WO2014005226A1

WO2014005226A1 - Bauxite residue neutralisation with enzymatically enhanced gas capture

Info

Publication number: WO2014005226A1
Application number: PCT/CA2013/050513
Authority: WO
Inventors: Jonathan Andrew CARLEY; Jingui HUANG
Original assignee: Co2 Solutions Inc.
Priority date: 2012-07-03
Filing date: 2013-07-03
Publication date: 2014-01-09

Abstract

Processes and system for neutralisation of a bauxite residue, including contacting a CO₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of carbonic anhydrase, to promote the hydration reaction of CO₂ into bicarbonate and hydrogen ions and produce a CO₂-depleted gas and an ion-rich solution; and contacting the ion-rich solution with the bauxite residue to produce a neutralised bauxite stream and an ion-depleted solution. The ion-rich solution may also be subjected to desorption for promoting release of the bicarbonate ions from the ion- rich solution and producing a CO₂ gas stream and an ion-depleted solution. The CO₂ gas stream may then be injected into the bauxite residue to produce a neutralised bauxite stream.

Description

BAUXITE RESIDUE NEUTRALISATION WITH ENZYMATICALLY ENHANCED

GAS CAPTURE

FIELD OF THE INVENTION

The present invention generally relates to bauxite residue neutralisation and more particularly to the neutralisation of bauxite residue integrated with enzymatically enhanced gas capture.

BACKGROUND

Bauxite residue is a by-product of processing bauxite which is aluminum ore that can be transformed into aluminum. In bauxite processing, the aluminum ore is usually refined and heated in a pressure vessel along with sodium hydroxide to promote dissolution of aluminate. Red mud, which is a ferruginous material also called bauxite residue (BR), is separated out and the remaining material undergoes further processing steps for production of aluminum. BR contains a mixture of minerals including iron and titanium oxides, silica, calcium carbonate, unrecovered alumina, as well as some caustic sodium hydroxide. BR thus is highly alkaline.

Each year, over 70 million tonnes of BR are generated, and more than 200 million tonnes of BR have accumulated worldwide, of which the majority is stored in tailing ponds. Due to caustic nature of BR, attempts have been made to neutralise or partially neutralise BR by sea water treatment or by injecting C0₂ into the red mud slurries, so as to improve its manageability, reduce its disposal costs and limit its potential environmental impacts (US 4,250,032; US 2006/0144797; US 7,922,792). BR neutralisation by sea water treatment will largely depend on location. This type of treatment is preferred where the sea water can be easily and cheaply sourced, which is, however, not the case in many aluminium refineries. BR treatment by C0₂ injection, wherein high concentration of C0₂ is generally required, could be affected by a number of factors such as location of a suitable C0₂ source and costs for the requested C0₂ concentration level. Furthermore, in a gaseous injection form, loss of C0₂ into the atmosphere would also be a concern.

Management of BR poses a number of economic, environmental and technical challenges. BR has also been relatively undervalued and may be treated to extract and recover valuable compounds including metal and/or rare earth elements (US 4,464,347; US 5,030,424). BR may be integrated into various industries for more useful applications, such as building materials, absorbents for gas and water treatment (US 20050087107; US 7,077,963) and other products. SUMMARY OF INVENTION

In some implementations, there is provided processes for neutralising bauxite residues with an ion-rich solution including bicarbonate ions that are derived from enzymatic C0₂ absorption from the gases derived from aluminum processing operations. This integrated approach uses enzymatically accelerated capture of significant quantities of C0₂ from smelter, refinery or other aluminum production operations, to produce a solution in effective amounts to neutralise bauxite residues.

In one aspect, there is provided a process for neutralising bauxite residues with an ion-rich solution including bicarbonate ions that are derived from enzymatic absorption of C0₂ from a C0₂-containing gas resulting of aluminum processing operations.

In another aspect, there is provided a process for neutralising bauxite residues with gaseous C0₂ derived from the desorption of an ion-rich solution including bicarbonate ions. According to optional embodiments, the ion-rich solution may be derived from absorption of C0₂ in presence of an enzyme. Optionally, the enzyme may be separated from the ion-rich solution before the desorption step. Alternatively, the desorption of the ion-rich solution may be performed in presence of an enzyme. Further optionally, the enzyme may be carbonic anhydrase or analogues thereof. In another aspect, there is provided a process for neutralisation of a bauxite residue, the process including:

(a) contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of carbonic anhydrase, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution; and (b) contacting the ion-rich solution with the bauxite residue to produce a neutralised bauxite stream and an ion-depleted solution.

According to optional embodiments, the process may include recycling the ion- depleted solution as at least part of the aqueous absorption solution. Optionally, the process may include recycling a portion of the ion-rich solution for contacting the ion-depleted solution for further neutralisation.

According to optional embodiments, step (a) may be performed in a packed reactor. Further optionally, the step (a) may be performed in a spray reactor, in a fluidized bed or in a bubble reactor. Optionally, the step (b) may be performed in a cell that is open to the atmosphere.

According to optional embodiments, in step (a), the carbonic anhydrase may be provided free in the aqueous absorption solution; dissolved in the aqueous absorption solution; immobilized on the surface of supports that are mixed in the aqueous absorption solution and flow therewith; immobilized on the surface of supports that are fixed within an absorption reactor; entrapped or immobilized by or in porous supports that are mixed in the aqueous absorption solution; entrapped or immobilized by or in porous supports that are fixed within the absorption reactor; as cross-linked enzyme aggregates (CLEA); and/or as cross linked enzyme crystals (CLEC); or a combination thereof. According to optional embodiments, the process may include a separation step, prior to step (b), wherein at least a portion of the carbonic anhydrase is removed from the ion-rich solution. Optionally, the separation step may include removing the at least a portion of the carbonic anhydrase by filtration, by centrifugal methods or any equivalents thereof. According to optional embodiments, a remaining portion of the carbonic anhydrase is present in step (b). Alternatively, the carbonic anhydrase may be completely removed from the ion-rich solution during the separation step.

According to optional embodiments, the separation step may be performed in a separation unit comprising a settler and another separation device which is a membrane, a hydrocyclone separator, a centrifugal decanter or any combination thereof. According to optional embodiments, the at least a portion of the carbonic anhydrase which is removed from the ion-rich solution may be recycled back into the step (a).

According to optional embodiments, the process may include monitoring neutralisation properties of the neutralised bauxite and adjusting operation of step (a) to achieve given neutralisation properties. Optionally, the step of adjusting operation of step (a) may include regulating a concentration of the ion-rich solution to achieve a given neutralisation reactivity in step (b).

According to optional embodiments, the process may further include treating the ion-depleted solution prior to recycling a treated solution back into step (a). Optionally, the treating of the ion-depleted solution may include removing fine particulate material from the ion-depleted solution. Optionally, the treating of the ion-depleted solution may include filtering the ion-depleted solution. Further optionally, the treating of the ion-depleted solution may include a pH adjustment of the ion-depleted solution.

According to optional embodiments, the process may include adding fresh carbonic anhydrase to the aqueous absorption solution prior to step (a).

In another aspect, there is provided a process for neutralisation of bauxite residue, the process including: (a) contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of carbonic anhydrase, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution;

(b) subjecting the ion-rich solution to desorption for promoting release of the bicarbonate ions from the ion-rich solution and producing a C0₂ gas stream and an ion-depleted solution; and

(c) injecting the C0₂ gas stream into the bauxite residue to produce a neutralised bauxite stream. According to optional embodiments, the process may include removing the carbonic anhydrase from the ion-rich solution prior to subjecting the ion-rich solution to the desorption. Alternatively, the process may include removing the carbonic anhydrase from the ion-rich solution after subjecting the ion-rich solution to the desorption.

In another aspect, there is provided a process for neutralisation of bauxite residue, the process including:

(a) subjecting an ion-rich solution to desorption for promoting release of bicarbonate ions from the ion-rich solution in presence of carbonic anhydrase and producing a C0₂ gas stream; and

(b) injecting the C0₂ gas stream into the bauxite residue to produce a neutralised bauxite stream.

In another aspect, there is provided a system for neutralisation of bauxite residue, the system including:

an absorption unit for contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of enzymes, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution;

a neutralisation unit for contacting the ion-rich solution with the bauxite residue to produce a neutralised bauxite stream and an ion-depleted solution; and

a separation unit arranged in between the absorption unit and the neutralisation unit, for removing at least some of the enzymes and produce an enzyme-depleted stream and an enzyme-rich stream.

According to optional embodiments, the enzyme-depleted stream may be supplied to the neutralisation unit and the enzyme rich stream may be recycled, in whole or in part, to the absorption unit.

According to optional embodiments, the separation unit may include one or more separators in series or parallel. Optionally, the separators may include filters or other types of separators, depending on the removal characteristics for the enzymes and the form of the enzymes or particles. In another aspect, there is provided a system for neutralisation of bauxite residue, the system including:

a desorption unit for subjecting the ion-rich solution to desorption for promoting release of bicarbonate ions from the ion-rich solution and producing a C0₂ gas stream and an ion-depleted solution; and a neutralisation unit for injecting the C0₂ gas stream into the bauxite residue to produce a neutralised bauxite stream; and

According to optional embodiments, the system may include a separation unit arranged in between the absorption unit and the neutralisation unit, for removing at least some of the enzymes and produce an enzyme-depleted stream and an enzyme-rich stream. Optionally, the separation unit may be arranged in between the absorption unit and the desorption unit.

According to optional embodiments, the enzymes may be carbonic anhydrase or analogues thereof. Optionally, the carbonic anhydrase may be provided free in the aqueous absorption solution; dissolved in the aqueous absorption solution; immobilized on the surface of supports that are mixed in the aqueous absorption solution and flow therewith; immobilized on the surface of supports that are fixed within an absorption reactor; entrapped or immobilized by or in porous supports that are mixed in the aqueous absorption solution; entrapped or immobilized by or in porous supports that are fixed within the absorption reactor; as cross-linked enzyme aggregates (CLEA); and/or as cross linked enzyme crystals (CLEC); or a combination thereof.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the present description. The objects, advantages and other features of the present invention will become more apparent and be better understood upon reading of the following non-restrictive description of the invention, given with reference to the accompanying drawings.

It should be understood that any one of the above mentioned optional aspects of each process of bauxite neutralisation may be combined with any other of the aspects thereof, unless two aspects clearly cannot be combined due to their mutually exclusivity. For example, the various operational steps and/or structural elements of the process including a separation step as described herein-above, herein-below and/or in the appended Figures, may be adapted to any of the process including a desorption step as appearing herein-above, herein-below and/or in the appended Figures.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments, examples and illustrations of some of the techniques described herein will be further understood in light of the following figures.

Fig. 1 is a process diagram according to an optional aspect of the present invention.

Fig. 2 is a process diagram according to another optional aspect of the present invention.

Fig. 3 is a process diagram according to another optional aspect of the present invention. Fig. 4 is a process diagram according to another optional aspect of the present invention.

DETAILED DESCRIPTION

Bauxite residue is issued from the digestion of bauxite with caustic liquor so as to further produce alumina, according to the well-known Bayer process. After crushing and milling the bauxite to enhance extraction, the bauxite is mixed with a caustic liquor to form another liquor, also referred to as the Bayer liquor. The Bayer liquor is then filtered for separation into a solid slurry (bauxite residue, also referred to as red mud) and an alkaline solution.

Bauxite residue (BR) consists of both solid and liquid phases, with solid concentration varying from 28% (for instance BR in Jamaica) to 65% (filtered mud cake) of the total weight of freshly produced BR. Solids of BR include sodalite [Na₆(AISi0₄)₆ ^■ 2NaOH] and cancrinite [Na₆(AISi0₄)₆ ^■ 2CaC0₃], that are characteristic solids produced during the pre-desilication and digestion steps of the Bayer process. The solid components of BR may also contain unextracted minerals from the treated bauxite such as hematite [Fe₂0₃], boehnite [γ-ΑΙΟΟΗ] and gibbsite [AI(OH)₃] , and some calcium-bearing byproducts that are formed during the process. One of the main calcium compounds in BR is tricalcium aluminate [3Ca(OH)₂-2AI(OH)₃].

The aqueous phase of BR is essentially a diluted solution of the Bayer liquor, includingcaustic soda (NaOH), sodium aluminate (NaAI(OH)₄), and sodium carbonate (Na₂C0₃), along with smaller amounts of sodium salt impurities such as sulfate, chloride, and fluoride, as well as traces of heavy metals. BR is a highly alkaline residue (with a pH value of 13 or higher) due to its content in alkaline anions in solution OH^", C0₃ ²7HC0₃ ^", AI(OH)₄7AI(OH)_3(aq) and H₂Si0₄ ²7H₃Si0₄ ^~.

The present invention relates to a process for neutralising BR. It should be understood that neutralising BR refers to reducing the alkalinity of the BR and producing a neutralised residue. The neutralised residue may have a pH between 8 and 10.5.

In one aspect of the process, BR may be neutralised by addition of a solution which is rich in bicarbonate ions. The bicarbonate ion-rich solution is issued from a C0₂ absorption process using aqueous absorption solution in presence of an enzyme. Each step of the process may be defined and adapted to each aspect of the system described herebelow and in the appended figures. Advantages of the process will become more apparent upon describing embodiments of the system.

For example, BR neutralization with a sodium bicarbonate solution could be described by the following reactions.

The caustic soda (NaOH) and the sodium aluminate (NaAI(OH)₄) included in the liquid phase of the BR could quickly react with the sodium bicarbonate (NaHC0₃) to form a sodium carbonate (Na₂C0₃) solution and dawsonite (NaAI(OH)₂C0₃) according to following reactions 1 and 2. NaOH + NaHC0₃→ Na₂C0₃ + H₂0 (1)

NaAI(OH)₄ (aq.) + 2 NaHC0₃→ NaAI(OH)₂C0₃ (s) + Na₂C0₃ + 2 H₂0 (2) The tricalcium aluminate (3Ca(OH)₂-2AI(OH)₃) and the sodalite (Na₆(AISi0₄)₆ ^■ 2NaOH) which are solids of the BR could also react with the sodium bicarbonateaccording to following reactions 3 and 4.

3Ca(OH)₂-2AI(OH)₃ (s) + 6 NaHC0₃ <→ 3 CaC0₃ (s) + 2 AI(OH)₃ (s) + 3 Na₂C0₃ + 6 H₂0 (3)

Na₆(AISi0₄)₆ ^■ 2NaOH + 2 NaHC0₃ <→ Na₆(AISi0₄)₆ + 2 Na₂C0₃ + 2 H₂0 (4)

Even though some BR solids, such as the above-mentioned tricalcium aluminate and sodalite, can be solubilized by reducing the pH with neutralisation, the kinetics of these solid reactions (3 and 4) is relatively slow. Solids may therefore not react completely within the available time in a neutralisation reactor and the pH of a neutralised BR could be thus increased after some time in a storage site. Thus, it may be advantageous to periodically add some bicarbonate solution to neutralize the caustic liquor of the neutralised BR in a storage site.

Referring to Fig. 1 , the overall system includes a C0₂ capture system 10 and a bauxite neutralisation system 1 1. The C0₂ capture system 10 enables to capture C0₂ from a C0₂ containing gas 12 while producing a bicarbonate solution which may be used for neutralising a bauxite residue (BR) in the bauxite neutralisation system 1 1. It should be understood that the C0₂ capture system and the bauxite neutralisation system may be operated in series so as to recycle various streams of the bauxite neutralisation system into the C0₂ capture system. Each of the C0₂ capture system and bauxite neutralisation system may further respectively include a plurality of C0₂ capture systems operated in parallel and a plurality of bauxite neutralisation system operated in parallel. The source of C0₂-containing gas may be a smelter, a refinery or another type of plant in an aluminum manufacturing operation.

The C0₂-containing gas 12 is supplied to an absorption unit 14, which is also fed with an aqueous absorption solution 16 for contacting the C0₂-containing gas 12 and absorbing the C0₂ therein. The C0₂-containing gas 12 may be fed into the absorption unit through a gas inlet port near the bottom of the absorption unit 14. The absorption unit 14 may be of various types, such as a packed reactor, a spray reactor, a fluidized bed, or a bubble column type reactor. There may be one or more reactors that may be provided in series or in parallel. The aqueous absorption solution 16 may include sodium carbonate (Na₂C0₃) or other carbonate salts such as potassium carbonate, ammonium carbonate or a combination thereof.

Upon contact with the aqueous absorption solution 16, the C0₂-containing gas can react with both hydroxide ions and water according to the following equations 5 and 6:

C0₂ ^■+ OH^~ → HCO3 (5)

C0₂ ^■+ 2H₂0→ HCO3 + H₃0⁺ (₆) For instance, upon contact with the C0₂-containing gas with an aqueous absorption solution 16 including sodium carbonate (Na₂C0₃), the aqueous absorption solution 16 absorbs the C0₂ of the gas and converts it into sodium bicarbonate (NaHC0₃) according to the following equation:

Na₂C0₃ + C0₂ + H₂0→ 2 NaHC0₃ (7) In one aspect of the present invention, the production of the sodium bicarbonate is enhanced in the presence of an enzyme. The enzyme may include carbonic anhydrase or analogs thereof. Carbonic anhydrase catalyses the hydration reaction of C0₂ into bicarbonate and hydrogen ions (equation 6) and thus a C0₂- depleted gas 18 and an ion-rich solution 20, including sodium bicarbonate, are produced. The ion-rich solution 20 may be collected near the bottom of the absorption unit 14, and the C0₂-depleted gas stream 18 may be discharged to the atmosphere near the top of the absorption unit 14. The aqueous absorption solution 16 may be supplied from an absorption solution make-up tank 22.

In an optional aspect, the enzyme is provided directly as part of a formulation or solution. There may also be enzyme provided in a reactor to react with incoming solutions and gases; for instance, the enzyme may be fixed to a solid non-porous packing material, on or in a porous packing material, on or in particles or as aggregates flowing with the absorption solution within a packed tower or another type of reactor. The carbonic anhydrase may be in a free or soluble state in the formulation or immobilized on or in particles or as aggregates, chemically modified or stabilized, within the formulation. It should be noted that enzyme used in a free state may be in a pure form or may be in a mixture including impurities or additives such as other proteins, salts and other molecules coming from the enzyme production process. Immobilized enzyme free flowing in the solutions could be entrapped inside or fixed to a porous coating material that is provided around a support that is porous or non-porous. The enzymes may be immobilized directly onto the surface of a support (porous or non-porous) or may be present as cross linked enzyme aggregates (CLEAs) or cross linked enzyme crystals (CLECs). CLEA include precipitated enzyme molecules forming aggregates that are then cross-linked using chemical agents. The CLEA may or may not have a 'support' or 'core' made of another material which may or may not be magnetic. CLEC include enzyme crystals and cross linking agent and may also be associated with a 'support' or 'core' made of another material. When a support is used, it may be made of polymer, ceramic, metal(s), silica, alumina, solgel, chitosan, nylon, cellulose, alginate, polyacrylamide, magnetic particles, titanium oxide, zirconium oxide and/or other materials known in the art to be suitable for immobilization or enzyme support. When the enzymes are immobilized or provided on particles, such as micro-particles, the particles are preferably sized and provided in a particle concentration such that they are pumpable with the solution throughout the process.

When the enzymes are provided on micro-particles, the micro-particles may be sized in a number of ways. The micro-particles may be sized to facilitate separation of the micro-particles from the ion-rich mixture. For instance, the micro-particles may be sized to have a diameter above about 1 μηι or above about 5 μηι. The micro-particles may also be sized to have a catalytic surface area including the biocatalysts having an activity density so as to provide an activity level equivalent to a corresponding activity level of soluble biocatalysts above about 0.05 g biocatalyst /L, optionally between about 0.05 g biocatalyst /L and about 2 g biocatalyst /L. Furthermore, the absorption solution and the C0₂ form a reactive liquid film having a thickness and the micro-particles may be sized so as to be within an order of magnitude of the thickness of the reactive liquid film. The micro-particles may also be sized so as to be smaller than the thickness of the reactive liquid film. The thickness of the reactive liquid film may be about 10 μηι. In another optional aspect, the micro-particles are sized between about 0.2 μηι and about 100 μηι. It should also be noted that precipitates may be formed in the ion-rich solution and the micro-particles may be sized to be larger or heavier than the precipitates or to be easily separable therefrom. In some optional aspects of the process, the particles may be sized so as to be nano-particles. The micro-particles may also be provided in the absorption solution at a maximum particle concentration of about 40% w/w. In some optional aspects, the maximum micro-particle concentration may be 35% w/w, 30% w/w, 25% w/w, 20% w/w, 15% w/w, 10% w/w, or 5% w/w. The micro-particles may be composed of support material(s) that is at least partially composed of nylon, cellulose, silica, alumina, silica gel, chitosan, polystyrene, polymethylmetacrylate, alginate, polyacrylamide, magnetic material, titanium oxide, zirconium oxide or a combination thereof. The support may preferably be composed of nylon or polystyrene. The density of the support material may be between about 0.6 g/cm³ and about 4 g/cm³.

Carbonic anhydrase is a very efficient catalyst that enhances the reversible reaction of C0₂ to HC0₃ ^". Carbonic anhydrase is not just a single enzyme form, but a broad group of metalloproteins that exists in three genetically unrelated families of isoforms, α, β and γ. Carbonic anhydrase (CA) is present in and may be derived from animals, plants, algae, bacteria, etc. The human variant CA II, located in red blood cells, is the most studied and has a high catalytic turnover number. The carbonic anhydrase includes any analogue, fraction and variant thereof and may be alpha, gamma or beta type from human, bacterial, fungal or other organism origins, having thermostable or other stability properties, as long as the carbonic anhydrase can be provided to function in the C0₂ capture or desorption processes to enzymatically catalyse the reaction:

carb onic Riinvcircts e „

C0₂ + H₂0 *- : >H^~ + HCO (₈) It should be understood that the expression "carbonic anhydrase or analogues thereof as used herein includes naturally occurring, modified, recombinant and/or synthetic enzymes including chemically modified enzymes, enzyme aggregates, cross-linked enzymes, enzyme particles, enzyme-polymer complexes, polypeptide fragments, enzyme-like chemicals such as small molecules mimicking the active site of the carbonic anhydrase enzyme and any other functional analogue of the enzyme carbonic anhydrase.

In an optional aspect, the aqueous absorption solution may be a carbonate-based solution, such as potassium carbonate solution, sodium carbonate solution, ammonium carbonate solution, promoted potassium carbonate solutions, promoted sodium carbonate solutions or promoted ammonium carbonates; or mixtures thereof. These carbonate-based solutions may be promoted with one or more of the above-mentioned chemical compounds.

In further optional aspects of the process, the ion-rich solution may contain from about 0.1 M to 10 M of bicarbonate ions. The carbonate loading of the solution will depend on the operating conditions, reactor design and the chemical compounds that are added. For instance, when potassium or sodium bicarbonate compounds are used in the absorption solution, the ion-rich solution may contain from about 0.2 M to 1 .5 M of bicarbonate ions. When the ion-rich solution is highly loaded with carbonate/bicarbonate ions, it may become much more viscous which can have a detrimental effect on mass transport within the solution. The presence of carbonic anhydrase flowing with the solution further enhances the mass transport along with the enzymatic reaction, thus improving the overall C0₂ capture, for instance by supersaturating the solution with bubbles of gaseous C0₂.

Still referring to Fig. 1 , the C0₂ capture system 10 may include a separation unit arranged in between the absorption unit 14 and the bauxite neutralisation system 1 1 , for removing at least some and possibly all of the biocatalyst in the event the enzyme is flowing with the ion-rich solution 20, e.g. when the enzyme is free in solution or provided with respect to particles. The ion-rich solution 20 may be a suspension of solid particles including heavier solid particles, such as the immobilized biocatalyst particles and/or aggregates. The separation unit separates the ion-rich solution into a sodium bicarbonate rich liquid stream 32 and an enzyme rich stream 52.

The sodium bicarbonate rich liquid stream 32 may be supplied to the bauxite neutralisation system 1 1.

The bauxite neutralisation system 1 1 may include a neutralisation unit 36 receiving a portion 33 of the sodium bicarbonate rich liquid stream 32 for contact and reaction with supplied BR 38. The neutralisation unit 36 aims at decreasing the pH of the alkaline BR by washing the BR with sodium bicarbonate to extract as much sodium hydroxide (NaOH) as possible while forming sodium carbonate (Na₂C0₃) (equation 1). A neutralised bauxite stream 37 and a sodium carbonate rich stream 40, also referred to as BR liquor 40, may thus be released from the neutralisation unit 36. Optionally, BR liquor 40 may be recycled to the C0₂ capture system 10. Some precipitation reactions may also occur in the neutralisation unit 36 as BR 38 may include some traces of heavy metals. The corresponding metallic ions may therefore react with the produced carbonate ions to form precipitates. Thus, the recycled BR liquor 40 may be advantageously cleaned from metallic ions which could poison the enzyme in the absorption unit 14 of the C0₂ capture system 10. The neutralised bauxite stream 37 may be sent to downstream storage sites or used as a construction material. The neutralised bauxite may be discharged safely and have a minimized impact on environment comparing to untreated bauxite residue.

Still referring to Fig. 1 , the bauxite neutralisation system 1 1 may also include a separate neutralisation tank 42 which is supplied with a sodium carbonate rich stream 40, also referred to as BR liquor, that may be pumped from the neutralisation unit 36. The sodium carbonate rich stream 40 is further neutralised with an aqueous solution of sodium bicarbonate which may be a recycled portion 44 of the sodium bicarbonate rich liquid stream 32 from the C0₂ capture system 10. A neutralised solution 46 is thereby formed and may be recycled from the neutralisation tank 42 to the absorption solution make-up tank 22.

The temperature in the bauxite neutralisation tank 42 may increase due to exothermal reaction of sodium hydroxide (remained in the BR liquor 40) and sodium bicarbonate (contained in the sodium bicarbonate rich liquid stream 32 and the bicarbonate solution 44). Optionally, a cold water cooling system may be combined to the bauxite neutralisation system. For example, when a large amount of the caustic BR liquor 40 is handled, a cold water cooling system may be adapted to the neutralisation tank 42. After cooling down to a predetermined temperature, optionally to 40°C or lower, the neutralised solution 46 is guided to the absorption solution make-up tank 22. In an optional aspect, the separation unit may include one or more separators in series or parallel. The separators may be filters or other types of separators, depending on the removal characteristics for the enzymes and the form of the enzymes or particles.

In an optional aspect, still referring to Fig. 1 , the separation unit may include a settler 24 which is supplied with the ion-rich solution 20 so as to precipitate heavier solid particles and separate them from the solution 20. Upon settling, the heavier particles may be collected near the bottom of the settler 24 as an enzyme rich stream 26 that may be recycled, in whole or in part, to the absorption unit 16, through the absorption solution make-up tank 22 for example. An enzyme- depleted stream 28 may be collected near the top of the settler 24. The separation unit may further include a membrane filtration system 30 which separates the enzyme-depleted stream 28 into a permeate 32 and a retentate 34. When free enzymes and/or enzymes immobilized on nanoparticles are used, the membrane filtration system 30 may optionally be an ultrafiltration system. When enzymes immobilized on microparticles or particle supports with larger particle size are used, the membrane filtration system 30 may optionally be a microfiltration system that is capable of removing fine particles from the system.

Advantageously, as the permeate 32 is rich in sodium bicarbonate (NaHC0₃), at least a portion 33 thereof may be supplied to the bauxite neutralisation system 1 1. The neutralisation unit 36 may receive the portion 33 of the permeate 32 for contact and reaction with BR 38.

A main portion 50 of the enzyme rich stream 26 and the retentate 34 from the membrane filtration system 30 are combined and recycled back as stream 52 to the absorption solution make-up tank 22. Additionally, some aggregates from the settler 24 can be partially removed out of the system, as a minor part of the enzyme rich stream 26. The absorption solution make-up tank 22 receives the neutralised solution 46 and recycled enzymes in stream 52. Optionally, fresh biocatalyst, such as carbonic anhydrase or analogues thereof, may be added to stream 52. The absorption solution make-up tank 22 aims at adjusting the concentration of the aqueous absorption solution 16 in enzymes and absorption compounds. The aqueous absorption solution 16 is then supplied to the absorption unit 14

In an optional aspect, the separation unit may include one or more membrane filtration system, also referred to as filtration modules. It is known that an average of about 40 kg of C0₂ per ton of BR by dry mass may be used for BR neutralisation into carbonates. Accordingly, the total C0₂ needed at any single alumina refinery site may be generally generated by C0₂ emissions of a 20 MW equivalent power plant. For example, according to an embodiment of the present invention, for a 20 MW equivalent power plant, an ion-rich solution flow rate of approximately 800 m³/h may be needed. Assuming that the separation unit includes membrane filtration systems that may have an average membrane flux of 400 LMH (liter per meter square per hour) for an ultrafiltration (UF) and 600 LMH for a microfiltration (MF), the total membrane filtration area needed would respectively be 2000 m², and 1333 m² for the UF and MF. The separation unit would have to respectively include 10 to 20 filtration modules for an UF filtration and 7 to13 filtration modules for a MF system with a membrane module size of 100-200 m²/per filtration module, so as to filtrate an ion-rich solution with a flow rate of 800 m³/h. In an optional aspect, the membrane filtration system may include membranes made of polypropylene (PP), polyamide (PA), polysulfone (PS), polyethersulfone (PES), polyvinylidene difluoride (PVDF), polyetherimide (PEI), polyimide (PI), polyvinylpyrrolidone or combination thereof. It should be understood that any membrane used for water and wastewater treatment could be used in the separation unit.

In another optional aspect, the separation unit may include a settler and a hydrocyclone separator. The separation unit may benefit from the advantages of the solid/liquid (S/L) separation in hydrocyclone separators. Hydrocyclone separators have a system configuration which is relatively simple and generally requires little maintenance. Additionally, the separation of the ion-rich solution may take place very quickly, with a residence time inferior to two seconds. The hydrocyclone separator may be suited when the C0₂ absorption is enhanced by enzymes immobilized on particles which size is superior to 10 μηι. Optionally, for smaller particles having a size inferior to 10 μηι, mini-hydrocyclones having a diameter ranging from 10 to 13 mm may be used.

Referring to Fig. 2, after sedimentation in the settler 24, the ion-rich solution 20 is separated into three streams, the enzyme rich stream 26, the enzyme-depleted stream 28 and intermediate enzyme-containing stream 53. At least a portion 50 of the enzyme rich stream 26 may be recycled to the absorption solution make-up tank 22. The remaining portion of the enzyme rich stream 26 may be removed from the system as discharge or sent to be regenerated, depending on the state of the biocatalysts being used. The enzyme-containing stream 53 is collected from an intermediate portion of the settler 24, and includes biocatalysts particles which have not yet settled. The enzyme-containing stream 53 is sent to a hydrocyclone separator 54 to be further separated into an enzyme-depleted liquid stream 55 collected at the top of the hydrocyclone separator 54, and an enzyme - concentrated stream 56 collected near the bottom of the hydrocyclone separator 54. As the enzyme-depleted streams 28 and 55 include a high concentration of bicarbonate ions (HC0₃ ^~), a portion 33 of which may be supplied to the neutralisation unit and another portion 44 of which may be supplied to the neutralisation tank 42.

In an optional aspect, the number and configuration of hydrocyclone separator included in the separation unit may vary according to the flow rate of ion-rich solution to be treated. For example, the feed capacity of a mini-hydrocyclone of 12,7 mm diameter may be about 0.7 GPM (2.65 L/min); each mini-hydrocyclone may occupy a surface area with an equivalent diameter of 20 mm; and for a given separation unit, the occupation density of the mini-hydrocyclones may be 80%. For a 1 -meter hydrocyclone module, the maximum small hydrocyclones (N) that may be installed is calculated according to the following equation:

N = 80% [(1/4) nD²/(nD₀ ²/4)], where D =1000 mm, D₀ = 20 mm;

N= 2000.

The flow rate of the ion-rich solution that may be treated for each 1 -m hydrocyclone module is calculated as follows: N feed capacity = 2000 ^χ 0.7 = 1400 GPM (317.5 m³/h)

Accordingly, for an ion-rich solution having a flow rate of 800 m³/h, a maximum of three 1 -m modules would be enough to recover and recycle the used biocatalyst particles.

In another optional aspect, the separation unit may include a settler and a centrifugal device. The use of the centrifugal device may be suited for biocatalysts having particle sizes of 10 μηι or larger. The biocatalysts may be separated from the ion-rich solution and recovered for reuse in the aqueous absorption solution by a centrifugal device including a decanter. Optionally, the decanter may be used to further separate the enzyme rich stream collected from the settler into an enzyme-concentrated stream and a liquid stream.

Referring to Fig. 3, the ion-rich solution 20 collected near the bottom of the absorption unit 14 is pumped into the settler 24, where the heavier biocatalyst particles are precipitated and settled. The enzyme rich stream 26 is collected near the bottom of the settler 24 and pumped for supply to a centrifugal decanter 60. The decanter 60 separates the enzyme rich stream 26 into an enzyme- concentrated stream 62 and an enzyme-depleted liquid stream 64. The enzyme- concentrated stream 62 includes solid particles of enzyme. At least a portion 63 of the enzyme-concentrated stream 62 may be re-dispersed into the absorption solution make-up tank 22, the remaining portion being discharged for regeneration or replacement when needed. An aqueous solution of fresh enzymes and bicarbonate ions (stream 23) may optionally be introduced into absorption solution make-up tank 22. The enzyme-depleted stream 28 (from the settler 24) and the enzyme lean liquid stream 64 (from the decanter 60) are combined so as to be recycled back for reuse. A liquid portion 33 of the combined streams 28 and 64 may be supplied to the neutralisation unit 36, and another liquid portion 44 may be supplied to the neutralisation tank 42. The remaining liquid portion maybe sent for use in other neutralisation systems or for other application purposes. This liquid portion may also be periodically sent to any BR storage pond or tank for neutralising the extracted BR during the storage process. Indeed, due to strongly alkaline buffered system of the BR, the pH of the storage ponds or tanks may increase after sometime. In an optional aspect, the decanter may be of GN Solids, LWF series (Tangshan Guanneng Machinery Equipment Co., Ltd. Hebei, China) that can separate particles with a particle separation point of 2-7μηι.

In an optional aspect, the separation unit may include one or more decanters according to the flow rate of the ion-rich solution to be treated. For example, the ion-rich solution entering the settler may have a flow rate of 800 m³/h and includes 10% (by volume) of biocatalyst particles. The enzyme rich stream collected from the settler may therefore be 80 m³/h. With a total particles concentration in the enzyme rich stream of 50% by volume, two decanters with a capacity of 60-80 m³/h and a particle separation point of 2-7 μηι would be suited to recover and recycle the biocatalyst particles contained in the ion-rich solution.

It should be noted that the biocatalyst may be provided in a number of ways. For instance, carbonic anhydrase may be provided to the aqueous absorption solution which flows through the absorption unit. In this scenario, the carbonic anhydrase may be introduced into the overall C0₂ capture system via an absorption solution make-up stream, which may be mixed with the recycled enzyme rich solution. The carbonic anhydrase may also be added to the absorption units via multiple enzyme feed streams. Depending on operating conditions and the thermal stability of the carbonic anhydrase strain, fraction, variant or analog that is used in the process, the carbonic anhydrase may be introduced at a given point in the process and spent enzyme may be replaced at a given point in the process. For example, when free enzyme is used as a component of the absorption solution, the process may include periodic or continuous removal of denatured enzyme or reduced-activity enzyme, which may be done as part of an absorption solution reclaiming or make-up technique. In an optional aspect, the particle size of the enzymes contained in the ion-rich solution may be between 0.05 μηι and 200 μηι, preferably between 0.2 μηι and 20 μηι. The enzyme concentration of the ion-rich solution may be between 0.02 g/L and 2.0 g/L, preferably between 0.2 g/L and 1.0 g/L, on free enzyme basis or equivalent free enzyme basis for the immobilized particles. The particle concentration of the enzyme rich stream may be between 0.2 (v/v)% and 20 (v/v)%, preferably between 1 (v/v)% and 10 (v/v)%. The particle concentration of the enzyme-depleted liquid stream may be inferior to at most 0.01 (v/v)%.

In an optional aspect, the neutralisation unit may be a large open pond or cell. Alternatively, the neutralisation unit may be a constructed vessel. It should be noted that there may be several different neutralised bauxite streams withdrawn from different locations of the neutralisation unit.

In another optional aspect, at least a portion of sodium bicarbonate rich stream may be added to an underflow stream of a last wash thickener of the Bayer process to neutralise BR online.

In another optional aspect, at least a portion of sodium bicarbonate rich stream may be periodically sent to BR storage ponds or tanks to neutralise the extracted BR during the storage process. Indeed, due to strongly alkaline buffered system of the BR, the pH of the storage ponds or tanks may increase after sometime. In one optional aspect, another portion of the sodium bicarbonate rich stream may be used for some other applications such as cleaning agent, bio-pesticides, pH level adjustment for garden ponds, or neutralisation of the acidic gases (e.g. HCI, HF, and S0₂ from the aluminum smelter) or other flue gas streams that need to be treated. In an optional aspect, the overall process combining C0₂ capture and BR neutralisation may be operated in a continuous mode.

The system may also include various other treatment units for preparing the ion- rich solution for the neutralisation unit and/or for preparing the ion deplete unit for recycling into the absorption unit. There may be pH adjustment units or various monitoring units.

The system may also include a measurement device for monitoring neutralisation properties of the neutralised bauxite and adjusting operation of the absorption unit to achieve desired neutralisation properties. For example, the step of adjusting may include regulating the concentration of the ion-rich solution to achieve the desired neutralisation reactivity, e.g. by increasing or decreasing the amount of bicarbonate in the ion-rich solution. This could be done by various methods including adjusting the liquid and/or gas flow rates, for example.

In another aspect, the present invention relates to a process for neutralising a bauxite residue by injecting a C0₂ gas stream into the bauxite residue. The C0₂ gas stream may be produced from desorption of an ion-rich solution including bicarbonate ions. The desorption may be performed in presence of an enzyme, such as carbonic anhydrase or analogues thereof. Optionally, the ion-rich solution may be derived from an enzymatic C0₂ capture system.

Referring to Fig. 4, the overall system includes a C0₂ capture system 10 and a bauxite neutralisation system 1 1. The C0₂ capture system 10 includes an absorption unit 14 enabling to capture C0₂ from a C0₂-containing gas 12 while producing an ion-rich solution 20. The C0₂ capture system 10 also includes a desorption unit 66 enabling to produce gaseous C0₂ 68 and a C0₂-depleted solution 70, from the ion-rich solution 20.

The enzymes contained in the ion-rich solution 20 exiting the absorption unit 14 may or may not be removed from the ion-rich solution 20. Enzymes may be removed from the ion-rich solution 20 before sending the ion-rich solution 20 to the desorption unit 66. Any of the above-mentioned separation system may be used to remove enzymes from the ion-rich solution 20: membrane filters, hydrocyclones, decanters, or any combination thererof, depending on the enzyme characteristics and configuration (e.g. free enzymes or immobilized enzymes on a particle support) according to above-described embodiments, may be applied.

Alternatively, the desorption may be performed in presence of enzymes, such as carbonic anhydrase or analogues thereof. The gaseous C0₂ 68 exiting near the top of the desorption unit 66 is then sent to the neutralisation unit 36 for neutralising the bauxite residue (BR) 38. The gaseous C0₂ is injected into the neutralisation unit 36 through an injection system 72 that bubbles through the BR 38. As the neutralized BR liquor 40 issued from the neutralization unit 36 is rich of carbonate ions, it may be recycled to the absorption solution make-up tank 22 to produce the absorption solution 16. The C0₂-depleted solution 70 may also optionally be recycled to the absorption solution make-up tank 22 so as to be mixed with the neutralized BR liquor 40 and a freshly introduced aqueous solution of enzymes and bicarbonate ions (stream 23). The C0₂-depleted solution 70 may optionally be used to pre-heat the ion-rich solution 20 through a heat-exchanger 74 before entering the desorption unit 66. Known BR treatment by C0₂ injection may be affected by a number of factors such as location of a suitable C0₂ source and costs for the requested C0₂ concentration level. The present invention provides a suitable source of highly concentrated gaseous C0₂ derived from enzymatic C0₂ absorption-desorption system or enzymatic ion-rich solution desorption system.

Claims

1. A process for neutralising bauxite residues with an ion-rich solution including bicarbonate ions that are derived from enzymatic absorption of C0₂ from a C0₂-containing gas resulting of aluminum processing operations.

2. A process for neutralising bauxite residues with gaseous C0₂ derived from the desorption of an ion-rich solution including bicarbonate ions.

3. The process of claim 2, wherein the ion-rich solution is derived from absorption of C0₂ in presence of an enzyme.

4. The process of claim 3, wherein the enzyme is separated from the ion-rich solution before the desorption step.

5. The process of claim 2 or 3, wherein the desorption of the ion-rich solution is performed in presence of an enzyme.

6. The process of any one of claims 3 to 5, wherein the enzyme is carbonic anhydrase or analogues thereof.

7. A process for neutralisation of a bauxite residue, the process comprising:

(a) contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of carbonic anhydrase, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution; and

(b) contacting the ion-rich solution with the bauxite residue to produce a neutralised bauxite stream and an ion-depleted solution.

8. The process of claim 7, comprising recycling the ion-depleted solution as at least part of the aqueous absorption solution.

9. The process of claim 8, comprising recycling a portion of the ion-rich solution for contacting the ion-depleted solution for further neutralisation.

10. The process of any one of claims 7 to 9, wherein step (a) is performed in a packed reactor.

1 1. The process of any one of claims 7 to 9, wherein the step (a) is performed in a spray reactor, in a fluidized bed or in a bubble reactor.

12. The process of any one of claims 7 to 1 1 , wherein the step (b) is performed in a cell that is open to the atmosphere.

13. The process of any one of claims 7 to 12, wherein, in step (a), the carbonic anhydrase is provided free in the aqueous absorption solution; dissolved in the aqueous absorption solution; immobilized on the surface of supports that are mixed in the aqueous absorption solution and flow therewith; immobilized on the surface of supports that are fixed within an absorption reactor; entrapped or immobilized by or in porous supports that are mixed in the aqueous absorption solution; entrapped or immobilized by or in porous supports that are fixed within the absorption reactor; as cross-linked enzyme aggregates (CLEA); and/or as cross linked enzyme crystals (CLEC); or a combination thereof.

14. The process of any one of claims 7 to 13, comprising a separation step, prior to step (b), wherein at least a portion of the carbonic anhydrase is removed from the ion-rich solution.

15. The process of claim 14, wherein the separation step comprises removing the at least a portion of the carbonic anhydrase by filtration, by centrifugal methods or any equivalents thereof.

16. The process of claim 14 or 15, wherein a remaining portion of the carbonic anhydrase is present in step (b).

17. The process of claim 14 or 15, wherein the carbonic anhydrase is completely removed from the ion-rich solution during the separation step.

18. The process of any one of claims 14 to 17, wherein the separation step is performed in a separation unit comprising a settler and another separation device which is a membrane, a hydrocyclone separator, a centrifugal decanter or any combination thereof.

19. The process of any one of claims 14 to 18, wherein the at least a portion of the carbonic anhydrase which is removed from the ion-rich solution is recycled back into the step (a).

20. The process of any one of claims 7 to 19, comprising monitoring neutralisation properties of the neutralised bauxite and adjusting operation of step (a) to achieve given neutralisation properties.

21. The process of claim 20, wherein the step of adjusting operation of step (a) 5 comprises regulating a concentration of the ion-rich solution to achieve a given neutralisation reactivity in step (b).

22. The process of any one of claims 7 to 21 , further comprising treating the ion- depleted solution prior to recycling a treated solution back into step (a).

23. The process of claim 22, wherein the treating of the ion-depleted solution 10 comprises removing fine particulate material from the ion-depleted solution.

24. The process of claim 22 or 23, wherein the treating of the ion-depleted solution comprises filtering the ion-depleted solution.

25. The process of any one of claims 22 to 24, wherein the treating of the ion- depleted solution comprises a pH adjustment of the ion-depleted solution.

15 26. The process may include adding fresh carbonic anhydrase to the aqueous absorption solution prior to step (a).

27. A process for neutralisation of bauxite residue, the process comprising:

(a) contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with 0 an aqueous absorption solution, in the presence of carbonic anhydrase, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution;

(b) subjecting the ion-rich solution to desorption for promoting release 25 of the bicarbonate ions from the ion-rich solution and producing a

C0₂ gas stream and an ion-depleted solution; and

(c) injecting the C0₂ gas stream into the bauxite residue to produce a neutralised bauxite stream.

28. The process of claim 27, comprising removing the carbonic anhydrase from the ion-rich solution prior to subjecting the ion-rich solution to the desorption.

29. The process of claim 27, comprising removing the carbonic anhydrase from the ion-rich solution after subjecting the ion-rich solution to the desorption.

30. The process of claim 27, wherein carbonic anhydrase is provided free in the aqueous absorption solution, thereby flowing with the ion-rich solution which is subjected to desorption.

31. A process for neutralisation of bauxite residue, the process comprising:

32. A system for neutralisation of bauxite residue, the system comprising:

33. The system of claim 32, wherein the enzyme-depleted stream is supplied to the neutralisation unit and the enzyme rich stream is recycled, in whole or in part, to the absorption unit.

34. The system of claim 32 or 33, wherein the separation unit comprises one or more separators in series or parallel.

35. The system of claim 34, wherein the separators comprise filters or other types of separators, depending on the removal characteristics for the enzymes and the form of the enzymes or particles.

36. A system for neutralisation of bauxite residue, the system comprising:

5 an absorption unit for contacting a C0₂-containing gas derived from a smelter, a refinery or a plant in an aluminum manufacturing operation, with an aqueous absorption solution, in the presence of enzymes, to promote the hydration reaction of C0₂ into bicarbonate and hydrogen ions and produce a C0₂-depleted gas and an ion-rich solution;

10 a desorption unit for subjecting the ion-rich solution to desorption for promoting release of bicarbonate ions from the ion-rich solution and producing a C0₂ gas stream and an ion-depleted solution; and a neutralisation unit for injecting the C0₂ gas stream into the bauxite residue to produce a neutralised bauxite stream.

15 37. The system of claim 36, comprising a separation unit arranged in between the absorption unit and the neutralisation unit, for removing at least some of the enzymes and produce an enzyme-depleted stream and an enzyme-rich stream.

38. The system of claim 37, wherein the separation unit is arranged in between the 20 absorption unit and the desorption unit.

39. The system of any one of claims 32 to 38, wherein the enzymes comprise carbonic anhydrase or analogues thereof.

40. The system of claim 39, wherein the carbonic anhydrase is provided free in the aqueous absorption solution; dissolved in the aqueous absorption solution;

25 immobilized on the surface of supports that are mixed in the aqueous absorption solution and flow therewith; immobilized on the surface of supports that are fixed within an absorption reactor; entrapped or immobilized by or in porous supports that are mixed in the aqueous absorption solution; entrapped or immobilized by or in porous supports that are fixed within the absorption

30 reactor; as cross-linked enzyme aggregates (CLEA); and/or as cross linked enzyme crystals (CLEC); or a combination thereof. The system of claim 36, wherein the enzymes are provided free in the aqueous absorption solution and flow through the absorption unit and the desorption unit.