METHOD AND APPARATUS FOR WASHING SHEETS OF PLASTIC MATERIAL

Abstract

A method and apparatus for washing sheets of plastic material originating from industrial and agricultural waste, for subsequent reuse thereof as a raw material. The sheets of plastic material, precut into soiled pieces of large dimensions, and a water flow are supplied to a washing tank containing a water bath; a screw stirrer generates intense stirring of the water bath and recirculation, inside the tank, of the soiled plastic pieces that causes the contaminating material to be detached and separated by sedimentation from the plastic pieces. The cleaned pieces of plastic material of large dimensions, devoid of contaminant, which float in the water bath are removed by dragging, in a floating condition, along a discharging channel by the combined action of a dragging device and of a water flow that exits the washing tank; the dragging device, consisting of an endless belt, extends from one side of the washing tank along the discharging channel, supported in a freely tiltable manner.

Claims

1-14. (canceled)

15. A method for washing film and/or sheets of plastic material and for the separation of contaminant substances, comprising the steps of introducing pre-cut soiled pieces of large dimension of the film or sheets of plastic material into a water bath of a washing tank and stirring the water bath to separate clean pieces of plastic material from contaminant substances; drawing the cleaned pieces of plastic material floating on the water bath along a discharging channel by an endless belt; and advancing and discharging the cleaned plastic pieces by drawing the cleaned plastic pieces, in a floating condition, along the discharging channel by a combined action of the endless belt and of the water flow exiting the washing tank along the discharging channel.

16. The method for washing film and/or sheets of plastic material according to claim 15, wherein the soiled plastic pieces are continuously fed into the water bath by a water flow having a flow rate substantially corresponding to the flow rate along the discharging channel, exiting the washing tank.

17. The method for washing film and/or sheets of plastic material according to claim 15, wherein bunching of cleaned pieces in the discharging channel is removed by a thrust and an upwardly tilting movement of the drawing belt.

18. The method for washing film and/or sheets of plastic material according to claim 15, and further comprising the steps of whirling the water bath and recirculating the soiled plastic pieces in the washing tank by a screw stirrer.

19. The method for washing film and/or sheets of plastic material according to claim 15, and further comprising the step of adjusting the time that the soiled plastic pieces remain in the water bath of the washing tank by changing the quantity of the cleaned plastic pieces drawn outside from the washing tank along the discharging channel.

20. The method for washing film and/or sheets of plastic material according to claim 19, wherein the time that the soiled plastic pieces remain in the washing tank is adjusted by changing the speed of the endless drawing belt.

21. The method for washing film and/or sheets of plastic material according to claim 19, wherein the time that the soiled plastic pieces remain in the washing tank is adjusted in relation to a contamination degree, respectively, to a required cleaning degree.

22. An apparatus suitable for washing pre-cut pieces of soiled film and/or sheets of plastic material and for separation of contaminant substances by the method according to claim 15, comprising: a washing tank containing a water bath having a prefixed level; a stirrer arranged in the water bath in spaced relation to a bottom of the washing tank; a discharging channel for cleaned pieces of plastic material and having a bottom wall upwardly extending upwardly from the washing tank; and an endless conveyor configured for drawing the cleaned plastic pieces from the washing tank, said endless conveyor extending along the discharging channel and having a lower branch provided with pads for drawing the cleaned plastic pieces, said pads extending beneath the level of the water bath along the discharging, channel; wherein the stirrer includes a screw stirrer which vertically extends in the water bath and wherein said endless conveyor is freely tiltably supported.

23. The apparatus according to claim 22, wherein said endless conveyor comprises a conveyor belt which is supported by a fore drum and a rear drum each having a rotation axis, in which the endless conveyor is tiltably supported around the rotational axis of the rear drum.

24. The apparatus according to claim 23, wherein the fore rotating drum of the conveyor belt comprises a rotational axis provided with sliding blocks movable in respective side guides in the discharging channel.

25. The apparatus according to claim 22, wherein said discharging channel comprises a first sloping bottom wall that extends from an opening of the washing tank, and a second sloping bottom wall having a slant angle less than a slant angle of said first bottom wall of said discharging channel.

26. The apparatus according to claim 22, wherein the washing tank comprises a hopper for charging the soiled plastic pieces, and a device for feeding and distributing a water flow.

27. The apparatus according to claim 22, wherein said washing tank comprises a conical bottom for sedimentation of contaminant substances, and a plurality of discharging valves selectively actuatable between an opened and a closed condition.

28. The apparatus according to claim 22, and further comprising a device to change the speed of the endless conveyor for drawing the cleaned plastic pieces along the discharging channel.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0019] A preferred embodiment of a washing apparatus for plastic film and sheets according to the invention, and the corresponding method, will be disclosed in greater detail below with reference to the example of the attached drawings, in which:

[0020] FIG. 1 is a perspective view of the washing apparatus;

[0021] FIG. 2 is a longitudinal section along the line 2-2 of FIG. 1, with the tilting dragging belt in the lowered work position;

[0022] FIG. 3 is a longitudinal section that is similar to that of FIG. 2, showing the tilting dragging belt in a raised position;

[0023] FIG. 4 is a top view of the apparatus, which is partially in a section along the broken line 4-4 of FIG. 2;

[0024] FIG. 5 is a flow chart illustrating the washing method of the apparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0025] As shown in FIGS. 1 and 2, the washing apparatus for film and sheets of plastics material, suitably precut into pieces 18 of large dimensions, having for example a dimension of the order of several decimetres and/or of a metre, for example dimensions between 50 and 150 cm, or more, comprises a washing tank 10 of cylindrical, square or polygonal shape, finishing with a conical bottom 11, or with walls tilting downwards; the bottom 11 of the tank is further provided with a large sediment-discharging pipe 12 for discharging the sediment, which sediment-discharging pipe 12 is normally closed by two valves 13, 14, which are selectively drivable by respective actuators or pneumatic cylinders 15, 16 for discharging polluting material that has separated from the soiled plastic pieces 18 during the washing step, and which has settled on the bottom 11.

[0026] The washing tank 10 comprises a screw stirrer 20 positioned eccentrically on a side of the tank 10, leaving in this manner a great stirring and recirculating zone for stirring and recirculating the soiled pieces 18 of plastic material in the washing tank 10, as shown in FIG. 4. The screw stirrer 20 extends vertically downwards, being immersed in a water bath 21 having an upper level 22 of preset height. The screw stirrer 20 can be of the type with one or more continuous screws, with a constant and variable pitch, and it is suitably configured for generating great stirring and preventing soiled plastic pieces 18 from being able to stick to the screw of the stirrer; the screw stirrer 20 is operationally connected to an electric motor 23 configured for driving the screw stirrer 20 at a high number of revolutions, which are suitable for generating intense stirring of the water bath 21 and a continuous upward recirculation, in the same water bath 21, inside the tank 10, of the soiled plastic pieces 18 that the heavy contamination tends to drag downwards, preventing the soiled plastic pieces 18 from floating and being buoyant in the water bath 21.

[0027] In the course of experimental tests conducted with a washing apparatus according to the invention, good results were obtained with plastic material 18 precut into pieces of large dimensions, as previously defined, using screw stirrers 20 having an outer diameter of the screw comprised between 500 and 600 mm, commanded to rotate at speeds comprised between 250 and 450 rpm, or higher.

[0028] The washing tank 10 is provided with a side hopper 24 for charging the soiled plastic pieces 18; the hopper 24 is provided with a water distributor 24 configured for supplying a washing water flow that is both suitable for dragging the soiled plastic pieces 18 into the washing tank 10, and for continuously replenishing the water bath in the washing tank. The soiled plastic pieces 18 are supplied continuously to the discharging hopper 24 with a washing water flow; nevertheless it is not excluded that in place of the hopper 24 it is possible to use any other means or system for loading the soiled plastic pieces 18 and the washing water flow, other than the one shown.

[0029] The washing tank 10, on the side opposite the charging hopper 24, is provided with a discharging channel 25 for discharging the cleaned plastic pieces 18 which float in the washing bath 21. The discharging channel 25 has a rear end in direct communication with a stirring zone of the water bath 21, through a wide side opening 10 of the tank 10, extending longitudinally to a front discharging end 26C for discharging the pieces 18 of plastic material, together with the overflow of a washing water flow originating from the tank 10, substantially corresponding to the washing water flow supplied to the charging hopper 24.

[0030] In particular, the discharging channel 25 has a bottom tilting upwards comprising a first greatly tilted bottom wall 26A, forming for example a corner comprised between 30 and 60 with the vertical axis of the tank 10 that is joined to a second bottom wall 26B, having a tilt that is less than that of the first bottom wall 26A, which terminates with a front edge 26C in a position below the level 22 of the water bath 21 in the washing tank 10 and in the discharging channel 25.

[0031] According to the present invention, the discharge of the cleaned plastic pieces 18 from the washing tank 10 along the channel 25 by dragging is obtained by the combined action of the washing water flow that exits in a controlled manner the discharging channel 25, the flow-rate of which substantially corresponding to the flowrate of the water flow dragging the soiled plastic pieces 18 that enter the charging hopper 24 and the tank 10, and of the action of continuous dragging means substantially consisting of an endless belt 27 that extends longitudinally from a rear end of the channel 25 communicating directly with the stirring zone of the tank 10, to a front end 26C of the discharging channel 25, as shown in FIG. 2.

[0032] The dragging endless belt 27 winds around a rear drum 28, the rotation axis 29 of which is operationally connected to an electric control motor 30, shown in FIG. 1, and a front drum 31; the rotation axis 29 of the rear drum 28 and the rotation axis 32 of the front drum 31 are positioned above the level 22 of the water bath 21, with the belt 27 slightly tilted upwards so that at least one rear portion of the lower branch 27A of the dragging belt 27 is partially immersed in the water flow 21 that exits continuously the washing tank 10, through overflow, from the front edge 26C of the channel 25.

[0033] In particular, the endless belt 27 dragging the cleaned plastic pieces 18 has a width substantially corresponding to that of the discharging channel 25, and is configured with a plurality of transverse pads 33, which are suitably spaced apart in the longitudinal direction of the belt 27.

[0034] The rotation axes 29 and 32 of the two drums 28, 31 of the dragging belt 27 are further positioned, with respect to the bottom wall 26B of the discharging channel 26, such that the dragging pads 33 of the lower branch 27A of the belt 27, protrude into the water flow 21; in this manner the cleaned plastic pieces 18 are advanced from the tank 10 and along the channel 25 by dragging by the endless belt 27 and by the water flow that continuously exits the discharging channel, maintaining the plastic pieces 18 constantly immersed in a buoyant condition.

[0035] The washing apparatus further comprises control means for controlling the motor of the endless belt 27 (which means is not shown) configured for varying the rotation speed of the motor and thus the speed of the endless belt 27, according to different work requirements.

[0036] The linear advancement speed of the belt 27 can be maintained constant or can be adjusted through increasing or decreasing during the same washing operating cycle, or during subsequent operating cycles, at a suitable speed, for example comprised between 30 and 40 metres/min; in this manner, by adjusting and/or varying the advancement speed of the endless belt 27 and consequently adjusting and/or varying the outlet flowrate of the cleaned plastic pieces 18, it is possible to control and consequently vary the washing and dwell time of the soiled plastic pieces 18 in the tank 10, increasing and/or decreasing the washing and dwell time according to the greater or lesser degree of contamination of the pieces 18 that are supplied to the hopper 27, and according to the greater or lesser degree of desired cleanness of the pieces 18 that exit the discharging channel 25.

[0037] In order to prevent the plastic pieces 18 dragged along the discharging channel 25 being able to get wedged under the pads 33, the tilt of the endless belt 27 is slightly less than the tilt of the bottom wall 26B of the discharging channel, taking care that the pads 33, along the lower branch 27A of the endless belt 27, are at least partially immersed in the bath 21 keeping at a distance that is spaced away from the bottom wall 26B of the discharging channel 25; in this manner, the plastic pieces 18 are always dragged in a buoyant condition.

[0038] Depending on the dimensions and on the features of the soiled plastic pieces 18, and on the constructional and functional features of the entire apparatus, cleaned pieces 18 of plastic material of large dimensions may stop and accumulate at the front edge 26C of the bottom wall of the discharging channel 25 causing, as indicated by 18 in FIG. 3, the locking and the arrest of the endless belt 27.

[0039] Another characteristic of the apparatus according to the invention, and of the manner of operation thereof, is that the endless belt 27 is supported by the channel 25, in a manner tilting freely upwards, as shown by 27 in FIG. 3, around the rotation axis 29 of the rear drum 28; accordingly, the rotation axis 32 the front drum 31 has at each end a pad 35 that is slidable in a guiding slot 36 near the front edge of the side walls of the channel 25. In this manner, if a bunching 18 of the plastic pieces 18 forms at the front edge 26C of the bottom of the discharging channel 25, the endless belt 27, moved by the control motor 37, lifts up freely, pushing and making the bunch 18 of plastic pieces 18 fall from the front edge of the discharging channel 25. Once the bunch 18 of plastic pieces 18 has been removed, the belt 27 lowers under its own weight again, readopting the operating position of FIG. 2, shown by a dashed line in FIG. 3.

[0040] The flow chart of FIG. 5 shows schematically the essential steps of the washing method for plastic pieces of large dimensions, during an operating cycle, by the apparatus according to the invention.

[0041] During the step S1 the soiled plastic pieces 18 are supplied continuously, together with a washing water flow, to the hopper 24 and consequently to the water bath 21 contained in the tank 10.

[0042] During the step S2, through the effect of the strong stirring of the water bath 21, caused by the stirrer 20, the contaminating material 34 which had stuck to the plastic pieces 18, separates, settling through decantation on the conical bottom 11 of the tank 10, during the step S3, from which it can be periodically discharged by opening and closing the valves 13 and 14 in sequence.

[0043] The cleaned pieces 18 of plastic material which float in the bath 21 are dragged during the step S4 along the discharging channel 25, being maintained in a floating condition by a combined action of the endless belt 27 and of the water flow that exits the discharging channel 25, as indicated for step S5.

[0044] If an obstruction 18 occurs of the discharging channel 25 by the plastic pieces 18, the endless belt 27 can tilt freely upwards, step S6, pushing forwards and making the bunch 18 of plastic pieces fall that obstructs the discharging channel, in this manner automatically restoring the normal operating conditions of the apparatus.

[0045] From what has been said and shown with reference to the example of the attached drawings, it is clear that an apparatus and a method is provided for washing sheets of plastic material precut into pieces of large dimensions, which are suitable for achieving the objects of the invention; accordingly other modifications or variations can be made to the entire apparatus or to parts thereof, in particular to the system of charging the plastic pieces and the washing water flow, to the system of stirring the water bath in the washing tank, using a different type of stirrer from the stirrer shown, or configuring otherwise the discharge channel and the continuous dragging devices for dragging the plastic pieces in a floating condition, without thereby falling outside the scope of the claims.