WO1996006043A1

WO1996006043A1 - Improved process for the extraction of alumina from bauxite

Info

Publication number: WO1996006043A1
Application number: PCT/AU1995/000526
Authority: WO
Inventors: Darren Paul Rodda; Raymond Walter Shaw
Original assignee: Comalco Aluminium Limited
Priority date: 1994-08-23
Filing date: 1995-08-23
Publication date: 1996-02-29
Also published as: CA2197457A1; AUPM761194A0; BR9508761A; EP0777628A1; EP0777628A4

Abstract

A process for the extraction of alumina from bauxite having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite over a period of about 2 to 2.5 minutes at a temperature of about 135° to 145 °C and at an A/C ratio of about 0.76, to achieve negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of DSP, subjecting the liquid fraction from the gibbsite digestion stage to post-desilication in the presence of hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite, subjecting the liquid fraction to secondary desilication using tricalcium aluminate produced externally by the addition of lime to spent liquor to minimise alumina losses from the pregnant liquor and to reduce the silica content in the spent liquor to reduce scaling, subjecting the residue from the biggsite digestion to an additional digestion step to digest the boehmite, said digestion of the boehmite fraction being formed at lower temperature and/or lower residence time than is normally the case.

Description

IMPROVED PROCESS FORTHE EXTRACTION OFALUMINA

FROM BAUXITE Field of the Invention

This invention relates to processes for the extraction of alumina from bauxite, and particularly to improvements in double digestion processes for the extraction of alumina from bauxite. Background of the Invention

Although there have been previous disclosures and patents (for example U.S. Patent 4994244 assigned to Alcan International Inc and International Patent Publication WO 94/02416, Comalco Aluminium Limited) for processes based on multi-stage lower temperature digestion of boehmite containing bauxites, none have yet been developed through to a commercial operation. The additional process steps necessary compared to a conventional high temperature single stage circuit make these processes unattractive unless there are major gains in processing efficiency. The targets for such a process therefore have to be major cost items such as caustic usage, high maintenance costs due to the high temperatures and scaling, improved liquor productivity and lower energy usage.

Digestion of gibbsite at temperatures around 140°C is standard technology for many bauxite deposits. However, digestion at this temperature is not commonly practised for boehmite containing deposits because the boehmite is not attacked and needs subsequent digestion at higher temperature for recovery. In this case there is little advantage in attacking the two minerals separately. There is also a potential problem known in the industry as boehmite reversion which can lead to unwanted precipitation of boehmite back out of the digestion liquor leading to losses of alumina and reduced productivity.

This reversion issue has been addressed in the patent literature by Alcan (U.S. Patent No 4994244) by developing a pressure decantation system to enable faster separation of the pregnant liquor from the solids residue without the need for cooling. This step is largely the basis of the above U.S. Patent for countercurrent digestion at lower temperatures. However, investigation of this process using conventional feed preparation and residence times for the digestion conditions disclosed in the U.S. Patent, revealed that the digestion step as proposed did not give liquor of as high as desired alumina content due either to insufficient solubility (at the lower temperatures) or due to significant losses of alumina from reversion at the higher temperatures. In a recent International Patent Publication WO 94/18122 Alcan have disclosed an alumina extraction process in which a stream of ground bauxite containing gibbsitic alumina is formed with a small portion of Bayer process spent caustic aluminate liquor used for digestion; the stream is mixed with the remainder of the spent liquor stream previously preheated to form a preheated slurry-liquor mixture; the preheated liquor-slurry mixture is passed through one or more parallel reaction tubes sized such that the slurry remains in the reaction tube for a residence time just sufficient to extract essentially all of the gibbsite from the slurry and no more than about four minutes; red mud solids, still containing undissolved boehmite and/or diaspore alumina, are separated from the pregnant liquor in a solid-liquid separator operating at substantially the same temperature and pressure as in the reaction tube; and dissolved silica is removed from the pregnant liquor by seeding the liquor with Bayer process desilication product.

In International Patent Publication WO 93/20251 Sumitomo disclose a similar Bayer process which pre-empts much of the subject matter of the above Alcan publication.

Each of the above prior art disclosures suffers from the disadvantage that the final silica level is either too high and/or the loss of caustic during desilication is too great for the process to be economically viable for bauxite with significant levels of reactive silica. Summary of Invention and Object

It is an object of the present invention to provide improvements in double digestion processes of the type described above to provide improved extraction of alumina from mixed gibbsitic boehmitic bauxite using acceptable processing conditions, whilst minimising the detrimental effects of DSP formation, including losses of caustic, and uncontrolled scale formation. In one aspect, the invention provides a process for the extraction of alumina from bauxite having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by relatively fast low temperature digestion of the gibbsite fraction to give a high A/C ratio liquor, negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of desilication product (DSP) in digestion, and a two stage post-desilication including the removal of some of the silica in a first stage followed by removal of substantially all of the remaining silica in a second stage by the use of tricalcium aluminate to reduce losses of alumina and lowering of the A/C ratio of the pregnant liquor.

In a presently preferred form of the invention, digestion of the gibbsite fraction is conducted over a period of about 0.5 minutes to 5 minutes, preferably about 2 to 2.5 minutes, at a temperature substantially falling within the range 120°C to 160°C, preferably about 135 to 145°C, to an A/C ratio of 0.70 to 0.80, preferably about 0.75. The process can operate at A/C ratios less than 0.70 or over a wider temperature range but efficiency is reduced.

The process is preferably carried out at caustic soda levels (expressed as Na_:C0₃) from 200 g/L to 450 g/L and most preferably from 300 g/L to 350 g/L. The use of higher caustic soda levels allows higher pregnant liquor A/C's to be achieved without reaching the solubility limit of the alumina. This assists in preventing gibbsite precipitation during the solid liquid separation and desilication stages particularly where these are carried out at atmospheric pressure. Use of lower temperatures for the DSP seeded post desilication can be advantageous in that the DSP formed at these temperatures has a lower caustic soda content and this reduces the loss of caustic soda from the circuit. This high caustic soda level also increases the solubility of the hydroxysodalite which allows more silica to remain in the liquor which can be detrimental to subsequent operations where the dilution needed to reduce the caustic soda level prior to precipitation also destabilises the silica. The use of the two stage post-desilication with TCA treatment overcomes this problem. Preferably a rapid solid/liquid separation process is applied to the product of the gibbsite digestion process to substantially prevent precipitation of DSP and gibbsite. This step can either be carried out at the digestion temperature (as taught in Alcan 's patent) or after cooling to boiling point in cases where the residue has sufficiently good settling characteristics to allow rapid separation.

While the bauxite processed according to the above definitions can be subjected to grinding to expose the gibbsite, it is preferred that the bauxite is only crushed or lightly ground if necessary and is not subjected to a fine grinding step, essential to other processes, such as that proposed by Alcan in WO 94/18122. The avoidance of fine grinding assists in the solid/liquid separation and is most beneficial where this separation is carried out at atmospheric pressure. The absence of the need for grinding can also simplify the equipment used and improve heat exchange efficiency particularly where the bauxite is not slurried prior to addition to the digester, as required by the Alcan process just referred to. In many instances, the bauxite as mined, particularly the highly reactive Weipa bauxite, is often suitable for processing without even a crushing step. It has been found that particle sizes up to about 10mm are capable of being subjected to satisfactory extraction processes. In the preferred two stage process small amounts of unreacted gibbsite can be extracted during the subsequent boehmite digestion and are recovered.

If the bauxite is subjected to crushing or grinding prior to the initial digestion, then a particle size of greater than 200μm and preferably between about 200μm and about 1mm has been found to be preferable to assist in subsequent removal of quartz prior to boehmite extraction if this step is desired. The slurry residue from the initial digestion stage may also be lightly ground to assist in providing a more efficient separation of the quartz and to assist in improving the boehmite digestion step.

The short residence times and relatively low temperatures stipulated above also have the advantage that none of the quartz present in the bauxite is attacked while the kaolin present is digested to give a high silica liquor. With the short time used this silica is retained in solution until after the boehmite containing solid residue is separated and thus can be treated separately allowing the potential for reducing the caustic losses. Aluterv and Sumitomo have disclosed in U.S. Patent No 5122349 and in International Patent Publication WO 93/20251, operation of the digestion process such as to avoid part of this kaolin dissolution. This has limited advantages for bauxite where the boehmite is to be leached subsequently unless the residual kaolin can be readily separated. There is no obvious commercially attractive technology for such a separation step at present.

The residue from the gibbsite digestion is treated to recover the alumina contained as boehmite, and any incompletely digested gibbsite, which has not been attacked. The exact process used depends upon the amount of boehmite present and the level and physical nature of any quartz present.

In another aspect, the invention provides a process for the extraction of alumina from bauxite having a gibbsite fraction and a boehmite fraction, including the steps of digestion of the gibbsite fraction, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by digestion of the boehmitic fraction by digestion of a boehmite containing mud slurry at a low temperature using spent liquor having a low A/C ratio and a caustic soda content of from 250-450 g/L and preferably from 300-350 g/L.

In a preferred form, the moderate A/C ratio liquor separated from the boehmite digestion stage is used for the low temperature digestion of the gibbsite fraction defined above.

The boehmite digestion stage is preferably conducted for a period of about 2 to 120 minutes at a temperature of about 150°C to 200°C to yield an A/C ratio of about 0.4 to 0.6. It has been determined that by using specialist equipment which improves the solid/liquid contact and increases the exposure of the boehmite to the liquor the recovery can become acceptable at temperatures as low as 150°C. One example of the equipment suitable is the stirred mill such as provided by Metprotech.

Alternatively very short residence times can be used at higher temperatures. The temperature chosen for the step is preferably set by the solubility requirement rather than the reaction kinetics through having the correct combination of equipment and physical properties of the boehmite containing residue to be digested. The solubility needed in turn depends upon the alumina level in the spent liquor being used and the total amount of alumina desired to be extracted from the solids. The use of the spent liquor to prepare TCA, as described further below, further helps in lowering the temperature needed.

To minimise the quartz dissolution in this step even at the lower temperatures it may be preferable to remove it via physical separation from the residue. The preferred method is to use hydrocyclones or similar separators to take out the quartz particles which are coarser than the boehmite minerals. This step is preferably carried out under pressure to avoid the need to cool the liquor. Part of the spent liquor can be mixed with the residue from the pressure decanter to give a slurry more suitable for the separation. As to whether this step is included or not depends upon the level of quartz present, its reactivity under the conditions chosen for the boehmite digestion, and the efficiency possible in the separation step. The quartz attack does not interfere with the process and therefore treatment to remove it is purely an economic consideration.

As already mentioned above, the pregnant liquor produced by the gibbsite extraction stage is much too high in silica, being already supersaturated, to be fed to precipitation for recovery of the alumina. Removal of this silica through formation of a desilication product such as sodalite by seeding with desilication product is conventionally practised and carrying this step out either at atmospheric pressure or under pressure at elevated temperatures is disclosed in the Alcan U.S. Patent. The limitation of the prior art technology is that the solubility of the silica is relatively high and is dependant upon the levels of caustic and alumina in the liquor. It has been found that at the caustic and alumina levels preferred to give the desired productivity from the digestion the residual silica levels from conventional desilication are unacceptably high. These silica levels can lead to excessive scaling and contamination in the precipitation circuit and during subsequent reheating of the spent liquor for use in digestion. Previous workers have employed lime additions for silica removal, for example, Crisp in WO 94/02416 proposes a two stage post-desilication treatment using desilication product seed and lime at the digestion temperature. The use of lime is unattractive as the hydrogarnets formed contain alumina and thus reduce the alumina content of the liquor as evidenced by a drop in A/C and hence the productivity.

In a further aspect, the invention provides a process for the extraction of alumina from bauxite containing gibbsite and boehmite by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by one or more desilication steps in which tricalcium aluminate is added to pregnant liquor to remove silica therefrom, said process being characterised by the step of producing tricalcium aluminate from spent liquor by the addition of a suitable calcium containing compound such as lime to the spent liquor.

In a preferred form of this aspect, the gibbsite and boehmite fractions are separated in separate process steps, and the pregnant liquor produced by the gibbsite extraction step is subjected to post-desilication in accordance with the above definition. In a particularly preferred form of the invention, the pregnant liquor is subjected to primary and secondary post-desilication, the primary desilication step preferably being achieved by subjecting the pregnant liquor to hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite. The secondary desilication step preferably involves the use of tricalcium aluminate to remove the majority of the remaining silica as hydrogarnet.

In a related aspect, the invention further provides a process for removing silica from Bayer process liquors comprising the step of introducing tricalciumaluminate into the liquor at a concentration necessary to convert at least some of the silica in the liquor to hydrogarnet. In a preferred form of the invention, the liquor to which tricalciumaluminate is introduced is to pregnant liquor, the tricalciumaluminate preferably being produced in the spent liquor to reduce the amount of alumina in the spent liquor and to reduce the amount of silica in the spent liquor.

In a still further aspect of the invention, the invention provides a process for the extraction of alumina from bauxite including the steps of digestion of the gibbsite fraction, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by subjecting the liquid fraction from the gibbsite digestion stage to post-desilication in the presence of hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

In a preferred form, the liquid fraction is subjected to a secondary desilication process in which tricalciumaluminate (TCA) is used to remove the majority of the remaining silica as hydrogarnet. The TCA is used to change the solubility of the silica species from being controlled by that of the hydroxysodalite to that of the hydrogarnet. Other calcium containing species can also achieve this (e.g. lime. Grossular, Dolomite) and can also be employed but do not appear to be as commercially attractive as TCA. The post-desilication stages are preferably carried out at a temperature substantially falling within the range 80° to 160°C. Temperatures in the higher range are preferred to maintain gibbsite solubility and to improve the kinetics of the desilication steps but may be detrimental in increasing the amount of caustic soda lost. The TCA is preferably prepared externally and in this way the alumina and calcium needed are being fed to the liquor so only the silica is being removed.

TCA can be prepared in a number of ways all of which involve reacting lime with alumina generally in a caustic solution. In the industry this has most commonly been done using pregnant liquor (for example U.S. Patent 4518571 assigned to VAMI) which has a high alumina content and the reaction readily goes to completion.

As defined above, it is preferred that spent liquor is used to produce the TCA. This avoids loss of potential product alumina in the pregnant liquor and also lowers the alumina level in the spent liquor to allow it to dissolve more alumina from the boehmite in the gibbsite digestion residue. A further advantage of preparing TCA in spent liquor is that residual silica can be removed thus reducing scaling during reheating of the liquor for the boehmite digestion stage. In certain configurations this procedure of adding lime to spent liquor may be applied solely to reduce this scaling rather than to provide TCA for desilicating pregnant liquor. In that application the resultant desilication product may be allowed to remain with the liquor and return to the digestion rather than be separated out. A complexity in using spent liquor to produce TCA is that the ratio of lime to liquor has to be carefully controlled to ensure complete reaction as if the liquor is depleted too much in alumina the TCA species become soluble and the transformation is incomplete. The situation is further complicated by the fact that high dissolved Na₂C0₃ concentrations in the liquor can cause formation of CaC0₃ rather than TCA.

This use of TCA prepared externally with spent liquor for control of the final residual silica level in pregnant liquor is applicable to any circuit but is particularly useful for operations running with high caustic and alumina levels where the silica solubility for the normal sodalite is high. In these circuits the solubility change across the precipitation stage is sufficiently high to cause problems of silica scaling and contamination. The addition point of this step will depend upon the plant arrangement but would most probably be during the final stages of the liquor cooling where the system is above boiling point and under pressure. The desilication reactions proceed more rapidly at higher temperatures.

It has been found that the preferred lime/spent liquor ratio depends upon the temperature and liquor composition with 10 grams of CaO per one litre of spent liquor being preferred at 100°C for liquor of CS * 240 g/L and A/C of 0.40. The upper limit is dependant upon the solubility of TCA and avoiding excess residual lime.

In the preferred embodiment of the invention around 75-80% of the silica is removed as sodalite with the remainder then being precipitated out as hydrogarnet using the TCA. The two silica containing residues are separated out using either decantation or filters and can be further treated to recover valuable constituents such as caustic soda if desired. Processes for this have been disclosed by other workers and include treatment with sulphur dioxide and/or carbon dioxide.

In a preferred arrangement, the tricalciumaluminate is separately produced from spent liquor and lime, although other processes may be used.

It is preferred that all of the above defined aspects of the invention are included in the extraction process to maximise the gains in processing efficiency.

In this form the invention provides an improved processing method for boehmite containing bauxites which does allow for operation at lower temperature whilst giving higher productivity, reduction in the caustic losses due to silica, and reduced scaling due to silica throughout the entire circuit. Brief Description of the Drawings In order that the invention may be more readily understood, reference is made to the following description which should be read in conjunction with the drawings in which:

Figure 1 is a process flowsheet which conceptually details the process steps of the preferred embodiment of the invention; Figure 2 is a graph of the liquor A/C achieved in experimental trial runs testing the process embodying the invention;

Figure 3 details the liquor caustic after digestion for the experimental trial runs referred to above;

Figure 4 details the gibbsite extraction achieved in the experimental trial runs referred to above;

Figure 5 details the boehmite mass in bauxite and in mud for the experimental trial runs referred to above;

Figure 6 details the kaolinite dissolution achieved in the experimental trial runs referred to above; Figure 7 details the liquor Si0₂/CS ratio after digestion for the experimental trial runs referred to above;

Figure 8 is a graph showing liquor A/C, liquor SiOVCS and Na₂0/Si0₂ mud ratio achieved in different experimental trials over the indicated time period;

Figure 9 is a graph showing the distribution of red mud components after digestion; and

Figure 10 is a graph showing changes in soda/silica ratio with temperature for various Na_;C0₃ levels. Description of Preferred Embodiment

Referring to the preferred flowsheet of Figure 1, it will be noted that the notable steps of the flowsheet are the short residence time, narrow temperature range gibbsite digestion at high caustic soda levels, the use of two stage post- desilication with the second step employing tricalciumaluminate and the silica containing residue being separated for further treatment if desired; the preparation of the TCA from spent liquor to minimise the alumina losses from the pregnant liquor, to provide a lower alumina content spent liquor for the boehmite digestion and to reduce the silica content of spent liquor and reduce scaling; boehmite digestion at lower temperature and/or lower residence times than is normally achieved; and separation of the quartz from the gibbsite digestion residue to avoid attack in the boehmite digestion. The novel steps can be applied either as a complete package or where circumstances are appropriate individual parts can be incorporated to improve an existing operation. Most of the steps in the flowsheet can be carried out under pressure to avoid the need to cool the liquor except where there are advantages in decreasing caustic soda losses by operating at lower temperatures below the boiling point. The operation and advantages of the preferred flowsheet will be better understood from the following description. Gibbsite Extraction

Gibbsite to Boehmite Transformation

Based purely on thermodynamic calculations, the gibbsite to boehmite transition temperature is predicted to be around 100°C. However, the rate of conversion is exceedingly slow at this temperature. A number of researchers claim that the upper temperature limit for the existence of gibbsite in aqueous or caustic solutions lies in the range 125-155°C. Results of our experiments are in general agreement with these findings with the transformation occurring at 140-150°C in water and 120-130°C in caustic solutions (seeded with boehmite). For shorter contact times, little transformation was observed below 140°C. The experimental data presented in Figure 8 supports the viability of a fast low temperature digestion of the gibbsite fraction to give a high A C ratio liquor, substantially complete kaolinite dissolution, and prevention of DSP precipitation. These digestion tests were conducted at 130°C in spent liquor of caustic concentration 260g/L and A/C ratio 0.32. The A C ratio (diamonds) reaches a maximum at «2 minutes digestion time as does dissolved silica (stars). Precipitation of DSP (circles), characterised by the Na_^O/SiO, ratio in mud is only «20% complete over the same period. Similar data is produced at 145°C digestion temperature.

Digestion

A comprehensive study was undertaken to identify conditions under which boehmite reversion was negligible during low temperature extraction of gibbsite to high liquor A/C ratios. The parameter ranges were:

Temperature 130-180°C

Time 0-30 minutes

Margin 0-0.25, from gibbsite solubility Results indicated that boehmite reversion was minimised if digestion was conducted at as low as possible a temperature, for a short time and maintaining as large as possible a margin with the equilibrium gibbsite solubility. Maintaining a margin was less important at lower digestion temperatures. The minimum digestion temperature, however, was also governed by the need to achieve high alumina concentrations in the liquor. A set of successful conditions were identified as the following:

Temperature : 140°C

Time 2 minutes

Caustic 300 g/L as Na^O_j Aim Final A C : 0.76 (equilibrium A C 0.79)

Results obtained from trials employing the above conditions (Figures 2-7) concluded that gibbsite extraction was complete and reversion of dissolved gibbsite to boehmite was negligible as the amounts of boehmite entering and leaving digestion were similar. Virtually all of the kaolin present dissolved. The practicality of suppressing kaolinite dissolution during gibbsite extraction is not appealing for mixed gibbsitic/boehmitic bauxites. During extraction of boehmite in a subsequent digestion stage, any remaining kaolinite would almost certainly dissolve. It is therefore preferable to operate the gibbsite extraction stage under conditions where all the kaolinite dissolves. The only likely exception would be in cases where this gibbsite digestion step is applied without any attempt to reduce soda consumption where a predesilication step is included to transform the reactive silica to DSP prior to digestion.

One of the major advantages of full silica dissolution but minimal DSP formation in the reactor is that this allows the initial post-desilication to be carried out at sufficiently low temperature (<100°) to enable formation of a lower sodium level DSP and hence reduce the caustic losses. The differences in soda/silica ratios in DSP as a function of temperature is shown in Figure 10. The secondary desilication step using TCA could then be carried out at either 100°C for simplicity, or after reheating to 135°C+ to improve the kinetics of the reaction. Silica Removal

Digestion of bauxite under conditions specified above is sufficient to dissolve the majority of kaolinite present but insufficient to allow significant formation of DSP provided a rapid solid/liquid separation is possible. Hence, bound soda losses to the red mud are minimised. As a consequence, however, the resulting liquor is very high in silica and requires post-desilication prior to precipitation.

Post-desilication of the liquor at 140°C in the presence of DSP (hydroxysodalite) seed has been found to be successful in reducing the Si0₂/CS ratio from 27x10^'3 («8 g/L) to 5xl0^'3 («1.5 g/L). This ratio remains too high for the liquor to be acceptable for precipitation. This is particularly the case when the pregnant liquor has high caustic and A/C ratios and where the solubility of hydroxysodalite is significantly higher than under the conditions described by Alcan in U.S. Patent No 4994244. This means that single stage post desilication with hydroxysodalite seed would be unsuccessful in reducing the silica concentration to acceptable levels. Dilution of this liquor prior to desilication would assist in lowering the silica levels but would also destabilise the gibbsite fraction. For this reason, the level of dilution which can be used to assist desilication is limited and is not sufficient to allow adequate desilication whilst maintaining high productivity. Even if excess silica was tolerable in precipitation, additional scaling in spent liquor heaters upon reheating prior to being used in digestion would be unacceptable. Hydroxysodalite Solubility

A stud) was undertaken to confirm the expected solubility trends in high caustic and A/C liquors and resulted in development of the following equation:

[SiO^ = 1.7 x l0^"5 [CS][A] This equation is in good agreement with data extrapolated from previous work. It will be appreciated that increasing the caustic and A/C ratio to improve liquor productivity will also result in increased dissolved silica levels. The conclusion is that increased liquor productivity and single stage post-desilication using hydroxysodalite seeding are not compatible. • This limitation led to the approach of using two stage post-desilication where as much silica as possible is removed in a conventional DSP seeded post- desilication and then using lime additions to remove most of the remaining silica from solution as hydrogarnet species (less soluble than hydroxysodalite) as disclosed in WO 94/02416. Two Stage Post-desilication

Lime was found to be very effective in removing silica. However, unacceptable reductions in the pregnant liquor A/C ratio were also observed due to the large amounts of alumina incorporated in hydrogarnets. To counter this the preferred approach of using externally prepared TCA to provide the calcium and alumina was developed. Alternatively desilication can be achieved using species such as Grossular (3CaO.Al₂0₃.3Si0₂) or the analogous MgO systems but these do not appear to offer any marked technical advantages and are not as economically attractive.

It has been shown that it is possible to produce an almost pure sample of TCA in synthetic liquor, however the formation of 4CaO.Al₂0₃.C0₂. l lH₂0 is preferred when carbonate is present.

In man}' plant liquors, carbonate will always be present in large amounts due to the reaction of caustic soda with carbon dioxide in the atmosphere:

2NaOH + C0₂ → Na,C0₃ + H₂0 Experiments were performed to determine whether (i) TCA can be formed when lime is added to plant liquor, and (ii) the species formed as a function of temperature on addition of lime to spent liquor and the results are detailed below. In each experiment, lOg of lime was added to spent liquor (240 g/L CS, 0.65 g/L Si0₂, A/C 0.40) and reacted at the desired temperature for one hour. The spent liquor may typically have an A/C ratio of 0.30 to 0.40, more particularly 0.35 to 0.40, and this provides sufficient alumina for the production of the required amounts of TCA.

Table 1: Liquor results

Trial # Temperature CS /L A/C Si0₂ Si0₂/CS

1 60°C 237.9 0.376 0.44 0.00185

2 100°C 233.5 0.381 0.13 0.00056

3 140°C 239.5 0.373 0.11 0.00046

4 180°C 261.2 0.388 0.10 0.00038

5 220°C 256.0 0.362 0.08 0.00031

6 250°C 235.9 0.368 0.09 0.00038

The XRD analysis of the solid products show that there are varying amounts of impurities in each of the trials (Table 2). The major phase in each of the samples is Ca₃Al₂(OH)₁₂.

Table 2: XRD Analysis

Trial # Ca_jAI-fOH),- Ca₃Al₂(OH), 4CaO.AI₂O_t.C0₂.l lH₂0 Ca(OH)₂ CaCO.

Tricalcium Tricalcium Calcium Aluminium Portlandite Calcite

Aluminate Aluminate Oxide Carbonate Hydrate

1 Major Reference Trace Trace Trace

2 Major Reference ND ND Trace

3 Major Reference Trace ND Trace

4 Major Reference ND Trace Trace

5 Major Reference Trace Minor Trace

6 Major Reference Trace Minor Trace The XRD results show that TCA was the major phase in the solid produced at all temperatures investigated. Only trace amounts of 4CaO.Al₂0₃.C0₂. l lH₂0 were present. The XRD pattern indicated that excess lime was also present in the solid product. From the experiments conducted it can be concluded that a high purity TCA can be formed in spent liquor. The main impurities are Ca(OH)₂, CaC0₃ and 4CaO.Al₂0₃.C0₂. l lH₂0. Temperature has no significant influence over the formation of TCA.

The formation of TCA also concurrently removes the silica from the liquor effectively desi Heating the spent liquor.

Quartz Removal

It will be noted from the preferred flowsheet of Figure 1 that quartz removal preferably takes place prior to the boehmite digestion stage. This is preferably achieved by the use of a conventional physical separation step. This is possible because the size distributions of the quartz and other mud components after the gibbsite digestion stage, as detailed in the graph of Figure 9, indicates that quartz remains in the course fraction. Therefore size separation using standard physical separation techniques, such as cyclones and screens, allows between 80% and 90% of the quartz to be removed with much less loss of the boehmite. Boehmite Extraction

Mechano Leaching

Extraction of boehmite from gibbsite free mud was first evaluated using a pressurised stirred ball mill as the reactor. Results were very encouraging in that boehmite could be extracted (95%) at 150°C over 15 minutes to an A/C ratio of 0.48 (initial A/C 0.31. equilibrium A/C 0.48) and final caustic strength of 340 g/L. In those samples the quartz was not removed prior to this step and approximately 70% of the quartz was attacked as a result of the milling where the mean particle size of the mud was reduced to <10μm.

Conventional Autoclaves Under the same digestion conditions used for the stirred mill trials, quartz attack was negligible in conventional autoclaves, however, boehmite extraction was only 64% and the final A/C ratio 0.42. Over a series of comparative trials, the final A/C was typically 0.05 units lower, boehmite extraction 25-30% lower and Si0₂ in liquor 35% higher in the autoclave compared to the stirred mill. On the positive side, quartz attack was negligible in autoclaves at these low temperatures. A f rther study in autoclaves was conducted where increasing the digestion residence time from 15 to 60 minutes increased the final A/C ratio to 0.45 and boehmite extraction to 78%. This was very similar to a result obtained at 30 minutes residence time so it was felt that no further gains would be made by increasing residence time beyond 60 minutes. It has been found that A/C ratios approaching equilibrium could best be obtained by overcharging the system (i.e. low extraction). Conversely, high extractions were possible when charging was done with a large margin (i.e. low final A/C). for which the TCA production from the spent liquor prior to its use in the desilication is therefore very advantageous. Increasing the temperature also assists as it increases the equilibrium solubility and therefore the A/C margin available as well as increasing the reaction rates.

By careful selection of conditions digestion times as low as 2 minutes were found to extract virtually all of the boehmite at 200°C without significant quartz attack. In this case as with the gibbsite digestion a simple pipe system would be effective.

The preferred equipment/temperature combination is therefore largely an economic decision which will depend upon the properties of the bauxite being treated and the cost of energy at the operating location. Preferred Flowsheet Based on the information presented above, it is possible to specify a flowsheet which represents a preferred sequence of operations for a dual stage/low temperature digestion circuit for processing mixed gibbsitic/boehmitic bauxite and this is provided in Figure 1. A summary of the main features of the flowsheet is given below: • A fast (»2 minutes) low temperature («140°C) digestion of bauxite is carried out to extract the gibbsite fraction. The resulting liquor has an A C ratio of about 0.75 or boehmite reversion is negligible and kaolinite dissolution is virtually complete. Insufficient time is allowed for DSP formation so the liquor is high in silica.

• A rapid solid/liquid separation at the digestion temperature is effected to prevent boehmite reversion and precipitation of DSP onto the mud.

• The liquor is subjected to post-desilication in the presence of hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

• The boehmitic mud slurry is digested at low temperature in spent liquor of low A/C ratio to a moderate final A/C ratio. The moderate A/C ratio liquor is used for the gibbsite extraction operation.

• The portion of DSP not recycled for liquor post-desilication is transferred to a soda recovery stage.

• Liquor from post-desilication is subjected to a secondary desilication using TCA to remove the majority of remaining silica as hydrogarnet.

• TCA to be used for secondary desilication is produced by reaction of spent liquor with lime. Traces of silica in spent liquor are removed and the A/C ratio is lowered which facilitates low temperature extraction of boehmite.

The perceived advantages and novelties of the flowsheet described above are as follows:

• Conducting the gibbsite digestion under conditions sufficient to extract both the gibbsite and kaolinite but insufficient to allow significant precipitation of DSP onto the red mud or reversion of gibbsite dissolved in the pregnant liquor to boehmite. • Rapid separation of the solid from the liquid to minimise the formation of DSP during this step facilitated by control of the size distribution of the initial bauxite charge.

• Two stage post-desilication of the pregnant liquor, where the second polishing stage is carried out using TCA to avoid reductions in pregnant liquor A/C ratio normally associated with lime and at temperatures high enough to avoid precipitation of alumina trihydrate. • Control of the A/C of the pregnant liquor to allow the initial desilication to be carried out below boiling point such that a lower soda content DSP is obtained in the seeded desilication without concurrent gibbsite phase precipitation occurring. • Preparation of TCA by reaction of lime with spent liquor where any residual silica in the spent liquor is removed which acts to reduce scaling during subsequent heating of the spent liquor and the A/C ratio of the spent liquor is reduced to facilitate subsequent boehmite digestion. This preparation of TCA and silica reduction from the addition of lime to spent liquor and the subsequent use of the TCA within the circuit can also be used in other arrangements of the Bayer process as well as in the preferred two stage digestion process.

• Extraction of boehmite at a temperature similar to that of the gibbsite extraction where the lower spent liquor A/C ratio caused by TCA production facilitates low temperature boehmite extraction. • Physical separation of quartz after the gibbsite digestion if economically attractive to allow either a stirred mill or high temperature to be used to further enhance boehmite extraction.

Claims

CLAIMS:

1. A process for the extraction of alumina from bauxite having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by relatively fast low temperature digestion of the gibbsite fraction to give a high A/C ratio liquor, negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of desilication product (DSP) in digestion, and a two stage post-desilication including the removal of some of the silica as DSP in a first stage followed by additional removal of part of the remaining silica in a second stage by the use of tricalcium aluminate (TCA).

2. The process of claim 1, wherein the first post-desilication stage is performed using hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

3. The process of claim 1 or 2, wherein said TCA is prepared externally of the pregnant liquor, the alumina and calcium needed to prepare the TCA are fed to the liquor, if necessary, so that only silica is removed from the pregnant liquor.

4. The process of claim 3, wherein said TCA is prepared using spent liquor thereby avoiding the loss of potential product alumina in the pregnant liquor and lowering the alumina level in the spent liquor to allow it to dissolve more alumina from the boehmite fraction in the gibbsite digestion residue and removing residual silica from the spent liquor.

5. The process of claim 4, wherein said TCA is produced by adding lime to the spent liquor in a concentration substantially falling within the range 1 g/L to 150 g/L.

6. The process of claim 5, wherein the ratio of lime to spent liquor is about 10 g L.

7. The process of any preceding claim, wherein digestion of the gibbsite fraction is conducted over a period substantially falling within the range of 0.5 minutes to 5 minutes, at a temperature substantially falling within the range 120°C to 160°C, and at an A/C ratio substantially falling within the range 0.70 to 0.80.

8. The process of claim 7. wherein digestion of the gibbsite fraction is conducted over a period of about 2 to 2.5 minutes, at a temperature substantially falling within the range 135°C to 145°C, and at an A/C ratio of about 0.75 to 0.76.

9. The process of any preceding claim, wherein said solid/liquid separation step is performed over a short time period and at a temperature substantially the same as the boehmite digestion temperature to substantially prevent boehmite reversion and precipitation of DSP onto the mud.

10. The process of any preceding claim, wherein said bauxite includes particle sizes up to about 10mm and said bauxite is not subjected to grinding.

11. The process of claim 5, wherein said bauxite is subjected to crushing to achieve a particle size no greater than about 2 to 10mm.

12. A process for the extraction of alumina from bauxite having a gibbsite fraction and a boehmite fraction including the steps of digestion of said gibbsite fraction, solid/liquid separation and digestion of said boehmite fraction, said process being characterised by digestion of said boehmite fraction by digesting a boehmite containing mud slurry at a low temperature using liquor having a low A/C ratio.

13. The process of claim 12, wherein said liquor is spent liquor.

14. The process of any preceding claim, wherein moderate A C ratio liquor separated from the boehmite digestion stage is used for the low temperature digestion of the gibbsite fraction.

15. The process of any preceding claim, wherein said boehmite digestion stage is conducted for a period substantially falling within the range 2 to 120 minutes at a temperature substantially falling within the range 150°C to 200°C to yield an A/C ratio substantially falling within the range 0.40 to 0.60.

16. The process of any preceding claim, further comprising the step of physically separating quartz from the residue of the gibbsite digestion stage.

17. The process of claim 16, wherein said physical separation step is carried out under pressure to avoid the need to cool the liquor.

18. The process of claim 16 or 17, further comprising the step of mixing part of the spent liquor with the residue to produce a slurry more suitable for the physical separation step.

19. A process for the extraction of alumina from bauxite containing a gibbsite fraction and a boehmite fraction by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by one or more desilication steps in which tricalcium aluminate (TCA) is introduced to the pregnant liquor to remove silica therefrom, said process further being characterised by production of the tricalcium aluminate for this purpose using spent liquor.

20. The process of claim 19, wherein said TCA is produced by the addition of lime to said spent liquor to produce a lime/spent liquor concentration substantially falling within the range 1 g L to 150 g/L.

21. The process of claim 22, wherein said lime/spent liquor concentration is about 10 g/L.

22. The process of claims 19, 20 or 21, wherein said desilication step is a post- desilication step.

23. The process of claim 22, wherein said post-desilication step is a secondary post-desilication step.

24. The process of claim 23, wherein said secondary post-desilication step is performed after a primary post-desilication step involving subjecting the liquid fraction from the gibbsite digestion stage to hydroxysodalite seed to reduce the silica concentration close to the solubility of hydroxysodalite.

25. A process for removing silica from Bayer process liquors, comprising the step of adding tricalciumaluminate produced externally of the pregnant liquor to the pregnant liquor at a concentration sufficient to convert at least some of the silica in the liquor to hydrogarnet.

26. The process of claim 25, wherein said liquor is spent liquor.

27. The process of claim 25 or 26, f rther comprising the step of producing said tricalciumaluminate in spent liquor to reduce the amount of alumina in the spent liquor and to reduce the amount of silica in the spent liquor.

28. The process of any one of claims 19 to 27, wherein the or each post- desilication step is carried out to produce a low soda DSP at a temperature less than the boiling point of the liquor.

29. A process for the extraction of alumina from bauxite containing gibbsite and boehmite by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by the physical separation of quartz from the residue of the digested gibbsite fraction.

30. A process for the extraction of alumina from bauxite containing a gibbsite fraction and a boehmite fraction by digestion of the gibbsite and boehmite fractions of the bauxite, characterised by said bauxite not being subjected to grinding and having particle sizes of up to about 1 to 10 mm.

31. A process for the extraction of alumina from bauxite having a gibbsite fraction and a boehmite fraction and having a relatively high boehmite content including the steps of digestion of the gibbsite fraction of the bauxite, solid/liquid separation and digestion of the boehmite fraction, said process being characterised by relatively fast low temperature digestion of the gibbsite fraction to give a high A/C ratio liquor, negligible boehmite reversion, substantially complete kaolinite dissolution and substantially no precipitation of desilication product (DSP), and by said bauxite not being subjected to grinding and having particle sizes of up to about 1 to 10mm.