The present invention relates ^" to apparatus for extraction of aluminium by hot processing of dross, metallics, turnings and shredded aluminium scrap, hereinaf er referred to as aluminium f agments. Aluminium is also defined as including aluminium alloys.

Hot Dross is the dross swimming on top of the aluminium bath in melting and casting furnaces. To continue proper furnace operation this dross has to be skimmed off regularly. Normally, it will be collected in dross bins. The hot dross referred to in this application is the hot dross from these furnaces and eventually after removal of any foreign matter, such as iron and steel, it is immediately charged into the distribution system while it is still hot.

Metallic is the coarse material that is produced by cold processing of aluminium dross after it has been cooled down. Cold processing of dross is mainly a process of repeated crushing, screening and separation by which the remaining more or less ductile metallics will have a much higher aluminium content than the cold dross had when it entered into the process. Principally, this cold processing is based on the fact that, the metallic inclusions in the dross are of a ductile nature, whereas the remaining

contaminants, mainly - aluminium oxide - and other metal oxides are brittle. By mechanical processing the brittle fractions are broken off and separated from the ductile metallic components. Size and finer aluminium contents of the metallics depend very much on the intensity by which the cold processing has taken place and on the consistency of the dross when it entered into the cold process. Normally metallics are smaller or larger grains or lumps, say between 3 and 25mm diameter but the sizes could be outside this range. The metallic inclusions are partially on the surf ce and partially inside the grain or lump each metal inclusion being partially or entirely separated from each other by metal oxide envelopes or bridges.

Turnings (sometimes called swarf or chips) represent aluminium turnings from the machining of aluminium components in way way or another. In some cases, the name turnings, for example in the case of sawdust and chips from milling, does not really apply, but these materials are still defined as such. -

Shredded aluminium scrap is the general term for wrought and cast aluminium scrap that went through a mechanical process of shredding, crushing and separation by which finally the lump size between approximately 10 and 100 mm was obtained with lowest possible contaminations of iron, steel and other unwanted matter. It could also have gone through an additional stage of thermal processing by

which the metal has been freed from most of the organic contaminations.

For many decades the above defined materials have been remelted through rotary furnaces, and through open and partially enclosed side well furnaces. In such furnaces, a tremendous amount of fluxing salt is used to enable the extraction of aluminium in an economical way. This has caused a serious problem of disposal or further processing of the remaining salt cakes. Dross from primary smelters and semi-producers sometimes contains certain amount of salt or salt cakes. Most of this will be removed during cold processing of the dross.

The functions of the fluxing salt are obvious:

1. The first function is that it encapsulates the grains or lumps to be melted during the melting process and so protects them from oxidation by direct flame impingement or other.

2. Secondly, it forms an eutecticum with the metal oxides that are then weakened at actual process temperatures. It so helps the aluminium inclusions to escape from the metal oxide envelopes and to coagulate so forming an aluminium bath underneath of the salt layer floating on top.

3. Thirdly, it also chemically reacts with some of the gaseous contaminants like hydrogen, and adsorbs most of the organic contaminants.

In most of the industrial countries the authorities have become aware of the serious effects that the gases, salt cakes and other remains coming from such salt bath furnaces, already have on the environment.

For many years, for example, the more developed nations could export their salt cakes to less developed nations, where it is thought to have been stored, for example, underground in former salt mines. In other countries remote former quarries, valleys, etc. have been used as storage facilities for the same. In extreme cases, even lakes, f ords and other open water has been polluted by just dumping the material.

It has to be emphasized that because of the undefined nature of aluminium scrap, any pollutant matter such as heavy metals, PCB's, dioxins, and other hazardous pollutants, could be present or could be generated during the course of any process. Especially the salt cakes as well as filter dust from bag houses downstream of salt bath furnaces, have been reported as environmentally highly suspicious matter.

Because of the great quantities of salt cake already

disposed of and still being produced in Switzerland, Germany, Italy and probably in other countries, huge sale cake processing plants have been built recently.

Most of the worlds primary aluminium is produced in the form of rolling ingots, extrusion billets and sows to be processed elsewhere.

Apart from production scrap circulating within the primary melters and the semi-producters, millions of tonnes of recoverable aluminium re-appear with secondary smelters and others yearly in the form of scrap, including dross and turnings.

The secondary smelters produce their own dross as well. Although an exact figure is hard to give, the total weight of aluminium to be recovered from hot dross, metallics, turnings and shredded aluminium to be remelted, can be estimated at several million tonnes annually.

In the order of more than 1 million tonnes of fluxing salt is used that results in approximately 1.5 times that weight of salt cakes annually.

The society in industrial countries has become more and more aware of the harm that salt bath aluminium remelting furnaces have done and are still doing to the environment.

It is an object of the present invention to avoid the use of salt fluxes or to reduce them to an absolute minimum. The most eye-catching feature is that by this alternative process the aluminium recovery rates are extended to be about 5% higher than in a salt bath rotary furnace, and the amount of salt used was as low or less than 2 o/oo of the charged metal input weight when compared to still presently between 16 to 35% in sale bath furnaces.

The present invention provides apparatus for the extraction of aluminium by hot processing of aluminium fragments.

The apparatus preferably includes a liquid aluminium container with a heat source and an adjacent hot processing system for the aluminium fragments.

The heat source is preferably operative to transfer heat to the liquid metal in the container. The hot liquid metal is preferably transported through the hot processing system for the aluminium fragments.

The means for transport of the liquid aluminium through the system may differ from case to case.

From the hot processing system liquid aluminium is preferably fed back into the container.

Normally the liquid metal level in both the container and in the hot processing system will be kept constant between allowable limits which implies that during operation the liquid aluminium extracted from the aluminium fragments will be either continually or periodically trapped.

The hot processing system preferably includes a first unit for feeding the aluminium fragments and for mixing the aluminium fragments with the heated liquid aluminium and for achieving a first stage of aluminium extraction. A second unit for major extraction of aluminium from the aluminium fragments, a third unit for final extraction of aluminium from the remaining fragments and a fourth unit to receive, cool and discharge the remaining dry dross dust.

In a first embodiment the mixing unit may comprise an Archimedes screw unit.

In a second embodiment, the mixing unit comprises a rotary drum furnace of the type described in our co-pending British Patent Application No. 9007319.8.

In a particular embodiment the heat source may comprise a closed well furnace of the type disclosed in our British Patent No. GB 2211159.0 and the Archimedes screw unit may be an adjoining unit.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows in block diagrammatic form a first apparatus according to the present invention;

Figure 2 shows in cross sectional plan jview a second apparatus according to the present invention;

Figure 3 shows the apparatus of Figure 2 in cross sectional elevation;

Figure 4 shows diagrammatically a double Archimedes screw arrangement;

Figure 5 shows a combined mixing unit and separator in cross sectional side elevation;

Figure 6 shows in side elevation the construction of a portion of a preferred Archimedes screw unit;

Figure 7 shows in end elevation an annulus of the screw unit of Figure 6;

Figure 8 shows the apparatus of Figure 2 in greater practical detail;

Figure 9 shows the apparatus of Figure 3 in greater practical detail;

Figure 10 shows the closed well furnace of Figures 8 and 9 in partial cross sectional side elevation; and

Figures 11 and 12 show possible arrangements for altering the axial positions of two helical screw conveyors.

With reference to Figure 1, the apparatus comprises a furnace 10 equipped with a suitable heat source 12. The furnace 10 holds a quantity of liquid metal 14 which in the preferred example is aluminium or aluminium alloy. The furnace 10 could be of the closed well type as shown in Figure 2. However, for this system there will be no requirement for a special furnace provided that sufficient hot liquid metal is available to circulate through the system to provide heat and to maintain the process. In this system the process runs continuously. A sensor 11 is preferably provided for monitoring the level of aluminium in container 10. The sensor may be suitably coupled to the infeed system to maintain the liquid level at a predetermined value or between predetermined limit values.

A connecting pipe or similar structure 16 connects the furnace 10 to a transport unit 18 which is connected by similar pipe or other means 20 to a pre-mixing unit 22 in

which an infeed 24 of aluminium fragments is fed into the liquid aluminium.

The mixture from pre-mixing unit 2"2 is fed to a rotary drum 26 via pipe or other means 28. The rotary drum 26 may be of the type described in the above mentioned co-pending British Patent Application.

The mixture in rotary drum 26 is processed as described in the above British Patent Application to produce mechanical pressure on the fragments thereby separating the waste material from the molten aluminium thereby allowing the molten aluminium to coalesce with the bulk of the liquid aluminium.

The mixture of liquid aluminium remaining fragments and waste matter is fed via pipe or other means 30 to a separator 32 from which dry skim may be removed by dry skim unit 34 and cooler 35.

The remainder liquid aluminium plus remaining fragments are fed via a pipe or similar means 36 back to the furnace 10.

Liquid aluminium may be tapped as shown at points 38 or 40 (from furnace 10).

As an alternative or in addition an infeed and

pre- ixer unit 42 may be included to mix fragments with the liquid aluminium directly in furnace 10. The fragments are then pre-heated in the furnace 10 prior to being transported to the rotary drum mixing unit 26 wherein it is subjected to mechanical forces to break up the small pieces of the fragments. Heating of the aluminium mixture could take place by for example induction heating 12' as the mixture passes along pipe 16 either as an addition to heat source 12 or instead. Heat could also be applied if required at other points but in most systems this will not be required.

The process through the apparatus is continuous and by feeding liquid metal and any remaining fragments through connection 36 back to the furnace 10 the aluminium can be further recovered if required by removing more aluminium from the fragments. The systems is, therefore, effective in recovering aluminium from dross with minimal addition of the salts which are used in many processes to break down the oxide coating. The use of less salt has a beneficial effect on both the economy of the system since the salt is expensive and also on the ecology since the salt creates exhaust-gases and further waste which must be processed.

The operation of the system is briefly described as follows:-

1. At the start of the process it has been ensured that the transport unit (aluminium pump) 18 maintains a sufficient hot liquid aluminium flow from the heated liquid aluminium buffer/furnace 10 through the rotating drum 26 via the final processing and separator unit 32, 34 back into the buffer/furnace 10 etc.

2. The liquid metal level in the system will be accurately kept at the required level throughout the process.

3. The material feeder 42, doses the material to be processed at a set constant rate into the liquid aluminium flow. Air entrainment is avoided by the design and operation of the feeder 42.

4. The mixture of hot metal coming from the buffer/furnace 10 and the material fed in by feeder 42 is preferably led underneath of a set of removable refractory blocks 6 to ensure through mixing prior to the entry of the mixture into the rotating drum 3.

5. The velocity of rotation of the drum 26, as well as its direction of rotation along its

longitudinal axis, can either periodically or at any instant be adjusted, resp. reversed.

6. The high density of the refractory balls inside the drum makes them sink into the liquid metal/infeed mixture.

7. Because of the drum's internal oval shape these balls will have a rather random pattern of movement. They will, however, mechanically process the solid particles moving through the drum. So oxide envelopes and bridges will be broken up to a large extent.

8. After the mixture of liquid metal and pre-processed infeed material leaves the rotary drum 26, it enters into the final processing and separator unit 32.

9. The separator unit 32 may be of similar design as the system installed on the side well as described hereinafter with reference to figure 2 etc. Before discharge of the remaining dross dust into the cooler, the recovered liquid metal flows into the buffer/furnace 10 as described above.

With reference now to figures 2 to 7, a process

similar to that of figure 1 may be carried out in a single unit, which comprises a furnace 100 of the closed well type having a main heating chamber 101 and a closed well chamber 102.

Equipment ^" is provided to intensely stir and mechanically process the fragments previously loaded into the well chamber of the CWF 100. It mainly consists of 2 parallel screw mixers 103, 104 that have their longitudinal axes parallel to the separating wall between the well chamber and the main heating chamber. The shafts of both screw mixers are at equal horizontal levels and also parallel with the liquid metal bath surface underneath. The pair of mixer screws 103, 104 rotate synchronously in mutually opposite directions and have the facility for their rotation to be reversed both ways, say clockwise or anti-clockwise.

On the side of the furnace 100 a sidewell 106 has been built that on one end has an open connection 108 to the liquid metal in the main heating chamber 101 and on the other end has a connection 110 to the metal in the closed well chamber.

A vertically sliding damper 112 can be let down to entirely or partially block the liquid metal connection between the closed well chamber 102 and the side well 106.

In the side well 106 there is another pair of screw mixers 114,116 (shown dotted) installed with their shafts at equal level and parallel to the liquid metal underneath and to the longitudinal walls of the side well. So in plan view the screw mixers in the closed well chamber are at right angles to the screw conveyors in the side well.

Each mixer may be provided with suitable drive means 120, 122 (shown dotted - see figures 2 and 3). The drive means may comprise an electric motor and gear box drive.

The drive means 120 comprises means for causing the screw 108 to be raised and lowered so that it can be entered into the closed well chamber 102 at a first height (shown dotted) via an entrance 124 which is sealed by door means 126. The means 110 may be mounted on wheels 128 to be removable from the second chamber. This removal enables maintenance to be carried out on the Archimedes screws 103, 104 and a separate closure door (not shown) can be used to close entrance 124 to enable the furnace to continue operation without using Archimedes screw 103, 104.

Characteristics and features of the screw conveyors in the side well 106 are:-

The spiral blades are not entirely continuous but preferably sub-divided into single blades of suitable size and shape.

2. The end of both screw conveyors 114, 116 are tapered 114, 116 (see figure 5) on the side that is closest to the liquid metal connection 108 with the main heating chamber.

3. Near the liquid metal connection to the main heating chamber the end wall 152 of the side well is partially sloped upwards, finally ending above a chute 156 (see figure 5).

4. It will be possible to vary the velocity of rotation of both screw mixers and also to reverse them as a pair, but always making sure that they are synchronously rotating in mutually opposite directions.

5. From the top in any suitable direction, a comb or scraper can be moved intermittently towards the shafts of the rotating screw conveyors to intensify their material processing action and also to keep them clean.

The mutual longitudinal position of the blades of each conveyor could also be changed either by moving the shafts longitudinally to move one conveyor with reference to the other or by allowing the relative mutual rotation mechanically, hydraulically or electrically. This

will then position the individual blades of the conveyors closer together and provide a means for cleaning the blades and also a variation on the mixing operation.

6. In the upwards slopes (152) underneath of each screw mixer it is also envisaged to install a row of teeth or a comb (not shown) to intensify the final separation of the remaining metal droplets from the dry dross discharged into the above mentioned chute.

It has been found, that false air entrainment into the process area very negatively affects the efficiency and the aluminium recovery. Tests have been done using a CF ₆ blanket over the processed dross layer. This worked out favourably, but it has also been found that flue gas from a fuel-fired furnace under controlled fuel air ratio ie, with nearly neutral atmosphere, did the job satisfactorily. This does not mean, that under all circumstances the use of any inert gas atmosphere or liquid metal treatment could be entirely avoided.

To achieve a controlled atmosphere in the entire processing system, the necessary provisions for encapsulating and sealing have to be made.

The described process has more or less to be seen as

a batch-type process. A process cycle can be seen in the following stages:

1. The CWF 100 as well as the side well 106, initially contain a liquid metal bath of sufficient temperature and which will be accurately maintained at a constant level throughout the entire process. The furnace doors 160, 162 on either front end of the CWF, as well as the openings 156 in the side well of the closed well chamber, and in the side well chamber 124, are completely closed. The melting burners (not shown) on the main heating chamber are set to maintain a constant liquid metal temperature throughout the process.

2. Through the charging door 162 opening on the closed well chamber 102 a load of the material to be processed is tipped into the liquid metal bath in the well chamber. It is presumed, that only dried materials and sometimes preheated materials are to be loaded into the well chamber in order to prevent any explosions. If the contents in the materials to be charged could be ensured, measures will be taken for sufficient preheating and drying prior to letting the material come into contact with the liquid metal 164 in the closed well chamber.

3. After charging, the charging door 162 will be entirely closed and sealed. Just before charging the separating liquid metal damper 112 between

- the closed well chamber and the side well was let down to prevent material floating into the side well 106 directly during charging and during the stirring process in the well chamber after charging.

4. Through the opening 124 in the side wall of the closed well chamber opposite of the liquid metal connection with the side well, the screw mixers 103, 104 move in at a level well above the previously loaded material that is now partially floating on top of the liquid metal bath (see dotted outline figure 3). The pair of screw mixers is entirely moved in so that they cover the whole width of the liquid metal bath in the closed well chamber. The screw mixers are running in a mutually opposite direction, which is periodically reversed during the first stage of the process.

Gradually, the screw mixers 103, 104 are moved downwards, finally reaching the materials underneath and mixing them thoroughly also with hot liquid metal. This stirring action causes extreme mechanical interaction amongst the solid

particles contained in the processed material, as well as with the hot liquid metal.

Heat is then released to each particle at such a level, that also any aluminium inclusions will liquify and the expansion forces, together with the external mechanical action, will enable the liquid metal inclusion to escape and move downwards and finally coagulate with the liquid metal underneath.

5. After a certain time the liquid metal separation damper 112 between the closed well chamber and the side well is entirely lifted up and at the same time, the 2 screw mixers 103, 104 remain running constantly in the appropriate direction, so moving the remaining hot fragments towards and into the side well.

Then, the liquid metal separation damper 112 goes down again, the pair of screw conveyors 103, 104 is withdrawn, the opening 124 in the side well of the closed well chamber is closed again, and the furnace 100 is ready to receive the following charge.

6. The already pre-processed fragments moved from the closed well chamber 102 into the side well

106 are then thoroughly processed by the pair of screw mixers 114, 116 as mentioned before. It is thought to only lift the screw mixers out of the liquid metal bath in the side well for maintenance or for any other interruptions of the process, so the screw mixers in the side well will be running accurately when pre-processed fragments from the closed well chamber are moved into the side well.

7. The pair of screw mixers 114, 116 in the side well 106, as this is the case with the screw mixers in the closed well chamber, will always run in a mutually opposite direction, which can be periodically reversed and of which the velocity of rotation can be adjusted to the needs of the process.

A smaller final processing, as it was described for the well chamber, takes place in the side well. However, at the end of the screw mixers closest to the liquid metal connection with the main heating chamber, the top layer of finally dry dross will be moved up the hot refractory slope 152 underneath of the tapered end 114, 116 of each of the screw mixers, while any droplets of liquid aluminium will still have a chance to drain backwards into the liquid metal bath in the

side well, before the remaining dross dust 151 will be discharged in a suitable cooler.

In figure 4 the twin Archimedes screw arrangement is shown in greater detail. The screws 14',16' may be rotated as shown by arrows 122-128 in the same directions 124, 128 to produce a flow effect or in opposite directions 122, 126 to produce turbulent conditions.

With reference to figures 4, 6, 7 the screws 14,16 may be constructed, as indicated schematically in figure 4, by means of individual blade elements 130. One element is shown in fig. 4 and the positions of others are indicated by dotted lines 130'.

Each blade element 130 may comprise an annulus 132 which fits over a drive shaft 134 of device means 110 and is keyed thereon at a defined position. The annulus 132 is provided with an upstanding preferably triangular shaped blade 136 which is preferably moulded integrally therewith.

The flat front face 138 of each blade 136 is in a preferred embodiment positioned to face the forward direction of movement (see arrow 140) so that it produces a hammer action when turning.

Each screw is, therefore, constructed from a plurality of identical such annuli, positioned to give the required

spiral effect. Each annulus is, therefore, removable to be replaceable when worn or damaged. ^Λ Y The system can be operated on a continuous basis, in which case the damper 112 would be permanently raised or 'not there at all. Aluminium fragment input must also be continuous (or effectively so) to provide a balance between input and output.

With reference now to Figures 8, 9 and 10 these show the apparatus of Figures 2 and 3 in greater practical detail. The same reference numerals are used to indicate the same parts of the apparatus.

In addition cooling fan arrangements 202, 204 are shown in Figure 8 which provide cooling air for the shafts of the screw conveyors.

In Figure 9 two positions are shown for the conveyors (one being dotted) and the main shaft drive motor arrangement 206 for the shafts is shown.

In Figures 8 and 9 various journals and bearings are shown for clarity. One possible arrangement comprising a scissor jack 208 is shown for raising the conveyors.

Figure 10 shows in side elevation the position of the conveyors relative to the closed well furnace and shows a collection bin 210 for the dross dust 151.

With reference to Figures 11 and 12 to move the conveyors 103, 104 axially an arrangement comprising a slot 220 and pin 222 or preferably as shown two slots 220, 224 and pins 222, 226 may be used.

Slots 220, 224 are cut in a gear wheel 230 driven by a shaft 232 to which pins 222, 226 are attached on a disc 234 for example welded onto the shaft at 236.

Shaft 232 may be used to drive one of the conveyors 103 and a co-operating gear wheel 240 may be used to drive the other conveyor 104.

It may be seen that by reversing the direction of rotation of shaft 232 the pins 222, 226 will move to the opposite ends of the slots 220, 224 hence altering the relative axial position of the conveyors 103, 104 by an angle determined by the length of the slots. Thus on reversal of drive the blades of the conveyors could be brought very accurately into close proximity.

In a further alternative embodiment as shown in figures 2,3 aluminium fragments and/or an aluminium fragment mixture may be fed continuously under the control of for example an Archimedes type screw conveyor directly into the closed well chamber (CWC) 102 of the closed well furnace. Alternatively the aluminium fragments and/or aluminium mixture of hot aluminium and fragments could be

fed directly in a similar manner into the side chamber 106.

In figures 2,3 a screw conveyor 300 is shown dotted and when operated this will feed the* fragments directly into closed well chamber 102 for processing by the screw arrangment 103. The speed of feeding can be controlled to maintain a correct throughput for the size of furnace.

As an alternative the infeed could be positioned on the top of the chamber shown at 300' (Fig.3). The screw conveyor 300 will preferably require to be gas tight to control the escape of gases. Thus with suitable sealing it is also possible to feed the fragments/mixture into one or both chambers 102,106 at a level underneath the aluminium level as indicated by dotted outline 300", 300'" Fig.3.

This embodiment enables the furnace to be operated more or less continuously with only occasional opening for clearing out any non-aluminium scrap which may collect e.g. stainless steel.