AU655466B2 - Improvements in or relating to processes and/or plant for recovery of aluminium metal from dross material - Google Patents

Description

AUSTRALIA

Patents Act COMPLETE SPECIFICATION

(ORIGINAL)

655 46 Int. Class Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority S Related Art: Name of Applicant: Bruce Stuart Haysom, Travers Robert Haysom, Ronald Frederick Ackroyd Actual Inventor(s): C I C 444 4 .441 Bruce Stuart Haysom Travers Robert Haysom Ronald Frederick Ackroyd Address for Service: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: IMPROVEMENTS IN OR RELATING TO PROCESSES AND/OR PLANT FOR RECOVERY OF ALUMINIUM METAL FROM DROSS MATERIAL Our Ref 289230 POF Code: 9379/173627,173635,173643 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): -1 6006 -2- The present invention relates to improvements in and/or relating to recovery of aluminium metal from dross material.

The recovery of aluminium from secondai, sources such as primary and secondary aluminium drosses is desirable because the production of aluminium from its primary source, bauxite, is extremely energy expensive. Primary and secondary aluminium drosses may be in the form of either low recovery dross residue or the output of dross coolers, and occur as by-products of the primary aluminium production process.

It is therefore desirable to provide an improved process of recovery of high grade aluminium from primary and secondary drosses. It is likewise desirable that such a process be faster than and economically advantageous over o a existing recovery processes.

It is therefore an object of the present invention to provide a method of recovering aluminium metal from aluminium dross material which goes at least 15 some way towards fulfilling at least some of the above objectives or which at least provides the public with a useful choice.

Accordingly, in a first aspect, the invention consists in a process for upgrading aluminium drosses and/or recovering aluminium metal comprising: using as a feed material a hot metallic aluminium dross fraction; separating and retaining a desired size range of particles from said dross fraction; milling the separated particles; separating and retaining particles of material above a desired size after milling; and i1

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1 J>7 j^ -3thereafter repeating the steps of milling and subsequent separation and retention, until the retained material comprises aluminium metal of a desired level of purity.

Preferably said seperated particles are milled in a high velocity autogenous mill.

Preferably said primary upgrading has a drying phase wherein the dross residue is heated so as to drive off moisture, and a milling phase during which aluminium oxide dust is drawn off, resulting in a hot partially enriched metallic dross fraction.

tc« 10 In a still further aspect the present invention consists in a secondary aluminium recovery process including a drying and primary upgrading process as in the preceding paragraph.

In a still further aspect the present invention consists in upgraded aluminium resulting from a process as aforesaid or substantially as hereinafter 15 described in more detail or substantially as defined hereinafter with reference to the accompanying drawings.

tt1 i 41 4 To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

Preferred forms of the present invention will now be described with xy^,'Y reference to the accompanying drawings of which;

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*l 4 I c 4 4 4 44 Figure 1 is a schematic diagram of an arrangement of plant for drying and primary upgrading of dross residue according to a preferred form of the present invention; Figure 2 is a schematic diagram of an arrangement of plant for recovery of metal from dross according to a preferred form of the present invention; Figure 3 is a flow diagram of one preferred form of the process of the invention.

With reference now to the drawings, in one preferred form of the 10 invention the feed material for the process of the invention may be derived from low recovery dross residue. Such low recovery dross residue such as that from waste heaps of conventional primary aluminium production plants will require drying and primary upgrading.

Initially, the dross is excavated from the heap and loaded into a hopper or feeder which feeds it onto a trommel screen (not shown) where the oversize fraction is screened off. The largest oversize material is removed by hand picking. The smaller fraction, which in the presently preferred form of the invention may be for example the minus 75mm fraction, although this can vary with the feed material characteristics and plant capacity, is fed 1 at a controlled rate from a hopper feeder 2 into a combined direct heat counterfiow rotary dryer and autogenous mill 3. The material is subjected to a drying phase, which removes moisture and preheats the material, to facilitate removal of aluminium oxide dust in the milling section. The moisture and dust 4 are drawn into a high 1 i-4- 4; temperature pulse jet dust collector 5. The moisture is exhausted through the stack 6, while the dust 7 may be retained for further processing in industry as a chemical additive.

Temperature probes 10 in the dust collector control the modulation of the burner to ensure that the dust collector temperature is maintained above the dew point. Material flow in the dryer is adjusted to the maximum level possible without cooling the drier below the dew point.

The hot partially enriched metallic fraction resulting from this primary upgrading and drying, is then conveyed 10 to a screen separator 11, for example an electro mechanical vibrating screen optionally incorporating a dust collector 12, and separated into three fractions. For example according to at least one mat presently preferred form of the invention these three fractions may comprise a 4% +57mm fraction, a -57mm +0.3mm fraction, and a -0.3mm fraction. The size ranges will be determined by plant capacity and by characteristics of the dross material. For example, very large plus 57mm) material contains sufficient metal to be furnaced without further upgrading, while attempting metal recovery uI o from very small fragments minus 0.3mm) and dust may not be cost or energy effective due to the very small amounts of metal. The size ranges given are not intended to set absolute limits but rather to give an indication of an operating range which is presently preferred and has been found effective at this time.

The largest e.g. the +57mm fraction is then r~moved 13 and hand picked.

The +57mm aluminium metal is fed to the furnace part of the plant, and any upradng nd dyin, i thn coveyd 1 toa sceensepratr 11 fo exmpl j I -F ir,_ 0 os 0 04*0a 0,0 0,04,0 00,40 other +57mm material may be discarded.

The intermediate size range e.g. the -57mm +0.3mm fraction is retained 14 for further processing according to the invention, as set out below.

The smaller fraction e.g. the -0.3mm fraction may be discarded 15 or subject to further independent processing.

The intermediate fraction or the hot partially enriched metallic fraction resulting from dross coolers at a primary aluminium smelting plant which with presently known methods may comprise for example 60-75 mm size ,articles, then proceeds to a sequence of treatments which will be referred to as the enrichment loop (Figure The material is fed from a storage hopper 20 into a high velocity autogenous mill 21. One example of a suitable mill is the vertical shaft impact crusher marketed under the trade mark BARMAC. At the mouth 22 of the mill the material can be passed through an annular random classifier 23 and the fine dust drawn away 24 to a dust collector (as in Figure As the material passes through the mill 21 the brittle fraction, comprising principally aluminium oxide, cryolite, carbon and similar materials, is broken free from the malleable aluminium and crushed.

The milled material is discharged 25 from the mill onto a multi-deck vibrating screen 26 where a desired smallest fraction e.g. the -0.3mm fraction is removed 27 and may be discarded or further processed independently. The larger size e.g. +0.3mm fraction is retained and fed to a surge hopper 28. As the material passes over the screen it is air washed for dust removal.

From the surge hopper 28 the larger size retained fraction 29 is fed back t 1 j i j i:- -6i 1 -i to the autogenous mill 21 by a variable rate feeder 31. The feeder 31 may be interlocked to and controlled by the load current drawn by the mill motor. The control circuit 32 ensures that the mill 21 is fed at an optimum enrichment rate.

The sequence of milling, and screening and discarding the smaller than desired fraction, is repeated, forming an enrichment loop process, until substantially all aluminium oxide and other extraneous matter is removed and the remaining aluminium reaches an acceptably high level of purity, The purified aluminium 33 is fed to a rotary furnace (see Figure 3) for processing back into ingot form.

10 Thus it can be seen that in accordance with the invention a process for the upgrading of primary and secondary aluminium drosses is provided. In w. contrast with known processes it uses a sequence of high velocity autogenous milling and screening in an enrichment loop process which has a number of advantages. For example, it has been found that the process of the invention S 15 recovers metal economically from dross material which is substantially smaller than has previously been processed, for example as small as 0.3mm. It is believed that metal may be recoverable from yet smaller material fractions by this process.

It has also been found that the processed metallic product obtained from the process of the invention is cleaner than that previously obtained by recovery processes. The processed metallic product is clean enough to be used as a steel deoxidant without smelting. There is higher ingot metal recovery from smelting the processed metallics compared to conventional recovery processes. The -7i'

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l processed metallics have been found to require less salt flux during secondary smelting when compared with the products of conventional recovery processes, which leaves a larger furnace metal capacity available for metal, thus improving efficiency and reducing energy costs. The process according to the invention can also improve the percentage of metal recovered and produces aluminium oxide dust and grit by-products suitable for other industrial uses. The improved recovery can reduce the volume of waste material from an aluminium plant.

Such waste would otherwise be disposed of in landfill dump sites which is environmentally undesirable. Also by reducing the production of salt slag during 10 secondary melting the amount of this waste presented to landfill dumps is reduced.

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Claims (9)

1. A process for upgrading aluminium drosses and/or recovering aluminium metal including: using as a feed material a hot metallic aluminium dross fraction; separating and retaining a desired size range of particles from said dross fraction; milling the separated particles separating and retaining particles of material above a desired size after milling; and thereafter repeating the steps of milling and subsequent separation and retention, until the retained material comprises aluminium metal of a desired level of purity.

2. A process as claimed in Claim 1 wherein the said separated particles are milled in a high velocity autogenous mill.

3. A process as claimed in Claim 1 wherein said feed material includes either low recovery dross residue which is subjected to a drying and primary -t upgrading process, or a hot partially enriched dross fraction resulting from primary aluminium production processes.

4. A process as claimed in either of Claims 1 and 3 wherein separation of particles in the method of the invention is performed using a vibrating screen separator.

A process as claimed in Claim 4 wherein material passing over the screen of said separator is air washed to at least partially remove dust. 1 1i 10

6. A process as claimed in any one of the preceding claims wherein the initial separation step includes separating the feed material into three fractions, of which the largest sized fraction is thereafter hand picked for furnaceable metal, the intermediate sized fraction proceeds to the milling stage, and the smallest size fraction is discarded or is separately processed,

7. A process as claimed in Claim 2 wherein said primary upgrading process has a drying phase wherein the dross residue is heated so as to drive off moisture, and a milling phase during which aluminium oxide dust is drawn off, resulting in a hot partially enriched metallic dross faction.

8. A process for upgrading aluminium drosses and/or recovering aluminium metal substantially as hereinbefore described with reference to the accompanying drawings.

9. Upgraded aluminium resulting from a process as claimed in any one of the preceding claims. DATED 28 September 1994 PHILLIPS ORMONDE FITZPATRICK ATTORNEYS FOR: BRUCE STUART HAYSOM, TRAVERS ROBERT HAYSOM and RONALD FREDERICK ACKROYD I S-. i 'Si I I I Lcr i-- l(p~ 999, 9. 9 6 ci e9r *O 4 4 91 9% (1 9-i 4 ~rAql ABSTRACT This invention relates to the recovery of aluminium metal from primary and secondary drosses, the residue of conventional aluminium smelting operations. The process involves using a hot metallic aluminium dross fraction as a feed material and using separation and retention apparatus to obtain particles of a desired size. These particles are then milled and sorted by size repeatedly until the retained material comprises aluminium metal of a desired level of purity. -1-