Such a metal can be aluminium or an aluminium alloy, but it must be understood that in principle the invention can be applied to the purification of all streams of molten metal having a melting point higher than 500 °C. Such metals frequently contain impurities that lead to undesired inclusions on solidification.

If, for example, aluminium is cast and a very thin aluminium foil is then produced form the cast product, the thickness of such as foil is frequently determined by the diameter of the largest possible impurity that may be present that is present as an inclusion in the aluminium. After all, in the case of foils the sealing characteristics are frequently of greater importance than the strength. Sealing can be guaranteed only if inclusions are surrounded by metallic material at least on one side.

This means that with certain types of foils the wall thickness is not determined by the strength requirements imposed on the foil but by the fact that relatively coarse inclusions are present.

The effect of such inclusions can be limited to some extent by the use of (ceramic) filters. However, it has been found that conventional filters do not work well for further restriction of inclusions and in particular the removal of very fine inclusions.

It has been found that if a smaller pore size is chosen for the filters, clogging and other problems arise during casting. Moreover, such filters are expensive and have to be replaced regularly. Therefore, in practice, particles larger than 20, um are retained by conventional filters and smaller particles are allowed to pass through. This means that in the abovementioned example of a foil material the minimum wall thickness of such a foil material must be greater than 20 um.

Inclusions are undesired for other applications as well. These inclusions can lead to pitting at the surface, which stands in the way of certain surface treatments. Moreover, it has been found that for certain metal alloys the fatigue strength decreases appreciably if relatively large inclusions are present.

Examples of such inclusions are aluminium oxides.

The aim of the present invention is to provide a method for purifying a stream of molten metal with which particles smaller than 20 llm present as impurity can be filtered out. Such particles are, in particular, solid particles or a fractionally solidified phase in the

stream of liquid metal.

This aim is achieved with a method for purifying a stream of molten metal having a melting point higher than 500 °C in that this method comprises generating a spiral flow of said molten metal at the introduction point and discharging said metal from said chamber at a point higher than said introduction point and removing any impurities to a point lower than said introduction point.

According to the present invention use is made of the cyclone principle for purifying a stream of metal.

The use of cyclones for removing dust particles or other impurities from gases is generally known in the state of the art. Cyclones are also used for purifying gases that are produced when casting metals. One example of this is found in US 3 790 143 A.

However, despite the need to obtain a metal melt of high purity, which has already existed for decades, the use of a cyclone has never been considered.

Use of a cyclone in combination with molten metal requires the cyclone to be able to withstand the relatively high temperatures prevailing in this case and the aggressive or non- aggressive characteristics of the metal concerned. Thus, in certain applications it is possible to make the wall of the cyclone from a metal having a high melting point, or it is possible to use a ceramic material for this that, obviously, does not release any particles under the operating conditions for the cyclone.

According to an advantageous embodiment of the invention, the cyclone is constructed as a cone with an inlet located between the ends. Preferably, the axis of the cone is essentially vertical in the use position and the outlet is above the inlet and the cone tapers towards the bottom. The impurities collect in the tapered section.

The flow when the metal is moving in the cone is preferably laminar.

The cone angle is dependent on the requirements imposed on the cyclone. With respect to the vertical axis of the cyclone, this angle is between 1 and 45°. That is to say if the cone angle is relatively large, that is to say closer to 90° (that is to say parallel to the axis of the cyclone), a longer path is obtained for the purification of the metal, but the pressure drop over the cyclone is greater. This pressure drop is important because it must be guaranteed under all circumstances that the metal emerges centrally through the outlet.

Moreover, for many applications a specific minimum casting speed is essential. Thus, in the case of directly cooled casting, or DC casting, which is used in combination with aluminium alloys to produce ingots that be rolled out for the production aluminium foil, the

speed of the metal is preferably 6 cm/min. The various aspects are, of course, dependent on the product concerned and the dimensions of the product concerned.

The construction according to the present invention is characterised in that there are no moving parts.

The abovementioned flow can be further stabilised by fitting a hollow ring at the bottom of the cone, that is to say close to the location where the impurities are collected, which hollow ring is furthermore some distance away from the wall of the cone. In addition, the surface of the ring is preferably perpendicular to the axis of the cone. The ascending and descending flow are separated by this means. That is to say the descending spiral flow moves between ring and wall of the cone, whilst the ascending linear flow to the outlet is unimpeded.

The inlet is arranged between the outlet and the bottom of the cone (when the latter is positioned vertically). Preferably, this inlet is at 1/4-1/4the total height from the bottom of the cyclone. More particularly, it is approximately halfway up the height of the cyclone.

If the inlet is made too high, that is to say too close to the outlet, there is a risk of short-circuiting, that is to say the desired spiral flow of the metal is not established or is inadequately established, as a result of which a short-circuiting effect is obtained from the inlet to the outlet. If, on the other hand, the inlet is positioned too"low", it is not possible to guarantee that there is sufficient space for settling of the particles to be separated off.

The device described above for purifying a stream of metal is preferably used in a casting installation. More particularly, there is a conventional filter as described above in such a casting installation. This filter is used upstream of the device according to the invention, so that the device according to the invention serves exclusively for filtering off the finest impurities. More particularly, these impurities have a particle size of between 10 and 20, um.

The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing: Fig. 1 shows, diagrammatically, a casting installation in which the device according to the present invention has been incorporated; Fig. 2 shows, diagrammatically and partially exposed, the cyclone according to the present invention; and Fig. 3 shows a cross-section m-in from Fig. 2. hi Fig. 1 a casting installation is indicated in its entirety by 1. This installation

consists of a casting crucible 2 that can be tilted and in which a metal can be melted in some way or other (burner/electrically). After melting, the metal is introduced into casting channel 3 and transferred via a preliminary filter 4 into device 5 according to the invention.

After having been purified in this device, said metal is fed to a direct chill casting installation 6 that is shown diagrammatically. This installation is shown as a single installation, but it must be understood that a number of such continuous casting installations can be used as a series. In such a direct chill casting installation solidified metal is moved downwards under accurately conditioned circumstances that are not shown in more detail.

The filter 4 is a conventional filter known in the state of the art.

Cyclone 5 is shown in more detail in Figure 2 and 3. This cyclone consists of a block of ceramic material 11 in which a conical chamber 12 has been made. The axis of this conical chamber is vertical and is indicated by 10. The inlet into the conical chamber is indicated by 13. This inlet is coupled to the filter 4 (Fig. 1). Above the inlet there is an outlet 14 which, in turn, is connected to the direct chill casting installation 6.

At the bottom of the conical chamber 12 there is a ring 15 that is held some distance away from the wall of the cone and is fixed thereto with the aid of fixing rods or the like.

The bottom section of the cone is indicated by 7.

It can be seen from Fig. 3 that inlet 13 is tangential to the cone, whilst the outlet is central. For this purpose there is an outlet sleeve 20 to stabilise the flow.

When a molten metal such as a molten aluminium alloy (for example Al-Cu or Al-Si) is poured in, this enters conical chamber 12 via inlet 13. The metal describes a spiral path towards the bottom, which is indicated by arrow 18. That is to say, after passing ring 15 the flow reverses and is converted into a stable, vertical ascending flow. Any impurities deposit in space 17. The purified metal is discharged via discharge 14.

It will be understood that the method and device described above can be used for purifying any type of molten metal and are not restricted to aluminium or the alloys thereof.

Moreover, it will be understood that variants of the cone described above are possible depending on the metal, the intended purification and the desired casting speed. These and further variants are obvious to those skilled in the art after reading the above description and fall within the scope of the appended claims.