GB1578396A

GB1578396A - Magnetic separator

Info

Publication number: GB1578396A
Application number: GB2575877A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-06-23
Filing date: 1977-06-20
Publication date: 1980-11-05
Also published as: JPS53475A; JPS6048209B2; DE2628095C3; FR2355545A1; FR2355545B1; DE2628095B2; DE2628095A1

Description

(54) A MAGNETIC SEPARATOR (71) We, SIEMENS AKTIENGESELL SCHAFT, a German company, of Berlin and Munich, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a magnetic separator suitable for separating magnetisable particles having a size below one millimetre down to below one micron from a fluid in which the particles are suspended.

According to the present invention, there is provided a magnetic separator suitable for separating magnetisable particles having a size below one millimetre down to below one micron from a fluid, which separator comprises: a) magnet means with two pole pieces for establishing a magnetic field between the pole pieces in a separating zone; b) means for guiding the fluid containing magnetisable particles through the separating zone in a direction substantially parallel or antiparallel to the magnetic field; and c) a filter structure and means for fastening it between the pole pieces, the filter structure incorporating at least ten substantially non-corrosive ferromagnetic wire gauzes disposed in a fixed position in the separating zone so that the fluid containing magnetisable particles flowing through the separating zone can pass substantially perpendicularly through a plurality of parts of the wire gauzes in turn, the mesh width of each gauze being at least twenty times as large as the diameter of the largest magnetisable particles to be separated when the separator is in use, the wire diameter of each gauze being less than 0.3 mm, and the magnetic flux density of the magnetic field in the separating zone in use being such that a plurality of magnetisable particles in the fluid are magnetised and attracted to the wires of the gauzes.

The filter structure preferably incorporates at least fifty gauzes.

Advantageously the mesh width of the or each gauze is at least fifty times as large as the diameter of the largest magnetisable particles to be separated by the separator in use. Thus clogging of the filter structure is to a considerable extent obviated and a high degree of loadability (i.e. the ability of the filter structure to retain captured magnetisable particles) and flushability (i.e. the ability of the filter structure to be flushed through with fluid to remove the magnetisable particles from the filter structure after separation) is achieved.

Support structures may be arranged between at least some of the parts of the gauze or at least some of the gauzes of the filter structure. Preferably each gauze is constituted by nickel-plated iron, nickel, or steel.

According to one embodiment of the invention for separating ferromagnetic particles less than 5ym in diameter, the filter structure incorporates at least 100 high grade steel gauzes having mesh widths between 0.1 mm and 0.3 mm.

Advantageously the filter structure comprises a plurality of gauzes and the mesh width of at least one gauze differs from the mesh width of the other gauze(s), in which case the gauze(s) having a smaller mesh width is (or are) disposed downstream of the gauze(s) having a larger mesh width.

Preferably the magnet means is adapted to establish a magnetic field of at least 0.7 Tesla in the separating zone.

Furthermore, the filter structure is arranged between two pole pieces, for example of at least one electromagnet. This is possible due to the relatively short filter length. The feed and discharge of fluid into and out of the filter structure is then expediently effected through the pole pieces, for example through appropriate bores. In this way, relatively high magnetic flux densities can be obtained within the filter volume with relatively small magnets.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows the filter structure of a magnetic separator according to the present invention; Figure 2 shows a graph of the efficiency of separation of various filter structures as a function of the applied magnetic flux density; Figures 3 and 4 show a vertical section and a cross-section respectively of a first embodiment of magnetic separator according to the present invention; and Figures 5 and 6 show a vertical section and a cross-section respectively of parts of a second embodiment of magnetic separator according to the present invention.

In a conventional magnetic separation process, use is made of the fact that in a suitable magnetic field a magnetisable particle is subjected to a force which displaces it or retains it in position relative to other forces, such as, for example, gravity, or displaces it or retains it in position in a flowing liquid against hydrodynamic friction forces. Such a separation process is utilized, for example, in steam or cooling water circuits in conventional and nuclear power stations. In the liquid or vapour in these circuits, particles are suspended which in general are produced by corrosion. These particles are partly ferromagnetic, such as, for example, magnetite (FeO4), and partly anti-ferromagnetic, such as, for example, haematite (a -Fe203), or paramagnetic, such as, for example, copper oxide (CuO).The magnetisable nature of these particles, which additionally occur in different sizes, may thus be utilized to magnetically separate the particles from the liquid or vapour. Large and/or strongly magnetisable (i.e. ferromagnetic) particles can, for example, be separated using magnetic ball filters. A known magnetic ball filter comprises a cylindrical filter container filled with soft iron balls which are arranged in a magnetic field generated by a magnet coil surrounding the filter container. Due to this magnetic field, in combination with the balls, adequately high magnetic field gradients are obtained to cause the ferromagnetic particles suspended in a liquid flowing through the filter to deposit at the magnetic poles of the balls. For cleansing the filter after separation, the balls can be de-magnetised.

Subsequently, with the magnetic field removed, the filter container can be flushed through with a liquid or with a gas (see German Auslegeschrift No. 1,277,488) to remove the magnetisable particles. Small ferromagnetic particles having diameters of the order of magnitude of lXLtm or weakly magnetisable (i.e. anti-ferromagnetic or paramagnetic) particles are, however, difficult to separate with this known filter, since the magnetic field gradients produced at the soft-iron balls are too small for this purpose.

A known filter for separating extremely small or paramagnetic particles contains as filter material ferromagnetic, non-corrosive steel wool which is arranged in a strong magnetic field, the magnetic flux density of which in the filter volume is larger than 1.2 Tesla. A suitable steel wool is, however, relatively difficult to manufacture for this purpose. Also substantially only those filaments of the wool which extend perpendicularly to the direction of the liquid passing near them and perpendicular to the direction of the applied magnetic field contribute to the separation of the particles. At the filaments of the wool extending parallel to the flow direction, the magnetic field gradients are so small that they capture substantially no weakly magnetisable particles.Furthermore, the flow channels formed between the individual filaments of the wool (the dimensions of which cannot be accurately controlled) must be sufficiently large to prevent clogging with separated particles and so as not to allow the flow resistance of the filter, and hence the pressure drop produced across it, to become excessively large. Thus, for a relatively high efficiency of separation, correspondingly large filter containers of large filter volume are necessary. The magnet coils for generating the high intensity magnetic field must therefore be selected to be correspondingly large. In the case of such a known filter, therefore, as a rule superconductive electromagnet coils are provided (see U.S. Patent Specification No. 3,567,026).

The filter structure 2 having a quadrangular cross-section shown diagrammatically and in oblique elevation in Figure 1 comprises a plurality of wire gauzes of which, for the sake of clarity, only seven gauzes 4 to 10 are shown. The number of gauzes is generally substantially higher than seven, the minimum number of gauzes being ten; for example, there may be approximately 100 gauzes. The gauzes have, for example, rectangular shape. They are arranged in a tubular retaining arrangement (not shown), for example a Plexiglass or Teflon tube, perpendicular to the flow direction of fluid M passing through the gauzes in use.

"Flexiglass" and "Teflon" are Registered Trade Marks. In the flow direction, the gauzes are stacked closely one behind the other to form a stack. The fluid M may be a liquid, a vapour or a gas and carries with it the magnetisable particles to be separated in the filter structure by magnetic means.

The flow direction of the fluid M is indicated by individual parallel arrows 12. The gauzes comprise thin wires or filaments 13 having a preset wire thickness and have a predetermined mesh width w. The spacing, designated h, between two adjacent gauzes is generally extremely small. The gauzes may abut each other directly (and may be joined together) or they may be spaced slightly by support structures.

As seen in the direction of flow, the filter structure 2 comprising the gauzes 4 to 10 has a length L. Its volume 15 has a magnetic field passing through it in use. The field lines of this magnetic field (which is generated by at least one magnet-not shown in the Figure) extend through the volume 15 parallel or anti-parallcl to the flow direction of the fluid M. These lines are diagrammatically indicated by a broken line 16 in the figure. Due to the arrangement of the gauzes perpendicular to the flow direction and to the magnetic field, all the filaments of the gauzes actively contribute to separation of the magnetisable particles from the fluid M passing through these gauzes in use.

The or each magnet may be an electromagnet or a permanent magnet. In particular, for separation of paramagnetic particles, the or each magnet may be a superconductive electromagnet.

The wires of the gauzes 4 to 10 are made from magnetisable and substantially noncorrosive material, for example from chromium steel or from iron which is nickelplated. The wire thickness is relatively small, i.e. less than 0.3 mm, and generally below 0.15 mm. The magnetic field gradients produced at the surfaces of these wires or filaments are then relatively high and the separation efficiency of these gauzes is thus correspondingly high. Thus, the filter length L can be kept relatively small. By special etching, for example by passing diluted hydrochloric acid through the entire gauze stack of a filter structure, the wire diameter of the gauzes can be further reduced. Thus, it becomes possible to produce gauzes having filament thicknesses below that of the smallest possible weavable filament thickness, that is below approximately 50 ,um or 0.05 mm.These gauzes are distinguished by especially high magnetic field gradients, and the filter structures comprising them may have high packing densities.

In the case of the fluid feed to the filter structure containing corrosion products, it is to be expected that the particle sizes of the corrosion products will vary considerably and that, depending on conditions, chemically different substances, such as, for example, haematite and magnetite, will be obtained simultaneously, although in varying quantities. Since the separation efficiency of a filter structure comprising wire gauzes, with which both small and weakly magnetisable particles may be separated, may become undesirably large from the point of view of strongly magnetisable or large particles, advantageously a filter structure having large mesh width (i.e. coarse) gauzes at the inflow side is employed.If appropriate, the use of two or more filter structures comprising gauzes of different mesh width disposed one behind the other is advantageous, and these filter structures can, if necessary, be scparately rinsed.

Additionally, it is expedient not to select the separation per gauze (which, for a given saturation magnetisation of the wires, is adjustable by means of the wire diameter and the mesh width w of the gauze) to be excessively high, so as to prevent clogging of the filter structure 2 at the inflow side, and to achieve good loading of the gauzes.

Thus, the mesh width w of the filament gauzes is selected to be at least twenty times, preferably at least fifty times, as large as the diameter of the largest particles to be separated from the fluid.

To clean the filter structure 2, the magnetic field which extends through it can be removed and replaced by an A.C. field. In this manner, the particles adhering to the filaments or wires of the gauzes can be loosened. The particles can then be removed from the filter structure by a flushing process, for example by passing flushing liquid through the filter in the direction opposite to the flow direction of the fluid M whilst admixing pressurised air with the flushing liquid.

There is described above a filter structure having rectangular gauzes. However, the filter structure may equally well be cylindrical in shape, its gauzes then being of circular form. Such gauzes are known as gauze "rounds".

The angles defined by the intersecting filaments or wires of a gauze are generally approximately 90 , so that the meshes of the gauzes are square or rectangular. However, it is also possible to employ gauzes having meshes of other shapes, for example of diamond shape.

Each wire of each gauze may be parallel to a corresponding wire of the or each adjacent gauze.

Furthermore, adjacent gauzes may be arranged so oriented relative to each other that each wire of each gauze is disposed at an angle (+ 0) to a corresponding wire of the or each adjacent gauze. In this way, the uniformity of loading of the gauzes can be increased.

Usually the throughflow velocity v of fluid through the filter structure is approximately between 1 and 10 cm/sec. Where appropriate, however, substantially higher throughflow velocities, of 100 cm/sec for example, may be expedient. An increase in the throughflow velocity generally results in a reduction in the separation efficiency.

Also, if the number of gauzes in the filter is increased, the throughflow velocity and the magnetic field being unchanged, an increase in the separation efficiency can be achieved.

Referring to Figure 2, the graph illustrates the separation efficiency q as a function of the magnetic flux density B for various filter structures. The separation efficiency q corresponds to the value i-p, where p is the permeability factor of the filter structure and is equal to the concentration of suspended magnetisable substances in a medium after it has passed through the filter structure divided by the corresponding concentration prior to entry of the medium into the filter structure. The magnetic flux extending through the filter structure is indicated in Tesla in the diagram.The individual curves of the graph were obtained with an -Fe203-H2O feed suspension having an output concentration of approximately 1.06 mg a-Fe203 per litre H2O and a magnetisable particle size between 2.5 "am and 0.25 slum. The velocity v at which the suspension was passed through the various filter structures was approximately 5 cm per second.

The curve a was obtained using a known ball filter having a cylindrical filter chamber, the length L (in the flow direction) of which was 15 cm and the cross-sectional diameter of which was 32 mm. The balls, made from magnetically soft iron, each had a diameter of 3.2 mm. Their total weight was 580 g.

The curves b to d were obtained with magnetic separators according to the present invention comprising gauze rounds having diameters of 19 mm. Employing a special filter structure comprising 100 nickel gauzes, according to the curve b a separation efficiency q which is substantially higher than that of the known ball filter was obtained. This filter structure had a filter length L of 2.3 cm. The diameter of the wires of its gauzes was 0.1 mm and the mesh width was 0.16 mm.

The curve c was obtained using a filter structure having two different nickel gauze types. Its total length L was approximately 2 cm. It was assembled from 100 gauze rounds having a mesh width of 0.056 mm and a wire diameter of 0.05 mm and also 25 coarser supporting gauzes, the mesh width of which was 0.23 mm and the wires of which had a diameter of 0.1 mm. In this arrangement, every fourth fine nickel gauze was followed by a coarser nickel gauze as a supporting grid for mechanical strengthening of the filter structure.

A still higher separation efficiency q according to the curve d was obtained using a filter structure comprising, over a total length L of 1.5 cm, 100 gauzes of highquality steel having a mesh width of 0.14 mm and a wire diameter of 0.067 mm.

Approximately the same curve was obtained using a filter structure having a filter length L of 2.5 cm and comprising 100 nickelplated iron gauzes, the mesh width of which was 0.15 mm and the wire diameter of which was 0.1 mm.

A further increase in the separation efficiency q to approximately 0.8 at a magnetic flux density of approximately 1 Tesla was achieved with a filter structure comprising 400 etched high-quality steel gauzes, the wires of which were etched down to a diameter of approximately 0.01 to 0.025 mm.

Their total length was only 8 mm and their mesh width was approximately 0.17 mm.

Due to the etching down of the wires, the magnetic field gradient was increased and thus the efficiency of separation was increased.

From the curves in the graph of Figure 2, it will be apparent that a substantial increase in the magnetic flux density B beyond 1 Tesla leads only to an unimportant increase in the separation efficiency q. Thus, it is not necessary to utilize magnetic flux densities far beyond 1 Tesla, at least in the cases given. Flux densities within the filter structures of the order of 1 Tesla can be achieved not only by the use of super-conductive electromagnets but also by the use of conventional electromagnets.

Referring to Figures 3 and 4, the figures show a magnetic separator according to the present invention utilizing two filter structures as described with reference to Figure 1. Figure 4 shows a cross-section of the separator taken along the line IV-IV in Figure 3. In use of the separator fluid M, for example a liquid, containing the magnetisable particles to be separated flows in a tube 18. Upstream of the separator, the liquid branches into two component streams indicated by arrows 19 and 20. The separator contains two parallel cylindrical separating chambers 21 and 22 which are arranged in juxtaposition and through which the two liquid component streams are guided in use. Arranged in each of these separating chambers, which are concentrically surrounded by a respective magnet coil 24 or 25, is a cylindrical filter structure 27 or 28.

These filter structures each correspond to the filter structure 2 according to Figure 1 and are disposed approximately in the centre of these chambers. They each separate an upper chamber part from a lower chamber part of substantially equal size. Each of these chamber parts contains a cylindrical pole piece made from magnetic iron provided for guiding the magnetic field produced by the coils 24 and 25 through the corresponding filter structure 27 or 28. The two pole pieces associated with the coil 24 are designated 30 and 31, and the two pole pieces associated with the coil 25 are designated 32 and 33. For guiding the liquid through the component chambers, the pole pieces 30 to 33 are each formed with bores 35. The liquid is introduced into, and is discharged from, the filter structures 27 and 28 by way of those bores 35.The parts of the pole pieces which the liquid contacts are advantageously made from corrosionresistant material or are covered with a corrosion-preventing coating. The cross-section of the bores 35 in each pole piece may be only a small percentage, for example 10%, of the pole cross-section, since the flow velocity of the medium M in these bores 35 may be substantially greater than the flow velocity in the filter structure. Thus the probability of premature separation of particles in the bores 35 at the inlet side of the chambers map be reduced. Additionally, a perforated plate (not shown in the Figure) may be arranged between each pole piece and the corresponding filter structure. The perforated plates serve to distribute uniformly the throughflow of liquid in the filter structure. If so required, lateral feed of liquid in the filter structure may also be provided.

For closing the magnetic circuit produced by the magnet coils 24 and 25, the two pole pieces 30 and 32, and also the two pole pieces 31 and 33, are connected with each other via a yoke made from magnetic iron. The yoke connecting the two pole pieces 30 and 32 is designated 37, and the yoke connecting the two pole pieces 31 and 33 is designated 38. The field lines of the magnetic field generated by the two magnet coils 24 and 25 are thus interconnected on either side of the filter structures 27 and 28. They are indicated by an arrowed line 40. The pole pieces 30 to 33 and the two yokes 37 and 38 thus serve to guide the magnetic field produced by the two magnet coils 24 and 25. Thus optimum utilisation of the magnetic field within the filter structures 27 and 28 is guaranteed, inasmuch as weakening of the field externally of the filter structures is to a considerable extent obviated.

The A.C. fields required for demagnetisation of the gauzes of the filter structures 27 and 28 at the end of a separation cycle can also be generated by the two magnet coils 24 and 25. With such an arrangement, conveniently the magnetic circuit closed by the two yokes 37 and 38 is opened during demagnetisation to reduce the losses produced by the demagnetisation process. For this purpose, for example in the two yokes 37 and 38, magnetic isolation zones (not shown) may be provided.

In addition to the embodiment shown in Figures 3 and 4 of a magnetic separator having two parallel filter structures, the associated magnets of which are magnetically coupled with each other by means of the yokes 37 and 38, it is also possible for a magnetic separator according to the present invention to comprise a single device or a plurality of devices which are not magnetically coupled together, each of which contains at least one filter structure and at least one magnet, the magnetic field of which is conveyed by magnetic guiding members and pole pieces to the filter structure(s).

Furthermore, it is also possible to arrange the or each magnet so it does not directly surround the corresponding filter structure and pole pieces, but so that it surrounds yokes for guiding the magnetic field.

Also a plurality of devices each having a guiding duct and a filter structure can be so arranged in juxtaposition that the magnetic fields generated by the magnet coils of these devices form, via yokes, an annular, closed magnetic circuit. A part of such a circuit is shown in cross-section in Figure 6. Figure 5 shows a single device of such a circuit in vertical section. Each device corresponds substantially to the separating chambers shown in Figure 3. The fluid M is introduced via a tube 42 and bores 35 in a cylindrical pole piece 30 into a cylindrical filter structure 27 according to Figure 1 and is discharged from this filter structure via bores 35 in a pole piece 31 and a tube 43.

Each of the pole pieces 30 or 31 is concentrically surrounded by a magnet coil 45 or 46. However, it is also possible to provide a single magnet coil about the two pole shoes. For closure of the magnetic circuit produced by the two magnet coils 45 and 46, via adjacently arranged separating chambers, the two pole shoes 30 and 31 are each connected via a yoke 48 or 49 with a corresponding pole shoe of an adjacent device of the circuit. The magnetic field guided through the device is indicated by an arrowed line 50.

The parts of the connecting yokes 49 covered by the magnet coils 45 in Figure 6 are indicated by broken lines. All the devices together therefore form a closed magnetic circuit of annular form.

The filter structure of the magnetic separator according to the present invention need not be in the form of a stack of at least ten discrete wire gauzes arranged one after the other. Instead an equivalent wire gauze roll or coil-i.e. one presenting at least ten filtration surfaces when in use-may be employed for the filter structure. In the case of such an embodiment, the gauze roll or coil is disposed between two tubular pole pieces which are arranged concentric to each other and provided with bores in the radial direction for the supply or discharge of the fluid for example. In the annular gap formed between these pole shoes there obtains a radial magnetic field which is directed parallel or antiparallel to the flow direction of the fluid flowing through the bores and the gaps between the pole pieces and the gauze coil.With this arrangement, the throughflow cross-section of the filter structure will be large and therefore a cor respondingly small flow velocity may be provided.

The above described magnetic separators according to the present invention are advantageous in that all the wires of the wire gauzes contribute to the separation of magnetisable particles. The efficiency of separation is therefore relatively large. The length of the filter structure in the flow direction can thus be selected to be correspondingly short.

Furthermore the separators are capable of separating anti-ferromagnetic or paramagnetic particles and magnetisable particles having a size down to below 1 jlm. Also the filter volume and the magnet surrounding it can be relatively small and nevertheless the efficiency of separation is high.

WHAT WE CLAIM IS:- 1. A magnetic separator suitable for separating magnetisable particles having a size below one millimetre down to below one micron from a fluid, which separator comprises: a) magnet means with two pole pieces for establishing a magnetic field between the pole pieces in a separating zone; b) means for guiding the fluid containing magnetisable particles through the separating zone in a direction substantially parallel or antiparallel to the magnetic field; and c) a filter structure and means for fastening it between the pole pieces, the filter structure incorporating at least ten substantially non-corrosive ferromagnetic wire gauzes disposed in a fixed position in the separating zone so that the fluid containing magnetisable particles flowing through the separating zone can pass substantially perpendicularly through a plurality of parts of the wire gauzes in turn, the mesh width of each gauze being at least twenty times as large as the diameter of the largest magnetisable particles to be separated when the separator is in use, the wire diameter of each gauze being less than 0.3 mm, and the magnetic flux density of the magnetic field in the separating zone in use being such that a plurality of magnetisable particles in the fluid are magnetised and attracted to the wires of the gauzes.

2. A separator according to claim 1, wherein the filter structure incorporates at least fifty gauzes.

3. A separator according to claim 1 or 2, wherein the mesh width of the or each gauze is at least fifty times as large as the diameter of the largest magnetisable particles to be separated by the separator in use.

4. A separator according to claim 1, 2 or 3, wherein adjacent gauzes of the filter structure abut each other directly.

5. A separator according to claim 1, 2 or 3, wherein support structures are arranged between at least some of the gauzes of the filter structure.

6. A separator according to any preceding claim, wherein the filter structure comprises a plurality of gauzes and each wire of each gauze is parallel to a corresponding wire of the or each adjacent gauze.

7. A separator according to any one of claims 1 to 5, wherein the filter structure comprises a plurality of gauzes and each wire of each gauze is disposed at an angle (+ 0) to a corresponding wire of the or each adjacent gauze.

8. A separator according to any preceding claim, wherein each gauze is constituted by a nickel-plated iron, nickel, or steel.

9. A separator according to any preceding claim, wherein the diameters of the wires of the or each gauze have been reduced by etching to a value less than 0.05 mm.

10. A separator according to any one of claims 1 to 7, wherein the filter structure incorporates at least 100 high grade steel gauzes having mesh widths between 0.1 mm and 0.3 mm.

11. A separator according to any preceding claim, wherein the filter structure comprises a plurality of gauzes and the mesh width of at least one gauze differs from the mesh width of the other gauze(s).

12. A separator according to claim 11, wherein the gauze(s) having a smaller mesh width is (or are) disposed downstream of the gauze(s) having a larger mesh width.

13. A separator according to any preceding claim, wherein the magnet means is adapted to establish a magnetic field of at least 0.7 Tesla in the separating zone.

14. A separator according to any preceding claim, wherein the supply and discharge of fluid into and out of the filter structure is effected at least partially through the pole pieces.

15. A separator according to claim 14, wherein the means for guiding the fluid comprise bores in the pole pieces for the passage of fluid therethrough.

16. A separator according to any preceding claim, which separator comprises a plurality of filter structures each disposed between two respective pole pieces, each pole piece being magnetically connected to a pole piece of an adjacent filter structure by means of a yoke of ferromagnetic material so as to form a closed magnetic circuit.

17. A separator according to any pre

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. respondingly small flow velocity may be provided. The above described magnetic separators according to the present invention are advantageous in that all the wires of the wire gauzes contribute to the separation of magnetisable particles. The efficiency of separation is therefore relatively large. The length of the filter structure in the flow direction can thus be selected to be correspondingly short. Furthermore the separators are capable of separating anti-ferromagnetic or paramagnetic particles and magnetisable particles having a size down to below 1 jlm. Also the filter volume and the magnet surrounding it can be relatively small and nevertheless the efficiency of separation is high. WHAT WE CLAIM IS:-

1. A magnetic separator suitable for separating magnetisable particles having a size below one millimetre down to below one micron from a fluid, which separator comprises: a) magnet means with two pole pieces for establishing a magnetic field between the pole pieces in a separating zone; b) means for guiding the fluid containing magnetisable particles through the separating zone in a direction substantially parallel or antiparallel to the magnetic field; and c) a filter structure and means for fastening it between the pole pieces, the filter structure incorporating at least ten substantially non-corrosive ferromagnetic wire gauzes disposed in a fixed position in the separating zone so that the fluid containing magnetisable particles flowing through the separating zone can pass substantially perpendicularly through a plurality of parts of the wire gauzes in turn, the mesh width of each gauze being at least twenty times as large as the diameter of the largest magnetisable particles to be separated when the separator is in use, the wire diameter of each gauze being less than 0.3 mm, and the magnetic flux density of the magnetic field in the separating zone in use being such that a plurality of magnetisable particles in the fluid are magnetised and attracted to the wires of the gauzes.

17. A separator according to any pre

ceding claim, wherein the magnet means comprises at least one superconductive electromagnet.

18. A magnetic separator suitable for separating magnetisable particles having a size down to below one micron from a fluid, which separator is substantially as hereinbefore described with reference to, and as illustrated in, Figures 1, 3 and 4 or Figures 1, 5 and 6 of the accompanying drawings.