Apparatus for treating a bath of liquid metal by injecting gas

The present invention concerns apparatus for treating a bath of liquid metal and in particular aluminum or alloys thereof by injecting gas, comprising a rotary agitator, the lower end of which, in the rest condition, rests on a gas injection plug disposed at the bottom of the bath and which, under the action of the pressure of the gas emitted by the plug, rises and, thus being supported by a fluid bearing, may, under the effect of an external couple, rotate freely about its axis and allow a multitude of regularly dispersed gas bubbles to escape through the space which separates the agitator from the plug, enabling intimate contact between the gas and the liquid metal and thus to improve the efficiency of the treatment. It may be used in treating metals from which hydrogen and non-metallic impurities are to be removed.

FIELD OF INVENTION 
The present invention relates to an apparatus for treating a bath of liquid 
metal and in particular aluminum or alloys thereof, by injecting gas. 
BACKGROUND OF THE INVENTION 
The man skilled in the art knows that, before proceeding to an operation of 
shaping semi-finished metallurgical products, the basic metal produced has 
to be subjected to treatment to remove therefrom dissolved gases and 
non-metallic impurities that it contains, the presence of which would 
detrimentally affect the desired properties and the easy solidification of 
the manufactured articles. 
Two main treatment processes are known at the present time: the first 
comprises passing the liquid metal through inert or active filtration 
agents which retain the impurities, either mechanically or as a 
consequence of chemical reactions or by combinations thereof, while the 
second method involves using inert or reactive gases or mixtures thereof 
which are mixed by stirring more or less intensively with the liquid 
metal, with or without substances, such as fluxes, being present. However, 
these two main methods may be used in combination with each other. 
In the second method, many alternative forms have been advanced, relating 
interalia to the manner of introducing ducing the gas into the bath of 
liquid metal, and the way of achieving greater or lesser dispersion of the 
gases in the metal. Thus, in French patent No. 1,535,953, the gas is 
introduced into the bath by a plunger arrangement, the lower part of which 
is provided with a rotary device for producing the stirring effect and 
distributing the gas over a large area of the bath. 
In French patent No. 2,063,906, the gas is blown into the molten metal by 
means of a lance having a water-cooled double jacket. 
In French patent No. 2,156,014, gases are injected in the form of small 
discrete bubbles by means of a device comprising a rotary shaft which is 
fixed with respect to a bladed rotor, and a fixed sleeve which is disposed 
around said shaft and which is connected at the lower end to a bladed 
stator; the shaft and the sleeve are separated by an axial-passage in 
which the gases are carried and introduced at the level of the blades 
where they are subdivided into small bubbles and brought into intimate 
contact with the metal which is agitated by the rotor. 
In French patent No. 2,200,364, the gas is introduced at the center of 
rotation of a turbine agitator and brought into contact with the liquid 
metal under conditions of agitation which are such as to avoid any 
emulsification. 
Many solutions have been proposed. However, each of those suffers from 
disadvantages. Thus, the apparatuses which involve injection of gas by way 
of an agitator have recourse to the use of gas distribution ducts which 
are disposed within the agitator, which complicates design of the 
arrangement and results in delicate systems which are expensive to 
maintain; moreover, such ducts are the cause of substantial pressure drops 
which limit the speed of injection of the gas into the bath. In addition, 
the speed of rotation of such arrangements and consequently their 
capability for dispensing the gases and agitating the bath is limited by 
the appearance of imbalance effects and abrasion phenomena. Certainly, it 
is possible to take steps against such imbalance phenomena by supporting 
the shafts of the agitators at their lower ends, but in that case it is 
necessary either to provide support bearings which operate in contact with 
liquid metal at a relatively high temperature or to extend the shafts to 
the outside of the metal container and provide them with rotary sealing 
means. These are solutions which give rise to technological difficulties 
that are often impossible to overcome. 
OBJECT OF THE INVENTION 
It is for this reason that the applicants sought to produce and develop 
apparatus for treating a bath of liquid metal by injecting gas, wherein 
the supply of gas by way of the shaft of the agitator and all the design 
complications resulting therefrom are eliminated; the imbalance and 
abrasion phenomena are limited; and a gas bubble lamination and dispersion 
effect is obtained, such that the efficiency of treatment of the metal is 
substantially enhanced thereby. 
BRIEF DESCRIPTION OF THE INVENTION 
The apparatus for treating a bath of liquid metal in a container by 
injecting gas is characterized in that it comprises a rotary agitator, the 
lower end of which, in the rest condition, rests on a gas injection plug 
disposed at the bottom of the bath and which, under the action of the 
pressure of the gas emitted by the plug, rises and, thus being supported 
by a fluid bearing, may, under the effect of an external couple, rotate 
freely about its axis and allow a multitude of regularly dispersed gas 
bubbles to escape through the space which separates the agitator from the 
plug. 
The apparatus according to the invention is therefore formed by the 
combination of two means: a rotary agitator on the one hand and a gas 
injection plug on the other hand. 
The rotary agitator comprises a solid member of cylindro-frustoconical 
shape, the upper portion of which is provided with a cylindrical opening 
which requires engagement with the shaft to which an external couple is 
applied. The lower portion of the agitator may have a smooth surface or 
may have a plurality of radial notches or grooves whose depth increases 
towards the periphery, so as to facilitate entrainment of the injection 
gases. The grooves are dimensioned to have a length, for example, which 
extends over approximately one third of the diameter of the agitator and 
may be extended onto the side face, forming kinds of right-hand or 
left-hand helices, which are of greater or smaller pitch, depending on the 
treatment operation conditions. 
The shaft of the agitator is provided with an anti-vortex system which is 
disposed at the bath-atmosphere interface so as to limit movements of the 
liquid metal which could detrimentally affect satisfactory dispersion of 
the gases. 
The external couple is preferably applied by a variable-speed motor which 
is fixed on a carrier disposed in the upper part of the container. 
The shaft of the motor is fixed with respect to the shaft of the agitator 
by way of any connecting system which permits the agitator to be able to 
describe a vertical translatory movement over a distance of several 
millimeters. 
The gas injection plug comprises a cylindrical member, the upper face of 
which is provided with apertures of small diameter, which may be disposed 
in rings and which are connected within the plug to ducts which come 
together at a central duct which opens into an expansion chamber that is 
supplied with gas by a conduit outside the container. 
In accordance with another embodiment of the invention, the mass of the 
plug, which is disposed above the expansion chamber, may be made of a 
porous material. 
The agitator and the plug are so disposed relative to each other that each 
of their vertical axes of symmetry coincide. 
The lower face of the agitator and the upper face of the plug are matched 
to each other so that, in the rest condition, they are in contact with 
each other and block the gas injection apertures. 
This matching configuration may be produced, for example, by the face of 
the plug being of a conical configuration and by forming a cavity of 
similar shape in the face of the agitator. 
In operation of the apparatus, the pressure of gas which is introduced at 
the level of the plug raises the agitator and produces a fluid bearing so 
that, when the drive motor is set in operation, the agitator may turn 
freely without contact with the plug. 
In this way, it is possible to achieve high speeds of rotation without the 
danger of wear to the bearing; in addition, the matching configuration 
which is achieved in particular with conical surfaces, combined with the 
injection of gas, ensures that the agitator is properly centered and 
limits imbalance. Under these conditions, there is provided a robust 
apparatus which produces intense stirring of the bath and lamination of 
the gas in the space between the plug and the agitator, so that the gas 
escapes into the metal in the form of a multitude of regularly dispersed 
bubbles. This results in intimate contact between the gas and the metal, 
and gives a remarkable level of efficiency in treatment. 
The agitator and the plug are preferably formed of graphite, but any other 
material which has sufficient resistance to the liquid metal is also 
suitable. 
The gas which is introduced by way of the plug is a neutral gas, such as 
argon or nitrogen, or a reactive gas, such as chlorine or mixtures 
thereof, or any other gas selected for treating the material. 
An arrangement of this nature is set in position on a container containing 
the bath of metal to be treated. The container may be, for example, a 
ladle through which the material passes continuously before being cast. 
The container may conventionally comprise a partitioning wall means and, 
in which case, the apparatus is disposed in the upstream compartment. The 
bottom of the ladle may or may not be provided with a filter bed. The 
ladle usually is provided with means which permit a neutral atmosphere to 
be maintained at the surface of the liquid metal, thus to avoid any 
oxidizing action by the outside air. 
The container may also be provided with heating means for maintaining the 
metal in a liquid state.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, illustrated therein are: 
the agitator 1 which is held by a screw thread 2 on the shaft 3 and the 
lower portion of which has a conical surface 4 provided with notches or 
grooves 5; 
an injection plug 6 which has a conical surface at which there open ducts 7 
which eminate from a central duct 8 extending from an expsnion chamber 9 
which is supplied by gas by way of a conduit 10. The plug is inserted into 
the lining 11 of a casting ladle, by means of an insulating seal 12, and 
held in place by a plate 13 which is bolted, as at 14, to the metal 
structure 15 of the ladle. 
FIG. 2 shows the arrangement of the grooves or notches 5 on the conical 
surface 4 of the lower portion of the agitator 1. 
Referring now to FIG. 3, shown therein is the apparatus according to the 
invention, on a casting ladle 16 which is divided into two compartments, 
namely, an upstream compartment 17 and a downstream compartment 18, by a 
partitioning wall means 19. A motor 21 rests on a bracket 20 which is 
disposed in the upper part of the ladle. The rotary shaft 22 of the motor 
21 is connected to the shaft 3 of the agitator by a system 23 which 
enables relative vertical movement of the shaft 3. The shaft may be 
provided with an anti-vortex system 24. 
The ladle is supplied with liquid metal by way of the runner channel 25. 
The liquid metal flows towards the bottom of the upstream compartment, and 
then below the partitioning wall means 19 to the downstream compartment, 
before leaving the ladle by way of the runner channel 27. 
As it passes through the upstream compartment, the metal of the bath which 
is delimited by the walls of the ladle and the surface 28, is intimately 
stirred by the agitator, with the formation of a multitude of finely 
dispersed gas bubbles which escape under the pressure of the gases, by way 
of the space 29 which is formed, between the lower portion of the agitator 
and the upper portion of the plug. 
The present invention is illustrated by the results obtained in the course 
of tests using the above-described apparatus; these results are given by 
way of illustration and not by way of limitation of the scope of the 
present invention. 
The metal to be treated was an alloy of aluminum of series 6CCO which was 
introduced into a casting ladle in a molten state at a temperature of 
close to 720.degree. C., at a flow rate of 5 tons per hour; the height of 
the bath in the ladle was 80 cm. 
The plug, of the type comprising ducts, was supplied with argon at a rate 
of about 2 m.sup.3 /h and at a pressure of between 1.2 and 1.4 bar. 
The agitator was rotated at a speed of 400 rpm. 
On entering the ladle, the alloy had 10 bubbles in the vacuum 
solidification test, while after passing through the ladle, the number of 
bubbles was zero. This shows the efficiency of the treatment which is 
achieved by means of the claimed apparatus. 
The present invention finds use in the treatment of liquid metals with a 
gas and in particular in the treatment of aluminum or alloys thereof for 
removing hydrogen and non-metallic impurities.