AU725438B2 - Apparatus and process for separating aluminium from a mixture of aluminium and aluminium dross - Google Patents

Description

AUSTRALIA Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Apparatus and process for separating aluminium from a mixture of aluminium and aluminium dross The following statement is a full description of this invention including the best method of performing it known to us:- 2 Field of the Invention This invention relates to an apparatus and a piocess for separating aluminium from a mixture of aluminium and aluminium dross. In particular, the invention relates to a process for recycling aluminium from dross produced during an aluminium melting processes, and to an apparatus for carrying out that recycling process.

Background of the Invention When aluminium is melted e.g. for manufacture of extrusions, because of the influence of oxygen from environmental air on the aluminium and the existence of impurities, particularly oxides, nitrides and carbides, in the molten aluminium, a layer of sludge, also known as dross rises to the surface of the molten aluminium. This layer of dross has to be removed from the molten aluminium before the molten aluminium can be extruded. This is *-*done by the use of a suitable ladle. In a rather crude process, known as 15 skimming, the ladle is dragged across the top of the molten aluminium and the dross is scraped into a suitable receptacle. During the skimming process, as well as removing sludge including oxides and other impurities, pure aluminium is also removed. The quantity of pure aluminium removed depends on the depth to which the ladle is inserted in the aluminium to ensure removal of all the dross and to a large extent depends on the skill of the furnace worker handling the ladle. However, typically 30 to 60% of the mixture/dross, by weight is aluminium.

The term dross, as used herein, refers to the impurities such as oxides which float to the surface of the molten aluminium, but the term is also used in the art to refer to the mixture of aluminium and the impurities.

Because of the amount of aluminium in the mixture, it is obviously desirable to remove as much aluminium from the dross/aluminium mixture as possible. Virtually all recycling is done by a process known as rotary salt furnace processing. In that process, the dross containing pure aluminium is first allowed to cool. The longer the aluminium spends hot, the more oxidation occurs and less aluminium is recovered in the recycling process, so often cooling is encouraged and accelerated by freeze drying. In some cases some initial separation of aluminium from the mixture is first carried out by one of two rather inefficient devices know as drip pans and dross presses, respectively. In the former the mixture is allowed to sit while molten and some of the aluminium will sink to, and agglomerate in, the bottom of the pan. In the latter, the mixture is compressed and the aluminium droplets tend to stick together. These processes are inefficient and have to be followed by rotary salt processing. Because the mixture is kept hot longer for the drip pan or dross press process, the recovery rate in the subsequent rotary salt process is reduced, so drip pans and dross presses are generally not commercially viable, and not often used.

Recycling is not generally done at the furnace, but is usually is done a specialist metal-recycling companies. In the rotary salt recycling process, the dross is heated and remelted and various salts and minerals are added in order to separate the aluminium from the oxides and other impurities. While the process is highly efficient in terms of the quantity of aluminium removed from the dross, removing approximately 95% of the available aluminium, the **waste product from the recycling process the mixture of salts and the oxides *is unpleasant, is very environmentally unfriendly, and is difficult to dispose of safely. Further, the process requires the transporting of the dross to the recycler in lorries or the like which is also undesirable from an environmental point of view, and inefficient in terms of fuel. Also, the dross has to be remelted in order to extract the aluminium in the recycling process which requires a substantial amount of energy. There are some smelting S 20 plants which have their own rotary salt recycling furnace, however, the process of cooling and transporting the cooled mixture to the furnace remains the same, although savings are made in total transport costs.

Proposals have been made for separating aluminium from dross in the past. GB 1533696 and US 3689049 disclose two different device for separating aluminium from dross. Neither device has had any commercial success, perhaps because they are over-complicated and too unreliable for the extreme environment in which they have to operate.

It is the object of the present invention to alleviate the above mentioned problems.

Summary of the Invention The applicants have made the surprising discovery that the accepted existing method of recycling aluminium from dross can be replaced with a much simpler method which can advantageously be carried at the aluminium melter/furnace and does not produce the environmentally unpleasant waste produced by the traditional recycling process using salts.

Thus in a first broad aspect, the invention involves a method for separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of: removing the mixture from a furnace containing molten aluminium; transferring the hot dross to an insulated crucible; transferring the crucible to a table means; inserting a paddle means into the mixture in the crucible; stirring the mixture with the paddle means and simultaneously vibrating the table and crucible to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce, such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and removing aluminium from the lower part of the crucible.

The pure aluminium can then be either cast as an ingot or recycled 15 straight back into the furnace containing molten aluminium.

The process is carried out without substantially cooling the dross, preferably at a temperature of about 7500.

Apparatus for carrying out the present invention may comprise a table for supporting a crucible which is adapted to vibrate in a generally vertical direction and mixing blades which are adapted to turn inside crucible to stir the contents of the crucible.

In particular, there is provided apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross S" comprising: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross, the table being adapted to vibrate so as to vibrate the crucible; and a frame adapted to support a paddle means or stirring means, means for rotating the paddle means and lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture.

The blades may be mounted on a shaft supported by a frame, the shaft being adapted to be raised or lowered so that the blades may be raised when a new crucible is placed on the table located underneath the blades.

The crucible may be made of a ceramic material having a metallic outer skin spaced from the ceramic material by a layer of insulation.

The table may be mounted on springs to assist in vibrating the crucible. The frame may consist of two or more upright pillars. A crossbeam may be supported by the frame and that cross-beam may carry the means for rotating the blades. A shaft may depend down from the crossbeam and the blades are mounted on the lower end of that shaft.

Panels may be provided covering three sides of the frame to protect against splashing when the blades are lowered into the crucible.

While the size of the crucible containing the dross is not important, typically the crucible should be large enough to carry between 800 kilograms to 1 tonne of dross. The process takes approximately 10 minutes and removes up to 95% of the aluminium in the dross, hence the present invention can produce approximately 500 kilograms of aluminium from dross every 10 minutes.

A specific embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of the present invention; and Figure 2 is a detailed drawing of apparatus embodying the present invention.

Detailed Description of the Preferred Embodiment Referring the to the drawings, Figure 1 shows a forklift truck transporting a crucible containing dross at a temperature about 750 0 C to an apparatus, generally indicated at 14, for removing aluminium from the dross.

The crucible is placed on a table 16 of the apparatus. Blades 18 are lowered into the molten dross, the table 16 is vibrated in a vertical direction, as illustrated by the arrows AA, the blades 18 rotate in the molten dross, and by virtue of that stirring and vibration, the aluminium droplets and particles in the dross coalesce to form larger droplets and gradually sink to the base of the crucible. The aluminium can then be drained out into a bucket 20 and either transferred directly back to the furnace or used to produce an aluminium ingot 22.

Figure 2 shows a more detailed drawing of the apparatus of the present invention. The device includes a frame comprised of a number of upright pillars or rails 50, a base 52 and cross beam 54. In plan view the frame includes four pillars located on the corners of a square. A table 54 is mounted on squat pillars 56 which rise up from the base 52. A number of springs of which only one 58 is illustrated, also extend between the base 52 6 and the table 54. On top of the table a crucible 59 is located. As illustrated the crucible is generally square in plan view and has sides which taper outwardly. The inside of the base of the crucible is shaped like a pyramid so that no sharp corners exist, so that the dross can be stirred properly and so that the crucible can be cleaned easily. A pipe or channel 60 is formed in the base of the crucible which, when unblocked, allows molten metal to flow out from the crucible.

The crucible is made of a ceramic material, having an outer skin of metal and an insulating layer disposed between the skin and the ceramic.

Although ceramic has insulating properties, the additional layer further helps to prevent the dross cooling as it is transported to the table. A shaft 62 ,depends from the cross-beam 54 and on the lower end of the shaft are a series of blades 64 for stirring the contents of the crucible. The shaft is movable S* relative to the cross-beam in the vertical direction to raise and lower the 15 blades 64. A motor is also provided on the shaft to rotate the shaft in the direction B to thus stir the contents of the crucible by means of the blades 64.

In use, dross is taken from the furnace and placed straight into a *"crucible 59 and the crucible moved by a forklift or the like and placed directly on the table 54. Once the crucible is in place on the table, the blades are lowered into the dross. The table on which the crucible sits is then vibrated in the vertical direction, at a rate of between 500 to 5,000 vibrations •go* per minute. The blades turn at a rate of between 3 to 40 rpms. The vibrations break the oxide layer surrounding the droplets of aluminium in the S. dross and allow the metal droplets to coalesce with the result that the droplets then become larger in volume and tend to sink to the bottom of the crucible where they can flow through the hole 60 into another bucket. The rate of vibration can change during the process and tends to start more slowly and then increase later for best results.

High vibrational rates are used to bond very small drops of liquid metal together and by utilising higher vibrational speeds a larger percentage of pure metal can be recovered. As discussed above, generally, the content of pure metal in dross from aluminium furnaces is usually between 30 to 6 0 and once the process described above has been carried out on the dross the content of the remaining aluminium in the dross tends to be in the range of 3 to Both stirring and vibrating is needed.

The frame may be provided with panels on 3 sides to protect against splashing when the blades are lowered into the dross.

The specific embodiment described above refers to vibrating the crucible in a generally vertical axis, and generally horizontal stirring, stirring and vibration could take place in other axes and by other methods. The important feature of the invention is that sufficient agitation, vibration, or stirring occurs at rates which cause the aluminium droplets to coalesce.

Although the above process has been described as being used with aluminium, it could be possible to employ the process in other processes having metals of similar properties to aluminium.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to 15 be considered in all respects as illustrative and not restrictive.

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Claims (13)

1. A method for separating molten aluminium from a mixture of aluminium and aluminium dross comprising the steps of: removing the mixture from a furnace containing molten aluminium; transferring the hot mixture to an insulated crucible; transferring the crucible to a table means; inserting a paddle means into the mixture in the crucible; stirring the mixture with the paddle means and simultaneously vibrating the table and crucible to cause the oxide skin on aluminium droplets in the dross to break, and to cause the aluminium droplets to coalesce, such that the droplets enlarge in volume and in weight and tend to sink to the lower part of the crucible; and removing aluminium from the lower part of the crucible.

2. A method as claimed in claim 1 characterised by recycling the aluminium removed from the crucible into the furnace in a molten state.

3. A method as claimed in claim 2 characterised in that process is carried .out without substantially cooling the dross, preferably at a temperature of greater than 7200 C.

4. A method as claimed in claim 1, characterised in that the paddle or S 20 stirring means rotates at a speed of 3 to 40 rpm and the table vibrates at a rate S of 500 to 5,000 vibrations per minute.

Apparatus for separating molten aluminium from a mixture of molten aluminium and aluminium dross comprising: a table for supporting an insulated crucible containing the mixture of molten aluminium and aluminium dross, the table being adapted to vibrate so as to vibrate the crucible; and a frame adapted to support a paddle means, means for rotating the paddle means and lowering the same into the mixture in the crucible for stirring the mixture with the paddle means enabling simultaneous vibration of the crucible and stirring of the mixture.

6. Apparatus as claimed in claim 5 characterised in that the blades are mounted on a shaft supported by the frame means, the shaft being adapted to be raised or lowered so that the blades may be raised when a crucible is placed on, or removed from, the table.

7. Apparatus as claimed in claim 5 or claim 6 characterised by including a crucible made of a ceramic material which includes a metallic outer skin which is spaced from the ceramic material by a layer of insulation.

8. Apparatus as claimed in claim 7 characterised in that the table is mounted on springs to assist in vibrating the crucible.

9. Apparatus as claimed in claim 8 characterised in that the frame consists of four upright pillars and a cross-beam supported by the at least two of the pillars, the cross-beam carrying the means for rotating the shaft.

Apparatus as claimed in claim 5 characterised in that panels are provided covering three sides of the frame to protect against splashing when the blades are lowered into the crucible.

11. Apparatus as claimed in any one of claims 6 to 10 characterised in that the table is adapted to vibrate along a generally vertical axis at a rate of between 500 and 5,000 vibrations per minute. 15

12. A method for separating molten aluminium from a mixture of aluminium molten aluminium and aluminium dross as claimed in any of claims 1 to 4 and substantially as hereinbefore described with reference to the accompanying drawings.

13. An apparatus for separating molten aluminium from a mixture of 20 aluminium molten aluminium and aluminium dross as claimed in any of *claims 5 to 11 and substantially as hereinbefore described with reference to, as shown in the accompanying drawings. DATED this twenty-third day of February 1998 Patent Attorneys for the Applicant: F.B. RICE CO. AL, 0 &iC