GB2087267A - Method for the electrostatic separation of solids - Google Patents
Method for the electrostatic separation of solids Download PDFInfo
- Publication number
- GB2087267A GB2087267A GB8131129A GB8131129A GB2087267A GB 2087267 A GB2087267 A GB 2087267A GB 8131129 A GB8131129 A GB 8131129A GB 8131129 A GB8131129 A GB 8131129A GB 2087267 A GB2087267 A GB 2087267A
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- United Kingdom
- Prior art keywords
- particles
- permittivity
- separated
- process according
- electrodes
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C7/00—Separating solids from solids by electrostatic effect
- B03C7/02—Separators
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- Electrostatic Separation (AREA)
Abstract
A process for the separation of particles 6 having different permittivities which comprises subjecting such particles in a fluid medium 7 having a permittivity different from that of at least some of the said particles to a non-uniform electric field under conditions such as to cause any particles of higher permittivity than the fluid to move towards the region of higher electrical field strength and any particles of lower permittivity than the fluid to move towards the region of lower electrical field strength. The non- uniform electric field is established between suitably shaped electrodes 1,2. <IMAGE>
Description
SPECIFICATION
Method for the separation of solids
This invention relates to the separation of solids, and in particular to the separation of solids having different permittivities (dielectric constants). The method is particularly useful for the separation of naturally occurring minerals having different permittivities.
It is well known that, in principle, solids having different permittivities can be separated by making use of this difference. The solids are suspended in a fluid medium having a permittivity between that of the substances to be separated, and an electrical field is applied to the medium. Solid having a higher permittivity than the fluid medium adheres to the electrodes through which the electrical field is applied, while solid having lower permittivity is repelled. In this manner, a separation of the solids is possible. The method is of potential interest because many valuable mineral materials have permittivities which are substantially different from those of the other minerals with which they are naturally associated and from which, therefore, it is desirable to.separate them.For example, the valuable mineral, ilmenite, has a very high permittivity (over 30) while the majority of mineral substances with which it is likely to be associated have a much lower permittivity (usually in the range of 6 to 10). Some mineral substances, for example diamond and quartz, have low permittivities and may similarly be separated from other mineral substances having substantially higher permittivities. Free metals have infinite permittivity and they also can be separated from most minerals and materials with which they are likely to be associated.
However, this method of separation has not heretofore found widespread use, principally because it has been found difficult or impossible to devise a large scale method of carrying out the separation which is not prohibitively expensive, and because previous proposals have required the use of very intense electrical fields which can easily cause breakdown of the liquid medium. It will be appreciated also that in many cases a valuable mineral constitutes less than 1% of the total material mined, so that the separation process used has to be capable of handling very large quantities of material to produce relatively small quantities of the desired mineral. No economical way of doing this has hitherto been devised.
The present invention provides an improved method of separation, which can much more readily be operated on a large scale than can the prior known methods of separating solids, making use of differences in their permittivities.
According to the present invention, particles of different permittivities are separated by subjecting such particles in a fluid medium having a permittivity different from that of at least some of the said particles to a non-uniform electric field under conditions such as to cause any particles of higher permittivity than the fluid to move towards the region of higher electrical field strength and any particles of lower permittivity than the fluid to move towards the region of lower electrical field strength.
In any electric field which is non-uniform, the particles experience a force which is proportional to the difference between their permittivity and that of the fluid medium in which they are placed. It will be appreciated also that the direction of the force depends on whether the permittivity of the particles is higher or lower than that of the fluid. Preferably the fluid is chosen so that its permittivity lies between that of the desired particles and that of the remaining particles so that they move in opposite directions. It is however, within the scope of the invention to use a fluid which has a lower or a higher permittivity than that of the particles to be separated so that the latter all move in the same direction but to different extents.
In order that predictable resuits can be obtained, it is desirable that the electrodes between which the electric field is established have a predetermined geometry so that the force on the particles can be calculated. For example, if the electrodes are planar and inclined to one another to make the field non-uniform then the force experienced by any particle in the field is inversely proportional to the cube of the distance of the particle from the intersection point of the two planes in which the electrodes lie. If one of the electrodes has a hyperbolic surface in one direction and the other electrode is planar and at right angles to the axis of the hyperbola, then the force experienced by a particle is inversely proportional to the distance of that particle from the point of closest approach of the hyperbolic electrode to the planar electrode.Again, if the electrodes are in the form of concentric circles, the force experienced by a particle is proportional to the natural logarithm of the ratio of the radii of the two electrodes.
Preferably, however, the present invention involves the use of a substantially isodynamic electric field, that is to say an electric field so constructed that the rate of change of intensity of the field, and therefore the force experienced by a particle, in at least one direction is substantially constant. Preferably the field is so constructed that in a plane at right angles to the field, the field strength is isodynamic in one direction in the plane and substantially constant in a direction in the plane at right angles to the direction in which the field is isodynamic. This result may be achieved inter alia by constructing the electrodes between which the field is created in a manner described more fully below.In an isodynamic field each of the particles to be separated experiences a constant force in the direction in which the field is isodynamic, the magnitude of that force depending upon the difference between the permittivity of the particle in question and the permittivity of the fluid in which it is placed, and the direction of the force depending upon whether the permittivity is higher or lower than that of the fluid. Particles of higher permittivity than the fluid experience a force in the direction of higher electrical field strength, while particles of lower permittivity than the fluid experience a force in the opposite direction.
The magnitude of the force experienced by the particles is, in typical circumstances, not very great, but it is nevertheless adequate to separate the particles if appropriate steps are taken, for example to minimize the gravitational and frictional forces acting on the particles. Thus, if the particles are agitated and moved in a direction at an angle to the electrostatic forces, the particles segregate in directions determined by their permittivities. The direction of the applied motion may be, for example, substantially horizontal, or downwards vertically or at an angle to the vertical.
It will be appreciated that the nearer in density are the particles to be separated to the fluid in which they are placed, the lower is the gravitational force acting on the particles, and therefore, in general, the lower the frictional and viscous forces tending to prevent their movement. However, in practice it is usually necessary to use a fluid substantially less dense than the particles to be separated.
The manner in which the particles are caused to move is not critical. They may simply be vibrated by means of an appropriate mechanical or electro-mechanical device; they may be caused to slide or roll gradually down an inclined slope under the influence of gravity; or they may be simply allowed to fall through the fluid (which must of course be less dense than the particles) under the influence of gravity. If desired, the fluid may be agitated and movement thereby transmitted to the particles. It is essential that any means used to cause the particles to move is not such as to impose appreciable directional motion on the particles contrary to the desired direction or directions of action of the electrical forces. Preferably, the particles are caused to moved in a direction substantially at right angles to the directions of action of the electrical forces on the particles.If this is done, then the original mixture of particles gradualiy separates into two (or more) streams of particles, one stream containing the particles of higher permittivity, and the other stream or streams containing particles of lower permittivity.
It has surprisingly been observed that in operation of the new process particles of higher permittivity than the fluid medium form chains extending between the electrodes. Where the electrodes are one above the other, the chains are vertical while if the electrodes are side by side, the chains extend generally horizontally but with the central part curved downwards to an extent governed by the difference between the density of the particles and the density of the fluid medium. It will be understood that in general the particles are of higher density than the fluid medium. Close observation of these chains shows that the individual particles are in rapid motion and that adjacent particles do not necessarily touch.The reason why such chains form has been worked out in detail and it is found that each particle becomes polarized in the electric field and then attracts neighbouring particles in a way which reflects the manner of polarization of each.
Particles of lower permittivity than the fluid medium do not form such chains. Thus the chains serve in practice to facilitate the separation achieved by the new process.
The fluid medium is preferably chosen so that its permittivity lies between that of the desired ingredient in the mixture of particles to be separated, and that of the remaining ingredients. In some cases, of course, this may not be possible, for example, because the mixture contains a proportion of material having substantially the same permittivity as that of the desired constituent. Sometimes, there may be no conveniently available liquid having a permittivity in the desired range. In other cases, the mixture may contain some ingredients having a higher permittivity than that of the desired constituent, and some ingredients having a lower permittivity. In this latter case, two separations may be necessary. However, in many cases the desired ingredient has either a higher permittivity or a lower permittivity than all the other ingredients in the mixture to be separated.In such cases, only a single separation step is in principle required.
The fluid medium must have a high dielectric strength so as not to break down under the electric field applied. It is usually most convenient to make up a liquid having the required permittivity by mixing together a liquid of high permittivity with a liquid of low permittivity. However in this connection it must be borne in mind that it is possible for a mixture of liquids to have a higher permittivity than either liquid by itself. For instance such liquids contain 4-methyl-pentan-2-one and nitrobenzene, which have high permittivities of about 15 and 36 respectively, di-n-butylphthalate and tetrabromoethane, which have intermediate permittivities of about 6, and kerosene, which has a permittivity of only about 2. A mixture of equal weights of nitrobenzene and 4-methyl-pentan-2-one has a permittivity as high as 51.Mixtures of di-n-butylphthalate and kerosene have proved very suitable for separating mixtures of solids, of which at least one ingredient has a permittivity lower than 6. In practice, in order to ensure a reasonable speed of movement of the particles of the mixture to be separated, it is desirable for the permittivity of the liquid medium to differ significantly (say, by at least 0.1 ) from that of the particles to be separated.
The electrical field applied to the mixture should be as high as possible having regard to the dielectric strength of the liquid. In practice, fields having an intensity of 5,000-50,000, preferably 10,000 to 20,000, volts per centimetre can easily be achieved and give good results.
As already indicated, in the method of the present invention, the particles to be separated are preferably subjected to a substantially isodynamic electric field. One convenient way of establishing such a field is between two electrodes, one of which is flat and the other of which is curved in a manner defined below. The nature of the curve required is illustrated in Figure 1 of the accompanying drawings which shows diagrammatically the relationship between the curved electrode, the curved surface of which is shown in section as the line BAXB', and the flat electrode the surface of which is shown in section as the line DOD'.
The position of any point X on the curve BAXB' may be defined by the equation:
where R is the length of the radius (OX in the Figure), 0 is the angle < COX, and K is a constant. It will be appreciated that if the angle 0 is 60", then R = K, so that K is numerically equal to the length of the radius ate = 60 (OA in the Figure). The closest approach of the electrode represented by BAXB' to the flat electrode represented by DOD' is the distance OA. The electrical force varies isodynamically in both directions from
OA. If a particle of higher permittivity than the liquid is placed in such a field, then it tends to move in the direction of the higher field strength, i.e. towards OA, while particles of lower permittivity than the liquid tend to move away from this line.
The curved electrode preferably preferably diverges from the flat electrode in both directions, as represented on the Figure 1. The curve AB is then a mirror image of the curve AB'. In this arrangement, the particles of higher permittivity collect at the point of closes approach of the electrodes, while the particles of lower permittivity than the liquid move towards the extremities of the electrodes.
A practical arrangement for operating the method of present invention is show diagrammatically in
Figures 2 and 3 which show, respectively, a cross-section and a plan view of an apparatus for operating the new method. In Figure 2, the flat electrode 2 has the particles to be separated, generally indicated as 6, resting thereon. The space between the curved electrode 1 and the electrode 2 is filled by an appropriate liquid 7 having a permittivity between that of the particles to be separated and that of the remaining particles. The walls 5 of the space filled by the liquid are constructed of electrically insulating material, and the electrical field is established between the electrodes 1 and 2, for example by earthing electrode 2 and applying a high voltage to electrode 1.Conveniently, the electrode 2 is fixed at a slight incline and provided with a vibrator, not shown, capable of keeping the particle 6 in continual motion. As shown in Figure 3, one end of the electrode 2 is provided with holes 3 and 4 through which particles which have migrated down the slope under the influence of the electrical field can be collected. The direction of the slope and the general direction of movement of the particles 6 is shown in Figure 3 by the arrow 8. It will be appreciated that the particles of higher permittivity than the liquid form chains in the manner already described and migrate towards the field of greater electrical intensity, and therefore towards the orifice 3, while the particles of lower permittivity do not form chains and migrate in the opposite direction, i.e. towards the orifice 4.
Figure 4 shows diagrammatically in vertical cross-section an apparatus for vertical operation of the process of the invention using an isodynamic electrical field. In the drawing the curved electrodes 11 establish an isodynamic field with the planar electrodes 12. The mixture of particles to be separated is fed into the centre of the electrode assembly and allowed to fall under the influence of gravity through the liquid between the electrodes. The degree of deflection of the particles from the centre of the electrode assembly depends on the difference between their permittivity and that of the liquid, the particles of lower permittivity being most deflected. Collectors (not shown) of the separated particles are placed underneath the assembly at appropriate points.
The accompanying Figure 5 shows in diagrammatic vertical cross-section another apparatus for vertical operation of the process of the invention. In this apparatus the electric field is established between the planar electrode 21 and the doubly curved electrode 22 shaped in the manner already described to provide an isodynamic field between the two electrodes. The space between the electrodes is filled with a non-conducting fluid medium 23 of appropriate permittivity. The particles to be separated are fed in at the top of the apparatus over the outlet 25 in the bottom. Particles of lower permittivity than the fluid medium are deflected to the center and fall through the fluid medium under the influence of gravity. They are deflected to the center since the electric field increases in both directions from the central axis of the apparatus.Particles of higher permittivity than the fluid medium are deflected into regions of more intense electric field and hence leave the apparatus through the outlets 26. To prevent particles from touching electrode 22, a membrance 24 of e.g. paper is provided. This acts purely as a mechanical barrier and does not affect the electric field. Such an apparatus may be used for example to separate diamond from kimberlite using a fluid medium comprising di-n-butyl phthalate.
In the drawing, the electrode 21 is shown as earthed while the curved electrode 22 is shown as connected to a negative high voltage, e.g. -70,000 volts. This is a convenient arrangement but the polarity of the electrodes is not significant, and it also would be possible to use an alternating field.
The size of the particles used in the new method is not critical as the force acting on each particle is proportional to its volume. Therefore, in principle, large particles migrate under the influence of the electrical field as easily as small particles, and indeed more easily since the viscous forces acting on the particles are proportionately less. However, it is, of course, essential that each particle to be separated consists substantially entirely of one of the ingredients of the mixture to be separated. In practice therefore it is necessary to comminute the mixture of materials to be separated sufficiently for each particle to consist substantially entirely either of the desired ingredient or of unwanted material. With many materials, this presents no particular problem.
The new method is particularly useful for separating diamond from kimberlite, as the permittivity of diamond is only about 5.7, while that of the kimberlite is appreciably higher. When therefore a mixture of diamond and kimberlite is treated in accordance with the present invention using a liquid having a permittivity above 5.7 but below that of the kimberlite, the kimberlite migrates in the direction of the higher electrical field, while the diamond migrates in the opposite direction. Separation of the diamond is thus possible.
The degree of separation which can be achieved by the new process is good, and in appropriate cases, a satisfactory separation of the desired productfrom unwanted materials may be achieved in a single pass. In this connection, it will be appreciated that the slower the movement of the particles, the better the degree of separation which is normally achieved. In practical terms, therefore, there may have to be a compromise between degree of separation and rate of throughput of the material to be separated, and in some cases it may be preferable to subject material to be separated to two treatments in accordance with the invention, the first designed to achieve a relatively rough separation of the desired material from the bulk of the unwanted matter, and the second designed to achieve substantially complete separation.
Since the force acting on the material depends on the difference between its permittivity and the permittivity of the liquid, in some cases the force may well be small, and this will adversely affect the degree of separation which can be obtained. In extreme cases, it may even be desirable to use a cascade of treatments in accordance with the present invention, in a manner similar to that employed with other methods of separation, such as fractional crystallisation and countercurrent liquid extraction processes, in which the degree of separation achieved at any one pass is relatively small.
The method of the present invention may be operated continuously or batchwise. Since it is well suited for continuous operation, for example in the manner already described, this is normally preferred, but in some cases, for example if the amount of material to be treated is small, batchwise operation may be preferable.
The invention includes within its scope apparatus for operating the aforesaid method. Such apparatus comprises a container for the liquid having a permittivity different from that of at least some of the particles in the mixture to be separated, electrodes shaped so as to be capable of establishing a non-uniform electric field within the said liquid on application of electric potential to said electrodes, and means for supplying the particles to be separated so that they move under the influence of the said electric field. Since normally both the particles to be separated and the liquid are electrically non-conducting, no particular electrical insulation problems are likely to arise. However, if some or all of the particles to be separated are electrically conducting, it may be desirable in order to prevent any risk of short circuit between the electrodes, to cover one or both of the latter with a thin layer of electrically insulating material adequate to prevent any direct electrical contact between the electrodes. This may be desirable when, for example, some of the particles to be separated are metallic.
Claims (19)
1. Process for the separation of particles having different permittivities which comprises subjecting such
particles in a fluid medium having a permittivity different from that of at least some of the said particles to a
non-uniform electric field under conditions such as to cause any particles of higher permittivity than the fluid to move towards the region of higher electrical field strength and any particles of lower permittivity than the fluid to move towards the region of lower electrical field strength.
2. A process according to Claim 1, in which the permittivity of the fluid medium is such as to lie between that of the desired particles and that of the remaining particles.
3. A process according to Claim 1 or 2, in which the non-uniform electric field is a substantially
isodynamic electric field.
4. A process according to Claim 3, in which the said electric field is so constructed that in a plane at right
angles to the field, the field strength is isodynamic in one direction in the plane and substantially constant in
a direction in the plane at right angles to the direction in which the field is isodynamic.
5. A process according to any of Claims 1 to 4, in which the particles to be separated are caused to move
in a direction substantially at right angles to the directions of action of the electrical forces on the particles.
6. A process according to Claim 5, in which the said particles are caused to move by the action of gravity.
7. A process according to any of Claims 1 to 6, in which the fluid medium comprises di-n-butyl-phthalate
alone or in a mixture with kerosene.
8. A process according to any of Claims 1 to 7, in which the electric field has a maximum intensity of
5,000-50,000 volts per centimetre.
9. A process according to Claim 8, in which the said field has a maximum intensity of 10,000-20,000 volts
per centimetre.
10. A process according to any of Claims 1 to 9, in which the particles to be separated comprise diamond
particles and kimberlite particles.
11. A process according to Claim 1, substantially as hereinbefore described with reference to Figures 2
and 3 of the accompanying drawings.
12. A process according to Claim 1, substantially as hereinbefore described with reference to Figure 5 of
the accompanying drawings.
13. A process according to Claim 1, substantially as hereinbefore described.
14. Apparatus for use in the process of Claim 1 comprising a container for the liquid medium, electrodes
shaped so asto be capable of establishing a non-uniform electric field within the said liquid medium on
application of an electric potential to said electrodes, and means for supplying the particles to be separated so that they move under the influence of the said electric field.
15. Apparatus according to Claim 14, in which the electrodes are shaped so as to be capable of establishing an essentially isodynamic electric field within the said liquid medium.
16. Apparatus according to Claim 14 or 15, arranged so that the particles to be separated can move by the action of gravity while they are under the influence of the said electric field.
17. Apparatus according to any of Claims 14 to 16 provided with means for the separate collection of the separated particles.
18. Apparatus as claimed in Claim 17, substantially as hereinbefore described with reference to Figures 2 and 3 or Figure 4 of the accompanying drawings.
19. Apparatus as claimed in Claim 18, substantially as hereinbefore described with reference to Figure 5 ofthe accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8131129A GB2087267B (en) | 1980-10-16 | 1981-10-15 | Method for the electrostatic separation of solids |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8033362 | 1980-10-16 | ||
| GB8131129A GB2087267B (en) | 1980-10-16 | 1981-10-15 | Method for the electrostatic separation of solids |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB2087267A true GB2087267A (en) | 1982-05-26 |
| GB2087267B GB2087267B (en) | 1985-02-13 |
Family
ID=26277229
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8131129A Expired GB2087267B (en) | 1980-10-16 | 1981-10-15 | Method for the electrostatic separation of solids |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2087267B (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2130921A (en) * | 1982-11-17 | 1984-06-13 | Blue Circle Ind Plc | Electrostatic separation of particulate materials |
| GB2130923A (en) * | 1982-11-17 | 1984-06-13 | Blue Circle Ind Plc | Electrostatic separation of particulate materials |
-
1981
- 1981-10-15 GB GB8131129A patent/GB2087267B/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2130921A (en) * | 1982-11-17 | 1984-06-13 | Blue Circle Ind Plc | Electrostatic separation of particulate materials |
| GB2130923A (en) * | 1982-11-17 | 1984-06-13 | Blue Circle Ind Plc | Electrostatic separation of particulate materials |
| US4556481A (en) * | 1982-11-17 | 1985-12-03 | Blue Circle Industries Plc | Apparatus for separating particulate materials |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2087267B (en) | 1985-02-13 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19941015 |