CA2226842A1 - High yield precipitation process - Google Patents
High yield precipitation process Download PDFInfo
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- CA2226842A1 CA2226842A1 CA002226842A CA2226842A CA2226842A1 CA 2226842 A1 CA2226842 A1 CA 2226842A1 CA 002226842 A CA002226842 A CA 002226842A CA 2226842 A CA2226842 A CA 2226842A CA 2226842 A1 CA2226842 A1 CA 2226842A1
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
- C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
- C01F7/00—Compounds of aluminium
- C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
- C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
- C01F7/14—Aluminium oxide or hydroxide from alkali metal aluminates
- C01F7/144—Aluminium oxide or hydroxide from alkali metal aluminates from aqueous aluminate solutions by precipitation due to cooling, e.g. as part of the Bayer process
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/50—Agglomerated particles
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
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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 ~ min~ by the Bayer process.
The Bayler process is widely used to recover alumina from bauxite ores. The 5 Bayer process involves contacting bauxite with a caustic liquor at elevated temp~ldLule to dissolve the alumina contained therein. lnsolubles, commonly called red mud, are separated from the resulting liquor. Dissolved impurities, such as silicates and organics, may also be removed ~om the liquor.
The dissolved alumina is recovered firom the liquor by precipitation. The 10 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 prom~ote precipitation of hydrate and produce particles of required size.
Precipitation trains at alumina refineries include a plurality of stages, usually in the 15 form of separate precipitation tanks, and the liquor is cooled as it moves through each successive tank. At the end of the 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 20 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 alllmin~te (NaAl02) 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 25 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/~ caustic, calculated 30 as Na2CO3. Throughout this specification, all caustic concentrations will be W O 97/03022 PCT/AU~5~ 5 calc~ ted 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. Tndee-l, alumina recovery ofup to 85g/~ A1203 can be obtained from high caustic precipitation. However, thisS 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 10 producing hydrate from a Bayer liquor comprising precipiL~tillg 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 precipiL~Ling alumina trihydrate from a pregnant caustic liquor cont~ining dissolved aluminium15 values, the process comprising the steps of - supplying the pregnant caustic liquor to a precipitation stage of a Bayer plant, - precipilaling 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 achievable30 levels.
It is preferred that the slurry obtained from precipitation in the high caustic e.lvirolmlent 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 Sconcentration. ][t 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 concentlration (A/C) stays substantially the same, because both the alumina and caustic coneentrations are reduced by equal amounts. However, at the lower 10caustic concenttation 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 la~ge amolLmt of dilution stream at a single point in the precipitation train.
15~lt~ tively, the dilution may occur as a plurality of smaller dilution steps, for exarnple, 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 200-350g/Q, 20calculated as Na~CO3 more preferably 240-275g/e. After completion of the dilution (which, as explained above, may be carried out in a single step or in a plurality of smaller steps), tlle caustic concentration is preferably reduced to from 200-250g/Q.
The alumina concentration of a Bayer liquor is usually measured by reporting the ~'C ratio, which is:
A/C = 2 3 9IQ
25The A/C ,~t the start of the 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 eft;ll~;d caustic concentration of 240-275g/Q.
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 plefeldbly 0.45 to 0.50. Addition of the dilution liquor does not significantly alter the A/C but it does reduce the caustic concentration. After completion of the low caustic precipitation, S 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 enco~ asses all seeding strategies within its scope. A cu~ tly preferred 10 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 ~m preferably 40-lOO,um. The fine seed charged to give 20 - 200g/e solids in feed liquor, and more preferably about 100 - 120g/e solids in feed liquor. This seed is preferably added to the agglomeration stage.
The present invention relates to an improved precipitation process used in the production of ~ min~ by the Bayer process.
The Bayler process is widely used to recover alumina from bauxite ores. The 5 Bayer process involves contacting bauxite with a caustic liquor at elevated temp~ldLule to dissolve the alumina contained therein. lnsolubles, commonly called red mud, are separated from the resulting liquor. Dissolved impurities, such as silicates and organics, may also be removed ~om the liquor.
The dissolved alumina is recovered firom the liquor by precipitation. The 10 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 prom~ote precipitation of hydrate and produce particles of required size.
Precipitation trains at alumina refineries include a plurality of stages, usually in the 15 form of separate precipitation tanks, and the liquor is cooled as it moves through each successive tank. At the end of the 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 20 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 alllmin~te (NaAl02) 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 25 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/~ caustic, calculated 30 as Na2CO3. Throughout this specification, all caustic concentrations will be W O 97/03022 PCT/AU~5~ 5 calc~ ted 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. Tndee-l, alumina recovery ofup to 85g/~ A1203 can be obtained from high caustic precipitation. However, thisS 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 10 producing hydrate from a Bayer liquor comprising precipiL~tillg 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 precipiL~Ling alumina trihydrate from a pregnant caustic liquor cont~ining dissolved aluminium15 values, the process comprising the steps of - supplying the pregnant caustic liquor to a precipitation stage of a Bayer plant, - precipilaling 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 achievable30 levels.
It is preferred that the slurry obtained from precipitation in the high caustic e.lvirolmlent 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 Sconcentration. ][t 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 concentlration (A/C) stays substantially the same, because both the alumina and caustic coneentrations are reduced by equal amounts. However, at the lower 10caustic concenttation 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 la~ge amolLmt of dilution stream at a single point in the precipitation train.
15~lt~ tively, the dilution may occur as a plurality of smaller dilution steps, for exarnple, 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 200-350g/Q, 20calculated as Na~CO3 more preferably 240-275g/e. After completion of the dilution (which, as explained above, may be carried out in a single step or in a plurality of smaller steps), tlle caustic concentration is preferably reduced to from 200-250g/Q.
The alumina concentration of a Bayer liquor is usually measured by reporting the ~'C ratio, which is:
A/C = 2 3 9IQ
25The A/C ,~t the start of the 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 eft;ll~;d caustic concentration of 240-275g/Q.
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 plefeldbly 0.45 to 0.50. Addition of the dilution liquor does not significantly alter the A/C but it does reduce the caustic concentration. After completion of the low caustic precipitation, S 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 enco~ asses all seeding strategies within its scope. A cu~ tly preferred 10 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 ~m preferably 40-lOO,um. The fine seed charged to give 20 - 200g/e solids in feed liquor, and more preferably about 100 - 120g/e 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 ,um, more preferably about 75 - 85 ~lm. Coarse seed would be charged to give a solids content of 200 - 700g/Q in the last precipitator, more preferably about 400 - 500g/e solids in the last precipitator. This seed is preferably added to the first tank of the growth stage of the precipitative process.
The tempe~dlul~ 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 WO 97r03022 PCT/AU96/0043S
precipitation su;,tably having a residence time of about 15 - 20 hours.
The i,~ oved precipitation process of the present invention allows a yield of up to 95 to l.?.Og/Q Al203 or higher. This is considerably higher than current best practice that obtains hydrate yields of about 85g/Q Al203.
The slur~ y from the last precipitator is treated to separate the solids from the liquor. After removing suitable quantities of the solids for see~ling, 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 r eturned 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 Na2CO3. 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 convelllional precipitation step there is too much water added to the circuit and as a consequence the evaporation capacity 5 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 10 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 15 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 the20 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 25 improved process may achieve higher yields by recovering more hydrate from the area where low soda hydrate is produced, i.e. within latter part of the 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 30 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 higl1 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 temp~ld~u,es 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 ofthose 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 embo~iment~ 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 of the 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 of the 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 skategy. The slurry resulting from the high caustic precipitation is diluted and a low caustic precipitation then W O 97t03022 PCT/AU96/0043S
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 S 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 10 precipitators Gl, G2,G Last. A Bayer liquor lO 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 throu~h 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 1~ and spent liquor stream 20. The tertiary seed is deliquored and washed at 22 to produce tertiary seed 12and spent liquor stream 24. The product hydrate is ~vashed 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 precipitatorsGl, G2,...G Last. Pregnant liquor 110 and tertiary seed 112 are supplied to 30 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 cl-~n~ing the A/C ratio and this increases the yield of hydrate in the overall process. The slur~ leaving the last precipitator 5 (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 thespent liquor to ia caustic concentration suitable for use in digesting bauxite.
10Figure 5 is another embodiment of the precipitation process of the present invention. The flowsheet of Figure S 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-1).
In the em~bodiments shown in Figures 3, 4 and 5, like features are denoted 15by 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 stre;am 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 20 stream to one precipitator, as two streams to two precipitators or as a number of streams to a nun~ber of precipitators. All such additions fall within the scope of the 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 25 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 30 for further seed ~with increased residence time (Case C). The results are shown in Table 1.
Base Case A B C D E
Caustic (in)g/Q 250 250 250 250 275 300 Caustic (out)g/Q 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 5 Yieldg/Q 86.7 90.3 94.2 95 .2 1 05 11 5 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 10 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 tempeldLu,e profiles detailed in Tables 2 and 3 below as the Profile 1 and Profile 2, respectively.
W O 97/03022 PCT/AU9~C'~S
TABL]E 2 Temperature Profile 1 Tank Temperature (~C) Holding Time (Hours) 1 &2 78 5.5 Total= 45.5 -W O 97/03022 PCT/A~'~5.'.~'~5 Temperature Profile 2 TankTemperature (~C)Holding Time (Hours) 1 &2 78 5.5 3 75.2 1.95 4 72.4 1.95 69.6 1.95 6 66.8 1.95 7 64 1.95 8 61.2 1.95 9 58.4 1.95 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 of the stage at 72.4~C. In each of Tables 5 and 6, the liquor was 20 diluted with the aim of obtaining a caustic concentration within the range of 200-250 g/Q 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.
W O 97/03022 PCT/AU95J~C~S
Single Pass Test Parameter Start End A/C 0.640 0.325 CS (g/Q as Na2CO3) 234 217 C/S 0.90 o.so Oxalate (g/e as Na2C204) n/a (~1) n/a Organics n/a (~10) n/a Yield (g/e as Al2O3) 74 TABLE S
Single Pass Test Pa~rameter Start End A/C 0.769 0.334 CS (gle as Na2CO3) 362 242 15 C/S 0.91 0.91 Oxalate (gl~ as Na2C204) n/a (~1) n/a Organics n/a (~10) n/a Yield (g/e as Al2O3) 158 25 Hour Single Pass Par,ameter Start End A/C 0.656 0.347 CS (g/~ as Na2CO3) 275 227 I~/S 0.91 0.90 ~= 25Oxalate (gle as Na2C204) 0.72 1.1 Organics n/a (~10~ n/a Yield (g/e 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:
SAverage Values from Dilution Recycle Test Parameter Start End A/C 0.656 0.3 14 CS (g/Q as Na2CO3) 275 247 C/S 0.91 0.91 10Oxalate (g/Q as Na2C2O4) 0 90 Organics 8.7 8.2 Yield (g/~ as Al2O3) 94 Average Values from High Organics Dilution Recycle l SParameter Start End A/C 0.716 0.339 CS (glQ as Na2CO3) 299 224 C/S 0.87 0.98 Oxalate (g/e as Na2C2O4) n/a n/a 20Organics 32.1 23.9 Yield (g/Q 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 25 given in Table 9.
W O 97t03022 PCTIAU9610043S
Single Pass Bottle Tests with No Dilutioll Test 1 Test 2 Test 3 Parameter Start Finish Start Finish Start Finish CS (gl~ 212.0 211.0 229.7 243.6 224.8 234.0 S Na~CO3) A/C 0.659 0.37 0.668 0.341 0.692 0.374 C/S 0.875 - 0.907 - 0.792 Yield (glp 61 70 68 Al203) In order to allow for easy comparison between the experimen~l 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/~
as Al203) obtained from the experiments detailed in Tables 4 to 9.
Comparison l[able Caustic Concentration Yield Comment Start Finish 4 234 217 74 Dilution 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 ~unmlal;sed in Table 10 show that the addition of water (dilution) during the precipitation process resulted in an increased yield of alumina.
Current precipitation cilcuil~ produce a yield of alumina in the range 60-80g/~. This is a low productivity, at approximately half the ~lllmin~ capacity of the 5liquor. The yield is ~letern ined by a number of solution and process conditions, chiefly the pregnant liquor A/C, caustic soda concentration, and inlet/outlet tempc.dlules. The tempcldlulc profile plays an important role in the quality of the product, influencing such aspects as soda percentage and strength. The liquor conditions det~rmine the equilibrium alumina concentration, and as Bayer liquors10are 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 15or a low strength caustic stream is added to the slurr- 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 islower at the lower caustic concentration and this increases the supersaturation of the liquor. This results in an increase in the driving force for precipitation.
20Those 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.
The tempe~dlul~ 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 WO 97r03022 PCT/AU96/0043S
precipitation su;,tably having a residence time of about 15 - 20 hours.
The i,~ oved precipitation process of the present invention allows a yield of up to 95 to l.?.Og/Q Al203 or higher. This is considerably higher than current best practice that obtains hydrate yields of about 85g/Q Al203.
The slur~ y from the last precipitator is treated to separate the solids from the liquor. After removing suitable quantities of the solids for see~ling, 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 r eturned 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 Na2CO3. 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 convelllional precipitation step there is too much water added to the circuit and as a consequence the evaporation capacity 5 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 10 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 15 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 the20 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 25 improved process may achieve higher yields by recovering more hydrate from the area where low soda hydrate is produced, i.e. within latter part of the 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 30 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 higl1 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 temp~ld~u,es 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 ofthose 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 embo~iment~ 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 of the 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 of the 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 skategy. The slurry resulting from the high caustic precipitation is diluted and a low caustic precipitation then W O 97t03022 PCT/AU96/0043S
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 S 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 10 precipitators Gl, G2,G Last. A Bayer liquor lO 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 throu~h 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 1~ and spent liquor stream 20. The tertiary seed is deliquored and washed at 22 to produce tertiary seed 12and spent liquor stream 24. The product hydrate is ~vashed 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 precipitatorsGl, G2,...G Last. Pregnant liquor 110 and tertiary seed 112 are supplied to 30 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 cl-~n~ing the A/C ratio and this increases the yield of hydrate in the overall process. The slur~ leaving the last precipitator 5 (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 thespent liquor to ia caustic concentration suitable for use in digesting bauxite.
10Figure 5 is another embodiment of the precipitation process of the present invention. The flowsheet of Figure S 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-1).
In the em~bodiments shown in Figures 3, 4 and 5, like features are denoted 15by 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 stre;am 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 20 stream to one precipitator, as two streams to two precipitators or as a number of streams to a nun~ber of precipitators. All such additions fall within the scope of the 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 25 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 30 for further seed ~with increased residence time (Case C). The results are shown in Table 1.
Base Case A B C D E
Caustic (in)g/Q 250 250 250 250 275 300 Caustic (out)g/Q 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 5 Yieldg/Q 86.7 90.3 94.2 95 .2 1 05 11 5 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 10 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 tempeldLu,e profiles detailed in Tables 2 and 3 below as the Profile 1 and Profile 2, respectively.
W O 97/03022 PCT/AU9~C'~S
TABL]E 2 Temperature Profile 1 Tank Temperature (~C) Holding Time (Hours) 1 &2 78 5.5 Total= 45.5 -W O 97/03022 PCT/A~'~5.'.~'~5 Temperature Profile 2 TankTemperature (~C)Holding Time (Hours) 1 &2 78 5.5 3 75.2 1.95 4 72.4 1.95 69.6 1.95 6 66.8 1.95 7 64 1.95 8 61.2 1.95 9 58.4 1.95 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 of the stage at 72.4~C. In each of Tables 5 and 6, the liquor was 20 diluted with the aim of obtaining a caustic concentration within the range of 200-250 g/Q 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.
W O 97/03022 PCT/AU95J~C~S
Single Pass Test Parameter Start End A/C 0.640 0.325 CS (g/Q as Na2CO3) 234 217 C/S 0.90 o.so Oxalate (g/e as Na2C204) n/a (~1) n/a Organics n/a (~10) n/a Yield (g/e as Al2O3) 74 TABLE S
Single Pass Test Pa~rameter Start End A/C 0.769 0.334 CS (gle as Na2CO3) 362 242 15 C/S 0.91 0.91 Oxalate (gl~ as Na2C204) n/a (~1) n/a Organics n/a (~10) n/a Yield (g/e as Al2O3) 158 25 Hour Single Pass Par,ameter Start End A/C 0.656 0.347 CS (g/~ as Na2CO3) 275 227 I~/S 0.91 0.90 ~= 25Oxalate (gle as Na2C204) 0.72 1.1 Organics n/a (~10~ n/a Yield (g/e 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:
SAverage Values from Dilution Recycle Test Parameter Start End A/C 0.656 0.3 14 CS (g/Q as Na2CO3) 275 247 C/S 0.91 0.91 10Oxalate (g/Q as Na2C2O4) 0 90 Organics 8.7 8.2 Yield (g/~ as Al2O3) 94 Average Values from High Organics Dilution Recycle l SParameter Start End A/C 0.716 0.339 CS (glQ as Na2CO3) 299 224 C/S 0.87 0.98 Oxalate (g/e as Na2C2O4) n/a n/a 20Organics 32.1 23.9 Yield (g/Q 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 25 given in Table 9.
W O 97t03022 PCTIAU9610043S
Single Pass Bottle Tests with No Dilutioll Test 1 Test 2 Test 3 Parameter Start Finish Start Finish Start Finish CS (gl~ 212.0 211.0 229.7 243.6 224.8 234.0 S Na~CO3) A/C 0.659 0.37 0.668 0.341 0.692 0.374 C/S 0.875 - 0.907 - 0.792 Yield (glp 61 70 68 Al203) In order to allow for easy comparison between the experimen~l 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/~
as Al203) obtained from the experiments detailed in Tables 4 to 9.
Comparison l[able Caustic Concentration Yield Comment Start Finish 4 234 217 74 Dilution 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 ~unmlal;sed in Table 10 show that the addition of water (dilution) during the precipitation process resulted in an increased yield of alumina.
Current precipitation cilcuil~ produce a yield of alumina in the range 60-80g/~. This is a low productivity, at approximately half the ~lllmin~ capacity of the 5liquor. The yield is ~letern ined by a number of solution and process conditions, chiefly the pregnant liquor A/C, caustic soda concentration, and inlet/outlet tempc.dlules. The tempcldlulc profile plays an important role in the quality of the product, influencing such aspects as soda percentage and strength. The liquor conditions det~rmine the equilibrium alumina concentration, and as Bayer liquors10are 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 15or a low strength caustic stream is added to the slurr- 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 islower at the lower caustic concentration and this increases the supersaturation of the liquor. This results in an increase in the driving force for precipitation.
20Those 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 (21)
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/~ 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/~ 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 returned to a Bayer process digestion in which thecaustic 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.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPN4107 | 1995-07-11 | ||
| AUPN4107A AUPN410795A0 (en) | 1995-07-11 | 1995-07-11 | High yield precipitation process |
| AUPN4785 | 1995-08-14 | ||
| AUPN4785A AUPN478595A0 (en) | 1995-08-14 | 1995-08-14 | High yield precipitation process |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2226842A1 true CA2226842A1 (en) | 1997-01-30 |
Family
ID=25644991
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002226842A Abandoned CA2226842A1 (en) | 1995-07-11 | 1996-07-11 | High yield precipitation process |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0839118A4 (en) |
| KR (1) | KR19990028922A (en) |
| BR (1) | BR9609429A (en) |
| CA (1) | CA2226842A1 (en) |
| WO (1) | WO1997003022A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU2008232309A1 (en) * | 2007-03-27 | 2008-10-02 | Alcoa Of Australia Limited | Method for preparing aluminium oxide |
| AU2018256247B2 (en) * | 2017-04-19 | 2020-11-12 | Korea Institute Of Geoscience And Mineral Resources | Method for smelting ilmenite using red mud |
| CN113608506B (en) * | 2021-06-18 | 2022-10-25 | 东北大学 | Intelligent detection device for alumina operation index |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3486850A (en) * | 1967-02-16 | 1969-12-30 | Kaiser Aluminium Chem Corp | Flash cooling of liquor during the continuous precipitation of alumina hydrate from bayer process liquor |
| US4051222A (en) * | 1971-09-21 | 1977-09-27 | Alcan Research And Development Limited | Coarsening of partially calcined alumina dust |
| NO135059C (en) * | 1975-04-16 | 1977-02-02 | Norsk Hydro As | |
| US4305913A (en) * | 1980-08-06 | 1981-12-15 | Kaiser Aluminum & Chemical Corporation | Alumina hydrate production from Bayer liquor by seeding |
| FR2591581B1 (en) * | 1985-12-17 | 1990-05-25 | Pechiney Aluminium | PROCESS FOR OBTAINING HIGH PRODUCTIVITY OF ALUMINUM TRIHYDROXIDE, HIGH PURITY AND MEDIAN DIAMETER LESS THAN 4 MICROMETERS, ADJUSTED ON REQUEST. |
| HU201276B (en) * | 1988-10-27 | 1990-10-28 | Magyar Aluminium | Process for producing alum earth from bauxite containing gibbsite |
| FR2712275B1 (en) * | 1993-11-09 | 1995-12-15 | Pechiney Aluminium | Process for the treatment of bauxites containing a mixture of alumina trihydrate and alumina monohydrate. |
-
1996
- 1996-07-11 CA CA002226842A patent/CA2226842A1/en not_active Abandoned
- 1996-07-11 KR KR1019980700224A patent/KR19990028922A/en not_active Application Discontinuation
- 1996-07-11 BR BR9609429-0A patent/BR9609429A/en not_active Application Discontinuation
- 1996-07-11 EP EP96922684A patent/EP0839118A4/en not_active Withdrawn
- 1996-07-11 WO PCT/AU1996/000435 patent/WO1997003022A1/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| WO1997003022A1 (en) | 1997-01-30 |
| EP0839118A4 (en) | 1998-12-09 |
| EP0839118A1 (en) | 1998-05-06 |
| BR9609429A (en) | 2000-04-25 |
| KR19990028922A (en) | 1999-04-15 |
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