PROCESS FOR EXTRACTING ALUMINA FROM BAUXITE

Abstract: A process for extracting alumina from bauxite comprising the steps of concentratring a portion of spent caustic liquor (18), combining said concentrated spent caustic liquor with a bauxite slurry (3) to provide a final alumina to caustic ratio (A/C) of greater than 0.7 in the slurry/liquor mixture, heating the mixture by means of a heat exchanger (5), subjecting the mixture to digestion 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 subjecting the mixture to high temperature digestion in a tank (11) to extract the remaining 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: PROCESS FOR EXTRACTING ALUMINA FROM BAUXITE Field of the Invention This invention relates to processes for the production of alumina from bauxite. Background of the Invention General Most alumina is produced using the Bayer process. A high volume of caustic liquor is circulated at a controlled concentration. In the digestion part of the circuit, the caustic is heated and reacted with bauxite to increase the alumina concentration (pregnant liquor) . After the bauxite residue is separated by settling, the caustic liquor is cooled, seeded, and alumina is precipitated as product. The spent liquor is recirculated. The quantity of circulating liquor can be as high as 20 m 3 /t alumina produced. 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 +80 g/1 yield today. For high yield, the digestion step has to achieve a high alumina concentration. A degree of supersaturation can be tolerated during the residue separation step. However, depending 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 added in a controlled manner to achieve a desired pregnant liquor alumina concentration. The alumina concentration is usually expressed as a ratio on caustic, A/C. As an example, the pregnant liquor can be 0.72 A/C (gAl 2 0 3 /gNa 2 C0 3 ) at say 105°C for 200 g/1 caustic expressed as Na 2 C0 3 (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 Caustic Usually the spent liquor to digestion is evaporated to some degree and fresh caustic make-up added at this point, such that the concentration may be 240 g/1 caustic at 0.36 A/C. The so called free caustic of this liquor is defined as: 240 - 240 x 0.36 x 106/102 = 150 g/1. (representing sodium hydroxide) Depending 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 diasporic bauxites the limit may be say 140 g/1 free caustic. Above this limit the heater tubes and piping may erode/corrode 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 overcome the free caustic. However, most plants use shell and tube exchangers for liquor heating, and adding a slurry is not desirable. Bauxite Types Bauxites are minerals rich in alumina and low in silica. The usual minerals containing 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/1 caustic, the digestion temperature required would be: Gibbsite, Al 2 0 3 .3H 2 0 : 140 - 145°C (3.2 bar(a) vapour pressure) Boehmite, A1 2 0 3 H-0 : 260 - 265 °C (41 bar(a) vapour pressure) Diaspore, A1 2 0 3 H 2 0 : impractical The digestion temperature for boehmite can be raised higher for higher A/C, however, 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 150°C. This is particularly so if the gibbsitic bauxite also contains boehmite to act as seed. Mixed gibbsitic-boehmitic bauxites containing above say 5% as boehmite alumina are usually processed in a single digestion step under boehmite extraction conditions. Digestion Productivity The digestion productivity is a measure of the alumina concentration increase due to xuxite for a given flowrate of liquor. If there is a limit to the flowrate, then potentially the liquor can be evaporated so that the mass flow of caustic is increased and more bauxite can be added in proportion. For a low temperature plant processing only gibbsitic bauxite, more evaporation is required to keep the circuit in balance, since less flashing occurs 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 because the caustic strength is above 200 g/1 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 concern for liquor heating in a low temperature plant. It becomes a concern 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 mechanical limitations to pumping and slurry heating, and consequently 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 cooling at say 170 - 150°C, 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 incorporate spent liquor trim. Sweetening for some plants can be a convenient retrofit, if the gibbsitic bauxite is available and 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 150°C. 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/1 caustic, 107°C 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 + 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 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. The strong caustic liquor to the gibbsite digestion may be concentrated by evaporation and/or by the addition of fresh caustic to achieve the desired high A/C. 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 is lower than that of the low temperature decanter to minimize autoprecipitation during subsequent residue washing. The invention also provides alumina when produced by the process defined in the preceding paragraphs. Brief description of the drawings In order that the invention may be more readily understood, two preferred embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic flow chart of a bauxite digestion process embodying 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 processes are applicable to mixed bauxit s containing a high proportion of gibbsite together with less soluble boehmite and diaspore. The processes can also be applied to a mixed bauxite of boehmite and diaspore. The processes 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 process 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 temperature 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 process is therefore particularly beneficial to bauxites having a high proportion 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 reversion 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 exchanger 5, fed with flash and low pressure live steam 6. The low temperature digestion conditions are controlled to limit the temperature to below 150°C to avoid reversion 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 added. The strong spent liquor after evaporation can then be heated up to the free caustic limitation, which depending on heater design can range from 130 - 150°C. If for example the limitation is 130°C, then the spent liquor can be heated from 85"C say to 130°C in shell and tube heat exchangers (not shown) using flashed steam. The bauxite slurry at 70°C say can be heated up to a similar temperature in slurry heaters. Both streams can then be combined for further 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 exchangers 5 is held at temperature in a well stirred vessel 7 or holding tube to achieve the required alumina 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 100°C 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 process has been successfully operated using a post- desilication step in which about 96% of the silica is removed. 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 remaining in liquor. Immediately following the low temperature digestion, the boehmite containing residue is separated in a pressure decanter 8 at digestion temperature using a suitable synthetic flocculent. Such a device has been described in US Patent 5080803 and elsewhere. High 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 seeding 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 unconcentrated spent liquor via a free caustic trim line 10 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 11, with or without further heating. Alternatively it can be mixed with the incoming unconcentrated spent liquor after an appropriate 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 11, or holding tube, is selected so as to extract the remaining boehmite, at the caustic concentration prevailing to achieve a medium to normal pregnant liquor A/C. Slurry from the high temperature digestion vessel 11 is flash cooled in flash tanks 13 to atmospheric conditions for residue removal by settling in a settler or separator 14 followed by washing 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 Figure 2, a pressure decanter 17 can be applied at the appropriate temperature down the flash train 13. The overflow can then join 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 washing and disposal. The high temperature digestion pressure decanter 17 will normally be at a lower A/C than for the main decanter. This will minimise autoprecipitation 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 capacity and efficiency. A large proportion of the previous high pressure live steam heating is replaced by low pressure steam. this allows better utilization and capacity 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. EXAMPLE Laboratory digestion tests were carried out using Weipa 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 reversion (autoprecipitation) to boehmite or gibbsite. The following results were obtained from one of the tests:- Bauxite: 54.6% Al 2 0 3 , 11.4% Fe 2 0 3 , 2.59% Ti0 2 5.92% Si0 2 ( incl . 1.3% quartz ) , 25.19% LOI (loss on ignition) Distribution of bauxite A1 2 0 3 : 3.93% in kaoiinite 0.48% in iron minerals 40.34% in gibbsite 9.85% in boehmite Synthetic spent liquor: 346.5 g/1 caustic soda as Na 2 C0 3 (C) 0.394 A/C 0.845 C/S (caustic/(caustic + carbonate) 2.5 g/kg Si0 2 /C Digestion conditions: 140°C 2 minute digestion time after 5-minute heat up 290 g dry bauxite/litre spent liquor Pregnant liquor liquor, post desilication was necessary. A 2- stage procedure was carried out using DSP seed followed by lime, both at 140"C. Pregnant liquor after DSP seeding: 313.0 g/1 caustic soda (C) 0.750 A/C 0.843 C/S 5.3 g/kg Si0 2 /C Pregnant liquor after lime treatment: 306.0 g/1 caustic soda (C) 1.5% CaO/ 0.735 A/C bauxite 0.848 C/S 3.5 g/kg Si0 2 /C

Claims: CLAIMS : 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 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. 2. The process of claim 1, further comprising concentrating a portion of spent caustic liquor and combining it with said bauxite slurry together with 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.

Inventors: CRISP ANTHONY JOHN, CRISP, ANTHONY, JOHN
Applicants: COMALCO ALU, CRISP ANTHONY JOHN, COMALCO ALUMINIUM LIMITED
Application year: 1993
Main classification: C01F7
IPC codes: C01F7