IE55499B1 - Process for the production of an aluminium trihydroxide of large granulometry - Google Patents

Abstract

Process for the decomposition of an over-saturated alkaline aluminate solution according to the Bayer process of alkaline etching of bauxites in order to obtain simultaneously a high yield and a large size grading aluminium trihydroxide of which 10 % at the most of the resultant particles have their smallest size lower than 45 microns, by introducing a primer, said process comprising contacting the whole of the primer used with the whole of the alkaline aluminate solution to be decomposed, characterized in that: a) in the decomposition area of the Bayer process comprising "n" stages in cascade, there is created a suspension with a high content of dry material of at least 700 g/l of alkaline aluminate solution to be decomposed in at least one stage by introducing the primer comprised of aluminium trihydroxide crystals of non-selected grading, b) after a dwell time in the decomposition area at a maximum temperature selected in the range of 50-75<o>C until a weight ratio of dissolved Al2O3/caustic Na2O at the most equal to 0.7 is obtained, a fraction comprised of 50 % by volume at the most of the dry material high content suspension circulating in the decomposition area is taken away, c) then, after the taking, said fraction is introduced in the classification area of which c1, the separated grained portion, is removed and represents the production of Al(OH)3 of large size grading and, c2, the other separate portion forming a suspension is withdrawn from the classification area and admixed to the remaining fraction of the circulating suspension in the decomposition area which has not been subjected to classification, d) the resultant suspension of the operation effected at c2 is subjected to a solid-liquid separation, the separated solid phase forming the unselected size grading aluminium trihydroxyde primer, recycled in the decomposition area of the Bayer process. [GB2123806A]

Description

PROCESS FOR THE PRODUCTION OF AN ALUMINIUM TRIHYDROXIDE OF LARGE GRANULOMETRY The invention concerns a process for precipitating Al(OH)^, at a high level of productivity, from a supersaturated solution of alkali metal aluminate, which solution is produced using the Bayer process for the alkaline attack on bauxites, for producing, by introducing a seed 5 crystal, an aluminium trihydroxide of large granulometry, in which 10% at most of the particles produced have their smallest dimension smaller than 45 microns.

The Bayer process which is widely described in the specialist literature and which is well known to the man skilled in the art 10 constitutes the essential process for the production of alumina which is 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, at a suitable level of concentration, thus rendering the alumina soluble and giving rise to a supersaturated solution of sodium 15 aluminate. After separation of the solid phase constituting the unattacked residue (red mud) of the ore, the supersaturated sodium aluminate solution is generally seeded with aluminium hydroxide, referred to hereinafter as the "seeding agent", in order to cause precipitation of an aluminium trihydroxide.

As is well known to the man skilled in the art, there are a number of different industrial processes for the production of aluminium trihydroxide by the Bayer process of alkaline attack on bauxites, and the custom is for such different processes to be classified in two categories, one being referred to as the European process and the other being referred to as the 25 American process.

In the European process, precipitation of the aluminium trihydroxide is performed in the course of the operation which is referred to as decomposition of an aqueous solution of sodium alufninate, at a high level of concentration in respect of caustic Na^O, generally having from 130 to 30 170 grams of Na20 per litre of sodium aluminate solution to be decomposed. 2 The expression "concentration in respect of caustic Na^O" is to be interpreted as meaning the total amount of Na20 expressed in grams per litre in the solution of sodium aluminate to be decomposed, occurring in the bound form of sodium aluminate and in the free form of sodium 5 hydroxide. That process comprises introducing into the sodium aluminate solution to be decomposed, an amount which is generally between 350 g/1 and 600 g/1 of A1(0H)3, acting as a seeding agent, decomposition of the solution generally being effected at a temperature which is at most 55°C. Such a process results in a high level of productivity of alumina, which 10 can attain 80 g of per litre of the sodium aluminate solution, but the aluminium hydroxide which is produced in that way is generally of fine granulometry and, by calcination, gives an alumina, the fine nature of which is at present considered to give rise to problems in regard to igneous electrolysis.

In the American process, precipitation of the aluminium trihydroxide is effected by decomposition of. an aqueous solution of sodium aluminate at a low level of concentration in respect of caustic Na.,0, which does not exceed 110 g of Na20 per litre of sodium aluminate solution to be decomposed. For that purpose, the American process comprises introducing 20 into the sodium aluminate solution to be decomposed, an amount of AlfOH)^, to act as a seeding agent, which is smaller than in the European process, being generally between 100 g/1 and 200 g/1 of the aluminate solution to be decomposed, the decomposition step being performed in contrast at a higher temperature such as for example 70°C. All those operating conditions, in 25 combination, result in the production of an aluminium trihydroxide of larger granulometry than that produced using the European process, which, after classification and calcination, gives an alumina which has the granulometry that is desired at the present time for igneous electrolysis and which is referred to as "sandy coarse". However, due to a contrary 30 effect, such operating conditions cause a drop in the level of productivity of Al203, which appears to be much lower thart in the European process, generally being about 50 g of A1203 per litre of aluminate solution, in the case of the production of a "sandy coarse" alumina. It is well known 3 that attempts to improve productivity by decreasing the decomposition temperature and by introducing a larger amount of AlfOHJ^ as a seeding agent, into the sodium aluminate solution to be decomposed, are balanced by the disappearance of the alumina of "sandy coarse" granulometry and 5 the occurrence of an alumina of smaller granulometry.

For a long period of time now, as is borne witness by the large number of publications in this field, many attempts have been made both in regard to the European process and in"regard to the American process, to find a process for producing aluminium trihydroxide of large granulometry, 10 which at the same time enjoys the level of productivity attained in the European process.

A first process which is described in US patent No 2 657 978, the aim of which is to promote the increase in productivity of aluminium hydroxide having a large granulometry, concerns the introduction of 15 aluminium trihydroxide as a seeding agent, in two periods, the first period comprising introducing just the amount of seeding agent required to produce crystals of large grain size, while a second period involves introducing a fresh amount of seeding agent. However, on the basis of the results which are set forth, the increase in productivity appears to be very low 20 and consequently of little attraction iron the industrial point of view.

Another process which is described in US patent No 3 486 850 and which pursues the aim of simultaneously increasing the level of productivity and the size of the particles of aluminium trihydroxide precipitated from the supersaturated sodium aluminate liquor comprises continuously intro-25 ducing aluminium trihydroxide to act as a seeding agent, into the sodium aluminate solution which is in a condition of circulation in a decomposition zone having a plurality of stages, and effecting intermediate cooling between two decomposition stages. However, that process is poorly suited to production on an industrial scale, by virtue of the narrow temperature 30 range in which it must take place and also by virtue of the low increase in productivity which is achieved with that process. 4 In addition, while simultaneously seeking to improve the precipitation yield and the granulometry of the aluminium hydroxide produced, another process as described in French patent No 2 440 916 proposes a two-phase procedure for decomposition of the supersaturated 5 sodium aluminate solution.

The first decomposition phase comprises introducing into the sodium aluminate solution, a controlled amount of a suspension of fine seeding agent, that phase taking place at a temperature of from 77°C to 66°C.

Then, the second decomposition phase comprises treating the cooled 10 suspension from the first phase by introducing a sufficient amount of seeding agent of larger granulometry, such that the combined amount of seeding agent introduced in the two phases represents at least 130 g of aluminium trihydroxide per litre of solution to be decomposed, the amount of seeding agent generally not exceeding 400 g/1. However, the improvement 15 which can be rioted in regard to the American process concerns more the increase in productivity than the attainment of a grain size v/hich is actually larger, and that improvement appears to be the simultaneous consequence of supersaturation of the sodium aluminate solution to be decomposed, which depends on the bauxite attack operation and its 20 particularly long residence time (from 45 to 100 hours) in the decomposition zone, while a larger overall amount of seeding agent introduced into that solution does not reveal a decisive action.

While the process described in French patent No 2 440 916 seems to recommend the use of a larger amount of recycled seeding agent, v/hile 25 however not exceeding 400 g/1 of sodium aluminate solution to be decomposed, this being in order to increase the level of productivity and the granulometry of the particles produced, US patent No 4 305 913 denounces the harm involved in using a large amount of seeding agent in the European process, even going so far as to say that the consequence thereof 30 is the production of an aluminium trihydroxide of small grain size. It is for that reason that that patent proposes another stepwise process for the decomposition of a supersaturated sodium aluminate solution, camprising a 5 first agglomeration stage, a second stage oi increasing the size oi the agglomerates, and finally a third stage comprising production of the seeding agent, the three stages being separate but related, while the temperature at which that process tabes place is between 74°C and 85°C 5 and tire amount of seeding agent introduced is between 70 and 140 g/1 of sodium aluminate solution to be decomposed. However, that process does not provide a solution which is more favourable from the point of view of the man skilled in the art since, while producing an alumina of a granulometry which is apparently favourable, it still suffers from a low 19 level of productivity, in comparison with a European process.

Thus, from the various known publications, it appears that many procedures have been followed in the attempt to arrive at a process for the decomposition of a supersaturated sodium aluminate solution,, which simultaneously presents the sole qualities which are to be found in the 15 American and European processes, that is to say, providing for the production of an alumina of large granulometry' (sandy coarse type), at a high level of productivity. However, the man skilled in the art is obliged to recognise that the processes proposed give incomplete and generally unsatisfactory' solutions since the production of an alumina of 20 acceptable granulometry' generally' involves a drop in the high level of productivity of alumina, which the man skilled in the art can no longer accept on an industrial scale.

Because of the above-indicated disadvantages further research has resulted in the discovery and development of a process for the decomposition of a supersaturated solution of alkali metal aluminate produced using the Bayer process of alkaline attack on bauxites, by introducing into the solution an amount of seeding agent that has never previously been used and that in the prior art is considered as harmful, with a view to achieving simultaneously, on an industrial .,q scale, a high level of productivity of aluminium trihydroxide by an improvement in the efficiency of decomposition of the alkali metal aluminate, and a crystallised aluminium trihydroxide of large granulometry, of which 10% at most of the particles produced have their smallest dimension at less than 45 microns.

In the process according to the invention, all the seeding agent used is brought into contact with all the supersaturated alkali metal aluminate solution produced with the Bayer process, and (a) in the decomposition zone of the Bayer process, comprising jn stages in a cascade configuration, there is produced a suspension having a high proportion of dry matter of at least 700 g/1 of alkali metal aluminate solution to be decomposed, in at least one stage, by the introduction of a seeding agent comprising crystals of aluminium trihydroxide of non-selected granulometry; (b) after a residence time in the decomposition zone at a maximum temperature in the range of from 50°C to 75°C, until the ratio by weight of dissolved A1203 to caustic Na20 obtained is at most equal to 0.7:1, a fraction comprising at most 50% by volume of the suspension with a high proportion of dry matter, circulating in the decomposition zone, is drawn off; (c) then, after the said fraction of suspension has been drawn off, it is introduced into a classification zone in which (1) the grainy portion separated is extracted and constitutes the production of A1(0H)3 of large granulometry, and (2) the other separated portion, which forms a suspension, is withdrawn from the classification zone and combined with the remaining fraction of the suspension circulating in the decomposition zone, which was not subjected to the classification step; and (d) the suspension resulting from the operation carried out in (c) (2) is subjected to a solid-liquid separation operation, the solid phase separated constituting the aluminium trihydroxide seeding agent of non-selected granulometry, which is recycled to the decomposition zone of the Bayer process. - 6 - 7 In order to facilitate the subsequent description of the invention, it should be recalled that the proportion of dry matter in the suspension generated by introducing the seeding agent into the supersaturated alkali metal aluminate solution to be decomposed is expressed in terms of grams of 5 dry aluminium trihydroxide per litre of the said solution, while the level of concentration of caustic tla20 in terms of grams per litre of the sodium aluminate solution expresses, as is well known, the total amount of Na20 present in that solution in the bound form of sodium aluminate and in the free form of sodium hydroxide. ]0 The above-indicated definitions having been recalled, the invention will ba describad by reference to a general diagrammatic view of an installation ior the production of aluminium trihydroxide, as shown in the single Figure of the accompanying drawings.

In the Figure, the sodium aluminate solution decomposition zone 15 comprises n. decomposition stages, formed by a first group A of £ stages and a second group B comprising (n.-£.) stages for the decomposition of the alkali metal aluminium solution.

Tne supersaturated alkali metal aluminate solution Lj to be decomposed ray be introduced in its entirety into one at least of the decomposition stages of groups A or B, for example L^, L^. - -, in the case of group (A). However, it nay also be introduced in respect of one part into at least one of the decomposition stages of group A and in respect of the other part into one at least of the decomposition stages of group B. Likewise, the aluminium trihydroxide, of non-sorted granulometry', which acts as a seeding agent, may be introduced, in its entirety or in part, in accordance with the same mode of distribution as the aluminate solution, for example in S ,, S , S , in hie case of group A. 3JL SZ Bp In the course of the research on which the present invention is based, it was found possible to produce a suspension having a high proportion of dry matter, of at least 700 g/1 of alkali metal aluminate solution to be decomposed, by introducing a seeding agent comprising crystals of aluminium trihydroxide of 8 non-selected granulometry, that is to say, a granulometry covering a wide range of distribution, such suspension simultaneously providing for the production of an alumina of large granulometry, of the "sandy coarse" type, and with a high level of productivity.

Preferably, the level of concentration of dry matter in the seeding agent suspension which is produced in at least one stage of the decomposition zone is between 800 and 2000 grams of AlfOH)^ of non-selected granulometry, per litre of sodium aluminate solution to be decomposed.

The seeding agent suspension, with its high proportion of dry matter, may preferably occupy at least (n-1) stages of the decomposition zone, and it may be desirable for the suspension to occupy the (n-1) last stages of that zone. Γη the latter case, the group A of the decomposition zone is then formed by a single stage.

However, it may be an attractive proposition for the seeding agent suspension produced, with its high proportion of dry matter, to occupy the n decomposition stages. That suspension is then produced by the simultaneous introduction into the first decomposition stage of the whole of the seeding agent and the whole of the alkali metal aluminate solution to be decomposed.

The seeding agent suspension produced, with its high content of dry matter, remains in the decomposition zone, once it is formed. It is maintained therein at a maximum temperature which is selected to fall in the range of from 50°C to 75°C for the period of time necessary to produce a weight ratio of dissolved A^O^ to caustic ^2*3, which is at most equal to 0.7. Preferably, the maximum temperature to which the seeding agent suspension is subjected in the decomposition zone is selected to lie in the range of from 50 to 68°C.

However, when the maximum temperature to which the suspension with its high proportion of dry matter is subjected in the decomposition zone is selected in the range of from 60 to 75°C, in one at least of the "n" decomposition stages, it may be found important to effect forced cooling of 9 said suspension which circulates in the other n-1 stages of the decomposition zone, as soon as it issues from the decomposition stage in question, so that the maximum temperature thereof, after cooling, is at most 60°C.

Since the suspension produced, having a high proportion of dry matter, which is circulating in the decomposition zone, remains therein for the period of time required, a fraction of said suspension, which is formed by at most 50% by volume and preferably at most 30% by volume, is taken off and introduced into the classification zone C in which the grainy portion 10 Lg is extracted, constituting the production of AMOH)^ of large granulometry, which is produced in accordance with the invention, while the other portion Lc which forms a suspension is withdrawn from the classification zone C and combined with the remaining fraction of the suspension circulating in the decomposition zone.

The suspension Ι·η from the decomposition zone, without passing through the classification zone C, is then subjected to a solid-liquid separation step in D, with the liquid phase L being passed to the following section of the Bayer process, while the solid phase S„, in accordance with the a invention, constitutes the seeding agent comprising aluminium trihydroxide 20 of non-selected granulometry, which is recycled to at least one stage of the zone for decomposition of the supersaturated alkali metal aluminate solution.

The solid phase S constituting the seeding agent, of non-selected a granulometry, may be introduced in that form into the alkali metal aluminate solution to be decomposed, or it may be introduced in the form of a 25 suspension which is previously prepared by dispersion in all or part of the alkali metal aluminate solution to be decomposed.

In accordance with an alternative form, a minor amount of the aluminium trihydroxide seeding agent is introduced into the first stage of the decomposition zone, and then the remaining amount of said seeding agent is 30 introduced into the second stage of the decomposition zone.

In general, the major amount of the aluminium trihydroxide seeding agent 10 introduced into the second decomposition stage is at least equal to 70% by weight of the whole of the recycled seeding agent.

In accordance with the same alternative form of the process, the alkali metal aluminate solution to be decomposed is introduced in its 5 entirety into the first stage of the decomposition zone. However, it has been found that it is also an attractive proposition to introduce into the first stage of the decomposition zone, at least 20% by volume of the alkali metal aluminate solution to be decomposed, while the remaining volume of said alkali metal aluminate solution is introduced into the second stage 10 of the decomposition zone.

Consequently, and in accordance with the above-mentioned alternative form of the process, the maximum temperature used in the first stage of the decomposition zone is selected in the range of from 65 to 80°C while the maximum temperature in the second stage of the decomposition zone is 15 selected in the range from 50 to 65°C.

Ihe process according to the invention, for decomposition of the supersaturated alkali metal aluminate solution, by bringing the whole of the seeding agent of non-selected granulometry into contact with the whole of said solution, may be effected both in a continuous mode and in a 20 discontinuous mode.

In the most widely encountered case of decomposition in a continuous mode, a decomposition stage is formed by a volume of suspension of the seeding agent in the alkali metal aluminate solution to be decomposed, which corresponds to a mean residence time desired in respect of said circulating 25 suspension, that volume being permanently supplied by the prior stage and permanently feeding the subsequent stage.

In the case of discontinuous mode decomposition, as may be used for example in a batch mode, a decomposition stage is formed by a volume of suspension of the seeding agent in the alkali metal aluminate solution to 30 be decomposed, which corresponds to the total period of time required for decomposition of that solution. 11 The essential characteristics of the invention will be better appreciated from the description of the examples set out below: EXAMPLE 1 This Example illustrates the possibility, in accordance with the 5 invention, of producing a suspension having a high proportion of dry matter, by the introduction of seeding agent formed by crystals of aluminium trihydroxide of non-selected granulometry, providing for the production of particles of AKOH)^ of large granulometry, while retaining a high level of productivity.

For that purpose, use was made of an industrial unit for the production of alumina using the Bayer process, in which an equal-weight mixture of French arid Australian bauxites, which were of the following 15 20 25 composition in percent by weight, was subjected to attack at a temperature of 235°C: Bauxites French Australian Firing losses 13.47 23.88 Si02 5.3 5.3 Al2°3 52.3 •54.8 Fe2°3 24.0 13.0 Ti°2 2.7 2.6 CaO 1.8 0.05 V2°5 0.08 0.04 % 0.20 0.08 Organic C 0.15 0.25 That results in a supersaturated sodium aluminate solution to be decomposed, of the following composition: Caustic Na20 160 g/1 Carbonated Na20 10 g/1 30 . A12°3 180 g/1 Organic C 8 g/1 12 The sodium aluminate solution to be decomposed was introduced, at a rate of 400 m3 per hour, into the decomposition zone which comprises 8 stages, each stage being provided with a mechanical agitator means.

The whole of the sodium aluminate solution to be decomposed was 5 introduced into the first stage of the decomposition zone, together with the whole of the seeding agent.

The temperature was 63°C in the first decomposition stage and 60°C in the last.

Three industrial tests in respect of decomposition in a continuous 10 mode were carried out over a three month period.

Test 1 illustrates the use of a suspension of Al(0H)y having a proportion of dry matter equivalent to that specified by the prior art but lower than the lower proportion which falls within the scope of the invention.

Test 2 shows the attraction of the substantial increase in the proportion of dry matter in the suspension of AllOH)^ in the supersaturated sodium aluminate solution to be decomposed.

Test 3 shows the influence of classification in accordance with the invention, by passing a fraction of the suspension circulating in the 20 decor position zone, through the classification zone.

In the three tests concerned, the seeding agent used was of non-selected or non-sorted granulometry.

The results obtained are set out in Table 1 below: 13 TABLE 1 Test 1 Test 2 Test 3 Seeding agent or A1(0H)3 in g/1 of the sodium aluminate solution to be decomposed 500 1400 1400 % by weight of the seeding agent below 45 microns 50 19 17 % by weight of the production of A1(0H)3 of smaller than 45 microns 50 19 9 % by volume of the suspension of A1(0H)3 circulating in the decomposition zone passing into the classification zone 0 0 17 Mean residence time in hours in the decomposition zone 50 35 35 Productivity of Al203 in g/1 of the sodium aluminate solution to be decomposed 80 80 80 Thus, the atove-indicated Table shows that the use of a high proportion of dry matter in the suspension of the seeding agent circulating in the decomposition zone (that suspension being produced by dispersion of the whole of the seeding agent of non-selected granulometry in the whole of the sodium aluminate solution to be decomposed) makes it possible to obtain a very substantial increase in the size of the circulating aluminium trihydroxide. The addition of a classification step results in the production of particles of aluminium trihydroxide of large grain size, while the productivity in grams of per litre of sodium aluminate solution still remains at a high level. ' EXAMPLE 2 Using an industrial unit, a supersaturated sodium aluminate solution to be decomposed was produced by a Bayer attack process at a temperature of 245°C on a French bauxite of the following composition in percent by weight: 14 Firing loss 12.02 Ti02 2.6 Si02 6.5 CaO 1.5 A!°3 52.8 MgO 0.2 24.0 Organic C 0.38 5 The supersaturated sodium aluminate solution to be decomposed was of the following composition: Caustic Na.,0 160 g/1 Al^O^ 176 g/1 Carbonated Na20 18 g/1 Organic C 4 g/1 The sodium aluminate solution to be decomposed was introduced at a 10 rate of 500 m3 per hour into the decomposition zone which comprises 8 stages, with each stage being provided with an agitation effect using air.

The whole of the sodium aluminate solution to be decomposed was introduced into the first stage of the decomposition zone, simultaneously with all the seeding agent.

The temperature was 58°C in the first decomposition stage and 49°C in the last.

Two industrial tests in respect of decomposition in a continuous mode were carried out over a three month period.

Test 4 illustrates the use of a suspension of AHOH)^ in which the 20 level of concentration of dry matter falls in the range which is claimed as preferable, in accordance with the invention.

Test 5 illustrates the use of the same suspension as in Test 4, while associating therewith the step of classification of a fraction of the suspension circulating in the decomposition zone.

In these two tests, the seeding agent used was of non-selected granulometry.

The results obtained are set out in Table 2 below: 15 TABLE 2 Test 4 Test 5 Seeding agent or AKOH)^ in g/1 of the sodium aluminate solution to be decomposed. 800 800 5 % by weight of the seeding agent below 45 microns 10 10 % by weight of the production of AHOH)^ of smaller than 45 microns 10 4 10 % by volume of the suspension of Al(OH)3 circulating in the decomposition zone passing into the classification zone O 28 Mean residence time in hours in the decomposition zone 46 46 15 Productivity of Al^O^ in g/1 of the sodium aluminate solution to te decomposed 78 78 Table 2 confirms the results obtained in Example 1, namely that the use of a high proportion of dry matter in the suspension of the seeding 20 agent circulating in the decomposition zone makes it possible to achieve a very substantial increase in the size of the aluminium trihydroxide circulating, as can be seen by comparison with Test 1 of Example 1.

Likewise, the addition of a classification step results in the production of particles of aluminium trihydroxide, of large granulometry.

Finally, the productivity in terms of grams of Al^O^ per litre of sodium aluminate solution remains at a high level.

EXAMPLE 3 This Example illustrates the case of producing particles of AHOH)^ of large granulometry, iron a supersaturated sodium aluminate solution to 30 te decomposed, produced by Bayer attack at a temperature of 260°C on a diaspore bauxite of the following composition in percent by v/eight: 16 Firing losses 14.4 sxo2 3.0 A1203 56.0 Fe2°3 22.0 Ti02 2.6 CaO 2.1 P2°5 0.06 Organic C 0.1 The supersaturated sodium aluminate solution resulting therefrom 10 was of the following composition: Caustic Na.,0 163 g/1 Carbonated NajO 26 g/1 A1203 177 g/1 Organic C 4 g/1 15 The sodium aluminate solution to be decomposed was introduced at a rate of 800 m3 per hour into the first stage of the decomposition zone which comprises 11 stages, each stage being provided with mechanical agitator means.

The whole of the sodium aluminate solution to be decomposed was 20 introduced into the first stage of the decomposition zone, with all the seeding agent.

The temperature was 58°C in the first decomposition stage and 56°C in the last.

An industrial test in respect of decomposition in a continuous mode 25 in an industrial production unit was performed over a three month period.

This test involves the use of a suspension of Al(0H)3, in which the concentration of dry matter falls in the preferred range of the process according to the invention.

The seeding agent used was of non-selected granulometry.

The results obtained are set out in Table 3 below: 17 TABLE 3 Test 6 Seeding agent or A1(0H)^ in g/1 of the sodium aluminate solution to be decomposed 1503 % by weight of the seeding agent below 45 microns 12 % by weight of the production of Al(OH)^ of smaller than 45 microns 7 % by volume of the suspension of Al(OH)3 circulating in the decomposition zone passing into the classification zone 15 Mean residence time in hours in the decomposition zone 32 Productivity of in g/l of the sodium aluminate solution to be decomposed 77 Thus, it can be found, as in the case of the preceding Examples, that using a suspension of AHOH)^, with a high level of concentration of dry matter in the supersaturated sodium aluminate solution, which circulates in the decomposition zone, makes it possible to produce an aluminium trihydroxide of large granulometry, while providing a high level of productivity, expressed in terms of grams of A^O^ per litre of sodium aluminate solution.

EXAMPLE 4 A supersaturated sodium aluminate solution was produced by Bayer attack at a temperature of 245°C on an equal-weight mixture of an African bauxite and a French bauxite, which were of the following compositions in percent by weight: 18 Bauxites French African Firing losses 14.4 30.0 sio2 7.0 1.0 A1-0-. 51.5 58.5 Fe203 22.4 6.8 TiO, 2.7 3.5 CaO 1.8 0.1 Organic C 0.2 o.i 10 The supersaturated sodium aluminate solution to be decomposed was of the following composition: Caustic Na.,0 155 g/1 Carbonated Na20 21 g/1 ai2o3 178 g/1 15 Organic C 14 g/1 The sodium aluminate solution to be decomposed was introduced at a rate of 200 m3 per hour into the decomposition zone which comprises 8 stages, each stage being provided with a mechanical agitator.

Three tests involving decomposition in a continuous mode of said 20 solution were performed in an industrial production unit over a three month period.

In Test 7, the sodium aluminate solution to be decomposed was introduced in its entirety into the first decomposition stage, together with the whole of the seeding agent.

The temperature was 60°C in the first stage, 59°C in the second stage and 50°C in the last stage.

In Test 8, 100 m3 per hoi^r of the sodium aluminate solution to be decomposed, the temperature of which was 75°C, and 10% by weight of the seeding agent, were introduced into the first decomposition stage. Then, 19 100 m3 per hour of said solution, at a temperature of 50°C, and 90¾ by weight of the seeding agent, were introduced into the second decomposition stage which also received the overflow from the first stage.

The temperature was 72°C in the first stage, 60°C in the second 5 stage and 51°C in the last stage.

Finally, Test 9 not only involved the same experimental procedure as in Test 8, but also an operation of classification in respect of 20¾ by volume of the suspension of A1(0H)^ issuing from the seventh stage, the grainy fraction being intended for the production of A1(0H)3 while the 10 other fraction was recycled to the last stage of the decomposition zone.

The results obtained are set out in Table 4 below: TABUS 4 Test 7 | Test 8 Test 9 15 Seeding agent or A1(0H)3 in g/1 of the sodium aluminate solution to be decomposed 1000 1000 1000 % by weight of the seeding agent below 45 microns 40 16 15 20 % by weight of the production of the Al(0H)3 of smaller than 45 microns'3 40 16 8 % by volume of the suspension of A1(0H)3 circulating in the decomposition zone passing into the classification zone 0 0 20 25 Mean residence time in hours in the decomposition zone 45 45 45 Productivity of Al-Ch in g/1 of the sodium aluminate solution to be decomposed 85 81 81 30 Consequently, it may be noted that the use of a suspension of A1(0H)3 having a high level of concentration of dry matter, in the decomposition zone, with the simultaneous introduction in two stages of fractions of the sodium aluminate solution and seeding agent, in accordance with the above-specified parameters, results in the production of an aluminium 35 trihydroxide of large granulometry, in Tests 8 and 9, while providing a high level of productivity.

Claims (5)

1. A process for the decomposition of a supersaturated alkali metal aluminate solution produced by means of the Bayer process of alkaline attack on bauxites with the aim of simultaneously obtaining a high level of productivity and an aluminium trihydroxide of large granulometry, of which 10% at most of the particles produced have their smallest dimension below 45 microns, by the introduction of seeding agent, in which the whole of the seeding agent used is brought into contact with all the supersaturated alkali metal aluminium solution produced with the Bayer process, and (a) in the decomposition zone of the Bayer process, comprising a stages in a cascade configuration, there is produced a suspension having a high proportion of dry matter of at least 700 g/1 of alkali metal aluminate solution to be decomposed, in at least one stage, by the introduction of a seeding agent comprising crystals of aluminium trihydroxide of non-selected granulometry; (b) after a residence time in the decomposition zone at a maximum temperature in the range of from 50°C to 75°C, until the ratio by weight of dissolved A1203 to caustic Na20 obtained is at most equal to 0.7:1, a fraction comprising at most 50% by volume of the suspension with a high proportion of dry matter, circulating in the decomposition zone, is drawn off; (c) then, after the said fraction of suspension has been drawn off; it is introduced into a classification zone in which (1) the grainy portion separated is extracted and constitutes the production of A1(0H)3 of large granulometry, and (2) the other separated portion, which forms a suspension, is withdrawn from the classification zone and combined with the remaining fraction of the suspension circulating in the decomposition zone, which was not subjected to the classification step; and (d) the suspension resulting from the operation carried out in (c) (2) is subjected to a solid-liquid separation operation, the solid phase separated constituting the aluminium trihydroxide seeding agent of non-selected granulometry, which is recycled to the decomposition zone of the Bayer process.

2. A process according to Claim 1, in which the suspension - 20 - having a high proportion of dry matter, which is produced in at least one stage of the decomposition zone, has a concentration of dry matter of from 800 to 2000 g/1 of aluminate solution to be decomposed.

3. A process according to Claim 1 or 2, in which the produced suspension with a high proportion of dry matter occupies at least n-1 decomposition stages.

4. A process according to Claim 3, in which the produced suspension having a high proportion of dry matter occupies at least the last n-1 decomposition stages. 5. A process according to any one of Claims 1 to 4, in which the suspension having a high proportion of dry matter occupies the n. decomposition stages and is produced by the simultaneous introduction into the first decomposition stage of the whole of the recycled seeding agent and the whole of the alkali metal aluminate solution to be decomposed. 6. A process according to Claim 5, in which the maximum temperature in the decomposition zone is in the range 50 to 68°C. 7. A process according to Claim 5, in which the maximum temperature employed in the decomposition zone is in the range 60 to 75° C. 8. A process according to Claim 7, in which the suspension having a high proportion of dry matter, circulating in the decomposition zone, is forcibly cooled. 9. A process according to Claim 8, in which the temperature of the suspension having a high proportion of dry matter, circulating in the decomposition zone, after forced cooling, is at most 60°C. 10. A process according to any one of Claims 1 to 4, in which the suspension with its high proportion of dry matter is produced by introducing a major amount of the seeding agent A1(H0)3 into the second decomposition stage while the remaining amount of the seeding - 21 - agent A1(0H)3 is introduced into the first decomposition stage. 11. A process according to Claim 10, in which the major amount of the seeding agent A1(0H)3 introduced into the second decomposition stage is at least equal to 70% by weight of the whole of the recycled seeding agent. 12. A process according to Claim 10 or 11, in which the whole of the alkali metal aluminate solution to be decomposed is introduced into the first stage of the decomposition zone. 13. A process according to Claim 10, in which 20% by volume of the alkali metal aluminate solution to be decomposed is introduced into the first stage of the decomposition zone, while the remaining volume of the aluminate solution to be decomposed is introduced into the second stage of the decomposition zone. 14. A process according to Claim 10, in which the maximum temperature in the first stage of the decomposition zone is in the range 65°C to 80°C and the maximum temperature in the second stage of the decomposition zone is in the range 50°C to 65°C. 15. A process according to Claim 1 substantially as hereinbefore described in any one of Examples 1 to 4. DATED THIS 7th day of July 1983 BY TOMKINS & CO.,

5. , Dartmouth Road Dublin, 6 - 22 -