U.S. patent number 3,834,629 [Application Number 05/281,984] was granted by the patent office on 1974-09-10 for method and means for shaping a stream of melt flowing from a tapping hole.
This patent grant is currently assigned to Stora Kopparbergs Bergslags Aktiebolag. Invention is credited to Per Ingvar Hellman, Jan Ivar Sondell.
United States Patent |
3,834,629 |
Hellman , et al. |
September 10, 1974 |
METHOD AND MEANS FOR SHAPING A STREAM OF MELT FLOWING FROM A
TAPPING HOLE
Abstract
A falling stream of a liquid tends to obtain a circular cross
sectional shape, owing to the surface tension of the liquid. In the
manufacture of metal powder by atomizing a falling stream of molten
metal it is desired, however, that the stream shall have a
flattened cross sectional shape. It has been found useful to make
the molten metal flow through an orifice having a rectangular cross
sectional area, and to make the molten metal follow the short sides
of the orifice for a longer distance than it follows the long sides
of the orifice.
Inventors: |
Hellman; Per Ingvar (Soderfors,
SW), Sondell; Jan Ivar (Soderfors, SW) |
Assignee: |
Stora Kopparbergs Bergslags
Aktiebolag (Falun, SW)
|
Family
ID: |
20292786 |
Appl.
No.: |
05/281,984 |
Filed: |
August 18, 1972 |
Foreign Application Priority Data
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|
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|
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Aug 24, 1971 [SW] |
|
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10748/71 |
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Current U.S.
Class: |
239/594; 239/597;
239/595; 239/601 |
Current CPC
Class: |
B22F
9/082 (20130101); B22D 41/50 (20130101) |
Current International
Class: |
B22D
41/50 (20060101); B22F 9/08 (20060101); B05b
001/04 () |
Field of
Search: |
;239/291,592,593,594,595,597,599,601,602 ;425/6,7,8 ;222/DIG.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wood, Jr.; M. Henson
Assistant Examiner: Love; John J.
Attorney, Agent or Firm: Curtis, Morris & Safford
Claims
What is claimed is:
1. A nozzle stone for a casting ladle or a casting box, having a
substantially rectangular outlet opening of a pair of long and
short edges for a molten stream of metal spread in a plane
extending in the longitudinal direction of the rectangular outlet,
said rectangular opening defined by, inwardly from the opening,
projecting walls of the rectangular outlet, and walls arranged in
pairs opposite each other, said walls forming a pair of short sided
walls and a pair of long sided walls, said long sided walls
defining narrow edges of said rectangle and wherein a projecting
boss at each of the short edges of the rectangular outlet opening
protrude outside the outer edges of the long walls whereby the
bosses provide an extension of the narrower edges of the
rectangular opening.
2. The nozzle stone according to claim 1 and wherein the long sided
walls of the rectangular opening diverge outwardly at the
rectangular opening.
3. The nozzle stone according to claim 1 and wherein the bosses are
convexly round towards each other.
4. The nozzle stone according to claim 1 and wherein the bosses are
convexly round towards each other and their faces divergent in
respect to each other.
Description
The present invention relates to processes for manufacturing or
granulated stock from a melt, in which a stream of molten metal is
atomized or disintegrated by one or more gas or liquid jets of some
suitable atomizing medium which is directed under high pressure
against the molten stream from specially shaped atomizing nozzles
in such a way that the stream is separated into free drops which
are in turn cooled to form the desired powder.
More specifically, the invention relates to a method and means of
shaping the molten stream to be atomized in such an atomization
process. The invention may also be defined as a method of delaying
the contraction to circular cross section to which a stream of melt
is subjected by its own surface tension as soon as it flows out
through the orifice of the gap-like opening, the melt being spread
in the longitudinal direction of the gap but otherwise in a
coherent flow. Since it is possible in the manner characteristic of
the invention to delay contraction of the stream of melt, it is,
therefore, possible to allow the jets of atomizing medium which are
to disintegrate the molten stream to intersect this stream while it
still has the most advantageous shape for the disintegration
process. This has greatly increased the production of powder per
quantity of atomizing medium as well as offering other advantages
which will be dealt with in the following.
The simplest way of performing the invention is to allow the melt
which is to be atomized to flow out through a specially constructed
outlet opening in nozzle stone so that the melt forms a coherent
stream with, at least to start with, a specified cross section. By
a nozzle stone is meant the actual lining around an outlet opening
in a casting box, for example. Since the method according to the
invention is fulfilled using outlet openings of varying
construction, the invention itself covers several special
embodiments for such outlet openings.
When manfacturing powder by atomizing such molten metals and metal
alloys which easily form stable oxides which are difficult to
reduce, it is necessary to use an inert atomizing medium and allow
the atomization to take place in an inert atmosphere. If,
furthermore, the particles must be spherical in shape it is usually
necessary to use a gaseous atomizing medium. In order to completely
avoid oxidation of the powder produced, the inert atomizing gas,
which may be argon, for example, must be extremely pure, i.e. it
must have a very low oxygen content. Such a pure inert gas,
however, is relatively expensive and the costs of the actual powder
production will therefore be greatly dependent on how effectively
the quantity of atomizing gas can be used. It is therefore
important that the quantity of gas used per quantity of atomized
metal is kept as low as possible.
The atomizing gas used can of course be used again if a
recirculation means is introduced in the system but such
recirculated atomizing gas must be cooled down after it has been
removed from the atomizing chamber and then cleaned from any
particles which have accompanied it. It must then be brought up to
the necessary pressure with the help of a compressor to enable the
atomizing nozzles to function. In order to prevent the gas from
becoming contaminated by oil and/or moisture when pressurizing in
the compressor, a special type of compressor must be used, which is
considerably more expensive than conventional compressors. Thus,
even with a recirculation system, it is important for the atomizing
nozzles from which the inert gas is directed towards the molten
stream to be able to operate with as low a gas pressure as possible
and for the quantity of gas necessary per atomized quantity of melt
to be as low as possible so that the smallest possible compressor
can be used without the splitting effect on the molten stream
becoming unsatisfactory.
The amount of gas necessary to atomize or separate a certain
quantity of melt into fine particles which can be cooled to a solid
powder to a great extent, but not entirely, dependent on how the
gas nozzles from which the jets of atomizing gas are directed
towards the molten stream are shaped. A number of different designs
are already known for such nozzles. However, the type which we have
found to be the most suitable is described in our U.S. Patent
application Ser. No. 94,148. In their most simple form the nozzles
described in this application consist of two parallel gaps arranged
one on each side of the molten stream and directed towards the
stream so that it is intersected at an angle of
25.degree.-60.degree. first by a flat, extremely thin gas jet which
causes the melt to change direction and follow the gas jet as a
layer of molten metal spread over this and partly divided into free
drops while a second flat, thin gas jet from a second nozzle
intersects the first gas jet at an angle of 30.degree.-60.degree.
and the layer of molten melt spread out over this gas jet and
accelerates it in a new direction, whereupon the melt is finally
divided into free drops which are cooled to form a solid powder.
The separating effect of the first gas jet on the molten stream
under otherwise identical conditions depends on the thickness of
the molten stream in the direction of movement of the gas jet.
Since the gas jet is a flat jet having certain extension laterally,
the effect in other words will be the same for a tapping stream
having circular cross section and a tapping stream having
rectangular or elliptical cross section and with the thickness (or
small axis) in the direction of movement of the gas jet, if this
axis is equal to the diameter of the circular molten stream,
obviously assuming that the width of the molten stream does not
exceed that of the gas stream. It has also been suggested to
atomize a stream of molten metal having an elliptical cross
sectional area. With an elliptical or rectangular stream of melt it
is thus possible to increase the quantity of atomized melt per time
unit without having to increase the quantity of gas. Similarly, an
unaltered quantity of melt can be atomized in a shorter time and
using less gas.
An elliptical or rectangular cross section of the molten stream can
be obtained if the melt is allowed to flow down into the atomizing
chamber through an outlet opening having the appropriate shape in a
nozzle stone which in turn is arranged in the lining of a casting
ladle or box. However, the surface tension of the melt causes the
molten stream to contract towards circular cross section if the
melt is brought to follow the inner wall of the opening at the
short sides of the opening of the nozzle stone.
In the simplest variant of the invention a nozzle stone is used,
the outlet opening of which towards the orifice has two inner long
walls which are parallel to the direction of flow of the melt or
slightly converging, opposite to each other and extending in the
spreading direction of the melt stream, while the inner short walls
of the outlet opening at least nearest the orifice diverge from
each other, i.e. the opening expands towards the orifice in the
spreading direction of the molten stream. Due to surface tension
conditions, the melt will then follow the diverging oblique short
walls at the short sides of the opening and its speed components
will therefore be directed towards the sides, which means that the
time is extended before the tapping stream noticeably begins to
contract towards the circular cross section.
So that the surface tension conditions will really make is possible
for the melt to follow the diverging oblique short walls of the gap
opening, the angle between these and the vertical should not be too
great. The angle permitted will therefore vary with the surface
tension conditions and viscosity of the melt and is most easily
determined by means of practical experiments.
In another variant of the invention the short sides at the opening
of the nozzle stone are made longer than the long sides so that
these short sides extend outside the long sides. This is most
easily achieved by applying a boss at the opposite short sides of
the tapping opening. In this latter variant no speed component is
achieved towards the side but the melt is guided by the bosses
along the short sides for a longer period and the result will
therefore be substantially the same as with the previous variant.
These bosses may therefore be rounded towards the melt so that a
somewhat more stable stream is obtained.
However, the contraction of the molten stream is most effectively
delayed by a combination of the means described above, i.e. by
applying a pair of bosses outside the orifice of the outlet gap,
forming an extension of the short walls of the gap opening, the
opposite sides, possibly rounded towards the melt, diverging from
each other at an angle which permits the melt to really follow
these diverging sides.
The invention will be described in more detail with reference to
the accompanying drawings, and is defined in the following
claims.
FIG. 1 shows a cross section through a means for atomizing a melt
and cooling the drops of melt obtained to a solid powder, while
FIGS. 2, 5, 8 and 11 shows nozzle stones according to the
invention, seen from the outlet side of the melt. FIGS. 3, 4, 6, 7,
9, 10, 12 and 13 show the longitudinal and transverse section,
designated by the same numbers in Roman numbers as in FIGS. 2, 5, 8
and 11.
The means shown in FIG. 1 consists of a vertical atomizing chamber
1, for example of stainless steel, at the upper end of which is a
casting box 2. This is filled with the molten metal 3 to be
atomized and cooled until it forms a solid powder. The casting box
is provided at the bottom with a nozzle stone 4 having a tapping
opening 5 inside it. The melt 3 flows gradually out of the tapping
opening 5 in the form of a molten stream 6. On each side of the
tapping opening 5 is an atomizing nozzle 7, 8 at the lower side of
the casting box, these having the shape of narrow slit nozzles
which run parallel to each other in a direction perpendicular to
the plane of the figure. Sharp, flat gas jets are directed from
these two nozzles on each side of the molten stream towards the
stream in such a way that a first gas jet 9 from the nozzle 7
intersects the molten stream 6 at an angle of, for example,
45.degree., thus causing the melt to alter direction and follow the
gas jet in the form of a layer spread out over this. A short
distance from this first intersection point the stream is
intersected by a second gas jet 10 from the nozzle 8 which
intersects the molten layer over the gas jet 9 and accelerates the
melt in a new direction so that this finally divided into free
drops 11 which are spread out inside the atomizing chamber and fall
freely through this, being cooled to form a solid powder 12 which
is collected at the lower end of the chamber and fed through a
rotating outlet valve 13. An inert gas, preferably argon, is used
as atomizing gas, this being supplied to the nozzles in compressed
state. The nozzle 7 is supplied with excess gas which is pumped
from the lower part of the atomizing chamber out through a pipe 14
and, when it has passed through a cooler 15, a gas cleaner 16 and a
compressor 17, is supplied to the nozzle at suitable pressure. The
nozzle 8 is supplied with atomizing gas through a tube 18 which
either comes from a similar circulation system as that for the tume
14 or from an external pressure source. The valve 19 on the wall of
the atomizing chamber is used for the supply or removal of gas from
the granulating chamber so that this is permanently filled witn an
inert atmosphere of suitable pressure. If an inert atmosphere is
permitted to flow continuously through the atomizing chamber, the
molten drops obtained can be cooled considerably more easily. The
granulation chamber is provided at the bottom with a watercooled
sheath 20 to which cooling water is supplied through a supply pipe
21 and removed through an outlet pipe 22.
FIG. 2 shows a nozzle stone 23 provided with the theoretically
simplest construction of a gap-like outlet opening for a melt. It
may replace the nozzle stone 4 in FIG. 1 for example. The nozzle
stone 23 is provided with a gap-like opening 24 limited by a pair
of short walls 25 and a pair of long walls 26 arranged opposite to
each other. FIG. 3 shows a section III--III of the nozzle stone 23
while FIG. 4 shows a longitudinal section of the same nozzle stone
along the line IV--IV. As is clear from these two figures, the
outlet opening 24 of the nozzle stone has an upper inlet part 27
which is funnel-shaped, becoming narrower downwardly towards an
outlet part where the opposite pairs of inner walls run parallel to
each other. If a melt is permitted to flow out through an outlet
opening of the type shown in FIGS. 2-4 a flat stream of melt is
formed which, because of the surface tension of the melt, starts to
contract towards a circular cross section immediately outside the
orifice of the outlet opening. The purpose of the inlet part 27 is
to see that the lower part of the outlet opening is kept
permanently filled with melt since it can otherwise only function
as a funnel section which will not even give the tapping stream the
desired shape from the start.
FIG. 5 shows from below a nozzle stone 28 provided with a gap-like
outlet opening 29 which is constructed in accordance with the
present invention. FIG. 6 shows a cross section through the nozzle
stone according to section VI--VI while FIG. 7 shows a longitudinal
section through the same stone along the section VII--VII. As can
be seen from FIGS. 5-7, the outlet opening of the nozzle stone 28
also has a funnel-shaped inlet part 32. From the inlet part the two
long sides 31 of the outlet opening 29 converge slightly towards
each other right up to the orifice of the outlet gap, whereas the
gap walls 30 limiting the short sides of the outlet opening, for
having slightly converged from the funnel-shaped inlet part 32,
start to diverge from each other towards the orifice. This means
that the gap opening expands in its longitudinal direction towards
the orifice and this in turn causes the melt when it flows through
the outlet opening 29 to follow the gap walls along their oblique
edges because of the surface tension conditions and the melt will
therefore acquire a speed component which is directed towards the
sides in the spreading direction of the molten stream which in turn
causes the contraction of the molten stream outside the orifice of
the gap to be considerably delayed.
The slight converging of the funnel-shaped inlet part 32 and the
long sides 31 towards each other right up to the orifice ensures
that the entire outlet opening is kept filled with melt. If the
device according to the present invention is to function it is
necessary for the orifice of the outlet opening, in spite of the
short sides 30 diverging from each other, to have the smallest
cross-sectional area of the outlet opening, or at least a
cross-sectional area which does not exceed the cross-sectional area
in any other section of the outlet opening. Should any other
section than the actual orifice be the narrowest section of the
outlet opening, the quantity of melt flowing out will not be
sufficient to be able to follow the diverging short walls in the
manner which is characteristic of the invention.
FIG. 8 shows a second variant 33 seen from below of a nozzle stone
according to the invention. This nozzle stone is provided with a
gap-like outlet opening 34 limited by long sides 35 and short sides
36. The nozzle stone is also provided on its lower side with two
bosses 37 and 38, the opposite sides of which form extensions of
the short walls 36 of the gap. FIGS. 9 and 10 show transverse and
longitudinal sections IX--IX and X--X, respectively, of the nozzle
stone 33. As can be seen from these figures, the nozzle stone 33
also has an upper funnel-shaped inlet part 39 to the outlet opening
34. The long edges 35 of the outlet opening 34 converge slightly
from the funnel-shaped inlet part and, at least nearest to the
inlet part, the short sides 36 may also converge. However, in this
example they run parallel to each other along the bosses 38 and 38.
Since the bosses 37 and 38 provide a direct continuation of the
short sides 36, the melt will be guided along the short sides for a
longer distance than along the long sides 35. This means that the
contraction to which the melt flowing through the outlet opening 34
is subjected will be considerably delayed in comparison with melt
flowing through a gap opening shaped as shown in FIGS. 2-4. If the
edges of the bosses 7 and 8 facing each other are rounded in the
manner shown in FIGS. 8-10, a more stable stream is obtained than
if these edges had been flat.
FIG. 11 shows from below the nozzle stone according to the
invention which has so far proved to be the most suitable. This
nozzle stone 40 has a gap-like outlet opening 41 limited by inner
longitudinal walls 42 opposite to each other and inner short walls
43 opposite to each other. FIGS. 12 and 13 represent transverse and
longitudinal sections, respectively, through the nozzle stone 40
along the dotted section lines XII--XII and XIII--XIII. As can be
seen from FIGS. 12 and 13, the outlet opening 41 is also provided
with a funnel-shaped inlet part 44. At the short sides 43 of the
funnel-shaped outlet opening 41 on the lower side of the nozzle
stone two bosses 45 and 46 are arranged extending below the bottom
of the nozzle stone in such a way that they provide a continuation
of the short walls 43. As can be seen from FIG. 13, the short walls
43 also diverge from each other along the bosses 45 and 46 at the
orifice of the outlet opening 41. Since the short walls 43 diverge
from each other towards the orifice of the gap and since these
short walls in the bosses 45 and 46 extend below the long walls 42,
the melt will be guided along the short sides of the gap-like
outlet opening for a long way and it will also acquire speed
components in the spreading direction of the molten stream. Both
these factors contribute to a further delay in the contraction of
the molten stream towards circular cross section. As previously
stated, the outlet opening may not be narrower at any section than
it is at the orifice in this case as well.
The nozzle stones shown in FIGS. 5-13 thus cause streams of a melt
spread in a certain direction to retain their spread shape for a
sufficient period of time for them to be disintegrated into fine
drops with the help of flat jets of some suitable atomizing medium,
for example an inert gas. The flat jets are directed in such a way
that their direction of movement intersects the spread stream of
melt substantially straight across its spreading direction. The
nozzle stones shown in FIGS. 2-13 should therefore be fitted in the
device according to FIG. 1 in such a way that the gap-like outlet
openings 24, 29, 34 and 41 extend perpendicular to the plane of
FIG. 1.
The invention is not limited to the examples shown above but can be
varied within the scope of the basic inventive idea.
* * * * *