WO2008116260A1

WO2008116260A1 - Method for preparing aluminium oxide

Info

Publication number: WO2008116260A1
Application number: PCT/AU2008/000422
Authority: WO
Inventors: Dean Ilievski; John Besida
Original assignee: Alcoa Of Australia Limited
Priority date: 2007-03-27
Filing date: 2008-03-27
Publication date: 2008-10-02
Also published as: AU2008232309A1; BRPI0808611A2; CN101663239A

Abstract

A method for preparing aluminium oxide from a Bayer process solution, the method comprising the steps of: precipitating a first alumina product and providing a first spent liquor; separating at least a portion of the first alumina product and the first spent liquor; treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the treated first spent liquor; precipitating a second alumina product from the treated first spent liquor and providing a second spent liquor; separating at least a portion of the second alumina product and the second spent liquor; calcining at least a portion of the first alumina product in a calciner; and calcining at least a portion of the second alumina product in the calciner, wherein the first alumina product is gibbsite or boehmite, or a combination thereof and the second alumina product is gibbsite or boehmite, or a combination thereof.

Description

Method for Preparing Aluminium Oxide

Field of the Invention

The present invention relates to a method for method for preparing aluminium oxide by the calcination of gibbsite and boehmite.

Background Art

The Bayer process is widely used for the production of alumina from alumina- containing ores such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions at elevated temperatures in a process commonly referred to as digestion. Solids are removed from the resulting slurry and the solution cooled to induce a state of supersaturation. The resulting solution is often referred to as green liquor.

Alumina is added to the green liquor as seed to induce precipitation of further aluminium hydroxide therefrom. The precipitated alumina is separated from the caustic aluminate solution (known as spent liquor), with a portion of alumina being recycled to be used as seed and the remainder recovered as product. The remaining caustic aluminate solution (often referred to as spent liquor) is recycled for further digestion of alumina-containing ore.

The alumina in most aluminium-containing ores is in the form of an alumina hydrate. In bauxite, the alumina is generally present as a trihydrate, i.e., AI₂O₃-SH₂O or AI(OH)₃. or as a monohydrate, i.e., AI₂O₃.H₂O or AIO(OH). The trihydrate, termed gibbsite, dissolves or digests more readily in the aqueous alkali solution than the monohydrate, termed boehmite. Thus, bauxite ores containing major proportions of gibbsite digest at lower temperatures and pressures than do bauxite ores containing major proportions of boehmite. Regardless of what form of alumina is present at digestion, under current practises, the majority of precipitated alumina is gibbsite. Thβ precipitation reaction can be generally represented by the following chemical equation:

AI(OH)₄- (aq) + Na⁺ (aq) ► AI(OH)₃ (S) + OH^" (aq) + Na* (aq)

As the precipitation reaction proceeds, the AJTC ratio of the liquor falls from about 0.7, typical of a green liquor, to about 0.4 (where A represents the alumina concentration, expressed as gl_^~1 L of AI₂O₃, and TC represents total caustic concentration, expressed as gl_^"1 sodium carbonate). At the lower value of AJTC, the rate of precipitation slows substantially due to a decrease in the level of supersaturation, and an increase in the level of "free caustic" in the liquor, as the system approaches equilibrium.

It is known that the TC and TA (where TA represents total alkali concentration, expressed as gL^'1 sodium carbonate) of Bayer process solutions affects the solubility of boehmite and gibbsite in those solutions.

The TC and TA in Bayer liquors are determined by the conditions in a number of processing steps including digestion, causticisation and precipitation.

The precipitation of gibbsite from Bayer liquors is induced and driven by first seeding the liquor with gibbsite and progressively cooling the suspension. The TC and TA are both changed during precipitation due to the changes in the liquor arising from the removal of the alumina from solution to form the aluminium hydroxide solid precipitate. Carbonation of Bayer liquors has also been used to induce precipitation of alumina. This step reduces the TC but does not affect the TA of the liquor. Further, carbonation results in loss of sodium hydroxide which must be recovered, and the steps associated therewith are costly and time consuming.

Although all commercial alumina production involves the precipitation of the aluminium trihydroxide, i.e. gibbsite, the calcination of boehmite requires less βnergy than the calcination of gibbsite, and consequently, it is desirable to precipitate digested alumina as boehmite.

The three principal types of methods previously used to produce boehmite can be summarized as follows:

a. Hydrothermal - treatment of aluminium trihydroxide at high temperature and steam pressure to produce boehmite;

b. Neutralization - aqueous solutions of aluminium salts such as aluminium chloride, aluminium sulfate and aluminium nitrate are neutralized by alkalis such as NaOH, KOH and NH4OH, or aluminates such as sodium aluminate are neutralized by an acid (e.g. HCI or H2SO4) or CO₂ to produce gelatinous boehmite; and

c. Hydrolysis - organic aluminium compounds such as aluminium alkylates are hydrolysed with water to produce gelatinous boehmite.

The energy cost of calcining boehmite to smelter grade alumina (SGA) is estimated to bθ between 1.45 GJ/t alumina and 1.8 GJ/t alumina, depending on the design and operation of the calciner. This is a potential saving of between 1.2 GJ/t and 1.8 GJ/t over a typical gas-fired calciner using gibbsite feed consuming 3.0 to 3.3 GJ/t. The energy savings arise from:

1. the enthalpy of the calcination reaction at 25 'C for boehmite is about 0.38 GJ/t. approximately half that for gibbsite (0.73 GJ/t);

2. two less water molecules are driven off, resulting in a substantially lower latent heat component; and

3. less sensible heat is lost as the amount of steam produced is lower. - A -

Other potential savings, more difficult to quantify, can arise from the use of lower calciner feed throughputs per tonne alumina produced, more efficient temperature profiles along the calciner, fewer combustion product gases and less air.

US 4595581 teaches a process for precipitating substantially pure boehmite by heating a seeded sodium aluminate suspension to a temperature of about 115 "C to 145 ⁰C and separating the boehmite precipitate from the suspension. These precipitation temperatures and pressures are substantially higher than for conventional Bayer gibbsite precipitation (typically 60 ⁰C to 80 ⁰C) and would require specialist equipment not currently used in Bayer precipitation.

WO1998/58876 teaches a process for precipitating boehmite from supersaturated sodium aluminate solutions at less than 100 ⁰C, with or without seed. The specification provides experimental data from batchwise seeded precipitation of boehmite from laboratory prepared pure synthetic sodium aluminate liquors. The specification reported, for a range of conditions, the increasing yield with time as their batch suspension gradually desupersaturated. The reported yields after 24 hr precipitation ranged from 35 gL^'1 as AI₂O₃ and 14.5 gl_^'1 as AI₂O₃; after 96 hr the reported yields ranged from 48 gL^"1 as AI₂O₃ and 24.6 gL^*1 as AI₂O₃.

Research from the National Technical School of Athens also shows that higher yields can be obtained at higher temperatures and solids loadings [Panias et al., Travaυx, 29(33), 94, 2002; Panias D. et al., Travaux, 26(30), 147, 1999; Panias and Paspaliaris, Erzmetall 56 (2), 75, 2003]. The highest yield after 24 hr precipitation reported in the literature [Panias et al., Light Metals, 97 (2001); and again in Panias and Paspaliaris, Erzmetall, 56 (2), 75, (2003) and Panias et al., Travaυx, 29(33), 94, (2002)] is 60 gL^"1 as AI₂O₃. This was achieved at a very high solids loadings of 1200 gL^"1 as boehmite. Other data from their research [Panias

D. et al., Travaux, 29(33), 94, (2002), Panias D. et al., Travaux, 26(30), 147,

(1999)] shows a yield of 35 gL^"1 as AI₂O₃ after 24 hr precipitation at 90 ⁰C in a synthetic sodium aluminate liquor (A/TC=0.64, TC=205 gL^'1) seeded with 230 gL^"1

^• boehmite AII the above boehmite precipitation studies have focussed on precipitating from synthetic and pure sodium aluminate solutions at high AfTC and TC values typical of a Bayer green liquor. They were conducted batch-wise, not in the continuous mode of operation usual in commercial Bayer plants. The yields are reported as batch yields after a specified holding time in the precipitator.

Loh ef a/. [Light Metals, 203 (2005)] concluded from their investigation that boehmite precipitation would be unlikely to compete or replace gibbsite precipitation due to the low yields, slow kinetics (up to 200 times slower) and findings of poor product quality, e.g. gibbsite in the product.

Thus, although boehmite is a thermodynamically more stable phase than gibbsite, with a lower solubility and, hence a higher theoretical yield potential, and its precipitation instead of gibbsite would provide energy savings, it is not considered to be commercially viable alternative.

A further issue holding back the development of commercial boehmite precipitation is that for operations processing a gibbsitic ore, the water in the liquor that would have reported to the gibbsite still needs to be removed, an energy costly step.

Vemon et at. [6^th AQW, 33, (2002)] investigating the precipitation kinetics of gibbsite in pure synthetic sodium aluminate liquors observed a phenomenon termed the solubility "gap". It appears to be a metastable state reached by a desupersaturating aluminate liquor in the presence of gibbsite solid, with concentrations remaining above the theoretical equilibrium solubility of gibbsite in sodium aluminate liquors for long periods of time. Vernon ef al. [ibid] also observed that the solubility gap decreases as the TC is lowered.

Skoufadis et al. [Hydrometallurgy, 68, 57-68, (2003)] studied the kinetics of boehmite precipitation in laboratory prepared pure synthetic sodium aluminate liquors and reported an apparent equilibrium stage at which the alumina concentration is much higher than the boehmite solubility at the same conditions. This metastable state for boehmite has also been reported by Loh et at. [ibid]; they report an apparent solubility after 216 hours of precipitation that is 2.3 times the boehmite solubility.

Skoufadis et al. [ibid] observed that the boehmite solubility gap in pure synthetic sodium aluminate liquors decreases with TC. Loh et al. [ibid] report that the precipitation rates for both boehmite and gibbsite increase with decreasing TC in pure synthetic sodium aluminate liquors.

Panias et el. [Light Metals, Minerals, Metals & Materials Society, 97-103, (2001)] reported experiments where boehmite was precipitated under conditions of constant free sodium hydroxide. This was achieved by, so^:called, carbonation, where carbon dioxide is used to neutralise the sodium hydroxide released by the decomposition of aluminate during precipitation. They observed higher yields, however, noted that this approach cannot be applied commercially because of the large consumption of sodium hydroxide by this process; As noted above, carbonation was used (n earlier times for the precipitation of gibbsite but has been replaced by modern seeded cooling Bayer precipitation processes because of the costs of sodium hydroxide consumption. Additionally, carbonation decreases the TC but not the TA₁ which continues to affect the solubility and precipitation kinetics.

Thus, though the literature teaches that achieving low TC in the feed liquor to boehmite precipitation would be beneficial, no practical method has been reported for doing this. For example, reducing the TC in the green liquor directly from digestion is unattractive because high TC is required to achieve efficient extraction of alumina values from the bauxite ore. Further, diluting a green liquor to lower TC has the penalty of an energy intensive evaporation step. Carbonation results in loss of hydroxide, an expensive raw material in Bayer operations, and does not change TA.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciatθd that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia, or anywhere else, as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the woπd "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.

It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein.

The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are hereby incorporated by reference.

In accordance with the present invention, there is provided a method for precipitating alumina from a Bayer process solution, the method comprising the steps of:

precipitating a first alumina product and providing a first spent liquor; separating at least a portion of the first alumina product and the first spent liquor;

treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the treated first spent liquor;

precipitating a second alumina product from the treated first spent liquor and providing a second spent liquor;

separating at least a portion of the second alumina product and the second spent liquor;

calcining at least a portion of the first alumina product in a calciner; and

calcining at least a portion of the second alumina product in the calciner,

wherein the first alumina product is gibbsite or boehmite, or a combination thereof and the second alumina product is gibbsite or boehmite, or a combination thereof.

Preferably, the first alumina product comprises substantially gibbsite and the second alumina product comprises substantially boehmite.

Without being limited by theory, it Is believed that where the first alumina product is gibbsite and the second alumina product is boehmite, energy and cost savings may be obtained, on top of the energy savings from the calcination of boehmite, by calcining boehmite in existing gibbsite calciners. Where boehmite is added to a gibbsite calciner, the boehmite may enter the calciner at a different point in the calciner resulting in not only energy savings due to the lower energy required to calcine boehmite, but energy savings due to the ability to use more effectively the heat flows and temperature profiles in the calciner for the thermal decomposition, and due to more favourable distribution of solids in the calciner. It is further believed that there lies opportunity to improve calciner designs. Advantageously, the method allows greater boehmite productivity from a green liquor coming from digestioπ than methods of the prior art by enabling further recovery of alumina values from the first spent liquor.

Advantageously, the present invention reduces hydroxide concentrations in the treated first spent liquor .

Advantageously, the present invention increases the A/TC of the first spent liquor, thereby increasing the precipitation efficiency of boehmite.

In one form of the invention, the treated spent liquor will have an A/TC approximating that of the green liquor.

It is known that the solubility of boehmite and of gibbslte decrease with TC and TA and advantageously, the present invention provides conditions more favourable to precipitation.

Without being limited by theory, it is believed that decreasing the TC will decease the gap between the metastable "apparent" solubility and the thermodynamic solubility for boehmite in industrial Bayer liquors and that as a result the actual driving force for boehmite precipitation will be increased. Advantageously this will improve the boehmite precipitation efficiency.

The present invention provides a method for creating low TC feed liquors to the boehmite precipitation stage without compromising the demands of the preceding digestion stage, where a higher TC is desired to maximize recovery of the alumina values in the bauxite ore, and avoiding dilution with water that subsequently requires an energy expensive evaporation step before return to the digestion stage.

Advantageously, the present invention enables the utilization of exisiting commercial Bayer gibbsite precipitation circuits and alumina calciners.

Preferably, the step of: treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor;

comprises decreasing the concentration of sodium ions in the treated first spent liquor.

Preferably, the step of:

comprises removing sodium ions from the first spent liquor.

Without being limited by theory, it is believed that boehmite precipitation from Bayer liquors is inhibited not only by the concentration of free hydroxide but also by the presence of sodium ions. Advantageously, removing sodium ions from the treated first spent liquor should have a positive effect on boehmite precipitation.

Preferably, the method comprises the further step of

recovering the sodium ions.

Preferably, the step of:

treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor;

comprises neutraiizing or removing hydroxide ions from the first spent liquor.

Preferably, the method comprises the further step of:

recovering the hydroxide ions. Advantageously, the recovered sodium and hydroxide values can be returned to the liquor circuit. In contrast, carbonation of the liquor consumes these caustic values and requires the expensive step of lime addition for caustic regeneration.

Advantageously, the media used for reducing the TC and TA of the spent liquor is regenerated for further use.

In one form of the invention, the method comprises the additional step of:

combining at least a portion of the treated first spent liquor with at least a portion of the green liquor; and

precipitating a first alumina product.

In one form of the invention, the method comprises the additional step of:

combining at least a portion of the green liquor with at least a portion of the treated spent liquor;

precipitating a second alumina product

It will be appreciated that the ratio of the treated first spent liquor and the green liquor may vary and will depend on many factors including temperature, digestion circuit operation, economic considerations, available plant infrastructure, the properties of the treated first spent liquor and the green liquor such as TC, TA, level of impurities, alumina concentration and the desired gibbsite and boehmite product ratio.

It will be appreciated that the quantities of green liquor or treated spent liquor can be different in the different stages in the precipitation circuit. Without being limited by theory, it is believed that by adjusting the amount of green liquor or the amount of treated spent liquor being distributed to the different precipitation stages, the TA, TC and supersaturation profiles along the whole precipitation stage can be controlled so as to improve overall yield and product quality. It will be appreciated that the appropriate amount and distribution of the green liquor bypass or treated spent liquor bypass will depend on the circuit configuration, operating conditions, the seed and liquor properties, the desired amounts of the gibbsite and boehmite products.

The ability to control the TC and TA of the spent liquor and consequently, the combined treated spent liquor and the green liquor, can provide greater yields of boehmite and greater liquor productivity.

Preferably, the method comprises the additional step of:

digestion of bauxite to provide the green liquor.

The bauxite may be provided in the form of gibbsitic bauxite, boehmitic bauxite, diasporic bauxite or any combination thereof.

It will be appreciated that the step of:

treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor,

may be performed on the entire first spent liquor or in a Bayer process side stream.

The method of the present invention can provide greater yields of precipitated boehmite than methods of the prior art for a given Bayer digestion circuit, by increasing the amount of the alumina precipitated from the green liquor.

It will be appreciated that the steps of:

treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the treated first spent liquor; precipitating a second alumina product from the treated first spent liquor and providing a second spent liquor

may be repeated.

It will be appreciated that the steps of:

combining the treated first spent liquor with a green liquor; and

precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor;

may be repeated.

It will be appreciated that the steps of:

combining the treated first spent liquor with a green liquor; and

precipitating gibbsitθ from the combination of the green liquor and the treated first spent liquor and producing a first spent liquor;

may be repeated. In a highly specific form of the invention, the method may comprise a plurality of treatment steps to decrease both the total caustic concentration and the total alkalinity of the first spent liquor.

In one form of the invention, the method comprises the further step of:

seeding the treated first spent liquor with the second alumina product.

It will be appreciated that the optimal seeding rate will depend on many factors, including the seed and liquor properties and the design of the precipitation circuit, and may be anywhere in the range of 50 to 1300 gl_^*\

It will be appreciated that the seed should be the same form as the desired alumina product. Where the seed is boehmite, the boehmite seed is preferably recycled from the boehmite precipitation circuit.

Preferably, the step of:

is conducted at a temperature less than about 105 ⁰C.

In a preferred form of the invention, the step of:

precipitating a second alumina product in the form of boehmite from the treated first spent liquor and providing a second spent liquor;

is conducted at an initial temperature of between about 95 ⁰C and 105 ⁰C.

In one form of the invention, the treated first spent liquor may be cooled after entering the precipitation circuit. Advantageously, as the boehmite precipitation is conducted at higher temperatures than gibbsite precipitation, less energy is lost to the process due to cooling from digestion temperatures and reheating to evaporation temperatures.

It will be appreciated that the precipitation holding time and flow rates can be adjusted to improve the boehmite yield and product quality. The skilled addressee will appreciate that decisions affecting holding times and flows may be made on a rational economic basis.

In one form of the invention, the step of:

is preceded by the step of:

sonication of the treated first spent liquor.

In one form of the invention, the method comprises the further step of:

adding a gibbsite precipitation inhibitor to the treated first spent liquor.

Advantageously, gibbsite co-precipitation with boehmite is reduced or eliminated by the addition to the pre-precipitation Bayer liquor of the gibbsite precipitation inhibitor.

The gibbsite precipitation inhibitor may be provided in the form of those certain organic compounds believed to inhibit gibbsite precipitation by reducing or blocking the number of active sites on the seed surface. Without being limited by theory, it is believed that as the crystal structure of boehmite is different to that of gibbsite, boehmite precipitation will be less affected by certain organic compounds than gibbsite precipitation. Given the recycling of the liquor in the Bayer circuit, it will be appreciated that the gibbsite precipitation inhibitor in the second precipitation circuit may impact on the precipitation of gibbsite in the first precipitation circuit-

In a preferred form of the invention, the gibbsite precipitation inhibitor is provided in the form of calcia. In the context of the present invention, the term calcia shall be understood to encompass any calcium compound which is compatible with the ionic species found in Bayer liquor and produces soluble calcium ions such as calcium oxide and hydrated forms thereof including calcium hydroxide, lime putty, calcium carbonate, tricalcium aluminate and hydrocalumite. It is known that calcia increases the gibbsite precipitation induction time and inhibits gibbsite precipitation from Bayer liquor. Advantageously, pre-precipitation liquor may contain calcia, which is known to affect the induction time of gibbsite precipitation. Without being limited by theory, it is believed that calcia does not affect the precipitation of boehmite to the same extent as gibbsite.

Whilst green liquors may contain calcia, spent liquors generally contain little or no calcia.

Where the steps of

are repeated, the method may comprise the further step of:

adding calcia to the treated first spent liquor.

It will be appreciated that where the steps of treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the treated first spent liquor;

precipitating a second alumina product from the treated first spent liquor and providing a second spent liquor

are repeated, it may not be necessary to add further gibbsite precipitation inhibitor to the treated first spent liquor after the step of precipitating boehmitθ from the combination of the green liquor and the treated first spent liquor.

It will be appreciated that the initial seed for starting up the boehmite precipitation circuit can be manufactured by a hydrothermal method to convert gibbsite to boehmite, such as described in US4534957 or any other method described in the literature for making boehmite.

In one form of the invention, hydrothermal process for converting gibbsite into boehmite is used to manage product quality excursions, where significant gibbsite is precipitated instead of boehmite, and to return the seed recycle to a predominantly boehmite form.

Preferably, the step of:

calcining at least a portion of the first alumina product in a calciner;

comprises the step of:

adding the first alumina product to the front end of a calciner.

In one embodiment of the invention, the step of:

calcining at least a portion of the second alumina product in a calciner; comprises the step of:

adding the second alumina product to the same location within the calciπer as the first alumina product.

In a second embodiment of the invention, the step of:

calcining at least a portion of the second alumina product in a calcineπ

comprises the step of: ^•

adding the second alumina product at a different location within the calciπer as the first alumina product.

In highly specific forms of the invention, the second alumina product is added to a later stage in a preheat and drying section, a furnace and holding vessel section, an early stage of a cooling section or a hydrate bypass system of a static calciner, or combinations thereof.

Advantageously, the invention produces two separate alumina product streams from the two stage precipitation circuit. The different thermal decomposition reaction temperatures for gibbsite and boehmite make it possible to distribute the separate product streams within a static calciner system in a manner that optimises calciner energy usage. It will be appreciated that the thermal decomposition of gibbsite occurs at about 200 ⁰C to 350 ⁰C, whereas the thermal decomposition of boehmite occurs at about 500 ⁰C. The skilled addressee will appreciate that the optimal entry locations for the second alumina product feed will depend on factors such as the relative amount of the second alumina product feed compared to the first alumina product feed, the gibbsite content, the calciner temperature profile, the air flows, the boehmite residence time, the calciner energy balance and the alumina product quality specifications. The separate gibbsite and boehmite product streams may require washing and filtering before entering the calciner. In one form of the invention, the step of:

comprises the step of:

contacting the first spent liquor with a substantially water-immiscible solution comprising an extractant; and

extracting at least a portion of the metal cations present in the first spent liquor into the substantially water-immiscible solution.

The present invention advantageously provides the ability to control the TC and TA of the treated spent liquor and hence the ability to precipitate further oxides of aluminium from the treated spent liquor. Said control may be affected by many factors including the number of repetitions of the contacting and extracting steps, as well as the nature, volume and concentration of the extractant and solvent, temperature, agitation, properties of the liquor and the presence of other species in the liquor.

It should be appreciated that the extraction of metal cations from the first spent liquor into the substantially water-immiscible solution will be accompanied by a charge transfer of a cation from the substantially water-immiscible solution into the first spent liquor. Preferably, the metal cation is a sodium ion.

Advantageously, water may be co-extractθd with metal cations from the first spent liquor into the substantially water-immiscible solution; thereby lowering the water in any spent liquor leaving the precipitation circuit and returning to the digestion circuit and thus reducing the spent liquor evaporation requirements.

Preferably, the extractant is provided in the form of a weak acid. Where the extractant is provided in the form of a weak acid, the extraction of a metal ion into the substantially water-immiscible solution will be accompanied by the transfer of a proton from the substantially water-immiscible solution into the first spent liquor.

Preferably, the weak acid extractant comprises at least one polar group with an ionisable proton with a pKa of between about 9 and about 13.

The extractant is preferably a straight chain, branched chain or cyclic hydrocarbon, a halogeπated hydrocarbon, an aliphatic or aromatic ether or alcohol with more than 6 carbon atoms. Preferably, the extractant comprises an alcohol or phenol functional group. Suitable extractants include IH.IH-perfluoranonanol, 1 H,1H,9H-hβxadecafluorononaπol, 1 ,1,1-trifluoro-3-(4-terf-octylphenoxy)-2- propanol, 1 ,1 ,1-trifluoro-2-(p-tolyl)/sopropanol, 1-(p-tolyl)-2,2,2-trifluoroethanol, hexafluoro-2-(p-tolyl)isopropanol₁ 2-(methyl)-2-(dodecyl)tetradecanoic acid, 3-(perfluorohexyl)propenol and 1-(1 Λ2,2-tetrafluoroethoxy)-3-(4-tert- octylphenoxy)-2-propanol, tø/f-octylphenyl, para-πonylpheπol, para-tert- butylphenol, para-terf-amylphenol, para-heptylphenol, parø-octylphenol, para- (alpha,alpha-dimethylbenzyl)phenol (4-cumylphenol), 2,3,6-trimethylphenol, 2,4-di-te/t-butylpheπol, 3,5-di-fert-butylphenol, 2,6-di-terf-butylphenol, 2,4-di-terf- pentylphenol (2,4-di-førf-amylphenol), 4-sec-butyl-2,6-di-te/t-butylphenol, 2,4,6-tri- fert-butylphenol, 2,4-bis(alpha,alpha-dimethylbenzyl)phenol (2,4-dicumylphenol) and other alkylated phenols or mixtures thereof.

It should be appreciated the substantially water-immiscible solution may form the extractant.

Preferably, the acidic form of the extractant is substantially insoluble in water.

Preferably, the deprotonated form of the extractant is substantially insoluble in water. It should be appreciated that partitioning of the extractant in the first spent liquor should be minimal.

It should be appreciated that the extractant concentration will depend on a number of factors including the intended amount of induced supersatu ration which in turn will be influenced by the temperature at which precipitation will be initiated.

It should be appreciated that the degree of deprotonation in the extraction step will depend on the acidity of the ionisable proton (as well as the pH and salt content of the first spent liquor.

Reactions between substances distributed in different phases can be slow because, in a reaction of first order with respect to each of the two components, the rate is maximised when the concentrations of the species in a given phase are maximised. The use of phase transfer catalysts may enhance extractions rates.

Suitable phase transfer catalysts may be selected from lipophilic quaternary ammonium or phosphonium salts or organic macrocycles such as crown ethers, calixarenes, calixarene-crown ethers, spherands and cryptands.

Specific complexing ligands may be added to the organic mixtures, to either synergistically enhance Na⁺ extraction, or to additionally extract impurities from the first spent liquor and enhance precipitation in a secondary manner.

Preferably, the substantially water-immiscible solution is an organic liquid, a combination of organic liquids or an ionic liquid.

Preferably, the organic liquid is substantially non-polar.

Preferably, the organic liquid is a high boiling organic liquid with a low vapour pressure and a relatively high flash point at Bayer process temperatures.

Preferably, the organic liquid is alkaline stable. The organic liquid is preferably a straight chain, branched chain or cyclic hydrocarbon, a halogenated hydrocarbon, an aliphatic or aromatic ether or alcohol with more than 4 carbon atoms. Suitable solvents include benzene, toluene, xylene, stilbene, 1-octanol, 2-octanol, 1-decanol, /sooctyl alcohol (such as that commercially available as Exxal 8 from ExxonMobil), iso-nonylalcohol (such as that commercially available as Exxal 9), Exxal 10, Exxal 11, Exxal 12 and Exxal 13 from ExxonMobil, /so-decanol, /sotridecanol, 2-ethyi-1-hexanol, kerosene and other hydrocarbons commercially available under the names Escaid 100, Escaid 110, Escaid 240, Escaid 300, lsopar L, lsopar M₁ Solvesso 150 and Exxsol D110 (from ExxonMobil) and mixtures thereof.

Preferably, the partitioning of the organic solvent in the first spent liquor is minimal. Preferably, the partitioning of the first spent liquor in the organic solvent is minimal.

Preferably, the organic solvent solvates the extractant in both its acid and sodium salt forms.

It should be appreciated that the volume of substantially water-immiscible solution relative to the volume of the first spent liquor may vary according to the manner in which both the first spent liquor and the substantially water-immiscible solution are contacted and the loading of the extractant in the substantially water-immiscible solution.

Preferably, the step of:

contacting the first spent liquor with a substantially water-immiscible solution comprising an extractant;

comprises agitating the first spent liquor and the substantially water-immiscible solution by any means known in the art including shaking, stirring, rolling and sparging. It will be appreciated that the contact time between the first spent liquor and the organic phase should be sufficient for reaction to occur between the extractaπt and the metal cations to form a metal cation-depleted aqueous phase and a hydrogen ion-depleted organic phase. Said contact time will be influenced by many factors including the pKa of the ionisable proton on the extractant, the pH of the aqueous phase, the volumes of the aqueous and organic phases, the temperature, the concentration of the extractant and sodium ions, the total alkalinity, the total caustic concentration, the extent of agitation and the presence of other species in the spent liquor.

It should be appreciated that the volumes of the Bayer process solution and the substantially water-immiscible solution need not be the same. It should be appreciated that where the method is performed as a countercurrent flow or continuous processing, volumes of the phases are less critical than with batch methods.

It will be appreciated that the steps of:

extracting at least a portion of the metal cations present in the first spent liquor into the substantially water-immiscible solution,

may be repeated.

Where the steps of:

extracting at least a portion of the metal cations present in the first spent liquor into the substantially water-immiscible solution, are repeated, the step of:

may be performed with different substantially water-immiscible solutions.

Preferably, the method comprises the further step of:

separating the first spent liquor and the substantially water-immiscible solution.

It should be appreciated that the step of separating the first spent liquor and the substantially water-immiscible solution may be performed by any method known in the art including centrifugatioπ.

Preferably, the method comprises the further steps of:

contacting the substantially water-immiscible solution with a stripping solution to provide an aqueous solution of sodium hydroxide.

The stripping solution may be provided in the form of water or a Bayer process stream including condensate or lake water Preferably, the stripping solution has a pH of at least 5.

Preferably, the method comprises the further steps of:

separating the stripping solution and the substantially water-immiscible solution.

Advantageously, the step of:

contacting the substantially water-immiscible solution with a stripping solution to provide an aqueous solution of sodium hydroxide protonates the weak acid extractant.

Advantageously, the substantially water-immiscible solution after contact with the stripping solution may be re-used in subsequent extraction steps.

The aqueous solution of sodium hydroxide may be re-used in other stages of the Bayer circuit such as for digestion of bauxite. Depending on the concentration of sodium hydroxide, the aqueous solution may need to be pre-treated prior to subsequent use.

Advantageously, the step of stripping the sodium ions and subsequent regeneration of hydroxide requires no further chemicals for recausticisation.

In one form of the invention, the step of:

comprises the step of:

contacting the first spent liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble, and

exchanging a sodium cation present in the first spent liquor with an ion on the solid support.

Preferably, the solid support comprising an extractant is an ion exchange resin.

Ion exchange resins are high molecular weight polymeric materials containing many ionic functional groups per molecule. Cation-exchange resins can be either a strong-acid type containing sulfonic acids groups (RSO₃H⁺) or a weak-acid type such as those containing carboxylic acid (RCOOH) or phenolic (ROH) groups. Anion exchange resins contain basic amine functional groups attached to the polymer molecule. Strong-base exchangers are quaternary amines (RN(CH₃)^OH^") and weak-base types contain secondary or tertiary amines.

Preferably, the ion exchange resin is a cation exchange resin and in highly preferred forms of the invention, the cation exchange resin is a weak-acid cation exchange resin.

Preferably, the exchangeable ion on the solid support is a proton.

It will be appreciated that the exchange of the sodium ion present in the Bayer process stream with a proton on the extractant will encompass the exchange of more than one sodium ion and more than one proton.

Preferably, the solid support has a pKa of about 9-13.

Examples of resins that may be used in the present invention include Amberlite IRC86 - hydrogen form, Amberlite IRC50 - hydrogen form and Lewatit CNP105- hydrogeπ form.

Advantageously, the exchange of the sodium ion present in the Bayer process stream with a proton on the extractant will be accompanied by a concomitant neutralisation of hydroxide ions in the Bayer process stream.

Preferably, the step of:

contacting the first spent liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble;

comprises agitating the first spent liquor and the solid support by any means known in the art including shaking, stirring, rolling and sparging. It will be appreciated that the contact time between the first spent liquor and the solid support should be sufficient for ion exchange to occur. Said contact time will be influenced by many factors including the pKa of the ionisable proton on the solid support, the pH of the first spent liquor, the volumes of the aqueous and solid phases, the temperature, the concentration of the sodium ions, the total alkalinity, the total caustic concentration, the extent of agitation and the presence of other species in the process stream.

Where the steps of:

contacting the first spent liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble; and

exchanging a sodium cation present in the first spent liquor with an ion on the solid support,

are repeated, the step of:

may be performed with different solid supports.

Preferably, the method comprises the further step of:

separating the treated first spent liquor and the solid support.

It should be appreciated that the step of separating treated first spent liquor and the solid support may be performed by any method known in the art including filtering and centrifugation.

Preferably, the method comprises the further steps of: contacting the solid support with a stripping solution to protonate the solid support after the step of:

The stripping solution may be provided in form of water or a Bayer process stream including condensate or lake water.

Advantageously, the stripping solution, after contact with the substantially water- immiscible solution can be re-used in subsequent steps in the Bayer process or in subsequent stripping steps. Depending on the sodium hydroxide concentration, the aqueous solution may need to be pre-treated prior to subsequent use.

Advantageously, the solid support, after contact with the stripping solution, can be used for further ion exchange with spent liquor.

In one form of the invention, the step of:

treating at least a portion of a first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor;

comprises the step of:

applying a potential between a first region comprising the first spent liquor and a second region comprising a catholyte, wherein the first spent liquor is an anolyte and wherein an ion permeable membrane is provided between the first region and the second region; and

causing transfer of a sodium ion across the ion permeable membrane from one region to another region.

It will be appreciated that more than two regions may be provided and that more than one ion permeable membrane may be provided. Where there is provided more than one ion permeable membrane, the ion permeable membranes will preferably be substantially coplanar such that adjacent ion permeable membranes will preferably permit the transfer of oppositely charged ions. In one form of the invention, there is provided an anion permeable membrane and a cation permeable membrane.

In one specific form of the invention, there is provided a plurality of ion permeable membranes, wherein the plurality of ion permeable membranes comprise a electrodialysis unit.

In one form of the invention, there may further be provided a bipolar membrane.

Where there is provided one ion permeable membrane, the ion permeable membrane is preferably a cation permeable membrane and the ion is a cation. Preferably, the cation is a sodium cation.

It will be appreciated that the transfer of the sodium ion from one region to another region will encompass the transfer of more than one sodium ion from the first region to the second region.

Preferably, one region is provided with an anode and another region is provided with a cathode.

It will be appreciated that the transfer of the sodium ion from one region to another second region will be accompanied by a concomitant neutralisation of hydroxide ions at the anode and generation of hydroxide ion in the second region.

It will be appreciated that the ion permeable membrane should be substantially resistant to corrosion or degradation under the electrolytic conditions.

It will be appreciated that the choice of ion permeable membrane will be dependant on many factors including the selectivity of ion transport, including the selectivity of sodium ion transport. Further factors include the conductivity and rate of ion transport, the mechanical, dimensional and chemical stability, resistance to fouling and poisoning and membrane lifetime.

In specific forms of the invention, the cation permeable membranes may comprise perfluorinated polymers such as a sulfonated tetrafluorethyleπe copolymer, carboxyiate polymer, polystyrene based polymer, divinylbenzene polymer, or sodium conducting ceramics such as beta-alumina or combinations thereof.

Perfluorinated membranes are known to have a high resistance to chemical attack under conditions of high pH. The stability and favourable physical properties are believed to be due to the substantially inert and strong backbone of the polymer which contains regular side chains ending with ionic groups. The choice of the ionic groups is important as they affect interactions with the migrating ions, the pKg of the ion exchange polymer, the solvation of the polymer and the nature and extent of interactions between the fixed ionic groups.

In highly specific forms of the invention, the cation permeable membrane is a Nafion 324 or Nafion 440 membrane.

It will be appreciated that the electrode material should exhibit high conductivity and low electrical resistance and be substantially resistant to corrosion under the electrolytic conditions. Spent liquor is highly caustic but H⁺ is produced at the anode. It will be appreciated that choice of electrode material will be within the ability and knowledge of the skilled addressee. Since spent liquor contains anions such as fluoride, sulphate etc. the production of hydrofluoric acid, sulfuric acid etc. occurs at the Interface between anode and solution (even though the solution Is highly caustic). Suitable anode materials include platinum coated niobium, platinum coated titanium or Monel.

It will be appreciated that base only is produced at the cathode so the choice of cathode material may be wider than anode material. Suitable cathodes include stainless steel or a gas diffusion electrode (oxygen depolarized cathode). It will be appreciated that the current density must be controlled as increasing the current density will increase the rate of product formation but it will also increase the energy consumption. For higher current densities, less membrane area may be required for a given quantity of caustic extracted. For a systems employing one cation exchange membrane, the preferred current density may be above 150 mA/cm².

In preferred forms of the invention, the catholyte is a caustic solution. Whilst it is advantageous to have the catholyte caustic concentration as high as possible, if it is too high, the current efficiency may be compromised due to back diffusion of ions from the catholyte to the anolyte.

Preferably, the catholyte caustic concentration is not greater than about 8M NaOH or 25% NaOH catholyte.

The method of the present invention may be performed as a batch process wherein the first region is provided in the form of a first compartment and the second region is provided in the form of a second compartment and the ion permeable membrane is provided between the first compartment and the second compartment. The first spent liquor anolyte is introduced into the first compartment and the catholyte is introduced into the second compartment and a potential is applied between the first compartment and the second compartment for a set period of time, after which the treated first spent liquor, depleted in sodium ions and in hydroxide ions is removed from the first compartment and the catholyte with an increased sodium hydroxide concentration is removed from the second compartment.

Alternatively, the method of the present invention may be performed as a continuous process wherein the first region is provided in the form of a first compartment and the second region is provided in the form of a second compartment and the ion permeable membrane is provided between the first compartment and the second compartment. First spent liquor anolyte is continuously introduced into the first compartment and catholyte is continuously introducβd into the second compartment with a potential continuously applied between the first compartment and the second compartment. Treated first spent liquor, depleted in sodium ions and in hydroxide ions is continuously removed from the first compartment and catholyte with an increased sodium hydroxide concentration is continuously removed from the second compartment.

Alternatively still, the method of the present invention may be performed as a continuous process with many compartments in a cell with adjacent compartments being alternately separated by cation permeable membranes and anion permeable membranes. Every second region contains a feed solution of first spent liquor anolyte and instead of hydroxide being neutralized by production of protons at the anode, it is removed from the feed solution through an anionic membrane to form pure caustic (sodium ions come in from the opposite side via a cationic membrane). The method is believed to consume less energy than electrolysis with a single ion permeable membrane because the amount of water that is electrolysed to form protons and hydroxide, with concomitant formation of hydrogen and oxygen, is minimized. Optionally, the arrangement could include bipolar membranes which split water directly, to produce hydroxide ions and protons, with no hydrogen or oxygen formation.

In highly specific forms of the invention, the anion permeable membrane is a Neosepta AHA membrane.

The catholyte containing sodium hydroxide may be re-used in other stages of the Bayer circuit such as for digestion of bauxite. Depending on the concentration of sodium hydroxide, and the impurity content, the catholyte may need to be pretreated prior to subsequent use.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to two embodiments thereof, and the accompanying drawings, in which:- Figure 1 is a schematic flow sheet of a part of a method in accordance with a first embodiment of the present invention utilised in a Bayer Process circuit;

Figure 2 is a schematic flow sheet of a part of a method in accordance with the present invention integrated with a split feed to a calcination stage;

Figure 3 shows the experimentally measured drop in TC of spent liquor after 1:1 contacting for 10 minutes at 60 ⁰C with different concentrations of the TOP extractant in the /so-octanol solvent; and

Figure 4 is a schematic flow sheet of a part of a method in accordance with a second embodiment of the present invention utilised in a Bayer Process circuit.

Best Mode(s) for Carrying Out the Invention

The invention focuses on recovering more of the alumina values from a Bayer green liquor, reducing the energy usage in the production of alumina and may be retrofitted to existing Bayer precipitation assets. The invention produces separate gibbsite and boehmite product streams, produced in a two stage precipitation arrangement that are introduced to different parts of the calcination circuit to minimize calcination energy usage. The boehmite is precipitated in the second stage of precipitation from a portion of the spent liquor from the first stage of precipitation, where gibbsite is produced, that has been treated to reduce the total alkalinity and total caustic concentration.

Figure 1 is a schematic flow sheet showing a part of a method in accordance with one embodiment of the present invention comprising the steps of:

digesting bauxite in a caustic solution 10;

separating the mixture 12 to residue 14 and green liquor 16 ;

precipitating gibbsite in a Bayer precipitation circuit 18 and forming a first suspension; separatiπg the first suspension in a classification circuit 20 into gibbsite product 22, gibbsite seed recycle 24 and spent liquor 26;

separating the spent liquor 26 into a first portion 28 and a second portion 30;

treating the second portion of the spent liquor 30 in a soda reduction unit

32 by contacting the second portion of the spent liquor 30 with a regenerated media containing an extractant 33 and forming a treated first spent liquor 34;

precipitating boehmite from the treated first spent liquor 34 in a boehmitβ precipitation circuit 36 and forming a second suspension;

separating the second suspension in a classification circuit 38 into boehmite product 40, boehmite seed recycle 42 and second spent liquor 44;

calcining at least a portion of the gibbsite product 22 in a calciner 41 ; and

calcining at least a portion of the boehmite product 40 in the calciner 41.

The spent liquor streams 28 and 44 are treated in the normal manner. The portion of the spent liquor 30 is contacted with the extractant 33 in the soda extraction media in apparatus 32 at a temperature less than the boiling point of the liquor. The used soda extraction media 46 is contacted with an aqueous solution in a recovery and regeneration unit 48 to back-extract sodium ions to the aqueous solution. The aqueous solution is then processed as required to produce a stream containing the recovered soda values 50 in a form suitable for return to the process; for example, it may be returned to the conventional spent liquor circuit 28 for further digestion of bauxite or used for washing seed or oxalate or for washing bauxite. Back extraction of the used soda extraction madia 46 results in regeneration of the protoπatθd form of the extractant in the media. The regenerated soda extraction media may then be re-used in further extraction steps and returned 33 to the soda extraction unit 32.

The treated spent liquor 34 is heated by heat exchanger 52 and passed to the second precipitator 36 for precipitation of boehmite at a temperature between about 95 ⁰C and 105°C. The treated spent liquor 34 may be cooled in the precipitator 36. The treated spent liquor 34 will have an A/TC in the range 0.77 to 0.55. The treated spent liquor 34 is seeded with boehmite 42 to facilitate boehmite precipitation. The seed charge is in the range from 50 g/L to 1200 g/L.

The treated spent liquor 34 may be sonicated to facilitate boehmite precipitation, with or without the presence of boehmite seed. Calcia may be added to the treated spent liquor 34 to reduce the proportion of gibbsite in the boehmite 40.

Figure 2 is a schematic flow sheet showing a part of a method in accordance with one embodiment of the present invention comprising the steps of:

feeding the gibbsite product 22 from the first stage of precipitation to the hydrate drying and preheating section 53 of a calciner 41 ;

feeding the boehmite product 40 from the boehmite precipitation stage to one or a number of later stages (53, 54, 56, 58 or 60) in the calciner 41.

The cooled alumina product 62 will be handled as usual in a Bayer operation. The air streams 64 will be adjusted to satisfy the calciner's combustion, transport, cooling, heat recovery, fines generation and temperature profile requirements. The exhaust gas stream 66 will be sent to dust collection and stacks.

The product 40 from the boehmite precipitator 36 may be sent either to the later stages of the hydrate preheat and drying section 53, to the holding vessel and furnace section 56 or to the early stages of the cooling section 60, or distributed across a combination of these sections. It will be appreciated that sections of static calciners are at temperatures under 330 ⁰C, and boehmite does not theπnally decompose at these temperatures. The boehmite will need sufficient holding time at temperatures above 540 ⁰C. The optimal entry location for the boehmite feed will depend on factors such as the size of stream 40, the gibbsite content of stream 40, the calciner temperature profile, boehmite residence time and the calciner energy balance. Streams 22 and 40 may require washing and filtering before entering the calciner 41

The following Examples serve to more fully describe the manner of using the above-described invention. It is understood that these Examples in no way serve to limit the true scope of this invention, but rather are presented for illustrative purposes.

Example 1: Soda extraction using ion exchange resins

Soda extraction using resins

Initial tests trial 3 different types of resins, all sourced from Sigma-Aldrich: (1) Ambβrlite IRC86 - hydrogen form - CAS#211811-37-9 (Weakly acidic resin 20-50 mesh); (2) Amberlitθ IRC50 - hydrogen form - CAS#9002-29-3 (Weakly acidic resin with carboxylic acid functionality) and (3) Lewatit CNP 105- hydrogen form (Weakly acidic resin). The results showed the Amberlite IRC86 and Amberlite IRC50 gave the best extraction, showing the larger relative drops in TC.

All resins were water conditioned prior to use by immersing the resins in Dl water in a 45:100 ^w/_v ratio at ambient temperature for 24 hr. The slurried resins were then collected on a Bϋchner funnel, drained free of water and allowed to air dry briefly (5-10 min).

Bayer spent liquor from one of Alcoa's Western Australian refineries was used in all the extraction and precipitation tests. The filtered spent liquor (a sub-sample was analysed by titration) was heated to the selected temperature (60 °C or 80 ⁰C) and contacted with the fresh water-conditioned resin in concentrations ranging from 165 to 220 gL^'1 at the selected temperature. In some tests, the spent liquor was subjected to two consecutive stages of contacting with the fresh water-conditioned resin; the liquor-resin slurry from the first contacting stage was filtered before the second stage of contacting.

Table 1. A/TC changes at 60 ⁰C with one contact at 21 δ gL^'1.

Table 2. A/TC changes at 80 ⁰C with two contacts with Amberiite IRC86 at loadings ranging from 165 to 200 gL^"1. The results show that the resins can extract sodium ions from refinery Bayer liquor with the concomitant transfer of protons to the liquor leading to neutralisation of the hydroxide.

The results show that soda extraction with resins can increase the A/TC of a spent liquor from 0.43 to 0.77, and reduce the TC from 224.5 to 118.7 gl_^'1 Na₂CC>₃. It would be expected that extra contacting stages and increased resin loadings would reduce the TC further still.

Laboratory Boehmite Batch Precipitation

The precipitation experiments were conducted using the resin treated spent liquor, seeded with boehmite particles prepared by a hydrothermal method and held at 95 ⁰C in polypropylene bottles rotating in a bath. The seed was added after the temperature of the treated spent liquor was equilibrated to 95 ⁰C. Seed loadings from 180 gL^"1 to 950 gL^"1 were used. After the designated precipitation holding time, the bottles were removed from the bath, the precipitation reaction was quenched using sodium gluconate, the contents filtered and liquor subsamples were analysed by titration at 25 ⁰C. The filtered solids were washed with hot de-ionised water and oven dried at between 60 ⁰C and 100 ⁰C. The solids were analysed by XRD.

The seed was prepared by a hydrothermal conversion of a commercial gibbsite to boehmite, conducted in a sealed, pressurised reactor at 200 ⁰C using de-ionized water . The material produced was analysed by XRD₁ TGA and DSC and found to be almost pure boehmite with about 0.2 % gibbsite. This boehmite seed was used for all the experiments reported below.

As a control, the untreated spent liquors were also seeded with boehmite and held in the rotating bath at 95 ⁰C. Neither the solids content nor the liquor composition showed any significant change after 6 hr and negligible yield of boehmite (- 1 gL^"1) after 48 hr.

Table 3. Boehmite precipitation from treated refinery spent liquor at 95 ⁰C and 500 gL^"1 seed loading.

Table 4. Boehmite precipitation from treated refinery spent liquor at 95 ⁰C₁ 500 gL ¹ seed loading and ~ 30 ppm gL^'1 calcia.

Table 5. Boehmite precipitation from treated refinery spent liquor at 95 ⁰C and 750 gL^"1 seed loading.

Table 6. Boehmite precipitation from treated refinery spent liquor at 95⁰C and 950 gL ¹ seed loading -

Refinery liquors contain significant organic and inorganic impurities, which can affect the precipitation behaviour. A number of experiments were conducted using a laboratory prepared sodium aluminate solution instead of the refinery liquors.

Table 7. Boehmite precipitation from a laboratory prepared sodium aluminate liquor of low TC at 95 ⁰C and 500 gL ¹ seed loading.

detected

Table 8. Boehmite precipitation from a laboratory prepared sodium alumiπate liquor of low TC at 95⁰C and 500 gl_^"1 seed loading with an excess of calcia (1 gl_^"1).

The batch boehmite precipitation data from treated refinery spent liquors shows that yields between 30 gL^*1 and 40 gL^'1 as AI₂O₃ can be readily achieved. Final A/TC values in the range 0.42 to 0.39 were observed. It is believed that optimisation of the precipitation conditions could raise the yields even further.

Boehmite precipitation from high purity sodium aluminate liquors can produce higher yields than equivalent precipitation from refinery Bayer liquors as is known for gibbsite precipitation. Yields as high as 50.6 gL^'1 as AI₂Oa were observed. Final A/TC values as low as 0.288 were observed.

The predominant product is boehmite, though gibbsite is present in some samples. It is clear the precipitation conditions can be adjusted to minimize any gibbsite co-precipitation.

Good reproducibility was observed in the precipitation data.

Expected productivity with soda extraction by ion exchange resins.

Models of the cases investigated of the invention were formulated, evaluated and refined using a combination of inhouse models built on chemical engineering first principles and tuned to existing Bayer unit operations, an extensive database of Bayer properties and thermodynamic data, Bayer operating experience and flowsheet models built within ASPEN Plus™ (ASPEN Technology Inc., a software process simulation software with state of the art physical properties packages, including added Bayer process properties and unit operations built inhouse, e.g. an Alcoa static calciner.)

Four cases were examined based on a 1100 kl_/h green liquor flow from digestion, with the following composition: A 146 gL^"1 AI₂O₃,

TC 205 gL-¹ as Na₂CO₃,

TA 251 gL^"1 as Na₂CO₃,

total organic carbon 22 gL^"1 as C,

chloride 11 gL¹ as NaCI₁

sulphate 22 gL^'1 as Na₂SO₄.

Case #1 considers gibbsite precipitation from the green liquor. There was no second stage precipitation of boehmite from treated spent liquor. The analysis assumes a gibbsite precipitation yield of 63 gL^'1, as A^O₃, typical of a modern low temperature Bayer plant processing Western Australian Bauxite.

Case #2 extends case #1 by including a second stage of boehmite precipitation from a treated spent liquor stream comprising 30% of the green liquor flow. The gibbsite precipitation circuit yield is still of 63 gL^"1 as AI₂O₃; The boehmite precipitation yield is 33 gL^"1 as AI₂O₃.

Case #3 is comparable to Case #2 but with a higher flow of treated spent liquor, i.e. 70% of green liquor flow. The boehmite precipitation yield was 30.5 gL^"1 as AI₂O₃.

Case #4 considers the treating all of the spent liquor from the first stage of precipitation. A boehmite precipitation yield of 30 gL^"1 as AI₂O₃ is assumed. Note: the analyses were constrained to ensure that the liquor compositions remained within the range observed in the experimental program and further optimization work is likely to identify conditions giving higher yields.

Table 9 provides estimated green liquor yields when utilising the invention with soda extraction by ion exchange resins and energy savings. The production is based on a green liquor flow from digestion of 110OkUh as described above. It should be noted that the following estimates are based on the models and data available on this system.

Table 9. Calculated Bayer circuit green liquor yields.

Cases 1 and 4 are examples of potential of the invention to both increase the yield of the process and to provide significant energy savings. The results suggest that the greater the portion of the spent liquor treated, the greater the benefits. However, actual optimum portion for treatment will depend a number of factors. These include:

• optimizing calciner performance, e.g. the optimum split between the gibbsite and boehmite product streams;

• the properties of the green liquor and the first spent liquor for example, with respect to impurities;

• Bayer plant operating conditions, e.g digestion TC and TA values;

• available gibbsite precipitation infrastructure and utilities; and economic considerations.

Table 10 shows one example of the predicted calciner performance for two cases:

Case #1 : the gibbsite and boehmite product streams both feed at the same location at the front of the calciner, and Case #2: two separate feed points, the gibbsite product is fed at the front of the calciner and the boehmite product is added to a line leading to the furnace.

The model predictions were made using an ASPEN Plus™ model of a Alcoa static calciner. The model is calibrated to refinery data for the calcination of gibbsite to alumina. The gibbsite product and boehmite product feed ratio was 4:1 on a mass basis.

Table 10. Predicted calciner performance.

The results show that an energy saving can be achieved by a split feed arrangement. The high stack temperature for Case #2 suggests an improved heat recovery. It will be appreciated that optimization of the calciner operating conditions and temperature profiles are likely to increase this energy benefit further. Acceptable temperatures were observed. Optimization of the split feed combinations and split feed ratios are expected to result in further energy benefits. Further, the extra degree of freedom available for adjusting the distribution of mass flows in the calciner may also be used to optimize the calciner circuit and increase throughput.

Example 2: Soda extraction by solvent extraction

Spent liquor from one of Alcoa's Western Australian operations was treated by solvent extraction with the organic solvent /so-octanol (Exxon-Mobil, 'Exxal 8') and the extractant 4-tert-octylphenol (97%, Sigma-Aldrich, TOP'). The practical upper loading limit of the TOP in the /so-octaπol solvent, with the refinery spent liquor system used in the test work, was determined to be between 1 M and 0.75M. The practical upper loading limit is not necessarily the solubility limit of the extractant in the solvent but is also determined by the behaviour of the resultant organic phase after contacting with the aqueous phase, .e.g. thickening/clouding, separation characteristics or crystallization. Figure 3 below shows the TC drops observed when a 0.5 A/TC spent liquor was contacted for 10 min at 60 ⁰C In 1 :1 ratio with the solvent containing various amounts of TOP.

A sample of filtered, refinery spent liquor was contacted with an equal volume of 0.75M 4-terf-octylphenol in /so-octanol, previously saturated with water, for 10 min at 60 ⁰C. The phases were settled and the treated liquor separated. The process was repeated with fresh solutions of 0.75M 4-terf-octylphenol in /so-octanol two further times.

Extraction kinetics for 4-tert-octylphenol in /so-octanol indicated that extraction was complete after less than one minute.

The resultant changes in TC and TA with each contact are shown in Table 11 for three spent liquor samples. The increase in alumina concentration is consistent with the reported water removal (US6322702) during extraction.

¹I-. Sample II... Contact .Ji.JL.. JTA₃L:: Ji jl _ ATTC .. .

¹I 1 Il 0"' ll 22472 il 271 63 -JfWSL 0.420 . . !

Il i 1" i • 196.32 il 244.86 W 98.20 !i 0.500 -J

L i ₂nα i| 172.28 !t _„ 223.75 . ...Jl .. 103.04 0.598 I il . _JL. 3™ _!L. 149.79 !l 202.87 ZJCZ 107.08 _JL_ 0.715 I il 2 il o^m !| 232 19 t| 281.08 102.19 i| 0.440 1 iL ■I r !| 209 39 'I 260.08 iL 108.39 __JCZ 0.517 I iL 'I ₂πo ! 184.66 l| 238.70 — ]I7Z 113.95 il 0.617 il H 3^ra Il 162.21 Il 218.94 il 115.72 IC 0.713 I

;| 3 il o^m •I 234.61 (I 285.12 103.55 Jl 0.441 _J il il r ι| 209.19 Il 259.77 107.48 il 0.514 .J

(I il 2™ il 185.20 il 239.04 J 113.74 Il JLSIl.. I

|L Il 3™ i| 162.42 il 219.16 J 115.80 ^~~![Z 0.713__ J

Table 11. A/TC changes at 60 "C with three contacts. Thβ analyses also show that between 3 and 10% of the water in the spent liquor is also transferred to the organic phase, depending on the number of contacts.

It is anticipated that increasing the number of contact stages would decrease the TC of the spent liquor to even lower values than shown in Table 10. Preferentially, instead of multiple stages of 1:1 contacting, using larger solvent to liquor ratio contacts should reduce the number of stages required to obtain the desired hydroxide removal and may reduce equipment costs.

It will be appreciated that other solvent and extractant combinations may provide different and improved results.

As in Example 1 , a number of cases were examined using a combination of inhouse models tuned to existing Bayer unit operations, an extensive database of Bayer properties and thermodynamic data, Bayer operating experience and flowsheet models built within ASPEN Plus. All of the models were based on a 1100 kL/h green liquor flow from digestion, with the following composition:

A 146 gL ¹ AI₂O₃,

TC 205 gL^'1 as Na₂CO₃,

TA 251 gL^*1 as Na₂CO₃, total organic carbon 22 gL^"1 as C,

chloride 11 gL^"1 as NaCI,

sulphate 22 gL^"1 as Na₂SO₄.

Green liquor yields ranging from 70 to 90 gL^'1 (g precipitated as AI₂O₃ per L of green liquor) are feasible, with energy savings from 0.4 to 1.25 GJ/t alumina. Higher yields are expected for Bayer liquor with fewer impurities (comparable 3, 4 and 8).

Example 3: Soda extraction by electrolysis Table 12 presents data from soda extraction experiments in a two compartment electrolytic cell containing Nafion 324 cation permeable membrane. The liquor is feed to the anolyte compartment. The electrolysis was conducted at 90 ⁰C with a current density of 350 mA/cm² and voltage 6.5V. The catholyte compartment produced NaOH (TC 400 gL^"1 as Na₂CO₃).

Table 12. A/TC changes at 90⁰C.

It will be appreciated that the above results could be optimised with respect to soda removal, in principle up to the metastable limit for heterogeneous nucleatioπ. Options include changing the cell configuration, current density, anode type and factors affecting current efficiency. Electrodialysis is also an option, requiring less power but with lower current densities.

Example 4 -Suppression of gibbsite precipitation by calcia addition

Spent liquor ex-precipitation having TC = 219.2, TA = 266.6, A = 94.4 and A/TC - 0.43 was contacted in two stages, each with 165 g/L of Amberlite IRC86 resin, to provide a final liquor composition of TC = 122.0, TA = 165.9, A = 85.9 and A/TC =

0.70. Boehmite seed containing 0.1 % w/w gibbsite at a charge of 180 gΛ., was added to the treated liquor and precipitation was conducted for a given time at 95

⁰C. All experiments were performed in triplicate and the results averaged and presented in Table 13 below.

Table 13. Suppression of gibbsite precipitation by calcia addition.

After 5 hr, both liquors precipitated similar amounts of alumina. However, the precipitate from the non calcia containing liquor consisted of 30 % gibbsite and 70 % boehmite compared to 2 % gibbsite and 98 % boehmite for the liquor containing calcia. Similar results were obtained for precipitates at 24 hr and 52 hr where the non calcia containing liquors precipitated 20 % and 31 % gibbsite respectively and the calcia containing liquors had precipitated 0 % and 1 % of gibbsite respectively. Notably, at 24 hr and 52 hr precipitation the non calcia containing liquors had slightly less overall precipitate which supports the evidence that gibbsite precipitation is suppressed. Clearly, the presence of excess calcia in the liquors inhibits the precipitation of gibbsite relative to boehmite.

Figure 4 is a schematic flow sheet of a part of a method in accordance with a second embodiment of the present invention utilised in a Bayer Process circuit. The methods shown in Figures 1 and 4 are substantially similar and like numerals denote like parts. As for the embodiment described above, method also comprises the key steps of:

digesting bauxite in a caustic solution 10;

separating the mixture 12 to residue 14 and green liquor 16 ;

separating the green liquor 16 into a green liquor 70 and a green liquor bypass 72;

precipitating gibbsite from the liquor 71 in a gibbsite precipitation circuit 18 and forming a first suspension; separatiπg the first suspension in a classification circuit 20 into gibbsite product 22, gibbsite seed recycle 24 and spent liquor 26;

separating the spent liquor 26 into a first portion 28 and a second portion 30;

treating the second portion of the spent liquor 30 in a soda reduction unit

32 by contacting the second portion of thθ spent liquor 30 with a regenerated media containing an extractant 33 and forming a treated first spent liquor 34;

separating the treated first spent liquor 34 into a treated first spent liquor 74 and a treated first spent liquor bypass 76;

adding at least a portion of the treated first spent liquor bypass 76 to the green liquor 70 to provide a liquor 71 ;

adding at least a portion of the green liquor bypass 72 to the treated first spent liquor 74 to provide a liquor 75;

precipitating boehmite from the liquor 75 in a boehmite precipitation circuit

36 and forming a second suspension;

calcining at least a portion of the gibbsite product 22 in a calciner 41 ; and

calcining at least a portion of the boehmite product 40 in the calciner 41.

The green liquor bypass 72 may be combined with the treated first spent liquor 74 and the liquor 75 fed to the beginning of the boehmite precipitation circuit 36, or added at later stages 78 of the boehmite precipitation circuit 36, or both. Thβ treated first spent liquor bypass 76 may be combined with the green liquor 70 and the liquor 71 fed to the beginning of the gibbsite precipitation circuit 18, or added at later stages 80 of the gibbsite precipitation circuit 18, or both.

It will be appreciated that streams 34, 72 and/or 74 may need to be heated to the desired temperature for boehmite precipitation (95 ⁰C to 105 ⁰C).

This arrangement allows greater flexibility to optimise the overall yield, product quality and energy usage. The optimal amount and distribution of the green liquor bypass 72 and the treated first spent liquor bypass 76 will depend on the precipitation circuit and calciner configurations, operating conditions, the performance of the soda extraction plant, the seed and liquor properties, desired gibbsite and boehmite product ratio and product quality requirements.

Example 5: Boehmite precipitation from combined refinery green liquor and treated refinery spent liquor

Batch boehmite precipitation experiments were conducted from a combined green liquor and treated spent liquor (60:40 ratio). The liquors were obtained from one of Alcoa's Western Australian refineries. The spent liquor was treated using water conditioned Amberlite IRC86 at 80 ⁰C in two stages of contacting as described previously.

The combined liquors were seeded with boehmite (500 gL^"1) and held at 95 ⁰C in polypropylene bottles rotating in a bath. The seed was added after the temperature of treated spent liquor was equilibrated to 95 ⁰C. After the designated precipitation holding time, the bottles were removed from the bath, the contents filtered and liquor subsamples were analysed by titration at 25 ⁰C and the precipitation reaction quenched using sodium gluconate. The filtered solids were washed with hot de-ionised water and oven dried at between 60 ⁰C and

100 ⁰C. The solids were analysed by XRD.

The seed was prepared by a hydrothermal conversion of a commercial gibbsite to boehmite, conducted in a sealed, pressurized reactor using de-ionized water and at 200 ⁰C. The material produced was analysed by XRD, TGA and DSC and found to be almost pure boehmite with about 0.2 % gibbsite. This boehmite seed is the same as used for the experiments reported in the tables above.

Table 14. Boehmite precipitation from combined green liquor and treated refinery spent liquor ( 60:40) at 95⁰C and 500 gL^"1 seed loading.

Table 15. Boehmite precipitation from combined green liquor and treated refinery spent liquor ( 60:40) at 95 ⁰C and 500 gL^"1 seed loading with an excess of calcia (- 1 gL^"1).

The results show that yields as high as 45 gL^"1 are feasible.

Claims

The Claims Defining the Invention is as Follows:

1. A method for preparing aluminium oxide from a Bayer process solution, the method comprising the steps of:

precipitating a first alumina product and providing a first spent liquor;

separating at least a portion of the first alumina product and the first spent liquor;

calcining at least a portion of the first alumina product in a calcine r; and

calcining at least a portion of the second alumina product in the calciner,

2. A method for preparing aluminium oxide in accordance with claim 1 , wherein the first alumina product comprises substantially gibbsite and the second alumina product comprises substantially boehmite.

3. A method for preparing aluminium oxide in accordance with claim 1 or claim 2. wherein the treated spent liquor has a A/TC approximating that of the green liquor.

4. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

5. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

comprises removing sodium ions from the first spent liquor.

6. A method for preparing aluminium oxide in accordance with claim 4 or claim 5, wherein the method comprises the further step of

recovering the sodium ions.

7. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

comprises neutralizing or removing hydroxide ions from the first spent liquor.

8. A method for preparing aluminium oxide in accordance with claim 7, wherein the method comprises the further step of:

recovering the hydroxide ions.

9. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the method comprises the additional step of:

precipitating a first alumina product.

10. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the method comprises the additional step of:

precipitating a second alumina product.

11.A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the method comprises the additional step of:

digestion of bauxite to provide the green liquor.

12. A method for preparing aluminium oxide in accordance with claim 11 , wherein the bauxite is provided in the form of gibbsitic bauxite, boehmitic bauxite, diasporic bauxite or any combination thereof.

13. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the method comprises the additional step of:

seeding the treated first spent liquor with the second alumina product.

14.A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

is conducted at a temperature less than about 105 ⁰C.

15.A method for preparing aluminium oxide in accordance with any one of claims 1 to 14, wherein the step of:

is conducted at an initial temperature of between about 95 ⁰C and

105 ⁰C.

16. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the method comprises the additional step of:

adding a gibbsite precipitation inhibitor to the treated first spent liquor.

17. A method for preparing aluminium oxide in accordance with claim 16, wherein the gibbsite precipitation inhibitor is calcia.

18. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

calcining at least a portion of the first alumina product in a calciner;

comprises the step of:

adding the first alumina product to the front end of a calciner.

19. A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

calcining at least a portion of the second alumina product in a calciner;

comprises the step of:

adding the second alumina product to the same location within the calciner as the first alumina product.

20.A method for preparing aluminium oxide in accordance with any one of claims 1 to 18, wherein the step of:

calcining at least a portion of the second alumina product in a calciner;

comprises the step of:

adding the second alumina product at a different location within the calciner as the first alumina product.

21. A method for preparing aluminium oxide in accordance with claim 20, wherein the second alumina product Is added to a later stage in a preheat and drying section, a furnace and holding vessel section, an early stage of a cooling section or a hydrate bypass system of a static calciner, or combinations thereof.

22.A method for preparing aluminium oxide in accordance with any one of the preceding claims, wherein the step of:

comprises the step of: contacting the first spent liquor with a substantially water-immiscible solution comprising an extractant; and

23.A method for preparing aluminium oxide in accordance with claim 22, wherein method comprises the further step of:

24.A method for preparing aluminium oxide in accordance with claim 23, wherein method comprises the further step of:

25.A method for preparing aluminium oxide in accordance with any one of claims 1 to 21 , wherein the step of:

comprises the step of:

26. A method for preparing aluminium oxide in accordance with claim 25, wherein the solid support comprising an extractant is an ion exchange resin.

27.A method for preparing aluminium oxide in accordance with claim 25, wherein the method comprises the further step of:

separating the treated first spent liquor and the solid support.

28.A method for preparing aluminium oxide in accordance with any one of claims 1 to 21 , wherein the step of:

comprises the steps of:

29.A method for preparing aluminium oxide in accordance with claim 28, wherein the sodium ion from one region to another second region is accompanied by a concomitant neutralisation of hydroxide ions at the anode and generation of hydroxide ion in the second region.

30. Aluminium oxide as prepared in accordance with any one of claims 1 to 28.

31. A method for preparing aluminium oxide from a Bayer process solution as hereinbefore described with reference to the accompanying Examples

32.A method for preparing aluminium oxide from a Bayer process solution as hereinbefore described with reference to the accompanying Figures.