WO1997023293A1

WO1997023293A1 - Magnetic separation in a magnetic fluid

Info

Publication number: WO1997023293A1
Application number: PCT/GB1996/003146
Authority: WO
Inventors: James Henry Peter Watson; Zhengnan Li
Original assignee: University Of Southampton
Priority date: 1995-12-21
Filing date: 1996-12-19
Publication date: 1997-07-03
Also published as: ZA9610790B; GB2308319A; AU1163697A; GB9526179D0

Abstract

There is disclosed a method of separating paramagnetic particles on the basis of their saturation magnetisation. A suspension of the particles in a paramagnetic fluid is subjected to a magnetic field gradient in a magnetic field sufficient to saturate them magnetically. By selecting the magnetisation of the fluid, the particles may be separated on the basis of how their individual saturation magnetisations compare to the magnetisation of the fluid. Preferably Vortex Magnetisation Separation is employed. The method is particularly suitable to sort synthetic diamonds, the impurities of which cause paramagnetic behaviour.

Description

MAGNETIC SEPARATION IN A MAGNETIC FLUID

The present invention relates to the magnetic separation of particles, and in particular of synthetic diamonds. Synthetic diamonds have previously been separated by a magnetic lifting process. Such a lifting process has poor selectivity and results in the separated groups having overlapping ranges of properties.

Though it has not previously been proposed for use with synthetic diamonds, a known magnetic separation technique is

High Gradient Magnetic Separation (HGMS) in which capture occurs on the front of a matrix arranged in a slurry flow and subjected to a magnetic field. The selectivity of HGMS is poor and a known improvement is Vortex Magnetic Separation (VMS) in which capture occurs on the downstream side of a wire extending perpendicular to the fluid flow, provided certain conditions are met. VMS capture is associated with vortices formed behind the wire, so one condition is that such vortices are indeed established. Vortices occur when the Reynolds number, Re, is greater than about 6, Re being defined by

Re » 2a.ρ.V₀/τ/ (1), where 2a is the diameter of the wire, Q is the density of the fluid, the V₀ is the velocity of the fluid and η is the viscosity of the fluid. Another condition is that the ratio of V_m/V₀ is less than about 1, where V„ is the slurry flow rate and V_ro is the magnetic velocity given by V_m = (2χb² M_sH₀)/(9na) (2), where χ is the susceptibility of particles of radius b, M_s is the saturation magnetisation of the matrix, H₀ is the applied field, η is the viscosity of the fluid and a is the radius of a circular cross-section matrix.

In accordance with the present invention there is provided a method of separating paramagnetic particles having differing saturation magnetisations, the method comprising preparing a suspension of the particles in a paramagnetic fluid, and subjecting the suspension to a magnetic field providing a region in which respective particles experience forces which depend on the difference between the values of the magnetisation of the respective particle and the selected magnetisation of the paramagnetic fluid to separate the particles according to that difference, wherein the magnetic field is of sufficient strength to saturate magnetically the particles so that the particles are separated according to their saturation magnetisation.

It is particularly advantageous to separate paramagnetic particles according to their saturation magnetism because this allows the selectively of the separation to be improved as compared to separation at low magnetic fields. By selecting the magnetisation of the paramagnetic fluid relative to the saturation magnetisation of a particular paramagnetic particle, it is possible to control whether that particle still behaves paramagnetically or starts to behave diamagnetically in the magnetic fluid and the particles may be separated on this basis. A further advantage of separating the particles in magnetic fields which saturate them is that, if the paramagnetic fluid is not saturated magnetically in those magnetic fields, then the magnetisation of the paramagnetic fluid may be selected by selection of the magnitude of the applied magnetic field. This in turn allows separation to occur according to the magnetic field. The magnetic field being easy to control, this allows a simple variation of magnetic field to control separation and hence improves selectivity. The method of the present invention is applicable (but not limited) to the separation of synthetic diamonds, because they have superparamagnetic impurities as a result of their manufacture. The benefit of separation according to saturation magnetisation is that the saturation magnetisation is dependent on the extent of impurity and hence the compressive strength of the diamonds. In general, the invention is expected also to be applicable where the particles themselves are paramagnetic, in particular where this is caused by impurities present therein, as in the case of synthetic diamond particles. Alternatively, if the magnetic fluid is not saturated magnetically in said magnetic field, and the magnetisation of the paramagnetic fluid may be selected by selection of the susceptibility thereof.

Any suitable magnetic separation technique can be used. However the present invention is particularly advantageous when used with HGMS or VMS, wherein said magnetic field is produced by magnetising a matrix by applying an externally-generated magnetic field to produce said region around said matrix, and said step of subjecting the suspension to the magnetic field comprises causing the suspension to flow past the matrix, so that separation occurs by virtue of whether or not the particles are attracted to and captured by the matrix.

In one embodiment, wherein said matrix is arranged to capture paramagnetically behaving particles having a saturation magnetisation more than the selected magnetisation of the paramagnetic fluid, the method comprises causing the suspension to flow repeatedly past the matrix while successively reducing the applied magnetic field, and collecting the particles captured on the matrix at successive values of the applied magnetic field as successive fractions.

In another embodiment, said step of causing the suspension to flow past the matrix comprises causing the suspension to flow past the matrix while an initial magnetic field is applied so that a group of particles are captured by the matrix, varying the applied magnetic field while a flow of the magnetic fluid is maintained so that successive particles captured by the matrix are swept off the matrix by the flow as the value of the magnetisation of the fluid passes the saturation magnetisations of respective particles, around which value the attractive force exerted by the matrix is insufficient to maintain capture, and collecting the particles which are swept off the matrix at successive values of the applied magnetic field as successive fractions. A solution of MnCl₂ in water is the preferable choice for the magnetic fluid.

It may be desirable to reject (or collect) particles having ferromagnetic inclusions. In the case of synthetic diamonds, for example, ferromagnetic inclusions can decrease particle strength. For this purpose, the particles may be subjected to an applied magnetic field, preferably lower than the magnetic field applied to separate paramagnetic particles according to their saturation magnetisation. As such low fields the effect of the ferromagnetic inclusions on a diamond exceeds the effect of any paramagnetic impurities. This low magnetic field separation according to the number and strength of ferromagnetic inclusions may be carried out in addition to the separation discussed above, but may also be used independently. According to another aspect of the present invention, there is provided wire for a matrix for magnetically capturing paramagnetic particles from a suspension flowing therepast, the wire comprising a magnetic core and a substantially non-magnetic material masking a partial surface area of the core to prevent capture of particles attracted to said partial surface area and to allow capture of particles attracted to the remaining surface area of the core.

The present invention is further explained by a non- limitative description given with reference to the accompanying drawings, in which :-

Fig. 1 shows a plot of the magnetisation of a group of diamonds at varying magnetic fields; Fig. 2 shows a plot of the compressive strength of groups of diamonds against the density of their superparamagnetic imparity grains;

Fig. 3 shows a related plot to Fig. 2 of strength against the average inter-grain distance;

Fig. 4 illustrates the magnetic behaviour of particles of magnetisation M_βat suspended in a paramagnetic fluid at different magnetic fields;

Fig. 5 illustrates a wire arranged in a fluid flow for Vortex Magnetic Separation;

Figs. 6A & 6B are cross-sectional views of the matrix wires used in the preferred method; and

Fig. 7 is a cross-sectional view of an alternative matrix wire; Fig. 8 illustrates a further embodiment; and

Fig. 9 is a perspective view of an alternative apparatus employing the separation method of the present invention.

To facilitate understanding, firstly the properties of synthetic diamonds will be discussed. To investigate the properties of synthetic diamonds, a population of diamonds of 30-35 mesh were carefully magnetically separated into twelve groups on the basis of their magnetic susceptibility. The magnetisation of the diamonds of each group was measured in a varying magnetic field (B) . Each group exhibited paramagnetic behaviour in that the magnetisation increased with B initially, but tended to a saturation value (M_βat) at high B. The measured value of M_βat is shown for each group in table 1 below. The filled-in points in Fig. 1 show the plot, for one of the groups, of the magnetisation (M„_βt) normalised by the high-B saturation magnetisation (M_βat) against B. Similarly shaped plots were obtained for each group. Synthetic diamonds may be produced by subjecting carbon

(eg. graphite) to high temperature and pressure in the presence of a catalyst such as Fe, Co or other metals or their compounds. The catalyst may remain as an impurity in the diamond. The magnetic properties of the studied groups can be attributed to the Fe-Co impurities in the samples on the basis that they form superparamagnetic grains.

The classical theory of paramagnetism of N grains of magnetic moment M (J/T) suggests that the magnetisation is given by the equation M = μμ₀ L(x) (3) , where L(x) is the Langevin function and x « μB/kT. By fitting the Langevin function to the measured results for each group, as shown by the hollow points in Fig. 1 for one of the groups, μ was determined for each group. The results are shown in Table 1 in the column headed Mu.

Assuming a value of 2.42 for the number n_B of Bohr magnetrons per atom of Fe-Co, the grain size can be calculated. As shown in the column of Table 1 headed Diam, the grain diameter is approximately 50 A. It is thought n_B for Fe-Co is fairly constant near the kinds of impurity component atoms expected. If n_B is smaller, the particle size would be larger, maybe to 80 A. At high x (high B) , L(x) tends to unity, so the classical theory suggests that the saturation magnetisation M_sat is given by the formula .«_t = μ₀ (4) . Using this formula and the measured values of M_#ac and μ, the number of grains N per unit volume was calculated for each group and is shown in the third column of table 1.

At low x, L(x) is approximately (x/3) , which means the low B susceptibility χ is given by χB = M_βat. (μ/3kT) . B (5) .

Using this formula, a value for the saturation magnetisation M_Bat(calc) was calculated for each group and is εhown in table 1. The agreement of the measured value of M_βat and M_sat(calc) gives a measure of confidence in the classical model for the studied diamonds.

Lastly, the compressive strength of the diamonds of each group was measured and is shown in the final column of table 1. The increasing strength can be explained by the decreasing number of grains of impurity through the groups, as shown in Fig. 2. Fig. 3 shows that there is a good linear fit between the strength of the groups and the average distance (r) between the N grains per unit volume.

Since the impurity grain size is constant, Mu is also (almost) constant through the groups. This indicates that separation of synthetic diamonds according to the parameter M_aat is a good way to separate them according to their compressive strength. Selection according to M_aat is also chosen in preference to selection at low B when there are present ferromagnetic inclusions which can cause anomalies in the magnetisation at such low fields. In general terms, magnetic separation is achieved by a combination of a magnetic field per se, which magnetises the particles, and a magnetic field gradient, which generates a force such that paramagnetic (and ferromagnetic) particles move towards higher B field regions and diamagnetic particles move towards lower B field regions. In general, the force F„ on a particle of volume V_p in a field of magnitude B which induces a magnetisation M is given by the equation

F_m = M.V_p. V(B)/μ₀ (6) .

In the present invention, the B field is chosen to saturate the magnetic particles, so their magnetisation is M_βat.

Furthermore, the particles are separated in a (preferably unsaturated) paramagnetic fluid. Thus, the resultant force F_m on the particles of the fluid in susceptibility χ_Uq is given by the equation F_m = (M_aat - χ_liq.B) . V_p.V(B)/μ₀ (7).

The magnitude of the force is proportioned to the difference between the saturation magnetisation of the particles (M_βat) and the magnetisation of the fluid (χ_in3.B) . Particles having a magnetisation higher than the fluid magnetisation are forced towards higher fields. Paramagnetic particles having a magnetisation lower than the magnetisation of the fluid M_Uq {= χ _<-.B) behave diamagnetically and are forced towards lower fields. Thus, the physical basis of the separation method of the present invention is that the direction of the force on the particles, and hence whether they are selected or rejected depends on how their saturation magnetisation compares to the magnetisation of the liquid, which may itself be selected. This force is substantially independent of the diamond size.

A particularly suitable magnetic fluid for the purpose is MnCl₂ dissolved in water. It is possible to use other magnetic fluids, for example other water-soluble substances having a magnetic moment. It is preferable (but not essential) to avoid fluids, such as a suspension of magnetite, which would saturate.

For the maximum amount of MnCl₂ soluble in 100 ml water at room temperature of 72g, the magnetic susceptibility is 0.78 x IO^*3. For the diamond groups 1 to 12 discussed above in such a solution, the B field required to convert them from paramagnetic to diamagnetic behaviour is shown in Table 2.

This is illustrated diagrammatically in Fig. 4, wherein particles of saturation magnetisation M_sat (plotted against the horizontal axis) in a fluid of magnetic susceptibility χ₁ and exposed to a magnetic field B (plotted against the vertical axis) exhibit paramagnetic behaviour in the shaded area below line χ_x and diamagnetic behaviour above.

Fig. 4 also makes it apparent how different fractions can be collected or rejected by varying the magnetisation of the fluid (χii_q.B) . Given a fluid of susceptibility χ_{l t} if the field experienced by the particles is raised from B_L to B„, then the fraction of diamonds between the lines M_L and M_H cease to behave - li ¬ as paramagnets and behave instead as diamagnets. Similarly at a given field B_L, if the selected susceptibility is increased from Xi to χ₂, then the fraction of diamonds between the lines M_L and M₂ cease to behave as diamagnets and behave instead as paramagnets.

A variety of techniques may be used to select and collect different fractions. Possible known separation techniques are High Gradient Magnetic Separation (HGMS) and Vortex Magnetic Separation (VMS) . In both HGMS and VMS, a suspension of particles is caused to flow past a, preferably ferromagnetic, magnetisable matrix subjected to a magnetic field. The necessary field gradient is produced around the matrix by the matrix becoming magnetised, causing the matrix to attract particles and physically to capture them. The theory and practice of HGMS and VMS are known to the skilled person and documented in various papers and articles (for example, Li and Watson: IEEE Trans.Mag. 30 (1994) 4662).

In HGMS capture occurs on the upstream side. HGMS allows magnetic particles to be manipulated on a large scale at high processing rates and has developed since its origin in the clay industry to have a large number of potential applications in fields as diverse as cleaning of human bone marrow, nuclear fuel reprocessing, sewage and waste water treatment, industrial effluent treatment, industrial and mineral processing, extracted metalogy and bio-chemical processing. However, HGMS has often been frustrated by a lack of selectivity mainly due to mechanical entrainment of unwanted particles as the particle size is reduced and the particle system becomes less monodisperse. Thus, a better separation technique for the present invention is Vortex Magnetic Separation (VMS) . In contrast to the upstream capture of HGMS, VMS involves the capture and retention of magnetisable particles on the downstream side of a matrix subjected to a magnetic field, in association with eddies formed behind the matrix.

Fig. 5 shows, as an illustrative example of a suitable matrix, a circular cross-section ferromagnetic wire 10 in the so-called longitudinal arrangement in which the wire is arranged perpendicularly to the parallel fluid flow V₀ and applied magnetic field H₀. The flow disturbance around the wire depends on the Reynolds number Re which is given by the formula Re = 2a . Q . V₀/η (1) , where 2a is the diameter of the wire, Q is the density of the fluid, the V₀ is the velocity of the fluid and η is the viscosity of the fluid. VMS occurs when Re > 6 at which point two symmetrical vortices or eddies 20, rotating in opposite directions, form and remain fixed to the rear of the wire with the flow closing behind them. The vortices are formed by the separation from the wire wall of the fluid boundary layer 30 caused by the frictional force of the wire wall on the fluid flowing past. When Re increases above about 40, VMS is no longer possible, because the vortices become unstable. Another factor which controls whether VMS occurs is the ratio V_m/V₀, where V_m is the so-called "magnetic velocity" and is given by V_m = <2M_s(M._at - χ_liq . B)b²/9 M_o-T?.a (8), where the variables have the same meanings as in equations (2) and (7) .

Above the field which magnetically saturates the wire 10, the effect of the field on the wire 10 is that the portions 40, 41 which are shaded in Fig. 5 are attractive to paramagnetic material, whereas the unshaded portions 50 centred on the Y-axis are attractive to diamagnetic particles. When V-,/V₀ > 1 paramagnetic particles are captured on the upstream portion 40 of the wire. Experiment and theory show that when V_m/V₀ < 1, paramagnetic particles are swept off the front of the wire, through the repulsive region and into the vortices which assist capture on the downstream portion 41 of the wire. V_m/V₀ must be sufficiently small that the particles are not deflected so far from the wire that they avoid capture..

The thickness δa of the boundary layer 30 also has a close relationship with the downstream magnetic capture. The boundary layer must be sufficiently thick that the particles can be kept within it, to be swept round into the vortices. Since the thickness of the boundary layer increases with the wire radius a, relatively thick wires are necessary for capture when the size range of the feed is wide.

Other relevant factors are the size of the upstream deposit on portion 40 and the size of the deposit of particles on the diamagnetic portion 50, which can sometimes disrupt the boundary layer on the wire, and the applied magnetic field which is required to magnetically saturate the wire. Around the wire, the field experienced by the particles in the fluid will be approximately (B₀ + M_w) , where B₀ is the applied field and M_w is the magnetisation of the wire. As discussed above, the field experienced controls whether or not the paramagnetic particles behave as paramagnetics or diamagnets. In fact the situation is slightly more complex. The magnetic field caused by the magnetisation of the wire is of course not uniform so their boundary between paramagnetic and diamagnetic behaviour is not a precise field but a range. Whilst VMS has been described above in terms of a single wire arranged in the fluid flow, any suitable magnetisable matrix may be arranged in the slurry flow. The matrix may take the form of a row of wires extending across a slurry flow channel, preferably perpendicular to the flow, or an array of wires. The wires may have circular cross-section but other cross-sections are equally applicable. Instead of wires, the matrix may take the form of one or more perforated plates arranged across the flow channel or a stainless steel wool.

For example, in one VMS embodiment, circular nickel wires of diameter between 125μm and 3mm were arranged in a flow channel of cross-section 5mm and 10mm. A larger scale embodiment uses 5mm diameter wires in an 8cm diameter channel. This larger-scale embodiment is intended to sort 35-40 mesh diamonds and with a slurry having 10% solids flowing at 40mm/s and an applied field of 2T, the processing rate may be 72 kg/h. The preferred method uses the wires illustrated in Figs. 6A and 6B or Fig. 7. The entirely ferromagnetic matrix is replaced by a composite matrix comprising a magnetic (preferably ferromagnetic) core disposed in a non-magnetic (or weakly magnetic) material which masks the attraction of some of the surface area of the core to prevent capture, whilst allowing capture by the attraction of the remaining surface areas of the core because they are exposed or covered only in a shallow layer of non-magnetic material. The non-magnetic material may be polythene or another plastic, neoprene or a ceramic, though any non-magnetic (or weakly magnetic) material will suffice. The wire is preferably formed by extrusion. For example, the matrix wires 104 shown in Figs. 6 comprise ferromagnetic wire 105 of circular cross-section as a core embedded eccentrically in non¬ magnetic strip 108 of circular cross-section. Other cross- sections for the core and matrix wire are equally possible. For example, the core may be disposed in a groove in the non¬ magnetic strip, for ease of manufacture.

A matrix of the wires 104 may then be positioned in a slurry flow arranged for VMS, with the ferromagnetic disposed on the downstream side of the wires 104 as shown in both Figs. 6. If the magnetic field is approximately parallel to the flow, as shown in Fig. 6A, then the downstream surface 106 of the ferromagnetic wires 105 is attractive to and captures paramagnetic particles. In contrast, non-magnetic strip 108 masks the surfaces 107 of ferromagnetic wire 105 which are attractive to diamagnetic particles to prevent their capture. Alternatively, if the magnetic field is disposed transversely to the wires (and the flow) , as shown in Fig. 6B, then the downstream surface 106 of the ferromagnetic wires 105 is attractive to and captures diamagnetic particles. In contrast, non-magnetic strip 108 masks surfaces 107 of ferromagnetic wire 105 which are attractive to paramagnetic particles to prevent their capture.

In the arrangements of both Figs. 6A and 6B, the non¬ magnetic material prevents capture of any particles on the upstream surface 109. This has the advantage that mechanical entrainment is reduced and VMS operation is enhanced, because the vortices are not disturbed.

It can thus be seen that the purpose of the non-magnetic material is to occlude some of the potential wells created around the wire 108 (for example, in the wire arrangement of Fig. 6A, the potential wells on the sides of the wires 108 which are attractive to diamagnetically behaving particles) whilst allowing attraction and capture at the remaining potential well (downstream of the wire in Figs. 6) .

Instead of the wires shown in Figs. 6, the ferromagnetic core may extend entirely through the non-magnetic material, so that capture occurs on both opposing sides of the wire. The wires 84 shown in Fig. 7 are an example and have a substantially circular cross-section formed by a ferromagnetic bar 85 of elongonate cross-section with flat or curved shorter sides 86 exposed and longer sides 87 sandwiched between two non-magnetic strips 88 of semi-circular cross-section. The wires 84 may be disposed perpendicular to the flow with bar 85 extending between strips 88 parallel thereto. If the magnetic field is parallel to the flow (or transverse to the flow and wires) it causes the exposed sides 86 to be attractive to paramagnetic (or diamagnetic) particles. The structure of wire 84 prevents diamagnetic particles from being captured, because strips 88 are non-magnetic and effectively mask the longer sides 87.

It is preferable not to use the wires 84 with a transvere magnetic field, because in this configuration, whilst the short sides 86 of the bar 85 are in their central parts attractive to diamagnetic particles, the corners of the bar 85 as seen in Fig. 7 are attractive to paramagentic particles.

In fact, the non-magnetic material will display weak diamagnetic behaviour, because it is disposed in a paramagnetic fluid. The origin of this force is the same as for the particles which are to be separated, as discussed above. This does not cause a great problem, because the forces concerned are small. To minimise them, it is possible to select a material which, instead of being non-magnetic, is slightly magnetic with a susceptibility similar or equal to that of the paramagnetic fluid, such as aluminium. Using either of the arrangements of Figs. 6A or 6B, the preferred method involves recirculating the suspension to flow repeatedly past the matrix. Since the arrangement of Fig. 6A captures paramagnetically behaving particles having a saturation magnetisation greater than that of the fluid, here the applied magnetic field is successively reduced, continuously or in steps. Thus at each value of the applied magnetic field a fraction of particles having a successively lower saturation magnetisation is captured. The particles captured at each value of the magnetic fields are collected as successive fractions, for example, by removing the matrix from the field, collecting the particles and replacing the matrix. On the other hand, with the arrangement of Fig. 6B which captures diamagnetically behaving particles, the magnetic field is successively increased and the particles captured at successive values are collected as successive fractions which have increasing saturation magnetisations. In fact, the preferred method for a given sample is to use both arrangements of Fig. 6A and 6B in succession, in either order, by altering the direction of the applied magnetic field in between. With the arrangement of Fig. 6A, the field is reduced to an intermediate value to separate the particles having saturation magnetisations higher than that of the fluid at the intermediate magnetic field value. The arrangement of Fig. 6B is used to separate the other particles by increasing the applied magnetic field from a minimum up to the intermediate value. A more complicated separation apparatus for diamonds will now be described with reference to Fig. 8. In a given solution of MnCl₂, a sample of diamonds may be grouped as follows:- Group 1 - Diamonds which always behave as paramagnets in fields up to maximum practically attainable field (H-_^) , because the magnetisation of the fluid (Xi_iq.B) does not reach their saturation magnetisation. Group 2 - Diamonds which behave as diamagnets or paramagnets depending on the field they experience. Group 3 - Diamonds which always behave as diamagnets, because they have a low saturation magnetism. As above, a lower saturation magnetism arises from a low impurity content, so the strongest diamonds are in group 3 and the weakest in group 1.

The overall apparatus is shown in Fig. 8. A suspension of diamonds is prepared and caused to flow in direction V₀ along a channel 80 through upper and lower sections 81 and 91 where magnetic fields H^ and H₀ are produced by independently controlled magnets 82 and 92, respectively.

The upper section 81 is intended to remove the diamonds of group 1. Magnet 82 produces the maximum practically attainable field H^_x in which only the group 1 diamonds behave as paramagnets and all the diamonds of groups 2 and 3 behave as diamagnets.

To capture the group 1 diamonds, there is disposed within upper section 81 a matrix 83 arranged for VMS comprising an array of wires 104 as shown in Fig. 6A or an array of wires 84 as shown in Fig.7. VMS capture of the paramagnetic group 1 diamonds occurs on the downstream side of wires 104 or 84, whereas the diamonds of groups 2 and 3 pass through upper section 81. Matrix 83 has a sufficient number and length of wires 104 or 84 to capture substantially all the diamonds of group 1. The lower section 91 houses a conventional VMS matrix 93, preferably comprising a array of wires 94 arranged perpendicular to the direction of the flow V₀ and the magnetic field H₀ produced by magnet 92. Though VMS is preferable, ordinary HGMS would be adequate.

To understand the operation of the lower section 82, it is first useful to consider the effect of the magnetic field on the wires 94 of matrix 93. As above, with reference to Fig. 5, portions 40, 41 of the wire centred on the axis of the magnetic field are attractive to paramagnetic material, whereas portions 50 centred on the axis perpendicular to the magnetic field are attractive to the diamagnetic material. The angular size 2 _x of the paramagentically attractive portions 40,41 (see Fig. 5) is given by the equation tan φ₁ = "/((l+lO/α-K)) (9), where K=M_β/2H₀, an M, is the saturation magnetisation of the matrix and H₀ is the applied field. This applies when the wire 94 is magnetically saturated, i.e. where the applied the applied field is greater than the saturation field H_s which occurs approximately in the region 0.8-1 T. If the matrix is not saturated, then K=l and = 90° and the diamagnetically attractive portion 50 vanishes. Lower section 91 is run initially with the applied field H₀ set at the maximum field H,^, well above H_B. Most of the diamonds of the groups 2 and 3 which have passed through upper section 81 are captured on the diamagnetic portions 50 of wires 94 of matrix 93. If any group 1 diamonds have passed uncaptured through upper section 81, they will be captured on the paramagnetic portion 41 of wires 94. At any given applied field H₀, there will be a sub-group of diamonds from group 2 which have a saturation magnetisation which is close to the magnetisation of the magnetic fluid. The consequence is that the force on these particles becomes very small, as given by equation (7) wherein the field B experienced is the applied field plus the magnetisation of the wires M_s. Thus for this sub-group of diamonds no capture occurs because the magnetic force is insufficient, as compared to the competing forces in the flow such as viscosity. At the initial maximum field H,-^ in lower section 91, the sub-group of the group 1 diamonds which pass through will be those at the boundary with group 1 with the higher saturation magnetism/impurity content.

Next, while the fluid flow is maintained at the same velocity, either by re-circulation or by reintroducing fresh magnetic fluid without new suspended diamonds, the magnetic field H₀ used in lower section 91 applied by magnet 92 is gradually and continuously reduced from H-^. Thus, sub-groups of the group 2 diamonds of successively lower saturation magnetisation cease to be attracted to the diamagnetic portions 50 of the wires 94 by which they were initially captured and are swept off and out of the lower section 91 of the apparatus. Fractions of diamonds of successively decreasing saturation magnetisation may be easily collected, for example by filtering the fluid output over successive ranges of the reducing magnetic field.

Clearly the range of saturation magnetisms of the sub¬ groups, and hence the size of the sub-groups, will vary a little here the competing forces vary, such as for different flow velocities or particle sizes.

It can occur that diamonds are physically prevented from being swept from wires 94 by other diamonds of lower saturation magnetisation which remain attracted. Eventually, after the applied magnetic field has been reduced further, these diamonds will become free, and be released. Instead of passing out of lower section 91, they are captured by portions 41 of the wires 91, because they are now acting as paramagnetics. This is a particularly advantageous feature of the method, because these late-released diamonds have a higher impurity content than the diamonds which are correctly released at a given field, so it would reduce the selectivity of the method to mix them in the collected fraction. This process of diamonds being swept of the wires is assisted by the reduction in size of the diamagnetic portion 50 which finally vanishes when the applied field lowers to the saturation field H_s of the wires. At lower fields the matrix 93 is not saturated and so no portion attracts diamagnetic material. The effect is that when the reducing applied field reaches H_s, then the diamonds of group 3 are released and pass out of lower section 91 as the final fraction. This leaves the matrix 93 holding the group 2 material which was released late and subsequently captured on the paramagnetic portion 41 of wires 94, and possibly also a very small quantity of group 1 diamonds which were not captured in the upper section 81. By subsequently reducing the field in lower section 91 to zero, this residual material is released and, instead of collection, is returned to the feed material and re-cycled in the subsequent run.

Lastly, the magnetic field in upper section 81 is reduced to zero and the strongly magnetic material of group 1 is collected. VMS separation works particularly well for diamonds having Fe-Co impurities which have a comparatively flat distribution of saturation magnetisations.

Of course, HGMS and VMS are not the only separation techniques in which the present invention may be employed. Shown in Fig. 9 is an alternative separate apparatus which uses the method of the present invention but which does not capture particles on a matrix. The arrangement comprises a first chamber 61, a first splitter 63, a second chamber 65 and a second splitter 67 adjacently disposed to form a fluid flow path, through which a suspension of diamonds is caused to flow in the direction of arrow F.

Gradient coils 62 create a high magnetic field in first chamber 61, sufficient to magnetically saturate the diamonds. The high magnetic field increases in strength vertically downwards in the direction of arrow H. Thus, the magnetisation of the fluid, which is not saturated, also increases in the direction of arrow H. For the reasons discussed previously, the resultant force on the diamonds in first chamber 61 acts to urge each individual particle towards a part of the high field region whereat the fluid has a magnetisation equal to the magnetisation of the given diamond. Thus, provided the field region in chamber 61 is sufficiently long, the diamonds are sorted to a horizontal level according to their saturation magnetisation.

Splitter 64 arranged downstream of chamber 61 is divided into collection passages 64 by shelves 68. Collection passages 64 are horizontal, so each receives a respective portion of the satisfied flow which has passed through a part of chamber 61 experiencing a respective part of the B field range. Thus, successive collection passages 64 collect fractions of the diamonds which have been sorted in chamber 61 to have successively increasing saturation magnetisation.

If the second chamber 65 and second splitter 67, which are optional, are omitted, then the flow from each collection passage 64 may be passed to a separate filtration system for extracting the respective fractions out of suspension. When second chamber 65 is used, there is produced within second chamber 65 by gradient coils 66, a low magnetic field lower than the high magnetic field. Preferably the low field is sufficiently low that the force acting on any ferromagnetic inclusions in a given diamond exceeds the force derived from superparamagnetic impurities in the diamond.

The magnitude of the low magnetic field increases in the direction of arrow L. Thus, the field acting on ferromagnetic inclusions existing within some diamonds urges those diamonds towards the higher field regions of the low magnetic field. Downstream of second chamber 65 is a second splitter 67 divided into an array of collection passages 69 by horizontal shelves 70 and one or more vertical walls 71. The rows 72 of collection passages 69 collect diamonds according to their saturation magnetisation, in a similar fashion to passages 68 of first splitter 63. On the other hand, wall 71 divides the second splitter 67 into a column 73 of passages 69 collecting diamonds according to the number and strength of ferromagnetic inclusions and a column 74 of passages 69 from which diamonds with inclusions have been rejected.

Of course, neither the linear apparatus arrangement nor the orientation shown in Fig 6 is essential. It is preferable, though, to arrange the low and high magnetic fields to increase in directions which are transverse, or even perpendicular, to one another.

As will be apparent, the method of the present invention may be exploited industrially to separate synthetic diamonds or other paramagnetic particles.

TABLE 1 - PARAMETERS OBTAINED FROM FITTING THE LANGEVIN CURVE

Meat (T) Mu(J/T) N/cu. metre Diam(A)

0.023 3.9335E-20 4.6530E+23 5.4100E+01

0.0092 3.5190E-20 2.0800E+23 5.2100E+01

0.00555 3.5200E-20 1.2500E+23 5.2100E+01

0.00249 3.7260E-20 5.3200E+22 5.3100E+01

0.001925 3.7260E-20 4.1100E+22 5.3100E+01

0.00125 3.7200E-20 2.6700E+22 5.3100E+01

0.000944 3.5200E-20 2.1400E+22 5.2100E+01

0.000835 3.0630E-20 2.1690E+22 4.9700E+01

0.000444 3.3120E-20 1.0670E+22 5.1100E+01

0.000429 3.3120E-20 1.0300E+22 5.1100E+01

0.000315 2.9810E-20 8.4100E+21 4.9300E+01

0.0002075 2.4800E-20 6.6600E+21 4.6400E+01

r dist Chi Meat(calc) 3 T

129 0.071 2.24E-02 1.2420E-20

169 0.028 9.88E-03 1.2420E-20

200 0.0155 5.47E-03 1.2420E-20

266 0.0068 2.27E-03 1.2420E-20

289 0.0048 1.60E-03 1.2420E-20

334 0.00315 1.05E-03 1.2420E-20

468 0.0023 8.12E-04 1.2420E-20

359 0.0019 7.70E-04 1.2420E-20

454 0.000885 3.32E-04 1.2420E-20

460 0.00078 2.92E-04 1.2420E-20

492 0.00058 2.42E-04 1.2420E-20

532 0.00035 1.75E-04 1.2420E-20 Mu/Chi Str'gth (N/mm)

1 361764E+18 2.7700E+03

2 770105E+17 2.6300E+03

3 909091E+17 2.8200E+03

4 216318E+17 3.2800E+03

5 505636E+17 3.8000E+03

6 483871E+16 3.5900E+03

7 090909E+16 4.6200E+03

8 712373E+16 4.2200E+03

9 275362E+16 4.8500E+03

10 768116E+16 5.5500E+03

11 194566E+16 4.5000E+03

12 580645E+16 4.9200E+03

Table 2

DIAMOND NO. FIELD (T) ABOVE WHICH PARTICLES ARE DIAMAGNETIC IN THE 0.78 X 10° SOLUTION

1 29

2 11.8

3 7.1

4 3.19

5 2.46

6 1.6

7 1.21

8 1.07

9 Always diamagnetic

10 Always diamagnetic

11 Always diamagnetic

12 Always diamagnetic

Claims

CLA S

1. A method of separating paramagnetic particles having differing saturation magnetisations, the method comprising preparing a suspension of the particles in a paramagnetic fluid, and subjecting the suspension to a magnetic field providing a region in which respective particles experience forces which depend on the difference between the values of the magnetisation of the respective particle and the selected magnetisation of the paramagnetic fluid to separate the particles according to that difference, wherein the magnetic field is of sufficient strength to saturate magnetically the particles so that the particles are separated according to their saturation magnetisation.

2. A method according to claim 1, wherein the paramagnetic fluid is not saturated magnetically in said magnetic field and the magnetisation of said paramagnetic fluid is selected by selection of the magnitude of said magnetic field.

3. A method according to claim 2, wherein said magnetic field is produced by magnetising a matrix by applying an externally-generated magnetic field to produce said region around said matrix, and said step of subjecting the suspension to the magnetic field comprises causing the suspension to flow past the matrix, so that separation occurs by virtue of whether or not the particles are attracted to and captured by the matrix.

4. A method according to claim 4, wherein said flow is arranged to create vortices downstream of said matrix which assist capture of said particles.

5. A method according to claim 3 or 4, wherein said matrix is arranged to capture paramagnetically behaving particles having a saturation magnetisation more than the selected magnetisation of the paramagnetic fluid, the method comprising causing the suspension to flow repeatedly past the matrix while successively reducing the applied magnetic field, and collecting the particles captured on the matrix at successive values of the applied magnetic field as successive fractions.

6. A method according to claim 5, wherein said step of reducing the applied magnetic field comprises reducing the applied magnetic field to a predetermined value, and the method additionally comprises causing the suspension to flow repeatedly past a matrix which is arranged to capture diamagnetically behaving particles having a saturation magnetisation less than the saturation magnetisation of the paramagnetic fluid, while successively increasing the applied magnetic field towards said predetermined value, and collecting the particles captured on the matrix at successive values of the applied magnetic field as successive fractions.

7. A method according to claim 3 or 4, wherein said matrix is arranged to capture diamagnetically behaving particles having a saturation magnetisation less than the selected magnetisation of the paramagnetic fluid, the method comprising causing the suspension to flow repeatedly past the matrix while successively increasing the applied magnetic field, and collecting the particles captured on the matrix at successive values of the applied magnetic field as successive fractions.

8. A method according to any one of claims 3 to 7, wherein said matrix comprises an array of wires disposed transversely to the flow of the suspension, the wires each comprising a magnetic core and a substantially non-magnetic material masking a partial surface area of the core to prevent capture of particles attracted to said partial surface area and to allow capture of particles attracted to the remaining surface area of the core.

9. A method according to claim 8, wherein the wires each have a substantially circular cross-section, with the cores disposed eccentrically therewithin so that said partial surface area is covered a greater thickness of said non-magnetic material than said remaining surface area.

10. A method according to claim 8 or 9, wherein said matrix is disposed in said flow with said remaining surface area downstream of said partial surface area, and said applied magnetic field is substantially parallel to said flow, whereby said matrix wires are arranged to capture paramagnetically behaving particles having a saturation magnetisation more than the selected magnetisation of the magnetic field.

11. A method according to claim 8 or 9, wherein said matrix is disposed in said flow with said remaining surface area downstream of said partial surface area, and εaid applied magnetic field is transverse to said flow, whereby said matrix wires are arranged to capture diamagnetically behaving particles having a saturation magnetisation less than the selected magnetisation of the magnetic field.

12. A method according to claim 3 or 4, comprising causing the suspension to flow past the matrix while an initial magnetic field is applied so that a group of particles are captured by the matrix, varying the applied magnetic field while a flow of the magnetic fluid is maintained so that successive particles captured by the matrix are swept off the matrix by the flow as the value of the magnetisation of the fluid passes the saturation magnetisations of respective particles, around which value the attractive force exerted by the matrix is insufficient to maintain capture, and collecting the particles which are swept off the matrix at successive values of the applied magnetic field as successive fractions.

13. A method according to claim 12, wherein the matrix is arranged to be attractive to diamagnetically behaving particles so that said group comprises particles having a saturation magnetism less than the magnetisation of the fluid at said initial magnetic field, and said step of varying the magnetic field is reducing the magnetic field so that the successive particles swept off the matrix have successively lower saturation magnetisations.

14. A method according to claim 3 to 13, further comprising causing the suspension to flow through an initial stage arranged to remove paramagnetically behaving particles which have a saturation magnetisation above that of the paramagnetic fluid in a predetermined maximum magnetic field.

15. A method according to any one of claims 3 to 14, wherein said matrix comprises an array of wires each of substantially circular cross section disposed transversly to the flow of suspension.

16. A method according to claim 1, wherein the magnetic fluid is not saturated magnetically in said magnetic field, and the magnetisation of the paramagnetic fluid is selected by selection of the susceptibility thereof.

17. A method according to claim 1, wherein the magnitude of the field varies across said region and the forces on the particles cause each respective particle to tend towards a part of said region in which the magnetisation of the magnetic fluid is the same as the magnetisation of the particle.

18. A method according to any one of the preceding claims, wherein at least some of the particles have ferromagnetic inclusions, and the suspension is caused to flow through a region provided by an applied field weaker than said first- mentioned applied field, in which weaker field respective particles experience a force which is dependent on the extent of ferromagnetic inclusions in the particle.

19. A method according to any one of the preceding claims, wherein the magnetic fluid comprises a solution or a suspension of a paramagnetic substance.

20. A method according to claim 19, wherein said magnetic fluid is a solution of MnCl₂ in water.

21. A method according to any one of the preceding claims, wherein said particles are synthetic diamonds.

22. A method of separating particles substantially as hereinbefore described.

23 . Diamonds εeparated by the method of any one of the preceding claims .

24. Wire for a matrix for magnetically capturing paramagnetic particles from a suspension flowing therepast, the wire comprising a magnetic core and a substantially non-magnetic material masking a partial surface area of the core to prevent capture of particles attracted to said partial surface area and to allow capture of particles attracted to the remaining surface area of the core.