WO2000065109A1 - Improved method and device for degasing and separation of inclusions in a liquid metal bath by injection of gas bubbles - Google Patents

Abstract

A device for injecting gas bubbles into a liquid metal (3, 23) contained in a treatment volume, preferably a treatment vessel, a circulation chute for liquid metal or an oven. The inventive device comprises at least one static inert-material injection part (1, 21) (1, 21) provided with a plurality of orifices (2, 22) and the material and/or lay-out of said orifices are such that the ratio between the diameter of the contact surface between each emitted bubble and the material at the exit of the orifice (2,22) on the diameter of said orifice, or the expansion ratio, is less than 5, preferably 3 or more advantageously 1.5 at the time when the bubble is detached from the material.

Description

 IMPROVED DEGASSING AND DEGASING METHOD AND DEVICE

SEPARATION OF INCLUSIONS FROM A LIQUID METAL BATH

BY INJECTION OF GAS BUBBLES

Technical area

The invention relates to a method and a device for improving the degassing treatment and the separation of inclusions from a bath of liquid metal, in particular aluminum, magnesium or their alloys, by injection and dispersion of a gas in the says liquid metal.

State of the art

It is known that before obtaining semi-finished metallurgical products by casting, such as aluminum, magnesium or their alloys, it is necessary to treat the liquid raw metal to rid it of dissolved gases (in particular hydrogen) , dissolved impurities (in particular alkalis) and solid or liquid inclusions which would adversely affect the quality of the castings.

This treatment is usually carried out by blowing in a suitable gas, for example an inert gas insoluble in liquid metal, of the Ar type which may contain a few percent of reactive gas of the chlorine type.

For this treatment to be effective, the bubbles must be of the smallest possible diameter so that there is a large contact surface between the gas and the metal.

It is known, for example from application FR 2727432 of the applicant, to inject the gas using a porous material inert with respect to the liquid metal generally based on graphite or alumina.

However, such a procedure does not make it possible to control the flow rate and the size of the gas bubbles emitted. In fact when the pores are too large, on the one hand the bubbles are too large, lack efficiency, the gas being insufficiently dispersed in the metal liquid, and cause harmful surface swirls, on the other hand it is necessary not to stop the passage of gas in the pores to prevent the liquid metal from entering it, in particular during the periods of rest between two flows. On the other hand when the pores are too small the bubbles spread out and remain large; moreover, it is difficult to introduce a high gas flow rate into the liquid metal.

Thus, the device of the aforementioned application tends to circumvent this difficulty of controlling the diameter of the bubbles and of obtaining bubbles of small diameter, by means of a particular arrangement of the gas emitting devices.

Likewise, a process for reducing the diameter of the bubbles emitted by a porous medium is described in US Pat. No. 4,714,494. This process consists in treating the liquid metal in a very long chute, the bottom of which is made of porous material used to introduce the gas wherein said liquid metal flows at a speed of at least 0.1 cm / sec and preferably at least 2.5 cm / sec. Even if this process makes it possible to reduce the diameter of the bubbles, it nevertheless remains important. Furthermore, the use of liquid metal at high speed is not simple, may present safety risks and may not be compatible with good quality of the liquid metal, given the eddies which occur within it.

Thus, several known methods using porous diffusers make it possible at best to obtain bubbles of the order of 30 to 50 mm in diameter even with very fine porosities, for example less than 1 mm, or circulation rates of the liquid metal of the order of magnitude of those described in US Pat. No. 4,714,494.

American patent US 4 290 590 describes a device for injecting gas bubbles comprising a plate of inert material and a series of protrusions provided with an orifice in their upper part and supplied by a gas source in their lower part. The opening of the protrusions must be as small as possible, which has the drawback of requiring a large number of protrusions to obtain a sufficient gas flow rate. The Applicant has continued its efforts to try to control and reduce the diameter of the bubbles emitted by a static gas blowing device and thus make it more efficient.

Description of the invention

The invention is a device for injecting gas bubbles into a liquid metal contained in a treatment volume, said device comprising at least one static injection part (also called emitter) made of substantially inert material, said static part comprising a plurality of orifices, said device being characterized in that the material and / or the location of the orifices are such that the ratio of the diameter of the contact surface between each bubble emitted and the said material at the outlet of the orifice , on the outlet diameter of the orifice, or spread ratio, is less than 5, preferably 3 or better than 1.5.

The treatment volume or container is generally a tank with one or more compartments, a liquid metal circulation chute, an oven, etc.

The diameter of the orifice is at most equal to the diameter of the bubble to be obtained and the spreading ratio is all the lower the more one wishes to obtain a small bubble diameter. The device according to the invention is especially useful when it is desired to obtain bubbles having a diameter of at most 20 mm and advantageously at most 10 mm even when the metal is calm or circulates at low speed. When the liquid metal flows at higher speed, the diameter of the bubbles can be even smaller.

The desired spreading ratio can be obtained by using a material wettable by liquid metal, that is to say one whose wetting angle is less than 90 °, and / or by geometrically limiting the spreading surface available. around the hole; this latter solution makes it possible to use diffusers made of materials which cannot be wetted by liquid metal.

Figure 1 (a and b) illustrates the difference in behavior of a wettable and non-wettable material in the context of the invention. In (1) we see the body of the static emitter, in (2) the gas inlet orifice where a bubble (9) is formed on the surface of the emitter, the said orifice (2) being supplied with gas using a small channel in the transmitter.

When the material is wettable by liquid metal (case of fig. La), the wetting angle (10) defined by the tangent to the bubble (9) at its point of contact with the emitter and by the emitter is less than 90 °. It can be seen that the metal wetting the material of the emitter well counteracts the spreading of the bubble (9) and limits its diameter. This mechanism, which occurs even if the surface surrounding the orifice (2) forms an angle different from 90 ° with the internal surface of the orifice, allows the gas to exit through protuberances made of material wettable by the metal having the shape , for example, drilled cones whose orifice passes through the axis of symmetry, that is to say that the orifices can be located at the top of conical protuberances. The use of a static emitter without protuberance, however, has the advantage of simplifying the construction of the device, of reducing the risks of evolution of the geometry by erosion and of limiting the fouling of the device. The protrusions may possibly be formed from separate parts which are fixed in place by mechanical means, such as by screws, which allows them to be easily changed in the event of damage.

When the material is not wettable by the liquid metal (case of fig. Lb), the wetting angle (10) is greater than 90 °. We see that the metal having difficulty wetting the emitter allows the bubble to spread; in this case it is important to mechanically or geometrically limit the spreading surface of said bubble, as will be seen below, so that it has a small diameter.

Said contact surface means the maximum contact surface A between each bubble emitted and said material at the outlet of the orifice. During the development of a bubble, the contact surface generally evolves fairly quickly towards its maximum value. The maximum contact surface can be measured using any means which allows the formation of gas bubbles, such as X-rays, to be visualized.

In the case of aluminum or magnesium or their liquid alloys, the wettable material of the diffuser can be chosen from certain refractory metals substantially inert with respect to said liquid metals, such as Mo, W, V, Ti, Cr, Fe, steels, ..., or their alloys, or among ceramics such as TiB, nitrides (AIN, BN), carbides (A1 ₄ C ₃ , TiCι_ _x ), .... We can note on this subject that normally graphite or alumina are not wettable by these liquid metals. ZrO and SiC are also non-wetting materials for aluminum and its alloys. The wetting behavior of a material also depends on the roughness and the oxidation state of its surface. The material is preferably wetting because it is then easier to obtain a low spreading ratio.

To physically limit the spreading surface, the diffuser may include a plurality of small protuberances, the top surface of which corresponds to said contact or spreading surface of the bubble and comprises at least one gas emission orifice.

With such a diffuser we see that it is then possible to use materials that cannot be wetted by liquid metal; in this case it is preferable to have only one orifice on the upper surface of the protuberance. These protrusions preferably have a height at least equal to their diameter and a shape which can be that of a straight or inclined or frustoconical cylindrical stud. The protrusions (32), or a part thereof, may be removable, that is to say that they form inserts, which makes it possible to replace them in the event of wear or damage. The removable protrusions (or studs) can be fixed to the body of the static part (21) by screws or any other means which allows easy replacement of the protrusions.

The diffuser may be in the form of a single piece or of an assembly of elementary pieces, generally of small thickness in which small channels have been drilled. The upper end of these channels constitutes the injection orifice, located on the surface of said diffuser in contact with the liquid metal, and their lower end constitutes the orifice receiving the flow of feed gas to be injected into the metal. liquid. The distance between two neighboring injection orifices is typically slightly greater than that corresponding to the diameter of the spreading surface and is such that the bubbles being formed do not touch. The channels can correspond to a pore system or communicate through a network of feed channels dug in the mass. To better control the diameter of the bubbles, it is important that the gas pressure at the outlet orifice, at the interface between the metal and the surface of the emitter, be substantially constant regardless of the gas flow rate. , especially during the formation and detachment of the bubble. For this it is advantageous to design the device in such a way that the buffer volume located between the gas outlet orifice and the nearest regulator of the gas supply (valve, flow meter, ...) is also reduced as possible, and / or to use a suitable mass flow meter, and / or to introduce a local pressure drop just upstream of the outlet orifice using for example a porous material.

In order to further reduce the diameter of the bubbles, it is advantageous to introduce shear energy into the liquid metal, for example by means of ultrasound or a rotary agitator, to promote the release of the bubbles.

The injection device according to the invention is advantageously used for the treatment of volumes of aluminum, magnesium or their liquid alloys. It can for example be installed in the bottom of liquid metal treatment tanks or of compartments of these same tanks, but also in the bottom of circulation channels of said liquid metal.

The size of the bubbles (1 1, 31) can be measured by a method consisting in irradiating the liquid metal bath (3, 23) in which the bubbles are emitted using X-rays, in visualizing said bubbles after recovery of the image by a camera and to measure them after calibration of the acquisition chain and to determine the spread ratio.

The invention also relates to any process for treating liquid metal using gas bubbles with a diameter of at most 20 mm, or preferably at most 10 mm, generated by a static diffuser, the products obtained by this process and the corresponding device. The tests carried out with the device according to the invention have shown that it is possible to achieve degassing efficiencies of up to 50% with bubbles with a diameter of the order of 5 mm, while they remained below 5 % with bubbles with a diameter of around 40 mm. The invention also relates to any method of treatment of liquid metal by gas injection using the static injection device of the present invention. For the implementation of the treatment process, the material and / or the implantation of the orifices of the static part (1, 21) can be chosen according to the nature of the liquid metal and, possibly, according to the composition of the gas and / or liquid metal temperature.

The method may include measuring the size of the bubbles being processed, for example using X-rays, sound probes or ultrasound.

The following drawings and examples illustrate the invention.

FIG. 2 represents a partial view, in section, of an example of a diffuser making it possible to obtain the spreading ratio according to the invention and FIG. 3 a view under the same conditions of another diffuser according to the invention.

In FIG. 2, we see in (1) the static diffuser in the form of a piece of material wettable by liquid metal, generally installed in the bottom of a liquid aluminum treatment volume (not shown), comprising a plurality injection ports (2) in contact with the liquid metal (3). On the lower face (4) of the part (1) opens an orifice (5) for supplying treatment gas, which will be conveyed to the injection orifice (2) via the buffer volume (8) . The diffuser rests on supports not shown and several diffusers can be installed in the same treatment volume as indicated in application FR 2727432 cited above. With a piece (1) of Ti (wettable by aluminum) of thickness 0.2 cm and orifices (2) of diameter 1.0 mm spaced from each other by 15 mm, the bubble in formation (9) shows a wetting angle (10) of approximately 70 ° and a spreading ratio of approximately 1. The diameter of the bubbles formed (1 1) was measured by an X-ray method consisting essentially in irradiating the metal bath liquid, in which the gas bubbles are emitted, and to visualize the said bubbles in clear on a dark background after recovery of the image by a camera; their diameter is then measured after a calibration of the acquisition chain. The diameter is 5 mm while the metal is calm and without any external shearing energy.

Figure 3 shows the detail of another means for limiting the spread ratio. The static diffuser (21) placed in the bottom of a liquid metal treatment volume (23) is in the form of a part comprising injection orifices (22) through which the treatment gas is diffused in the liquid metal (23). These orifices are located at the top of protuberances (32) whose apex diameter, in combination with the diameter of the orifice, is used to calculate said spread ratio. The injection orifices (22) are connected to the supply orifice (25) located on the underside (24) of the part (21) via the buffer volume (28) as small as possible. Upstream of said bottom face (24) is, as before, the constant pressure supply device (26).

With a graphite part (not wettable by aluminum) having orifices (22) of diameter 2 mm located at the top of small cylinders of diameter 10 mm, thus representing a spreading ratio of 5, having a height of 10 mm above the surface of the rest of the diffuser and spaced from each other by 40 mm, it is possible to obtain bubbles (31) having a diameter of approximately 10 mm. It can be seen that the bubble being formed (29) does not exceed the periphery of the well-individualized, non-wettable cylindrical stud (32) on which it is formed.

Upstream of the lower face (4, 24), and typically at the orifice (5, 25), is the device for supplying gas at constant pressure (6, 26) comprising, for example, a felt graphite introducing a pressure drop between the feed gas flow (7, 27) and the buffer volume as small as possible (8, 28).

The tests carried out with the device of the invention have shown that the choice of the material and the location of the orifices alone makes it possible to effectively control the size of the bubbles, even if this choice can be made according to the nature of the metal. to be treated and / or, in certain cases, depending on the composition of the gas and / or the temperature of the liquid metal.

Claims

1. Device for injecting gas bubbles into a liquid metal (3, 23) contained in a treatment volume, preferably a treatment tank, a liquid metal circulation chute or an oven, said device comprising at least a static injection piece (1, 21) made of inert material, said static piece (1, 21) comprising a plurality of orifices (2, 22), said device being characterized in that the material and / or the implantation orifices are such that the ratio of the diameter of the contact surface between each bubble emitted and the said material leaving the orifice (2, 22) over the diameter of the orifice, or spreading ratio, is less to 5, preferably to 3 or more advantageously to 1.5. 2. Device according to claim 1, characterized in that the spreading ratio is obtained by using a static part (1, 21) in a material wettable by the liquid metal (3, 23). 3. Device according to claim 2, characterized in that, when the liquid metal (3, 23) is aluminum, magnesium or their alloys, the wettable material is chosen from refractory metals, such as W, Mo, Ti , V, Cr, Fe or steels, or their alloys, or among refractory ceramics, such as TiB ₂ , nitrides (AIN, BN) or carbides (A1 ₄ C, TiC]. _X ). 4. Device according to claim 2 or 3, characterized in that the orifices are located at the top of conical protuberances. 5. Device according to any one of claims 1 to 4, characterized in that the spreading ratio is obtained by mechanically or geometrically limiting said contact surface. 6. Device according to claim 5, characterized in that, to obtain geometrically the limitation of the contact surface, the orifices (2, 22) are located at the top of protuberances (32) located on the static part (1, 21) . 7. Device according to claim 5 or 6, characterized in that, when the material of the static part (21) is made of non-wettable material, each protuberance (32) has only one orifice (22) for emitting gas . 8. Device according to claim 6 or 7, characterized in that at least one of the 5 protrusions (32) is removable. 9. Device according to any one of claims 1 to 8, characterized in that it comprises means so that the gas pressure at the outlet orifice is essentially constant regardless of the gas flow. 10. Device according to claim 9, characterized in that the said means o comprise a buffer volume located between the gas outlet orifice and the closest gas supply adjustment member as small as possible, and / or a suitable mass flow meter and / or a porous means introducing a local pressure drop just upstream of the gas outlet orifice. 1 1. Device according to any one of claims 1 to 10, characterized in that 5 is introduced into the liquid metal (3, 23) a shearing energy, preferably using ultrasound or a rotary agitator. 12. Device according to any one of claims 1 to 1 1, characterized in that the orifices (2, 22) are separated from each other by a distance such that the bubbles in formation do not come into contact. 0 13. Device according to any one of claims 1 to 12, characterized in that the static injection part (1, 21) is in one or more assembled elements. 14. A method of treating a liquid metal (3, 23) by injecting a gas using a static gas injection device (1, 21) according to any one of claims 1 to 13 , characterized in that the bubbles (1 1, 31) of the treatment gas 5 introduced into the liquid metal (3, 23) have a diameter of less than 20 mm and preferably of 10 mm, the liquid metal (3, 23) being at rest. 11 15. A method of treating a liquid metal (3, 23) by injecting a gas using the static gas injection device of any one of claims 1 to 13. 16. A method of treatment of any one of claims 14 or 15, characterized in that the size of the bubbles (11, 31) is measured by a method consisting in irradiating the bath of liquid metal (3, 23) in which the bubbles are emitted using X-rays, to visualize said bubbles after recovery of the image by a camera and to measure them after calibration of the acquisition chain.