WO1997003022A1

WO1997003022A1 - High yield precipitation process

Info

Publication number: WO1997003022A1
Application number: PCT/AU1996/000435
Authority: WO
Inventors: Gregory Patrick Brown
Original assignee: Comalco Aluminium Limited
Priority date: 1995-07-11
Filing date: 1996-07-11
Publication date: 1997-01-30
Also published as: EP0839118A4; EP0839118A1; CA2226842A1; BR9609429A; KR19990028922A

Abstract

A process for precipitating alumina trihydrate from a pregnant caustic liquor comprising supplying the pregnant caustic liquor (10) to a precipitation stage of a Bayer plant. The precipitation stage includes an agglomeration stage (A1, A2) and a growth stage (G1, G2... Glast). Precipitation of alumina trihydrate takes place in the precipitation stage to form a slurry of precipitated alumina trihydrate in caustic liquor. During the precipitation process, the slurry is diluted by addition of an aqueous stream (115). This dilutes the caustic concentration of the liquor in the slurry, which increases the driving force for precipitation, thereby increasing yields from the precipitation process. The dilution stream may be added to the slurry as a single stream or a plurality of streams.

Description

HIGH YIELD PRECIPITATION PROCESS

The present invention relates to an improved precipitation process used in the production of alumina by the Bayer process.

The Bayer process is widely used to recover alumina from bauxite ores. The Bayer process involves contacting bauxite with a caustic liquor at elevated temperature to dissolve the alumina contained therein. Insolubles, commonly called red mud, are separated from the resulting liquor. Dissolved impurities, such as silicates and organics, may also be removed from the liquor.

The dissolved alumina is recovered from the liquor by precipitation. The precipitation stage of the Bayer process involved passing a supersaturated Bayer liquor to a series of precipitation tanks, which are generally arranged in agglomeration and growth sections. Seed hydrate is typically added to both sections to promote precipitation of hydrate and produce particles of required size.

Precipitation trains at alumina refineries include a plurality of stages, usually in the form of separate precipitation tanks, and the liquor is cooled as it moves through each successive tank. At the end ofthe precipitation stage, the precipitated hydrate particles are separated and classified. Smaller particles are generally retained as seed particles whilst particles in the desired size range are recovered and calcined to produce alumina. The Bayer process is widely used throughout the world and is well known to those involved in the production of alumina.

As will be known to the skilled person, the dissolution of alumina in caustic solution results in sodium aluminate (NaAlO₂) and other dissolved aluminium values being formed as the dissolved species. Throughout this specification, this will be referred to as alumina in solution. Moreover, the precipitation stage results in the precipitation of alumina trihydrate (Al₂O₃ ^«3H₂O). This will be referred to as hydrate or precipitated hydrate. The precipitated hydrate is calcined to remove the water of hydration to form the final product alumina.

Current practice at most alumina refineries utilises Bayer liquors having high caustic concentrations, often in the range of from 200 - 450g/l? caustic, calculated as Na-,CO₃. Throughout this specification, all caustic concentrations will be calculated as Na₂CO₃. At these high caustic concentrations, large amounts of alumina are extracted into solution during digestion of the bauxite and this allows for good recovery of alumina from the Bayer liquor. Indeed, alumina recovery of up to 85g/ A1₂0₃ can be obtained from high caustic precipitation. However, this does appear to represent the upper limit of alumina recovery using known precipitation processes.

The present invention provides an improved precipitation process that has the potential to increase alumina recovery.

The present invention provides an improved precipitation process for producing hydrate from a Bayer liquor comprising precipitating hydrate from a Bayer liquor having a high caustic concentration, diluting the Bayer liquor to reduce the caustic concentration and precipitating further hydrate.

In another aspect, the present invention provides a process for precipitating alumina trihydrate from a pregnant caustic liquor containing dissolved aluminium values, the process comprising the steps of supplying the pregnant caustic liquor to a precipitation stage of a Bayer plant, precipitating alumina trihydrate from the pregnant liquor to form a slurry of precipitated alumina trihydrate in caustic liquor, - adding an aqueous stream to the slurry to thereby dilute the caustic liquor and precipitating further alumina trihydrate. The present invention is particularly suitable for the production of smelter grade alumina. In the present invention, a Bayer feed liquor having high caustic concentration and a high dissolved alumina concentration is fed to a precipitation train. After precipitation in a high caustic environment, a dilution stream is added to reduce caustic concentration which thereby decreases alumina solubility and promotes further precipitation. This increases yields above currently achievable levels.

It is preferred that the slurry obtained from precipitation in the high caustic environment is not deslurried before dilution, which means that the dilution stream is added to the slurry of precipitated hydrate in the caustic liquor.

The dilution stream is preferably water or wash water which has been produced elsewhere in the Bayer process. Wash water may have a low caustic concentration. It will be appreciated that any liquid stream that has a lower caustic concentration than the Bayer liquor will be suitable for use as a dilution stream.

When the Bayer liquor is diluted, the ratio of alumina concentration to caustic concentration (A/C) stays substantially the same, because both the alumina and caustic concentrations are reduced by equal amounts. However, at the lower caustic concentration resulting from the dilution, the equilibrium A/C is reduced and thus the supersaturation of the liquor is increased. This promotes further precipitation of hydrate.

Dilution of the liquor may occur in a single step, for example, by addition of a large amount of dilution stream at a single point in the precipitation train. Alternatively, the dilution may occur as a plurality of smaller dilution steps, for example, by adding the dilution stream at two or more stages of the precipitation train.

In a preferred embodiment, the Bayer liquor that is used as a feed stream to the high caustic precipitation has a caustic concentration in the range of 200-350gA£, calculated as Na₂CO₃ more preferably 240-275g/ . After completion ofthe dilution

(which, as explained above, may be carried out in a single step or in a plurality of smaller steps), the caustic concentration is preferably reduced to from 200-250g/-f .

The alumina concentration of a Bayer liquor is usually measured by reporting the A/C ratio, which is:

A!C - ^{AΑ B}"

N^CO_Q glu

The A/C at the start ofthe high caustic precipitation is preferably within the range of 0.65-0.80, more preferably within the range of 0.72 to 0.75 at the preferred caustic concentration of 240-275 g/i?. The high caustic precipitation preferably proceeds until the A/C ratio in the precipitation liquor falls within the range of 0.40 to 0.60, more preferably 0.45 to 0.50. Addition ofthe dilution liquor does not significantly alter the A/C but it does reduce the caustic concentration. After completion ofthe low caustic precipitation, the A/C is preferably within the range of 0.25 to 0.35, more preferably 0.30 to 0.33.

As with all Bayer process precipitation processes, it is generally necessary to seed the liquor in order to promote precipitation of hydrate. The present invention encompasses all seeding strategies within its scope. A currently preferred seeding strategy uses a double seeding strategy which is similar to that practiced at many alumina refineries throughout the world. This strategy includes:

1) Washed, fine seed free of solid phase organic matter. Medium particle size in the range of 40 - 150 μm preferably 40-100 μm. The fine seed charged to give 20 - 200g/ solids in feed liquor, and more preferably about 100 - l20g/2 solids in feed liquor. This seed is preferably added to the agglomeration stage.

2) A coarse seed with a medium particle size in the range of 50 - 110 μm, more preferably about 75 - 85 μm. Coarse seed would be charged to give a solids content of 200 - 700g/£ in the last precipitator, more preferably about 400 - 500g/ solids in the last precipitator. This seed is preferably added to the first tank of the growth stage of the precipitative process.

The temperature profile used in the precipitation train may also be any suitable profile. Suitably, the feed liquor to the precipitation has a temperature of from 65 - 85°C, more preferably 75 - 80°C. Precipitators would be progressively cooled to achieve 45 - 55°C in the last of the precipitators. It is preferred that each precipitator is cooled by about 1-3°C relative to the adjacent upstream precipitator, in accordance with the disclosure in our co-pending Australian Patent Application

No. 36212/93 entitled "Improvements in Alumina Plants". The total residence time for the precipitation process may be in the range of about 30 to 50 hours, more preferably about 40 - 45 hours, with the high caustic precipitation suitably having a residence time of about 15 - 20 hours.

The improved precipitation process of the present invention allows a yield of up to 95 to 120g £ A1₂0₃ or higher. This is considerably higher than current best practice that obtains hydrate yields of about 85g/f A1₂0₃. The slurry from the last precipitator is treated to separate the solids from the liquor. After removing suitable quantities of the solids for seeding, the particles of the desired size range are calcined to form the alumina product.

The liquor recovered from the last precipitator of the precipitation train is conventionally returned to the digestion step in which the liquor is contacted with bauxite to extract alumina into solution. However, this liquor has a lower caustic concentration due to the dilution carried out during the precipitation process.

Accordingly, it is likely to be necessary to treat this liquor to increase its caustic concentration prior to re-using the liquor in the extraction step. Preferably, this is achieved by evaporating off some of the water from the liquor equivalent to the dilution added. However, any other process that increases the caustic concentration of the liquor may also be used.

The process of the present invention adds another degree of freedom to the precipitation phase of the Bayer process. Conventional Bayer precipitation processes control the inlet A/C ratio, feed caustic concentration, seeding parameters and temperature profile. The process of the present invention also allows for control of the caustic concentration of the liquor during the precipitation process by providing for dilution during precipitation.

The process of the present invention provides increased production and improved efficiency. Hydrate quality may be improved by reducing soda pick-up. Moreover, stand-alone ancillary processes that are normally uneconomic may be attached to Bayer processes in an economic way, e.g. recovery of soda from DSP in mud. If an evaporation plant is used to concentrate the diluted caustic liquor after precipitation, power generation by high efficiency co-generation power stations becomes possible. Processes which recover soda from DSP in mud usually produce a dilute caustic stream of say 10-50 gpl Na^O*,. Although this stream contains valuable caustic it also contains water. To reuse (recover) the caustic, the stream must be re-introduced to the Bayer process circuit. If the stream is introduced as a dilute stream in a Bayer process circuit having a conventional precipitation step there is too much water added to the circuit and as a consequence the evaporation capacity of the refinery has to be increased. This requires capital and increased energy. Such processes do not provide for economic recovery of caustic when the cost of caustic and energy (fuel) are considered. With the improved precipitation process of the present invention the dilute caustic stream can be introduced to the Bayer circuit to give increased precipitation yield. It is still necessary to evaporate the additional water but when the additional alumina production from increased yield is considered the process of caustic recovery may produce favourable economics.

With regard to power generation, Bayer refineries require electrical power.

The power can be purchased from a State authority if the refinery is suitably located but has to be generated by the refinery if located in a remote area. In such a case power is usually generated by steam turbines. In a refinery which has a requirement for low pressure steam, e.g. to run an Evaporation Plant, the power can be generated by operating back pressure or let-down turbines, so producing low pressure steam and power from the feed high pressure steam from the Boilerhouse (co-generation). However, in a refinery that does not use low pressure steam the power is generated by condensing turbines where cooling water is used to condense the steam. The fuel efficiency of the co-generation power station is 70-80% whereas in the power station using condensing turbines it is 25-35%.

Soda pick-up in hydrate occurs primarily in areas of the process with high alumina supersaturation, i.e. in front-end of the precipitation process. The improved process may achieve higher yields by recovering more hydrate from the area where low soda hydrate is produced, i.e. within latter part ofthe process where supersaturations are lower. While the addition of dilution liquor increases the supersaturation to promote precipitation the increase is not sufficient to increase soda pick-up, so the high soda hydrate produced at the front-end gets diluted by the increase of low soda hydrate.

The present process is also especially useful for modern alumina refineries. Such refineries typically utilise very high caustic concentration to digest the bauxite. Although use of liquors having very high caustic concentrations should enable dissolution of large quantities of alumina to give a pregnant caustic liquor having a large amount of dissolved alumina therein, caustic liquors having a high caustic concentration are very aggressive, corrosive liquors that can cause severe corrosion of the process vessels used in digestion, especially at higher temperatures used for boehmite digestion. As a consequence, limitations may be put on the digestion process (in terms of either or both of residence time and digestion temperatures). Accordingly, although the content of dissolved alumina in the pregnant liquors may be high, the supersaturation of those liquors may be low. The dilution step or steps included in the present invention act to increase the supersaturation of the liquor and allow recovery of a larger proportion of the dissolved alumina content of the caustic liquor.

The present invention will now be described in more detail with reference to the following Figures. It will be appreciated that the accompanying Figures illustrate embodiments of the present invention and the Figures should not be construed as limiting the invention. In the Figures:

FIGURE 1 is a schematic diagram of the general flowsheet ofthe process of the present invention; FIGURE 2 is a schematic diagram showing a general flowsheet of a conventional precipitation process used in the Bayer process; FIGURE 3 shows an expanded flowsheet ofthe conventional precipitation process of FIGURE 2; FIGURE 4 shows an expanded flowsheet of a precipitation process according to the present invention, and

FIGURE 5 shows another expanded flowsheet of a precipitation process according to the present invention.

The process of the present invention is schematically depicted in Figure 1.

As can be seen by reference to Figure 1, feed liquor is supplied to a high caustic precipitation process which includes a double seeding strategy. The slurry resulting from the high caustic precipitation is diluted and a low caustic precipitation then occurs to precipitate further hydrate. The slurry bearing the low caustic precipitation stages is subsequently sent to classification.

Figure 2 shows a schematic diagram of a prior art precipitation process. In this process, feed liquor is supplied to a high caustic precipitation process which includes a doubled seeding strategy. The slurry levering the high caustic precipitation process is then sent to classification.

Figure 3 is an expanded flowsheet of the conventional precipitation process shown in Figure 2. In Figure 3, the precipitation train includes two agglomeration precipitators Al and A2 and a growth stage having a multiplicity of growth precipitators Gl, G2,...G Last. A Bayer liquor 10 is fed to the first agglomeration precipitator Al. Washed tertiary seed 12 is also supplied to agglomeration precipitator Al.

After a suitable residence time in precipitator Al, the slurry of seed and liquor (which will also include some agglomerated or precipitated hydrate) then passes to precipitator A2 and then into the first of the growth precipitators Gl. Deliquored secondary seed 14 is also fed to first growth precipitator Gl . The slurry of liquor and hydrate sequentially passes through the growth precipitators and it is slowly cooled. The A/C ratio of the liquor gradually reduces but the caustic concentration remains essentially constant. After leaving the final growth precipitator (G Last), the slurry is classified

(16) into secondary seed, tertiary seed and product. The secondary seed is deliquored at 18 to produce a deliquored secondary seed 14 and spent liquor stream 20. The tertiary seed is deliquored and washed at 22 to produce tertiary seed 12 and spent liquor stream 24. The product hydrate is washed at 26 which produces a wash water/spent liquor stream 28 and a washed product hydrate 30.

Figure 4 is an expanded flowsheet showing one embodiment of the precipitation process of the present invention. This flowsheet includes agglomeration precipitators Al and A2 and a multiplicity of growth precipitators Gl, G2,...G Last. Pregnant liquor 110 and tertiary seed 112 are supplied to agglomeration precipitator Al. The slurry passes sequentially through precipitator A2 to precipitator Gl, wherein secondary seed 114 is added. Up to this point, the flowsheet of Figure 4 is essentially identical to the flowsheet of Figure 3. However, at precipitator G3, a dilution stream 115 is added to the slurry. This reduces the caustic concentration without changing the A/C ratio and this increases the yield of hydrate in the overall process. The slurry leaving the last precipitator (G Last) is classified into secondary seed, tertiary seed and product hydrate. The respective spent liquor streams 120, 124 and 128 are combined and evaporated in an evaporation plant 132. Evaporation plant 132 is required in order to remove the dilution water added during the precipitation process to thereby concentrate the spent liquor to a caustic concentration suitable for use in digesting bauxite. Figure 5 is another embodiment of the precipitation process of the present invention. The flowsheet of Figure 5 is similar to that shown in Figure 4, with the exception that the dilution stream is added as a series of dilution streams 215a, 215b ... 215x to the respective growth precipitators Gl, G2,... G(Last-l).

In the embodiments shown in Figures 3, 4 and 5, like features are denoted by like reference numerals that differ by 100. For example, Features 10 in Figure 3 corresponds to Feature 110 in Figure 4 and Feature 210 in Figure 5.

The embodiments shown in Figures 4 and 5 respectively show addition of the dilution stream in one step and addition of the dilution stream in a number of steps. It will be appreciated that the dilution stream may be added as a single stream to one precipitator, as two streams to two precipitators or as a number of streams to a number of precipitators. All such additions fall within the scope ofthe present invention. The embodiments of Figures 4 and 5 also show seed additions to precipitators Al and Gl. Other seeding strategies may be used that vary the number of seed additions and/or the precipitators to which the seed is added. All such seeding strategies fall within the scope of the present invention.

In order to demonstrate the advantages of the present invention, preliminary modelling of the process was carried out. In particular, the process was modelled without dilution (Base Case-prior art), with dilution after the 7th precipitator (Case A), with dilution after the first precipitator (Case B) and with dilution and addition for further seed with increased residence time (Case C). The results are shown in Table 1. TABLE 1

Base Case A B C D E

Caustic (in)g/f 250 250 250 250 275 300

Caustic (out)g/ 254.6 205.6 205.7 205.7 220 240

Dilution No Yes Yes Yes Yes Yes P-7 P-l +

Add'n

Seed

+ Add'n Time

Yieldgtf 86.7 90.3 94.2 95.2 105 115

As can be seen from Table 1, the process in accordance with the present invention produces significantly higher yields than the base case.

A series of experimental runs were conducted to determine the effect of the present invention on Bayer process precipitation. The tests that were conducted were bottle tests in which a pregnant caustic liquor and seed were placed in a bottle. The caustic liquor was then cooled in accordance with one of two temperature profiles detailed in Tables 2 and 3 below as the Profile 1 and Profile 2, respectively.

TABLE 2 Temperature Profile 1

Tank Temperature (°C) Holdmg Time (Hours)

1 & 2 78 5.5

3 76 4

4 74 4

5 72 4

6 70 4

7 68 4

8 66 4

9 64 4

10 61 4

11 58 4

12 55 4

Total = 45.5

TABLE 3 Temperature Profile 2

Tank Temperature (°C) Holding Time (Hours)

1 & 2 78 5.5

3 75.2 1.95

4 72.4 1.95

5 69.6 1.95

6 66.8 1.95

7 64 1.95

8 61.2 1.95

9 58.4 1.95

10 55.6 1.95

11 52.8 1.95

12 50 1.95

Total = 25

The results for the bottle precipitation tests are given in Tables 4 to 6.

Tables 4 and 5 detail the experimental results obtained using the profile 1 in which dilution occurred just before the commencement of the stage at 72°C. Table 6 details the results obtained using Profile 2 in which dilution took place just prior to commencement ofthe stage at 72.4°C. In each of Tables 5 and 6, the liquor was diluted with the aim of obtaining a caustic concentration within the range of 200- 250 g/£ following dilution. This was a calculated value and the actual caustic concentration obtained after dilution is similar to the end liquor caustic concentrations given in Tables 4, 5 and 6. TABLE 4 Single Pass Test

Parameter Start End

A/C 0.640 0.325

CS (g/l as Na^O*,) 234 217

C/S 0.90 0.90

Oxalate (g/l as Na^O n/a (-1) n/a

Organics n/a (-10) n/a

Yield (g/l as AI₂0₃) 74

TABLE 5 Single Pass Test

Parameter Start End

A/C 0.769 0.334

CS (g/l as Na^O- 362 242

C/S 0.91 0.91

Oxalate (g/l as Na^O n/a (~1) n/a

Organics n/a (-10) n/a

Yield (g/l as Al₂O₃) 158

TABLE 6 25 Hour Single Pass

Parameter Start End

A/C 0.656 0.347

CS (g/l as Na^O;*) 275 227

C/S 0.91 0.90

Oxalate (g/l as Na^O 0.72 1.1

Organics n/a (-10) n/a

Yield (g/l as Al₂O₃) 85 Tables 7 and 8 provide the results of similar tests to those of Tables 4 and

5, but using recycled liquor having higher organics content. The results were as follows:

TABLE 7 Average Values from Dilution Recycle Test

Parameter Start End

A/C 0.656 0.314

CS (g/l as Na₂CO₃) 275 247

C/S 0.91 0.91

Oxalate (g/l as Na^O 0.90 0.90

Organics 8.7 8.2

Yield (g/l as Al₂O₃) 94

TABLE 8 Average Values from High Organics Dilution Recycle

Parameter Start End

A/C 0.716 0.339

CS (g/l as Na_jCO- 299 224

C/S 0.87 0.98

Oxalate (g/l as Na^O^ n/a n/a

Organics 32.1 23.9

Yield (g/l as Al₂O₃) 112

In order to compare the above results obtained using a process in accordance with the present invention with conventional precipitation technology, a series of bottle tests using Profile 1 but no dilution were conducted. The results thereof are given in Table 9. TABLE 9 Single Pass Bottle Tests with No Dilution

Test l Test 2 Test 3

Parameter Start Finish Start Finish Start Finish

CS (g/l 212.0 211.0 229.7 243.6 224.8 234.0 Na₂CO₃)

A/C 0.659 0.37 0.668 0.341 0.692 0.374

C/S 0.875 - 0.907 - 0.792 -

Yield (g/p 61 70 68 A1₂0₃)

In order to allow for easy comparison between the experimental results obtained using a process in accordance with the present invention with the experimental results obtained using conventional precipitation, Table 10 below tabulates the start and finish caustic soda contents of the liquor and the yield (in g/l as Al₂O₃) obtained from the experiments detailed in Tables 4 to 9.

TABLE 10 Comparison

Table Caustic Concentration Yield Comment

Start Finish

4 234 217 74 Dilution

5 362 242 158 Dilution

6 275 227 85 Dilution

7 275 247 94 Dilution

8 299 224 112 Dilution

9(1) 212 211 61 No Dilution

9(2) 230 244 70 No Dilution

9(3) 225 234 68 No Dilution The results summarised in Table 10 show that the addition of water

(dilution) during the precipitation process resulted in an increased yield of alumina.

Current precipitation circuits produce a yield of alumina in the range 60-

S0g/l. This is a low productivity, at approximately half the alumina capacity ofthe liquor. The yield is determined by a number of solution and process conditions, chiefly the pregnant liquor A/C, caustic soda concentration, and inlet/outlet temperatures. The temperature profile plays an important role in the quality ofthe product, influencing such aspects as soda percentage and strength. The liquor conditions determine the equilibrium alumina concentration, and as Bayer liquors are supersaturated, the difference between the actual and equilibrium alumina concentration (the supersaturation) is also dependent on the liquor condition. This difference is generally accepted as the driving force for precipitation, and is the main influence on the yield obtained in any given process configuration.

The process of the present invention includes a dilution step in which water or a low strength caustic stream is added to the slurry of liquor and hydrate. This decreases the caustic concentration of the liquor, and although the A/C ratio of the liquor remains unchanged, the equilibrium alumina concentration of the liquor is lower at the lower caustic concentration and this increases the supersaturation ofthe liquor. This results in an increase in the driving force for precipitation. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically disclosed. It is to be understood that the invention is considered to encompass all such variations and modifications that are all within its spirit and scope.

Claims

CLAIMS:

1. A process for producing hydrate from a Bayer liquor having a high caustic concentration comprising precipitating hydrate from the Bayer liquor having high caustic concentration, diluting the Bayer liquor to reduce the caustic concentration and precipitating further hydrate.

2. A process as claimed in claim 1 wherein the Bayer liquor comprises a pregnant caustic liquor obtained by digestion of bauxite in a caustic liquor.

3. A process as claimed in claim 1 or claim 2 wherein the Bayer liquor is cooled during precipitation.

4. A process for precipitating alumina trihydrate from a pregnant caustic liquor containing dissolved aluminium values, the process comprising the steps of supplying the pregnant caustic liquor to a precipitation stage of a Bayer plant, precipitating alumina trihydrate from the pregnant liquor to form a slurry of precipitated alumina trihydrate in caustic liquor, adding an aqueous stream to the slurry to thereby dilute the caustic liquor and precipitating further alumina trihydrate.

5. A process as claimed in claim 4 further comprising recovering the alumina trihydrate and calcining the alumina trihydrate to produce smelter grade alumina.

6. A process as claimed in claim 4 or claim 5 wherein the precipitation stage includes an agglomeration stage and a growth stage and the pregnant caustic liquor is supplied to the agglomeration stage.

7. A process as claimed in claim 6 wherein the growth stage comprises a plurality of precipitation tanks and the slurry passes through each of the precipitation tanks.

8. A process as claimed in claim 7 wherein the slurry in each successive downstream precipitation tank of the growth stage is about 1°C to 3°C cooler than the adjacent upstream precipitation tank.

9. A process as claimed in claim 6 wherein the agglomeration stage is operated at a temperature of about 75°C to 85°C.

10. A process as claimed in any one of claims 6 to 9 wherein the slurry in a first tank of the growth stage is 1°C to 3°C cooler than the slurry exiting the agglomeration stage.

11. A process as claimed in any one of claims 4 to 10 wherein the aqueous stream is added to the slurry in a single step.

12. A process as claimed in any one of claims 4 to 10 wherein the aqueous stream is added to the slurry at a plurality of points.

13. A process as claimed in any one of claims 4 to 12 wherein the aqueous stream comprises water, wash water or a caustic liquor having low caustic concentration.

14. A process as claimed in any one of the preceding claims wherein the caustic liquor has a caustic concentration of 200-350g/l before dilution and a caustic concentration of 200-250g/£ after dilution.

15. A process as claimed in any one of claims 4 to 14 wherein the pregnant caustic liquor has an initial A/C ratio of 0.65-0.80 and an initial caustic concentration of 240-275g/ .

16. A process as claimed in claim 15 wherein the slurry is diluted when the A/C ratio in the caustic liquor has fallen to 0.40 to 0.60.

17. A process as claimed in claim 6 wherein a seed slurry is supplied to the agglomeration stage, the seed slurry having a median particle size in the range of 40-150 μm, the seed slurry being fed to give a solids constant of 20-200g/f in the caustic liquor in the agglomeration stage.

18. A process as claimed in claim 6 wherein a seed slurry is supplied to the growth stage, the seed slurry having a medium particles size of 50-110 μm, the seed slurry being fed at a rate to give a solids content of 200-700g/£ at the end of the growth stage.

19. A process as claimed in claim 5 wherein caustic liquor recovered following the recovery of alumina trihydrate from the slurry is treated to increase its caustic concentration and retumed to a Bayer process digestion in which the caustic liquor is contacted with bauxite.

20. A process as claimed in claim 19 wherein part of the water is evaporated from the caustic liquor to increase its caustic concentration.

21. A process as claimed in claim 20 wherein the caustic liquor is heated by low pressure steam from electricity generation to thereby evaporate part of the water therefrom.

22. A process for precipitating hydrate from a caustic liquor containing dissolved aluminium values, the process including diluting the caustic liquor and thereafter precipitating hydrate.