Aluminium Purification

Abstract

A method for separating iron from an aluminium alloy comprises providing a first zone of an aluminium alloy at a first temperature at which the aluminium alloy is partially melted and any iron-containing particles therein are fully molten, and providing a second zone of the alloy at a second temperature at which the aluminium alloy is fully molten, such that a temperature gradient is created between the first zone and the second zone. By applying a static homogeneous magnetic field to the alloy, and maintaining the temperature gradient and the magnetic field for a period of time, the iron content of the first and/or second zone can be reduced.

Claims

1. A method for separating iron from an aluminium alloy, the method comprising: providing a first zone of an aluminium alloy at a first temperature at which the aluminium alloy is partially melted and any iron-containing particles therein are fully molten, and providing a second zone of the alloy at a second temperature at which the aluminium alloy is fully molten, such that a temperature gradient is created between the first zone and the second zone, applying a static homogeneous magnetic field to the alloy; and maintaining the temperature gradient and the magnetic field for a period of time sufficient to reduce the iron content of the first and/or second zone to below a predetermined level.

2. The method of claim 1, wherein the first temperature is from 450° C. to 650° C.

3. The method of claim 1, wherein the second temperature is from 500° C. to 700° C.

4. The method of claim 1, wherein the magnetic field strength is from 0.1 to 16 T.

5. The method of claim 1, wherein the alloy is heated up to the first and second temperatures, then the heating and the magnetic field are maintained together for a period of time sufficient to reduce the iron content of the first and/or second zone.

6. The method of claim 5, wherein the heating and magnetic field are maintained for a period of time of from 10 minutes to 10 hours.

7. The method of claim 1, wherein the alloy is fully melted, then cooled until the first zone reaches the first temperature and the second zone reaches the second temperature, and wherein the magnetic field is applied while the alloy is cooling.

8. The method of claim 7, wherein the alloy is cooled at a controlled rate.

9. The method of claim 1, wherein the application of the magnetic field results in the formation of an iron-enriched layer, the method further comprising separating the iron-enriched layer from the alloy.

10. The method of claim 9, wherein the iron-enriched layer is separated from the alloy while it is in liquid form.

11. The method of claim 9, wherein the method further comprises solidifying the alloy prior to separating the iron-enriched layer from the alloy.

12. The method of claim 11, wherein the iron-enriched layer is separated from the alloy by machining.

13. The method of claim 1, wherein the temperature gradient is formed by at least two heaters, one which heats the first zone of the alloy to the first temperature, and another which heats the second zone of the alloy to the second temperature.

14. An apparatus for separating iron from an aluminium alloy, the apparatus comprising: at least one heater arranged to heat an aluminium alloy in a first zone to a first temperature at which the alloy is partially melted, and to heat the alloy in a second zone to a second temperature at which the aluminium alloy is fully molten; and a magnetic field generator for generating a homogenous magnetic field across the alloy.

15. The apparatus of claim 14, wherein the magnetic field generator comprises a pair of permanent magnets.

16. The apparatus of claim 14, wherein the apparatus comprises a first heater arranged to heat the alloy in the first zone and a second heater arranged to heat the alloy in the second zone.

17. The apparatus of claim 14, further comprising a vessel for containing the molten alloy.

18. The apparatus of claim 14, further comprising a thermal insulating layer disposed between the heaters and the magnetic field generator.

19. The apparatus of claim 18, further comprising a water cooling plate disposed between the thermal insulating layer and the magnetic field generator.

20. The method of claim 10, wherein the iron-enriched layer is separated from the alloy by pouring, ladling or pumping.

Description

[0060] Embodiments of the invention will now be described with reference to the accompanying figures in which:

[0061] FIG. 1 is a schematic diagram of an apparatus in accordance with an embodiment of the invention, prior to heating the alloy;

[0062] FIG. 2 shows the apparatus of FIG. 1, after the alloy is heated;

[0063] FIG. 3 shows the apparatus of FIGS. 1 and 2, after the alloy has been held under a temperature gradient and a magnetic field for a period of time;

[0064] FIG. 4 is a schematic diagram of an apparatus in which an elongate alloy is processed in accordance with embodiments of the invention;

[0065] FIG. 5a is a vertical section of an X-ray tomographic image of an aluminium alloy after the alloy has been held under a temperature gradient and a magnetic field for a period of time, in accordance with an embodiment of the method of the present invention; and

[0066] FIG. 5b is a microscope image of the aluminium alloy of FIG. 5a, after cooling.

[0067] FIG. 6 is a microscope image of the Al-4Cu-1 Fe aluminium alloy after re-processing.

[0068] FIG. 1 shows an apparatus 10 for separating iron (Fe) from an aluminium alloy. The apparatus 10 comprises a crucible 12 which contains the aluminium alloy 14. The aluminium alloy 14 contains Fe contaminants in the form of Fe-enriched particles or intermetallics 11 around the grain boundaries of the alloy.

[0069] On either side of the crucible 12 there is a lower heating element 16 and an upper heating element 18. The apparatus 10 further comprises a magnetic field generator 20 comprising an opposing pair of permanent magnets that will generate a transversal magnetic field across the sample. The magnetic field generator 20 is placed outside of the heating elements 16, 18 in the embodiment shown. To keep the magnetic field generator 20 below its working temperature, it is separated from the heating elements 16, 18 by a high performance thermal insulating layer 22. A water cooling plate can also be inserted between the insulating layer 22 and the magnetic field generator 20 if needed (not shown).

[0070] The apparatus will now be described in use with reference to FIG. 2. The heating elements 16, 18 are turned on in order to heat the aluminium alloy 14 within the crucible 12. The lower heating elements 16 heat the alloy in a first zone 24 to a first temperature which is sufficient to keep the aluminium alloy 14 in a semi-solid condition in which the solid grains and liquid coexist. The upper heating elements 18 heat the alloy 14 in a second zone 26 to a second temperature which fully melts the alloy 14. The temperature in the first zone 24 is also high enough to melt the Fe-enriched particles 11 into the liquid which surrounds the solid grains 13 within the alloy 14. Thus, a temperature gradient is established across the alloy forming a liquid zone 26 and a semi-solid zone 24 in which Fe-particles 11 are fully re-melted into the liquid.

[0071] With reference to FIG. 3, a static homogenous magnetic field is provided by the magnetic field generator 20, as indicated by the arrows. The alloy 14 is held under the temperature gradient and the magnetic field for a period of time, causing Fe to move from the molten and semi-molten zones 24, 26 to the interface of the zones 24, 26. This results in the formation of an Fe-enriched layer 28 between the two zones 24, 26 and a consequent reduction in the amount of Fe present in these zones. The Fe-enriched layer 28 can then be removed directly from the liquid alloy, for example by pouring, ladling or pumping. Alternatively, the heating elements 16, 18 can be switched off or powered down in a controlled manner, allowing the alloy to cool to room temperature and solidify. The resulting Fe-enriched layer can then be separated from the rest of the alloy, e.g. by machining.

[0072] FIG. 4 shows an apparatus 100 in which an elongate sample of alloy 114 is processed in stages. The apparatus comprises a crucible 112 in which the elongate alloy sample 114 is received. On each side of the crucible 112 there is a lower heating element 116 and an upper heating element 118. A pair of opposing permanent magnets (not shown) is disposed outside of the heating elements 116, 118. The crucible 112 with the sample 114 within is moveable relative to the apparatus 100, as indicated by the arrow. It will be appreciated that the crucible 112 containing the sample 114 may move while the heating elements 116, 118 and magnets are stationary, or that the heating elements 116, 118 and magnets may move along the length of the crucible 112.

[0073] In use, the heating elements 116, 118 heat a portion of the sample 114 that is disposed between them, thereby creating first and second zones in the alloy as previously described. A static homogenous magnetic field is applied, causing the formation of a Fe-enriched layer between these zones. The crucible 112 is then moved relative to the heaters 116, 118 and magnets so that the treated portion of the sample 114 is no longer subject to heating or the magnetic field and is allowed to cool, while the next portion of the sample 114 is received between the heating elements 116, 118 and magnets and treated in the same way. The process is repeated until the whole length of the sample is treated. This results in a Fe-enriched band across the full length of the sample, which may then be separated from the rest of the solidified sample as previously described.

EXAMPLE 1

[0074] The method of the invention was tested using the alloy Al-7Si-3.5Cu-0.8Fe (weight percent). The alloys formed plate-shape β (Al.sub.5SiFe) intermetallics around grain boundaries. The sample (1.8 mm diameter) was partially melted under two heaters. The temperature in the upper region of the sample (fully molten zone) was around 620° C. while the temperature in the lower region of the sample (partially molten zone) was around 580 to 590° C. While the temperatures of the zones were maintained, the sample was held in a steady and homogeneous transverse magnetic field of 0.5 T for 25 min.

[0075] As shown in FIG. 5a, three distinctive layers were observed: (i) the top layer—fully molten alloy; (ii) the middle layer—enriched with iron; and (iii) the bottom layer—semi-solid alloy (a zone where liquid and solid co-exist).

[0076] After the holding period, the sample was cooled down to room temperature under the same magnetic field. As shown in FIG. 5b, after solidification the bottom part of the sample (corresponding to the partially molten zone (iii) of FIG. 5a) was almost free of plate-shape β (Al5SiFe) intermetallics (volume fraction 0.002). There were significantly more β intermetallics formed within the top region of the sample (corresponding to the top liquid zone (i) and the iron-rich layer (ii) of FIG. 5a), as indicated by the arrows (volume fraction 0.024). This demonstrates the successful separation of the iron-containing β phase in Al—Cu—Si based alloys.

EXAMPLE 2

[0077] The method of the invention was tested using the alloy Al-4Cu-1Fe (weight percent). The sample (1.8 mm diameter) was fully melted at a temperature gradient of 20° C./mm and held for 5 min for temperature homogenization. Afterwards, a 1 T transversal magnetic field was applied, and the sample was cooled down within the 1 T magnetic field at 6° C./min. The results show that Fe-containing intermetallics (Al.sub.3Fe and Al.sub.7Cu.sub.2Fe) were aggregated on one side of the sample (FIG. 6). This demonstrates that Fe can be successfully separated from Al—Cu based alloys.