Method for Precipitating Boehmite from Pre-precipitation Bayer Liquors

Field of the Invention

The present invention relates to a method for precipitating boehmite from a pre- precipitation Bayer liquor.

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 and provide a 'pre-precipitation liquor', also known as green liquor or pregnant liquor.

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

The 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 A/TC ratio of the liquor falls from about

0.7 to about 0.4 (where A represents the alumina concentration, expressed as gl_ ^~1 L of AI ₂ O ₃ , and TC represents total caustic concentration, (concentration of sodium hydroxide plus sodium aluminate; expressed as gl_ ^"1 sodium carbonate).

At the lower value of A/TC, 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, i.e. sodium hydroxide plus sodium aluminate plus sodium carbonate; expressed as gl_ ^'1 sodium carbonate)expressed as gL ^"1 sodium carbonate) of Bayer process solutions affects the solubility of boehmite and gibbsite in those solutions in a number of ways.

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

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.

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., AI ₂ O ₃ -SH ₂ O or AI(OH) ₃ , or as a monohydrate, i.e., AI ₂ O ₃ -H ₂ O 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.

There are significant energy savings associated with calcining boehmite over gibbsite arising from the enthalpy of the calcination reactions, the number of water molecules driven off, resulting in a substantially lower latent heat component and less sensible heat being lost as the amount of steam produced is lower. Consequently, it is desirable to precipitate digested alumina as boehmite.

Other potential benefits associated with precipitating boehmite instead of gibbsite, that 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.

The three principal types of methods previously used to produce boehmite can be summarised 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 NH ₄ OH, or aluminates such as sodium aluminate are neutralized by an acid (e.g. HCI or H ₂ SO ₄ ) or CO ₂ to produce gelatinous boehmite; and

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

US4595581 teaches a process for precipitating substantially pure boehmite by heating a seeded sodium aluminate suspension to a temperature between about 115 ⁰ C to 145 ⁰ C and separating the boehmite precipitate from the suspension. US4595581 reports boehmite precipitation experimental results from a range of liquors with compositions characteristic of Bayer refinery pre-precipitation liquors. The liquors were seeded with boehmite and the tests conducted at 125 °C. Both crystalline and non-crystalline boehmite was tested as seed. Yields after 6 hr

precipitation were reported in the range from 11.3 to 32 gl_ ^"1 as AI ₂ O ₃ . US4595581 recommends the preferred seed material to be amorphous boehmite gel rather than crystalline boehmite on the basis of higher yields achieved. Separating and recycling boehmite gel as part of the seeding circuit for a continuous commercial scale Bayer operation introduces significant operating difficulties and possibly a new classification technology when compared to using large crystalline aluminium hydroxide as seed. US4595581 reports yields of less than 20 gl_ ^"1 when crystalline boehmite seed is used. Whilst US4595581 teaches the precipitation of boehmite, the author fails to recognise that increased yields may be obtained by treating the liquor to drive the precipitation reaction.

Loh et al. [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 saving, it is not considered to be a commercially viable alternative.

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.

In the context of the present invention, the term pre-precipitation liquor shall be understood to indicate any liquor in the Bayer circuit post digestion and prior to aluminium hydroxide precipitation.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated 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 word "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.

Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing undissolved and/or dissolved solids.

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 boehmite from a pre-precipitation Bayer liquor, the method comprising the steps of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor; and

precipitating boehmite from the treated pre-precipitation liquor,

wherein at least a portion of the boehmite is precipitated at a temperature of at least 105 ⁰ C.

Preferably, the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

comprises decreasing the concentration of sodium ions in the pre-precipitation liquor.

Preferably, the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

comprises removing sodium ions from the pre-precipitation liquor.

Advantageously, the step of treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor favours the precipitation of alumina and should result in increased alumina yields.

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 pre-precipitation liquor should have a positive effect on boehmite precipitation.

Preferably, at least a portion of the boehmite is precipitated from the treated pre- precipitation liquor at a temperature of at least 105 ⁰ C and a pressure greater than atmospheric pressure. In a highly specific form of the invention, boehmite is precipitated from the treated pre-precipitation liquor at a temperature of at least 120 ⁰ C.

Without being limited by theory, it is believed that precipitation of boehmite at elevated temperatures can decrease the likelihood and extent of co-precipitation with gibbsite due to a number of factors including the increase of the relative kinetic constants for boehmite precipitation and the preferential dissolution of gibbsite nuclei at higher temperatures.

Preferably, the boehmite is precipitated from the treated pre-precipitation liquor at a pressure greater than atmospheric pressure.

Preferably, the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

is conducted at a temperature of at least 80 °C.

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

- -

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

is conducted at a temperature of at least 105 ⁰ C.

In a highly specific form of the invention, the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

is conducted at a temperature at about the boiling point of the Bayer liquor at that pressure.

Preferably, the method comprises the steps of:

clarification and filtration of the pre-precipitation liquor prior to the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor.

Pre-precipitation liquors that have been clarified and filtered may be known in the art by various terms including green liquors and pregnant liquors.

It will be appreciated that the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

may be performed on the entire pre-precipitation Bayer liquor or on a portion of the Bayer pre-precipitation liquor.

In one form of the invention, the steps of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor; and

precipitating boehmite from the treated pre-precipitation liquor,

comprises precipitating boehmite from a combination of the treated pre- precipitation liquor and untreated pre-precipitation liquor.

The combination of the treated pre-precipitation liquor and untreated pre- precipitation liquor may contain the liquors in any ratio. It will be appreciated that the ratio will depend on many factors including Bayer circuit throughput, treated pre-precipitation liquor properties (e.g. A, TC, TA) and untreated pre-precipitation liquor properties (e.g. A, TC, TA).

The method of the present invention maybe performed as a batch process or a continuous process.

Advantageously, the present invention reduces hydroxide concentrations in Bayer liquors and reduces both the TC and TA of the Bayer process liquor. Further, the present invention increases the A/TC of the Bayer process liquor, thereby increasing the precipitation efficiency of boehmite.

It will be appreciated that the steps of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor; and

precipitating boehmite from the treated pre-precipitation 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 pre-precipitation liquor.

It will be appreciated that there may be provided a plurality of boehmite precipitation steps and that they may be conducted at different temperatures.

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

adding a gibbsite precipitation inhibitor to the green 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.

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.

Where the steps of

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor; and

precipitating boehmite from the treated pre-precipitation liquor,

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

adding calcia to the pre-precipitation liquor after the step of precipitating boehmite from the pre-precipitation liquor.

- -

In another form of the invention, the gibbsite precipitation inhibitor is provided in the form of organic compounds such as gluconate and tartrate. Certain organic compounds are believed to inhibit gibbsite precipitation by reducing the number of active sites on the seed surface. As the crystal structure of boehmite is different to that of gibbsite, it is believed that boehmite precipitation will be less affected by organic compounds than gibbsite precipitation.

It will be appreciated that where the steps of

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor; and

precipitating boehmite from the treated pre-precipitation liquor,

are repeated, it may not be necessary to add further gibbsite precipitation inhibitor to the pre-precipitation liquor after the step of precipitating boehmite from the pre- precipitation liquor.

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

seeding the pre-precipitation liquor with boehmite.

It will be appreciated that the step of:

seeding the pre-precipitation liquor with boehmite,

may be conducted prior to, during, or subsequent to the step of:

treating the Bayer liquor to decrease the concentration of sodium ions in said liquor.

- -

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_ ^"1 .

Many methods of producing boehmite seed exist. Preferably, the boehmite seed is recycled from the Bayer precipitation circuit.

It will be appreciated that the boehmite seed particle characteristics (e.g. shape, size and inter-surface forces) may affect the particle characteristics of the precipitated boehmite. Preferably, the particle characteristics of the precipitated boehmite will make them suitable for classification by commercially available bulk processing technologies such as cyclones, thickeners and classifiers.

In one form of the invention, the step of:

precipitating boehmite from the treated pre-precipitation liquor,

is preceded by the step of:

sonication of the treated pre-precipitation liquor with or without the presence of boehmite seed.

In one form of the invention, the step of:

treating the pre-precipitation liquor to decrease both the total caustic concentration and the total alkalinity of the pre-precipitation liquor,

comprises the step of:

contacting the pre-precipitation liquor with a substantially water-immiscible solution comprising an extractant; and

_ ^ _

extracting at least a portion of the sodium ions present in the pre- precipitation liquor into the substantially water-immiscible solution.

It should be appreciated that the extraction of sodium ions from the pre- precipitation 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 pre-precipitation liquor.

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 sodium ion into the substantially water-immiscible solution will be accompanied by the transfer of a proton from the substantially water-immiscible solution into the pre-precipitation 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 halogenated 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 1 H,1 H-perfluorononanol,

1 H,1 H,9H-hexadecafluorononanol, 1 ,1 ,1-trifluoro-3-(4-ferf-octylphenoxy)-2- propanol, 1 ,1 ,1 -trifluoro-2-(p-tolyl)/sopropanol, 1 -(p-tolyl)-2,2,2-trifluoroethanol, hexafluoro-2-(p-tolyl)isopropanol, 2-(methyl)-2-(dodecyl)tetradecanoic acid,

3-(perfluorohexyl)propenol and 1-(1 ,1 ,2,2-tetrafluoroethoxy)-3-(4-ter£- octylphenoxy)-2-propanol, te/i-octylphenyl, para-nonylphenol, para-tert- butylphenol, para-terf-amylphenol, para-heptylphenol, para-octylphenol, para-

(alpha,alpha-dimethylbenzyl)phenol (4-cumylphenol), 2,3,6-trimethylphenol, 2,4-di-terf-butylphenol, 3,5-di-ferf-butylphenol, 2,6-di-ferf-butylphenol, 2,4-di-terf- pentylphenol (2,4-di-fert-amylphenol), 4-sec-butyl-2,6-di-terf-butylphenol, 2,4,6-tri-

- -

terf-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 Bayer liquor should be minimal.

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

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

_ _{(| g} _

Preferably, the organic liquid is substantially non-polar.

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

Preferably, the organic liquid has a flash point above process temperature.

Preferably, the organic liquid is a 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, /so-octyl alcohol (such as that commercially available as Exxal 8 from ExxonMobil), iso-nonylalcohol (such as that commercially available as Exxal 9 from ExxonMobil), /so-decanol, iso- tridecanol, 2-ethyl-1-hexanol, kerosene and other hydrocarbons commercially available under the names Escaid 100, Escaid 110, Escaid 240, Escaid 300, lsopar L, lsopar M, Solvesso 150, Exxsol D110 from ExxonMobil) and mixtures thereof.

It should be appreciated that partitioning of the organic solvent and the extractant in the pre-precipitation liquor is minimal. Preferably, the partitioning of the pre- precipitation 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 pre-precipitation liquor may vary according to the manner in which both the Bayer 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 pre-precipitation liquor with a substantially water-immiscible solution comprising an extractant;

comprises agitating the pre-precipitation 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 pre-precipitation liquor and the organic phase should be sufficient for reaction to occur between the extractant and the sodium cations to form a sodium 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 pre-precipitation liquor.

It should be appreciated that the volumes of the pre-precipitation liquor 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:

contacting the pre-precipitation liquor with a substantially water-immiscible solution comprising an extractant; and

extracting at least a portion of the sodium cations present in the pre- precipitation liquor into the substantially water-immiscible solution,

- -

may be repeated.

Where the steps of:

contacting the pre-precipitation liquor with a substantially water-immiscible solution comprising an extractant; and

extracting at least a portion of the sodium cations present in the pre- precipitation liquor into the substantially water-immiscible solution,

are repeated, the step of:

contacting the pre-precipitation liquor with a substantially water-immiscible solution comprising an extractant;

may be performed with different substantially water-immiscible solutions.

Preferably, the method comprises the further step of:

separating the pre-precipitation liquor and the substantially water- immiscible solution.

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

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. Depending on the concentration of sodium hydroxide, the aqueous solution may need to be pre-treated prior to subsequent use.

In one form of the invention, the step of:

treating the pre-precipitation liquor to decrease the concentration of sodium ions in said liquor;

comprises the step of:

applying a potential between a first region comprising the pre-precipitation liquor and a second region comprising a catholyte, wherein the pre-

- -

precipitation 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.

Preferably, the pre-precipitation liquor is not directed to the second region.

Advantageously, by not directing the Bayer process liquor to the second region, a relatively pure caustic stream may be produced from the second region. Such a stream could be used within the Bayer circuit for, for example, washing bauxite (to extract impurities) or any other application where clean caustic is useful. In addition, the Bayer process liquor, after a precipitation step may be used elsewhere in the Bayer circuit. For example, it would have a lower TC/TA content and a lower alumina content than normal spent liquor which should enable more efficient causticisation with lime.

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.

- -

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 cation from one region to another region will either be accompanied by a concomitant neutralisation of hydroxide ions within the Bayer process liquor and generation of hydroxide ion in the second region or, in the case of an electrodialysis unit, the transfer of hydroxide from one region to another region, and in the opposite direction to the cation transfer to maintain solution charge balance.

The present invention offers distinct advantages over methods employing carbonation to reduce hydroxide concentrations in Bayer process solutions, as carbonation reduces TC without affecting TA, but the present invention reduces both the TC and TA of the Bayer process liquor. Further, the present invention increases the A/TC of the Bayer process liquor, thereby increasing the precipitation efficiency of alumina.

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 tetrafluorethylene copolymer,

carboxylate polymer, polystyrene based polymer, divinylbenzene polymer, or sodium conducting ceramics such as beta-alumina or combinations thereof.

In highly specific forms of the invention, the cation permeable membrane is a Nafion 115, Nafion 117, Nafion 324, Nafion 440, Nafion 350, Nafion 900 series, Fumatech FKB, Fumatech FKL membrane, Astom CMB or Astom CMX membrane.

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 pK _a of the ion exchange polymer, the solvation of the polymer and the nature and extent of interactions between the fixed ionic groups.

In the case of an electrodialysis unit, the anion permeable membrane is preferably a Neosepta AHA membrane or a Fumatech FAP 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. Pre-precipitation 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 pre-precipitation 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 systems employing one cation exchange membrane, the preferred current density may be between 20 mA/cm ² and 600 mA/cm ² . More preferably, the current density is between 150 mA/cm ² and 350 mA/cm ² .

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. The caustic solution may be sourced from the Bayer circuit. It will be appreciated that where the caustic solution is sourced from the Bayer circuit, the solution should have a caustic concentration below that of the Bayer process liquor. Non-limiting examples include Bayer lake water or condensate.

Preferably, the catholyte caustic concentration is not greater than about 8M NaOH or 25% NaOH catholyte. It will be appreciated that if the caustic concentration is too low then the current density may drop due to lower conductivity.

Preferably, the catholyte has a maximum alumina concentration of 20 gl_ ^"1 as AI ₂ O ₃ .

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 pre-precipitation 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 pre-precipitation 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. Pre-precipitation liquor anolyte is continuously introduced into the first compartment and catholyte is continuously introduced into the second compartment with a potential continuously applied between the first compartment and the second compartment. Treated pre- precipitation 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 pre-precipitation 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 one form of the invention, the step of:

treating the pre-precipitation liquor to decrease the concentration of sodium ions in said liquor;

comprises the step of:

contacting the pre-precipitation 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 pre-precipitation liquor with an ion on the solid support.

Preferably, the solid support comprising an exchangeable ion 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 (RSO ₃ ^' H ⁺ ) or a weakly-acidic 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(CH ₃ ) ₃ ⁺ 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 weakly-acidic 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 pre- precipitation liquor 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 the following ion exchange resins all in their hydrogen form: Amberlite IRC-86, Amberlite IRC- 50,Lewatit CNP-105, Amberlite CG-50, Lewatit CNP-80, Lewatit CNP-80 WS, Purolite C115KMR, Purolite C115E, Diaion WT01S, Dowex Mac-3, Duolite® CS- 100, RF Resin and SRL Resin.

Advantageously, the exchange of a cation present in the pre-precipitation liquor with a proton on the resin will be accompanied by a concomitant neutralisation of hydroxide ions in the pre-precipitation liquor according to the following equation where RH represents the hydrogen form of the resin.

NaOH + RH ► R-Na ⁺ + H ₂ O

In a second form of the invention, the solid support and the extractant are provided in the form of an ion aqueous biphasic extraction chromatography resin. Examples of this type of resin are ABEC-2000 and ABEC-5000. Such resins contain polyethylene glycol (PEG) chains tethered to a polymer backbone, such as polystyrene divinylbenzene backbone. As the PEG chains are not endowed with ion-exchange capability, the expected extraction mechanism is by transfer of NaOH ion pairs into the resin. The high concentrations of sodium and hydroxide ions in the Bayer liquor drive the uptake of the ion pairs into the resin, making possible caustic recovery by elution with water by a simple mass-action reversal of the uptake. ABEC resins operate best at high ionic strengths with hydrophilic inorganic salts, such as NaOH, making Bayer liquor an appropriate medium for use with such resins.

- -

Preferably, the step of:

contacting the pre-precipitation liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble,

comprises agitating the pre-precipitation 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 pre-precipitation 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 aqueous phase, 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 liquor.

Where the steps of:

contacting the pre-precipitation 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 pre-precipitation liquor with an ion on the solid support,

are repeated, the step of:

contacting the pre-precipitation liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble,

may be performed with different solid supports.

- -

Preferably, the method comprises the further step of:

separating the Bayer process solution and the solid support.

It should be appreciated that the step of separating the Bayer process solution 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 regenerate the solid support after the step of:

exchanging the sodium ion present in the pre-precipitation process solution with an ion on the solid support;

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

It will be appreciated that the pH of the stripping solution will be influenced by the type of resin employed. For example, carboxylic acid ion exchange resins may require a stripping solution of pH <3 , whilst for a phenolic based ion exchange resin or an aqueous biphasic extraction chromatography resin, a stripping solution of pH 5 or higher should be sufficient.

It will be appreciated that the pKa of the ion exchange resin will influence the step of exchanging a metal cation present in the Bayer process solution with an ion on the solid support. Preferably, the ion exchange resin has a pKa of about 9-13.

Preferably, the weak-acid cation exchange resin comprises a phenolic group or a hydroxyl group attached to an aromatic ring.

_ ^ _

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

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.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to one embodiment thereof, and the accompanying drawings, in which:-

Figure 1 is a schematic flow sheet of a Bayer Process circuit;

Figure 2 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised in a Bayer Process circuit; and

Figure 3 is an example of an X-ray diffraction pattern of the product from the experiment shown in Table 5.

Best Mode(s) for Carrying Out the Invention

The invention focuses on the precipitation of boehmite from a pre-precipitation Bayer liquor by treating the pre-precipitation liquor to reduce the total alkalinity and total caustic concentration of the spent liquor.

Figure 1 shows a schematic flow sheet of the Bayer process circuit for a refinery using a single digestion circuit 10 comprising the steps of:

digestion 12 of bauxite 14 in a caustic solution 16;

- -

liquid-solid separation 17 of the mixture to residue 18 and a pre- precipitation liquor 20;

aluminium hydroxide precipitation 22 from the seeded 24 pre-precipitation liquor 20;

separation of aluminium hydroxide 26 and spent liquor 28; and

recycling spent liquor 28 to digestion 12.

In accordance with an embodiment of the present invention and best seen in Figure 2, at least a portion 30 of the pre-precipitation liquor 20 is contacted in a solvent extraction apparatus 32 with a solution of an organic solvent 34 comprising an extractant. Up to 100 % of the pre-precipitation liquor 20 may be processed in the extraction apparatus 32. The aqueous layer 36 and the organic layer 38 are separated and the aqueous layer 36 combined with the portion of the untreated pre-precipitation liquor 40 and seeded 24 to induce boehmite precipitation 22. It will be appreciated that there may be provided multiple precipitation vessels connected in any arrangement, for example, series or parallel or a combination thereof. Further, it will be appreciated that there may be a temperature profile along the precipitation circuit.

It will be appreciated that the solvent extraction apparatus 32 may be replaced with an ion exchange apparatus or an electrolytic membrane apparatus or other apparatus to remove sodium ions from the process stream.

On removal of the precipitated aluminium hydroxide 26, the spent liquor stream 42 may be recycled back through the Bayer circuit or recontacted with a further solvent extraction step to include further precipitation of aluminium hydroxide. It will be appreciated that there may be provided a number of solvent extract apparatus 32 operating in series. Notably, the precipitation of aluminium hydroxide does not need rely on changes in temperature to induce states of supersaturation.

- -

The organic layer 38 is contacted 44 with an aqueous solution 46 to back extract sodium ions from the organic layer 38 to the aqueous solution 46. The aqueous solution of increased causticity 48 may then be used in the causticisation of further bauxite or in other places in the circuit as appropriate such as, for example, as a pre-treatment step in the washing of bauxite before digestion to remove impurities or in the washing of seed or oxalate. Back extraction of the organic layer 38 results in regeneration of the protonated form of the extractant. The extractant may then be re-used in further extraction steps.

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: Treatment of pre-precipitation liquor using ion exchange resins

Soda extraction tests were performed using Amberlite IRC86 - hydrogen form - CAS#211811 -37-9, weakly acidic resin 20-50 mesh, sourced from Sigma-Aldrich.

All resins were water conditioned prior to use by immersing the resins in de- ionized (Dl) water at ambient temperature for 24 hr. The slurried resins were collected on a Bϋchner funnel, drained free of water and allowed to air dry briefly (5-10 min).

Bayer pre-precipitation liquors (also known as green liquor or pregnant liquor) from the Applicant's Western Australian refineries were used in the extraction tests. The filtered green liquor (a sub-sample was analysed by titration) was heated to 80 ⁰ C and contacted with the water-conditioned resin at various concentrations ranging from 100 to 200 gL ^"1 as shown in Table 1. The liquor- resin slurry was filtered to produce the "treated green" liquor. Sub-samples were titrated for alumina concentration (A), the total caustic concentration (TC) and the total alkali concentration (TA).

- -

The results of single contact tests, shown in Table 1 , demonstrate that the resins can extract sodium ions from refinery Bayer pre-precipitation liquor with the concomitant transfer of protons to the liquor leading to neutralisation of the hydroxide. The resultant treated liquors were stable for at least 30 minutes.

Table 1. TC, TA and A/TC changes after contacting refinery green liquor with Amberlite IRC86. Note: both the TC and TA are reduced, unlike carbonation which decreases TC but TA remains constant.

The stability of the treated green liquor may be prolonged by the addition of gibbsite precipitation inhibitors such as calcia, sodium gluconate, or other inhibitors.

Table 2 presents the results of further soda extraction tests on a different Bayer pre-precipitation liquor using Amberlite IRC86 resin. The liquor was contacted with the resin at 80 ^° C for 2 min. Gluconate (5 drops per 10 ml_) was added to stabilize the treated liquor. All the treated liquors were stable at room temperature.

Table 2. TC, TA and A/TC changes after contacting a refinery green liquor with Amberlite IRC86. Note: the moisture content of the conditioned resin is 50 to 60% by weight.

The tests demonstrate that increased A/TC ratios and reduced TC concentrations can be achieved in treated refinery green liquors by increasing the extractant charge or increasing the number of contacts with fresh resin. The TC/TA concentration varies very little with treatment. Other tests have produced stable treated liquors with 150 gl_ ^"1 TC concentrations.

Contacting can be conducted at other temperatures, including higher temperatures with suitable contacting and separation equipment. It will be appreciated that the contact temperature may impact on the choice of resin.

Example 2: Boehmite precipitation from a combined liquor comprising treated and untreated refinery pre-precipitation liquor

Laboratory Batch Boehmite Precipitation Experiments

Batch boehmite precipitation experiments were conducted from a liquor prepared by combining a treated refinery pre-precipitation (green) liquor and the same refinery green liquor untreated (75:25 ratio). The liquor was obtained from one of the Applicant's Western Australian refineries.

The treated green liquor at 95 ^° C was injected into a 1 L autoclave containing boehmite seed (324 gl_ ^"1 ), calcia and untreated green liquor at 145 ^° C and under pressure. The temperature control setting on the autoclave was reset to 125 ^° C and the temperature of the mixture rapidly adjusted to this value.

Boehmite seed was prepared by a hydrothermal conversion of a commercial gibbsite to boehmite in a sealed, pressurised reactor at 200 ^° C using de-ionized water. 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 was used for all the experiments reported below.

The refinery green liquor was filtered through a Pall A/B glass filter paper; calcium hydroxide added (5 gl_ ^'1 ) and held for 30 min at 80 ^° C in a rotating water bath before adding to the autoclave.

The treated green liquor was prepared using water conditioned Amberlite IRC86 at 80 C in a single stage of contacting as described above. The filtrate was heated to 95 ^° C and added to a sealed cylinder connected to an autoclave.

The cylinder containing the treated green liquor was pressurized with argon and the contents injected into the autoclave containing the boehmite slurry at temperature and pressure and the temperature controller set to 125 ^° C.

The contents were stirred at 300 rpm for 6 hr at 125 C. The autoclave was rapidly cooled to -80 ⁰ C and contents filtered through Pall A/B glass filter paper. The filtrate was sampled, gluconate added and titrated at 25 C for the alumina concentration (A), TC and TA. The solids were washed thoroughly with hot Dl water and dried for 24 hr in a 60 C laboratory oven. The solids were analysed by XRD. The run was repeated using another green liquor sample from the same refinery. The normal variation in refinery liquor composition with time resulted in slightly different but comparable liquor compositions for the two runs.

In performing a control run, untreated green liquor was injected into the autoclave instead of the treated green liquor and the procedure described above repeated.

The liquor compositions from the precipitation experiments are shown in Tables 3 to 5. The precipitation yields, expressed as gl_ ^"1 AI ₂ O ₃ , are shown in Table 6.

- -

Table 3. Results of the control experiment; untreated green liquor injected into autoclave.

Table 4. Results of the first precipitation experiment with combined treated and untreated green liquor; treated green liquor injected into autoclave containing untreated green liquor slurried with boehmite seed.

Table 5. Results of the repeat precipitation experiment with combined treated and untreated green liquor.

Table 6. Precipitation yields.

The precipitation yields from the combined treated and untreated liquors are significantly higher than for the control run precipitating from untreated green liquor. Note that the duration of the control run was half an hour longer than the other runs with precipitation continuing during the extra period, hence the relative yield benefit from the treated liquor is expected to be even greater. The yields from the combined treated and untreated liquors are significantly higher than those reported in US 4595581 , also after 6 hr precipitation at 125 ^° C. Further, the best yields reported in US 4595581 were using amorphous boehmite gel as seed which would pose separation and transport problems in a commercial scale continuous process. In contrast, the results reported in tables 4 to 6 were obtained using large crystalline boehmite as seed.

Circuit Productivity Potential

The potential productivity of a boehmite precipitation circuit with a feed comprising of treated and untreated pre-precipitation liquor was investigated using an extensive database of Bayer properties and thermodynamic data, Bayer precipitation experience and in-house models built on chemical engineering principles.

The laboratory precipitation experiments described above were restricted to about 6 hr duration due to of site health and safety considerations. It is well established that greater precipitation yields can be achieved by longer residence times and larger seed surface areas. The solubility of boehmite at 125 C in an equivalent refinery liquor to that used in the above experiments is 0.345 (expressed as an A/TC ratio, g AI ₂ O ₃ per g Na ₂ CO ₃ ;). Thus, a final tank A/TC ratio of 0.4 is realistic, particularly given the relatively rapid desupersaturation observed in Tables 4 and 5. In the present case, the yield from an isothermal boehmite precipitation circuit with a combined treated and untreated green liquor feed of the same composition as Table 5 would be ~69 gl_ ^"1 (as AI ₂ O ₃ ), ignoring scale and solids overflow losses. This is comparable to commercial gibbsite precipitation circuits using similar refinery liquor. The theoretical yield in this case is ~78 gl_ ^"1 , as AI ₂ O ₃ . If

- O -

this boehmite precipitation circuit had inter-stage cooling down to 105 C, yields up to -89 gl_ ^'1 would be theoretically possible.

Example 3: Boehmite precipitation from impurity-free laboratory prepared Bayer liquors

Laboratory Batch Boehmite Precipitation Experiments

Laboratory batch precipitation tests were conducted in an autoclave using boehmite seed (500 gl_ ^"1 liquor), prepared as described above and laboratory prepared Bayer liquors. The autoclave was a gas-fired bomb that could be heated very rapidly, taking <5 min to reach 125 C from room temperature and cooled rapidly. The total time taken for cooling and filtering was ~ 2 minutes. In some runs, calcia was added to inhibit gibbsite precipitation. At higher temperatures it was found that calcia was not needed and the product was all boehmite. XRD analyses from initial tests showed that the solids produced during the short heat up and cool down was boehmite. The charged autoclave was rotated at temperature for 7 hr, then cooled rapidly to 110 C and filtered. The product was filtered through a 0.45 μm membrane and washed with hot deionised water. The filtrate was titrated for A, TC and TA values. The solid was dried overnight in an oven (60 ⁰ C) and characterized by XRD and DCS-TGA.

A range of precipitation temperatures (105 ^° C to 160 ^° C) and liquor compositions were tested. The liquors were prepared so as to allow comparison between equivalent refinery green liquor and various combinations of a treated green liquor and untreated green liquor. As a reference, the equivalent treated and untreated liquors were prepared to match the compositions for the 0 gl_ ^"1 and 200 gl_ ^"1 resin charges in Table 1 , respectively. All tests were of 7 hr duration and charged with 500 gl_ ^"1 boehmite seed from the same batch.

Table 7 compares the results from laboratory boehmite precipitation experiments at 125 ^° C for three different liquors: (1) a control, i.e. equivalent to a refinery green liquor; (2) an equivalent to a 50:50 treated and untreated green liquor and (3)

- -

100% treated green liquor. The results show that significant improvements to the precipitation yield can achieved by increasing the ratio. XRD and DCS-TGA identified the product solids as boehmite.

Table 7. Boehmite precipitation from laboratory prepared Bayer liquors at 125 C and 500 gL ^"1 seed loading.

Table 8 compares the results from laboratory boehmite precipitation experiments at 105 ^° C for a control liquor (i.e. equivalent to a refinery green liquor) and a 100% treated green liquor. XRD and DCS-TGA identified the product solids as boehmite.

Table 8. Boehmite precipitation from laboratory prepared Bayer liquors at 105 C and 500 gL ^"1 seed loading.

- -

The solubility of boehmite at 105 ^° C in an impurity free Bayer liquor of the same composition as used in the above experiments is ~0.265 (expressed as an AfTC ratio, g AI ₂ O ₃ per g Na ₂ CO ₃ ), estimated using models available in-house that have been built on an extensive Bayer properties data base. Thus for a boehmite precipitation circuit using impurity free Bayer liquor and with inter-stage cooling down to 105 C, yields up to ~89 gl_ ^"1 , as AI ₂ O ₃ , are theoretically possible.

Example 4: Effect of temperature on boehmite precipitation from impurity- free laboratory prepared Bayer liquors

Comparison of the results in Tables 7 and 8 suggest a temperature effect on the kinetics of boehmite precipitation that favours, on the basis of yield, operating at 125 ^° C rather than 105 C. Table 9 compares the precipitation yields at different temperatures from a low soda Bayer liquor, i.e. treated liquor. The results indicate that the kinetic temperature benefit, for the precipitation and extraction conditions investigated, plateaus between 115 C and 140 ^° C and drops slightly by 160 C. It will be appreciated that the temperature-yield profile will vary depending on factors such as the extraction temperature, initial liquor composition and other process variables. The decision on the optimal precipitation temperature and temperature profile should primarily be made on the basis of overall circuit yield, energy efficiency and optimizing the plant energy balance.

- -

Table 9. Temperature effect on boehmite precipitation from low soda, laboratory prepared Bayer liquors (500 gL ^"1 seed loading). * 95 ⁰ C precipitation experiments conducted under atmospheric conditions in a rotating water bath. * Yield includes significant gibbsite.