WO2000040336A1

WO2000040336A1 - Method of magnetically separating particulate materials

Info

Publication number: WO2000040336A1
Application number: PCT/NL2000/000012
Authority: WO
Inventors: Peter Carlo Rem; Shunli Zhang; Otto Norbert Fraunholcz
Original assignee: Bakker Magnetics B.V.
Priority date: 1999-01-07
Filing date: 2000-01-07
Publication date: 2000-07-13
Also published as: NL1010977C2; AU3083400A

Abstract

The present invention relates to a method of separating non-ferro metal particles using a rotating magnetic field. According to the invention the particles are surrounded by a fluid restricting the fall-velocity significantly, suitably a fluid having a density of at least 0.1 kg/l. The presence of such a medium causes the particle, if it is a non-ferro metal particle, to experience a hydrodynamic lifting force allowing for an excellent separation.

Description

METHOD OF MAGNETICALLY SEPARATING PARTICULATE MATERIALS

The present invention relates to a method of separating non-ferro metal particles from non-magnetic, nonconducting particles, which particles are part of a stream of particles, wherein the stream of particles is passed through a generated rotating magnetic field such that the axis of rotation of the magnetic field and the direction of movement of the stream of particles are at an angle, yielding at least one stream enriched in non-ferro metal particles and a stream depleted in non-ferro metal parti- cles, which streams are discharged separately.

Such a method is generally known in the art, for example for separating non-ferro metal particles from a waste stream such as scrap material from cars. For example, the European patent application 0 305 881 describes separation using a fast rotating magnetic drum. Particles are supplied using a conveyor belt and passed through the magnetic field. The rotating magnetic field induces eddy-currents in the metal srap particles resulting in an oppositely directed magnetic field as a result of which different metals can be separated from each other.

A limitation of the known method is that it can separate only relatively large particles (5 mm or larger; and with considerable technical effort 3 mm or larger) . Apart from the repelling action by the induced magnetic field, there is another, though smaller effect caused by the rotating magnetic field which may interfere with the separation. This effect, starting metal particles to rotate, may cause a rotating particle when coming in con- tact with a part of the apparatus, to go in an unwanted direction. This adversely affects the separation, and thus the quality of the streams of particles.

The object of the present invention is to provide a method facilitating the separation of small particles, of a size of such as 0.5 - 5 mm or smaller, depending on the material of which the particles consist. A particular object is to provide a method for performing such a separation on an industrial scale.

To this end, the method according to the present invention is characterized in that the non-ferro metal particles of the stream of particles are passed through the magnetic field in a fluid and substantially without contact with a solid carrier at a velocity limited by the flow resistance in the fluid, the non-ferro metal particles are caused to rotate by the rotating magnetic field yielding at least one stream enriched in non-ferro metal particles in the fluid and a stream depleted in non-ferro metal particles in the fluid, which particle-containing fluid streams are discharged separately.

Surprisingly it has been found that the weak effect causing electrically conducting (metal) non-ferro particles to rotate can be used for performing a separation in the fluid. In such a medium a hydrodynamic lifting force (Magnus effect) is created by the combination of i) the translational movement of the particle and ii) the rotation of the particle caused by the rotating magnetic field, which lifting force can reach a much larger, significant value due to the chosen velocity reduction in the fluid and can result in an excellent separation. This turns out to be highly independent of the particle size. Because the separation is achieved in the fluid, the method allows for an excellent throughput . The width of the stream of particles may, for example, be 3 cm.

When, in the present invention, mention is made of a velocity limited by the flow resistance, this is under- stood to be a condition in which at least 90% of the non- ferro metal particles have a velocity which is smaller or equal to the free fall velocity of those particles in said fluid of at most 3 m/sec. Preferably the velocity is smaller or equal to a free fall velocity of 2 m/s. When, in the present application, mention is made of a solid carrier, this means the surface of an apparatus for performing the method, which surface may or may not move, such as a conveyor belt. Substantially without contact with a solid carrier means surrounded by a fluid, such as a gas or liquid, contact with other particles of the stream of particles being excluded from the term solid carrier. Substantially without contact means that from the moment the particles in the rotating magnetic field are subjected to separation until the moment of separation of the particle-containing fluid streams, at least 50% (preferably at least 80%, and more preferably at least 90%) of all particles of the stream of particles have not been in contact with a surface of the separation apparatus. From WO 98/06500 a method and an apparatus are known for performing a separation. According to this publication use is made of a rotating magnetic field, in which separation occurs by non-ferro magnetic, electrically conducting particles rolling over a surface. According to a first embodiment the direction of rotation of the magnet and the direction of movement of the stream of particles are chosen such that the at least one stream enriched in non-ferro metal particles in the fluid moves away from the magnet. By placing baffles between each of the resulting streams, such an embodiment allows for obtaining more than one stream enriched in non-ferro metal particles.

When it is passed trough the magnetic field, the stream of particles is preferably surrounded by a fluid having a density of at least 0.1 kg/1.

The fluid having a density of at least 0.1 kg/1 may be a particulate solid substance fluidized by means of vibration and/or passing gas through, a gas under elevated pressure, and according to a preferred embodiment a liquid, such as water.

The use of a liquid, in particular an inert liquid such as water, has the advantage that the apparatus necessary for the separation may be relatively inexpensive. Also, in those cases where the streams of particles do not (any longer) need to be dried, the costs for energy are reduced.

According to a favourable embodiment the value of the density is chosen to be close to that of one of the particulate materials to be separated. In practice, the density will always be chosen lower than that of the particulate materials to be separated. Because of this, gravity may cause the movement of the particle. Choosing the density as indicated here, maximizes the difference in speed with which gravity causes the particles to move through the generated rotating magnetic field, as a result of which the velocity of each particle depends on the specific type of particle. Thus there is an extra parameter for controlling and optimizing the separation.

According to an interesting embodiment, the fluid is passed in a direction parallel with the trajectory of the particles.

Passing the fluid in a direction that is the same as or opposite to the direction of movement of the particle through the generated rotating magnetic field, also allows for an effect on the separation and optimization.

Depending on their size, the particles are, in accordance with an advantageous embodiment, supplied par- allel with the axis of rotation of the magnetic field .

The method according to the present invention is already distinguished from the methods according to the state of the art by the fact thaat the particle size has less effect on the separation. By supplying the particles, depending on their size parallel to the axis of rotation, for example using a sieve having a graduate retention, the quality of the separated streams of particles may be enhanced further.

Preferably the rotating magnetic field is generated by means of a rotating magnetic cylinder. This rotating magnetic cylinder usually has 2 - 32 poles, preferably 2 - 8, more preferably 2 - 4, and most preferably 2 poles. The use of a low number of poles has the advantage that the magnetic field has an extended range away from the rotating cylinder and is more homogeneous, which benefits the separation. However, with a low number of poles, the speed of rotation should be higher, which involves higher demands with respect to the construction and may result in a higher consumption of energy. The cylinder will usually rotate with a pole frequency between 3,000 and 20,000 rpm. The invention will be elucidated with reference to the drawing in which Fig. 1 shows the principle of action of the method according to the invention; and

Fig. 2 schematically depicts an apparatus suitable for applying the method according to the invention.

In Fig. 1 a particle A is shown moving in a down- ward direction, for example due to the force of gravity. The particle A is surrounded by a medium B with a density of at least 0.1 kg/1. Preferably the density is higher, such as at least 0.3 kg/1, and more preferably at least 0.6 kg/1. Owing to a rotating magnetic field generated by a magnetic drum 1, the particle A, provided it comprises a non-ferro metal will also rotate (the axis of rotation is perpendicular to the plane of the drawing) . This results in a hydrodynamic lifting force (F_lift) which is perpendicu- lar to the direction of movement of the particle A (and also perpendicular to the axis of rotation of the particle A) . Because of the lifting force exerted on particle A, the direction of movement changes and the desired separation is obtained. Reference is made to Figure 2, where particles are supplied to a reservoir 2 under the influence of the force of gravity, which reservoir 2 contains water as the fluid. As a result of the rotating magnetic field generated by the rotating magnetic drum 1, non-ferro metals move to the right, whereas non-conducting materials are not deflected.

By means of (optionally adjustable) guiding baffles 3 the stream of particles supplied may be divided in one or more streams of particles of non-ferro metals. Non-metal ends up in stream R. Ferro-magnetic particles which are poss- ibly present, and which move in the direction of the magnet, may be discharged using a conveyor belt (not shown) provided in the reservoir near the magnet.

In a first experiment, 33.1 kg bottom-ash from a waste incineration plant, having a particle size of 0-15 mm is sieved and the fraction of 2-15 mm is subjected to a conventional top belt magnetic separator for the separation of (ferro) magnetic particles. The remaining fraction (15.4 kg) is supplied to a eddy-current separator accord- ing to the invention. The eddy-current separator had a magnetic dipole-rotor (width 0.5 m; diameter 120 mm) of FeNdB and was operated at a rotational speed of 9600 rpm. A water-filled reservoir 2 was used and the stream of particles to be subjected to the separation was supplied at a centre-to-centre distance from the rotor-axis of 70-100 mm. This resulted in 0.469 kg of particles enriched in non-ferro metal being separated (after sorting by hand: 0.161 kg Al, 0.179 kg Cu/brass; and 0.017 kg zinc. Non- ferro metal content: 76%, considered to be easily saleable) . The effective through-put (calculated for use of both sides of the wet eddy-current separator) was 8.5 ton per hour.

The method according to the present invention may be used for separating very small non-ferro metal par- tides from a stream. The method according to the present invention may, for example, be used for separating gold from river deposits (wet) or dry sand (such as gold dust- comprising desert sand) . The method according to the present invention may also be used for cleaning up shooting ranges or recovering non-ferro metals from electronics scrap.

For separating in more than one stream of non-ferro metal fractions using a clock-wise rotating magnet, the stream of particles may best be supplied to the right of the magnet. In such a case the separation that can be achieved at the left will be less. There, in practice, a stream of particles will be supplied lower and/or at a slightly larger centre-to-centre distance from the magnet axis. Above or alternatively, the left side may be used for performing a pre-separation of non-ferro metal and non-metal. The mixture of non-ferro metal obtained may be added to the stream supplied to the right side for achieving the separation desired. If it is only desired to have only one non-ferro metal, this stream may be advantageously be introduced, in the condition described above where the magnet rotates clock-wise, left of the magnet.

Claims

1. Method of separating non-ferro metal particles from non-magnetic, non-conducting particles, which particles are part of a stream of particles, wherein the stream of particles is passed through a generated rotating magnetic field such that the axis of rotation of the magnetic field and the direction of movement of the stream of particles are at an angle, yielding at least one stream enriched in non-ferro metal particles and a stream depleted in non-ferro metal particles, which streams are discharged separately, characterized in that the non-ferro metal particles of the stream of particles are passed through the magnetic field in a fluid and substantially without contact with a solid carrier at a velocity limited by the flow resistance in the fluid, the non-ferro metal particles are caused to rotate by the rotating magnetic field yielding at least one stream enriched in non-ferro metal particles in the fluid and a stream depleted in non- ferro metal particles in the fluid, which particle-containing fluid streams are discharged separately.

2. Method according to claim 1, characterized in that the direction of rotation of the magnet and the direction of rotation of the stream of particles is chosen such that at least one stream in the fluid enriched in non-ferro metal particles moves away from the magnet.

3. Method according to claim 1 or 2 , characterized in that the particles are, when they are passed through the magnetic field, surrounded by a fluid having a density of at least 0.1 kg/1.

4. Method according to claim 3, characterized in that the fluid is a liquid.

5. Method according to claim 4, characterized in that the value of the density is chosen to be close to that of one of the particulate materials to be separated.

6. Method according to any of the preceding claims, characterized in that the fluid is passed in a direction parallel to the trajectory of the particles.

7. Method according to any of the preceding claims, characterized in that depending on their size the particles are supplied parallel with the axis of rotation of the magnetic field.

8. Method according to any of the preceding claims, characterized in that the rotating magnetic field is generated using a rotating magnetic cylinder.

9. Method according to claim 8, characterized in that the rotating magnetic cylinder has 2-32 poles, pre- ferably 2-8, more preferably 2-4, and most preferably 2 poles .