Process for forming high purity alumina

The present invention claims priority from AU2021901141 entitled “Process for forming high purity alumina” filed on 19 April 2022, the contents of which are hereby incorporated by reference in their entirety.

Technical field

The present invention relates to a process for forming high purity alumina (HPA).

Background

High purity Alumina (HPA) is the pure form of aluminium oxide (AI203). The size and morphology of the crystallites of the aluminium oxides are an important trait of these aluminas. Nearly half of the 30,000 tonnes of alumina that is produced annually is used to manufacture sapphires. There is a market demand for HPA for the formation of sapphires as well as for the manufacture of e.g. LED and semiconductor substrates, for the formation of lamps, and in polishing, and in the formation of lithium-ion battery separator coatings. The price and performance of HPA varies according to the degree of purity.

The purity of HPA can be categorised as 99.9% (3N), 99.99% (4N), 99.999 (5N) and 99.9999 (6N). Impurities of 0.1% in 3N HPA, and 0.01% in 4N HPA, (and so on) depend on the original source of the aluminium used to create the HPA product. In 6N HPA, there is 1 g of impurity for every tonne of material. The impurities found alongside Al in most mined ores are reasonably consistent. Where mineral ores are the feedstock for the formation of HPA, the most common impurities that need to be removed are iron and silicon.

The purer the HPA the more valuable it is since most applications have a low tolerance for the impurities in the material. However, the purer the HPA, the more expensive it is to produce due to the intensive chemical processes that the raw material must be subject to in order to achieve that purity. The market, therefore, has to be prepared to pay for the advantages of the purer product. In recent times, some manufacturers have switched to cheaper 3N HPA materials believing that they will save costs while providing an adequate outcome. However, this is not always the case. In the lithium-ion battery industry, high grade 4N alumina (99.99%) was initially adopted as the standard coating material for separator sheets, especially where battery safety was paramount. Scientific tests recently completed by the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) plainly vindicate the initial choice of 4N alumina by the battery industry. In its tests, the Institute exposed various commercially available lower grades of alumina / boehmite powders to lithium battery electrolyte solution under controlled battery type conditions. What was observed was extremely concerning - the severe leaching of sodium from the lower grade alumina's into the organic electrolyte solution, which resulted in significant electrolyte contamination. Specifically, the research reported that in its test of 3N alumina (99.9% alumina) the sodium content within the electrolyte solution rose from an acceptable 0.5 ppm up to a potentially catastrophic level of 40 ppm (an 80-fold increase). Similar leaching was observed for boehmite (99.7% alumina), where the level of sodium in the electrolyte jumped 20-fold. As a base line, sodium leaching from 4N alumina (99.99%) into the electrolyte is negligible, as there is virtually no sodium present in the 4N product. In the case of the alumina coating on battery separator sheets, it is potentially catastrophic that many in the industry appear to be attempting to move to lower quality material.

Common feedstocks for the formation of 4N (and higher) HPA include aluminium ingots (metal) and aluminium containing minerals such as kaolin. The chemical processes used to form the HPA from these feedstocks can include various of solvent extraction, acid leaching, precipitation/crystallisation and calcination.

China is currently the largest supplier of HPA to the manufacturing industry with over 80% of HPA being produced by Chinese processes. Four manufacturing processes of ultra-pure aluminas are currently in use, either using Bayer gibbsite or using aluminium as starting materials.

• The most common process for the formation of HPA, consists in dissolving Bayer gibbsite in an excess of sulphuric acid, neutralizing it in ammonia in the form of NhUAI (S0 ₄ ) ₂ ,12H ₂ 0, crystallizing it by cooling, then calcining it at more than 1000°C leaving a very friable residue of AI ₂ O ₃ .

• The alcoholate process produces an even purer alumina if the aluminium isopropylate is obtained by attack of pure aluminium then purified by distillation.

• The chloride process, consists of dissolving pure alumina in hydrochloric acid (HCI) and precipitating the hexahydrated aluminium chloride, the calcination of which leaves alumina as a residue.

• The last process is simply a re-dissolution in soda then a re-precipitation of the gibbsite, either by the Bayer precipitation, or by neutralization. Gibbsite is obtained and the soda content is then lowered by hydrothermal treatment

All of these processes involve calcination in a tunnel kiln without any mineraliser in common. This produces alpha alumina crystallites that remain sub-micronic.

The current demand for HPA outstrips supply. While demand is increasing, customers are always looking to get more for less. With a driver to using the cheapest possible HPA, but with a requirement for the highest purity, most stakeholders are now focused on how to decrease the price of HPA without compromising on quality. Accordingly, there exists a need for improved HPA processes that provides a commercially viable 4N (or more) HPA product when compared to the products that are currently provided.

Summary of invention

In a broadest aspect the present invention provides a method of manufacturing high purity alumina of at least 4N using de-coated aluminium cans as the only source of aluminium in the feedstock.

Whilst previous processes have focused on the use of aluminium ore and aluminium ingots as a feedstock for HPA, the present inventors have found that other sources of aluminium previously thought to be waste or scrap can be used to produce HPA provided the scrape or waste aluminium is treated in a certain way. The present inventors tried various approaches to treating the aluminium cans and found that the alloy content in the cans presented an opportunity to use scrap beverage cans as a raw material for HPA production. The inventors also found that some approaches to removing impurities resulted in the coatings on the cans themselves contributing to the impurity load, with the plastic lining of the cans presenting a particular challenge. The inventors found that de-coating the cans prior to production or smelting the cans to produce Al ingots was required..

In another broad aspect of the invention there is provided a method of manufacturing high purity alumina of at least 4N from aluminium cans, the method comprising the steps of increasing the temperature of the aluminium cans in a non-oxidising atmosphere to form a molten material; separating molten metal from the molten material and allowing the molten metal to cool to a cooled metal material; forming discrete particles of the cooled metal material optionally by milling; dissolving discrete particles of the cooled metal material thereby forming a solution of the cooled metal material; precipitating an aluminium compound from the solution of the cooled metal material and recovering precipitate of the aluminium compound; washing the precipitate of the aluminium compound to form a washed product; and calcining the washed product to produce a high purity alumina.

In an embodiment, the step of increasing the temperature of the aluminium cans in a non-oxidising atmosphere comprises hearing to the cans to above the melting point of aluminium. The increase in temperature de-coats the cans by melting the polymer layer. The polymer coating waste in the molten material is separated/removed from the molten metal,

In an embodiment, the aluminium cans are shredded prior to increasing their temperature. The shredded can material can be heated to around 500 degrees Centigrade in order to de-coat the cans.

The step of dissolving the discrete particles (sometimes referred to as de-coated particles) can be undertaken in an alkaline digest or in an acid digest.

Dealing first with the alkaline digest, there is provided a method of manufacturing high purity alumina of at least 4N from aluminium cans, the method comprising the steps of increasing the temperature of the aluminium cans in a non-oxidising atmosphere to form a molten material; separating molten metal from the molten material and allowing the molten metal to cool to a cooled metal material; forming discrete particles of the cooled metal material; dissolving discrete particles of the cooled metal material in an alkaline digest thereby forming an alkaline solution of the cooled metal material; precipitating alloy metals from the alkaline solution of the cooled metal material by adjusting the pH towards a more acidic pH, optionally between about 9.5 to 10, to produce precipitated impurities and a filtrate; preferably removing the precipitated impurities. The removal could be by filtration or other means, e.g. centrifuging, to separate the precipitated impurities from the clear liquor; precipitating aluminium hydroxide trihydrate (gibbsite) from the filtrate and recovering precipitate of the aluminium hydroxide trihydrate (gibbsite); washing the precipitate of aluminium hydroxide trihydrate (gibbsite) to form a washed product; and optionally treating the washed product by either or both of dihydroxylation and an acid digest to form a further treated washed product. The washing can be to partially dehydroxylate the washed product, with further washing of the intermediate. The washing can be conducted with hot deionised water and may be pressurised to obtain a water temperature over 100 and up to 250 degrees Centigrade; calcining the further treated washed product to produce a high purity alumina. In one aspect, the process is undertaken with an acid digest. In this aspect there is provided a method of manufacturing high purity alumina of at least 4N from aluminium cans, the method comprising the steps of increasing the temperature of the aluminium cans in a non-oxidising atmosphere to form a molten material; separating molten metal from the molten material and allowing the molten metal to cool to a cooled metal material; forming discrete particles of the cooled metal material; dissolving discrete particles of the cooled metal material in an acid digest thereby forming a solution of the cooled metal material; evaporating the acid and redissolving the remainder in a minimum amount of deionised water to provide a clear liquid phase, precipitating aluminium chlorohexahydrate from the clear liquid phase by sparging with hydrochloric acid gas, and recovering precipitate of the aluminium compound; heating the aluminium chlorohexahydrate to recover the hydrochloric acid and precipitate aluminium oxide; other steps may include washing the precipitate of aluminium compound to form a washed product; and calcining the washed product to produce a high purity alumina.

In an embodiment, the washed product from the alkaline digest is subject to the acid digest process. Thus, following the step of washing the precipitate of aluminium hydroxide trihydrate (gibbsite) to form a washed product, the washed product is: dissolved in an acid digest thereby forming a solution of the washed product; the method further includes the steps of evaporating the acid of the acid digest and redissolving the remainder in a minimum amount of deionised water to provide a clear liquid phase, precipitating aluminium chlorohexahydrate from the clear liquid phase by sparging with hydrochloric acid, and recovering precipitate of the aluminium compound; washing the precipitate of aluminium compound to form a further treated washed product; and calcining the further treated washed product to produce a high purity alumina.

The present method makes use of aluminium cans (sometimes referred to as aluminium containers) as a starting material for the production of HPA. The aluminium cans may be scrap aluminium cans. In an embodiment, the scrap aluminium cans were previously used or intended for use as beverage cans. In an embodiment, the scrap aluminium cans were previously used or intended for use as food cans. It should be understood that the scrap aluminium can may have been a container for any material, but the container is now empty and is available for recycling. The container may have been used, or it might be a reject from a manufacturing line, so it was intended for use, but never actually progressed to use. In embodiments, mixtures of different types of cans from different sources are used as a feedstock for the process.

The impurities in a man-made aluminium can is well-known. Accordingly, the present inventors have found that a certain sequence of processing steps is required in order to properly prepare the cans for treatment, and then to ensure complete removal of the impurities (including unexpected impurities) from the can material. The inventors have found that if the cans are not treated according to the present method, the plastic lining of the cans can contaminate the resultant product and may contribute to the impurity levels in the resultant product.

Aluminium cans comprise aluminium alongside other impurity elements such as magnesium and iron. The different parts of the aluminium can may have different requirements including strength requirements. The aluminium can lid, for example, needs to be more rigid than the can body, so the elemental composition likely differs. Any aluminium ring pull/tag in the lid of the can must be the most rigid part of the can, so will comprise e.g. more magnesium together with the aluminium to provide the require structural rigidity. Typically, the aluminium cans are principally made of three alloys. Can lid: Alloy 5052

• %Si 0.25, Fe 0.40, Cu 0.10, Mn 0.10, Mg 2.8, Zn 0.10, Ti 0.05, Cr 0.15,

Can ring and tag: Alloy 5182

• %Si 0.20, Fe 0.35, Cu 0.10, Mn 0.10, Mg 4.5, Zn 0.10, Ti 0.20, Cr 0.35,

Can body: Alloy 3104

• %Si 0.60, Fe 0.80, Cu 0.05, Mn 0.8, Mg 1.0, Zn 0.10, Ti 0.10, Cr O.10,

Some manufacturers may use slightly different alloys. In theory, there is no need to separate the can lids from the body of the can prior to use in the process, as the only significant alloy variation is the Mg content. However, in embodiments, it may be necessary to remove the can lids prior to subjecting the aluminium cans to an embodiment of the method of the present invention.

The aluminium cans as provided may, in embodiments, have an inner food grade plastic lining. The cans may have printing or decoration on their outside surfaces. The inside and or outside surface of the cans may comprise one or more of paint, ink, paper, plastic and oil. If the can has been used it may also contain residue of the material that was in the can. Depending on where the can was sourced from, it may also be covered in dirt, sand or other material that contacted the can while it was treated as rubbish/scrap.

Prior to heating, the aluminium cans can be shredded. The cans may then be de- coated by thermal de-coating involving heating and melting. In an embodiment, the aluminium cans are heated in a furnace or kiln. The temperature of the cans in the furnace is increased to a temperature above the melting point of the aluminium (Al melting point 660.3 degrees Centigrade). The temperature of the cans in the furnace can be increased to above 650 degrees Centigrade. Preferably, the temperature is increased to at least about 700 degrees Centigrade. More preferably, the temperature is increased to at least about 750, 800 or 850 degrees Centigrade. As the temperature increases, the molten material formed can include the metal materials and the lining and or coating materials on the cans surfaces. The lining and coating materials can be removed as volatile organic carbons (VOCs) thereby separating molten metal from the other molten materials. The VOCs generated can be collected and treated according to environmental regulations. Beverage can coatings have steadily been altered to mininimise VOC’s on recycling by the introduction of water based resins.

In an attempt to avoid formation of dross (solid oxides floating on the surface of the material as it melts) the increase in temperature is undertaken in a non-oxidising atmosphere. In an embodiment, the atmosphere during the increase in temperature is oxygen free. There may be some oxygen such as less than 15, 10 or 8 % oxygen but it is preferred that there is no oxygen.

The molten metal that forms following the increase in temperature is a mixture of molten aluminium, and molten impurities including but not necessarily limited to Magnesium (Mg), Iron (Fe), Manganese (Mn), Zinc (Zn), Silicon (Si), Copper (Cu), Titanium (Ti) and Chromium (Cr). Other impurities include Gallium (Ga), Vanadium (V) and Yttrium (Y). The molten material (which can include metalloids such as silicon) can be cooled into handleable solid pieces of metal such as bars or ingots.

The ingots that form on cooling of the molten metal will form according to the size and shape of a mould into which the molten metal is directed from the furnace. The molten metal can run into the mould when released from the furnace and as it cools, it takes the shape of the mould and can subsequently be removed from the mould for further processing. In an embodiment, the mould comprises multiple small discrete areas for the molten metal, so that each ingot is essentially a small chip of cooled metal material (cm or mm in size). Thus, the formation of discrete particles of the cooled metal material can comprise forming the discrete particles in a mould. However, whilst it is feasible to mould small particles, such a mould would be expensive to prepare and difficult to handle. Accordingly, it is more common for moulds to be provided that form larger ingots into which the molten material will cool. The large ingots (2-3 cm x 3-8 cm x 6-12 cm) of metal are then broken, chopped, cut, milled or otherwise treated to form discrete particles of cooled metal sometimes referred to as chips (mm or cm in size). The chips can be sized in a range of from about 1 mm to about 2 mm. There can be smaller and larger chips, so the average size distribution is about 80 % in the range stated. The formation of discrete particles of the cooled metal material can comprise milling the ingots in a ball mill. The formation of discrete particles of the cooled metal material can comprise milling the ingots in a hammer mill. The formation of discrete particles of the cooled metal material can comprise moving the ingots against a multi-bladed or pointed cutting tool in a lathe.

At least some of the discrete particles of cooled metal material once formed are dissolved in order to from a solution. Preferably, all of the discrete particles are dissolved. In some embodiment at least 99, 95, 90, 80 or 85 % of the discrete particles are dissolved with the remainder being undissolved. Any undissolved particles can be removed (e.g., by filtration) and added to a subsequent batch being processed by the method. There can be any number of batches being processed in a factory in a sequence.

The discrete particles of cooled metal material can be dissolved in an alkaline digest. 2AI°+2Na0H+2H ₂ 0 ® 2(NaAI0 ₃ ) +3H ₂

Any hydrogen generated during the alkaline dissolution process can be captured for recycle or sale. Generally, there will be about 111 kg of hydrogen generated per tonne of Al.

Producing alumina from scrap aluminium is of interest only if alumina of high purity can be produced with a target of 99.99 as a minimum. Hence, in an embodiment it is advantageous if a high purity sodium aluminate can be produced, from which a high purity aluminium hydroxide trihydrate (gibbsite) can be produced. Thus, in an embodiment, the discrete particles of the cooled metal material are dissolved in a caustic solution containing sodium. The caustic solution containing sodium can be sodium hydroxide (caustic soda).

In an embodiment, the particles of metal material are dissolved at about 30g/L in a 20% caustic soda solution. The fastest dissolution of Al is said to take place in a solution of 5.5N NaOH which contains 220 grams per litre of flake caustic soda. Alternatively, 30, 40 or 50 % liquid caustic can be used for dissolution.

The dissolution of the molten metal will produce some insoluble hydroxides and de- silication products. These can be removed at various points as would be understood by the skilled person. The main portion of the hydroxides will be magnesium plus some iron, manganese, chromium, and copper It has been found that silicon co-precipitates with the hydroxides.

The pH of the caustic solution of cooled metal material can be adjusted using an acid. The pH can be adjusted to a pH of about 10.. The acid can be hydrochloric acid (HCI). Alternatively carbon dioxide can be bubbled into the alkaline solution to lower the pH . Ionic species Fe, Cu, Mn, Mg, Cu will precipitate at high pH whilst those such as Zn will precipitate below Ph 10..These precipitates can be removed from the filtrate. The precipitates can be removed by decanting and or filtration or centrifuge. The solids recovered as precipitate can be washed to recover any residual aluminium and sodium that might have value in other industrial processes. Tests will establish the composition of the removed precipitates and determine if the waste has any commercial value or waste cost. The filtrate is recovered for further processing.

In order to precipitate aluminium hydroxide trihydrate (gibbsite) from the filtrate seeds of gibbsite can be added to the solution as nucleation centres. The following is a description of the Bayer process precipitation:

The yield of hydrate through the precipitator train is an important efficiency parameter. The greater the yield the more efficient the refinery. Typically, the pregnant/green liquor entering precipitation (at 75-80°C) will have an aluminium content of 140 g/L (expressed as AI2O3) and the spent liquor exiting precipitation (at 55°C) a value of 50 g/L, giving a yield of 90 g/L. The mechanism of precipitation involves a combination of agglomeration and growth. The first stage of precipitation involves the sticking together of small seed particles to form agglomerates. This is a relatively rapid process, which is favoured by high alumina supersaturations and high temperatures.

It therefore is the predominant mechanism in the first stage of precipitation. The agglomerates so formed are of random shapes, and the overall strengths of the agglomerated particles are low. This is also the step in which most of the chemically included soda is incorporated into the hydrate crystals. It is therefore essential that the high yielding agglomeration stage is followed by a period of controlled growth. In this growth stage, as well as increasing in size the particles are strengthened, and their shapes are regularised. In addition, the overall soda content of the particles is diluted.

The Precipitator used to effect the precipitation can be similar to those used in the older plants i.e. tall vessels with a fluted bottom. However, the precipitation could be undertaken in a flat bottomed circular tank fitted with impellers for a slow stirring action as would be understood by the skilled person.

The gibbsite precipitate is recovered e.g. by filtration. Some precipitate from the process can be reserved for the provision of seeding material in subsequent precipitation.

In an embodiment, the step of precipitating aluminium hydroxide trihydrate (gibbsite) from the filtrate is undertaken more than once by recovering the filtrate and adding further seed to the filtrate and then filtering the solution again. In an embodiment, the step of precipitating aluminium hydroxide trihydrate (gibbsite) from the filtrate is undertaken more than once such as 2, 3 or 4 times. In a preferred embodiment the precipitation is undertaken twice.

The gibbsite produced by precipitation of alumina produced from aluminium cans will have to be washed free from at least sodium (Na) to attain the 99.99% required. The first step in this washing purification is to wash the gibbsite precipitate with water. Preferably, the water is deionised to avoid adding any impurities to the solution with the water. It is also preferred that all material in which reactions are undertaken are lined with non-contaminating surfaces such as Teflon to minimise the transfer of contamination into solution.

The washed precipitate can then be processed in various ways.

• In a first embodiment, the washed precipitate is dehydroxylated at about 400, 450 or 500 degrees Centigrade, and then acid washed at high temperature under pressure and then filtered before being calcined.

• In an alternative embodiment, the washed precipitate is subject to the acid digest aspect of the invention as set out below. The washed precipitate treated by the acid digest process is treated in the same way as the discrete particles of the cooled metal material obtained fresh from the furnace.

Above, it is described that the discrete particles of cooled metal material obtained from the furnace can be dissolved in an alkaline digest (such as NaOH).

Dissolution in an alkaline digest such as caustic soda is preferred because of the generation of a high pH solution which precipitates Fe, Mn, Cu and Mg as hydroxides, which can be removed by filtration. However, alternatively, the discrete particles of cooled metal material can be dissolved in an acid digest. The acid of the digest can be a strong acid. The acid can be hydrochloric acid (HCI). Again, any hydrogen generated during the acid dissolution process can be captured for resale or reuse. Generally, there will be about 111 kg of hydrogen generated per tonne of Al.

The acid digest can take the solution to near dryness, at which point the solution is redissolved in a minimum quantity of deionised water and then filtered to provide a clear liquid phase. The clear liquid phase can then be sparged with a chlorinated acid until precipitation of the chloride hexahydrate is complete. The acid can be HCI. The result is a precipitate of aluminium chlorohexahydrate (ACH) which can be filtered and collected.

Optionally, the ACH precipitate can be redissolved in a minimum amount of water and sparged again to repeat the precipitation of the ACH with increased purity. The precipitation, hydration and sparging cycle can be undertaken 1 , 2, 3 or more times. Once the cycle of precipitation and sparing is considered complete, the product can be calcined at about 350, 400 or 450 degrees Centigrade. Any HCI recovered during this calcination step can be recovered for reuse in the process. The product can then be washed and passed to the final calcination step.

The final calcination step comprises elevating the temperature of the purified precipitate to a temperature in the range of about 500 to about 1200 degrees Centigrade, preferably 500 to 1000 degrees Centigrade. Preferably the calcination is undertaken at least about 1000 degrees Centigrade. Without wishing to be limited by theory, it is thought that the final calcination step assists in reorganising the oxide structure of the HPA. The calcined product can be acid and water washed and then dried.

In one aspect there is provided a system of manufacturing high purity alumina of at least 4N using de-coated aluminium cans as the only source of aluminium in the feedstock, the system comprising the steps of sourcing coated aluminium cans; de-coating the coated aluminium cans in a furnace by increasing the temperature of the aluminium cans in a non-oxidising atmosphere to form a molten material; separating molten metal from the molten material and allowing the molten metal to cool to a cooled metal material; subjecting the cooled metal material to a method or process optionally in accordance with any one of claims 1 to 10, wherein that method or process removes impurities from the cooled metal material, thereby leaving only highly pure aluminium in the resultant product.

Brief Description of the Figures

Embodiments of the invention will now be described with reference to the accompanying drawings which are not necessarily drawn to scale, where some numbering is shown only on some Figures for clarity and which are exemplary only and in which:

Figure 1 is a flow diagram showing the alkaline digest embodiment of the present invention.

Figure 2 is a flow diagram showing the acid digest embodiment of the present invention.

Figure 3 is a flow diagram showing the hybrid embodiment of the present invention.

Figure 4 is a table showing the alloy content of sample aluminium cans.

Detailed Description of Embodiments of the Invention

A process according to an embodiment of the present invention is shown in Figure 1 . The aluminium cans are acquired and brought to site 111. The cans can be from any source. The aluminium cans are optionally shredded and compressed and are then delivered into a furnace or kiln for heating 112. The temperature of the cans in the furnace is increased to about 500, 600 or 700 degrees Centigrade for about 20, 30 or 60 minutes or longer The molten material (which includes alloying metals and metalloids) can be cooled 113 into handleable solid pieces of metal such as bars or ingots. The ingots are then milled using a lathe to form discrete particles 114. At least some of the discrete particles of cooled metal material once formed at 114 are dissolved to form a solution 115. In Figure 1 , the process shows that the discrete particles of cooled metal material are dissolved in an alkaline digest.

2AI°+2Na0H+2H ₂ 0 ®' 2(NaAI0 ₃ ) +3H ₂

Any hydrogen generated during the alkaline dissolution process can be captured for recycle or sale.

In the embodiment of Figure 1 , particles of metal material are dissolved at about 30g/L in a 20% caustic soda solution 115. The pH of the caustic solution of cooled metal material is then adjusted using HCI acid 116. The pH is adjusted to just below 10 Metals such as Fe, Cu, Mn, Mg, Cu will already have precipitated above pH 10 , Zn and other minor impurities will precipitate below 10 117. These precipitates can be removed from the filtrate by filtration 118. The solids recovered as precipitate can be washed to recover any residual aluminium and sodium that might have value in other industrial processes 119.

Seeds of aluminium hydroxide trihydrate (gibbsite) are added to the filtrate 120.

The first stage of gibbsite precipitation involves the sticking together of small seed particles to form agglomerates. The agglomerates so formed are of random shapes, and the overall strengths of the agglomerated particles are low. The gibbsite precipitate is recovered e.g. by filtration 121. The step of precipitating aluminium hydroxide trihydrate (gibbsite) from the filtrate is undertaken more than once 122 by recovering the filtrate and adding further seed to the filtrate and then filtering the solution again 123.

The gibbsite precipitate is washed with deionised water 124. The washed precipitate can then be processed in various ways.

• In a first embodiment, the washed precipitate is dehydroxylated at about 450 Centigrade 235, and then acid washed 236 before being calcined 130.

• The acid may be hydrochloric, acetic or citric or a combination thereof.

• The washing will be at elevated temperature and may be conducted when pressurised; the wash water will reach temperatures of up to 250 degrees Centigrade.

• In an alternative embodiment (shown in Figure 3), the washed precipitate is subject to the acid digest. The washed precipitate treated by the acid digest process (125 to129) .

Above, it is described that the discrete particles of cooled metal material obtained from the furnace can be dissolved in an alkaline digest (such as NaOH). This is represented by steps 115 to 124 of e.g. Figure 1 . However, alternatively, the discrete particles of cooled metal material can be dissolved in an acid digest as shown in Figure 2 as steps 125 to 129 and in Figure 3 using like numbering (325 to 339). The acid digest shown in Figure 2 (and also in part in Figure 3) can take the solution to near dryness 337, at which point the solution is redissolved in a minimum quantity of deionised water 338 and then filtered to provide a clear liquid phase 339. The clear liquid phase can then be sparged with a HCI gas 326/126. The result is a precipitate of aluminium hexachlorohydrate (ACH) which can be filtered and collected 327/127.

Optionally, the ACH precipitate can be redissolved in a minimum amount of water 338’ and sparged 3267 126’ again to repeat the precipitation of the ACH with increased purity. Once the cycle of precipitation and sparing is considered complete, the product can be calcined at 450 degrees Centigrade 328/128. Any HCI recovered during this calcination step can be recovered for reuse in the process as shown by the arrow in Figure 3. The product can then be washed 329/129 and passed to the final calcination step 130.

The final calcination step 130 comprises elevating the temperature of the purified precipitate to a temperature in the range of about 800 to about 1200 degrees Centigrade. The calcined product can be acid 131 and water washed 132 and then dried 133.

Aluminium cans comprise aluminium alongside other impurity elements such as magnesium and iron. The Table of Figure 4 shows the elemental analysis of an aluminium can that contained Coke ® and an aluminium can that contained beer. The elemental analysis of a standard aluminium ingot is also provided.

Purity of the product will be confirmed by a combination of ICP-mass spec and XRF analytical techniques.

Examples

Embodiment of the invention will now be described with reference to the following non-limiting examples. One tonne of Aluminium theoretically can produce 1 .889 tonnes of HPA. Since one beverage can weighs about 13.44 grams; At 95% this equates to 12.77 grams of pure aluminium. It follows therefore that the equivalent to one tonne of Al requires about 78,308 cans.

To produce alumina from the metal the preferred pathway is in summary.

1. Shred the aluminium cans.

2. De-coat the aluminium cans by thermal de-coating at 500 degrees Centigrade under air oxidising conditions for approximately 20 minutes.

3. Dissolve 10gr/I of the cans in 20% NaOH. The stoichiometric reaction of one tonne of Al° and NaOH requires 1 .48 tonnes of NaOH. It is preferred that at least a 10 % excess is added to ensure complete dissolution. Thus, 1.6 tonnes of caustic are required or 3.2 tonnes of a 50 % solution. The caustic can be recycled after precipitation and modern Bayer plants recover about 96%. In our calculations we could use a conservative 90 % recycle factor which brings the actual use down 0.32 tonnes of 50% liquid caustic per tonne of aluminium (Al°).

4. Capture emitted H2

5. Adjust the pH to 9.5-10 6. Filter the solution to remove precipitated impurities

7. Hydrolyse to precipitate gibbsite

8. Filter the Gibbsite

9. Deionised water wash the Gibbsite to remove sodium

10. Calcine to 400 degrees C 11. Hot or pressurised deionised water wash the dehydroxylated Gibbsite to remove sodium traces 12. Calcine at 1000 degrees C

Alternative method 2.

1 to 9 as above.

10. Dissolve the Gibbsite in HCI

11. Sparge with HCI to precipitate AICI3.6H20 (ACH)

12. Filter AICI3.6H20 13. Heat to drive off and recycle HCI

14. Calcine at 1000 degrees C

It will of course be realized that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. Any promises made in the present description should be understood to relate to some embodiments of the invention and are not intended to be promises made about the invention as a whole. Where there are promises that are deemed to apply to all embodiments of the invention, the applicant/patentee reserves the right to later delete them from the description and does not rely on these promises for the acceptance or subsequent grant of a patent in any country.