CA1191665A - Method for the recovery of alumina - Google Patents

Abstract

ABSTRACT
Improved method for recovering coarse-crystalline aluminum oxide from bauxite according to the Bayer method with which, in the presence of fine seed crystals in a supersaturated sodium aluminate solution at 80 to 65°C, agglomerates are first of all formed, which are then, if necessary while introducing medium-coarse seed crystals, led through a first cascade of crystallizers and, after cooling to 55 to 45°C, through a second crystallizer cascade. The drawn off crystal-suspension is fractionated into fine seed crystals, if necessary also medium-coarse seed crystals, and coarse product crystals which are ultimately calcined.

Description

ti5 Improved Method for the Recovery of Alumlna Subject of the present invention is an improved method for recovering of alumina ~A12O3~ from bauxite according to the Bayer method and subsequent calcination, with which both the properties of the coarse aluminum oxide crystals and the yield, with respect to time, are improved. The seed crystal quantity, which is required for the precipitation of a specific aluminum hydroxide quantity, can also be reduced.
In the European Bayer method, ground bauxite is heated in known manner in an autoclave with a lye, which is circulated and which contains sodium hydroxide and sodium aluminate, whereby alumina dissolves and red mud remains in undissolved form. The lye coming out of the autoclave, is diluted with wasing liquor and/or stirred out sodium aluminate liquor and the red mud is separated from the lye. From the separated lye, the alumina hydrate is precipitated at temperatures of 50 to 70C after cooling and seeding with aluminum hydrate crystals.
An essential fea-ture of this European method is the use of concentrated sodium hydroxide solution (280 to 450 g/l, calculated as Na2CO3) for treatment and the stirring of the produced aluminum oxide hydrate after dilution at lower temperatures. An aluminum oxide hydrate, stirred under the given conditions, accumulates in fine~crystalline form. A
portion of this product is calcined to alumina and another portion is used for seeding. However, due to considerable losses of dust during calcination, transport and electrolysis, such a product is undesirable todayO FurthermDre, the fine-grained aluminum oxide does not c~bsorb readily the fluorine gas, which forms during electrolysis, whereas coarse-grained aluminum oxide is ~ery well suited therefor. The result of the latter is that the losses of fluorine gas are reduced and considerably less fluorine gas is released into the atmosphere, which, in turn, facilitates adherence to the environmental protection regulations which continue to ~ecome mDre stringent. Mbreover~
in various fields of appli~ation, e.g. use as abrasive and catalyst, the use of coarse crystalline al~Lnum oxide is preferred in many cases.
In con~ast hereto, in the American Bayer me~od, the concentration of the sodium hydroxide in the lye is apprDximately 160 to 225 gJl, that is, substantially below the .~
~

~9~
~2--concentration in European digestion lyes. With this method, however, alumlnum oxide hydrates of different grain size are produced, whereby the coarse crystals are drawn off and calcinated as product, while the fine and medium coarse crystals are used as seed crystals.
Due to the differences in the bauxites, which are treated according to the variants of the above-described Bayer method, the American Bayer method cannot, however, be easily applied to the bauxites processed in Europe. Moreover, in the American method, diluted lye is used in the production of aluminum oxide hydrates of difEering grain size, which has the disadvantage that the lye productivity is lower than when concentrated lye is used. When using diluted lye, more liquid must be worked and circulated than when using concen-trated lye, which necessitates correspondingly more equipmentand technical resources and, in addition, requires more energy.
It was, therefore, the object to avoid the described disadvantages of the Bayer method and to find a method which allows the production of coarse-grained aluminum oxide hydrate, also in more concentrated mother liquors.
To solve this object, Canadian Patent No. 1,098,284 proposes that the production of the product crystals and seed crystals be conducted und~r different conditions in separate stirrers. In this way, essentially coarse crystalline alumina 2S hydrate is to be produced in a product cascade, during continuous parallel introduction of fresh ~not yet stirred), supersaturated mother liquor into the individual stirrer - tanks in the presence of medium-coarse seed crystals, and fine - crystalline alumina hydrate is to be produced in a seed cascade from the simultaneously drawn off overflows of the product cascade. The seeding material and the aluminum oxide hydratel formed while passing through the product cascade, are led in series through the stirrer tanks of the cascade. It is essential to this process that there are high solids concentrations ~solid aluminum hydroxide~ in the product cascade in highly supersaturated sodium aluminate liquors r~ and that one works at temperatures ~f 75 to 90C. Under these`
..

conditions, crystal enlargement by crystal growth prevails and nucleation and seed crystal formation are reduced. In contrast thereto, the formation of fine~crystalline aluminum oxide hydrate in partially spent sodlum al~inate liquors is favored in the seed cascade at temperatures o~ 50 to 65C.
With the previously known method, coarse-crystalline aluminum oxide hydrate in amounts of 72 to 85 g/l, expressed as A12O3, is obtained from the supersaturated mother liquor.
The product is said to contain at the most, 1% crystals which are smaller than ~5 ~mO
The sodium hydroxide contained in the supersaturated sodium aluminate liquor is present, in the previously known method, in amounts of 200 to 300 g/l, in particular 240 g/l, calculated as Na2CO3.
A further characteristic of the method according to Canadian Patent No. 1,098,2~4 is the fact that 3-phase sti~rers are used in the product cascade. In such stirrers, the lowest phase I is a settling zone with high solids content (40 to 65%), the middle zone II is the stirring zone with a solids content of 10 to 20% and the upper zone II~ is a stabilizing zone with `
a solids content of 1 to 5~. The phase division, which can be attained without difficulty with aid of the stirrer device described in said Canadian patent, leads to the formation of coarse-crystalline aluminum oxide hydrate in concentrated sodium aluminate liquors (the NaOH content i5 higher than in the American method and is approximately at the same level as in the ~uropean Bayer method~, however, it has been shown that the improved lye productivity is still not sufficient for solving the stated object. Stirring out times of at least 50 hours are in practice often too long. In particular, it has been observed -that the coarse hydrate crystals, pro-duced in accordance with the cited prior art, are less stable and break easily, in many instances already in the fluid bed ~` 35 of the calcining furnace.
It is ~he main object of the present in~ention to `~ produce coarse aluminum oxide hydrate crystals, which are : `~

.

largely stable against influence of mechanical forces and resistant under stress, for example during transportation.
It is a fu~her object of the present invention to shorten the stirring out time and improve lye productivity.
The solution to these ob~ects ls based on the finding that the mechanical properties of coarse aluminum oxide hydrate crystals are, to a great extent, influenced by the conditions under which the crystal enlargement takes place. It has been conclusively shown that aluminum oxide hydrate crystals, which are primarily formed by crystal growth, have different pro-perties from crystals which are predominantly formed by agglomeration or agglomeration and crystal growth. Only aluminum oxide hydrate particles, formed by crystal growth, break easily and are obtained with the method of Canadian 15 Patent No. 1,0g8,284. Crystals, which are formed by agglo-meration of seed crystals and subsequent crystal growth, are substantially more stable.
Accordingly the present in~ention provides a continuous method for recovery of aluminum oxide from bauxite according to the Bayer process comprising:
a) forming a supersaturated sodium aluminate solution by treating ground bauxite with aqueous sodium hydroxide;
b) passing the supersaturated sodium aluminate solution having a sodium hydroxide concentration of about 200 to 300 g/l, calculated as Na2CO3, and a temperature of from about 80 to 65C through a cascade of agglomerators connected in series, introducing a suspension of fine seed crystals into the first agglomerator of said agglomerator cascade such that a solids content of between about 10 and 50 g/l, calculated as A12O3, is reached in the first agglomerator, and stirring out a por-tion of the dissolved aluminum oxide as aluminum oxide hydrate agglomerates;
c) passing the sodium aluminate solution containing said agglomerates through a first cascade of crystallizers connected in series, stirring out a portion of the aluminum hydroxide such that in each crystalliæer at least about 80~ of the filling volume has a high solids content and the ov~rflow of the remaining ~illing~,volume has a solids content of not greater than about 20 g/l, underflow and overflow of each 5~

crystallizer being combined prior to introduction into the following crystallizer;
d) combining the discharges from the last crystallizer of said first crystallizer cascade, said discharges comprising the inflow into the first crystallizer of said first crystalli-zer cascade and the aluminum oxide hydrate formed in said first crystallizer cascade, and cooling said combined discharges to a temperature of from about 55 to ~5C;
e) transferring said cooled combined discharges ~rom said first crystallizer cascade into a second cascade of crystallizers connected in series and stirring out said discharyes such that in each crystallizer at least about 80% of the filling volume has a high solids content and the overflow of the remaining ~illing volume has a solids content not greater than about 20 g/l; and f~ drawing off a crystal suspension from ~he last crystallizer of the second crystallizer cascade and fractiona-ting said crystal suspension into fine and medium coarse seed crystals and coarse product crystals.
The agglomerators are the standard stirrers known in the alumina industry. At temperatures between 80 and 65~C, in particular 78 and 75C and in the presence of fine-crystalline seed material and an A/C-ratio (this means the ratio of the dissolved aluminum oxide in grams per liter to the dissolved sodium hydroxide in grams per liter, calculated as Na2CO3) of approximately 0O700 to 0.650, in particular 0.690 to 0.660 in the first agglomerator and 0.550 to 0.450, in particular 0.500 to 0.490, in the last agglomerator, primarily agglomerate formation takes place. In the first agglomerator :25 to 60% of the seed crystal particles, preferably 35 to 45% should have a size of under 45 ~m, whereby it should be pointed out that the seed crystal quantity is very small with 10 to 50 g/l, prefer~
ably 15 to 25 g/l calculated as Al2O3. ~t has been shown that the seed crystal quantity in the agglomerator cascade must not be greater than app~oximately 52 g/l, calculated as A12O3, (or approximately 80 g~l, calculated as hydrate). If there are greater quantities, the crystal growth prevails and the agglomerate formation p~actically comes to a standstill.
The concentration of sodium hydroxide in the sodium aluminate solution is, at the beginn.ing and end of the agglomerator cascade, 200 to 300 g/l, preferably 240 to 250 g/l, calculated as Na2CO3. The time for passing through the agglomerator cascade can be 6 to 10 hours, mostly however, amounts to 7.5 to 8.5 hours. Under these conditions, hardly anything but agglomerate formation occurs in the agglomerators.
It is a further essential feature of the method according to the invention that sodium oxalate crystals must not be present in the lye. Seed nuclei formation is furthered and agglomerate formation disturbed by oxalate crystals. According to screen analysis 80 to 90% of the particles in the end product of the agglomerator cascade are larger than ~5 ~m.
In accordance with the invention, ~n the following cascade (first crystallizer cascade), the partially stirred out sodium aluminate liquor is stirred out in each crystallizer in such a way that at least 80% by volume, preferably 85 to 90 by volume of the filling volume, have a high solids content and the overflow of the remaining filling volume has a solids content of, at the most, 20 g/l, calculated as A12O3. It is especially advantageous to work in such a way that a solids content of, at the most, 3 g/l is contained in the overflow.
It is a characteristic of this procedural step that in the crystallizers enlargement of the agglomerate particles, formed in the agglomerator cascade i5 provided exclusively by crystal growth.
A seed crystal addition can, however, be practical in the first crystallizer cascade for maintaining a constant crystal formation e~uilibrium. If medium-coarse seed crystals are added ~n the first crystallizer, then the quantity should be between 100 and 300 gjl, preferably between 150 and 170 g/l, calculated as aluminum oxide~ 70 to 90% oF the seed crystals should be larger than 45 ~m, preferably 78 to 83%.
In this embodimen~ of the invention, the decanters are designed practically in such a way ~hat the total medium-coarse crystal 6~

fraction is used as seed material in the second cascade and the fine crystal fraction as seed material in the agglomerator cascade. If in the first crystallizer cascade the temperature is 70 to 69C in the first crystallizer and 68 to 62C, preferably 65C, in the last crystallizer, durations of 7 to 14 hours, preferably of 8 to 10 hours are required for passage of the first crystallizer cascade. The higher the temperature and the degree of supersaturation are in this first crystallizer cascade, the greater is the rate of crystal growth. Agglomer-ator cascade and first crystallizer cascade can therefore, incontrast to the second crystallizer cascade, also be designated as hot cascade. For the already mentioned reasons, sodium oxalate crystals should be absent. Neither seed nuclei formation nor agglomeration is provided.
The aluminum oxide hydrate concentration should be high in the lower part of the crystallizers. It may be between 300 and 300 g/l. The agglomeration can best be avoided, if the lower phase, the "solids bed", occupies a large space and has a high solids c~ntent. According to the invention, the "solids bed" comprises approximately 80 to 85~ by volume of the filling volume. The A/C-ratio which is between 0.550 and 0.450, preferably however between 0.500 and 0.490, when entering the first crystallizer cascade, is reduced to 0.430 to 0.380, preferably 0.420, by the time it leaves the last crystallizer.
The solids content of at least 80% by volume of the suspension is preferably 600 to 700 g/l, expressed as aluminum oxide hydrate.
The total discharge quantity (underflow and overflow) from the last crystallizer of the first crystallizer cascade is equal to the inflow quantity into the first crystallizer, increased by the essentially grown aluminum oxide hydrate formed in the crystallizers. The volumes with different solids content which are formed in the crystallizers are, after separate withdrawal from the crystallizers, mixed with one another before feeding into the following crystallizer and subsequently again separated into volumes with high and verY low solids content.
Of the crystals withdrawn from ~he last crystallizer 88 to 92%
of the particles are larger than 45 ~m according to screen analysis.
The aluminum oxide hydrate suspension coming from the last crystallizer is cooled to a temperature of 55 to 45C prior :, :

..

s to entry into the second crystallizer cascade, whereby heat exchangers as well as expansion coolers can be used as suit-able cooling units. ~s in the first crystallizer cascade, in the second crystallizer cascade, primarily crystal growth takes place, of course, at substantially lower temperatures and smaller A/C ratio. A seed crystal supplement is not required in the cold cascade.
The A/C-ratio should be between 0.430 and 0.380, preferably 0.420, at entry into the last cascade. When leaving the second crystallizer cascader the A/C-ratio has fallen to 0.350 to 0.320, preferably to 0.340 to 0.330. The discharge temperature is indicated with 55 to 45~C, preferably 50C.
90 to 92% of the formed crystals are larger than 45 ~m.
In this connection, it is pointed out that the preceding results are only attained if impurities which may be present in the process liquor have been, for the most part, removed.
Purifying processes are known and are~ for example, described in German Patent No. 25 18 431 and other publications. A
purified process liquor may still contain approximately the following impurities:
1) Less than 0.05 g/l solid oxalates, expressed as sodium oxalate,

2) less than 110 mg/l humic acid derivatives,

3) less than 8 g/l long-chain organic compounds, expressed as carbon,

4) less than 4 g/l inorganic salts, expressed as Na2CO3, and

5~ less than 10~ Na2CO3, relati~e to the dissolved sodium h~droxide in the process liquor.

The subject of the invention will now be described in greater ~etail with reference to the two drawings.

Figure 1 shows a flow chart of the method according to the invention for the production of stable and, at the same time, coarse aluminum oxide hydrate crystals; and Figure 2 shows a flow chart in accordance with the prior art for the production of coarse~crystalline aluminum oxide hydrate, which, however shows only a low stability and is easily broken in the calcining furnace.
Of course, the subject of the invention is not restricted to the embodiment shown in Figure 1. The number of agglomerators and crystallizers can, for example, be smaller or greater.
In accordance with Figure 1, hot, supersaturated mother liquor is introduced into ~he agglomerator 1 of the "hot;' cascade.
Agglomerator 1 may, for example,consist of five agglomerators. The fine seed crystal q~tity introduced into the agglomerator 1 from the ~tirrer 15 tank 14 by means of pump 15 is 20 g/l, calculated as A12O3. ~ile in agglorrerators 1 and 2 at 78C and an A/C-ratio of 0.660 primarily nucleation takes place, in the subsequent agglomerators pri~arily agglomerates are fonmed, that is 40 g/l, so that the suspension, when leaving this cascade, contains, in allr 60 g solids per liter, expressed as A12O3. When leaving 20 the last agglomerator, the A/C-ratio is 0.500 and the lye concentration is 250 g/l/ expressed as Na2CO3.
The suspension leaves the last agglomerator having the stated solids content, an A/C-ratio of 0.500 and a temperature of 70C for the crystallizer 6, to which crystallizer 7 is 25 connected. The crystallizers 6 and 7 form the first crystalli-zer cascade (second hot cascade) of the process according to the invention. In practice, the number of crystallizers will be higher. For the sake of simplicity~ only two are specified here.
The time for passing through the cascade is apprcximately 8 hours 30 and the amount of crystallizing aluminum oxide hydrate is 20 gfl, expressed as A12O3. The suspension, which leaves the second cascade at 65C, therefore contains 80 g/l, calculated as A12O3.
At the end of the cascade, the A/C-ratio has fallen to 0.420.
In the crystallizers 6 and 7, according to the invention, are 35 set up in such a way that at least 80~ by volume of the filling volume has a high solids content and '`10--the remaining filling volume has a very low solids content.
The solids content in the o~erflow should not be higher than 20 g/l, preferably not greater than 3 g/lO After cooling of the suspension to 55C in the heat exchanger 8, fl~rther stirring out of the sodium aluminate liquor follows in the second crystallizer cascade, also referred to as "cold" cascade. It consists of the three crystallizers 9, 10 and 11. In pract:ice, this number can vary. The combined outflow from the crystalli-zer 11 has an A/C-ratio of 0.340 and a temperature of 50C.
The solids content (expressed as A12O3) is 100 g/l. Accordingly, 20 g A12O3/1 are formed in the cold cascade, and in all three cascades together 80 g/l are formed. The time for passing through the third cascade is approximately 15 hours, through the second cascade approximately 8 hours and through the first cascade approximately 7 hours. In total, therefore, about 30 hours are required.
In accordance with the invention, the combined outflow from the crystallizer 11 is separated in a two-step process, whereby the coarse-grained crystals are separated as product in the decanter 12, while the fine-grained crystals are removed as seed crystals from filter 13 and, after mixing with process liquor, are reintroduced into the circulation. 80 g are removed from decanter 12 as product of the process. As already noted, only 8 to 10~, preferably 2 to 5~, of these crystals have a grain size smaller than 45 ~m. After calcina-tion, a coarsely grained aluminum oxide is obtained the crystals of which are only insignificantly damaged during calcination.
It can be seen from the data relating to the method shown in Figure 1 that/ for production of 80 g coarse aluminum hydroxide, calculated as A12O3, 20 ~ fine seed material are xequired. Accordingly, only 20 g/l seed material, reintro-duced into the process, circulated through all 3 cascades.
The circulating seed crystal quantity is very small. To produce 1 ton coarse crystalline aluminum oxide hydrate, mereIy 0.25 t (20/80) fine-crystalline seed material is required. This seed quantity can, as was shown, fluctuate . . . ...

--11~

within certain limits. Thus, good results can be obtained when 0.2 - 1.0 t of fine seed material is added per ton of produced aluminum oxide.
If according to a special embodiment medium-coarse seed crystals are reintroduced or channelled into the first crystallizer of the first crystallizer cascade of the process, thenquantities of 100 to 300 g/l, calculated as A1203, are advantageous, preferably quantities of 150 to 170 g/l. 70 to 90%
of the medium-coarse seed crystals should be larger than ~5 ~m.
The flow chart shown in Figure 2 describes an embodiment of the method according to Canadian Patent No. 1,098,284, which can be compared with the method according to this invention in several points. Thus, in the illustrated embodiment, three cascades are used and the suspension which leaves the second cascade is cooled to a temperature of 65 to 50C prior to entry into the third cascade. The essential difference between the mathods of Figures 1 and ~, which are being compared, is that, with the method according to the invention, agglomerators are used in the first cascade, whereas, in the previously known ~o method, 3-phase stirrers are used in the first cascade. The methods also differ in that in the method according to the invention the stir~ers in the second and third cascade are not 3-phase stirrers, but are crystallizers in which crystal growth occurs almost exclusively and in which at least 80% by volume of the filling volume have a very high solids content.
In Figure 2, hot, supersaturated mother liquor is conveyed to the 3-phase stirrers 16 and 18 of the produc-t cascade(first cascad~ via duct 23 and seed crystals via duct 89. The lye concentration is approximately 200 to 300 g/l.
The overflow, that is, the partially spent liquor from the stirrer tanks of the product cascade is passed via ducts 29/31 into the first stir~er tank 36 of the second casca~e which is also constructed of a number of stirrer tanks, e.g. eight to tweIve. Illustra~ed are only the two stirrer tanks 36 and 38, which can be 3~phase stirrPrs. Additionally, fresh (not yet spent~ hot mother liquors are conveyed to the first stirrer tank 36 of this cascade ~ia duct 67 which branches off from ~12-duct 23. Moreover, the crystal slurry~ draw~ off the last stirrer tank 18 of the product cascade, is introduced into stirrer tank 36 via duct 69. These crystals are conveyed through stirrer tanks 16 and 18 of the product cascade, which are connected in series, while the partially spent liquors are removed from the stirrers. The concentration o~ solids,.
being discharged from stirrer tank 18, is between about 40 and 65%u In addition, medium-coarse seed crystals are introduced into the stirrer 36 via duct 85 from the bottom of decanter 46. The suspension is now agitated further in the stirrers 36 and 38. The entire mass, that is, the spent liquor and the cryst~ls suspended in it, is passed through stirrer tanks 36 and 38 of the cascade, which are connected in series and are connected to one an~ther by way of duct 71. The liquor and crystals leave the tank 38 via duct 73. The liquor/
crystal mixture is conveyed through heat exchanger 40 and cooled in it to a temperature of about 65 to 50~C. Subsequentl~, the mixture is introduced by way of duct 75 into stirrer tank 42, which is the first stirrer tank of a further cascade or which may be only one single stirrer tankO To simplify matters, only one stirrer tank 42 is illustrated. However, if in ~ cascade several tanks are connected in series, then the entire mass, that is the liquor and crystals are passed through the stirrer tanks in series and drawn off from the last stirrer tank~
Medium-coarse seed crystals are introduced into stirrer tanks 42, 36 and 38 via duct 85 and duct 87.
The mixture is agitated in the stirrer tanks 36, 38 and 42 and introduced into the first decanter 44 via duct 77.
Coarse crystals are removed as product from the decanter 44 via duct 79, while the overflow is introduced into a second decanter 46 via duct 81~ Medium~coarse seed crystals are drawn off from decanter 46 ~ia duct 85 and these are as already noted, introduced into stirrer tanks 36 and 42. The overflow from decanter 46 is introduced via duct 8~ into the third decanter 48. ~ine seed crystals are drawn o~f from de-canter 48 via duct 89 and introduced into the first stirrer tank 16 of the product cascade. The overflow of the decanter 48 is returned to the autoclave (bauxite treatment) via duct 91. The temperature in the cascade, formed by the stirrer tanks 36 and 38, is, as in the product cascade, about 90 to 75C, whereby the temperature decreases in direction of flow from the upper value to the lower value~ The temperature in stirrer t~c 42 is approximately 65 to 50C, preferably about 60 to 55C. The quantity of seed crystals introduced into the first stirrer tank 16 of the product cascade, is between about 30 and 200 g/l. (The quantities and weight indications with respect to the seed crystals and the lye refer to one liter of the total mixture).
The A/C-ratio of the mother liquor introduced into the stirrer tank 16 is approximately 0.660, the NaOH-content, calculated as Na2CO3, is about ~40 g~l and the content of aluminum oxide hydrate, calculated as A12O3, is about 157.4 g/l.
The quantity of fine seed crystals, which is fed into tank 16, is 36 g/l. When leaving the tank 18, the aluminum oxide hydrate concentration, calculated as A12O3, is 121~4 g/l. Thus, 36 g/l are produced in the product cascade. The partially spent sodium aluminate solution which is introduced into the first stirrer tank of the second cascade containing 121.4 g A12O3/1 leaves the last stirrer of this cascade with an A12O3-content of 99.4 g/l (A/C-ratio 0.414). ~ spent lye with an aluminum oxide hydrate content of 77.4 g/l, calculated as A12O3, passes from stirrer tank 42 of the third cascade via duct 77 into decanter 44. The A/C-ratio is 0.322. Thus, in all, with the prior art method 80 g/l aluminum oxide hydrate cxystals are formed.
As shown, the amount in grams per ldter of aluminum hydrate crystals produced is of no significance for assess-ment of the two methods which are being compared. The technical advance of the method according to the in~ention is apparen~
in the shorter time required for the production of the coarse crystals. The production time can be reduced by 30 to 40~ of the time re~uired for the method according to the Canadian patent. Moreover, the quantity o circulating seed crystals can be substantially decreased t S that smaller amounts have to be recycled and les~ equipment and technical resources are needed. Additionally, the crystals produced according to the :inventive method are substantially more stable, which is a further, important object of the present method.

Claims (28)

1. A continuous method for recovery of aluminum oxide from bauxite according to the Bayer process comprising:
a) forming a supersaturated sodium aluminate solution by treating ground bauxite with aqueous sodium hydroxide;
b) passing the supersaturated sodium aluminate solution having a sodium hydroxide concentration of about 200 to 300 g/l, calculated as Na2CO3, and a temperature of from about 80 to 65°C through a cascade of agglomerators connected in series, introducing a suspension of fine seed crystals into the first agglomerator of said agglomerator cascade such that a solids content of between about 10 and 50 g/l, calculated as Al2O3, is reached in the first agglomerator, and stirring out a portion of the dissolved aluminum oxide as aluminum oxide hydrate agglomerates;
c) passing the sodium aluminate solution containing said agglomerates through a first cascade of crystallizers connected in series, stirring out a portion of the aluminum hydroxide such that in each crystallizer at least about 80%
of the filling volume has a high solids content and the over-flow of the remaining filling volume has a solids content of not greater than about 20 g/l, underflow and overflow of each crystallizer being combined prior to introduction into the following crystallizer;
d) combining the discharges from the last crystallizer of said first crystallizer cascade, said discharges comprising the inflow into the first crystallizer of said first crystal-lizer cascade and the aluminum oxide hydrate formed in said first crystallizer cascade, and cooling said combined discharges to a temperature of from about 55 to 45°C;
e) transferring said cooled combined discharges from said first crystallizer cascade into a second cascade of crystallizers connected in series and stirring out said dis-charge such that in each crystallizer at least about 80% of the filling volume has a high solids content and the overflow of the remaining filling volume has a solids content not greater than about 20 g/l; and f) drawing off a crystal suspension from the last crystallizer of the second crystallizer cascade and fraction-ating said crystal suspension into fine and medium coarse seed crystals and coarse product crystals.

2. Method according to claim 1, characterized in that in steps c) and e) said overflow of the remaining filling volume has a solids content of not greater than about 3 g/l.

3. Method according to claim 1, characterized in that the fine seed crystals in the agglomerator cascade are intro-duced in amounts of between about 10 and 50 g/l, calculated as Al2O3, and about 25 to 60% of the crystals have a particle size of below 45 µm.

4. Method according to claim 3, characterized in that the fine seed crystals are introduced in amounts of between about 15 and 25 g/l, calculated as Al2O3.

5. Method according to claim 4, characterized in that about 35 to 45% of the fine seed crystals have a particle size of below 45 µm.

6. Method according to claim 1, characterized in that the sodium aluminate solution passes through the agglomerator cascade in about 6 to 10 hours.

7. Method according to claim 1, characterized in that the sodium aluminate solution passes through the agglomerator cascade at a temperature of about 78 to 75°C.

8. Method according to claim 1, characterized in that the sodium aluminate solution passes through the agglomerator cascade with a ratio of dissolved aluminum oxide to dissolved sodium hydroxide, (A/C-ratio), calculated as Na2CO3, of between 0.700 and 0.650 in the first agglomerator and of between 0.550 and 0.450 in the last agglomerator.

9. Method according to claim 1, characterized in that (1) said fine seed crystals in the agglomerator cascade are introduced in amounts of between about 10 and 50 g/1, calculated as Al2O3, and 25 to 60 percent of said crystals have a particle size of below 45 µm;
(II) the sodium aluminate solution passes through the agglomerator cascade with an A/C-ratio of between about 0.700 and 0.650 in the first aqglomerator and of between about 0.550 and 0.450 in the last agglomerator; and (III) said sodium aluminate solution passes through the agglomerator cascade in about 6 to 10 hours at a temperature of about 78 to 75°C.

10, Method according to claim 1 characterized in that the discharge from the agglomerator cascade passes through the first crystallizer cascade at temperatures of from about 70 to 62°C.

11. Method according to claim 1, characterized in that the discharge from the agglomerator cascade passes through the first crystallizer cascade in about 7 to 14 hours.

12. Method according to claim 8, characterized in that the discharge from the agglomerator cascade passes through the first crystallizer cascade in about 7 to 14 hours at a temperature of from about 70 to 62°C.

13. Method according to claim 1, characterized in that the sodium aluminate solution leaves the first crystallizer cas-cade with an A/C-ratio of between about 0.420 and 0.380.

14. Method according to claim 1, characterized in that at least 80% of the filling volume of each crystallizer has a solids content of between about 300 and 900 g/l, calculated as aluminum hydroxide.

15. Method according to claim 12, characterized in that at least about 80% of the filling volume of each crystallizer has a solids content of between about 300 and 900 g/l, cal-culated as aliminum hydroxide, and the overflow of the remaining filling volume has a solids content of not greater than about 3 g/l.

16. Method according to claim 9, characterized in that at least about 80% of the filling volume of each crystallizer has a solids content of between about 300 and 900 g/l, calculated as aluminum hydroxide, and the overflow of the remaining filling volume has a solids content of not greater than about 3 g/l.

17. Method according to claim 1, characterized in that the discharge from the last crystallizer of the first crystallizer cascade is cooled in a heat exchanger.

18. Method according to claim 1, characterized in that the cooled discharge from the last crystallizer of the first crystallizer cascade passes through the subsequent second crystallizer cascade in about 13 to 23 hours.

19. Method according to claim 15, characterized in that the discharge from the last crystallizer of the first crystallizer cascade is cooled in a heat exchanger and the cooled discharge passes through the subsequent second crystallizer cascade in about 13 to 23 hours.

20. Method according to claim 9 or 12, characterized in that the discharge from the last crystallizer of the first crystallizer cascade is cooled in a heat exchanger and the cooled discharge passes through the subsequent second crystallizer cascade in about 13 to 23 hours.

21. Method according to claim 1, characterized in that the sodium aluminate solution leaves the second crystallizer cascade with an A/C-ratio of between about 0.350 and 0.320.

22. Method according to claim 19, characterized in that the sodium aluminate solution leaves the first crystallizer cascade with an A/C-ratio of between about 0.420 and 0.380 and leaves the second crystallizer cascade with an A/C-ratio of between about 0.350 and 0.320.

23. Method according to claim 9, 12 or 15 characterized in that the sodium aluminate solution leaves the first crystallizer cascade with an A/C-ratio of between about 0.420 and 0.380 and leaves the second crystallizer cascade with an A/C-ratio of between about 0.350 and 0.320.

24. Method according to claim 1, characterized in that medium-coarse seed crystals are introduced into the first crystallizer of the first crystallizer cascade in amounts of between about 100 and 300 g/l, calculated as Al2O3.

25. Method according to claim 24, characterized in that medium-coarse seed crystals are introduced into the first crystallizer of the first crystallizer cascade in amounts of between about 150 and 170 g/l, calculated as Al2O3.

26. Method according to claim 24 or 25, characterized.
in that about 70 to 90% of the medium-coarse seed crystals introduced are larger than 45 µm.

27. Method according to claim 22, characterized in that medium-coarse seed crystals are introduced into the first crystallizer of the first crystallizer cascade in amounts of between about 100 and 300 g/l, calculated as Al2O3, and about 70 to 90 % of the medium-coarse seed crystals introduced are larger than 45 µm.

28. Method according to claim 5, 12 or 15, characterized in that medium-coarse seed crystals are introduced into the first crystallizer of the first crystallizer cascade in amounts of between about 100 and 300 g/l, calculated as A1203, and about 70 to 90 % of the medium-coarse seed crystals introduced are larger than 45 µm.