U.S. patent number 3,740,043 [Application Number 05/173,683] was granted by the patent office on 1973-06-19 for apparatus for vaporizing molten metal.
This patent grant is currently assigned to Republic Steel Corporation. Invention is credited to Frank J. Cole, William A. Reed.
United States Patent |
3,740,043 |
Reed , et al. |
June 19, 1973 |
APPARATUS FOR VAPORIZING MOLTEN METAL
Abstract
Apparatus and method for vaporizing molten metal without
entrainment of liquid droplets includes charging a pool of molten
metal into a chamber in which there is a block having a plurality
of passages. A finger is disposed in each passage of the block, a
space being left between the inner surface of the passage and the
outer surface of the finger to create a shell of fluid metal within
the passage. Heating the shells of fluid metal generates metal
vapor. The metal vapor is passed through a tortuous path and then
directed by a nozzle toward a moving substrate to be coated.
Inventors: |
Reed; William A. (West
Richfield, OH), Cole; Frank J. (Parma, OH) |
Assignee: |
Republic Steel Corporation
(Cleveland, OH)
|
Family
ID: |
26717115 |
Appl.
No.: |
05/173,683 |
Filed: |
August 20, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
40500 |
May 26, 1970 |
3640762 |
|
|
|
Current U.S.
Class: |
266/149; 266/208;
118/726 |
Current CPC
Class: |
C23C
14/243 (20130101) |
Current International
Class: |
C23C
14/24 (20060101); C21c 007/10 () |
Field of
Search: |
;118/48-49.5
;117/106-107.2 ;219/275 ;266/24,34 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kaplan; Morris
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a division of my co-pending application Ser.
No. 40,500, filed May 26, 1970, now U.S. Pat. No. 3,640,762 for
APPARATUS AND METHOD FOR VAPORIZING MOLTEN METAL.
Claims
We claim:
1. Apparatus for vaporizing molten metal, comprising:
a. a chamber for containing a pool of molten metal;
b. a block disposed within said chamber having a plurality of
passages therethrough into which said molten metal may flow;
c. a plurality of fingers disposable in respective passages in said
block leaving spaces between the inner surfaces of the respective
passages and the outer surfaces of the respective fingers to create
a plurality of shells of fluid metal within said respective
passages; and
d. heating means associated with said block for heating said shells
of fluid metal to generate metal vapor.
2. Apparatus according to claim 1, including means for continuously
charging said chamber with said molten metal.
3. Apparatus according to claim 2, wherein said charging means
comprises:
a. a reservoir for containing a supply of molten metal and having
an outlet therefrom communicating with said chamber; and
b. valve means for controlling the flow of molten metal through
said outlet from said reservoir to said chamber.
4. Apparatus according to claim 1, including means for maintaining
said chamber under vacuum to provide for the production of metal
vapor under vacuum.
5. Apparatus according to claim 1, wherein said chamber, said block
and said fingers are fabricated of material not attacked by said
molten metal.
6. Apparatus according to claim 5, wherein said material is
graphite when said molten metal is zinc.
7. Apparatus according to claim 1, including nozzle means extending
from said chamber to direct said metal vapor therefrom.
8. Apparatus according to claim 7, including means defining a
tortuous path through which said metal vapor passes between said
chamber and said nozzle means.
9. Apparatus according to claim 7, wherein said nozzle means
extends horizontally from said chamber.
10. Apparatus according to claim 1, including means for directing
said generated metal vapor against a continuously moving substrate
to be coated.
11. Apparatus according to claim 1, wherein said block and said
fingers are spaced from the bottom of said chamber so that a thin
layer of said molten metal is located at the bottom of said
chamber.
12. Apparatus according to claim 1, wherein said chamber has a
substantially rectangular cross-sectional area of vaporization.
Description
BACKGROUND OF THE INVENTION
This invention relates to vaporizing molten metal, particularly
zinc, while preventing liquid droplets from being entrained in the
vapor.
Vaporized zinc coatings on steel offer many advantages over
conventional electroplated zinc coatings and hot dip galvanized
coatings. A smooth, non-splattered coating can be produced by a
vapor deposition while coating thickness can be varied by
controlling the zinc evaporation rate and/or the speed of a moving
steel substrate. A one-side coating can be easily produced by
properly shielding the strip. Composite coatings, e.g. a thin
aluminum coating over a thin zinc coating, can be produced by vapor
deposition, the steel thus coated having excellent corrosion
resistance. Production speed can be considerably faster for vapor
deposition than for an electroplating line or a hot dip galvanizing
line because the restrictions of current density and coating roll
speed do not apply. There is no inherent limitation on the gage of
steel that might be coated; and is unlike galvanizing lines where
line speed must be lowered as thickness and/or width of the strip
is increased in order to allow for increased heat demand on the
galvanizing pot. Furthermore, the cost of vapor deposition is
believed to be less than the cost of either electroplating or hot
dip galvanizing.
Traditionally, metals have been evaporated from crucibles or
chambers simply by heating the metal to a temperature above its
vaporization point and then removing the vapor from the crucible or
chamber, e.g. by vacuum methods. At a high rate of vaporization,
solid or liquid particles may become entrained in the vapor. It is
not necessary to distinguish between liquid and solid particles as
both have the same deleterious effect when present in the form of
splatter upon a substrate. Several methods of avoiding the splatter
problem have been tried. One involves locating a baffle plate or
other baffle arrangement between the surface of the pool of molten
metal to be evaporated and the area where the vapor is to be
collected. Providing a tortuous path is somewhat effective in
removing large inclusions from the vapor, but it has not been found
effective in completely preventing splatter on the substrate to be
coated.
Another attempt to provide particle-free metal vapor involves
locating within the crucible or chamber a means whereby the vapor
bubbles generated by heating might be obviated or at the least
permitted to escape without bursting at the surface of the liquid.
Elimination of the bubbles in the metal vapor substantially reduces
the opportunity for particles to be retained by the generated
vapor.
Abbott U.S. Pat. No. 1,265,863 discloses forming thin shells of
liquid within a chamber. The thin shells of liquid are heated by
steam contained within the central core of each shell to vaporize
the liquid. The generated vapor is permitted to escape from each
shell through a small annular space formed at the top of each
shell. When an evaporator is constructed of steel or the like, it
makes little difference how heat is transmitted to the thin shells
of molten material. However, when the evaporator is fabricated of a
material such as graphite or silicon carbide in order to negate
reaction with molten metal, it is necessary that heat be
transmitted to the molten metal from as great a contact surface as
possible. In constructing a useful evaporator it must be kept in
mind that the vapor must be completely free of liquid, be generated
at a controllable rate, and be generated at a high rate. Prior art
devices such as shown in Abbott do not provide particle-free metal
vapor at a controllable and high production rate as is necessary
for commercial operation. Thus, to be commercially successful it is
necessary that an evaporator be constructed which provides
liquid-free vapor at a rapid and controllable rate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide apparatus for
vaporizing molten metal without entrainment of particles in the
metal vapor.
It is a further object of the invention to provide apparatus for
coating a substrate with metal vapor at a high rate of production
without liquid inclusions or splatter in the coating. Still another
object of the invention is to provide for the vaporization of
molten metal at a controllable rate.
To these and other ends, the present invention contemplates the use
of metal vaporizing apparatus in which molten metal is continuously
charged into a vacuum chamber. A block having plural passages is
disposed in the chamber and plural fingers are disposed in the
respective passages of the block. As spaces are left between the
inner surfaces of the respective passages and the outer surfaces of
the respective fingers, plural thin shells of fluid metal are
created within the passages. Heating the thin shells of fluid metal
generates metal vapor.
In accordance with the invention a reservoir contains a supply of
molten metal and has an outlet extending from its bottom portion. A
valve is disposed in the outlet and has an aperture extending
therethrough such that upon alignment of the aperture with the
outlet of the reservoir molten metal is permitted to flow through
the aperture to the chamber. This allows continuous charging of the
chamber with molten metal in order that metal vapor may be
generated at a controllable rate. The metal can also be charged
continuously from the outside of the vacuum system, if desired. The
chamber has a substantially rectangular area of vaporization and is
under vacuum so that metal vapor may be produced under vacuum. The
base which forms the chamber is preferably fabricated of a material
such as graphite or the like that is not attacked by the molten
metal.
The block having a plurality of passages therethrough is located
within the chamber in a manner whereby molten metal may flow into
the passages. The block is also spaced from the bottom of the
chamber so that a thin layer of molten metal is located at the
bottom of the chamber during the vaporization process; thus, there
is always a constant supply of molten metal in the chamber for
vaporization. Disposing a plurality of fingers in respective
passages in the block establishes a plurality of shells of fluid
metal within the respective passages. The shells of fluid metal
form in the spaces between the inner surfaces of the respective
passages and the outer surfaces of the respective fingers. Both the
block and the fingers are fabricated of a material such as graphite
that is not attacked by the molten metal.
The shells of fluid metal are heated (by means either external or
internal of the chamber) in order to generate metal vapor.
Preferably, heat is transmitted to the shells through both the
block and the fingers to rapidly vaporize the molten metal. The
metal vapor is then passed through a tortuous path to remove any
particles entrained in the vapor and is then directed horizontally
by a nozzle from the chamber. The nozzle directs the metal vapor
against a vertically moving substrate to form a coating thereon. As
the metal vapor is generated at a rapid and constant rate, a
coating of constant thickness is deposited on the continuously
moving substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional side view of representative apparatus in
accordance with the present invention.
FIG. 2 is an exploded perspective view of the block and fingers of
the present invention.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawings, there is shown a shell 10 in
which metal vaporizing apparatus 12 is disposed. The shell 10 is
fabricated of stainless steel or the like and is maintained under
vacuum so that metal vapor may be produced under vacuum. The shell
10 is placed within a furnace 14 so that the shell and hence the
metal vaporizing apparatus 12 may be heated to a temperature
sufficient to vaporize molten metal. In operation the shell 10
should be capable of withstanding an atmospheric pressure
difference at a temperature of about 1,500 to 1,800.degree.F. It
should be understood that the furnace 14 may be of any suitable
type, such as electrical resistance heating graphite rods, furnace
heater tubes enclosing burning gas or the like. Insulation material
16 is preferably disposed at all openings into the shell 10, while
the shell itself is surrounded by refractory material 17 comprising
the furnace 14.
Metal vaporizing apparatus 12 includes a reservoir 18 which is
adapted to contain a supply of molten metal. The entire metal
vaporizing apparatus 12 including the reservoir 18 is fabricated of
a material that is not attacked (corroded or reactive with) by the
molten metal, e.g. graphite or the like when the molten metal is
zinc. Although it is possible to transfer molten metal directly
into the reservoir 18, especially in a commercial installation, it
is presently preferred to locate an ingot of the metal or metal
granules within the reservoir so that upon heating of the shell 10
the ingot or granules will melt to form fluid metal. Continuous or
batch type feeding of molten metal from outside the shell 10 also
may be employed.
At the base of the reservoir 18 is an outlet 20 through which
molten metal flows naturally. The outlet, however, is blocked by a
valve 22 which allows or prevents molten metal from leaving the
reservoir 18. Normally, the valve 22 is in closed position until
the metal ingot or granules within the reservoir 18 has melted and
until the temperature of the metal vaporizing apparatus 12 has
reached the desired level. The valve 22 is variably controllable
and may be of the type disclosed in Iezzi et al. Application Ser.
No. 778,609, filed Nov. 25, 1968, now U.S. Pat. No. 3,559,662,
entitled "Graphite Variable Control Valve," and assigned to the
assignee of the present application. Thus, the valve 22 comprises a
valve body 24, an operator structure (not shown) which may be
either manual or automatic, and an aperture 26 in the valve body
alignable with the outlet 20 upon movement of the valve body 24 by
the operator structure. In FIG. 1, the valve 22 is shown in open
position. As the valve 22 is also fabricated of a non-attacked
material, e.g. graphite when the molten metal is zinc, a
liquid-to-liquid seal may be achieved about the valve body 24 so
that there will be no liquid leakage through the various clearances
about the valve components.
When the aperture 26 is aligned with the outlet 20, molten metal
may flow from the reservoir 18 through the outlet 20 and the
aperture 26 into a chamber 28 adapted to contain a pool of molten
metal. The chamber 28 may be continuously charged with molten metal
from the reservoir 18 so that a continuous supply of metal vapor
may subsequently be generated. For convenience, both the reservoir
18 and the chamber 28 may be formed in a crucible 29. This prevents
vapor and liquid leakage. The crucible 29 which forms the chamber
28 is fabricated of material not attacked by the molten metal, e.g.
graphite or the like when the molten metal is zinc. Graphite has
been found to be a suitable material for fabricating the crucible
29 as well as the other elements of metal vaporizing apparatus 12
as it does not react with molten zinc, is easily fabricated,
maintains its strength at high temperatures, and is efficient in
conducting heat from the furnace 14 to the molten zinc to be
vaporized.
The chamber 28 is substantially rectangular in cross-section. It is
believed that a chamber 28 having a rectangular cross-section is
more efficient than one having a circular cross-section. A more
consistent and controllable evaporation rate can be more readily
achieved in a horizontal rectangular cross-section evaporator than
in a horizontal cylindrical evaporator where the evaporator-molten
metal interface area and the liquid-vapor interface area vary
significantly with the amount of zinc in the evaporator. The
evaporation rate is to a considerable degree dependent on these two
interface areas.
Disposable within and substantially filling the chamber 28 is a
block 30, as best seen in FIG. 2. The block 30 is cubic in shape
and has a number of cylindrical passages 32 therethrough between
the top and bottom thereof into which molten metal may flow when
the block 30 is located within the chamber 28. Of course, the block
30 may be cylindrical or any other suitable shape where necessary
to fit within the chamber 28. Preferably, the passages 32 are
equally spaced about the block 30 in a geometric pattern. Again,
the block 30 is fabricated of a material not attacked by the molten
metal, such as graphite when the molten metal is zinc. Legs 34 are
formed at the bottom of the block 30 so that the block may rest
within the chamber 28 with the major portion of the bottom of the
block 30 being slightly spaced from the bottom of the chamber 28.
This allows a thin layer of molten metal to be present on the
bottom of the chamber 28 so that a continuous local supply of
molten metal is available to enter into the passages 32 in the
block 30. It should be understood that the block 30 may be integral
with the crucible 29, but in order to practically fabricate the
apparatus, it is desirable that the chamber 28 first be formed in
the crucible 29 and then a separately formed block 30 be disposed
therein.
Within each of the passages 32 in the block 30 is disposed a
cylindrical finger 36. Each finger is slightly smaller in
cross-section than the respective passage 32. Thus, when a finger
36 is located in a passage 32, a concentric space 38 (FIG. 1) is
formed between the inner surface 32a of the passage 32 and the
outer surface 36a of the finger 36. Fluid metal enters the
concentric space 38 from the bottom of the chamber 28 to form a
shell of fluid metal within the passage. Preferably, the ends of
the fingers 36 do not reach to the bottom of the chamber 28 so that
the thin layer of molten metal thereat will be undisturbed. The
plurality of fingers 36 suspended in the molten metal in the
respective passages 32 increase the surface area in contact with
the molten metal, which is necessary for efficient vaporization
when the apparatus is fabricated of graphite. When the shell 10 is
heated to a temperature of about 1,700.degree.F., the temperature
of the block 30 and the fingers 36 will be only about 1,500.degree.
F.
The fingers 36 are equally spaced on and extend from a plate 40 in
a geometric pattern corresponding to the pattern of the passages 32
in the block 30 (see FIG. 2). The plate 40 also serves as a cover
for the crucible 29 in which the chamber 28 and the reservoir 18
are formed. A protruding section 42 at the bottom of the plate 40
fits within the chamber 28 such that an annular passage 44 (FIG. 1)
is left between the perimeter of the protruding section 42 and the
inner wall of the chamber 28. Through this passage 44 generated
metal vapor may pass, as explained below. The protruding section 42
at the bottom of the plate 40 does not contact the upper surface of
the block 30 so that a gap 46 is left between the protruding
section 42 and the block 30. Generated metal vapor may pass through
the gap 46, as explained in more detail below. As noted above, both
the fingers 36 and the plate 40 from which the fingers extend are
fabricated of graphite or the like which is not attacked by molten
zinc.
Forming a plurality of relatively thin shells of fluid metal in the
concentric spaces 38 has definite advantages in vaporizing the
molten metal. First, the thinness of the shells either reduces the
size of vapor bubbles which form in the molten metal upon heating
or obviates them altogether. Eliminating bubbles from the vapor
consequently reduces the liquid inclusions or particles in the
vapor. Second, by forming relatively thin shells of fluid, a
relatively large surface area is provided for the application of
heat to those volumes of fluid, which increases the efficiency of
the transfer of heat for vaporization. The efficiency of heat
transfer is also increased by the high velocity of bubble or fluid
movement through the thin shells of fluid. Thus, vaporization of
the molten metal may occur at a high rate so that the apparatus may
be used for commercial coating of continuously moving strip
material or the like.
But is is also within the scope of this invention to make block 30
integral and continuous with crucible 29 so that liquid zinc is
forced to percolate to the upper edges of block 30 from one passage
38 (which now becomes a hole) to communicate to another passage 38.
In this manner it is possible to have substantially total vapor in
the region of the most remote passage 38, free of liquid zinc due
to this progressive screening effect.
The vaporization rate of molten metal depends primarily on four
factors. First, the most critical, is the temperature of the
evaporator, as the vapor pressure of the metal depends on
temperature. The second and third factors are the liquid-vapor
interface area and the liquid-evaporator contact area. Generally,
the higher the temperature of the vaporizing apparatus 12, the
higher the vaporization rate. However, although the vaporization
rate remains constant for a period of time, it tends to decay when
the amount of molten metal in the chamber 28 begins to fall, even
though the temperature of the vaporizing apparatus 12 reaches an
extremely high level, i.e. about 1,800.degree. F., since the second
and third factors are now influencing the evaporation rate. The
fourth factor affecting the vaporization rate is the provision of
adequate vapor transport from the vaporizing apparatus 12. If
excessive pressure drops exist in the vapor stream, causing the
pressure to rise in the evaporator, the temperature of the molten
metal will rise and the evaporation rate will drop.
After the fluid metal has been heated to generate metal vapor, the
metal vapor accumulates in the gap 46 between the protruding
section 42 of the plate 40 and the top of the block 30. The metal
vapor passes from the gap 46 through an aperture 48 in the crucible
29 into a trap 50, which also may be formed in the crucible. The
gap 46, the aperture 48 and the trap 50 form a tortuous path
wherein any splatter entrained in the metal vapor is removed
therefrom. The entrained splatter removed from the metal vapor
collects at the base of the trap 50 from which it may later be
removed.
A nozzle 52 transfers the metal vapor from the tortuous path in a
direction horizontally from the chamber 28. The nozzle 52 may be
fabricated of the same material as the other elements of the metal
vaporizing apparatus 12, e.g. graphite or the like, and must be
sufficient in size to permit ready transport of the vapor from the
chamber 28. The nozzle 52 directs the metal vapor against a
substrate (not shown) continuously moving transversely thereto so
that the vapor condenses to form a coating thereon. The shell 10
preferably encases the entire metal vaporizing apparatus 12
including the nozzle 52 so that no pressure leakage will occur in
the system. As the metal vapor is generated at a rapid and constant
rate, a coating of substantially equal thickness may be deposited
about the surface area of the continuously moving substrate.
In operation of the apparatus of the present invention, a supply of
molten metal, e.g. zinc, is disposed in the reservoir 18. At the
same time the block 30 is inserted in the chamber 28 of the
crucible 29 and the plate 40 containing the fingers 36 is secured
in place to the crucible. The entire vaporizing apparatus 12
including the nozzle 52 is located inside the stainless steel shell
10. The valve 22 is placed in the closed position, i.e. where
aperture 26 is not in alignment with the outlet 20 from the
reservoir 18. The shell 10 is then evacuated to a low pressure and
heated by the furnace 14. A pressure of less than 30 microns is
necessary to produce a bright coating on the substrate. Heating
results in the metal in the reservoir 18 reaching a temperature of
approximately 1,150.degree. to 1,200.degree.F. This temperature is
sufficient to melt zinc ingot or granules in the reservoir. The
valve 22 is then fully opened so that molten zinc will be permitted
to flow into the chamber 28, the molten metal rising within the
concentric spaces 38 surrounding the respective fingers 36 to form
thin shells of fluid metal. The thin shells of fluid metal are
heated by means of the furnace 14, which may be of the natural gas
or electrical resistance type, to form metal vapor. Internal
heating may also be employed. In such a case the walls of the
vacuum shell 10 would be "cold", and the crucible 29 would be
heated by gas-fired tubes or electric resistance elements, e.g.
Passing through the tortuous path defined by the gap 46, the
aperture 48 and the trap 50, the metal vapor is directed by the
nozzle 52 away from the chamber 28. The vapor condenses upon a
substrate, e.g. copper-plated steel strip, which moves transversely
to the end of the nozzle 52. It is also possible to divert some of
the metal vapor into a collecting box (not shown) rather than all
of the vapor onto the moving strip when a thinner coating is
desired.
EXAMPLES
In one particularly successful operation of representative
apparatus used in the present invention, the shell 10 was placed
horizontally in a furnace 14. The shell was evacuated to a pressure
of about one micron and heated to a temperature of 1,750.degree.F.
A zinc ingot weighing about 13 lbs. was disposed in the reservoir
18 to be melted. The zinc in the reservoir 18 reached a temperature
of 1,100.degree.F. which was sufficient to liquefy the zinc. Before
the liquid zinc was introduced into the chamber 28 through the
valve 22, the temperature of the chamber was 1,710.degree.F. The
steady-state temperature of the chamber 28 after the liquid zinc
entered the chamber was 1,680.degree. F. Each of the passages 32
was 1.0 in. in diameter while each of the fingers 36 was 0.75 in.
in diameter so that the thickness of each thin shell of fluid metal
was 0.125 in. The molten zinc was vaporized and the metal vapor
produced upon continued heating of the molten zinc in the chamber
28 was passed through the tortuous path and directed through a
one-inch nozzle onto a moving steel strip. The strip had previously
been prepared by cleaning in an alkali cleaner and pickling in a 5
percent hydrochloric acid solution; the strip, which was 0.015 in.
thick, was then electroplated with a copper coating about 5 to 7
microinches thick to promote adherence of the zinc vapor. As the
strip prior to coating was maintained at a temperature of only
about 70.degree. to 110.degree. F., the zinc vapor condensed on the
cool substrate rather than being dissipated into the warmer
environment. During vapor condensation, the temperature of the
strip reached 450.degree. - 600.degree. F. About 500 feet of 6 inch
wide strip were coated to a thickness of about 0.5 to 1.0 mils at a
rate of up to 75 ft. per minute. The average condensation rate for
the run was 0.55 lbs. zinc per minute while the maximum
condensation rate was found to be 0.74 lbs. per minute or 2.24 lbs.
per minute per square foot of evaporating area. Upon visual
examination for coating appearance it was determined that little or
no splatter occurred upon the substrate while the coating
appearance was excellent.
The shell temperature should be maintained at no more than about
1,750.degree. F. to avoid design problems in the shell 10 and
furnace 14. Thus, in another operation using representative
apparatus, the shell 10 was evacuated to a pressure of about one
micron and maintained at a temperature of about 1,700 F. The
temperature of the chamber 28 before introduction of 13 lb. of
molten zinc was 1,620.degree. F. and the temperature of the chamber
after introduction of the zinc was 1,520.degree. F. The average
condensation rate upon a steel strip of the type used in the first
example at these temperatures was 0.45 lbs. zinc per minute while
the maximum condensation rate was 0.62 lbs. per minute. The shell
10 was maintained within the maximum operating temperature. Little
or no splatter was present in the substrate coating and the coating
appearance was excellent.
Thus, the present invention provides for vaporization of molten
metal, particularly zinc, without entrainment of particles in the
metal vapor. The invention further provides for coating of a
substrate, particularly copper-plated steel strip, with a metal
vapor at a high rate of production without the entrainment of
liquid particles or the inclusion of splatter in the coating.
Furthermore, the invention allows for vaporization of molten metal
at a controllable rate so that a coating of equal thickness may be
deposited on a continuously moving substrate.
* * * * *