Method and apparatus for the separation of solid particles having different densities

The invention relates to a method and apparatus for separating solid particles of different densities, using a magnetic process fluid.

Such a method is known from the Dutch patent 1 030 761. This patent describes a method and apparatus for separating solid particles in a magnetic process fluid, wherein the magnetic fluid is conducted through a magnetic field, generated by means of permanent magnets .

It should be noted that this known method and apparatus is indeed suitable for separating solid particles of greatly differing densities, wherein the density difference of the solid particles may be 1000 kg/m ³ or more as for example, copper at 8900 kg/m ³ in comparison with aluminium at 2700 kg/m ³ . Such particles are separated from each other by strong forces with the result that turbulence in the process fluid, or the possibility of clustering particles due to sedimentation, hardly influence the separation of the solid particles.

When separating solid particles such as plastic particles, seeds and diamonds of slight differences in density, in the order of up to 10 kg/ ³ , turbulence in the process fluid or clustering of particles due to sedimentation have been shown to be very disadvantageous .

The known methods and apparatuses are not suitable for the separation of solid particles of slight differences in den- sity, in the order of up to 10 kg/ ³ , such as solid polypropylene and solid polyethylene particles.

It is the object of the invention to provide a method and apparatus with which the drawbacks of the known method and apparatus are removed in an effective manner. Surprisingly, it was shown that this problem can be solved by conducting two separate partial flows of process fluid into the magnetic field, with the considerably larger partial flow consisting of the magnetic process fluid without particles, flowing in under laminar conditions, whereas the second, consid- erably smaller partial flow, is added to the process fluid in a turbulent state and mixed with the particles to be separated.

It has been shown that through the present invention the turbulence of the total fluid stream in the magnetic field is limited to a minimum, while in addition allowing the particles to start at or near the height of the splitter, such that the distance they have to travel {in the vertical direction) in order to be recovered at the desired side of the splitter, is minimal .

The present invention fulfils the ever increasing need to separate solid particles of small density differences such as plastic materials, seeds, diamonds etc. having a density difference of only up to 10 kg/ ³ .

To this end the present invention provides a method for the separation of solid particles of different densities in a magnetic process fluid, wherein the solid particles that differ little in density are separated by first thoroughly mixing the solid particles to be separated in a small partial flow of the process fluid, which small turbulent partial flow is added to a large laminar partial flow of the process fluid, after which the obtained mixture of the respective partial process fluids is conducted over, under, or through the middle of two magnetcon- figurations, wherein the particles are separated into lighter particles at the top of the laminar process fluid and heavier particles at the bottom of the laminar process fluid, each of which are subsequently removed with the aid of a splitter, wherein furthermore the materials of low density and the materials of high density are separated from the respective process streams, dried and stored and finally, the process streams are returned to the original starting process fluid streams .

According to the present method it is essential that the solid particles of little density difference to be separated are separately mixed with each other in a significantly smaller partial process fluid stream before being added to the process fluid, which is in a laminar flow condition. The combined proc ^¬ ess fluids are subsequently conducted over, under, or through the middle of two magnetconfigurations, with the lighter particles ending up in the laminar process fluid, while the heavier particles move to a lower stratum of the laminar process fluid. The thus separated particles are subsequently removed with the aid of a splitter. The separated solid particles are then with-

drawn from the respective process fluids and after drying they are collected and stored.

The process fluid from which the solid particles have been removed is then conducted back into the system for reuse. The present method is especially suitable, for example, for separating polypropylene particles having a density of 880- 920 kg/m ³ and solid polyethylene particles having a density of 930-960 kg/m ³ . In the plastics industry there is an increasing need for the recovery of such materials, which can then be used anew in the plastic processing industry.

The process fluid according to the invention usually consists of a suspension of iron-oxide particles.

The partial process fluid to which the solid particles to be separated have been admixed, generally constitutes ap- proximately 10% of the total process fluid.

In contrast with the Dutch patent 1 030 761, in which only the use of permanent magnets is mentioned, good separation results are obtained in accordance with the present method, by using permanent magnets, electromagnets or superconducting mag- nets.

The invention further relates to an apparatus for separating solid particles of little density difference in a magnetic process fluid, wherein the apparatus 1 is provided with a mixing vessel 2 for the solid particles to be separated in a small portion of the magnetic process fluid, which mixing vessel 2 is provided with a stirrer 3, wherein 4 denotes the turbulent small process fluid stream containing the particles, 5 and 6 are laminators for obtaining laminar process fluid, 8, 9 denote a rotating endless belt, 10 represents a splitter for dividing and removing the process fluid stream 11 containing the lighter particles on the one hand, and the process fluid stream 12 containing the heavier particles on the other hand. A simultaneously moving trough-shaped endless belt 13 serves to remove settled heavy particles and to maintain the laminar flow. The mixing vessel 2 is, usually funnel-shaped, that is to say it tapers, and comprises a stirrer 3 for mixing the particles of small density difference with a small portion of the process fluid.

It is particularly useful to pre-moisten the solid particles, for example, with the aid of steam so as to, when mixing the particles into the turbulent fluid stream, prevent the adherence to the particles of air bubbles, which would make the particles effectively lighter and heavy particles would incorrectly be separated into the lighter product stream. The contact between the cool particles and the hot steam produces a microscopically thin layer of condensation on the entire surface of the particles, so that air bubbles are unable to adhere to the solid surface, which would interfere with the separation.

The laminators 5 and 6 are provided before the magnet 7. The laminators 5 and 6 generate a laminar process fluid stream 8, with the result that there is no, or hardly any, turbulence in the laminar process fluid stream 8, allowing an ade- quate separation to take place between the light particles and the heavier particles.

According to the invention, the magnet 7 may be a permanent, electro- or superconducting magnet.

The invention is further elucidated by means of the ac- companying figures 1-3.

Fig. 1 shows a preferred embodiment of the apparatus 1 according to the invention.

The apparatus 1 is provided with a tapering mixing vessel 2, in which a standard stirrer 3 is provided for thoroughly mixing the solid particles to be separated that have slightly differing densities, with the black particles being polyethylene (PE) particles and the white particles representing polyethylene (PP) particles. The process fluid 4 that is in the turbulent condition and containing the solid particles to be separated passes the laminators 5 and 6 and ends up in the laminar process fluid 8 between the magnets 7, in this case an electromagnet. In order to realise a suitably laminar effect, the laminators 5 and 6 are preferably provided at the feed side of the fluid stream. Examples of laminators include a porous material having a homogeneous permeability and a material having parallel channels oriented in the direction of flow.

Under the influence of the magnetic field a separation takes place between the polyethylene particles of higher density

and the polypropylene particles of lower density. Approximately at the end of the magnets 7 the splitter 10 is located, preferably at the same level as the inlet opening of the turbulent process fluid stream. The splitter 10 ensures that the separated PP and PE particles 11 and 12, respectively, are removed and, after drying, stored for further use.

The process fluid containing the particles to be separated moves via an equidirectionally moving endless channel- shaped belt 13, which subsequently removes the settled particles and maintains the laminar flow.

Fig. 2 is a schematic representation of the particle distribution during the prior art separating process.

According to the prior art separating process as described in the Dutch patent 1 030 761, a slurry of plastic par- tides (PE) and (PP) and magnetic fluid are mixed and in turbulent condition introduced into the magnetic field between the magnets 1. The black particles 4 are heavier PE particles and the white particles 3 are the lighter PP particles.

The process fluid runs from left to right, as shown by the arrows 5. The splitter 6 is located at the end of the magnets 1.

The separation results show that the PP particles are not completely recovered in the light fraction, although in a laminar flow this ought to be the case . Apparently the flow is not sufficiently laminar in one part of the magnetic field, and/or from some of the starting positions, the particles have to travel too great a vertical distance from the position at which the particles flow into the field to the level of the splitter. In accordance with the invention this problem is solved by conducting two separate fluid stream into the magnetic field. By far the largest fluid stream consists for approximately 90% of magnetic fluid without particles, being introduced under laminar conditions, while the second much smaller flow has a turbulence of approximately 10%, into which are mixed the particles to be separated.

Fig. 3 shows the simulated trajectories of three pairs of PP and PE particles at laminar conditions in a fluid process stream from left to right. The solid lines are PE particles and

the dotted lines represent PP particles. The results show that the separation is most efficient if the particles to be separated are introduced into the process fluid stream in a small turbulent flow of approximately 10%, roughly at the height of the splitter, which provides a particularly good separation of the PP and PE particles.

The invention will now be further elucidated by way of the following examples.

Example 1

A mixture of approximately 70% PP and approximately 30% PE is obtained by means of floatation-sedimentation separation in water of a quantity of automotive shredder residue, ground into particles of approximately 10 mm diameter, and subsequently moistened with steam (10 kg steam per ton of plastics) . The moistened plastics are then mixed with a magnetic process fluid on a basis of water and iron-oxide particles with a magnetisation saturation of approximately 300 A/m at a ratio of 10 kg of plastics to 100 litres of process fluid. This mixture is stirred and injected at the height of the splitter, between two strata of laminar flow, in the field below a magnet as in Fig. 1, with the magnetic field under the magnet more or less exponentially decreasing with the distance to the lower surface of the magnet. The (horizontal) rate of the fluid streams and the conveyor belts is 0.3 m/s and the lingering time of the particles in the field up to the splitter is approximately 2 seconds. Above and below the splitter PP and PE products are removed at a purity better than 95%.

Example 2

A mixture of diamond and mineral particles with grain sizes between 0.5 mm and 2.0 mm is moistened with steam and subsequently mixed with a magnetic process fluid on a base of water and iron-oxide particles having a magnetisation saturation of approximately 6000 A/m at a ratio of 10 kg of mixture to 100 litres of process fluid. This mixture is stirred and injected at the height of the extractor opening for the diamond-enriched stream, between two laminar stream strata, in the field above a magnet as in Fig. 1, wherein the magnetic field above the magnet

in a good approximation exponentially decreases with the distance to the upper surface of the magnet. The (horizontal) rate of the fluid streams and the conveyor belts is 0.3 m/s and the lingering time of the particles in the field up to the splitter is approximately 2 seconds . The diamond-enriched stream is extracted by means of the extractor opening under the splitter.

Attention is drawn to the fact that the invention is in no way limited to the above described embodiments .