EP0080407A2 - Process and apparatus for removing coatings from objects - Google Patents

Abstract

The invention relates to the stripping of coated objects. The coating is dissolved in a chamber 10 (11) inserted with nitrogen (V3 - V4) with a solvent and the solvent-coating product mixture is transferred to a thickening tank (31) subjected to a current. of nitrogen (32) heated in (33). The nitrogen-solvent mixture is separated by cooling in (36) (38) and condensation of the solvents extracted in (41). Application to the stripping of objects coated with paints and varnishes.

Description

The invention relates to a method of pickling objects provided with a coating, in particular painted or varnished objects, by soaking in a solvent bath.

There are several methods of pickling painted or varnished objects, one of which is based on pyrolysis, that is to say the incineration of the varnish layers at temperatures between 500 ° C and 700 ° C. Its drawbacks are high energy consumption, a thermal load imposed on the objects, difficulties in handling the objects, as well as the diffusion of large quantities of harmful substances into the atmosphere. Another known method uses sandblasting to remove the layers of varnish. This process is relatively time consuming and poses problems in removing the sanding product from the varnish. In addition, the storage of the varnish requires a special deposit. Another known method consists in pickling the varnish by subjecting the objects to a jet of high pressure steam. In this case too, the amount of energy required is relatively large and a special deposit is necessary for storage.

Pickling is also known using ultrasound, with objects to be stripped immersed in a solvent bath which is carried out under a normal atmosphere. In this case, the energy required is not as great as in the case of the processes described above, but there is also a drawback on the environment due to the deposition of the detached varnish which is still partly charged with solvent. Since the solvent is flammable at relatively low temperatures, the danger of fire or explosion of the solvent is very great.

Finally, hot pickling as well as cold pickling on the basis of processes using special chemicals, which are themselves the source of storage problems, have been developed. In addition, the quantities of chemicals required are considerable and the duration of the process is relatively long.

Consequently, the object of the invention is a method for removing the separable layers from the objects covered therewith, in particular for stripping varnished objects which, despite the low operating costs, produces no waste, only requires 'a low consumption of raw material and produces only a small and negligible emission into the atmosphere.

According to the invention, this task is solved, in a process according to which the objects are introduced into a closed tank and treated with a liquid fluid, the mixture of treatment fluid and pickled material being then collected, the treatment fluid being separated from the pickled material, inert gas being introduced into the tank after the removal of the layers has been completed and the gas leaving the tank being condensed to separate the released treatment fluid, in that coated articles are introduced into the hermetically sealed tank, that inert gas is introduced into said tank until a residual oxygen content of 3% by volume is obtained, which the gas leaving the inerting tank heats up by heat exchange. inert gas introduced into the tank, only after inerting a pickling product is introduced into the tank, in which the objects to be pickled are soaked, only after the pickling operation and before t the drying of the pickled objects the mixture of coating and pickling product is pumped out of the tank, that the gas mixture leaving the tank during drying is cooled by the inert gas introduced into the tank by heat exchange before the condensation of the pickling product and the inert gas is discharged into the atmosphere after the pickling product has condensed.

In the case of the process which is the subject of the invention, the removal of the layers, for example the pickling of a varnish, takes place with a solvent which is known per se but, according to the invention, the process has therefore takes place under an inert atmosphere. This avoids any risk of fire or explosion when stripping the coating. Once the pickling has been carried out, the inert gas can be evacuated into the atmosphere, possibly passing through an appropriate separation device, without harmful substances being entrained. The inert gas used in the layer removal process is also used for the separation of the solvent and the material forming the protective coating. For this purpose, the inert gas evacuates the vaporized solvent and brings it to condensation in a suitable device. The separated solvent can then be reused in the operating process, the use of polluting and expensive chemicals being therefore kept within very low limits during the implementation of the process.

When the solvent is separated from the solvent-coating material mixture, an appropriate concentration of this mixture can be obtained in order to make it reusable. This is for example the case of removing the varnish. The varnish mixed in the solvent can be reused with the desired concentration for a new varnishing process, for obtaining pigments, etc. In this way, it is avoided to direct it on a particular discharge and the disadvantage which results for the environment is removed.

In a version of the process which is the subject of the invention, nitrogen is used as inert gas. As an easy-to-handle industrial gas, nitrogen can be used particularly favorably in the process which is the subject of the invention.

It is not entirely possible to avoid inert gas escaping from the bath entraining vaporized solvent. Consequently, according to one embodiment of the invention, it is provided that the gas escaping from the bath is cooled by heat exchange with the inert gas which is introduced. If, for example, nitrogen gas is removed from a pressure tank containing liquid nitrogen, the temperature of the nitrogen gas drops, because the heat of evaporation is removed. If the nitrogen which is introduced by heat exchange is now heated, this leads to cooling of the nitrogen-vapor mixture of the solvent. Thereby the solvent vapor can condense and the liquid condensate can be collected while pure nitrogen escapes into the atmosphere.

When the solvent-coating material mixture is extracted from the bath, the pickled objects are still impregnated with solvent. For this reason, according to another embodiment of the invention, it is proposed, when the solvent-coating material mixture is removed from the bath, to dry the pickled objects with inert gas and for the solvent to be separated from the inert gas stream and solvent vapor and be collected.

The process step indicated last is carried out as described above.

As already explained, inert gas is used to separate the solvent from the coating material. To this end, according to one embodiment of the invention, provision is made to reheat the inert gas for the evaporation of the solvent. Alternatively, the tank into which the solvent-coating material mixture is pumped can be reheated. If nitrogen is used as the inert gas and the usual solvent, reheating involves a temperature of around 60 ° C.

In order to ensure better use of the energy of the separation process based on condensation, according to another version of the invention, provision is made to cool the solvent vapor-inert gas mixture by heat exchange with the inert gas more cold released after condensation of solvent vapor.

The inert gas used for the evaporation and condensation of the solvent is preferably kept in a closed circuit, the quantity of inert gas required being thereby reduced to a minimum. Before the start of the separation, the product circulating in the circuit will preferably be made inert by means of an inert gas.

As is apparent from the preceding explanations, the process which is the subject of the invention is characterized by optimal reuse of the raw materials used. With very low operating costs, there is no production of waste. The quantities of products emitted with the process which is the subject of the invention are clearly below the authorities' recommendations. The amounts of energy necessary for implementing the process which is the subject of the invention are very low. Due to the handling of the solvent, which takes place only in an inert atmosphere, work safety is clearly better than average, the danger of fire and explosion being greatly reduced. The influences exerted on workstations with regard to noise and other nuisances are minimal. Finally, the process which is the subject of the invention can be largely automated.

Various suitable installations are possible for implementing the process which is the subject of the invention. An installation according to the invention with a tank receiving the objects and which can be connected to an inert gas tank via a supply line and a shut-off valve, a tank of pickling product connected to the tank is characterized in that the tank can be hermetically sealed and can be connected to the atmosphere via a second shut-off valve, an exchanger and a condenser, as the inert gas tank is connected to the tank via the exchanger, that the pickling product tank is connected to the tank via a pump and that another pump for pumping the pickling product-varnish mixture out of the tank is planned.

The installation which is the subject of the invention makes it possible to render the pickling tank inert before it is filled with solvent and drying of the pickled objects when the solvent-coating material mixture is removed. Between these phases of the process, the shut-off valves are closed, which makes it possible to maintain at a relatively small value the quantity of inert gas necessary for such a process. Leakage of inert gas to the atmosphere, in particular during the drying phase, does not represent any disorder for the environment, since appropriate precautions make it possible to separate the solvent which is contained therein in the form of vapor.

The heat exchanger of the installation which is the subject of the invention is preferably an evaporator of a refrigeration unit. This evaporator ensures sufficient cooling of the gaseous mixture constituted by the solvent vapor and by the inert gas, so that the solvent vapor can be condensed and separated, the cooled inert gas being able, according to one embodiment of the invention, to be brought by via a pipe in the thickening tank and which, according to another embodiment of the invention, can be used for cooling in a recuperator brought to the evaporator.

The separated solvent can be returned to the solvent tank via suitable pumps and can be reused. The solvent mixture and the coating materials concentrated in the thickening tank can also be reused.

An exemplary embodiment is explained below with reference to the single figure which shows, schematically, an installation making it possible to implement the method which is the subject of the invention.

Two tanks, 10 and 11, hermetically closed by a cover 12 or 13, are provided. The tanks 10 and 11 can be connected to a nitrogen tank 16, via the supply lines 14 and 15, and stop valves V3 or V4. The outlet of the nitrogen tank 16 can be closed directly using a shut-off valve V1. The tanks 10 and 11 can be connected to the atmosphere by means of the valves V6 or V5, a pipe 17, an exchanger 18, a separator 19 and a pipe 20. In the exchanger 18 is a coil 21, which constitutes a part of the supply line 14.

The tanks 10 and 11 are pickling tanks which are further equipped with ultrasonic generators, as indicated in 22 or 23. The tanks 10 and 11 can be connected to a solvent tank 27 by means of a pump 24, pipes 25 or 26 and valves V9 and V10.

The tanks 10 and 11 can be connected to a thickening tank 31 by means of the shut-off valves V8 and V7, the pipes 28 or 29 and a pump 30. The supply pipe 32 to the tank d thickening 31 is provided with a heating device 33. A fan 35 is arranged on an evacuation pipe 34, coming from the thickening tank 31. From the blowing side of the fan 35, the pipe 34 is directed towards the input of a recuperator 36, one of the outputs of which is connected to the evaporator 38 of a refrigeration unit designated, in general, by 39, by means of a pipe 37. An outlet pipe 40 of the evaporator 38 is connected to a separator 41. A pipe 42 leaving the separator 41 passes through the recuperator 36 and is connected to line 32, upstream of the heating device 33. Line 14 is connected to line 37 or 32, via a line 43.

The separators 19 and 41 are provided with outlet pipes 44 or 45 which lead to a collecting pipe 46 which comprises a pump 47 and which is connected to the solvent tank 27.

At the lower part, a pipe 48 comprising a valve V11 is connected to a thickening tank 31.

• Le couvercle 12 étant ouvert, les pièces à décaper sont accrochées à la main dans le réservoir de décapage 10. Après son remplissage, le réservoir 10 est fermé hermétiquement. On ouvre alors les soupapes V1, V3 et V6. L'azote pénètre dans le réservoir 10, ressort de celui-ci à l'air libre en passant par l'échangeur 18 et le séparateur 19. En même temps, de l'air est extrait. Dès que la teneur en oxygène atteint une valeur inférieure à 3 % de volume, les soupapes V3 et V6 sont fermées. En même temps, ou juste après, la pompe 24 est enclenchée et la soupape V9 couverte, du solvant étant pompé dans le réservoir de décapage 10, comme indiqué en 45 a. Lorsqu'un degré de remplissage prédéterminé est atteint, la soupape est à nouveau fermée et la pompe 24 arrêtée.

The installation described works as follows:

• The cover 12 being open, the parts to be stripped are hung by hand in the pickling tank 10. After filling, the tank 10 is hermetically closed. The valves V1, V3 and V6 are then opened. The nitrogen enters the reservoir 10, leaves it in the open air passing through the exchanger 18 and the separator 19. At the same time, air is extracted. As soon as the oxygen content reaches a value of less than 3% by volume, the valves V3 and V6 are closed. At the same time, or just after, the pump 24 is started and the valve V9 covered, solvent being pumped into the pickling tank 10, as indicated in 45 a. When a predetermined degree of filling is reached, the valve is closed again and the pump 24 stopped.

The stripping procedure then begins with the help of ultrasound.

At the same time, or later, the valve V2 is open, which makes it possible to render the condensation circuit consisting of the heating device 33, the thickening tank 31, the recuperator 36, the evaporator 38, the inert separator 41 as well as corresponding pipes.

When a predetermined oxygen concentration is reached the valve V2 is closed again. The heater 33 is turned on until a predetermined temperature is reached.

When the pickling phase is complete, the valve V8 opens and the pump transports the mixture consisting of dissolved varnish and solvent into the thickening tank 31. In the thickening tank 31 the mixture is designated by 46.

At the same time, or later, the valves V3 and V6 open. The stream of nitrogen passing through the tank 10 dries the pickled parts. The solvent-nitrogen vapor mixture passes through the exchanger 18. The solvent condenses in this exchanger and is separated in the separator 19. The purified nitrogen escapes into the atmosphere through line 20. The solvent coming from the separator 19 enters the solvent tank 27 via line 44 and using the pump 47.

After completion of the drying phase, the reservoir 10 is filled with air. As soon as the oxygen content reaches at least 19% of volume, the cover 12 opens automatically and the pickled and cleaned parts can be extracted. The pickling phase can then be repeated as described above.

Meanwhile, the thickening phase takes place as follows in the tank 31. The tempered nitrogen evaporates at a temperature of for example 60 ^° C. part of the solvent in the thickening tank 31. The gaseous mixture of solvent and d nitrogen undergoes preliminary cooling in the recuperator 36 via the fan 35 and is sent to the evaporator 38 where the solvent vapor condenses. In the separator 41 the solvent and the nitrogen are separated. The nitrogen is again reheated in the recuperator 36 and is brought into the heating device 33 to the desirable temperature for its introduction into the thickening tank 31. The thickening phase lasts until the desirable viscosity of the varnish is reached. When the thickening phase is complete, the valve V11 opens so that the varnish can be transferred to suitable containers via the line 48.

The temperatures used in the thickening circuit are shown in the figure. However, they should not be considered as limiting.

It is obvious that a pickling phase can also be carried out in parallel, or with offset, in the tank 11.

The example of execution shows that the process which is the subject of the invention can be implemented with usual components and apparatuses and that expensive special manufacturing is avoided.

Claims (16)

1. - Method for pickling objects provided with a coating according to which said objects are introduced into a closed tank and treated with a liquid fluid, the mixture of treatment fluid and pickled material then being collected, the treatment fluid being separated from the pickled material, inert gas being introduced into the tank after the removal of the layers has been completed and the gas leaving the tank being condensed to separate the released treatment fluid, characterized in that coated articles are introduced into the sealed tank, that inert gas is introduced into said tank until a residual oxygen content of 3% by volume is obtained, that the gas leaving the inerting tank heats by exchange of heat the inert gas introduced into the tank, only after inerting a pickling product is introduced into the tank, in which the objects to be pickled are soaked, only after the pickling operation and before the drying of the pickled objects the mixture of coating and pickling product is pumped out of the tank, that the mixture of gases leaving the tank during drying is cooled by the inert gas introduced into the tank by heat exchange before the condensation of the pickling product and that the inert gas is discharged into the atmosphere after the pickling product has condensed. 2. - Method according to claim 1, characterized in that nitrogen is used as inert gas. 3. - Method according to claim 1 or 2, characterized in that the mixture of pickling product and pickled coating is collected in a separation tank, that heated inert gas is introduced into the separation tank for the evaporation of pickling product and the evaporated pickling product is then condensed until a sufficient concentration of the pickled coating is obtained. 4. - Method according to claim 3, characterized in that the inert gas is heated to about 60 ° C. 5. - Method according to claim 3 or 4, characterized in that the inert gas used for evaporation and condensation of the pickling product is maintained in a closed circuit. 6. - Method according to claim 5, characterized in that the vapor mixture of the pickling product and the inert gas leaving the separation tank undergoes preliminary cooling by heat exchange with the cooler inert gas released after the condensation of the pickling product vapor. 7. - Method according to one of claims 1 to 6, characterized in that the circuit for passing the inert gas or the inert gas-vapor mixture of the pickling product is previously rendered inert with inert gas for the condensation of the vapor pickling product. 8. - Method according to one of claims 1 to 7, characterized in that the pickling bath is excited by ultrasound. 9. - Installation for the stripping of objects coated with varnish or paints with at least one tank receiving the objects and which can be connected to a tank of inert gas by means of a supply pipe and a valve off, a tank of pickling product connected to the tank, in particular for implementing the method according to one of claims 1 to 8, characterized in that the tank (10, 11) can be hermetically closed and can be connected to the atmosphere via a second shut-off valve (V6, V5), an exchanger (21, 18) and a condenser (19), that the inert gas tank (16) is connected to the reservoir (10, 11) via the exchanger (18, 21), that the pickling product reservoir is connected to the reservoir (10, 11) via a pump (24) and that another pump to pump the mixture of pickling and varnish out of the tank (10, 11) is provided. 10. - Installation according to claim 9, characterized in that at least one thickening tank (31) is provided, which can be connected to the discharge side of the other pump (30), and to the inert gas tank (16 ) via a supply line (43) and a third shut-off valve (V2) and in that a discharge line (34) connected to the thickening tank (31) is directed to a separator (41) via a heat exchanger (38) supplied with refrigerant. 11. - Installation according to claim 9 or 10, characterized in that the heat exchanger is an evaporator (38) of a refrigeration unit (39). 12. - Installation according to one of claims 9 to 11, characterized in that the inert gas released from the condensed pickling product is returned to the thickening tank (31) via a pipe (37 or 32 ). 13. - Installation according to claim 12, characterized in that a heating device (33) is arranged on the pipe (37 or 32). 14. Installation according to one of claims 9 to 13, characterized in that a fan (35) is disposed on the discharge pipe (34) coming from the thickening tank (31). 15. - Installation according to one of claims 13 or 14, characterized in that the discharge pipe (34) coming from the thickening tank (31) and the pipe (37) going to the thickening tank (31 ) passes behind the separator (41) through a recuperator (36) to carry out a heat exchange. 16. - Installation according to one of claims 9 to 15, characterized in that the outlets of the separators (19, 41) are connected to the tank of stripping product (27) via a third pump (47) .

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