PURIFYING AN ALLOY MELT

Abstract

Device and method for melt treatment of aluminium alloys having excessive inclusions, impurities and unwanted gases to be removed, by (a) cooling the melt at an appropriate cooling rate to a temperature below the liquidus by shearing the melt associated with the introduction of at least one type of inert gases into the melt to form fine bubbles and high shear in the melt, and (b) purifying inclusions in the melt by floating them to the top surface, degassing the undesirable gases by reacting with the inert gas, and forming solid intermetallics containing impurity elements and transferring the melt mixture by the shearing device into a holding furnace, and (c) maintaining the melt in the holding furnace at a temperature below the liquidus and above the solidus temperature to settle the solid intermetallics formed by impurity elements as sediment at the bottom of the holding furnace while flowing the melt with much reduced inclusions, impurities and unwanted gases out of the holding furnace as applicable materials. The method is advantageously applicable for upgrading aluminium alloys from recycled and/or scrapped materials.

Claims

1.-11. (canceled)

12. A device for separating impurities from an alloy melt, including: (a) a rotor comprising a shaft having a first end and a second end, the shaft having a longitudinal channel with an inlet proximate the first end of the shaft and an outlet proximate the second end of the shaft to allow a fluid to flow into the channel through the inlet, through the shaft and out of the channel at the outlet, (b) a motor to rotate the shaft about its longitudinal axis, and (c) a stator in the form of a sleeve proximate the second end of the shaft, the sleeve having a wall with a plurality of apertures therein, wherein the rotor is encircled by a round stator, whereby in use fluid exiting the longitudinal channel of the shaft is able to pass through said apertures.

13. A device as claimed in claim 12, wherein the sleeve is a hollow cylinder.

14. A device as claimed in claim 12, wherein the apertures are distributed evenly around the sleeve wall.

15. A device as claimed in claim 12, wherein the motor is configured to rotate the shaft at rates from 1 rpm to 10000 rpm.

16. A method for separating impurities from an alloy melt, including the steps of: (a) providing a melt of the alloy in a first vessel, and including the following steps in any order: (b) deploying a device as recited in claim 1 and rotating the rotor in order to shear the melt, (c) deploying a device as recited in claim 1 and supplying to the alloy melt an inert gas by causing said gas to flow along the longitudinal channel of said device from the first end to the second end of the shaft in order that the gas exits the shaft, forms gas bubbles in the alloy melt, and passes through the aperture in the sleeve wall, and (d) reducing the temperature of the alloy melt to below its liquidus but above its solidus, in order to cause impurities to collect at the top surface of the melt, at the bottom of the vessel or to dissolve in the gas bubbles.

17. A method as claimed in claim 16, additionally including the step of: (e) after steps (a) to (d), transferring at least some of the alloy melt to a second vessel and leaving at least some impurities in the first vessel.

18. A method as claimed in claim 17 wherein the temperature of the alloy in the second vessel is maintained above its solidus but below its liquidus.

19. A method as claimed in claim 17, including the step of: (f) after step (e), removing at least some of the alloy from the second vessel and leaving at least some impurities in the second vessel.

20. A method as claimed in claim 16, wherein the alloy is an aluminium alloy.

21. A method as claimed in claim 20, wherein the alloy additionally includes at least one element chosen from Si, Mn, Mg, Cr, Co and Ni.

Description

[0045] A number of preferred embodiments of the present invention will now be illustrated with reference to the drawings in which:

[0046] FIG. 1 is a schematic representation of an embodiment of a device according to the present invention;

[0047] FIG. 2A is a schematic representation of a stirrer for a device in accordance with the present invention; and

[0048] FIG. 2B is a schematic representation of an alternative stirrer for a device in accordance with the present invention.

[0049] In FIG. 1, the dirty aluminium alloy melt with excessive inclusions, undesirable gases and impurity elements is melted first. A furnace 11 is usually regarded as melting furnace to supply aluminium melt 12 through a transfer tube 13 at a temperature above the liquidus to a shearing crucible 1. Then the aluminium melt 12 become the melt 2 by cooling down to a temperature below its liquidus in the shearing crucible 1, during which the sold intermetallics 3 is created in the melt.

[0050] A stirrer 5 located in a stator 8 is rotated inside the melt 2. The stator 8 is manufactured with several holes 4 on its wall and the inert gas 9 is supplied through the pipe 6 in the centre of the stirrer 5. The mechanism (not shown in FIG. 1) at one end of the stirrer 5 is used to supply shear and to break the inert gas 9 into small bubbles 7 through the holes 4, which are immediately dispersed into the melt 2. The gas bubbles 7 combine with the high shear to provide an appropriate cooling rate to create the solid intermetallics 3 in the aluminium melt 2.

[0051] The gas bubbles 7 react with dissolved gas during floating to the top surface, while affixing the inclusion 14, resulting the formation of sludge 15 on the top surface of the melt.

[0052] The simultaneous shearing and pumping action delivers the melt 2 with solid intermetallics into the holding furnace 21 through the connecting tube 22. At least one separator 23 is located in the middle of the holding crucible 21. The solid intermetallics 3 are settled as sediment in the holding crucible 21 to leave the melt 25 with much less iron content in comparison with the original melt 12.

[0053] The purified melt 25 is flown out of the groove 24 for casting. If the original melt 12 is charged continuously into the shearing crucible 1, the purified melt 24 can be continuously flown out from the holding furnace 21.

[0054] Turning to FIG. 2A, this depicts in more detail a stirrer 5 for use in the device of FIG. 1. Stirrer 5 includes a shaft 30 (with the pipe 6 of FIG. 1 not shown) having at the distal end of shaft 30 four vanes 35 protruding radially from shaft 30 and equally distributed about the periphery of shaft 30.

[0055] FIG. 2B shows an alternative embodiment of stirrer 5 having shaft 30 and vanes 35 as shown in FIG. 2A but also having flange 40 disposed radially about shaft 30 and capping the top surface of vanes 35.

[0056] In use, rotation of stirrer 5 generates an eccentric flow against the wall of stator 8 and this results in high shear and localised turbulence which generates fine bubbles of the inert gas as the gas passes through holes 4.

[0057] In the following example, an aluminium alloy with a nominal composition of 9 wt. % Si, 4 wt. % Cu, 3 wt. % Zn and 0.3 wt. Mn is provided with different levels of Fe and Mn contents to demonstrate use of the method of the present invention for iron removal.

[0058] Table 1 shows the variation of individual elements in the original alloy before and after processing of iron removal. It can be seen that the concentration of the elements except Fe and Mn is very consistent and there is no apparent variation in the original alloy and in the alloy after iron removal. However, the change in the levels of Fe and Mn are different during processing. The Mn concentration is adjusted at different levels in order to promote the formation of (Fe, Mn)-rich intermetallics. After the processing of iron removal, the Fe concentration is consistently reduced to a low level in each alloy, although it is different from one to the other. However, the final Mn concentration is close to the level or within the levels required by the international standard specification, although the higher initial Mn concentration leads to a higher final Mn concentration. The ratio of Fe/Mn has clearly confirmed the changes of the relationship between Fe and Mn in the original alloy, before deferrization and after deferrization.

[0059] Furthermore, the partition coefficient confirms that the Mn has significant effect on the final Fe concentration in the purified alloy. A higher Mn concentration leads to a lower final Fe concentration in the final purified alloy, which corresponds to a lower partition coefficient. In other words, a high Fe/Mn ratio in the original alloy leads to a high iron residue in the purified aluminium alloy and therefore a lower effectiveness of iron removal. It is important to point out that the iron removal process is achieved after preferably not more than 3 minutes of shearing, more preferably not more than one minute.

TABLE-US-00001 TABLE 1 The effectiveness of iron removal and the associated Mn content in Al-9 wt. % Si-4 wt. % Cu-3 wt. % Zn alloy. Test Partition number Fe Mn Si Cu Zn Al Fe/Mn coefficient Before ex. 1 1.03 0.28 9.24 3.36 1.72 84.37 3.68 iron ex. 2 1.15 0.88 9.17 3.43 1.73 83.64 1.30 removal ex. 3 1.15 1.45 9.24 3.31 1.71 83.14 0.79 ex. 4 1.05 1.95 9.29 3.42 1.75 82.54 0.54 After ex. 1 0.70 0.15 8.81 3.46 1.73 85.15 4.63 0.68 iron ex. 2 0.48 0.29 8.86 3.48 1.72 85.18 1.66 0.41 removal ex. 3 0.36 0.31 8.84 3.41 1.73 85.36 1.17 0.31 at 590 C. ex. 4 0.29 0.44 8.78 3.49 1.74 85.27 0.65 0.27 Note: all the compositions are in wt. %