PROCESS FOR PRODUCTION OF MONOHYDRATE ALUMINA FROM SUPERSATURATED ALUMINATE SOLUTIONS

Abstract: Process for production of monohydrate alumina (boehmite) with precipitation from supersaturated sodium aluminate solutions starting at temperatures lower than 100 DEG C. The current invention is referred to a new aluminium precipitation process from supersaturated sodium aluminate solutions at temperatures lower than 100 DEG C in which aluminium is precipitated with the form of monohydrate alumina (boehmite, Al2O3.H2O). The process for production of monohydrate alumina (boehmite, Al2O3.H2O), from supersaturated sodium aluminate solutions, is characterised by precipitation at temperatures lower than 100 DEG C. The process comprises a highly innovative variation of the precipitation step in the current Bayer process in which it is achieved 40 % energy reduction in the calcination stage and more than 10 % total energy savings in the Bayer process.

Description: PROCESS FOR PRODUCΗON OF MONOHYDRATE ALUMINA FROM SUPERSATURATED ALUMINATE SOLUTIONS The current invention is referred to a new aluminium precipitation process from supersaturated aluminate solutions at temperatures lower than 100°C in which aluminium is precipitated with the form of monohydrate alumina (Al 2 O 3 .H 2 O). Bayer process is used almost exclusively for the extraction of alumina from bauxites. The essential steps of the Bayer process are: • Digestion of bauxite by using sodium hydroxide at elevated temperatures and pressures in autoclaves. Al(OH) 3(s) + NaOH (aq) = NaAl(OH) 4( „ q) • Precipitation of aluminum hydroxide (gibbsite, AJ O 3 .3H 2 O) from the pregnant liquor according to the following equation. Precipitation is effected by cooling the solution to 55 - 60°C, thus causing supersaturation, and by introducing small gibbsite particles as seeds. 2Al(OH) 4 (1) = Al 2 O 3 .3H 2 O (5) + 2OH- (aq) • Calcination of gibbsite at about 1000- 1200°C to produce smelter-grade alumina according to the following equation: Al 2 O 3 .3H 2 O ( .) = Al2θ 3(s , + 3H 2 O Energy cost of the Bayer process constitute about 30% of the alumina price. Total energy requirements in modern alumina plants amounts to about 12 GJ/t of alumina produced. 3 GJ/t Al O? of this, representing 25% of the total energy consumption, is used for calcination. The new process, which is analytically described in the current invention, is a highly innovative variation of the Bayer process whereby, at the precipitation step, alumina monohydrate (Al 2 O 3 H 2 O) rather than tπhydrate is precipitated at atmospheric conditions The mam feature of this method is the production of a material needed substantially lower amounts of energy during the calcination stage The technical characteristics of the new process are the following • Precipitation of the monohydrate alumina (Boehmite) from supersaturated aluminate solutions is carried out at temperatures lower than 100°C, namely under atmospheric conditions • Precipitation of the monohydrate alumina (Boehmite) from supersaturated aluminate solutions is carried out with cooling, starting the procedure at temperatures close to but lower than 100°C • In a process variation, precipitation of the monohydrate alumina (Boehmite) from supersaturated aluminate solutions is carried out with cooling, starting the procedure at temperatures close to but lower than 100°C and with simultaneous use of ultrasounds with frequency varied, typically but not exclusively, in the region 20-50kHz • Alumina concentration in supersaturated aluminate solution is varied, typically but not exclusively, in the region 60-250g/l • Caustic concentration in supersaturated sodium aluminate solution, expressed as grams of Na 2 O per liter of solution, is varied, typically but not exclusively, in the region 50-250g/l • The precipitation of the monohydrate alumina can be carried out without boehmite seed addition in the aluminate solution In this case, a time consuming induction period is observed • In a process variation, precipitation of the monohydrate alumina (Boehmite) from supersaturated sodium aluminate solutions is carried out with the presence of boehmite seed In this case, the precipitation process is drastically accelerated The initial seed ratio (SR), grams of added boehmite/grams of dissolved alumina, is varied, typically but not exclusively, in the region 0-5 • The precipitation of the monohydrate alumina can be carried out with or without glycerol or any other alcohol addition at concentration varied, typically but not exclusively in the region 0-5mol/kg boehmite seed In the following, examples of the process application are presented Example 1 A detailed example concerning the application of the process at temperatures below 100 C, is the precipitation of monohydrate alumina (boehmite) from a supersaturated aluminate solution at 90°C The aluminate solution is similar to the one that is used by the Bayer process The concentrations of Al 2 O~, and Na 2 O are 132g/l and 120g/l respectively Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 1,76 The resulting pulp is mantained by mechanical agitation at 300mιn ' for 96 hours During the test, systematic sampling is carried out and the samples are analysed for Al 2 O~, The results of this test are given in Table 1 Table 1 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution at 90°C. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 2 Another detailed example concerning the application of the process at temperatures below 100°C, is the precipitation of monohydrate alumina (boehmite) from a supersaturated aluminate solution at 90°C The concentrations of Al 2 O and Na 2 O in the aluminate solution are 99g/l and 90g/l respectively Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 1,76 The resulting pulp is mantained by mechanical agitation at 300min " ' for 96 hours During the test, systematic sampling is carried out and the samples are analysed for Al O 3 The results of this test are given in Table 2 Table 2 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution at 90°C. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 3 Another detailed example concerning the application of the process at temperatures below 100°C, is the precipitation of monohydrate alumina (boehmite) from a supersaturated aluminate solution at 90°C The concentrations of Al 2 O 3 and Na 2 O in the aluminate solution are l lOg/l and lOOg/l respectively Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 0,4 The resulting pulp is mantained by mechanical agitation at 300min " ' for 96 hours During the test, systematic sampling is carried out and the samples are analysed for Al 2 O 3 The results of this test are given in Table 3 Table 3 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution at 90°C. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 4 A detailed example concerning the application of the process at starting temperature near but always under 100°C is the following The concentrations of Al 2 O 3 and Na 2 O in the aluminate solution are 132g/l and 120g/l respectively Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 1,76 The resulting pulp is mantained by mechanical agitation at 300min " ' for 96 hours at 95°C The temperature of the pulp is lowered by 10°C, every 24 hours, until the final value of 60°C During the test, daily sampling is carried out and the samples are analysed for Al 2 O~, The results of this test are given in Table 4 Table 4 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution with cooling from 95°C to 60°C. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 5 Another detailed example concerning the application of the process at starting temperature near but always under 100°C is the following The concentrations of Al 2 O 3 and Na 2 O in the aluminate solution are 179g/l and 160g/l respectively Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 2,10 The resulting pulp is mantained by mechanical agitation at 300min _1 for 48 hours at 90°C Afterwards, the pulp is cooled at 70°C, where it remains for 24 hours and finally the pulp is cooled at 60°C, where it remains for 24 hours During the test, daily sampling is carried out and the samples are analysed for Al 2 O 3 The results of this test are given in Table 5 Table 5 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution with cooling from 90°C to 60°C. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 6 A detailed example concerning the application of the process at starting temperature near but always under 100°C, with the addition of alcohol, is the following The concentrations of Al 2 O, and Na 2 O in the aluminate solution are 179g/l and I60g/l respectively Glycerol is also added at concentration of 0,9mole/kg of monohydrate alumina (boehmite) seeds Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 2, 10 The resulting pulp is mantained by mechanical agitation at 300min " ' for 48 hours at 90°C Afterwards, the pulp is cooled at 70 ( C, where it remains for 24 hours and finally the pulp is cooled at 60°C, where it remains for 24 hours During the test, daily sampling is carried out and the samples are analysed for Al 2 O 3 The results of this test are given in Table 6 Table 6 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution with cooling from 90°C to 60°C and addition of glycerol. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) Example 7 A detailed example concerning the application of the process at starting temperature near but always under 100 ϋ C, with the addition of alcohol and use of ultrasounds, is the following The concentrations of Al 2 O 3 and Na 2 O in the aluminate solution are 155 g/1 and 160g/l respectively Glycerol is also added at concentration of 0,4mole/kg of monohydrate alumina (boehmite) seeds Monohydrate alumina (boehmite) seeds are added in the solution with a ratio of 2,42 The resulting pulp is mantained by mechanical agitation at 300mm "1 for 48 hours at 90 υ C Afterwards, the pulp is cooled at 70°C, where it remains for 48 hours Ultrasounds with frequency of 35kHz are applied only in the last 48 hours During the test, daily sampling is carried out and the samples are analysed for Al 2 O 3 The results of this test are given in Table 7 Table 7 Process for production of monohydrate alumina (boehmite) by precipitation from supersaturated aluminate solution with cooling from 90°C to 60°C, addition of glycerol and use of ultrasounds. The precipitate was mineralogicaly characterized as pure monohydrate alumina (boehmite) The above method comprises a highly innovative variation of the precipitation step in the current Bayer process It has a lot of advantages, the most important of them is the significant energy savings during the calcination stage for the following reasons (a) the enthalpy of dehydration of boehmite to alumina is significantly lower than that of gibbsite A1 2 0 3 .3H 2 0 → A1 2 0 3 + 3H 2 0 ΔH = 187 KJ/moleAl 2 O 3 A1 2 0 3 . H 2 0 → A1 2 0 3 + H O ΔH = 72 KJ/ ole Al 2 O 3 This means energy savings about 1 1 GJ/t Al 2 O 3 (b) As boehmite contains one mole of water against three for gibbsite, the amount of the material to be calcined will be reduced by 360 kg/t of alumina produced This will result in additional energy savings of the order of 0 7 GJ/t Al 2 O 3 as well as in increase in the calciner productivity The energy consumption during the calcination stage is reduced to 1.2 GJ/t Al 2 O , representing more than 40% energy reduction in the calcination stage and more than 10% total energy savings in the Bayer process.

Claims: CLAIMS 1 A process for production of monohydrate alumina (boehmite, Al 2 O H 2 O), from supersaturated sodium aluminate solutions, characterised by precipitation starting at temperatures lower than 100°C 2 The process as defined by the claim 1, wherein precipitation takes place isothermally 3 The process as defined by the claim 1, wherein precipitation starts at temperature close to but lower than 100°C and afterwards the supersaturated sodium aluminate solutions is cooled to successively lower temperatures 4 The process as defined by the claims 1, 2 or 3, wherein precipitation is carried out with boehmite seed addition in the supersaturated sodium aluminate solutions The initial seed ratio (SR), grams of added boehmite/grams of dissolved alumina, is varied, typically but not exclusively, in the region 0-5 5 The process as defined by the claims 1, 2 or 3, wherein precipitation is carried out with simultaneous application of ultrasounds with frequencies varied, typically but not exclusively, in the region 20-50kHz 6 The process as defined by the claims 1 , 2 or 3 wherein the precipitation is carried out with addition of glycerol or any other alcohol, at concentration varied, typically but not exclusively, in the region 0-5mol/kg boehmite seed 7 The process as defined by the claims 1 -6, wherein the alumina concentration in the supersaturated sodium aluminate solution is varied, typically but not exclusively, in the region 60-250g/l 8 The process as defined by the claims 1-6, wherein the caustic concentration in the supersaturated sodium aluminate solution, expressed as grams of Na O per liter of solution, is varied, typically but not exclusively, in the region 50-250g/l.

Inventors: BALLAS DIMITRIOS, COPPIETERS ARNOLD, KALAITZOGLOU KOSTAS, KONTOPOULOS ANTONIOS DI, PANIAS DIMITRIOS, PANOU GEORGE, PASPALIARIS IOANNIS, RIGAS GEORGE, SKOUFADIS CHRISTOFOROS, BALLAS, DIMITRIOS, COPPIETERS, ARNOLD, KALAITZOGLOU, KOSTAS, KONTOPOULOS, ANTONIOS +DI, PANIAS, DIMITRIOS, PANOU, GEORGE, PASPALIARIS, IOANNIS, RIGAS, GEORGE, SKOUFADIS, CHRISTOFOROS
Applicants: BALLAS DIMITRIOS, COPPIETERS ARNOLD, HELLENIC ALUMINA INDUSTRY S A, KALAITZOGLOU KOSTAS, KONTOPOULOU ELIZA & HF, KONTOPOULOU KONSTANTINA & HF, KONTOPOULOU MARIANNA & HF, NAT TECHNICAL UNIVERSITY OF AT, PANIAS DIMITRIOS, PANOU GEORGE, PASPALIARIS IOANNIS, RIGAS GEORGE, SKOUFADIS CHRISTOFOROS, UNIV BRUXELLES, BALLAS, DIMITRIOS, COPPIETERS, ARNOLD, HELLENIC ALUMINA INDUSTRY S.A. (E.L.V.A.), KALAITZOGLOU, KOSTAS, KONTOPOULOU, ELIZA +HF, KONTOPOULOU, KONSTANTINA +HF, KONTOPOULOU, MARIANNA +HF, NATIONAL TECHNICAL UNIVERSITY OF ATHENS (NTUA), PANIAS, DIMITRIOS, PANOU, GEORGE, PASPALIARIS, IOANNIS, RIGAS, GEORGE, SKOUFADIS, CHRISTOFOROS, UNIVERSITE LIBRE DE BRUXELLES
Application year: 1998
Main classification: C01F7
IPC codes: C01F7