DE3750226T2 - MAGNETIC CUTTER. - Google Patents

Abstract

A magnetic separator comprises a magnet (2) positioned at an angle (3) to the vertical and means (4) for feeding a mixture of magnetic and non-magnetic particulate material at or closely or adjacent the magnet in the region of high magnetic field. The non-magnetic particles fall under the action of gravity only whereas the magnetic particles are diverted towards the magnet until the gravitational force exceeds that exerted by the magnet. The arrangement provides a clean separation between the magnetic and non-magnetic particles and may be employed to separate the particules according to degree of magnetic susceptibility. A magnet (2) for a magnetic separator is also described comprising a linear superconducting magnet having a coil (36) with two generally straight portions (38) joined by curved ends (40). The coil is supported by a clamp (42) in a cryostat vessel (35) with the longest axis arranged horizontally and so as to provide a magnetic separation zone on one side of the coil. The magnet is powerful, robust and has a long range.

Description

The invention relates to a magnetic separator for minerals.

The invention particularly relates to systems in which a strong magnet is used to separate magnetic particles from non-magnetic particles. In the simplest form of such a system, the magnet is moved across or back and forth over a layer of ore so that the magnetic particles are attracted in its direction and attached to the magnet. Thus, the magnetic components of the minerals in an ore can be removed, but this method is not a continuous process and several passes are necessary to complete the separation.

A continuous process has been developed whereby a stream of minerals can pass at some distance from the high field region of the magnet, thereby deflecting the magnetic constituents towards the higher field region while the non-magnetic constituents fall relatively unaffected. Such a process is described and claimed in British Patent 2064377.

However, continuous processes of this type have the disadvantage that the mineral stream never experiences the maximum magnetic force and the separation is therefore not completely pure, especially when it is required to separate only very weakly magnetic particles. Furthermore, to prevent the magnetic particles from being caught on the magnetic wall, the mineral must fall from such a height that the vertical momentum is sufficient to transport the strongest magnetic particles through the high field region. This reduces the degree of deflection of the weakly magnetic particles and in turn prevents a pure separation.

Another problem found with the known magnetic separators has to do with the magnet itself. To separate the magnetic particles from the non-magnetic particles, a strong magnet with a long range is required, and superconducting or very strong conventional magnets have been used for this. Known linear magnets have had two coils carried side by side and enclosed in a single refrigeration controller. Several disadvantages have been found in practice with this design.

Firstly, since the two coils attract each other with a considerable force, inaccuracies in the manufacture of the support structure will result in the surfaces not engaging perfectly, which can lead to degradation of the magnet. This problem is exacerbated by the expansion and contraction of the components of the magnet, which occurs when the temperature in the refrigeration controller varies.

Second, each coil experiences a strong repulsive force along its length, requiring a very robust support structure. This is difficult to achieve in practice.

Third, because the coils are positioned relatively close together to create a strong magnetic field on both sides of the magnet, only a small space is available behind each coil for the cooling system and the support structure between the coils. In practice, it has been found that due to these related problems, a remarkable reduction in the theoretical magnetic force that can be realized by many coil magnets takes place.

The general aspect of the invention is to provide a magnetic separator which will operate more efficiently and produce a purer separation than has been possible heretofore.

British Patent Application 2102702 describes a magnetic separator having a magnet which is said to comprise a single coil. However, the embodiment of the separator described and illustrated comprises a magnet having two coils of the type discussed above.

A magnetic separator according to the invention comprises a linear superconductive magnet having a single magnetic coil with two substantially straight parallel sections connected by curved ends, a cleat surrounding both longitudinal straight outer edges of the coil and one end thereof, the cleat receiving the coil in a cryogenic tank so that the major axis of the coil is oriented substantially horizontally therein and so that the other end of the magnetic coil is disposed adjacent a first outer wall of the cryogenic tank, at least one radiation shield between the cleat and the walls of the cryogenic tank and means for supplying a mixture of magnetic and non-magnetic singulated material to a magnetic separation zone of the magnet so that the singulated material mixture is separated into fractions and subsequently falls freely under the action of gravity, characterized in that the or each radiation shield surrounds the cleat and the magnetic coil and defines a reservoir between the cleat and the outer walls of the cryogenic regulator vessel which is filled with helium, creating a single magnetic separation zone on the outer wall of the cryogenic regulator vessel.

The advantage of using a single magnet separation zone is that only one half of the magnet is exposed, so that heat losses are reduced, the coil is firmly held by the clamp which is not subject to expansion and contraction to the same extent as the many coil designs and the manufacture of the magnet is simplified because there is no need to create exactly identical coils.

Preferably, an embedding medium is provided around the turns of the coil.

The magnet may be positioned with its sides horizontal and the magnetic separation zone below the magnet, and a belt conveyor may be provided which moves horizontally through the magnetic separation zone adjacent to the magnet and substantially at right angles to the longitudinal axis of the magnet, whereby the particles are fed horizontally so that the non-magnetic particles fall due to gravity, the magnetic particles being picked up by the belt conveyor which transports them through the magnetic separation zone until the gravitational force exceeds that of the magnet and the particles fall off the belt conveyor.

This arrangement is particularly suitable when a high capacity process is required.

Suitably, the belt conveyor may move in the same or in the opposite direction to that in which the particles are fed.

Preferably, the magnet is supported such that the coil side angle is at an acute angle to the vertical, the feeding means feeding the particular material into the region of the high magnetic field from the magnet separation zone so that the non-magnetic particles fall down only under the influence of gravity, the magnetic particles being deflected towards the magnet until the gravitational force exceeds the force exerted by the magnet.

This arrangement has the advantage that the feed point is close to the magnet. This means that all particles can pass through the area or areas of highest magnetic field, allowing separation of weaker magnetic parts than is possible with conventional separation methods. The ore feed position also results in the particles experiencing the highest magnetic field over a greater distance, increasing separation efficiency.

An additional advantage of the arrangement is that the vertical inclination of the magnet ensures that the magnetic and non-magnetic parts are physically well separated.

Conveniently, the angle at which the coil minor axis is inclined to the vertical can be adjusted for a particular ore so that the most strongly magnetic particles follow a path parallel and close to the magnet. This prevents clogging on the magnet surface.

Preferably, however, a belt conveyor is provided which moves beyond the magnet in close proximity to it. The belt conveyor acts to remove any strongly magnetic particles which are caught by the separation zone on the belt conveyor, thereby preventing clogging. This is particularly suitable for an ore whose constituents are not known with great precision.

Preferably, a divider plate is provided in the lower region of the path of the falling particles in order to separate the flow of magnetic and non-magnetic particles.

Preferably, the particles are fed in such a way that they fall behind the belt conveyor and not on the belt conveyor surface This prevents weaker magnetic parts from jumping off the belt conveyor with sufficient momentum to fall into the flow of non-magnetic particles. Suitably, the particles are fed at a speed lower than that of the belt conveyor, thereby reducing the risk of non-magnetic parts becoming trapped in the magnetic layer because only a thin layer of strongly magnetic particles is collected on the surface of the belt conveyor.

Suitably, the angle at which the coil minor axis is inclined to the vertical can be adjusted for a particular ore so that the least magnetic parts within the ore follow a path which is parallel and close to the belt conveyor. This ensures that the separation between the magnetic and non-magnetic particles is completely pure.

Preferably, one or more dividers may be provided in the lower region of the path of the falling particles in order to separate the particles according to their degree of magnetic receptivity.

Preferably, the mixture of magnetic and non-magnetic material is fed from a container having an outlet wall adjacent to the magnet inclined vertically at the same angle as the magnet. The specified material may alternatively be fed from a container onto a plate inclined vertically at the same angle as the magnet, from which the material falls and is fed adjacent to the magnet. These feeding methods ensure that the particles have a long residence time within the high magnetic field zone.

The invention will now be described by way of example with reference to the accompanying figures, in which:

Fig. 1 is an outline drawing of a magnetic separator;

Fig. 2 is a second outline drawing of a magnetic separator according to Fig. 1, showing the approximate positioning of the components;

Fig. 3 is a vector diagram of the forces experienced by a magnetic particle in the magnetic separator of Fig. 1;

Figures 4a, 4b, 4c and 4d are fragmentary outline drawings of the magnetic separator of Figure 1 showing various embodiments of the feeding means;

Fig. 5 is an outline drawing of a magnet forming part of a magnetic separator according to the invention;

Fig. 6 is an outline drawing of the coil of a magnet shown in Fig. 5;

Fig. 7 is a drawing of a typical force profile of the magnet of Fig. 5;

Fig. 8 is an outline drawing of an embodiment of a magnetic separator incorporating the magnet of Fig. 5; and

Fig. 9 is an outline drawing of another embodiment of the magnetic separator incorporating the magnet of Fig. 5.

With reference to Figures 1 and 2, a magnetic separator comprises a magnet, generally numbered 2, and a feeding means 4. The magnet 2 is inclined at an angle 3 in the vertical.

The magnet 2 is arranged to generate a strong magnetic field in such a way that any magnetic particles experience a force at right angles to and in the direction of the magnet, e.g. in the direction of arrow 6.

Dry specified material to be separated is fed through a feeding means 4 to a Point which is closely adjacent to the magnet but separated from it by a gap 8. The feed means 4 described in more detail below is preferably adjustable towards and away from the magnet 2. Preferably the material is fed at a low speed and the non-magnetic particles fall straight from the feed means 4 in a vertical path under the influence of gravity.

The magnetic particles are attracted towards the magnet 2 and are deflected by the non-magnetic parts. They therefore fall down from the ore feed point in a parabolic path. The magnetic parts pass through the magnetic field until the gravitational force exceeds the magnetic attraction, at which point the parts fall down under the influence of the gravitational force. The inclination of the magnet 2 in the vertical causes the path of the magnetic parts to be physically well separated from the path of the non-magnetic parts.

The ore feed point can be located so that the material is fed within the highest magnetic field. If the constituents of the mineral ore are well defined, the inclination of the magnet can be set so that the strongest magnetic particles fall parallel and close to the magnetic face to avoid clogging.

Alternatively, if the mineral or constituents are not precisely known, a belt conveyor 10 is provided which moves beyond and close to the face of the magnet 2, as shown in Figures 1 and 2. The belt conveyor 10 is supported by rollers 12. Any strongly magnetic particles are caught by the belt conveyor and carried away from the magnet, preventing clogging of the magnetic separation zone. The belt conveyor preferably moves at a relatively high speed so that only a thin layer of strong magnetic particles is trapped on the belt, reducing the risk of non-magnetic particles becoming trapped within the magnetic particles. Even if the feed point is close to the magnet, the inclination of the magnet ensures clean separation. In previous magnetic separation methods, the ore had to be fed at a certain distance from the region of highest field, and the resulting force acting on the weak magnetic particles was insufficient to separate them from the non-magnetic parts.

The angle 3 by which the magnet is inclined to the vertical is preferably set for a particular ore so as to cause the particles with the least magnetic receptivity within the ore to move in a path parallel to and closely adjacent to the magnet and/or the belt conveyor. In this way it can be ensured that all magnetic particles are removed from the particulate material.

Another advantage of tilting the magnet at an angle in the vertical is that a non-magnetic particle which has become interspersed in the magnetic stream still has a chance of getting out. It can be seen that the greater the inclination of the angle, the greater the chance of separating non-magnetic particles. Although this might suggest that the best arrangement is one in which the magnetic and gravitational forces are arranged in direct opposition to each other, e.g. a lifting operation as described above, this is not the case because in such an operation the material and the magnet must be physically well separated in order to achieve separation of the particles. Therefore weakly magnetic particles are not separated purely from the ore. Tilting the magnet in the vertical creates a compromise. between exposing the material to a large area of high magnetic field to allow any trapped non-magnetic particles to be released and ensuring that the particle-containing material is well separated.

The angle 3 can be calculated from a simple vector diagram. One such is shown in Fig. 3. The magnetic force on a particle with the least magnetic receptivity in a given ore, represented by Fm, is the product of the magnetic receptivity of the particle, the magnetic field strength, the field gradient and the particle mass. Fg represents the gravitational force acting vertically downwards and is the product of the acceleration due to gravity and the particle mass. The resulting force is represented by Fr and is at right angles to the magnetic force. By simple geometrical or mathematical determination of the forces acting on the particles in the direction perpendicular to the magnet or conveyor belt, it can be shown that the sine of the angle 3 is directly proportional to the magnetic receptivity of the particle. Therefore, the angle 3 can be easily calculated and the inclination of the magnet and hence the magnetic field direction can be arranged such that the least magnetic material will inevitably move along the magnetic surface or belt conveyor, thus ensuring that all magnetic particles are removed from the ore.

The calculation shown above is an oversimplification in which the decrease of the magnetic force with distance from the magnetic surface or other effects such as collisions have not been taken into account. The assumption has been made that the magnetic force is uniform and acts at right angles to the magnetic surface. A further assumption is that the "optimal" separation is achieved when the magnetic particles are dispersed along the surface fall from the inclined magnet or belt conveyor. The latter assumption is based on the fact that separation in this way will firstly increase the probability that any non-magnetic parts caught in the magnetic stream will fall out and secondly that a long physical separation will be established between magnetic and non-magnetic parts.

Therefore, the calculation gives a minimum limit on the value of the angle 3 which ensures "optimal" separation. The separator can, for example, be designed to produce an excess resultant force, in which case the resultant force will no longer be at right angles to the magnetic force. Alternatively, and as described above, the separator can be designed so that the most strongly magnetic parts follow a path which is parallel to the face of the magnet. It will be seen that for normal operation the inclination in the vertical for a particular ore can be easily calculated and arranged to achieve satisfactory results.

All magnetic particles will therefore be removed from the ore, either by being transported by the belt conveyor or by being made to follow a path parallel to the belt conveyor. However, the point at which the gravitational force will exceed the magnetic attraction for a particular particle will depend on its magnetic receptivity. Because the belt conveyor is inclined at an angle to the vertical, under the influence of gravity particles with different magnetic receptivity will follow different vertical paths. Divider plates 11 can be arranged to separate the ore not only into magnetic and non-magnetic fractions but also according to receptivity. This is shown schematically in Fig. 1, where section A represents the strongly magnetic parts, Section B represents the weakly or moderately magnetic parts and section C represents the non-magnetic parts or residual parts. Separation according to the degree of receptivity is extremely difficult to achieve using previous separation methods.

The following separation examples, carried out with a magnetic separator shown in Fig. 1 and 2, have been included to illustrate the efficiency of the separation and the improvements over the known magnetic separators. In each case, the mixture was allowed to fall over a suitably inclined magnet.

example 1

Potassium permanganate (magnetic receptivity 1.75·10⁻⁷ emu/g) was separated from nonmagnetic quartz, giving an 80% grade magnetic product in a single run with a yield of over 90%.

Example 2

A mixture of nickel sulphate (magnetic receptivity of 1.6·10⁻⁶ emu/g) and copper sulphate (magnetic receptivity of 6·10⁻⁶ emu/g) and glass sand (non-magnetic) was separated into almost perfect individual fractions in a single pass.

Example 3 (comparison)

A mixture of bauxite ore and ferrous impurities was separated using a magnetic separator with a vertical magnet with a double coil with an inclined plane arranged in such a way that the particles were fed at some distance from the magnetic surface, with the particles being trickled down onto the inclined plane from a certain height. After After two passes, the non-magnetic product contained 2% Fe₂O₃. With the tilted magnet, the non-magnetic product after a single pass contained 1.7% Fe₂O₃, an improvement of 15%. In addition, the tilted magnet was found to give consistently better results over a range of particle sizes.

With reference to Figs. 4a, b, c and d, various embodiments of the feeding means are shown. In Fig. 4a, a container 13 is used with an outlet wall 14 which is inclined in the vertical at the same angle as the magnet. This feeding means ensures that the particles are fed closely adjacent to the magnet surface and is suitable for an arrangement in which the inclination of the magnet in the vertical is small. If the inclination in the vertical is large, a container of the type in Fig. 4a cannot be used because the particulate material will not fall out of the outlet. For this, the arrangements according to Figs. 4b and 4c are preferably used. In both figures, the feeding means 4 comprises a container 13 which feeds the particulate material to a flat plate 16 which is inclined in the vertical and onto which the material falls downwards and which is inserted adjacent to the magnet. The arrangement shown in Fig. 4b is particularly successful because the plate is inclined at the same angle as the magnet and because it is positioned such that the magnetic material on the plate falls through at least part of the magnetic field region before being released. This ensures that the particulate material falls over a long distance close to the magnet and therefore has a long residence time in the high magnetic field region, resulting in a cleaner separation.

Fig. 4d shows a further embodiment of the feeding means 4, in which a container 13 feeds the material consisting of particles onto a slowly moving belt conveyor 18. The belt conveyor then feeds the material to a point close to the magnet. As an alternative to the belt conveyor, a vibrating table can be used as shown in Fig. 1.

In all embodiments of the feed means, the particulate material is preferably fed at a relatively low speed so that it will have a long residence time in the high magnetic field region. This was not possible in known methods where the magnet was kept in a vertical position because the particles had to be thrown from a certain height to give them sufficient momentum so that the more strongly magnetic parts were not caught by the magnetic surface and clogged. This is not necessary with the inclined arrangement because the magnet can be arranged so that the most strongly magnetic parts follow a path parallel to the magnet or a belt conveyor can be used to carry the most strongly magnetic parts away.

With reference to Figs. 5 and 6, a magnet 2 according to the invention is shown.

The magnet 2 comprises a linear race track solenoid coil disposed in a cryogenic tank 35. The solenoid coil 36 is shown in Fig. 6, from which it can be seen that the coil 36 has two parallel straight sections 38 connected by curved ends 40. Another, smaller coil may be provided within the coil 36. The solenoid coil is held by a G or C shaped cleat 42 in a helium tank 41, suitably maintained at a temperature of 4K, the gap between the coil turns being filled with an encapsulating medium such as epoxy resin. The coil is positioned with its long axis horizontal, and the cleat 42 surrounds one side and two corners of the coil to the free side of the coil to create a magnet separation zone. The helium vessel 41 is surrounded by two radiation shields 46, the inner radiation shield preferably maintained at 16K and the outer radiation shield preferably maintained at 60K. The radiation shields are maintained at these temperatures by cooling tubes 48 and are enclosed by a layer of super-insulating material. The cryogenic vessel is closed by a front cover plate 52 which is made as thin as practical, a rear cover plate 54 and two corner plates 56 to form a substantially rectangular shaped magnet.

The gap 56 between the straight sections 38 of the solenoid coil is 50mm, while the total distance 60 between the outer turns of the solenoid coil is 180mm. The distance from the side of the solenoid coil to the front of the refrigeration control vessel is 7 to 20mm. The total length 58 of the coil can typically vary between 150mm and 4m. All these values are given as examples only.

Magnet 2 is powerful, robust and has a long range. Because there is only one coil, the cooling system can be arranged so that only minimal heat losses occur on all sides of the magnet, except the side that forms the magnet separation zone. The cleat can hold the coil firmly and its full theoretical field strength can be realized.

Fig. 7 shows a typical force profile of magnet 2, where the Y-axis represents the distance from the magnet surface and the X-axis represents the distance from the centerline of the magnet. Lines 62 and 64 represent contours of the force of constant magnitude expected from a particle of certain mass and magnetic receptivity when it approaches the magnet. , with the force at 64 being less than that at 62.

The magnet 2 shown in Fig. 5 can be used in a magnetic separator according to Figs. 1 and 2.

When the magnet is used in the inclined arrangement, the feed position shown in Figures 1 and 2 means that the material is fed at 66 in Figure 7 and that the ore experiences a much greater field than if it is fed at 68 as has been done in previous magnetic separation processes. It will also be seen that this greater field strength acts over a longer distance. The inclined position of the magnet therefore allows the separation of even very weakly magnetic particles. In addition, any magnetic particles caught in the non-magnetic stream have a greater chance of being deflected because the force acting on them acts over a much longer range of their path.

The magnet 2 shown in Fig. 5 can also be used in the magnetic separators according to Figs. 8 and 9, where the magnet is supported with its sides horizontal. The particulate material is fed into the magnetic separation zone via a belt conveyor 70 below the magnet. In the arrangement of Fig. 8, the feeding direction is the same as the direction of movement of the belt conveyor 10. The magnetic particles are attracted vertically upwards and caught on the belt conveyor 10, which carries them over the magnet and away from the non-magnetic parts which simply fall down due to gravity. Divider plates 11 can be provided, either for separating the magnetic and non-magnetic particles, or, as shown in the drawing, for separating the particles into fractions A, B and C, depending on the degree of magnetic receptivity.

The arrangement according to Fig. 9 differs from that in Fig. 8 only in that the movement of the belt conveyor is in the opposite direction to that in which the particles are fed. The direction of movement of the particles is therefore reversed and they are transported away again by the belt conveyor until they fall down in the direction of arrow 74. The non-magnetic parts fall in the direction of arrow 76.

The magnetic separators shown in Figs. 8 and 9 are particularly suitable where a high capacity process is required because the long reach and high force of the magnet designed as described above ensures that the magnetic particles are lifted out even from a large mass of mineral ore.

The magnetic separators described above are not limited to separating magnetic particles from an ore and can be equally successfully applied to other particulate mixtures from which it is desirable to remove magnetic components.

Claims (14)

1. A magnetic separator comprising a linear superconductive magnet (2) having a single magnetic coil (36) with two substantially straight parallel sections (38) connected by curved ends (40), a cleat (42) surrounding both longitudinal straight outer edges of the coil and one end thereof, the cleat (42) receiving the coil (36) in a cryogenic tank (35) so that the major axis of the coil is oriented substantially horizontally therein and so that the other end of the magnetic coil (36) is disposed near a first outer wall of the cryogenic tank (35), at least one radiation shield (46) between the cleat (42) and the walls of the cryogenic tank (35), and means (4) for supplying a mixture of magnetic and non-magnetic separated material to a magnetic separation zone of the magnet (2), so that the isolated mixture of materials is separated into fractions and subsequently falls freely under the action of gravity, characterized in that the or each radiation shield (46) surrounds the cleat (42) and the magnetic coil (36) and a reservoir (41) filled with helium is created between the cleat (42) and the outer walls (54, 56) of the cryogenic regulator vessel (35), whereby a single magnetic separation zone is created on the outer wall (42) of the cryogenic regulator vessel (35). 2. Magnetic separator according to claim 1, wherein the feeding means (4) is arranged so that the mixture of material is fed close to the magnet (2) in the region of the strong magnetic field and the mixture of material can fall under the effect of gravity. 3. A magnetic separator according to claim 2, wherein the feeding means (4) is arranged so that the mixture of magnetic and non-magnetic singulated material behind the face of the magnet (2) is moved through at least part of the region of the strong magnetic field before the mixture can fall under the action of gravity. 4. Magnetic separator according to one of claims 2 or 3, wherein the mirror axis of the coil of the magnet (2) is arranged at an acute angle (3) to the vertical, such that the magnetic separation zone lies under the magnet (2), whereby, when the mixture of material falls under gravity, the non-magnetic particles (C) alone fall vertically under the effect of gravity, while the magnetic particles (A, B) are deflected towards the magnet and follow a substantially parabolic trajectory until the gravitational force exceeds the force of the magnet (2). 5. Magnetic separator according to one of claims 2 or 3, wherein the magnet is arranged so that the mirror axis of the coil is horizontal and that the magnetic separation zone is under the magnet (2). 6. Magnetic separator according to one of the preceding claims, wherein the feed means (4) comprises a belt conveyor (10). 7. Magnetic separator according to claim 6, wherein the belt conveyor (10) conveys the mixture of separated material through at least a portion of the strong magnetic field in a direction parallel to the main axis of the coil and the material can then fall under the effect of gravity. 8. Magnetic separator according to one of the preceding claims, wherein a belt conveyor (10) is provided which moves past the magnet (2) close to its front face, there where the magnetic separation zone is provided, between the magnet (2) and the separated material. 9. Magnetic separator according to claim 8, wherein the speed of movement of the belt conveyor (10) is greater than the feed speed of the separated material. 10. Magnetic separator according to one of claims 8 or 9, wherein the belt conveyor (10) moves in the same direction as the material is fed. 11. Magnetic separator according to one of claims B or 9, wherein the belt conveyor (10) moves in the opposite direction to the supply of the material. 12. Magnetic separator according to one of the preceding claims, wherein the feed means (4) has a filling funnel (13). 13. Magnetic separator according to one of the preceding claims, wherein at least one divider plate (11) is provided in the lower region of the path of the falling particles in order to separate them according to the degree of their magnetizability. 14. Magnetic separator according to one of the preceding claims, wherein a shielding medium is provided around the turns of the coil.