GB2308319A - Magnetic separationin a magnetic fluid - Google Patents

Abstract

Paramagnetic particles of different saturation magnetisation are separated by subjecting a suspension of the particles in a paramagnetic fluid to a magnetic field of sufficient strength to saturate them magnetically. By selecting the magnetisation of the fluid, the particles are separated on the basis of how their individual saturation magnetisation compares to the magnetisation of the fluid; thus they may behave paramagnetically or diamagnetically. The magnetisation of the fluid may be controlled by altering the size of the applied field or the susceptibilty of the fluid. Preferably Vortex Magnetisation Separation is employed. The method is particularly suitable to sort synthetic diamonds using an aqueous solution of MnCl 2 , the impurities in the diamonds causing paramagnetic behaviour which aids separation. Diamonds with ferromagnetic impurities are removed by a sequential or simultaneous application of a magnetic field.

Description

MAGNETIC SEPARATION 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. The present inventor has appreciated that 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.Q.V0/ (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 17 is the viscosity of the fluid.

Another condition is that the ratio of Vm/VO is less than about 1, where V0 is the slurry flow rate and Vm is the magnetic velocity given by Vm = (2Xb2 MsHO)/(9Xa) (2) ,where X is the susceptibility of particles of radius b, Ms is the saturation magnetisation of the matrix, Ho is the applied field, P is the viscosity of the fluid and a is the radius of the matrix.

In accordance with the present invention there is provided a method of separating paramagnetic particles having differing saturation magnetisation, the method comprising subjecting a suspension of the particles in a paramagnetic fluid to an applied magnetic field of sufficient strength as to saturate magnetically the particles, the field providing a region in which each particle experiences a force dependent on the difference between the value of the saturation magnetisation of the particle and the selected magnetisation of the paramagnetic fluid.

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. This allows separation according to the magnitude of the saturation magnetisation of the individual particles with good selectivity. Particles having saturation magnetisations at or close to the required value become concentrated in the above-mentioned region and, once "captured" there, they can be removed by any suitable technique.

The magnetisation of the paramagnetic fluid may be selected (i) if the fluid is not magnetically saturated, by altering the size of the applied magnetic field or the susceptibility of the fluid, and/or (ii) by using a solution or a suspension of a paramagnetic substance as the fluid and altering the concentration thereof.

The present invention may be used within a Vortex Magnetic Separation technique. Alternatively, the method may be applied by varying the field across the region, whereby the magnetisation of the non-saturated fluid also varies across the region. Thus the system may be arranged to force respective particles towards the particular part of the region at which the magnetisation of the fluid, as controlled by the local magnetic field, is the same as the magnetisation of the particle.

A solution of MnCl2 in water is a possible choice for the magnetic fluid.

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. In general, the invention is expected to be applicable where, as in the case of synthetic diamond particles, impurities present therein cause the particles to act as paramagnets, making them susceptible to the present method.

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 imparities. 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.

The present invention is further explained by a nonlimitative 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 Nut suspended in a paramagnetic fluid at different magnetic fields; Fig. 5 illustrates a wire arranged in a fluid flow for Vortex Magnetic Separation; and Fig. 6 is a perspective view of an alternative apparatus employing the separation method of the present invention.

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 (Nut) at high 8. The measured value of Nut 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 q ) normalised by the high-B saturation magnetisation (M@at) 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 = NASo L(x) (3) where L(x) is the Langevin function and x = B/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, 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 nB 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 nB for Fe-Co is fairly Table 1 Parameters obtained from fitting the Langevin Curve

Dia No Msat(T) Mu(J/T) N/cu. metre Diam(A) r dist Chi 1 0.023 3.9335E-20 4.6530E+23 5.4100E+01 129 0.071 2 0.0092 3.5190E-20 2.0800E+23 5.2100E+01 169 0.028 3 0.00555 3.5200E-20 1.2500E+23 5.2100E+01 200 0.0155 4 0.00249 3.7260E-20 5.3200E+22 5.3100E+01 266 0.0068 5 0.001925 3.7260E-20 4.1100E+22 5.3100E+01 289 0.0048 0.00125 3.7200E-20 2.6700E+22 | 5.3100E+01 | 334 j 0.00315 7 0.000944 3.5200E-20 2.1400E+22 j 5.2100E+01 i 468 0.0023 8 0.000835 3.0630E-20 2.1690E+22 I 4.9700E+01 ! 359 i 0.0019 9 0.000444 3.3120E-20 1.0670E+22 5.1100E+01 454 0.000885 10 0.000429 3.3120E-20 1.0300E+22 5.1100E+01 460 0.00078 11 0.000315 2.9810E-20 8.4100E+21 4.9300E+01 492 0.00058 1 2 0.0002075 2.4800E-20 6.6600E+21 I 4.6400E+01 | 532 0.00035 Msat(calc) 3kT Mu/Chi Str'gth(N/mm 1 2.24E-02 1.2420E-20 361764E+18 2.7700E+03 2 9.88E-03 1.2420E-20 770105E+17 2.6300E+03 3 5.47E-03 1.2420E-20 909091E+17 2.8200E+03 4 2.27E-03 1.2420E-20 216318E+17 3.2800E+03 5 1.60E-03 1.2420E-20 505636E+17 3.8000E+03 6 1.05E-03 1.2420E-20 483871E+16 3.5900E+03 7 8.12E-04 1.2420E-20 090909E+16 4.6200E+03 8 7.70E-04 1.2420E-20 712373E+16 4.2200E+03 9 3.32E-04 1.2420E-20 275362E+16 4.8500E+03 10 2.92E-04 1.2420E-20 768116E+16 5.5500E+03 11 2.42E-04 1.2420E-20 194566E+16 4.5000E+03 I 2 1.75E-04 1.2420E-20 | 580645E+16 4.9200E+03 constant near the kinds of impurity component atoms expected. If nB 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@at is given by the formula = = N;0 (4) Using this formula and the measured values of M@at and , 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 X is given by XB = M;,.(M/3kT). B (5) Using this formula, a value for the saturation magnetisation M,(calc) was calculated for each group and is shown in table 1.

The agreement of the measured value of M@at and Mi,(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@at is a good way to separate them according to their compressive strength.

Selection according to M@at 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, which magnetises the particles, and a 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 Fm on a particle of volume Vp in a field of magnitude B which induces a magnetisation M is given by the equation Fm =MVp V(B)/po (6) In the present invention, the B field is chosen to saturate the magnetic particles, so their magnetisation is Ant Furthermore, the particles are separated in a paramagnetic fluid.

Thus, the resultant force Fm on the particles of the fluid in susceptibility X is given by the equation Fm = (M@at - Xliq#B) # Vp##(B)/ o (7) If the magnetisation of the fluid Mliq (= Xliq#B) is greater than the particle magnetisation, then the particle behaves diamagnetically and is forced towards lower fields. If the liquid magnetisation is lower than the particle magnetisation, then the particle is forced towards the higher 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 be selected. This force is substantially independent of the diamond size.

A particularly suitable magnetic fluid for the purpose is Bit12 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 103. 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.

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 This is illustrated diagrammatically in Fig. 4, wherein particles of saturation magnetisation M;; (plotted against the horizontal axis) in a fluid of magnetic susceptibility Xi and exposed to a magnetic field B (plotted against the vertical axis) exhibit paramagnetic behaviour in the shaded area below line X1 and diamagnetic behaviour above.

A possible known separation technique is High Gradient Magnetic Separation (HGMS) which allows magnetic particles to be manipulated on a large scale at high processing rates. HGMS 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 more preferable 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 socalled longitudinal arrangement in which the wire is arranged perpendicularly to the parallel fluid flow V0 and applied magnetic field HO. The flow disturbance around the wire depends on the Reynolds number Re which is given by the formula Re = 2a.Q.V0/ (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 P 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/V0, where Vm is the so-called "magnetic velocity" and is given by Vm = (2(Mz - Xliq . B)b2/9 i0..a (8), where the variables have the same meanings as in equations (2) and (7).

The portions 40, 41 of the wire which is shaded in Fig. 3 are attractive to paramagnetic material, whereas the unshaded portions 50 around the Y-axis are repulsive thereto. When V=/V0 > 1 the particles are held on the upstream side of the wire.

Experiment and theory show that when Va/V0 < 1 the particles are swept off the front of the wire, through the repulsive region and into the vortices which assist capture on the rear of the wire.

Vm/V0 must be sufficiently small that the particles are not deflected so far from the wire that they avoid capture.

The thickness Sa 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 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 particlesupporting fluid will be approximately (BO + Mw), where BO is the applied field and Mw is the magnetisation of the wire. As discussed above, the field experienced by the paramagnetic particles (BO + Mw) controls whether or not they behave paramagnetically and are caught by the wire (because they are suspended in a paramagnetic fluid).Thus, at an applied field B0 such that the field experienced by the particles is BH, in general terms a wire will capture particles which are paramagnetic at BH. With reference to Fig. 4 this is particles for which M, > MH in a fluid of susceptibility X1 In fact the situation is slightly more complex. The magnetic field caused by the magnetisation of the wire is of course not uniform so it is not possible to define a single field experienced by the particles. The result is that slightly diamagnetic particles may be captured or slightly paramagnetic particles rejected, so that is some variability at the boundary between whether a particle is captured or not.

If the particles are synthetic diamonds, their compressive strength increases in the direction of the decreasing M, shown by arrow S. By decreasing the applied field B0 so that the field experienced by the particles is reduced to BL, an additional fraction of stronger diamonds (those for which ML > Nut > MH) may be captured so that the diamonds which pass are stronger, but in a more dilute suspension.

Similarly, to pass stronger diamonds, the susceptibility of the paramagnetic solution may be lowered (for example by using a different salt solution or altering the solution strength), so that at the same applied field an additional fraction of diamonds are captured which are stronger than those captured with a solution of higher susceptibility. This is illustrated in Fig. 4 by the dotted line X2 which represents the diamagneticparamagnetic boundary for a solution of susceptibility X2 (X2 < Xl)- Thus (at the field BL) diamonds for which Nut > M2 fall to be captured rather than only particles for which Nut > ML.

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. The wires may have circular cross-section but other cross-sections are equally applicable. Instead of wires, the matrix may take the form of a perforated plate arranged across the flow channel.

For example, in one VMS embodiment, circular nickel wires of diameter between 125m and 3mm were arranged in a flow channel of cross-section 5mm and lOmm. 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.

Of course, VMS is not the only separate technique in which the present invention may be employed. Shown in Fig. 6 is an apparatus which uses the method of the present invention but not VMS. 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 magnetisations.

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.

Claims (14)

1. A method of separating paramagnetic particles having differing saturation magnetisation, the method comprising subjecting a suspension of the particles in a paramagnetic fluid to an applied magnetic field of sufficient strength as to saturate magnetically the particles, the field providing a region in which each particle experiences a force dependent on the difference between the value of the saturation magnetisation of the particle and the selected magnetisation of the paramagnetic fluid.

2. A method according to claim 1, wherein the magnetic fluid is not magnetically saturated in said magnetic field to which the suspension is subjected.

3. A method according to claim 2, wherein the magnetisation of the paramagnetic fluid is selected by selection of the susceptibility thereof.

4. A method according to either one of claims 2 or 3, wherein the magnetisation of the paramagnetic fluid is selected by alteration of the strength of the applied magnetic field.

5. A method according to claim 4, 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.

6. A method according to claim 5, wherein the suspension is caused to flow through said region.

7. A method according to any one of claims 1 to 4, using Vortex Magnetic Separation, wherein the field gradient is produced by a magnetisable matrix arranged in the magnetic field and past which the suspension is caused to flow, such that capture on the downstream side of the matrix occurs.

8. A method according to claim 7, wherein the matrix has a Reynolds number in the range from 6 to 40.

9. A method according to any one of the preceding claims, wherein the magnetic fluid comprises a solution or a suspension of a paramagnetic substance, and the magnetisation of the paramagnetic fluid is selected by alteration of the concentration of the paramagnetic substance.

10. A method according to any one of the preceding claims, wherein said magnetic fluid is a solution of MnCl2 in water.

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

12. 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.

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

14. Diamonds separated according to the method of any one of the preceding claims.