EP0916066B1 - Rotary gas dispersion device for treating a liquid aluminium bath - Google Patents

Description

FIELD OF THE INVENTION

The invention relates to a rotary gas dispersing device for treatment of a liquid bath of aluminum or its alloys. In the following text the word "aluminum" will be used in the generic sense of "Aluminum and its alloys".

STATE OF THE ART

Liquid aluminum leaving electrolytic cells or reflow ovens contains dissolved or suspended impurities. The most important of these impurities are hydrogen, alkaline elements such as sodium or calcium and the oxides, in particular the aluminum oxide itself.

In order to remove these harmful impurities for the subsequent properties of the half product, liquid aluminum is subjected to various treatments to eliminate impurities. The most common of these treatments, which uses a combination of chemical reactions and flotation phenomena, involves introducing into the bath in the form of small bubbles an inert or reactive gas. For example a bubble of argon will bring with it on the surface of the bath a solid inclusion in suspension. Likewise a chlorine bubble will react with the sodium contained and give a sodium salt which will also be transported to the surface of the bath. Such treatments by the action of gases can be carried out in discontinuous in an oven or in a crucible. But they are most generally carried out continuously between the furnace and the casting machine in a treatment of the type that is schematically represented in the figure 1.

The liquid metal to be treated enters the first compartment (2) of the pocket by an inlet spout (1). It is treated on the way by gas bubbles (4) dispersed by the rotary device (3). The metal thus treated overflows in a outlet compartment (5) equipped with a baffle (6) and pocket spring by the outlet spout (7).

The gas to be dispersed in the liquid bath is still injected a few times with simple canes but the most common technique is to use a rotary dispersing device composed of a hollow axis of rotation through which arrives the gas and a rotor of the most suitable form to disperse the gas bubbles in the bath. The effectiveness of the treatment is obviously maximum when the exchange surface between the bath and the gas is itself maximum. This is achieved by designing the rotor to obtain very small bubbles, project these bubbles throughout the volume (the least dead volume possible) and create recirculations of the bath itself so that it comes in contact with bubbles (always as little dead volume as possible).

This search for the greatest effectiveness of treatment by agitation intense in the volume of the bath results in permanent agitation in surface often called "surface waves", by bath projections by rising large bubbles and by a vortex phenomenon around the axis of rotation. These three phenomena risk reintroducing into the bath inclusions and generate annoying oxidation of liquid aluminum.

We tried to eliminate or reduce these drawbacks.
American patent US 4,618,427, for example, proposes a radical change in the technology of gas dispersing devices. Admittedly, such a device does not have the aforementioned drawbacks, but such a rotor generates only a very low recirculation of the liquid metal, which amounts to reducing the metal / gas interface and consequently the efficiency of the process.

Patent application EP 0347108 proposes to combine gas treatment and filtration in the same device. A filter layer is interposed between the gas injection rotor and the surface of the liquid metal. Gas bubbles cross the filter up to the surface and it is understandable that the surface turbulence must be very reduced, the filter playing a role distribution of the bubbles and breaking any large broths. These measures has serious drawbacks, however: filter, which becomes clogged and must be periodically renewed, is a device expensive and difficult to operate; on the other hand, the size of the rotor is necessarily reduced to facilitate passage through the filter layer and ensure tightness at this level. The conical shape of the bubble distribution of this rotor, if it ensures a good distribution of the bubbles under the layer filter, leaves a large part of the tank out of reach of bubbles what does not compensate for the toroidal recirculation of the liquid metal itself. The efficiency of gas treatment is therefore significantly reduced, which may not be unacceptable in a mixed treatment device gas / filtration as described in this application, but is not satisfactory for a gas treatment device only.

Patent application EP 0611830 proposes to provide at the bottom of the tank for treats a baffle across its entire width. This chicane goes to the right of the rotor (s) and, by modifying the bubble distribution fields and metal circulation, would reduce the disturbances of surface or, which amounts to the same thing, to increase the quantity of gas injected and the speed of rotation of the rotor without increasing these surface disturbances. This solution has a considerable practical drawback. As and when as liquid metal passes through the tank, dross builds up around of the privileged area that constitutes the chicane and it is necessary to clean very often the chicane in particularly difficult conditions.

Japanese patent application JP 06-273074 very specifically targets the decrease in surface agitation and teaches an improved rotor for this purpose. Experience shows that the use of such a rotor effectively reduces the permanent phenomenon of “surface waves” but that it occurs periodically and inadvertently projections on the surface of the bath which have negative consequences on the resumption of inclusions.

PROBLEM

The Applicant has sought to develop a rotary dispersing device of gas which makes it possible to reduce both the agitation phenomena of surface, episodic projections and vortex without requiring modifications to the tank itself such as a filter layer or a chicane and without reducing the effectiveness of the treatment.

DESCRIPTION OF THE INVENTION

The object of the invention is a rotary gas dispersing device for the continuous treatment of a bath of liquid aluminum in a tank treatment comprising a drive shaft serving as a gas inlet and a rotor, said rotor consisting of an even number of blades arranged in a star around a central hub and a substantially flat disc covering the star formed by the blades, the gas being injected into the bath through orifices located between the blades, the ratio between the outside diameter of the rotor and the diameter of its central hub being between 1.5 and 4, characterized in that are alternating complete blades having a given contact surface with the bath and reduced blades having a larger contact surface with the bath low delO at 30% compared to the contact surface of the complete blades.

The drive shaft has at its lower end a threaded part or a threaded part intended for fixing the rotor. The rotor itself has a central hub and a threaded tube for fixing the rotor to the part or threaded part of the drive shaft. At this central hub are fixed blades arranged in spokes. The number of these blades can be variable, even or odd. If the number of blades is too low, agitation and therefore the effectiveness of the treatment may prove to be insufficient. A number of blades too high leads to a more difficult assembly and therefore more expensive. The choice depends on the case by case of the volumes of metal to be treated in a given time, the size of the tank which can be a compartment or with several compartments, etc ... Under the usual conditions of aluminum treatment, a number of blades between 6 and 8 constitutes a good compromise.

The blades generally have a rectangular shape but one can also use trapezoidal blades where the blade height is lower at the outer end thereof only at its connection to the hub central and even triangular blades where the height of the blade is zero at its outer end. The shape of the blade must be such that, taking into account its height and the configuration of the injection orifices which will be discussed more far, most of the injected gas is taken up and dispersed by the blade.

The rotor has a substantially horizontal disc whose diameter is equal or close to the outside diameter of the star constituted by the blades. This disc is positioned above the star formed by the blades. It is advantageous to give the upper side of the disc a slightly tapered shape in order to facilitate the flow of liquid metal when the rotor is removed vertically from the tank. It is not recommended to choose a diameter smaller than diameter defined by the star formed by the blades. As soon as the end of the blades exceeds the diameter of the disc, the anti-wave effect of the device decreases considerably. In the other direction, on the other hand, the anti-wave effect is maintained even if the diameter of the disc is greater than the diameter defined by the star of the blades. There is little point, however, in adopting such a configuration. And, in the preferred version of the invention, we do substantially coincide disc diameter and outer diameter of the star that are the blades.

The external diameter of the rotor according to the invention is variable. As for rotors of the prior art, it depends on the volume to be treated, on the size of the tank, of the shape of the tank with one or more compartments.

The rotor according to the invention is characterized by a high lift of the blades. The lift of the blades can be defined by the ratio between the outside diameter of the rotor and the diameter of its central hub. The rotors of the prior art have a weak blade lift because an increase in lift would cause an increase in surface agitation. A typical example of a rotor of the prior art with low lift of blades is given by rotor A from the example given below. The increase in the lift of the blades has however limits. Below a certain ratio, the rotor is difficult to to manufacture, fragile and expensive. Beyond a certain ratio, the beneficial effect of lift of the blades becomes negligible. Under the usual conditions of aluminum industry tanks, a range for the ratio of 1.5 to 4 represents a good compromise.

The rotor according to the invention comprises an even number of blades, blades "Complete" alternating with blades whose surface in contact with the bath is reduced from 10% to 30% compared to the surface of the complete blade.

The arrangement between the disc and the set of blades can be achieved in several ways. A first solution consists in making the rotor by machining in one block. Disc, blades and central hub make up a single unit. Another solution is to make the rotor in two parts: on the one hand the disc with, in its center, its own thread fixing hub to the drive shaft, on the other hand all of the blades with its central hub. The rotor is then obtained by successive adjustments of the disc and blades on the drive shaft. The advantage of a two-piece assembly is that the rotor can be made of different materials. For example, the blades which are subjected to greater stresses than the disc, can be made of a material harder than the disc.
In general, the device according to the invention can be produced in all materials compatible with the conditions of use (mechanical resistance held at high temperature, wear, etc.) and in particular with all the materials already known for be used in similar equipment (graphite, boron nitride, alumina, silicon nitride, ceramics from the SIALON family, etc.). the three parts (drive shaft, disc and blades) can possibly be made of different materials.

The gas injection ports are drilled radially in the central hub on which the blades are fixed. The connection of these ports on the arrival of gas via the drive shaft will be discussed later.

The gas injection ports are positioned and constructed in such a way that the gas jet is generally located at the height of the central zone of the blade which, during the rotation, will disperse it.

FIGURES

Figure 1 shows in section the diagram of a conventional tank of continuous processing of liquid aluminum with a rotary device gas injection.

Figure 2 shows a rotary gas injection device of the prior art.

Figure 3a shows a rotary gas injection device with 8 identical blades.

Figure 3b shows a rotary gas injection device according to the invention alternating full blades and reduced surface blades.

Figure 4 shows two possible variants (4a and 4b) for assembly different elements of a device according to the invention and for feeding injection ports.

DETAILED DESCRIPTION OF THE INVENTION

In its simplest, most rational and efficient version, the rotor according to the invention comprises an injection of gas between each blade by a single orifice which is positioned vertically halfway up the blade, which is oriented radially in such a way that its axis roughly corresponds to the bisector of the angle formed by the two blades and which is drilled along a horizontal axis. A rotor of this type is shown FIG. 3a in which the drive shaft (1) is distinguished, the upper disc (4), the blades (5) and a gas injection orifice (10).

However, numerous variants are possible within the framework of the invention. We can for example not inject gas between each blade but in front only one blade in two. The effectiveness of the whole will be reduced but under certain circumstances the volume to be treated or the quality required of the metal, this may be sufficient. You can also position the orifice vertically not halfway up the blade but higher or higher down and / or tilt the hole down or up relative to the horizontal. The important point is that the gas jet must be for the most part dispersed by the blade avoiding that a significant part of the gas escapes below or above the blade without being dispersed.

It is preferable that the diameter of the orifices is between 1 and 5 mm. Below 1 mm in diameter, there is a risk of blockage. Above 5 mm. the diameter of the bubbles becomes too large, the exchange surface metal / gas decreases and the effectiveness of the treatment may be compromised. In certain configurations, depending on the volume to be treated, the size of the rotor and of its speed, of the volume of gas to be dispersed, it may be advantageous to replace the single orifice located between the blades with two or more smaller diameter.

The holes thus described, drilled in a star in the central hub of the rotor, can be connected to the gas supply through the hollow shaft training by all kinds of means. These means depend on choices also made for the mechanical arrangement of the rotor and the shaft, in depending on the materials, the size of the rotor, etc. These different means possible which can be very numerous are compatible with the invention as long as they provide a sufficiently regular and good gas flow distributed in the different orifices.

Without this constituting in any way a limitation of the scope of the invention, two possible solutions can be cited for the supply of rotor orifice gas:

One of the solutions is shown in Figure 4a. A drive shaft (1) has at its lower end a threaded cylindrical hole (2) which will be the female part of a screw connection. The rotor itself (3) manufactured in one piece has an upper disc (4), a number of blades (5) and a cylindrical central core (6). This central core (6), full in its part lower (6a), has a cylindrical cavity (7a) which plays the role of gas distributor. From this cavity, the holes are drilled radially (10) which diffuse the gas between the blades. Crossing the disc (4) and the part upper (6b) of the central core, a cylindrical threaded hole (8) with a diameter identical to that of the threaded cylindrical hole (2) of the drive shaft, intended also to serve as female part for screw connection, opens into the central gas distribution cavity. Finally, the whole comprises a screw (9) of cylindrical shape and pierced in its center with a channel gas passage. During assembly, we first fix the screw to the rotor in the threaded cylindrical hole (8) provided for this purpose. Then we fix the rotor to the shaft driving by screwing in the threaded cylindrical hole (2) provided in the shaft, the upper part of the screw (9) which protrudes above the disc. Once the assembled assembly, the passage of gas and its distribution are ensured through the central channel of the drive shaft, the central channel provided in the screw (9), the distribution chamber (7) and the lateral orifices (10).

Another solution for rotor / shaft assembly and gas distribution is shown in Figure 4b. The drive shaft (1) has a hole cylindrical threaded (2) which will be the female part of the screw connection. The rotor is in two parts: the upper disc (4) is manufactured separately and joined to the assembly constituted by the blades and the central core during assembly only. The upper disc (4) has on its lower face grooves (4a) intended to receive the upper part of the blades at the time of mounting. The center of the disc is drilled with a threaded cylindrical hole intended to receive the connecting screw. The central core (6) of the rotor proper is drilled with a threaded cylindrical hole (8) intended to receive the screw connection. Halfway up the blades, is also hollowed out in this core central a circular cavity (7b) which will act as a gas distributor. Of this cavity radially leaves the orifices (10) for injecting the gas between the blades. Finally, the assembly includes a screw (9) pierced in its center with a channel gas passage. This channel which, at the top of the screw, is will connect to the drive shaft channel, ends at the party lower into a series of small radial channels which, once assembled, will open into the gas distribution chamber. When mounting, the screw (9) is introduced at the bottom of the central core. Thanks to the parties threaded from the upper part of the central core, the disc and the lower of the drive shaft, the screw (9) assembles the three rooms. Once the assembly is complete, the complete gas circuit is constituted from the central channel of the drive shaft, passing through the channel central of the screw, the small lateral channels inside the screw, the distribution hollowed out inside the central core and the injection orifices between the blades.

The rotor according to the invention comprises an even number of blades, blades "Complete" alternating with blades whose surface in contact with the bath is reduced from 10% to 30% compared to the surface of the complete blade. The reduction in surface area of one blade in two at the bottom can be performed in several ways, depending, among other things, on the chosen form for the "complete" blade. We can for example rotate blades Rectangular “complete” with reduced surface blades where we have simply decreased the height of the rectangle. We can also do alternate rectangular blades with trapezoidal blades of the same height at the hub but lower height at the blade end. Other configurations are possible, the important thing being that, for the blade to reduced surface as for the "complete" blade, the combination forms the blade / position of the orifices is such that the gas jet is at its greatest share supported and dispersed by the blade. This can lead in the case of blades alternating at a different position of the orifices in front of the surface blades reduced compared to their position in front of the "complete" blades. But we can also choose the respective shapes of the “complete” blades and the blades with reduced surface to be able to use orifices positioned so identical for all blades.

The important thing to obtain the maximum result is that the surface of the blades is sufficient and that alternating "complete" blades and blades to reduced surface. The favorable effect of alternating blades on the appearance of surface waves, projections and vortex, which remains for the moment unexplained, begins to make itself felt when every second pay has a area reduced by 10%. When the reduction in area of one blade in two reaches 30% the effectiveness of the treatment, all other things being equal, probably starting to decrease because the agitation is insufficient.

EXAMPLE

• un dispositif A suivant l'art antérieur, couramment utilisé dans les installations industrielles actuelles et représenté à la figure 2. Le diamètre extérieur du rotor était de 250 mm et comportait 8 pales identiques de forme rectangulaire de hauteur 100 mm dans le sens vertical et de largeur 30 mm dans le sens horizontal. Le moyeu central était de diamètre 190 mm. Le ratio entre le diamètre extérieur du rotor et le diamètre de son moyeu (la portance des pales) était de 1,3. L'injection de gaz était effectuée suivant le principe de ce rotor conventionnel par 8 trous de diamètre 2,5 mm débouchant en extrémité de pale.
• un dispositif B représenté figure 3a. Ce dispositif comportait un disque d'épaisseur 15 mm et de diamètre extérieur 250 mm. Il comportait 8 pales rectangulaires identiques de hauteur constante 85 mm dans le sens vertical et de largeur 75 mm dans le sens horizontal. Le moyeu central était de diamètre 100 mm. Le ratio entre le diamètre extérieur du rotor sur le diamètre de son moyeu central était de 2,5. L'injection de gaz conformément à l'invention était réalisée par 8 orifices situés dans un même plan horizontal, diffusant horizontalement des jets de gaz approximativement dirigés suivant les bissectrices des angies formés par deux pales successives et ceci approximativement à mi-hauteur des pales. Ces orifices avaient un diamètre identique de 2,5 mm.
• un dispositif C suivant l'invention et représenté figure 3b de mêmes dimensions que le dispositif B mais comportant en alternance des pales "complètes" et des pales à surface réduite. 4 pales, identiques aux pales du dispositif B, avaient une hauteur constante de 85 mm dans le sens vertical. Les 4 autres pales, alternées avec les précédentes avaient une hauteur variable de 85 mm à leur raccordement au moyeu central à 65 mm en extrémité de pale. L'injection de gaz, comme pour le dispositif B, était effectuée par des orifices de 2,5 mm situés sur un même plan horizontal diffusant horizontalement des jets à mi-hauteur des pales que celles-ci soient complètes ou tronquées.

In a tank with internal dimensions 800 mmx800 mmx800 mm filled with a load of liquid aluminum of 1200 kg were successively tested:

• a device A according to the prior art, commonly used in current industrial installations and represented in FIG. 2. The outside diameter of the rotor was 250 mm and included 8 identical blades of rectangular shape with a height of 100 mm in the vertical direction and width 30 mm horizontally. The central hub was 190 mm in diameter. The ratio between the outside diameter of the rotor and the diameter of its hub (the lift of the blades) was 1.3. The gas injection was carried out according to the principle of this conventional rotor by 8 holes of 2.5 mm diameter opening at the end of the blade.
• a device B shown in FIG. 3a. This device included a 15 mm thick disc with an outside diameter of 250 mm. It included 8 identical rectangular blades of constant height 85 mm in the vertical direction and width 75 mm in the horizontal direction. The central hub was 100 mm in diameter. The ratio between the outside diameter of the rotor and the diameter of its central hub was 2.5. The gas injection in accordance with the invention was carried out by 8 orifices situated in the same horizontal plane, horizontally diffusing gas jets approximately directed along the bisectors of the angies formed by two successive blades and this approximately halfway up the blades. These holes had an identical diameter of 2.5 mm.
• a device C according to the invention and represented in FIG. 3b of the same dimensions as the device B but comprising alternating "complete" blades and blades with reduced surface. 4 blades, identical to the blades of device B, had a constant height of 85 mm in the vertical direction. The other 4 blades, alternating with the previous ones, had a variable height of 85 mm at their connection to the central hub at 65 mm at the end of the blade. The injection of gas, as for device B, was carried out by 2.5 mm orifices situated on the same horizontal plane horizontally diffusing jets at mid-height of the blades whether these were complete or truncated.

• le nombre de projections a été observé pour un débit de gaz de 6 Nm³/h et une vitesse de rotation de 250 tours/minute. Le nombre de projections par unité de temps a été réduit respectivement d'un facteur 2 avec le dispositif B et d'un facteur 3 avec le dispositif C par rapport au nombre de projections par unité de temps constatées avec le dispositif A de référence.
• les mesures de la profondeur du vortex (en cm) ont été effectuées volontairement sans injection de gaz. Les résultats sont résumés dans le tableau 1. Vitesse de rotation en tours/minute 250 300 350 Dispositif A 2 4 7 Dispositif B 1 3 5 Dispositif C 1 3 5
• l'amplitude des vagues de surface, très difficilement mesurable a été évaluée à l'oeil pour un débit de gaz de 6 Nm³h et deux vitesses de rotation. Les observations sont rassemblées dans le Tableau 2. Vitesse de rotation (en tours/minute) 250 350 Dispositif A (art antérieur) moyenne forte Dispositif B faible moyenne Dispositif C (suivant l'invention) très faible faible
• L'efficacité du traitement a été mesurée par le taux de réduction de la teneur en H₂ du métal liquide après un traitement de 6 minutes avec un débit de gaz de 6 Nm^3//h. Les résultats obtenus au cours des essais étaient du même ordre pour les trois rotors testés avec des taux de réduction compris entre 60 et 75%.

During the tests, the following parameters were measured or observed: frequency of projections, depth of the vortex, amplitude of the surface waves, effectiveness of the treatment. The results obtained were as follows:

• the number of projections was observed for a gas flow rate of 6 Nm ³ / h and a rotation speed of 250 revolutions / minute. The number of projections per unit of time was reduced by a factor of 2 with device B and by a factor of 3 with device C, respectively, compared to the number of projections per unit of time observed with the reference device A.
• the vortex depth measurements (in cm) were made voluntarily without gas injection. The results are summarized in Table 1. Rotation speed in revolutions / minute 250 300 350 Device A 2 4 7 Device B 1 3 5 Device C 1 3 5
• the amplitude of the surface waves, which is very difficult to measure, was evaluated by eye for a gas flow of 6 Nm ³ h and two speeds of rotation. The observations are collated in Table 2. Rotation speed (in revolutions / minute) 250 350 Device A (prior art) average strong Device B low average Device C (according to the invention) very weak low
• The effectiveness of the treatment was measured by the rate of reduction of the H ₂ content of the liquid metal after a treatment of 6 minutes with a gas flow rate of 6 Nm ^{3 /} / h. The results obtained during the tests were of the same order for the three rotors tested with reduction rates of between 60 and 75%.

Claims (4)

1. Rotary gas dispersion device for the continuous treatment of a liquid aluminum bath in a treatment ladle comprising a drive shaft used for the inlet of gas and a rotor, the said rotor being composed of an even number of blades laid out in a star formation around a central hub and an approximately flat disk covering the star formed by the blades, the gas being injected into the bath through orifices located between the blades, the ratio of the outside diameter of the rotor to the diameter of its central hub being between 1.5 and 4, characterized in that complete blades with a given contact surface area with the bath are alternated with blades with a contact surface area with the bath 10% to 30% less than the contact surface are of the complete blades.
2. Device according to claim 1, characterized in that the number of blades is between 6 and 8.
3. Device according to any one of claims 1 or 2, characterized in that the vertical position of the gas injection orifices is approximately at the mid-height of the blade, that they are drilled approximately horizontally and that their center line is approximately along the bisector of the angle formed by the two blades.
4. Device according to any one of claims 1 to 3, characterized in that the diameter of the orifices is between 1 and 5 mm.

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