The present invention relates to a vessel for the treatment
of molten metal and to a process for the manufacture thereof. More particularly,
the invention relates to a vessel for gas treatment of a molten metal.
Gases are often injected into molten metal in vessels such
as ladles, crucibles or tundishes for diverse purposes. For instance, a gas may
be introduced into the bottom part of a vessel to clear the relatively cool bottom
area of solidification products, e.g. to remove them from the vicinity of a bottom
pour outlet where the vessel has such an outlet. In steel making for example, the
use of slow injection of a fine curtain of gas bubbles in the tundish assists in
inclusion removal; the inclusions being attracted to the fine gas bubbles and rising
upwards through the melt to the surface where they are conventionally captured by
the tundish cover powder or flux. Gas may also be introduced for rinsing or to homogenise
the melt thermally or compositionally, or to assist in dispersing alloying additions
throughout the melt.
Usually, an inert gas is used but reactive gases may also
be employed, e.g. reducing or oxidising gases, when the melt compositions or components
thereof needs modifying. For example, it is customary to inject gases such as nitrogen,
chlorine, freon, sulphur hexafluoride, argon, and the like into molten metal, for
example molten aluminium or aluminium alloys, in order to remove undesirable constituents
such as hydrogen gas, non-metallic inclusions and alkali metals. The reactive gases
added to the molten metal chemically react with the undesired constituents to convert
them into a form such as a precipitate, a dross or an insoluble gas compound that
can be readily separated from the remainder of the melt. These gases (or others)
might also be used for example with steel, copper, iron, magnesium or alloys thereof.
In order to efficiently carry out a gas injection operation,
it is desirable that the gas be introduced into the molten metal, preferably from
the bottom of the recipient, in the form of a very large number of extremely small
bubbles. As the size of gas bubbles decreases, the number of bubbles per unit volume
increases. An increase in the number of bubbles and their surface area per unit
volume increases the probability of the injected gas being utilised effectively
to perform the expected operation.
disclose crucibles for the treatment of molten metal according to the
preamble of claim 1. These crucibles are provided with a porous block or plug extending
through the bottom wall of the crucible. The treatment gas is injected into the
molten metal from the bottom of the crucible and achieves efficiently its treatment
purpose. However, such kind of arrangement is problematic for the following reasons.
- i) The hole in the bottom wall of the crucible which is required for the insertion
of the porous plug or block represents a significant source of danger. Molten metal
infiltration through the joint between the crucible bottom wall and the porous plug
are possible if the joint has not been set properly or if, for whatever reason,
the joint has become deteriorated during use of the crucible.
- (ii) In addition, the erosion of the porous plug or block can also represent
a significant source of molten metal leakage if particular and expensive measures
are not taken. Consequently, even though the crucible is still in perfect shape,
if some wear of the purge plug of block has been detected, it is necessary to stop
the operation and to replace the purge plug of block, resulting in loss of time
and production. (iii) The presence of the purge plug or block extending through
the bottom wall of the crucible and particularly of its gas supply line, makes it
also difficult to move the crucible when it is fully loaded with molten metal. (iv)
Further, the furnace where the crucible is to be used must be specially adapted
to accommodate the purge plug or block extending through the bottom wall of the
crucible and particularly of its gas supply line.
As an altemative to these crucibles provided with a purge
plug or block extending through the bottom wall of the crucible, it is also possible
to use gas treatment means plunging directly into the molten metal bath through
the surface of the molten metal bath. Gas lancing or rotary degassers are examples
of this technique. While solving some of the above mentioned problems, these arrangements
are still problematic for the following reasons. (i) Since the gas treatment is
not performed from the bottom surface of the crucible, there remains a zone of the
molten metal bath which is never in contact with the gas and the treatment is not
perfectly efficient. (ii) Techniques involving a mobile lance are often not reliable
since the positioning of the lance may largely vary from one melt to the next. (iii)
Altemately, techniques involving a fixed gas diffusing means such as a rotary degasser
require a rigging or support system for the gas diffusing means which makes it uneasy
to move the crucible and require a particular adaptation of the furnace where the
crucible is to be used to accommodate this support or rigging system.
The object of the present invention is therefore to provide
a crucible for the treatment of a molten metal which would not present the inconveniences
of the prior art crucibles. In particular, it would be desirable to provide a crucible
(i) that would be safer than a crucible with a hole in the bottom wall; (ii) that
would possess the advantage of a gas diffusing means performing its gas diffusing
action from the bottom of the crucible; (iii) that would not require the immediate
termination of the operation when the purge plug or block is wom or has been damaged;
i.e. that would allow to pursue the operations even with a reduced ability to diffuse
gas into the molten metal; (iv) that would allow easy displacement of the crucible
when required; (v) that would not require particular adaptation of the furnace to
accommodate bottom diffusing means or some rigging/support system; (vi) that would
be reliable; (vii) that would not require expensive and cumbersome rigging or support
This objective is reached with a crucible as defined in
Indeed, it has been found that a refractory crucible for
the treatment of molten metal comprising side wall(s) having an inner surface and
an outer surface and a bottom wall, the crucible having gas diffusing means arranged
near the bottom of the crucible and gas supply means for conveying a gas from a
gas source to the gas diffusing means, wherein the gas supply means is fixedly arranged
adjacent to the inner surface of the side wall or within the side wall solves the
above mentioned problems.
First, it is to be noted that the crucible according to
the invention can have any conventional shape; i.e. it can be polygonal, for example
square or rectangular (trough-shaped) with a plurality of side walls or more conventionally,
rounded with a circular single wall. By "adjacent to the inner surface of the side
wall or within the side wall", it is meant that the gas supply means does not extend
through the bottom surface of the crucible. However, it would not be excluded to
have the gas supply means extending thorough a region of the side wall which is
normally not in contact with molten metal; i.e. which is above the normal level
of molten metal, for example near the upper surface of the crucible.
The crucible according to the invention can be a crucible
used for the processing of metal, including melting and/or treating the metal or
it can be a crucible which is temporary used, for example for treating, transporting
or pouring the molten metal.
The crucible according to the invention (i) is safer than
a crucible with a hole in the bottom wall; indeed, the gas supply means being fixedly
arranged adjacent to the inner surface of the side wall or within the side wall,
there is no need for a hole in the bottom wall; (ii) possesses the advantage of
a gas diffusing means performing its gas diffusing action from the bottom of the
crucible; (iii) does not require the immediate termination of the operation when
the purge plug or block is wom or has been damaged since there is no risk of leakage
through the gas diffusing means; (iv) allows easy displacement of the crucible when
required since there is no complicated connection with a gas supply means or rigging
or support system to take into account; (v) does not require particular adaptation
of the furnace; (vi) is reliable; (vii) does not require expensive and cumbersome
rigging or support system.
The connection of the gas supply means to a gas source
can be performed by any known technique. For example, advantage can be taken from
the particularly useful gas-tight connection system as disclosed in
Advantageously, the gas supply means comprises a duct fixedly
arranged adjacent to the inner surface of the side wall. Preferably, this duct is
made of refractory material, preferably of the same refractory material as the crucible.
The duct can advantageously be formed by an inward protrusion extending from the
inner surface of the side wall. It can be made integral with the crucible or formed
separately and then glued or cemented into place. Such a protrusion can be shaped
manually or mechanically from soft material such as clay graphite or other ceramic
and/or metal or can be copressed together with the crucible.
The gas supply means might also comprise an inlet orifice
arranged in the upper surface of the crucible or in the outer surface of the crucible
side wall at a level which is higher than the maximum level of the molten metal.
The gas diffusing means comprises advantageously a porous
refractory plug or brick for injecting gas into molten metal through a molten metal-contacting
surface with a porous refractory body substantially encased in a refractory substantially
non-porous body except at the molten metal-contacting surface.
According to one embodiment of the present invention, the
non-porous bodies is constituted from the crucible wall itself and the porous and
non porous bodies have been co-pressed.
According to another embodiment, the porous and non porous
bodies form a monobloc entity which is manufactured separately and then fixedly
attached at the bottom of the crucible. The document
discloses a purge plug or bubbling block particularly suitable for use
in a crucible according to the invention.
According to another of its aspects, the present invention
also relates to a process for the manufacture of a crucible which comprise the steps
- a) forming the crucible,
- b) forming the gas supply means adjacent to the inner surface of the side wall
or within the side wall, and
- c) arranging the gas diffusing means at the bottom of the crucible.
Advantageously, steps a), b) and c) are carried out at
once, for example by isostatic pressing. Altemately, the gas diffusing means can
be formed separately and then fixedly arranged near the bottom of the crucible.
The voids present in the gas supply means (plenum chamber under the porous body
for example or the gas passage bore in the duct can be produced according to known
and conventional techniques (wax, combustible material, low melting point metal
or metal alloys,...).
In order to enable a better understanding of the invention,
it will now be described with reference to the figures illustrating particular embodiments
of the invention, without however limiting the invention in any way.
Figs. 1 and 3 show cut view of two embodiments of crucibles
according to the invention (cut in the middle in a vertical plane);
- Figs. 2 and 4 are top views of the crucible of Figs. 1 and 3 respectively;
- Fig. 5 is a perspective view of a cut of the crucible of Figs. 2 and 4.
On these Figs., crucibles have been represented with the
reference '1. Each of these crucibles has a bottom wall 13 and a side wall 11 with
an inner surface 12, an outer surface 15 and a top surface 14. The gas diffusing
means 2 has a molten metal contacting surface 21 and a porous body 22 encased in
a substantially non-porous (tight) body. This non porous body can be the crucible
itself (Figs. 3, 4, 5) or a separate body 23 fixedly attached at the bottom 13 of
the crucible (Figs. 1 and 2).
The gas supplying means 3 are formed of a duct 31 extending
from an orifice 32 at the top surface 14 of the wall 11 to the gas diffusing means
2. This duct can be a protrusion formed adjacent to the inner surface 12 of the
wall as depicted on Figs. 1 to 5 or can be formed directly within the wall.