WO2005103335A1

WO2005103335A1 - A device and a method for cleaning and purifying molten aluminium

Info

Publication number: WO2005103335A1
Application number: PCT/CA2005/000626
Authority: WO
Inventors: Sylvain P. Tremblay
Original assignee: Les Produits Industriels De Haute Temperature Pyrotek Inc.
Priority date: 2004-04-23
Filing date: 2005-04-25
Publication date: 2005-11-03

Abstract

The present invention relates to a prefabricated refining device and a method for cleaning molten aluminium. The invention also relates to the process of fabrication of the device. The prefabricated refining device comprises a metallic tube made of a metal that melts in molten aluminium and a refining salt flux solidified in the metallic tube. When the refining device is fed into molten aluminium, both the tube and the refining salt flux melt, whereby the melted refining salt flux is released into the molten aluminium and reacts with contaminants contained in the aluminium to provide clean molten aluminium.

Description

A DEVICE AND A METHOD FOR CLEANING AND PURIFYING MOLTEN ALUMINIUM

FIELD OF THE INVENTION

The present invention relates generally to the field of devices and processes for treating molten metals. More particularly, the present invention relates to a device and a method for cleaning and purifying molten contaminated aluminium.

BACKGROUND OF THE INVENTION

In the aluminium industry, treatment of molten metal has been usually made with particulate refining agents such as halide salts to remove inclusions and alkali metals. The most commonly known of these halide salts is MgCI₂ that is usually coupled with NaCI, KCI and/or cryolite to remove solid inclusions from molten aluminium.

Also known in the prior art are a variety of methods to add these particulate refining agents to molten aluminium. Some of these methods will be briefly reviewed herein below.

One of the prior art methods consists in shoveling the refining agent at the surface of molten metal or throwing bags of such refining agent directly into the furnace on the bath surface. This method is not very efficient and a lot of material is lost in the collection system. Steel tools attached to a lift truck can improve the efficiency if the melted bath is stirred.

Another prior art method consists in putting some refining agent at the bottom of a transfer trough and pour molten aluminium onto it. The turbulence generated during the transfer is the mixing mechanism.

A solid particulate injection system can also be used. The solid particulate can either be injected at the molten aluminium surface or below the surface using a long tube. In US patent no. 5,080,715, (PROVENCHER, R. et al.) describe a rotary injector consisting of a material feeder coupled to a hollow shaft turning at a determined speed. A carrier gas is then needed to propel the solid particulate material into the hollow shaft. This results in the formation of gas bubbles, each surrounded by a layer of liquid salt.

Finally, as described in US patent no. 6,602,318 B2 (BILODEAU, J.F. et al.), the solid particulate material can be gas fed to the molten metal through an injection tube below the surface of the molten metal, adjacent to the axis of the impeller used to mix the molten metal.

The apparatus disclosed in US 6,602,318 B2 mainly consists of an injection tube for the refining agent, which extends downwardly in the vicinity of the top face of the impeller. More specifically, the upper end of injection tube is connected by way of a flexible tube to a reservoir for the particulate refining agent. Treatment agent delivered to the upper end of the injection tube falls to the bottom under gravity. Moreover, a small flow of gas is maintained through the tube to prevent metal from flowing back up the tube.

In both US patent nos. 5,080,715 and 6,602,318 B2, the salt bubbles collide with wetted contaminating particles, thus dewetting the latter. Thereafter, the contaminating particles and the salt flux rise to the surface of the melt, forming an easily removable layer of dry powder.

As can be appreciated, most of the prior art devices and processes rely on feeding the refining agents to the molten metal to be treated by way of gas flow. This type of feeding creates a positive flow toward the molten metal. Use of gas to feed refining agents to molten metal is also explained by the fact that the commonly used refining agents are hygroscopic, meaning that th^y readily absorb moisture, as from the atmosphere. Therefore, they need to be carried by gas to their site of action without contacting moisture beforehand.

However, when using gas to inject a refining agent, a high number of gas bubbles can be created upon injection. The refining agent thus injected can be directly carried by the gas bubbles to the surface of the molten metal without even having exerted its action.

One solution to gas feeding of refining agents is to assemble a purifying device near the bath of molten contaminated metal, before purification thereof. One such example is disclosed in US patent no. 4,702,768, wherein AREAUZ et al. disclose the compaction and the strip formation of a metal salt flux before its introduction into a bath of molten contaminated metal. The metal salt flux is formed as it enters the molten contaminated metal. This specific formation/introduction sequence insures a more intimate contact between the flux and the metal to be cleaned. The gradual introduction of the flux avoids any considerable disturbance in the pool of molten contaminated metal. However, the purifying device of AREAUZ et al. requires the presence of bulky instruments to carry its non-compacted components near the cleaning site.

The known devices and methods often resort to the use of chlorine flux to clean molten contaminated metal. Consequently, gas emissions such as HCI gas emissions are produced in the purification process. These gas emissions being health hazardous, have to be removed from the casthouse. Understandably, use of chlorine compounds should be kept to a minimum during purification of molten contaminated metal.

Furthermore, the introduction of gas along with the purifying agent into the molten contaminated metal leads to a decrease in liquid-liquid contact areas between the refining agent and the molten metal and thus to a decrease in the efficient purification of the molten metal by the refining agent .

Also known in related prior art is US 5,781 ,846 (JOSSICK) which discloses a prefabricated mobile tubular instrument, made of an external layer of metal that covers a central flux, this instrument is intended for brazing and not cleaning molten contaminated aluminium. SUMMARY OF THE INVENTION

One object of the present invention is to propose a device and a method to efficiently and easily clean and purify molten contaminated aluminium, in order to solve several of the inconveniences associated with prior art devices.

Accordingly, this object is achieved by a prefabricated refining device for cleaning molten contaminated aluminium. This device comprises:

- a metallic tube made of a metal that melts in molten aluminium; and

- a refining salt flux solidified in the metallic tube.

The present invention also relates to a process for prefabricating the above- described device. More particularly, this process comprises the steps of: a) heating a metallic tube; b) melting a mixture of refining salt flux to obtain a melted refining salt flux mixture; c) filling the heated metallic tube of step a) with the melted mixture of step b) to obtain a heated refining device; and d) letting the heated refining device cool down to ambient temperature to obtain the prefabricated refining device.

Preferably, the metallic tube is made of aluminium, the same metal as the one to be cleaned. Preferably also, the refining salt flux is a mixture of salts. The flux may be any mixture of at least two fused halide salts, characterized in that they are hygroscopic. Preferably, the process of the present invention further comprises, between steps c) and d), a step of sealing the heated refining device to keep moisture away from the refining salt flux mixture.

The present invention also provides a method for- cleaning and purifying molten aluminium. This method comprises the steps of: a) providing a prefabricated refining device as defined hereinabove;and b) feeding the refining device into molten aluminium so that, upon contact with the molten aluminium, both the tube and the refining salt flux melt;

whereby the melted refining salt flux is released into the molten aluminium and reacts with contaminants contained in the aluminium to provide clean molten aluminium.

Preferably, the step b) of feeding in the method of the invention is performed by a rod feeder that more preferably works at a predetermined feeding rate for efficient cleaning.

More preferably also, the rod feeder further comprises an auxiliary feeding apparatus to facilitate a trickle flow of gas around the prefabricated refining device and thus, to prevent backflow of molten aluminium into the rod feeder.

In a preferred variant, the auxiliary feeding apparatus comprises a pipe made of a refractory material, having an upper end opposite a lower end and a gas inlet located in an upper section of the pipe; a connector for sealingly connecting a feeding end of the rod feeder to the upper end the pipe. A gas injecting means is operatively connectable to the gas inlet for injecting an inert gas around the prefabricated refining device while it crosses the pipe, thereby preventing a backflow of molten aluminum.

As can be appreciated from the above features, the present invention presents many advantages, some of which are presented hereinbelow. First, the present device and method alleviate the need to use an injector to feed the flux into the molten contaminated metal.

Furthermore, owing to the prefabrication of the refining device, the present invention is easily achieved and readily put into practice on site. There is no need to bring large volumes of flux on site since it is already precompacted in an easy-to-carry refining device.

Another advantage of the present invention is that it provides a high control level of the refining device, even in situations where low feed rate additions are required, depending on the level of contamination of the molten aluminium.

Finally, the device of the invention presents the additional advantage of being protected from moisture. More particularly, the prefabrication of the refining device prevents the hygroscopic flux from coming into contact with any surrounding moisture, a jeopardizing factor in the efficient removal of contaminating particles by the flux.

These and other features, objects and advantages of the present invention will be better understood upon reading the following nonrestrictive description of preferred embodiments thereof, made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will become apparent upon reading the detailed description and upon referring to the drawings in which :

Figure 1 is a cross-sectional view of the prefabricated refining device, according to a preferred embodiment of the present invention.

Figure 2 is a perspective view of the prefabricated refining device shown in Figure 1 , in coiled form. Figure 3 is a front schematic view of an auxiliary feeding apparatus for use with a rod feeder.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to- such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents which may be envisioned.

DESCRIPTION OF PREFERRED EMBODIMENTS

Refining device and its process of fabrication

According to a first preferred embodiment of the present invention shown in Figures 1 and 2, the prefabricated refining device (10) comprises a metallic tube (20) made of a metal that melts in molten aluminium and a refining salt flux (30) solidified in the metallic tube.

The metallic tube (20) is preferably made of the same metal as the one to be cleaned, namely aluminium. This prevents introducing additional, eventually contaminating elements into the molten aluminium. This preferred aspect of the invention also has the advantage that both the metallic tube (20) and the molten aluminium melt at essentially the same melting point.

The metallic tube (20) can alternatively be made of a metal like steel or any equivalent thereof. It should be understood that, given the tubular shape of the device (10), the amount of steel introduced into the molten aluminium is minimal. For instance, a steel tube (20), according to a preferred aspect of the invention, when used for the treatment of a typical 100 metric ton aluminium bath, increases the iron content in aluminium by .a mere 5.6 ppm. It is clear for any person skilled in the art of the invention that this relative contamination is negligible.

Referring back to Figure 2, the aluminium tube (20) is preferably coil-shaped for easy shipment of the prefabricated refining device (10), In use, the tube (20) is wound on a reel and is gradually fed into the molten aluminium by gradually unwinding the reel. The aluminium tube (20) is preferably of defined dimensions for the purpose of the invention. Any person skilled in the art of the present invention will understand that the expression "defined dimensions" preferably encompasses the diameter, wall thickness and total length of the aluminium tube (20). Preferably also, the diameter is about 0.45" (or 11.5 mm), with a very thin wall thickness. By the expression "very thin wall thickness", it is meant a tube thick enough to efficiently enclose the flux while being thin enough to melt upon contact with the molten contaminated aluminium, as described hereinbelow. Preferably, the expression "very thin wall thickness" indicates a thickness of about 0.025" (or 0.6 mm).

According to a preferred aspect of the present invention, the just-mentioned preferred dimensions are preferably efficient with an aluminium tube (20) of about 10,600 feet (or 5,000 m) in total length.

As mentioned above, the prefabricated refining device (10) of the invention also comprises a refining salt flux (30) that is preferably a mixture of salts. By the term "refining" in the expression " refining salt flux ", it will be understood that the device (10) of the present invention cleans and purifies molten contaminated aluminium. According to a preferred aspect of the invention, the refining salt flux (30) can be any mechanical mixture of at least two (2), preferably two (2), and more preferably three (3) refining agents. Moreover, these refining agents preferably comprise halide salts that can be chosen from any one of such known salts, as is apparent to any person skilled in the art of the present invention. Halide salts to be preferably used at predetermined proportions according to the present invention, can be chosen from the group of MgCI₂, KCI, NaCI, AICI₃ , cryolite and any other known equivalent salt.

In this connection, the flux (30) of halide salts can be obtained by the fusion and thereafter the solidification of the above-mentioned halide salts at the predetermined proportions, under the form of an eutectic compound having a melting point clearly lower than the original salts. A fused salt composition is thus obtained. According to a preferred aspect of the invention, the mixture of halide salts can be any different salt mixture, preferably from 20% to 60% MgCI₂ and from 40% to 80% KCI, but also preferably different MgCI₂/ KCI mixtures to which cryolite may be added. Preferably also, a mixture of MgCI₂ and AICI₃, at proportions ranging from 25% to 50% MgCI₂ and from 50% to 75% AICI3, and more preferably at proportions of 50% MgC-₂/50% AICI₃, may be used. Halide salt combinations that comprise aluminium provide the further advantage of using, within the refining device, the same metal, namely aluminium, as the one to be treated, thereby decreasing the percentage of impurities added to the molten aluminium.

Alternatively, the flux (30) of the invention can be in powder form, preferably in the form of a ground solid refining particulate material. According to this preferred aspect, the refining agents can comprise a mixture of refining salts powders.

The refining device (10), with its aluminium tube (20) and refining salt flux (30) as described hereinabove is further characterized in that it is prefabricated. In other words, this device (10) is fabricated before being transported to the purification site. Such a prefabrication alleviates the need to bring bulky equipment and non compacted flux near the purification site. The fabrication process that can be performed at any manufacturing location, comprises the steps of: a) heating the metallic tube (20), preferably at a temperature of about 400°C to 450°C; b) melting the mixture of refining salt flux (30) to obtain a melted refining salt flux mixture; c) filling the heated metallic tube of step a) with the melted mixture of step b) to obtain a heated refining device; and d) letting the heated refining device cool down to ambient temperature to obtain the prefabricated refining device (10). The step b) of melting the mixture of refining salt flux is preferably achieved at the melting point temperature of the refining salt flux (30) mixture. More preferably, this refining salt flux (30) mixture that can be in the form of a fused salt composition or in powder form, presents a melting point in the range of around 400°C to 450°C. Of course, the melting point of the refining salt flux (30) mixture depends on the specific mixture of salts and their respective proportions, as can be appreciated by any person skilled in the art of the invention.

The step c) of filling the heated metallic tube with the melted salt mixture is preferably done by any suitable means that can efficiently compact a salt composition into a container, namely the metallic tube. According to a preferred aspect of the present invention, this filling step is achieved by means of an air injector or under vacuum.

As can be appreciated by any person skilled in the art of the present invention, although the refining salt flux (30) of the present invention has indeed the potential of cleaning and purifying contaminated aluminium, this flux is also hygroscopic, owing to its chemical composition. In other words, the refining salt flux readily absorbs moisture. Thanks to the tubular shape of the refining device (10), as best depicted in Figure 2, areas of contact between the refining salt flux (30) and the atmosphere are kept to a minimum. However, transport of the refining device from the site of fabrication to the purification site may increase the possibility of such a contact between flux and atmosphere. In order to alleviate any remote possibility of undesirable contact between the hygroscopic flux and surrounding moisture that could jeopardize efficient removal of contaminants, the prefabrication process may preferably comprise, between steps c) and d), a further step of closing, preferably sealing the prefabricated refining device (10) to keep moisture away from the refining salt flux (30) mixture. Therefore, moisture blockage of the refining device is immediately achieved during the fabrication process.

Method for cleaning and purifying molten aluminium

The present invention also provides a method for cleaning and purifying molten aluminium, by using any preferred embodiment of the above-described prefabricated refining device (10). This method is preferably carried out in a purifying container that can withstand relatively high levels of temperature such as those needed to melt a metal like aluminium. This container can be a crucible, a ladle, a furnace either tilting or stationary, a transfer trough, a distribution trough or any equivalent thereof presenting similar characteristics.

The above-mentioned prefabricated refining device (10) is fed into molten aluminium so that, upon contact therewith, both the tube (20) and the refining salt flux (30) melt. Consequently, the melted refining salt flux (30) is released into the molten aluminium and reacts with contaminants within the aluminium to provide clean molten aluminium.

Feeding of the refining device into the molten aluminium is performed at a predetermined feeding rate, preferably provided manually or by a rod feeder, and more preferably by a conventional rod feeder (not illustrated). An advantage of such a feeding apparatus is that rod feeders are commonly found at plant sites. No additional equipment is thus required for feeding. For example, a grain refiner rod feeder provides a wide range of addition rates, from very low feeding rates to very high feeding rates. More specifically, this type of rod feeder works at a speed ranging from about 3 cm/min to about 60 cm/min; and is more preferably used at 5 cm/min or 6.25 cm/min. Moreover, a 50 metric tons/hour cleaning/purification treatment requires a feeding rate of 6.6 cm/min, which is well within the range of any usual grain rod feeder.

After feeding, proper mixing of the melted refining salt flux (30) mixture with the molten aluminium is also sought. Such mixing increases the probability of contact between the salt mixture and the metallic/non-metallic inclusions that contaminate the molten aluminium, thereby increasing the efficiency of removal of the metallic/non-metallic inclusions from the molten contaminated metal. In preferred situations wherein the flux is made of KMgCI₃ and the metallic tube (20) is made of aluminium, upon contact with molten aluminium, KMgCI₃ goes from solid phase to liquid phase, in the form of droplets. The droplets of KMgCI₃ can persist in the molten metal for some time before being eliminated by floating out. Throughout their presence in the bath of molten aluminium, the droplets of KMgCI₃ can react with alkali/alkaline earth metals, such as Na, Li or Ca, usually found as contaminants in molten aluminium and efficiently remove them. Upon collision between KMgCI₃ droplets and alkali/alkaline earth metals, the following chemical reactions take place, namely:

KMgCI₃ + 2Na → 2NaCI + KCI + Mg

KMgCIs + 2Li → 2LiCI + KCI + Mg

KMgCI₃ + Ca → CaCI₂ + KCI + Mg.

As can be appreciated from the chemical reactions above, the endpoints of the present method are efficient alkali metal reduction and, consequently, aluminium quality.

According to yet another aspect of the invention, to ensure efficient completion of chemical reactions as the ones presented above, the prefabricated refining device can be fed into the molten aluminium by a number of means. These feeding means are characterized in that they preferably provide proper mixing of the salt mixture and the metallic/non-metallic inclusions contaminating the molten metal to be treated.

According to a preferred aspect of the invention, proper mixing may be provided by any rotor such as a high shear energy rotor, a degassing rotor, a rotary flux injector or any equivalent thereof as can be envisioned by any person skilled in the art of the present invention; or by the vortex created by a typical metal pump. Any person skilled in the art will understand that any suitable mechanical mixing means can be used in the context of the present invention, without departing from the purpose thereof.

More specifically, mixing of the melted refining flux of the device with the molten aluminium can preferably be achieved by feeding the prefabricated refining device

(10) close to a rotor or into a rotor shaft. Since the rotor shaft preferably reaches the bottom area of the purifying container, feeding the refining device therein ensures a sustained contact between the melted refining flux and the bulk of molten aluminium.

Preferably also, the prefabricated refining device (10) can be fed into a transfer trough and proper mixing of the melted refining flux of the device with the molten 5 aluminium is generated by the transfer thereof.

Yet preferably also, the prefabricated refining device can be fed into a molten metal pump system. A metal pump is a typical device found at any plant. This pump usually comprises an impeller driven by a motor at its base, to pump and push back the molten aluminium under the metal surface and thus increase the probability of o contact between the molten aluminium to be treated and the melted refining salt flux. It is a very efficient way to move molten aluminium with minimum surface turbulence and thus minimum formation of air bubbles. Such a feature is of particular importance in the present context since efficient cleaning and purification of molten aluminium may be impeded by the presence of air bubbles that may transport the 5 melted refining salt flux, in the form of salt droplets, to the surface even before it has exerted its action on the molten contaminated aluminium. In this connection, the prefabricated refining device can be fed at the metal pump exit to maximize shearing and distribution of the salt droplets into the molten aluminium bath.

As can be appreciated by any person skilled in the art of the invention, the o prefabricated refining device can be fed into the purifying container before any other typical metal treatment steps, such as degassing. Accordingly, the refining device can be preferably fed just before an in-line degassing box, such as a SNIF (Spinning Nozzle Inert Floatation) degasser, used to remove exhaust hydrogen gas. Alternatively, the refining device may be fed into the purifying container before a5 compact degasser, such as the Alcan compact degasser, where the degassing rotors shear the melted refining agent into very small droplets. By then, the inclusions are removed from the molten aluminium, without initial use of the health-hazardous chlorine gas. According to yet another preferred embodiment of the present invention, the rod feeder further comprises an auxiliary feeding apparatus (70), shown in Figure 3. This auxiliary feeding apparatus (70) preferably aims at facilitating a trickle flow of preferably inert gas around the prefabricated refining device, and at assisting the 5 refining device into the molten aluminium at the desired depth. Provision of this inert gas around the refining device prevents molten aluminium to backflow into the rod feeder and thus the precarious melting of the refining salt flux into the rod feeder even before it reaches the bath of molten aluminium.

Referring to Figure 3, one can appreciate that the auxiliary feeding apparatus (70) l o comprises a rod seal fitting (72) to be connected to the end of the rod feeder (50), a metering valve (74) to control the supply of inert gas (76) and a pipe (78), preferably ceramic coated or made of graphite, to facilitate a trickle flow of inert gas around the refining device (10), as it is fed under the molten aluminium. Inert gas according to the present invention is to be understood by any person skilled in the art of the

15 present invention as any of the elements in Group O of the periodic table which are monatomic and, with limited exceptions, are chemically inert. Inert gases (76) in the context of the present invention may be chosen from the group of helium, neon, argon, krypton, xenon, and radon. Preferably, helium or argon is used as an inert gas and more preferably, argon. According to another preferred aspect, nitrogen

20 may also be used instead of the inert gas.

EXAMPLE

TEST PROCEDURE

Two tests were conducted to evaluate the performance of a refining device according to the invention, hereinafter referred to as the Flux In Tube (FIT). The auxiliary 25 feeding apparatus, hereinafter referred to as the FIT Underpour Apparatus, was mounted on the roof of the first chamber of a 2-stage degassing unit. A single AA- 5052 alloy DC ingot of approximately 18" x 60" was cast during each test.

The exit of the FIT was positioned at 24" under the molten aluminium bath at steady- state, just slightly above the rotor. The FIT was fed manually through the FIT Underpour Apparatus during the tests by marking the FIT and using a stopwatch.

The FIT Underpour Apparatus was provided with a "sealing valve" and a "union fitting" to allow for easier insertion of the FIT into the molten aluminium bath and connection to the rod feeder supply piping.

Feeding rates of 2.5 ipm (inches per minute) and 2.0 ipm were used for the first and second tests, respectively.

As stated above, the AA-5052 alloy was cast. Alkali metals ( Na and Ca) were added to the furnace to simulate plant conditions. Limca readings and Podfa samples were taken upstream and downstream of the degassing unit. Metal chemistry samples were also taken to measure the alkali metal removal.

RESULTS

The FIT fed easily through the FIT Underpour Apparatus. The FIT mixed well in the degassing unit chamber, as observed when the chamber was opened and quickly assessed at the onset of the test. The typical "fireflies" could be observed "dancing" on the melt surface above the rotor.

The degassing unit chamber then remained closed for the duration of the test to maintain the inert gas cover. The dross skimmed from the degassing unit appeared to be "dry" at the end of the cast.

The Limca readings and Podfa samples showed that the use of the FIT, as an alternative to the conventionally-used argon and chlorine gas in degassing units, efficiently removed alkali metals and decreased the level of inclusions in the molten aluminium.

Although preferred embodiments of the present invention have been described in detail herein and illustrated in the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments and that various changes and modifications may be effected therein without departing from the scope or spirit of the present invention.

Claims

WHAT IS CLAIMED IS:

1. A prefabricated refining device for cleaning molten aluminium, said device being characterized in that it comprises:

- a metallic tube made of a metal that melts in molten aluminium; and - a refining salt flux solidified in the metallic tube.

2. The prefabricated refining device of claim 1, wherein the metallic tube is made of aluminium.

3. The prefabricated refining device of claim 1 or 2, wherein the refining salt flux is a mixture of salts.

4. The prefabricated refining device of any one of claims 1 to 3, wherein the refining salt flux is a mixture of at least two fused halide salts.

5. The prefabricated refining device of claim 4, wherein the at least two halide salts are chosen from the group consisting of MgCI₂, KCI, NaCI, AICI₃ and cryolite.

6. The prefabricated refining device of claim 5, wherein the at least two halide salts are MgCI₂ and KCI.

7. The prefabricated refining device of claim 6, wherein the mixture comprises:

-from 20% to 60% of MgCI₂; and -from 40% to 80% of KCI .

8. The prefabricated refining device of claim 5, wherein the at least two halide salts

9. The prefabricated refining device of claim 8, wherein the mixture comprises: -from 25%> to 50% of MgCI₂; and

-from 50% to 75% of AICI₃.

10. The prefabricated refining device of any one of claims 1 to 3, wherein the refining salt flux is in powder form.

11. The prefabricated refining device of any one of claims 1 to 10, wherein the refining salt flux has a melting point around 400 to 450°C.

12. The prefabricated refining device of any one of claims 1 to 11, wherein the metallic tube is coil-shaped.

13. A process for prefabricating a refining device as defined in claim 1, the process being characterized in that it comprises the steps of: a) heating a metallic tube; b) melting a mixture of refining salt flux to obtain a melted refining salt flux mixture; c) filling the heated metallic tube of step a) with the melted mixture of step b) to obtain a heated refining device; and d) letting the heated refining device cool down to ambient temperature to obtain the prefabricated refining device.

14. The process of claim 13, wherein the step a) of heating the metallic tube is achieved at a temperature of about 400 to 450°C.

15. The process of claim 13 or 14, comprising between steps c) and d), an additional step of sealing the prefabricated refining device to keep moisture away from the refining salt flux mixture.

16. The process of any one of claims 13 to 15, wherein the step c) of filling is performed by means of an air injector.

17. The process of any one of claims 13 to 15, wherein step c) is performed under vacuum.

5 18. A method for cleaning and purifying molten aluminium, said method being characterized in that it comprises the steps of: a) providing a prefabricated refining device as defined in claim 1 ; and b) feeding said refining device into molten aluminium so that, upon contact with said molten aluminium, both the tube and the refining salt flux melt;

l o whereby the melted refining salt flux is released into the molten aluminium and reacts with contaminants contained in the aluminium to provide clean molten aluminium.

19. The method of claim 18, wherein the step b) of feeding is performed at a predetermined feeding rate.

20. The method of claim 19, wherein the feeding rate ranges from about 3 cm/min to 15 about 60 cm/min.

21. The method of any one of claims 18 to 20, wherein the step b) of feeding is performed by a rod feeder comprising a tube into which said refining device is introduced, said tube having a feeding end from which the refining device exits.

22. The method of claim 21 , wherein the rod feeder is a grain refiner rod feeder.

20 23. The method of claim 21 or 22, wherein the rod feeder further comprises an auxiliary feeding apparatus comprising:

-a pipe made of a refractory material, having an upper end opposite a lower end and a gas inlet located in an upper section of the pipe; - a connector for sealingly connecting the feeding end of the tube to the upper end the pipe; and

-a gas injecting means operatively connectable to the gas inlet for injecting an inert gas around the prefabricated refining device while it crosses the pipe, and the method further comprises the step of injecting an inert gas into the pipe to prevent a backflow of molten aluminium within the pipe.

24. The method of claim 23, wherein the pipe is made of ceramic.

25. The method of claim 23 or 24, wherein the inert gas is argon.