WO2019074444A1

WO2019074444A1 - Process for treating waste streams containing bauxite tailings

Info

Publication number: WO2019074444A1
Application number: PCT/SG2018/050511
Authority: WO
Inventors: Charles Dean JAQUAYS; John Franklin SMALL
Original assignee: Ecologistics Pte Ltd
Current assignee: Ecologistics Pte Ltd
Priority date: 2017-10-11
Filing date: 2018-10-09
Publication date: 2019-04-18
Anticipated expiration: 2020-04-11

Abstract

The invention provides a solution to the problems encountered by conventional red mud produced by the Bayer process. The invention includes a process for treating waste streams containing bauxite tailings to recover alkali metal hydroxide(s) present, to obtain them as refined mineral values such as sodium hydroxide and aluminum hydroxide that will increase the efficiency of the Bayer process. Further, the process leads to an environmentally beneficial a calcium carbonate product that can be incorporated into building materials or filtration media.

Description

PROCESS FOR TREATING WASTE STREAMS

CONTAINING BAUXITE TAILINGS

FIELD OF THE INVENTION

[001] The invention relates generally to a chemical process for utilizing bauxite tailings, and more specifically, to a process for treating waste streams containing bauxite tailings to recover alkali metal hydroxide(s) present as refined mineral values such as aluminum hydroxide.

BACKGROUND OF THE INVENTION

[002] The Bayer process is an industrial process for refining bauxite to produce alumina (aluminium oxide). Over 95% of the alumina produced globally is a product of the Bayer process. Bauxite, the ore of aluminium and starting material in the process, contains 30-60% aluminium oxide (AI2O3). The remainder is a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be purified before it can be refined to aluminium metal.

[003] In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution. The aluminum is penetrated as sodium aluminate in an extraction process. The aluminum compounds in the bauxite can be present as gibbsite (AI(OH)s), boehmite (AIHO2) or diaspore (AIO(OH)). The different forms of the aluminium component and their respective concentration can be used to determine the extraction conditions. After separation of the residue by filtering, gibbsite (aluminium hydroxide) is precipitated when the liquid is cooled and then seeded with fine-grained aluminium hydroxide.

[004] The Bayer process results in the formation of large quantities of bauxite tailings, often referred to as "red mud." Bauxite tailings are a by-product in the production of alumina (aluminium oxide). For every ton of alumina produced, approximately 1 to 1 .5 tons of bauxite tailings/residue are also produced. The aluminum industry generates more than a hundred million tons of this waste per year. The tailings are both a source of pollution and a waste of a potentially valuable mineral resource. [005] A small amount of the sodium hydroxide used in the process remains with the tailings, causing the material to have a high pH/alkalinity (usually >12). The tailings have a high concentration of iron oxide which gives the product a characteristic red colour. Scientists in the aluminum industry have attempted to recycle the

environmentally problematic red mud. However, proposed applications have not been safe or economical.

[006] Various stages in the solid/liquid separation process have been introduced to recycle as much sodium hydroxide as possible from the residue back into the Bayer Process to make the process more efficient and reduce production costs. This also lowers the final alkalinity of the tailings making it easier to handle. Despite

improvements in efficiency, the tailings are a major source of pollution. They are typically dumped into storage pounds where they gradually seep into water supplies or the ocean.

[007] In early years, tailings were often discharged into rivers, estuaries, or the oceans via pipelines or barges. In some instances the residue was shipped out to sea in deep ocean trenches offshore. It was also common to pump the tailings into lagoons or ponds sometimes created in former bauxite mines or depleted quarries. In other operations, impoundments were constructed with dams, levees or valleys for long-term storage areas of the tailings.

[008] Because of environmental concerns, approaches to the storage of tailings have changed in recent decades. As these practices have largely been stopped, dry stacking has been increasingly adopted. In this method, tailings are thickened to a high density slurry (48-55% solids or higher), and then deposited in a way that it consolidates and dries.

[009] Recent efforts have continued to focus on improving the efficiency and yield of the Bayer process as well as reducing the amount of red mud produced. Some have attempted to treat the red mud to reduce its toxicities. For example, U.S. Patent No. 3,985,567 to Iwu describes a process for combining treated bauxite tailings with clay and heating in an oven to obtain a brick product. U.S. Patent No. 4,133,866 to Lokatos et al. describes a process for separating bound sodium from red mud residue in which ferric sulfate is used to extract the sodium content. U.S. Patent No. 5,554,352 to Jaques et al. describes treating virgin bauxite to produce pozzolan for use in concrete products.

[0010] Conventional approaches often use a molar equivalent of acid to effectively neutralize the alkali remnant that remains entrained within the tailings. Thus, salts of the acid used are produced that are water soluble and can then be washed free from the solids. This can yield a stable neutral solid with a neutral pH and a supernatant liquid of similar pH consisting mainly of sodium sulfate.

[001 1 ] While these approaches can reduce the toxicity of red mud or reduce the volume that is dumped, they have their shortcomings. They do not recover aluminum or sodium hydroxide. Further, they are generally impractical for large scale use.

[0012] Accordingly, there is a need for a method to recover aluminum from red mud to improve the yield of industrial Aluminum production. It would also be beneficial to have a process that efficiently utilizes by-products and recovers the chemical values for industrial use that make tailings so detrimental to the environment and living organisms.

INTRODUCTION

[0013] The invention relates to a process for treating waste streams containing bauxite tailings to recover alkali metal hydroxide(s) and obtain them as refined mineral values such as aluminum hydroxide that will increase the efficiency of the Bayer process. The results are improved yields and efficiency. Further, the process yields an environmentally beneficial finished calcium carbonate product that can be incorporated into building materials and/or filtration media. Building materials such as bricks can be produced with enhanced properties. Filtration systems can have improved heavy metal and other contaminant uptake properties. The invention further relates to enhanced building materials containing tailing material treated in accordance with the process of the invention. [0014] More specifically, the invention includes a process for recovering aluminum oxide from a slurry containing bauxite tailings comprising the steps of (a) washing and/or rinsing the slurry with a basic solution, (b) removing a supernatant fluid from the slurry, (c) exposing the supernatant fluid to acidic gas, thereby lowering the pH of the solution to form an acidic salt and an alkali metal hydroxide, (d) extracting the alkali metal hydroxide and (f) filtering the extracted alkali metal hydroxide. The acidic gas can be carbon dioxide; the acidic salt can be sodium bicarbonate; and the alkali metal hydroxide can be aluminum hydroxide. The aluminum hydroxide can be recycled to an Aluminum refiner.

[0015] The invention also includes a process for recovering calcium carbonate from bauxite tailings comprising the steps of (a) adding water to bauxite tailings to form a slurry, (b) washing and/or rinsing the slurry with a basic solution, (c) removing a supernatant fluid from the slurry, (d) exposing the supernatant fluid to acidic gas, thereby lowering the pH of the solution to form an acidic salt and an alkali metal hydroxide, (e) allowing the alkali metal hydroxide to settle and then extracting the alkali metal hydroxide and (f) recovering calcium carbonate from the supernatant. The acidic gas can be carbon dioxide; the acidic salt can be sodium bicarbonate; and the alkali metal hydroxide can be aluminum hydroxide.

[0016] The process can also include the step of the treating calcium carbonate for use in filtration media and/or treating the calcium carbonate to form a component of building material (e.g. a brick or cementitious material).

SUMMARY OF THE INVENTION

[0017] A first aspect of the invention is a method of recovering aluminum and sodium hydroxide from bauxite tailings.

[0018] A second aspect of the invention is a method of converting bauxite tailings into useful industrial compounds. [0019] A third aspect of the invention is a method of reducing the toxicity of bi-products of the Bayer process.

[0020] A fourth aspect of the invention is a method of isolating aluminum oxide from bauxite ore with zero effluent.

BRIEF DESCRIPTION OF FIGURES

[0021 ] FIG. 1 is a flowchart that depicts the steps involved in the Bayer process.

[0022] FIG. 2 is a flowchart that depicts the steps involved in treating red mud according to one aspect of the invention.

[0023] FIG. 3 is a flowchart that depicts the processes used in a preferred method of treating red mud according to one aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0024] Reference in this specification to "one embodiment/aspect" or "an

embodiment/aspect" means that a particular feature, structure, or characteristic described in connection with the embodiment/aspect is included in at least one embodiment/aspect of the disclosure. The use of the phrase "in one

embodiment/aspect" or "in another embodiment/aspect" in various places in the specification are not necessarily all referring to the same embodiment/aspect, nor are separate or alternative embodiments/aspects mutually exclusive of other

embodiments/aspects. Moreover, various features are described which may be exhibited by some embodiments/aspects and not by others. Similarly, various requirements are described which may be requirements for some embodiments/aspects but not other embodiments/aspects. Embodiment and aspect can be in certain instances be used interchangeably.

[0025] The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Certain terms that are used to describe the disclosure are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the disclosure. For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term is the same, in the same context, whether or not it is highlighted. It will be appreciated that the same thing can be said in more than one way.

[0026] Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein. Nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and is not intended to further limit the scope and meaning of the disclosure or of any exemplified term. Likewise, the disclosure is not limited to various embodiments given in this specification.

[0027] Without intent to further limit the scope of the disclosure, examples of

instruments, apparatus, methods and their related results according to the embodiments of the present disclosure are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the disclosure. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. In the case of conflict, the present document, including definitions, will control.

[0028] The term "aluminum" or "aluminium" refers to a chemical element with symbol Al and atomic number 12. It is the most widely used non-ferrous metal. Aluminum is the most widely used non-ferrous metal.

[0029] The term "alumina," "aluminium oxide" or "aluminum oxide" is a chemical compound of aluminium and oxygen with the chemical formula Al₂0₃. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium(lll) oxide.

[0030] The term "bauxite" refers to a rock formed from a laterite soil that has been severely leached of silica and other soluble materials in a wet tropical or subtropical climate. It is the primary ore of aluminum and is mixture of hydrated aluminium oxides and compounds of other elements such as iron.

[0031 ] The term "bauxite tailings" refers to a by-product in the production of alumina (aluminium oxide) by the Bayer Process. The tailings have a high concentration of iron oxide which gives the product a characteristic red color. A small residual amount of the sodium hydroxide remains with the tailings, causing the material to have a high pH/alkalinity.

[0032] The term "calciner" refers to a steel cylinder that rotates inside a heated furnace and performs indirect high-temperature processing (550-1 150 °C, or 1000-2100 °F) within a controlled atmosphere.

[0033] The term "causticize" refers to a process of making a solution caustic, and more particularly, to converting (alkaline carbonate) into a hydroxide by the use of lime.

[0034] The term "Hall-Heroult process" refers to a method of conversion of alumina to aluminum metal. In this energy-intensive process, a solution of alumina in a molten (950 and 980 °C (1 ,740 and 1 ,800 °F)) mixture of cryolite (NasAIFe) with calcium fluoride is electrolyzed to produce metallic aluminium: Al³⁺ + 3 e-→ Al. The liquid aluminum metal sinks to the bottom of the solution and is tapped off, and usually cast into large blocks called aluminum billets for further processing. Carbon dioxide is produced at the carbon anode: 2 O^2" + C→ CO2 + 4 e-

[0035] The term "polishing" refers to a process that removes small (usually microscopic) particulate material, or removes very low concentrations of dissolved material from a liquid or water. [0036] The term "sodium aluminate" refers to an important commercial inorganic chemical. It works as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as NaAIO2 or NaAI(OH)₄ (hydrated).

[0037] The term "sodium hydroxide," also known as lye and caustic soda, refers to an inorganic compound with formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH-.

[0038] The term "red mud," "tailings," "bauxite residue" or "alumina refinery residues (ARR)" refers to a toxic byproduct of the industrial process that refines bauxite, raw aluminum ore, into aluminum oxide, or alumina. The main constituents of the residue after the extraction of the aluminium component are unreacted metallic oxides. The percentage of these oxides produced by a particular alumina refinery will depend on the quality and nature of the bauxite ore and the extraction conditions.

[0039] The term "thixotropy" refers to a time-dependent shear thinning property. Certain gels or fluids that are thick, or viscous, under static conditions will flow (become thin, less viscous) over time when shaken, agitated, sheared or otherwise stressed (time dependent viscosity).

[0040] Other technical terms used herein have their ordinary meaning in the art that they are used, as exemplified by a variety of technical dictionaries.

Description of Preferred Embodiments

[0041 ] The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.

[0042] The invention includes a process for the conversion of a mining industry's operation to become fully sustainable, with zero effluent, while at the same time, creating the opportunity to remediate other industrial mining sites to make those water supplies that were once rendered toxic, suitable for repopulation of the affected bio- system and put back on the path to natural rehabilitation.

Bayer Process

[0043] FIG. 1 depicts the steps involved in the Bayer process. Bauxite ore 105 is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. The bauxite ore is heated in a pressure vessel 110 with a sodium hydroxide solution 105.

[0044] The soluble part of the bauxite ore is dissolved using sodium hydroxide under conditions of high temperature and pressure. The insoluble part of the bauxite (the residue) is removed, giving rise to a solution of sodium aluminate, which is then seeded and allowed to cool and aluminium hydroxide precipitates from the solution.

[0045] Sodium aluminate (NaAI02) is soluble in strongly alkaline water and the other components of the ore are not. Sometimes lime is added to precipitate the silica as calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a flocculant such as starch, to remove the fine particles. The undissolved waste after the aluminium compounds are extracted (i.e. red mud) 120 contains iron oxides, silica, calcia, titania and some unreacted alumina

[0046] The aluminum is penetrated as sodium aluminate in an extraction process. After separation of the residue by filtering, gibbsite (aluminium hydroxide) is precipitated when the liquid is cooled and then seeded with fine-grained aluminium hydroxide, (AI(OH)s) 125.

[0047] The aluminum hydroxide seeds stimulate the precipitation/crystallization of solid aluminum hydroxide crystals 130. The aluminum hydroxide settles at the bottom of the tank and is removed. Finally, the aluminum hydroxide is washed 135 of any remaining caustic soda and heated to remove excess water 140. After this process, alumina (aluminum oxide) emerges as a fine white powder 145. [0048] The process converts the aluminium oxide in the ore to soluble sodium aluminate (NaAI02). This treatment also dissolves silica, but the other components of bauxite do not dissolve. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a flocculant such as starch, to remove the fine particles. The undissolved waste after the aluminium compounds are extracted (i.e. bauxite tailings) contains iron oxides, silica, calcia, titania and some unreacted alumina.

[0049] The left-over or "spent" sodium aluminate solution can then be recycled. This, however, allows gallium and vanadium impurities to build up in the liquors, so these can be extracted. The end result is Alumina Oxide (AI2O3) 145. However, the process results in the formation of large quantities of "red mud" 120 or tailings.

Red Mud

[0050] In accordance with the invention, a process is provided for treating waste streams containing bauxite tailings to render them suitable for incorporation into useful materials, such as bricks and filtration media that are formed using classic concrete formulations or sintering to form ceramic materials. The invention further provides an improved building material in which bauxite tailings which have been treated with an alkaline solution to remove previously fugitive alkali metal hydroxides currently lost by the Bayer-based alumina refining industry, and rendering the resultant tailings environmentally benign and concurrently creating a cost and performance enhancing adjunct for building materials and filtration media. This new by-product can then be combined with concrete based materials and cured to provide superior construction materials. The treated tailings can also be incorporated into ceramic formulations that result in cost and performance enhancements of ceramic building materials and even be utilized as a filtration media with unparalleled sorbent characteristics. Prior to combining with cementitious or other material, water and all or a portion of the alkali metal salts native to classic Bayer processes are removed. [0051 ] The tailings, while they do contain a small amount of "free sodium hydroxide," the reality is that, in the Bayer process, the aluminum present in the ore body has been leached out using concentrated caustic soda (sodium hydroxide) which forms aqueous sodium aluminate NaAI(OH)₄ when contacted with the aluminum rich bauxite ore.

Sodium aluminate only remains stable at high pH levels. The refiners usually recover some of that liquid as that represents the means by which the aluminum values are separated from the solid remnant of the ore. The liquid is separated from the ore by filtration, or sedimentation and then is allowed to cool in its supersaturated condition, often times "seeded" with some nucleation material (aluminum hydroxide) harvested from previous batches to stimulate and accelerate the precipitation of aluminum hydroxide from the solution.

[0052] The aluminum hydroxide is allowed to settle, is drawn off the bottom of the settling tank and is then filtered and washed in preparation to be further processed into aluminum oxide using thermal decomposition by the refiner. The depleted bauxite is pumped out to catchment ponds where any surplus liquid is drawn off the surface and recycled back into the process. The loss of value occurs when one considers the volume of liquid that is required to make one ton of dry bauxite fluid enough to pump.

[0053] A 50% humidity content in bauxite tailings represents a dry friable solid that is neither plastic nor fluid. The typical humidity content of concentrated depleted bauxite tailings that have had days or weeks to settle in containment ponds is between 60% - 70%. This means that for every tonne of depleted bauxite sent to the containment pond, there are about three tonnes of liquid trapped in those tailings that can hold as much as 18% by weight of sodium aluminate, plus some "free" unbound sodium hydroxide.

Treatment of Red Mud

[0054] The invention takes advantage of a little-known attribute of sodium hydroxide, that being, "a little bit goes a long way" and the fact that sodium aluminate will spontaneously "break" during the rinsing process of a slurry if plain water is used and the pH of the slurry being washed falls below nine. When that happens, if the sodium aluminate is still comingled with the tailings slurry, (which it invariably is) the aluminum hydroxide will precipitate out of solution as a solid and will form a colloidal suspension. When that happens, the slurry will spontaneously transition from a fluid slurry to a thixotropic gel resembling a red hydroscopic and plastic clay. At this point it becomes very difficult to remove those three parts liquid from the one part of tailings, the minimum amount needed to render the suspension, fluid enough to pump and or filter, further confounding the dewatering process.

[0055] Instead of rinsing with an acid (to neutralize the slurry) or water (in an effort to rinse out the remaining values) the new process displaces the basic sodium aluminate using a displacement solution that has been raised to a pH of at least 10 by adding about 100 PPM of sodium hydroxide to the rinse water solution. Up to two

displacement volumes are used to wash the tailings using a filtration means that allows for a rinsing step to be incorporated in the dewatering process. In this way, the sodium aluminate is prevented from "breaking" while "in situ" with the ore, so that the aluminum, while still in its soluble state during the filtration and rinsing process, is simply displaced by the rinsing solution, which allows for the recovery of those values from the displaced liquid (once it has been filtered) and recovery of those values at a high degree of purity (99.9%) and at a very high rate of recovery (99.9%).

[0056] The mechanism by which those values are recovered are as follows: Once the aqueous solution of sodium aluminate has been separated from the bauxite tailings, it is microfiltered to remove any impurities carried over from the tailings. The clear liquid is then contacted with a sufficient Molar amount of CO2 to form a stoichiometrically balanced amount of sodium bicarbonate with the sodium found in the solution, which, once exposed to the surplus water in the solution will form carbonic acid. That acid will lower the pH of the solution, at which point the sodium aluminate will react with the carbonic acid to form (soluble) sodium bicarbonate and (less soluble) aluminum hydroxide. The aluminum hydroxide will immediately begin to form crystals that will precipitate out of the solution, and if left to settle, will accumulate at the bottom of the reactor vessel in which it is contained. The aluminum hydroxide can then be drawn off the bottom of the vessel, filtered and rinsed with distilled water to yield 99.9% pure aluminum hydroxide.

[0057] FIG. 2 depicts the steps involved in treating red mud according one aspect of the invention. The process can be used to convert red mud 120 to obtain 99.9% pure Aluminum Hydroxide (AI(OH3)) 180.

[0058] The bauxite tailings (red mud 120) are delivered to the process usually suspended in (roughly) three to four parts of water by an alumina refinery. Slurry is delivered directly to a filtration means that allows for a washing or rinsing cycle to occur during the filtration and dewatering process to produce a filter cake. This step is depicted at 150. In one embodiment the filtration means may be one or more of a pressure filter, polishing filter, red mud filter or any combination thereof.

[0059] The filter cake is washed with an aqueous solution of water and sodium hydroxide with a pH of 10-12 using a volume of the solution sufficient to displace the resident liquor that remains entrained within the dewatered slurry 155. To successfully proceed, the filter cake should be simultaneously dewatered and rinsed. Once the filter cake has been sufficiently depleted of its hydroxide(s) rich liquid, the filtrate cake is rejected from the dewatering/filtration system 200 and termed as depleted red mud.

[0060] The hydroxide(s) rich liquid or supernatant fluid is then re-filtered using a microfiltration system to remove any microparticles of the "red mud" from the solution (not shown). The fluid which is now rich with sodium aluminate, is sent to a contacting platform. In the platform, the fluid is exposed to CO2 160 that, on contact with the free water in the solution, will form carbonic acid 165. As the acid is formed it lowers the pH of the solution, and ultimately, selectively forms sodium bicarbonate and aluminum hydroxide. The aluminum hydroxide is insoluble in water and so precipitates out of solution 170. The mixture is transferred to a conical settling tank where the aluminum hydroxide is allowed to settle to bottom of the tank, is extracted as a suspended slurry and sent to a filtration means that allows the filtrate to be rinsed so it is free from sodium bicarbonate impurities 180. The rinsed dry cake of 99.9% aluminum hydroxide can then be recycled back to the refiner so that it can be calcined along with refiner's "hydrate" to supplement their production of aluminum oxide "alumina" 185.

[0061 ] The remaining sodium bicarbonate solution 190 can then be "causticized" by using classic causticizing means typically found on-site at Bayer-based alumina refineries. The by-products of this reaction are sodium hydroxide and calcium carbonate 195. The calcium carbonate is also insoluble in water and so precipitates out of solution. That mixture can once again be sent to a conical settling tank where the calcium carbonate is allowed to settle to the bottom of the tank, where it is extracted as a slurry, sent to a filtration means that also provides rinsing capabilities to provide a 99.9% pure filter cake of calcium carbonate that is then used for decomposition back into calcium oxide and gaseous carbon dioxide that get recycled back into the carbonization and caustization process respectively. The filtrate, a dilute solution of sodium hydroxide can then be recycled back to the refiner. It can be used at the front end of the process where it is added with the raw material to reach the required pH required during the preliminary grinding and digestion process.

[0062] It is a metathesis reaction between calcium hydroxide and the sodium

bicarbonate where the calcium ion exchanges places with the sodium ion to form calcium carbonate. The hydroxyl ion exchanges places from the calcium ion to the sodium ion to form sodium hydroxide. The calcium carbonate is relatively insoluble and so therefore precipitates out of the solution to settle to the bottom of the reactor vessel. That material can then be drawn off the bottom of the vessel, washed with clean (or distilled water) to yield up to 99.9% pure precipitated calcium carbonate. The material can be dried, bagged and sold as an industrial chemical that has a myriad of applications. In the alternative, it can be recycled back into the process by thermally decomposing the calcium carbonate, back into two of the primary reagents used in the process (calcium oxide & carbon dioxide) to close the loop of the process.

[0063] The solution of sodium hydroxide that results from the "causticizing" step will also be of high purity (~ 99.9%) and can be recycled back to the refiner. This allows the recovery of the fugitive component of the refiner's process flow and radically reduces the recurring cost of replacing the lost sodium hydroxide that is at the root of the safe bauxite tailings storage and recycling challenge that faces the industry.

[0064] The tailings, having now been effectively "scrubbed" of the valuable remnants of caustic and alumina. They also now have a comparatively neutral at a pH (-10) which is low enough to be used "as is" for some recycling applications like building materials and acidic water run-off remediation from other mining efforts that use acids for the enrichment of their ores. The pH can be lowered further to 7, if an application requires it, by simple titration with any other mineral acid or additional carbonic acid. This issue of tailings storage and its isolation from the environment and public water supplies has always been a challenge, most often unmet, by the mining industry.

[0065] As a by-product of the foregoing preparation procedure of the tailings and subsequent incorporation into classic concrete and ceramic media, the finished modified tailings product has enhanced efflorescence resistance that is superior to the resistance levels of concrete products not incorporating untreated bauxite tailings as well as those that do.

WORKING EXAMPLE

Recovery of Aluminum and Sodium Hydroxide from Red Mud

[0066] FIG. 3 further depicts the steps involved in treating red mud according one aspect of the invention.

Red Mud - Step 1

[0067] Red mud waste material 120 is obtained from a final settling tank by any conventional refinery plant for use in the process described herein ("the process"). Transport water is added to the red mud waste to form a red mud suspension for the purpose of transport to the process, via a pipeline for example. The red mud

suspension is composed of approximately 40% solids, which contains Na, Al, and O, plus Fe and other leftover components. Filtration - Step 2

[0068] The red mud suspension is subjected to pressure filtration 205, potentially through the use of a hyperbaric mechanism. The filtration further includes a washing step that utilizes a mild caustic solution of water and sodium hydroxide to displace the sodium aluminate that is in suspension. This step results in the red mud solids being separated from the liquid suspension components. The solids are separated out as a dry waste component referred to as depleted red mud 200.

Carbonation - Step 3

[0069] The liquid (supernatant) sodium aluminate solution is filtered and is then exposed to carbon dioxide within a contacting platform, such as a carbonation reactor where the carbon dioxide, once exposed to the free water, forms carbonic acid 210. The carbonic acid then changes the pH of the solution and causes the sodium aluminate to react with the acid to form sodium bicarbonate and aluminum hydroxide. The aluminum hydroxide, which is not water soluble, precipitates out as a solid.

Filtration - Step 4

[0070] The aluminum hydroxide solids are sent to a filter press for filtration 215 to squeeze all the liquids out of the aluminum hydroxide, which are also rinsed with distilled water, for the sake of providing a pure product of aluminum hydroxide. The rinsing water having a remnant of sodium bicarbonate forms a liquid (supernatant) sodium bicarbonate solution.

Caustization - Step 5

[0071 ] The liquid (supernatant) sodium bicarbonate solution is then subjected to caustization 220. In this regard, the supernatant solution is exposed to a suspended solution of calcium hydroxide. The calcium hydroxide with the sodium bicarbonate produce a metathesis reaction (i.e. dual ion exchange reaction) whereby the calcium ion bonds to the carbonate ion, and the hydroxyl ion bonds to the sodium ion. Under these circumstances, a calcium carbonate forms that is insoluble and precipitates out of the mixed solution, to leave an aqueous solution of sodium hydroxide 240. [0072] The calcium carbonate 235 is filtered and washed 230 with distilled water for the sake of purifying the calcium carbonate product. The calcium carbonate product is set aside and reserved to be recycled back into the process by thermal decomposition in a calciner to form calcium oxide and carbon dioxide to perpetuate the process at the appropriate stage and step.

[0073] Finally, the dilute sodium hydroxide is recycled back to the host facility to be admixed to the refiner's primary grinding, which is the initial digestion step in the alumina refining process.

[0074] It will be appreciated that variations of the above disclosed compact evaporative cooler system and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

[0075] Although embodiments of the current disclosure have been described comprehensively, in considerable detail to cover the possible aspects, those skilled in the art would recognize that other versions of the disclosure are also possible.

[0076] The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof.

Claims

CLAIMS What is claimed is:

1 . A process for recovering aluminum oxide from a slurry containing bauxite tailings comprising the steps of:

a) washing and/or rinsing the slurry with a basic solution;

b) removing a supernatant fluid from the slurry;

c) exposing the supernatant fluid to acidic gas, thereby lowering the pH of the solution to form an acidic salt and an alkali metal hydroxide;

d) extracting the alkali metal hydroxide from the supernatant fluid; and e) filtering the extracted alkali metal hydroxide.

2. The process of claim 1 wherein the acidic gas is carbon dioxide.

3. The process of claim 1 wherein the acidic salt is sodium bicarbonate.

4. The process of claim 1 wherein the alkali metal hydroxide is aluminum hydroxide.

5. The process of claim 1 wherein the aluminum hydroxide is recycled to an

Aluminum refiner.

6. A process for recovering calcium carbonate from a slurry containing bauxite

tailings comprising the steps of:

a) washing and/or rinsing the slurry with a basic solution;

b) removing a supernatant fluid from the slurry;

d) extracting the alkali metal hydroxide from the supernatant fluid; and e) recovering calcium carbonate from the supernatant.

7. The process of claim 6 wherein the acidic gas is carbon dioxide.

8. The process of claim 6 wherein the acidic salt is sodium bicarbonate.

9. The process of claim 6 wherein the alkali metal hydroxide is aluminum hydroxide.

10. The process of claim 6 further comprising the step of the treating calcium

carbonate for use in filtration media.

1 1 .The process of claim 6 further comprising the step of treating the calcium

carbonate to form a component of building material.

12. The process of claim 1 1 wherein the building material is cementitious.

13. The process of claim 12 wherein the cementitious material is a brick.