TECHNICAL FIELD

[0001 ] The present invention relates to a method for producing valuable products from bauxite residue produced via the Bayer process, waste known as red mud. More particularly, the present invention relates to a method for recovering valuable products including but not limited to, aluminium and iron from red mud using a combination of pyrometallurgy and hydrometallurgy.

BACKGROUND ART

[0002] Red mud is a very significant waste stream from the production of aluminium by the Bayer Process. Red mud is the result of the precipitation of, predominantly, iron from the alkaline solution which retains the majority of the aluminium. In addition to iron some Bayer Process operations also precipitate a significant fraction of their aluminium and silica making the red mud a poorly crystalline mixture of iron (hydr-)oxides, aluminium (hydr-)oxides and silica. Also reporting to red mud are any insoluble phases present in the original bauxite.

[0003] There have been many attempts to reuse, recycle or otherwise valorise the red mud but thus far, none has been successful. This is considered to be due to the low value of the products, high reagent consumption and high cost of production failing to generate sufficient revenue.

[0004] Prior patent AU 2014339746 taught a method whereby red mud could be converted to high purity alumina (HPA), high purity iron oxides and high purity silica by a process of calcination, leaching, precipitation of iron, solvent extraction of aluminium, precipitation of aluminium and calcination to HPA. The process taught in AU 2014339746 removes the iron from the solution by adjusting pH using a base, resulting in precipitation of the iron, but also the loss of a significant fraction of the dissolved aluminium. The loss of the aluminium fraction from the process stream has been considered as an impediment to widespread adoption of the process taught in AU 2014339746.

[0005] Any process which can overcome these impediments would have a significant impact on the aluminium industry and allow recycling of waste red mud. [0006] The present invention has as one object, to provide a method which can overcome the abovementioned problems, or to at least provide a useful alternative.

[0007] Throughout this 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.

[0008] The discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.

SUMMARY OF INVENTION

[0009] It has been found that it is possible to extract valuable aluminium and iron products from red mud using a process comprising the steps of leaching the red mud, selectively extracting the iron, which can be further processed to produce iron oxide, selectively extracting the aluminium, precipitating the aluminium and calcining the aluminium precipitate to HPA. There does not appear to have been any previous reports of this occurring.

[0010] In accordance with the present invention, there is provided a method for recovering iron and aluminium from red mud, the method comprising the steps of:

Leaching the red mud in hydrochloric acid, removing the insoluble material, sequentially extracting the iron and aluminium from solution, separately treating the iron-rich solution and aluminium -rich solution to produce high value products.

[0011 ] In accordance with the present invention, there is provided a method for recovering iron values and aluminium values from a residue comprising: optionally adjusting the pH of the residue from alkaline toward neutral; separating a solid component therefrom and calcining said solid component to produce a calcined solid; leaching the calcined solid in aqueous acid to dissolve said iron values and said aluminium values, and separating a solubilised leachate therefrom; performing on said solubilised leachate, a first solvent extraction of iron values using an iron values preferencing immiscible organic extractant to produce an iron values loaded organic extractant and a raffinate stripped of iron values; stripping iron values from said iron values loaded organic extractant into aqueous solution to form an iron values loaded aqueous solution; optionally recovering acid from or neutralising acid in said raffinate; performing a second solvent extraction of aluminium values on said raffinate using an immiscible organic extractant to produce an aluminium values loaded organic extractant; and stripping aluminium values from said aluminium values loaded organic extractant into aqueous solution to form an aluminium values loaded aqueous solution. This may subsequently be followed by recovery of said aluminium values from the aluminium values loaded aqueous solution.

[0012] Preferably said step of separating said solid component and calcining said solid component are performed separately.

[0013] Preferably said step of separating said solid component from the residue is performed using a filtration process.

[0014] Preferably said step of calcining said solid component is performed under conditions sufficient to convert aluminium values contained therein substantially to amorphous phases.

[0015] Preferably said step of calcining said solid component is performed at a temperature of from 350 to 600 degrees Celsius, for a period of at least 0.5 hours.

[0016] Preferably the residue is alkaline, and the step of adjusting the pH of the residue comprises adjusting the pH within a range of 7 to 10, by mixing with seawater and/or acid. The acid may be selected from sulphuric acid, nitric acid, phosphoric acid or hydrochloric acid, although hydrochloric acid is most preferred.

[0017] Preferably the step of leaching the calcined solid is performed using aqueous acid in sufficient quantity to produce said solubilised leachate containing free acid in solution.

[0018] Preferably the step of leaching the calcined solid is performed using hydrochloric acid.

[0019] Preferably said iron values preferencing organic extractant is selected from the group comprising liquid organophosphorus extractants, amine extractants and/or combinations thereof. [0020] Preferably the step of stripping iron values from said iron values loaded organic extractant into aqueous solution is performed using water. This may be followed by recovery of iron values therefrom.

[0021 ] Preferably the step of recovering acid from said raffinate is performed using a nanofiltration array, wherein said acid is recovered as permeate and retentate forms said raffinate depleted of acid.

[0022] Preferably said immiscible organic extractant is an organophosphorus extractant.

[0023] Preferably, prior to the step of stripping aluminium values from said aluminium values loaded organic extractant, said aluminium values loaded organic extractant is subjected to treatment with an aqueous scrub solution to remove impurities.

[0024] Preferably the aqueous scrub solution comprises a weak acid solution and/or a solution of an aluminium salt.

[0025] Preferably the step of stripping aluminium values from said aluminium values loaded organic extractant into aqueous solution is performed using an acidic aluminium depleted aqueous solution.

[0026] Preferably the step of recovery of said aluminium values comprises adjusting the pH of said aluminium values loaded aqueous solution using an alkali in optimum quantity to produce an aluminous precipitate. This may be performed at elevated temperature or elevated temperature and elevated pressure. This may also be assisted with the introduction of aluminous seed crystals.

[0027] The aluminous precipitate and solution is preferably aged, preferably at elevated temperature in order to agglomerate and settle the precipitate, prior to its separation by filtration from the aqueous phase.

[0028] The solid residue can be calcined to recover purified alumina.

[0029] The residue may be a red mud waste stream from the production of aluminium by the Bayer Process.

Pre-treatment of red mud - Neutralisation [0030] Preferably, the red mud is neutralised such that the pH of the slurry remains below around 10.5, 10.0, 9.5, 9.0, 8.5, 8.0, 7.5, or 7.0.

[0031 ] Preferably, the red mud is neutralised to a pH between 7.0 and 1 1 , more preferably between 7.5 and 10.5, yet more preferably between 8.0 and 10.0. Most preferably, the red mud is neutralised to a pH between 8.5 and 9.5.

[0032] Preferably, the red mud is neutralised by mixing with sea water. Preferably, the red mud is neutralised by adding an acid. Preferably, the acid used selected from the list: sulphuric acid, nitric acid, phosphoric acid or hydrochloric acid. In the most preferable version, the acid used is recycled from a subsequent stage within the process.

[0033] As will be recognised by those skilled in the art, there will be red muds which do not required any neutralising pre-treatment and may be used unadulterated.

Calcination preferences

[0034] After solid-liquid separation the neutralised red mud is calcined to render the aluminium soluble in the subsequent leaching stage. Without wishing to be bound by theory, the calcination converts unleachable aluminium phases to amorphous phases which are soluble during leaching.

[0035] Preferably, the neutralised red mud is calcined at least at 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, or 700°C.

[0036] Preferably, the neutralised red mud is calcined at up to 800°C.

[0037] Preferably, the neutralised red mud is calcined at between 300 and 800°C, and more preferably between 400 and 700°C. Most preferably, the neutralised red mud is calcined at between 400 and 600°C.

[0038] Preferably, the neutralised red mud is calcined for a period between 0.25 and 24.0 hours, more preferably between 0.25 and 12.0 hours, more preferably between 0.25 and 6.0 hours, and yet more preferably between 0.25 and 4.0 hours. More preferably, the neutralised red mud is calcined for a period between 0.25 and 3.0 hours. Most preferably, the neutralised red mud is calcined for a period between 0.25 and 2.0 hours.

[0039] Preferably, the neutralised red mud is calcined in air. [0040] As will be recognised by those skilled in the art, for each and every neutralised red mud there will be an optimum combination of calcination conditions, particularly time and temperature which will result in the highest recovery during subsequent processing.

Leaching of the calcined red mud

[0041 ] After calcination, the calcined neutralised red mud is leached in acid to dissolve the aluminium and the iron.

Acid preferences

[0042] A person skilled in the art will understand that if the concentration of the acid is too dilute, there is too much needless fluid to handle, and too concentrated might lead to equipment or handling issues.

[0043] Preferably, the concentration of the hydrochloric acid used is between 100 g/L and saturation. Preferably, the concentration of the hydrochloric acid used is between 1 00 g/L and 350g/L saturation. More preferably, the concentration of the hydrochloric acid used is between 150 g/L and 300g/L. Most preferably, the concentration of the hydrochloric acid used is between 200g/L and 300g/L

[0044] Preferably, the leaching occurs between 50°C and the boiling point of the acid solution, more preferably between 60°C and the boiling point of the acid solution, more preferably between 70°C and the boiling point of the acid solution, more preferably between 80°C and the boiling point of the acid solution, and yet more preferably between 90°C and the boiling point of the acid solution. Most preferably, the leaching occurs between 100°C and the boiling point of the acid solution.

[0045] Preferably, the leaching takes place at atmospheric pressure.

[0046] Preferably, the leaching takes place for between 0.05 and 6h, more preferably between 0.1 and 4h, more preferably between 0.5 and 4h, and yet more preferably, between 1 .0 and 4h. Most preferably, the leaching takes place for between 1 .0 and 3h.

[0047] Preferably, after leaching the solution will contain between 10 and 250g/L of free acid, more preferably between 10 and 150g/L of free acid, more preferably between 10 and 10Og/L of free acid, and yet more preferably, between 20 and 75g/L of free acid. Most preferably, after leaching the solution will contain between 20 and 50g/L of free acid. A person skilled in the art will understand that in the leaching process, a slurry comprising acid solution and calcined red mud is formed. The density/viscosity of the slurry is a function of the solubility of target aluminium values in the calcined red mud, and slurry density/viscosity needs to be tailored to maximise the leaching of those aluminium values.

[0048] As will be recognised by those skilled in the art, for each and every calcined neutralised red mud there will be an optimum combination of leaching conditions, particularly acid strength, leaching time and leaching temperature which will result in the highest extent of dissolution of the target metal.

[0049] After leaching, the slurry is separated using conventional means into a solution containing the soluble elements and a solid material composed of insoluble phases. Without wishing to be bound by theory, the solid phase will be predominantly silica-based.

Iron solvent extraction

[0050] After separation, the solution is treated to selectively remove iron using solvent extraction.

[0051 ] Preferably, the concentration of iron extractant in the organic solution is between 0.5 and 95vol%, more preferably between 1 and 80vol%, more preferably between 5 and 70vol%, more preferably between 10 and 60vol%, and yet more preferably, between 20 and 60vol%. Most preferably, the concentration of iron extractant in the organic solution is between 30 and 60vol%.

[0052] In one form of the invention, the iron extractant is selected from the group: liquid organophosphorus extractants, amine extractants and/or combinations thereof. While untested, it is considered that some carboxylic acids may prove suitable as iron preferencing extractants.

[0053] In a preferred form of the invention, the organophosphorus extractant is a derivative of phosphoric, phosphonic, phosphinic or dithiophosphinic acid.

[0054] In a highly preferred form of the invention, the amine extractant is a trialkyl amine or a quaternary amine or a derivative thereof.

[0055] In a highly preferred form of the invention, the organophosphorus extractant is tributyl phosphate, also known as TBP, with the formula OP(OC4Hg)3 and has the CAS Number 126-73-8. [0056] In a highly preferred form of the invention, the organophosphorus extractant is trioctylphosphine oxide, also known as TOPO, with the formula OP(CsHi7)3 and has the CAS Number 78-50-2.

[0057] In a highly preferred form of the invention, the amine extractant is N,N-dioctyl-1 - octanamine, also known as Alamine 3365; Alamine 336S; Farmin 08; Octanamine; Tricaprylamine; Tridioctylamine; Tri-n-octylamine; Tri-n-caprylylamine; Trioctylamine with the CAS number 1 1 16-76-3. Most commonly, the extractant is the majority component of a mixture of tri C8-10 Alkyl Amines known as Alamine 336, CAS number 57176-40-6

[0058] In a highly preferred form of the invention, the amine extractant is N-Methyl-N,N,N- trioctylammonium chloride, also known as Aliquat 336, Starks' catalyst; Tricaprylmethylammonium chloride, Methyltrioctylammonium chloride, CAS number 63393-96-4

[0059] In a preferred form of the invention, 1 -decanol, CH ₃ (CH ₂ ) ₉ OH, CAS number 1 12- 30-1 , is added at 2-20volume% to the organic phase. In a highly preferred form of the invention 1 -decanol is added at 8-12volume% to the organic phase.

[0060] In a preferred form of the invention the kerosene used contains 50-100% aliphatic hydrocarbons. In a preferred form of the invention the kerosene used contains 80-100% aliphatic hydrocarbons. In a highly preferred form of the invention the kerosene used contains 90-100% aliphatic hydrocarbons.

Iron stripping

[0061 ] The organic solution from the solvent extraction iron is treated to recover the iron. This is achieved by contacting the iron-loaded organic solution with an iron-depleted aqueous solution such that the iron transfers back in to the aqueous phase from the organic phase giving an iron-depleted organic phase and an iron-enriched aqueous phase.

[0062] The iron-depleted aqueous solution is selected from the group: water and acid.

[0063] In a preferred form of the invention the iron-depleted aqueous solution is water.

[0064] In a preferred form of the invention the acid is selected from the group of mineral acids: hydrochloric, sulphuric, nitric and phosphoric, or mixtures thereof. [0065] Preferably, the concentration of the acid used contains up to around 5g/L, 10g/L, 20g/L, or 50g/L of free acid.

[0066] In a preferred form of the invention, the iron-depleted aqueous solution is a solution obtained from a previous contact between an iron-depleted aqueous solution and an iron- loaded organic solution. In this arrangement the aqueous and organic phases in the solvent exchange run counter-current.

[0067] The iron-enriched aqueous solution can be treated to produce an iron-bearing product by one, or more of the following methods: electrowinning, oxidation, precipitation and crystallisation.

Acid recovery

[0068] The aluminium-bearing, iron-depleted solution remaining after the iron is extracted, the raffinate, is optionally treated to recover acid from the solution using one, or more, of: ultrafiltration, nanofiltration, reverse osmosis, distillation, evaporation, and solvent extraction. Whether it is justified to recover the acid, depends on the concentration of the acid in the raffinate. If the acid is too dilute, it may be more cost effective to neutralise it before sending it to tails, or simply to send it to the stockpile of red mud awaiting processing.

[0069] Preferably the acid is removed using nanofiltration.

[0070] Preferably, the acid may be further concentrated using one, or more, of: ultrafiltration, nanofiltration, reverse osmosis, distillation and evaporation.

[0071 ] Most preferably, the acid recovered is recycled to the, optional, initial neutralisation or the leach stage.

Aluminium Solvent extraction

[0072] After iron removal, the solution is treated to selectively remove aluminium from the solution using one or more of ion exchange and solvent extraction.

[0073] Preferably, the aluminium is removed using solvent extraction. The aluminium is transferred from the aqueous phase into an immiscible organic phase containing an extractant. [0074] Preferably, the concentration of aluminium extractant in the organic solution is greater than 0.1vol%, 1vol%, 5vol%, 7vol%, 10vol%, 15vol%, 20vol%, 30vol%, 40vol%, or 50vol%.

[0075] Preferably, the concentration of aluminium extractant in the organic solution is between 0.5 and 50vol%, more preferably between 1 and 50vol%, more preferably between 5 and 50vol%, more preferably between 10 and 50vol%, more yet preferably between 10 and 30vol%. Most preferably, the concentration of aluminium extractant in the organic solution is between 15 and 25vol%.

[0076] Preferably, the temperature at which the Al SX operates is greater than 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, or 50°C.

[0077] In a preferred form of the invention the temperature at which the Al SX operates is between 10 and 60°C, more preferably between 20 and 50°C, and more preferably between 20 and 40°C. In a most preferred form of the invention the temperature at which the Al SX operates is between 30 and 40°C.

[0078] In a preferred form of the invention the diluent is 50-100% aliphatic, more preferably 60-100% aliphatic, more preferably 70-100% aliphatic, and more preferably 80-100% aliphatic. In a highly preferred form of the invention the diluent is 90-100% aliphatic.

[0079] In one form of the invention, the aluminium extractant is selected from the group: liquid organophosphorus extractants or combinations thereof.

[0080] In a preferred form of the invention, the organophosphorus extractant is a derivative of phosphoric, phosphonic, phosphinic or dithiophosphinic acid.

[0081 ] In a highly preferred form of the invention, where there is an organophosphorus extractant, the organophosphorus extractant is Bis(2-ethylhexyl) hydrogen phosphate (also known as, Bis(2-ethylhexyl) phosphoric acid, Bis(2-ethylhexyl) phosphate, Bis(2- ethylhexyl) hydrophosphoric acid, BEHPA, BEHP, BEHHPA, BEHHP, Di-(2-ethylhexyl) phosphoric acid, DEHPA, D2EHPA, (CsHnO^PC H and has the CAS Number 298-07- 7).

Phase Modifier Preferences [0082] In one preferred form of the invention a phase modifier is added to prevent formation of three phases during the extraction process.

[0083] In one preferred arrangement of the invention, the phase modifier is present at 1 - 50vol%, more preferably at 2.5-40vol%, more preferably at 5-30vol%, and more preferably at 5-20vol%. In a most preferred form of the invention the phase modifier is present at 5-15vol%.

[0084] In a preferred form of the invention the phase modifier is an organic alcohol. In a highly preferred form of the invention the phase modifier is 1 -decanol (isodecanol, n-decanol, CAS 112-30-1 , C10H22O). In a most highly preferred form of the invention the phase modifier is tributyl phosphate (TBP, CAS 126-73-8, C12H27O4P).

[0085] As will be understood by a person skilled in the art, a base may be added to overcome protons evolved during the solvent extraction.

Aluminium scrubbing

[0086] The organic solution from the solvent extraction aluminium is treated to recover unwanted impurities. This is achieved by contacting the aluminium-loaded organic solution with an aqueous scrub solution such that the impurities transfer into the aqueous phase from the organic phase giving an impurity-depleted organic phase and an impurity- enriched aqueous phase.

[0087] Preferably, the scrub solution is a weak acid. Preferably, the acid is selected from the group: sulphuric, hydrochloric, nitric, phosphoric acid. More preferably, the acid is sulphuric acid. Most preferably, the acid is hydrochloric acid. Preferably, the scrub solution is a solution of aluminium which is substantially free of the impurities. Most preferably, the scrub solution is the aluminium solution obtained by stripping the loaded organic.

[0088] As would be recognised by those skilled in the art, the optimum conditions for scrubbing will vary according to the level and nature of the impurities present within the organic phase. The pH of the acid scrub solution should be sufficient to selectively extract the impurities whilst leaving the aluminium within the organic phase.

[0089] As would be recognised by those skilled in the art, the optimum aluminium concentration in the scrub solution will vary according to the level and nature of impurities present within the organic phase. The aluminium concentration should be sufficient to selectively extract the impurities whilst leaving the aluminium within the organic phase.

[0090] Typically, and preferably, the pH of the impurity-enriched aqueous phase is between 0.5 and 4. The pH range is defined by the solvent extraction isotherm, and the most suitable narrow pH range will vary according to the ionic strength of the aqueous phase and the particular ions that are present as impurities.

[0091 ] Preferably, the aluminium concentration in the scrub solution is between 0.1 g/L and saturation of the relevant salt, more preferably between 1 .Og/L and saturation of the relevant salt, more preferably between 5. Og/L and saturation of the relevant salt, more preferably between 10g/L and saturation of the relevant salt, more preferably between 25g/L and saturation of the relevant salt, more preferably between 50g/L and saturation of the relevant salt, more preferably between 75g/L and saturation of the relevant salt, and more preferably between 10Og/L and saturation of the relevant salt. Most preferably, the aluminium concentration in the scrub solution is between 150g/L and saturation of the relevant salt.

Aluminium stripping

[0092] The organic solution from the solvent extraction aluminium is treated to recover the aluminium. This is achieved by contacting the aluminium-loaded organic solution with an aluminium-depleted aqueous solution such that the aluminium transfers back in to the aqueous phase from the organic phase giving an aluminium-depleted organic phase and an aluminium-enriched aqueous phase.

[0093] The aluminium-depleted aqueous solution is an acidic solution.

[0094] In a preferred form of the invention the acid is selected from the group of mineral acids: hydrochloric, sulphuric, nitric and phosphoric, or mixtures thereof.

[0095] More preferably, the acid is sulphuric acid. Most preferably, the acid is hydrochloric acid.

[0096] Preferably, the concentration of the acid used will be between 10 and 1000g/L of free acid, more preferably between 20 and 500g/L of free acid, more preferably between 25 and 250g/L of free acid, and yet more preferably between 50 and 200g/L of free acid. Most preferably, the concentration of the acid used will be between 50 and 150g/L of free acid.

[0097] Preferably, the pH of the aluminium-enriched aqueous solution after contact will be between 0 and 3, more preferably between 0.5 and 3, more preferably between 1 .0 and 3, more preferably between 1 .5 and 3, and more preferably between 2.0 and 3. Most preferably, the pH of the aluminium-enriched aqueous solution after contact will be between 2.0 and 2.5.

[0098] The aluminium-enriched aqueous solution can be treated to produce an aluminium-bearing product by one, or more of the following methods: precipitation and crystallisation.

Aluminium precipitation

[0099] In a preferred form of the invention, the aluminium is recovered by precipitation.

[00100] Preferably, the precipitation is achieved by raising the pH to a range of 4- 11 , more preferably?- 10, and yet more preferably to a range of 8-10. Most preferably, the precipitation is achieved by raising the pH to a range of 9-10.

[00101] As will be recognised by those skilled in the art, aluminium is amphoteric and is soluble in both acid and alkaline solutions. Thus, the optimum pH is below that where the solubility substantially increases due to formation of the tetrahydroaluminate ion, AI(OH) ₄ -.

[00102] The precipitating agent is selected from the list: sodium hydroxide, potassium hydroxide, calcium oxide, calcium hydroxide, ammonia, magnesium hydroxide, magnesium oxide.

[00103] In a preferred embodiment, the precipitating agent is soluble in water.

[00104] In a most preferred embodiment, the precipitating agent is aqueous ammonia, NH4OH.

[00105] In a preferred embodiment the precipitating agent is anhydrous ammonia.

[00106] In a preferred embodiment, the precipitation produces pseudoboehmite.

[00107] Preferably the precipitation will be performed at elevated temperature. [00108] Preferably the precipitation will be performed between 30°C and the boiling point of the solution, more preferably between 40°C and the boiling point of the solution, more preferably between 50°C and the boiling point of the solution, more preferably between 60°C and the boiling point of the solution, more preferably between 70°C and the boiling point of the solution, and yet more preferably between 80°C and the boiling point of the solution. Most preferably the precipitation will be performed between 90°C and the boiling point of the solution.

[00109] As will be recognised by those skilled in the art, the precipitation may also take place in an autoclave utilising an elevated pressure to prevent the boiling of the solution above its boiling point at atmospheric pressure.

[00110] In a preferred embodiment the precipitate will be aged at elevated temperature.

[0011 1] Preferably the aging will be performed between 30°C and the boiling point of the solution, more preferably between 40°C and the boiling point of the solution, more preferably between 50°C and the boiling point of the solution, more preferably between 60°C and the boiling point of the solution, more preferably between 70°C and the boiling point of the solution, and yet more preferably between 80°C and the boiling point of the solution. Most preferably the aging will be performed between 90°C and the boiling point of the solution.

[00112] Preferably the aging will have a duration of more than 30min.

[00113] Preferably the aging will have a duration between 60min and 72h, more preferably between 2h and 72h, and yet more preferably between 4h and 48h. Most preferably the aging will have a duration between 8h and 24h.

[00114] As will be recognised by those skilled in the art, the aging of the precipitate may also take place in an autoclave utilising an elevated pressure to prevent the boiling of the solution above its boiling point at atmospheric pressure.

[00115] As will be recognised by those skilled in the art, aging the precipitate will allow it to form more crystalline phases. Aging will also allow larger particles to form which will aid solid liquid separation. [00116] In an alternative preferred embodiment, or additionally, seed crystals are added to the precipitation stage. The seed crystals may be derived by recycling the finer particle sizes separated from the product precipitate.

[00117] In a preferred form of the invention, the precipitate is washed prior to drying and/or calcination. In a preferred form, the washing solution is high purity water.

Calcination of precipitate

[00118] The precipitate is separated from the solution, and calcined to drive off water and produce alumina, AI2O3.

[00119] In a preferred form of the invention, the precipitate is dried prior to calcination.

[00120] In a preferred form of the invention, calcination occurs in two stages. In the first stage, the precipitate is dehydrated to AI2O3. In the second stage, the AI2O3 is transformed to the alpha-A Os crystalline form. In a highly preferred form, both stages of calcination occur within a single piece of equipment through which the precipitate proceeds or is transported.

[00121] Preferably, the precipitate is calcined at above 300°C, more preferably at above 400°C, and more preferably at above 500°C.

[00122] Preferably, the precipitate is calcined between 600 and 1400°C, more preferably between 800 and 1400°C, more preferably between 1000 and 1400°C, more preferably between 1000 and 1300°C, and yet more preferably between 1100 and 1300°C. Most preferably, the precipitate is calcined between 1 150 and 1250°C.

[00123] Preferably, the precipitate is calcined for between 15 minutes and 6 hours, more preferably between 30 minutes and 6 hours, more preferably between 45 minutes and 6 hours, and yet more preferably between 1 hour and 4 hours. Most preferably, the precipitate is calcined for between 2 hours and 4 hours.

[00124] As will be recognised by those skilled in the art, a single calcination stage may incorporate both calcination stages simply by moving the precipitate through the calciner. The initial stage of dehydration will occur whilst the precipitate is heating up to the temperature required for the second stage. However, there may be advantages in separating the two calcination stages into independent calciners.

DESCRIPTION OF EMBODIMENTS

[00125] A method for valorising red mud in accordance with a preferred embodiment of the present invention is now described with reference to the flowsheet shown in Figure 1.

[00126] The red mud from tails 1 is fed into a tank 2 where it slurried with hydrochloric acid fed from feed line 3 in order to reduce the pH to around 9.5. Without wishing to be bound by theory, neutralisation of the red mud is believed to reduce the free sodium hydroxide present in the red mud by a simple acid-base reaction. Were significant amounts of sodium hydroxide to remain present there is potential for it to melt within the calcination stage which may lead to sintering of the calcined mass necessitating the addition of a grinding operation after calcination. The sodium hydroxide is also likely to undergo different chemical reactions reducing the subsequent solubility of aluminium.

[00127] The neutralised slurry is pumped into a filter 4 where the excess solution is drained to a waste holding tank 5 for disposal. The filter cake 6 is recovered from the filter 4 and fed into a rotary kiln calciner 7 where it is heated to 500°C for one hour in order to increase the solubility of the aluminium. Whilst not wishing to be bound by theory it is believed that calcination converts boehmite to an amorphous phase which is more readily acid soluble.

[00128] The calcined product 8 is fed into a reactor 9 along with 20% hydrochloric acid provided by feed line 10 and heated for 3h at 100°C such that the aluminium and iron dissolve whilst the silica remains largely insoluble. The leach slurry is pumped into a filter 1 1 , the aluminium bearing solution proceeds via feed line 13 to the iron loading stage reactor 14 of the Fe Solvent Extraction (FeSX) circuit, whilst the leach residue is discarded to tails 12.

[00129] In the iron loading stage 14, the iron-bearing solution is mixed with a solution of iron extractant comprising 50 vol% Alamine 336 and 10% 1 -decanol dissolved in kerosene input 15 from the FeSX strip stage reactor 19 of the FeSX circuit. Mixing takes place for 60 seconds during which the iron transfers from the aqueous phase into the organic phase whilst aluminium remains in the aqueous phase. The phases are allowed to settle, the iron-depleted aqueous phase proceeds via feed line 16 to acid recovery 17 whilst the iron-loaded organic phase 18 moves into the FeSX strip stage reactor 19 where it was mixed with water provided by water feed line 20 for 60 seconds transferring the iron from the organic to the water resulting in regenerated Alamine 336 which is recycled to the FeSX loading stage via feed line 15 and an iron bearing aqueous solution which proceeds to further processing 21 .

[00130] As would be recognised by those skilled in the art, solvent extraction processes are most commonly conducted in several stages with the organic and aqueous phases moving in a counter-current manner. This arrangement is the most efficient at maximising the recovery of the metal of interest whilst also maximising the concentration of metal within the organic phase. The aqueous feed is contacted with an organic phase within which there is also a high, but not maximised, concentration of metal, a small amount of metal is transferred into the organic maximising the concentration of metal within the organic. The fully loaded organic phase then proceeds to scrub or stripping stages. The slightly depleted aqueous phase proceeds to a second stage where it is contacted with an organic phase which is less concentrated and further metal transfers increasing the concentration within the organic. Such stages continue until the largely depleted aqueous phase is contacted with freshly regenerated organic from the strip stages.

[00131] As would be recognised by those skilled in the art, the conditions required for optimum solvent extraction of Fe and Al will need to be determined by testwork for each individual red mud. Typical parameters investigated are concentration of the extractant, concentration of phase modifiers, organic to aqueous ratio during mixing and temperature of operation. For example, a lower concentration of iron in the leach solution would typically require a lower concentration of extractant and/or a lower ratio of aqueous to organic in the mixing unit.

[00132] The acid recovery unit 17 is a nanofiltration array which reduces the acid concentration in the aluminium-bearing solution resulting in hydrochloric acid permeate which is recycled within the process to feed line 3 and 10. Additional hydrochloric acid reagent may be added from a reagent tank (not shown) to compensate for losses and for initial plant start up. The retentate comprising acid-depleted aluminium-bearing solution proceeds via feed line 22 into a mixer 23 being part of an Al Solvent Extraction (Al SX) circuit along with a 20 vol% solution of aluminium extractant D2EHPA and 10 vol% of tributyl phosphate in kerosene 24.

[00133] In the mixer 23, the aluminium-bearing aqueous solution and the organic phase are mixed for 120 seconds during which the aluminium transfers from the acid-depleted aluminium-bearing solution into the organic phase releasing protons. An ammoniacal solution 25 is added to prevent lowering of the pH.

Al ³⁺ + 3RH = AIR3 + 3H ⁺ the organic phases are italicised

3H ⁺ + 3NH ₃ = 3NH ₄ ⁺

[00134] The then aluminium-depleted aqueous phase is separated from the aluminium-loaded organic phase and disposed to waste 26. The aluminium-loaded organic phase is pumped via feed line 27 into a reactor 28 along with a concentrated aluminium solution using feed line 29. The concentrated aluminium solution may be a small volume derived from the aluminium strip solution 33. Any impurities co-extracted with the aluminium in the loading stage 23 will be displaced from the organic phase by the aluminium in the aqueous solution 29. The impurity enriched solution 30 is disposed of, or recycled within the process.

2AI ³⁺ + 3CaR2 = 2AIR3 + 3Ca ²⁺ the organic phases are italicised

[00135] The impurity-depleted organic phase 31 is then mixed with hydrochloric acid from feed line 16 in a reactor 32 forming part of the Al SX circuit. The aluminium is transferred from the organic 31 to an aqueous solution 33 regenerating the extractant which is recycled using feed line 24.

AIR3 + 3H ⁺ = Al ³⁺ + 3RH the organic phases are italicised

[00136] As with the Fe SX circuit, and as would be recognised by those skilled in the art, the Al SX circuit is also conducted in several stages with the organic and aqueous phases moving in a counter-current manner.

[00137] The aluminium solution 33 is fed into a precipitation unit 34 along with an ammonia solution 35 to form an aluminium precipitate. Precipitation may be assisted by adding seed crystals derived from solids 39 from the following separation stage.

Al ³⁺ + 3NH ₄ OH = AI(OH) ₃ (S) + 3NH ₄ ⁺ [00138] The resultant slurry 36 is fed into a filter 37 where the solution 38 and solids 39 are separated. The solid filter cake within the filter 37 may be washed with water 40 prior to discharge, the wash water being combined with the filtrate 38. The solids 39 are fed into a calciner 41 and heated at 1250°C for 4 hours to convert the precipitate to high purity alumina 42.

2AI(OH) ₃ = AI2O3 + 3H ₂ O

[00139] Various parts of the process according to the preferred embodiment will now be illustrated with reference to a series of experimental examples. The description of the examples should not be understood to be limiting the generality of the preceding description of the invention.

EXAMPLE 1

[00140] A sample of red mud was obtained from a tailings facility. The major element analysis is given in Table 1 .

Table 1 . XRF analysis of the untreated and treated red mud

[00141] The red mud had an initial pH of ~10.5. The red mud was slurried using a minimum volume of sea water and further seawater was added until the pH was ~9.5. As would be recognised by those skilled in the art, the volume of neutralising agent added, in this case seawater, will vary according to the red mud being processed. After filtering and drying the neutralised red mud the analysis in Table 1 was obtained. The mass of the red mud increased due to the presence of the salts in the seawater remaining in the solid after neutralisation, as indicated by the increase in Na and Mg.

[00142] The dry neutralised red mud was then calcined in air for 4h at 550°C. The calcined red mud was analysed as per Table 1 . The calcination was accompanied by a mass loss of ~6%. The data in Table 2 shows the relative phase abundances as determined by semiquantitative X-ray diffraction, the error is typically 1 -2%. As is clear, the major effect of the calcination stage decreases the boehmite and increases the amorphous and hematite content.

Table 2. XRD phase abundance in untreated and treated red mud.

[00143] Without wishing to be bound by theory, it is believed that the increase in amorphous content is largely due to the decomposition of boehmite, the increase in hematite also suggests that some crystallisation of amorphous iron oxides has also occurred during calcination. The high fraction of aluminium present in the amorphous phase is believed to be the major reason for the increase in aluminium dissolution during subsequent leaching.

EXAMPLE 2

[00144] A sample of neutralised and calcined red mud was prepared using the methodology outlined in Example 1 . 6.2L of 32% hydrochloric acid were heated to ~80°C and 333g of calcine added, further batches of 333g were added at 10 and 20minutes for a total of 1000g of calcine. The solution was heated for a total of 60min at which time the solution was allowed to cool, filtered and analysed. The major elements in solution (>1 .Og/L) were 21.6g/L Al, 22.35g/L Fe, 3.35g/L Ca and 2.67g/L Na. The aluminium recovery was found to be 71 % and iron recovery was 84%. The final acid concentration was 210g HCI/L.

[00145] Without wishing to be bound by theory, it is believed that the aluminium recovery was limited by the solubility of Al in HCI. It is well known by those skilled in the art that the solubility of Al decreases from 62.6g/L in the absence of HCI to 2.1 g/L in 32wt% HCI. A lower slurry density is likely to lead to higher extents of aluminium dissolution. EXAMPLE 3

[00146] A solution was prepared by leaching red mud in concentrated hydrochloric acid for 1 h at 95°C and filtering. The solution contained 22.35g/L Fe and 21.60g/L Al. 2000mL of this solution was mixed for 60s with 1997mL of a water-immiscible solution containing 50vol% Alamine 336 dissolved in kerosene. Analysis of the aqueous phase after the phases were settled and separated showed 0.008g/L of Fe and 21 ,6g/L Al.

[00147] Clearly, >99.96% of the iron had been extracted from the solution whilst there was no discernible extraction of the aluminium.

EXAMPLE 4

[00148] A solution of iron-free aluminium was obtained from leaching a sample of red mud in hydrochloric acid for 1 h at 95°C, filtering and removing iron by precipitation. The solution contained 7.52g/L of Al. 500mL of this solution was mixed with 500mL of a water-immiscible solution containing 20vol% D2EHPA and 10vol% TBP dissolved in kerosene. 84mL of a sodium hydroxide solution was added to maintain the pH at ~2.0. Once the pH was stable, the phases were settled and separated, the aqueous solution contained 2.32g/L Al. Clearly 64% of the aluminium had been transferred from the aqueous solution to the organic solution in a single extraction stage.

[00149] The Al-loaded organic solution was contacted with l OOmL of water to remove any entrained solution thereby reducing the level of impurities in the organic phase. After settling, the aqueous was tapped off and discarded.

[00150] The organic phase was then mixed with 100mL of ultrapure water and 55.5mL of ultrapure HCI added to give a pH of 1.0, sufficient for the aluminium to be largely extracted from the organic back into the aqueous phase. The resultant aqueous phase contained 9.53g/L of aluminium, thus 57% of the Al was stripped in a single stage.

[00151] As would be recognised by those skilled in the art, further stages of extraction and stripping can be reasonably expected to increase the initial loading of the organic and the stripping of the organic. Operation in a counter current manner is most efficient.

EXAMPLE 5 [00152] A solution of aluminium containing 9.53g/L Al was obtained by stripping an Al-loaded organic phase. The solution was heated to 80°C and 25% NH3 solution added slowly until the pH was 6-7. The slurry was held at >80°C for 1 h prior to recovering the precipitate by filtration. The aqueous filtrate was analysed and found to contain 0.0003g/L of Al, >99.99% of the aluminium had been precipitated indicating the effectiveness of the process for recovering aluminium.

[00153] The precipitate formed was calcined for 4h at 1250°C and analysed. X-ray diffraction showed that the final product was a-alumina. The solid product was dissolved and analysed to show that total impurities were <100ppm indicating that the HPA was >99.99% AI2O3.