A method for the preparation of alumina

Abstract

The present invention relates to a method for the preparation of alumina from an alunite material, the method comprising the steps of: subjecting the alunite material to an alkaline hydroxide leach step; subjecting the product of the alkaline hydroxide leach step to a solid liquid separation to produce a liquid product comprising potassium sulfate and a solid residue comprising alumina; subjecting the solid residue containing alumina to an acid leach step to produce an aluminous leach solution and separating undissolved solids; subjecting the aluminous leach solution to a crystallisation step to crystallise an aluminium salt; and subjecting the aluminium salt to a calcination step to produce alumina.

Description

“A Method for The Preparation of Alumina”Field of the Invention

[0001] The present invention relates to a method for the preparation of alumina from alunite materials. More particularly, the method of the present invention is intended to prepare high purity alumina from alunite materials.Background Art

[0002] Traditionally, highly purified form of alumina (AI2O3), also referred to as High Purity Alumina (HPA) has been prepared by methods such as refining of aluminabearing minerals (aluminium hydroxides) and hydrolysis of aluminium alkoxide. These traditional methods involve high costs due to the energy intensive process to produce hydroxides from bauxite as well as high consumption of aluminium metal and alcohol.

[0003] Presently, the primary method of preparing HPA is treating kaolin ores with hydrochloric acid, then refining the treated ores using traditional technologies developed in the 1920-40s, namely HCI gas sparging or aluminium chloride hexahydrate (ACH) crystallisation.

[0004] Australian Innovation Patent Application 2019100630 describes a method for the preparation of alumina from aluminous material, the method comprising the steps of treating the aluminous material to reduce particle size and increase alumina content, calcining the aluminous material, and subsequently leaching the aluminous material with HCI to provide a pregnant liquor. The pregnant leach is then precipitated to ACH by adding HCI gas and subsequently, the precipitated ACH is calcined to provide alumina, whereby HCI is recycled. The aluminous material is preferably kaolin. Despite lower production costs compared to the traditional method(s) from bauxite, producing HPA from kaolin requires substantial HCI consumption. This is an economic concern given the price of HCI and generally low availability in large volumes.

[0005] With a growing demand for HPA, economic and environmental concerns are becoming of increasing importance, in addition to being able to produce HPA in sufficient quantities.

[0006] Hence, there has been much research directed to methods utilising various alumina-bearing minerals to prepare HPA, whilst being economical.

[0007] Alunite is an alumina-bearing mineral represented by the formula K2SO4 AI2(SO4)3-2AI2O3. The use of Alunite has typically been limited to the preparation of Smelter Grade Alumina (SGA) or metallurgical grade alumina. Typically, the potassium component is separated and directed to produce muriate of potash (MOP or KCI) or sulfate of potash (SOP or K2SO4). SOP is the preferred potassium fertiliser chemical as it is chlorine-free and comprises both, potassium and sulfur.

[0008] Historical processes for the treatment of alunite include reductive pyrometallurgy, the potash-alkaline method and the soda alkali method.Reductive pyrometallurgy

[0009] In this method, the ore after crushing, grinding and roasting is subject to a recovery roasting at a temperature greater than 560-580°C with a reducing agent:K2SO4AI2(SO4)3-2AI2O3 + 0.5C2H43AI2O3+ K2SO4+ 3SO2+CO2+H2O

[0010] The preferred reducing agents for the reaction are diesel fuel or gaseous sulfur and the SO2resulting from the reaction is used to produce sulfuric acid. The complete process of recovery takes place at temperatures over 580°C.

[0011] This method had several major drawbacks including low yield of alumina in the product and significant damage to the environment, particularly by production of excess sulfur in the form of sulfuric acid as sulfur remains in net oversupply.Potash-alkaline Method

[0012] The potash-alkaline method comprises roasting the alunite at 550°C, and subsequently leaching the roasted alunite with a potassium carbonate solution:K2SO4 AI2(SO4)3-2AI2O3 + 3K2CO34K2SO4+ 3AI2O3+ 3CO2

[0013] This reaction effectively converts potassium carbonate to potassium sulfate (SOP) and neutralises or uses the excess sulfur in the alunite as a by-product. The insoluble residue contains active alumina (Y-AI2O3) which is then processed using the Bayer out-of-autoclave process to produce alumina and quartz sand. Despite the high yield of alumina in the product (up to 90%), this method uses high volumes of potassium carbonate, a scarce and expensive auxiliary raw material.Soda-alkaline Method

[0014] More recently, US 2021 / 0071281 A1 describes an alternative method of treating alunite ore. This method modifies the potash alkali method by substituting the potassium carbonate solution with a sodium carbonate solution, and subsequently reacting the produced sodium sulfate with kainite (KCI) to produce additional SOP and by-product, sodium salt:K2SO4-Al2(SO4)3-2Al2O3 + WR + 3Na2CO3 K2SO4 + 3Na2SO4+ (3AI2O3+ WR) + 3CO2K2SO4 + 3Na2SO4+ 6KCI 4K2SO4+ 6NaCI*WR - waste rock

[0015] Alumina is recovered by treating the AI2O3 via the Bayer process and the yield of alumina in the product is up to 90%.

[0016] Other methods to recover alumina from alunite has been explored, including:• Bayer process pressure leach with NaOH to produce SOP and alumina;• Potassium hydroxide leaching to produce SOP and alumina (CN 101913633 B);• CaF smelting to produce cryolite, alumina and SO2 gas;• Chloride salt pyrometallurgy to produce SOP and alumina; and• Sulfuric or hydrochloric acid with potassium fluoride or fluoro silicic acid addition.

[0017] Whilst the methods described above are able to provide alumina, uneconomical and environmentally hazardous steps, such as high temperaturecalcination and the Bayer process, are necessary due to the high content of sulfur trioxide (SO3) and silica in most alunite ores.

[0018] The method of the present invention has as one object thereof to overcome substantially the abovementioned problems of the prior art, or to at least provide a useful alternative thereto.

[0019] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. This discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0020] Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0021] Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirely by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of brevity.Disclosure of the Invention

[0022] In accordance with the present invention there is provided a method for the preparation of alumina from an alunite material, the method comprising the steps of: subjecting the alunite material to an alkaline hydroxide leach step; subjecting the product of the alkaline hydroxide leach step to a solid liquid separation to produce a liquid product comprising potassium sulfate and a solid residue comprising alumina; subjecting the solid residue containing alumina to an acid leach step to produce an aluminous leach solution and separating undissolved solids;subjecting the aluminous leach solution to a crystallisation step to crystallise an aluminium salt; and subjecting the aluminium salt to a calcination step to produce alumina.

[0023] In one form of the present invention, the alumina is in the form of high purity alumina (HPA). Preferably, the purity of the high purity alumina (HPA) is at least 99.99% (mass%).

[0024] Throughout the specification, unless the context requires otherwise, the term “alunite material” will be understood to refer to a material that comprises alunite. The alunite material may be an ore containing alunite or it may be a concentrate formed from such an ore. In one form of the present invention, the alunite material is an aluminous clay. Preferably, the alunite material is a lacustrine aluminous clay.

[0025] In one form of the present invention, the method comprises treating the alunite material to reduce particle size prior to the alkaline hydroxide leach step. Preferably, the alunite material is treated in an attrition step. Preferably, the alunite material is homogenised during the attrition step.

[0026] In one form of the present invention, the alunite material subjected to a pulping step prior to the alkaline hydroxide leach step. Preferably, the pulping step comprises the mixing of the alunite material with water to produce an alunite material slurry.

[0027] Preferably, the pulp density of the alunite material slurry is between 10% solids and 30% solids. More preferably, the pulp density of the alunite material slurry is between 10% solids and 20% solids.

[0028] The alkaline hydroxide leach step comprises the contact of the alunite material with an alkaline hydroxide solution. Throughout the specification, unless the context requires otherwise, the term “alkaline hydroxide”, will be understood to refer to chemical compound composed of an alkali group metal or alkaline earth group metal cation and a hydroxide anion. Preferably, the alkaline hydroxide is selected frompotassium hydroxide, sodium hydroxide or a combination thereof. More preferably, the alkaline hydroxide is potassium hydroxide.

[0029] In one form of the present invention, the solid residue resulting from the alkaline hydroxide leach step comprises alumina and silica gangue. In one form of the present invention, the undissolved solids resulting from the acid leach step comprises undissolved silica gangue.

[0030] In one form of the present invention, the concentration of the alkaline hydroxide solution ranges between about 150 to 1 ,000 kg / tonne of alunite material.

[0031] In one form of the present invention, the alkaline hydroxide leach step utilises potassium hydroxide at a molar ratio of 0.1 to 4 moles of KOH per mole of alumina in the alunite material.

[0032] In forms of the present invention where the alkaline hydroxide leach utilises potassium hydroxide at a molar ratio of 3 or more moles of KOH per mole of alumina in the alunite material, alumina is dissolved into solution. In this form of the invention, the solution is contacted with carbon dioxide to recover alumina. Preferably the carbon dioxide gas sparging is performed at 45°C to 90°C. Still preferably the carbon dioxide gas sparging occurs for 15 minutes to 2 hours. Still preferably the sparging is performed for one hour.

[0033] In one form of the present invention, the alkaline hydroxide leach step is conducted at a temperature between about 25°C to 160°C. Preferably, the alkaline hydroxide leach step is conducted at a temperature between about 25°C to 100°C. More preferably, the alkaline hydroxide leach step is conducted at a temperature between about 80°C to 160°C.

[0034] In one form of the present invention, the alkaline hydroxide leach step is conducted at atmospheric pressure. In an alternative form of the present invention, the alkaline hydroxide leach step is conducted at elevated pressure. Preferably, the alkaline hydroxide leach step is conducted at a pressure up to 2.5 Bar.

[0035] Preferably, the alkaline hydroxide leach step is conducted over a period of about 15 minutes to 24 hours.

[0036] Preferably, the alkaline hydroxide leach step has a pulp density between about 5 to 30% alunite material in solution. More preferably, the pulp density is between 10% to 20%.

[0037] In one form of the present invention, the alkaline hydroxide leach utilises sodium hydroxide at a molar ratio of 0.1 to 6 moles of NaOH per mole of alumina.

[0038] In this form of the invention the alkaline leach occurs under atmospheric pressure is preferably conducted at a temperature of about 90°C. Still preferably, the residence time ranges between about 1 to 4 hours. Still yet preferably, pulp density of slurry after the alkaline leach ranges between about 10 to 30%.

[0039] In one form of the present invention, the method further comprises the step of: recovering potassium from the liquid product comprising potassium sulfate

[0040] In one form of the present invention, the liquid product comprising potassium sulfate is subjected to a crystallisation step to recover a sulphur of potash product. Preferably, the crystallisation step comprises evaporative crystallisation.

[0041] The acid leach step comprises the contact of the solid product containing alumina with an acidic solution. Preferably, the acidic solution is selected from hydrochloric acid, sulphuric acid, nitric acid or a combination thereof. More preferably, the concentration of the acidic leach solution in excess of the stoichiometric amount required to dissolve aluminium hydroxides.

[0042] In embodiments where hydrochloric acid is utilised in the acid leach step, the concentration of the hydrochloric acid ranges between about 3 mol / L to 6 mol / L. Preferably, the acid leach step is conducted at a temperature between about 80 to 95°C. More preferably, the acid leach step is conducted at atmospheric pressure. Still preferably, the acid leach step has a pulp density between about 10 to 40% solids.

[0043] In embodiments where nitric acid is utilised in the acid leach step, the concentration of the nitric acid ranges between about 3 mol / L to 6 mol / L. Preferably, the nitric acid leach step is conducted at a temperature between about 80 to 95°C. More preferably, the nitric acid leach step is conducted at atmospheric pressure. Still preferably, the residence time ranges between about 1 to 4 hours. Still preferably, the nitric acid leach step has a pulp density between about 15 to 20% solids.

[0044] In embodiments where sulphuric acid is utilised in the acid leach step, the concentration of the sulphuric acid ranges between about 10 mol / L to 15 mol / L. Preferably, the sulphuric acid leach step is conducted at a temperature between about 60 to 95°C. More preferably, the sulphuric acid leach step is conducted at atmospheric pressure. Still preferably, the residence time ranges between about 1 to 4 hours. Still preferably, the sulphuric acid leach step has a pulp density between about 15 to 20% solids.

[0045] In one form of the present invention, the undissolved solids from the acid leach step are neutralised prior to disposal. Preferably, the undissolved solids are neutralised with an alkaline earth carbonate. More preferably, the alkaline earth carbonate is selected from one or more of CaCOs or NaCOs. Still preferably, the alkaline earth carbonate is NaCOs.

[0046] In one form of the present invention, the aluminous leach solution is subjected to one or more impurity removal steps prior to the crystallization of the aluminium salt. Preferably, the one or more impurity removal steps are selected from electrodialysis, ion exchange and solvent extraction.

[0047] In forms of the invention where the acidic solution is hydrochloric acid, the crystallisation step comprises the crystallisation of a chloride hexahydrate (ACH) salt. Preferably, the crystallisation step comprises contacting the aluminous leach solution with gaseous hydrogen chloride. More preferably, gaseous hydrogen chloride is sparged through the aluminous leach solution. Preferably, the concentration of the gaseous hydrochloric acid to produce the aluminium salt is greater than or equal to about 360 g / L in STP conditions.

[0048] In forms of the invention where the acidic solution is nitric acid, the crystallisation step comprises the crystallisation of an aluminium nitrate nonahydrate salt. Preferably, the crystallisation step comprises contacting the aluminous leach solution with nitric acid or gaseous nitric acid to precipitate aluminium nitrate nonahydrate.

[0049] An alternative method of precipitating alumina from the nitric acid solution containing alumina is via the synthesis of aluminium ammonium carbonate hydroxide, otherwise known as dawsonite. Preferably the crystallisation step comprises contacting the aluminous leach solution with ammonium carbonate to precipitate dawsonite. Ammonium carbonate is added at a rate of 1 mole per mole of alumina, at a temperature ranging from 25°C to 95°C.

[0050] In forms of the invention where the acidic solution is sulphuric acid, the crystallisation step comprises the crystallisation of an aluminium sulfate salt. This may take the form of any commonly available aluminium sulfate, such as ammonium alum or basic aluminium sulfate. Preferably ammonium aluminium sulfate is synthesised via addition of ammonium sulfate in the ratio of 0.6 to 1 mole of ammonium sulfate per mole of alumina in solution, at any temperature between 4°C and 95°C.

[0051] In one form of the present invention, the aluminium salt is subjected to a purification step to produce a purified aluminium salt prior to the calcination step. Preferably, the purification step comprises: dissolving the aluminium salt in water; and crystallising a purified aluminium salt from solution.

[0052] In one form of the present invention, the calcination step is conducted at a temperature of about 1200°C. Preferably, the comprises the steps of: a. initial heating to about 350°C; and b. final calcination to about 1200°C.

[0053] Preferably, off-gases produced in the calcination step are treated to recover acid products. More preferably, the acid products are recycled to the crystallisation step or the acid leach step.Description of the Drawings

[0054] The present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:-Figure 1 is a flow sheet depicting a method for the preparation of alumina from alunite in accordance with the present invention.Best Mode(s) for Carrying Out the Invention

[0055] The present invention relates to the recovery of alumina from alunite minerals. The method generally comprising the steps of subjecting the alunite material to alkaline hydroxide leach step to recover a liquid product comprising potassium sulfate and a solid residue containing alumina. The solid residue is further treated in an acid leach step to produce an aluminous leach solution. The aluminous leach solution is treated in a crystallisation step to crystallise an aluminium salt, which is then calcined to produce alumina. The inventors have found that the alkaline hydroxide leach will leach a significant portion of the potassium sulphate from the alunite material, allowing the separation of a solid alumina product. The acid leach step is then used to leach alumina into solution, allowing a purified aluminium salt to be recovered. The purified aluminium salt can then be calcined to produce alumina.

[0056] In Figure 1 there is shown a method 10 for the preparation of alumina, for example a high purity alumina (HPA), from an alunite material in accordance with one embodiment of the present invention.

[0057] Alunite materials are typically associated with aluminous clays. The Applicant has identified that lacustrine alunite, in particular alunite from salt lakes in Western Australia, may be a potential ore deposit suited to produce highly purified alumina. Based on metallurgical test work on a deposit of alunite from a salt lake in Western Australia, the content of the deposit is 27% aluminium, 5.8-6.3% potassium,20-23% silica, 2.5% Fe20s, 12% sulfur and 2.5-3.5% sodium. The lacustrine alunite is found to be higher in aluminium and potassium content, lower in silica, Fe2Os, sulfur and sodium content, in comparison to hard rock from other places around the world such as Azerbaijan, North America, and China.

[0058] A raw alunite clay 12 is first treated in an attrition step 14. Attrition step 14 is conducted to disaggregate the mineral particles. The attrition step 14 reduces the particle size, allow efficient washing and chemical treatment that follow. The attrition step 14 is preferably conducted in an agitated washing apparatus, such as a drum scrubber or wet attrition cell. The resulting material is passed through a screen to remove oversize materials. The screening of the alunite materials will remove a portion of the gangue materials, such as quartz minerals, from the alunite material, thereby increasing the concentration of aluminium in the screened material. In one embodiment, the screen will remove particles larger than 0.5 mm.

[0059] It is envisaged that the alunite material may be subjected to one or more beneficiation steps (not shown) to remove further gangue materials. The one or more beneficiation steps can include one or more of a gravity classification, magnetic classification and flotation.

[0060] In one embodiment, the alunite material has a minimum aluminium content of 15%.

[0061] In one embodiment, the alunite material has a maximum iron content of 5%.

[0062] In one embodiment, the alunite material has a maximum calcium content of0.25%.

[0063] Feed slurry 16 from the attrition step may be mixed with water 18 in a pulping step (not shown) to reach the desired pulp density. In one embodiment, the pulp density is 10-30%. In one embodiment, the pulp density is 10-20%. The Applicant has found that the pulp density will impact the water balance of the overall circuit and the cost. It is generally impractical to operate below 10% solids in liquid and the upper limit may be defined by the thickness of the slurry.

[0064] Feed slurry 16 is directed to alkaline hydroxide leach step 20 in which it is contacted with an alkaline hydroxide solution 22 to leach potassium sulphate from the alunite material. It is envisaged that the alkaline hydroxide solution 22 may comprise any alkali group metal hydroxide or alkaline earth group metal hydroxide. In one embodiment, the alkaline hydroxide leach step 20 utilises a potassium hydroxide solution, a sodium hydroxide solution or a combination thereof. In one embodiment, the alunite material is not subjected to a high temperature treatment step or a calcination step prior to the alkaline hydroxide leach step 20. In one embodiment, the alunite material is not subjected to an acid leach step prior to the alkaline hydroxide leach step 20.

[0065] The alkaline hydroxide leach step 20 is conducted using conventional equipment, preferably including leach vessel(s) having suitable chemical / temperature resistance, an agitation means and a suitable ventilation system with off-gas scrubbing. In embodiments where the alkaline hydroxide leach step is conducted at elevated pressure, appropriate pressurised autoclaves are utilised. The alkaline hydroxide leach step may be conducted using two or more leach vessels operating in series or in a counterflow arrangement.

[0066] In one embodiment, the alkaline hydroxide leach step is conducted at ambient temperature. In one embodiment, the alkaline hydroxide leach step is conducted at elevated temperature. Preferably, the leach step is conducted at a temperature between about 25°C to 160°C. Steam is preferably used to maintain the desired temperature. Operating at temperatures over 100°C requires the alkaline leach step to be operated at elevated pressure to prevent boiling. Preferably, the alkaline hydroxide leach step is conducted at a temperature between 80°C and 100°C.

[0067] In one embodiment, the alkaline hydroxide leach step is conducted at atmospheric pressure. In an alternative embodiment, the alkaline hydroxide leach step is conducted at elevated pressure. In one embodiment, the alkaline hydroxide leach step is conducted at a pressure up to 2.5 Bar.

[0068] The alkaline hydroxide leach step is conducted at a suitable pulp density. In one embodiment, the pulp density of the alkaline hydroxide leach step is 10-50%. In one embodiment, the pulp density of the alkaline hydroxide leach step is 20 - 40 %.

[0069] In one embodiment, the alkaline hydroxide leach step is conducted for at least 15 minutes. In one embodiment, the alkaline hydroxide leach step is conducted over a period of about 15 minutes to 24 hours.

[0070] In a preferred embodiment, the concentration of the alkaline hydroxide is based on the amount of alumina in the alunite material being treated. In a preferred embodiment, the concentration of the alkaline hydroxide is based on a ratio of alkaline hydroxide in relation to the amount of alumina in the alunite material being treated.

[0071] Where potassium hydroxide is used in the alkaline hydroxide leach step, the alkaline hydroxide leach step comprises contact of the alunite material with 0.1 to 4 moles of potassium hydroxide per mole of alumina in the alunite material. Methods to calculate the amount of alumina present in the alunite material are known to those in the art, including atomic absorption spectroscopy (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES or ICP-AES), or other appropriate analytical techniques.

[0072] In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 0.1 -3 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 0.8-3 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 1-3 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 1.5-3 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 2-3 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 2-2.8 moles of potassium hydroxide per mole of alumina in the alunite material. In oneembodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 2.4-2.6 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 0.8-1 .5 moles of potassium hydroxide per mole of alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step comprises the contact of the alunite material with 1.1 -1.3 moles of potassium hydroxide per mole of alumina in the alunite material. In embodiments where the alkaline hydroxide leach step comprises the contact of the alunite material with less than 3 moles of potassium hydroxide per mole of alumina in the alunite material, it is understood that the concentration of the potassium hydroxide is not sufficient to leach a substantial part of the alumina from alunite material. The preferred ratio of potassium hydroxide to alumina is that which will produce a leach product that contains a liquid product comprising potassium sulfate with minimal alumina in solution and a solid residue comprising alumina with minimal amounts of sulphate and potassium. This ratio is largely dependent on the amount of alumina in the alunite material, but may also vary according to the nature and varied chemistry of the alunite material. As would be appreciated, any variation from the preferred ratio of alkaline hydroxide to alumina may result in more or less alumina in solution, and potassium and sulphate in the solid residue. To assist in the production of high purity alumina, the ratio of alkaline hydroxide to alumina in alunite should minimise the sulphur and potassium content in the solid product, while maximising the alumina content. As shown in Figure 1 , this allows the product of the alkaline hydroxide leach step 22 to be transferred directly to the solid liquid separation step 24.

[0073] In one embodiment, the alkaline hydroxide leach step is operated at a pH<11 . In one embodiment, the alkaline hydroxide leach step is operated at a pH <10. It is understood that limiting the pH of the alkaline hydroxide leach step may reduce the amount of alumina that is leached in the alkaline hydroxide leach step. The pH of the alkaline hydroxide leach step can be controlled, for example, by the addition of the alkaline hydroxide solution 22.

[0074] In one embodiment, the alkaline hydroxide leach step leaches less than 10 wt% of the alumina in the alunite material. In one embodiment, the alkaline hydroxide leach step leaches less than 5 wt% of the alumina in the alunite material.

[0075] In an alternative embodiment of the present invention, the alkaline hydroxide leach step comprises the contact of the alunite material with 3 or more moles of potassium hydroxide per mole of alumina in the alunite material. In this embodiment of the present invention, the concentration of the potassium hydroxide will leach at least a portion of the alumina from alunite material. This will produce a leach product that contains a liquid product comprising potassium sulfate and alumina. In such an embodiment, the process can optionally further include the recovery of alumina from the liquid product. This can be achieved by is contacting the liquid product with carbon dioxide to recover alumina. Carbon dioxide gas sparging is preferably performed at 45°C to 90°C for 15 minutes to 2 hours, preferably an hour. The resulting slurry is transferred to the solid liquid separation step 24.

[0076] Where sodium hydroxide is used in the alkaline hydroxide leach step, the alkaline hydroxide leach step comprises contact of the alunite material with 0.1 -6 moles of sodium hydroxide per mole of alumina in the alunite material. In a preferred embodiment, the alkaline hydroxide leach step comprises contact of the alunite material with 4-6 moles of sodium hydroxide per mole of alumina in the alunite material.

[0077] The product of the alkaline hydroxide leach step 22 is directed to a solid liquid separation step 24 to separate a liquid product 26 comprising potassium sulfate and a solid residue 28 comprising alumina and silica gangue. Wash water is used in the solid liquid separation step 24 to ensure entrained liquids are fully separated from the solid residue 28. Wash filtrate 30 is recycled to the alkaline hydroxide leach step 20. It is envisaged that the solid liquid separation step 24 will be conducted in a suitable filtration apparatus known in the art, such as a filter press. Alternative solid liquid separation devices may be utilised in solid liquid separation step 24. It is envisaged that a pre-filter thickener may also be incorporated upstream of the filtration apparatus.

[0078] The liquid product 26 is directed to a potassium recovery step 32. Potassium recovery step 32 preferably subjects the liquid product to a crystallisation step to crystallise a sulphur of potash product 34. The Applicant has found that evaporative crystallisation may be used to crystallise a sulphur of potash product. Water stream 36 may be recovered using, for example, mechanical vapourrecompression and recycled to other parts of the process. Bleed stream 38 containing sodium chloride may be separated and evaporated to the solid salt for disposal.

[0079] The solid residue 28 from the alkaline hydroxide leach step comprises aluminium hydroxides, iron hydroxides and silicates. The solid residue 28 may further comprises refractory oxides including those of zirconium and titanium. The solid residue 28 is directed to acid leach step 40 where it is contacted with an acidic solution 42 to leach aluminium into solution. The acidic solution 42 is selected from hydrochloric acid, sulphuric acid, nitric acid or a combination thereof. While the acidic solution 42 shown in Figure 1 is recycled from other parts of the flowsheet, it is envisaged that fresh acid solution may be used in the acid leach step 40.

[0080] The acid leach step 40 is conducted using conventional equipment, preferably including leach vessel(s) having suitable chemical / temperature resistance, an agitation means and a suitable ventilation system with off-gas scrubbing. In embodiments where the acid leach step 40 is conducted at elevated pressure, appropriate pressurised autoclaves are utilised. The acid leach step 40 may be conducted using two or more leach vessels operating in series or in a counterflow arrangement.

[0081] The acid leach step 40 is operated under suitable conditions to leach a substantial amount of the aluminium in the solid residue 28. Those skilled in the art would appreciate that the pulp density, temperature, residence time and reactor configuration may all be modified to achieve the desired leaching of the solid residue 28. The concentration of the acidic solution should be in excess of the stoichiometric amount required to leach the aluminium in the solid residue 28. It will be appreciated that the acid concentration used will depend on the type of acid itself, as well as the properties of the solid residue 28.

[0082] In one embodiment, the acid leach step is conducted using hydrochloric acid. In this embodiment, the concentration of the hydrochloric acid is at least 3 mol / L. Preferably, the concentration of the hydrochloric acid ranges between about 3 mol / L to 6 mol / L. In this embodiment, the acid leach step is conducted at a temperature above ambient temperature. Preferably, the acid leach step is conducted at a temperaturebetween about 80 to 95°C, at atmospheric pressure. In this embodiment, the pulp density is between about 10 to 40% solids. In this embodiment, the residence time is at least 1 hour. Preferably, the residence time ranges between about 1 to 4 hours.

[0083] In one embodiment, the acid leach step is conducted using nitric acid. In this embodiment, the concentration of the nitric acid is at least 3 mol / L. Preferably, the concentration of the nitric acid ranges between about 3 mol / L to 6 mol / L. In this embodiment, the acid leach step is conducted at a temperature above ambient temperature. Preferably, the acid leach step is conducted at a temperature between about 80 to 95°C, at atmospheric pressure. In this embodiment, the pulp density is between about 10 to 40% solids. In this embodiment, the residence time is at least 1 hour. Preferably, the residence time ranges between about 1 to 4 hours.

[0084] In one embodiment, the acid leach step is conducted using sulfuric acid. In this embodiment, the concentration of the sulphuric acid is at least 10 mol / L. Preferably, the concentration of the sulphuric acid ranges between about 10 mol / L to 15 mol / L. In this embodiment, the acid leach step is conducted at a temperature above ambient temperature. Preferably, the acid leach step is conducted at a temperature between about 80 to 95°C, at atmospheric pressure. In this embodiment, the pulp density is between about 10 to 40% solids. In this embodiment, the residence time is at least 1 hour. Preferably, the residence time ranges between about 1 to 4 hours.

[0085] The product of the acid leach step 40 is directed to a solid liquid separation step 43 to separate an aluminous leach solution 44 from undissolved solids 46. It is envisaged that the solid liquid separation step 43 will be conducted in a suitable filtration apparatus known in the art, such as a filter press. Alternative solid liquid separation devices may be utilised in solid liquid separation step 43. It is envisaged that a pre-filter thickener may also be incorporated upstream of the filtration apparatus.

[0086] Undissolved solids 46 from the acid leach step are neutralised prior to disposal. In the embodiment shown in Figure 1 , the undissolved solids are directed to residue treatment step 48 in which they are contacted with an alkaline carbonate 50. In one embodiment, the alkaline carbonate 50 is selected from one or more of CaCOs or NaCOs.

[0087] In one embodiment, the aluminous leach solution 44 is subjected to one or more impurity removal steps (not shown) prior to the crystallization of the aluminium salt. The one or more impurity removal steps may be selected based on the impurities present in the aluminous leach solution 44. Suitable impurity removal steps include electrodialysis, ion exchange and solvent extraction.

[0088] The aluminous leach solution 44 is directed to a crystallisation step 52 to crystallise an aluminium salt. The crystallisation step 52 allows aluminium to be separated, leaving sulfur, iron, calcium, sodium, potassium and minor contaminants in solution.

[0089] The aluminium salt recovered in crystallisation step 52 is dependent on the acidic solution utilised in acid leach step 40. In the embodiment shown in Figure 1 , the acidic solution is hydrochloric acid. The aluminium salt produced from the resulting aluminous leach solution is aluminium chloride hexahydrate (ACH) salt. In this embodiment, the crystallisation step 52 comprises the contact of the aluminous leach solution 44 with gaseous HCI 54 to precipitate ACH. The target HCI concentration to achieve precipitation is greater than or equal to about 360 g / L in STP conditions.

[0090] In embodiments where the acidic solution is nitric acid, the produced aluminium salt is aluminium nitrate nonahydrate salt. In this embodiment, the crystallisation step 52 comprises contacting the aluminous leach solution 42 with nitric acid or gaseous nitric acid to precipitate aluminium nitrate nonahydrate.

[0091] In embodiments where the acidic solution is sulphuric acid, the produced aluminium salt is ammonium aluminium sulphate dodecahydrate salt. In this embodiment, the crystallisation step 52 comprises contacting the aluminous leach solution 42 with ammonium sulphate to precipitate ammonium aluminium sulphate. Preferably, the 0.6 to 1 moles of ammonium sulphate are added per mole of aluminium in the aluminous leach solution.

[0092] The product of the crystallisation step 52 is directed to solid liquid separation step 56 to separate an aluminium salt 58 from barren solution 60. It is envisaged that the solid liquid separation step 56 will be conducted in a suitable filtrationapparatus known in the art, such as a filter press. Alternative solid liquid separation devices may be utilised in solid liquid separation step 56.

[0093] The aluminium salt 58 may be directed to calcination or may be subjected to further purification. Further purification may be utilised to improve the purity of the final alumina product. In the embodiment shown in Figure 1 , the aluminium salt 58 is contacted with fresh water 61 to re-dissolve the aluminium salt into a secondary aluminous solution 62. Secondary aluminous solution 62 is treated in a secondary crystallisation step 64 to re-crystallise a purified aluminium salt. The secondary crystallisation step 64 is operated similarly to crystallisation step 52. In the embodiment shown in Figure 1 , the secondary crystallisation step 64 comprises the contact of the secondary aluminous solution 62 with gaseous HCI 54 to precipitate ACH. It is envisaged that additional purification step(s) may be utilised to produce an aluminium salt with a desired purity.

[0094] The product of the secondary crystallisation step 64 is directed to solid liquid separation step 66 to separate a purified aluminium salt 68 from barren solution 70. Barren solution 70 comprises hydrochloric acid and may be recycled to other parts of the process. It is envisaged that the solid liquid separation step 66 will be conducted in a suitable filtration apparatus known in the art, such as a filter press. Alternative solid liquid separation devices may be utilised in solid liquid separation step 66.

[0095] The purified aluminium salt 68 is subjected to a calcination step 72 to produce an alumina powder product 74. Preferably, the calcination step 70 comprises of two steps, initial heating to about 350°C and final calcination to about 1200°C. The initial heating to about 350°C releases off-gases 76. The final calcination to about 1200°C produces alumina powder 74.

[0096] The off-gases 76 from the calcination step 72 may be scrubbed through water to regenerate acid product (not shown). The composition of the off-gases 76 will depend on the purified aluminium salt 68. In the embodiment shown in Figure 1 , the purified aluminium salt 68 is ACH. In this embodiment, the off-gases 76 comprise HCI. The off-gases 76 may be recycled to crystallisation step 52 or secondary crystallisation step 64.

[0097] In the embodiment shown in Figure 1 , the process comprises an electrolysis step 78. An alkaline salt 80 in the form of the brine solution is treated in electrolysis step 78 to produce an alkaline hydroxide solution 82 and a hydrochloric acid 84. Alkaline hydroxide solution 82 is directed to the alkaline hydroxide leach step 20 and hydrochloric acid solution is directed to acid leach step 42, crystallisation step 52 and crystallisation step 64. Electrolysis step 78 is not essential to the process of the present invention and it is envisaged that hydrochloric acid and alkaline hydroxide may be obtained from any source.

[0098] It is to be understood that other processes for the regeneration of acid may be utilised without departing from the scope of the present invention.

[0099] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.EXAMPLE 1

[0100] 1 ,000 kilograms of alunite ore was prepared as a feed stock. The alunite ore comprising 28% AI2O3, 7.2% K2O, 3.2% Na2O, 3% Fe2O3, 27.5% SO3 and 15.8% SiC>2, being 280kg of AI2O3, 275kg SO3, 72kg of K2O and 158kg SiO2.

[0101] The feed stock was slurried in 5,000L of water, at 90°C, prior to addition of 250kg of KOH on an anhydrous basis. KOH mass will depend upon the water content of the KOH added but in this example comprises 295kg of 85% KOH pearl. The resulting slurry was filtered and the aluminous residue is collected, with a mass of 680kg which assays approximately 40.4% AI2O3, 2.31 % K2O, 0.18% Na2O, 5.08% Fe2Os, 3.15% SO3, and 30.59% SiO2. This represents 274kg of AI2O3, 21 kg SO3, 34kg Fe2Os, 12kg Na2O, 206kg SiO2 and 15kg K2O.

[0102] The filtrate containing potassium sulphate comprises a solution containing 27kg Na2O, 254kg SO3 and 350kg K2O. This forms approximately 500kg of K2SO4 product once crystallised, with waste of a sodium chloride salt of approximately 80kg.

[0103] The aluminous residue was leached in sufficient 32% HCI, resulting in extraction of 235kg of AI2O3, 32kg Fe2Os, 14kg K2O, and 20kg SO3, with negligible SiO2.The resultant liquor is filtered and gas sparged with hydrogen chloride, recovering 230kg of AI2O3, and <0.1 Kg Fe2Os; the remaining metals then being disposed in waste acid.

[0104] The 230kg of AI2O3 was further purified into pure aluminium hexachlorohydrate (ACH) at 15% AI2O3, comprising 1 ,530kg of ACH. The 1 ,530kg of ACH was then calcined to produce 230kg of AI2O3 with a purity exceeding 99.99% AI2O3. The remainder of the mass is water and chlorine, recovered as HCI.

[0105] The process produced a silicate waste totalling 206kg or 20.6% of the initial mass of ore. This comprises 158kg SiC>2, 44kg AI2O3, 2kg Fe2Os, 1 kg K2O, and 1 kg Na2O. This is then disposed of appropriately in any method as may be required.

Claims

Claims1. A method for the preparation of alumina from an alunite material, the method comprising the steps of: subjecting the alunite material to an alkaline hydroxide leach step; subjecting the product of the alkaline hydroxide leach step to a solid liquid separation to produce a liquid product comprising potassium sulfate and a solid residue comprising alumina; subjecting the solid residue containing alumina to an acid leach step to produce an aluminous leach solution and separating undissolved solids; subjecting the aluminous leach solution to a crystallisation step to crystallise an aluminium salt; and subjecting the aluminium salt to a calcination step to produce alumina.

2. The method according to claim 1 , wherein the alumina is in the form of high purity alumina (HPA).

3. The method according to claim 2, wherein the purity of the high purity alumina (HPA) is at least 99.99%4. The method according to any one of the preceding claims, wherein the alkaline hydroxide leach step comprises the contact of the alunite material with an alkaline hydroxide solution.

5. The method according to claim 4, wherein the alkaline hydroxide is selected from potassium hydroxide, sodium hydroxide or a combination thereof.

6. The method according to claim 4, wherein the alkaline hydroxide leach step utilises potassium hydroxide at a molar ratio of 0.1 to 4 moles of KOH per mole of alumina in the alunite material.

7. The method according to claim 4, wherein the alkaline hydroxide leach step utilises potassium hydroxide at a molar ratio of 0.1 to 3 moles of KOH per mole of alumina in the alunite material.

8. The method according to claim 4, wherein the alkaline hydroxide leach step utilises potassium hydroxide at a molar ratio of 2 to 3 moles of KOH per mole of alumina in the alunite material.

9. The method according to any one of the preceding claims, wherein the method further comprises the step of: recovering potassium from the liquid product comprising potassium sulfate10. The method according to claim 9, wherein the liquid product comprising potassium sulfate is subjected to a crystallisation step to recover a sulphur of potash product.

11. The method according to any one of the preceding claims, wherein the acid leach step comprises the contact of the solid product containing alumina with an acidic solution.

12. The method according to claim 11 , wherein the acidic solution is selected from hydrochloric acid, sulphuric acid, nitric acid or a combination thereof.

13. The method according to any one of the preceding claims, wherein the aluminous leach solution is subjected to one or more impurity removal steps prior to the crystallization of the aluminium salt.

14. The method according to any one of the preceding claims, wherein the aluminium salt is subjected to a purification step to produce a purified aluminium salt prior to the calcination step.

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