JPH02291B2 - - Google Patents

Description

Claim 1 By introducing starting materials, a high production rate and up to 10% of produced particles with a minimum dimension of less than 45 microns can be achieved.
A method for decomposing a supersaturated alkali aluminate solution obtained according to the Bayer method of alkali corrosion of bauxite in order to obtain coarse-grained aluminum trihydroxide containing a) aluminum trihydroxide crystals of non-selective particle size in the decomposition zone of the Bayer process comprising n cascade stages; b) producing a suspension with a high dry matter content, comprising at least 700 g of dry matter per alkaline aluminate solution to be decomposed in at least one stage, by introducing starting materials containing dissolved Al; ₂ O ₃ / Caustic Na ₂ O weight ratio up to 0.7
Consisting of up to 50% by volume of a dry matter-rich suspension that is circulated through the decomposition zone after a period of residence in the decomposition zone at a maximum temperature selected between 50°C and 80°C until reaching c) Then, after said pre-emption, said fraction is introduced into a classification zone, c ₁ - extracting the separated granular part and dividing it into coarse-grained Al
removing the remaining separated fraction devoted to the production of (OH) ₃ and forming a c ₂ -suspension from the classification zone and adding it to the remaining fraction of the suspension circulating unclassified through the decomposition zone; , d) solid-liquid separation of the suspension resulting from the treatment of c ₂ and from the separated solid phase forming aluminum trihydroxide starting material of non-selective particle size which is reused in the decomposition zone of the Bayer process. A method for decomposing a supersaturated alkaline aluminate solution, comprising the steps of: 2. The suspension with a high dry matter content produced in at least one stage of the decomposition zone preferably has a dry matter content of from 800 to
2. Process for decomposing supersaturated alkaline aluminate solutions according to claim 1, characterized in that the dry matter concentration is selected within the range of 2000 g. 3. Process for the decomposition of supersaturated alkaline aluminate solutions according to claim 1 or 2, characterized in that the resulting suspension with a high dry matter content is present in preferably at least n-1 decomposition stages. 4. Process for the decomposition of supersaturated alkaline aluminate solutions according to claim 3, characterized in that the resulting suspension with a high dry matter content is present in preferably at least the last n-1 decomposition stages. 5. A product suspension with a high dry matter content is present in n decomposition stages, said suspension containing the total amount of starting material to be reused and the total amount of alkaline aluminate solution to be decomposed in the first stage. The method for decomposing a supersaturated alkali aluminate solution according to any one of claims 1 to 4, characterized in that the method is obtained by simultaneously introducing the solution into the second decomposition stage. 6. The method for decomposing a supersaturated alkaline aluminate solution according to claim 5, wherein the maximum temperature used in the decomposition zone is selected within the range of 50 to 68°C. 7. The method for decomposing a supersaturated alkali aluminate solution according to claim 5, wherein the maximum temperature used in the decomposition zone is selected within the range of 60 to 75°C. 8. Process for decomposing supersaturated alkali aluminate solutions according to claim 7, characterized in that forced cooling is applied to the resulting suspension with a high dry matter content circulating through the decomposition zone. 9. The method for decomposing a supersaturated alkali aluminate solution according to claim 8, characterized in that the temperature after forced cooling of the suspension with a high dry matter content circulating through the decomposition zone is a maximum of 60°C. 10 A large amount of Al(OH) ₃ during the second decomposition step
The method according to claim 1, characterized in that a suspension with a high dry matter content is produced by introducing the starting material and by introducing the remaining amount of the starting material Al(OH) ₃ during the first decomposition stage. A method for decomposing a supersaturated alkali aluminate solution according to any one of scopes 1 to 4. 11. According to claim 10, the amount of starting material Al(OH) ₃ introduced into the second decomposition stage is equal to at least 70% by weight of the total amount of starting material to be reused. Method for decomposing supersaturated alkaline aluminate solution. 12. Process for decomposing a supersaturated alkali aluminate solution according to claim 10, characterized in that the entire amount of the alkaline aluminate solution to be decomposed is introduced into the first stage of the decomposition zone. 13 20 of the alkaline aluminate solution to be decomposed
% by volume into the first stage of the decomposition zone;
11. Process for decomposing a supersaturated alkaline aluminate solution according to claim 10, characterized in that the remaining amount of the alkaline aluminate solution to be decomposed is introduced into the second stage of the decomposition zone. 14 The maximum temperature used in the first stage of the decomposition zone is selected in the range 65°C to 80°C, and the maximum temperature used in the second stage of the decomposition zone is selected in the range 50°C to 65°C. The method for decomposing a supersaturated alkali aluminate solution according to claim 10, characterized in that the method is selected from among the following. Description The present invention relates to a method for precipitating Al(OH) ₃ at a high production rate from a supersaturated alkaline aluminate solution obtained according to the Bayer method using alkaline corrosion of bauxite, according to which the starting material Coarse-grained aluminum trihydroxide can be obtained with up to 10% of product particles having a smallest dimension of less than 4 microns. The Bayer process is widely disclosed in the specialized literature and well known to those skilled in the art, and constitutes the main technology for producing alumina for conversion to aluminum by pyrolytic electrolysis. According to this method, bauxite is heat treated with an aqueous sodium hydroxide solution of an appropriate concentration, thereby solubilizing the alumina and obtaining a supersaturated solution of sodium aluminate. After separating the solid phase constituting the non-corroding residue (red day) from the mineral, the supersaturated solution of sodium aluminate is mixed with aluminum hydroxide, generally referred to hereinafter as "starting material", in order to precipitate aluminum trihydroxide. The seeds are decomposed. As is well known to those skilled in the art, there are several industrial methods for producing aluminum trihydroxide using Bayer's alkaline corrosion of bauxite, which are generally classified into two types: the so-called European method and the American method. . According to the European method, precipitation of aluminum trihydroxide occurs during the so-called decomposition process of a sodium aluminate aqueous solution with a high concentration of caustic Na ₂ O containing approximately 130 to 170 grams of Na ₂ O per liter of sodium aluminate to be decomposed. arise. “Caustic _Na2O
"Concentration" is the total amount, in grams, of Na ₂ O present in one liter of the sodium aluminate solution to be decomposed, either in the form associated with sodium aluminate or in the free form from sodium hydroxide. According to this method, almost all of the starting material is added to the sodium aluminate solution to be decomposed.
Introducing 350g/~600g/Al(OH) ₃ ,
Decomposition of the solution is generally carried out at temperatures up to 55°C.
If such a method is followed, a high alumina production rate of 80 g of Al ₂ O ₃ per liter of sodium aluminate solution can be obtained, but the aluminum trihydroxide produced in this way generally has a fine particle size, and the alumina obtained by calcination is It is recognized that the particle size is so fine that it actually hinders pyrolytic decomposition. According to the American method, the precipitation of aluminum trihydroxide is carried out by decomposition of an aqueous sodium aluminate solution with a low concentration of caustic Na ₂ O, said concentration not exceeding 110 g of Na ₂ O per liter of sodium aluminate solution to be decomposed. . For this, the American method contains a smaller amount of the starting material Al(OH) ₃ in the sodium aluminate solution to be decomposed than the European method,
Approximately 100% per aluminate solution to be decomposed
g to 200 g, while the decomposition is carried out at a higher temperature, for example 70°C. By combining the processing conditions in this way, a coarser grained aluminum trihydroxide is produced than that obtained by the European process, and the grain size of the alumina obtained after classifying and calcining said coarse grained aluminum trihydroxide is actually It is suitable for ignited electrolysis and is known to have a “sandy coarseness”.
However, on the other hand, according to the above processing conditions,
The production rate of Al ₂ O ₃ is much lower than that of the European method, and when producing alumina with “sand-like roughness”, generally about 50 g of Al ₂ O ₃ per liter of aluminate solution is required. It just stops generating. As is well known, attempts to improve the production rate by lowering the decomposition temperature and introducing a larger amount of the starting material Al(OH) ₃ into the sodium aluminate solution to be decomposed result in "sandy coarseness". Small-grained alumina is lost and finer-grained alumina is created. As disclosed in numerous publications in the field, efforts have long been made to the American and European processes to find ways to produce coarse-grained aluminum trihydroxide while maintaining the production rates of the European process. Many attempts have been made. First, in the method disclosed in US Pat. No. 2,657,978 for the purpose of improving the production rate of coarse-grained aluminum hydroxide, aluminum trihydroxide, which is a starting material, is introduced in two stages. 1
In the numbered stages, as much starting material as is necessary to obtain coarse-grained crystals is introduced, and in the second stage fresh starting material is introduced. However, the degree of increase in production rate according to the results described is very small and is insufficient for industrial use. The method disclosed in U.S. Pat. No. 3,486,850
The purpose is to simultaneously increase the production rate and particle size of aluminum trihydroxide precipitated from a supersaturated sodium aluminate solution. It consists of a continuous introduction of aluminum trihydroxide and an intermediate cooling between the two decomposition stages. However, this method is inconvenient for production on an industrial scale because of the narrow temperature range in which the method is run and because of the low yields obtained as a result of using this method. On the other hand, the method disclosed in French Patent No. 2440916 for the purpose of simultaneously improving the precipitation yield and the particle size of the aluminum hydroxide produced involves decomposing a supersaturated sodium aluminate solution in two stages. is suggesting. The first decomposition step involves introducing a controlled amount of the finely divided starting material suspension into the sodium aluminate solution, and this step is carried out at a temperature in the range of 77°C to 66°C. A second decomposition stage then treats the cooled suspension introduced in the first stage by introducing a sufficient amount of coarser-grained starting material.
The total amount of starting material introduced in these two stages is:
It is chosen such that there are at least 130 g and generally not more than 400 g of aluminum trihydroxide per liter of solution to be decomposed. However, the perceived improvement over the American method lies in the increased production rate rather than the actual coarser particle size; Supersaturation of the sodium aluminate solution and extra long residence time of bauxite in the decomposition zone (45 to
100 hours), while the amount of starting material finally introduced into the solution is so large that no decisive effect is obtained. The method disclosed in French Patent No. 2 440 916 requires a larger amount of recycled starting material, not exceeding 400 g per sodium aluminate solution to be decomposed, in order to increase the production rate and the size of the particles produced. However, U.S. Patent No. 4,305,913 states that using a large amount of starting material as in the European method is harmful and that the trihydroxide produced as a result is harmful. It states that the grain size of aluminum becomes finer. And the latter patent proposes another stepwise decomposition method for supersaturated sodium aluminate solutions, consisting of a first coagulation stage, a second condensate coarsening stage, and a third starting material production stage. There is. It should be noted that the three steps are separate but related, the temperature at which the process is carried out is 74° C., and the amount of starting material introduced ranges from 70 to 140 g per liter of sodium aluminate solution to be decomposed. selected. Although following this method clearly produces alumina of a preferred particle size, the yield is still low compared to the European method and does not provide a preferred solution to those skilled in the art. Moreover, according to various known publications, it simultaneously has the advantages found only in American and European methods, respectively. i.e. coarse grain size (sandy coarseness)
It appears that many methods are being used to decompose supersaturated sodium aluminate solutions that can produce high yields of alumina. However,
These proposed methods provide only incomplete and insufficient solutions, since in order to obtain alumina of acceptable grain size one generally has to give up the high alumina production rates that are industrially acceptable to those skilled in the art. Those skilled in the art must recognize this point. Therefore, based on the above-mentioned disadvantages, the applicant has conducted extensive research and has attempted to solve the problem by introducing into the supersaturated alkaline aluminate solution an amount of starting material that has not been previously used and was considered harmful in the prior art. We have discovered a method for decomposing a supersaturated alkali aluminate solution obtained by the Bayer method of alkali corrosion of bauxite. The purpose of the process is to increase the production rate of aluminum trihydroxide by improving the decomposition yield of alkali aluminates and to increase the coarse particle size, i.e., crystalline trihydroxide with up to 10% of the particles produced having dimensions smaller than 45 microns. The aim is to simultaneously realize the production of aluminum hydroxide on an industrial level. The method according to the invention consists in contacting the entire amount of the starting material used with the entire amount of a supersaturated alkaline aluminate solution obtained according to the Bayer method,
The process comprises a) decomposition in at least one stage by introduction of a starting material consisting of aluminum trihydroxide crystals of non-selective particle size in a decomposition zone of the Bayer process comprising n cascade stages; b) producing a suspension with a high content of dry matter of _at least ₇₀₀ g in 1 liter of _alkaline aluminate solution;
After residence in the decomposition zone at a maximum temperature selected within the range of 50°C to 80°C until reaching 0.7;
c) preempting the flanks containing up to 50% by volume of a suspension with a high content of dry matter circulating in the decomposition zone; c) then, after said preemption, introducing said flanks into a classification zone; , c ₁ - extraction of the separated granular part, which constitutes coarse-grained Al(OH) ₃ ; c ₂ - another separated part forming a suspension is removed from the classification zone and classified. d) solid-liquid separation of the suspension obtained as a result of the treatment of _c.2 . , the separated solid phase constitutes a non-selective particle size aluminum trihydroxide starting material that is reused in the decomposition zone of the Bayer process. For the sake of clarity in the following description of the invention, the dry matter content of the suspension produced by introducing the starting materials into the supersaturated alkali aluminate solution to be decomposed is defined as dry matter content per liter of said solution. It shall be expressed in grams of aluminum trihydroxide, while the caustic Na ₂ O concentration is, as is well known, present in sodium aluminate solutions in the form bound to sodium aluminate and in the free form from sodium hydroxide. Let the total amount of Na ₂ O be expressed in grams. Based on the above definition, the present invention will be explained with reference to FIG. 1, which is a schematic diagram of an apparatus for producing aluminum trihydroxide according to the present invention. In the drawing, the decomposition zones of the sodium aluminate solution are divided into a first group A consisting of "p" stages and a second group A comprising (n-p) alkali aluminate solution decomposition stages. It includes "n" decomposition stages consisting of B. Supersaturated alkaline aluminate solution to be decomposed
L _d is applied to at least one of the decomposition stages of group A or B, e.g. L _d1 for group A,
The total amount can be introduced according to L _d2 ..., L _dp . However, a portion of the solution may be introduced into at least one of the decomposition stages of group A, and the remaining part may be introduced into at least one of the decomposition stages of group B.
It is also possible to follow the same distribution as an aluminate solution starting with all or part of aluminum trihydroxide of non-selective particle size, e.g. for group A.
Sa ₁ , Sa ₂ ... May be introduced according to Sa _p . Through its research, the applicant has discovered that the alkaline aluminate solution to be decomposed 1.
It is possible to produce suspensions with a high dry matter content of at least 700 g per liter, and from such suspensions it is possible to obtain alumina with a coarse grain size of the "sandy" type but with high yields. I have proven that I can get it. Preferably the dry matter concentration of the starting material suspension produced in at least one stage of the decomposition zone is:
The amount of non-selective particle size Al(OH) ₃ contained per liter of sodium aluminate solution to be decomposed is selected in the range of 800 to 2000 grams. The starting material suspension of said dry matter content is preferably present in at least (n-1) stages of the decomposition zone, and preferably said suspension is present in at least (n-1) stages from the end of the decomposition zone. It exists at the stage of In the latter case, group A of decomposition zones consists of only one stage. However, it is advantageous if the resulting starting material suspension with a high dry matter content is present in n decomposition stages. In this case, the suspension is obtained by simultaneously introducing into the first decomposition stage the total amount of starting material and the total amount of the alkaline aluminate solution to be decomposed. The resulting starting material suspension with a high dry matter content accumulates in the decomposition zone as soon as it is formed. The suspension is allowed to remain in the zone for the time necessary for the dissolved Al ₂ O ₃ /caustic Na ₂ O weight ratio to reach a maximum of 0.7.
A selected maximum temperature is maintained within the range of 50°C to 75°C. Preferably, the maximum temperature to which the starting material suspension is subjected in the decomposition zone is selected within the range from 50°C to 68°C. provided that the maximum temperature to which the product suspension with a high dry matter content is subjected in the decomposition zone is between 60°C and 75°C in at least one of the "n" decomposition stages.
, the suspension circulating through the other n-1 decomposition stages is subjected to forced cooling at the same time as it leaves the corresponding decomposition stage, and the suspension after cooling is It is recognized that it is essential to ensure that the maximum temperature of the suspension is at most 60°C. As soon as the product suspension with a high content of dry matter circulating through the decomposition zone has remained in the decomposition zone for the required time, up to 50% by volume of said suspension, preferably up to A flank L _{o-1 consisting} of 30 vol. A portion L _g is extracted, while the remaining portion L c forming the suspension is removed from the classification zone C and added to the remaining fraction of the suspension circulating in the decomposition zone. The suspension Ln that exits the decomposition zone without passing through the classification zone C is then subjected to solid-liquid separation in D, and the liquid phase La
is sent to the next section of the Bayer method, while the solid phase
Sa becomes a non-selective particle size aluminum trihydroxide starting material according to the invention and is reused in at least one stage of the decomposition zone of the supersaturated alkali aluminate solution. The solid phase Sa constituting the starting material of non-selective particle size can be introduced as a solid phase into the alkaline aluminate solution to be decomposed or dispersed in all or part of the alkaline aluminate solution to be decomposed. It can also be introduced as a pre-formed suspension. According to a variant, a small amount of aluminum trihydroxide starting material is introduced into the first stage of the decomposition zone, and then the remaining amount of said starting material is introduced into the second stage of the decomposition zone. Generally, the greater amount of aluminum trihydroxide starting material introduced during the second cracking stage is at least 70% by weight of the total amount of starting material recycled. According to said variant, the alkaline aluminate solution to be decomposed is introduced entirely into the first stage of the decomposition zone. However, equally at least 20% by volume of the alkaline aluminate solution to be decomposed is introduced into the first stage of the decomposition zone and the remaining volume of said alkaline aluminate solution is introduced into the second stage of the decomposition zone. It has been found advantageous to introduce it in stages. Therefore, according to the above modification, the first decomposition zone
The maximum temperature used in the second stage is between 65°C and 80°C.
C., while the maximum temperature of the second stage of the decomposition zone is selected within the range of 50.degree. C. to 65.degree. The inventive process of decomposing a supersaturated alkaline aluminate solution by contacting the entire amount of starting material of non-selective particle size with said solution can be carried out either continuously or discontinuously. In continuous cracking, which is more common, the cracking stage is formed from a volume of suspension of the starting material in the alkaline aluminate solution to be cracked; The volume of the suspension corresponds to the average residence time determined from the circulating suspension, which volume is continuously fed from the previous stage and continuously fed to the next stage. Discontinuous decomposition can be carried out, for example, in a "batch" manner, but in the case of discontinuous decomposition the decomposition stage is formed from a volume of suspension of the starting material in the acid salt solution to be decomposed; The volume of the suspension corresponds to the total time required for the dissolution of the solution. The essential features of the present invention will be better understood from the description of the following examples. Example 1 This example shows that by introducing a starting material consisting of aluminum trihydroxide crystals of non-selective particle size according to the invention, a suspension with a high dry matter content can be produced and a high production rate can be achieved. coarse grain while maintaining
We show that Al(OH) ₃ particles can be obtained. For this purpose, an industrial alumina production apparatus was used according to the Bayer method, in which an equal weight mixture of French and Australian bauxite having the composition (% by weight) shown in the table below was corroded at 235°C.

[Table] As a result, a supersaturated sodium aluminate solution to be decomposed having the following composition was obtained. 160 g of caustic Na ₂ O / 10 g of Na ₂ O carbonate / 180 g of Al ₂ O ₃ / 8 g of organic C / This sodium aluminate solution to be decomposed is
A rate of 400 m ² per hour was introduced into a decomposition zone comprising 8 stages each equipped with mechanical stirring means. The entire amount of sodium aluminate solution to be decomposed was introduced into the first stage of the decomposition zone together with the entire amount of starting material. The temperature was 63°C in the first zone stage and 60°C in the final decomposition stage. Three industrial continuous degradation tests were conducted over a period of three months. Test 1 shows the use of an Al(OH) ₃ suspension with a dry matter content equal to the dry matter content proposed by the prior art but lower than the low content within the framework of the invention. Test 2 shows the case where the dry matter content of the Al(OH) ₃ suspension in supersaturated sodium aluminate solution to be decomposed was significantly increased. Test 3 shows the influence of classification within the framework of the invention by passing the suspension fraction circulating in the decomposition zone through a classification zone. In the three tests mentioned above, the particle size of the starting material used was a non-selective particle size. The results obtained are shown in Table 1 below.

[Table] As is clear from the above table, the starting material suspension circulating in the decomposition zone (suspension obtained by dispersing the total amount of starting material of non-selective particle size in the total amount of the sodium aluminate solution to be decomposed) The use of high dry matter contents in the liquid) results in circulating aluminum trihydroxide of very coarse particle size. By adding classification it is possible to produce aluminum trihydroxide particles of coarse particle size, while the number of grams per liter of sodium aluminate solution
The production rate of Al ₂ O ₃ is maintained at a high rate. Example 2 A supersaturated sodium aluminate solution to be decomposed in an industrial apparatus was produced by Bayer corrosion of French bauxite having the following composition (weight percent) at 24°C. Dissolution loss 12.02 SiO ₂ 6.5 Al ₂ O ₃ 52.8 Fe ₂ O ₃ 24.0 TiO ₂ 2.6 CaO 1.5 MgO 0.2 Organic C 0.38 The composition of the supersaturated sodium aluminate solution to be decomposed was as follows. Caustic Na ₂ O 160g / Na ₂ O carbonate 18g / Al ₂ O ₃ 176g / Organic C 4g / The above sodium aluminate solution to be decomposed is as follows:
A rate of 500 m ³ per hour was introduced into a decomposition zone comprising 8 stages each equipped with pneumatic stirring means. The entire amount of sodium aluminate solution to be decomposed was introduced into the first stage of the decomposition zone together with the entire amount of starting material. The temperature was 58°C in the first decomposition stage and 49°C in the last stage. Two types of industrial continuous decomposition tests were carried out over a period of three months. Test 4 preferably consists of Al with a dry matter concentration falling within the range claimed within the framework of the invention.
An example using a (OH) ₃ suspension is shown. Test 5 shows an example in which the same suspension as in test 4 is combined with classification of the suspension fraction circulating in the decomposition zone. In both tests, the particle size of the starting material used was a non-selective particle size. The results obtained are shown in Table 2 below.

【table】

Table The above table confirms the results obtained in Example 1. That is, by using a high dry matter content in the starting material suspension circulating through the cracking zone, a circulating aluminum trihydroxide with a much larger particle size can be obtained compared to Test 1 of Example 1. Also, adding classification may produce coarse-sized aluminum trihydroxide particles. Furthermore, the production rate in terms of grams of Al ₂ O ₃ per liter of sodium aluminate solution remains high. Example 3 This example shows how coarse-grained Al
(OH) The case where ₃ particles are generated is shown. Dissolution loss 14.14 SiO ₂ 3.0 Al ₂ O ₃ 56.0 Fe ₂ O ₃ 22.0 TiO ₂ 2.6 CaO 2.1 P ₂ O ₅ 0.06 Organic C 0.1 The composition of the obtained supersaturated sodium aluminate solution was as follows. Caustic Na ₂ O 163g / Na ₂ O carbonate 26g / Al ₂ O ₃ 177g / Organic C 4g / The above sodium aluminate solution to be decomposed is as follows:
A rate of 800 m ³ per hour was introduced into the first stage of a decomposition zone comprising 11 stages, each equipped with mechanical stirring means. The entire amount of sodium aluminate solution to be decomposed was introduced into the first stage of the decomposition zone together with the entire amount of starting material. The temperature was 58°C in the first decomposition stage and 56°C in the last stage. One type of industrial continuous decomposition test was conducted in an industrial production facility for 3 months. The test shows an example using an Al(OH) ₃ suspension with a dry matter concentration that falls within the preferred range of the method according to the invention. The particle size of the starting material used was a non-selective particle size. The results obtained are shown in Table 3 below.

[Table] Thus, similar to the previously described examples, when using a suspension of Al(OH) ₃ with a high concentration of dry matter in a supersaturated sodium aluminate solution circulating in the decomposition zone, the sodium aluminate solution Coarse-grained aluminum trihydroxide can be produced while maintaining a high production rate in terms of grams of Al ₂ O ₃ per liter. Example 4 A supersaturated sodium aluminate solution was produced by Bayer corrosion of an equal weight mixture of African bauxite and French bauxite having the following composition (weight percent) at 245°C.

【table】

[Table] The composition of the supersaturated sodium aluminate solution to be decomposed was as follows. Caustic Na ₂ O 155g / Na ₂ O carbonate 21g / Al ₂ O ₃ 178g / Organic C 14g / The above sodium aluminate solution to be decomposed is as follows:
A rate of 200 m ³ per hour was introduced into a decomposition zone comprising 8 stages each equipped with mechanical stirring means. Three consecutive decomposition tests were carried out on the solution in an industrial production facility for a period of three months. In test 7, the entire amount of sodium aluminate solution to be decomposed was introduced into the first decomposition stage along with the entire amount of starting material. 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 m ³ /h of sodium aluminate solution to be decomposed at a temperature of 75° C. and 10% by weight of starting material were introduced into the first decomposition stage. Then the temperature
100 m ³ /h of this solution at 50° C. and 90% by weight of starting material were introduced into the second decomposition stage, to which the overflow of the first stage was also added. The temperature was 72°C in the first stage, 60°C in the second stage, and 51°C in the last stage. Furthermore, in Test 9, not only the same experimental conditions as Test 8 were used, but also 20% by volume of the Al(OH) ₃ suspension from the seventh stage was subjected to a classification treatment to remove the particulate fraction. It was used to generate Al(OH) ₃ while the remaining fraction was reused in the last stage of the cracking zone. The results obtained are shown in Table 4 below.

[Table] Therefore, the concentration of dry matter in the decomposition zone is high.
Test 8 _and It can be demonstrated that aluminum trihydroxide of coarse particle size can be produced as shown in Figure 9.