Process for the manufacture of coarse aluminum hydroxide

Process for the manufacture of coarse aluminum hydroxide, containing a maximum of 15 weight percent of the particles in a particle size under 45 micrometers, with a productivity that may be higher than 60 grams of precipitated Al.sub.2 O.sub.3. The process consists of a decomposition in two stages (an agglomeration phase and a growth phase); each stage proceeding within defined temperature ranges with seed material of different constitution. The first (fine) seed amount is added at the start of the decomposition in such an amount, that the ratio of the supersaturation of the aluminate liquor to the surface area of the seed hydroxide per liter of the aluminate liquor presented for decomposition exhibits a value of 7 to 25 grams per square meter. The process exhibits the advantage of the high productivity of the so-called European process and the advantage of the so-called American process by obtaining a coarse product hydroxide.

BACKGROUND OF THE INVENTION 
The invention concerns a process for the manufacture of coarse aluminum 
hydroxide, for example via the Bayer process, in which bauxite is digested 
with an aqueous sodium aluminate liquor, so that the aluminum oxide 
contained in the bauxite goes into solution. The crystallization 
(hereinafter referred to as decomposition) of aluminum hydroxide results 
from the seeding of the filtered, supersaturated sodium aluminate solution 
with finely divided aluminum hydroxide. 
In particular, it concerns a process for the crystallization of an aluminum 
hydroxide of coarse particle size, which contains a maximum of 
approximately 15 percent by weight of the particles of a particle size 
under 45 micrometers, from a supersaturated, alkaline aluminate solution 
obtained for example from the familiar Bayer process. 
The decomposition proceeds in two stages through the addition of aluminum 
hydroxide seed of different constitution in the referred to aluminate 
solution in each of the two stages. 
Two processes for the manufacture of aluminum oxide by the Bayer process 
find large scale application today, namely that usual in European plants 
and that practiced in American plants. 
The process usual in European plants employs a high Na.sub.2 O 
concentration, up to 140 g/l, for the decomposition. So that a good 
productivity may be obtained at this high liquor concentration, the 
decomposition is carried out using a sufficiently great amount of seed 
hydroxide, for example 200-250 g Al(OH).sub.3 /l (and more) at a 
sufficiently low enough temperature, for example at 55.degree. C. or less. 
Productivities of up to 80 g of aluminum hydroxide per liter are thereby 
obtained. However, the precipitated hydroxide in such plant is finer than 
that produced in American plants. As long as the fine hydroxide from these 
plants is calcined at high temperatures, an oxide with little tendency 
towards dustiness is obtained. However, the introduction of the dry 
scrubbing of waste gases demands an oxide with a BET-surface area between 
30 and 60 square meters per gram, which can only be obtained by a weak 
calcination of the aluminum hydroxide. However, a weak calcination of the 
fine aluminum hydroxide produced in European plants gives an oxide with a 
strong tendency to dustiness, which is not readily acceptable to the 
consumer. 
The process practiced in American plants is so designed to produce a coarse 
hydroxide, which, under the weak calcination usual in these plants, 
results in an oxide with little tendency to dustiness. In order to 
manufacture a coarse hydrate, a liquor concentration normally under 110 
grams Na.sub.2 O caustic per liter is chosen in the American process. The 
starting temperature is high, for example 70.degree. C., and the amount of 
seed material low, for example 50-120 grams Al(OH).sub.3 per liter. If too 
low a starting temperature for the decomposition is chosen, and the amount 
of seed hydroxide too high, then a fine product is obtained. The 
conditions in the American process for the production of the desired 
coarse product are in opposition to a good liquor productivity. The lower 
liquor productivity of this process is shown in that at best about 55 
grams of aluminum oxide per liter of liquor is produced in opposition to a 
productivity of up to 80 grams per liter from the European process. 
Expressed in another way, under the American process, 18-20 cubic meters 
of liquor need to be decomposed to produce a ton of aluminum oxide, 
compared to only about 13 cubic meters for the European process. 
As already referred to above, a coarse aluminum hydroxide, as produced in 
American plants and not produced in European plants, is required for the 
production of a weakly calcined aluminum oxide with a BET surface area of 
30 to 60 square meters per gram. European plants could very well adapt the 
American practice, however, the productivity of the European plants would 
fall about 30-40 percent, with a corresponding rise in heat consumption 
per ton of aluminum oxide. It is therefore very desirable that the 
European plants have a process which allows the production of a coarse 
aluminum hydroxide, without however having to accept a reduction in 
capacity of the plant with a corresponding increase in the specific heat 
consumption of the manufactured product. 
On the other hand, it is very desirable to lift the productivity of 
American plants to the level of European plants while maintaining the 
coarse product quality. Such an improvement in the American plants would 
mean an increase in capacity, accompanied by a reduction in the specific 
heat consumption per ton of manufactured aluminum oxide. 
To the present time, there has been no lack of proposals as to how this aim 
of a coarse product and high productivity may be achieved. In U.S. Pat. 
No. 2,657,978, it is suggested to modify the American process so that the 
seed material addition proceeds in two steps. In the first step, only 
sufficient seed is added to promote a strong coarsening. This is then 
followed by a second addition so that a good productivity is achieved. By 
the fixing of the referred to caustic concentration at about 85 grams per 
liter Na.sub.2 O, a productivity of about 48 grams Al.sub.2 O.sub.3 per 
liter of liquor to be decomposed may be arrived at for this process. This 
may be compared with a productivity of about 45 grams per liter for the 
unmodified process with a single addition of seed, where a reaction time 
of 35 hours is employed in both cases. The increase in productivity is 
thus calculated to be about 6.5 percent. 
In FR Pat. No. 1,391,596 a two-stage process with two decomposing chains is 
described, which, with a decomposition time of 30-40 hours, results in a 
productivity increase of 6.4% and a coarser product than that obtained 
with the normal one-stage American process. Although no absolute figures 
as to the productivity are given in this patent, this would not greatly 
exceed that in the previously quoted U.S. Pat. No. 2,657,978. The process 
consists of two decomposition chains, wherein one recieves fine seed in an 
amount and under such conditions which allows agglomeration to occur, and 
where the other is treated with coarse hydroxide in an amount and under 
such conditions that growth of the crystals results. Following the 
separation of the coarse product hydroxide and the coarse seed, the 
partially exhausted aluminate liquor from both chains is treated with 
further fine seed in the second stage, in order to exhaust the aluminate 
liquor still further, and to raise the productivity of the precipitated 
aluminum hydroxide. The main feature of this process is a coarse, abrasion 
resistant product obtained at an improved productivity. 
In U.S. Pat. No. 3,486,850, a process is projected, where the increase in 
productivity of the American process is obtained by intermediate cooling 
during the decomposition, while maintaining the production of a coarse 
product. However, this must be carried out in a narrowly defined 
temperature region, in order not to obtain too fine a product. In one 
example with this process, a productivity of 51 grams of Al.sub.2 O.sub.3 
per liter of liquor to be decomposed is quoted, with a decomposition time 
of approximately 40 hours. 
In Light Metals 1978, Volume 2 (Proceedings of sessions 107th AIME Annual 
Meeting, Denver, Colo., page 95) the conversion of a European process 
alumina plant to the American process is described. The process selected 
is similar, with only minor deviations, to the previously referred to FR 
Pat. No. 1,391,596. The productivity thus achieved is 56.3 g Al.sub.2 
O.sub.3 per liter of aluminate liquor to be decomposed, with a reaction 
time between 40 and 50 hours. Other processes are also referred to in this 
publication, which, although definitely giving a coarse product, exhibit 
lower productivity than the described process employed. 
Summarizing, previously known proposals for the improvement of the 
productivity of the American process do not yield much more than 
approximately 55 grams Al.sub.2 O.sub.3 per liter of the aluminate liquor 
to be composed. 
This value is naturally subject to certain deviations above and below, and 
is dependent on the initial supersaturation of the aluminate liquor and 
the time of reaction selected. 
When the productivity is compared with that of up to 80 grams Al.sub.2 
O.sub.3 per liter from the European process a very considerable difference 
exists. Accordingly, the purpose of the invention is to improve the 
decomposition yield (productivity) of aluminum hydroxide in terms of grams 
of Al.sub.2 O.sub.3 per liter from the clarified, supersaturated sodium 
aluminate liquor to be decomposed, while obtaining an aluminum hydroxide 
of coarse particle size (American type) whose fine fraction (less than 45 
micrometers) does not exceed 15 percent by weight, and normally exhibits a 
range of between 4 to 8 percent by weight. 
SUMMARY OF THE INVENTION 
According to the invention, this purpose is accomplished by means of the 
following process steps. 
1. The quantity of aluminum seed material is distributed as follows: 
1. An initial addition of fine seed (primary seed) at the beginning of the 
decomposition, where the amount is so calculated that the ratio between 
the supersaturation of the aluminate liquor as grams per liter of Al.sub.2 
O.sub.3 and the surface area of the above mentioned seed, expressed as 
square meters per liter of aluminate liquor, lies between 7 and 25 grams 
per square meter. 
1.2 a second addition of coarse seed (secondary seed) after an interval of 
at least approximately two hours following the first addition, where the 
total amount of seed (primary and secondary seed) is at least 130 grams of 
Al(OH).sub.3 per liter of aluminate liquor and 
2. that the temperature is regulated in the following manner: 
2.1 the first stage of the referred to decomposition which corresponds to 
the first addition of seed material is carried out in a temperature range 
of 77 degrees Celsius to 66 degrees Celsius and 
2.2 the second stage of the referred to decomposition which mainly 
corresponds to the second addition of seed material is carried out at 
reduced temperature which can be down to approximately 40 degrees Celsius. 
The process according to the invention is a combination of individual 
operations, which are individually more or less well known, but which by 
themselves or under insufficiently employed conditions have never (as the 
state of the art demonstrates) attained the results which may be achieved 
with the invention presented here.

DETAILED DESCRIPTION 
FIG. 1 substantially illustrates a production arrangement for production of 
American type aluminum hydroxide. It has been correspondingly adapted in 
order to be able to carry out the process according to the invention, and 
that, among other items, with the possibility to carry out the 
decomposition in two stages with the appropriate distribution of the seed 
material. The schematic flow diagram in FIG. 1 shows only two decomposers, 
1 and 6, arranged in series. However, normally more than one of each of 
decomposers 1 and 6 are installed, which are connected together either in 
series or parallel in batch operation, but are mostly operated 
continuously. 
As is schematically shown, decomposer tank 1 is supplied by pipeline 2 with 
sodium aluminate liquor supersaturated with aluminum oxide. Measured 
amounts of a fine seed suspension are delivered through pipeline 3 into 
decomposer tank 1. The temperature, amount of seed and the molar ratio are 
so adjusted to the characteristics of the seed material and the plant 
conditions that the desired degree of agglomeration of the fine seed 
material occurs in decomposer tank 1, so that the equilibrium of the fine 
seed inventory may be maintained if necessary through the addition of a 
certain amount of coarse seed through pipeline 8--8'. 
This agglomeration proceeds relatively rapidly in a temperature range of 77 
degrees Celsius to 66 degrees Celsius. After a reaction time of two hours, 
it is already considerably advanced, and practically complete after six 
hours (see following). The suspension can now be cooled with a device 4, 
and then delivered by means of pump 5 into decomposer tank 6, where the 
decomposition proceeds to completion. In this decomposer 6, the suspension 
from decomposer 1 is then seeded with sufficient coarser seed material 
from secondary thickener 16, so that the decomposition proceeds anew to 
completion with a larger seed surface area and a newly raised 
supersaturation obtained by cooling. The cooling can also result due to 
contact with the surroundings through the uninsulated walls of the 
decomposer tank or tanks. According to the process of the invention, a 
sufficiently large quantity of seed material is transferred from the 
secondary thickener 16 through pipeline 8 into decomposer 6, and, if 
necessary small excess quantities of fine seed material from tertiary 
thickener 19 through pipeline 3--3'. This second process stage permits the 
seed hydrate to grow further, and according to the degree of 
supersaturation of the aluminate liquor, results in the formation of fine 
particles of hydroxide by secondary nucleation and by mechanical 
detachment of fine crystals. Due to the relatively high amount of 
secondary seed, the secondary nucleation effect is held within bounds. 
The suspension is then pumped by pump 7 through pipeline 9 into primary 
thickener 10. In this primary thickener 10, the thickened underflow 
consists of product hydroxide, which is pumped through pipeline 11 by pump 
12 into filtration plant 13, from which the hydroxide filter cake is sent 
to the calcining kiln (not shown). 
The overflow of the primary thickener 10 is sent through pipeline 14 into 
the secondary thickener 16. The thickened underflow of the secondary 
thickener 16 consists of coarse seed hydroxide, which is pumped by pump 17 
through pipeline 8 into decomposer tank 6. The overflow of the secondary 
thickener is sent through pipeline 18 into the tertiary thickener 19. The 
thickened underflow of the tertiary thickener 19 contains the fine seed 
material, which is pumped as such by pump 20 through pipeline 3 to be 
agglomerated into decomposer tank 1. The overflow of the tertiary 
thickener 19 consists of clarified, decomposed aluminate liquor, which is 
directed back for a new digestion operation. The plant 21 permits a 
washing of the fine seed material if required in order to reduce the 
content of organic substances, particularly sodium oxalate. The operation 
in question is well known. 
Pipeline 15 serves to return the production hydrate in the case that a 
compensation in the production hydrate inventory should be shown to be 
necessary. 
As already mentioned, under continuous operations, the process according to 
the invention is carried out in more than one decomposer connected in 
series in the place of one single decomposer 1, and after the cooling 
device 4, further carried out in more than one decomposer connected in 
series in the place of one single decomposer 6. 
With sufficient cooling from the air, the cooling device 4 may be omitted, 
or be replaced or augmented by internal cooling in the decomposers by 
cooling coils, cooling jackets or such like. 
The cooling of the suspension may proceed either continually or stepwise. 
In the latter case, each step corresponds to a cooling device. The final 
temperature is dependent on the sought after degree of decomposition, 
among other items, and may be certainly reduced to about 40 degrees 
Celsius. The fine seed washing system may be omitted if the fine seed is 
of sufficient purity, that is, little contamination of the fine seed with 
organic substances. The type, the behavior and the amount of these organic 
substances determine the necessity for washing the fine seed. 
In FIG. 2, the percentage degree of agglomeration is expressed as a 
function of the quotient "supersaturation of the liquor to be decomposed 
in grams Al.sub.2 O.sub.3 per liter of liquor to the surface area of the 
seed material used in square meters per liter of liquor." The 
supersaturation of the liquor is determined using the thermotitration 
procedure, for example, and the specific surface area, for example, by 
means of the well-known Fisher Sub-Sieve Sizer. 
The degree of agglomeration in percent is thus defined as 
EQU (I-A.multidot.100/I) 
I=Fraction of seed (percent) less than 45 micrometers in size 
A=Fraction of agglomeration product (percent) less than 45 micrometers in 
size 
The diagram illustrated in FIG. 2 considers a temperature range from 66 
degrees Celsius to 77 degrees Celsius and a range of liquor concentrations 
from 70 to 150 grams Na.sub.2 O caustic per liter of liquor. Agglomeration 
certainly occurs outside of these ranges, however the realizable results 
according to the invention are only partially attained. 
The degrees of agglomeration represented in FIG. 2 are attained after a 
residence time of 6 hours in decomposer 1. Good degrees of agglomeration 
are also attained at residence times shorter than 6 hours, as represented 
in FIG. 3 (degrees of the agglomeration as function of the residence time) 
with different seed surface areas (square meters of seed per liter or 
liquor), where temperature, aluminate liquor concentration (grams per 
liter Na.sub.2 O) and degree of supersaturation are practically the same. 
It may be derived from this representation that after only 2 to 3 hours, 
approximately 50% of the total degree of agglomeration is attained. It may 
be further seen from FIG. 3 that after a residence time of approximately 
six hours, close to the maximum degree of agglomeration has been attained. 
(Some of this knowledge was obtained with operation batches of 600 cubic 
meters of supersaturated aluminate liquor). 
The knowledge illustrated in FIGS. 2 and 3 and described above is used to 
carry out the process according to the invention for the first stage of 
the process, that is, for the carrying out of the agglomeration in 
decomposer 1. 
The process in the first decomposer is carried out according to the 
conditions illustrated in diagrams 2 and 3, such that the fine seed 
hydrate added becomes coarser by means of agglomeration, so that a 
sufficiently coarse product results at the end of the total decomposition 
cycle. 
The investigations in the laboratory and in the plant have shown that the 
necessary degree of agglomeration may be attained without effort, when the 
amount of fine seed in the first stage of decomposition is so fixed that 
the relationship of the supersaturation of the aluminate liquor to be 
decomposed (grams per liter Al.sub.2 O.sub.3) to the surface area of this 
fine seed (square metres per liter) is between 7 and 25 grams per square 
meter, preferably between 7 and 16 grams per square meter. 
It is advantageous if the duration of this first processing step is chosen 
to be as short as possible, however, to be long enough so that the 
necessary coarsening results, in order that as long as possible a 
residence time is available for the second stage of decomposition. 
According to the invention, this second stage of decomposition is carried 
out under conditions which are normal in European plants and which lead to 
high productivities, that is, at relatively low temperatures and high 
amounts of seed. 
The investigations have shown that the temperature in this second 
decomposition stage must be lowered. This lowering of the temperature may 
be carried out continuously or in one or more successive steps. The final 
temperature depends on a number of factors, among which are the duration 
of the decomposition, the amount of fine particles formed, etc; for 
example, it can be reduced to about 40 degrees Celsius. 
The amount of secondary seed which is added to this second stage of 
decomposition is less critical than that of the amount of fine seed added 
to the first stage of decomposition. However, it must be large enough so 
that a good factor at the completion of decomposition is attained, and so 
that secondary nucleation is held within bounds. The tests have 
demonstrated that the amount of this secondary seed must be large enough, 
so that the total amount of seed (primary and secondary seed) is at least 
130 grams per liter of Al(OH).sub.3. Generally, 400 grams per liter is not 
exceeded. 
It has also been determined that it is advantageous if the secondary seed, 
which, as has already been referred to, is coarser than the primary seed, 
is all added together at once. The examples which are presented in the 
following, were all carried out using this procedure. 
It is obvious that the addition of the secondary seed may also be repeated, 
that is, in a number of portions of the total amount, without deviating 
from the process according to the invention. 
As already mentioned, a further growth of the seed results during the 
second stage of decomposition (illustrated as decomposer 6), as well as 
the formation of fine particles of hydroxide by means of secondary 
nucleation and by mechanical detachment of fine crystals, these in turn 
occurring due to the renewed increased supersaturation of the aluminate 
liquor and the continuous agitation of the suspension. However, this 
formation of fine hydrate particles is of no significant disadvantage to 
the process according to the invention, in opposition to previously known 
processes, as, in the first processing stage of the process according to 
the invention, (the agglomeration in decomposer 1), even a gross 
occurrence of fine particles may be processed according to the 
agglomeration conditions depicted in FIGS. 2 and 3. The conditions in the 
second processing stage may therefore be chosen to obtain a maximum liquor 
productivity, where the associated formation of fine particles may be 
tolerated without representing an impairment to the process. 
A precipitation of aluminum oxide of up to 80 grams of aluminum oxide per 
liter of liquor is attained. That is the process according to the 
invention achieves the productivity of the European process, and with it, 
a coarse aluminum hydroxide which is separated as product hydroxide is 
separated in the primary thickener, whose fine portion normally lies 
between 4 and 6 weight percent smaller than 45 micrometers. 
This productivity (precipitated Al.sub.2 O.sub.3 in grams per liter of 
liquor presented for decomposition) is also naturally dependent on the 
caustic liquor concentration (grams per liter Na.sub.2 O) of the liquor to 
be decomposed. If the process according to the invention is also to be 
considered alone for the improvement of the productivity of the aluminate 
liquor--regardless of which liquor concentration--then in order to attain 
a high productivity, the caustic liquor concentration should be 
correspondingly high. This is the reason why it is stated that the process 
is to be carried out at liquor concentrations (expressed as grams per 
liter Na.sub.2 O caustic) which are at least 100 grams per liter, 
preferably at least 120 grams per liter. European plants are not normally 
provided with classification devices for the separation of the product, 
secondary and tertiary hydroxides. 
On the conversion of European plants to the process according to the 
invention, appropriate classification devices are necessary, which however 
need not necessarily be gravity classifiers as in the American process, 
but may be any suitable known classification devices. 
The American plants are furnished with the necessary classification 
devices, and FIG. 1 features a representation of such an arrangement. 
According to the process according to the invention, the conversion of 
American plants consists of the introduction of the agglomeration phase 
and the second seeding stage as well as a possible raising of the caustic 
liquor concentration and the introduction of cooling following the 
agglomeration steps. 
The decomposer suspension withdrawn from the last decomposer 6 could 
possibly exhibit too high a solids content which renders the 
classification in the primary thickener more difficult or even makes it 
impossible. By dilution of this suspension, for example with the clear 
overflow liquor from tertiary thickener 19, the solids content may be 
adjusted, if necessary. 
The following experimental examples illustrate the main aspects of the 
process according to the invention, however without limiting the extent of 
the invention. 
EXAMPLE 1 
1000 liters of supersaturated Bayer aluminate solution from a production 
plant, with an initial concentration of 120.2 grams Na.sub.2 O caustic per 
liter and 142.3 grams Al.sub.2 O.sub.3 per liter were placed in a vessel 
of 1.5 cubic meters capacity which had been provided with air agitation. 
This aluminate liquor exhibited a supersaturation of 69.9 grams of 
Al.sub.2 O.sub.3 per liter at 71 degrees Celsius. After the addition of 50 
kilograms Al(OH).sub.3 primary seed material (60.8 weight percent less 
than 45 micrometers) the reaction mass was adjusted to a starting 
temperature of 71 degrees Celsius according to a temperature profile 
adapted from large scale industrial operations. 
The primary seed exhibited a specific surface area of 0.1148 square meters 
per gram, so that a surface area per liter of aluminate liquor of about 
5.75 square meters per liter of aluminate liquor was employed. The 
relationship of the supersaturation (grams per liter Al.sub.2 O.sub.3) to 
the seed surface area (square meters per liter) thus employed was about 
12.1 grams per square meter. 
After six hours, 156 kilograms of secondary seed (16.4 weight percent less 
than 45 micrometers) was added to the reaction mass which was then rapidly 
cooled by 7.5 degrees Celsius. The decomposition was allowed to proceed 
for a further six hours, following which a second intermediate cooling of 
7.5 degrees Celsius was undertaken. The decomposition was then allowed to 
proceed for a further 33 hours to the end of the test. The final 
temperature was 50 degrees Celsius. The resultant suspension was filtered, 
and the aluminum hydroxide so obtained was washed and dried. 
The dried filter cake, consisting of seed material and precipitated 
aluminum hydroxide, contained a fine portion of 14.9 weight percent less 
than 45 micrometers. By subtraction of the weight of the seed from the 
total weight of the dried filter cake, and conversion to an Al.sub.2 
O.sub.3 basis, a yield of 71.1 kilograms of Al.sub.2 O.sub.3 was obtained. 
This corresponds to a specific yield of 71.1 grams of Al.sub.2 O.sub.3 per 
liter of liquor to be decomposed. 
The test results reproduced in the following table 1 are average values of 
two parallel tests carried out at the same time. 
EXAMPLE 2 
A further test was carried out as described in Example 1 with a Bayer 
aluminate liquor of higher starting concentration (124.6 grams Na.sub.2 O 
caustic and 146.4 grams Al.sub.2 O.sub.3 per liter). In this case, the 
liquor supersaturation was 70.2 grams of Al.sub.2 O.sub.3 per liter. 
Primary seed of the same quality and amount was added. Contrary to the 
first test, the secondary seed was considerably finer than in Example 1 
(156 kilograms with 24.9 weight percent less than 45 micrometers). The 
profile of the reaction mass temperature, the parameters and the point in 
time of the intermediate cooling were also identical to those in Example 
1. The handling of the suspension and the evaluation proceeded in the same 
manner as described in Example 1. The dried filter cake consisting of seed 
material and precipitated aluminum hydroxide, contained a fine fraction of 
20.1 weight percent less than 45 micrometers. A value of 72.3 grams 
Al.sub.2 O.sub.3 per liter for the specific yield was obtained from the 
liquor presented for decomposition. These values are averages from three 
parallel tests. 
EXAMPLE 3 
In this test, a Bayer aluminate liquor with a concentration of 120.3 grams 
Na.sub.2 O caustic per liter and 142.4 grams Al.sub.2 O.sub.3 per liter 
was used. The reaction mass was mechanically agitated. The primary seed 
contained 54.3 weight percent less than 40 micrometers, its specific 
surface area being 0.1148 square meters per gram. The secondary seed 
contained 23.5 weight percent less than 40 micrometers. The amount of the 
primary seed was 50 kilograms, that of the secondary seed 156 kilograms. 
The supersaturation of the aluminate liquor amounted to 69.9 grams of 
Al.sub.2 O.sub.3 per liter, so that a ratio of the supersaturation to the 
surface area of the primary seed of 12.1 grams per square meter was 
computed. The temperature profile was distinguished from that in Example 
1, in that the intermediate cooling was carried out in one step of 15 
degrees Celsius before the addition of the secondary seed. The final 
temperature was 49 degrees Celsius. The working up and evaluation was 
performed in the same manner as described in Example 1. 
The dried filter cake consisting of seed and precipitated aluminum 
hydroxide contained a fine fraction of 18.9 weight percent less than 40 
micrometers. The specific yield attained a value of 72.1 grams Al.sub.2 
O.sub.3 per liter of aluminate liquor presented for decomposition. 
EXAMPLE 4 
In this test, an aluminate liquor with a lower concentration than those in 
tests 1 to 3 was used, namely 111.7 grams Na.sub.2 O caustic and 130.5 
grams Al.sub.2 O.sub.3 per liter. The supersaturation of the aluminate 
liquor mounted to 65.6 grams of Al.sub.2 O.sub.3 per liter at 71 degrees 
Celsius. The amount and quality of primary and secondary seed were 
identical to those in Example 2, so that a supersaturation to primary seed 
surface area ratio of 11.4 grams per square meter was calculated. The same 
temperature profile as in Example 3 was chosen, with the intermediate 
cooling accomplished in one step of 15 degrees Celsius prior to the 
addition of the secondary seed. The final temperature was 49 degrees 
Celsius. The dried filter cake, consisting of seed material and 
precipitated aluminum hydroxide, contained a fine fraction of 19.5 weight 
percent less than 45 micrometers. The specific yield attained was 67.8 
grams of Al.sub.2 O.sub.3 per liter of aluminate liquor presented for 
decomposition. 
EXAMPLE 5 
This test was carried as described in Example 1, using a Bayer aluminate 
liquor with a starting concentration of 130.6 grams Na.sub.2 O and 163.2 
grams Al.sub.2 O.sub.3 per liter. In this example, the supersaturation of 
the liquor amounted to 80.6 grams of Al.sub.2 O.sub.3 per liter at 70 
degrees Celsius. The amount of primary seed was 125 kilograms (38.6 weight 
percent less than 45 micrometers). The starting temperature was 70 degrees 
Celsius. The primary seed exhibited a specific surface area of 0.0885 
square meters per gram, so that a surface area of about 11 square meters 
per liter was available. The ratio, supersaturation (grams per liter 
Al.sub.2 O.sub.3) to the seed surface area (square meters per liter) thus 
amounted to about 7.3 grams per square meter. 
After 6 hours, the reaction mass was cooled by 7.5 degrees Celsius and 105 
kilograms of coarser secondary seed (14.1 weight percent less than 45 
micrometers) added. The decomposition was carried out for a further 3 
hours, when a second intermediate cooling of 7.5 degrees Celsius was 
carried out. The decomposition proceeded at this temperature for a further 
3 hours. A third and last intermediate cooling of 7.5 degrees Celsius then 
followed. The decomposition then proceeded for a further 58 hours to the 
completion of the test. The final temperature was 41 degrees Celsius. The 
resultant suspension was filtered and the aluminum hydroxide so obtained 
was washed and dried. The dried filter cake consisted of seed material and 
precipitated aluminum hydroxide, and contained a fine portion of 18.6 
weight percent less than 45 micrometers. By subtraction of the seed 
hydrate weight from the total weight of the dried filter cake, and on 
conversion to an Al.sub.2 O.sub.3 basis, a yield of 83.0 kilograms of 
Al.sub.2 O.sub.3 was obtained. This corresponds to a specific yield of 
83.0 grams of Al.sub.2 O.sub.3 per liter of aluminate liquor presented for 
decomposition. 
EXAMPLE 6 
In this test, a similar test procedure was used as in Example 5. The Bayer 
aluminate liquor had a starting concentration of 136.8 grams Na.sub.2 O 
caustic and 174.5 grams of Al.sub.2 O.sub.3 per liter. In this case, the 
supersaturation of the liquor amounted to 84.6 grams Al.sub.2 O.sub.3 per 
liter at 70 degrees Celsius. The primary seed amount used was 125 
kilograms (38.6 weight percent less than 45 micrometers), and the starting 
temperature was 70 degrees Celsius. 
The primary seed exhibited a specific surface area of 0.0885 square meters 
per gram, so that a surface area of 11 square meters per liter was 
available. The ratio supersaturation (grams per liter Al.sub.2 O.sub.3) to 
the seed surface area (square meters per liter) thus amounted to about 7.7 
grams per square meter. 
After six hours, the reaction mass was cooled by 7.5 degrees Celsius, and 
105 kilograms of coarser secondary seed (14.1 weight percent less than 45 
micrometers) added. The decomposition was carried out for a further 3 
hours, when a second intermediate cooling of 7.5 degrees Celsius was 
carried out. 
The decomposition proceeded at this temperature for a further 3 hours. A 
third and last intermediate cooling of 7.5 degrees Celsius then followed. 
The decomposition then proceeded for a further 88 hours to the completion 
of the test. The final temperature was 41 degrees Celsius. The resultant 
suspension was filtered, and the aluminum hydroxide so obtained was washed 
and dried. The dried filter cake consisted of seed material and 
precipitated aluminum hydroxide, and contained a fine portion of 16.5 
weight percent less than 45 micrometers. By subtraction of the seed 
hydrate weight from the total weight of the dried filter cake, and on 
conversion to an Al.sub.2 O.sub.3 basis, a yield of 91.7 kilograms of 
Al.sub.2 O.sub.3 was obtained. This corresponds to a specific yield of 
91.7 grams of Al.sub.2 O.sub.3 per liter of aluminate liquor presented for 
decomposition. The coarsening and the high yields which characterize the 
process, are listed in the following Table 1. 
TABLE 
__________________________________________________________________________ 
FINE PORTION LESS THAN 45 MICROMETERS 
PRODUCT 
seeded plus 
SEED MATERIAL precipitated 
YIELD PRIMARY SECONDARY .SIGMA. 
material 
Al.sub.2 O.sub.3 
Al(OH).sub.3 
Al(OH).sub.3 
Al(OH).sub.3 
Al(OH).sub.3 
Example 
g/l % g/l % g/l g/l % g/l 
__________________________________________________________________________ 
1 71.1 60.8 30.4 16.4 25.6 56.0 14.9 46.9 
2 72.3 60.8 30.4 24.9 38.3 69.2 20.1 63.6 
3 .sup.+ 72.1 
.sup.+ 54.2 
.sup.+ 27.1 
.sup.+ 23.5 
.sup.+ 36.7 
.sup.+ 63.8 
.sup.+ 18.9 
.sup.+ 59.8 
4 67.8 60.8 30.4 24.9 38.8 69.2 19.5 60.4 
5 83.0 38.6 48.2 14.1 14.8 63.0 18.6 66.3 
6 91.7 38.6 48.2 14.1 14.8 63.0 16.5 61.0 
__________________________________________________________________________ 
.sup.+ FRACTION LESS THAN 40 MICROMETERS 
It may be derived from the table, that after the return of primary and 
secondary seed hydroxides of the same amount and similar constitution to 
those used, a product hydroxide with a very low fine portion can be 
produced, for example 3 to 5 weight percent less than 45 micrometers, as 
is required for the production of sandy aluminum oxide. Further, the 
productivity (yield) of the aluminate liquor is extremely high, and has 
never been achieved on a practical basis for the manufacture of aluminum 
oxide of coarse particle size.