GB2102702A - Magnet for magnetic separation - Google Patents

Abstract

A superconducting magnet system for use in magnetic separation wherein the magnet is of linear shape having a coil or coils with two approximately straight parallel sections 10 joined by curved ends 12, the magnet being arranged so that the longest axis is horizontal and the sides of the coils are vertical to provide a flat substantially rectangular magnetic separation zone on one or both sides. A supporting and enclosing structure for the coils is described (Fig. 3, not shown). In use, material to be separated falls in a stream past the magnet (Fig. 4, not shown). <IMAGE>

Description

SPECIFICATION Improvements in and relating to magnetic separators This invention relates to magnet systems for use in minerals separation and to methods of minerals separation.

The invention is particularly concerned with a separation system in which the material to be separated is allowed to fall freely past a high strength magnet. The relatively magnetic material is attracted towards the magnet and the relatively non-magnetic material continues in a relatively straight path. Splitter members may be used to separate the two streams.

Previous magnetic separators proposed by us have employed a reversed pair of superconducting circular coils to provide a high magnetic field and a high gradient of field in the separation channel. This has necessitated building annular separation channels. If the channels are of complex form for the purpose of minerals separation, the requirement to make them annular increases the complexity and expense of the channel system.

According to the present invention, the coil or coils are linear, and provide a high magnetic field the gradient of which can be adjusted by appropriate design and while it may be as high as the previously mentioned system it may be lower and thus provide a force over a much larger volume.

Preferably two coils are provided aligned side by side arranged with one longitudinal edge of each coil above the other edge of the same coil so that its long axis is horizontal and its sides vertical in a rectangular section cryostat. With such a dipole magnet of high field strength, good depth of field is achieved and a separation zone which is rectangular and flat. Hence a straight separation channel may then be arranged at each side of the pair of coils. The use of a straight channel enables the position of splitter plates within the channels much more easily to be adjusted especially towards and away from the magnet(s) as compared with curved or annular plates.

More than one pair of coils may be used, one pair positioned above another pair to form the cryogenic magnet.

If required, several pairs of coils, in separate cryostats or a single cryostat, can be cascaded one above the other. The coils may be energised in either the same direction or reverse directions, so as to vary the field modulus and gradient in the separation zone.

However, the forces between adjacent coils should be supported by a rigid structure and if the coils are mounted in close proximity they are preferably enclosed by a single cryostat and carried on a common yoke.

Advantages of a system of this invention also include reduced stress on the supercooling arrangements, more efficient generation of magnetic field for a given mass of superconductors, and use of both sides of the coil.

The design of large-scale machines is also simplified by using a linear design. In a circular magnet, thermal contraction of the coils moves the coil in a radial direction away from the outside of the cryostat when in operation.

In a linear magnet this movement is very much smaller.

To separate the minerals a vertical feed channel is used which may be of the cascade type or free fall type as previously described but preferably is as described below.

For example a mineral of susceptability 10-5 cgs units per unit mass in a field times gradient product of 50 X 106 gauss2 cm-1 the force due to the magnetic field is half that of gravity. If the ore is fed as a stream down a vertical wall the magnetic force will retain all magnetic mineral against the wall. Friction against the wall then reduces the falling velocity and ore separation takes place in this condition. It is a preferred feature of the invention that the wall should be so configured as to cause the material retained against or adjacent the wall, and especially the non-magnetic fraction, to be diverted horizontally away from the magnet and the wall.

In an advantageous configuration the wall is arranged to have at least one, and if space permits, preferably several, humps or ridges to give the mineral momentum away from the magnet. Substantially non-magnetic mineral is diverted away from the rest of the stream while the magnetic mineral follows the surface of the wall. The relatively non-magnetic mineral is collected by a splitter set below each ridge and thereby separated from the remaining mineral.

The invention will now be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a sketch side elevation of a linear magnet coil illustrating the shape thereof, Figure 2 is a section of the coil of Fig. 1, Figure 3 is a more detailed section of a pair of coils, and Figure 4 is a sketch of a portion of a magnetic separator in accordance with the invention adjacent the magnet.

Referring to the drawings, each cryogenic magnet coil consists of two approximately straight parallel winding sections 10, typically 2 metres long with ends 1 2 of approximately semi-circular shape (Figure 1). The separation of the straight sections will be typically 50 to 1 50 mm and the cross-section 25 x 40 mm.

In use two linear coils 10, 10' such as are shown in Figs. 1 and 2 are placed back to back with their long sides horizontal and their sides vertical as can be seen in Fig. 3. The two coils are separated by a distance of about 5 to 10 mm. To sustain the forces between the straight sections, a yoke 14 of glass fibre reinforced material or metal is provided. The yoke is placed between the two identical coil windings so as to provide the highest field at each flat surface 15, 15' of the cryostat instead of surrounding a single coil.

Super insulation and radiation screens 1 8 are provided between the coils and the sides of the cryostat. The walls 1 6 are held spaced apart by a support member 20 and the top and bottom of the member is closed by caps 22. The magnet is employed with a conventional refrigeration system to provide supercooled super-conducting coils.

The field from such a linear dipole magnet as is shown in Fig. 4 extends out on either side, and a separation channel S can be placed on each side of the magnet member.

The magnet member as illustrated in Fig. 3 is employed in a magnetic separator as shown generally in Fig. 4 which is only of the right hand side. The left hand side is similar.

The material to be separated is fed from a hopper 21 through an adjustable choke 23 feed to fall adjacent the wall surface 1 5 of the magnet in a stream about 10 mm thick.

The magnetic force is adjusted, depending on the ore to be separated so that the ore 25 falls down the side of the magnet under the influence of gravity the magnetic portion of the ore being drawn towards the magnet and held against the wall. This tends to reduce the falling velocity and the separation achieved.

Hence a smooth bump 24 (or its equivalent) is provided on the wall 1 6 which causes the ore falling against or adjacent the wall and especially the non-magnetic fraction, to be diverted horizontally away from the wall.

Substantially non-magnetic mineral is diverted away from the magnetic mineral which tends to be re-attracted by the magnet back towards the wall surface 1 5.

Several bumps may be provided below each other. The concept of using such bumps forms the subject of our co-pending application No. 8219258, filed simultaneously herewith.

Finally the relatively magnetic material falls adjacent the magnet and the relatively nonmagnetic material away from the magnet, the two streams M and NM being separated by an adjustable flat splitter member 26, whose position can readily be adjusted towards or away from the wall surface 1 5. Typically, the stream of ore is 3 to 6 mm in thickness and the ridge or bump 24 projects 4 10 mms from the wall surface 1 5. It is desirable that the shape is smooth on the upper side so as to avoid remixing of the mineral. A sharp step causes mineral to be bounced at random and this may cause a degradation in the quality of separation.

The materials are reseparated at each successive ridge or bump.

The feed channel can, if desired, be divided into a horizontal series of thin vertical channels, e.g. each 20 mm wide, each receiving a stream of crushed ore to be separated, instead of one broad channel, given that the magnetic field is of sufficient extent (say 100 mm) to encompass all the channels.

For example if a second channel is used. on both sides, this will be positioned outwardly of the channel S as shown on one side at S' in Fig. 4, where the magnetic field is weaker.

Channel S' is bounded on the magnet side by a wall 16' provided with a ridge or bump 24' similar to that shown at 24. A first pass of the material may be made through this second channel S' and then a final or second pass through the first channel S adjacent the magnet where the field is stronger.

As an example of the separation achieved tests were made on phospate mineral containing about 14% apatite mineral and analysing as 5.8% P205. In a separation at a modest magnetic field of 24,000 gauss at a flow rate of 9 ton/hour per metre of magnet length ore was passed over two bumps of 10 mm projection from the magnet face. The ore had a free fall of 100 mm from the linear hopper during which fall it was held against the face of the channel adjacent to the magnet by the magnetic field. Below each bump the ore was split into magnetic and non-magnetic fractions. The magnetics from the first bump were passed over the second bump the two non-magnetic fractions were combined for retreatment at a higher field. The splitter below each bump was positioned 30 mm away from the magnet face and 70 mm below the centre of the bump. The non-magnetic product was 36% of the mass. The magnetic product was discarded as waste mineral. The recovery of apatite was 77% in the non-magnetic product. This product was then retreated at a higher field of 31,000 gauss.

Again the mineral was passed over two bumps of 10 mm projection after a 100 mm free fall. The splitter was set at 20 mm frorn the magnet wall and 70 mm below the bump.

The non-magnetic product from the first bump analysed at 38.3% P2Os or 90.3% phosphate. Magnetic measurement of the susceptability indicated 93% phosphate. The nonmagnetic product from the second bump represented 32.4% P206 or 76% apatite. The recovery of this second double stage of separation was 78%.

The final product is of sufficient commercial grade.

Claims (8)

1. A magnetic separator comprising a superconducting magnet the horizontal field force of which exceeds the vertical force and the vertical component of the force being not greater than gravity, means for feeding a stream of ore adjacent the magnet wherein the magnet is a linear dipole magnet having a coil or coils having two approximately straight parallel sections joined by curved ends, the longest axis being arranged horizontally and its sides vertical to provide a flat, substantially rectangular magnetic/separation zone on one or both sides, a flat splitter plate being provided in the or each zone.

2. A magnetic separator as claimed in Claim 1 in which the magnet has two coils aligned side by side.

3. A magnetic separator as claimed in Claim 1 or 2 wherein a yoke is provided to hold the straight section of the coil(s) straight against the self force of the coils.

A. A magnetic separator as claimed in Claim 3 when dependent on Claim 2 in which the yoke is located between the coils of the pair.

5. A magnetic separator as claimed in Claim 3 or 4 in which the coils are enclosed in a cryostat the flat walls of which are held apart by a support and which form one side of a separation channel.

6. A magnetic separator as claimed in any of the preceding claims in which at or adjacent the inner wall of a separation channel is a means to cause the particles to move out away from the magnet.

7. A magnetic separator substantially as hereinbefore described with reference to the accompanying drawings.

8. A linear magnet substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.