CA2140773A1 - Process for extracting alumina from bauxite - Google Patents
Process for extracting alumina from bauxiteInfo
- Publication number
- CA2140773A1 CA2140773A1 CA002140773A CA2140773A CA2140773A1 CA 2140773 A1 CA2140773 A1 CA 2140773A1 CA 002140773 A CA002140773 A CA 002140773A CA 2140773 A CA2140773 A CA 2140773A CA 2140773 A1 CA2140773 A1 CA 2140773A1
- Authority
- CA
- Canada
- Prior art keywords
- liquor
- alumina
- bauxite
- caustic
- mixture
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- 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
-
- 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
- C01F7/062—Digestion
Abstract
A process for extracting alumina from bauxite comprising the steps of concentration a portion of spent caustic liquor (18), combining said concen-trated spent caustic liquor with a bauxite slurry (3) to provide a final aluminato caustic ratio (A/C) of greater than 0.7 in the slurry/liquor mixture, heatingthe mixture by means of a heat exchanger (5), subjecting the mixture to diges-tion in a tank (7) at a temperature below that at which reversion of dissolved gibbsite to boehmite may occur to thereby extract alumina from the gibbsite bauxite in the slurry, separating the high alumina pregnant liquor (9) from the boehmite containing residue in a pressure decanter (8), mixing the residue with the remaining portion of unconcentrated spent caustic liquor (18) and subject-ing the mixture to high temperature digestion in a tank (11) to extract the re-maining alumina from the residue, subjecting the digested mixture to flash cooling in flash tanks (13), and separating the alumina rich liquors from the high alumina pregnant liquor and from the digested mixture in an atmospheric separator (14) or a pressure decanter (17).
Description
WO94/0~16 21 4 0 7 7 3 PCT/AU93/00369 PROCESS FOR EXTRACTING ALUMINA FROM BAUXITE
Field of the Invention This invention relates to processes for the production of alumina from bauxite.
Ba~ o~ n~ of the Invention r.n~al Most alumina is pro~l~ce~ using the Bayer process. A high volume of caustic liquor is circulated at a controlled ~o~entration. In the digestion part of the circuit, the caustic is heated and reacted with bauxite to increase the alumina co~c~ntration (pregnant liquor). After the bauxite residue is separated by settling, the caustic liquor is cooled, ~ e~, and alumina is precipitated as product. The spent liquor is recirculated.
The quantity of circulating liquor can be as high as 20 m3/t alumina proA~ce~. Expressed in terms of productivity, this is only 50 g/litre. To increase production of existing plants or to reduce the capital cost of new plants, it is important to achieve high yield. Only a few alumina plants are achieving l80 g/l yield today.
For high yield, the digestion step has to achieve a high alumina cQ~-~ntration. A de~ e_ of supersaturation can be tolerated during the residue separation step. Hos~e~e~, depen~ing on certain factors, there is a limiting supersaturation above which alumina will be lost in increasing amounts to the residue.
Usually the digestion system operates at the highest flowrate that the equipment and steam heating will allow. Bauxite is then ~ in a controlled manner to achieve a desired pregnant liquor alumina cQnc~ntration.
The alumina ro~c~ntration is usually expressed as a ratio on caustic, A/C. As an example, the pregnant liquor can be 0.72 A/C (gAl2O3/gNa2CO3) at say 105C for 200 g/l caustic expressed as Na2CO3 (representing sodium hydroxide associated with the dissolved alumina as well as free sodium hydroxide). The spent liquor A/C will depend on precipitation conditions but can be say 0.37 A/C.
Free r ~ic Usually the spent liquor to digestion is evaporated to some degree and fresh caustic make-up added at this point, such that W094/0~16 PCT/AU93/~369 the conc~ntration may be 240 g/l caustic at 0.36 A/C. The so called free caustic of this liquor is defined as: 240 - 240 x 0.36 x 106/lOZ = 150 g/l. (representing sodium h~oxide) Depen~; ng on the velocity and temperature of the liquor heating to digestion, there will be an upper limit for free caustic. For a high temperature process required to digest boehmitic or AiAcporic bauxites the limit may be say 140 g/l free caustic. Above this limit the heater tubes and piping may erode/co G~e rapidly.
One way of dropping the free caustic is to-add bauxite that will partially dissolve as the liquor is heated so as to raise the A/C. Some modern high temperature plants use special slurry heaters, or tube digestion as it is known, to ove - the free caustic. Hcs~e~_ , most plants use shell and tube ~Y~h~n~Jc~s for liquor heating, and ~dA;ng a slurry is not desirable.
Bau~ite ~.
Bauxites are minerals rich in alumina and low in silica.
The usual minerals contA;n;ng only alumina are gibbsite, boehmite and diaspore. This is their respective order of solubility, for example, to achieve 0.72 A/C at say 200 g/l caustic, the digestion temperature required would be:
Gibbsite, Alz03.3H20 : 140 - 145C
(3.2 bar(a) vapour pressure) Boehmite, Al203H20 : 260 - 265C
(41 bar(a) v~o~r pressure) Diaspore, Al203H20 : impractical The digestion L~ ,_ ature for boe~ ite can be raised higher for higher A/C, hcwe~e., if the temperature for gibbsite is raised higher, there is a risk of the dissolved alumina precipitating out as boehmite, which is the stable species above say 150C. This is particularly so if the gibbsitic bauxite also contains boehmite to act as seed.
Mixed gibbsitic-boehmi.ic bauxi~es contAining above say 5%
as boehmite alumina are usually processed in a single digestion step under boehmite extraction conditions.
Diqe~tion PloductivitY
The digestion productivity is a measure of the alumina WO94/0~16 21 4 0 7 7 3 PCT/AU93/00369 co~c~ntration increase due to bauxite for a given flowrate of liquor. If there is a limit to the flowrate, then potentially the liquor can be e~d~p,ated so that the mass flow of caustic is increased and more bauxite can be added in ~,v~-rtion.
For a low temperature plant processing only gibbsitic bauxite, more e~a~G,ation is required to keep the circuit in h-l. e, since less fl~Ch ing o~u-~ on cooling than with a high temperature digestion plant.
For gibbsite digestion, it is possible therefore to achieve an A/C greater than 0.72 h~C~l~ce the caustic strength is above 200 g/l while remaining at 145~C. As well as the evaporation aspect, indirect live steam heating is also used. For high temperature digestion, indirect heating is not so easy and direct steam injection is more common although counter-productive.
Free caustic is not such a ~onc~rn for liquor heating in a low temperature plant. It becomes a cQn~rn in a high temperature plant processing boehmitic bauxite in which the spent liquor is heated separately and the bauxite added direct to the digester (two stream process). In a two stream process, free caustic will impose a limit to the productivity.
In a high temperature tube digestion design or similar single stream process (liquor plus bauxite slurry heating) free caustic is not a limitation to the productivity. Although tube digestion has desirable process features, there are some mech-n;cal limitations to pumping and slurry heating, and co~s~,~,tly the production rate of such a unit is limited.
If the digestion A/C is higher than the degree of supersaturation allowable for residue separation, then some spent liquor trim can be added. this bypass of spent liquor around digestion represents the productivity gain achieved.
One way of achieving digestion productivity with a boehmitic bauxite is to add a gibbsitic bauxite downstream during flash cooiing at say 17G - 150C, and to increase the achieved boehmite A/C. This approach is known as "sweetening" and is practiced by a number of plants in various forms. Sweetening can also in~.~.ate spent liquor trim. Sweet~ning for some plants can be a ~,lv~,ient retrofit, if the gibbsitic bauxite is available and 21~ 0 7 7 3 R~C~ 2 8 FEB 1~''' handling of two bauxites is practical.
Irrespective of the source of alumina, the alumina precipitates out as gibbsite, as the temperature is usually well below 150C. with good residue settler and washer design and operation, the degree of supersaturation can be as high as 25%
of equilibrium solubility of gibbsite. Some alumina loss will occur (known as autoprecipitation) but is usually about 0.015 A/C.
For example, if the equilibrium solubility of gibbsite is 0.58 A/C (220 g/l caustic, 107C and impure plant liquor) then the allowable A/C is 0.58 x 1.25 = 0.725 A/C. Allowing for autoprecipitation, the digestion A/C = 0.725 ~ 0.015 = 0.74 A/C.
If the digestion unit can achieve higher than 0.74 A/C then trim liquor has to be added. Usually the digestion conditions of caustic strength and temperature have to be such that the equilibrium solubility of boehmite or gibbsite from the bauxite is 0.74 1 0.025 margin = 0.765 A/C.
SummarY of Invention and Object The object of the invention is to provide an improved process for extracting alumina from bauxite in which the problems discussed above are at least ameliorated.
The invention provides a process for extracting alumina from bauxite comprising the steps of combining caustic liquor with a bauxite slurry to provide an alumina to caustic ratio (A/C) of greater than about 0.70 in the slurry/liquor mixture, subjecting the mixture to digestion at a temperature below that at which reversion of dissolved gibbsite to boehmite may occur, to extract alumina from the gibbsite bauxite in the slurry, separating the high alumina pregnant liquor from the boehmite cont~ining residue, ;Y;ng the residue with caustic liquor and subjecting the mixture to a high temperature digestion process to extract the rema;n;ng alumina from the residue, and separating the alumina rich liquor, said alumina to caustic ratio of the slurry/liquor mixture being achieved in part by the step of evaporating spent caustic liquor supplied to the low temperature digestion step.
IPI~A/AU
2140773 RECÉIVED 2~ FEB 1994 4a If desired fresh caustic may be added to the slurry to achieve the desired high A/C.
~PP.A/AI I
If desired, the bauxite slurry may be subjected to a known predesilication process prior to heating. Alternatively, two stage post-desilication using lime has been found to be satisfactory.
The atmospheric settler or pressure decanter following the high temperature digestion step is preferably operated at an A/C
which i8 lower than that of the low Le ,-rature decanter to minimize auLG~.e_ipitation during subsequent residue w-shing.
The invention also provides alumina when ~,o.l...-~A by the process defined in the pr~ce~ing paragraphs.
Brief descriPtion of the drawings In order that the invention may be more readily understood, two preferred emho~; - ts of the invention will now be described with refe,~,ce to the accompanying drawings in which:
Figure 1 is a schematic flow chart of a bauxite digestion ~ess ~mhodying the invention, and Figure 2 is a similar schematic flow diagram of a modified bauxite digestion process embodying the invention.
DescriPtion of Preferred Embodiments The preferred ~o~ se5 are applicable to mixed bauxites cont~ining a high ~.G~o-Lion of gibbsite together with less soluble boehmite and diaspore. The proces~es can also be applied to a mixed bauxite of boehmite and diaspore. The ~-~--~sses can also be applied to gibbsite bauxite where an over-charged digestion (for high A/C) can be followed by a re-digestion for complete extraction. The following description relates to gibbsitic-boehmitic bauxite and refers to Figs. 1 and 2 of the drawings, but the ~G~e-S is equally applicable to other bauxites.
The preferred process involves two sequential bauxite digestion steps 1 and 2 to extract gibbsite at low temperature and high caustic, followed by boehmite at high temperature and low caustic to produce alumina at high liquor productivity. The process is particularly suited to retrofitting a two stream high t~m~erature digestion plant, as will be described further below.
The low temperature digestion step 1 is designed to extract essentially all the gibbsite from the bauxite feed, and the WO94/0~16 PCT/AU93/00369 ~4~'l3 6 process is therefore particularly beneficial to bauxites having a high ~-o~o-Lion of gibbsite (eg Comalco bauxite containing about 75% of available alumina as gibbsite). The conditions are designed to achieve a very high A/C liquor~with minimal eve-~ion and without free caustic limitations.
In this step 1, bauxite slurry 3 is fed with spent liquor 18 and caustic make-up 4 to a heat ~vch~nger 5, fed with flash and low pressure live steam 6. The low temperature digestion conditions are controlled to limit the temperature to below 150C
to avoid eve~ion of dissolved gibbsite to boehmite. To achieve the required high A/C of greater than 0.70 A/C strong caustic liquor is required. This is achieved by evaporation 21 of the spent liquor 18 and by indirect heating in 5. Caustic make-up 4 can also be a~e~.
The strong spent liquor after evaporation can then be heated up to the free caustic limitation, which depPn~ng on heater design can range from 130 - 150C. If for example the limitation is 130C, then the spent liquor can be heated from 85C say to 130C in shell and tube heat PY~h-n~srs (not shown) using flash~
steam. The bauxite slurry at 70C say can be heated up to a similar temperature in slurry heaters. Both streams can then be combined for ru~ L~e~ heating using flash and indirect low pressure live steam (not shown). Alternatively, both liquor and bauxite slurry can be combined for all the heating as a single stream in suitable heaters 5 as shown.
The heated slurry from the heat eY~h~ngers 5 is held at temperature in a well stirred vessel 7 or holding tube to achieve the required al~ ; n~ extraction.
Depending on the nature of the bauxite, some predesilication may be necessary prior to heating the slurry. This requires heating the ground bauxite caustic liquor slurry up to say 100C
and holding it in stirred vessels (not shown) to allow any silica such as kaoiinite to dissolve and then to reprecipitate as desilication product (sodium aluminium silicate). Without predesilication, there is the risk of desilication product scaling out and fouling the heaters. As indicated in the Example, the ~lo~ess has been successfully operated using a post-WO94/0~16 21 ~ 0 7 7 3 PCT/AU93/00369 desilication step in which about 96% of the silica is ,.- ~ed.
As well as for gibbsite extraction, some of the digestion holding time may be required for further desilication so as to avoid excessive silica levels rem~;n;ng in liquor.
Immediately following the low temperature digestion, the boehmite con~ining residue is ~eparated in a pressure decanter 8 at digestion temperature using a suitable synthetic flocculent.
Such a device has been described in US Patent 5080803 and el~ew~-e. H$gh underflow density and good overflow clarity can be achieved.
The underflow at high density and temperature is pumped with suitable pumps (not shown) to the high temperature digestion step 2.
The pressure decanter overflow 9 at high A/C pregnant liquor is flash cooled in flash tanks 13 to atmospheric boiling point for dilution, filtering, cooling and se~;ng to produce alumina product. If required, post-desilication can be carried out on the overflow at temperature before flash cooling.
Some of the desilicated decanter overflow may be diverted to the remaining 1~nçonç~ntrated spent liquor via a free caustic trim line lO to control the free caustic to allow high temperature heating for the high temperature digestion step 2.
The low temperature digestion pressure decanter underflow is pumped direct to a high temperature digester vessel ll, with or without further heating. Alternatively it can be mixed with the incoming ~lnconçentrated spent liquor after an a~ Gpriate spent liquor heating and final pumping stage. The single stream can then be further heated in suitable heaters by flash steam and high pressure live steam or other heat transfer medium (not shown). High pressure steam 12 is added direct to the digester as required.
The temperature and time of the high temperature digestion vessel ll, or holding tube, is selected so as to extract ~he ~ ~;ning boehmite, at the caustic conç~ntration prevailing to achieve a medium to normal pregnant liquor A/C.
Slurry from the high temperature digestion vessel ll is flash cooled in flash tanks 13 to atmospheric conditions for WO94/0~16 PCT/AU93/00369 2~4~13 residue removal by settling in a settler or separator 14 followed by w~h;ng and disposal of red mud via 16. The settler overflow pregnant liquor 15 is diluted, filtered, cooled and seeded in a known manner not shown for alumina production.
Alternatively, as shown in Fi-gure 2, a pressure decanter 17 can be applied at the appropriate temperature down the flash train 13. The overflow can then 3Oin the overflow of the main low temperature digestion decanter for common flash cooling.
The pressure decanter underflow of final residue or "red mud" can be cooled indirectly or directly by wash water prior to hi ng and disposal.
The high temperature digestion pressure decanter 17 will nG~ -lly be at a lower A/C than for the main decanter. This will minimise au~G~,ecipitation during subsequent red mud wAshing.
As a further refinement (not shown), the high A/C and normal A/C pregnant liquor streams 9 and 15 can be cooled and diluted separately for a more optimised precipitation of alumina.
If necessary, flash tank sweetening and spent liquor trim can still be applied to the high temperature digestion step (not shown).
The above embodiments of the invention are particularly suitable for retrofitting of existing alumina plants processing a high gibbsite, moderate boehmite bauxite using a two stream digestion process.
The bulk of the digestion is carried out at low temperature freeing up the high temperature equipment for potentially greater p~;ty and efficiency.
A large ~.o~o~ion of the previous high pressure live steam heating is replaced by low pressure steam. this allows better utilization and rApAçity of existing boilers and electrical generators.
The high A/C pregnant liquor from the low temperature digestion has the potential to replace part or all of any sweetening bauxite previously used.
-~L~
Laboratory digestion tests were carried out using Weip bauxite and synthetic Bayer liquor at various caustic strengths, temperature, time and bauxite charge, without Predesilication.
The aim was to find conditions for high A/C from gibbsite dissolution with minimal e~c ~ion (auLo~ e_ipitation) to boehmite or gibbsite. The following results were obt~ine~ from one of the tests:-Bauxite: 54.6% Al20" 11.4% Fe20" 2.59% Ti02 5.92% Si02 (incl. 1.3% quartz), 25.19% LOI (loss on ignition) Distribution of bauxite Al20,: 3.93% in kaolini'~
0.48% in iron minerals 40.34% in gibbsite 9.85% in boehmite Synthetic spent liquor: 346.5 g/l caustic soda as Na2C03 (C) 0.394 A/C
0.845 C/S (caustic/(caustic + carbonate) 2.5 g/kg Si02/C
Digestion conditions: 140C
2 minute digestion time after 5-minute heat up 290 g dry bauxite/litre spent liquor Pregnant liquor result: 301.2 g/l caustic soda (C) 0.778 A/C
0.843 C/S
28.6 g/kg Si02/C
Kaolinite reacted: 93%
Gibbsite reacted: 92%
Post Desilication: Because of the high residual silica in pregnant liquor, post desilication was necessary. A 2-stage procedure was carried out using DSP seed followed by lime, both at 140C.
W O 94/02416 P(~r/AU93/00369 ~4~ 3 Pregnant liquor after DSP s~e~;ng: 313.0 g/l caustic soda (C) 0.750 A/C
0.843 C/S `;`
5.3 g/kg SiO2/C
Pregnant liquor after lime treatment: 306.0 g/l caustic soda (C) 1.5% CaO/ 0.735 A/C
bauxite 0.848 C/S
Field of the Invention This invention relates to processes for the production of alumina from bauxite.
Ba~ o~ n~ of the Invention r.n~al Most alumina is pro~l~ce~ using the Bayer process. A high volume of caustic liquor is circulated at a controlled ~o~entration. In the digestion part of the circuit, the caustic is heated and reacted with bauxite to increase the alumina co~c~ntration (pregnant liquor). After the bauxite residue is separated by settling, the caustic liquor is cooled, ~ e~, and alumina is precipitated as product. The spent liquor is recirculated.
The quantity of circulating liquor can be as high as 20 m3/t alumina proA~ce~. Expressed in terms of productivity, this is only 50 g/litre. To increase production of existing plants or to reduce the capital cost of new plants, it is important to achieve high yield. Only a few alumina plants are achieving l80 g/l yield today.
For high yield, the digestion step has to achieve a high alumina cQ~-~ntration. A de~ e_ of supersaturation can be tolerated during the residue separation step. Hos~e~e~, depen~ing on certain factors, there is a limiting supersaturation above which alumina will be lost in increasing amounts to the residue.
Usually the digestion system operates at the highest flowrate that the equipment and steam heating will allow. Bauxite is then ~ in a controlled manner to achieve a desired pregnant liquor alumina cQnc~ntration.
The alumina ro~c~ntration is usually expressed as a ratio on caustic, A/C. As an example, the pregnant liquor can be 0.72 A/C (gAl2O3/gNa2CO3) at say 105C for 200 g/l caustic expressed as Na2CO3 (representing sodium hydroxide associated with the dissolved alumina as well as free sodium hydroxide). The spent liquor A/C will depend on precipitation conditions but can be say 0.37 A/C.
Free r ~ic Usually the spent liquor to digestion is evaporated to some degree and fresh caustic make-up added at this point, such that W094/0~16 PCT/AU93/~369 the conc~ntration may be 240 g/l caustic at 0.36 A/C. The so called free caustic of this liquor is defined as: 240 - 240 x 0.36 x 106/lOZ = 150 g/l. (representing sodium h~oxide) Depen~; ng on the velocity and temperature of the liquor heating to digestion, there will be an upper limit for free caustic. For a high temperature process required to digest boehmitic or AiAcporic bauxites the limit may be say 140 g/l free caustic. Above this limit the heater tubes and piping may erode/co G~e rapidly.
One way of dropping the free caustic is to-add bauxite that will partially dissolve as the liquor is heated so as to raise the A/C. Some modern high temperature plants use special slurry heaters, or tube digestion as it is known, to ove - the free caustic. Hcs~e~_ , most plants use shell and tube ~Y~h~n~Jc~s for liquor heating, and ~dA;ng a slurry is not desirable.
Bau~ite ~.
Bauxites are minerals rich in alumina and low in silica.
The usual minerals contA;n;ng only alumina are gibbsite, boehmite and diaspore. This is their respective order of solubility, for example, to achieve 0.72 A/C at say 200 g/l caustic, the digestion temperature required would be:
Gibbsite, Alz03.3H20 : 140 - 145C
(3.2 bar(a) vapour pressure) Boehmite, Al203H20 : 260 - 265C
(41 bar(a) v~o~r pressure) Diaspore, Al203H20 : impractical The digestion L~ ,_ ature for boe~ ite can be raised higher for higher A/C, hcwe~e., if the temperature for gibbsite is raised higher, there is a risk of the dissolved alumina precipitating out as boehmite, which is the stable species above say 150C. This is particularly so if the gibbsitic bauxite also contains boehmite to act as seed.
Mixed gibbsitic-boehmi.ic bauxi~es contAining above say 5%
as boehmite alumina are usually processed in a single digestion step under boehmite extraction conditions.
Diqe~tion PloductivitY
The digestion productivity is a measure of the alumina WO94/0~16 21 4 0 7 7 3 PCT/AU93/00369 co~c~ntration increase due to bauxite for a given flowrate of liquor. If there is a limit to the flowrate, then potentially the liquor can be e~d~p,ated so that the mass flow of caustic is increased and more bauxite can be added in ~,v~-rtion.
For a low temperature plant processing only gibbsitic bauxite, more e~a~G,ation is required to keep the circuit in h-l. e, since less fl~Ch ing o~u-~ on cooling than with a high temperature digestion plant.
For gibbsite digestion, it is possible therefore to achieve an A/C greater than 0.72 h~C~l~ce the caustic strength is above 200 g/l while remaining at 145~C. As well as the evaporation aspect, indirect live steam heating is also used. For high temperature digestion, indirect heating is not so easy and direct steam injection is more common although counter-productive.
Free caustic is not such a ~onc~rn for liquor heating in a low temperature plant. It becomes a cQn~rn in a high temperature plant processing boehmitic bauxite in which the spent liquor is heated separately and the bauxite added direct to the digester (two stream process). In a two stream process, free caustic will impose a limit to the productivity.
In a high temperature tube digestion design or similar single stream process (liquor plus bauxite slurry heating) free caustic is not a limitation to the productivity. Although tube digestion has desirable process features, there are some mech-n;cal limitations to pumping and slurry heating, and co~s~,~,tly the production rate of such a unit is limited.
If the digestion A/C is higher than the degree of supersaturation allowable for residue separation, then some spent liquor trim can be added. this bypass of spent liquor around digestion represents the productivity gain achieved.
One way of achieving digestion productivity with a boehmitic bauxite is to add a gibbsitic bauxite downstream during flash cooiing at say 17G - 150C, and to increase the achieved boehmite A/C. This approach is known as "sweetening" and is practiced by a number of plants in various forms. Sweetening can also in~.~.ate spent liquor trim. Sweet~ning for some plants can be a ~,lv~,ient retrofit, if the gibbsitic bauxite is available and 21~ 0 7 7 3 R~C~ 2 8 FEB 1~''' handling of two bauxites is practical.
Irrespective of the source of alumina, the alumina precipitates out as gibbsite, as the temperature is usually well below 150C. with good residue settler and washer design and operation, the degree of supersaturation can be as high as 25%
of equilibrium solubility of gibbsite. Some alumina loss will occur (known as autoprecipitation) but is usually about 0.015 A/C.
For example, if the equilibrium solubility of gibbsite is 0.58 A/C (220 g/l caustic, 107C and impure plant liquor) then the allowable A/C is 0.58 x 1.25 = 0.725 A/C. Allowing for autoprecipitation, the digestion A/C = 0.725 ~ 0.015 = 0.74 A/C.
If the digestion unit can achieve higher than 0.74 A/C then trim liquor has to be added. Usually the digestion conditions of caustic strength and temperature have to be such that the equilibrium solubility of boehmite or gibbsite from the bauxite is 0.74 1 0.025 margin = 0.765 A/C.
SummarY of Invention and Object The object of the invention is to provide an improved process for extracting alumina from bauxite in which the problems discussed above are at least ameliorated.
The invention provides a process for extracting alumina from bauxite comprising the steps of combining caustic liquor with a bauxite slurry to provide an alumina to caustic ratio (A/C) of greater than about 0.70 in the slurry/liquor mixture, subjecting the mixture to digestion at a temperature below that at which reversion of dissolved gibbsite to boehmite may occur, to extract alumina from the gibbsite bauxite in the slurry, separating the high alumina pregnant liquor from the boehmite cont~ining residue, ;Y;ng the residue with caustic liquor and subjecting the mixture to a high temperature digestion process to extract the rema;n;ng alumina from the residue, and separating the alumina rich liquor, said alumina to caustic ratio of the slurry/liquor mixture being achieved in part by the step of evaporating spent caustic liquor supplied to the low temperature digestion step.
IPI~A/AU
2140773 RECÉIVED 2~ FEB 1994 4a If desired fresh caustic may be added to the slurry to achieve the desired high A/C.
~PP.A/AI I
If desired, the bauxite slurry may be subjected to a known predesilication process prior to heating. Alternatively, two stage post-desilication using lime has been found to be satisfactory.
The atmospheric settler or pressure decanter following the high temperature digestion step is preferably operated at an A/C
which i8 lower than that of the low Le ,-rature decanter to minimize auLG~.e_ipitation during subsequent residue w-shing.
The invention also provides alumina when ~,o.l...-~A by the process defined in the pr~ce~ing paragraphs.
Brief descriPtion of the drawings In order that the invention may be more readily understood, two preferred emho~; - ts of the invention will now be described with refe,~,ce to the accompanying drawings in which:
Figure 1 is a schematic flow chart of a bauxite digestion ~ess ~mhodying the invention, and Figure 2 is a similar schematic flow diagram of a modified bauxite digestion process embodying the invention.
DescriPtion of Preferred Embodiments The preferred ~o~ se5 are applicable to mixed bauxites cont~ining a high ~.G~o-Lion of gibbsite together with less soluble boehmite and diaspore. The proces~es can also be applied to a mixed bauxite of boehmite and diaspore. The ~-~--~sses can also be applied to gibbsite bauxite where an over-charged digestion (for high A/C) can be followed by a re-digestion for complete extraction. The following description relates to gibbsitic-boehmitic bauxite and refers to Figs. 1 and 2 of the drawings, but the ~G~e-S is equally applicable to other bauxites.
The preferred process involves two sequential bauxite digestion steps 1 and 2 to extract gibbsite at low temperature and high caustic, followed by boehmite at high temperature and low caustic to produce alumina at high liquor productivity. The process is particularly suited to retrofitting a two stream high t~m~erature digestion plant, as will be described further below.
The low temperature digestion step 1 is designed to extract essentially all the gibbsite from the bauxite feed, and the WO94/0~16 PCT/AU93/00369 ~4~'l3 6 process is therefore particularly beneficial to bauxites having a high ~-o~o-Lion of gibbsite (eg Comalco bauxite containing about 75% of available alumina as gibbsite). The conditions are designed to achieve a very high A/C liquor~with minimal eve-~ion and without free caustic limitations.
In this step 1, bauxite slurry 3 is fed with spent liquor 18 and caustic make-up 4 to a heat ~vch~nger 5, fed with flash and low pressure live steam 6. The low temperature digestion conditions are controlled to limit the temperature to below 150C
to avoid eve~ion of dissolved gibbsite to boehmite. To achieve the required high A/C of greater than 0.70 A/C strong caustic liquor is required. This is achieved by evaporation 21 of the spent liquor 18 and by indirect heating in 5. Caustic make-up 4 can also be a~e~.
The strong spent liquor after evaporation can then be heated up to the free caustic limitation, which depPn~ng on heater design can range from 130 - 150C. If for example the limitation is 130C, then the spent liquor can be heated from 85C say to 130C in shell and tube heat PY~h-n~srs (not shown) using flash~
steam. The bauxite slurry at 70C say can be heated up to a similar temperature in slurry heaters. Both streams can then be combined for ru~ L~e~ heating using flash and indirect low pressure live steam (not shown). Alternatively, both liquor and bauxite slurry can be combined for all the heating as a single stream in suitable heaters 5 as shown.
The heated slurry from the heat eY~h~ngers 5 is held at temperature in a well stirred vessel 7 or holding tube to achieve the required al~ ; n~ extraction.
Depending on the nature of the bauxite, some predesilication may be necessary prior to heating the slurry. This requires heating the ground bauxite caustic liquor slurry up to say 100C
and holding it in stirred vessels (not shown) to allow any silica such as kaoiinite to dissolve and then to reprecipitate as desilication product (sodium aluminium silicate). Without predesilication, there is the risk of desilication product scaling out and fouling the heaters. As indicated in the Example, the ~lo~ess has been successfully operated using a post-WO94/0~16 21 ~ 0 7 7 3 PCT/AU93/00369 desilication step in which about 96% of the silica is ,.- ~ed.
As well as for gibbsite extraction, some of the digestion holding time may be required for further desilication so as to avoid excessive silica levels rem~;n;ng in liquor.
Immediately following the low temperature digestion, the boehmite con~ining residue is ~eparated in a pressure decanter 8 at digestion temperature using a suitable synthetic flocculent.
Such a device has been described in US Patent 5080803 and el~ew~-e. H$gh underflow density and good overflow clarity can be achieved.
The underflow at high density and temperature is pumped with suitable pumps (not shown) to the high temperature digestion step 2.
The pressure decanter overflow 9 at high A/C pregnant liquor is flash cooled in flash tanks 13 to atmospheric boiling point for dilution, filtering, cooling and se~;ng to produce alumina product. If required, post-desilication can be carried out on the overflow at temperature before flash cooling.
Some of the desilicated decanter overflow may be diverted to the remaining 1~nçonç~ntrated spent liquor via a free caustic trim line lO to control the free caustic to allow high temperature heating for the high temperature digestion step 2.
The low temperature digestion pressure decanter underflow is pumped direct to a high temperature digester vessel ll, with or without further heating. Alternatively it can be mixed with the incoming ~lnconçentrated spent liquor after an a~ Gpriate spent liquor heating and final pumping stage. The single stream can then be further heated in suitable heaters by flash steam and high pressure live steam or other heat transfer medium (not shown). High pressure steam 12 is added direct to the digester as required.
The temperature and time of the high temperature digestion vessel ll, or holding tube, is selected so as to extract ~he ~ ~;ning boehmite, at the caustic conç~ntration prevailing to achieve a medium to normal pregnant liquor A/C.
Slurry from the high temperature digestion vessel ll is flash cooled in flash tanks 13 to atmospheric conditions for WO94/0~16 PCT/AU93/00369 2~4~13 residue removal by settling in a settler or separator 14 followed by w~h;ng and disposal of red mud via 16. The settler overflow pregnant liquor 15 is diluted, filtered, cooled and seeded in a known manner not shown for alumina production.
Alternatively, as shown in Fi-gure 2, a pressure decanter 17 can be applied at the appropriate temperature down the flash train 13. The overflow can then 3Oin the overflow of the main low temperature digestion decanter for common flash cooling.
The pressure decanter underflow of final residue or "red mud" can be cooled indirectly or directly by wash water prior to hi ng and disposal.
The high temperature digestion pressure decanter 17 will nG~ -lly be at a lower A/C than for the main decanter. This will minimise au~G~,ecipitation during subsequent red mud wAshing.
As a further refinement (not shown), the high A/C and normal A/C pregnant liquor streams 9 and 15 can be cooled and diluted separately for a more optimised precipitation of alumina.
If necessary, flash tank sweetening and spent liquor trim can still be applied to the high temperature digestion step (not shown).
The above embodiments of the invention are particularly suitable for retrofitting of existing alumina plants processing a high gibbsite, moderate boehmite bauxite using a two stream digestion process.
The bulk of the digestion is carried out at low temperature freeing up the high temperature equipment for potentially greater p~;ty and efficiency.
A large ~.o~o~ion of the previous high pressure live steam heating is replaced by low pressure steam. this allows better utilization and rApAçity of existing boilers and electrical generators.
The high A/C pregnant liquor from the low temperature digestion has the potential to replace part or all of any sweetening bauxite previously used.
-~L~
Laboratory digestion tests were carried out using Weip bauxite and synthetic Bayer liquor at various caustic strengths, temperature, time and bauxite charge, without Predesilication.
The aim was to find conditions for high A/C from gibbsite dissolution with minimal e~c ~ion (auLo~ e_ipitation) to boehmite or gibbsite. The following results were obt~ine~ from one of the tests:-Bauxite: 54.6% Al20" 11.4% Fe20" 2.59% Ti02 5.92% Si02 (incl. 1.3% quartz), 25.19% LOI (loss on ignition) Distribution of bauxite Al20,: 3.93% in kaolini'~
0.48% in iron minerals 40.34% in gibbsite 9.85% in boehmite Synthetic spent liquor: 346.5 g/l caustic soda as Na2C03 (C) 0.394 A/C
0.845 C/S (caustic/(caustic + carbonate) 2.5 g/kg Si02/C
Digestion conditions: 140C
2 minute digestion time after 5-minute heat up 290 g dry bauxite/litre spent liquor Pregnant liquor result: 301.2 g/l caustic soda (C) 0.778 A/C
0.843 C/S
28.6 g/kg Si02/C
Kaolinite reacted: 93%
Gibbsite reacted: 92%
Post Desilication: Because of the high residual silica in pregnant liquor, post desilication was necessary. A 2-stage procedure was carried out using DSP seed followed by lime, both at 140C.
W O 94/02416 P(~r/AU93/00369 ~4~ 3 Pregnant liquor after DSP s~e~;ng: 313.0 g/l caustic soda (C) 0.750 A/C
0.843 C/S `;`
5.3 g/kg SiO2/C
Pregnant liquor after lime treatment: 306.0 g/l caustic soda (C) 1.5% CaO/ 0.735 A/C
bauxite 0.848 C/S
3.5 g/kg SiO2/C
Claims (9)
1. A process for extracting alumina from bauxite comprising the steps of combining caustic liquor with a bauxite slurry to provide an alumina to caustic ratio (A/C) of greater than about 0.70 in the final slurry/liquor mixture, subjecting the mixture to low temperature digestion at a temperature below that at which reversion of dissolved gibbsite to boehmite may occur, to extract alumina from the gibbsite bauxite in the slurry, separating the high alumina pregnant liquor from the boehmite containing residue, mixing the residue with caustic liquor and subjecting the mixture to a high temperature digestion process to extract the remaining alumina from the residue, and separating the alumina rich liquor, said alumina to caustic ratio of the slurry/liquor mixture being achieved in part by the step of evaporating spent caustic liquor supplied to the low temperature digestion step.
2. The process of claim 1, further comprising adding to the bauxite slurry caustic liquor make-up as required to achieve an A/C of greater than 0.70.
3. The process of claim 1 or 2, wherein the high alumina pregnant liquor is separated from the boehmite containing residue in a pressure decanter.
4. The process of claim 1 or 2 wherein the boehmite containing residue is mixed with the remaining portion of spent caustic liquor before high temperature digestion.
5. The process of claim 3, wherein the boehmite containing residue is mixed with the remaining portion of spent caustic liquor before high temperature digestion.
6. The process of claim 4, wherein the digested mixture is cooled and the alumina rich liquor is separated in an atmospheric settler or pressure decanter.
7. The process of claim 5, wherein the digested mixture is cooled and the alumina rich liquor is separated in an atmospheric settler or pressure decanter.
8. The process of any preceding claim, further comprising the step of desilicating the slurry before the first digestion step.
9. The process of any one of claims 1 to 7, further comprising the step of desilicating the digested mixture after the low temperature digestion process.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPL3754 | 1992-07-24 | ||
| AUPL375492 | 1992-07-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2140773A1 true CA2140773A1 (en) | 1994-02-03 |
Family
ID=3776319
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002140773A Abandoned CA2140773A1 (en) | 1992-07-24 | 1993-07-22 | Process for extracting alumina from bauxite |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0651728A4 (en) |
| KR (1) | KR950702506A (en) |
| BR (1) | BR9306780A (en) |
| CA (1) | CA2140773A1 (en) |
| WO (1) | WO1994002416A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AUPM761194A0 (en) * | 1994-08-23 | 1994-09-15 | Comalco Aluminium Limited | Improved process for the extraction of alumina from bauxite |
| US6528028B2 (en) * | 1999-12-28 | 2003-03-04 | Showa Denko K.K. | Process for treating bauxite in which a desilication product and an insoluble residure are separately precipitated |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4324769A (en) * | 1979-10-01 | 1982-04-13 | Alumina Development Corporation | Extraction and production of alumina containing less than 0.03 percent iron oxide |
| JPS57145029A (en) * | 1981-02-27 | 1982-09-07 | Sumitomo Alum Smelt Co Ltd | Extracting method of alumina from mixed type bauxite |
| AU551019B2 (en) * | 1981-05-15 | 1986-04-17 | James W. McDaniel | Reducing iron content of bayer process solutions |
| US4446117A (en) * | 1983-04-04 | 1984-05-01 | Alumina Development Corporation | Double digestion system to reduce formation of ferrous iron |
| GB8906500D0 (en) * | 1989-03-21 | 1989-05-04 | Shell Int Research | Process for the production of aluminium hydroxide from bauxite |
-
1993
- 1993-07-22 WO PCT/AU1993/000369 patent/WO1994002416A1/en not_active Application Discontinuation
- 1993-07-22 KR KR1019950700276A patent/KR950702506A/en not_active Application Discontinuation
- 1993-07-22 CA CA002140773A patent/CA2140773A1/en not_active Abandoned
- 1993-07-22 BR BR9306780A patent/BR9306780A/en not_active Application Discontinuation
- 1993-07-22 EP EP93915547A patent/EP0651728A4/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| BR9306780A (en) | 1998-12-08 |
| KR950702506A (en) | 1995-07-29 |
| WO1994002416A1 (en) | 1994-02-03 |
| EP0651728A1 (en) | 1995-05-10 |
| EP0651728A4 (en) | 1995-09-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |