Process for effecting seeding in two phases for producing large-grain alumina

The invention concerns a process for the precipitation of aluminium hydroxide by seeding in two phases, which is intended to produce at a high level of productivity alumina with large grains, referred to as `sandy coarse`, by precipitation in a succession of tanks in cascade relationship of a supersaturated solution of sodium aluminate coming from alkaline attack on bauxite in accordance with the BAYER process. The process comprises three stages: PA0 in the first agglomeration stage, the supersaturated aluminate liquor is introduce into the tank, with an equivalent amount of terms of Na.sub.2 O of between 110 and 175 g/liter, at a temperature of between 65.degree. and 80.degree. C., and seeding is effected with an amount of washed seed of between 20 and 120 g/l of aluminate, PA0 in the second stage, selective retention of the largest crystals of aluminium trihydrate formed is effected in the tanks, so as to produce a content of dry matter of between 300 and 800 g/liter of suspension, PA0 the whole of the aluminium trihydrate which is intended for the production of alumina is extracted in the course of or at the end of the second stage, PA0 in a third stage, referred to as a liquor depletion stage, an amount of seed of between 400 and 800 g/l of suspension, that is to say between 480 and 1200 g/liter of liquor, is introduced, PA0 at the end of the third stage, separation is effected on the one hand of a depleted aluminate liquor which is recycled in known fashion and on the other hand aluminium trihydrate with very fine grains, a small portion of which is recycled after washing as a seed for the agglomeration stage while the remainder is recycled as a seed to the liquor depletion stage.

TECHNICAL FIELD OF THE INVENTION 
The present invention concerns a process for effecting precipitation of 
aluminium hydroxide by seeding in two phases to produce, at a high level 
of productivity, large-grain alumina in which less than 10% of the grains 
have their smallest size less than 45 micrometers, from supersaturated 
solutions of alkali metal aluminates which are provided by the BAYER 
process for alkaline attack on bauxites. 
The Bayer process which is widely described in the specialist literature is 
the essential process for the production of alumina which is intended to 
be converted into aluminium by igneous electrolysis. In that process, the 
bauxite is treated in a hot condition by means of an aqueous solution of 
sodium hydroxide, thus causing solubilisation of the alumina and the 
production of a supersaturated solution of sodium aluminate. After 
separation of the solid phase constituting the residue of the ore which 
has not been attacked (red mud), the supersaturated solution of sodium 
aluminate is generally seeded with aluminium hydroxide which is referred 
to hereinafter `seed`, in order to cause precipitation of an aluminium 
trihydroxide Al(OH).sub.3. 
There are a number of industrial variations of the process for the 
production of aluminium trihydroxide by Bayer alkaline attack on bauxites, 
and they are usually put into two categories, one being known as the 
European process and the other being known as the American process. 
In the European process, precipitation of the aluminium trihydroxide is 
effected in the course of the operation of decomposing an aqueous solution 
of sodium aluminate with a high level of concentration of caustic Na.sub.2 
O, titrating from 130 to 170 grams of Na.sub.2 O per liter of sodium 
aluminate solution to be decomposed. The expression `concentration in 
respect of caustic Na.sub.2 O` is used to denote the total amount of 
Na.sub.2 O expressed in terms of grams per liter in the sodium aluminate 
solution to be decomposed, both in the form of sodium aluminate and in the 
form of sodium hydroxide. In accordance with that process, an amount of 
trihydrate which is generally between 350 g/l and 600 g/l of suspension, 
acting as seed, is introduced into the sodium aluminate solution to be 
decomposed, decomposition of the solution generally being carried out at a 
temperature which is at most 55.degree.. Such a process provides a high 
level of productivity of alumina, which may attain 80 g of Al.sub.2 
O.sub.3 per liter of the sodium aluminate solution, but the aluminium 
trihydroxide which is produced in that way is generally of fine grain size 
and, by roasting, gives an alumina, the fine nature of which is considered 
at the present time to be a nuisance in regard to igneous electrolysis, 
because of the dust that it produces. 
In the American process, precipitation of the aluminium trihydroxide is 
effected by decomposition of an aqueous solution of sodium aluminate with 
a low level of concentration of caustic Na.sub.2 O, not exceeding 100 g of 
Na.sub.2 O per liter of a solution of sodium aluminate to be decomposed. 
An amount of Al(OH).sub.3 acting as a seed is introduced into the sodium 
aluminate solution, the amount used being less than in the European 
process and being of the order of 100 g/l to 200 g/l of the aluminate 
solution to be decomposed; in contrast, decomposition in the American 
process is carried out at a higher temperature, for example 70.degree. C. 
All those operating conditions in combination result in the production of 
an aluminium trihydroxide which is of larger grain size than that produced 
by the Euorpean process, and the large grains produced, after 
classification and roasting, give an alumina which is referred to as 
`sandy coarse`, with the grain size required for the production of 
aluminium by means of igneous electrolysis. However, due to a contrary 
effect, the above-indicated operating conditions give rise to a drop in 
the yield, which thus appears to be much lower than with the European 
process, generally being around 50 g of Al.sub.2 O.sub.3 per liter of the 
aluminate solution when producing a sandy coarse alumina. Attempts to 
improve the level of productivity by a reduction in the decomposition 
temperature and introducing a larger amount of Al(OH).sub.3 as a seed into 
the solution of sodium aluminate to be decomposed, gave the result that 
the alumina of sandy coarse grain size was no longer produced and an 
alumina of smaller grain size was obtained instead. 
STATE OF THE ART 
For a long time now, as can be witnessed from the large number of 
publications in this art, many attempts have been made both in regard to 
the American process and the European process to arrive at a process for 
the production of aluminium trihydroxide of large grain size, while 
enjoying the benefit of the level of productivity of the European process. 
Among the processes which have been put forward, some involve using the 
whole of the seed at the beginning of decomposition, as is the case with 
U.S. Pat. Nos. 3,607,113 and 3,649,184 and our patent application FR-A-2 
529 877, but most of such processes provide for seeding in two phases, a 
small part of the seed being used at the beginning of the decomposition 
operation and the balance being added after a time that varies between 6 
and 24 hours. The present invention falls into the category of 2-phase 
seeding processes. 
A first two-phase seeding process is disclosed in U.S. Pat. No. 2,657,978, 
the purpose of which is to promote an increase in the level of 
productivity of aluminium hydroxide of large grain size, and which uses a 
procedure involving introducing the aluminium hydroxide acting as the seed 
in two periods, the first period involving introducing just the amount of 
seed required to produce crystals of large grain size while the second 
period involves introducing a fresh amount of seed. However from the 
results which are listed, the increase in productivity appears to be 
slight and is therefore not a very attractive proposition on an industrial 
scale. 
Another two-phase seeding process (FR-A-2 440 916) provides two-phase 
decomposition of the supersaturated solution of sodium aluminate: the 
first phase of the decomposition procedure comprises introducing a 
controlled amount of a suspension of fine seed into the sodium aluminate 
solution, that phase taking place at a temperature of from 77.degree. C. 
to 66.degree. C. Then, in the second decomposition phase, a sufficient 
amount of seed of larger grain size is introduced into the cooled 
suspension produced in the first phase, in such a way that the combined 
amount of seed introduced in the two phases represents at least 130 g of 
aluminium trihydroxide per liter of solution to be decomposed, and at most 
400 g/l. 
The essential part of the improvement in productivity claimed is due to a 
cause which a totally independent of the decomposition process: it was 
assumed that the procedure resulted in the decomposition of more highly 
supersaturated liquors. In actual fact, this process cannot claim to 
approach the level of productivity in decomposition in the European 
process for many reasons: according to the examples given in the 
above-indicated patent, the amount of seed used is substantially lower: 
less than 400 g/liter of liquor as against at least 600 g in the European 
processes and up to 2000 g/liter of liquor in our application FR-A-2 529 
877. In addition, the alumina contained in the decomposing vessels 
includes a large proportion of large grains and accordingly constitutes a 
seed with a low specific surface area. 
Finally, the level of concentration of the liquors involved in the 
decomposition procedure is lower: 125 g of Na.sub.2 O/liter as against 150 
to 170 g/l in the European processes, the level of productivity varying 
almost proportionally with the level of concentration. 
French patent FR-A-1 391 596 and U.S. Pat. No. 305,913 propose two 
relatively similar processes for decomposition in a step-wise fashion of a 
supersaturated solution of sodium aluminate at a temperature of from 
74.degree. C. to 85.degree. C., the amount of seed introduced being from 
70 to 140 g/l of solution of sodium aluminate to be decomposed. However, 
although those processes produce an alumina of grain size which is 
apparently favourable, they have only a low level of productivity in 
comparison with the European process. There are at least two reasons for 
that: 
the amount of seed used is very small: less than 200 g/liter of aluminate 
as against 600 g/l in most of the European processes and up to 2000 g in 
our application FR-A-2 529 877; 
the level of concentration of soda is low: 140 g of Na.sub.2 O (that is to 
say, 240 g expressed in terms of Na.sub.2 CO.sub.3 /liter) as against 150 
to 170 g/l in the European decomposition processes. 
In addition, the above-indicated process comprise two circuits in parallel 
in relation to the liquor and in series in relation to the solids, there 
are a number of qualities of seed, all of which requires many grading and 
liquid/solid separation apparatuses. That gives rise to burdensome levels 
of capital investment and a relatively complicated mode of operation. 
It thus seems that many paths have been followed in the attempts to arrive 
at a process for the decomposition of a supersaturated solution of sodium 
aluminate, which concurrently enjoys the qualities which are to be found 
in the American processes and the European processes alone, that is to 
say, which permits the production of an alumina of coarse grain size 
(sandy coarse type), with a high level of productivity. However, the man 
skilled in the art is forced to recognise that the proposed processes give 
incomplete solutions which are generally unsatisfactory since, in order to 
arrive at an alumina of acceptable grain size, it is generally necessary 
to suffer a drop in the high level of productivity of alumina, that the 
man skilled in the art is no longer able to accept on an industrial scale. 
Thus, in French patent No. 2 318 113 (=CA 1 098 284), there is claimed the 
production of large-grain alumina with a level of productivity which 
approaches that obtained in the European processes. Nonetheless, as that 
process does not involve any agglomeration phase, the increase in the size 
of the crystals is essentially produced by crystalline growth. Now, it has 
been irrefutably established that the alumina which is produced by 
crystalline growth is much more fragile than that which is produced by the 
agglomeration of the seed crystals and the crystalline growth which 
results therefrom. 
In our patent application FR-A-2 529 877 (PECHINEY), we proposed a process 
for the production of aluminium trihydroxide in which 10% at most of the 
particles are smaller than 45 micrometers, and with an enhanced level of 
productivity, comprising, in the decomposition zone of the Bayer process 
comprosing a succession of stages in cascade relationship, creating a 
suspension with a high proportion of dry matter of at least 700 g/l and 
preferably 800 to 2000 g/l of alkali metal aluminate, and seeding, in the 
decomposition zone, by means of crystals of aluminium trihydroxide of 
non-selected grain size, and separating, in the final classification zone, 
the portion of large grain size which constitutes the production and the 
remaining suspension which, after a fresh separation operation, 
constitutes the non-selected seed which is recycled to the decomposition 
zone. 
Finally, European patent application EP-A-102403 describes a process 
involving five successive steps, an alternative embodiment of which may 
comprise seeding in two phases: 
In accordance with that process, the supersaturated solution of sodium 
aluminate, which is mixed at a level of concentration of sodium hydroxide 
of from 200 to 300 g/l, considered as Na.sub.2 CO.sub.3, passes at 
temperatures of between 80.degree. and 65.degree. C. through a cascade 
array of agglomerators which are connected in series, and a part of the 
aluminium oxide is added as aluminium oxide hydrate and in the course of 
that procedure; in the first agglomerator, a suspension of fine seed 
crystals is added, and the first agglomerator has a solid matter content 
of from 10 to 50 g/l of aluminate, considered as aluminium oxide (Al.sub.2 
O.sub.3); the solution of sodium aluminate containing the agglomerates 
then passes through a first cascade array of crystallisers which are 
connected in series and a part of the aluminium hydroxide is agitated in 
such a way that, in each crystalliser, at least 80% of the total volume 
has a high proportion of solid matter and the upper part of the residual 
volume has a proportion of solid matter considered as aluminium oxide 
(Al.sub.2 O.sub.3) that does not exceed 20 g/l and in particular does not 
exceed 3 g/l, and in which the lower and upper parts of the solution, only 
after being mixed together, pass into the following crystalliser; the 
combined flows which are found in the last crystalliser and which are 
formed by the flow passing into the first crystalliser and by the 
aluminium oxide hydrate formed in the first cascade array of 
crystallisers, are cooled, reducing the temperature from 55.degree. C. to 
45.degree. C.; the flows are agitated in a second cascade array of 
crystallisers which are disposed in series in such a way that, in each 
crystalliser, at least 80% of the total volume has a high proportion of 
solid matter and the upper part of the residual volume has a proportion of 
solid matter, expressed as aluminium oxide (Al.sub.2 O.sub.3), that does 
not exceed 20 g/l and in particular does not exceed 3 g/l and finally, the 
crystals in suspension which are extracted from the last crystalliser of 
the second cascade array of crystallisers are split up into fine or medium 
inocculation crystals, as appropriate, and into coarse crystalline 
rejects. That process claims a level of productivity of the order of 80 
grams per liter (expressed as Al.sub.2 O.sub.3) of aluminium trihydrate 
crystals. 
Although the level of productivity is an attractive proposition, it is 
limited for the following reason: the alumina which circulates in the 
decomposing vessels comprises the fraction which will be taken off for the 
production. Accordingly, it is necessarily of large grain size and has a 
low specific surface area. It therefore constitutes a seed which is of low 
effectiveness, bearing in mind that the decomposition kinetics are 
directly proportional to the surface area of the seed used. 
SUBJECT OF THE INVENTION 
The subject-matter of the present invention is a continuous process for the 
production of aluminium trihydrate which combines both a high level of 
productivity (which can attain 85 to 87.5 g/l expressed in terms of 
Al.sub.2 O.sub.3), a granulometry which is in accordance with the 
requirements of the producers of aluminium (substantially less than 10% of 
grains smaller than 45 micrometers) and `solidity` of the grains, which 
reduces the undesirable formation of fines in the operations of roasting, 
transportation and use of the alumina, in particular in electrolysis 
works. 
This process is characterised by three successive stages which are carried 
out in a cascade array of tanks disposed in series: an agglomeration stage 
in the course of which the fine particles of alumina are agglomerated to 
form grains of larger size, a second stage which provides for 
consolidation of the grains and an increase in granulometry by selective 
retention of the large grains, at the end of which the whole of the 
production is removed, and, finally, a stage involving depletion of the 
liquors with the massive addition of fine seed with a large specific 
surface area, the last stage producing both the seeds for the first stage 
and the whole of the seed for the liquor depletion stage.

It will be noted that the selective retention tanks are supplied by way of 
their bottoms to effect the formation of a fluidised bed which is one of 
the ways of effecting selective retention of the large grains. 
In the first stage, referred to as the agglomeration stage, a conventional 
aluminate liquor is introduced into the tank B1, the liquor having an 
equivalent content in terms of Na.sub.2 O of between 110 and 175 g/liter, 
and an alumina/caustic Na.sub.2 O weight ratio of the order of 1.10 to 
1.20 at a temperature of between 65.degree. and 80.degree. C. and 
generally between 70.degree. and 75.degree. C. 
The aluminate solution is seeded with a fine seed (50%&lt;45 micrometers for 
example) with a high specific surface area, which comes from the third 
stage, referred to as the `liquor depletion stage`, as will be described 
hereinafter. The agglomeration phase is carried out at a temperature of 
between 65.degree. and 80.degree. C. (on average 70.degree. C.), for a 
period of from 6 to 20 hours (on average from 8 to 10 hours) and the 
amount of seed used is between 20 and 120 g of trihydrate per liter of 
aluminate, with an average of the order of 30 to 50 g/l. 
The above-indicated conditions in respect of temperature and seeding result 
in the production of an alumina with large grains, as from the early hours 
of the decomposition procedure, by virtue of a low rate of nucleation and 
a high rate of agglomeration of the nuclei. 
In the tank B1, the aluminate liquor is circulated in a downward direction. 
That circulation tends to `expand` the suspension in the tank in such a 
way that the concentration of the aluminium trihydrate suspension in the 
tank is lower than the concentration at the intake and at the outlet. In 
addition, the large particles have a residence time and therefore a level 
of concentration which is reduced in comparison with the fines, which, 
combined with the absence of any form of agitation, operates in such a way 
as to promote agglomeration. 
The second stage is carried out at a lower temperature, so as to increase 
the speed of decomposition of the aluminate, of the order of 50.degree. to 
65.degree. C. (on average 60.degree. C.) for a period of from 10 to 25 
hours (on average 15 hours), and the aim is to produce a proportion of 
`dry matter` in the tanks that is greater than 300 g/l and preferably 
between 400 and 800 g/liter (the term `dry matter` is used to denote the 
dry aluminium trihydrate expressed in terms of Al(OH).sub.3). 
The essential feature of the second stage is that selective retention of 
alumina is produced therein, in relation to the granulometry thereof, by 
providing the rapid circulation of the finest particles of trihydrate and 
extending the residence time of the largest particles, so as to provide 
for both an increase in the size thereof and consolidation thereof. 
Selective retention of the large grains is achieved by suitable means such 
as for example the procedure referred to as the fluidised bed procedure as 
described in French patent No. 1 187 352 (in the name of Societe 
d'Electrochimie-d'Electrometallurgie et des Acieries Electriques d'Ugine). 
In the course of this second stage for the selective retention of the large 
grains which is achieved by an increase in the length of the residence 
time in a zone in which the aluminate decomposition mechanism is still 
operating rapidly, the alumina selectively precipitates on the large 
grains, which causes deformation of the granulometric histogram, to the 
favour of the large grains of alumina, and consolidation of the grains by 
an improvement in their shape factor: they go from a fragile irregular 
shape to a more rounded shape which is better able to resist attrition. It 
is known in fact that the grains produced by an agglomeration process 
followed by nourishing are particularly solid. 
To achieve that double improvement in regard to the size and the shape of 
the grains, selective retention is effected in respect of the grains of 
aluminium trihydrate which are of a mean size of at least 50 to 60 
micrometers so as to ensure therefor a mean residence time in the tank 
that is at least equal to twice and preferably from 5 to 10 times the mean 
residence time of the aluminate liquor, which is achieved by providing for 
a speed of rise of the liquor in the upper part of the tank that is 
between 0.5 and 3 meters per hour and preferably between 1 and 2 meters 
per hour. 
It is important to note that the latter mechanism for selectively 
increasing the size of the large grains of alumina is not sensitive to 
deposits of oxalate and to other organic impurities. That is an advantage 
in comparison with granulometric control processes which are based 
exclusively on the agglomeration mechanism alone which is sensitive to 
such impurities. Moreover, the deposit of alumina improves the `solidity` 
of the grains, by imparting thereto more rounded forms which make them 
less vulnerable to handling: it is that that we have referred to as 
`consolidation`. 
In practice, the successive tanks B3 in the second phase are supplied by 
way of the bottom, from the overflow of the previous tank, except in the 
case of the first cementation tank B2 which is supplied with the liquor 
that is taken off in the lower part of the germination tank B1. In 
accordance with that procedure, the rising flow, in contrast to what 
happened in the tank B1, contracts the suspension, the level of 
concentration of which substantially increases, which is a factor 
operating to produce an elevated level of productivity. 
Irrespective of the procedure used for providing for selective retention of 
the large grains, the production (P)--and this is one of the original 
points of the process--is taken off in its entirety in this stage. It is 
characterised by a proportion of fine product of smaller than 45 .mu.m 
that is less than 5% and by a yield which can exceed 85 kg (expressed in 
terms of Al.sub.2 O.sub.3) per cubic meter of aluminate liquor which 
passes into tank No 1. 
The production can be removed either from each of the tanks in the second 
phase or from the last tank of the second phase; in the latter case, it is 
appropriate to ensure that the large grains circulate from one tank to the 
following tank. 
The third stage, referred to as the liquor depletion or exhaustion stage 
(tanks B4, B5 and B6) is characterized by a massive injection of fine 
seeds (from 400 to 800 g of trihydrate per liter of suspension and on 
average 750 g/l), with a high specific surface area. The temperature is 
fixed at between 45.degree. and 55.degree. C. (on average 50.degree. C.) 
and the duration of the third phase is from 10 to 20 hours (on average 15 
hours) and agitation is effected in the usual fashion. 
Removed at the discharge from the last tanks (B6) is the suspension which 
contains about 800 g/l of dry matter per liter of suspension, comprising 
approximately 40% of fines (smaller than 45 .mu.m), which, after 
separation by filtration, in the usual fashion, provides on the one hand a 
depleted liquor (in which the sodium aluminate content has fallen to a 
ratio by weight of approximately 0.57) which is concentrated and recycled 
to the bauxite attack stage, and on the other hand, aluminium trihydroxide 
of which a small portion (approximately 1/10th) is used, after washing (to 
remove the organic compounds and in particular sodium oxalate), as a seed 
in the first seeding stage, the remainder thereof being recycled as the 
seed to the head of the liquor depletion zone. The washed fraction of seed 
may be added in its entirely at the beginning of the first stage or it may 
be graded into fine seed which is added at the beginning of the first 
stage and medium seed which is added at the beginning of the second stage. 
EMBODIMENT 
Using an industrial BAYER alumina production unit, a supersaturated 
solution of sodium aluminate which was produced by the attack at a 
temperature of 245.degree. C. on a mixture of French and Australian 
bauxites of the following composition, expressed in percent by weight, was 
introduced: 
______________________________________ 
BAUXITE 
COMPOSITION FRENCH AUSTRALIAN 
______________________________________ 
firing loss 13.47 23.88 
SiO.sub.2 5.3 5.3 
Al.sub.2 O.sub.3 
52.5 54.8 
Fe.sub.2 O.sub.3 
24.0 13.0 
TiO.sub.2 2.7 2.6 
CaO 1.8 0.05 
V.sub.2 O.sub.5 
0.08 0.04 
P.sub.2 O.sub.5 
0.20 0.08 
Organic C 0.15 0.25 
______________________________________ 
The composition of the sodium aluminate solution to be decomposed was as 
follows: 
______________________________________ 
Caustic Na.sub.2 O: 160 g/l 
Carbonated Na.sub.2 O 18 g/l 
Al.sub.2 O.sub.3 181 g/l 
Al.sub.2 O.sub.3 /caustic Na.sub.2 O 
1.13 
Organic C 12 g/l 
______________________________________ 
The sodium aluminate solution to be decomposed was introduced into the 
agglomeration tank at a rate of 120 m3 per hour and at a temperature of 
70.degree. C., with the simultaneous addition of 45 kg per m3 of seed 
coming from the depletion phase and previously washed to remove the 
organic compounds (oxalate). 
The whole of the aluminate liquor which was extracted by way of the bottom 
of the agglomeration tank in which it passed a residence time of on 
average 8 hours was adjusted to a temperature of 60.degree. C. by passing 
it through an exchanger, and then passed to the lower part of the first 
tank of the second zone. 
That zone comprised three tanks in series, providing the liquor with a 
residence time of 18 hours; the tanks were supplied by way of their lower 
part and operated on the basis of the fluidised bed principle described in 
FR-A-1 187 352. The apparent rate of rise of the liquid in the upper part 
of the tank was 1.6 m/h. That rate does not permit the grains which are 
larger in diameter than 60 .mu.m to be entrained towards the tank 
overflow; they are therefore retained therein until they are removed as 
the production alumina. The amount of alumina in the tanks being 650 g/l 
of suspension, the mean residence time of the grains of a diameter of 
greater than 60 .mu.m is between 4 and 5 times greater than that of the 
liquor. In contrast, grains with a diameter of less than 60 .mu.m have a 
residence time which increasingly approaches the residence time of the 
liquor, in proportion to decreasing diameter. The alumina taken off as the 
production is removed by means of a battery of cyclone separators. 
The liquor issuing from that zone contains about 20 g/l of fine alumina in 
suspension; it is cooled to 50.degree. C. and mixed with 1090 kg/m3 (that 
is to say 750 g/l of suspension) of fine seed, containing 40% of grains 
smaller than 45 .mu.m and with a specific surface area of 820 cm2/g. That 
suspension is agitated in tanks providing a residence time of 16 hours, 
and then filtered. The alumina is recycled as a seed to the beginning of 
the third phase after the portion which, after washing, is intended to be 
used as a seed in the first phase has been taken off. The liquor issuing 
from the filtration operation, in which the Al.sub.2 O.sub.3 /caustic 
Na.sub.2 O ratio has fallen from 1.13 to 0.57 is recycled to the 
evaporation shop and then to the operation for attacking a fresh amount of 
bauxite. 
The results obtained are summarised below: 
productivity of the aluminate liquors: 87.5 kg of Al.sub.2 O.sub.3 /m3 of 
aluminate liquor introduced at the head end, 
granulometry of the alumina produced: cumulative material passing at: 
______________________________________ 
45 .mu.m 64 .mu.m 96 .mu.m 128 .mu.m 
3% 14% 58% 92% 
______________________________________ 
attrition index of the roasted product, in accordance with the modified 
version of the Forsythe-Hertwig test: 11; (N.B. this method is described 
in the following publication: FORSYTHE W L & HERTWIG W R Attrition 
characteristics of fluid cracking catalysts. Ind and Engr Chem 41, pages 
1200-1206. 
CONCLUSION 
Carrying the invention into effect made it possible to produce an alumina 
of excellent granulometric quality since it contains only 3% of substance 
smaller than 45 .mu.m. 
The level of productivity of the liquors is the same as that which is 
obtained in European processes for producing fine alumina.