TW202421582A - Process for preparing alumina - Google Patents

Abstract

The present disclosure relates generally to processes for preparing high purity alumina. More specifically, the present disclosure relates to processes for preparing high purity alumina utilising an acid regeneration system for recycling acid throughout the process.

Description

Method for preparing aluminum oxide

The present invention relates to a method for preparing aluminum oxide, and more particularly to an acid regeneration system and method for preparing high-purity aluminum oxide.

High-purity alumina is used in a wide range of technological applications, including 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, aerospace and aviation industry components, and high-strength ceramic tools. It can also be used in lithium-ion batteries as an electrical insulator between the anode and cathode cells. High purity specifications are particularly necessary in the latter application, as any significant impurities will promote undesirable electron transfer between the cells.

High purity alumina can be made directly from aluminum metal by reacting high purity aluminum metal with an acid to produce an aluminum salt solution, which is then concentrated and the concentrated salt solution is spray-baked to provide alumina powder. This method is based on the premise of preparing high purity alumina from high purity aluminum raw materials to reduce the possibility of impurity contamination.

Alternatively, alumina can be prepared from other raw materials, however, each raw material presents challenges in processing to the appropriate purity level due to the presence of impurities in the raw materials.

Smelting grade or metallurgical grade alumina can be made by direct calcining aluminum hydroxide produced from alumina ores using the Bayer process. However, these calcined grades of alumina may have a soda content of 0.15-0.50%, which is too high for the applications discussed above.

Aluminous clays such as kaolin contain aluminum oxide and a relatively high silicon content in the form of silicon oxide. During leaching of such aluminous clays, various impurities such as iron, titanium, calcium, sodium, potassium, magnesium and phosphorus (which are found as oxides in aluminous clays) are leached into the solution along with the aluminum.

Therefore, there is a need to develop alternative, more efficient methods to consistently prepare high purity alumina from a variety of aluminum sources.

The inventors have conducted research and development to develop a method for producing high purity alumina. Specifically, the inventors have discovered that process hydrochloric acid, which is typically considered a "waste" stream in the industry, can be unconventionally regenerated and recycled to digest aluminum chloride hexahydrate solids to produce high purity alumina. By using acid rather than conventional water to re-digest the solids, the inventors unexpectedly reduced the overall reagent volume and energy consumption of the process while maintaining the formation of high purity alumina.

In one embodiment, a method for preparing high purity alumina from aluminum chloride hexahydrate solid is provided, the method comprising: a) digesting aluminum chloride hexahydrate solid having one or more inorganic impurities in hydrochloric acid to produce an aluminum chloride liquid containing aluminum chloride and the inorganic impurities in solution; b) precipitating aluminum chloride hexahydrate solid from the liquid in one or more crystallization stages such that at least some of the inorganic impurities remain in the liquid; c) separating the precipitated aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; d) treating the separated aluminum chloride hexahydrate solid to form high purity alumina; e) reducing the concentration of hydrogen chloride in the hydrochloric acid process stream as appropriate; 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).

In another embodiment, a method for preparing high purity alumina from an aluminum-containing material is provided, the method comprising: a) digesting the aluminum-containing material in hydrochloric acid to provide an aluminum chloride liquid comprising aluminum chloride and one or more impurities in solution; b) precipitating an aluminum chloride hexahydrate solid from the liquid in one or more crystallization stages such that at least some of the inorganic impurities remain in the liquid; c) separating the precipitated aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; d) treating the separated aluminum chloride hexahydrate solid to form high purity alumina; e) optionally reducing 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 for digesting the aluminum-containing material in step a).

In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration above the azeotropic point of the stream, and the method includes step e) reducing the hydrogen chloride concentration in the hydrochloric acid process stream before removing the one or more inorganic impurities. In another embodiment, step e) includes heating the super-azeotropic hydrochloric acid process stream under pressure to reduce the azeotropic point to generate a hydrogen chloride gas stream and reduce the hydrogen chloride concentration in the hydrochloric acid process stream before removing the one or more inorganic impurities. In one embodiment, heating the hydrochloric acid process stream reduces 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 heating the hydrochloric acid process stream in step e) is recycled for use as a source of hydrogen chloride gas in one or more crystallization stages in step b).

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

In another aspect, a high purity alumina prepared by a method according to any aspect, embodiment or example described herein is provided.

In another embodiment, a system for preparing high-purity alumina from aluminum chloride hexahydrate solid having one or more impurities is provided, the system comprising: One or more acid digesters for digesting aluminum chloride hexahydrate solid in hydrochloric acid to produce aluminum chloride liquid containing the inorganic impurities; A crystallization vessel associated with each acid digester for receiving the aluminum chloride liquid from the acid digester and for precipitating aluminum chloride hexahydrate solid from the liquid so that at least some of the inorganic impurities remain in the liquid; Optionally, one or more subsequent crystallization vessels for recrystallizing the aluminum chloride hexahydrate solids; a separation component associated with the one or more crystallization vessels for separating the aluminum chloride hexahydrate solid formed from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; a stripping column, if applicable, for receiving the hydrochloric acid process stream from the separation component to generate a hydrogen chloride gas stream and reduce the concentration of hydrogen chloride in the hydrochloric acid process stream; a distillation component 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 recirculating the hydrochloric acid recycle stream to the one or more acid digesters; and Heat treatment components for heat treating the aluminum chloride hexahydrate solid to provide high purity aluminum oxide.

In another embodiment, a system for preparing high-purity alumina from an aluminum-containing material containing one or more impurities is provided, the system comprising: One or more acid digesters for digesting the aluminum-containing material to provide an aluminum chloride liquid containing the inorganic impurities; A crystallization vessel for receiving the aluminum chloride liquid from the acid digester and for precipitating aluminum chloride hexahydrate solid from the liquid so that at least some of the inorganic impurities remain in the liquid; Optionally, one or more subsequent crystallization vessels for recrystallizing the aluminum chloride hexahydrate solids; a separation member associated with the one or more crystallization vessels for separating the aluminum chloride hexahydrate solid formed from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; an optional stripping column for receiving the hydrochloric acid process stream from the separation member to generate a hydrogen chloride gas stream and reduce 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 recirculating the hydrochloric acid recycle stream to the one or more acid digesters for digesting the aluminum-containing material; and Heat treatment components for heat treating the aluminum chloride hexahydrate solid to provide high purity aluminum oxide.

Other aspects and embodiments related to the present invention are described herein. It will be understood that each example, aspect, and embodiment of the present invention described herein applies mutatis mutandis to each other example, aspect, or embodiment unless otherwise specifically stated. The scope of the present invention is not limited to the specific examples described herein, which are intended for illustrative purposes only. Functionally equivalent products, compositions, and methods are clearly within the scope of the disclosure described herein.

The present invention is described below with various non-limiting examples relating to research conducted to develop a method for preparing high purity alumina.

In the following description, reference is made to the accompanying drawings, which form a part of the following description and show several embodiments by way of illustration. It should be understood that other embodiments may be used and structural changes may be made without departing from the scope of the invention.

With respect to the definitions provided herein, unless otherwise stated or implied from the context, the defined terms and phrases include the meanings provided. Unless otherwise expressly stated or apparent from the context, the terms and phrases used below do not exclude the meanings that one skilled in the art would have acquired for the term or phrase. These definitions are provided to aid in describing particular embodiments and are not intended to limit the claimed invention, as the scope of the invention is limited only by the scope of the patent application. In addition, unless the context requires otherwise, a singular term shall include the plural and a plural term shall include the singular.

All publications discussed and/or referenced herein are incorporated by reference in their entirety.

Any discussion of documents, acts, materials, devices, articles or the like included in this specification is for the sole purpose of providing background for the present invention. It should not be regarded as an admission that any or all of these matters form part of the prior art basis or are common knowledge in the field relevant to the present invention simply because they existed before the priority date of the various technical solutions of this application.

Throughout this disclosure, references to a single step, composition of matter, group of steps, or group of compositions of matter are to be construed to cover both one and plural (i.e., one or more) of such steps, composition of matter, group of steps, or group of compositions of matter, unless specifically stated otherwise or the context requires otherwise. Thus, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. For example, reference to "a" includes one as well as two or more; reference to "an" includes one as well as two or more; reference to "the" includes one as well as two or more, and so forth.

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

The term "and/or", such as "X and/or Y", should be understood to mean "X and Y" or "X or Y" and should be regarded as providing clear support for both or either meanings.

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 such terms refer. Furthermore, reference to a "second" item does not require or preclude the existence of lower-numbered items (such as a "first" item) and/or higher-numbered items (such as a "third" item).

As used herein, the phrase "at least one of...", when used with a list of items, means that different combinations of one or more of the listed items may be used and that only one of the items in the list may be required. The item may be a specific object, thing, or category. In other words, "at least one of..." means that any combination or number of items in the list may be used, but not all items in the list are 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 but not limited to, two items A, one item B, and ten items C; four items B and seven items C; or some other suitable combination.

As used herein, unless stated to the contrary, the term "about" generally refers to +/- 10%, eg, +/- 5%, of a specified value.

It should be understood that certain features described in the context of separate embodiments herein for the purpose of clarity may also be provided in combination in a single embodiment. Conversely, various features described in the context of a single embodiment for the sake of brevity may also be provided separately or in any sub-combination.

Throughout this specification, various aspects and components of the present invention may be presented in a range format. The inclusion of a range format is for convenience and should not be construed as an inflexible limitation on the scope of the invention. Therefore, unless specifically indicated, the description of a range should be considered to have specifically disclosed all possible sub-ranges and individual numerical values within that range. For example, unless an integer is required or implied by the context, a 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 range, such as 1, 2, 3, 4, 4.5, and 5. This applies regardless of the breadth of the disclosed range. Where specific values are required, they will be indicated in this specification.

Throughout this specification, the word "comprise" or variations such as "comprises" or "comprising" should be understood to imply the inclusion of stated elements, integers or steps, or groups of elements, integers or steps, but not the exclusion of any other elements, integers or steps, or groups of elements, integers or steps. The phrase "consisting of" means the listed elements and no other elements. Specific terms

The term " _alumina " as used herein refers to aluminum oxide ( _Al2O3 ), specifically, the crystalline polymorphs α, γ, θ and κ. High purity alumina refers to _Al2O3 with a purity of about 99.99%, such as purity >99.99% ( _4N ) or purity >99.999% (5N), which is suitable for use as a key material in a variety of applications including but not limited to the following applications: high intensity discharge lamps, LEDs, sapphire glass for precision optical devices, handheld devices, TV screens and dials, synthetic gemstones for lasers, aerospace and aviation industry components, high strength ceramic tools, or electrical insulators in lithium ion batteries.

As used herein, the term "aluminum-containing material" refers to any material having a content (wt% equivalent Al ₂ O ₃ ) greater than 10%. Examples of such aluminum-containing materials include, but are not limited to, acid-soluble aluminum hydroxide compounds such as aluminite (γ-Al(OH) ₃ ), trihydroxyaluminite (α-Al(OH) ₃ ), norite, triclinic aluminite or natrium aluminite (NaAl(OH) ₂ .CO ₃ ); acid-soluble aluminum oxide (hydrogen) compounds such as alumina (α-AlO(OH)) or aluminite (γ-AlO(OH)), tricalcium aluminate hexahydrate (TCA); or Al-substituted hydroxy iron oxides such as aluminum-containing goethite (Fe(Al)OOH). The term covers naturally occurring materials, for example aluminium-containing clays such as kaolin or alumina ores, or products or by-products of a process. As an example, aluminium-containing materials may be by-products of alumina production from the Bayer process, such as calciner dust, DSP and red mud, which typically have an aluminium content of > 10 wt% (equivalent _{Al2O3 )} .

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

The calcination of alumina in alumina production produces fine particles which can be emitted as calciner dust. Calciner dust emissions can be mitigated and controlled to low levels by using various collection techniques, such as using electrostatic precipitators on the calciner pile. ESP dust is the fine particle residue captured by the electrostatic precipitator. Calciner dust particles can contain alumina as well as various (oxy)alumina hydroxides and alumina compounds, which are contaminated with occluded and surface soda.

DSP is a collective term used to describe several acid-soluble silica-containing compounds that precipitate in the Bayer process. DSP is primarily Bayer sodium ore, with the general formula [NaAlSiO ₄ ] ₆ .mNa ₂ X.nH ₂ O, where "mNa ₂ X" represents the sodium salts intercalated within the cage structure of the zeolite, and X can be carbonate (CO ₃ ^{2 -} ), sulfate (SO ₄ ^{2 -} ), chloride (Cl ^- ), aluminate (AlO ₄ ) ^- ). DSP is formed in the Bayer process "desiliconization" loop prior to the digestion loop and also in the digestion loop itself. DSP ultimately becomes part of alumina slag (e.g. red mud). In addition, those skilled in the art will appreciate that, although the silica content is reduced in the desiliconization loop, silica may be supersaturated in solution throughout the Bayer process. Therefore, DSP may also form as deposits on the inner surfaces of tanks, pipes and heaters.

As used herein, the terms "soda" and "soda content" refer to _Na2O and the amount of _Na2O present in a material, reported as a weight percent (wt%) of the total weight of the material. It will be appreciated that the soda content of high purity alumina must be low. References to "surface soda" relate to the presence of _Na2O adsorbed on the surface of the particles, while references to "occluded soda" relate to soda encapsulated in another material.

Calcination is a heat treatment process in which a solid is heated in the absence of air or oxygen or in the presence of a controlled supply of air or oxygen, usually resulting in the decomposition of the solid to remove carbon dioxide, water of crystallization or volatiles, or to effect a phase change, such as the conversion of aluminum hydroxide to aluminum oxide. Such heat treatment processes can be carried out in furnaces or reactors such as vertical furnaces, rotary kilns, multi-bed furnaces and fluidized bed reactors.

The term 'normal boiling point' is used to refer to the temperature at which a liquid or slurry boils under atmospheric pressure. It will be appreciated that the boiling point may also vary depending on the various solutes and their concentrations in the liquid or slurry. Atmospheric pressure (also known as ambient pressure) is considered to be approximately 1 bar (e.g. 1.01325 bar).

The term "wt.%" refers to the percentage content of a substance in a stream or composition described herein by weight (ie, w/w). Azeotropes and Azeotropic Points

As used herein, an "azeotrope" refers to a hydrochloric acid/water mixture containing two or more liquids in proportions that cannot be varied or changed by simple distillation. This occurs because when the azeotrope is boiled, the composition of the vapor is in the same proportions as the unboiled mixture. The concentration at which this occurs is often referred to as the "azeotropic point" of the mixture. For example, referring to FIG. 4 for a hydrochloric acid/water mixture, an azeotrope at atmospheric pressure is formed when the concentration of hydrogen chloride in the mixture is at or near about 19-20 wt.%. However, as described herein, this concentration may increase or decrease depending on the pressure.

In practice, boiling a hydrochloric acid/water mixture at a given pressure in which the concentration of hydrogen chloride is below the azeotropic point will result in more water evaporating than hydrochloric acid evaporating before the composition of the boiling solution reaches the azeotropic point at that pressure. On the other hand, boiling a hydrochloric acid/water mixture at a given pressure in which the concentration of hydrogen chloride is above the azeotropic point will result in more hydrogen chloride gas evaporating than water evaporating before the azeotropic point is reached at that pressure.

The azeotropic point of a mixture may vary depending on the pressure. For example, with respect to the hydrochloric acid stream described herein, at higher pressures (i.e., pressures above atmospheric pressure), the azeotropic point of the stream decreases, and at lower pressures (i.e., pressures below atmospheric pressure), the azeotropic point of the stream increases. Once at or near the azeotropic point, the concentration of hydrogen chloride in the mixture cannot be varied or changed by simple distillation. The azeotropic point will be understood to be related to the pressure and/or temperature of the stream or mixture. Acid Regeneration Method and System for Preparing High Purity Alumina

The present inventors have developed a method for preparing high purity alumina. Specifically, the present inventors have developed a method that recycles hydrochloric acid, which is usually regarded as "waste" in the industry, for use in digesting aluminum chloride hexahydrate solids and/or aluminum-containing materials prior to crystallization to prepare high purity alumina. According to some embodiments or examples described herein, by recycling such "waste" hydrochloric acid into the process as a liquid for preparing high purity alumina, the present inventors unexpectedly reduced the total reagent amount and energy consumption of the process while maintaining the formation of high purity alumina. In addition to recycling the hydrochloric acid as a liquid, the process described herein also allows a portion of the acid to be regenerated as hydrogen chloride gas for use in crystallizing aluminum chloride hexahydrate solid from the digested aluminum chloride liquid.

Referring to FIGS. 1 to 3 , a system for preparing high purity alumina is provided. The system includes: one or more acid digesters for digesting aluminum chloride hexahydrate solid/aluminum-containing material in hydrochloric acid to produce aluminum chloride liquid; a crystallization vessel associated with each acid digester for receiving aluminum chloride liquid from the acid digester and for precipitating aluminum chloride hexahydrate solid from the liquid so that at least some of the inorganic impurities remain in the liquid; a separation member associated with the crystallization vessel for separating the formed aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing inorganic impurities; and optionally, HCl. a regeneration apparatus (e.g., a stripper/distiller) for receiving a hydrochloric acid process stream from a separation component to generate a hydrochloric acid gas stream and to reduce the concentration of hydrochloric acid 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 recirculating the hydrochloric acid recycle stream to one or more acid digesters (e.g., for digesting aluminum chloride hexahydrate solids and/or aluminum-containing materials); and a heat treatment component for heat treating the aluminum chloride hexahydrate solids to provide high purity alumina. High Purity Alumina

The methods of the present invention produce high purity alumina. As described herein, high purity alumina can be prepared from various aluminum-containing materials (e.g., aluminum-containing clays such as kaolin) or products or by-products of processes such as the Bayer process. However, many of these materials have high levels of inorganic impurities relative to the high purity threshold (about 99.99%) of the final desired product. Removing or controlling inorganic impurities to achieve the high purity threshold is technically difficult.

The types and levels of impurities throughout the described process will depend on many factors, primarily on the source of the aluminum-containing material, but it will be appreciated that, although the process steps described are intended to provide a reduction in impurity levels at each step, new inorganic impurities may be introduced during the various process steps employed in the production of high purity alumina.

The term inorganic "impurity" or "impurities" is intended to cover any non-aluminum compounds that are present. Specifically, with respect to the final high purity alumina product, inorganic "impurity" or "impurities" means any material other than aluminum oxide (Al ₂ O ₃ ). The grade of high purity alumina is based on the total level of inorganic impurities (regardless of composition) in the final product, with products with purity >99.99% Al ₂ O ₃ (i.e., less than 0.01% inorganic impurities) being graded "4N" and products with purity >99.999% Al ₂ O ₃ (i.e., less than 0.001% inorganic impurities) being graded "5N".

By way of non-limiting example, at least one inorganic impurity may be calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), sodium (Na), phosphorus (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), phosphorus (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, such as a chloride salt (e.g., NaCl).

Individual or total inorganic impurities in the final high purity alumina product may be less than about 1000 ppm, 500 ppm, 400 ppm, 300 ppm, 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.

In one example, the amount of inorganic impurities 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 one example, the impurity 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 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 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 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 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 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 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 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 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 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 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 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 zinc (Zn) is less than about (in ppm) 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1. Method for preparing high purity alumina

As will be described herein, recycling and regeneration of hydrochloric acid, which is typically considered a waste in the industry, closes the acid loop, thereby reducing overall acid consumption, thereby creating an optimized loop with the potential to reduce capital and/or operating costs.

In one aspect or embodiment, a method for preparing high-purity alumina from aluminum chloride hexahydrate solid is provided, the method comprising: a) digesting aluminum chloride hexahydrate solid having one or more inorganic impurities in hydrochloric acid to produce an aluminum chloride liquid containing aluminum chloride and the inorganic impurities in solution; b) precipitating aluminum chloride hexahydrate solid from the liquid by spraying hydrogen chloride gas into the aluminum chloride liquid in one or more crystallization stages, so that at least some of the inorganic impurities remain in the liquid; c) separating the precipitated aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; d) treating the separated aluminum chloride hexahydrate solid to form high purity alumina; e) reducing 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 hydrochloric acid in step a).

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

In one embodiment, reducing the concentration of hydrogen chloride in the hydrochloric acid process stream in step e) includes heating the hydrochloric acid process stream to generate a hydrogen chloride gas stream and reducing the concentration of hydrogen chloride in the hydrochloric acid process stream.

In one embodiment, reducing the concentration of hydrogen chloride in the hydrochloric acid process stream in step e) comprises heating the hydrochloric acid process stream under pressure to generate a hydrogen chloride gas stream and reducing the concentration of hydrogen chloride in the hydrochloric acid process stream.

In one embodiment, in step e), the hydrochloric acid concentration in the hydrochloric acid process stream is reduced to a concentration at or near the azeotropic point of the stream. In another 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).

In one embodiment, in step e), the superazeotropic hydrochloric acid process stream obtained from step c) is heated under pressure to reduce the azeotropic point to generate a hydrogen chloride gas stream and reduce the concentration of hydrogen chloride in the hydrochloric acid process stream.

In one embodiment, at least a portion of the hydrogen chloride gas stream generated by heating the hydrochloric acid process stream in step e) is recycled for use as a source of hydrogen chloride gas in one or more crystallization stages in step b).

In some examples, the hydrochloric acid concentration in the hydrochloric acid process stream is reduced, preferably to or near the azeotropic point, by heating to generate a hydrochloric acid gas stream, at least a portion of which can be recycled to crystallize aluminum chloride hexahydrate, the hydrochloric acid process stream can then be easily handled to allow impurities to be removed from the process and the remaining hydrochloric acid is recycled for use in digesting aluminum chloride hexahydrate and/or aluminum-containing materials. In addition, the recycling of hydrochloric acid as both a liquid and a gas in the process described herein reduces energy consumption and operating costs relative to methods known in the art by combining the hydrochloric acid gas obtained by reducing the concentration of the hydrochloric acid process stream and the hydrochloric acid gas generated from the thermal decomposition and/or calcining furnace described below.

In one embodiment, removing impurities from the hydrochloric acid process stream in step f) comprises distilling the hydrochloric acid process stream. In another embodiment, removing 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.

Advantageously, distilling a hydrochloric acid process stream at or near the azeotropic point (i.e., azeotropic distillation) produces an impurity-rich "brine" stream in which most of the impurities from the hydrochloric acid process stream are concentrated; and a "clean" hydrochloric acid stream having significantly lower impurity levels than "brine" hydrochloric acid. Distilling a hydrochloric acid process stream at or near the azeotropic point evaporates and recondenses hydrochloric acid of known concentration, and the recondensed "clean" hydrochloric acid stream advantageously has a low impurity content and consistent concentration and can be used as the hydrochloric acid recycle stream described herein.

In order to fully understand the optimized hydrochloric acid loop provided by some embodiments of the present invention and one or more advantages associated therewith, the method can be described with reference to the following steps: digesting aluminum chloride hexahydrate solid in hydrochloric acid to form aluminum chloride liquid; precipitating aluminum chloride hexahydrate solid from the aluminum chloride liquid, including by sparging hydrogen chloride gas; separating the precipitated aluminum chloride hexahydrate solid from the liquid to obtain a hydrochloric acid process stream; Regenerating and recycling hydrochloric acid from a hydrochloric acid process stream, comprising optionally reducing 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 hydrochloric acid in step a); and treating the separated aluminum chloride hexahydrate solid to form high purity alumina. Digestion of aluminum chloride hexahydrate solid in hydrochloric acid

The process comprises digesting an aluminum chloride hexahydrate solid including one or more inorganic impurities. The one or more impurities present in the precipitate arise from various aluminum-containing materials (e.g., aluminum-containing clays such as kaolin), or are products or by-products of processes such as the Bayer process. However, many of these materials have a high content of inorganic impurities relative to the high purity threshold (about 99.99%) of the final desired product. Removing or controlling the inorganic impurities to achieve the high purity threshold is technically difficult.

In one embodiment, aluminum chloride hexahydrate solid is provided as a precipitate from an aluminum chloride liquid via one or more crystallization stages described herein. The aluminum chloride liquid may be a digestion product from an acid digestion of an aluminum-containing material described herein. In this embodiment, solid digestion may also be referred to as re-digestion.

It will be understood that although crystallization and precipitation of aluminum chloride hexahydrate solid from aluminum chloride liquid as described herein may leave at least a portion of the impurities in the liquid, one or more inorganic impurities are still present in the aluminum chloride hexahydrate solid. Accordingly, in order to further reduce the concentration of inorganic impurities in the final high purity alumina, the aluminum chloride hexahydrate solid is subjected to one or more digestion and crystallization steps to form high purity alumina prior to thermal decomposition and calcination. For example, the aluminum chloride hexahydrate solid is further digested in hydrochloric acid to form an aluminum chloride liquid comprising aluminum chloride and any impurities (e.g., any impurities remaining from the first crystallization stage used to prepare the initial aluminum chloride hexahydrate precipitate). This liquid may then be crystallized as described below to produce a precipitated aluminum chloride hexahydrate solid, leaving additional impurities in the remaining liquid.

Thus, the method comprises digesting an aluminum chloride hexahydrate solid having one or more inorganic impurities in hydrochloric acid to produce an aluminum chloride liquid comprising aluminum chloride and the inorganic impurities in solution.

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

Acid digestion of aluminum chloride hexahydrate solids can be carried out in a single reactor (e.g., an acid digester) or in a plurality of reactors configured in series (e.g., up to 5 acid digesters).

The acid digestion can be carried out at a temperature ranging from ambient temperature to the atmospheric boiling point of the resulting aluminum chloride liquid. The acid digestion can be carried out 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 can be carried out 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 can be carried out at a temperature between any two of these higher and lower amounts, such as about 25° C. to 100° C., 50° C. to 95° C., 70° C. to 90° C., or 75° C. to 85° C., for example, about 80° C.

It will be appreciated that the rate of acid digestion will depend on the temperature, solid concentration, and acid concentration of the resulting digestion mixture. Acid digestion can be performed for a period of at least about 15 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. Acid digestion can be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, 2 hours, or 1 hour. Solid precipitation can be performed within a range provided by any two of the higher amount and/or the lower amount, such as a period in the range of about 15 minutes to 6 hours or about 2 to 4 hours. In a specific example, the period is about 3 hours.

In one embodiment, the hydrochloric acid used to digest the aluminum chloride hexahydrate solids may also be referred to as a hydrochloric acid digestion stream.

In one embodiment, the hydrochloric acid used to digest the aluminum chloride hexahydrate solids may comprise or consist of a hydrochloric acid recycle stream as described herein. The embodiments or examples described herein for the hydrochloric acid recycle stream are also applicable below to the hydrochloric acid used to digest the aluminum chloride hexahydrate solids. Precipitation of aluminum chloride hexahydrate solids from aluminum chloride liquid

After acid digestion to produce an aluminum chloride liquid containing inorganic impurities in solution, aluminum chloride hexahydrate solid is then precipitated from the aluminum chloride liquid such that at least some of the inorganic impurities remain in the liquid.

The precipitated aluminum chloride hexahydrate solid obtained in this step can be referred to as "reprecipitated" aluminum chloride hexahydrate solid. For example, since the initial aluminum chloride hexahydrate solid digested in step a) can be a precipitate obtained from the aluminum chloride liquid described herein, which is then subjected to acid digestion to produce aluminum chloride liquid, the subsequent precipitation described herein produces reprecipitated aluminum chloride hexahydrate solid.

The method comprises precipitating an aluminum chloride hexahydrate solid from the liquid in one or more crystallization stages such that at least some of the inorganic impurities remain in the liquid. For example, an aluminum chloride liquid comprising aluminum chloride and one or more impurities in solution may be subjected to one or more crystallization stages in order to precipitate an aluminum chloride hexahydrate solid.

Prior to precipitation, the aluminum (Al) concentration in the solution of the aluminum chloride liquid 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. Prior to precipitation, the Al concentration in the solution of the aluminum chloride liquid may be less than about 100 g/L, about 90 g/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 one embodiment, before precipitation, the Al concentration in the aluminum chloride liquid solution may be in the range of about 1-100 g/L, for example, in the range between any two of the above-mentioned higher concentrations and/or lower concentrations, such as about 10-90 g/L or 50-85 g/L or about 60-80 g/L. In order to facilitate precipitation/crystallization, the Al concentration in the aluminum chloride liquid is preferably equal to or slightly lower than the saturation concentration of the solution.

An aluminum chloride liquid comprising aluminum chloride and one or more impurities in solution undergoes a crystallization stage in order to precipitate aluminum chloride hexahydrate solid and leave at least a portion of the impurities in the liquid. It will be appreciated that crystallization can be performed in a batch mode or a continuous mode. Additionally, crystallization can be performed in a single reactor (e.g., a crystallization vessel) or in a plurality of reactors configured in series such that the concentration of aluminum chloride hexahydrate solid precipitated in each vessel increases.

In the precipitation reactor, the chloride concentration in the liquid is increased to a saturated concentration or higher relative to aluminum chloride hexahydrate, thereby causing aluminum chloride hexahydrate to precipitate from the solution. For example, the initial chloride concentration can be increased to at least about 6 M. In another example, the initial chloride concentration can be increased to at least about 7 M, 8 M, 9 M, 10 M, or 11 M. The initial chloride concentration can be increased to provide less than about 12 M, 11 M, 10 M, 9 M, 8 M, or 7 M. The initial chloride concentration can be increased to provide an amount in the range between any two of these higher and lower amounts, such as between about 6 M to 12 M, 7 M to 11 M chloride, or 8 M to 10 M. In a specific example, the initial chloride concentration is about 9 M.

In one embodiment, the precipitation in step b) comprises spraying hydrogen chloride gas into the liquid. For example, the chloride concentration in the liquid can be easily increased by spraying hydrogen chloride gas. In certain embodiments, the chloride concentration is increased by continuously spraying hydrogen chloride gas. Alternatively, the spraying can be suspended periodically during the precipitation process. The spraying of the liquid can be suspended after the initial portion of the hydrogen chloride gas has been introduced into the liquid, for example, the spraying can be suspended after 50% of the hydrogen chloride gas has been introduced into the liquid. Advantageously, spraying hydrogen chloride gas instead of liquid can reduce the possibility of contaminating the liquid with undesirable impurities.

In one embodiment, the hydrochloric acid gas used to sparge the aluminum chloride liquid comprises or consists of hydrochloric acid gas that is regenerated and recycled from a hydrochloric acid process stream as described herein.

It will be appreciated that the use of multiple reactors in series for precipitation, and therefore the need to process smaller volumes of solution, may allow for improved control of acid concentration, temperature and other precipitation conditions, thereby improving control over the crystallization rate of the aluminum chloride hexahydrate solid.

The corrosive nature of hydrochloric acid and hydrogen chloride gases and aluminum chloride liquids can result in impurities being introduced into the process by corrosion of processing equipment. Therefore, care should be taken to ensure that processing equipment components are formed of materials that are inert to hydrochloric acid and hydrogen chloride gases whenever possible and/or to protect processing equipment components from acid attack.

Solid precipitation can be carried out 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. Solid precipitation can be carried out 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. Solid precipitation can be carried out at a temperature between any two of these higher and lower amounts, such as between about 25° C. and 100° C., 30° C. and 90° C., or 40° C. and 80° C.

Solid precipitation can be performed for a period of at least about 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. Solid precipitation can be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, or 2 hours. Solid precipitation can be performed within a range provided by any two of the higher amounts and/or lower amounts, such as a period in the range of about 1 hour to 6 hours or about 2 to 4 hours. In a specific example, the period is about 3 hours.

The concentrated liquid may be seeded to assist the crystallization kinetics and to increase the purity of the resulting product. The composition of the seed crystals may be any suitable material that promotes the crystallization of aluminum chloride hexahydrate from the aluminum chloride liquid, for example, the concentrated liquid may be seeded with aluminum-containing seed crystals such as aluminum chloride hexahydrate or alumina crystals. The aluminum chloride hexahydrate or alumina crystals used to seed the crystals may be recycled from other stages of the process.

The aluminum chloride liquid may be seeded with aluminum 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 aluminum chloride liquid may be seeded with aluminum 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 aluminum chloride liquid may be seeded with aluminum chloride hexahydrate crystals in an amount provided by any two of the higher amount and/or the lower amount, such as 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 amount of aluminum chloride hexahydrate crystals seeded may be in the range of 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, such seeding amounts including ranges may be provided for other suitable seeding materials.

Seed crystals may be added to the aluminum chloride liquid prior to introduction into the crystallization vessel (i.e., precipitation reactor). In this step, additional soluble aluminum-containing material may be added to the aluminum chloride liquid to increase the Al concentration to a desired level prior to seeding and crystallization.

When crystallization is performed in a plurality of reactors, one or more of the reactors may be seeded with aluminum chloride hexahydrate crystals. In one embodiment, when crystallization is performed in a plurality of reactors in series, liquid containing precipitated aluminum chloride hexahydrate solids fed from one reactor to a subsequent reactor in the series may be used to seed precipitation in the subsequent reactor.

In the case of performing multiple crystallization stages, seeding can be performed in some or all of the crystallization stages, but need not be performed in all of the crystallization stages. In one example, seeding can be performed only in the first crystallization stage of the multiple crystallization stages to generate aluminum chloride hexahydrate solid while most of the impurities are present in the aluminum chloride liquid produced in the hydrochloric acid digestion of the aluminum-containing material.

In one embodiment, the aluminum-containing seed crystals contain greater than 90%, 95%, 98%, or 99% aluminum compound to minimize the introduction of impurities into the liquid. In another embodiment, the aluminum-containing seed crystals are aluminum hexahydrate solids, which are greater than 90%, 95%, 98%, 99%, 99.9%, 99.99%, or 99.999% aluminum hexahydrate solids to minimize the introduction of impurities into the liquid. In other examples, the total amount of impurities in the aluminum-containing seed crystals is less than 1%, 0.1%, 0.01%, 0.001%, or 0.0001%.

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

Individual impurities or the total impurities in the seeds may be less than about 1000 ppm, 500 ppm, 400 ppm, 300 ppm, 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.

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 one 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.

In a process where there are multiple digestion and crystallization stages, operating conditions need not be the same in each stage and may vary in response to increased product purity. In one example, as discussed above, the presence of seed crystals may vary across crystallization stages. In addition, when seed crystals are provided in multiple crystallization stages, the amount of seed crystals or the type of seed crystals may be different in different stages, for example, the amount of seed crystals may be reduced in subsequent crystallizations.

In another example, the hydrochloric acid concentration may be different at different crystallization stages. Although a higher hydrochloric acid concentration will increase the amount of aluminum chloride hexahydrate solid precipitated from the liquid, this may also result in a higher impurity concentration in the precipitated solid. Conversely, a lower concentration will leave more aluminum in the liquid, but provide a purer precipitate.

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

In one embodiment, the or each crystallization stage is carried out in a plurality of reactors arranged in series. In such embodiments, the concentration of hydrochloric acid in the series reactors can be gradually increased to reach the concentration in the final reactor in the series as described above. In one embodiment, the or each crystallization stage comprises an aluminum chloride liquid that has undergone two or more precipitation stages, wherein the precipitation stages are connected in series.

In one embodiment, step b) comprises precipitating aluminum chloride hexahydrate solid from aluminum chloride liquid in two or more crystallization stages, wherein between each crystallization stage, the separated aluminum chloride hexahydrate is digested in a hydrochloric acid recycle stream to produce aluminum chloride liquid.

The resulting aluminum chloride hexahydrate solid is separated from the liquid to produce a hydrochloric acid process stream comprising inorganic impurities. It will be appreciated that the hydrochloric acid process stream comprises liquid and impurities after separation of the solid. The separated solid may form a solid stream that is separated from the hydrochloric acid process stream.

Any suitable known separation technique may be used, such as filtration, gravity separation, centrifugation, fractionation, and the like. It will be appreciated that the solids may undergo one or more washes during separation.

Prior to heat treatment to high purity alumina, the separated aluminum chloride hexahydrate solid can be washed to remove any liquid/impurities entrained in the solid. The solid can be washed with hydrochloric acid. The hydrochloric acid used to wash the solid can be recycled from the regenerated hydrochloric acid as described herein. In one embodiment, the hydrochloric acid used to wash the solid can be generated during the HCl regeneration process described herein, for example as a concentrated hydrochloric acid stream generated during the condensation of the hydrogen chloride gas stream described herein. Alternatively or in addition, the separated liquid and combined wash liquid can be recycled for use as a wash medium for filtering aluminum chloride hexahydrate solids generated upstream.

To further reduce the impurity concentration, the separated precipitated aluminum chloride hexahydrate solid may optionally undergo one or more additional digestion and recrystallization steps prior to thermal decomposition and calcination into high purity alumina. For example, the separated precipitated aluminum chloride hexahydrate solid may be digested in hydrochloric acid as described herein to form an aluminum chloride liquid comprising aluminum chloride and any impurities remaining from the crystallization stage. This liquid may then be subjected to one or more subsequent crystallization steps in the manner described above to produce a precipitated aluminum chloride hexahydrate solid, leaving additional impurities in the remaining liquid.

It will be appreciated that the acid digestion and crystallization stages in hydrochloric acid may be repeated as many times as necessary to achieve a suitably pure aluminum chloride hexahydrate solid concentration prior to treating the solid to form high purity alumina. However, in those embodiments where the residual impurities in the solid are sufficiently low that the alumina produced from thermal decomposition and calcination of the solid collected after filtration will meet the purity requirements for high purity alumina, repeated acid digestion and crystallization may not be required. Regeneration and Recycling of Process Hydrochloric Acid

As described above, a hydrochloric acid process stream containing impurities is produced after separation of the precipitated aluminum chloride hexahydrate solids from the hydrochloric acid process stream. In contrast to a "waste" stream, the inventors have discovered that the hydrochloric acid remaining in the liquid after acid digestion and crystallization can be recycled back into the process as a liquid stream for use in digesting aluminum chloride hexahydrate solids and/or aluminum-containing materials.

The hydrochloric acid process stream has a hydrogen chloride concentration. The hydrogen chloride content of the process stream results from one or more previous acid digestion steps and one or more crystallization stages described herein.

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

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.% by weight of the process stream. In one embodiment, the hydrochloric acid process stream has a hydrogen chloride concentration between about 25 wt.% and about 40 wt.% by 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 by weight of the stream. The hydrochloric acid process stream may have a hydrogen chloride concentration (in wt.%) of at least 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 higher and/or lower values, such as a range between about 28 wt.% to about 37 wt.%.

In one embodiment, the hydrochloric acid process stream has a concentration of hydrogen chloride above the azeotropic point of the stream, and the method includes step e) reducing the concentration of hydrogen chloride in the hydrochloric acid process stream prior to removing impurities.

To reduce the concentration, a hydrochloric acid process stream having a hydrochloric acid concentration above the azeotropic point of the stream may be heated to evaporate and generate hydrochloric acid gas. As described above, heating and boiling a hydrochloric acid process stream having a hydrochloric acid concentration above the azeotropic point evaporates more hydrochloric acid gas than water before the azeotropic point is reached. Thus, by heating a "super-azeotropic" hydrochloric acid process stream, generating hydrochloric acid gas, the hydrochloric acid concentration of the process stream is then reduced until the azeotropic point is reached.

In one embodiment, the hydrochloric acid process stream is heated at a pressure between atmospheric pressure (e.g., about 1 bar) and about 16 bar. The pressure of the hydrochloric acid process stream may be 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 bar. 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 higher and/or lower values, such as in a range between about 3 bar to about 10 bar, such as in a range between about 3 bar to about 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 reduced, which can generate hydrogen chloride gas at a steady-state concentration. In addition, the pressure during the heating step also facilitates the fluid transfer of the generated hydrochloric acid recycle stream.

In one embodiment, a hydrochloric acid process stream is heated at a pressure effective to reduce the azeotropic point (e.g., at a pressure above atmospheric pressure) to generate a hydrochloric acid gas stream and reduce the concentration of hydrochloric acid in the hydrochloric acid process stream.

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

In one embodiment, heating the hydrochloric acid process stream reduces the concentration of hydrogen chloride to between about 10 wt.% and about 25 wt.% based on the weight of the stream. Heating the hydrochloric acid process stream can reduce the concentration of hydrogen chloride (in wt.%) to less than about 25, 20, 15, or 10 based on the weight of the stream.

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

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 that may be recycled and used to wash the separated aluminum chloride hexahydrate solid (not shown).

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 a source of hydrogen chloride gas in one or more of the crystallization stages described herein.

In one embodiment, reducing the hydrogen chloride concentration of the hydrochloric acid process stream may be performed in a stripper/distiller column (see FIG. 3 ).

Those skilled in the art will appreciate that the concentration of the hydrogen chloride gas stream generated from the heating of the hydrochloric acid process stream may vary depending on the processing unit used and the processing conditions used in the unit. In one embodiment, the hydrogen chloride gas stream generated from the heating of the hydrochloric acid process stream has a hydrogen chloride concentration between about 20 wt.% and about 100 wt.% based on the weight of the stream. The hydrogen chloride gas stream may have a hydrogen chloride concentration 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 (in wt.%). 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 higher and/or lower values, such as between about 20 wt.% and about 100 wt.%, or between about 40 wt.% and about 100 wt.%, between about 60 wt.% and about 100 wt.%, or between about 80 wt.% and about 100 wt.%.

The hydrochloric acid process stream contains one or more inorganic impurities that remain in the liquid after precipitation and separation of the aluminum chloride hexahydrate solid. In order to close the hydrochloric acid loop of the method of the present invention, it is necessary to remove at least some or all of the inorganic impurities from the process stream in order to avoid reintroduction of the impurities. Removing the impurities from the process stream produces the hydrochloric acid recycle stream described herein.

In one embodiment, the hydrochloric acid process stream comprises between about 0.1 wt.% and about 10 wt.% inorganic impurities based on the weight of the stream. The concentration of inorganic impurities in the hydrochloric acid process stream (in wt.%) 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 the hydrochloric acid process stream (in wt.%) can be 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 can be a range provided by any two of these higher and/or lower values, such as a range between about 0.5 wt.% to about 1.5 wt.%.

Impurities may be removed from the hydrochloric acid process stream by any suitable method, such as using a suitable impurity removal unit. Examples of suitable methods for removing impurities from the hydrochloric acid process stream include distillation, ion exchange, and precipitation.

In one embodiment, removing impurities from a hydrochloric acid process stream comprises distilling the hydrochloric acid process stream. In one embodiment, removing impurities from a 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 by azeotropic distillation) evaporates and recondenses the hydrochloric acid stream and produces an inorganic impurity stream containing impurities as a separate stream. It will be understood that the recondensed hydrochloric acid stream is a hydrochloric acid recycle stream as described herein.

Concentrating an inorganic impurity stream (i.e., a purified stream) having 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.

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., a purified stream).

In one embodiment, the hydrochloric acid recycle stream has a hydrogen chloride concentration of between about 5 wt.% and about 40 wt.% based on the weight of the stream. The hydrochloric acid recycle stream may have a hydrogen chloride concentration 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 (in wt.%). The hydrochloric acid recycle stream may have a hydrogen chloride concentration 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 (in wt.%). The hydrogen chloride concentration may be in a range provided by any two of these higher values and/or lower values, such as a range between about 15 wt.% to about 35 wt.% based on the weight of the stream.

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, as described herein, the concentration of hydrogen chloride in the process stream is reduced before removing one or more inorganic impurities. In one embodiment, the hydrochloric acid recycle stream has a hydrogen chloride concentration between about 5 wt.% and about 30 wt.% based on the weight of the stream. The concentration of hydrogen chloride in the recycle stream (in wt.%) can be less than about 35, 30, 25, 20, 18, 15, 10, or 5 based on the weight of the stream. Ranges may be provided between any two of these equivalent values, such as between about 5 wt.% to 25 wt.%, about 10 wt.% to about 25 wt.%, such as between about 18 wt.% to about 20 wt.%, based on the weight of the stream.

In some embodiments, at least a portion of the hydrogen chloride recycle stream is then recycled for use as hydrochloric acid for digesting the aluminum chloride hexahydrate solids in step a). Such recycling of the hydrochloric acid stream for digestion (compared to using water) is unusual because it reduces the solubility of aluminum chloride hexahydrate and reduces the amount of aluminum chloride hexahydrate solids that can be dissolved and digested due to the same ion effect. This explains why such acid streams are not conventionally recycled and reused in the process. However, according to some embodiments or examples described herein, the inventors unexpectedly determined that hydrochloric acid can be regenerated and recycled while reducing the operating costs of the process and maintaining a high purity alumina production rate. Specifically, recycling the hydrochloric acid stream for use in step a) can reduce the extent to which the hydrochloric acid process stream must be distilled to produce HCl gas (see, for example, Table 1). This can reduce capital and energy requirements in the acid regeneration loop and can completely offset any losses incurred by reducing the amount of aluminum chloride hexahydrate solid that can be dissolved in step a). In addition, by increasing the acid concentration of the digested aluminum chloride liquid, in some embodiments, less hydrogen chloride gas is required per liter of liquid during subsequent spraying for crystallization and precipitation of aluminum chloride.

Alternatively or additionally, at least a portion of the hydrochloric acid recycle stream may be recycled for use as hydrochloric acid for digesting aluminum-containing materials, as described herein.

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

In some embodiments, before 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 the hot gases generated by the thermal decomposition and/or calciner (described below). The resulting enriched hydrochloric acid stream containing hydrogen chloride absorbed from the thermal decomposition and/or calciner gases can then be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, the uncondensed gases from the absorption unit can be separated and transferred to a tailing tower into which the recycle stream has been introduced, the tailing tower acting as a degasifier for absorbing the gas including the uncondensed gases before being introduced into the absorption unit (see Figure 3).

In some embodiments, before or in addition to being sent to the impurity removal step f), at least a portion of the hydrochloric acid process stream is introduced into an absorption unit to condense and absorb hydrogen chloride from hot gases generated by the thermal decomposition and/or calciner (described below). In one embodiment, the enriched hydrochloric acid stream containing hydrogen chloride absorbed by the thermal decomposition and/or calciner gases can be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, the uncondensed gases from the absorption unit can be separated and transferred to a tail tower into which a recycle stream has been introduced, which serves as a degasifier for absorbing gases including uncondensed gases before being introduced into the absorption unit (see Figure 3).

In some embodiments, before or in addition to being sent to the impurity removal step f), at least a portion of the lower concentration hydrochloric acid process stream obtained from step e) is introduced into an absorption unit to condense and absorb hydrogen chloride (described below) from hot gases generated by the thermal decomposition and/or calciner (see FIG. 3 ). In one embodiment, the uncondensed gas from the absorption unit can be separated and transferred to a tailing tower into which a recycle stream has been introduced, the tailing tower acting as a degasifier for absorbing the gas including the uncondensed gas before being introduced into the absorption unit (see FIG. 3 ).

In one embodiment, the enriched hydrochloric acid stream comprising hydrogen chloride absorbed 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 in a range provided by any two of these higher values and/or lower values, for example, in a range between about 28 wt.% to about 37 wt.%.

In one embodiment, an enriched hydrochloric acid stream comprising hydrogen chloride absorbed from the thermal decomposition gases may be combined with the hydrochloric acid process stream (see FIG. 3 ). In one embodiment, an enriched hydrochloric acid stream comprising absorbed hydrogen chloride generated by thermal decomposition may be combined with the hydrochloric acid process stream before the hydrochloric acid process stream is fed to the stripping column (see FIG. 3 ).

In one embodiment, the enriched hydrochloric acid stream comprising hydrogen chloride absorbed 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 in a range provided by any two of these higher values and/or lower values, for example, in a range between about 28 wt.% to about 37 wt.%.

In one embodiment, the enriched hydrochloric acid stream comprising hydrogen chloride absorbed from the calciner can be combined with the hydrochloric acid process stream (see Figure 3). In one embodiment, the enriched hydrochloric acid stream comprising hydrogen chloride absorbed from the calciner can be combined with the hydrochloric acid process stream before the hydrochloric acid process stream is fed into the stripper (see Figure 3).

In one embodiment, a water stream may be introduced into the hydrochloric acid recycle stream to dilute the hydrogen chloride concentration. High purity alumina is produced from the precipitated aluminum chloride hexahydrate solid .

The precipitated aluminum chloride hexahydrate solid separated from step c) can then be treated to form high purity alumina. In one embodiment, treating the separated aluminum chloride hexahydrate solid to form high purity alumina comprises heat treating the separated aluminum chloride hexahydrate solid in one or more heating stages.

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

Any hydrogen chloride gas emitted during the thermal decomposition can be regenerated as hydrochloric acid for digesting the aluminum chloride hexahydrate solid described herein and/or hydrogen chloride gas for precipitating the aluminum chloride hexahydrate solid in one or more crystallization stages described herein, as summarized in Figure 3. In some embodiments, the hydrochloric acid gas emitted during the thermal decomposition has a hydrogen chloride concentration of between about 30 wt.% and about 70 wt.% based on the weight of the stream. The hydrochloric acid gas emitted during the thermal decomposition may have a hydrogen chloride concentration of at least about 30, 35, 40, 45, 50, 52, 55, 60, 65, or 70 based on the weight of the stream (in wt.%). The hydrochloric acid gas emitted during the 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 emitted during the thermal decomposition may be a range provided by any two of these higher values and/or lower values, such as a range of about 30 wt.% to about 70 wt.%, about 40 wt.% to about 60 wt.%, or about 45 wt.% to about 55 wt.%, based on the weight of the stream. In one embodiment, the hydrogen chloride concentration of the hydrochloric acid gas emitted during the thermal decomposition may be greater than the hydrochloric acid concentration in the hydrochloric acid process stream.

In one embodiment, the pyrolyzed solid after the first heating stage is then calcined at a second temperature of from 900° C. to 1300° C. to produce high purity alumina. The pyrolyzed solids may be calcined at a second temperature (in ° C.) of at least about 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, or 1300. The pyrolyzed solids may be calcined at a second temperature (in ° C.) of less than about 1300, 1250, 1200, 1150, 1100, 1050, 1000, 950, or 900. The calcination temperature may be a range provided by any two of the higher and/or lower values.

Any hydrogen chloride gas that may be emitted during calcination may be regenerated as hydrochloric acid for digesting the aluminum chloride hexahydrate solids described herein and/or for precipitating the aluminum chloride hexahydrate solids in one or more crystallization stages described herein, as summarized in Figure 3. In some embodiments, the hydrogen chloride gas emitted during calcination has a hydrogen chloride concentration of between about 30 wt.% and about 70 wt.% based on the weight of the stream. The hydrochloric acid gas emitted during calcination may have a hydrogen chloride concentration of at least about 30, 35, 40, 45, 50, 52, 55, 60, 65, or 70 based on the weight of the stream (in wt.%). The hydrochloric acid gas emitted 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 emitted during calcination may be a range provided by any two of these higher values and/or lower values, such as a range of about 30 wt.% to about 70 wt.%, about 40 wt.% to about 60 wt.%, or about 45 wt.% to about 55 wt.%, based on the weight of the stream. In one embodiment, the hydrogen chloride concentration of the hydrochloric acid gas emitted during calcination may be greater than the hydrochloric acid concentration in the hydrochloric acid process stream.

By controlling the removal of impurities from the initial aluminum chloride liquid formed from the aluminum-containing material and taking steps to reduce the impurity content in the aluminum chloride hexahydrate solid by performing one or more digestion steps in hydrochloric acid according to the present invention, higher purity alumina of greater than 99.99% purity (4N) or greater than 99.999% purity (5N) can be reliably achieved from a variety of aluminum-containing materials.

To help minimize the introduction of impurities during crystallization, a portion of the high purity alumina formed may optionally be recycled to seed the aluminum chloride liquid in one or more upstream precipitation steps.

The initial aluminum chloride hexahydrate solid in step a) can be provided by digesting an aluminum-containing material in hydrochloric acid to provide an aluminum chloride liquid, and precipitating and separating the aluminum chloride hexahydrate solid from the aluminum chloride liquid to obtain the aluminum chloride hexahydrate solid and produce a hydrochloric acid process stream. Precipitating the aluminum chloride hexahydrate solid from the aluminum chloride liquid is described herein.

In one embodiment, the hydrochloric acid recycle stream described herein is recycled for use as hydrochloric acid for digesting aluminum-containing materials.

In one embodiment, a method for preparing high purity alumina from an aluminum-containing material is provided, the method comprising: a) digesting the aluminum-containing material in hydrochloric acid to provide an aluminum chloride liquid containing aluminum chloride and one or more impurities in solution; b) precipitating an aluminum chloride hexahydrate solid from the liquid in one or more crystallization stages such that at least some of the inorganic impurities remain in the liquid; c) separating the precipitated aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; d) treating the separated aluminum chloride hexahydrate solid to form high purity alumina; e) optionally reducing 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 hydrochloric acid for digesting the aluminum-containing material in step a).

In one embodiment, step b) comprises precipitating aluminum chloride hexahydrate solid from aluminum chloride liquid in two or more crystallization stages, wherein between each crystallization stage, the separated aluminum chloride hexahydrate is digested in a hydrochloric acid recycle stream to produce aluminum chloride liquid, as described herein.

It will be understood that, unless otherwise stated, the embodiments and examples described herein with respect to hydrochloric acid process streams and/or hydrochloric acid recycle streams that are recycled for use as hydrochloric acid to digest aluminum chloride hexahydrate solids may be equally applicable to hydrochloric acid process streams and/or hydrochloric acid recycle streams that are recycled for use as hydrochloric acid to digest aluminum-containing materials described herein.

The aluminum-containing material may undergo several pretreatment and treatment steps in order to form an aluminum chloride liquid comprising aluminum chloride and one or more inorganic impurities in solution.

The aluminum-containing material as received may be subjected to one or more pre-treatment steps before being subjected to digestion to form an aluminum chloride liquid. The pre-treatment step(s) may be any one or more mineral processing steps including, but not limited to, concentration, gravity separation to remove veins such as sand or quartz from the material, or crushing to a particle size of 1 μm to 200 μm.

It will be appreciated that certain aluminum-containing materials such as calciner dust may include occluded soda and surface soda. The surface soda may be readily removed from the calciner dust by washing the calciner dust with carbon dioxide to remove the surface soda as sodium bicarbonate before the calciner dust enters the process loop. The washed calciner dust may then be filtered and washed with water to remove residual sodium bicarbonate before entering the process loop.

Alternatively, the soluble surface soda can be at least partially removed from the calciner dust by washing with water. The washed calciner dust can then be filtered before entering the process loop.

In one embodiment, the alumina feed may be provided from a Bayer process loop, wherein the alumina feed may optionally be subjected to one or more steps of recrystallization from an alkaline solution included in the Bayer process loop, thereby removing one or more impurities in the feed, in particular soda.

In another embodiment, an aluminum-containing clay feed such as kaolin may be provided.

The method of preparing high purity alumina may include digesting an aluminum-containing material with hydrochloric acid to produce an aluminum chloride liquid. The hydrochloric acid may have a concentration of from 5 M to 12 M HCl, 6 to 11 M HCl, 6 to 10 M HCl, or 7 M to 9 M HCl.

The HCl concentration of the resulting aluminum chloride liquid may range from 0 M to 2 M. For example, the HCl concentration of the resulting aluminum chloride liquid may be about 0 M, 0.5 M, 1 M, 1.5 M, or 2 M. 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 in a plurality of reactors (e.g., up to 5 vessels, such as 3 vessels) configured in series, such that the HCl concentration in the liquid in each vessel in the series decreases from about 10 M to about 2 M in a cascade order.

In one embodiment, the hydrochloric acid recycle stream described herein may be introduced into one or more of the reactors configured 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 in addition, the hydrochloric acid stream may be introduced into the hydrochloric acid recycle stream to concentrate the hydrogen chloride concentration prior to digesting the aluminum-containing material in one or more reactors described herein.

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.

The acid digestion can be carried out at a temperature ranging from ambient temperature to the atmospheric boiling point of the resulting aluminum chloride liquid. The acid digestion can be carried out 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 can be carried out 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 can be carried out at a temperature between any two of these higher and lower amounts, such as about 25° C. to 100° C., 50° C. to 95° C., 70° C. to 90° C., or 75° C. to 85° C., for example, about 80° C.

It will be appreciated that the rate of digestion will depend on the temperature, solid concentration, and acid concentration in the resulting digestion mixture. Acid digestion can be performed for a period of at least about 15 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, or 7 hours. Acid digestion can be performed for a period of less than about 7 hours, 6 hours, 5 hours, 4 hours, 3 hours, 2 hours, or 1 hour. Solid precipitation can be performed for a period of time within a range provided by any two of the higher amount and/or the lower amount, such as from about 15 minutes to 6 hours or about 2 to 4 hours. In a specific example, the period of time is about 3 hours.

After dissolution of the acid-soluble compound is complete, the resulting aluminum chloride liquid is separated from any remaining solids by any suitable known separation technique, such as filtration, gravity separation, centrifugation, etc. It will be understood that the solids may undergo one or more washes during the separation.

The separated aluminum chloride liquid may be subjected to another pretreatment, for example in the manner described above, before being subjected to crystallization for precipitating aluminum chloride hexahydrate solid.

For example, one such pretreatment may include contacting an aluminum chloride liquid with an ion exchange resin, particularly a cation exchange resin.

Alternatively, another example of such pretreatment may include contacting the aluminum chloride liquid with an adsorbent to adsorb 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 zeolite, and polymer adsorbents.

Another example of pretreatment may include selectively precipitating one or more chloride salts of impurities. For example, the liquid may be cooled and sparged with HCl gas to promote salt precipitation of sodium chloride.

Another example of such pretreatment may include reacting the liquid with a complexing agent, wherein the complexing agent is capable of selectively forming a complex with one or more impurities. In this way, when the aluminum chloride hexahydrate solid is produced, the complexed impurities can be retained in the solution. The complexing agent 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 with good selectivity for sodium include 15-crown 5, 12-crown 4 and 18-crown 6. Such crown ethers are soluble in aqueous solution. Suitable complexing agents for iron include, but are not limited to, polypyridine ligands, such as bipyridine ligands and tripyridine ligands, polyazamacrocyle. 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.

Another example of such pretreatment may include solvent extraction. Suitable carriers may be non-polar solvents, including but not limited to halogenated alkanes, such as methyl chloride, dichloromethane, chloroform, and long-chain alcohols, such as 1-octanol. Crown ether complexing agents, as discussed above, are generally more soluble in water than non-polar solvents. Therefore, by modifying the crown ether complexing agents, as discussed above, by adding hydrophobic groups such as benzo groups and long-chain aliphatic functional groups, the distribution of the crown ether complexing agents in non-polar solvents can be improved.

In some embodiments where the impurity is sodium, the aluminum chloride liquid can be purified by passing the aluminum chloride liquid through a semipermeable cation-selective membrane, specifically, a sodium-selective membrane, to separate the sodium impurity from the liquid.

After undergoing any pretreatment as described above, the resulting aluminum chloride liquid may be concentrated in an evaporator to increase the Al concentration in the solution. The Al concentration in the solution of the aluminum chloride liquid 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 the solution of the aluminum chloride liquid after evaporation may be less than about 100 g/L, about 90 g/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 one embodiment, the Al concentration in the aluminum chloride liquid solution after evaporation may be in the range of about 1-100 g/L, such as a range between any two of the above-mentioned higher concentrations and/or lower concentrations, such as about 10-90 g/L or 50-85 g/L or about 60-80 g/L. In order to facilitate crystallization, the Al concentration in the aluminum chloride liquid after evaporation is preferably at or slightly below the saturation concentration of the solution.

The concentrated liquor is then treated, for example, in the manner described in detail above, to precipitate the aluminum chloride hexahydrate solid from the aluminum chloride liquid.

It should be understood that the embodiments and examples described herein also encompass a process that operates in a continuous manner that is as close to a steady state as is practically achievable. With this understanding, one skilled in the art will appreciate that during startup and/or shutdown, operating conditions and/or feed requirements may deviate significantly from those outlined herein. For example, it may be required to start a unit and/or line with a solution whose composition is outside the scope of what is described herein.

Those skilled in the art will also appreciate that one or more top-up streams may be required to maintain the desired composition and flow rate throughout the system. Such streams may be required due to changes in environmental conditions, leaks or other unexpected losses, changes in raw materials, etc.

This application claims priority to AU2022902444 filed on August 26, 2022, the entire contents of which are incorporated herein by reference .

In order to more clearly understand the present invention, the following further describes the specific embodiments of the present invention in detail by referring to the following non-limiting experimental materials, methods and examples. Example 1 : Regeneration of hydrochloric acid for digestion of aluminum chloride hexahydrate solid

A non-limiting example of the production of high purity alumina (HPA) is provided in the flow charts of Figures 1 and 2. Digestion of aluminum-containing materials

Aluminum-containing materials are digested with hydrochloric acid (HCl (aq)) in a continuous agitated HCl acid digester unit. The digestion can be carried out in a single reactor or in a plurality of reactors (not shown) arranged in series. As described herein, the molar (M) concentration of HCl in the liquid in one or more HCl acid digester units can be varied. The resulting slurry is then separated from a purified aluminum chloride liquid (AlCl ₃ ) comprising aluminum chloride and several impurities in solution. Precipitation and digestion of aluminum chloride hexahydrate ( ACH ) solid

Hydrogen chloride gas (HCl (g)) is bubbled through the aluminum chloride liquid in a continuously stirred crystallization vessel (see "Precipitation" in Figures 1 and 2) to precipitate aluminum chloride hexahydrate solid. From Figure 2, it can be seen that HCl (g) can be derived from the HCl regeneration described herein. The crystallization vessel may comprise two or more crystallization tanks (e.g., three tanks) connected in series, in which the HCl level is gradually increased to the concentration in the final tank. Alternatively, one crystallization tank may be used. The resulting precipitate product stream is then subjected to a separation step to obtain an aluminum chloride hexahydrate (ACH) solid stream and a "super-azeotropic" process HCl (aq) stream comprising about 32 wt% HCl.

The ACH solid may then undergo one or more additional digestion/crystallization processes. Referring to the dashed box in FIG. 2 , the separated ACH solid is fed to the acid digester unit and re-digested with HCl (aq) to produce an aluminum chloride liquid. This liquid is fed to a second crystallization vessel (i.e., a second "precipitation") to crystallize by bubbling HCl (g) through the aluminum chloride liquid to re-precipitate the aluminum chloride hexahydrate solid. The resulting precipitate stream then undergoes a separation step to obtain an aluminum chloride hexahydrate (ACH) solid stream and a "super-azeotropic" process HCl (aq) stream containing about 32 wt% HCl.

It will be appreciated that the dashed box surrounding the ACH digestion, precipitation and separator units in FIG. 2 indicates that the ACH digestion/precipitation process may be repeated more than once. For example, as indicated by the vertical upward dashed arrow in FIG. 2, the ACH solids exiting the separator unit (within the dashed box) may be fed to a subsequent acid digester unit for further purification, emphasizing that the digester units may be arranged in series and the above process may be repeated multiple cycles until the purity of the ACH solids reaches the desired level. Calcination of ACH to Prepare High Purity Alumina

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 produced as a product. Although not shown in FIG. 2 , low-quality HCl (g) containing about 52 wt % HCl may also be produced in the calciner and may be fed to an HCl regeneration unit as described herein. Recycling and Regeneration of HCl

Hydrochloric acid (HCl (aq)) used to digest the aluminum-containing material and ACH solids can be provided by the hydrochloric acid (HCl) recycle stream described herein. In addition, hydrogen chloride gas (HCl (g)) used to precipitate the ACH solids can be regenerated from the super-azeotropic HCl process stream described herein. The HCl recycle stream and the regenerated HCl (g) can be generated by the HCl regeneration and impurity removal unit.

The operation of HCl regeneration and impurity removal is depicted in FIG3 . Hot gases from the calciner with increased HCl concentration are fed to an absorber unit where they are quenched by applying external cooling and mixing with a portion of the azeotropic 19 wt % HCl process stream initially from the stripping column as described below. Any uncondensed gases are separated from the liquid phase at the bottom and transferred to the tail column, which is a degasser into which the aforementioned azeotropic 19 wt % HCl process stream first enters. Any residual HCl is removed from the gas stream, and inert gases from the calciner (e.g., leaked air or combustion gases) may also be released.

The liquid phase from the absorber unit has a HCl concentration of about 32 wt% and is mixed with the superazeotropic 32 wt% HCl process stream produced after one or more separation steps described herein. This mixture is transferred to the stripping column.

In the stripper, the above-mentioned super-azeotropic 32 wt.% HCl process stream is heated under pressure, so that HCl (g) is generated and leaves the column from the top and is recycled for use in one or more ACH precipitation steps. The HCl (g) leaving the stripper can also be combined with water to produce an HCl wash stream, which can be recycled and used to wash the separated aluminum chloride hexahydrate solid (not shown).

The remaining solution leaves the stripper at the bottom as an azeotropic 19 wt% HCl process stream and contains inorganic impurities that were introduced into the system. A portion of the solution is returned to the absorber unit while the remaining azeotropic 19 wt% HCl process stream is discharged.

In order to approach the HCl equilibrium of the current system, the azeotropic 19 wt.% HCl process stream containing inorganic impurities generated by the process is purified by subjecting the azeotropic 19 wt.% HC process stream to an impurity removal step (e.g., distillation) (see Figure 3). Distillation of the azeotropic 19 wt.% HC process stream produces an impurity-rich "brine" stream that is enriched with substantially all impurities, and a separate clean 19 wt.% HCl recycle stream for recycling back to the HCl digester unit and/or ACH digester unit, as shown in Figure 2.

Table 1 summarizes various outputs predicted based on modeling of the HCl (aq) recycle process described herein compared to a HCl (g) regeneration process utilizing known pressure swing (PS) technology. The amount of HCl is shown relative to the amount of HCl in the HCl (g) regenerated utilizing known pressure swing (PS) technology. The energy consumption is shown relative to the predicted heating requirements utilizing known pressure swing (PS) technology. The cost is shown relative to known pressure swing (PS) technology. As highlighted below, the present method of recycling an azeotropic HCl (aq) recycle stream can provide various savings in terms of HCl (g) consumption, energy consumption, and capital expenditures as highlighted below. Table 1 : Comparison of HCl ( aq ) Recycle ( Present Method ) with Known Pressure Swing Technology Output Pressure Swing (PS) HCl (g) Regeneration HCl (aq) recycle ( method of the present invention ) HCl (gas) 100 ~58.6 HCl (aq) - ~34.7 Energy Heating 100 25.4 Energy Cooling 93.2 16.9 Special cooling 1.7 - Electricity 1.1 - Relative total energy 196.0 42.4 Relative cost 100 31.3

The following is a further description and explanation of the embodiments of the present invention by way of example only with reference to the accompanying drawings, wherein:

FIG1 : A representative flow chart of an embodiment of a method for preparing high purity alumina;

FIG2 : A representative flow chart of an embodiment of a method for preparing high purity aluminum oxide using hydrochloric acid and hydrogen chloride gas regeneration;

Figure 3 : Representative flow diagram of hydrochloric acid and hydrogen chloride gas regeneration; and

Figure 4 : Examples of HCl vapor pressure and H ₂ O vapor pressure of HCl/H ₂ O solutions at different temperatures (A. Schmidt, Chem. Ing. Techn. 25, 445/62 [1953]).

Claims (25)

A method for preparing high purity alumina from aluminum chloride hexahydrate solid, the method comprising: a) digesting aluminum chloride hexahydrate solid having one or more inorganic impurities in hydrochloric acid to produce an aluminum chloride liquid containing aluminum chloride and the inorganic impurities in solution; b) precipitating aluminum chloride hexahydrate solid from the liquid in one or more crystallization stages so that at least some of the inorganic impurities remain in the liquid; c) separating the precipitated aluminum chloride hexahydrate solid from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; d) treating the separated aluminum chloride hexahydrate solid to form high purity alumina; e) reducing the concentration of hydrogen chloride in the hydrochloric acid process stream as appropriate; and f) removing at least some of the inorganic impurities from the hydrochloric acid process stream to produce a hydrochloric acid recycle stream, the hydrochloric acid recycle stream being recycled for use as the hydrochloric acid in step a). The method of claim 1, wherein the hydrochloric acid recycle stream has a hydrogen chloride concentration between about 5 wt.% and about 30 wt.% based on the weight of the stream. The method of claim 1 or claim 2, wherein the concentration of hydrogen chloride in the hydrochloric acid recycle stream at or near the azeotropic point of the stream is, for example, between about 18 wt.% and about 20 wt.% based on the weight of the stream. The method of any one of claims 1 to 3, wherein removing the impurities from the hydrochloric acid process stream in step f) comprises distilling the hydrochloric acid process stream. The method of any of claims 1 to 4, wherein the hydrochloric acid recycle stream comprises less than about 0.05 wt.% inorganic impurities based on the weight of the stream. A method as claimed in any one of claims 1 to 5, wherein the hydrochloric acid process stream has a concentration of hydrogen chloride above the azeotropic point of the stream, and the method comprises step e) reducing the concentration of hydrogen chloride in the hydrochloric acid process stream before removing the one or more inorganic impurities. A method as claimed in claim 6, wherein a superazeotropic hydrochloric acid process stream is heated under pressure to lower the azeotropic point to generate a hydrogen chloride gas stream and reduce the concentration of hydrogen chloride in the hydrochloric acid process stream. The method of claim 6 or claim 7, wherein prior to step e), the hydrochloric acid process stream has a hydrogen chloride concentration of at least about 25 wt.% based on the weight of the stream. The method of claim 7 or 8, wherein the heating of the hydrochloric acid process stream is at a pressure between about 1 bar and about 10 bar. The method of any of claims 7 to 9, wherein the heating of the hydrochloric acid process stream is at a temperature between about 100°C and about 200°C. The method of any of claims 7 to 10, wherein the heating of the hydrochloric acid process stream reduces the hydrogen chloride concentration to less than about 25 wt. % based on the weight of the stream. A method as in any of claims 7 to 11, wherein the heating of the hydrochloric acid process stream reduces the hydrogen chloride concentration to at or near the azeotropic point of the stream. The method of any one of claims 1 to 12, wherein the precipitation in step b) comprises spraying hydrogen chloride gas into the aluminum chloride liquid. A method as claimed in claim 13, wherein 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 a source of hydrogen chloride gas in the one or more crystallization stages. The method of any one of claims 1 to 14, wherein the precipitation of aluminum chloride hexahydrate solid from the aluminum chloride liquid is carried out at a temperature of from about 40°C to about 80°C. A method as claimed in any one of claims 1 to 15, wherein step b) comprises precipitating aluminum chloride hexahydrate solid from the aluminum chloride liquid in two or more crystallization stages, wherein between each crystallization stage, the separated aluminum chloride hexahydrate is digested in the hydrochloric acid recycle stream to produce the aluminum chloride liquid. A method as in any one of claims 1 to 16, wherein treating the separated aluminum chloride hexahydrate solids to form high purity alumina comprises heat treating the separated aluminum chloride hexahydrate solids in one or more heating stages. The method of claim 17, wherein heat treating the separated aluminum chloride hexahydrate solids comprises heating the separated aluminum chloride hexahydrate solids at a first temperature from about 200°C to about 900°C, and calcining the thermally decomposed solids at a second temperature from about 1000°C to about 1300°C. The method of any one of claims 1 to 18, wherein the aluminum chloride hexahydrate solids in step a) are provided by the following operations: Digesting the aluminum-containing material in hydrochloric acid to provide aluminum chloride liquid; and Precipitating and separating the aluminum chloride hexahydrate solids from the aluminum chloride liquid to obtain the aluminum chloride hexahydrate solids and produce the hydrochloric acid process stream. The method of claim 19, wherein the hydrochloric acid recycle stream is recycled to provide the hydrochloric acid for use in digesting the aluminum-containing material. A method as in any one of claims 1 to 20, wherein the high purity alumina formed contains greater than 99.99% or 99.999% alumina. A high purity aluminum oxide prepared by the method of any one of claims 1 to 21. The high-purity alumina of claim 22, wherein the high-purity alumina contains greater than 99.99% of alumina. The high-purity alumina of claim 22, wherein the high-purity alumina contains greater than 99.999% of alumina. A system for preparing high-purity alumina from aluminum chloride hexahydrate solid having one or more impurities, the system comprising: One or more acid digesters for digesting aluminum chloride hexahydrate solid in hydrochloric acid to produce aluminum chloride liquid containing the inorganic impurities; A crystallization vessel associated with each acid digester for receiving aluminum chloride liquid from the acid digester and for precipitating aluminum chloride hexahydrate solid from the liquid so that at least some of the inorganic impurities remain in the liquid; Optionally, one or more subsequent crystallization vessels for recrystallizing the aluminum chloride hexahydrate solids; a separation member associated with the one or more crystallization vessels for separating the aluminum chloride hexahydrate solid formed from the liquid to produce a hydrochloric acid process stream containing the inorganic impurities; an optional stripping column for receiving the hydrochloric acid process stream from the separation member to generate a hydrogen chloride gas stream and reduce 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 recirculating the hydrochloric acid recycle stream to the one or more acid digesters; and Heat treatment components for heat treating the aluminum chloride hexahydrate solids to provide high purity aluminum oxide.