Abstract:
A method for recycling polymeric materials, including a mechanical separation step (B), a first density separation step (C), an intensive cleaning step (E), and a second density separation step (G, V) is described. Additional separation steps, such as a third density separation step (M, W), and optionally a fourth density separation step (G a , G b ). Further less essential steps such as wetting (A), grinding (D), air flow separation (K), drying (I), electrostatic sorting (Z) and optional sorting (Z a ) steps may also be carried out to achieve optimized and refined separation. Polymeric materials from refuse or crushed motor vehicles may thus be separated, and polyethylene, polypropylene, polystyrene and acrylonitrile-butadiene-styrene resins may be recycled.]

Description:
[0001]    The present invention relates to a method for separating polymer materials originating from waste.  
           [0002]    The invention also relates to a plant intended to carry out the method for separating polymer materials.  
           [0003]    The recycling industry concerns the selective recovery of polymer materials, for example such as polyethylene (PE), polypropylene (PP), acrylonitrile-butadiene-styrene (ABS), polystyrene (PS), polyvinyl chloride (PVC), polyurethanes (PU) and others. The invention relates to a method by steps and a corresponding plant which ensure the upgrading of spent polymer materials originating from all types of wastes and more particularly from motor vehicles. It is hence a matter of achieving a very selective separation of mixtures of polymer materials according to their components, and also the conversion thereof into materials, which can be reused or reintroduced into a new cycle of manufacture.  
         PRIOR ART  
         [0004]    The sorting of polymer materials can be achieved by several ways It exists the manual sorting, the sorting after cryogenic grinding, the sorting with electrostatic processes, the sorting by infrared analysis or by laser radiation, the density sorting, and the sorting according to colour and shapes. Interest is shown more particularly here in the sorting of large amounts of polymer materials of different natures.  
           [0005]    A plant for recycling polymer materials is known from DE-A4,329,270, and comprises, in particular, a grinder, a discharge basin for removing the heaviest materials, an air-flow separator for separating the polymers in the form of sheets, and then density separators. On the first hand, in the discharge basin, polymer materials with a density greater than 1 can be removed despite their advantages, and, on the other hand, there is no cleaning apparatus for polymer materials. This cleaning apparatus proves to be essential in the case of wastes originating from public garbage dumps or from motor vehicle breakers.  
           [0006]    U.S. Pat. No. 4,728,045 describes a method for recovering synthetic materials originating from bottles of polymer materials. A grinding, an air-flow separation in order to remove light materials such as paper and PP in film form, two flotation separations in medium of specific density in order to separate the PE from the polyethylene-terephthalate (PET), and a further aerodynamic separation are successively achieved. This method is applicable only to bottles of polymer materials whose polymer composition is precisely known at the start.  
           [0007]    A method for separating polymer materials originating from containers is known from WO-A-92/22,380. This method comprises a grinding step, an airflow separation step, a cleaning step of materials, and several density separation steps. This method is applicable only to one type of starting substrate whose polymer composition is precisely known.  
           [0008]    Document Kunststoffberater, 38, June 1993, No. 6, pages 26 to 30, describes a plant comprising a grinder, a washer and density separators. The washer has a rotating-drum device. However, the starting substrate contains from 95 to 100% PVC and from 0 to 5% of impurities. This plant is only designed for the separation of PVCs.  
           [0009]    Document Kunststoffe 80, April ]990, No. 4, pages 493 to 495, describes a plant for separating polymer materials generally like to the previous one, comprising a grinder, a cleaning device in the form of a rotating drum with nozzles, and density separators.  
           [0010]    A process for separating polymer materials is known from FR-A-2,599,279. After grinding and washing, there is a centrifugation in a hydrocyclone, then sorting with a shaking screen.  
           [0011]    AT-363,05 1 presents a process for recovering synthetic materials, which consists of a grinding, a first flotation, a washing and a further grinding, followed directly by a second flotation operation. These methods give insufficient results in terms of quality of the polymer materials obtained after separation.  
           [0012]    However, none of these cited methods give satisfactory results. They are slow or require a very large initial investment. They are also inapplicable to sorting at high rate and therefore cannot be adapted to industrial scale.  
           [0013]    Thus, it appears that no separation process of the prior art can be used for sorting mixtures of a wide variety of polymer materials originating from the grinding of motor vehicles or from other sources of wastes.  
         STATEMENT OF THE INVENTION  
         [0014]    The problem posed is to provide a method and a corresponding plant for separating complex mixtures of polymer materials of all types, which is effective at industrial scale and which gives a degree of purity of the polymer materials close to the degree of purity of the polymer materials of initial melt.  
           [0015]    The aim of the invention is to overcome the lack of efficacy of existing techniques, by using and appropriately placing a new additional mechanical separation phase in a sequence of cleaning and density separation steps.  
           [0016]    According to the invention, the method for separating polymer materials originating from waste comprises a grinding step and is characterised by the following four steps, which can be carried out in any order: a mechanical separation step, a cleaning step, and at least two density separation steps.  
           [0017]    These four main steps can be organised according to 24 different ways. It is possible to start with a mechanical separation step, followed, in any order, by the other steps envisaged Alternatively, the method can also start with a density separation step, followed, in any order, by the other steps envisaged.  
           [0018]    Preferably, a mechanical separation step is followed by a first density separation step, then a step of cleaning polymer materials and a second density separation step. A mechanical separation step followed by a first density separation step, then a second density separation step and a step of cleaning polymer materials is also proposed. Or alternatively, a mechanical separation step is followed by a step of cleaning polymer materials, then a first density separation step and a second density separation step. Or alternatively, a first density separation step is followed by a mechanical separation step, then a step of cleaning polymer materials and a second density separation step. Or alternatively, a first density separation step is followed by a mechanical separation step, then a second density separation step and a step of cleaning polymer materials. But also, a first density separation step is followed by a step of cleaning polymer materials, then a mechanical separation and a second density separation.  
           [0019]    The mechanical separation step is achieved in particular with screening means comprising a calibrated grid, and allowing the removal of all materials with a too large size.  
           [0020]    All density separation steps preferably are achieved by a flotation operation in liquid medium. The mixed polymer materials are immersed in a bath. The bath density is checked and controlled, which allows separation of a material that will float, in having a density less than that of the liquid medium, from another material that will sink, in having a density greater than that of the liquid medium. The materials of interest are recovered and then subjected to further treatments. The density separations can also be achieved with a mechanical device comprising a densimetric table.  
           [0021]    The step of cleaning polymer materials allows them to regain a surface like to the surface of materials of initial melt, i.e. a surface free of damaged layers, paint, grease, etc. The cleaning is achieved in liquid medium, preferably with vigorous stirring, at elevated temperature, in liquid medium of precise compositions.  
           [0022]    In the method, the separation steps allow the separation of, on the first hand, polymer materials having a density of less than 1, in particular polyolefins, and more particularly PP and PE, and, on the other hand, polymer materials having a density of greater than 1, in particular styrenes, and more particularly PS and ABSs.  
           [0023]    In order to further refine the separation, to remove the maximum amount of impurities of all sizes and of all materials, and to obtain an optimised method, other additional purification steps can be inserted among the main steps. A wetting step, a grinding step, an airflow separation step to remove the lightest particles, an electrostatic sorting step, an optical sorting step, a step of density separation with a mechanical device, a rinsing and dripping step, a centrifugation step to remove any liquid, a drying step and a step of storage in a silo are appropriately placed in the line.  
           [0024]    In order to obtain separated materials of better quality and so as to improve their respective mechanical properties, the polymer materials are centrifuged and dried if the polymers are wet, they are then homogenised, extruded, centrifuged, calibrated and homogenised again, and the purified polymer materials obtained are finally bagged.  
           [0025]    With the invention, the precise sequence of each of the separation steps makes it possible to obtain highly purified polymer materials at the end of the method.  
           [0026]    According to a second aspect of the invention, a plant is characterised in that it preferably includes the devices for grinding, for density separation with flotation, for mechanical separation, for electrostatic separation, for separation by optical sorting, for air-flow separation, for cleaning, for density separation with a mechanical device, for wetting, for rinsing and dripping, for centrifugation, for drying, for homogenisation, for storage, for extrusion, for calibration and for bagging. These devices follow each other in the plant in the preferred order given by the separation method. The preceding devices feed the following devices. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Other advantages of the method and of the plant according to the invention will become apparent on reading the detailed embodiment of the invention, with reference to the drawings given for illustrative purposes, in which:  
         [0028]    [0028]FIG. 1 is a flow chart of the method of the invention, comprising a first and a second part corresponding, respectively, to a first and to a second part of the plant;  
         [0029]    [0029]FIG. 2 is a flow chart of the method of the invention, comprising a first and a third part corresponding, respectively, to a first and to a third part of the plant;  
         [0030]    [0030]FIG. 3 shows a first part of the plant carrying out the method of the invention;  
         [0031]    [0031]FIG. 4 shows a second part of the plant carrying out the method of the invention, more particularly intended to separate polymer materials with a density of less than 1;  
         [0032]    [0032]FIG. 5 is a third part of the plant carrying out the method of the invention, more particularly intended to separate polymer materials with a density of greater than 1. 
     
    
       [0033]    For a car, a truck or a bus, fluids are removed and it is crushed and then ground. All the free pieces of metal are separated from the non-metallic parts by usual techniques. The inorganic materials, glass and soil are removed by screening. A large deposit 2 of polymer materials with a density, for example, of less than 1.25 is then recovered. These polymer materials are ground between 8 and 100 mm, preferably to 25 mm. If need, grinding to 25 mm is repeated, and a pre-selection operation is repeated in order to remove everything which has a density of greater than 1.25.  
         [0034]    It is at this stage that the steps for recycling the polymer materials begin. Pieces of wood, of soil adhering to the polymer materials, tars, paper, etc. remain. Throughout the plant 1 which will be described below, conveyor belts, screw systems and pneumatic carriers, represented solely in the form of black-headed arrows, allow continuous and high-speed transportation of the materials from one device to the next. All the steps from A to Z and their corresponding devices operate continuously, or also by batches according to the respective flow rate of each of the devices. The sequence of steps given below has given particularly advantageous results.  
         [0035]    Firstly, in a first part of the plant  1 , wood present among the polymer material is densified, so as to give it a density of greater than 1, in order subsequently to remove it by a separation according to the criterion of density. For this,.a wetting operation A is achieved by a device  3  which sprays water onto the piles of waste  2 . It can also be proceed by immersing the wastes or alternatively a mechanical defibration of the wood, by trituration, for example using a turbomixer. An alkaline solution based on KOH or NaOH can be used for the soaking, the aim being to start a hydrolysis of the cellulose fibres of the wood. The target is to destroy this material in order to give it a density of greater than 1.  
         [0036]    The parts  4 , such as foams, alveolar PUs, alveolar rubbers, textiles, threads, expanded PSs, film wastes of polymer materials, and wood, are then removed by a mechanical selection B. This operation is a form-factor separation. A screening device allows this sorting to be performed. It comprises, for example, a rotating drum  6  having a calibrated grid with a mesh size of 10 mm in width by 25 cm in length, for example. Everything which does not have a plate shape, and thus everything which has not been correctly shredded during a preceding grinding, is retained for removal. The treatment is continued with: all materials that pass through this sorter  6 .  
         [0037]    A first density separation operation C with flotation is achieved in a hydraulic separator  7 . The density chosen is more or less equal to 1, the flotation medium being water.  
         [0038]    The floating materials  8  and the polymers not filled with minerals, with d≦1, are recovered and treated in a second part of the plant  1  which will be described below, in particular:  
         [0039]    PE (d=0.92 to 0.95),  
         [0040]    PP (d=0.9),  
         [0041]    ethylene-vinyl acetate EVA,  
         [0042]    ethylene-propylene, propylene-ethylene-propylene rubber, ethylene-propylene-diene monomer EPDM copolymers,  
         [0043]    PE foams, PP foams,  
         [0044]    PU foams (d=0.02 to 0.035).  
         [0045]    All the non-expanded polymers and materials  9  with a density of greater than 1 are directed into and treated in a third part of the plant 1 which will be described last. These materials are, in particular.  
         [0046]    unfilled PS (d=1.05);  
         [0047]    unfilled ABS (d=1.07);  
         [0048]    PVC  
         [0049]    polycarbonates PC  
         [0050]    rubbers, except thermosetting alveolar rubbers;  
         [0051]    filled polyolefins with d&gt;1, PP filled with 20% talc (d=1.05);  
         [0052]    filled PUs (d=1.21);  
         [0053]    filled PE;  
         [0054]    unsaturated polyesters (d=1.10 to 1.3);  
         [0055]    saturated polyesters (d≦1.2), whether or not filled with glass fibres;  
         [0056]    polyamides, PA 6 (d=1.13), PA 6,6 (d=1.14); PA 6,10 (d=1.08), PA 11 (d=1.04), PA 12 (d=1.02) whether or not filled;  
         [0057]    poly(methyl methacrylate) PMMA (d=1.18).  
         [0058]    The supernatant is treated in the second part of the plant  1 , also comprising unwanted materials such as wood, foams, fabrics, fibres and textiles.  
         [0059]    The floating materials are thus directed into a grinder  11  for a reduction step D. The grids have a size from 8 to 100 mm with an optimum of 12 mm. The first object of the previous operations B and C is to avoid grinding just any material, for example nails, stones, etc., and the second object is to achieve energy savings by reducing the amount of material to be ground.  
         [0060]    A wetting step totally identical to the wetting step A and to its variants described above can also be performed after this grinding. The wetting step A must precede a hydraulic separation as closely as possible.  
         [0061]    An intense cleaning step E takes place at this stage. This operation is fundamental since it allows the polymer materials in substrate form to regain their native surface. In fact, the latter are very often soiled with tars, soil, lead residues, oils, coolant fluid, acid, gazole and petrol. Furthermore, the surfaces of the polymer materials are either covered with paint (in the case of PP bumpers), or etched with chemical agents, or oxidised, or degraded by UV rays or alternatively aged. All of these factors modify the surface tensions and do not allow full efficacy of the subsequent flotation separations. The scouring treatment is necessary if it is desired that the polymer materials obtained at the end of separation have the same physicochemical properties as native polymer materials. Accordingly, the pieces are introduced into one or more washing and triturating machines  12 . The latter comprise, for example, one or more fixed tanks, rotating paddles and powerful motor means which can develop 0.05 to 0.5 kWh per kg of material to be treated, for example of the trade name Wemco®. A liquid medium composed of water, cleaning agent and optionally abrasive agent is added in the tanks. The washing operation E is performed in a medium which is as concentrated as possible, at room temperature or, better still, hot, it being possible for the water to be brought to boiling by means of the heat dissipated by the motors.  
         [0062]    The composition of the cleaning medium can be:  
         [0063]    mixture of polymer materials from 50 to 70% by weight;  
         [0064]    liquid phase from 30 to 50% by weight comprising:  
         [0065]    from 50 to 100% by weight of water,  
         [0066]    from 0 to 90% by weight of cleaning agent, for example sodium hydroxide, potassium hydroxide, sodium carbonate, non-foaming biodegradable soaps of trade name Akypo MB 2621 S® supplied by the Chemy Company,  
         [0067]    wetting agent (based on fatty alcohols modified with ethylene oxide and/or propylene oxide), and  
         [0068]    from 0 to 30% by weight of abrasive agent, for example minerals such as calcium carbonate powder, talc, silica or alumina.  
         [0069]    At the outlet, the polymer materials are rinsed with water and drained during step F, for example in the extraction screw  13 . The water recovered can be re-injected into the washing machine  12 .  
         [0070]    A second step of density separation with flotation G in a separator  14  then takes place. This is performed in a liquid medium with a density more or less equal to 1, the liquid being water. The material which sinks  16 , with a density of greater than 1, is removed. This sorting makes it possible to improve the selection already started during the first flotation separation C. The surfaces of the polymer materials are cleaned during step E, which makes it possible to dive materials  16 , artefacts of which gave them false densities of less than 1. A finer density sorting operation can be achieved at this stage. The various types of polymer materials are separated by this process, if the density of the liquid medium is precisely controlled. There is also elimination by sinking of materials  16 , residual sands stuck beforehand to the PE and to the PP, and especially most of the wood. This material has been extensively eroded, disintegrated and soaked with liquid during the previous trituration step, thus giving it a density of greater than 1.  
         [0071]    The sorting device  6  of the step B can also be placed at other places in the upgrading line, for example after the flotation step C (arrows  17  showing the sequence A, C, B, D).  
         [0072]    Next, a rinsing and a centrifugation H in a centrifuge  18 , to remove the remaining liquid, are performed for the supernatant polymer materials.  
         [0073]    A continuous drying I is then performed. This operation can be achieved using various apparatuses. For example, a fluidised bed dryer  19 , a flash dryer, a drying in a Wemco® type triturator, in which the temperature increase is obtained by means of the mechanical stirring energy, or a Voom® type counter-current dryer can be used.  
         [0074]    The resulting mass is stored J in a buffer silo  21 , which is useful if the line breaks down or in the event of stop for servicing, and maintenance.  
         [0075]    An air flow separation step K, which may be optional, is placed at this stage. This step must be placed after a drying step I. The separator  22  allows the removal of all light materials  23 , such as dusts remaining adhered, small fibres and foams, as well as the remaining fine particles of wood.  
         [0076]    A mechanical density sorting step L makes it possible to enhance the separation. It is preferably performed in a mechanical device  24  of densimetric shaking table coming, for example, from the Herbold Company (Germany) or from the Eldan Company (Denmark), on which the circulating charge  26  is rejected. All the remaining circulating light materials  26 , wood, foams, rubber and impurities are removed.  
         [0077]    A final density separation step M is placed in the final sorting phase, to separate two particularly advantageous and high-value materials among the densities less than 1: these are the two polyolefins, PE and PP. Preferably, a flotation M, like to the flotation G, is achieved. The density of PP is 0.9 and the density of PE is from 0.92 to 0.95. The separation takes place in a separator  27  in liquid medium of a very precisely controlled density and, in fact, between 0.9 and 0.95. The liquid medium preferably comprises water and an alcohol chosen from methanol, ethanol, a wine alcohol, a denatured alcohol or methylated alcohol. The degree of the water-alcohol mixture used is in the range 55 to 60°. The monitoring is preformed using an alcohol gauge. The adjustment of the degree of alcohol makes it possible to optimise the medium density in order to obtain a separation as selective as possible. The intensive washing G which was previously achieved allows optimal contact between the native surface of the polymer materials to be treated and the medium of chosen density. A wetting agent is also added to the medium, in a proportion from 0.1 to 0.2% by weight. The presence of this wetting agent substantially improves the separation. Antarox-FM3® from the Rhone-Poulenc Company was preferably used. The advantage of the preceding steps of centrifugation H and of drying I, in order for the finely adjusted density of the liquid medium in the separator  27  not to be modified by an interference addition of water, is easier to appreciate. The PP-PE mixture introduced is almost fully separated. The supernatant material  28 , recovered at the top, is composed of PP and the sinking material  29 , recovered in the bottom part, is composed of PE.  
         [0078]    The process can be stopped at step J of storage in a silo  21 , if it is desired to sell only a polyolefins mixture. It should be noted that step M can be placed directly after the centrifugation step H when the wastes are free of light products; if steps I to L are suppressed, H is followed by M, according to the arrow  30 .  
         [0079]    Step M can be suppressed. The mechanical density sorting L with a vibrating table  24  can serve to separate the two above-mentioned polymer materials according to their intrinsic density. The method can thus be completed at this stage by the obtaining of pure PE and PP,  31  and  32  respectively.  
         [0080]    The PP  28 ,  32  and the PE  29 ,  31  sorted by means of this plant can either be sold for direct use, or can immediately undergo further treatment steps N to U. In particular, an extrusion is achieved which makes it possible to obtain, for the polymer materials thus treated, mechanical properties which are almost identical to those of the native polymer materials. At this stage, FIG. 4 represents only the plant required for the treatment of a single material, PP  28 , it being understood that the other material, PE  29 , is treated in the same way, by following the same steps from N to U carried out with the same devices (represented with dashed lines in FIG. 4).  
         [0081]    The PP phases  28 ,  32  and PE phases  29 ,  31  are thus separately centrifuged N in a centrifuge  33 , in order to bring the amount of liquid to about 1 to 2%. They are dried O in a dryer  34 , for example one which is identical to the dryer used in step I.  
         [0082]    They are homogenised P by circulation in a silo  36  which also serves for the storage.  
         [0083]    They are extruded Q in an extruder  37  to produce pellets. There is also an in-process filtration of 20 to 300 [μm in order to remove the last impurities. This is a test, to determine whether or not the product to be extruded is pure. Various agents can also be introduced: dyes, anti-UV agents, etc. An air-flow separation step can optionally be placed between storage P and extrusion Q, in order to remove fibres and dusts (not represented).  
         [0084]    The PE and PP phases are centrifuged R in a centrifuge  38  in order to remove the cooling water for the pellets.  
         [0085]    The phases exiting in the form of pellets are graded S in a grader  39  in order to remove the fines and aggregates.  
         [0086]    They are re-homogenised T in a homogenisation and storage silo  41 . In the final step U, they are placed in bags of about 50 l in a bagging machine  42 .  
         [0087]    The PP  43  and PE  44  exit as finished products of excellent quality, and can be used in the usual applications of these polyolefins.  
         [0088]    In a third part of the plant  1 , the polymers with a density of greater than 1, which it is desired to recover from motor vehicle crushing residues, are in particular styrenes, and more particularly PS and ABS. Other wastes are also found among these: stones, nails, etc.  
         [0089]    From the separator  7 , the sinking materials  9  are passed, by a density separation operation V, with flotation into a hydraulic separator  46 . The chosen density is about equal to 1.05. The PS  47  floats, but so do polypropylenes filled with talc to 10-20%, wood and fibres. The ABS  48  flows, but so do poly(methyl methacrylate), other polymers, wood, dense foams and all materials up to a density of 1.25. The liquid medium of hydraulic separator contains water, wetting agents and inorganic compounds such as clays, bentonite, or soluble compounds such as salts, etc., compounds used to increase the density of water. Quarry clay is preferably used. This clay is suspended in water and the heaviest particles, which accumulate at the bottom of the container, are removed in order to obtain a homogeneous medium. The density is then controlled by adding water. The role of the wetting agent is also to keep the clay in suspension. The agent used is Coatex SP 30 S®.  
         [0090]    The ABS purifying line  48  will be now described. The ABS and the sinking  48  are conveyed to a third flotation operation W in a hydraulic separator  49 . This takes place in a liquid medium with a density of about 1.1. The liquid has a like composition to the composition described for the n medium in the separator  46  of step V, except that the amount of clays per unit volume is greater. The material which sinks  51  comprises stones, soil, and 30-40% filled polypropylenes. At this density, the ABS floats, but so does the wood.  
         [0091]    Next, floating ABS is rinsed X by spraying it in the transport screw  52 , and is drained and centrifuged in order to completely remove the adhering clay therefrom.  
         [0092]    The ABS is then directed into a grinder  53  for a reduction step D, which is identical to the grinding D already described. The grids are from 8 to 100 mm in size, with an optimum of 12 mm. The first object of the preceding operation W is to avoid grinding just any material, for example nails, stones, etc., and its second object is to achieve energy savings by reducing the amount of material to be ground.  
         [0093]    A wetting step entirely identical to the wetting step A and to its variants described above can also be achieved after this grinding D. The wetting step A must precede a hydraulic separation as closely as possible.  
         [0094]    Next, a step of intense cleaning E for the materials which float  48  (ABS) takes place at this stage. This operation, which allows the ABSs to regain their native surface, is identical to the operation E already described above, with the use of one or more washing apparatuses  54  and a cleaning medium of identical composition.  
         [0095]    At the outlet, polymer materials are rinsed with water and are drained during step F, for example in the extraction screw  56 . The water recovered can be re-injected into the washing apparatus  54 .  
         [0096]    Then a fourth density separation step with flotation G a  in a separator  57  takes place. This is achieved into a liquid medium with a density more or less equal to 1.07. The liquid has an identical composition to the composition described for the medium in the separator  46  of step V, except that the amount of clays per unit volume is greater. The material which floats  58  (wood and other wastes) is removed. At this density, ABS  48  sinks. This sorting makes it possible to improve the selection already made during the second separation by flotation W in the third part of the plant  1 . The surfaces of the polymer materials are cleaned during step E, which makes it possible to float the materials  58 , artefacts of which gave them false densities of greater than 1.07.  
         [0097]    Then a rinsing operation Y by spraying the polymers in the transport screw  59  in order to remove any clay, dripping, and a centrifugation H of the supernatant polymer material ABS is achieved in a centrifuge  61  in order to remove the remaining liquid.  
         [0098]    A continuous drying I is then performed with the same  62  or with apparatuses different from the dryer described for polyolefins.  
         [0099]    The resulting mass is stored J in a buffer silo  63 .  
         [0100]    An air-flow separation step K, which may be optional, is placed at this stage. This step must be placed after a drying step I. The separator  64  allows the removal of all light materials  66 , such as the dusts remaining adhered, small fibres, and foams, as well as the remaining fine particles of wood.  
         [0101]    A mechanical density sorting step L allows the separation to be improved. It is preferably performed in a mechanical device  67  of densimetric shaking table, described above, on which the circulating charge  68  is rejected. All the remaining circulating light materials  68 , wood, foams, rubber and impurities are removed. PP filled to 30% with talc is also removed by this device.  
         [0102]    This last interference product  69  can also be removed by adding an optional step Z of electrostatic sorting with electrostatic sorting table  71 , for example from the Hamos Recycling Technique Company (Germany).  
         [0103]    The ABS 72 obtained after L and/or Z can be sold directly.  
         [0104]    [0104]FIG. 5 represents only the plant required to treat ABS material  48 . However, during the above step V, PS  47  was recovered in the floating material, and the purifying line for it will now be described.  
         [0105]    The PS  47  is treated in more or less the same way, according to the same steps X, D, E, F, G, Y and H to L carried out by the same devices (represented with dashed lines in FIG. 5). Only the density separation step W and its corresponding device  49  is suppressed. A second line of devices  52 ,  53 ,  54 ,  56 ,  57 ,  59 ,  61 ,  62 ,  63 ,  64  and  67  can be mounted in parallel with the first, or else these steps can be carried out in batches and alternatively stored ABS  48  or PS  47  polymer materials. This last solution is advantageous if there is a large difference in the entry rate of polymer materials between ABS  48  and PS  47 .  
         [0106]    Thus, after grinding D, after cleaning E and dripping F, a step G b  of density separation with flotation in a liquid medium is achieved in order to refine the purification of PS  47 . The density is more or less equal to 1.05, which is identical to the density of the flotation step V. Materials such as wood, mixed products with metals, etc., and any products having a density modified by the cleaning E sink in  58 , to be removed. PS is rinsed in Y in the transport screw  59 , to remove all the clay, and is then dripped.  
         [0107]    Steps H to L remain identical to those described. PS exiting still contains polyolefins charged to a maximum of 20% with 1&lt;d≦1.05. Usually, filled PE and PP are black. In contrast, PS is white, especially when it is obtained from wastes of household appliances. An optical sorting Z a  is thus achieved in a bench  73 , coming, for example, from the Sortex Company, in order to remove these filled polymer materials  74 . This separation Z a  can optionally be achieved by electrostatic sorting.  
         [0108]    The PS  76  obtained after L and/or Z a  can be sold directly.  
         [0109]    In certain specific cases, certain steps, among the preceding steps of the second or of the third part of the plant  1 , are not essential. Mention may be made, for example, of the step D for finer grinding. The process can be stopped at step J of storage in silo  63 . It should be noted that steps Z and Z a  can be placed directly after the drying step I when the wastes are free of light products; if the steps J to L are suppressed, I is followed by Z or Z a , according to the arrow  40 .  
         [0110]    The hydraulic separators  7 ,  14 ,  27 ,  46 ,  49 ,  57  used are so-called static separators. They can be replaced with so-called dynamic hydraulic separators or those comprising a pump and a cyclone in order to produce a circulating sorting flow. All the hydraulic floating separators  7 ,  14 ,  27 ,  46 ,  49  and  57  can be replaced by shaking separation tables  24 ,  67 , but the materials introduced therein must be dry. All the hydraulic flotation separators  7 ,  14 ,  27 ,  46 ,  49 ,  57  can also be replaced with electrostatic separators, for example from the Hamos Recycling Technique Company (Germany). The purification of the PE-PP and PS-ABS fractions can also be achieved or refined by means of a cryogenic grinding operation giving particles of different sizes depending on the polymer material. All the possibilities of use for the devices mentioned above in order to achieve a separation according to a mechanical or chemical property of polymer materials are possible.  
         [0111]    The ABS  72  and the PS  76  can undergo further treatment steps P to U, in order to further improve their purity degree and their mechanical properties. In a similar manner to that for the polyolefins separated in the second part of the plant, an extrusion is achieved in particular, which makes it possible to give the polymer materials thus treated physical properties almost identical to those of the native polymer materials.  
         [0112]    At this stage, FIG. 3 represents only the plant required to treat an ABS material  72 , it being understood that the other material PS  76  is treated in the same way, by following the same steps from P to U carried out with the same devices (represented with dashed lines in FIG. 5).  
         [0113]    ABS  72  and PS  76  phases are thus separately homogenised P by circulation in a silo  77 . They are extruded Q in an extruder  78  with filtration from 20 to 300 μm. An air-flow separation step can optionally be placed between the storage P and the extrusion Q, in order to remove fibres and dusts (not represented). The advantage of all the above rinsing steps inserted to avoid the introduction of clay into the extruder  78  can be appreciated. ABS and PS phases are then separately centrifuged R in a centrifuge  79 . The phases exiting in the form of pellets are graded S in a grader  81  for removal of fines and aggregates. They are re-homogenised T in a silo  82 . In the final step U, they are placed in bags of about 50 l in a bagging machine  83 .  
         [0114]    ABS  84  and PS  86  exit as finished products of excellent quality, and can be used in the usual applications of these styrenes.  
         [0115]    All of the devices intended for carrying out the common steps D to L, P to U, in the second and in the third part of the plant I can be provided just once, the method then being carried out in batches, or they can be provided several times, for example two of each device respectively in the second and the third part of the plant. Thus, the separation optimisation parameters of these apparatuses will change (density, extrusion temperatures, etc.).  
         [0116]    Results  
         [0117]    Table 1 gives an evaluation of the percentages of various products, of PP  28  and of PE  29  recovered with the method and the plant  1 , from 100% of motor vehicle grinding residues (MVGR).  
                           TABLE 1                                   Products obtained   % recovery                           Foams and light fractions   19           Foams and heavy fractions    2           Polymers with d &gt; 1   64           PP-PE mixture with d &lt; 1   15               (100%)           PP   67           PE   33                      
 
         [0118]    Table 2 gives mechanical properties of native PE and of PE  29 ,  44  recovered with the method and the corresponding plant  1 , from motor vehicle grinding residues.  
                                                         TABLE 2                                   Characteristics   Units   Native   MVGR                                        Melt index MFI   g/10 min.   1.9   1.7           2.16 kg/230° C.           Density       0.925   0.928           Strength at the flow   MPa   22   20.3           threshold 25 mm/s           Tensile strength   MPa   18   15           Modulus of   MPa   700   650           Elasticity           Notched IZOD   J/m   160   148           impact +23° C.           Notched IZOD   J/m   60   56           impact −18° C.                      
 
         [0119]    Table 3 fives mechanical properties of native PP and of PP  28 ,  43  recovered with the method and the corresponding plant  1 , from motor vehicle grinding residues.  
                                                         TABLE 3                                   Characteristics   Units   Native   MVGR                                        Melt index MFI   g/10 min.   4.1   4           2.16 kg/230° C.           Density       0.90   0.901           Strength at the flow   MPa   29   22.5           threshold 25 mm/s           Tensile strength   MPa   30   20           Elongation at break   %   500   304           Modulus of   MPa   1200   1000           Elasticity           Notched IZOD   J/m   100   170           impact +23° C.           Notched IZOD   J/m   45   55           impact −18° C.                      
 
         [0120]    In the case of motor vehicle grinding residues, the values obtained are the result of a study achieved on test pieces injected directly after sorting PE  28  and PP  29  (step M).  
         [0121]    It is seen from these two tables that the quality, and thus the commercial value of the PE and of the PP is more or less equivalent to the quality of first-melt PE and PP.  
         [0122]    This is likewise the case for the styrene materials ABS and PS.  
         [0123]    Taking into account the solvents which may be used, the plant satisfies the flameproof standards and the various items of apparatus are as leakproof as possible. Throughout the industrial plant described above the washing and rinsing waters, the waters recovered after dripping and centrifugation, the condensation waters from the dryers and the waters in the flotation media either return to the device using them or are collected by a pipe  87  in order to be stored and retreated in a purification plant  88 .  
         [0124]    The invention is not limited by the details of the embodiments and the examples chosen to illustrate it. Modifications may be made without, however, departing from the scope of the invention. For example, the polymer materials can also come from household waste or industrial waste. The densities of the liquids used in the flotation separation devices can be modified according to the nature of the polymer materials which it is desired to recover and separate.