Processing sulphate matrices

Abstract

A method is described for controlling the amount of aluminium in a sulphate-based aluminium containing matrix, the method comprising adding ammonium sulphate in stochiometric ratio of from about 0.1: 1 to greater than 1: 1 to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, and separating the ammonia alum.

Description

[0001] PROCESSING SULPHATE MATRICES

[0002] This document claims priority to AU 2024901876 filed on 19 June 2024, the contents of which are hereby incorporated by reference in their entirety.

[0003] Technical field

[0004] The present invention relates to the removal of aluminium from a solution comprising aluminium and sulphate. The aluminium removed can be used to form high purity alumina. The solution having the aluminium removed therefrom can be used in a process that benefits from the depleted aluminium content.

[0005] Background

[0006] Complex sulphate-containing matrices are systems or materials that incorporate sulphate ions along with a variety of other components. These matrices can be found in numerous contexts, such as industrial processes, environmental systems and advanced materials. Sulphate matrices can result from industrial processes such as sulphuric acid production. The matrices can also form from leaching, either when sulphuric acid is used as the leaching solution and or where sulphide minerals react with air and water.

[0007] The residual material left behind after a specific extraction or processing step can be referred to as a raffinate. Some raffinates are sulphate containing matrices. After the desired components have been extracted or separated from a material, the raffinate remains. Typically, raffinates are recycled or reused if there is further economic value. Oftentimes, however, where further processing is not commercially viable, the raffinate is destroyed or discarded. Some raffinates contain valuable aluminium. For example, in the refining of certain ores, such as bauxite ore, which is the primary source of aluminium, the raffinate may contain residual aluminium compounds. Similarly, in processes like solvent extraction or leaching where aluminium-containing materials are being processed, the raffinate could potentially retain some aluminium content. Some ore processing results in a raffinate containing aluminium, but the aluminium is contained within a complex sulphate matrix. This type of aluminium may be more difficult to extract particularly if there are other impurities present in the raffinate. In order to extract the aluminium, the pH of the raffinate could be adjusted in an attempt to selectively precipitate the desired compounds. However, precise control over which compounds will precipitate can be difficult and the result is often a combination of solids and hydrogels which are difficult to process and almost impossible to dewater. The result is an increase in processing costs and reduced yields and purity.

[0008] There exists a need for a process that can extract aluminium from an aluminium bearing raffinate or any other complex aluminium containing sulphate compound or solution.

[0009] Summary of invention

[0010] In a first aspect, there is provided a method for controlling the amount of aluminium in a sulphate-based aluminium containing matrix, the method comprising: adding ammonium sulphate in stochiometric ratio of from about 0.1 :1 to greater than 1 :1 to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum.

[0011] In embodiments, the separated ammonia alum is subject to further processing to form high purity alumina.

[0012] In embodiments, the sulphate-based aluminium containing matrix depleted in aluminium is recycled in a process that requires a sulphate containing acid source.

[0013] In a second aspect, there is provided a method for producing alumina from a sulphate-based aluminium containing matrix, the method comprising: adding ammonium sulphate in a stochiometric excess to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum, optionally washing the separated ammonia alum to remove one or more of organics, entrained and soluble contaminants, dissolving separated / washed ammonia alum to form a solution, sparging the solution with HCI to crystallise aluminium chloride hexahydrate (ACH), calcining the separated ACH to form alumina.

[0014] In a third aspect, there is provided a method for recycling a sulphate-based aluminium containing matrix, the method comprising: obtaining a sulphate-based aluminium containing matrix from a process; adding ammonium sulphate at less than a stochiometric excess to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum to provide an aluminium depleted sulphate- based aluminium containing matrix; recycling the aluminium depleted sulphate-based aluminium containing matrix into the process.

[0015] Any further description applies to all aspects unless the context makes clear otherwise.

[0016] Ammonium alum precipitation has been identified as a highly selective method for the extraction and recovery of aluminum from the acidic, sulphate-rich aqueous media containing a diversity of dissolved metals in high concentrations. One advantage of the present ammonium alum process, in embodiments, is that good recovery of aluminum may be achieved at very low pH. This has a distinct advantage over neutralization / precipitation in mineral processes where acid recycling is economically desirable as the acid-rich, aluminium-depleted stream may be returned to the upstream process.

[0017] In embodiments, the ammonium sulphate is added at stoichiometry or in a stoichiometric excess to the aluminium sulphate concentration of the sulphate- based aluminium-containing matrix to form ammonia alum. In embodiments, the ammonium sulphate is added in a sub-stoichiometric ratio to the aluminium sulphate concentration of the sulphate-based aluminium-containing matrix to form ammonia alum and an aluminium depleted liquid.

[0018] In processes where the sulphate-based aluminium-containing matrix is recycled for its acid content, it can be useful to maintain a balanced ammonium load. While ammonium sulphate is added to precipitate aluminium as ammonia alum, an excess of ammonium beyond stoichiometric requirements can lead to accumulation of free ammonium ions in the recycled acid stream. This may compromise downstream process compatibility or require additional treatment. Accordingly, in recycling pathways, sub-stoichiometric or near-stoichiometric dosing of ammonium sulphate is preferred to enable selective aluminium removal while preserving the acid-rich matrix with minimal ammonium contamination.

[0019] Any downstream high purity alumina (HPA) process is somewhat independent of the process decisions made at the alum precipitation stage. Regardless of the stoichiometry, the ammonium alum products are substantially similar and fit for processing to HPA if desired.

[0020] Nevertheless, management of the stoichiometric ratio of ammonium sulphate to the aluminium sulphate concentration may be used to optimise downstream aluminium recovery and the recycling of acid streams from the extraction process.

[0021] Ammonia alum, also known as ammonium alum or ammonium aluminium sulphate, is a type of chemical compound with the formula (NH4)AI(SO4)2 12H2O. It is a double sulphate salt of aluminium and ammonium. The present process can in embodiments transition from a sulphate process to a chloride process with the aim of selective recovery of aluminium.

[0022] As described herein, the present process in embodiments is a hybrid method combining the precipitation of ammonium alum from the sulphate-based aluminium containing matrix (pre-concentration) followed by the formation and then recrystallisation of aluminium chloride hexahydrate (ACH) (purification). The recrystallisation of ACH is the subject of applicants own WO / 2023 / 235913 entitled: A METHOD FOR PRODUCING AN ALUMINOUS MATERIAL filed 3 June 2023. Without wishing to be bound by theory, since about 98% of the aluminium can be recovered by the precipitation of ammonium alum it is expected that the present process will require fewer ACH crystallization steps to achieve 4N alumina. It is thought that the benefit of the first process is that it produces a highly pure starting material for the second process. This may represent a significant commercial cost saving since precipitation of the ammonia alum is likely a cheaper process than the ACH recrystallisation step.

[0023] The sulphate-based aluminium containing matrix can be from any source. The sulphate-based aluminium containing matrix can be provided by a third party. The sulphate-based aluminium containing matrix can be any sulphate-based aluminium containing feedstock or waste stream. In an embodiment, the sulphate-based aluminium containing matrix is the result of a leaching process such as the sulphuric acid leaching of e.g. kaolin clay. The sulphate-based aluminium containing matrix can be a raffinate. In an embodiment, the raffinate is a vanadium raffinate.

[0024] Each sulphate-based aluminium containing matrix feedstock is different and will have a unique contaminant profile. The contamination profile may change the process as described below. In embodiments, the process has two purification stages:

[0025] (1) Controlled precipitation of high-purity ammonia-alum, to be feed for (2).

[0026] (2) Controlled recrystallisation of high-purity ACH.

[0027] All other steps of the process are undertaken to minimise contamination of the product. Washing can remove entrained liquor in the solids, which contains the liquid impurities. For impurities that are not sufficiently removed in the precipitation or recrystallisation step, there is the option of repeating the precipitation / ACH step multiple times and or introducing ion-exchange and or solvent extraction steps. The skilled person will appreciate, based on the teachings herein, whether ionexchange or solvent extraction is required. Regardless of whether the ammonium alum is formed using stoichiometric, sub- stoichiometric, or excess ammonium sulphate, the separated ammonium alum can be further processed to produce high purity alumina (HPA). This can involve dissolution, crystallisation of aluminium chloride hexahydrate and calcination.

[0028] In any case, the ammonium sulphate can be added as a solid. The ammonium sulphate can be added directly into the sulphate-based aluminium containing matrix. The sulphate-based aluminium containing matrix can be added to the ammonium sulphate. If required, the ammonium sulphate can be dissolved prior to adding it to the sulphate-based aluminium containing matrix.

[0029] The stoichiometric ratio is determined in accordance with the formula below:

[0030] AI2(SO4)3 +(NH4)2SO4 +24H2O^2NH4AI(SO4)2-12H2O where there is a 1 to 1 molar ratio of the reactants aluminium sulphate and ammonium sulphate. The starting concentration of aluminium sulphate can be known or calculated by any means. For example, the concentration of the aluminium in the feed can be determined by spectroscopy. One suitable spectroscopic method is ICP-OES (Inductively coupled plasma - optical emission spectrometry). The elemental composition of the feed samples can be determined, while acid titration can be used to determine the free acid composition.

[0031] The aluminium in the sulphate-based aluminium containing matrix is dissolved in solution with the counter ion being sulphate. All aluminium detected is used to determine the required ammonium alum concentration.

[0032] The amount of ammonium sulphate added to the solution can be adjusted. In an embodiment, the ammonium sulphate can be added in a stochiometric ratio of from about 0.1 : 1 to greater than 1 : 1.

[0033] In embodiments, there can be greater than 1 : 1 stoichiometric excess of ammonium sulphate relative to the aluminium sulphate. In one or all embodiments, there is at least about 3x (3: 1), 2.5x (2.5: 1), 2.25x (2.25: 1) or 2x (2: 1) stochiometric excess of ammonium sulphate relative to the aluminium sulphate.

[0034] In other embodiments, there can be a less than 1 : 1 stoichiometric ratio of ammonium sulphate relative to the aluminium sulphate. In one or all embodiments, there is at least about or at most about 0.1x (0.1 : 1) sub-stoichiometric ratio of ammonium sulphate relative to the aluminium sulphate. In one or all embodiments, there is at least about or at most about 0.5x (0.5: 1 ) or 0.8x (0.8: 1 ) sub- stoichiometric ratio of ammonium sulphate relative to the aluminium sulphate.

[0035] The ammonium sulphate can be added in a stochiometric ratio in the range of from about 0.1 :1 to about 2.5:1 , such as about 0.5:1 to about 1.5:1 , such as about 0.8:1 to about 1 :1.

[0036] Preferably, a balance is struck between aluminium recovery and amount of ammonium sulphate added. While adding ammonium sulphate in stoichiometric excess maximises aluminium precipitation as ammonium alum, it introduces surplus ammonium into the system. Conversely, sub-stoichiometric addition limits ammonium loading but results in partial aluminium recovery, with some aluminium remaining in solution. Process optimisation therefore can involve weighing the economic and operational costs of ammonium loss against the value of unrecovered aluminium.

[0037] The ammonium sulphate can be added in one single dose meaning that all the ammonium sulphate can be all added during the same reaction period to achieve one reaction outcome. However, the ammonium sulphate can be added in small amounts over the reaction period to ensure good mixing and or reaction. Each of batch (add all at once), semi batch (add a bit at a time) and continuous processing (add a bit at a time, and products are removed at the same time) can be used.

[0038] The ammonium sulphate can be added to the matrix with heating. The heating can be to any temperature that results in dissolution of the ammonium sulphate. In an embodiment the temperature at this step is at least about 60, 70 or 80 degrees Centigrade. The elevated temperature is required to ensure good dissolution. As the solution cools, the ammonium alum will precipitate from solution. The solution can be allowed to cool. The temperature of the solution can be monitored during cooling. The cooling of the solution allows for the precipitation of the ammonium alum from the solution. In some embodiments, the majority of the aluminium bearing compounds have precipitated by the time the solution cools to about 30°C. Accordingly, the solid ammonium alum can all be removed from solution before further cooling to avoid or at least reduce any contamination of the precipitate with e.g. iron.

[0039] The ammonium alum precipitate can be recovered by any means. In one or more embodiments, the precipitate is removed via vacuum filtration.

[0040] The supernatant can be recycled or reused. As expected with a large excess ammonium sulphate being added during the initial alum precipitation, there is a high concentration of ammonia present in the alum supernatant.

[0041] The sulphate-based aluminium containing matrix as supplied may comprise some contaminants, such as organics that result from the process from which it was produced. Optionally, organics can be removed prior to or during any processing in the present method. Organics can be removed via solvent extraction or any other suitable known method. Organics can be removed with an activated carbon stage to remove the organics before progressing down any HPA processing stage. If present, the removal of organics is recommended to avoid the potential for polychlorinated biphenyls (PCBs), furans and dioxins forming. These harmful side products form when organics are reacted in hot hydrochloric acid solutions.

[0042] In some embodiments, the ammonium alum precipitate is subsequently dissolved. The precipitate can be dissolved in any solvent including water. The precipitate can be dissolved in an acid. The acid can be Hydrochloric Acid (HCI). An acid other than HCI can be used; however, a greater amount of HCI sparging gas will be required to effect the reaction. The ammonium alum precipitate can be dissolved in a 1 :1 ratio with the acid where for every 1 Kg of ammonium alum precipitate, 1 litre (L) of HCI is used. Other ratios can work, and the skilled person can change the ratio using normal schooling to the effect that the ammonium alum dissolves. The HCI can be a 20% solution. The ammonium alum precipitate can be dissolved into the HCI at an elevated temperature. The temperature can be at least about 50, 60 or 70 degrees Centigrade.

[0043] Once the saturation limit of ammonium alum in HCI is reached no further ammonium alum may be added or the mixture may not remain stable.

[0044] The solution of ammonium alum in HCI and be cooled and then sparged with gaseous hydrochloric acid to increase the concentration of chloride ions in solution. Generally, the sparging may be sufficient to form a slurry of ACH crystals. Any suitable type of gaseous hydrochloric acid may be used for sparging. For instance, the gaseous hydrochloric acid may be purified, enriched, or the like. In one or all embodiments, the hydrochloric acid is anhydrous.

[0045] The solution can be sparged with gaseous hydrochloric acid until the saturated solution attains a hydrochloric acid concentration suitable for the crystallization of the chloride product. When the solution is saturated with chlorides, it starts to precipitate selectively as aluminium chloride hexahydrate (ACH). The end point can be flexible depending on the outcome desired. Generally, sparging is undertaken until HCI approaches saturation in the solution. Sparging can be ceased at any point after visual crystal formation.

[0046] The precipitate crystal slurry may be separated using any suitable technique known in the art. Preferably, the separation technique may be sufficient to separate the precipitate from the spent liquor. For instance, the separation technique may include gravity settling clarifiers, sedimentation, decanting, centrifugation, filtration, or the like.

[0047] The precipitate may be washed. The washing can be to separate the crystals from impurities in the precipitate. The washing stage can be to remove any entrained liquor that may contain the impurities. The precipitate may be washed with any suitable wash liquid. Generally, the wash liquid may be sufficient to redissolve soluble contaminants or the like from the crystals. The wash liquid may be sufficient to also displace the entrained contaminated supernatant and replace with the less-contaminated wash liquid. In some embodiments, the wash liquid may be hydrochloric acid, aluminium chloride hexahydrate in a hydrochloric acid solution, or spent liquor.

[0048] The precipitate can be re-dissolved in HCI for a second or a re-crystallisation process with sparging as outlined above.

[0049] In one or all embodiments, the method further include one or more pyrolysis steps selected from the group consisting of:

[0050] (a) drying the aluminium chloride hexahydrate crystals under at least a partial vacuum;

[0051] (b) heating the crystals under controlled air flow; and

[0052] (c) decomposing the heated crystals; wherein the above one or more pyrolysis steps occur after the step of separating the crystals from the spent liquor and before the step of calcining the crystals.

[0053] In the method there can be: drying of the aluminium chloride crystals under at least a partial vacuum; and heating the aluminium chloride hexahydrate crystals under controlled air flow before calcining of the crystals to obtain aluminium oxide.

[0054] The drying of the crystals can be undertaken under at least a partial vacuum. This occurs after the separating the crystals from the spent liquor and before the heating of the crystals under the controlled air flow .Typically, the crystals may be dried under a partial vacuum to aid in the evaporation of liquids by reducing the boiling point of liquids.

[0055] The crystals may be dried at any temperature that removes solvent. In one or all embodiment, the temperature is in the range of from about 50 °C and about 150 °C, or between about 60 °C and about 140 °C, such as between about 80 °C and about 130 °C.

[0056] In one or all embodiments, the crystals comprise residual chloride levels of about 30% by weight to about 45% by weight of the crystals after the step of drying. In one or all embodiments, the crystals comprise substantially no residual moisture content.

[0057] In use it is envisaged that low temperature heating under at least a partial vacuum may assist in reducing entrained liquor, including water, within the crystals to produce stabilised dehydrated chloride crystals.

[0058] Advantageously, low temperature drying under a partial vacuum to reduce entrained liquor before the crystals are dried at higher temperatures may improve the energy efficiency of the drying stages as compared to drying only at higher temperatures.

[0059] The chloride crystals may be dried using any suitable technique known in the art. For example, using a microwave drier, a vacuum drier, a microwave-assisted vacuum drier, or any other suitable indirect drying techniques under vacuum.

[0060] The method may further include: heating the chloride crystals under controlled air flow before calcining the chloride crystals to obtain the metal oxide.

[0061] The step of heating the chloride crystals under controlled air flow may occur after the separating of the chloride crystals from a spent liquor and before the calcining of the chloride crystals.

[0062] The heating of the chloride hexahydrate crystals under controlled air flow produces metal oxyhydroxychlorides. The term “metal oxyhydroxy chloride” refers to an intermediate product in the formation of metal oxides formed by heating metal chloride crystals at low temperatures. Metal oxyhydroxy chlorides typically comprise a mixture of metal oxychloride species with a low concentration of free chlorides, which are generally dry, chemically stable and with good flowing properties.

[0063] Where ACH is used as a starting material, the aluminium chloride crystals may be heated under controlled air flow conditions using any suitable technique that dries the aluminium chloride and produces aluminium oxyhydroxychlorides. For example, the dried chloride crystals may be heated in a forced air-drying oven, a flash dryer, or a fluidised bed dryer. Suitably, under the controlled air flow conditions, air is directed into a vessel such that it contacts the metal chloride crystals and assists in removal of water and hydrochloric acid vapour from the vessel.

[0064] The dried metal chloride crystals may be heated in a heated vessel under a heated and / or dry air flow. Generally, the metal chloride crystals may be gradually heated to a hold temperature, typically using a ramp rate (rate of temperature change over time) that reaches the hold temperature as quickly as possible. As used herein, the term “hold temperature” may refer to a substantially constant temperature or a range of temperatures to which a material is heated to and held at during a step of a process. Any suitable hold temperature may be used. For example, for the aluminium crystals, the hold temperature may be between about 150 °C and about 300 °C, between about 160 °C and about 280 °C, or between about 170 °C and about 250 °C, preferably between about 180 °C and about 230 °C. Likewise, any suitable ramp rate may be used to reach the hold temperature. For example, the ramp rate may be about 10 °C per minute, about 20 °C per minute, about 30 °C per minute, about 40 °C per minute, about 50 °C per minute, about 75 °C per minute, about 100 °C per minute or greater. The metal chloride crystals may be heated at the hold temperature for any suitable period of time. For example, the crystals may be heated at the hold temperature for at least about 30 minutes, at least 60 minutes, at least about 90 minutes, at least about 120 minutes, at least about 150 minutes, at least about 180 minutes, at least about 210 minutes, at least about 240 minutes, at least about 270 minutes, at least about 300 minutes, at least about 330 minutes or even at least about 360 minutes. The metal chloride crystals may be agitated while being heated under the controlled air flow conditions.

[0065] In use, it is envisaged that agitating the crystals while heating may break up any aggregates formed and assist in particle size reduction of the crystals. In addition, heating the chloride crystals under controlled air flow conditions may assist in deagglomeration and / or particle size reduction of the crystals through the introduction of high velocity air into the vessel. In other embodiments, the metal chloride crystals may undergo a particle size reduction process before, during, or after heating under the controlled air flow conditions. The method may further include: decomposing the chloride crystals before crystals are calcined.

[0066] The chloride crystals may be decomposed by means of a rotary kiln or fluidised bed at high temperatures to transition phases to obtain alumina. The means of undertaking the phase transformation is not limited. Generally, the decomposition temperature may be sufficient to remove the majority of the remaining chlorides. For example, the decomposition temperature may be between about 600 °C and about 1 ,800 °C, such as between about 700 °C and about 1 ,400 °C or between about 800 °C and about 1 ,000 °C.

[0067] The chloride crystals may be heated at a decomposition temperature of about 800 °C. The crystals may be heated at the decomposition temperature for a period of at least about 30 minutes, at least 60 minutes, at least about 90 minutes, at least about 120 minutes, at least about 150 minutes, at least about 180 minutes, at least about 210 minutes, at least about 240 minutes, at least about 270 minutes, at least about 300 minutes, at least about 330 minutes, at least about 360 minutes, or more.

[0068] In some embodiments, the decomposing step may comprise controlling the humidity in the vessel. It is envisaged that controlling the humidity of the vessel may assist in promoting chloride removal prior to calcination.

[0069] In some embodiments, the metal oxide formed by the decomposing comprises residual chloride levels of less than about 1.5% by weight, preferably less than about 1 .0% by weight, more preferably less than about 0.4% by weight of the metal oxide. Advantageously, lowering the residual chloride levels of the metal oxide reduces a potential cause of corrosion in the vessel during calcination. As a result, this allows a wider selection of materials used in the construction of the vessels, kilns, calciners, and the like in which the pyrolysis occurs.

[0070] In addition, splitting the decomposition process into a lower temperature heating and a higher temperature decomposition stage effectively splits the process across two pieces of equipment that can each be designed for a tighter range of operating conditions thereby reducing the stress placed on each piece of equipment and potential equipment failure. The chloride crystals can be heated through a number of steps as described before being calcined at a temperature of between about 800 °C and about 1 ,750 °C to obtain a metal oxide material.

[0071] The metal oxide may have a purity of at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 99%.

[0072] The metal chloride crystals may be heated at any temperature and for any length of time that results in calcination of the material. The term “calcining” means a high temperature heating process whereby a mineral-containing material is converted to its oxide form. It is also commonly referred to as ignition, heating, decomposition, pyrolysis or hydro-pyrolysis. As an example only, the calcination temperature can be between about 800 °C and about 1 ,350 °C or between about 950 °C and about 1 ,300 °C, such as between about 1 ,100 °C and about 1 ,250 °C. The heating may occur for any suitable period of time. For example, the crystals may be heated for a period of at least about 30 minutes, at least 60 minutes, at least about 90 minutes, at least about 120 minutes, at least about 150 minutes, at least about 180 minutes, at least about 210 minutes, at least about 240 minutes, or more.

[0073] Brief Description of the Figures

[0074] Embodiments of the invention will now be described with reference to the accompanying drawings which are not drawn to scale and which are exemplary only and in which:

[0075] Figure 1 : A schematic process flow.

[0076] Figure 2: A more detailed schematic process flow.

[0077] Figure 3: Initial Raffinate 1 and Raffinate 1 supernatant post alum precipitation with percentages recovered.

[0078] Figure 4: ICP-OES results in mg / L for stoichiometric ratio testing of ammonium sulphate using Raffinate 1. Figure 5: Elemental concentration of solid samples synthesised from the alum process with concentrations being presented as mg / kg.

[0079] Figure 6: Stoichiometric ratio testing of ammonium sulphate to aluminium sulphate on percentage aluminium recovered and ammonia carryover in the Alum process.

[0080] Figure 7: Block flow diagram for the Alum process including recycle streams.

[0081] Detailed Description of Embodiments of the Invention

[0082] The present examples make use of a raffinate as a sulphate-based aluminium containing matrix however the invention is not so limited and other feedstocks could be processed by the present method.

[0083] Example 1

[0084] Each raffinate sample was provided in 3 separate containers (~4L each container). To form a bulk sample (~12L), the 3 smaller raffinate splits were mixed into a single raffinate sample after the composition of each split was analysed to ensure there were no significant variations in composition.

[0085] Two raffinates were tested:

[0086] • Beneficiation - leach - solvent extraction: Raffinate 1

[0087] • Beneficiation - roast - leach - solvent extraction: Raffinate 2

[0088] Organics have been noted in the raffinate samples provided both in smell and ALS results provided. For this study, no activated carbon stage will be undertaken to remove the organics before progressing down the HPA processing stage. The removal of organics prior to evaporation is recommended to avoid the potential for polychlorinated biphenyls (PCBs), furans and dioxins forming. These harmful side products form when organics are reacted in hot hydrochloric acid solutions.

[0089] Raffinate 1 Spiking

[0090] The raffinate supplied may be a recirculated leach liquor raffinate. As a result, aluminium (and other impurities) are likely to have accumulated in the process. About 10% of the raffinate can be bled from the system as a feed to the present HPA process. In order to emulate the level of aluminium (and other impurities) accumulated in the raffinate due to recirculation, the raffinate was spiked. Aluminium and iron were spiked into the provided raffinate to replicate a proposed concentration of aluminium that most likely reflects the amount of aluminium in the raffinate to be treated. Iron was added with the aluminium to maintain a constant Al / Fe ratio.

[0091] The targeted total Al and Fe concentrations in the spiked raffinate were 25g / L Al and 15g / L Fe.

[0092] The compounds used to spike are AI2(SO4)3.18H2O and FeSO4.7H2O respectively.

[0093] The calculated masses of each Al and Fe salt were added to a 100mL sample of each respective bulk raffinate at the same time and mixed thoroughly to check if co-precipitation occurred. If there was co-precipitation the raffinate sample was heated to about 50°C to check if the salts would dissolve. If complete dissolution was achieved at the 100mL scale, the bulk raffinate samples were spiked to the targeted Al and Fe concentrations. ICP-OES (Inductively coupled plasma - optical emission spectrometry) was used to confirm the targeted concentrations had been achieved (to within 5%)

[0094] Formation of Aluminium Alum

[0095] The spiked raffinate was filtered, and then heated to 80°C, where 386.5 g of ammonium sulphate was added. This represents a 5x stochiometric excess. Once the ammonium alum had completely dissolved, the heating was turned off. The cooling of the solution allowed for the precipitation of the ammonium alum from the synthetic raffinate.

[0096] The ammonium alum was recovered via vacuum filtration the following day. As can be seen in the table of Figure 3, 98% of the aluminium was recovered by the precipitation of ammonium alum.

[0097] The benefit of this method is it resulted in less than 21% recovery for all the other elements including iron (16.29%).

[0098] The ammonium sulphate intermediate still had a slight organic smell, likely as the precipitate was not washed post filtration. However ammonium alum is not soluble in alcohols and acetone. Therefore, the precipitate was washed with acetone / ethanol to remove the organics. (Some of the elements in Figure 3 in the percent extracted column display negative percentages, likely due to impurities in the ammonium alum and / or uncertainties in ICP-OES due initial low concentrations and dilution errors).

[0099] The table of Figure 4 summarises the results from experimenting with the stoichiometric ratio to reduce the amount of ammonium sulphate that is required to be added to the raffinate feed solution.

[0100] The results indicate that there is minimal increase in aluminium recovery with the increase of ammonium sulphate (98.1 - 98.7%). The results indicate that a 3x excess would be sufficient.

[0101] Recrystallisation

[0102] The precipitate of ammonium alum was dissolved in HCI at 1 :1 ratio 20% at 60 °C (e.g. 1 kg alum: 1 L 20% HCI). The solution was subjected to a crystallization process. The crystallization process included adjusting a temperature of the liquor and sparging the liquor with gaseous hydrochloric acid to form a slurry of metal chloride crystals.

[0103] Recrystallisation Step 1 had a relatively high yield at 261.4 g, however this is hypothesised due to the sample not completely drying during the washing stage. When looking at the projected alumina purity, 4N alumina would have been achieved after 2 recrystallisations. There was minimal increase in the purity between Recrystallisation Step 2 and Recrystallisation Step 3 (99.996 - 99.997). Raffinate 2

[0104] The spiking step was repeated as for Raffinate 1 . The sample of Raffinate 2 was spiked with Al and Fe sulphates to represent the concentrations expected in the plant with the exception of lithium and molybdenum (due to their initial low concentration), all the elements had minimal extraction in the alum. Two important elements to focus on are iron and chromium with 5.08 and 15.14% respectively, which will carry over into the crystallisation stages. Finally, 97.1% of aluminium was successfully recovered in the alum intermediate step.

[0105] The table of Figure 5 summarises the solid samples synthesised including the ammonium alum intermediate to RC3. Iron was the largest contaminate in the crystalliser feed at 1818 mg / kg. This allows for a relatively clean starting product. The resulting aluminium chloride hexahydrate synthesised from the alum had a projected purity of 99.993% after a single crystallisation, however this ACH is likely to have a higher sulphate component that will create sulphuric acid during thermal decomposition. It is proposed that a secondary crystallisation will reduce the sulphate concentration in the ACH crystals. The second recrystallisation had just over half the impurity concentration with a projected alumina of 99.996% being achieved.

[0106] Cooling

[0107] A 2 L solution of 27000 ppm Al and 14000 ppm Fe was prepared with duplicate 1 L tests being performed. Once the ammonium sulphate was added, the sample was allowed to cool down from 80°C, and samples were taken at 5°C increments through a syringe filter to remove any precipitated aluminium. The results from the initial test work revealed that the majority of the aluminium precipitated at 30°C with minimal iron precipitation being observed.

[0108] Example 2 - Sub-stoichiometric

[0109] The table of Figure 6 summarise the effect of adjusting the stoichiometric ratio of ammonium sulphate to aluminium sulphate on aluminium recovery and ammonium carry over (contamination) in the Alum process. In laboratory scale trials, a good recovery of aluminium was demonstrated (76.3%) with a 0.78:1 sub-stoichiometric ratio of ammonium sulphate to aluminium sulphate concentration with a low ammonium carryover (88.3 mg / L) in the alum supernatant. A high recovery of aluminium was demonstrated (97.8%) with a stoichiometric excess of 2.27:1 ammonium sulphate to aluminium sulphate concentration with a high ammonium carryover (16,828 mg / L) in the alum supernatant.

[0110] Figure 7 illustrates a block flow diagram for the overall process for the extraction and recovery of aluminium from acidic, sulphate-rich aqueous media such as vanadium process raffinates. In Figure 7, the vanadium process raffinate undergoes an alum process (Block ID: ALUM PROCESS) where ammonium sulphate is added to the raffinate in a stoichiometric deficiency. The ammonia alum is allowed to cool and then separated using vacuum filtration to produce alum supernatant and alum solids (precipitate). The alum supernatant (Block ID: H2SO4 RECYCLE) is recycled for use in vanadium extraction.

[0111] The alum solids undergo a hydrochloric acid leaching process (Block ID: HPA PROCESS (RC-1) to produce aluminium chloride hexahydrate (ACH) crystals. The alum solids are dissolved in hydrochloric acid to form a saturated solution, the saturated solution is then cooled and sparged with gaseous hydrochloric acid to precipitate crystals of ACH. The crystal slurry is separated to produce ACH crystals and supernatant. The ACH crystals are calcined to form alumina.

[0112] The supernatant from the ACH crystallisation stage can then be passed to a process (Block ID: HCI RECOVERY) to recover the hydrochloric acid. The recovered hydrochloric acid can be recycled into the hydrochloric acid leaching process. The supernatant from the ACH crystallisation stage can be passed to a process (Block ID: AMMONIUM SULPHATE RECYCLE) to recover the ammonium sulphate for reuse in the Alum process (Block ID: ALUM PROCESS).

[0113] Ammonium sulphate stoichiometry has a significant impact on the outcomes of the process. Excess ammonium sulphate will result in substantial carryover of ammonium into the ALUM SUPERNATANT stream which may be undesirable for the acid recycling step (Block ID: H2SO4 RECYCLE). As shown in Figure 6, a good recovery of aluminium with negligible ammonium carryover can be possible with sub-stoichiometric ammonium sulphate dosages which may be desirable for acid recycling step(s).

[0114] 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.

[0115] 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.

[0116] 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.

Claims

Claims1 . A method for controlling the amount of aluminium in a sulphate-based aluminium containing matrix, the method comprising: adding ammonium sulphate in stochiometric ratio of from about 0.1 : 1 to greater than 1 : 1 to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum.

2. A method of claim 1 , wherein the ammonium sulphate is added in a stochiometric ratio of greater than 1 : 1 , and further wherein the separated ammonium alum is subject to further processing to form high purity alumina.

3. The method of claim 2, wherein the stochiometric excess of ammonium sulphate relative to the aluminium sulphate is at most about 3: 1 .

4. A method of claim 1 , wherein the ammonium sulphate is added in a sub- stochiometric ratio in the range of from about 0.1 : 1 to less than 1 : 1 , and further wherein the step of separating the ammonia alum provides an aluminium depleted sulphate-based aluminium containing matrix which is recycled into a process.

5. The method of claim 4, wherein the sub-stochiometric excess of ammonium sulphate relative to the aluminium sulphate is about 0.8: 1 .

6. A method for producing alumina from a sulphate-based aluminium containing matrix, the method comprising: adding ammonium sulphate in a stochiometric excess to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum, optionally washing the separated ammonia alum to remove one or more of organics, entrained and soluble contaminants, dissolving separated / washed ammonia alum to form a solution,sparging the solution with HCI to crystallise aluminium chloride hexahydrate (ACH), calcining the separated ACH to form alumina.

7. The method of claim 6, further comprising re-crystallising the crystalised aluminium chloride hexahydrate (ACH) one or more times to increase purity of the product.

8. The method of claim 6 or 7, wherein in the step of dissolving separated / washed ammonia alum comprises dissolving in hydrochloric acid.

9. The method of any one of claim 6 to 8, further comprising(a) drying the aluminium chloride hexahydrate crystals under at least a partial vacuum;(b) heating the crystals under controlled air flow; and(c) decomposing the heated crystals; wherein the above one or more pyrolysis steps occur before the step of calcining.

10. A method for recycling a sulphate-based aluminium containing matrix, the method comprising: obtaining a sulphate-based aluminium containing matrix from a process; adding ammonium sulphate at less than a stochiometric excess to the aluminium sulphate concentration of the sulphate-based aluminium containing matrix to form ammonia alum, separating the ammonia alum to provide an aluminium depleted sulphate-based aluminium containing matrix; recycling the aluminium depleted sulphate-based aluminium containing matrix into the process.

11. The method of claim 10 wherein the separated ammonium alum is subject to further processing to form high purity alumina.

12. The method of claim 10 or 11 , wherein the process is a process ofextracting vanadium and the sulphate-based aluminium containing matrix is a vanadium raffinate.

13. The method of any one of the preceding claims, wherein the method further comprises the step of determining the stochiometric excess by determining the concentration of the aluminium by spectroscopy.

14. The method of any one of the preceding claims, wherein the ammonium sulphate is added in one single stochiometric excess all at the same time during the same reaction period.

15. The method of any one of the preceding claims, wherein the ammonium sulphate is added with heating to at least about 60, 70 or 80 degrees.

16. The method of any one of the preceding claims, further comprising the step of cooling the solution to precipitate the ammonia alum for separation.

17. A method for producing alumina from a sulphate-based aluminium containing matrix, the method comprising two purification stages:(1) controlled precipitation of high-purity ammonia-alum, to be feed for (2).(2) controlled recrystallisation of high-purity ACH from the ammonia alum.

18. Alumina when prepared by the method of any one of the preceding claims.

19. Aluminium depleted sulphate-based aluminium containing matrix when obtained by any one of claims 10 to 16.

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