gradient will provide adequate .DELTA.f to offset the
competing forces of viscosity, turbulence, and gravitation. If
.DELTA.f drops to zero, then the forces due to any background field
on the respective north and south poles induced in the particle
would be equal and opposite.
It can be seen from the foregoing explanation that the particle
size, susceptibility, and fluid flow velocity each play a part in
determining background field strength and gradient; however, it can
be said than no meaningful separation in the context of the present
disclosure is obtained in a separator operated below the saturation
level of the stainless steel matrix, and, in a great number of
situations of interest, a background field of at least about 12,000
gauss is needed to provide removal on a commercially acceptable
basis.
It is possible, using the apparatus described in the present
application, to distinguish one paramagnetic particle from another
in terms of size alone even though both particles are of
substantially the same magnetic susceptibility; or it is possible
to distinguish particles of one substance from particles of another
substance even though both have about the same magnetic
susceptibility but have the distinguishing feature that one will
flocculate, under the influence of a high field and gradient of the
type described and claimed herein, whereas the other will not. This
mechanism allows separation or filtration by magnetic means of
particle sizes which ordinary filtration cannot separate.
The previous explanation relates to liquid carriers such as, for
example, kaolin slurries. The slurries are passed through the
separator at about 35 feet per minute, and the volume of matrix is
quite large to pass the tons of liquid per hour necessary in a
commercial industrial installation. Such large volume is necessary
not only to effect removal at all, but to provide the large number
of collection stations necessary if numerous cleanout shutdowns are
to be avoided. In order to provide substantially uniform high level
magnetization through the large volume of stainless steel matrix on
an economical basis, the steel wool is placed within the center of
the coil 6, as best shown in FIG. 2. It should be noted in this
connection, that the steel wool, if too loosely packed, will shift
toward the axial center point of the coil 6, particularly above
about 18,000 gauss, and should the background or inducing field be
orthogonal to fluid flow, as is done in some prior art separators,
rather than parallel thereto, as herein shown, the strands tend to
pack at circumferential locations due to the influence of
divergence of the background field, thereby to open paths through
the wool containing little or not steel wool.
THe discussion herein has chiefly concerned situations in which the
fluid carrier is a slurry, but the fluid carrier can be air, also.
For example, certain fly-ash particles in smoke from utility
generator station stacks cannot be removed by electrostatic
precipitation but do exhibit slight magnetic properties which
render them separable using the herein described apparatus. The
background field values given are useful, but it is contemplated
that economics will require background fields on the order of
20,000 gauss and above, in view of the high throughput required in
such installation, the slight magnetic dipole moments of the
fly-ash, and the very small size thereof. The volumes of removables
can be as high as 1,500 tons per day at which, in one installation,
40 tons would be removed by magnetic means. In view of the high
fields necessary, superconductor coils should be used and the
matrix will pass through the center of the superconductor coil in
conveyor fashion, thereby removing the collected fly-ash to remote
locations for shaking, etc. to remove the ash from the matrix.
Further modifications will occur to those skilled in the art.
* * * * *