Process for the manufacture of coarse aluminium hydroxide

A process for the manufacture of coarse aluminum hydroxide via the Bayer Process. The process involves the decomposition of a supersaturated alkaline aluminate liquor in a two stage precipitation process having an agglomeration stage and a growth stage. The agglomeration stage takes place in agglomeration tanks, where the liquor is seeded with fine aluminum hydroxide to induce precipitation and formation of a suspension, followed by a growth stage, where the suspension is seeded with a coarse aluminium hydroxide. Typically, the feed of the supersaturated alkaline aluminate liquor is split over a first agglomeration tank at a higher liquor temperature of 70.degree. to 100.degree. C. and a second agglomeration tank or one or more of a series of second agglomeration tanks at a lower liquor temperature of 50.degree. to 80.degree. C.

The invention concerns a process for manufacturing coarse aluminium
 hydroxide by decomposition of a supersaturated alkaline aluminate liquor
 in a two stage precipitation process having an agglomeration stage in
 agglomeration tanks connected in series, where the liquor is seeded with
 fine aluminium hydroxide to induce precipitation and formation of a
 suspension, followed by a growth stage, where the suspension is seeded
 with coarse aluminium hydroxide.
 The invention relates to the manufacture of aluminium hydroxide
 Al(OH).sub.3 for the purpose of manufacturing metal grade alumina which
 meets the requirements of the modern aluminium smelters.
 In particular the invention relates to a process for the manufacture of
 coarse aluminium hydroxide via the Bayer Process in which the alumina
 values of the alumina containing ore are solubilized at a relatively high
 temperature in an aqueous liquor of caustic soda and sodium aluminate and
 in which these alumina values are crystallized later on at a lower
 temperature in the form of aluminium hydroxide. The aluminium hydroxide,
 also called product hydrate, is calcined to yield a sandy alumina.
 This crystallization procedure, hereinafter referred to as precipitation,
 is enhanced by the seeding of the supersaturated sodium aluminate liquor
 (also called the pregnant liquor) with aluminium hydroxide (also simply
 called "hydroxide" or even "hydrate").
 The invention concerns more particularly a mode of application of the Bayer
 process in which the precipitation is achieved in two stages, namely an
 agglomeration stage followed by a growth stage.
 In the U.S. Pat. No. 4,234,559 an agglomeration stage is characterized by
 the seeding of the pregnant liquor with a controlled amount of relatively
 fine hydrate referred to as the fine seed allowing to achieve the control
 of the product hydrate granulometry. A following growth stage is
 characterized by the seeding of the suspension leaving the agglomeration
 stage with a high amount of coarser hydrate referred to as the coarse seed
 allowing to achieve a high precipitation yield. In order to make it
 operational, this precipitation process in two stages must be complemented
 with the classification of the separated hydrate into fine seed, coarse
 seed and product hydrate, and with the separation of the exhausted liquor
 (also called the spent liquor) from the hydrate.
 This two-stage precipitation process allows the production of a product
 hydrate with a particle size distribution showing a proportion of
 particles with the diameter smaller than 45 micrometers not exceeding 15%
 by weight and which may be as low as 3% by weight.
 This two-stage precipitation process also allows to achieve a high liquor
 productivity i.e. a high yield of precipitation of aluminium hydroxide per
 unit volume of pregnant liquor. Thus, liquor productivities of typically
 70 to 85 g Al.sub.2 O.sub.3 per liter of liquor can be commonly achieved
 under industrial operating conditions. A yield of 91.7 g Al.sub.2 O.sub.3
 was achieved in a test procedure.
 As far as the chemical composition of the metallurgical alumina is
 concerned, the modern smelters are favoring nowadays an alumina with a
 relatively low soda content.
 In the AIME-Report of 1988, pages 125 to 128, "Operation of the Alusuisse
 precipitation Process at Gove" by S. G. Howard, a precipitation process
 layout had been presented, showing cooled pregnant liquor seeded in two
 phases. The pregnant liquor flow is split between the first two
 precipitation tanks of the agglomeration phase.
 It is an objective of the instant invention to reduce the occlusion of soda
 in the aluminium hydroxide (also called "product hydrate") produced in a
 two-stage precipitation process while maintaining a high liquor
 productivity and a good control of the granulometry and strength of the
 product.
 Under occlusion of soda is meant the incorporation in the aluminium
 hydroxide crystal lattice of soda values which cannot be removed by the
 thorough washing with water of the product hydrate. For example an
 occluded soda content of the product hydrate could be as high as 0.4% or
 even 0.45% calculated in weight percent Na.sub.2 O on an Al.sub.2 O.sub.3
 basis, depending on the liquor purity. With the process according to the
 instant invention it is possible to reduce the occluded soda content for
 example by 0.05% to 0.15% Na.sub.2 O, thus allowing to achieve an occluded
 soda content of for example below 0.35% or even of 0.25% and below.
 According to the present invention, the feed of the supersaturated alkaline
 aluminate liquor to the agglomeration stage is split into a first
 substream and said first substream is fed to a first agglomeration tank or
 to the first and more of a series of first agglomeration tanks at a higher
 liquor temperature of 70 to 100.degree. C. and a second substream and said
 second substream is fed to a second agglomeration tank or to two or more
 of second agglomeration tanks at a lower liquor temperature of 50 to
 80.degree. C., at the end of precipitation yielding a strong coarse
 product hydrate, giving after calcination a sandy alumina with low
 occluded soda content, compatible with a high liquor productivity.
 In a preferred embodiment of the instant invention the supersaturated
 alkaline aluminate liquor is split into a first substream representing 30
 to 60% of the total supersaturated alkaline aluminate liquor stream. Said
 first substream is fed to the first agglomeration tank or a series of
 first agglomeration tanks of the agglomeration stage. The first substream
 of the supersaturated alkaline aluminate liquor is fed at a relatively
 high temperature of for example 70 to 90.degree. C. and preferably 80 to
 90.degree. C. One first agglomeration tank can be present or a series of
 two or more first agglomeration tanks can be present. The first substream
 of the supersaturated alkaline aluminate liquor can be fed to the one
 first agglomeration tank or can be fed and distributed to each of two,
 preferably two, or more of the series of first agglomeration tanks. If a
 series of two of first agglomeration tanks are present, the first
 substream of the supersaturated alkaline aluminate liquor can be
 distributed to the first and the second agglomeration tank of the series
 of first agglomeration tanks. The fine aluminium hydroxide, also called
 the fine seed, can be fed into the one first agglomeration tank or can be
 fed in the first agglomeration tank of the series of two or more first
 agglomeration tanks or can be fed and distributed over the first two
 agglomeration tanks of the series of two or more first agglomeration
 tanks. The supersaturated alkaline aluminate liquor and the fine seed form
 a suspension with a solids content in the one first agglomeration tank or
 in the series of first agglomeration tanks of 100 to 500 g/l, preferably
 150 to 450 g/l and especially 300 to 400 g/l.
 Connected in series with the first agglomeration tank or the series of
 first agglomeration tanks are the second agglomeration tank or a series of
 second agglomeration tanks.
 The balance of 70 to 40% of the supersaturated alkaline aluminate liquor
 forms the second substream. Said second substream is typically fed to the
 one second agglomeration tank or fed to one of the series of second
 agglomeration tanks or fed and distributed to two or more or all of the
 series of second agglomeration tanks. The second substream of the
 supersaturated alkaline aluminate liquor is preferably fed to the second
 agglomeration tank or to the series of second agglomeration tanks at a
 relatively low temperature of preferably 60 to 80.degree. C. and
 especially 60 to 70.degree. C.
 For example the second substream of liquor representing 70 to 40% of the
 total supersaturated alkaline aluminate liquor flow may be sent to the one
 second agglomeration tank, or to one of two or more of the series of
 second agglomeration tanks, or to two or more of two or more of the series
 of second agglomeration tanks. If two or more agglomeration tanks are
 present in the series of second agglomeration tanks, the second
 agglomeration tanks are connected in series. The one first agglomeration
 tank or the last of the series of first agglomeration tanks is connected
 in series with the one second agglomeration tank or the first tank of the
 series of second agglomeration tanks.
 In another embodiment the second substream of liquor representing 70 to 40%
 of the total supersaturated alkaline aluminate liquor flow may be sent to
 and distributed over two of the series of second agglomeration tanks.
 In another embodiment the second substream of liquor representing 70 to 40%
 of the total supersaturated alkaline aluminate liquor flow may be sent to
 and distributed over three of the series of second agglomeration tanks.
 The second substream of the supersaturated alkaline aluminate liquor is
 typically fed to the second agglomeration tank or the series of second
 agglomeration tanks at a temperature of for example 5 to 50.degree. C.,
 preferably 15 to 40.degree. C. and especially 20 to 30.degree. C., below
 the temperature of the first substream.
 Preferably the supersaturated alkaline aluminate liquor split over the
 first substream represents 40 to 60% of the total supersaturated alkaline
 aluminate liquor stream. Accordingly the second substream represents in
 the preferred manner the balance of 60 to 40% of the total supersaturated
 alkaline aluminate liquor stream.
 All numbers given in percentage are related to the weight, if not indicated
 otherwise.
 Typically the first substream is fed to the one first agglomeration tank
 and the second substream is fed to one tank of the series of second
 agglomeration tanks. Or, in an alternative preferred embodiment, the first
 substream is fed to the one first agglomeration tank and the second
 substream is fed to the one second agglomeration tank or to 2, 3 or 4 of
 the series of second agglomeration tanks of 2 or more, 3 or more or 4 or
 more of the series of second agglomeration tanks respectively.
 As for example the first substream is fed to the one first agglomeration
 tank and the second substream is fed to the first in series of the second
 agglomeration tanks. Or, the first substream is fed to the one first
 agglomeration tanks and the second substream is fed to the first of two or
 more of the series of second agglomeration tanks. Or, the first substream
 is fed to the one first agglomeration tank and the second substream is fed
 to the two of the series of second agglomeration tanks following the one
 first agglomeration tank. Or, the first substream is fed to the one first
 agglomeration tank and the second substream is fed to the three of the
 series of second agglomeration tanks following in series the first
 agglomeration tank. Or, the first substream is fed to the one first
 agglomeration tank and the second substream is fed to the second or the
 third or the second and the third of the series of second agglomeration
 tanks, arranged in series.
 The total residence time for the precipitation in the first agglomeration
 tank or the series of first agglomeration tanks and the second
 agglomeration tank or all of the series of second agglomeration tanks in
 series is typically 3 to 12 hours. The total residence time for the
 precipitation in the first or first series and in the second or second
 series of agglomeration tanks is achieved in a first or a series of first
 and a second or a series of second agglomeration tanks, the number of
 which being a total of for example 2 to 10 and preferred 2 to 6.
 Typically the one first agglomeration tank or the series of first
 agglomeration tanks and the one second agglomeration tank or the series of
 second agglomeration tanks are arranged in series. In an alternative
 embodiment one or more of the tanks of the series of second agglomeration
 tanks can be substituted by tanks in parallel. In such an arrangement the
 feed of suspension from the first agglomeration tank or the series of
 first agglomeration tanks is divided and fed to the second agglomeration
 tanks in parallel and the second substream is divided and fed to the
 second agglomeration tanks in parallel as well.
 The suspension leaving the agglomeration stage then enters a first growth
 tank of the growth stage where it meets the coarse seed in an amount
 allowing to achieve a solids content in the end of precipitation
 suspension of 300 to 900 g/l and preferably 350 to 550 g/l. The total
 residence time in the growth stage is normally 20 to 50 hours and is
 achieved in one or more growth tanks or in one or more series of growth
 tanks. The total number of growth tanks is 1 to 200 and typically 5 to 30.
 The temperature profile along the growth tanks of the growth stage can be
 adjusted by one or several steps of forced cooling as to achieve an end of
 precipitation temperature of typically 50 to 65.degree. C.
 The advantages of the invention in comparison with the previous art are
 manifold:
 The hot supersaturated alkaline aluminate liquor which enters in contact
 with a high amount of fine seed (in terms of solids content) offers the
 best conditions (high temperature, low supersaturation) for a lower soda
 occlusion.
 The cooler supersaturated alkaline aluminate liquor which is added further
 downstream over one or more tanks of the agglomeration stage allows to
 re-establish the conditions for a good agglomeration of the fine seed and
 hence for a satisfactory control of the product hydrate granulometry.
 The temperature which can be achieved at the beginning of the growth stage
 by cooling part of the supersaturated alkaline aluminate liquor allows to
 obtain favorable conditions for a high productivity and this more
 economically than by cooling the whole precipitation suspension.
 The growth precipitation stage having a minor influence on the soda
 inclusion mechanisms can be run as to maximize the productivity, in
 particular, a high seed charge can be applied. Furthermore, forced cooling
 in one or several intermediate steps can also be applied to increase the
 supersaturation and hence the liquor productivity.
 Therefore it can be seen that the method of introducing the liquor to the
 agglomeration stage of the two-step precipitation process to which the
 invention applies can actually permit the achieving of a lower soda
 content in the product hydrate while maintaining the product quality
 (strong sandy alumina) and high liquor productivity advantages of the
 two-stage precipitation process as per the prior art.

The arrangements shown in these figures are not limitating, but constitute
 only a selection among many possible arrangements.
 The following descriptions illustrate the main aspects of the process
 according to the instant invention, however, without limiting the extent
 of the invention.
 In the process depicted under FIG. 1, the agglomeration stage of the
 precipitation procedure is taking place in the first agglomeration tank
 1.1 and the series of second agglomeration tanks 1.2, 1.3, 1.4, . . . ,
 1.N. The agglomeration stage is fed continuously with the supersaturated
 alkaline aluminate liquor 2, also called pregnant liquor, and a controlled
 amount of fine aluminium hydroxide constituting the fine seed stream 12.
 The amount of fine seed is adjusted taking into account the active
 specific surface area of the seed and the liquor characteristics so as to
 allow an efficient control of the granulometry and strength of the final
 product aluminium hydroxide. The pregnant liquor 2 and the fine seed
 stream 12 are forming by precipitation of aluminium hydroxide the
 suspension 6. The pregnant liquor 2 is divided in the substreams 2.1 and
 2.2. The direction of streams is indicated by the arrows.
 The pregnant liquor substream 2.1, fed to the agglomeration stage into the
 first agglomeration tank 1.1, is at a temperature of 70 to 100.degree. C.,
 typically 80 to 90.degree. C., and represents 30 to 60% of the total
 liquor stream 2.
 The balance of the liquor constituting the substream 2.2 is cooled in the
 cooling operation 3 which can be achieved by surface heat exchangers or by
 flash cooling units. Most of the heat of the pregnant liquor substream 2.2
 taken by the surface heat exchangers or by the flash cooling units may be
 transferred to the spent liquor recovered from the end of precipitation
 suspension and/or to any other cooling fluid.
 For example the cooled pregnant liquor leaving the cooling operation 3 can
 be fed to one of the three tanks of the series of second agglomeration
 tanks 1.2, 1.3 or 1.4 of the series of second agglomeration tanks the
 series ending with tank 1.N, or can be distributed over two or all three
 of the tanks 1.2, 1.3 and 1.4 of the series of second agglomeration tanks
 the series ending with tank 1.N.
 According to one possible embodiment of the process according to the
 instant invention the cooled pregnant liquor can form only one stream 4.1
 or 4.2 or 4.3 fed to the agglomeration tank 1.2 or 1.3 or 1.4
 respectively.
 In another preferred alternative the cooled pregnant liquor can be split
 into two substreams 4.1 and 4.2 or 4.1 and 4.3 or 4.2 and 4.3 fed to the
 agglomeration tanks 1.2 and 1.3 or 1.2 and 1.4 or 1.3 and 1.4
 respectively.
 In another preferred alternative the cooled pregnant liquor can be split in
 three substreams 4.1, 4.2 and 4.3 fed to the agglomeration tanks 1.2, 1.3
 and 1.4.respectively.
 The temperature of the individual cooled pregnant liquor substream or
 substreams 4.1, 4.2 and 4.3 is 50 to 80.degree. C. or typically 60 to
 70.degree. C., and can be different for each substream in case there is
 more than one substream.
 The amount of fine seed 12 which is added to the first agglomeration tank
 1.1 is typically in such an amount as to achieve a solids content in said
 agglomeration tank 1.1 of 100 to 500 g/l, typically 150 to 450 g/l or
 preferably 300 to 400 g/l.
 The number N agglomeration tanks is typically 2 to 10, preferably 2 to 6. N
 may represent a number of 2, 3, 4, up to 10, with the proviso that the
 number N is the same or higher than the number of all agglomeration tanks
 fed with any pregnant liquor. The capacity of the agglomeration tanks is
 such as to achieve a total residence time in the agglomeration stage of 3
 to 12 hours.
 The agglomeration suspension 6 leaving the agglomeration stage is fed to
 the first tank 5.1 of the growth stage or also called precipitation tank
 5.1 of the growth stage, together with the coarse seed stream 15.
 In an alternative embodiment the agglomeration suspension 6 leaving the
 agglomeration stage can be used partly or totally as substream 6.1 for
 reslurrying the coarse seed if said coarse seed is filtered in the
 optional filtration section 14, resulting in the coarse seed suspension
 15. In this case the agglomeration suspension balance substream 6.2 is fed
 directly to the first growth tank 5.1.
 The growth precipitation stage takes place in the precipitation tanks 5.1,
 5.2, . . . 5.M, allowing a residence time of typically 20 to 50 hours. M
 represents a number of typically 2 to 50. Preferably the growth tanks are
 arranged in series or in two or more series in parallel and are operated
 continuously. However, batch precipitation tanks or a combination of batch
 and continuous tanks can also be used.
 The amount of coarse seed 15 added to the first growth precipitation tank
 5.1 is typically in such an amount as to achieve a solids content in the
 end of precipitation suspension 7 of 300 to 900 g/l, typically 350 to 550
 g/l.
 The end of precipitation suspension stream 7 is classified in the
 classification operation 8. This operation can be achieved by means of
 hydrocyclones, conventional gravity thickeners and/or any other suspension
 classification equipment. The purpose of the classification operation 8 is
 to separate:
 A coarse aluminium hydroxide fraction as product hydrate suspension stream
 9. This product hydrate suspension stream is then fed to the product
 filtration and washing operations followed by the calcination operation.
 A fine aluminium hydroxide fraction as fine seed suspension stream 10
 containing the maximum possible amount of the finest particles contained
 in the end of precipitation suspension 7. This fine seed suspension is
 usually filtered, and possibly washed in the fine seed filtration section
 11, giving the resulting fine seed stream 12 fed to the first
 precipitation tank 1.1 of the agglomeration stage. The filtered and
 possibly washed fine seed can also be reslurried with a part of the liquor
 substream 2.1 in a dedicated tank not shown on the drawing and the
 resulting suspension fed to the first agglomeration tank 1.1. Optional is
 the possibility of reslurrying the fine seed stream 12 with a side stream
 of spent liquor 16. Spent liquor means liquor separated from the slurry at
 the end of precipitation after classification.
 The rest of the aluminium hydroxide as coarse seed suspension stream 13
 which is usually filtered in the coarse seed filtration section 14 giving
 the resulting coarse seed stream 15 fed to the first precipitation tank
 5.1 of the growth stage. If the filtered coarse seed is reslurried with
 the agglomeration substream 6.1, the coarse seed stream 15 is then the
 suspension resulting from this operation. In the absence of a filtration
 operation 14, the coarse seed stream 15 is the same as the coarse seed
 suspension stream 13. The filtration operation 14 can be replaced by any
 other thickening operation. Optional is the possibility of reslurrying the
 coarse seed with a sidestream of cooled pregnant liquor.
 FIG. 2 provides a schematic view of the agglomeration stage of the
 precipitation process in the one first agglomeration tank 1.1 and the one
 second agglomeration tank 1.2. A controlled amount of fine aluminium
 hydroxide constituting the fine seed stream 12 and the pregnant liquor
 substream 2.1, are fed into the one first agglomeration tank 1.1. The
 resulting suspension is continuously transferred to the one second
 agglomeration tank 1.2. In tank 1.2 the balance of the liquor constituting
 the cooled pregnant liquor substream 4.1 is added to the suspension. The
 agglomeration suspension 6 leaving the agglomeration stage is fed to the
 first precipitation tank of the series of precipitation tank of the growth
 stage. The remaining process steps are for example the same as mentioned
 above.
 FIG. 3 provides a schematic view of the agglomeration stage of the
 precipitation process in a series of two first agglomeration tanks 1.1.1
 and 1.1.2 and in a series of N-1 second agglomeration tanks 1.2, 1.3, 1.4
 to 1.N. A controlled amount of fine aluminium hydroxide constituting the
 fine seed stream 12 and the pregnant liquor substream 2.1, are distributed
 and fed into the two first agglomeration tanks 1.1.1 and 1.1.2. The tanks
 1.1.1. and 1.1.2 are connected in series and the resulting suspension is
 transferred to tank 1.2 of the series of second agglomeration tanks. The
 balance of the pregnant liquor 2.2 is cooled in the cooling operation 3
 constituting the cooled pregnant liquor substreams 4.1 or 4.2 or 4.3. In
 tanks 1.2 or 1.3 or 1.4 the cooled pregnant liquor substream 4.1 or 4.2 or
 4.3 is added to the suspension accordingly. Or, in an alternative
 embodiment, the pregnant liquor substream 2.2 is cooled in the cooling
 operation 3 and can be split in substreams 4.1 and 4.2 or 4.1 and 4.3 or
 4.2 and 4,3 or 4.1, 4.2 and 4.3 and said substreams are added to the tanks
 1.2 and 1.3 or 1.2 and 1.4 or 1.3 and 1.4 or 1.2 and 1.3 and 1.4
 accordingly. The second agglomeration tanks 1.2 to 1.N are arranged in
 series and the agglomeration suspension flow is indicated by the arrows.
 The agglomeration suspension 6 leaving the agglomeration stage is fed to
 the first precipitation tank of the growth stage. The remaining
 process-steps are for example the same as mentioned above.
 FIG. 4 provides a schematic view of the agglomeration stage of the
 precipitation process in the one first agglomeration tank 1.1 and in the
 series of second agglomeration tanks 1.2.1 and 1.2.2. The first
 agglomeration tank 1.1 is connected in series with the following second
 agglomeration tanks 1.2.1 and 1.2.2. Said second agglomeration tanks 1.2.1
 and 1.2.2 are in parallel. A controlled amount of fine aluminium hydroxide
 constituting the fine seed stream 12 and the pregnant liquor substream
 2.1, are fed into the one first agglomeration tank 1.1. The resulting
 suspension is transferred to the one second agglomeration tank 1.2. In
 tank 1.2.1 and 1.2.2 the balance of the liquor constituting the cooled
 pregnant liquor substream 4.1 is added to the suspension, preferably in
 equal amounts. The suspension leaving the tanks 1.2.1 and 1.2.2 forms
 either the agglomeration suspension 6 leaving the agglomeration stage or
 the suspension is fed to the following second agglomeration tanks in
 series with tanks 1.2.1 and 1.2.2. In the later case the suspension forms
 after tanks 1.N, 1.N' respectively, the agglomeration suspension 6 flowing
 to the first precipitation tank of the growth stage. The remaining
 process-steps are for example the same as mentioned above.
 FIG. 5 provides a schematic view of the agglomeration stage of the
 precipitation process in the one first agglomeration tank 1.1 and the
 second agglomeration tanks 1.2., 1.3.1 and 1.3.2. The first agglomeration
 tank 1.1 is connected in series with the following second agglomeration
 tank 1.2 and subsequently the second agglomeration tanks 1.3.1. and 1.3.2.
 Said second agglomeration tanks 1.3.1 and 1.3.2 are in parallel. A
 controlled amount of fine aluminium hydroxide constituting the fine seed
 stream 12 and the pregnant liquor substream 2.1 are fed into the one first
 agglomeration tank 1.1. The resulting suspension is transferred to the
 second agglomeration tank 1.2. The cooled pregnant liquor substream is
 divided into the cooled pregnant liquor substreams 4.1 and 4.2. Substream
 4.1 is added to the suspension in tank 1.2. Substream 4.2 is added,
 preferably in equal amounts, to the suspension in tanks 1.3.1 and 1.3.2.
 The suspension leaving the tanks 1.3.1 and 1.3.2 forms either the
 agglomeration suspension 6 leaving the agglomeration stage or the
 suspension is fed to the following second agglomeration tanks. In the
 later case the suspension forms after tanks 1.N, 1.N' respectively, the
 agglomeration suspension 6 flowing to the first precipitation tank of the
 growth stage. The remaining process-steps are for example the same as
 mentioned above.
 In a preferred process the supersaturated alkaline aluminate liquor is
 seeded with fine aluminium hydroxide to induce precipitation and said fine
 seed 12 is prepared by reslurrying a side stream of the supersaturated
 alkaline aluminate liquor 2 or of spent liquor 16 prior to being fed to
 the agglomeration stage.
 In another preferred process embodiment the coarse aluminium hydroxide
 suspension 13, emerging the classification operation 8, is filtered in the
 filtration operation 14, whereby a coarse seed filter cake is formed and
 said filter cake is fed to the first precipitation tank 5.1 of the growth
 stage.
 In another preferred process embodiment the coarse aluminium hydroxide
 suspension 13, emerging the classification operation 8, is filtered in the
 filtration operation 14, whereby a coarse seed filter cake is formed said
 filter cake is reslurried in a side stream of the agglomeration suspension
 6.1 or of cooled supersaturated alkaline aluminate liquor and fed to the
 first precipitation tank 5.1 of the growth stage.
 In another preferred process the growth stage is made of precipitation
 tanks 5.1 to 5.M in series or precipitation tanks in parallel or a
 combination of precipitation tanks in series and parallel.
 For the various process operations presented above with the support of the
 attached drawings FIGS. 1 to 5, the normal equipment allowing their
 implementation such as pipes, lines, tanks, washers, filters, classifiers,
 coolers and so on, are not described in detail because there exists a wide
 flexibility in the design of the installations capable of operating them.
 The process described above can be equipped with additional equipment such
 as intermediate cooling facilities which can be inserted within the growth
 precipitation equipment in order to further optimize the temperature
 profile along precipitation.
 EXAMPLES
 A plant test is carried out over two precipitation lines (A) and (B)
 running in parallel, both lines with five agglomeration tanks in series,
 allowing a total residence time in the agglomeration stage of each line of
 4.5 hours. The tanks of each line are numbered from tank 1 for the first
 tank to tank 5 for the last tank. The total duration of the test is 54
 days. For comparison purposes the agglomeration stage of line (A) is run
 with a pregnant liquor flow splitting of 50% to tank 1, with a liquor
 temperature of 74 to 75.degree. C. and 50% to tank 3 with a liquor
 temperature of 74 to 75.degree. C. This temperature of 74 to 75.degree. C.
 is the temperature observed along the whole agglomeration stage.
 In accordance to the instant invention, the agglomeration stage of line (B)
 is run with a pregnant liquor flow splitting of 50% to tank 1 with a
 liquor temperature of 84.degree. C. and 50% to tank 3 with a liquor
 temperature of 62 to 64.degree. C. The temperature achieved in the last
 three tanks (tanks 3-5) is 74 to 75.degree. C. The liquor flow and
 temperature profile in the growth stage are the same for both lines (A)
 and (B).
 It must be mentioned that the two precipitation lines (A) and (B) used for
 the test are not totally independent as at the end of precipitation the
 suspensions of both lines (A) and (B) are combined. Therefore, the fine
 seed and coarse seed qualities are the same for both lines and the product
 hydrate quality is the one resulting from the operation of both lines.
 The quality of the liquor during the plant test are as follows:

Pregnant Liquor (Industrial)
 g/l total carbon 15.0
 g/1 Na.sub.2 O caustic 135.0 to 136.0
 molar ratio Na.sub.2 O/Al.sub.2 O.sub.3 1.33 to 1.35
 The other particular test conditions and the results obtained are presented
 in the following table:
 TABLE
 Results of plant trial on liquor splitting,
 splitting and cooling respectively:
 Line A Line B
 Reference Inventive
 Fine seed
 % Na.sub.2 O occluded at test start 0.46
 % Na.sub.2 O at test end 0.42
 Agglomeration suspension
 g/l solids (average) 189 187
 % Na.sub.2 O occluded at test start 0.51 0.49
 % Na.sub.2 O occluded at test end 0.48 0.43
 End of precipitation suspension
 g/l solids (average) 425
 % - 44 micron fraction at test start 5.7 5.9
 % - 44 micron fraction at test end 6.3 6.0
 Productivity in g Al.sub.2 O.sub.3 /1 liquor
 at test start 78 78
 at test end 78 78
 Product hydrate
 % Na.sub.2 O occluded at test start 0.44
 % Na.sub.2 O occluded at test end 0.40
 The term "test start" refers to the begin of the test after one day, the
 term "test end" refers to the end of the tests after 54 days.
 It can be seen that the productivity and granulometry are not significantly
 affected by the inventive process.
 After the 54 days of the test, the occluded soda in the product hydrate is
 decreased from 0.44 to 0.40% Na.sub.2 O. Two substreams of pregnant liquor
 having different temperatures are applied in line (B) only. This means
 that with both lines (A) and (B) operated under the temperature conditions
 of line (B) then an occluded soda content of 0.36% Na.sub.2 O could be
 reached after 54 days. Since at the end of the test the decreasing
 occluded Na.sub.2 O trend is still significant, one can conclude that a
 product hydrate with less than 0.35% Na.sub.2 O to compare with 0.44%
 Na.sub.2 O initially, could be reached under liquor flow splitting and
 temperature difference conditions. This with the proviso the same
 conditions as for line (B) would have been applied for line (A) and more
 time would have been allowed. This represents a decrease in the order of
 0.1% Na.sub.2 O occluded for the liquor and for the plant under
 consideration. In other words, a relative decrease of more than 20%
 concerning occluded soda can be reached.