Production of aluminium compounds from clay TECHNICAL FIELD

[0001] The present invention relates to a method for the production of aluminium compounds from clay. In some embodiments, the present invention relates to a method for the production of high purity aluminium compounds, such as high purity alumina, from kaolin.

BACKGROUND ART

[0002] Kaolinite (also called kaolin, which is the terminology that will be adopted throughout this specification) is a widely occurring clay mineral having a chemical composition of Al ₂ Si ₂ O ₅ (OH) ₄ in its pure form. Most natural deposits of kaolin are mixed with other compounds, including iron oxides, titanium dioxide, magnesium oxide, potassium dioxide and smaller amounts of other oxides including oxides of calcium, sodium, phosphate, sulphate, barium, chromium, manganese, strontium and zinc. Kaolin occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates. Kaolin is mined in a large number of countries.

[0003] Kaolin has a number of industrial uses, including as a feed material for the production of ceramics, in cosmetics, as a filler in paper to provide a smoother surface to the paper, and as a filler in paint.

[0004] Alumina is used as a feed material for the production of aluminium metal. It is also used as an adsorbent, as a support for catalysts, as an abrasive and as a refractory material. Most alumina that is produced on an industrial scale is manufactured using the Bayer process. This process involves dissolution of bauxite in hot sodium hydroxide. The pregnant leach liquor containing dissolved aluminium is separated from the red mud solid residue. The pregnant leach liquor is clarified. Aluminium hydroxide is precipitated by adding seed particles and cooling. The aluminium hydroxide is dried and then calcined to form alumina. Alumina from the Bayer process is mainly used as a feed material to produce aluminium metal. Alumina for making aluminium is specified as smelter grade alumina and the purity is on the order of 98-99%. Typical impurities in smelter grade alumina are Na, Cl, Ca, Fe, Ga, Si, for example.

[0005] Some efforts have been made to produce alumina from kaolin, usually by countries that have limited access to high quality bauxite ore. Existing methods have involved contacting the kaolin with acid. Of the acid processes, hydrochloric acid is generally regarded as the most cost effective due to energy considerations and the relative ease of recycling the acid. Although a concept for such a manufacturing process was first outlined in the 1940s and piloted extensively in the subsequent years, such processes have not been commercially successful to date as they cannot compete with the conventional Bayer process for smelter grade alumina.

[0006] High purity alumina, which may be 99.9% pure, or 99.95% pure, or 99.7% pure, or 99.99% (4N) pure, is a high-value material relative to smelter grade alumina that is used in manufacturing battery materials, advanced ceramics, LEDs and synthetic sapphires.

[0007] Throughout this specification, the term “kaolin” will be used to refer to kaolin- containing materials as well as more pure forms of kaolin. Examples include kaolin clay, mining tailings, solid residues from hydrometallurgical treatment processes that contain kaolin. Most kaolin ore deposits contain a significant amount of other mineral impurities, especially quartz, feldspar and muscovite. Most mining tailings contain significant amount of clay materials, including kaolin minerals.

[0008] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

SUMMARY OF INVENTION

[0009] The present invention is directed to a process for manufacturing aluminium compounds from kaolin, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

[0010] With the foregoing in view, the present invention in a first aspect, resides broadly in a method for preparing aluminium compounds from kaolin, the method comprising the steps of: a) leaching the kaolin with sulphuric acid; b) separating a pregnant leach solution from a solid residue; c) precipitating an alum containing solid from the pregnant leach liquor; d) redissolving the alum; e) treating the dissolved alum solution to separate aluminium from potassium and to form a higher purity aqueous phase containing dissolved aluminium; and f) forming one or more aluminium compounds from the higher purity aqueous phase.

[0011] In one embodiment, the present invention provides a method for preparing aluminium compounds from kaolin, the method comprising the steps of: a) leaching the kaolin with sulphuric acid; b) separating a pregnant leach solution from a solid residue; c) precipitating an alum containing solid from the pregnant leach liquor; d) redissolving the alum; e) subjecting the dissolved alum solution to solvent extraction to produce a higher purity aqueous solution containing dissolved aluminium; and f) forming one or more aluminium compounds from the higher purity aqueous phase.

[0012] In another embodiment, the present invention provides a method for preparing aluminium compounds from kaolin, the method comprising the steps of: a) leaching the kaolin with sulphuric acid; b) separating a pregnant leach solution from a solid residue; c) precipitating an alum containing solid from the pregnant leach liquor; d) redissolving the alum; e) subjecting the dissolved alum solution to an ion exchange treatment process to produce a higher purity aqueous solution containing dissolved aluminium; and f) forming one or more aluminium compounds from the higher purity aqueous phase.

[0013] The one or more aluminium compounds may be precipitated from the higher purity aqueous phase.

[0014] In one embodiment, the method of the present invention comprises the step of converting the kaolin to metakaolin and then leaching the metakaolin with sulphuric acid. In this embodiment, step (a) comprises: a)(i) heating the kaolin to form metakaolin, a)(ii) leaching the metakaolin with sulphuric acid.

[0015] The present inventors have found that converting the kaolin to metakaolin and then leaching the metakaolin with sulphuric acid results in improved dissolution of aluminium from the metakaolin and hence a higher recovery from the process. Therefore, it is expected that preferred commercial embodiments of the present invention will involve converting the kaolin to metakaolin prior to the leaching step. However, it will be understood that the present invention also encompasses leaching kaolin.

[0016] Kaolin may be converted to metakaolin by heating the kaolin to an elevated temperature. In one embodiment, the kaolin is placed in a furnace or oven at a temperature above 500°C, or at a temperature of greater than 550°C, or greater than 600°C, or greater than 650°C, or greater than 700°C, or greater than 750°C, or at a temperature of from 550°C to 900°C, or from 600°C to 850°C, or from 650°C to 750°C. In general terms, if a higher temperature is used in this step, lower treatment times can be used. For example, if a temperature of 650°C was used, a heating time of 1.5 hours is suitable. For a temperature of 750°C, a shorter heating time of 25 minutes can be used. In general terms, is expected that a total heating time of less than 5 hours, or less than 4 hours, or less than 3 hours, or less than 2 hours or from 15 minutes to 5 hours, or from 20 minutes to 3 hours, or from 25 minutes to 1.5 hours, should produce acceptable conversion of kaolin to metakaolin. The kaolin may be substantially completely converted to metakaolin in this step.

[0017] The feed material may be subjected to a washing step prior to heating to convert the kaolin to matter kaolin. This washing step will be referred to as a pre-washing step. The pre- washing step may remove some contaminants, such as iron, from the feed material prior to sending it to me step. In one embodiment, the pre- washing step comprises contacting the feed material with an acidic solution. In another embodiment, the pre-washing step comprises mixing the feed material with an acidic solution and subjecting the mixture to ultrasonication or other agitation.

[0018] The leaching step involves contacting the kaolin or metakaolin with sulphuric acid. The leaching step may be conducted at elevated temperature in order to increase kinetics of the leaching reactions. For example, the leaching step may be conducted at a temperature of from 50°C up to the boiling point of the liquid/slurry in the leaching step, or from 60°C to 95°C or from 70° to 90°C. Desirably, the leaching step is conducted at a temperature at or below the boiling temperature of the sulphuric acid solution used as the leachant.

[0019] The leaching step may be conducted at atmospheric pressure, thereby avoiding the need to conduct the leaching step in one or more pressure vessels.

[0020] In another embodiment, the leaching may be conducted at a temperature of up to 300°C. In embodiments where leaching is conducted at a temperature above the atmospheric boiling point of the leaching solution, leaching will be conducted in a pressure vessel, such as an autoclave. Leaching at temperatures above the atmospheric boiling point of the leaching solution may be useful in embodiments where the kaolin is leached without initially converting it to metakaolin. However, the present inventors believe that it is likely that it will be preferred to first convert the kaolin to metakaolin and subsequently conduct leaching at a temperature below the atmospheric boiling point of the leaching solution.

[0021] The sulphuric acid solution used in the leaching step may have a concentration of from 0.5 to 5M, or from 0.5 to 4M, or from 0.5 to 3M, or from 0.5 to 2M. The present inventors have found that increasing the acid concentration from 0.5 to 2M increased this total aluminium recovery, with no improvement in recovery seen by increasing further to 3M. However, although aluminium extraction is not increased by increasing the sulphuric acid concentration to greater than 2M, there may be benefits in using higher concentrations as the amount of silica in solution is decreased as acid concentration increases. It is expected that commercial embodiments of the present invention will operate at sulphuric acid concentrations of from 0.5 to 3M, or 0.5 to 2M, in the leaching step. An acid concentration of 2M or greater, suitably 2 to 3M, may be used in order to limit the amount of dissolved Si in solution.

[0022] The leaching step may have a contact time or residence time of up to 6 hours, or up to 5 hours, or to 4 hours, or from 30 minutes to 4 hours. Longer contact times or residence times may be used. In some embodiments, a residence time of 2 hours or greater may be used in the leaching step.

[0023] In the leaching step, aluminium present in the kaolin or metakaolin is leached into solution. Most other impurities, including potassium and iron, also dissolve proportionally. However, silica and some silicates either remain insoluble or re-precipitate as silica or amorphous silica. As a result, the leaching step selectively separates aluminium from silica.

[0024] The pregnant leach solution containing dissolved aluminium, dissolved potassium, dissolved iron and other dissolved metals, but only a low level of dissolved silica, is separated from the solid leaching residue. This solid/liquid separation step may be achieved using any technique known to the person skilled in the art, including filtration, centrifugation, settling, thickening, clarification, hydrocyclone separation or the like. The solid residue may be sent for disposal processed further, or sold as a by-product. The solid residue may be washed prior to disposal to minimise residual acid content in the solids and to recover some of the acid.

[0025] The pregnant leach liquor contains dissolved aluminium and other impurities that have also been extracted from the kaolin or metakaolin. These impurities include potassium, iron, magnesium, calcium and a low level of silica. Smaller amounts of other impurities may also be present.

[0026] Step (c) of the present invention involves precipitating an alum containing solid from the pregnant leach solution. In one embodiment, ammonium alum (aluminium ammonium sulphate) is precipitated by adding an ammonium compound, such as ammonium sulphate, to the pregnant leach liquor. Addition of ammonium sulphate to the leach solution produces aluminium ammonium sulphate, which is less soluble than aluminium sulphate and can be crystallised from the leach solution by cooling without requiring evaporation. This crystallisation step rejects the majority of impurities in solution, with the exception of potassium which crystallises as potassium aluminium sulphate. The majority of the other impurities remain in solution. Aluminium ammonium sulphate and potassium aluminium sulphate have similar solubilities and form a mixed crystalline structure, making separation of the aluminium ammonium sulphate and potassium aluminium sulphate not possible at this stage. Impurities other than potassium can be significantly reduced by re-dissolution and recrystallisation of the alum containing solids one or more times. Advantageously, precipitating aluminium ammonium sulphate also provides an efficient way to separate aluminium from sulphuric acid, thereby negating the need to neutralise before further processing.

[0027] In another embodiment, ammonia (ammonium hydroxide) may be used to precipitate the aluminium ammonium sulphate. It has been found that using ammonia improves precipitation yield and potassium rejection slightly, when compared with using ammonium sulphate, while also providing some neutralisation of the acid in the filtrate.

[0028] The precipitated alum containing solids are separated from the liquid phase. The aluminium ammonium sulphate that has been formed in this stage can be directly calcined to form alumina. However, the potassium contamination at this point is significant and further processing will be required to produce high purity aluminium compounds. However, a significant reduction in all other impurities, especially iron, has been achieved at this stage of the method.

[0029] In order to reduce the potassium contamination at this point, the precipitated alum is re-dissolved in an aqueous solution, such as water or an acidic solution. This forms an aqueous solution containing dissolved aluminium and some dissolved potassium. In some embodiments, steps (c) and (d) may be repeated. For example, when the alum is dissolved, alum may be re-precipitated and the re-precipitated alum may then be re-dissolved again.

This is effectively a washing and re-precipitation step. Multiple cycles of alum precipitation and re-dissolution may be required, depending on the final purity requirements of the product. In experimental work conducted to date, the alum precipitation and re-dissolution steps have taken place 2 or 3 times.

[0030] The aqueous solution formed in step (d) is subsequently treated to form a higher purity aqueous solution containing dissolved aluminium and very little dissolved potassium.

[0031] In embodiments of the present invention, the treatments currently envisaged by the present inventors to form the higher purity aqueous solution include solvent extraction treatment and ion exchange treatment. [0032] In one embodiment, in order to reduce or substantially remove potassium from the alum containing solids, the alum containing solids are subjected to a solvent extraction process. In one embodiment, the alum containing solids are dissolved in an aqueous solution, such as water or an acidic solution. The aqueous solution is then contacted with an organic solvent extraction agent. The organic solvent extraction agent selectively extracts dissolved aluminium from the aqueous solution into the organic phase. The organic solvent extraction agent may comprise a cation exchange extractant. Aluminium preferentially loads onto cation exchange extractants owing to its relatively high charge density compared with most divalent cations and all monovalent cations. Dissolved potassium remains with the aqueous phase.

The organic phase is then separated from the liquid phase. Aluminium may be stripped from the organic phase by mixing the organic phase with an aqueous stripping solution that strips the aluminium from the organic phase into the stripping solution. The stripping solution may then be separated from the organic phase. The aqueous stripping solution represents a high purity aqueous solution containing dissolved aluminium.

[0033] In one embodiment, the organic solvent extraction agent comprises an organo- phosphoric acid, such as D ₂ EHPA. Such organic solvent extraction agents at acidic pH allow loading of aluminium whilst avoiding loading of potassium. The present invention encompasses the use of any organic solvent extraction agent that can selectively strip aluminium from solution whilst leaving potassium in the aqueous solution. There is expected to be a large number of possible organic solvent extraction agents that can be used as any organic solvent extraction agent that preferentially separates the trivalent aluminium ions from the monovalent potassium ions in solution will be able to be used in this embodiment of the present invention.

[0034] Aluminium may be stripped from loaded organic solvent extraction agents by contacting the loaded organic phase with an acid solution, such as sulphuric acid, hydrochloric acid or nitric acid. Organic acids may also be used. Examples of organic acids that may be used include versatic acid, dialkyl phosphinic acid, naphthenic acid, hydroxyoximes are other examples. It is expected that a large number of possible stripping agents can be used because any stripping agent that operates on the principle of generally being more selective for the trivalent metals such as aluminium in this case will be suitable.

[0035] In some embodiments, the aqueous solution containing alum that is to be contacted with the organic solvent extraction agent may contain a buffer. The buffer suitably comprises a buffering agent that does not load into the organic phase. Glycine is an example of a suitable buffering agent. Other potential buffering agents may include citrate, acetate and ammonia. It has been found that the buffering agent counters the release of hydrogen ions from the organic phase during metal ion loading, therefore avoiding the need to readjust the pH. It will be understood that adding an alkali metal base to adjust the pH could result in ions from the alkali or base material entering the organic phase and this could result in additional impurities entering the organic phase. Also, the addition of a stronger base could result in aluminium hydrolysis and precipitation during the solvent extraction which could cause operational problems.

[0036] In other embodiments, pH is controlled during solvent extraction by adding a base or an alkali material. In one embodiment, ammonia is used for pH control during solvent extraction. In some embodiments, a combination of an initial glycine dose followed by pH control by ammonia addition may be used.

[0037] In some embodiments of this embodiment of the present invention, the loaded organic phase is washed with wash water to strip any potassium, sodium or silicon that has also transferred to the organic solvent. This washing step will occur prior to the step of stripping aluminium from the loaded organic solvent extraction agent.

[0038] In some embodiments, the solvent extraction step is conducted in a number of stages: for example, as a multistage counter current operation or as a multistage co-current operation.

[0039] The aqueous liquid phase arising from stripping of the loaded organic extraction agent in the solvent extraction step comprises an aqueous solution containing dissolved aluminium ions. If the solution also contains dissolved sulphate ions, as would be the case if sulphuric acid was used as the stripping agent, alum (aluminium sulphate) can be precipitated from the aqueous liquid phase. This will result in the formation of high purity alum/aluminium sulphate. Alum may be precipitated by using any precipitation method known to be suitable, such as evaporation or addition of a counter- solvent. In one embodiment, an organic solvent, such as an alcohol, for example, ethanol, is added to the aqueous liquid phase to cause precipitation of alum. Other alcohols could also be used to cause precipitation. [0040] The precipitated alum is of high purity (greater than 99%, or 99.5% or greater, or even 99.9% or greater purity, or even 99.97% or greater purity, or even 99.99% or greater purity and can form a valuable product in its own right. However, in some embodiments, the precipitated alum is subject to a calcination step to form high purity alumina. In one embodiment, the calcination step includes heating the alum to a temperature in excess of 900°C, or from 950°C to 1200°C, or to about 1000°C, to convert the alum to alumina. Initial experimental work conducted by the present inventors has shown that high purity alumina having a purity of greater than 99%, or 99.5% or greater, or 99.97% or greater, can be achieved.

[0041] In another embodiment, the re-dissolved alum solution is treated by ion exchange to selectively adsorb aluminium ions onto the resin whilst leaving potassium ions in solution. The loaded ion exchange resin may then be separated from the solution. The loaded ion exchange resin may be optionally washed and then treated to remove aluminium therefrom to form an aqueous phase comprising an aluminium-containing solution of higher purity. The higher purity aqueous phase containing dissolved aluminium can then be treated to form aluminium compounds. The aluminium compounds may be formed by precipitation. The aluminium compounds can be separated from the aqueous phase using any known solid/separation technique. The aluminium compounds are of high purity.

[0042] In one embodiment of this embodiment of the present invention, a cation exchange resin is used. In one embodiment, a cation exchange resin that contains an iminodiacetate functional group is used.

[0043] In one embodiment, the ion exchange resin selectively adsorbs aluminium ions from solution whilst leaving potassium ions in solution. After separating the loaded ion exchange resin from the solution containing dissolved potassium ions, the aluminium ions are stripped from the loaded ion exchange resin. In one embodiment, the loaded ion exchange resin is contacted with an acid to strip the aluminium ions therefrom.

[0044] Stripping the aluminium ions from the loaded resin results in the formation of a solution containing dissolved aluminium ions. This solution is of high purity. High purity aluminium compounds may be made from this solution. The high purity aluminium compounds may be made using similar conditions and processes to that as described above. [0045] In a second aspect, the present invention provides a method for producing high purity alumina having a purity of greater than 99%, the method comprising: a) leaching the kaolin with sulphuric acid; b) separating a pregnant leach solution from a solid residue; c) precipitating an alum containing solid from the pregnant leach liquor; d) redissolving the alum; e) treating the dissolved alum solution to separate aluminium from potassium and to form a higher purity aqueous phase containing dissolved aluminium; f) forming one or more aluminium compounds from the relatively pure aqueous phase, and g) calcining the one or more aluminium compounds to produce alumina.

[0046] In some embodiments, the high purity alumina has greater than 99% purity, or 99.5% or greater purity, or even 99.7% or greater purity, or even 99.9% or greater purity, or even 99.97% or greater purity, or even 99.99% or greater purity.

[0047] In one embodiment, step (a) comprises a)(i) heating the kaolin to form metakaolin, a) (ii) leaching the metakaolin with sulphuric acid;

[0048] In some embodiments of the second aspect of the present invention, steps (c) and (d) may be repeated. For example, when the alum is dissolved, alum may be re-precipitated and the re-precipitated alum may then be re-dissolved again. Multiple cycles of alum precipitation and re-dissolution may be required, depending on the final purity requirements of the product.

[0049] Embodiments of the second aspect of the present invention may use the same processing steps as those described with reference to the embodiments of the first aspect of the present invention. For brevity of description, these will not be repeated. [0050] Embodiments of the second aspect of the present invention may use similar operating parameters to embodiments as described with reference to the first aspect of the present invention.

[0051] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0052] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge.

BRIEF DESCRIPTION OF DRAWINGS

[0053] Various embodiments of the invention will be described with reference to the following drawings, in which:

[0054] Figure 1 comprises a flowsheet of an embodiment of the present invention for producing high purity alumina via the sulphate route;

[0055] Figure 2 shows a graph of impurity levels against Al level, obtained from leaching metakaolin with sulphuric acid at varying conditions;

[0056] Figure 3 is a graph showing the effect of acid concentration on dissolved aluminium with calculated recovery;

[0057] Figure 4 is a graph showing the effect of acid concentration on solution Si concentration during leaching;

[0058] Figure 5 is a graph showing the effect of temperature variation on dissolved aluminium with calculated recovery;

[0059] Figure 6 is a graph showing the effect of solid loading variation on dissolved aluminium with calculated recovery;

[0060] Figure 7 is a graph showing impurities (as calculated oxide wt%) in precipitated ammonium alum, with Figure 7A showing Mg, Na, Si and Ca impurities and Figure 7B showing Fe and K impurities; [0061] Figure 8 is a graph showing dissolved aluminium concentration stripping solutions using acids as the stripping solution to extract aluminium from the organic phase of the solvent extraction step;

[0062] Figure 9 shows a plot of impurities in ammonium alum filtrate from OF (mg/L) for example 2;

[0063] Figure 10 shows a plot of impurities in ammonium alum filtrate from 1:1 OF:UF mix (mg/L);

[0064] Figure 11 shows a plot of impurities in ammonium alum filtrate from UF (mg/L);

[0065] Figure 12 shows a graph showing the effect of temperature and ultrasonic power on relative extraction of Fe from OF in example 3. At these conditions 100% extraction would equate to approximately 10.4g/L Al and 0.5g/L Fe;

[0066] Figure 13 shows a graph of elffect of temperature and ultrasonic power on relative extraction of Al from OF. At these conditions 100% extraction would equate to approximately 10.4g/L Al and 0.5g/L Fe;

[0067] Figure 14 shows a graph showing effect of temperature and ultrasonic power on ratio of Fe (%) to Al (%) relative extraction efficiency. Higher values indicate more selective extraction of Fe over Al;

[0068] Figure 15 shows the solid yield (oven dried at 60°C) using either ammonium sulphate or ammonia as a precipitating agent; and

[0069] Figure 17 shows the effect of ammonium dose on alumina content of ammonium alum, plotted as calculated oxide grade.

DESCRIPTION OF EMBODIMENTS

[0070] The drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited solely to the features as shown in the drawings. [0071] Figure 1 shows a flowsheet suitable for use in the present invention. The process begins with a kaolin feed 10. The kaolin is heat-treated at 12 to convert kaolin to metakaolin, which is more responsive to the subsequent sulphuric acid leaching stage. The metakaolin 14 is transferred to leaching stage 16 where it is mixed with sulphuric acid 18. During sulphuric acid leaching, aluminium is largely dissolved whilst silica remains largely insoluble, then re- precipitates as amorphous silica. Most other impurities dissolved during leaching were found to dissolve proportionally to the aluminium dissolution extent.

[0072] A liquid/solid separation step is used to separate the solid leach residue 18 from the pregnant leach solution 20. Pregnant leach solution 20 is fed to ammonium alum precipitation step 22. This acts as the first impurity removal step. In this step, the pregnant leach solution is mixed with ammonium sulphate 24 which precipitates the aluminium in solution as aluminium ammonium sulphate. This precipitation step is selective for aluminium over most impurities and also provides an efficient way to separate aluminium from sulphuric acid, negating the need to neutralise before further processing. Washing stages (not shown) can be performed by re-dissolving the aluminium ammonium sulphate in clean water or sulphuric acid solution, allowing for a cleaner re-precipitation. This can be carried out multiple times, depending on the extent of impurity separation that is required. Potassium follows aluminium by precipitating as potassium aluminium sulphate in this stage.

[0073] A solid/liquid separation step (not shown) is used to separate saturated ammonium alum solution 26 from the precipitated ammonium alum 28. The precipitated ammonium alum is sent to a solvent extraction step. In step 30, the precipitated ammonium alum is dissolved into water or acid solution. The acid solution is then mixed with an organic solvent extraction agent that preferentially takes aluminium from the aqueous phase to the organic phase. Potassium remains with the aqueous phase. The aqueous phase raffinate 32, which contains most of the dissolved potassium, is separated from the loaded organic phase 34. The loaded organic phase 34 is sent to solvent extraction stripping step 36 where the loaded organic phase is mixed with sulphuric acid 38. This strips aluminium ions from the organic phase into the aqueous strip solution. The organic phase is recycled at 40 to the solvent extraction loading step. A wash stream 42, which may arise from washing of the organic phase prior to recycle, is removed. The organic phase could be stripped with a stronger acid periodically in the event that strongly bonded impurities build up. [0074] Once aluminium is stripped from the organic phase, the aluminium-containing strip solution 44 is of relatively high purity aluminium, which permits high purity product to be obtained. The loaded strip solution 44 is sent to precipitation step 46 in which alum is precipitated. In one embodiment, aluminium sulphate is precipitated by precipitation with ethanol. Other precipitation methods may be used. It will be appreciated that if a different acid other than sulphuric acid 38 is used in the solvent extraction stripping step 36, other aluminium compounds besides aluminium sulphate will be formed in the precipitation step. The alum 48 that is formed in the alum precipitation step 46 is of high purity and it can be converted to high purity alumina by high temperature calcination 50.

Example 1

[0075] A number of experiments were undertaken to demonstrate working of the various unit operations of the flowsheet shown in Figure 1. The examples were conducted using kaolin samples taken from a kaolin deposit located west of Bundaberg, Queensland. The kaolin samples were provided as 2 feed samples which had been processed by hydrocycloning. The samples comprised a fine fraction (the overflow fraction, less than 10 pm) and a coarse fraction (the underflow fraction, 10 to 38 μm). The samples are composed of SiO ₂ , AI ₂ O ₃ , Fe ₂ O ₃ , K ₂ O and TiO ₂ , with a small portion of MgO, Na ₂ O and CaO, represented as oxides as is the convention with XRF analysis. The key impurities in terms of concentration being K, Si, Fe, Ti, Mg and Ca. The analysis of the feed samples are set out in Table 1:

Table 1: XRF and FOI analysis of feed samples

[0076] As the fine sample had a higher aluminium grade than the coarse sample, further test work was conducted using the fine sample. [0077] The major phases present in the samples were identified by XRD. An estimate of the crystalline phase composition of the overflow/fine feed material is 80% kaolinite, 13% quartz, 2.2% muscovite and 0.96 anatase.

[0078] The kaolin feed material was then heated to converted to metakaolin. During the heat treatment of kaolin, it is converted to metakaolin in accordance with the following reactions:

[0079] Reaction (1) takes place at temperatures of greater than 100°C. Reaction (2) normally takes place at temperatures greater than 600°C. In the experimental work conducted by the present inventors, the samples of kaolin were treated in a muffle furnace at 700 to 750°C for 0.5 to 1.5 hours with the goal of achieving total conversion of kaolin to metakaolin.

[0080] The experimental work conducted to date shows that for the samples tested, transition from kaolin to metakaolin is occurring at 600°C and that higher temperatures require shorter heating time to effect the transformation.

Leaching

[0081] The metakaolin was leached with sulphuric acid in order to dissolve the aluminium into solution. The effects of sulphuric acid concentration (0.5-3M), temperature (70 to 90°C) solid loading (60 to 150g/L) and residence time (0.5 to 4 hours) were investigated. Across all conditions tested most impurities leached to a similar extent as aluminium. Fe, and K are shown in Figure 2. Results for Ca, Mg, Na and Zn show similar trends. The only exception is Si, which is released by the dissolution of metakaolin and then precipitates out of solution in the leaching step.

[0082] Varying acid concentration from 0.5M to 2M increased total aluminium recovery, with no improvements seen by increasing further to 3M. This is shown in Figure 3. There was however a reduction in Si concentration when increasing sulphuric acid concentration to 3M due to a continued reduction in Si solubility, as shown in Figure 4. [0083] Increasing temperature of the leaching step was primarily found to affect kinetics and, to a lesser extent, the final aluminium concentration. This is shown in Figure 5. Increasing solids loading increased final aluminium concentration, with similar recovery (about 90%) reached after 4 hours. This is shown in Figure 6.

Impurity rejection by ammonium crystallisation

[0084] The metakaolin leach filtrate contains appreciable quantities of dissolved aluminium and significantly less but still noticeable quantities of dissolved K, Fe and Mg, with lesser amounts of dissolved Si and other impurities. Tables 2 and 3 hereunder provide an analysis of the pregnant leach liquor from the metakaolin lid step, as well as assays for other important steps of the overall process.

[0085] Addition of ammonium sulphate to the leach solution produces aluminium ammonium sulphate. This is less soluble than aluminium sulphate and can be crystallised from the leach solution by cooling from leach conditions to 20 to 25 °C without requiring evaporation. Some aluminium (~6g/L Al for a 2M sulphuric acid leach) remains in solution and is one of several major candidates for recycling or lower grade co-products.

[0086] The primary crystallisation of aluminium ammonium sulphate rejects the majority of impurities in solution, with the exception of potassium which crystallises as potassium aluminium sulphate. Potassium aluminium sulphate and aluminium ammonium sulphate are similarly soluble and form a mixed crystalline structure, making separation not possible at these conditions. Impurities other than potassium can be significantly reduced by re- dissolution and recrystallisation. This is shown in Figure 7. This also produces a similar but cleaner (>95%) solution of saturated ammonium alum to the initial precipitation.

[0087] While the solid ammonium alum precipitate can be directly calcined to alumina, the potassium contamination at this point is too high to form high purity alumina and further processing is required. However, this step of aluminium ammonium sulphate crystallisation causes a reduction in all other impurities, especially iron, and provides a much cleaner feed for the subsequent solvent extraction step.

Potassium removal by solvent extraction

[0088] Solvent extraction is used in order to remove potassium carried over into the solids from the ammonium alum crystallisation stage. Previous studies have shown that using an organic-phosphoric acid such as D ₂ EHPA at acidic pH allows loading of aluminium whilst avoiding loading potassium. In order to ensure the feasibility of recovering high purity alumina using solvent extraction, several considerations were addressed with regards to certain process parameters such as the pH adjustment agent, stripping acid in contact times.

[0089] In the solvent extraction steps used in this experiment, D ₂ EHPA was loaded using an aqueous solution of alum at an Al concentration of 10 g/L and an equilibrium pH of 3.0. In a commercial implementation, the ammonium alum precipitate will be dissolved into water or an acid solution at the same pH and the resulting solution contacted with D ₂ EHPA. The loaded organic solvent was then contacted with acid over a range of concentrations. The acid acted as an aqueous stripping solution to strip Al from the loaded organic phase. Sulphuric acid, hydrochloric acid and nitric acid were each tested as possible stripping agents. Kinetic testing determined that a contact time of around 10 minutes between the stripping acid solution and the loaded organic phase was sufficient to reach equilibrium.

[0090] Due to the high purity required in the product, sodium hydroxide cannot be used to adjust the pH of the aqueous phase during loading due to the high affinity of sodium with the organic solvent. If sodium hydroxide was used, there would be considerable sodium transfer to the product, which would produce unacceptably high impurity levels. A potential solution to the requirement of adjusting the pH lies in the use of a buffering agent such as glycine. Glycine is dissolved in the aqueous phase and acts as a buffer, countering the release of H ⁺ ions from the organic phase during metal ion loading and therefore avoiding the need to readjust the pH.

[0091] The results shown in Figure 8 demonstrates that hydrochloric acid and sulphuric acid were the most efficient at stripping Al from the organic solvent and performed similarly with stripping recoveries of 86% and 78% respectively. Nitric acid was much less efficient at stripping Al, with much lower Al concentration detected over these stripping solutions and a lower total aluminium mass transferred with the recovery of 27%.

Extraction from redissolved ammonium alum

[0092] To avoid Fe contamination in the solvent extraction circuit, the solvent extraction process can be fed using an aqueous feed solution generated by dissolving the ammonium alum into water. In the experimental work conducted to date, the feed contained 5g/L Al, due to the lower saturation point of ammonium alum vs alum in water. Glycine was used as a buffer during organic phase loading to avoid sodium contamination.

[0093] The solvent extraction process was successful in rejecting potassium transfer from the feed solution to the product by 99.9%, while extracting 60% of aluminium in the feed solution and recovering 55% of aluminium loaded in the organic. This resulted in an overall recovery of 29% in the product streams from the feed. This is for a single pass and recoveries would improve significantly in a multistage counter current operation.

[0094] A small amount of contamination in the form of iron, potassium, sodium and silicon was also transferred to the organic solvent. However, a water wash was successful in removing any excess potassium before the stripping step.

[0095] Calculations regarding the composition of the products stream so that the purity of the high purity alumina product after calcination is currently at 99.95%. At this point the most important contaminants in the product which are hindering a higher purity are iron, sodium and silicon. Mass balance suggest that the sodium may have been impurity in the extracted and/or diluent rather than in the feed solution.

Ethanol precipitation from solvent extraction strip

[0096] Crystallisation of alum (aluminium sulphate) using ethanol was initially investigated on untreated leach filtrate. This provided some selectivity, calculated AI ₂ O ₃ was around 94% after leaching and around 97% in a single stage ethanol produced alum, with Fe and K being primary impurities in both cases. However, this approach can also be used for the precipitation of high purity alum from the high purity stripping solution from the solvent extraction stage. Initial testing confirms that ethanol precipitation of solvent extraction strip solution is possible at aluminium concentrations of at least 1.6g/L and above. However, lower concentrations require a higher ethanol ratio so optimising the stripping stages to maximise aluminium concentration in the solvent extraction stripping liquor will reduce ethanol requirements significantly.

[0097] The alum produced in the ethanol precipitation stage is a high purity aluminium sulphate that can itself be a valuable product. If it is desired to produce high purity alumina, the alum can be calcined. Experimental data shows that alum dehydrates at relatively low temperature and by 400°C will convert to anhydrous aluminium sulphate. By 1000°C, this is converted to alpha-alumina. The following reactions are involved:

[0098] The high purity alumina obtained is calculated to have a purity of 99.95%. Improvements can be obtained by redissolving and re -precipitating of the ammonium aluminium alum in step (c) and by conducting the solvent extraction process as a multi-stage counter current extraction process.

[0099] As mentioned above, an example of that scale investigation of key process steps is described in table 2 with assays in table 3. ICP data for liquid samples is adjusted for sample dilution only. ICP of water-soluble (alum) samples is converted to mg/kg of hydrate. Both are converted to AI ₂ O ₃ by converting each assayed element to the oxide formed by molar ratio, removing the sulphur component in dividing each oxide by the total.

[00100] It can be seen that the key impurities in the loot solution are Fe, K> Mg> Si, Na, Ca > Zn, Mn, Ti and Cu. After the ammonium aluminium sulphate precipitation -re- dissolution process, the impurities in the solvent extraction feed our K»Fe>Mg, Si, Na, Ca, Zn, Mn, Ti, Cu. After solvent extraction the key impurities are Na>Fe>Ca. Some Na likely comes off from the solvent extraction reagent rather than the feed kaolinite. For the final solid alum sample, the purity is approximately 99.97% pure on an AI ₂ O ₃ basis. The key remaining impurities are Na, Si»Fe, Ca>Cu, Mg, Zn. It is likely that most of the Na and Si is from contamination during the final experiments from the organic extractant and dissolution of the lab glassware.

Table 2: Description of key samples Table 3: ICP (top) and calculated calcined grade (bottom) for key samples

Example 2

[00101] In order to determine sensitivity to varying feed, three samples have been processed in laboratory batch tests. The three samples used are the cyclone overflow (OF) which was used for Example 1, cyclone underflow (UF) and a 1 : 1 mass ratio blend of the two (OF:UF). As in example 1, samples of each were thermally activated for 2 hrs at 750°C, the 1:1 OF:UF mixture was blended before activation. Original sample grades are shown in Table 4 below.

Table 4: XRF and LOI analysis of hydrocyclone products O O

[00102] Using the following assumptions (as used in the mass balance model), the approximate feed grade is shown in Table 5.

• All potassium present is muscovite K(Al ₄ Si ₂ O ₉ (OH)3)

• All aluminium not present as muscovite is kaolinite Al ₂ Si ₂ O ₅ (OH) ₄

• All silicon not present as muscovite or kaolinite is quartz (SiO ₂ )

Table 5: Calculated OF and UF grade

Leaching

[00103] The leach conditions were kept identical to those used in Example 1, namely 120g/L solids, 2M H ₂ SO ₄ for 4 hours at 90°C. As expected leached aluminium was proportionally lower in the UF and OF:UF samples due to the lower starting grade with the same quantity of solids. The most significant implication of this is lower yield during ammonium alum precipitation, since the aluminium concentration will start lower and still reach the same saturation point in the first stage.

[00104] Apart from aluminium leached the most significant difference between the OF, UF and UF:OF leaches was the presence of copper in samples containing UF. The overall concentration is relatively low, 40ppm in the pure UF leach, and 24ppm in the OF:UF mix making copper a minor impurity, but still a significant addition compared to the 2ppm found in the pure OF leach. Table 6 shows a comparison of leach assays from each feed. Other than copper all impurities are present at comparable levels, however due to lower aluminium in the UF and mix leaches, the ratio of impurities to aluminium is lower. Table 6: Impurities in leach filtrate from differing feeds

Ammonium Alum precipitation

[00105] Figure 9 through to Figure 11 show impurities in filtrate from the initial ammonium alum precipitation and from the 3 stages of redissolution/reprecipitation washing. Since these are filtrate samples, they represent impurities removed from the product solids rather than the solids themselves. Final solids from stage 3 washing are then redissolved in fresh water again to prepare the feed for solvent extraction.

[00106] As per previous findings all impurities other than K are reduced each wash cycle to achieve similar final values. While total impurity level after the first stage is similar regardless of feed, the higher impurities to aluminium ratio in UF and mix results in higher contamination in the initial precipitate, so higher impurities in the wash 1 filtrate.

Solvent extraction and processing

[00107] Due to the similar final results after ammonium alum precipitation and washing, no significant differences between feed materials were noted during solvent extraction. The aluminium and potassium behaved in the same manner as for the OF derived solution, with no other elements at high enough levels to draw reliable conclusions from ICP-AES. Final calcined results were also similar.

Calcination

[00108] The high purity alumina in example 1 was analysed by ICP only, and the only major impurity was sodium at 320ppm. The source was later determined to likely be contamination from the alumina which was used to line the crucible prior to calcination. The intention was that an existing layer of alumina would protect the sample from contamination from impurities in the crucible itself, however the sodium level in the alumina used was higher than anticipated (~3000ppm) and likely contaminated the sample.

[00109] In example 2 larger magnesia crucibles were used to reduce the potential for contamination from Na ₂ O and allow larger calcination batches. Magnesia was selected due to the undetectable levels of magnesium seen throughout lab testing and the low level seen in the solid assay from example 1. The high magnesium in the assay of example 2is therefore attributable to reaction with the crucible.

Solid assays

[00110] All final calcined solids produced in example 1 and example 2 have been analysed. The complete list of assayed elements was Ag, Al, As, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, In, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, Re, S, Sb, Sc, Se, SiO2, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, Y, Yb, Zn, Zr. For brevity all elements which were below the LLD for all 4 samples have been excluded from Table 7 to Table 10.

Table 7: XRF and SiO2 results for re-assayed alumina O O

[00111] The major contaminants in all three produced aluminas were magnesium, phosphorus and sulphur. These are shown in Table 8, as well as the predicted grade if all three were removed entirely. All three of these elements have known sources in the current lab process. Magnesium is from the crucibles used. Phosphorous is likely from entrained DE2EHPA from solvent extraction, and sulphur is from the calcination feed being alum.

[00112] The magnesium can be avoided by changing crucible material. Sulphur was unexpected since it was not present in the solids from example 1. It is possible that the lids on the magnesia crucibles seal too well, and/or larger sample volumes do not allow all sulphur to cleanly escape as SO ₂ or SO ₃ gas. Phosphorus my behave in a similar way, although it is possible that an additional washing step may be required after solvent extraction to remove any entrained solvent. Table 8: As assayed grade calculation, and predicted grade with total removal of Mg, P, S

[00113] There were minor levels of a number of rare earth elements detected in all samples, those detected in the commercial 4.5N or the produced samples are shown in Table 9 and Table 10. There has been no work done on attempting to remove or control these to date, however it is probable that they are following aluminium through solvent extraction. However with the exception of gallium they all assayed at similar or lower levels than was detected in the commercial 4.5N.

Table 9: All assayed elements registering above LLD (As to Sb)

Table 10: All assayed elements registering above LLD (Si to Y)

Example 3 - Pre-wash of feed material prior to calcination

[00114] In this example, a prewash of the feed material was conducted prior to calcining to convert the kaolin to metakaolin. In this example, 60g/L solid loading and a 2M sulphuric acid wash was used and ultrasonication was used to agitate the mixture of the feed material and acid. As shown in Figure 18 the Al and Fe leaching extent increased as both temperature and ultrasound intensity increased. The maximum measured extraction of Fe that did not also extract a significant amount of Al was at 60°C and 60% ultrasonication intensity. While this was the highest Fe extraction seen, the ratio of Fe to Al extracted was identical to the 60°C test with no ultrasonic probe, see Figure 14. At lower ultrasonic intensities the Fe to Al extraction ratio increased but the total Fe removal was lower.

Example 4 - ammonium sulphate vs ammonium hydroxide for precipitation

[00115] In the example 1, ammonium sulphate was dissolved into hot (>70°C) leach liquor, followed by cooling to crystallise aluminium as ammonium alum. This example was performed to compare ammonium sulphate to ammonium hydroxide (liquid ammonia) as way of increasing ammonium in the solution. The use of ammonia has the additional benefit of neutralising some of the sulphuric acid from leaching, see reactions (1) and (2) below:

(1) Al ₂ (SO ₄ ) _3(aq) + (NH ₄ )2SO _4(s) → 2(NH ₄ )Al(SO ₄ ) _2(aq)

(2) Al ₂ (SO ₄ ) _3(aq) + 2NH ₄ OH(aq)+ H ₂ SO _4(aq) → 2(NH ₄ )Al(SO ₄ ) _2(aq) + 2H ₂ O

[00116] In this example ammonium hydroxide was added in the form of liquid ammonia (28-30wt% NH ₄ OH in water, AR grade) which adds some dilution. The dilution was accounted for when interpreting results, however would be undesirable industrially. Instead ammonium hydroxide could be formed in situ by sparging ammonia gas into the leach liquor. The neutralisation may be beneficial depending on the final destination of the filtrate stream (i.e. waste, recycling or alternative products) however it must be controlled to acceptable levels. With a typical 2M sulphuric acid leach liquor at around 0.9M aluminium, the stoichiometric dose brought the liquor to pH 1.2 without issues arising. The next highest at 150% stoichiometric dose raised to pH 4.2 and formed an unidentified precipitate, most likely a mix of gibbsite and iron sulphate which contaminated the alum. In operation the dose and pH would need to be kept below this point.

[00117] It is possible that a combination of ammonia gas and ammonium sulphate could be used to increase yield without raising pH to the point at which precipitation of impurities occurs. The optimal combination will depend on the price of the ammonium sulphate and ammonia as well as the purity of the ammonium sulphate. The process mass balance can be used to predict the best reagent or combination of reagents. The total solids yield increased with ammonium dose for both sources, but was always higher with hydroxide than sulphate at equivalent doses (Figure 15). At high dose the ratio of aluminium to potassium did improve slightly (Figure 17), however it did not improve sufficiently to avoid requiring further treatment by solvent extraction. Example 5 - Ammonia for pH control in solvent loading

[00118] The use of ammonia (NH ₃ OH 29% w/w solution) as a method of pH control during the solvent extraction loading stage was investigated experimentally using solvent extraction methods previously outlined. With regards to the overall loading capacity, aluminium loaded on to the organic and reached equilibrium in a similar manner to using NaOH, avoiding the addition of excess sodium which introduces the risk of contaminating the HPA. Observations from this investigation noted that as the loading stage tends towards equilibrium, an emulsion forms between the aqueous and organic phase after contact, leading to phase separation times of several hours. Additionally, over 15 phase contacts were required to reach loading equilibrium. Increasing the temperature of the solvent extraction process or addition of a phase modifier to the organic solution could potentially help with phase disengagement. This should be investigated in the future as the process progresses towards industrial implementation.

Table 11: Solvent extraction feed and raffinate solution Al concentrations

[00119] A second experiment was conducted to investigate the effect of glycine at a concentration of 50g/L in the aqueous phase and dosing with NH ₃ solution for pH adjustment. The glycine was added to the aqueous phase after the first contact and separation and NH ₃ was used to adjust the pH of subsequent contacts. The effect of glycine was obvious, requiring only 5 phase contacts to reach loading equilibrium and a much lower overall mass of ammonia used to pH dose. The ICP analysis of the raffinate solutions show that the extraction extent was the same for both experiments, with glycine improving the overall loading kinetics. Aluminium concentrations for the aqueous streams before and after solvent extraction for NH ₃ only and Glycine + NH ₃ can be found in Table 11.

[00120] Other embodiments of the present invention use ion exchange resins to extract aluminium from the pregant leach solution. Possible ion exchange resins that could be used in these embodiments are listed in Table 12 below.

Table 12: Cation exchange resins as potential candidates for Al purification

[00121] In the present specification and claims (if any), the word ‘comprising’ and its derivatives including ‘comprises’ and ‘comprise’ include each of the stated integers but does not exclude the inclusion of one or more further integers. [00122] Reference throughout this specification to ‘one embodiment’ or ‘an embodiment’ means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases ‘in one embodiment’ or ‘in an embodiment’ in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[00123] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.