Electrostatic particle separation process and associated installation

Abstract

The invention relates to a method for the electrostatic separation of particles from a mixture of particles, said mixture comprising particles of a first material and particles of at least one second material, the method comprising at least: a preliminary conditioning step in which the mixture of particles is carried through a dry turbulent airflow passing through a tube (40) of the same material as the first material, the tube extending over a length greater than 50 m, preferably greater than 100 m, so as to cause the particles of the mixture of particles to collide with each other and with the tube and to electrically charge at least the particles of the second material by triboelectric effect; and then a separation step in which the mixture of particles is dropped into an electric field (E) generated by a set of electrodes (62), so as to separate the particles of the first material from the particles of the second material. (Fig. 1)

Description

Title of the invention: Electrostatic particle separation process and associated installation. Technical field of the invention

[0001] The invention relates, in general, to the technical field of separating particles from a mixture of particles for the purpose of recycling them.

[0002] The invention relates more specifically to a method for the electrostatic separation of particles from a mixture of particles comprising particles of a first material and particles of at least a second material, for example, a mixture composed mainly of two materials, and to the installation associated with such a method. In this description, "particles" means small pieces, preferably having a mass of less than 0.1 g, preferably greater than 0.01 g, and which may have any shape, regular or irregular (grains, flakes, etc.). Prior art

[0003] Waste recovery, and in particular its reuse and recycling, represents a significant economic and environmental challenge for manufacturers. It makes it possible to reduce the proportion of raw materials extracted and the energy consumption associated with the production of an object, and to decrease the environmental impact of production.

[0004] In particular, the recovery of plastic waste is all the more important as the use of plastics increases and diversifies. There are, moreover, different plastics (thermoplastic polymers for the scientific name), referred to hereafter as "plastic materials", among which we can cite, by way of example, polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, polypropylene or polystyrene.

[0005] The most common approach to reusing and / or recycling plastic waste using a mechanical recycling method consists first of all in sorting and separating the different plastic materials that compose it. Each plastic material is shredded into particles, then the particles are washed and dried and transformed into granules that can subsequently be reused as raw material for the production of new objects.

[0006] However, separating the different plastic materials in waste is complicated when at least two plastic materials are mixed to form combined plastics. Indeed, in such objects, the plastic materials cannot be disassembled and are difficult to differentiate by processes of conventional separation methods. In this case, simple manual sorting cannot separate the different plastic materials.

[0007] To remedy this problem, it is known in the prior art to perform post-shredding sorting. The mixture of plastics is reduced to particles and then sorted by various processes, such as optical sorting, flotation sorting, or density sorting, which allows the plastic materials to be separated according to their physical properties. However, such separation methods are not satisfactory when the plastic materials have similar sizes or densities.

[0008] A solution proposed in patent no. CN1 14734554 consists of an installation capable of separating, by tribo-electrostatics, a mixture of particles comprising particles of a first material and particles of a second material. The installation comprises an inlet duct connected to an S-shaped tube having an inner wall of constant cross-section, and, at the outlet of the tube, an intense electric field created by two electrodes of opposite polarities. To perform the separation, the plastic particles are conveyed into the inlet duct. A turbulent flow of hot air carries the particles into the tube, and the S-shape of the tube promotes collisions between the particles and against the inner wall of the tube, and their triboelectric charging by friction. The first and second materials of the particles then possess opposite electrical charges.The particles then fall due to gravity into the electric field of the electrodes and are attracted to the electrode of opposite polarity to their own, which allows them to be separated according to the material they are made of.

[0009] However, such an installation is not suitable for all types of materials and particles. Indeed, when the first and second materials are said to be close or "at a short distance" from each other on a triboelectric scale, the friction between the particles of the first and second materials does not create a sufficiently large charge difference between the two materials to separate them. Moreover, this insufficient polarization of the particles is accentuated when the particles are essentially two-dimensional flakes rather than three-dimensional granules. Description of the invention

[0010] The invention aims to remedy all or part of the drawbacks of the prior art by proposing in particular a solution enabling efficient separation of particles from a mixture of particles, the mixture comprising particles of a first material and particles of at least a second material, in particular when the materials are close on a triboelectric scale, and in particular when the particles are flakes.

[0011] To this end, according to a first aspect of the invention, an electrostatic separation method for particles of a mixture of particles is proposed, said mixture comprising particles of a first material and particles of at least a second material, the method comprising at least: • a preliminary conditioning step in which the particle mixture is subjected to triboelectric loading so as to electrostatically charge the particles of the first material and / or the particles of the second material, the preliminary conditioning step comprising driving the particle mixture through a dry turbulent airflow passing through a tube of the same material as the first material, the tube extending over a length greater than 10 m, preferably greater than 50 m, preferably greater than 100 m, so as to collide the particles of the particle mixture with each other and with the tube and to charge at least the particles of the second material by triboelectric effect; then • a separation step in which the mixture of particles is dropped into an electric field generated by a set of electrodes, so as to separate the particles of the first material from the particles of the second material.

[0012] Passing the mixture of particles through the tubing made of the first material offers the technical advantage of creating a sufficiently large charge difference between the particles of the first material and the particles of the second material to effectively separate the mixture of particles in the electric field, even if the first material and the second material are close to each other on a triboelectric scale.

[0013] A triboelectric scale is defined as a classification of materials according to the negative or positive electrical charges they acquire through friction with other materials. In the remainder of this application, two materials will be said to be close, or "at a short distance," from each other on a triboelectric scale when, when rubbing against each other under standardized conditions, they tend to exchange a small number of electrical charges. Two materials will be said to be far apart, or "at a great distance," from each other on a triboelectric scale when, when rubbing against each other under standardized conditions, they tend to exchange a large number of electrical charges.

[0014] Using a tubing material located at a large distance from both the first and second materials on a triboelectric scale does not allow sufficient differentiation of the electrostatic charge of the particles of the first material and the electrostatic charging of the particles of the second material. Due to the large distance between the tubing material and the first and second materials on a triboelectric scale, the tubing material tends to charge the first and second materials similarly during particle collisions against the tubing walls, so that the difference in charges between the first and second materials resulting from their collisions with the tubing material is not sufficient to differentiate them during the separation step.

[0015] Furthermore, since the first and second materials are close to each other on a triboelectric scale, it is difficult to find a tubing material located between said first and second materials on a triboelectric scale that would be sufficient to charge the particles of the first material and the particles of the second material with different polarities. Moreover, such a tubing material would not allow sufficient electrostatic charging of the first and second materials.

[0016] Using a tubing material identical to the first material thus makes it possible to strongly charge the particles of the second material (during collisions both with the particles of the first material and with the tubing) and to weakly charge the particles of the first material (primarily during collisions with the particles of the second material), or even to keep it electrically neutral. The length of the tubing allows for multiple collisions of the particles with each other and against the tubing.

[0017] Thus, thanks to the judicious choice of tubing material, the step of separating the particles in the electric field is more efficient.

[0018] Advantageously, the tubing includes bends that deflect the trajectories of the particles within the tubing to increase the number of collisions between the particles and against the tubing. These bends can be created by winding the tubing in a spiral or helix, or by sinuous curves. However, the bends preferably have large radii of curvature to avoid excessive pressure loss in the tubing. Radius values ​​greater than 1 meter, and preferably greater than 2 meters, are therefore preferred.

[0019] According to one embodiment, the particle conditioning step is carried out when the ratio of the mass of particles moving in the tubing to the surface area of ​​the inner wall of the tubing is greater than 20 kg / m², preferably greater than 25 kg / m², for example equal to 29 kg / m². Such a ratio ensures efficient loading of the particles into the tubing, as well as satisfactory separation of the particles of the first material from the particles of the second material during the separation step.

[0020] According to one embodiment, the process includes, prior to the particle conditioning step, a drying step, consisting at least of: drying the particles of the particle mixture to obtain a relative humidity of the particle mixture of less than 0.4%, preferably less than 0.1%. This drying step may follow a washing step which removes impurities and reduces the number of foreign particles present in the particle mixture, thus limiting collisions and / or electrification of the particles in the tubing. Preferably, the particles are washed several times before drying.

[0021] According to one embodiment, the large number of particle collisions in the tubing generates thermal energy that increases the temperature of the particles and thus dries them as they pass through the tubing. This phenomenon is all the more noticeable as the length of the tubing increases. In practice, the particle collisions in the tubing make it possible to obtain a relative humidity of the particle mixture of less than 0.2%, preferably less than 0.1%. The drying of the particle mixture in the tubing limits the appearance of parasitic charges caused by the presence of water molecules in the tubing. As a result, the separation of the particles in the electric field is improved.

[0022] According to one embodiment, the process includes a step of driving the particles of the particle mixture into an electrostatic charging path formed between the tubing and the electrode assembly, the electrostatic charging path allowing the electrical charges of the particles of the particle mixture to be retained.

[0023] The electrostatic charging path may include, in particular, a particle recovery hopper; and / or a rotary valve; and / or a dust collector. Where applicable, the electrostatic charging path may include a finned drum or other stirring device that increases the electrical charge of the particles while conveying them to the electrode assembly.

[0024] In particular, the recovery hopper and the dust collector may include suction means for removing air present in the particle mixture. Preferably, the suction means are turbines.

[0025] According to another embodiment, the electrostatic charging path includes any other mechanism or set of mechanisms for increasing and / or maintaining the electric charges of the particles to promote their separation in the electric field of the electrode set.

[0026] According to one embodiment, the particles of the particle mixture are made of plastic materials. Preferably, the first material is high-density polyethylene and the second material is polypropylene. According to a preferred embodiment, the particle mixture contains between 25% and 45% particles of polypropylene and between 55% and 75% high-density polyethylene particles. This particle distribution ensures satisfactory particle separation despite the proximity of the first and second materials on the triboelectric scale.

[0027] The process is particularly well-suited to a mixture whose particles are flakes, that is, flat, elongated particles with a high surface area to volume ratio. Collisions between flakes are very different from collisions between more or less spherical three-dimensional granules, particularly with regard to their triboelectric efficiency. Generally, flakes are more difficult to charge than granules, and experience shows that the specific steps of the process according to the invention, which uses a long tube made of a suitable material and preferably incorporates a thorough drying step, make it possible to obtain good separation performance for these flakes.

[0028] According to another aspect of the invention, it relates to an installation for the electrostatic separation of particles from a mixture of particles, said mixture comprising particles of a first material and particles of at least a second material, the installation comprising: • a portion of the particle mixture conditioning, in which the particle mixture is subjected to triboelectric loading, so as to electrostatically charge at least the particles of the second material; • a set of electrodes generating an electric field capable of separating the particles of the first material from the particles of the second material of particles; • a pipe longer than 10m, preferably longer than 50m, preferably longer than 100m, made of the same material as the first material; and • a turbine to drive the mixture of particles in a dry turbulent airflow passing through the tubing.

[0029] The use of a long tube has the advantage of electrically charging the particles through collisions with each other and with the tube. Repeated friction between the particles promotes their heating, which contributes to their drying. Thus, such an installation serves both as a drying device and as a particle pre-charging device.

[0030] According to a preferred embodiment, the tubing is made of polyethylene. In this configuration, the polyethylene particles acquire negligible charges, while the polypropylene particles acquire significant electrical charges through collisions with the polyethylene particles and with the tubing wall. Thus, the polypropylene particles will be deflected more by the electric field than polyethylene particles. The tubing generally allows for the different loading of the particles of the first material from the other particles in the particle mixture.

[0031] According to another embodiment, the tubing is made of polypropylene.

[0032] According to one embodiment, the tubing is a pipe of constant cross-section and comprises an inlet portion, an electrostatic charging portion, and a particle outlet portion, at least the inlet and outlet portions extending parallel to a principal axis of the tubing. The electrostatic charging portion comprises an inner lateral wall of the tubing.

[0033] Preferably, the tubing comprises curves formed in the electrostatic charging portion of the tubing, allowing the particles to collide more frequently with each other and against the lateral wall of the tubing to become electrically charged. Alternatively, the tubing is straight, with the inlet, electrostatic charging, and outlet portions extending parallel to the main axis of the tubing.

[0034] According to one embodiment, the turbine of the installation is a suction turbine located downstream of the outlet portion of the manifold. Alternatively, the turbine of the installation may be a blower turbine located upstream of the inlet portion of the manifold. The turbine imposes a constant airflow, which ensures a constant flow rate of particles passing through the manifold. Furthermore, the use of a dry airflow contributes to the drying of the particles.

[0035] According to a preferred embodiment, the electrode set is formed of two electrodes of opposite electrical charges creating an intense electric field allowing the separation of the particles of the first material from the particles of the second material.

[0036] According to one embodiment, each electrode in the electrode set has a large surface area and forms a predetermined angle with the vertical to promote the attraction and deflection of the particles in the particle mixture and optimize the separation.

[0037] Optionally, the installation includes one or more dehumidifiers for reducing the humidity of the ambient air in the installation. In particular, the dehumidifiers reduce electrical exchange between particles and water molecules in the tubing and in the ambient air of the installation.

[0038] According to one embodiment, the installation includes a particle drying line enabling the relative humidity of the particle mixture to be less than 0.4%, preferably less than 0.1% upstream of the tubing.

[0039] According to various embodiments, the drying line follows a washing line comprising one or more pieces of equipment for removing impurities from the particle mixture upstream of the tubing, for example: • a wet shredder; and / or • a magnetic separator; and / or • a float sort line; and / or • an underwater grinder.

[0040] Preferably, to obtain a suitable relative humidity, the drying line includes one or more pieces of equipment for drying and sorting the particle mixture upstream of the tubing, for example: • a centrifuge; and / or • a thermal dryer; and / or • a mechanical dryer; and / or • an air separator.

[0041] According to one embodiment, the installation includes an electrostatic charging path formed between the tubing and the electrode assembly, the electrostatic charging path allowing the electrical charge of the particles of the particle mixture exiting the tubing to be maintained or increased, the path including a particle recovery hopper; and / or a rotary valve.

[0042] According to one embodiment, the recovery hopper is connected to the outlet portion of the tubing. The hopper has an inverted frustoconical shape with a vertical axis, so that the particles exiting the tubing fall into the hopper by gravity. The hopper allows the particles in the tubing to be recovered and immobilized, thus changing their trajectory.

[0043] According to one embodiment, the recovery hopper includes a suction cyclone enabling the particles to undergo a circular motion in the hopper in order to prolong collisions between the particles and increase the electrostatic charge of the particles.

[0044] According to one embodiment, the rotary valve makes it possible to keep the particles dust-tight and to control the flow of particles arriving in the dust collector.

[0045] According to one embodiment, the dust collector allows dust to be removed from the particles.

[0046] According to one embodiment, after passing through the electrostatic charging path, the particles fall by gravity onto an inclined vibrating table, allowing the particles to advance and gradually fall into the magnetic field created by the electrode array. The vibrating table allows also to distribute the particles over the entire surface of the electrodes of the electrode assembly and to present them in a monolayer at the entrance of the electrodes.

[0047] According to one embodiment, the installation includes at least a first and a second recovery tanks to collect respectively the particles of the first material and the particles of the second material after their passage through the magnetic field of the electrode assembly.

[0048] Preferably, the installation includes a third recovery tank for collecting particles of the first and second materials that are not sufficiently charged to be attracted to the electrodes. These particles can then be returned to the inlet of the installation for a second separation treatment. brief description of the figures

[0049] Other features and advantages of the invention will become apparent from the following description, with reference to the attached figures.

[0050] [Fig-1]: [Fig.1] is a schematic view of an installation for a process of electrostatic separation of particles from a mixture of particles according to a first embodiment of the invention.

[0051] [Fig.2] : [Fig.2] is a schematic view of the installation according to a second embodiment of the invention.

[0052] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED description of a method of implementation

[0053] Figures 1 and 2 illustrate an installation 1 for carrying out an electrostatic separation process for particles in a mixture of particles according to a first and a second embodiment of the invention, respectively. In particular, the installation 1 enables the separation of a mixture of particles in plastic materials comprising particles of a first material and particles of a second material, the first material being located at a short distance from the second material on a triboelectric scale.

[0054] Preferably, the particle mixture contains between 25% and 45% polypropylene particles and between 55% and 75% high-density polyethylene particles, these proportions having been empirically found to be the best for optimal electrostatic charge of the particles. Preferably, the particles are flakes. By way of example, the flakes may have a thickness of less than 2 mm, for example less than 1.5 mm for a length and width of approximately 8 to 10 mm.

[0055] The installation 1 shown in Figures 1 and 2 comprises at least one particle preparation line 2, a conveying line 3, a drying device and a pre-charge 4, an electrostatic loading path 5 for the particles and a separation zone 6 for the particles to carry out the separation process, which includes at least the following steps: • Step I: sort, wash and dry the particles from the particle mixture in preparation line 2 of installation 1; • Step II: convey the particles through a conveying line 3 of the installation 1; • Step III: load and dry the mixture of particles in the drying and pre-charging device 4 of the installation 1; • Step IV: move and charge the particles in the electrostatic charging path 5 of the installation 1; • Step V: separate the particles in separation zone 6 of installation 1.

[0056] To carry out step I of the separation process, the mixture of particles is introduced into the preparation line 2 of the installation 1.

[0057] According to the first and second embodiments of the invention, the particle preparation line 2 comprises at least one washing line 20 for removing impurities from the particle mixture and a drying line 22 for reducing the relative humidity of the particles before electrostatic charging. The washing line 20 and the drying line 22 also serve to sort the particles from the particle mixture to retain only the polypropylene particles and the high-density polyethylene particles.

[0058] The washing line 20 may include, for example: • a water shredder designed to remove large foreign objects from the particle mixture; and / or • a magnetic separator to remove ferrous elements from the particle mixture; and / or • a flotation sorting line, allowing particles heavier than water to be separated by gravity; • a wet grinder to wash the particles from the particle set.

[0059] In general, the washing line 20 includes all equipment enabling the sorting, washing and cleaning of the particle mixture in order to obtain the cleanest possible particle mixture.

[0060] The particle drying line 22 can operate simultaneously and / or successively with the washing line 20. The drying line 22 may include, for example, a centrifuge and / or a thermal or mechanical dryer for reducing the amount of water in the particle mixture and partially drying it. The drying line 22 may also include an air separator for remove particles with masses lighter than polypropylene and high-density polyethylene particles.

[0061] In practice, tests carried out have shown that the relative humidity of the particles at the outlet of the preparation line 2 should preferably be less than 1%, preferably less than 0.4% to obtain effective separation of the particles.

[0062] The mixture of particles exiting the preparation line 2 is then recovered and conveyed into the conveying line 3 of the installation 1 to carry out step II of the process.

[0063] According to the embodiment of figures 1 and 2, the conveying line 3 comprises a blowing turbine 30, a storage silo 32, a screw conveyor 34, a discharge hopper 36 and a rotary distribution lock 38.

[0064] The particle mixture exiting the preparation line 2 is conveyed to the storage silo 32 by means of a pipe 31 supplied with air by the turbine 30. Preferably, the turbine 30 provides a flow of dry air which promotes the drying of the particles. [Do you have any information regarding the turbine's power?]

[0065] The storage silo 32 is preferably a conical-bottomed silo that allows for the temporary storage of a large quantity of particles, protected from ambient humidity and external dust, before they are conveyed to the drying and pre-charging device 4. The storage silo 32 can also serve as a particle flow regulator. For example, as long as the particle mixture is less than a predetermined quantity that allows for particle separation, said particle mixture remains in the storage silo 32.

[0066] At the outlet of the storage silo 32, the mixture of particles is recovered by the screw conveyor 34, which allows the particles to be progressively conveyed into the discharge hopper 36 in a dust-tight manner.

[0067] The conveyor 34 advantageously has an angle of elevation relative to the horizontal such that the particles fall by gravity into the hopper 36 and collide with each other. The collisions thus created electrify, at least to a small extent, the particles in the particle mixture. Preferably, the inner walls of the discharge hopper 36 are made of polyethylene, so as to strongly charge the polypropylene particles through collision with the polyethylene particles and with the polyethylene inner walls of the hopper 36.

[0068] The particles exiting the hopper 36 fall by gravity into the rotary distribution valve 38. Preferably, the valve 38 is a rotary paddle valve, which allows the particles to be introduced into an inlet portion 41 of the drying and pre-charging device 4, without depressurizing the drying and pre-charging device 4. The rotation speed of the paddles of the valve 38 is regulated and allows a desired mass flow rate of particles to be imposed at the inlet of the device 4.

[0069] Generally, to carry out step II of the separation process, the conveying line 3 includes any apparatus or set of apparatus enabling the mixture of particles from the preparation line 2 to be brought into the , in an airtight and dust-tight manner and with a regulateable mass flow rate at the inlet of the device 4.

[0070] Step III of the process is carried out using the drying and pre-charging device 4 of the installation 1. The device 4 comprises at least one long tube 40 of constant cross-section, the tube 40 comprising the inlet portion 41, an electrostatic charging portion 42, and an outlet portion 43 for the particles. The inlet portion 41 and outlet portion 43 of the tube 40 extend parallel to a principal axis of the tube 40.

[0071] The inlet 41 and outlet 43 portions of the tubing are straight cylinders that respectively allow the admission of particles into the electrostatic charging portion 42 and their expulsion from the tubing 40. According to the embodiment of [Fig. 1], the tubing 40 comprises an inner side wall made of polyethylene. The electrostatic charging portion 42 of the tubing 40 includes bends formed by sinuosities, the bends having large radii of curvature.

[0072] In practice, the length of the tubing 40 is greater than 50 m, preferably greater than 100 m, preferably greater than 150 m, for example equal to 180 m, and the diameter of the inner wall of the tubing 40 is greater than 50 mm, preferably greater than 100 mm, preferably greater than 150 mm, for example equal to 160 mm.

[0073] According to the embodiment of [Fig. 1], the mixture of particles which is at the inlet of the inlet portion 41 of the pipe 40 is drawn in by a suction turbine 44 located at the outlet of the pipe 40, so as to carry the particles in a turbulent dry air flow, from the inlet portion 41 to the outlet portion 43 of the pipe 40. In practice, the suction turbine 44 has sufficient power to carry a flow of particles from the inlet portion 41 to the outlet portion 43 of the pipe 40, for example a power of 20 kW.

[0074] Preferably, in steady state, the suction turbine 44 allows a large and constant flow of particles to be driven into the tube 40. The ratio of the mass of particles moving in the tube 40 to the surface of the inner wall of the tube 40 is greater than 20 kg / m2, preferably greater than 25 kg / m2, for example equal to 29 kg / m2.

[0075] By moving turbulently in the tube 40, and in particular by moving in any curves of the electrostatic charging portion 42, the particles of the particle mixture collide with each other and with the wall inside the tube 40. The polypropylene particles become positively charged through repeated friction with the polyethylene particles and with the inner wall of the tube 40, while the polyethylene particles become negatively charged through friction with the polypropylene particles, or through contact with the inner polyethylene wall of the tube 40. It should be noted that a small particle coming into contact or rubbing against a wall of the same material acquires a slightly negative charge, while the wall itself becomes positively charged. Thus, a significant charge difference is created between the polypropylene particles, which have very strong positive charges, and the polyethylene particles, which have very weak negative charges.

[0076] In particular, for satisfactory loading of the particles of the particle mixture, the ratio of the mass of particles moving in the tube 40 to the surface of the inner wall of the tube 40 must be greater than 20 kg / m2, preferably greater than 25 kg / m2, for example equal to 29 kg / m2.

[0077] Preferably, the airlock 38 is kept in the closed position and the suction turbine 44 is stopped until the aforementioned value of the ratio of the mass of particles to the surface area of ​​the inner wall of the tubing 40 is reached. The particle mixture then remains in the discharge hopper 36.

[0078] Optionally, the outlet portion 43 of the tubing 40 includes a trap which, in the closed position, retains the particles in the tubing 40 so as to increase the residence time of the mixture of particles in the tubing 40 and to prolong their electrostatic charging, in particular the electrostatic charging of the polypropylene particles.

[0079] Furthermore, by colliding with each other, the polypropylene and polyethylene particles generate thermal energy inside the tube 40, which allows the particle mixture to dry out and the relative humidity of the particles to decrease, thus limiting the presence of liquid water molecules that can slow down and alter the normal electrostatic charge of the particles. In practice, at the outlet of the tube 40, the relative humidity of the particles is less than 0.4%, preferably less than 0.1%.

[0080] According to the embodiment of [Fig. 1], the drying and pre-charging device 4 may further include a dehumidifier 39 placed upstream of the inlet portion 41 of the tubing 40, and used to reduce the humidity of the air entering the tubing 40. This reduction in the humidity of the air contributes to the drying of the particles in the tubing 40.

[0081] According to another embodiment of the invention, the inner wall of the tube 40 is made of polypropylene, and the polyethylene particles become highly charged by friction with the polypropylene particles and with the inner polypropylene wall.

[0082] The drying and pre-charging device 4 shown in [Fig.2] differs from device 4 in [Fig.1] in that the turbine 44 is a blowing turbine, placed upstream of the inlet portion 41 of the tube 40 and blowing a flow of dry and preferably hot air inside the tube 40 to move the particles in the tube 40. According to this embodiment, the blowing turbine 44 is also stopped until the value of the ratio of the mass of particles to the surface of the inner wall of the tube 40 is reached.

[0083] The outlet portion 4 3 of the tubing 40 is directly connected to the electrostatic loading path 5 of the particles, said path 5 enabling step IV of the separation process to be carried out.

[0084] According to the embodiment of [Fig.1], the electrostatic loading path 5 includes a recovery hopper 52, a rotary airlock 54 and a dust collector 56.

[0085] The charged particles in the tube 40 are ejected by the suction turbine 44 from the outlet portion 43 of the tube 40 and fall by gravity into the hopper 52. The hopper 52 includes a sealed suction cyclone that subjects the particles to a high-speed cyclonic motion inside the hopper 52. The particles collide with each other and against the hopper 52, which increases their electrical charges. The particles then fall by gravity into the rotary valve 54 located below the hopper before reaching the dust collector 56. The dust collector 56 removes any remaining dust from the particle mixture using an air jet.

[0086] According to the embodiment shown in [Fig.2], the hopper 52 further includes a suction turbine 58 for removing the air contained in the particle mixture and amplifying the cyclonic movement of the particles in the hopper 52. In addition, to amplify the drying of the particles and the removal of dust, the dust collector 56 is connected to a drying turbine 57 generating an additional airflow inside the dust collector 56.

[0087] In general, the electrostatic charging path 5 includes an apparatus or set of apparatuses capable of retaining or increasing the electrical charges accumulated by the particles in the device 4.

[0088] For example, the electrostatic charging path 5 may include a finned drum. The finned drum is a rotating shaft whose axis of rotation is preferably inclined with respect to the horizontal. Advantageously, the finned drum rotates at a predetermined rotational speed conducive to the electrostatic charging of the particles. The drum includes an inner side wall on which are The fins are arranged not parallel to the axis of rotation, allowing the direction of the particles to be deflected. Thus, the particles are set in rotation by the movement of the drum and collide with each other due to the presence of the fins, which increases their electrical charges.

[0089] The electrostatic charging path 5 leads to the separation zone 6 of the installation 1, in which step V of the particle separation process is carried out.

[0090] According to the embodiment of [Fig. 1], the separation zone 6 comprises a vibrating table 60, a pair of electrodes 62 and a set of recovery trays 64.

[0091] The vibrating table 60 is inclined relative to the horizontal so that it comprises an upper and a lower portion relative to the horizontal. In particular, the table 60 vibrates with an adjustable amplitude. The particles of the electrostatic loading path 5 fall by gravity onto the upper portion of the vibrating table 60 and advance in a controlled manner, thanks to the vibrations of the table 60, to the lower portion of the table 60.

[0092] Once at the end of the lower portion of the table 60, the particles fall in a single layer by gravity into an intense electric field E generated by the pair of electrodes 62. Preferably, the particle flow falling into the intense electric field E is very low and controllable. To better control this particle flow, the lower portion of the table can be connected to a chute allowing the particles to fall gradually.

[0093] The electrode pair 62 comprises two voltage-supplied thin plates positioned at a distance from each other and with opposite charges, so as to create the intense electric field E. Preferably, each thin plate forms a predetermined angle with the vertical to orient the electric field E in a desired direction. In particular, the electrode voltages are adjustable to vary the electric field E as required. According to one embodiment: • the voltage of one of the electrodes of the electrode pair varies between -125kV and 0V; • the voltage of the other electrode of the electrode pair varies between 0V and 125kV.

[0094] When falling into the electric field E of the electrode pair 62, the polypropylene particles, which have strong positive charges, are deflected one by one by the intense electric field E and are strongly attracted to the negatively charged plate, so as to fall on the side of the negatively charged plate.

[0095] On the contrary, the polyethylene particles, which have almost negligible or at least weak negative electric charges, are deflected very little, or not at all, by the intense electric field E and fall by gravity to the center of the pair of electrodes 62 or on the opposite side to the side where the polypropylene particles fall. If necessary, the anode is inclined and away from the point of impact, to accentuate the deflection of the polypropylene particles that it attracts, while the cathode can be almost vertical, close to the point of impact of the particles.

[0096] The collection tray assembly 6, located below the electrode pair 62, comprises a first and a second collection tray (66, 68) separated from each other, allowing the collection of polypropylene and polypropylene particles, respectively. In this way, the particles of the particle mixture are separated according to their material. The assembly 6 also includes a third collection tray 67 for collecting particles that are not sufficiently charged to be separated. The particles recovered in the third collection tray 67 are reinjected into the storage silo 32 of the conveying line 3 to be reloaded into the tubing 40.

[0097] According to another embodiment, the electrode assembly 62 comprises a plurality of negatively charged electrodes facing a plurality of positively charged electrodes so as to generate a very intense electric field E.

[0098] According to the embodiment of [Fig. 2], the separation zone 6 is surrounded by a separation chamber 7. The separation chamber 7 maintains warm ambient air at controlled humidity and temperature, so as to optimize the separation of particles from the particle mixture. Furthermore, the separation chamber 7 includes a heater 70 and is connected to an air humidification and dehumidification unit 72, allowing the humidity and temperature to be regulated within the separation chamber 7 and consequently within the separation zone 6. Advantageously, the warm air from the separation chamber 7 passes through the tubing 40, entering the blower turbine 44 of the drying and pre-charging device 4 and exiting the suction turbine 58 of the recovery hopper 52 of the electrostatic charging path 5. Thus, • The blower turbine 44 recovers the hot air contained in the separation chamber and discharges it into the pipe 40 so as to draw the particle mixture into the pipe 40 and to heat said particle mixture; and / or • the suction turbine 58 of the hopper 52 draws in the hot air contained in the hopper 52 to reinject it into the separation chamber 7.

[0099] This method has the advantage of not requiring any additional air beyond that contained in the separation chamber 7 to operate the separation process, thus limiting energy consumption. Furthermore, the separation process as shown in [Fig. 2] allows to overcome the ambient humidity of installation 1, which can be variable, by using the controlled air of the separation enclosure 7.

[0100] Naturally, the invention is described above by way of example. It is understood that a person skilled in the art is able to carry out different embodiments of the invention without departing from the scope of the invention.

[0101] For example, the tubing 40 of the drying and precharging device may be straight, with the inlet 41, electrostatic charging 42, and outlet 43 portions of the tubing 40 all parallel to the main axis of the tubing 40. Alternatively, the electrostatic charging portion 42 of the tubing may comprise a helix with a vertical central axis, between the inlet 41 and outlet 43 portions of the tubing 40, the outlet 43 preferably being located vertically above the inlet 41 portion. This helical configuration may allow for greater electrostatic charging of the particles in the particle mixture, for example, if these particles are larger than flakes. Other configurations with bends in the tubing 40 of constant or variable radius may be considered.

[0102] According to another example, the particle mixture may comprise more than two particle materials, the installation 1 allowing the flakes to be charged and separated according to the triboelectric properties of the materials which compose them.

[0103] It is emphasized that all features, as they are apparent to a person skilled in the art from the present description, drawings and attached claims, even if in practice they have only been described in relation to other specific features, both individually and in any combinations, can be combined with other features or groups of features disclosed herein, provided that this has not been expressly excluded or that technical circumstances make such combinations impossible or meaningless.

Claims

Demands

1. A method for the electrostatic separation of particles from a mixture of particles, said mixture comprising particles of a first material and particles of at least a second material, the method comprising at least: - a preliminary conditioning step in which the mixture of particles is subjected to triboelectric loading, so as to electrostatically charge the particles of the first material and / or the particles of the second material, and then - a separation step in which the mixture of particles is dropped into an electric field (E) generated by an array of electrodes (62), so as to separate the particles of the first material from the particles of the second material;the process being characterized in that the prior conditioning step comprises: - driving the mixture of particles in a dry turbulent airflow through a tube (40) of the same material as the first material, the tube (40) extending over a length greater than 10 m, preferably greater than 50 m, preferably greater than 100 m, so as to collide the particles of the mixture of particles with each other and with the tube (40) and to electrically charge at least the particles of the second material by triboelectric effect.;

2. Separation method according to claim 1, characterized in that the particle conditioning step is carried out when the ratio of the mass of particles moving in the tubing (40) to the surface of the inner wall of the tubing (40) is greater than 20 kg / m2, preferably greater than 25 kg / m2, for example equal to 29 kg / m2.

3. A separation process according to claim 1 or 2, characterized in that it comprises, prior to the particle conditioning step, a drying step consisting at least of: - dry the particles of the particle mixture so as to obtain a relative humidity of the particle mixture of less than 0.4%, preferably less than 0.1%.

4. A separation method according to any one of the preceding claims, characterized in that it comprises a step of entraining the particles of the particle mixture into an electrostatic charging path (5) formed between the tubing (40) and the electrode assembly (62), the path (5) comprising: - a particle recovery hopper (52); and / or - a rotary airlock (52); and / or - a dust collector (54); the electrostatic charging path (5) allowing the electrostatic charge of the particles of the particle mixture to be retained.

5. Separation method according to any one of the preceding claims, characterized in that the particles of the particle mixture are made of plastic materials.

6. Separation method according to the preceding claim, characterized in that the first material is high-density polyethylene and the second material is polypropylene.

7. Separation process according to the preceding claim, characterized in that the mixture of particles contains between 25% and 45% polypropylene particles and between 55% and 75% high-density polyethylene particles.

8. Separation method according to any one of the preceding claims, characterized in that the particles of the particle mixture are flakes.

9. Installation (1) for the electrostatic separation of particles of a mixture of particles, said mixture comprising particles of a first material and particles of at least a second material, the installation comprising: - a conditioning portion (4, 5) of the mixture of particles, in which the mixture of particles is subjected to triboelectric loading, so as to electrostatically charge at least the particles of the second material; - a set of electrodes (62) generating an electric field (E) capable of separating the particles of the first material from the particles of the second material of particles; - a tube (40); and - a turbine (50) for driving the mixture of particles in a dry turbulent airflow through the tube (40); the installation (1) being characterized in that: - the tube (40) has a length greater than 50 m, preferably greater than 100 m; and - the tube is of the same material as the first material.

10. Installation (1) according to the preceding claim, characterized in that the tubing (40) is made of polyethylene, preferably high-density polyethylene.

11. Installation (1) according to claim 9 or 10, characterized in that the tubing (40) comprises bends.

12. Installation (1) according to claim 9 or 10, characterized in that the tubing (40) is straight.

13. Installation (1) according to any one of claims 9 to 12, characterized in that it comprises a particle drying line (20) enabling the relative humidity of the particle mixture to be obtained below 0.4%, preferably below 0.1%.

14. Installation (1) according to any one of claims 9 to 13, characterized in that it comprises an electrostatic charging path (5) formed between the tubing (40) and the electrode assembly (62), the path (5) comprising: - a particle recovery hopper (52); and / or - a rotary airlock (54); and / or - a dust collector (56).

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