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1. (WO1979000622) IMPROVEMENTS IN OR RELATING TO METHODS AND APPARATUS FOR SEPARATING MIXTURES OF PARTICULATE SOLIDS
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IMPROVEMENTS IN OR RELATING TO METHODS AND APPARATUS FOR SEPARATING MIXTURES OF PARTICIPATE

SOLIDS
This invention relates to methods and apparatus for
separating mixtures of non-magnetic solids in particula±e form.
It is known that colloidόϋ. suspensions of magnetic particles in suitable carrier-liquids, for instance water or paraffin, can behave as magnetic fluids. When they are placed in travelling magnetic fields, they experience a magnetic force in addition to the normal force of gravity., and thus their effective density relative to other materials within the liquid is either enhanced or diminished. Usually such magnetic fields are produced by an electromagnet supplied with direct current, and the pole pieces are shaped so as to produce a high field gradient. By suitably selecting such a magnetic fluid, and operating parameters such as the magnitude of the current and the number of windings of the electromagnet, the effective density of such a fluid can be raised higher than that of certain solids, so that when mixtures containing such solids are introduced to the liquid separation will take place, those solids floating while heavier ones sink.
Unfortunately colloidal suspensions of magnetic particles, in which the typical j-taritijele size is of the order of O.Olμ , are expensive to produce. They are also expensive in use, since the fine particles tend to adhere to the materials being separated and thus to get carried out of the suspension with them. Washing followed by very high-gradient magnetic separation, or centrifugin is then necessary to recover the fine magnetic particles.
Non-colloidal suspensions of magnetic powders such as ground magnetite or ferrosilicon, with an average particle size of up to say lOμm, are much cheaper to produce, and efficient recovery of the powder after use is possible by means of settling tanks or low-gradient magnetic separation. Unfortunately suspensions of magnetic powders of this sort of particle size are known to flocculate when placed in an ordinary constant or alternating magnetic field, and because of this behaviour the use of such suspensions for the separation of particulate mixtures has hithert been ignored. The present invention is based upon the realisation that such flocculation can be inhibited, and t at when this is done the suspensions may become suitable for use in separation processes.
According to this invention a process for separating a mixtur of particulate, non-magnetic materials includes introducing the mixture to a fluid comprising a suspension of magnetic particles in a suitable liquid, these particles being of such size that they would tend to flocculate if exposed to a simple constant or alternating magnetic field. Flocculation is however inhibited by causing the magnetic particles to spin vigorously, and a magnetic field is applied to enhance the density of the fluid relative to the mixture so that at least one constituent of that mixture tends to float at the surface or some other level of the fluid while at least one other constituent sinks. Such floating and sinking fractions are then separately collected.
The mixture may be introduced to the separation vessel pre-mixed with the fluid, the heavy fraction may simply sink to the bottom of the fluid and be removed from it either continuously or periodically, and a mixture of the fluid and the floating fraction may be removed, preferably continuously, by means of an outlet within the vessel and located substantially at the level of float. The spin may be imparted to the magnetic particles by
exposing the fluid to a travelling or rotating magnetic field,.
This field may also serve as the means for creating the enhanced relative density of that fluid, or a constant magnetic field could be added, the latter creating at least part of the enhanced density effect while the sole or primary effect of the rotating or travelling field is to spin the magnetic particles.
The invention includes apparatus for carrying out such a separation process.
A travelling magnetic field, suitable both to spin the fluid particles and to create the necessary density effects, may be provided by at least one linear magnetic stator carrying polyphase windings excited from a suitable AC supply, arranged around a simple trough with a single inlet for the combined mixture and fluid, and two vertically-separated exits, one for fluid and the light
(floating) separated mixture fraction, and the other for fluid and the heavy (sinking) mixture fraction. Alternatively a form of polyphase cylindrical induction motor stator can be used, preferably surrounding a vessel arranged with its axis vertical. Mixture and fluid, which may be pre-mixed, may be introduced from the top, the heavy separated fraction may sink to the bottom of th vessel and the floating (light) fraction together with a
proportion of magnetic fluid may be withdrawn through an outlet with its mouth located close to the flotation level; the outlet may be close to the wall of the vessel, and the inlet located on the vertical axis of the vessel. Pre-mixing is not essential and in one alternative system extracted fluid could be returned to the bottom of the vessel, when its upward motion through the vessel could then enhance the floating action.
The invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic vertical section through a separation apparatus;
Figure 2 is a diagrammatic perspective view illustrating relative motions, and
Figure 3 includes two graphs, (a) and (b).
Figure 1 shows a separation vessel 1, made of non-magnetic plastics material, having a blind-end cylindrical base 1 with a funnel mouth 2. This vessel is placed, with its axis 3 vertical, inside a tank 4 containing an oil bath 5« A polyphase cylindrical induction-motor-type stator 6 is mounted within bath 5 so as to

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. SAfa WiPO surround cylinder l, and is fed with alternating current from a polyphase supply represented diagrammatically at 7« Tank 4 and the oil bath within it is shown being cooled by water flowing through a spiral cooling unit 8; in practice hollow water-cαόled conductors would probably be preferred.
An ore, or mixture of particles to be separated, for instance barite and galena having specific gravities of 4.5 and 7-5 respectively, is introduced to the vessel through an inlet tube 10, pre-mixed with a 10% suspension of magnetite in water, the • magnetite particles having an average size of the order of 10μm. The tube 10 may be placed close to the wall of vessel 1, as shown in Figure 1, or alternatively it could be placed on the axis 3» An outlet tube 11, adjustable as to position both vertically and horizontally, is placed close to the wall of cylinder 1. The open bottom end of this member, and a cut-away ι2 in its side wall, are located at that level within the vessel at which the light fraction of the ore will tend to float' when stator 6 is energised to create an enhanced density gradient within the fluid. This light fraction, together with fluid, will then be withdrawn, through outlet 12 at a rate to match what enters the vessel through inlet 10, and after such withdrawal the magnetic particles will typically be recovered from the light fraction by washing, followed by magnetic separation, gravity separation or the like. The recovered magnetic particles will then be re-used as a constituent of another charge of material for inlet 10.

A possible advantage of locating the inlet tube 10 on the axis 3J instead of displaced from it as shown in Figure 1, is that the slight centrifugal action of the rotating fluid should tend to carry the light fraction towards the outlet tube 11; such an arrangement also makes essentially more use of the cross-section of the separator.
Various theories have been put forward as to how magnetic particles, suspended in suitable fluid, may be caused to spin by being subjected to a travelling magnetic field. One such
explanation is given by the present inventor in his article
"Travelling Magnetic Waves in Electrical Machines described by Rotating Vectors" in Proc. IEE, Vol. Il6, No. 6, June 1969.
In another article, on pages 235-236 of "Electrical Review" l4th February 1969, he explains in detail how the directional travel of the field may be related to the directions of spin of the particles and of movement of the fluid as a whole. Figure 2 illustrates these directions. Thus if the direction of rotation of the electrical wave through the windings of the stator 6 of Figure 1 is clockwise when viewed from above (arrow- 21 ), then the direction of spin of the magnetic particles within the fiuid will be clockwise (arrows 22). At the surface 23 of vessel 1 the friction of the vessel walls permits only negligible relative movement of the fluid, but a short distance radially inward from those walls (arrows 24) observations suggest that the fluid as a whole is rotating clockwise also. However, on moving yet closer

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O to axis > the direction of rotation of the fluid is usually seen to change to anti-clockwise (arrow 25 )• If the vessel, instead of being fixed as already described with reference to Figure 1, is mounted on a platform freely rotatable about the axis ~ , then the contrast between the various rotary motions will be even more
marked since the' platform (26, Figure 2) and the vessel will both tend to move clockwise with the outermost fluid 24, while the
inner fluid layers will initially move anti-clockwise as at 25 •
The rotary motions of the fluid as a whole are caused by the spinning of the magnetic particles which, as already explained, is caused by the travelling magnetic fi.eld. The articles in the
Proceedings of the IEE and in "Electrical Review", already quoted, go into more detail as to the nature of these causes, but the
reason for the spinning of the particles may be summarized by
stating that travelling or rotating magnetic fields- are
characterised locally, i.e. in the vicinity of all the individual magnetic particles, by pairs of contra-rotating field vectors.
It may be shown that where a rotating field is set up within a
vessel surrounded by an annular stator, one of these vectors
almost always predominates and so induces spin. If the vectors were to become almost equal in magnitude the particles would not spin and flocculation would then be possible. If the magnetic
field were to be set up by two identical linear stators, one to either side of the vessel, then such equality of vectors could
occur on the centre line between the faces of the two stators;

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O PI however, in that case turbulence of the fluid could well then prevent such limited vector equality from causing flocculation.
The several graphical functions of Figure 3 show how the magnitude of various quantities, measured at the axis 3> varies along the length of that axis when a rotating magnetic field is established around the vessel 1 of Figure 1. If the stator 6 is short in axial length, as shown in Figure 1 and as
represented by dimension A in Figure 3ι then a magnetic field as shown by function 30 will be set up, rising to a maximum around the central transverse plane 31 of the stator, together with the corresponding field' gradient pattern as shown by
function 32. At the higher field strengths required for a practic separator the magnetic particles of the fluid tend to be saturated. The "enhanced density" of the fluid, and hence the separation force which the fluid will exert on particles of non-magnetic mixtures placed within it, is then proportional to the product of the field gradient function 32 and the saturation magnetisation level of the fluid. In the saturated condition the "enhanced density" is lower than it would have been in the absence of saturation but is independent of the shape of the particles of material to be separated. A practical separator will have both saturated and unsaturated regions of magnetic fluid and a curve of enhanced density as shown by function 33 i the enhancement being relative to the density 34 of the unmagnetised fluid. Constituents of the unseparated mixture which have a density corresponding to

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r- W1P0 level 35 will float within a zone 36, while those with densities greater than level 37 will sink. The shaded area bounded by graph 33 and level 35 is also of interest: this represents the maximum kinetic energy of incoming ore particles of density level 35 that can be absorbed. Even though these particles of ore have a density below that indicated by level 37, if nevertheless they enter the separation vessel with a kinetic energy greater than that represented by the shaded area they will not have come to a halt by the time they reach level 38 along axis 3, and so they will therefore then continue to fall to the bottom of the vessel with the "heavy" fractions because after level 38 their density will be greater than that of the fluid. Inlet 10 should be designed to minimise this energy therefore.
The useful region of the separator is that corresponding to the flotation zone, which 'is at one end of the stator only. Increasing the stator length A beyond a certain value results in only a minor increase in enhanced density but a major increase in electrical supply -.loading .
In practice, the performance of the apparatus is also of course affected by other factors, especially dynamic quantities related to the rotation of the fluid, the suction at the mouth of outlet 11, and the horizontal entry velocity of fluid and ore into vessel 1 from inlet 10. The mouths of inlet 10 and outlet 11 may be shaped to minimise such effects also, which is why in Figure 1 the cut-away 12 is shown on the reverse face of the tube relative to the rotation of the fluid within vessel 1.

Further polyphase windings above and close to the stator shown in Figure 1 can be used to increase the field-gradient,.. and hence the "enhanced density", by forcing the field to fall off to zero more quickly. Iron can also be used in association with, or in place of, these windings but this iron could give rise to a flocculation problem by causing near-equality of the spinning vectors in particular regions.

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