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Publication numberUS3676337 A
Publication typeGrant
Publication dateJul 11, 1972
Filing dateJul 9, 1970
Priority dateJul 9, 1970
Publication numberUS 3676337 A, US 3676337A, US-A-3676337, US3676337 A, US3676337A
InventorsKolm Henry H
Original AssigneeMassachusetts Inst Technology
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Process for magnetic separation
US 3676337 A
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.
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United States Patent Kolm 5] July 11, 1972 54] PROCESS FOR MAGNETIC OTHER PUBLICATIQNS SEPARATION Polgreen, G. R., New Applications of Modern Magnets, Mac- [72] Inventor: Henry H. Kolm, Wayland, Mass. 'z London, 1966, P -QC 5 P6 73 H. H. Kolm and A. J. Freeman, Intense Magnetic Fields, I 1 Asslgnee- Q Imam fl Scientific American, p. 66, Vol. 212, No. 4, Apr. 1965 Cambridge, Mass.

22 Filed; July 9 970 Primary Examiner-Reuben Friedman Assistant ExaminerT. A. Granger [21] APPLNO': 53,497 Attorney-ThomasCooch, Martin M. Santaand Robert Shaw Related U.S. Application Data 57] ABSTRACT [63] Conunuanonm'pan of Sept Process for separating colloidal and sub-colloidal ceramic 1968, Pat. No. 3,567,026.

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 sufiicient 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.

all

PROCESS FOR MAGNETIC SEPARATION 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.

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 magneticcomponent 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 mag netic 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 sub stances 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 ferromagnetic 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 slury;

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 efi'ect 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 l5 (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 l5 and 16 are designated N,, S, and N 8,, respectively, and the induced poles in a magnetic component 14 therebetween are designated N 8,. The poles N S have equal magnitude; so if the values of S, and N are also equal and if the particle 14 is located half way between S, and N,, no movement will occur. Any change in the position toward either S, or N, 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, or N;.. Only if the gradient is high and the forces due to S, and N 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 II 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 dc 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.

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 gradehas 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. lJIn FIG. 6A the demagnetizing field is provided by a separate coil 27 energized by the ac 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. 68 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 1r/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 ac 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 ob ject 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 112 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 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 subcolloidal 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 commer cial 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 S: 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 X IOemu/m 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, and N for example, is countered by a force in the opposite direction between S, and S, leaving a difference Af to provide movement toward 8,. The difference Af is proportional to the field gradient between the N and the S, portions of the particle, i.e., the change in field intensity between N, and 5,. If the particle is quite small in cross dimensions, as colloidal and subcolloidal particles are, then only the existence of a high gradient will provide adequate Af to ofiset the competing forces of viscosity, turbulence, and gravitation. If Af 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.

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.

I I t I. i

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Classifications
U.S. Classification210/695
International ClassificationG01N27/74, B03C1/02, B03C1/00, G01N27/72, B03C1/033
Cooperative ClassificationG01N27/74, B03C1/0335, B03C1/00, G01N27/72
European ClassificationG01N27/74, B03C1/033C, G01N27/72, B03C1/00
Legal Events
DateCodeEventDescription
Jun 13, 1991ASAssignment
Owner name: STROMBERG-CARLSON CORPORATION (FORMERLY PLESUB INC
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST.;ASSIGNOR:UNITED TECHNOLOGIES CORPORATION;REEL/FRAME:005733/0537
Effective date: 19850605