US3447951A

US3447951A - Cyclone separation of particles in vapor coating

Info

Publication number: US3447951A
Application number: US498301A
Authority: US
Inventors: Lennie L Ashburn
Original assignee: Pennsalt Chemical Corp
Current assignee: Pennwalt Corp
Priority date: 1965-10-20
Filing date: 1965-10-20
Publication date: 1969-06-03
Anticipated expiration: 1986-06-03

Description

.. L. ASHBURN 3,447,95l

CYCLONE SEPARATION OF FARTICLES IN VAPOR COATING June 3, 1969 Filed Oct. 20, 1965 INVENTOR.

LENNE L. SHBURN ATTORNEY United States Patent O 3,447,951 CYCLONE SEPARATION OF PARTICLES IN VAPOR COATING Lenne L. Ashburn, Rockledge, Pa., assignor to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 20, 1965, Ser. No. 498,301 Int. Cl. C23c 13/02; B44d 1/02 U.S. Cl. 117-101 17 Claims ABSTRACT OF THE DISCLOSURE Apparatus and method for vacuum coating of metal in which metal vapor evaporated from a bath is exposed to contrifugation, such as by a cyclone classifier, to separate the vapor into a stream of heavy metal particles, which had been entrained during evaporation, and a stream of clarified vapor. The clarified vapor is drawn off and deposited on an article in the vacuum chamber while the heavy metal particles are returned to the bath whereby the rate of vaporization may be increased without splattering.

This invention relates to continuous vacuum coating of metallic or non-metallic substrates. The invention is concerned particularly with means to apply the coating evenly to the substrate at high evaporation rates.

In the prior art it is well known to apply a coat to a strip of material such as paper, plastic or metal by directing metal vapors in a vacuum chamber at the surfaces of the substrate. Often in the past this has been done by having disposed within the chamber a roll of the uncoated strip material and a Wind-up roll. The chamber is evacuated and the strip is transferred from the full roll to the empty roll. In the transfer the strip is passed over a source of metal vapor. The vapor forms the desired coating on the surface of the sheet. For coatings on both sides the process has simply been reversed exposing the opposite face of the material on the rewinding of the strip, or the strip has been zig-Zaggged on a single pass so that the opposite surface has been exposed.

To increase the rate at which the vaporized metal may be applied to the running strip, a simple expedient has been to increase the rate of vaporization of the metal. While this has increased the rate at which the coating can be applied, it has decreased the quality of the end product, 'because particles of the evaporant are projected into the vapor stream and are deposited on the substrate as small but discrete particles, having not gone into the vapor phase. The deposition of these particles onto very thin strips (0.0002") has, because the particles are quite large and have a high heat content, caused pin-holing of the substrate. For a considerable time the metallizing industry has been searching for means to apply coatings entirely in the gaseous state without the particle entrainment. The present invention solves the problem successfully, It is especially useful in the application of a coating on a metal strip such as, for example, in the making of a zinc-coated steel product.

In accordance with the invention the evaporant is passed through a centrfugal classifier so that particles, being heavier than the vapor, are separated out and the rest of the vapor then passes onto the coating site. The particles may be returned to the source of vapor until they themselves are vaporized.

It is, therefore, a feature of this invention to provide means for avoiding the pin-hole and other defect condition of a substrate in a vacuum coating operation.

Other features of the invention will be obvous to those skilled in the art after reading the following specification including the attached drawings wheren a preferred embodiment of the invention is disclosed. In the drawngs:

FIGURE 1 represents a partly schematic flow diagram and apparatus embodying the invention;

FIGURE 2 represents a sectional View taken on the line 2-2 of FIGURE 1; and

FIGURE 3 represents a modified portion of FIGURE 1 showing the trap for particles.

Briefiy, then, the invention involves the centrifugal separaton of particles of the coating material from the vapor of the coating material in a vacuum coating operation.

Referring more specifically to the drawings, an apparatus embodying the invention is generally designated 10 in FIGURE 1. It comprises a standard crucible 12 heated by gas firing or other means. In the crucible the material to be vaporized is reduced to the molten state from a solid. In an embodiment in which the coating is to be of Zinc, the temperature of the liquid in the crucible 12 might be in the area of about 850 F.

A vacuum tank 14 is provided and it contains a graphite vaporizing pot or boiler 16. At its lower end the pot is connected by a tube 18 to a submerged location in the crucible 12. The tube may provide a continuous supply to the boiler 16. An appropriate seal 20 is provided where the feed tube 18 penetrates the wall of the vacuum tank 14.

A heater 22 may be disposed along the line 18 to raise the temperature of the molten material as it moves up into the pot 16. In operation on Zinc metallizing the heater may heat the line 18 to a temperature roughly of 900 F. Of primary importance for obvious reasons is that the molten material not be allowed to cool and solidify in the tube 18. The entire tube 18 may be of corrosion resistant alloy such as molybdenum, 30% tungsten, or alternatively the tube may be of low carbon silicon steel depending on the evaporant being processed.

It should be understood at this point that the molten material is elevated through the tube 18 from the crucible 12 and into the pot 16 by the pressure difference between atmospheric pressure working on the surface of the materal in the crucible 12 and the vacuum in the vacuum tank 14. Whereas atmospheric pressure is approximately 14.7 lbs. per square inch, pressure within the tank 14 may be on the order of 1 to mm. Hg.

About and above the pot 16 the walls of the tank 14 comprise a castable aluminum oxide or other refractory material 24. This inner wall 24 is surrounded by electric induction heating coils 26 which supply appropriate heat energy to drive the temperature of the molten metal in the pot 16 up to vaporizing temperature and to superheat the graphite or metal cyclone above the znc vapor temperature to prevent condensation. As an example the temperature of molten zinc may be 1350-1400 F. Shunts 26a are provided outside the coils 26 The inner wall 24 is closed at its upper end by a pair of appropriately shaped refractory annuli 27. These annuli as shown support a cyclone Separator 28 which may be of graphite or metal composition. The transition tube 30 comprising the upper opening of the cyclone extends through the wall of the tank 14 in a penetration that is sealed by a seal 32 similar to the sea] 20 and not comprising part of this invention. The transition tube is then connected to a tube 34 which delivers the purified vapor through an appropriate seal 36 to a nozzle 38 in the vacuum chamber 40. The tube 34 may be provided with a heater 42 to further elevate the temperature of the vapor up to, for instance, 1450 F. in the coating with Zinc, and to keep the vapor from condensing on the walls of the tube. A valve 44 may be provided in the vapor line and can be used to control the pressure in the tank 14. Nozzle heating means 46 may be provided within the vacuum coating charnber and directed at the underside of the strips.

It should be understood that the vacuum in the tank 14 is not so great as in the vacuum chamber 40, but that the pressure in the tank is substantially below atmospheric. As an example the pressure in the tank may be in the range of 1 to 100 mm. Hg, while the pressure in the chamber 40 may be in the range of 1 1O- to 1 10 mm. Hg. Means to provide these pressures are not shown.

FIGURE 2 indicates that the tangential opening 48 of the cyclone 2-8 is exposed to the inside of the chamber defined by the walls 24. Thus the vapor rising from the surface of the molten material in pot 16 is free to enter the tangential opening 28 due to the flow induced by the lower pressure in the vacuum chamber 40 which communicates with transition tube 30. Vaporized material, together with the unvaporized particles, enter the cyclone through the opening 48 and swirl around the wall of the cyclone in conventional cyclone manner. The particles being heavier than the vapor stay adjacent the wall on the inside of the cyclone and eventually settle and return to the pool in the pot 16 through central discharge opening 50 at the lower end of the cyclone. Vapor, being lighter, is displaced inward toward the center of the cyclone and moves upwardly to the transition tube 30 and into the pipe 34. It should be noted that the opening 50 should be small Compared to the surface area of the pool.

FIGURE 3 shows a modified form of lower end of the cyclone 28' which comprises trap 52. 'Ihe trap is provided with upwardly slanted discharge passages 54 for the molten coating material. Since the passages 54 angle upward from the lower end of the trap and the molten coating material discharges from the passages at a point above the inlet, there is constantly maintained a liquid layer in the trap precluding the passage of vapor through the passages 54. In this embodiment the lower end of the cyclone is preferably disposed above the surface of the molten material in the pot 16.

By virtue of the arrangement disclosed superior results are obtained in the vacuum coating of strips. At high vaporization rates the unvaporized metal particles above the pot 16 discharge back into the pot 16 through the lower opening of the cyclone. It should be understood that while the specification has used the disposition of Zinc on a steel substrate as an example, the invention has other uses. Generally the invention will find its most advantageous uses with the more highly volatile coating materials including such metals as bismuth, calcium, lead, lithium, selenium, strontium and Zinc and non-metals as fluorocarbons and other -plastics and some of the elastomers.

I claim:

1. The process of vacuum coating an article with metal comprising the steps of vaporizing a portion of the batch of the metal in a first zone, passing the vapor through a zone of centrfugation spaced apart a'nd distinct from said first zone and exposing the vapor in the zone of centrifugation to cyclone classification treatment to separate out any unvaporized metal particles, passing the clarified vapor from the zone of centrifugation to a vacuum chamber and depositing said clarified vapor on the article therein, and returning the unvaporized metal particles to the batch of metal whereby the rate of vaporization can be increased without splattering.

2. The process of claim 1 wherein the zone of centrifugation is heated.

3. The process of claim 1 wherein the batch of metal .4 is drawn into the first zone from a heated molten metal bath.

4. A process for vacuum coating an article at high rates of evaporation without spattering comprising the steps of evaporating metal from a bath thereof so as to produce a vapor with relatively heavier metal particles entrained therein, swirling the evaporated metal in a zone spaced apart and distinct from the bath and by cyclone classification in said zone separating the evaporated metal into a stream of heavier metal particles which had become entrained during vaporizing and a stream of clarified vapor, and exposing the article under vacuum only to the stream of clarified vapor so that uniform deposition of the coating is elfected.

5. The process of claim 4 wherein the stream of heavier metal particles is returned to the bath.

6. The process of claim 5 wherein the zone of separation is exposed to heat.

7. A Vacuum coating apparatus comprising a vacuum coating chamber, a source holding molten metal, the surface of which source is open to the atmosphere, conduit means connecting the source of molten metal and the vacuum coating chamber, a heated vaporizing chamber disposed intermediate the ends of the conduit means, the vaporizing chamber including a heated vaporizing pot, a cyclone stationed above the pot having a central upper opening and a lower opening and a tangential opening, the tangential opening being exposed to the inside of the chamber, the central upper opening being connected to the conduit through which it communicates to the vacuum coating charnber, the lower central opening permitting drain to the pot, and means in the vacuum chamber and attached to the conduit means for directing the vapor toward the object to be vacuum coated.

8. The apparatus as described in claim 7 wherein the conduit means includes valve means between the vaporizing chamber and the vacuum coating chamber.

9. The metallizing apparatus as described in claim 7 wherein the lower opening of the cyclone comprises an upward discharge passage portion to constitute a liquid trap precluding passage of vapor therethrough.

10. Vacuum coating apparatus comprising a first chamber, stationary evaporation means for vaporizing metal in said first charnber, a vacuum chamber in communication with said first chamber for holding an article to be coated, classifying means constituting a cyclone classifier intermediate said evaporating means and said vacuum chamber for separating the evaporated metal into a stream of metal particles previously entrained therein and a clarified vapor stream, and means for exposing the article only to the clarified vapor stream whereby high rates of evaporation can be achieved without spattering.

11. The apparatus of claim 10 including means to melt the metal outside of said first chamber, conduit means connecting said means to melt with said evaporation means, and means for evacuating said vacuum chamber whereby charging of metal into the first chamber may be accomplished without breaking the vacuum.

12. The apparatus of claim 10 including means to return the stream of separated metal particles back to said evaporating means.

13. The apparatus of claim 10 wherein said cyclone classifier includes a conical tube having a lower end spaced above the metal in said evaporation means, and a trap at said lower end comprising a cup member having upwardly slanted radial passages therein for collecting the stream of heavy metal particles and returning molteu metal to said evaporation means while precluding vaporized metal from being transmitted through the passages.

14. The apparatus of claim 10 including means to heat said first chamber to prevent deposition of vaporized metal therein.

15. The apparatus of claim 14 wherein said first clan ber is inductively heated.

16. The apparatus of claim 14 wherein said means to 5 6 beat comprses an inducton col surrounding said fir st OTHER REFERENCES cha-mbar and further Supplying thermal energy to Sald Weed, IBM Technical Dsclosure Bulletin, Vol. 2, NO. 3, evaporaton means. 195 2 28 d 17. The apparatus of claim 16 wherein the walls of said October PP 7 and Te le upon first Chamber are graphlte- 5 RALPH S. KENDALL, P''mary Exam'ner.

References Cted A. GOLIAN, Assistant Exam'ner.

UNITED STATES PATENTS U.S. CI. X.R. 2,439,983 4/ 1948 Morgan et al 117-107 X 3,373,050 3/1968 Paul et al. 117-106 10 113-49