FIELD

[0001] The present disclosure relates to a process for preparing alumina, in particular to an acid regeneration system and process for preparing high purity alumina.

BACKGROUND

[0002] High purity alumina is used in a broad range of technology applications, including use as a key material in high intensity discharge lamps, LEDs, sapphire glass for precision optics, handheld devices, television screens and watch faces, synthetic gemstones for lasers, components in the space and aeronautics industry and high strength ceramic tools. It may also be used in lithium ion batteries, acting as an electrical insulator between the anode and cathode cells. A high purity specification is particularly necessary in this latter application because any significant impurities would encourage undesirable electron transport between the cells.

[0003] High purity alumina may be made directly from aluminium metal by reacting a high purity aluminium metal with an acid to produce an aluminium salt solution, subsequently concentrating the solution and spray roasting the concentrated salt solution to provide aluminium oxide powder. This method is based on the premise of preparing the high purity alumina from a high purity aluminium feedstock to reduce potential for contamination with impurities.

[0004] Alternatively, alumina may be prepared from other feedstocks, however each feedstock presents a challenge to process to a suitable level of purity as a result of impurities present in the feedstocks.

[0005] Smelter or metallurgical grade alumina may be manufactured by direct calcination of aluminium hydroxide produced from bauxite by the Bayer process. However, these calcined grades of alumina may have soda content from 0.15-0.50%, which is too high for the applications discussed above.

[0006] Aluminous clays such as kaolin comprise aluminium oxide and a relatively high silicon content in the form of silicon oxide. During leaching of such aluminous clays, a number of impurities such as iron, titanium, calcium, sodium, potassium, magnesium, and phosphorus, which are found as oxides in the aluminous clay, are leached into solution along with the aluminium.

[0007] Thus, there is a need to develop alternative and more efficient processes for consistent preparation of high purity alumina from a variety of sources of aluminium.

SUMMARY

[0008] The present inventors have undertaken research and development into developing a process for preparing high purity alumina. In particular, the inventors have identified that process hydrochloric acid typically considered by industry as a ‘waste’ stream can be counterintuitively regenerated and recycled to digest aluminium chloride hexahydrate solids for preparing high purity alumina. By using acid to redigest the solids instead of conventional water, the inventors have surprisingly reduced the overall amount of reagents and energy consumption of the process while maintaining the development of alumina having high purity.

[0009] In one aspect, there is provided a process for preparing high purity alumina from aluminium chloride hexahydrate solids, the process comprising: a) digesting aluminium chloride hexahydrate solids having one or more inorganic impurities in hydrochloric acid to produce an aluminium chloride liquor comprising aluminium chloride and the inorganic impurities in solution; b) precipitating aluminium chloride hexahydrate solids from the liquor in one or more crystallisation stage(s) such that at least some of the inorganic impurities remains in the liquor; c) separating the precipitated aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; d) processing the separated aluminium chloride hexahydrate solids to form high purity alumina; e) optionally lowering the concentration of hydrogen chloride in the hydrochloric acid process stream; and f) removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream at least a portion of which is recycled for use as the hydrochloric acid in step a).

[0010] In another aspect, there is provided a process for preparing high purity alumina from an aluminium-bearing material, the process comprising: a) digesting an aluminium-bearing material in hydrochloric acid to provide an aluminium chloride liquor comprising aluminium chloride and one or more impurities in solution; b) precipitating aluminium chloride hexahydrate solids from the liquor in one or more crystallisation stage(s) such that at least some of the inorganic impurities remains in the liquor; c) separating the precipitated aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; d) processing the separated aluminium chloride hexahydrate solids to form high purity alumina; e) optionally lowering the concentration of hydrogen chloride in the hydrochloric acid process stream; and f) removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream at least a portion of which is recycled for use as the hydrochloric acid in step a) to digest the aluminium- bearing material. [0011] In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration above the azeotropic point of the stream, and the process comprises step e) lowering the concentration of the hydrogen chloride in the hydrochloric acid process stream prior to removal of the one or more inorganic impurities. In a further embodiment, step e) comprises heating the over-azeotropic hydrochloric acid process stream under pressure to lower the azeotropic point to generate a hydrogen chloride gas stream and lower the concentration of hydrogen chloride in the hydrochloric acid process stream prior to removal of the one or more inorganic impurities. In one embodiment, the heating of the hydrochloric acid process stream lowers the hydrogen chloride concentration of the hydrochloric acid process stream to at or near the azeotropic point of the stream. In one embodiment, at least a portion of the hydrogen chloride gas stream generated by the heating of the hydrochloric acid process stream in step e) is recycled for use as the source of hydrogen chloride gas in the one or more crystallisation stage(s) in step b).

[0012] In one embodiment, the hydrochloric acid recycle stream has a hydrogen chloride concentration of between about 5 wt.% to about 30 wt.% based on the weight of the stream.

[0013] In another aspect, there is provided a high purity alumina prepared by a process according to any aspects, embodiments or examples described herein.

[0014] In another aspect, there is provided a system for preparing high purity alumina from aluminium chloride hexahydrate solids having one or more impurities, the system comprising: one or more acid digesters for digesting aluminium chloride hexahydrate solids in hydrochloric acid to produce an aluminium chloride liquor comprising the inorganic impurities; a crystallisation vessel associated with each acid digester for receiving the aluminium chloride liquor from the acid digester, and for precipitating aluminium chloride hexahydrate solids from the liquor such that at least some of the inorganic impurities remains in the liquor; optionally one or more subsequent crystallisation vessels for recrystallising the aluminium chloride hexahydrate solids; a separation means associated with the one or more crystallisation vessels for separating formed aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; optionally a stripping column for receiving the hydrochloric acid process stream from the separation means to generate a hydrogen chloride gas stream and to lower the concentration of hydrogen chloride in the hydrochloric acid process stream; a distilling means for removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream; a conduit for recycling the hydrochloric acid recycle stream to the one or more acid digesters; and a thermal treatment means for thermally treating the aluminium chloride hexahydrate solids to provide high purity alumina.

[0015] In another aspect, there is provided a system for preparing high purity alumina from an aluminium-bearing material comprising one or more impurities, the system comprising: one or more acid digesters for digesting an aluminium-bearing material to provide an aluminium chloride liquor comprising the inorganic impurities; a crystallisation vessel for receiving the aluminium chloride liquor from the acid digester, and for precipitating aluminium chloride hexahydrate solids from the liquor such that at least some of the inorganic impurities remains in the liquor; optionally one or more subsequent crystallisation vessels for recrystallising the aluminium chloride hexahydrate solids; a separation means associated with the one or more crystallisation vessels for separating formed aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; optionally a stripping column for receiving the hydrochloric acid process stream from the separation means to generate a hydrogen chloride gas stream and to lower the concentration of hydrogen chloride in the hydrochloric acid process stream; an impurity removal unit for removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream; a conduit for recycling the hydrochloric acid recycle stream to the one or more acid digesters for digesting the aluminium-bearing material; and a thermal treatment means for thermally treating the aluminium chloride hexahydrate solids to provide high purity alumina.

[0016] Other aspects and embodiments relating to the present disclosure are described herein. It will be appreciated that each example, aspect and embodiment of the present disclosure described herein is to be applied mutatis mutandis to each and every other example, aspect or embodiment unless specifically stated otherwise. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and processes are clearly within the scope of the disclosure as described herein.

BRIEF DESCRIPTION OF FIGURES

[0017] Embodiments of the present disclosure are further described and illustrated as follows, by way of example only, with reference to the accompanying drawings in which:

[0018] Figure 1: Representative flow sheet of an embodiment of the process for preparing high purity alumina;

[0019] Figure 2: Representative flow sheet of an embodiment of the process for preparing high purity alumina utilising hydrochloric acid and hydrogen chloride gas regeneration;

[0020] Figure 3: Representative flow sheet of the hydrochloric acid and hydrogen chloride gas regeneration; and [0021] Figure 4: Example of HC1- and EEO-vapor pressures for an HCI/H2O solution at different temperatures, pressures, and acid concentrations (A. Schmidt, Chem. Ing. Techn. 25, 445/62 [1953]).

DETAILED DESCRIPTION

[0022] The present disclosure describes the following various non-limiting embodiments, which relate to investigations undertaken to develop a process for preparing high purity alumina.

General terms

[0023] In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure.

[0024] With regards to the definitions provided herein, unless stated otherwise, or implicit from context, the defined terms and phrases include the provided meanings. Unless explicitly stated otherwise, or apparent from context, the terms and phrases below do not exclude the meaning that the term or phrase has acquired by a person skilled in the relevant art. The definitions are provided to aid in describing particular embodiments, and are not intended to limit the claimed invention, because the scope of the invention is limited only by the claims. Furthermore, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular.

[0025] All publications discussed and/or referenced herein are incorporated herein in their entirety.

[0026] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

[0027] Throughout this disclosure, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e., one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms “a”, “an” and “the” include plural aspects unless the context clearly dictates otherwise. For example, reference to “a” includes a single as well as two or more; reference to “an” includes a single as well as two or more; reference to “the” includes a single as well as two or more and so forth.

[0028] Those skilled in the art will appreciate that the disclosure herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the disclosure includes all such variations and modifications. The disclosure also includes all of the examples, steps, features, methods, compositions, coatings, processes, and coated substrates, referred to or indicated in this specification, individually or collectively, and any and all combinations or any two or more of said steps or features.

[0029] The term “and/or”, e.g., “X and/or Y” shall be understood to mean either “X and Y” or “X or Y” and shall be taken to provide explicit support for both meanings or for either meaning.

[0030] Unless otherwise indicated, the terms “first,” “second,” etc. are used herein merely as labels, and are not intended to impose ordinal, positional, or hierarchical requirements on the items to which these terms refer. Moreover, reference to a “second” item does not require or preclude the existence of lower-numbered item (e.g., a “first” item) and/or a higher-numbered item (e.g., a “third” item). [0031] As used herein, the phrase “at least one of’, when used with a list of items, means different combinations of one or more of the listed items may be used and only one of the items in the list may be needed. The item may be a particular object, thing, or category. In other words, “at least one of’ means any combination of items or number of items may be used from the list, but not all of the items in the list may be required. For example, “at least one of item A, item B, and item C” may mean item A; item A and item B; item B; item A, item B, and item C; or item B and item C. In some cases, “at least one of item A, item B, and item C” may mean, for example and without limitation, two of item A, one of item B, and ten of item C; four of item B and seven of item C; or some other suitable combination.

[0032] As used herein, the term “about”, unless stated to the contrary, typically refers to +/- 10%, for example +/- 5%, of the designated value.

[0033] It is to be appreciated that certain features that are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination.

[0034] Throughout the present specification, various aspects and components of the invention can be presented in a range format. The range format is included for convenience and should not be interpreted as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range, unless specifically indicated. For example, description of a range such as from 1 to 5 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 5, from 3 to 5 etc., as well as individual and partial numbers within the recited range, for example, 1, 2, 3, 4, 4.5, and 5, unless where integers are required or implicit from context. This applies regardless of the breadth of the disclosed range. Where specific values are required, these will be indicated in the specification. [0035] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The phrase “consisting of’ means the enumerated elements and no others.

Specific terms

[0036] The term “alumina” as used herein refers to aluminium oxide (AI2O3), in particular the crystalline polymorphic phases a, y, 0 and K. High purity alumina refers to AI2O3 with a purity of about 99.99%, e.g. a purity of >99.99% (4N) or a purity of >99.999% (5N) suitable for use as a key material in various applications including, but not limited to, high intensity discharge lamps, LEDs, sapphire glass for precision optics, handheld devices, television screens and watch faces, synthetic gemstones for lasers, components in the space and aeronautics industry, high strength ceramic tools, or electrical insulators in lithium ion batteries.

[0037] The term ‘aluminium -bearing material’ as used herein refers to any material with a greater than 10% content (by wt% eq. AI2O3). Examples of such aluminium- bearing materials include, but are not limited to, an acid-soluble aluminium hydroxide compound such as gibbsite (y-Al(OH)3), bayerite (a-Al(0H)3), nordstrandite, doyleite or dawsonite (NaAl(OH)2.CO3), an acid-soluble aluminium oxyhydroxide compound such as diaspore (a-AlO(OH)) or boehmite (y-AlO(OH)), tricalcium aluminate hexahydrate (TCA), or Al -substituted iron hydroxy oxide such as aluminous goethite (Fe(Al)OOH). The term encompasses naturally occurring materials, for example aluminous clays such as kaolin or bauxite, or products or by-products of processes. As an example, the aluminium-bearing material may be a by-product of alumina production originating from the Bayer process such as calciner dust, DSP and red mud which typically have an aluminium content of > 10 wt % (equiv. AI2O3).

[0038] As used herein, crystallisation refers to the precipitation of a solid material (the precipitate) from a liquid solution. Precipitation of the solid material occurs by converting the material into an insoluble form and/or changing the properties of the solution to reduce the solubility of the material.

[0039] The calcination of aluminium hydroxide in alumina production creates fine particulates which can be emitted as calciner dust. Calciner dust emissions may be mitigated and controlled to low levels by the use of various collection techniques such as electrostatic precipitators on the calciner stacks. ESP dust is the fine particulate residue captured by electrostatic precipitators. Calciner dust particles may comprise alumina and various aluminium (oxy)hydroxide and aluminium hydroxide compounds contaminated with occluded and surface soda.

[0040] DSP is a collective term used to describe several acid-soluble silica containing compounds which precipitate within the Bayer process. DSP is mainly Bayer-sodalite having a general formula of [NaAlSiO436.mNa2X.nH2O, in which “mNa2X” represents the included sodium salt intercalated within the cage structure of the zeolite and X may be carbonate (COs ² '), sulfate (SO4 ² '), chloride (CT), aluminate (A1O4)'). DSP forms in the ‘desilication’ circuit of the Bayer process prior to digestion circuit and also in the digestion circuit itself. DSP ultimately becomes part of bauxite residues (e.g. red mud).

Further, it will be appreciated by those skilled in the art that silica may be supersaturated in solution throughout the Bayer process, despite reducing silica content in the desilication circuit. Consequently, DSP may also form as scale on the internal surfaces of tanks, pipes and heaters.

[0041] The term ‘soda’ and ‘soda content’ as used herein refers to Na2O and the amount of Na2O present in a material, reported as a percentage by weight (wt %) per total weight of the material. It will be appreciated that the soda content of high purity alumina must be low. A reference to ‘surface soda’ relates to the presence of adsorbed Na2O on the surface of a particle, while a reference to ‘occluded soda’ relates to soda encapsulated in another material.

[0042] Calcination is a thermal treatment process in which solids are heated in the absence of, or controlled supply of, air or oxygen, generally resulting in the decomposition of the solids to remove carbon dioxide, water of crystallisation or volatiles, or to effect a phase transformation, such as the conversion of aluminium hydroxide to alumina. Such thermal treatment processes may be carried out in furnaces or reactors, such as shaft furnaces, rotary kilns, multiple hearth furnaces and fluidized bed reactors.

[0043] The term ‘atmospheric boiling point’ is used to refer to the temperature at which a liquid or slurry boils at atmospheric pressure. It will be appreciated that the boiling point may also vary according to the various solutes in the liquid or slurry and their concentration. Atmospheric pressure (also referred to as ambient pressure) is taken to be about 1 bar (e.g. 1.01325 bar).

[0044] The term ‘wt.%’ refers to the percentage amount of a substance in a stream or composition described herein on a weight basis (i.e. w/w).

Azeotrope and azeotropic points

[0045] The term ‘azeotrope’ as used herein, refers to a hydrochloric acid/water mixture comprising two or more liquids whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. The concentration at which this occurs is often called the ‘azeotropic point’ of the mixture. For example, referring to Figure 4 in relation to a hydrochloric acid/water mixture, an azeotrope at atmospheric pressure forms when the concentration of hydrogen chloride in the mixture is at or near about 19-20 wt.%. However, this concentration can increase or decrease depending on pressure as described herein.

[0046] In practice, boiling a hydrochloric acid/water mixture having a hydrogen chloride concentration below the azeotropic point at a given pressure evaporates more water than hydrochloric acid until the composition of the boiling solution reaches the azeotropic point at that pressure. On the other hand, boiling a hydrochloric acid/water mixture having a hydrogen chloride concentration above the azeotropic point at a given pressure evaporates more hydrogen chloride gas than water until the azeotropic point is reached at that pressure.

[0047] The azeotropic point of a mixture can shift depending on pressure. For example, in relation to a hydrochloric acid stream described herein, at higher pressures (i.e. pressures above atmospheric pressure) the azeotropic point of the stream is lowered, and at lower pressures (i.e. pressures below atmospheric pressure) the azeotropic point of the stream is raised. Once at or near the azeotropic point, the concentration of hydrogen chloride in the mixture cannot be altered or changed by simple distillation. The azeotropic point will be understood as being relative to the pressure and/or temperature of the stream or mixture.

Add regeneration process and system for preparing high purity alumina

[0048] The present inventors have developed a process for preparing high purity alumina. In particular, the present inventors have developed a process which recycles hydrochloric acid typically considered as ‘waste’ by industry for digesting aluminium chloride hexahydrate solids and/or aluminium-bearing material prior to crystallisation to prepare high purity alumina. According to some embodiments or examples described herein, by recycling such ‘waste’ hydrochloric acid back into the process as a liquid for preparing high purity alumina, the present inventors have surprisingly reduced the overall amount of reagents and energy consumption of the process while maintaining the development of alumina having high purity. Along with recycling the hydrochloric acid as a liquid, the process described herein can also regenerate a portion of the acid as hydrogen chloride gas for crystallising aluminium chloride hexahydrate solids from digested aluminium chloride liquors.

[0049] With reference to Figures 1 to 3, a system used to prepare high purity alumina is provided. The system comprises one or more acid digesters for digesting aluminium chloride hexahydrate solids/aluminium-bearing material in hydrochloric acid to produce an aluminium chloride liquor, a crystallisation vessel associated with each acid digester for receiving the aluminium chloride liquor from the acid digester and for precipitating aluminium chloride hexahydrate solids from the liquor such that at least some of the inorganic impurities remains in the liquor, a separation means associated with the crystallisation vessel for separating formed aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities, optionally an HC1 regeneration apparatus (e.g. a stripping column/distiller) for receiving the hydrochloric acid process stream from the separation means to generate a hydrogen chloride gas stream and to lower the concentration of hydrogen chloride in the hydrochloric acid process stream, an impurity removal unit for removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream, a conduit for recycling the hydrochloric acid recycle stream to the one or more acid digesters (e.g. for digesting the aluminium chloride hexahydrate solids and/or aluminium-bearing material), and a thermal treatment means for thermally treating the aluminium chloride hexahydrate solids to provide high purity alumina.

High purity alumina

[0050] The process of the present disclosure produces high purity alumina. As described herein, high purity alumina may be prepared from various aluminium-bearing materials for example aluminous clays such as kaolin, or products or by-products of processes such as the Bayer process. Many of these materials, however, have a high inorganic impurity content relative to the high purity threshold (about 99.99%) of the final desired product. Removal or control of the inorganic impurities to achieve the high purity threshold is technically difficult.

[0051] The type and levels of impurities throughout the described process will depend on a number of factors, primarily the source of the aluminium-bearing material, although it will be appreciated that, while the described process steps aim to provide reduction in the levels of impurities at each step, new inorganic impurities may be introduced during the various process steps undertaken in the production of the high purity alumina. [0052] The term inorganic ‘impurity’ or ‘impurities’ is intended to cover any nonaluminium compounds present. In particular, with regard to the final high purity alumina product, the inorganic ‘impurity’ or ‘impurities’ denote any material that is not aluminium oxide (AI2O3). The grade of the high purity alumina is based on total levels of inorganic impurities (regardless of composition) in the final product, with a product having a purity of >99.99% AI2O3 (i.e. less than 0.01% inorganic impurities) being graded “4N” and a purity of >99.999% AI2O3 (i.e. less than 0.001% inorganic impurities) graded “5N”.

[0053] By way of non-limiting example, the at least one inorganic impurity may be calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), phosphorous (P), silicon (Si), titanium (Ti), copper (Cu), molybdenum (Mo), chromium (Cr), gallium (Ga), zinc (Zn) or a combination thereof. In one example, the impurity is provided by one or more of calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), phosphorous (P), silicon (Si), titanium (Ti), copper (Cu), molybdenum (Mo), chromium (Cr), gallium (Ga), and zinc (Zn). The inorganic impurity may be present in the form of an inorganic salt, for example a chloride salt (e.g. NaCl).

[0054] The individual or total inorganic impurity in the final high purity alumina product may be less than about 1000 ppm, 500 ppm, 400 ppm, 300ppm, 200 ppm, 100 ppm, 90 ppm, 80 ppm, 70 ppm, 60 ppm, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, or 5 ppm.

[0055] In one example, the amount of inorganic impurity of any one impurity present in the high purity alumina is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In an example, the impurity of potassium (K) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of phosphorus (P) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of sodium (Na) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of silicon (Si) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of calcium (Ca) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of iron (Fe) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of magnesium (Mg) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of titanium (Ti) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of copper (Cu) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of molybdenum (Mo) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of chromium (Cr) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of gallium (Ga) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5,

4, 3, 2, or 1. In another example, the impurity of zinc (Zn) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1.

Process for preparing high purity alumina

[0056] As will be described herein, the recycling and regeneration of hydrochloric acid typically considered as a waste by industry closes the acid loop thereby reducing overall acid consumption, creating an optimized loop than has the potential to reduce capital and/or operational costs.

[0057] In one aspect or embodiment, there is provided a process for preparing high purity alumina from aluminium chloride hexahydrate solids, the process comprising: a) digesting aluminium chloride hexahydrate solids having one or more inorganic impurities in hydrochloric acid to produce an aluminium chloride liquor comprising aluminium chloride and the inorganic impurities in solution; b) precipitating aluminium chloride hexahydrate solids from the liquor by sparging the aluminium chloride liquor with hydrogen chloride gas in one or more crystallisation stage(s) such that at least some of the inorganic impurities remains in the liquor; c) separating the precipitated aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; d) processing the separated aluminium chloride hexahydrate solids to form high purity alumina; e) lowering the concentration of hydrogen chloride in the hydrochloric acid process stream; and f) removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream at least a portion of which is recycled for use as the hydrochloric acid in step a).

[0058] In one embodiment, the hydrochloric acid process stream obtained from step c) has a hydrogen chloride concentration above the azeotropic point of the stream.

[0059] In one embodiment, the lowering the concentration of hydrogen chloride in the hydrochloric acid process stream at step e) comprises heating the hydrochloric acid process stream to generate a hydrogen chloride gas stream and lower the concentration of hydrogen chloride in the hydrochloric acid process stream.

[0060] In one embodiment, the lowering the concentration of hydrogen chloride in the hydrochloric acid process stream at step e) comprises heating the hydrochloric acid process under pressure to generate a hydrogen chloride gas stream and lower the concentration of hydrogen chloride in the hydrochloric acid process stream.

[0061] In one embodiment, at step e) the concentration of hydrochloric acid in the hydrochloric acid process stream is lowered to at or near the azeotropic point of the stream. In a further embodiment, step f) comprises distilling the hydrochloric acid process stream at or near the azeotropic point of the stream to remove at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream at least a portion of which is recycled for use as the hydrochloric acid in step a).

[0062] In one embodiment, at step e) the over-azeotropic hydrochloric acid process stream obtained from step c) is heated under pressure to lower the azeotropic point to generate a hydrogen chloride gas stream and lower the concentration of hydrogen chloride in the hydrochloric acid process stream.

[0063] In one embodiment, at least a portion of the hydrogen chloride gas stream generated by the heating of the hydrochloric acid process stream in step e) is recycled for use as the source of hydrogen chloride gas in the one or more crystallisation stage(s) in step b).

[0064] In some examples, by lowering the concentration of hydrochloric acid in the hydrochloric acid process stream, preferably to at or near the azeotropic point, by heating to generate a hydrochloric acid gas stream, at least a portion of which may be recycled to crystallize aluminium chloride hexahydrate, the hydrochloric acid process stream may subsequently be easily treated to allow for impurities to be purged from the process and the remaining hydrochloric acid to be recycled for digestion of aluminium chloride hexahydrate and/or aluminium bearing material. In addition, in conjunction with hydrochloric acid gas by lowering the concentration of the hydrochloric acid process stream and hydrochloric acid gas generated from thermal decomposition and/or calciner described below, the recirculation of hydrochloric acid as both a liquid and a gas in the process described herein reduces both energy consumption and operating costs relative to processes know in the art.

[0065] In one embodiment, the removing of the impurities from the hydrochloric acid process stream in step f) comprises distilling the hydrochloric acid process stream. In a further embodiment, the removing of the impurities from the hydrochloric acid process stream in step f) comprises distilling the hydrochloric acid process stream at or near the azeotropic point of the stream.

[0066] Advantageously, distillation of the hydrochloric acid process stream at or near the azeotropic point (i.e. azeotropic distillation) produces an impurity rich ‘brine’ stream, in which the majority of impurities from hydrochloric acid process stream are concentrated, and a ‘clean’ hydrochloric acid stream which has a substantially lower impurity level compared to the ‘brine’ hydrochloric acid. Distilling the hydrochloric acid process stream at or near the azeotropic point evaporates and recondenses the hydrochloric acid at a known concentration and the recondensed ‘clean’ hydrochloric acid stream, which is advantageously both low in impurity and of a consistent concentration, may be used as the hydrochloric acid recycle stream described herein.

[0067] To fully appreciate the optimized hydrochloric acid loop provided by some embodiments of the present disclosure and the associated one or more advantages, the process can be described with reference to the steps of: digestion of aluminium chloride hexahydrate solids in hydrochloric acid to form an aluminium chloride liquor; precipitation of aluminium chloride hexahydrate solids from the aluminium chloride liquor, including by sparging with hydrogen chloride gas; separating precipitated aluminium chloride hexahydrate solids from the liquor to obtain a hydrochloric acid process stream; regeneration and recycling of hydrochloric acid from the hydrochloric acid process stream, comprising optionally lowering the concentration of hydrogen chloride in the hydrochloric acid process stream and removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream at least a portion of which is recycled for use as the hydrochloric acid in step a); and processing the separated aluminium chloride hexahydrate solids to form high purity alumina.

Digestion of aluminium chloride hexahydrate solids in hydrochloric acid

[0068] The process comprises digesting aluminium chloride hexahydrate solids which comprise one or more inorganic impurities. The one or more impurities present in the precipitate arises from the various aluminium-bearing materials for example aluminous clays such as kaolin, or products or by-products of processes such as the Bayer process. Many of these materials, however, have a high inorganic impurity content relative to the high purity threshold (about 99.99%) of the final desired product. Removal or control of the inorganic impurities to achieve the high purity threshold is technically difficult.

[0069] In one embodiment, the aluminium chloride hexahydrate solids are provided as a precipitate from an aluminium chloride liquor via one or more crystallisation stages described herein. The aluminium chloride liquor may be digestions product from the acid digestion of an aluminium bearing material described herein. In this embodiment, the digestion of the solids may also be called a re-digestion.

[0070] It will be appreciated that while crystallisation and precipitation of aluminium chloride hexahydrate solids from an aluminium chloride liquor as described herein may leave behind at least a portion of the impurities in the liquor, one or more inorganic impurities may still be present in the aluminium chloride hexahydrate solids. As such, to further reduce the concentration of inorganic impurities reporting to the final high purity alumina, the aluminium chloride hexahydrate solids under go one or more digestion and crystallisation steps prior to thermal decomposition and calcination to form high purity alumina. For example, the aluminium chloride hexahydrate solids is re-digested in hydrochloric acid to form an aluminium chloride liquor comprising aluminium chloride and any impurities (e.g. any impurities that were remaining from the first crystallisation stage used to prepare the initial aluminium chloride hexahydrate precipitate). This liquor can then undergo crystallisation in a manner as described below to produce precipitated aluminium chloride hexahydrate solids, leaving further impurities behind in the remaining liquor.

[0071] Accordingly, the process comprises digesting aluminium chloride hexahydrate solids having one or more inorganic impurities in hydrochloric acid to produce an aluminium chloride liquor comprising aluminium chloride and the inorganic impurities in solution.

[0072] In one example, the acid digestion of the aluminium chloride hexahydrate solids occurs in a suitable reactor/vessel (e.g. an acid digester) and the hydrochloric acid is introduced into the reactor to digest the aluminium chloride hexahydrate solids. The acid digestion of the aluminium chloride hexahydrate solids may be performed in the reactor in a batch mode or a continuous mode. The hydrochloric acid may be introduced as a continuous hydrochloric acid stream into a reactor for digesting the aluminium chloride hexahydrate solids.

[0073] The acid digestion of the aluminium chloride hexahydrate solids may be performed in a single reactor (e.g. an acid digester) or a plurality of reactors (e.g. up to 5 acid digesters) arranged in series.

[0074] The acid digestion may be performed at a temperature of from ambient temperature to atmospheric boiling point of the resulting aluminium chloride liquor. The acid digestion may be performed at a temperature of at least about (in °C) 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95. The acid digestion may be performed at a temperature of less than about (in °C) 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, or 30. The acid digestion may be performed at a temperature between any two of these upper and lower amounts, such as between about 25 °C to 100 °C, 50 °C to 95 °C, 70 °C to 90 °C, or 75 °C to 85 °C, for example at about 80 °C.

[0075] It will be appreciated that the rate of acid digestion will depend on the temperature, concentration of solids and acid concentration in the resulting digestion mixture. The acid digestion may be performed for a period of at least about 15 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. The acid digestion may be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours,

2 hours, or 1 hour. The solids precipitation may be performed for a period in a range provided by any two of the upper and/or lower amounts, such as from about 15 minutes to 6 hours, or about 2 to 4 hours. In one particular example the period of time is about

3 hours.

[0076] In one embodiment, the hydrochloric acid used to digest the aluminium chloride hexahydrate solids may also be called a hydrochloric acid digestion stream. [0077] In one embodiment, the hydrochloric acid used to digest the aluminium chloride hexahydrate solids may comprise or consist of a hydrochloric acid recycle stream described herein. The embodiments or examples described herein for the hydrochloric acid recycle stream below equally apply for the hydrochloric acid used to digest the aluminium chloride hexahydrate solids.

Precipitation of aluminium chloride hexahydrate solids from the aluminium chloride liquor

[0078] Following acid digestion to produce the aluminium chloride liquor comprising inorganic purities in solution, aluminium chloride hexahydrate solids are then precipitated from the aluminium chloride liquor such that at least some of the inorganic impurities remains in the liquor.

[0079] The precipitated aluminium chloride hexahydrate solids obtained in this step may be called “re-precipitated” aluminium chloride hexahydrate solids. For example, as the initial aluminium chloride hexahydrate solids digested at step a) may be a precipitate obtained from an aluminium chloride liquor described herein which then undergoes acid digestion to produce an aluminium chloride liquor, the subsequent precipitation described herein produces re-precipitated aluminium chloride hexahydrate solids.

[0080] The process comprises precipitating aluminium chloride hexahydrate solids from the liquor in one or more crystallisation stage(s) such that at least some of the inorganic impurities remains in the liquor. For example, an aluminium chloride liquor comprising aluminium chloride and one or more impurities in solution may undergo one or more crystallisation stages in order to precipitate aluminium chloride hexahydrate solids.

[0081] The aluminium (Al) concentration in solution of the aluminium chloride liquor prior to precipitation may be at least about 1 g/L, about 10 g/L, about 20 g/L, about 30 g/L, about 40 g/L, about 50 g/L, about 60 g/L, about 70 g/L, about 80 g/L, or about 90 g/L. The Al concentration in solution of the aluminium chloride liquor prior to precipitation may be less than about lOOg/L, about 90g/L, about 80 g/L, about 70 g/L, about 60 g/L, about 50 g/L, about 40 g/L, about 30 g/L, about 20 g/L, or about 10 g/L. In an embodiment, the Al concentration in solution of the aluminium chloride liquor prior to precipitation may be in a range of between about 1-100 g/L, for example a range between any two of the above upper and/or lower concentrations, such as about 10-90 g/L, or 50-85 g/L, or about 60-80 g/L. To facilitate precipitation/crystallisation, the Al concentration in the aluminium chloride liquor is preferably at or just below the saturation concentration for the solution.

[0082] The aluminium chloride liquor comprising aluminium chloride and one or more impurities in solution undergoes a crystallisation stage in order to precipitate aluminium chloride hexahydrate solids and leave behind at least a portion of the impurities in the liquor. It will be appreciated that the crystallisation may be performed in batch mode or a continuous mode. In addition, crystallisation may be performed in a single reactor (e.g. a crystallisation vessel) or a plurality of reactors arranged in series such that the concentration of precipitated aluminium chloride hexahydrate solids increases in each vessel.

[0083] In the precipitation reactors, the chloride concentration in the liquor is raised to saturation concentration or above with respect to aluminium chloride hexahydrate, thereby encouraging aluminium chloride hexahydrate to precipitate from solution. For example, the initial chloride concentration may be raised to at least about 6 M. In another example, the initial chloride concentration may be raised to at least about 7 M, 8 M, 9 M, 10 M, or 11 M. The initial chloride concentration may be raised to provide less than about 12 M, 11 M, 10 M, 9 M, 8 M, or 7 M. The initial chloride concentration may be raised to provide an amount in a range between any two of these upper and lower amounts, such as between about 6 M to l2 M, 7 M to l l M chloride, or 8 M to 10 M. In one particular example the initial chloride concentration is about 9 M.

[0084] In one embodiment, the precipitating at step b) comprises sparging the liquor with hydrogen chloride gas. For example, the chloride concentration in the liquor can be readily raised by sparging with hydrogen chloride gas. In some embodiments, the chloride concentration is raised by continuous sparging with hydrogen chloride gas. Alternatively, the sparging may be periodically paused during the precipitation process. Sparging of the liquor may be paused after an initial portion of the hydrogen chloride gas has been introduced into the liquor, for example sparging may be paused after 50% of the hydrogen chloride gas has been introduced to the liquor. Advantageously, sparging with hydrogen chloride gas rather than a liquid can reduce the potential for contaminating the liquor with undesirable impurities.

[0085] In one embodiment, the hydrogen chloride gas used to sparge the aluminium chloride liquor comprises or consists of hydrogen chloride gas which is regenerated and recycled from the hydrogen chloride process stream, as described herein.

[0086] It will be appreciated that the use of a plurality of reactors in series for precipitation, and therefore having smaller volumes of solution to be treated, may allow for an improved control of acid concentration, temperature and other precipitation conditions and therefore provide an improved control of the rate of crystallisation of aluminium chloride hexahydrate solids.

[0087] The corrosive nature of the hydrochloric acid and hydrogen chloride gas and the aluminium chloride liquor can lead to the introduction of impurities into the process through the corrosion of process equipment. As such, care is taken to ensure process equipment parts are where possible formed of materials inert to hydrochloric acid and hydrogen chloride gas and/or to protect the process equipment parts from acid attack.

[0088] The solids precipitation may be performed at a temperature of at least about (in °C) 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95. The solids precipitation may be performed at a temperature of less than about (in °C) 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, or 30. The solids precipitation may be performed at a temperature between any two of these upper and lower amounts, such as between about 25 °C to 100 °C, 30 °C to 90 °C, or 40 °C to 80 °C. [0089] The solids precipitation may be performed for a period of at least about 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. The solids precipitation may be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, or 2 hours. The solids precipitation may be performed for a period in a range provided by any two of the upper and/or lower amounts, such as from about 1 hour to 6 hours, or about 2 to 4 hours. In one particular example the period of time is about 3 hours.

[0090] The concentrated liquor may be seeded to assist the kinetics of crystallisation and improve the purity of the resulting product. The composition of the seed may be any suitable material for promoting crystallisation of aluminium chloride hexahydrate from the aluminium chloride liquor, for example the concentrated liquor may be seeded with an aluminium-bearing seed such as aluminium chloride hexahydrate or alumina crystals. The aluminium chloride hexahydrate or alumina crystals for seeding the crystallisation may be recycled from other stages of the process.

[0091] The aluminium chloride liquor may be seeded with aluminium chloride hexahydrate crystals in an amount of at least about 0.1 g/L, about 1 g/L, about 5 g/L, about 10 g/L, about 15 g/L, about 20 g/L, about 25 g/L, about 30 g/L, about 35 g/L, about 40 g/L, about 45 g/L, or about 50 g/L. The prepared aluminium chloride liquor may be seeded with aluminium chloride hexahydrate crystals in an amount of less than about 60 g/L, about 55 g/L, about 50 g/L, about 45 g/L, about 40 g/L, about 35 g/L, about 30 g/L, about 25 g/L, about 20 g/L, about 15 g/L, about 10 g/L, or about 5 g/L.

The prepared aluminium chloride liquor may be seeded with aluminium chloride hexahydrate crystals in a range provided by any two of the upper and/or lower amounts, for example between about 0.1 g/L to 60 g/L, about 1 g/L to 50 g/L, or about 10 g/L to 55 g/L. In other examples, the range amount of seeded aluminium chloride hexahydrate crystals may be 0.1-1 g/L, 1-5 g/L, 5-10 g/L, 10-15 g/L, 15-20 g/L, 20-25 g/L, 25-30 g/L, 30-35 g/L, 35-40 g/L, 40-45 g/L, or 45-50 g/L. In other examples, these seeding amounts including ranges may be provided for other suitable seeding materials. [0092] The seed may be added to the aluminium chloride liquor prior to introduction to the crystallisation vessel (i.e. precipitation reactors). In this step, additional soluble aluminium-bearing material may be added to the aluminium chloride liquor in order to increase the Al concentration to a desired level prior to seeding and crystallisation.

[0093] Where crystallisation is performed in a plurality of reactors, one or more of the reactors may be seeded with the aluminium chloride hexahydrate crystals. In an embodiment, where crystallisation is performed in a plurality of reactors in series, the liquor comprising precipitated aluminium chloride hexahydrate solids fed from one reactor to the subsequent reactor in the series may act to seed precipitation in the subsequent reactor.

[0094] Where multiple crystallisation stages are performed, seeding may be performed in some or all of the crystallisation stages, but need not be performed in all crystallisation stages. In an example, seeding may be performed in the first of the multiple crystallisation stages only to generate aluminium chloride hexahydrate solids while leaving the bulk of impurities present in the aluminium chloride liquor produced in the hydrochloric acid digestion of the aluminium -bearing material.

[0095] In an embodiment, the aluminium-bearing seed comprises greater than 90%, 95%, 98%, or 99%, of aluminium compounds to minimise the introduction of impurities into the liquor. In another embodiment, the aluminium-bearing seed is an aluminium hexahydrate solid of greater than 90%, 95%, 98%, 99%, 99.9%, 99.99%, or 99.999% of aluminium hexahydrate solid to minimise the introduction of impurities into the liquor. In other examples the total amount of impurities in the aluminium bearing seed is less than 1%, 0.1%, 0.01%, 0.001%, or 0.0001%.

[0096] By way of non-limiting example, impurities present in the seed may include calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), phosphorous (P), silicon (Si), titanium (Ti), copper (Cu), molybdenum (Mo), chromium (Cr), gallium (Ga), zinc (Zn) or a combination thereof. In one example, the impurity is provided by one or more of calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), phosphorous (P), silicon (Si), titanium (Ti), copper(Cu), molybdenum (Mo), chromium (Cr), gallium (Ga), and zinc (Zn).

[0097] The individual or total impurity in the seed may be less than about 1000 ppm, 500 ppm, 400 ppm, 300ppm, 200 ppm, 100 ppm, 90 ppm, 80 ppm, 70 ppm, 60 ppm, 50 ppm, 40 ppm, 30 ppm, 20 ppm, 10 ppm, or 5 ppm.

[0098] In one example, the impurity of any one impurity is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In an example, the impurity of potassium (K) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of phosphorus (P) is less than about (in ppm) 50, 40, 30, 20, 10,

9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of sodium (Na) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of silicon (Si) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of calcium (Ca) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of iron (Fe) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of magnesium (Mg) is less than about (in ppm) 50, 40, 30, 20,

10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of titanium (Ti) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of copper (Cu) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of molybdenum (Mo) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of chromium (Cr) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of gallium (Ga) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. In another example, the impurity of zinc (Zn) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1.

[0099] In processes where multiple digestion and crystallisation stages are present, the operating conditions need not be the same in each stage and may be varied in response to the increasing purity of the product. In one example, as discussed above, the presence of seeding can be varied across crystallisation stages. Additionally, where seeding is provided in multiple crystallisation stages, the amount or type of seed may be varied for different stages, for example the amount of seed may be decreased for subsequent crystallisations.

[0100] In another example, the hydrochloric acid concentration may be varied for different crystallisation stages. While a higher hydrochloric acid concentration will increase the amount of aluminium chloride hexahydrate solids that precipitates from the liquor, this may also result in higher concentrations of impurities in the precipitated solids. Conversely, lower concentration may leave behind more aluminium in the liquor, however provide a more pure precipitate.

[0101] In an embodiment, the process comprises two or more crystallisation stages, in particular three crystallisation stages. The concentration of hydrochloric acid in the first crystallisation stage is lower than at least one of the subsequent crystallisation stages. For example, the concentration of hydrochloric acid in the first crystallisation stage may be less than about 10 M, 9 M, or 8 M, and the concentration of hydrochloric acid in at least one of the subsequent crystallisation stages may be greater than 11 M, 10 M, or 9 M, respectively. In another example, where three crystallisation stages are provided, the concentration of hydrochloric acid in the first crystallisation stage may be around 9 M, the hydrochloric acid concentration in the second crystallisation stage may be around 10.5 M and the hydrochloric acid concentration in the third crystallisation stage may be around 10 M.

[0102] In an embodiment, the or each crystallisation stage is performed in a plurality of reactors arranged in series. In such an embodiment, the concentration of hydrochloric acid may be progressively increased in the reactors in series to achieve a concentration in the final reactor in the series as described above. In one embodiment, the or each crystallisation stage comprises the aluminium chloride liquor undergoing two or more precipitation stages, wherein the precipitation stages are in series.

[0103] In one embodiment, step b) comprises precipitating aluminium chloride hexahydrate solids from the aluminium chloride liquor in two or more crystallisation stages, wherein between each crystallisation stage, separated aluminium chloride hexahydrate is digested in the hydrochloric acid recycle stream to produce the aluminium chloride liquor.

Separation of aluminium chloride hexahydrate solids to produce a hydrochloric acid process stream

[0104] The resulting aluminium chloride hexahydrate solids are separated from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities. It will be appreciated that the hydrochloric acid process stream comprises the liquor and impurities following separation of the solids. The separated solids may form a solids stream separate to the hydrochloric acid process stream.

[0105] Any suitable conventional separation technique, such as filtration, gravity separation, centrifugation, classification and so forth, may be used. It will be appreciated that the solids may undergo one or more washings during separation.

[0106] Prior to undergoing thermal treatment into high purity alumina, the separated aluminium chloride hexahydrate solids may be washed to remove any liquor/impurities entrained within the solids. The solids may be washed with hydrochloric acid. The hydrochloric acid used to wash the solids may be recycled from regenerated hydrochloric acid as described herein. In one embodiment, the hydrochloric acid used to wash the solids may be generated during the HC1 regeneration process described herein, for example as a concentrated hydrochloric acid stream generated during the condensing of the hydrogen chloride gas stream described herein. Alternatively or additionally, the separated liquor and combined washings may be recycled for use as a washing medium for filtration of aluminium chloride hexahydrate solids produced upstream.

[0107] To further reduce the concentration of impurities, the separated precipitated aluminium chloride hexahydrate solids may optionally undergo one or more further digestion and recrystallization steps prior to thermal decomposition and calcination to high purity alumina. For example, the separated precipitated aluminium chloride hexahydrate solids may be digested in hydrochloric acid as described herein to form an aluminium chloride liquor comprising aluminium chloride and any impurities remaining from the crystallisation stage. This liquor can then undergo one or more subsequent crystallisation steps in a manner as described above to produce precipitated aluminium chloride hexahydrate solids, leaving further impurities behind in the remaining liquor.

[0108] It will be appreciated that the acid digestion in hydrochloric acid and crystallisation stages can be repeated as many times as necessary to arrive at a suitably pure aluminium chloride hexahydrate solids concentration prior to processing the solids to form high purity alumina. However, repeated acid digestion and crystallisation may not be required in those embodiments where the remaining impurities in the solids are sufficiently low such that the alumina which would be produced from thermal decomposition and calcination of the solids collected after filtration would meet the purity requirements for high purity alumina.

Regeneration and recycling of process hydrochloric acid

[0109] As described above, a hydrochloric acid process stream comprising impurities is produced following separation of the precipitated aluminium chloride hexahydrate solids therefrom. As opposed to a “waste” stream, the inventors have discovered that the hydrochloric acid remaining in the liquor following acid-digestion and crystallisation can be recycled back into the process as a liquid stream for digesting aluminium chloride hexahydrate solids and/or aluminium-bearing material.

[0110] The hydrochloric acid process stream has a hydrogen chloride concentration. The hydrogen chloride content of the process stream arises from the previous one or more acid-digestion steps and one-or more crystallisation stages described herein.

[0111] In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration of between about 5 wt.% to about 40 wt.% based on the weight of the stream. The hydrochloric acid process stream may have a hydrogen chloride concentration (in wt.%) of at least about 5, 8, 10, 12, 15, 17, 19, 20, 21, 22, 25, 28, 30, 32, 35 or 40, based on the weight of the stream. The hydrochloric acid process stream may have a hydrogen chloride concentration (in wt.%) of less than about 40, 35, 32, 30, 28, 25, 22, 21, 20, 19, 17, 15, 12, 10, 8 or 5 based on the weight of the stream. The hydrogen chloride concentration may be a range provided by any two of these upper and/or lower values, for example between about 15 wt.% to about 35 wt.% based on the weight of the stream.

[0112] In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration above the azeotropic point of the stream. For example, the hydrogen chloride concentration may be greater than the azeotropic point and less than about 40 wt.% based on the weight of the process stream. In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration of between about 25 wt.% to about 40 wt.% based on the weight of the stream. The hydrochloric acid process stream may have a hydrogen chloride concentration (in wt.%) of at least about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 or 40 based on the weight of the stream. The hydrochloric acid process stream may have a hydrogen chloride concentration (in wt.%) of less than about 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26 or 25 based on the weight of the stream. The hydrogen chloride concentration of the hydrochloric acid process stream may be a range provided by any two of these upper and/or lower values, for example between about 28 wt.% to about 37 wt.%.

[0113] In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration above the azeotropic point of the stream, and the process comprises step e) lowering the concentration of hydrogen chloride in the hydrochloric acid process stream prior to removal of the impurities.

[0114] To lower the concentration, the hydrogen chloride process stream having a hydrogen chloride concentration above the azeotropic point of the stream may be heated to evaporate and generate hydrogen chloride gas. As described above, heating and boiling the hydrochloric acid process stream having a hydrogen chloride concentration above the azeotropic point will evaporate more hydrogen chloride gas than water until the azeotropic point is reached. As such, by heating the ‘over- azeotropic’ hydrochloric acid process stream, hydrogen chloride gas is generated which subsequently lowers the hydrogen chloride concentration of the process stream until the azeotropic point is reached.

[0115] In one embodiment, the heating of the hydrochloric acid process stream is at a pressure of between atmospheric pressure (e.g. about 1 bar) to about 16 bar. The pressure of the hydrochloric acid process stream may be at a pressure (in bar) of at least about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5 or 16. The pressure of the hydrochloric acid process stream may be at a pressure (in bar) of less than about 16, 15.5, 15, 14.5, 14,

13.5, 13, 12.5, 12, 11.5, 11, 10.5, 10, 9.5, 9, 8.5, 8, 7.5, 7, 6.5, 6, 5.5, 5, 4.5, 4, 3.5, 3,

2.5, 2, 1.5 or 1. The pressure may be in a range provided by any two of these upper and/or lower values, for example between about 3 bar to about 10 bar, e.g. between about 3 bar to 3.5 bar. According to some embodiments or examples described herein, by increasing the pressure of the heating step, the azeotropic point of the hydrochloric acid process stream is lowered which can provide the generation hydrogen chloride gas at a steady state concentration . Additionally, the pressure during the heating step may also facilitate fluid transfer of the generated hydrochloric acid recycle stream.

[0116] In one embodiment, the hydrochloric acid process stream is heated under pressure (e.g. at a pressure above atmospheric pressure) effective to lower the azeotropic point to generate a hydrogen chloride gas stream and lower the concentration of hydrogen chloride in the hydrochloric acid process stream.

[0117] In one embodiment, the heating of the hydrochloric acid process stream to generate hydrogen chloride gas is at a temperature of between about 100°C to about 200°C. The heating of the hydrochloric acid process stream may be at a temperature (in °C) of at least about 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200. The heating of the hydrochloric acid process stream may be at a temperature (in °C) of less than about 200, 190, 180, 170, 160, 150, 140, 130, 120, 110 or 100. The heating temperature may be a range provided by any two of these upper and/or lower values.

[0118] In one embodiment, the heating of the hydrochloric acid process stream lowers the hydrogen chloride concentration to between about 10 wt.% to about 25 wt.% based on the weight of the stream. The heating of the hydrochloric acid process stream may lower the hydrogen chloride concentration (in wt.%) to less than about 25, 20, 15 or 10 based on the weight of the stream.

[0119] In one embodiment, the heating of the hydrochloric acid process stream lowers the hydrogen chloride concentration to at or near the azeotropic point of the stream. In one embodiment, the heating of the hydrochloric acid process stream lowers the hydrogen chloride concentration to at or near the azeotropic point up to about 25 wt.% based on the weight of the stream.

[0120] In one embodiment, at least a portion of the hydrogen chloride gas stream may also be combined with an aqueous stream (e.g. water) to produce a hydrochloric acid wash stream which may be recycled and used to wash the separated aluminium chloride hexahydrate solids (not shown).

[0121] In one embodiment, at least a portion of the hydrogen chloride gas stream generated by the heating of the hydrochloric acid process stream is recycled for use as the source of hydrogen chloride gas in the one or more crystallisation stage(s) described herein.

[0122] In one embodiment, the lowering of the hydrogen chloride concentration of the hydrochloric acid process stream may be performed in a stripper/distiller column (see Figure 3).

[0123] It will be appreciated by the persons skilled in art that the concentration of the hydrogen chloride gas stream generated from heating of the hydrochloric acid process stream may vary depending on the process unit used and the process conditions used in said unit. In one embodiment, the hydrogen chloride gas stream generated from heating of the hydrochloric acid process stream has a hydrogen chloride concentration of between about 20 wt.% to about 100 wt.% based on the weight of the stream. The hydrogen chloride gas stream may have a hydrogen chloride concentration (in wt.%) of at least about 20, 25, 30, 35, 40, 45, 50, 52, 55, 60, 65, 70 75, 80, 85, 90, 92.5, or 95 based on the weight of the stream. The hydrogen chloride gas stream may have a hydrogen chloride concentration (in wt.%) of less than about 100, 99, 97.5, 95, 92.5 90, 85, 80, 75 70, 65, 60, 55, 52, 50, 45,40, 35, or 30, based on the weight of the stream. The hydrogen chloride gas stream may be a range provided by any two of these upper and/or lower values, for example between about 20 wt.% to about 100 wt.%, or between about 40 wt.% to about 100 wt.%, between about 60 wt.% to about 100 wt.%, or between about 80 wt.% to about 100 wt.%.

[0124] The hydrochloric acid process stream comprises one or more inorganic impurities which remain in the liquor following precipitation and separation of the aluminium chloride hexahydrate solids. In order to close the hydrochloric acid loop of the present process, at least some or all of the inorganic impurities needs to be removed from the process stream in order to avoid re-introduction of the impurities. Removal of the impurities from the process stream produces the hydrochloric acid recycle stream described herein.

[0125] In one embodiment, the hydrochloric acid process stream comprises between about 0.1 wt.% to about 10 wt.% of inorganic impurities based on the weight of the stream. The concentration of inorganic impurities (in wt.%) in the hydrochloric acid process stream may be at least about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.4, 2.8, 3.2, 3.6, 4, 4.5, 5, 6, 7, 8, 9 or 10 based on the weight of the stream. The concentration of inorganic impurities (in wt.%) in the hydrochloric acid process stream may less than about 10, 9, 8, 7, 6, 5, 4.5, 4, 3.6, 3.2, 2.8, 2.4, 2, 1.8, 1.6, 1.4, 1.2, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2 or 0.1 based on the weight of the stream. The concentration may be a range provided by any two of these upper and/or lower values, for example between about 0.5 wt.% to about 1.5 wt.%. [0126] The impurities may be removed from the hydrochloric acid process stream by any suitable process, for example using a suitable impurity removal unit. Examples of suitable processes for removing the impurities from the hydrochloric acid process stream include distillation, ion-exchange and precipitation.

[0127] In one embodiment, the removing of the impurities from the hydrochloric acid process stream comprises distilling the hydrochloric acid process stream. In one embodiment, the removing of the impurities from the hydrochloric acid process stream comprises distilling the hydrochloric acid process stream at or near the azeotropic point of the stream. Distilling the hydrochloric acid process stream (such as via azeotropic distillation) evaporates and recondenses the hydrochloric acid stream and produces an inorganic impurity stream comprising the impurities as a separate stream. It will be appreciated that the recondensed hydrochloric acid stream is the hydrochloric acid recycle stream as described herein.

[0128] The inorganic impurity stream (i.e. the purge stream) is concentrated with the one or more inorganic impurities. In one embodiment, the inorganic impurity stream has an inorganic impurity concentration (in wt. %) of at least about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 8 or 10 based on the weight of the stream.

[0129] In one embodiment, the hydrochloric acid recycle stream comprises less than about 0.5, 0.1, 0.05, 0.01, 0.005 or 0.001 wt.% of inorganic impurities based on the weight of the stream. In one embodiment, the hydrochloric acid recycle stream is substantially free of inorganic impurities (e.g. is a purified stream).

[0130] In one embodiment, the hydrochloric acid recycle stream has a hydrogen chloride concentration of between about 5 wt.% to about 40 wt.% based on the weight of the stream. The hydrochloric acid recycle stream may have a hydrogen chloride concentration (in wt.%) of at least about 5, 8, 10, 12, 15, 17, 19, 20, 21, 22, 25, 28, 30, 32, 35 or 40, based on the weight of the stream. The hydrochloric acid recycle stream may have a hydrogen chloride concentration (in wt.%) of less than about 40, 35, 32, 30, 28, 25, 22, 21, 20, 19, 17, 15, 12, 10, 8 or 5 based on the weight of the stream. The hydrogen chloride concentration may be a range provided by any two of these upper and/or lower values, for example between about 15 wt.% to about 35 wt.% based on the weight of the stream.

[0131] In one embodiment, the concentration of hydrogen chloride in the hydrochloric acid recycle stream is lower than the concentration of hydrogen chloride in the hydrochloric acid process stream. For example, the concentration of hydrogen chloride in the process stream is lowered prior to removal of the one or more inorganic impurities, as described herein. In one embodiment, the hydrochloric acid recycle stream has a hydrogen chloride concentration of between about 5 wt.% to about 30 wt.% based on the weight of the stream. The hydrogen chloride concentration in the recycle stream (in wt. %) may be less than about 35, 30, 25, 20, 18, 15, 10 or 5 based on the weight of the stream. A range may be provided by any two of these values, for example between about 5 wt.% to 25 wt.%, about 10 wt.% to about 25 wt.% based on the weight of the stream, e.g. between about 18 wt.% to about 20 wt.%.

[0132] In some embodiments, at least a portion of the hydrogen chloride recycle stream is then recycled for use as the hydrochloric acid to digest the aluminium chloride hexahydrate solids at step a). Such recycling of a hydrochloric acid stream for digestion (as opposed to using water) is counterintuitive, as it decreases the solubility of the aluminium chloride hexahydrate and reduces the amount of aluminium chloride hexahydrate solid that is able to be dissolved and digested due to the common-ion effect. This explains why such acid streams are not conventionally recycled and re-used in the process. However, according to some embodiments or examples described herein, the present inventors have surprisingly identified that the hydrochloric acid can be regenerated and recycled while simultaneously reducing the operating cost of the process and maintaining high purity alumina production rates. Specifically recycling of a hydrochloric acid stream for use in step a) can reduce the extent to which the hydrochloric acid process stream must be distilled to produce HC1 gas (see for example Table 1). This can reduce both the capital and energy requirement in the acid regeneration circuit and more than offsets any penalty accrued by reducing the amount of aluminium chloride hexahydrate solid that can dissolve in step a). Additionally, by raising the acid concentration of the digested aluminium chloride liquor, in some embodiments less hydrogen chloride gas per litre of liquor is required during subsequent sparging to crystallise and precipitate the aluminium chloride

[0133] Alternatively or additionally, at least a portion of the hydrochloric acid recycle stream can be recycled for use as hydrochloric acid to digest the aluminium-bearing material, as described herein.

[0134] In one embodiment, following impurity removal, the hydrochloric acid recycle stream may have a temperature of between about 20°C about 100°C. The hydrochloric acid recycle stream may have a temperature (in °C) of at least about 20, 30, 40, 50, 60, 70, 80, 90 or 100. The hydrochloric acid recycle stream may have a temperature (in °C) of less than about 100, 90, 80, 70, 60, 50, 40, 30 or 20. A range may be provided by any two of these upper and/or lower values. According to some embodiments or examples described herein, a heated recycle stream provides one or more advantages such as reduces the temperature required to re-digest the aluminium chloride hexahydrate solids, and/or increases the dissolution rate of the aluminium chloride hexahydrate solids.

[0135] In some embodiments, prior to or in addition to recycling the hydrochloric acid recycle stream to step a), at least a portion of the hydrochloric acid recycle stream is introduced into an absorption unit to condense and absorb hydrogen chloride from hot gas generated from the thermal decomposition and/or calciner (described below). The resulting enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the thermal decomposition and/or calciner gas can then be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, uncondensed gas from the absorption unit can be separated and transferred to a tail tower, where the recycle stream has been introduced, which acts as a scrubber to absorb gases including from the uncondensed gas, prior to introduction to the absorption unit (see Figure 3). [0136] In some embodiments, prior to or in addition to being sent to the impurity removal step f), at least a portion of the hydrochloric process stream is introduced into an absorption unit to condense and absorb hydrogen chloride from hot gas generated from the thermal decomposition and/or calciner (described below). In one embodiment, enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the thermal decomposition and/or calciner gas may be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, uncondensed gas from the absorption unit can be separated and transferred to a tail tower, where the recycle stream has been introduced, which acts as a scrubber to absorb gases including from the uncondensed gas, prior to introduction to the absorption unit (see Figure 3).

[0137] In some embodiments, prior to or in addition to being sent to the impurity removal step f), at least a portion of the lower concentration hydrochloric process stream obtained from step e) is introduced into an absorption unit to condense and absorb hydrogen chloride from hot gas generated from the thermal decomposition and/or calciner (described below) (see Figure 3). In one embodiment, uncondensed gas from the absorption unit can be separated and transferred to a tail tower, where the recycle stream has been introduced, which acts as a scrubber to absorb gases including from the uncondensed gas, prior to introduction to the absorption unit (see Figure 3).

[0138] In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the thermal decomposition gas has a hydrogen chloride concentration (in wt.%) of at least about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 or 40 based on the weight of the stream. The enriched hydrochloric acid stream may have a hydrogen chloride concentration (in wt.%) of less than about 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26 or 25 based on the weight of the stream. The hydrogen chloride concentration of the enriched hydrochloric acid stream may be a range provided by any two of these upper and/or lower values, for example between about 28 wt.% to about 37 wt.%.

[0139] In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the thermal decomposition may be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride generated by thermal decomposition may be combined with the hydrochloric acid process stream prior to the hydrochloric acid process stream being fed to the stripping column (see Figure 3).

[0140] In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the calciner gas has a hydrogen chloride concentration (in wt.%) of at least about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,

37, 38, 39 or 40 based on the weight of the stream. The enriched hydrochloric acid stream may have a hydrogen chloride concentration (in wt.%) of less than about 40, 39,

38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26 or 25 based on the weight of the stream. The hydrogen chloride concentration of the enriched hydrochloric acid stream may be a range provided by any two of these upper and/or lower values, for example between about 28 wt.% to about 37 wt%.

[0141] In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the calciner may be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, the enriched hydrochloric acid stream comprising absorbed hydrogen chloride from the calciner may be combined with the hydrochloric acid process stream prior to the hydrochloric acid process stream being fed to the stripping column (see Figure 3).

[0142] In one embodiment, a water stream may be introduced to the hydrochloric acid recycle stream to dilute the hydrogen chloride concentration.

Production of high purity alumina from precipitated aluminium chloride hexahydrate solids

[0143] The separated precipitated aluminium chloride hexahydrate solids from step c) may then be processed to form high purity alumina. In one embodiment, the processing the separated chloride hexahydrate solids to form high purity alumina comprises thermally treating the separated chloride hexahydrate solids in one or more heating stages.

[0144] For example, the aluminium chloride hexahydrate precipitate is heated to a first temperature from 200 °C to 900 °C to thermally decompose the solids The aluminium chloride hexahydrate precipitate may be heated to a first temperature (in °C) of at least about 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850 or 900. The aluminium chloride hexahydrate precipitate may be heated to a first temperature (in °C) of less than about 900, 850, 800, 750, 700, 650, 600, 550, 500, 450, 400, 350, 300, 250 or 200. The first temperature may be a range provided by any two of these upper and/or lower values.

[0145] Any hydrogen chloride gas evolved during thermal decomposition may undergo regeneration into hydrochloric acid for digesting aluminium chloride hexahydrate solids described herein and/or hydrogen chloride gas for precipitation of aluminium chloride hexahydrate solids in the one or more crystallisation stage(s) described herein, as outlined in Figure 3. In some embodiments, the hydrochloric acid gas evolved during thermal decomposition has a hydrogen chloride concentration of between about 30 wt.% to about 70 wt.% based on the weight of the stream. The hydrochloric acid gas evolved during thermal decomposition may have a hydrogen chloride concentration (in wt.%) of at least about 30, 35, 40, 45, 50, 52, 55, 60, 65, or 70, based on the weight of the stream. The hydrochloric acid gas evolved during thermal decomposition may have a hydrogen chloride concentration (in wt.%) of less than about 70, 65, 60, 55, 52, 50, 45,40, 35, or 30, based on the weight of the stream. The hydrochloric acid gas evolved during thermal decomposition may be a range provided by any two of these upper and/or lower values, for example between about 30 wt.% to about 70 wt.%, between about 40 wt.% to about 60 wt.%, or between about 45 wt.% to about 55 wt.%, based on the weight of the stream. In one embodiment, hydrochloric acid gas evolved during thermal decomposition may have a hydrogen chloride concentration greater than the concentration of hydrochloric acid in the hydrochloric acid process stream. [0146] In one embodiment, the thermally decomposed solids following the first heating stage are subsequently calcined at a second temperature from 900 °C to 1300 °C to produce high purity alumina. The thermally decomposed solids may be calcining at a second temperature (in °C) of at least about may be at least about 900, 950, 1000, 1050, 1100, 1150, 1200, 1250 or 1300. The thermally decomposed solids may be calcining at a second temperature (in °C) of at least about may be less than about 1300, 1250, 1200, 1150, 1100, 1050, 1000, 950 or 900. The calcining temperature may be a range provided by any two of these upper and or lower values.

[0147] Any hydrogen chloride gas that may be evolved during calcination may undergo regeneration into hydrochloric acid for digesting aluminium chloride hexahydrate solids described herein and/or hydrogen chloride gas for precipitation of aluminium chloride hexahydrate solids in the one or more crystallisation stage(s) described herein, as outlined in Figure 3. In some embodiments, the hydrochloric acid gas evolved during calcination has a hydrogen chloride concentration of between about 30 wt.% to about 70 wt.% based on the weight of the stream. The hydrochloric acid gas evolved during calcination may have a hydrogen chloride concentration (in wt.%) of at least about 30, 35, 40, 45, 50, 52, 55, 60, 65, or 70, based on the weight of the stream. The hydrochloric acid gas evolved during calcination may have a hydrogen chloride concentration (in wt.%) of less than about 70, 65, 60, 55, 52, 50, 45,40, 35, or 30, based on the weight of the stream. The hydrochloric acid gas evolved calcination may be a range provided by any two of these upper and/or lower values, for example between about 30 wt.% to about 70 wt.%, between about 40 wt.% to about 60 wt.%, or between about 45 wt.% to about 55 wt.%, based on the weight of the stream. In one embodiment, hydrochloric acid gas evolved during calcination may have a hydrogen chloride concentration greater than the concentration of hydrochloric acid in the hydrochloric acid process stream.

[0148] By control of the removal of impurities from the initial aluminium chloride liquor formed from the aluminium-bearing material and taking measures to reduce the inclusion of impurities in the aluminium chloride hexahydrate solids through the implementation of the one or more digestions steps in hydrochloric acid according to the present disclosure, a higher purity alumina of greater than 99.99% purity (4N) or greater than 99.999% purity (5N) may be reliably achieved from a wide range of aluminium-bearing materials.

[0149] To assist in minimising the introduction of impurities during crystallisation, a portion of the formed high purity alumina may be optionally recycled to seed the aluminium chloride liquor in one or more precipitation steps upstream.

Preparation of aluminium chloride liquor from an aluminium-bearing material

[0150] The initial aluminium chloride hexahydrate solids at step a) may be provided by digesting an aluminium-bearing material in hydrochloric acid to provide an aluminium chloride liquor, and precipitating and separating aluminium chloride hexahydrate solids therefrom to obtain the aluminium chloride hexahydrate solids and producing the hydrochloric acid process stream. The precipitating of the aluminium chloride hexahydrate solid from the aluminium chloride liquor is described herein.

[0151] In one embodiment, the hydrochloric acid recycle stream described herein is recycled for use as the hydrochloric acid to digest the aluminium -bearing material.

[0152] In one aspect or embodiment, there is provided a process for preparing high purity alumina from an aluminium-bearing material, the process comprising: a) digesting an aluminium-bearing material in hydrochloric acid to provide an aluminium chloride liquor comprising aluminium chloride and one or more impurities in solution; b) precipitating aluminium chloride hexahydrate solids from the liquor in one or more crystallisation stage(s) such that at least some of the inorganic impurities remains in the liquor; c) separating the precipitated aluminium chloride hexahydrate solids from the liquor to produce a hydrochloric acid process stream comprising the inorganic impurities; d) processing the separated aluminium chloride hexahydrate solids to form high purity alumina; e) optionally lowering the concentration of hydrogen chloride in the hydrochloric acid process stream; and f) removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream which is recycled for use as the hydrochloric acid in step a) to digest the aluminium-bearing material.

[0153] In one embodiment, step b) comprises precipitating aluminium chloride hexahydrate solids from the aluminium chloride liquor in two or more crystallisation stages, wherein between each crystallisation stage, separated aluminium chloride hexahydrate is digested in the hydrochloric acid recycle stream to produce the aluminium chloride liquor, as described herein.

[0154] It will be appreciated that the embodiments and examples described herein in relation to the hydrochloric acid process stream and/or hydrochloric acid recycle stream recycled for use as the hydrochloric acid to digest the aluminium chloride hexahydrate solids can equally apply to the hydrochloric acid process stream and/or hydrochloric acid recycle stream recycled for use as the hydrochloric acid to digest the aluminium- bearing material described herein, unless otherwise stated.

[0155] The aluminium-bearing material may undergo a number of pre-treatment and treatment steps in order to form an aluminium chloride liquor comprising aluminium chloride and one or more inorganic impurities in solution.

[0156] As-received aluminium-bearing materials may undergo one or more pretreatment steps prior to undergoing digestion to form an aluminium chloride liquor. Said pre-treatment step(s) may be any one or more beneficiation processes including, but is not limited to, concentration, gravity separation to deplete the material of gangue such as sand or quartz, or comminution to a particle size of 1 pm to 200 pm. [0157] It will be appreciated that certain aluminium -bearing materials, such as calciner dust, may include occluded and surface soda. Prior to calciner dust entering the process circuit, surface soda may be readily removed from the calciner dust by scrubbing the calciner dust with carbon dioxide to remove surface soda as sodium bicarbonate. The scrubbed calciner dust may then be subsequently filtered and washed with water to remove residual sodium bicarbonate before entering the process circuit.

[0158] Alternatively, soluble surface soda may be at least partially removed from the calciner dust by washing with water. The washed calciner dust may then be subsequently filtered before entering the process circuit.

[0159] In an embodiment, gibbsite feed may be provided from a Bayer process circuit in which the gibbsite feed may have, optionally, been subjected to one or more recrystallization steps from an alkali solution including within the Bayer process circuit, thereby depleting the feed of one or more impurities, in particular soda.

[0160] In another embodiment, an aluminous clay feed such as kaolin may be provided.

[0161] The process for preparing high purity alumina may include digesting the aluminium-bearing material with hydrochloric acid to produce an aluminium chloride liquor. The hydrochloric acid may have a concentration of from 5 M to 12 M HC1, 6 to 11 M HC1, 6 to 10 M HC1, or 7 M to 9 M HC1.

[0162] The concentration of HC1 of the resulting aluminium chloride liquor may range from 0 M to 2 M. For example the concentration of HC1 of the resulting aluminium chloride liquor may be about 0 M, 0.5 M, 1 M, 1.5, M, or 2M. It will be appreciated that the digestion step may be performed in a batch mode or a continuous mode.. The digestion step may be performed in a single reactor (vessel) or a plurality of reactors (e.g. up to 5 vessels, such as 3 vessels) arranged in series such that the concentration of HC1 in the liquor in each vessel in the series decreases in cascading order from about 10 M to about 2 M. [0163] In one embodiment, the hydrochloric acid recycle stream described herein may be introduced into one or more of the reactors arranged in series, wherein the concentration of hydrogen chloride in the hydrochloric acid recycle stream is at or near the concentration of hydrogen chloride in the one or more reactors. Alternatively or additionally, a hydrochloric acid stream may be introduced to the hydrochloric acid recycle stream to concentrate the hydrogen chloride concentration prior to digesting the aluminium-bearing material in the one or more reactors described herein.

[0164] The resulting mixture may have an initial solids content of up to 50% w/w, although it will be appreciated that the solids content of the mixture will decrease as digestion progresses.

[0165] The acid digestion may be performed at a temperature of from ambient temperature to atmospheric boiling point of the resulting aluminium chloride liquor. The acid digestion may be performed at a temperature of at least about (in °C) 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95. The acid digestion may be performed at a temperature of less than about (in °C) 100, 95, 90, 85, 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, or 30. The acid digestion may be performed at a temperature between any two of these upper and lower amounts, such as between about 25 °C to 100 °C, 50 °C to 95 °C, 70 °C to 90 °C, or 75 °C to 85 °C, for example at about 80 °C.

[0166] It will be appreciated that the rate of digestion will depend on the temperature, concentration of solids and acid concentration in the resulting digestion mixture. The acid digestion may be performed for a period of at least about 15 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. The acid digestion may be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, 2 hours, or 1 hour. The solids precipitation may be performed for a period in a range provided by any two of the upper and/or lower amounts, such as from about 15 minutes to 6 hours, or about 2 to 4 hours. In one particular example the period of time is about 3 hours.

[0167] After dissolution of the acid-soluble compounds is complete, the resulting aluminium chloride liquor is separated from any remaining solids by any suitable conventional separation technique, such as filtration, gravity separation, centrifugation and so forth. It will be appreciated that the solids may undergo one or more washings during separation.

[0168] The separated aluminium chloride liquor may undergo further pre-treatment prior to undergoing crystallisation to precipitate aluminium chloride hexahydrate solids, for example in the manner described above.

[0169] For example, one such pre-treatment may include contacting the aluminium chloride liquor with an ion exchange resin, in particular a cation exchange resin.

[0170] Alternatively, another example of such a pre-treatment may include contacting the aluminium chloride liquor with an adsorbent to adsorb the one or more impurities, optionally in combination with a complexing agent. Suitable adsorbents include, but are not limited to, activated alumina, silica gel, activated carbon, molecular sieve carbon, molecular sieve zeolites and polymeric adsorbents.

[0171] Yet another example of a pre-treatment may include selectively precipitating chloride salts of the one or more impurities. For example, the liquor may be cooled and sparged with HC1 gas to encourage salting out of sodium chloride.

[0172] A further example of such a pre-treatment may include reacting the liquor with a complexing agent, wherein the complexing agent is capable of selectively forming a complex with one or more impurity. In this way, the complexed impurity may remain in solution when aluminium chloride hexahydrate solids are produced. The complexing agents may be selective for Na, Fe or Ti. Suitable complexing agents for Na include, but are not limited to, macrocyclic polyethers such as crown ethers, lariat crown ethers, and cryptands. Suitable crown ethers which demonstrate good selectivity for sodium include 15-crown 5, 12-crown 4 and 18-crown 6. Such crown ethers are soluble in aqueous solutions. Suitable complexing agents for Fe include, but are not limited to, polypyridyl ligands such as bipyridyl and terpyridyl ligands, polyazamacrocyles. Suitable complexing agents for Ti include, but are not limited to, macrocyclic ligands incorporating O, N, S, P or As donors. Other metal complexing agents may include heavy metal chelating agents such as EDTA, NTA, phosphonates, DPTA, IDS, DS, EDDS, GLDA, MGDA.

[0173] Still another example of such a pre-treatment may include solvent extraction. Suitable carriers may be non-polar solvents including, but not limited to, haloalkanes such as chloromethane, dichloromethane, chloroform, and long-chain alcohols such as 1-octanol. The crown ether complexing agents discussed above are generally more soluble in water than non-polar solvents. Accordingly, modification of the crown ether complexing agents discussed above by addition of hydrophobic groups such as benzo groups and long chain aliphatic functional groups may improve the partitioning of the crown ether complexing agent in the non-polar solvent.

[0174] In some embodiments wherein the impurity is sodium, the aluminium chloride liquor may be purified by passing it through a semi-permeable cation selective membrane, in particular a sodium selective membrane to separate sodium impurities from the liquor.

[0175] After undergoing any pre-treatments such as described above, the resulting aluminium chloride liquor may be concentrated in an evaporator to increase the Al concentration in solution. The Al concentration in solution of the aluminium chloride liquor after evaporation may be at least about 1 g/L, about 10 g/L, about 20 g/L, about 30 g/L, about 40 g/L, about 50 g/L, about 60 g/L, about 70 g/L, about 80 g/L, or about 90 g/L. The Al concentration in solution of the aluminium chloride liquor after evaporation may be less than about lOOg/L, about 90g/L, about 80 g/L, about 70 g/L, about 60 g/L, about 50 g/L, about 40 g/L, about 30 g/L, about 20 g/L, or about 10 g/L. In an embodiment, the Al concentration in solution of the aluminium chloride liquor after evaporation may be in a range of between about 1-100 g/L, for example a range between any two of the above upper and/or lower concentrations, such as about 10-90 g/L, or 50-85 g/L, or about 60-80 g/L. To facilitate crystallisation, the Al concentration in the aluminium chloride liquor after evaporation is preferably at or just below the saturation concentration for the solution. [0176] The concentrated liquor is then treated, for example in the manner described in detail above, in order to precipitate aluminium chloride hexahydrate solids from the aluminium chloride liquor.

[0177] It is to be understood that the embodiments and examples described herein also encompass a process operating in a continuous method as close to steady state as practically achievable. With this understanding, it would be appreciated by a person skilled in the art that during start up and/or shut down, the operating conditions and/or feed requirements can substantially deviate from those outlined here. For example, it may be required that units and/or lines are primed with solutions the compositions of which fall outside that described here.

[0178] It will also be appreciated by a person skilled in the art that there may be one or more top up streams required to maintain the desired compositions and flow rates throughout the system. These streams could be required due to, amongst other things, changes in environmental conditions, leaks or other unexpected losses, raw material variations.

[0179] The present application claims priority from AU2022902444 filed on 26 August 2022, the entire contents of which are incorporated herein by reference.

EXAMPLES

[0180] In order that the disclosure may be more clearly understood, particular embodiments of the invention are described in further detail below by reference to the following non-limiting experimental materials, methodologies and examples.

Example 1: Regeneration of hydrochloric acid for digesting aluminium chloride hexahydrate solids

[0181] A non-limiting example of the production of high purity alumina (HP A) is provided in the flowsheet in Figures 1 and 2. Digestion of aluminium-bearing material

[0182] Aluminium-bearing material is digested with hydrochloric acid (HC1 (aq)) in an HC1 acid digester unit which is continuously stirred. The digestion may be performed in a single reactor or in a plurality of reactors (not shown) arranged in series. The molar (M) concentration of HC1 in the liquor in the HC1 acid digester unit(s) may vary as described herein. The resulting slurry is then separated from a clarified aluminium chloride liquor (AlCh) comprising aluminium chloride and a number of impurities in solution.

Precipitation and digestion of aluminium chloride hexahydrate (ACH) solids

[0183] Hydrogen chloride gas (HC1 (g)) is bubbled through the aluminium chloride liquor in a continuously stirred crystallisation vessel (see “Precipitation” in Figures 1 and 2) in order to precipitate aluminium chloride hexahydrate solids. From Figure 2, it can be seen that HC1 (g) can be sourced from the HC1 regeneration described herein. The crystallisation vessel can comprise two or more crystallisation tanks (e.g. three tanks) in series, with the HC1 level increasing progressively to a concentration in the final tank. Alternatively, one crystallisation tank may be used. The resulting precipitation product stream then undergoes a separating step to obtain an aluminium chloride hexahydrate (ACH) solids stream and an ‘over-azeotropic’ process HC1 (aq) stream comprising about 32 wt% HC1.

[0184] The ACH solids can then undergo one or more additional digestion/crystallisation processes. Referring to the dashed box in Figure 2, an acid digester unit is fed with the separated ACH solids and re-digested with HC1 (aq) to create an aluminium chloride liquor. This liquor is fed to a second crystallisation vessel (i.e. a second “Precipitation”) to undergo crystallisation by bubbling HCl(g) through the aluminium chloride liquor to re-precipitate aluminium chloride hexahydrate solids. The resulting precipitation product stream then undergoes a separating step to obtain an aluminium chloride hexahydrate (ACH) solids stream and an ‘over-azeotropic’ process HC1 (aq) stream comprising about 32 wt% HC1. [0185] As will be appreciated, the dashed box surrounding the ACH digest, precipitation, and separator units in Figure 2 indicates that there may be more than one iteration of the ACH digestion/precipitation process. For example, as indicated by the upwards dashed vertical arrow in Figure 2, ACH solids leaving the separator unit (within the dashed box) can be fed to a subsequent acid digester unit for further purification, highlighting that the digester units can be set up in series and that the process described above can be repeated for any number of cycles until the purity of the ACH solids reaches the desired level.

Calcination of ACH to prepare high purity alumina

[0186] When the purity of the ACH solids reaches the desired level, the ACH solids leaving the separator are fed to a calciner where HPA is generated as a product. Although not shown in Figure 2, low quality HC1 (g) comprising about 52 wt% HC1 may also be generated in the calciner and this can be fed to the HC1 regeneration unit as described herein.

Recycling and regeneration ofHCl

[0187] Hydrochloric acid (HC1 (aq)) used in the digestion of the aluminium -bearing material and ACH solids can be provided by the hydrochloric acid (HC1) recycle stream described herein. Additionally, hydrogen chloride gas (HC1 (g)) used to precipitate ACH solids can be regenerated from the over-azeotropic HC1 process stream described herein. The HC1 recycle stream and regenerated HC1 (g) can be generated by a HC1 regeneration and Impurity Removal Unit.

[0188] The operation of the HC1 regeneration and Impurity removal are shown in Figure 3. Hot gas from the calciner, which has an increased HC1 concentration, is fed to an absorber unit where it is quenched by applying external cooling and mixing with a portion of an azeotropic 19 wt% HC1 process stream that originally comes from the stripping column described below. Any uncondensed gas is separated at the bottom from the liquid phase and transferred to the tail tower, which is a scrubber that the aforementioned the azeotropic 19 wt% HC1 process stream enters first. Any remaining HC1 is removed from the gas stream, and inert gases coming from the calciner (e.g. leaked air or combustion gases) can also be released.

[0189] The liquid phase from the absorber unit has an HC1 strength of about 32 wt% and is mixed with the over azeotropic 32 wt% HC1 process stream generated following the one or more separation steps described herein. This mixture is transferred to the stripping column.

[0190] In the stripping column, the over-azeotropic 32 wt.% HC1 process stream described above is heated under pressure such that HC1 (g) is generated and leaves the column from the top and is recycled for use in the one or more ACH precipitation steps. The HC1 (g) exiting the stripping column may also be combined with water to produce a HC1 wash stream which may be recycled and used to wash the separated aluminium chloride hexahydrate solids (not shown).

[0191] The remaining solution leaves the stripper column at the bottom as a azeotropic 19 wt% HC1 process stream and contains the inorganic impurities brought to the system. Part of the solution is returned to the absorber unit, while the remaining azeotropic 19 wt% HC1 process stream is discharged.

[0192] To close the HC1 balance of the current system, the azeotropic 19 wt% HC1 process stream - that contains inorganic impurities - generated by the process is purified by subjecting the azeotropic 19 wt.% HC1 process stream to an impurity removal step, for example by distillation (see Figure 3). Distillation of the azeotropic 19 wt% HC1 process stream produces an impurity rich ‘brine’ stream concentrated in substantially all the impurities and a separate clean 19 wt% HC1 recycle stream for recycling back to the HC1 digester unit and/or ACH digester unit(s) as shown in Figure 2.

[0193] Table 1 summarises various outputs based on modelling projections performed on the HC1 (aq) recycling process described herein compared to an HC1 (g) regeneration process utilising conventional pressure swing (PS) technology. The amount of HC1 is shown relative to the amount of HC1 in HC1 (g) regenerated utilising conventional pressure swing (PS) technology. Energy consumption is shown relative to the projected heating requirement utilising conventional pressure swing (PS) technology. Cost is shown relative to conventional pressure swing (PS) technology. As highlighted below, the present process recycling the azeotropic HC1 (aq) recycle stream can provide various savings in terms of HC1 (g) consumption, energy consumption and capital expenditure as highlighted below.

Table 1: Comparison of HC1 (aq) recycling (present process) to conventional pressure swing technology