WO2003022466A1

WO2003022466A1 - Plant and method for reconditioning structural elements

Info

Publication number: WO2003022466A1
Application number: PCT/SE2002/001613
Authority: WO
Inventors: Tomas HÅKANSSON
Original assignee: Premier Manufacturing Support Services Limited Partnership, Usa, Filial Sverige
Priority date: 2001-09-12
Filing date: 2002-09-10
Publication date: 2003-03-20
Also published as: SE0103012D0; SE0103012L

Abstract

A plant for reconditioning structural elements which, in an industrial process, have obtained a coating of a surface-finishing material, which plant comprises a main station (5) for removal of the coating from the structural elements, said main station including a tank (51) containing a treating liquid with a solvent capable of softening the coating, and a transport device for immersing the structural elements in the treating liquid, wherein the tank has a plurality of nozzles (67) arranged inside the tank below the surface of the treating liquid. In accordance with the invention the nozzles are arranged to emit jets of said treating liquid to generate turbulence, which jets of liquid are arranged to disintegrate the coating to particle form and remove the particles from the surfaces of the structural elements. The invention also relates to a method for reconditioning structural elements using such a plant.

Description

Plant and method for reconditioning structural elements

The present invention relates to a plant for reconditioning structural elements which, in an industrial process, have obtained a hard, water-insoluble coating of a surface-finishing material such as a varnish with or without pigment, or the like, which plant comprises a main station for removal of the coating from the structural elements, said main station including

- at least one tank containing a treating liquid including a solvent capable of softening the coating, and

- a transport device for immersing the structural elements in the treating liquid, wherein the tank includes a plurality of nozzles arranged inside the tank below the surface of the treating liquid.

The invention also relates to a method for reconditioning structural elements which, in an industrial process, have been subjected to a surface-finishing material such as a varnish with or without pigment, or the like, and have thus formed a hard, water-insoluble coating on the surfaces of the construction elements, which method comprises a main step in which the structural elements are exposed to a treating liquid including a solvent capable of softening the coating, and in which treating liquid the structural elements are immersed.

The invention relates particularly to a plant and a method for removing varnish with or without pigment from a re-usable metal fixture of the type used in manufacturing automobiles. Such fixtures are used to facilitate assembling automobile parts to finished vehicles by holding or securing vehicle parts in the desired position in relation to each other during one or more production stages. However, when painting vehicle parts it is extremely difficult to avoid certain fixtures receiving an undesired coating of paint, which renders the re-use of these fixtures more difficult. The fixtures must therefore regularly undergo treatment in which the paint deposits are removed.

Several methods are known for removing a coating of paint from fixtures. One method is to dip them in a bath with lye. However, this method entails drawbacks associated with the injurious effect of the lye on both health and environment . Another method is to burn the coating off the fixtures. With this method, however, there is a risk of the fixtures being damaged by thermal stresses and become unusable. Yet another method is to blast the fixtures but this has the drawback that the blasting material may accompany the fixtures to the production line and contaminate the painting or varnishing stations when the fixtures are re-used.

Yet another known method for removing a varnish coating from fixtures is to immerse the fixtures in a bath of varnish-softening chemical, such as a chemical based on N-methyl-2-pyrrolidone so that the coating is softened. The fixtures are then removed from the bath and the coating removed by rinsing with water, brushing or high-pressure washing. The final step to remove the coating is generally performed manually, i.e. by a person with a water hose, brush or high-pressure cleaning jet removing the chemically loosened coating from the fixtures. Although, provided prescribed safety precautions are adhered to, it is permitted to handle certain varnish-softening chemicals manually, such as the above-mentioned N-methyl-2-pyrrolidone, the manual supplementary work entailed in the step to remove the coating means that said person is exposed to the chemical, which should preferably be avoided. Manual supplementary work is also a time-consuming operation.

The object of the present invention is to provide a plant and a method for automatic reconditioning of structural elements, such as fixtures.

The plant in accordance with the invention is characterized in that the nozzles are arranged to emit jets of said treating liquid to generate turbulence, which jets of liquid are arranged to disintegrate the coating to particle form and remove the particles from the surfaces of the structural elements.

The method in accordance with the invention is characterized in that in said main step the structural elements are also subjected to jets of said treating liquid to generate turbulence, which jets are caused to flow out into the treating liquid surrounding the structural elements in order to disintegrate the coating to particle form and remove the particles from the surfaces of the structural elements.

In accordance with the invention the liquid jets "strip" the loosened coating from the structural elements, i.e. tear or pull the coating off mechanically. Preferably at least two of the nozzles are so placed and directed that their jets of liquid cross each other so that vortex cavitation occurs and effectively removes the coating from the structural elements.

The main station preferably includes a filtering device arranged to separate loosened coating particles from the treating liquid.

The main station preferably also includes a sedimentation means arranged to separate the majority of particles remaining after filtration, from the treating liquid by means of sedimentation.

The invention will be described in more detail in the following with reference to the drawings.

Figure 1 shows schematically a plant for removing a varnish coating from structural elements in the form of fixtures of the type used in the automobile industry.

Figure 2 shows a flow chart for said plant illustrating the main steps in the process for removing the coating.

Figure 3 shows schematically a pre-wash station in said plant.

Figures 4 and 5 show schematically a main station in said plant.

Figure 6 shows schematically an after-wash station in said plant .

Figure 1 shows schematically a plant for removing a coating of varnish from structural elements in the form of fixtures of the type used in the automobile industry. The plant includes a plurality of stands 1 designed to support the fixtures (not shown) , a line-up surface 2 on which stands 1 with fixtures contaminated with varnish are placed, and a set-down surface 3 on which stands 1 with clean fixtures from which the varnish has been removed are placed. The plant also includes a pre-wash station 4, a main station 5 and an after-wash station 6. The plant also includes a transport system arranged to convey each stand 1 from the line-up surface 2 to the pre-wash station 4, on to the main station 5, then the after-wash station 6 and finally to the set-down surface 3. In the present case the transport system consists of a pair of rails 7 arranged horizontally and in parallel above the stations 4-6, and a carriage 8 movable along the rails 7. The carriage 8 comprises a gripping and hoisting member 9 arranged to grip each stand 1 at its upper end and lift it to enable horizontal transport of the stand 1 between the stations 4-6 and the surfaces 2, 3.

Figure 2 shows a flow chart for the plant . The main steps of the process comprise in order a loading step 11, a first washing step 12, a main step 13 for removing the coating, a second wash step 14 and an after-treatment step 15. The process also comprises a collecting and pre-treatment step 16, a reprocessing and destruction step 17 and a ventilation step 18. These steps will be described further in the following.

In the loading step 11 the contaminated fixtures are loaded onto said stands 1 which are then placed on the line-up surface 2.

After loading, each stand 1 is conveyed by the carriage 8 to the pre-wash station 4 where the stand 1 undergoes the first washing step 12. The purpose of the first washing step 12 is to remove any corrosion-preventing oil from the fixtures. In the following the washing step 12 will be described more fully with reference to Figure 3 showing schematically the pre-wash station 4. The pre-wash station 4 comprises a cylindrical washing tank 21 and a cylindrical collecting vessel 22. The washing tank 21 is provided with a spillway that overflows into the collecting vessel 22 via a pipeline 23. The washing tank 21 also has a bottom outlet discharging into the collecting vessel 22 via a pipeline 24. A pipeline 25 consisting of pipe sections 45 and 46 opening into nozzles 26 inside the washing tank 21, leads from the collecting vessel 22 to the washing tank 21, as well as a pipeline 27 consisting of pipe sections 45 and 47 that emerges at the side wall of the washing tank 21. The washing station 4 also comprises an oil separator 28 connected via pipe sections 29 and 30 to the collecting vessel 22, and an evaporator 31 connected via pipe sections 32 and 33 to the collecting vessel 22.

Prior to a washing operation the washing tank 21 is filled with hot water from the collecting vessel 22 via the pipeline 27. The water has a predetermined temperature lying preferably within the interval 70-90 degrees Celsius. To ensure that the water is kept at the predetermined temperature the washing tank 21 and collecting vessel 22 are preferably surrounded individually by a casing 34 containing pipes (not shown) in which a hot medium circulates. The stand 1 is lowered into the washing tank 21 and a lid 37 is then placed over the washing tank 21. The supply of water from the collecting vessel 22 to the washing tank 21 then continues via the pipe 25 and nozzles 26 with the aid of a pump 35 in the pipe section 46, so that the oil on the fixtures melts and floats to the surface in clumps. Thanks to continuous surface separation most of the oil is washed out over the spillway of the washing tank 21 and is carried via the pipeline 23 and a pump 36 to the collecting vessel 22. Inside the lid 37 is a fan (not shown) intended to carry water vapour mixed with oil to the collecting vessel 22 via a pipe 38.

In the collecting vessel 22 the oil is separated from the water by a pump 39 pumping the water/oil mixture to the oil separator 28 via the pipe section 29. The cleaned water is returned to the collecting vessel 22 via the pipe section 30 and the separated oil is collected in a waste vessel 40 for processing in said collecting and pre-treatment step 16 (see Figure 2) . When most of the oil has been removed from the fixtures the washing tank 21 is emptied through the bottom outlet and the pipe 24 with the aid of a pump 41. The remaining oil is then removed from the fixtures by thorough flushing from the nozzles 26, after which the stand 1 is lifted out of the washing tank 21. To minimize the quantity of water transferred to the following step 13 for removing the coating, water remaining on the stand 1 and fixtures when they are lifted out of the tank is removed by means of a strong flow of air from an air-knife (not shown) located at the upper edge of the washing tank 21. In the collecting vessel 22 the oil is removed from the water by means of a pump 42 continuously pumping water through the evaporator 31.

The oil separated out in the evaporator 31 is collected in a waste vessel 43 and, like the oil from the oil separator 28, is then processed in the collecting and pre-treatment step 16. To compensate for the water lost during the oil separation, clean water is supplied through a filling pipeline 44 discharging into the collecting vessel 22. As can be seen from the flow chart in Figure 2, only water is supplied in the first washing step and only corrosion-preventing oil is removed from the first washing step. The first washing step is otherwise closed.

After the washing step the stand 1 is conveyed by the carriage 8 to the main station 5. Said step 13 for removing the coating will be described in more detail in the following with reference to Figures 4 and 5 showing schematically the main station 5. The station 5 comprises four identical cylindrical tanks 51, only one of which is shown in Figure 5. The station 5 also comprises a cylindrical collecting vessel 52 and a cylindrical discharge vessel 53. Each tank 51 has a conical bottom and a bottom outlet 54 leading into the collecting vessel 52 via a pipeline 55 consisting of pipe sections 56 and 57. Alternatively the bottom outlet 54 may be caused to open into the discharge vessel 53 through a pipeline 58 consisting of pipe sections 56 and 59, by switching valves 60 and 61 in the pipe sections

57 and 59. A pipeline 62 leads from the side wall of the tank 51 and opens into the same side wall by way of a pump 63 and a continuous flow heater 64. A pipeline 65 also emerges from the side wall of the tank 51 and discharges through nozzles 67 into the tank 51 via a pump 66. A lateral outlet 68 is provided in the side wall of the collecting vessel 52 which, via a pipeline 69 consisting of pipe sections 70-75, opens into the tank 51. Similarly a lateral outlet 76 is provided in the side wall of the discharge vessel 53 which, via a pipeline 77 consisting of pipe sections 78 and 71-75, also opens into the tank 51. Through a pipeline 79 consisting of pipe sections 78, 71, 72 and 80 the lateral outlet 76 of the discharge vessel 53 can be caused to discharge into the collecting vessel 52 by switching valves 81-83 in the pipe sections 74, 73 and 80. The collecting vessel 52 has a conical bottom and a bottom outlet 84 which, via a pipeline 85, opens into a first waste vessel 86. Similarly the discharge vessel 53 has a conical bottom and a bottom outlet 87 which, via a pipeline 88, opens into a second waste vessel 89. The main station 5 also comprises two storage tanks 90, 91 for varnish-softening chemicals in liquid form. A pipeline 92, 93 leads from each storage tank 90, 91 to the collecting vessel 52.

In the initial stage of step 13 for removing the coating the stand 1 is lowered into one of the tanks 51, after which the opening of the tank 51 is closed by means of a lid, not shown. At this stage the tank 51 is filled with a treating fluid including a solvent for softening the coating. The treating liquid has a predetermined temperature at which it most efficiently causes the varnish coating to come loose from the surfaces of the fixtures. Said solvent preferably includes N-methyl-2-pyrrolidone. The treating liquid may, for instance, be the chemical known under the trade name STRIPP-T, which is a mixture of the chemicals known under the trade names STRIPP-B and STRIPP-E. If the treating liquid is STRIPP-T the predetermined temperature should preferably lie within the interval 80-85 degrees Celsius. To maintain this temperature the tank 51 and collecting vessel 52 are preferably surrounded individually by casings 94, 95 containing a pipe (not shown) in which a hot medium circulates . For fine control of the temperature the treating liquid is caused to flow through said continuous flow heater 64 with the aid of the pump 63. When the stand 1 has been lowered into the tank 51 the treating liquid is pumped round in the pipeline 65 with the aid of the pump 66, and out through the nozzles 67 which are then below the surface of the treating liquid. The composition of the treating liquid is regularly checked and more treating liquid can be supplied from the storage tanks 90 and 91 via the pipelines 92 and 93 by means of pumps 101 and 102 if this is deemed necessary. The nozzles 67 are preferably ejector nozzles so designed that the treating liquid flowing from each nozzle draws with it treating liquid from the surrounding treating liquid in the tank 51 so that the total flow from the nozzles 67 is multiplied and liquid jets with high pressure are obtained. The nozzles 67 thus emit jets of said treating liquid that generate turbulence, which jets of liquid disintegrate the coating chemically loosened from the fixtures to particle form and remove the particles from the surfaces of the fixtures. In other words the liquid jets "strip" the loosened coating from the fixtures, i.e. tear or pull the coating off mechanically. Preferably at least two of the nozzles 67 are so placed and directed that their jets of liquid cross each other so that vortex cavitation occurs and effectively removes the coating from the fixtures. The "stripped" coating particles also contribute with a certain blasting effect as they accompany the treating liquid and are pumped out through the nozzles 67.

In accordance with a preferred embodiment the nozzles 67 are mounted on vertical ramps 96 arranged inside the tank 51 on its side wall. The ramps 96 are uniformly distributed around the periphery of the tank 51. If there are four ramps 96, for instance, they will occur every quarter of a turn. Figure 4 shows only one of the ramps 96. The vertical extension of the ramps 96 substantially coincides with the height of the tank 51. Six nozzles 67 are mounted on each ramp 96 and are uniformly distributed along the length of the ramp 96. During the stripping process the pressure in the nozzles 67 lies preferably within the interval 6-8 bar. The flow through each nozzle 67 is preferably approximately

100 litre per minute and about 2400 litres of treating liquid are therefore pumped through the nozzles 67 every minute .

During step 13 for removing the coating the treating liquid is continuously wide-mesh filtered by a part of the liquid being pumped through the pipeline 55 to the collecting vessel 52 by a pump 97. Larger particles are collected in the collecting vessel 52 in a wide-mesh filter 98 which may be a jute sack filter with a mesh width of about 0.5 mm. An equivalent quantity of filtered treating liquid is simultaneously pumped back into the tank 51 through the pipeline 69 by pumps 99 and 100 in the pipe sections 72 and 74. The pipeline 55 with pump 97, wide-mesh filter 98, collecting vessel 52 and the pipeline 69 with pumps 99 and 100 thus form a filtering arrangement that continuously filters the treating liquid during step 13 for removing the coating.

When the stripping process has continued long enough for substantially all of the coating to have been removed from the fixtures - which may take 20-40 minutes - pumping of the treating liquid is discontinued and the stand 1 is lifted out of the tank 51 with the aid of the carriage 8. The stand 1 and fixtures are rinsed with treating liquid with the aid of a pump 103 and a pipeline 104 leading from the side wall of the tank 51 and emerging in nozzles at the opening of the tank 51, while the stand 1 is being lifted out of the tank 51, after which the treating liquid is blown off the stand 1 and fixtures by an air-knife (not shown) located at the upper edge of the tank 51.

Practical experience has shown that it is difficult to separate particles using a filter since the filter easily becomes clogged. Said wide-mesh filtering, in which larger particles are separated off, is therefore preferably combined with sedimentation where remaining particles, i.e. smaller flakes and particles of the coating, are separated from the treating liquid. During a stay in the coating-removal process the particles remaining in the treating liquid sink to the bottom of the tank 51 and the collecting vessel 52. Experiments have shown that 40-50 % of the particles settle within one hour and 90-95 % settle within 24 hours. After an extended stay in the coating-removal process, e.g. in the morning before the process is started up, or after a week-end break, the sediment in the tank 51 is pumped by the pump 97 to the discharge vessel 53 via the pipeline 58. The particles are allowed to settle a second time in the discharge vessel 53, after which the remaining, clean treating liquid in the discharge vessel 53 is sucked up and returned to the collecting vessel 52 via the pipeline 79. The particles that have settled at the bottom of the discharge vessel 53 are then emptied into the waste vessel 89 and conveyed first to said collecting and pre-treatment step 16 (see Figure 2) where remaining treating liquid is separated out as far as possible and returned to the coating-removal step 13. The particles and the treating liquid remaining after the separation are then conveyed to said reprocessing and destruction step 17 to be reprocessed for re-use or to be destroyed. The tank 51, discharge vessel 53, pipeline 58 with pump 97, pipeline 79 with pump 99, collecting vessel 52, waste vessel 89 and pipeline 88 thus form a sedimentation means arranged to separate most of the particles remaining after the filtering, from the treating liquid. In the collecting vessel 52 the particles in the treating liquid are allowed to settle on the bottom, after which the clean treating liquid above the sedimented flakes of varnish is sucked up and conveyed to the tank 51 via the pipeline 69. The sedimented particles at the bottom of the collecting vessel 52 are then emptied into the waste vessel 86 and conveyed to said collecting and pre-treatment step 16 and thereafter to said reprocessing and destruction step 17. To facilitate sedimentation the conical bottoms of the tank 51, collecting vessel 52 and discharge vessel 53 preferably have an angle of inclination exceeding 30 degrees.

The main station 5 and washing stations 4, 6 are enclosed in a building (not shown) in which ventilation

(not shown) is provided to clean the air from vaporized treating liquid in said ventilation step 18. The ventilation means include equipment for condensing the treating liquid, in which equipment the treating liquid is caused to condense. Precipitated and purified treating liquid is returned to the coating-removal step 13. If the treating liquid includes N-methyl-2-pyrrolidone, the property of this chemical of completely mixing with water can be exploited to remove any treating liquid remaining from the air after the condensation. In this case the air is washed with clean water after the condensation so that any

N-methyl-2-pyrrolidone remaining is effectively separated from the air. The water and the chemical dissolved in the water are then conveyed to the second washing step 14. To increase the efficiency of the ventilation step 18 the ventilation means is preferably connected to exhausting ramps placed at the opening of the tank 51 so that vaporized treating liquid and air-borne particles of the treating liquid can be efficiently sucked up.

The three tanks not shown are connected to the collecting and discharge vessels in the same way as the tank 51 shown. The main station 5 can thus handle four stands 1 in parallel.

After the coating-removal step 13 each stand 1 is conveyed by the carriage 8 to the after-wash station 6 where the stand 1 undergoes the second washing step 14. The purpose of the second washing step is to remove any treating liquid and any remaining coating particles from the fixtures. In the following the washing step 14 will be described more fully with reference to Figure 6 showing schematically the after-wash station 6. The after-wash station 6 comprises a cylindrical washing tank 105 and a cylindrical collecting vessel 106. The washing tank 105 is provided with a bottom outlet 107 discharging via a pipeline 108 into a wide-mesh filter 109 in the collecting vessel 106. A pipeline 110 leads from the side wall of the collecting vessel 106 to nozzles 111 inside the washing tank 105. The collecting vessel 106 has a bottom outlet 112 leading via a first pipeline 113 to the waste vessel 114 and via a second pipeline 115 to an evaporator 116. From the evaporator 116 the pipeline 115 continues to the collecting vessel 106.

In the second washing step 14 the stand 1 is lowered into the washing tank 105 and rinsed with hot water from the nozzles 111 so that treating liquid and any particles remaining are washed off the fixtures. To heat the water the collecting vessel 106 is preferably surrounded by a casing 117 containing pipes (not shown) in which a hot medium circulates . After being washed clean the stand 1 is removed from the washing tank 105 and any water remaining on the stand 1 and fixtures when they are lifted out of the tank is removed by means of a flow of air from an air-knife (not shown) located at the opening of the washing tank 105. The mixture of water, particles and treating liquid in the washing tank 105 is conveyed via the pipeline 108 to the wide-mesh filter 109 which may be a jute sack filter where larger particles are caught. Smaller particles may be permitted to settle at the bottom of the collecting vessel 106 after completion of the washing step, where they are tapped off through the bottom outlet 112 of the collecting vessel 106 and the pipeline 113 to the waste vessel 114 to be conveyed to the collecting and pre-treatment step 16. To deal with the treating liquid that accompanies the water, the treating liquid mixed with water is continuously conveyed to the evaporator 116 via the bottom outlet 112 of the collecting vessel 106 and the pipeline 115. The treating liquid and the water are separated in the evaporator 116, after which the water is returned to the collecting vessel 106. The treating liquid separated out then goes to the reprocessing and destruction step 17. To compensate for the water lost during the evaporation, clean water is supplied through a filling pipeline 118 'discharging into the collecting vessel 106. After the second washing step 14 the after-treatment step 15 takes over, in which the stand 1 is moved by the carriage 8 to the set-down surface 3 where the fixtures are removed from the stand 1 and prepared for transport from the plant .

The invention has been described above with reference to a plant for reconditioning fixtures of the type used in the automobile industry. However, it will be understood that the invention can be used for reconditioning other structural elements . A plant in accordance with the invention could be used, for instance, prior to a repainting step in order to remove inadequate coatings of paint from various types of goods. An example of such goods is aluminium hub caps on which the coating of varnish is uneven or too thick. It will also be understood that the plant can be modified without departing from the principle of the invention. The first washing step 12 can be omitted, for instance, if the structural elements are not contaminated with corrosion-protective oil.

Claims

C L A I M S

1. A plant for reconditioning structural elements which, in an industrial process, have obtained a hard, water-insoluble coating of a surface-finishing material such as a varnish with or without pigment, or the like, which plant comprises a main station (5) for removal of the coating from the structural elements, said main station (5) including - at least one tank (51) containing a treating liquid including a solvent capable of softening the coating, and - a transport device for immersing the structural elements in the treating liquid, wherein the tank (51) includes a plurality of nozzles

(67) arranged inside the tank (51) below the surface of the treating liquid characterized in that the nozzles

(67) are arranged to emit jets of said treating liquid to generate turbulence, which jets of liquid are arranged to disintegrate the coating to particle form and remove the particles from the surfaces of the structural elements .

2. A plant as claimed in claim 1, characterized in that at least two of said plurality of nozzles (67) are directed so that their jets of liquid cross each other and cause turbulence in the form of vortex cavitation.

3. A plant as claimed in claim 1 or claim 2, characterized in that the tank (51) is cylindrical and in that the nozzles (67) are mounted on vertical ramps (96) arranged inside the tank (51) on the side wall of the tank (51) .

4. A plant as claimed in any one of claims 1-3, characterized in that the main station (5) includes a filtering device arranged to separate the particles from the treating liquid.

5. A plant as claimed in claim 4, characterized in that the filtering device comprises

- a collecting vessel (52) ,

- a first pipeline (55) leading from the tank (51) and discharging into a wide-mesh filter (98) in the collecting vessel (52) , which first pipeline (55) is arranged, with the aid of a pump (97) to carry unfiltered treating liquid from the tank (51) to the collecting vessel (52) through the wide-mesh filter (98) , and

- a second pipeline (69) leading from the collecting vessel (52) and discharging into the tank (51) , which second pipeline (69) is arranged, with the aid of at least one pump (99 or 100) , to carry filtered treating liquid from the collecting vessel (52) back to the tank (51) .

6. A plant as claimed in claim 4 or claim 5, characterized in that the main station (5) also includes at least one sedimentation means arranged to separate the majority of particles remaining after filtration, from the treating liquid by means of sedimentation.

7. A plant as claimed in claim 6, characterized in that the sedimentation means comprises

- said tank (51) , in which the particles remaining after filtration are permitted to settle a first time,

- a discharge vessel (53) and a first pipeline (58) leading from a bottom outlet (54) in the tank (51) into the discharge vessel (53) , which first pipeline (58) is arranged, with the aid of a pump (97) , to convey the sedimented particles to the discharge vessel (53) , in which the particles are permitted to settle a second time, - a second pipeline (79) leading from the discharge vessel (53) to the collecting vessel (52) which is arranged, with the aid of a pump (99) , to convey the clean treating liquid above the sedimented particles to the collecting vessel (52) , and

- a waste vessel (89) into which a bottom outlet (87) in the discharge vessel (53) opens via a third pipeline (88) , which waste vessel (89) is arranged to receive the particles that have settled in the discharge vessel (53) .

8. A plant as claimed in claim 7, characterized in that the tank (51) and the discharge vessel (53) have conical bottoms, preferably with an angle of inclination exceeding 30 degrees, in order to facilitate said sedimentation.

9. A plant as claimed in any one of claims 1-8, characterized in that the structural element is a metal fixture re-usable in the industrial process and in that said solvent includes N-methyl-2-pyrrolidone.

10. A method for reconditioning structural elements which, in an industrial process, have been subjected to a surface-finishing material such as a varnish with or without pigment, or the like, and have thus formed a hard, water-insoluble coating on the surfaces of the construction elements, which method comprises a main step (13) in which the structural elements are exposed to a treating liquid including a solvent capable of softening the coating, and in which treating liquid the structural elements are immersed, characterized in that the structural elements in said main step (13) are also subjected to jets of said treating liquid to generate turbulence, which jets are caused to flow out into the treating liquid surrounding the structural elements in order to disintegrate the coating to particle form and remove the particles from the surfaces of the structural elements .

11. A method as claimed in claim 10, characterized in that at least two of said jets of liquid are caused to cross each other and generate turbulence in the treating liquid surrounding the fixtures.

12. A method as claimed in claim 10 or claim 11, characterized in that during said step (13) the treating liquid is continuously filtered in order to separate the removed particles from the treating liquid.

13. A method as claimed in claim 12, characterized in that the particles remaining after filtering are separated from the treating liquid by means of sedimentation.

14. A method as claimed in any one of claims 10-13, characterized in that said step (13) is preceded by a first washing step (12) in which the structural elements are immersed in a washing tank (21) and rinsed with hot water.

15. A method as claimed in any one of claims 10-14, characterized in that said step (13) is followed by a second washing step (14) in which the structural elements are immersed in a second washing tank (105) and rinsed with hot water, so that the treating liquid and any remaining particles are removed from the surfaces of the structural elements.

16. A method as claimed in any one of claims 10-15, characterized in that the structural element is a metal fixture re-usable in the industrial process and in that said solvent includes N-methyl-2-pyrrolidone.