Patent Publication Number: US-8541709-B2

Title: Method for electrostatically separating a granule mixture made of different materials, and device for implementing same

Description:
The invention relates to a method for electrostatically separating granular materials and to a device for implementing the method. 
     Electrostatic separation methods are already used to sort mixed granular materials which originate, for example, from the grinding of industrial waste. Preferably, these materials are insulating materials. 
     Thus, the recycling of electrical and/or electronic waste entails separating the different components before the materials obtained can be valued. This separation must be as effective as possible to obtain a substantially constant quality of the materials obtained. It is therefore worth considering creating and future-proofing a downstream line for valuing these materials. For example, the plastic materials retrieved from the electrical and/or electronic waste may be used in the manufacture of terrace outline boards. To future-proof this activity, the boards need to have a quality and a color that are substantially constant. 
     There is also a need to be able to separate and recover plastic materials of different types, effectively and automatically. 
     A number of types of methods have been proposed, such as optical or float-based methods. However, these methods are not accurate enough and generate too many impurities. 
     Another solution is to grind the insulating materials so as to turn them into granules and, in a first step, to charge these granules by triboelectric effect in a vibration or rotary device. In a second step, the charged granules are conveyed to an electrostatic sorting device in which they are separated by an electric field. 
     To this end, the granules are injected from the top of the sorting device where they fall by gravity between two parallel and vertical electrodes. 
     Hereinafter in this application, the term “vertical” will be understood to mean the direction substantially parallel to the force of gravity. Similarly, the term “horizontal” will be understood to mean the direction substantially perpendicular to the force of gravity. 
     The positively charged granules are attracted by the anode (the negative electrode), whereas the negatively charged granules are attracted by the cathode (the positive electrode). 
     The granules which are thus deflected in their fall are separated and fall into two different collectors, arranged at the bottom of the device and in line with the electrodes. 
     The granules which have not been attracted by the electrodes fall into a third central collector where they are recovered. They can then be recirculated into the sorting device. 
     These granules may have lost their charge during conveying between the triboelectric charging device and the sorting device. They may also have acquired too weak a charge to be attracted by an electrode. 
     In fact, the electrical charge acquired by the granules in the abovementioned devices is not uniform. Some granules manage to be charged appropriately and can therefore be separated in a fairly intense electric field, whereas others leave the triboelectric charging devices with a charge level that is insufficient to enable them to be separated. The result is that a significant quantity of unseparated granules must be recovered and then returned to the triboelectric charging device. The productivity of the method is low since the return of the granules into the triboelectric charging device limits the charging of new granules. 
     The state of charge of the granules could be improved by increasing the duration of the triboelectric charging process. However, the productivity of the method would not be improved since the granules would remain longer in the triboelectric charging device, which consumes time and energy. 
     Furthermore, for a fixed charging duration, the quantity of charge actually acquired by the granules may vary significantly with the surface condition of the granules and, more particularly, their size. As it happens, when two granules of different sizes collide, they acquire two opposite electrical charges of the same value. However, while this value is sufficient for the smallest granule to be attracted by one electrode, it is insufficient for the largest granule to be attracted by the other electrode. It is then removed and redirected to the charging device. 
     To improve the quality of the triboelectric charging of the granules, the known installations therefore have, preferably, means for screening by granule size, arranged upstream of the triboelectric charging device. Next, each type of granule is charged and then electrically separated. 
     The quantity of charge actually acquired by the granules may also vary significantly with ambient temperature and humidity. 
     To resolve the problem of atmospheric conditions, it is desirable to use means for controlling the humidity and the temperature of the ambient atmosphere and of the granules. 
     However, these additional plants greatly complicate the management of the overall installation and significantly increase the cost of the method. 
     The productivity of the known installations for separating granular insulating materials is fairly low and the quality of the products obtained does not always meet the requirements of the clients. The current methods are too sensitive to the random variations in the ambient conditions and in the physico-chemical properties of the granules to be separated. 
     The present invention aims to overcome the above drawbacks and proposes a method for electrostatically separating granular insulating materials and a device for implementing same, which are efficient in terms of triboelectric charging and sorting quality and productivity. They are also multipurpose, economical in terms of energy and can easily be adapted to the ambient atmospheric conditions and to the physico-chemical properties of the granules to be separated. 
     To this end, the invention proposes a method and a device that make it possible, simultaneously, within one and the same enclosure, to electrically charge the granules and electrostatically separate them. 
     Thus, the subject of the invention is a method for electrostatically separating a mixture of granules of different materials, comprising the following steps:
     a) injection, between two electrodes in a separation chamber delimited by walls and provided with an air inlet and outlet, of a current of fluidizing air;   b) introduction of the mixture of granules of different materials into the current of fluidizing air;   c) control of the current of fluidizing air, so that the granules levitate in the current of air in a turbulent mode and become electrically charged by contacts between them and/or with the walls of the separation chamber;   d) generation of an electric field between the two electrodes, substantially perpendicularly to the direction of the current of air, such that the granules charged in step c) are displaced either in the direction of the electric field if they are charged positively or in the opposite direction if their charge is negative;   e) adhesion of the charged granules to the surface of the electrodes;   f) removal and collection of the granules adhered to each electrode.   

     According to other embodiments:
         in step a), the current of fluidizing air can be injected substantially vertically upward, and in step b), the granule mixture is introduced by free fall and in counterflow relative to the current of fluidizing air;   the current of fluidizing air, injected into the separation chamber in step a), may exhibit a negative pressure gradient in the vertically upward direction;   the introduction of the granule mixture in step b) can be carried out at a rate, expressed in terms of weight of granules introduced per unit of time, regulated to a value substantially equal to the weight of granules collected in step f) per unit of time;   the current of air can be previously heated before entering into the separation chamber;   the current of air can be homogenized on entering into the separation chamber;   step f) can be implemented by means of conveyor belt-type electrodes made of electrically conductive material, the removal of the granules being performed by translating the conveyor belts, and the collection being carried out by scraping; and/or   the method may also include a step g), after step f), of cleaning the electrodes.       

     Also the subject of the invention is a device for electrostatically separating a mixture of granules of different materials, characterized in that it comprises:
         a separation chamber delimited by walls and provided with an air inlet and outlet;   two electrodes extending into the separation chamber between the air inlet and outlet;   a means for injecting, between the two electrodes, a current of fluidizing air in a determined direction;   a means for introducing the mixture of granules into the current of fluidizing air;   a means for controlling the current of fluidizing air such that, in use, the granules levitate in the current of air in a turbulent mode and are electrically charged by contacts between them and/or with the walls of the separation chamber;   a means for generating, between the two electrodes, an electric field substantially perpendicularly to the direction of the current of air;   a means for removing and for collecting the granules adhered to each electrode.       

     According to other embodiments:
         the air inlet can be arranged so that the current of air is, in use, substantially vertically upward;   the means for introducing the mixture of granules can be arranged to introduce the granules, into the separation chamber, by free fall and in counterflow relative to the current of fluidizing air;   the electrodes can be arranged so as to diverge from the air inlet toward the air outlet;   the separation device may include a means for heating the current of air arranged upstream of the air inlet of the separation chamber;   the separation device may include an air chamber arranged downstream of the air inlet of the separation chamber and including means for homogenizing the current of air;   the means for homogenizing the current of air may be glass balls;   the separation device may include a means for controlling the rate of introduction of the granules;   the separation device may include a means for measuring the weight of granules collected, linked to the means for controlling the rate, the latter being adapted to control the rate of introduction of the granules according to the weight measured by the measurement means;   the means for collecting the granules may be a scraper;   the separation device may include a means for cleaning the electrodes;   the electrodes may be of conveyor belt type; and/or   the means for generating the electric field may be adjustable.       

     The method and the device according to the invention make it possible to remedy the abovementioned drawbacks by simultaneously carrying out the charging of the granules by triboelectric effect and their separation in an electric field. Thus, the granules cannot lose their charge between the moment when they are charged and the moment when they are subjected to the electric field. 
     Moreover, the current of air separates the granules by size, so that the triboelectric charging is optimal since it is done on granules substantially of the same size. 
     Furthermore, each granule remains in the current of air only for the minimum time needed to acquire a sufficient triboelectric charge for it to be attracted by one of the electrodes. The uncharged granules cannot leave the current of air, which ensures the purity of the granules collected. Thus, the method and the device according to the invention optimize the sorting efficiency and adapts naturally to each granule. 
     Finally, because the charging and the separation are simultaneous and take place in one and the same enclosure, it is possible to easily, and economically, control the ambient atmospheric conditions. 
     Thus, a device according to the invention offers a sorting efficiency and quality that are significantly enhanced compared to a device of the state of art of equivalent useful dimensions. 
    
    
     
       Other features of the invention will be stated in the following detailed description given with reference to the figures which represent, respectively: 
         FIG. 1 , a schematic view in longitudinal cross section of a first embodiment of an electrostatic separation device according to the invention; and 
         FIG. 2 , a schematic view in longitudinal cross section of a second embodiment of an electrostatic separation device according to the invention. 
     
    
    
     With reference to  FIG. 1 , an electrostatic separation device according to the invention comprises a separation chamber  100  delimited by lateral walls  101  (of which only two are illustrated) and provided with an air inlet  102  and an air outlet  103  respectively allowing for the intake and exhaust of compressed air. 
     Preferably, the air inlet  102  is provided with an air diffuser  102   a , and the air outlet  103  is provided with a filter  103   a.    
     Two electrodes  105 - 106  extend into the separation chamber between and on either side of the air inlet and outlet. Thus, the current of air circulating between the air inlet and outlet is located between the electrodes  105 - 106 . These electrodes are linked to a high DC voltage generator  107 , preferably adjustable: the electrode  105  is linked to the negative terminal of the generator  107 , and the electrode  106  is linked to the positive terminal of the generator  107 . This arrangement generates an electric field between the two electrodes  105 - 106  when the current flows. 
     Preferably, as illustrated in  FIGS. 1 and 2 , the electrodes are arranged so as to diverge from the air inlet toward the air outlet. 
     The device also includes a means  108  for injecting, between the two electrodes  105 - 106 , a current of air in a determined direction represented by the arrow F 1 . The current of air therefore passes through the separation chamber  100  between the air inlet  102  and the air outlet  103 . This current of air forms a fluidized bed. The air inlet  102  is arranged, advantageously, so that the current of air is, in use, substantially vertically upward. 
     A means  109  is arranged to allow the introduction of a mixture M of granules into the current of fluidizing air. 
     Preferably, the means  109  for introducing the mixture of granules M is arranged to introduce the granules, into the separation chamber  100 , by free fall and in counterflow relative to the current of fluidizing air. 
     The means  109  is, preferably, a variable rate means controlled by a rate control means (not illustrated). 
     The mixture M comprises at least two different materials M 1 -M 2 , illustrated in the figures by white disks M 1  and black disks M 2 . The granules may be of different sizes. In the figures, two sizes (small size: M 1   p  and M 2   p , and large size: M 1   g  and M 2   g ) are illustrated, but in practice, the method and the device according to the invention can effectively separate granules of numerous sizes. 
     The means  108  for injecting the current of fluidizing air is linked to a means for controlling the current of fluidizing air such that, in use, the granules levitate in the current of air in a turbulent mode and are electrically charged by contacts between them and/or with the walls  101  of the separation chamber  100 . 
     The device according to the invention makes it possible to implement the method for electrostatically separating the mixture of granules of different materials according to the invention. It comprises the following steps. 
     In a step a), a current of fluidizing air is injected between the two electrodes. This current of air comes from the air inlet  102  and is removed through the air outlet  103 . In the advantageous configuration illustrated in  FIGS. 1 and 2 , the current of fluidizing air is injected roughly vertically upward. Combined with this upward current, the divergent configuration of the electrodes creates a negative pressure gradient in the vertically upward direction. In other words, the air pressure decreases in the direction of the current of air. Thus, the pressure of the air at the air outlet  103 , at the top of the chamber  100 , is lower than the pressure of the air at the air inlet  102 , at the bottom of the chamber  100 . 
     In a step b), the mixture of granules M of different materials is introduced into the current of fluidizing air. In the abovementioned advantageous configuration, the mixture of granules is introduced by free fall and in counterflow relative to the current of fluidizing air. 
     Simultaneously, in a step c), the current of fluidizing air is controlled so that the granules levitate in the current of air in a turbulent mode and are electrically charged by contacts between them and/or with the walls of the separation chamber. 
     The negative pressure gradient makes it possible to distribute the granules at different heights, relative to their dimensions: the larger or heavier granules remain at the bottom, whereas the smaller or lighter granules rise more into the fluidized bed. The upper limit of the fluidized bed is established by the smallest or lightest granules, but the current of air is controlled so that this upper limit does not exceed, preferably, two thirds of the height of the separation chamber  100 . 
     The method and the device according to the invention therefore allow natural distribution of the granules according to their weight actually within the chamber. There is therefore no need for any screening by size of the mixture M before introduction into the separation chamber  100 . The characteristic diameter of the granules of the mixture M may lie between, advantageously, 0.5 and 5 mm. 
     The method and the device according to the invention therefore make it possible to obtain a dimensional uniformity of the granules which come into contact with one another. This ensures the best triboelectric charging conditions because two granules roughly of the same weight, but of different materials, acquire opposite charges of the same value. This enables the granules each to be attracted by an electrode. 
     While the granules M 1   p -M 2   p , M 1   g -M 2   g  levitate in the current of fluidizing air and are charged by triboelectrification, in a step d), an electric field E is generated between the two electrodes, roughly perpendicularly to the direction F 1  of the current of air, and directed from the cathode to the anode. 
     The electric field needed to implement the invention is greater, preferably, than 1 kV/cm. It is typically between 4 and 5 kV/cm. 
     Thus, the granules charged in step c) are displaced, either in the direction of the electric field if they are positively charged, or in the opposite direction if their charge is negative. In  FIGS. 1 and 2 , the granules M 1   p  and M 1   g  are negatively charged and are displaced toward the cathode  106 , in the direction opposite to the electric field E. The granules M 2   p  and M 2   g  are positively charged and are displaced toward the anode  105 , in the same direction as that of the electric field E. 
     When subjected to the action of the electrical image force, the positively charged granules M 2   p  and M 2   g  adhere, in step e), to the anode  105 . Similarly, the negatively charged granules M 1   p  and M 1   g  adhere, in a step e), to the cathode  106 . 
     The method according to the invention includes a step f) for the removal and collection of the granules adhered to each electrode. 
     According to a preferred embodiment, this step f) is implemented using conveyor belt-type electrodes, advantageously made of an electrically conductive material such as a metal. Preferably, the conveyor belt is made of stainless steel with a smooth surface. The use of conveyor belts made of plastic materials with metallic insertions may also be envisaged. 
     According to the embodiments illustrated in  FIGS. 1 and 2 , the electrodes  105  and  106 , of conveyor belt type, are translated to remove the granules deposited on their surface in a direction diagrammatically represented by the arrow F 2 , roughly the same direction as the current of air. It is also possible to drive the conveyor belts in the reverse direction, that is to say, roughly in counterflow relative to the current of air. However, the granules adhered to the surface of the conveyor belts run the risk of being unstuck by the current of air. 
     The conveyor belts remove the granules opposite to the current of air relative to the electrodes. Next, the granules are collected on the conveyor belts by scraping, using scrapers  110 . These unstick the granules from the conveyor belts and direct them to a collector  111 - 112 . 
     The speed of the belt is correlated with the rate of granules coming from the means  109  for introducing the mixture M of granules, the initial composition of the granular mixture to be separated and the width of the belt. 
     It has to be sufficient for the granules attracted by the electrode to form only a single layer on the surface of the belt. Otherwise, the electrical image force is not great enough to cause the granules to adhere to the belt. 
     Moreover, by using a speed that is too low, the granules would remain in contact with the belt of the electrodes for long enough for them to discharge. This has the effect of reducing the electrical image force which causes the granules to adhere to the surface of the belt. The granules then run the risk of being detached from the belt before being able to be recovered by the collectors  111 - 112 , and of falling back to the base of the electrodes. It the current of air is as wide as the distance separating the base of each electrode, the granules which fall back may be returned to circulation in the current of air. Otherwise, the granules drop into the bottom of the chamber  100  and have to be recovered and then reintroduced into the chamber via the means  109 . 
     As an example, with the plastic materials originating from computer waste, a rate of approximately 300 kg/hour and a belt with a width of 1 m, a speed of around 5 m/min may be sufficient. 
     The method according to the invention may also include a step g) for cleaning the electrodes, after step f). To this end, the separation device according to the invention includes a means for cleaning the electrodes, diagrammatically represented in  FIGS. 1 and 2  by brushes  113 . These are used to unstick the granules which may not have been unstuck by the scrapers  110 . They are used in particular to clear the conveyor belts of the dust P inevitably generated by the implementation of the method. In fact, the impacts of the granules on one another, during the triboelectric charging, give rise to a certain erosion of these granules which takes the form of dust. This builds up on the conveyor belts and may reduce the adhesion of the granules by the electric image force. The brushes  113  are used to clear the belts of this dust and to maintain the attraction and adhesion power throughout the duration of operation of the device. 
     Preferably, as illustrated in  FIG. 1 , the collectors  111 - 112  are in tight contact with the corresponding conveyor belt  106 - 105  to gather the dust and remove it from the chamber  100 . Alternatively, as illustrated in  FIG. 2 , the dust P may be removed by a dedicated collector  114 . 
     Other means can be used, provided that they allow the removal and the collection of the granules adhered to each electrode. It is possible, for example, to use rotary electrodes combined with a scraper positioned opposite the current of fluidizing air relative to the electrodes. It is also possible to use a removal and collection means that is mobile relative to an immobile electrode. 
     With the method and the device according to the invention, the charging is done actually within the separation chamber, so that the granules do not run the risk of losing their charge before being subjected to the electric field. 
     Furthermore, as soon as a granule is charged, it is attracted by the electrode of opposite polarity. Each granule therefore remains in the current of triboelectric charging air only for the time needed to acquire a sufficient charge for it to be attracted by an electrode. This allows optimum efficiency, by leaving space for other granules and by using only the mechanical energy of the current of air strictly necessary for the acquisition of the triboelectric charge. 
     Finally, the fact that the granules are immediately removed as soon as they adhere to an electrode also optimizes the efficiency, since the granules do not have time to lose their charge, and leaves space for other granules to adhere to the electrodes. 
     Preferably, the device includes a means for controlling the rate of introduction of the granules, linked to a means (not represented) for measuring the weight of granules collected by the collectors  111 - 112 . 
     Thus, the introduction of the mixture of granules in step b) is carried out at a rate, expressed in terms of weight of granules introduced per unit of time, regulated to a value substantially equal to the weight of granules collected in step f) per unit of time. In other words, the means for controlling the rate is adapted to control the rate of introduction of the granules according to the weight measured by the measurement means. 
     According to the embodiment illustrated in  FIG. 2 , the current of air is previously heated before entering into the separation chamber. To this end, the electrostatic separation device according to the invention includes a means  120  for heating the current of air, arranged upstream of the air inlet  102  of the separation chamber  100 . This heating means  120  can be used to adjust the temperature of the fluidizing air to an optimum temperature to reduce the surface humidity of the granules and improve the conditions of electrification by triboelectric effect. For example, with a mixture of granules of ABS (acrylonitrile butadiene styrene) and HIPS (high impact polystyrene) materials, with a size of between 1.5 and 3 mm, this optimum temperature is between 35° C. and 45° C. 
     The electrostatic separation device according to the invention may also include an air chamber  130 , arranged downstream of the air inlet  102  of the separation chamber  100 , and including means for homogenizing the current of air entering into the separation chamber  100 . Preferably, this air chamber  130  is arranged upstream of the air diffuser  102   a  and it is connected to a compressor  131 . 
     The means for homogenizing the current of air are, for example, glass balls  132 . Their distribution in the air chamber  130  makes it possible to divide the current of compressed air, so that the current of air is uniform over its entire width when it enters into the chamber  100  and ensures a uniform horizontal pressure in the separation chamber  100 . 
     According to other embodiments, the introduction of the granules can be done by projection from the bottom of the separation chamber, with the current of air (and, possibly, a complementary current of air) so that the granules projected upward levitate in the current of air in a turbulent mode and are electrically charged by contacts with one another and/or with the walls of the separation chamber.