Title:
METHOD AND APPARATUS FOR REMOVING COATINGS FROM METALS
United States Patent 3770501


Abstract:
Thermochemical procedure and equipment for the substantially airless stripping of organic base coatings from objects by volatilization of the coating material as immersed in a molten salt bath. The procedure is carried out in a shroud-like object-receiving retort. This retort, when immersed at least in part at an open bottom thereof penetrating in said bath, is substantially sealed hermetically against entry of external atmospheric air to its interior. The bath is heated in a conventional pot by conventional immersion heating means; and the interior of the retort is conditioned during immersion, as by being placed and held under partial vacuum or by introducing an inert gas, to greatly reduce air content in the space therein above the bath, thus creating a nonoxidizing atmosphere. The retort is shown as being by preference sealed at its top by a removable gas tight cover. Volatilization continues for a relatively very brief time period, the resulting vapor product and possible gas arising to the conditioned retort space above the bath level. During this volatilizing period, or directly before and thereafter and with the retort still sealed and bottom-immersed in the bath, its interior above the salt level is evacuated, as by a vacuum pump. The volatilization vapor, minimal product of combustion, smoke and the like are piped in a closed system to conventional incinerator, filter and/or scrubber means prior to discharge to atmosphere. The retort is then unsealed and placed in position for rinsing and removal of the stripped object or objects therefrom.



Inventors:
KEMPER E
Application Number:
05/211580
Publication Date:
11/06/1973
Filing Date:
12/23/1971
Assignee:
UPTON IND INC,US
Primary Class:
Other Classes:
134/2, 134/21, 134/38, 134/85
International Classes:
B44D3/16; C23G1/28; (IPC1-7): C23G1/28; B08B7/00
Field of Search:
134/2,19,21,38,85,121,135,137,143 118
View Patent Images:



Primary Examiner:
Wolk, Morris O.
Assistant Examiner:
Hagan, Tim
Claims:
I claim

1. A method of removing coating material from an object, comprising immersing said object in a molten volatilization bath, creating and maintaining a reduced oxygen atmosphere in a space above the level of the bath by evacuating said space, which space is sealed against entry of external air thereto when said object is so immersed, maintaining said immersion for a time period and at an elevated temperature sufficient to cause volatilization of said coating material and escape of the volitilization product to said space, and withdrawing the escaped product from the space.

2. A method of removing organic base coating material from an object, comprising disposing said object in a partially exposed but otherwise sealed retort, immersing the retort and the thus disposed object in a molten salt bath, with the object being substantially totally immersed and the retort at least sufficiently immersed that the partial exposure thereof is sealed by a portion of said bath, the bath being at a temperature sufficiently elevated to volatilize said material, creating and maintaining a reduced oxygen atmosphere in a space in the thus sealed retort above the level of said bath portion therein, maintaining said immersion for a time period sufficient to cause volatilization of said coating material and escape of its volatilization product to said space, and voiding the escaped product from said space prior to removing the object from the retort.

3. The method of claim 2, in which the volatilization product is voided under a condition of partial vacuum.

4. A method of removing coating material from an object, comprising immersing said object in a molten volatilization bath, creating and maintaining a reduced oxygen atmosphere in a space above the level of the bath, which space is sealed against entry of external air thereto when said object is so immersed, maintaining said immersion for a time period and at an elevated temperature sufficient to cause volatilization of said coating material and escape of the volatilization product to said space, and withdrawing the escaped product from the space in which the immersed object is disposed in a retort which is also at least partially immersed in the volatilization bath, being filled to a predetermined level with a portion of the bath in which the object is immersed, the sealed space being in said retort and above said predetermined level therein.

5. The method of claim 1, in which said reduced oxygen atmosphere is created by placing said sealed space under a substantially sub-atmospheric pressure.

6. The method of claim 2, in which said reduced oxygen atmosphere in the retort is created by placing said sealed space in the latter under a substantially sub-atmospheric pressure.

7. The method of claim 4, in which said reduced oxygen atmosphere in the retort is created by placing said sealed space in the latter under a substantially sub-atmospheric pressure.

8. The method of claim 2, in which said reduced oxygen atmosphere in the retort is created by introducing a normally substantially non-oxidizing gas into said sealed space of the retort.

9. The method of claim 4, in which said reduced oxygen atmosphere in the retort is created by introducing a normally substantially non-oxidizing gas into said sealed space of the retort.

10. The method of claim 1, in which the object is suspended during the volatilizing immersion thereof in said retort portion of said bath.

11. The method of claim 2, in which the object is suspended during the volatilizing immersion thereof in said retort portion of said bath.

12. The method of claim 4, in which the object is suspended during the volatilizing immersion thereof in the retort portion of said bath.

13. The method of claim 2, in which said bath portion enters the retort from the bottom of the latter as the object and retort become immersed, said sealed retort space to which the volatilization product escapes being at the top of the retort.

14. The method of claim 4, in which said bath portion enters the retort from the bottom of the latter as the object and retort become immersed, said sealed retort space to which the volatilization product escapes being at the top of the retort.

15. The method of claim 11, in which said bath portion enters the retort from the bottom of the latter as the object and retort become immersed, said sealed retort space to which the volatilization product escapes being at the top of the retort.

16. The method of claim 12, in which said bath portion enters the retort from the bottom of the latter as the object and retort become immersed, said sealed retort space to which the volatilization product escapes being at the top of the retort.

17. Apparatus for removing by volatilization a coating material from an object, comprising an object-receiving retort having means to seal the interior thereof as at least partially immersed, along with an object received therein, in a volatilizing bath, said retort having an exposed portion at which the bath enters the thus immersed retort, means for maintaining the bath at a temperature sufficiently elevated to volatilize said material, means for creating a reduced oxygen atmosphere in a space of the retort above the bath portion therein, the volatilization product escaping to said space, and means for voiding said product from said space after volatilization.

18. The apparatus of claim 17, in which said retort has an open bottom affording said closed portion, and means sealing the top of the retort above the level of the bath entering the same.

19. The apparatus of claim 17, in which said retort has an open bottom affording said closed portion, and means releasably sealing the top of the retort above the level of the bath entering the same.

20. The apparatus of claim 19, in which said sealing means is a cover removable from the remainder of the retort before and after volatilization, the cover being provided with a fitting-type connection assisting in the creation of said reduced oxygen atmosphere.

21. The apparatus of claim 17, in which said retort has means for lowering and raising the same relative to the bath respectively before and after immersion of the retort.

22. The apparatus of claim 19, in which said retort has means for lowering and raising the same relative to the bath respectively before and after immersion of the retort.

23. The apparatus of claim 17, in which said retort is in fixed relation to the bath before, during and after immersion thereof.

24. A method of treating an object to separate organic and inorganic materials thereof as subjected to treatment, comprising immersing said object in a molten volatilization bath, creating and maintaining a reduced oxygen atmosphere in a space above the level of the bath by evacuating said space, which space is sealed against entry of external air thereto when said object is so immersed, maintaining said immersion for a time period and at an elevated temperature sufficient to cause volatilization of the organic material, escape of the organic volatilization product to said space and said separation of said materials, and withdrawing the escaped product from the space.

25. Apparatus for treating an object to separate organic and inorganic materials thereof as subjected to treatment, comprising an object-receiving retort having means to seal the interior thereof as thus subjected, along with an object therein, to a volatilizing environment, means for maintaining said environment at a temperature sufficiently elevated to volatilize said organic material, a vacuum pump for creating a reduced oxygen atmosphere in a space of the retort adjoining said atmosphere, the gaseous product of the organic material escaping to said space, and means for voiding said product from said space.

26. Apparatus for treating an object to separate organic and inorganic materials thereof as subjected to treatment, comprising an object-receiving retort having means to seal the interior thereof as at least partially immersed, along with an object therein, in a volatilizing bath, means for maintaining said bath at a temperature sufficiently elevated to volatilize said organic material, means for creating a reduced oxygen atmosphere in a space of the retort above the bath therein, the gaseous product of the organic material separating from the inorganic material and escaping to said space, and means for voiding said product from said space.

Description:
BACKGROUND OF THE INVENTION - FIELD

The method and equipment of the invention will find use, whether in a small, structurally simple embodiment such as is schematically illustrated herein or on a larger scale form for mass stripping of small or large objects, in a dip or spray coating department of a plant in which, for example, periodic strippings to remove incrustations of paint from conveyor work holders are in order. Such a build-up, for example, adversely affects the conductivity of a work holder which serves as an electrode in an electrostatic paint spray operation. An equally common application will be in the stripping of work products supported by a foraminated carrier in a retort, as well as in the stripping of massive objects in which a special carrier is not used.

SUMMARY OF THE INVENTION

As used industrially in the stripping from workpieces and the like of a large number and variety of varnishes, enamels, lacquers, synthetic resin compositions, and organic base-coatings in general, it is a commonly accepted fact that, in order to comply even reasonably well with existing atmospheric anti-pollution laws and ordinances, industrial stripping installations of the thermo-chemical salt bath volatilization type must employ an elaborate ventilation system in order to vent from the plant's stripping area the voluminous vapors, gases, smoke and soot which have inescapably been evolved. This necessarily must in turn be followed by a large scale incineration, filtering and/or scrubbing of such vapor and/or gas products prior to their discharge to the external atmosphere.

Other known volatilization-type systems, such as the fluidized bed type, do not employ a salt bath as such, but in none of them is an effort made to conduct the operation in a substantially oxygen-free atmosphere.

Specifically in relation to a molten salt bath operation, despite all conventional attempts to seal the bath's enclosure against entry of combustion-supporting atmospheric air prior to and during stripping, it inevitably occurs at necessary access openings, and in relatively large volume, all things considered. The result is an extensive and violent combustion of the volatilized organic base coating materials in the stripping pot zone, with attendant generation in a major degree of soot and other contaminants, in turn calling for resort to the extensive, expensive and space-consuming gas handling and disposal system heretofore thought unavoidable.

By contrast, the method and equipment of the present invention, while thermochemically relying as before upon the principle of vaporization of organic constituents of the coating, also ordinarily to a degree on an ultimate incineration and/or scrubbing of the product of volatilization, operate in a manner to vastly diminish in all respects the size, complexity and expense of the installation. Needless to say, to the extent that the method and apparatus simplify a solution to a serious atmospheric problem they are well attuned to ecological aims which increasingly occupy the attention of the public.

Typically, the objects to be stripped, as carried in a foraminate container appropriately supported within and from the top of a shroud-like vaporization retort, open at its bottom, are immersed in a molten salt bath of the stripping pot, but prior to this immersion, the interior of the retort is tightly sealed from the external atmosphere, which may be readily accomplished in view of the relatively small size of the retort, particularly as herein shown. In being thus immersed in the bath (which typically may be a mixture of sodium hydroxide and sodium nitrate or alkaline nitrates), the molten material of said bath increasingly enters the retort at its open bottom; however, well before the work to be stripped becomes immersed the air space in the retort above the level of the bath is either reduced to a substantially sub-atmospheric pressure or has introduced into this space a low pressure non-combustible gas, such as carbon dioxide or nitrogen. Likewise, in some cases the process may well tolerate the presence of a small volume of air and resultant small oxidation at the outset and conclusion of the operation.

Further pursuant to the invention, and after volatilization for an appropriate time interval at an appropriate immersion bath temperature, the products of the volatilization, including vapor and whatever small amount of gas and smoke that have been generated, and voided from the retort prior to opening the latter or other exposure of its interior, such as for removal of the container and its stripped content. The conditioning of the retort's interior, as under the instanced vacuum, may be done once per cycle under valve control, only at the commencement of a stripping cycle, but is preferably continued throughout the duration of the cycle, i.e., a continuous removal of the products of vaporization during their continued generation from within the salt bath. More generally stated, and more important, the understanding will be that the invention contemplates a hermetic sealing of the zone above the bath against the entry of atmospheric air during the stripping cycle. This, it will be noted, is the antithesis of operations and procedures of the present day, in which there is an inevitable fan or blower introduction of a large volume of air as an incident to the disposing of products of vaporization, and an equally inescapable discharge of a large percentage of such products to pollute the atmosphere. This contrasts with the wholly sealed nature of the system disclosed herein, commencing at the retort and ending at the discharge end of an incinerator or scrubber.

While the equipment, as herein illustrated and later specifically described, involves the use of a vapor-confining retort at least periodically sealed at its top and periodically immersed along with work objects housed therein and to be volatilization-stripped in the salt bath, the invention also contemplates a continued fixed immersion of the retort within the bath, the molten content of the latter desiredly entering the retort through an open retort bottom or some other immersed area. In one respect this fixed arrangement presents an advantage in point of eliminating repeated relative coolings and reheatings of the retort proper.

Similarly, the equipment as here shown and described features a cover which is removable from the retort's top, being hermetically sealed to the latter during the stripping cycle and carrying certain retort-conditioning fittings. However, the cover need not necessarily be removable from the retort, other provisions for removing the work or its container from the retort and for conditioning the retort interior being obviously practical.

In general, the invention essentially involves the separation of organic-base substances from inorganic pieces or substances which are ordinarily of metallic or metallic-base composition. However, use of the invention is not confined to the separation of an organic substance from a three-dimensional object such as a workpiece. Further applications are contemplated, for example, in the recovery, by a volatilizing separation treatment, of a metallic component, whether the metal itself or a salt thereof, from a volatilizable organic part or substance, these organic and inorganic materials being present on or in an object as subjected to such treatment. Likewise, while the invention is shown and described in terms of a batch procedure, continuing operations are contemplated in respect to the voiding of the volatilized product, the conditioning of the retort and/or the placement and removal of the treated object.

It is further to be understood that while the equipment herein described is, in the interest of simplicity, of a very elemental nature, any or all of the usual, and many times necessary, refinements present in industrial stripping equipment may be incorporated. There are, for example, sludge removal means, attendant or operator protection means, bath agitation means and the like. However, it is, by the same token, to be noted that the method of the invention contemplates a shrouded hence quiescent bath, so that the mentioned agitation means will normally not be necessary.

Most importantly, a blower installation for ventilation of the zone immediately external of the volatilization stripping pot is or may be non-existent.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a very schematic perspective view showing the several components of the improved equipment, as disposed in a relatively spread or vertically extended relationship to one another, comparable to that of FIG. 4, the retort cover and article container being shown in dotted line in FIG. 1 as swung to a position at one side for rinsing, container unloading and reloading;

FIG. 2 is a fragmentary view in side elevation, partially broken away and in vertical cross-section on a diameter of the stripping pot and cover-sealed retort, illustrating the latter as fully immersed with the article container while stripping volatilization of the container-held parts proceeds;

FIG. 3 is a view sectioned similarly to FIG. 2, but with the still sealed retort and enclosed container in an elevated position following the volatilizing phase, the several means for elevating the retort and its cover to this vertically extended position also being better shown; and

FIG. 4 is a further similar view showing the retort as returned downwardly into the pot for an ensuing cycle, the cover and article container being left elevated for rinsing, unloading and reloading, as swung to one side in the manner suggested in dotted line in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

The equipment schematically illustrated in FIGS. 1-4, and generally designated by the reference numeral 10, is preferred only in the sense that it represents a typical and simple, if not generic or all-encompassing, illustration of the procedural and structural principles of the invention. Thus, the apparatus 10 comprises an appropriately insulated pot, vat or tank 11 which is shown as cylindrical in cross-section and fixedly mounted upon either a floor or elevated support S. It is open at its top, through which it may receive one or more conventional immersion heaters 12 of either the electrode or internally gas-fired type, although external heating may be employed; and such heater means is conventionally controlled to maintain a salt bath B in the pot 11 at a typical molten temperature of, say, 900° F. However, temperatures as low as 300° F. and as high as 1,300° F. or above are contemplated; the composition and temperature of bath B will, of course, vary in nature with the product and its coating to be stripped, and specifically as such constitute no part of the present invention.

A cylindrical and vertically elongated, imperforate-walled shroud-like retort 13, wholly open at its bottom 13', is mounted coaxially of the pot 11 in a manner to be described, being equipped at its top with an annular bead or flange 14 at which a mating circular and mildly domed cover 15 will have a hermetically sealed connection during the volatilization phase of the method. This seal may be effected through the agency of an annular compressible gasket 15' interposed between the retort flange 14 and an inturned annular flange 15" of cover 15, as when the latter is vacuum drawn downwardly to compress gasket 15'. In the alternative the seal may be effected by knife edge or other known equivalent means. The seal may also be maintained by positive clamping of the cover in place or by gravity alone.

In any case, it is of the essence of the invention that the access of external atmospheric air into the interior of the shrouded retort 13 during the volatilization phase, and preferably for short periods prior and subsequent thereto, be prevented. During the treatment in question, the retort is preferably lowered well down into the bath B, its lower exposed rim edge 13' being but slightly elevated above the floor of vat 11. However, since it is only necessary to obtain a hydrostatic seal at that rim, the immersion, though desirably, need not necessarily be so fully complete, thus enabling a retort of lesser axial length to be used.

For the purpose of thus immersedly lowering the retort 13 and subsequently elevating it and its content, the invention illustratively contemplates the use of a horizontal support arm 16 fixed to and extending radially outwardly of a circular lift and lower collar 17 underlying the retort flange 14. Said arm 16 is fixed at its outer end to the vertically acting plunger of a conventional pneumatic or hydraulic type cylinder 18 having pressure entry and exhaust connections 19 adjacent its upper and lower ends. The cylinder's plunger is designated 20.

Cylinder 18 is illustrated schematically as being received between parallel upright legs of a fixed upright guide frame 21. Such legs are of lesser and greater height, being therefore designated 21' and 21" respectively; and the retort's radial support arm 16 is guided between said legs for up and down movements of retort 13 on the axis of pot 11.

A generally similar, but differently operated, cross-arm or cantilever support is provided for the removable sealing cover 15 of the retort, i.e., one which will permit a suitable lateral swing of the cover from its elevated solid line, retort-centered position of FIG. 1 to the rinsing, unload and reload position appearing in dotted line in that figure. To this end, a radially extending cross arm 22 is schematically suggested as being secured to the top center of cover 15. The outer end of this arm is secured to the top of a vertically elongated guide rod 23 which is mounted for vertically sliding action and limited turn in relation to the legs 21', 21" of guide frame 21, the rotational component of arm 22 being in a horizontal plane above the shorter frame leg 21'. A rod 23 which carries the arm is stably guided for vertical action in an upright supplemental bracket structure 24 which fixedly abuts a side of the retort guide frame 21.

An elongated flexible cable 25 is secured to arm 22 directly above the latter's radially outer connection to rod 23, the cable being trained horizontally outwardly and then downwardly about a pair of pulleys 26 which are journalled on a top and side extension 27 of the taller retort guide leg 21", it being again noted that the shorter comparable leg 21' terminates at an elevation sufficiently beneath the fully raised cover-hanging cross arm 22 to permit the swing of arm 22 indicated in FIG. 1. Cable 25 carries a cover-balancing counterweight 25'.

Referring to FIGS. 2, 3 and 4, a straight or spider-like cross piece 28 is fixedly mounted within retort cover 15 to extend diametrically thereof; and piece 28 fixedly carries a depending hook 29, which serves as a support for the bail of a removable article container 30, shown as being of cylindrical cross-section. The wall of container 30 is plentifully perforated at 31, or otherwise meshed or foraminated to permit free access to and full immersion of articles to be stripped in the portion of bath B which enters retort 13 from the latter's open shroud bottom 13'.

Cover 15 is preferably equipped with a conventional automatic pressure-regulated vent fitting 33 at its top, through which an excessive vapor and/or gas pressure in retort 13 during the volatilization phase may be vented and/or forwarded externally, as through an appropriate line 34. More importantly, cover 15 carries a fitting 35 at which an elongated flexible conduit or hose 36 has an air-tight connection, the hose leading to a schematically shown multi-port valve 37. This hose and valve will optionally connect fitting 35, as through a line 35', to a source of an inert gas such as CO2 or N2 for the low pressure charging of the sealed interior of retort 13. This is in one possible adaptation of the invention mentioned above. In another and preferred embodiment, valve 37 will connect through a suitable conduit 38 to a vacuum pump (not shown) for the evacuation of the retort interior prior to, during and shortly after volatilization, there being by preference appropriate passaging in the valve to break such vacuum after volatilization.

In accordance with the preferred vacuum-based procedure pursuant to the invention, articles to be paint or coating-stripped are either individually suspended from the hook 29 of retort cover 15, or are placed in the container or carrier 30, being similarly hook-suspended in the latter. With the retort 13 in the fully lowered position of FIG. 2, or sufficiently lowered to liquid-seal its bottom rim 13', valve 37 is operated to drop the pressure beneath cover 15 and above the salt bath level, once the cover has been lowered onto the sealing bead or flange 14 of the retort. External atmospheric pressure thus causes the cover to have a hermetic seal atop the retort 13, preventing access of any atmospheric air to the retort interior. Stripping volatilization now occurs for the contemplated limited time interval at the indicated elevated salt bath temperature.

Assuming the evacuation of any significant air content above the salt bath level, as designated L in FIGS. 2, 3 and 4, the level will rise somewhat within retort 13, but in no significant degree sufficient to alter the extent of sub-atmospheric pressurization of the interior; and the volatilization proceeds to termination without a sustained accumulation of any volume of vapor, gas or suspended matter in any degree corresponding to the accumulation inevitable to result in the absence of a seal of the retort against entry of external atmospheric air.

Upon completion of volatilization and stripping, valve 37 may be operated to void the products of volatilization to the incinerator, filter and/or scrubber unit; but by preference the evacuation is continuously maintained, with continuous voiding of the primarily vaporous product of volatilization as it arises, thus diminishing the load on a diminished scale incinerating and/or scrubbing adjunct or system.

The retort 13, with its cover 15 still sealed thereto, is now elevated through the agency of the lift-lower cross arm 16 as powered by fluid pressure cylinder 18, to a position approximately as shown in FIG. 3, yet with the lower open rim 13' of the retort remaining beneath the level of the bath B. The article container 30 is then well above the salt level in pot 11; and at any time in or prior to this phase the retort interior may be de-conditioned through valve 37, as by breaking the vacuum or alternatively discontinuing the inert gas supply.

Retort 13 alone is then restored downwardly to the fully lowered position of FIG. 4, ready for a next cycle of operation. Article carrier or container 30 and its stripped content remain in the counter weighted, cable-elevated position of FIG. 4, to be swung laterally to the dotted line position of FIG. 1 for spray rinsing, emptying and recharging, then return of the loaded cover to coaxiality with and seating on retort 13 as described above.