Note: Descriptions are shown in the official language in which they were submitted.
~ 21 92750
METHQD AND INSTALLATION FOR CONFINEMENT OF A LIOUID
MASS BY A GAS LAYER
DESCRIPTION
The invention relates to a method and an
installation for confinement of a liquid mass by a gas
layer.
The need may arise to maintain a liquid in a
container such as a mould without allowing it to touch
the walls of such container. Numerous liquids may react
with the container material, in particular many metal
alloys, glasses, crystals and semiconductors. Contact
with the container may deteriorate the chemical
composition of the product, or the state of the product
obtained after solidification or nucleation during this
solidification.
This has led to the idea of using various means
to prevent contact between the container and the liquid
mass, but the so-called gas layer confinement methods
are particularly suitable under earth gravity and for
liquids. For this method the container is fitted with a
porous wall through which the gas is injected to push
back the liquid and flow over the length of the wall.
An important application of these confinement
methods, whose effect is to confine the liquid in the
centre of the content of the container, is described in
French Patent N 89 09123 and concerns the continuous
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manufacture of an ingot in the form of a bar: the
liquid is poured into the mould cavity whose bottom end
is made of a valve that is gradually lowered as the
liquid is added, leaving the lower part of the ingot to
solidify. It is unfortunately observed that it is
impossible to produce thin bars since possible height
is limited to approximately four times the diameter in
existing installations: thereafter, excessive shape
instability becomes apparent that can be seen in the
form of curving of the meniscus formed on the upper
surface of the bar; it ultimately causes contact of the
mass liquid with the container wall by overcoming the
gas thrust.
The invention arises through the desire to
remedy this height insufficiency of the confined liquid
mass, and through the observation that the, essentially
dynamic, pressure produced by the flow of gas between
the liquid mass and the porous wall, towards the ends
of the cavity, is insufficient and that the application
of higher absolute static pressure, of at least two
bars, in the cavity would give much better results.
The characteristic installation of the
invention is therefore characterised in that an
upstream enclosure, partially bordered by the porous
wall and into which the gas is first insufflated before
crossing through this wall to support the liquid, is
completed by a downstream enclosure surrounding the
mould or confinement cavity of the liquid so that the
pressure therein may reach the req~ired value. Means of
pressure adjustment are added so that the difference in
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21 92750
pressure between the enclosure and the cavity is also
maintained at an acceptable value (from 40 to
300 mbar); it has already been noted that a difference
in pressure that is too great to shape instabilities of
the liquid mass.
For a more concrete description of the
invention, the following Figures shall be commented
upon, which are appended for illustrative purposes and
are in now way restrictive:
- Figure 1 is a schematic view of the
characteristics of the installations of the
prior art,
- Figure 2 is a view of an installation of
the invention,
- Figure 3 is a diagram of the gas supply
system,
- Figure 4 is another diagram of the gas
supply system, and
- Figure 5 sets forth experimental results.
Figure 1 represents a mould 1 whose wall
contains a ring-shaped enclosure 2 whose inner surface
forms a porous wall 3 also ring-shaped, which surrounds
a cavity 4 in which a bar 5 whose moulded part is
liquid is contained. Gas arriving from source 6 and a
duct 7 is insufflated into enclosure 2, crosses through
porous wall 3, pushes away from it the liquid of bar 5
and flows on forming a gas layer 8 along the length of
porous wall 3 towards the ends of cavity 4. Bar 5 is
placed on a platform 9 that is lowered as the liquid
solidifies. A bar with regular section and increasing
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length is thus obtained, but, as previously mentioned,
the height of the liquid portion cannot exceed
approximately four times its diameter.
A mould 10 of the invention, represented in
Figure 2, comprises in particular a porous wall 11
separating a cavity 12 from an upstream enclosure 13
which are closely similar to those of the prior art; a
bar 14, placed on a support (here simply a sliding rod
15) is again gradually moulded. But cavity 12 is closed
at its two ends; porous wall 11 finishes at its upper
end by a stopper 16 on top of which is fixed a closed
relay box 17 which may be used to observe the moulding
process by means of instruments which are passed
through chamber 18 which it forms. Also, the top of
relay box 17, in the extension of cavity 12, is
designed as a porthole 19 beyond which is a control
camera 20; a semi-reflective mirror 21 lies ahead of
camera 20 and its oblique positioning allows the inside
of cavity 12 and the upper meniscus 22 of bar 14 to be
lit up by radiation from a source of light 23 reflected
by mirror 21 along the observation axis of camera 20,
through chamber 18, the upper stopper 16 (which is
pierced) and cavity 12.
The opposite end of porous wall 11 is extended
by a lower stopper 24 fitted with a guide bore 25 for
bar 14. Cavity 12 thus forms an insulated downstream
enclosure.
Upstream enclosure 13 is bordered chiefly, in
addition to porous wall 11, by qu~artz sleeve 26 which
seals stoppers 16 and 24. It is surrounded by high
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~_ 5
frequency heating turns 27 at the top of bar 14 to make
or maintain it liquid at this level. Stoppers 16 and 24
are cooled by canals 28 and 29 extending inside them
and along which runs a cooling liquid arriving from
circuit 30. A thermocouple 31 stretches across lower
stopper 24 and its end is housed in the bottom end of
porous wall 11 to check the proper functioning of the
process.
Finally there are several ducts intended for
gas entry, namely a supply duct 32 which arrives in
upstream enclosure 13, a first measuring duct 33 also
arriving in upstream enclosure 13, and finally a gas
release duct 34 and a second measuring duct 35, both
crossing through upper stopper 16 and connecting with
cavity 12.
These ducts (with reference to Figure 3) are
connected to a gas supply under pressure, here
consisting of a bottle 36 of compressed argon, on whose
exit duct 37 is connected the end of supply duct 32,
whereas the end of exit duct 37 leads to gas release
duct 34. An absolute pressure manometer 38 is connected
to the second measuring duct 35, whereas a differential
pressure manometer 39 is connected to the two measuring
ducts 33 and 35; absolute manometer 38 commands a
control valve 40 placed on exit duct 37, upstream from
supply duct 32, to adjust the gas supply, whereas the
differential manometer commands another control valve
41, installed on supply duct 32 which restricts access
to upstream enclosure 13. Therefo~e, control valve 40
allows the two enclosures to be maintained at over-
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,
pressure in relation to the atmosphere, by imposing thepressure value in gas release duct 34 and the
downstream enclosure, and the other control valve 41
allows adjustment of the over-pressure produced in
upstream enclosure 13 in relation to the downstream
enclosure. A manually operated valve 42 is installed at
the end of gas release duct 34 to produce the required
outlet flow at the duct end. This flow, which may be
found experimentally, may be checked by installing flow
meters (given the common reference D) on various parts
of the gas circuit. Valves (non referenced) may also be
installed on the ducts to disconnect them or, on the
contrary to connect the enclosures if required; closing
of the circuit is particularly helpful at a preliminary
stage of the process when a vacuum is made by means of
a vacuum pump 43 connected by a suction pipe 44 to one
of the enclosures.
A diversion 45 may be set up between the ends
of the downstream enclosure, either side of bar 14, to
produce uniform -pressure thereat, if necessary, or to
cause a pressure difference if needed. Finally, in
order to provide for testing possibilities, a water
reservoir 46 is provided so that its contents may be
injected into cavity 12, in place of bar 14: the water
can be used to simulate the confinement with no risk to
the apparatus and the column it forms is observed to
ensure that it remains stable and does not wet porous
wall 11. The viscosity of water is lower than that of
most other liquids, which makes~ confinement of the
water column more difficult and allows for cautious
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evaluations to be made of process quality. A diversion
47 of exit duct 37 can be used to insufflate gas from
bottle 36 into reservoir 46 to empty it, and a water
injection pipe 48 connects reservoir 46 to cavity 12.
Here, as elsewhere, valves are provided for so that
connections can only be made when required. Pipe 48 is
fitted into lower stopper 24, in place of rod 15,
during this preliminary test; a rigid nozzle 49 may be
fitted to its top end whose diameter matches that of
bore 25. When no test is required, pipe 48 may be
withdrawn. A manually adjustable valve 50 controls
operations at diversion 47.
Figure 4 proves that other suitable
arrangements are also possible; pressure here is
controlled on the gas release side, which is why
control valve 40 of absolute pressure manometer 38 is
placed at the end of gas release duct 34, and control
valve 41 of differential pressure manometer 39 is
placed as previously upstream from enclosure 13 on
supply duct 32. Control valve 40 restricts air outlet
flow until sufficiently high pressure is obtained in
the two enclosures 12 and 13. In this embodiment, as in
the previous one, manual valves are provided for that
can be opened or closed at will, to permit normal
functioning of the installation or to be used under
exceptional conditions. But here it is not necessary
for exit duct 37 to lead into gas release duct 34 .
Figure 5 shall now be commented upon. It is a
series of curves representing, for various pressure
values in cavity 12, the water column height than can
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be reached ( X-axis in cm) in relation to the pressure
difference between the two enclosures (Y-axis in
millibars). The resulting information is twofold:
first, and in accordance with the invention, the
maximum height that can be reached is much higher with
pressures of at least two bars, as - in particular from
five bars upwards - it is nearly twice the height that
can be reached with a pressure of one bar (that is to
say at atmospheric pressure, all pressure values given
in this description relating to absolute pressures);
and secondly, which is known in itself, that the
maximum possible height is reached with relatively low
pressure differences between the enclosures, as too low
pressure is unable to set up confinement and an
excessive difference aggravates the disturbances sited
in the liquid mass; it must, however, be noted that the
value to be found is more or less the same in all
cases, approximately 40 or 50 millibars, which
simplifies adjustments. This value may be higher in
other situations. In practice, the pressure difference
is set at a higher value at the outset, and it is
gradually reduced as the liquid mass is injected, until
the value mentioned above is reached which is chosen
for a stability phase of the confinement during which
2~ the moulding process is carried out.
The invention may be applied to containers,
moulds, crucibles, etc. of any shape to surround flat,
hemispherical, conical, cylindrical etc., liquid
masses; the invention may be applied to horizontal
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bottom walls of containers to lift up the liquid. The
liquid mass may be as heavy as a hundred or so grams.
The invention may be considered for application
to the manufacture of optic or laser fibres, or glass
of great purity, and/or with good surface condition,
and generally with the materials mentioned at the start
of this description. Among the different tests, the
glass with the following composition could be moulded
with one hundred times less defects than with a
conventional method: ZrF4: 54%, BaF2: 19%, LaF3: 4%,
AlF3: 4% and NaF: 20%. Another example to be mentioned
concerns glass in silicate oxide, borosilicate or
silico-calcic glass for which high concentrations of
CaO could be used which conventional techniques do not
allow: an example of composition is SiO2: 40%
approximately, CaO: 30%, LiO: 27.3%, Al2O3: 2.5%, Nd2O3:
0.5% and C2: 0.16%.
The confinement gas layer may be approximately
ten micrometers thick and the gas may be of any
composition. Generally inert in relation to the product
to be treated, the gas chosen may, in certain cases, be
reactive: the refining of fluoride glasses for example
requires the presence in the gas phase of a certain
quantity of SF6 or BF3.
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