AU2008232308A1 - Method for precipitating boehmite - Google Patents

Description

WO 2008/116259 PCT/AU2008/000421 Method for Precipitating Boehmite Field of the Invention The present invention relates to a method for precipitating boehmite from a Bayer process solution. 5 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 10 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 15 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., 20 A1 2 0 3 .3H 2 0 or AI(OH) 3 , or as a monohydrate, i.e., A1 2 0 3

.H

2 0 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 25 of alumina is present at digestion, under current practises, the majority of precipitated alumina is gibbsite.

WO 2008/116259 PCT/AU2008/000421 -2 The precipitation reaction can be generally represented by the following chemical equation: AI(OH)t (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 5 0.7, typical of a green liquor, to about 0.4 (where A represents the alumina concentration, expressed as gL 1 L of A1 2 0 3 , and TO represents total caustic concentration, 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 10 system approaches equilibrium. It is known that the TC and TA (where TA represents total alkali concentration, expressed as gL 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 15 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 20 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. 25 Although all commercial alumina production involves the precipitation of the aluminium trihydroxide, i.e. gibbsite, the calcination of boehmite requires less WO 2008/116259 PCT/AU2008/000421 -3 energy than the calcination of gibbsite, and consequently, it is desirable to precipitate digested alumina as boehmite. The there principal types of methods previously used to produce boehmite can be summarized as follows: 5 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 10 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. The energy cost of calcining boehmite to smelter grade alumina (SGA) is 15 estimated to be between 1.45 GJIt 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 20 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.

WO 2008/116259 PCT/AU2008/000421 -4 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 5 heating a seeded sodium aluminate suspension to a temperature of about 115 *C to 145 "C and separating the boehmite precipitate from the suspension. These precipitation temperatures and pressures are substantially higher than for conventional Bayer gibbsite precipitation (typically 60 *C to 80 "C) and would require specialist equipment not currently used in Bayer precipitation. 10 W01 998/58876 teaches a process for precipitating boehmite from supersaturated sodium aluminate solutions at less than 100 *C, 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 15 their batch suspension gradually desupersaturated. The reported yields after 24 hr precipitation ranged from 35 gL 1 as A1 2 0 3 and 14.5 gL as At 2

O

3 ; after 96 hr the reported yields ranged from 48 gL' as A1 2 0 3 and 24.6 gLi as A1 2 0 3 . Research from the National Technical School of Athens also shows that higher yields can be obtained at higher temperatures and solids loadings [Panias at al., 20 Travaux, 29(33), 94, 2002; Panias D. et a/., Travaux, 26(30), 147, 1999; Panias and Paspaliaris, Erzmetal/ 56 (2), 75, 2003]. The highest yield after 24 hr precipitation reported in the literature [Panias et at, Light Metals, 97 (2001); and again in Panias and Paspaliaris, Erzmetall, 56 (2), 75, (2003) and Panias at al., Travaux, 29(33), 94, (2002)] is 60 gt- 1 . This was achieved at a very high solids 25 loadings of 1200 gL'. Other data from their research [Panias D. ot al., Travaux, 29(33), 94, (2002), Panias D. at at., Travaux, 26(30), 147, (1999)] shows a yield of 35 gL' after 24 hr precipitation at 90 *C in a synthetic sodium aluminate liquor (AfTC=0.64, TC=205 gL') seeded with 230 gL 1 boehmite WO 2008/116259 PCT/AU2008/000421 -5 All the above boehmite precipitation studies have focussed on precipitating from synthetic and pure sodium aluminate solutions at high ATC 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 5 batch yields after a specified holding time in the precipitator. 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. 10 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 commercially viable alternative. A further issue holding back the development of commercial boehmite 15 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. Vernon et al. [6th AQW, 33, (2002)] investigating the precipitation kinetics of gibbsite in pure synthetic sodium aluminate liquors observed a phenomenon 20 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 et al, [ibid] also observed that the solubility gap decreases as the TC is lowered. 25 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.

WO 2008/116259 PCT/AU2008/000421 -6 This metastable state for boehmite has also been reported by Loh et al. [ibid]; they report an apparent solubility after 216 hr of precipitation that is 2.3 times the boehmite solubility. Skoufadis et a/. [(bid] observed that the boehmite solubility gap in pure synthetic 5 sodium aluminate liquors decreases with TC. Loh et al. ibidd] report that the precipitation rates for both boehmite and gibbsite increase with decreasing TC in pure synthetic sodium aluminate liquors. Panias et al. [Light Metals, Minerals, Metals & Materials Society, 97-103, (2001)] reported experiments where boehmite was precipitated under conditions of 10 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, 15 carbonation was used in earlier times for the precipitation of gibbsite but has been replaced by modem seeded cooling Bayer precipitation processes because of the costs of sodium hydroxide consumption. Additionally, carbonation decreases the TC but not the TA, which continues to affect the solubility and precipitation kinetics. 20 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 TO 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 25 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 WO 2008/116259 PCT/AU2008/000421 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. The preceding discussion of the background to the invention is intended to 5 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. Disclosure of the Invention 10 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 15 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 20 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 25 precipitating boehmite from a Bayer process solution, the method comprising the steps of: WO 2008/116259 PCT/AU2008/000421 -8 treating at least a portion of a first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor; combining at least a portion of the treated first spent liquor with at least a portion of green liquor 5 precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor; and separating at least a portion of the boehmite and the second spent liquor. Advantageously, the method allows greater boehmite productivity from a green liquor coming from digestion than methods of the prior art by enabling further 10 recovery of alumina values from the first spent liquor. Advantageously, the present invention reduces hydroxide concentrations in the treated first spent liquor and in the combined treated first spent liquor and green liquor, thus reducing both the TC and TA of these liquors. Advantageously, the present invention increases the A/TC of the first spent liquor, 15 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 decreases with TC and TA and advantageously, the present invention can provide a combined treated spent 20 liquor and green liquor with a lower TC and TA than green liquor, thereby providing conditions more favourable to boehmite 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 WO 2008/116259 PCT/AU2008/000421 -9 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 5 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. Preferably, the step of: 10 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 at least the portion of the first spent liquor. Preferably, the step of: 15 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 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 20 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.

WO 2008/116259 PCT/AU2008/000421 -10 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 neutralizing or removing hydroxide ions from the first spent liquor. 5 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. 10 Advantageously, the media used for reducing the TC of the spent liquor is regenerated for further use. Preferably, the step of: combining at least a portion of the treated first spent liquor with at least a portion of the green liquor, 15 comprises the step of: combining the treated first spent liquor with the green liquor in a ratio of between about 1:100 and 3:1. More preferably, the step of: combining at least a portion of the treated first spent liquor with at least a 20 portion of the green liquor; comprises the step of: WO 2008/116259 PCT/AU2008/000421 -11 combining the treated first spent liquor with the green liquor in a ratio of between about 1:5 and 2.5:1. 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 5 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 and alumina concentration. In one form of the invention, the step of combining at least a portion of the treated first spent liquor with at least a 10 portion of the green liquor comprises the steps of: providing a first portion of green liquor and a second portion of green liquor combining the first portion of the green liquor with the treated spent liquor; 15 directing the second portion of the green liquor to a later stage in the boehmite precipitation circuit. In an alternative form of the invention, the step of combining at least a portion of the treated first spent liquor with at least a portion of the green liquor 20 comprises the step of providing a first portion of treated spent liquor and a second portion of treated spent liquor; WO 2008/116259 PCT/AU2008/000421 -12 combining the first portion of the treated spent liquor with the green liquor; directing the second portion of the treated spent liquor to a later stage in the boehmite precipitation circuit. It will be appreciated that different quantities of green liquor or treated spent liquor 5 can be different to different stages in the boehmite 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 boehmite yield and quality. It will be 10 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 and the seed and liquor properties. 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 15 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. 20 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.

WO 2008/116259 PCT/AU2008/000421 - 13 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: 5 treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor; combining at least a portion of the treated first spent liquor with at least a portion of a green liquor; and precipitating boehmite from the combination of the green liquor and the 10 treated first spent liquor and producing a second 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. 15 In one form of the invention, the method comprises the further step of: seeding the combination of the green liquor and the treated first spent liquor with boehmite. In a second form of the invention, the method comprises the further step of: seeding the green liquor with boehmite. 20 In a third form of the invention, the method comprises the further step of: seeding the treated first spent liquor with boehmite.

WO 2008/116259 PCT/AU2008/000421 -14 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-'. Many methods of producing boehmite seed exist. Preferably, the boehmite seed 5 is recycled from the Bayer precipitation circuit. Preferably, the step of: precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor; is conducted at a temperature less than about 105 *C. 10 In a preferred form of the invention, the combined green liquor and treated spent liquor is at a temperature of between about 95 0 C and 105 C when entering the precipitation circuit. In one form of the invention, the combined green liquor and treated first spent liquor may be cooled after entering the precipitation circuit. 15 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 20 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: precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor; WO 2008/116259 PCT/AU2008/000421 -15 is preceded by the step of: sonication of the treated first spent liquor and/or the combination of the treated first spent liquor and the green liquor with or without the presence of boehmite seed. 5 In one form of the invention, the method comprises the further step of: adding a gibbsite precipitation inhibitor to the treated first spent liquor or the combination of treated first spent liquor and 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 10 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 15 gibbsite, boehmite precipitation will be less affected by certain organic compounds than gibbsite precipitation. In preferred forms of the invention, the gibbsite precipitation inhibitor is selected from the group comprising gluconate and tartrate. Alternatively, the gibbsite precipitation inhibitor may be provided in the form of calcia. In the context of the present invention, the term calcia shall be understood 20 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 25 precipitation from Bayer liquor. Advantageously, pre-precipitation liquor may contain calcia, which is known to affect the induction time of gibbsite precipitation.

WO 2008/116259 PCT/AU2008/000421 -16 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. 5 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 liquor; combining at least a portion of the treated first spent liquor with at least a portion of the green liquor and 10 precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent 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 combination of the green liquor and the treated first 15 spent liquor and producing a second 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 liquor; combining at least a portion of the treated first spent liquor with at least a 20 portion of the green liquor; and precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor; WO 2008/116259 PCT/AU2008/000421 -17 are repeated, it may not be necessary to add further gibbsite precipitation inhibitor to the treated first spent liquor after the step of precipitating boehmite 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 5 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 10 is precipitated instead of boehmite, and to return the seed recycle to a predominantly boehmite form. 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; 15 comprises the step of: contacting at least a portion of 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. 20 The present invention advantageously provides the ability to control the TC and TA of the treated spent liquor and hence the combination of the green liquor and 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, 25 agitation, properties of the liquor and the presence of other species in the liquor.

WO 2008/116259 PCT/AU2008/000421 - 18 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. 5 Advantageously, water may be co-extracted 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 retuming to the digestion circuit and thus reducing the spent liquor evaporation requirements. Preferably, the extractant is provided in the form of a weak acid. 10 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 15 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 I H,1 H-perfluorononanol, 20 1 H, 1 H ,9H-hexadecafluorononanol, 1,1,1-trifluoro-3-(4-tert-octylphenoxy)-2 propanol, 1,1,1-trifluoro-2-(p-tolyl)/sopropanol, 1-(p-toly)-2,2,2-trifluoroethanol, hexafluoro-2-(p-tolyl)isopropanoi, 2-(methyl)-2-(dodecyl)tetradecanoic acid, 3-(perfluorohexyl)propenol and 1-(1,1,2,2-tetrafluoroethoxy)-3-(4-tert octylphenoxy)-2-propanol, tet-octylphenyl, para-nonylphenol, para-tert 25 butylphenol, para-tert-amylphenol, para-heptylphenol, para-octylphenol, para (alpha,alpha-dimethylbenzyl)phenol (4-cumylphenol), 2,3,6-trimethylphenol, 2,4-di-tert-butylphenol, 3,5-di-tert-butylphenol, 2,6-di-tert-butylphenol, 2,4-di-tert pentylphenol (2,4-di-tert-amylphenol), 4-sec-butyl-2,6-di-tert-butylphenol, 2,4,6-tri- WO 2008/116259 PCT/AU2008/000421 - 19 tert-butylphenol, 2,4-bis(alphaalpha-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. 5 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. 10 It should be appreciated that the extractant concentration will depend on a number of factors including the intended amount of induced supersaturation 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 lonisable proton (as well as the pH and salt content of 15 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. 20 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 25 the first spent liquor and enhance precipitation in a secondary manner.

WO 2008/116259 PCT/AU2008/000421 -20 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 5 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, 10 xylene, stilbene, 1-octanol, 2-octanol, 1-decanol, iso-octyl 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, iso-decanol, iso-tridecanol, 2-ethyl-1-hexanol, kerosene and other hydrocarbons commercially available under the names Escaid 100, Escaid 15 110, Escaid 240, Escaid 300, Isopar L, Isopar M, Solvesso 150 and Exxsol DI 10 (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. 20 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 25 contacted and the loading of the extractant in the substantially water-immiscible solution.

WO 2008/116259 PCT/AU2008/000421 -21 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 5 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 extractant and the metal cations to form a metal cation-depleted aqueous phase and a 10 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 15 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 20 methods. It will be appreciated that the steps 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 25 liquor into the substantially water-immiscible solution, WO 2008/116259 PCT/AU2008/000421 -22 may be repeated. Where the steps of: contacting the first spent liquor with a substantially water-immiscible solution comprising an extractant; and 5 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: contacting the first spent liquor with a substantially water-immiscible solution comprising an extractant; 10 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 15 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. 20 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.

WO 2008/116259 PCT/AU2008/000421 -23 Preferably, the method comprises the further steps of: separating the stripping solution and the substantially water-immiscible solution. Advantageously, the step of 5 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. 10 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 15 regeneration of hydroxide requires no further chemicals for recausticisation. In one form of the invention, 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 the step of: 20 contacting at least a portion of the first spent liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble; and WO 2008/116259 PCT/AU2008/000421 -24 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 5 many ionic functional groups per molecule. Cation-exchange resins can be either a strong-acid type containing sulfonic acids groups (RSOjH*) 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 10 (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 weak-acid cation exchange resin. Preferably, the exchangeable ion on the solid support is a proton. 15 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 20 IRG86 - hydrogen form, Amberlite IRC50 - hydrogen form and Lewatit CNP105 hydrogen 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.

WO 2008/116259 PCT/AU2008/000421 -25 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; 5 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 10 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: 15 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, 20 are repeated, the step of: contacting the first spent liquor with a solid support having an exchangeable ion, wherein the solid support is substantially water insoluble; may be performed with different solid supports.

WO 2008/116259 PCT/AU2008/000421 -26 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 5 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: exchanging a sodium cation present in the first spent liquor with an ion on 10 the solid support. 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 15 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: 20 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: WO 2008/116259 PCT/AU2008/000421 -27 applying a potential between a first region comprising at least a portion of 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 5 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 10 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 15 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. 20 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 25 with a cathode.

WO 2008/116259 PCT/AU2008/000421 -28 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 psrmeable membrane should be substantially 5 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, 10 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. 15 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 20 pK 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 25 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 WO 2008/116259 PCT/AU2008/000421 -29 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, 5 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 10 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/cm 2 15 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 20 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 25 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 WO 2008/116259 PCT/AU2008/000421 - 30 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 5 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 10 introduced 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. 15 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 20 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 25 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.

WO 2008/116259 PCT/AU2008/000421 - 31 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. 5 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 method in accordance with a first 10 embodiment of the present invention; Figure 2 shows the experimentally measured drop in TC of spent liquor after 1:1 contacting for 10 minutes at 60 "C with different concentrations of the TOP extractant in the iso-octanol solvent; and Figure 3 is a schematic flow sheet of a method in accordance with a second 15 embodiment of the present invention. Best Mode(s) for Carrying Out the Invention The invention focuses on the precipitation of boehmite from a Bayer process solution by treating spent liquor to reduce the total alkalinity and total caustic concentration of the spent liquor and then recombining at least a portion of the 20 treated spent liquor with at least a portion of the green liquor to precipitation. The remaining portions of the treated spent liquor and green liquor can be distributed along the precipitation stage as is determined to be most beneficial. Figure 1 is a schematic flow sheet showing a method in accordance with a first embodiment of the present invention comprising the steps of: WO 2008/116259 PCT/AU2008/000421 -32 digesting bauxite in a caustic solution 10; separating the mixture 12 to residue 14 and green liquor 16; combining the green liquor 16 the treated spent liquor 18; seeding the combination of the green liquor 16 and treated spent liquor 18 5 with boehmite recycled from a classification circuit 20 and precipitating boehmite in a precipitation circuit 22 and forming a suspension; separating the suspension in the classification circuit 20 into boehmite product 24, boehmite seed recycle 26 and spent liquor 28; separating a portion of the spent liquor 28 into a first portion 30 and a 10 second portion 32; treating the second portion of the spent liquor 32 in a soda reduction unit 34 with a regenerated media containing an extractant 36; and combining the treated spent liquor 18 with green liquor 12 as described above. 15 The first portion of the spent liquor 30 is treated in the normal manner and the second portion of the spent liquor 32 is contacted at a temperature less than the boiling point of the liquor with the soda extraction media In apparatus 34. The used soda extraction media 38 is contacted with an aqueous solution in the recovery and regeneration unit 40 to back extract sodium ions to the aqueous 20 solution. The aqueous solution is then processed as required to produce a stream containing the recovered soda values 42 in a form suitable for return to the process; for example, it may be returned to the conventional spent liquor circuit 30 for further digestion of bauxite or used for washing seed or oxalate or for washing bauxite. Back extraction of the used soda extraction media 38 results in 25 regeneration of the protonated form of the extractant in the media. The WO 2008/116259 PCT/AU2008/000421 -33 regenerated soda extraction media may then be re-used in further extraction steps and returned 36 to the soda extraction unit 34. The treated spent liquor 18 is heated by heat exchanger 44, combined with the green liquor 16 and passed to the precipitator 22 for precipitation of boehmite at a 5 temperature between about 95 "C and 105 *C. The combined green liquor 16 and treated spent liquor 18 may be cooled in the precipitator 22. The ratio of the green liquor to treated spent liquor is in the range from 5:1 to 1:2.5. The combined green liquor and treated spent liquor will have an A/TC in the range 0.77 to 0.55. The combination of the treated spent liquor 18 and the green liquor 10 16 is seeded with boehmite to facilitate boehmite precipitation. The seed charge is in the range from 50 g/L to 1200 g/L. The treated spent liquor 18 may be sonicated 46 to facilitate boehmite precipitation, with or without the presence of boehmite seed. Alternatively, or in addition to, the combination of the green liquor 16 and treated spent liquor 18 may 15 be sonicated 46 to facilitate boehmite precipitation. Calcia may be added to the treated spent liquor stream 18 to reduce the proportion of gibbsite in the boehmite product. 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 20 to limit the true scope of this invention, but rather are presented for illustrative purposes. Example 1: Soda extraction by ion exchange resins Soda Extraction Using Resins Initial tests trial 3 different types of resins, all sourced from Sigma-Aldrich: (1) 25 Amberlite IRC86 - hydmgen form - CAS#211811-37-9 (Weakly acidic resin 20-50 mesh); (2) Amberlite IRC50 - hydrogen form - CAS#9002-29-3 (Weakly acidic resin with carboxylic acid functionality) and (3) Lewatit CNP105- 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.

WO 2008/116259 PCT/AU2008/000421 -34 All resins were water conditioned prior to use by immersing the resins in DI water in a 45:100 "/y ratio at ambient temperature for 24 hr. The slurred resins were then collected on a BOchner funnel, drained free of water and allowed to air dry briefly (5-10 min). 5 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 *C) and contacted with the fresh water-conditioned resin in concentrations ranging from 165 to 220 gL-' at the selected temperature. In some tests, the 10 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. TC, as gL' TA, as gL~ Alumina, as A/TC Na 2 CO% Na 2 COs gL' A1,0 3 initial spent 208.43 250.88 89.44 0.4291 liquor _______ AR5te 137.25 179.15 87.00 0.6339 Amberlite 140,61 182.80 88.72 0.6310 IRC86 ______ Lewat CNP 165.00 203.16 82.71 0.5013 105 ______ _____ _ Table 1. A/TC changes at 60 "C with one contact at 215 gL- 1 . Sample TC, as gL~ TA, as gLJ Alumina, as gL A/TC Na 2 CO3 Na 2

CO

3 Al203 Refiner spent 226.83 274.13 98.14 0.4327 liquor I Treated Refinery Spent Liquor 1 129.92 174.54 91.62 0.7052 (165 gL~') Refine Spent 221.26 267.94 93.75 0.4237 Liquor #2 ___ ____ Treated Refinery Spent Liquor #2 126.42 171.83 89.15 0.7052 (165 gL'') Refinery Spent 224.51 272.21 96.62 0.4304 Liquor #3____ ____ Treated Refinery Spent Liquor #3 118.69 165.53 91.05 0.7671 (200 gL 1 )________ ________ ____ ____ WO 2008/116259 PCT/AU2008/000421 - 35 Table- 2. A/TC changes at 80 *C with two contacts with Amberlite IRC86 at loadings ranging from 165 to 200 gL- . The results show that the resins can extract sodium ions from refinery Bayer 5 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 AITC of a spent liquor from 0.43 to 0.77, and reduce the TC from 224.5 to 118.7 gL Na 2

CO

3 . It would be expected that extra contacting stages and increased resin 10 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 "C in polypropylene bottles rotating in a bath. The seed was added 15 after the temperature of the treated spent liquor was equilibrated to 95 *C. Seed loadings from 180 gL 1 to 950 gL- 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 *C.. The filtered solids were washed 20 with hot de-ionised water and oven dried at between 60 "C and 100 "C. 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 "C using de-ionized water. The material produced was analysed by XRD, TGA and DSC and found 25 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 aC. Neither the solids content nor the liquor WO 2008/116259 PCT/AU2008/000421 - 36 composition showed any significant change after 6 hr and negligible yield of boehmite (- 1 gL-1) after 48 hr. TC TA A120 3 AlIO Average Gibbsite Description as gL' as gL as gL-1 AITC Yield Al203 i Na 2 C0 3 NaCO A1 2 0 3 (gL) Ld Preipitate Green Liquor - 203.0 245.2 146,1 0.720 - .. Initial _ _ _ _ 5hr R1 204.0 247.0 138.8 0.680 8.03 5hr R2 204.9 247.8 139.8 0.682 7.64 24hr R1 205.6 248.9 120.9 0.588 26.8 26.8 33% 24hr R2 206.1 249.1 121.2 0.588 26.7 48hr RI 207.5 250.2 115.9 0.558 32.8 32.9 31% 48hr R2 205.9 249.1 114.7 0.557 33.1 Table 3. Boehmite precipitation from treated refinery green liquor at 95 "C and 180 gL 1 6 seed loading. TC TA Al 2 0 AJ 2 0 3 Average Gibbsite In Description as 9L as guL as gLf' AlTC Yield Yiel3 Precipitate Na 2

CO

3 Na 2

CO

3 A1 2 03 (gL") -1i(%) Initial Green 205.0 251.0 145.5 0.710 - Liquor I_____ I____ I__ _ _ _ _ __ _____ 24hr R1 207.2 253.2 114.4 0.552 32.3 32.0 2% 24hr R2 206.4 252.6 114.5 0.555 31.7 2 2 Table 4. Boehmite precipitation from treated refinery green liquor at 95 *C and 600 gL 1 seed loading. TC TA A1 2 0 3 A1 2 03 Gibbsite Description as gL as gL" as gL- A/TC Yield In Na 2

CO

3 Na 2

CO

3 A1 2 0 3 (gL) Precipitate spet quor 126.4 171.8 89.1 0.705 - 3hr ppt 125.2 169.1 77.7 0.620 10.7 14% 5hr ppt 124.8 168.4 73.6 0.590 14.6 0% 24hr ppt 125.6 168.6 60.7 0.483 28.1 6% 51hr ppt 125.5 168.6 52.8 0.421 36. - 0% 10 Table 5. Boehmite precipitation from treated refinery spent liquor at 95 *C and 500 gL 1 seed loading.

WO 2008/116259 PCT/AU2008/000421 -37 TC TA A1 2 03 A1 2 0 3 Gibbsite Description as gL' as gL~' as gL'' A/C Yild in Na 2

CO

3 Na 2

CO

3 Al 2 03 (L~I) Precipitate spnt eq or 125.2 170.9 88.8 0.704 3hr ppt 126.9 171.7 78.4 0.618. 10.9 24% 5hr ppt 126.6 171.6 74.7 0.590 14.4 2% 24hr ppt 127.4 170.9 60.9 0.478 28.5 6% 51hr ppt 127.5 171.5 53.7 0.421 35.7 1% Table 6. Boehmite precipitation from treated refinery spent liquor at 95 "C, 500 gL' seed loading and - 30 ppm gL-' calcia. TC TA Al 2 03 A' 2 0 Average Gibbsite Description as gL-1 as gL-1 as gL~ A/TC Yield Yield ip Na 2

CO

3 Na2CO 3 A1 2 0 (gL') dNpre)tate initial LXP 226.8 274.1 98.1 0.433 - - LXP-Na 129.9 174.5 91.6 0.705 - - Extn 2 Stage____ 750 seed, 132.7 179.1 76.2 0.575 17.0 50r, Ri 17.0 2% 750seed, 132.4 179.0 76.0 0.574 17.0 6hr, R2 750 seed, 132.9 179.3 54.3 0.409 38.6 48hr, R1 38.6 3% 750 seed, 131.4 177.5 53.6 0.408 38.6 4Bhr,_R2 ______ ______ ____ ____ ____ ____ _____ Table 7. Boehmite precipitation from treated refinery spent liquor at 95 "C and 750 gL-1 5 seed loading. TC TA A1 2 0 3 A1 2 0 3 Average Gibbsite Description as gL' as gL- as gL' AITC Yield A1 2 0 3 in Na 2 COs Na 2

CO

3 A1 2 0 3 (gL) Yield precipitate gi) (gU'*) N% initial LXP 226.8 274.1 98.1 0.433 - - Exn 2 Sage 129.9 174.5 91.6 0.705 - 950 seed, 131.9 178.9 74.0 0.561 18.8 5hr, RI1____ 19.2 0% 950 seed, 129.6 175.4 71.8 0.654 19.6 Shr,_R2 ____;1_____ 950 seed. 120.8 163.1 47.1 0.390 4. 090 48hr,_R2 __ _ _ _ _ _ _ __ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ WO 2008/116259 PCT/AU2008/000421 - 38 Table 8. Boehmite precipitation from treated refinery spent liquor at 95 *C and 950 9L seed loading TC TA A1 2 0 A1 2 O Gibbsite Description as gL' as gL as gL~1 A/TC Yield in Na2CO 3 Na 2

CO

3 A1 2 0 3 (gL') pre Pltat Initial Green 205.0 251.0 145.5 0,710 - Liquor LTP _____ ____ _________ Initial Spent 220.3 266.5 94.7 0.430 - Liquor LXP ____ ____ Na Extn LXP 130.5 174.7 89.0 0.682 - 60:40 Green and Spent 175.0 220.0 122,0 0.697 - Liquor Blend 24hr R1 177.3 222.0 93.6 0.528 29.6 1% 92hr R1 178.6 222.9 77.7 0.435 45.8 0% Table 9. Boehmite precipitation from combined green liquor and treated refinery spent 5 liquor ( 60:40) at 95 *C and 500 gL-1 seed loading. TC TA A120 A1203 Average Gibbsite Description as gL- as gL- 1 as gL'' A/TC Yiel d precipitate Na2CO 3 Na 2

CO

3 A1 2 0 3 (gL) gl) (%) Initial Green 205.0 251.0 145.5 0.710 - - Initial Spent 220.3 266.5 94.7 0.430 - - Liquor______ __________ ____ __ Treated Spent 130.5 174.7 89.0 0.682 - Liquor 60/40 Green and Treated 175.0 220.0 122.0 0.697 - - Liquor Blend 222.5 24hr R1 177.9 222.5 93.3 0.524 30.2 24hr R2 178.0 222.6 94.9 0.533 28.7 0 92hr R1 176.9 221.7 76.5 0.433 46.3 45.5 1% 92hr R2 178.6 223.3 78.8 0.441 44.7 Table 10.. Boehmite precipitation from combined green liquor and treated refine spent liquor ( 60:40) at 95 "C and 500 gL) seed loading with an excess of calcia (~ 1 gL).

WO 2008/116259 PCT/AU2008/000421 -39 A number of experiments were also conducted using a laboratory prepared sodium aluminate solution instead of the refinery liquors. Refinery liquors contain significant organic and inorganic impurities. TC,as gL~' TA, as gL A10 3 , as Average Gibbsite in Description Na2CO NN20 L-A23 ATC A1 2 0 3 Yield Prdc Na 2 COs gL AO(gL) Product D 130.4 169.3 87.8 0.673 0 detected 3 133.1 172.9 72.8 0.547 16.43 trace 6 133.6 172.7 66.1 0.494 23.34 trace 18 135.1 177.1 56.8 0.421 32.86 trace 24 136.1 175.4 56.3 0.414 33.77 trace 96 136.2 176.0 44.8 0.329 44.86 not 5 Table 11. Boehmite precipitation from a laboratory prepared sodium aluminate liquor of low TC at 95 *C and 500 gL- 1 seed loading. TCas gL~' TA, as gL~1 A1 2 0 3 , as Average Gibbsite in Description Na 2 CO Na 2

CO

3 gL'2 AA2C -1i Product 0 129.5 168.2 87.9 0.679 0.00 dete ed 3 132.3 172.9 70.7 0.534 18.78 trace 6 131.3 171.6 64.2 0.489 24.61 trace 18 135.5 173.9 60.6 0.447 30.04 trace 24 133.3 173 54.8 0.411 34.71 trace 96 139.4 179.2 40.2 0.288 50.63 detected Table 12. Boehmite precipitation from a laboratory prepared sodium aluminate liquor of low TC at 95 *C and 500 gL- 1 seed loading with an excess of calcia (1 gL 1 ). 10 The batch boehmite precipitation results show that for refinery liquors yields between 30 gL' and 40 gL~1 as A1 2 0 3 can be readily achieved; yields as high as 45 gL~ as A1 2 0 3 were also observed using plant liquors. Final A/TC values in the range 0.42 to 0.39 were observed. It is believed that optimisation of the 15 precipitation conditions could raise the yields even further.

WO 2008/116259 PCT/AU2008/000421 -40 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' as Al 2 0 3 were observed. Final ATC values as low as 0.288 were observed. 5 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. 10 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 PlusTm, ASPEN Technology Inc. software 15 process simulation software with state-of -the-art physical properties packages, including added Bayer process properties and unit operations built inhouse. Six cases were examined based on a 1000 kLh green liquor flow from digestion, with the following composition: A 146 gL A1 2 0 3 , 20 TC 205 gLi as Na 2

CQ

3 , TA 251 gLO as Na 2

CO

3 , total organic carbon 22 gL as C, chloride 11 gLO as NaCl, sulphate 22 gL1 as Na 2

SO

4 . 25 Case #1 considers gibbsite precipitation from the green liquor. There was no second stage precipitation of boehmite from treated spent liquor. The analysis WO 2008/116259 PCT/AU2008/000421 - 41 assumes a yield of 32 gL" as A1 2 0 3 and is based on the kinetic data from Table 4 after 24 hours precipitation. Case #2 is comparable to Case #1, with the addition that a treated spent liquor stream is generated and combined with the green liquor flow in a ratio of 0.667:1. 5 The lower yield of 29.6 gVL as A1 2 0 3 is used in this case and is based on data in Table 9 after 24 hours precipitation. In both experimental data sets, the boehmite seed loading was 500 gL'. Note: the lower A concentration into precipitation. Case #3 is comparable to Case #2 but with a higher flow of treated spent liquor being returned. The treated spent liquor flow was increased to the point where 10 the feed to the precipitator was similar to that in table 5. For the analysis, a yield of 28.1 gL as A1 2 0 3 was used, i.e. after 24 hrs precipitation in Table 5. Case #4 considers the plausible scenario of 40 gL 1 as A1 2 0 3 in precipitation and a green liquor to treated spent liquor flow split of 60:40. Case #5 is an extension of Case 4#, with a green liquor to treated spent liquor 15 flow ratio increased to 1:1.5. Case #6 represents a best case scenario on the limited data available, it assumes 51 gL' yield as A1 2 0 3 in precipitation, consistent with the data for pure sodium aluminate in Table 12 and a green liquor to treated spent liquor flow ratio of 100:85. This flow ensures that the composition of the combined feed falls 20 within the range of experimental data and that the final A/TC is >0.29, i.e. the lowest observed in our experimental data. Note: the analyses were constrained to ensure that the liquor compositions remained within the range observed In the experimental program; further optimization studies are likely to identify conditions giving higher yields. 25 Table 13 provides estimated green liquor yields when utilising the invention with soda extraction by ion exchange resins. The production is based on a green liquor flow from digestion of 10OOkL/h as described above. It should be noted that the following estimates are based on the models and data available on this system.

WO 2008/116259 PCT/AU2008/000421 -42 Green Liquor Yield, (g Production (as t/h Case precipitated as Al2% boehmite) based on per L of green liquor) 1OOOKU green liquor from digestion Case #1 32 37.6 t/h Case #2 49.3 68.1 t/h Case #3 84.3 99.2 Case #4 66.7 78.4 Case #5 100 117.6 Case #6 94.4 111 Table 13. Calculated Bayer circuit green liquor yields. Cases #1 to #3 illustrate the benefits and flexibility of the invention. 5 Case #4 shows a case with yields comparable to commercial Bayer plants precipitating gibbsite and using a Western Australian Bauxite. In this case, the energy benefit would be of the order of 1.5 GJ/t alumina. As demonstrated by the laboratory results, this scenario is expected to be feasible. Cases #5 and #6 are examples of potential of the invention to both increase the 10 yield of the process and to make significant energy savings. Example 2: Soda extraction by solvent extraction Spent liquor from one of Alcoa's Westem Australian operations was treated by solvent extraction with the organic solvent iso-octanol (Exxon-Mobil, 'Exxal 8') and 15 the extractant 4-tert-octylphenol (97%, Sigma-Aldrich, 'TOP'). The practical upper loading limit of the TOP in the iso-octanol solvent, with the refinery spent liquor system used in the test work, was determined to be between IM 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 20 resultant organic phase after contacting with the aqueous phase, .e.g. thickening/clouding, separation characteristics or crystallization. Figure 2 below shows the TC drops observed when a 0.5 A/TC spent liquor was contacted for 10 min at 60 *C in 1:1 ratio with the solvent containing various amounts of TOP.

WO 2008/116259 PCT/AU2008/000421 -43 A sample of filtered, refinery spent liquor was contacted with an equal volume of 0.75M 4-tert-octylphenol in iso-octanol, previously saturated with water, for 10 min at 60 "C. The phases were settled and the treated liquor separated. The process was repeated with fresh solutions of 0.75M 4-tert-octylphenol in iso-octanol two 5 further times. Extraction kinetics for 4-tert-octylphenol in iso-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 14 for three spent liquor samples. The increase in alumina concentration is consistent 10 with the reported water removal (US6322702) during extraction. . . ... ...... ........ . .. . .1. A....AI.. I .7... L..... ~ ~ J-715 39 _0.----. fE7Z ZL " ~ 17.2 I 223.75 _[ 1o.04 LI 0.598. L 2..ZL- 12 4929L IL 20.87r 107.08 .JL_ 0715 2 2-g.?9 1 281.08 JL_102.19_JLo44o -L.. L 1Z 2-i. e.9s IL 260.08-_JL_10.339_JL_.517 ;[hZ.:2Z~t~~3 162.21 21_8.9_____115.72L_1 0.713 3 o 37IZ.Z9~ ?4z.61L._28s12__ ~L .103.55 4 0.41 . . ... L P91 _2 2 A.107.8__L 0.514 i~ZZI.F~27L.5~ 0 _3.04_ L1137.4_t__o .6?At 21916 ._115.80 __ 0.7_13 Table 14. A/TC changes at 60 "C with three contacts. The 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. 15 it is anticipated that increasing the number of contact stages will decrease the TC of the spent liquor to even lower values than shown in the above Table. 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.

WO 2008/116259 PCT/AU2008/000421 -44 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 5 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 A1 2 0 3 , 10 TC 205 gL 1 as Na 2

CO

3 , TA 251 gL 1 as Na 2

CO

3 , total organic carbon 22 gL 1 as C, chloride 11 gL 1 as NaCI, sulphate 22 gL 1 as Na 2

SO

4 . 15 Case #1 considers only green liquor only with no treated spent liquor. The analysis assumes a yield of 38 gL', i.e. and based on longer holding times in Table 4. Case #2 is comparable to Case #1, with the addition that a treated spent liquor stream is generated and combined with the green liquor flow in a ratio of 0.9:1. 20 The lower yield of 35 gUL as A1203 is used in this case. Case #3 is comparable to Case 2# but with a higher flow of treated spent liquor being returned. The treated spent liquor flow was increased to the point where the feed to the precipitator was similar to that in table 5. For the analysis, a yield of 28.1 gL 1 was used, i.e. after 24 hrs precipitation in table 5. 25 Table 15 provides estimated green liquor yields for embodiment of the invention shown in Fig 1 with soda extraction by solvent extraction. The production is based on a green liquor flow from digestion of 1 OOOkLh as described above. It WO 2008/116259 PCT/AU2008/000421 -45 should be noted that the following estimates are based on the limited models and data available on this system, and do not necessarily capture the complexities of an commercial precipitation operation. Key criteria were that the final A/TC>0.4 and that the feed compositions were in the range of the extraction data in the 5 table above. Green Liquor Yield, ( Production (as t/h Case precipitated as Aid ( boehmite) based on Cas prciptatd a A 2 0 3 IOOOKL~h green per L of green liquor) 10K/gre per liquor from digestion Case #1 38 44.7 t/h Case #2 66.5 78.2 t/h Case #3 84 98.82 Table 15. Calculated Bayer circuit green liquor yields. Case #2 indicates that it is feasible to get green liquor yields comparable to commercial Bayer plants precipitating gibbsite and using a Westem Australian 10 Bauxite. In this case, the energy benefit would be of the order of 1.5 GJ/t alumina. As demonstrated by the laboratory results, this scenario is expected to be feasible. Case #3 indicates attractive green liquor yields are feasible. 15 Example 3 -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 = 20 0.70. Boehmite seed containing between 0.1 and 0.2 % w/w gibbsite at a charge of 180 g/L, was added to the treated liquor and precipitation was conducted for a given time at 95 *C. All experiments were perfomied in triplicate and the results averaged and presented in Table 16 below.

WO 2008/116259 PCT/AU2008/000421 -46 Precipitation Ca addition as Yield % gibbsite time CaCO3 (g/L Al203) precipitated (1 g/L) from solution 5 hour No 5.60 30 'P 5 hou r 4% Y7esl]i. &aw 2 i 24 hour No 20.5 20 24 hour-t Ye is.& :if5.9 0 \W,: 52 hour No 25.7 31 52 houig Ys_ i22.4 z || Table 16. 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 5 % 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 10 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. Example 4: Embodiment of the present invention with soda extraction by 15 Electrolysis Table 17 presents data from soda extraction experiments in a two compartment electrolytic cell containing Nafion 324 cation permeable membrane. The liquor is fed to the anolyte compartment. The electrolysis was conducted at 90 *C with a current density of 350 mA/cm 2 and voltage 6.5V. The catholyte compartment 20 produced NaOH (TC 400 gLf as Na 2

CO

3 ). Description TC, gL-' as Na 2 C0 3 A, gL' as A 2 0 3 A/TC Initial Spent Liquor 244 116 0.475 Treated Spent Liquor 174 113 . 0.650 Table 17. A/TC changes with electrolysis.

WO 2008/116259 PCT/AU2008/000421 -47 These results could be optimized further with respect to soda removal, in principle up to the metastable limit for heterogeneous nucleation. 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 5 current densities. Figure 3 is a schematic flow sheet showing a method in accordance with a second embodiment of the present invention. The methods shown in Figures 1 and 3 are substantially similar and like numerals denote like parts. As for the embodiment described above, method also comprises the key steps of: 10 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 first green liquor 50 and a second green liquor 52; combining the first green liquor 50 and treated spent liquor 54; 15 adding the combined first green liquor 50 and treated spent liquor 54 to the beginning of the precipitation circuit 22; distributing the treated spent liquor 56 and the second green liquor 52 along the precipitation circuit 22 at various intervals 58; seeding the combination of the first green liquor 50 and treated spent liquor 20 54 with boehmite recycled from a classification circuit 20 and precipitating boehmite in a precipitation circuit 22 and forming a suspension; separating the suspension in the classification circuit 20 into boehmite product 24, boehmite seed recycle 26 and spent liquor 28; WO 2008/116259 PCT/AU2008/000421 -48 separating a portion of the spent liquor 28 into a first portion 30 and a second portion 32; treating the second portion of the spent liquor 32 in a soda reduction unit 34 with a regenerated media containing an extractant 36; and 5 combining the treated spent liquor 54 with green liquor 50 as described above. The first portion of the spent liquor 30 is treated in the normal manner and the second portion of the spent liquor 32 is contacted at a temperature less than the boiling point of the liquor with the soda extraction media in apparatus 34. The 10 used soda extraction media 38 is contacted with an aqueous solution in the recovery and regeneration unit 40 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 42 in a form suitable for return to the process; for example, it may be returned to the conventional spent liquor circuit 30 15 for further digestion of bauxite or used for washing seed or oxalate or for washing bauxite. Back extraction 40 of the used soda extraction media 38 results in regeneration of the protonated form of the extractant in the media. The regenerated soda extraction media may then be re-used in further extraction steps and returned 36 to the solvent extraction unit 34. 20 The treated spent liquor 18 is heated by heat exchanger 44. The combination of the green liquor 50 and the treated spent liquor 54 and the bypass liquor streams 52 and 56 are sent to the precipitator 22 for precipitation of boehmite at temperatures between about 105 0 C and 60 9 C. The mixed green liquor 50 and treated spent liquor 54 is sent to the beginning of the boehmite precipitation circuit 25 in a ratio of the green liquor to treated spent liquor that is in the range from 1000:1 to 1:1000. The combined green liquor and treated spent liquor will have an A/TC in the range 0.77 to 0.50. The remainder of the green liquor 52 that is not mixed with the treated spent liquor is sent to later stages of the precipitation circuit. Similarly, remainder of the treated spent liquor stream 56 that is not mixed with WO 2008/116259 PCT/AU2008/000421 -49 green liquor is sent to later stages of the precipitation circuit. The optimal amount and distribution of the green liquor bypass 52 and the treated spent liquor bypass 56 will depend on the circuit configuration, operating conditions and the seed and liquor properties. The combination of the treated spent liquor 64 and the green 5 liquor 50 is seeded with boehmite to facilitate boehmite precipitation. The seed charge is in the range from 50 g/L to 1200 g/L. The treated spent liquor 54 may be sonicated 46 to facilitate boehmite precipitation, with or without the presence of boehmite seed. Alternatively, or in addition to, the combination of the green liquor 50 and treated spent liquor 54 in 10 may be sonicated 46 to facilitate boehmite precipitation.

Claims (33)

1. A method for precipitating boehmite from a Bayer process solution, the method comprising the steps of: treating at least a portion of a first spent liquor to decrease both the total 5 caustic concentration and the total alkalinity of the liquor; combining at least a portion of the treated spent liquor with at least a portion of a green liquor; precipitating boehmite from the combination of the green liquor and the treated spent liquor and producing a second spent liquor; and 10 separating at least a portion of the boehmite and the second spent liquor.

2. A method for precipitating boehmite from a Bayer process solution in accorndance with claim 1, wherein the treated spent liquor has an A/TC approximating that of the green liquor. 15

3. A method for precipitating boehmite from a Bayer process solution in accordance with claim 1 or claim 2, wherein 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 at least the 20 portion of the first spent liquor.

4. A method for precipitating boehmite from a Bayer process solution in accordance with claim 3, wherein the step of WO 2008/116259 PCT/AU2008/000421 -51 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 removing sodium ions from the first spent liquor.

5. A method for precipitating boehmite from a Bayer process solution in 5 accordance with claim 3 or claim 4, wherein the method comprises the further step of: recovering the sodium ions.

6. A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of: 10 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 neutralising or removing hydroxide ions from the first spent liquor.

7. A method for precipitating boehmite from a Bayer process solution in 15 accordance with claim 6, wherein the method comprises the further step of: recovering the hydroxide ions.

8. A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of: combining at least a portion of the treated first spent liquor with at least 20 a portion of the green liquor; comprises the step of: WO 2008/116259 PCT/AU2008/000421 - 52 combining the treated first spent liquor with the green liquor in a ratio of between about 1:100 and 3:1.

9. A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of: 5 combining at least a portion of the treated first spent liquor with at least a portion of the green liquor; comprises the step of: combining.the treated first spent liquor with the green liquor in a ratio of between about 1:5 and 2.5:1.

10 IO.A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of: combining at least a portion of the treated first spent liquor with at least a portion of the green liquor, comprises the steps of: 15 providing a first portion of green liquor and a second portion of green liquor; combining the first portion of the green liquor with the treated spent liquor; and directing the second portion of the green liquor to a later stage in the 20 boehmite precipitation circuit.

11.A method for precipitating boehmite from a Bayer process solution in accordance with any one of claims 1 to 9, wherein the step of: WO 2008/116259 PCT/AU2008/000421 - 53 combining at least a portion of the treated first spent liquor with at least a portion of the green liquor, comprises the step of: providing a first portion of treated spent liquor and a second portion of 5 treated spent liquor; combining the first portion of the treated spent liquor with the green liquor and directing the second portion of the treated spent liquor to a later stage in the boehmite precipitation circuit. 10

12.A method for precipitating boehmite from a Bayer process solution 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.

13.A method for precipitating boehmite from a Bayer process solution in 15 accordance with claim 12, wherein the bauxite is provided in the form of gibbsitic bauxite, boehmitic bauxite, diasporic bauxite or any combination thereof.

14.A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of 20 treating at least a portion of the first spent liquor to decrease both the total caustic concentration and the total alkalinity of the liquor, is performed on the entire first spent liquor or in a Bayer process side stream. WO 2008/116259 PCT/AU2008/000421 - 54

15.A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the method comprises the further step of: seeding the combination of the green liquor and the treated first spent 5 liquor with boehmite.

16.A method for precipitating boehmite from a Bayer process solution in accordance with any one of claims I to 14, wherein the method comprises the further step of: seeding the green liquor with boehmite. 10

17.A method for precipitating boehmite from a Bayer process solution in accordance with any one of claims 1 to 14, wherein the method comprises the further step of: seeding the treated first spent liquor with boehmite.

18.A method for precipitating boehmite from a Bayer process solution in 15 accordance with any one of the preceding claims, wherein the step of: precipitating boehmite from the combination of the green liquor and the treated first spent liquor and producing a second spent liquor is conducted at a temperature less the 105 C.

19.A method for precipitating boehmite from a Bayer process solution in 20 accordance with any one of claims 1 to 17, wherein the combined green liquor and treated spent liquor is at a temperature of between about 95 'C and 105 'C when entering the precipitation circuit. WO 2008/116259 PCT/AU2008/000421 -55

20.A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the method comprises the further step of: adding a gibbsite precipitation inhibitor to the treated first spent liquor or 5 the combination of treated first spent liquor and green liquor.

21.A method for precipitating boehmite from a Bayer process solution in accordance with claim 20, wherein the gibbsite precipitation inhibitor is from the gluconate ortartrate family of compounds.

22.A method for precipitating boehmite from a Bayer process solution in 10 accordance with claim 20, wherein the gibbsite precipitation inhibitor is calcia.

23.A method for precipitating boehmite from a Bayer process solution in accordance with any one of the preceding claims, wherein the step of: treating at least a portion of a first spent liquor to decrease both the 15 total caustic concentration and the total alkalinity of the liquor; comprises the steps of: contacting at least a portion of 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 20 spent liquor into the substantially water-immiscible solution.

24.A method for precipitating boehmite from a Bayer process solution in accordance with claim 24, wherein method comprises the further step of: separating the first spent liquor and the substantially water-immiscible solution. WO 2008/116259 PCT/AU2008/000421 -56

25.A method for precipitating boehmite from a Bayer process solution in accordance with claim 24, wherein the method comprises the further step of: contacting the substantially water-immiscibie solution with a stripping 5 solution to provide an aqueous solution of sodium hydroxide.

26.A method for precipitating boehmite from a Bayer process solution in accordance with any one of claims I to 22, wherein 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; 10 comprises the step of: contacting at least a portion of 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 15 on the solid support.

27.A method for precipitating boehmite from a Bayer process solution in accordance with. claim 26, wherein the solid support comprising an extractant is an ion exchange resin.

28.A method for precipitating boehmite from a Bayer process solution in 20 accordance with claim 27, wherein the method comprises the further step of: separating the treated first spent liquor and the solid support.

29.A method for precipitating boehmite from a Bayer process solution in accordance with any one of claims 1 to 22, wherein the step of: WO 2008/116259 PCT/AU2008/000421 -57 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 5 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. 10

30.A method for precipitating boehmite from a Bayer process solution according to claim 29, 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.

31. Boehmlte as prepared in accordance with any one of claims 1 to 29. 15

32.A method for precipitating boehmite from a Bayer process solution as hereinbefore described with reference to the accompanying Examples

33.A method for precipitating boehmite from a Bayer process solution as hereinbefore described with reference to the accompanying Figures. 20