HU201276B - Process for producing alum earth from bauxite containing gibbsite - Google Patents

Abstract

Process for the production of alumina from "gibbsitic" bauxites. The process comprises mixing "gibbsitic" bauxites with sodium hydroxide-sodium aluminate digestion liquor, digestion between 100 and 180 degree C, dilution of the suspension, separation of the red sludge, precipitation of aluminium hydroxide and calcination, characterised by control of the process in which the concentration of reactive hydroxides in the liquid phase leaving the reactor, defined by the formula <IMAGE> is maintained below 6 g/l, preferably between 0 and 2 g/l. Desirably this control is achieved by controlling the bauxite/digestion liquor ratio.

Description

The present invention relates to a process for the production of alumina from bauxite of the gibbsite type by further developing the Bayer process for the extraction of alumina. More particularly, the present invention relates to a process wherein the bauxite is digested with 100% to 180 ° C of alkali hydroxide alkaline aluminate solution, after dilution from the digested slurry, the red sludge is separated and washed, and the resulting sodium aluminate solution is separated by cooling and stirring. and calcine it to a mile.

In the Bayer process, bauxite is decomposed in sodium unsaturated sodium hydroxide solution in sodium hydroxide solution, depending on the mineral composition of the bauxite, at a temperature of -100 to 300 ° C, i.e. the soluble aluminum minerals are converted to dissolved sodium aluminate. After digestion, the slurry is cooled to an atmospheric boiling point using the heat content, after dilution and removal of insoluble residues, the aluminum hydroxide (alumina hydrate) is further crystallized from the supersaturated solution and, after separation, the mother liquor is evaporated, possibly partially evaporated. After washing, alumina hydrate is calcined into alumina, technically pure AhO3, at temperatures above 1000 ° C.

Bauxites always contain a certain amount of silicon-containing material, in addition to the aluminum minerals to be digested (gibbsite, boehmite, diapers). The SiO2 content occurs mainly in the form of kaolinite (Al2O3S1O2-2H2), in particular in the form of quartz (S1O2), chamosite, illite, halloysite in the brownates. In addition to the ingredients listed above, minor amounts of Fe2O3-TiO2-containing constituents are also present in the browns. Bauxites are considered gibbsites having a gibbsite content of at least 90% by weight of recoverable aluminum minerals (gibbsit + mugmit + diapers).

It is known in the literature that SiOa minerals in the brownates are converted to sodium aluminum hydrosilicates during processing.

The Na2O / SiO2 weight ratio of sodium aluminum hydrosilicate formed in a 2.5 to 3.5 molar solution of 80-200 g / l Na2Ok at a temperature of 140-150 ° C is reported by various authors as 0.62-0.79 (Adamson , AN: Alumina Production: Principles and Practice, The Chemical Engineer, Huné, 1970, pp. 156-171; Z. Sartowski, Zs. Vargáné Kiss, D, Bulkai: Deacidification in the Bayer Process Mining and Metallurgy Papers, Metallurgy, 114, 1981, 2, pp. 79-85; Yamada, K. et al., U.S. Patent No. 4,426,363 to Process Foal Extraction Alumina Front Aluminum. This ratio makes it possible to calculate the chemical loss of caustic soda, which is a significant cost element in the Bayer process for alumina production.

Caustic Na 2 O, represented by the formula Na ² O k, means the amount of Na ⁺ ions in NarO equivalent to the free OH 'ions in the liquid phase and the OH' ions present as ((/ ΟΗφΟ 'complex anions).

z \ caustic molar ratio is the quotient of the caustic NarO and AI2O3 moles in solution.

In American terminology, the caustic concentration as defined above is expressed in terms of Na2CO3. Also, in US terminology, the ratio of dissolved Al2O3 concentration to caustic concentration in Na2CO3, known as the A / C ratio, is widely used.

Gibbsite bauxites are typically 140-150 Con (exceptionally at 105 ° C). low temperature digestion. In a typical embodiment, the bauxite is frozen in 10-20% of the solution in digestion-soluble Al2O3-poor. The slurry of about 80 'C is usually mixed with the digestion fluid in heat exchangers for about 1 hour after storage without preheating for up to 1-2 hours. 160 ° C for the main current. By direct injection of steam, the temperature of the slurry obtained after mixing is stabilized at the desired 140-150 ° C. The solution outlined above is called a dual-current explorer. Typically, the digestion of gibbsite bauxites is after use of a caustic Na2O concentration of 90-120 g / l, 2.6-2.8 molar (as used in the literature for alumina production: returlgae). The measurement of bauxite in the digestion solution (ratio control of the bauxite / digestion solution) is controlled so that the caustic molar ratio of the solution reactor leaving solution is 1.34-1.45 (A / C ratio 0.66-0.72). For low temperature exploration, a residence time of 40-100 minutes is used, which is accomplished in 3-5 serial autoclaves. Gibbsite dissolves in the first 10-20 minutes of the excavation.

Along with the gibbsite reaction, kaolinite dissolves in the liquid phase. The dissolved S1O2, which is believed to be a complex ion, is converted into a solid phase sodium aluminum hydrosilicate by reaction. The formation of nuclei, the formation and growth of crystals takes longer than the dissolution of gibbsite, and therefore a residence time of 40-100 minutes is usually used in the reactor (Carlos, S., Interalumina Bauxite Grinding and Digestion, May 1983). pp. 29-94; Kotte, J. J.: Bayer Digestion and Prcdigestion Desilication Reactor System. Light Metals. Proc. of ΑΙΜΕ Annual Conference, 1981, pp. 45-79.).

The solution leaving the digestion reactor at 140-150 ° C, 1.34-1.45 molar solution, is supersaturated with respect to boehmite. Adjust the ratio of bauxite to digestion solution so that the AhOs concentration in the solution does not approach the equilibrium value by more than 18-20 g / l. If the equilibrium concentration of AI2O3 were approached, the increase in supersaturation to boehmite would result in an increase in the secondary boehmite accumulation during normal residence time of 0.5-2% AbO3, which would be economically unacceptable.

The purpose of red mud washing is to minimize the concentration of dissolved Na 2 O and Al 2 O 3 in the liquid phase accompanying the red mud leaving the factory by countercurrent washing. In the settler, the liquid phase is supersaturated with respect to AhO3, and therefore, under the given conditions, the equilibrium phase, gibbsite, begins to precipitate. In wash 1, ki-2EN 201276 Due to the higher Na ₂ O concentration and temperature, the supersaturation is higher than in the settler, so secondary gibbsite formation is more potent. A loss of 2-3% of bauxite-fed AI2O3 occurs during sedimentation washing due to secondary gibbsite formation.

No. 203,873. European Patent Application Publication Publication No. 1697/86 and Publication No. 1697/86, Publication No. T / 41326. Hungarian patent application relates to the processing of low SiO2-containing gibbsite bauxites. According to the method of Lepetite and Mordini, the brown slurry is milled in a solution of 50-120 g / l Na ₂ O, which is at least partially obtained by washing red mud or alumina hydrate, the resulting slurry being treated at 80-100 ° C for at least 85% kaolinite. -convert to sodium aluminum hydrosilicate. The disadvantage of the process is that by grinding in the wash water, the digestion operation is burdened with the addition of excess water, which evaporator increases the energy consumption of the production.

A process for the processing of high kaolinite-containing gibbsite bauxites is disclosed in the Grubbs invention (U.S. Patent No. 4,614,641). According to the present invention, it is broken from brown to classified at 105 µm (50 mesh), the fine fraction rich in kaolinite is decomposed in a solution of sodium aluminate containing more than 140 g / l Na ₂ O at 80-130 ° C, and the solid phase is removed by settling , the liquid phase rich in dissolved aluminum and silicic acid is mixed with the coarse fraction which has been digested on a separate branch at 140-150 ° C.

A disadvantage of the well-known technologies widely used for the processing of gibbsite bauxites is that reactive SiO ₂ entering the alumina plant is ca. 0.65-0.69 Na ₂ O / SiO ₂ causes loss of caustic soda. The solubility of the digestate is not fully utilized, thus wasting heat and electricity on heating and pumping part of the digestate stream. All this decreases the throughput of the exploration unit and increases the specific energy consumption. A further disadvantage of the typical low temperature digestion technology is the residence time of 0.5-2 hours, which requires the installation of costly pressurized equipment (autoclaves). In autoclaves it is difficult to solve the problem of material transfer. preventing slurry settling. Another disadvantage of widespread low temperature digestion technology is secondary boom formation, which in many cases causes a loss of 0.5-2% AI2O3. Current technologies also have the disadvantage of secondary gibbsite formation in the settling and washing system, which usually results in a loss of 2-3% AI2O3.

SUMMARY OF THE INVENTION The object of the present invention is to reduce the drawbacks of known technologies by a simple process which allows to reduce the bound caustic soda losses caused by reactive SiO2, to fully utilize the solubility of the digestate and equipment capacity, significantly reduce the digestion reaction time and to achieve a favorable SiO ₂ concentration in the solution for the production of grade alumina meeting market requirements. It is a further object of the present invention to reduce and prevent secondary loss-inducing secondary warts.

Deacidification experiments with 100 to 150 g / l, 3.0 to 3.4 molar ratios of solids at various solids contents have shown that at concentrations of 100 to 150 g / l Na ₂ kaolinite conversion 4-6 hours does not achieve the 50-70% conversion achieved in 4-6 hours.

Previously, the relationship between gibbsite dissolution and kaolinite conversion has not been extensively studied.

The mechanism of the reaction was investigated by means of kinetic modeling. Kinetic modeling has led to the novel (recognition that reactive OH 'concentration is the common driving force for the solubility of both kaolinite and gibbsite in the complex reaction mechanism; kaolinite and gibbsite compete for free OH' ions in the system. Definition of OH'concentration:

AH = 34 / ^ Al2O3, g- ^C Al2O3 ^c SiO2 ' ⁷ 102 60 ¹ where:

c ^eq Al2O3, gi against the gibbsite tel equilibrium existing Al ₂ O 3 concentration, g / l 'Abos current _Al2 O3 concentration, g / l ^c the current _{SiO2 SiO2} concentration, g / l

The formula 102 represents the molecular weight of Al2O3, 60 of S1O2, and 34 of two OH '.

It does not cause a significant change if the dissolved SiO ₂ concentration is ignored in the calculation of AH, since it is usually 2 to 3 orders of magnitude lower than the Al ₂ O 3 concentration.

According to the literature, during the transformation process, the dissolution of kaolinite takes place in a very short time (Adamson, AN: Principles and Practice of Alumina Production, June, 1970, pp. 156-171; J, Kotte, Bayer Digestion and Predigestion Desilication Reactor System. Light Metals. Proc. Of ΑΙΜΕ Annual Conference, 1981, pp.45-79.).

According to the literature, the relationship between the rate of reaction of gibbsite and kaolinite in alkali hydroxide alkaline aluminate solutions and the relationship between these reactions has not been investigated.

In kinetic modeling, it is not expected from the literature that gibbsite dissolution rate is approximately 2 to 8 times higher than that of kaolinite.

As a result, it was realized that if enough gibbsite was present, it would consume the reactive OH 'present, and therefore some, possibly significant, amount of kaolinite would "survive" up to 140-160 ° C.

Our investigations have confirmed this recognition. In the presence of a sufficient amount of gibbsite, a ratio of 0.4-0.5 Na ₂ O / SiO _{2 was} significantly lower than the theoretical value of 0.69 in the digestion sludge. X-ray diffractometer kimu3

-3H 201276 It was possible to react with unreacted kaolinite after digestion for 20 minutes or even 60 minutes at 150 ° C. These results were not expected based on previous knowledge.

The kinetic model that best describes the results of the measurements in sodium hydroxide sodium aluminate solutions of approximately the same molar ratio but with different NaOH concentrations suggests that the sodium aluminum hydrosilicate formed during the reaction slows down and then stops further conversion of kaolinite. This phenomenon is explained by the fact that the sodium aluminum hydrosilicate formed during the conversion of some of the kaolinite acts as a protective layer to prevent further conversion of kaolinite. The presence of sodium aluminum hydrosilicate on the surface of the partially transformed kaolinite in alumina alkaline solution has been confirmed by recent research (Roach GID, White, AJ. Dissolution Kinetics of Kaolin in Caustic Liquors. Proc. Of ΑΙΜΕ Annual Conference, 1988, 4147). .).

The invention is based on the discovery that if the reactive OH concentration of the solution phase of the slurry optionally containing partially deacidified bauxite is kept below 6 g / l, preferably less than 2 g / l during digestion at 140-180 ° C. conversion of kaolinite to sodium aluminum hydrosilicates.

Mixed gibbsites Based upon this finding it bauxitokból alkálialuminát exploratory alkali hydroxide solution and 100 to 180 ^e C and exposing temperature is preferably 140-160 ° C, the digested slurry of red mud after dilution resulting sodium aluminate by separation by sedimentation and washing solution from the hydrate by cooling and stirring and calcining the alumina hydrate, the alumina is prepared by desulfurizing the bauxite introduced into the digestion in a known manner and optionally in a known manner, and containing bauxite and at least 80-180 g / l Na2Ok. By controlling the ratio of the digestion solution having a molar ratio of 2.1 molar to less than 1 g / l in the liquid phase of the slurry outlet slurry, the reactive hydroxide ion concentration of the above formula is less than 6 g / l, preferably 0-2. g / l.

The exploration equipment has three main parts:

(a) a part for heating the digestion fluid or the bauxite slurry to digestion temperature

(b) the reactor part

(c) a part for reducing the temperature and pressure of the sludge leaving the reactor.

If the SiO2 concentration is neglected in the calculation of the reactive hydroxide ion concentration, the driving force of the reaction will be proportional to the difference between the equilibrium and the actual Al2O3 concentrations.

Therefore, the process according to the invention is preferably carried out in such a way that by controlling the ratio of the bauxite to the digestion solution, the equilibrium concentration of AhCb is at least 18 g / l, ⁴ preferably 0-6 g / l, typical of the liquid phase of the digestion effluent slurry. until we approach it.

When mixing the bauxite and the digestion solution, the ratio of the two components is adjusted in a known manner. Preferably, the mass flow rate of the bauxite and the volume flow rate of the digestion solution are measured and the reactive hydroxide ion concentration is kept within the limits given above by controlling their proportion. This otherwise known operation is analogous to what is known in the literature as slurry adjustment, A / C ratio or molar ratio control.

In the practice of the invention, it is advantageous to calculate the controlled characteristic and to determine the concentrations taken into account in the calculation using the process model. Control can also be easily accomplished by measuring the electrical conductivity of the digested slurry.

In the practice of the invention, the bauxite is preferably rested by mixing with at least 10 of the digestion solution. This operation, called dehydrogenation, is carried out at 80-120 ^° C, preferably 90-100 ° C with gentle agitation, using 0.03-0.2 kW / m ³ slurry mixing energy, converting kaolinite into 0.5-20, preferably After 2-4 hours, i.e. after conversion of 1080, preferably 20-50%, of the kaolinite. This prevents the conversion of unreacted kaolinite into sodium aluminum hydrosilicate in subsequent operations. At the same time, the formation of sodium aluminum hydrosilicate nodules in the solid phase entering the digestion reactor facilitates the rapid precipitation of the S1O2 sodium aluminum hydrosilicate which is dissolved in the liquid phase during digestion.

According to the invention, the digestion of gibbsite brunates can be carried out in a much shorter time than usual, 1-60 minutes, preferably 3-20 minutes. The operation is also economically feasible in a tubular reactor which is more technically advantageous than the so-called autoclave series.

It is further preferred that a solution of a poor alkaline hydroxide alkaline aluminate in dissolved AhCl2 is subjected to a dilution and / or red mud separation (settling) and / or red mud washing operation. In this way, the AbO over-saturation of the scrubbing solution phase can be reduced, thereby reducing secondary gibbsite formation. A solution of alumina having a caustic molar ratio of greater than 2.5, preferably greater than 2.7, is considered poor in dissolved Al ₂ O ₃ . Not only is the solution in AkOy soluble in alkali hydroxide alkaline aluminate solution a solution derived from red mud causticisation, caustic soda causticisation, washing of alumina hydrate, hydrothermal treatment of red mud. it can also be a mixture of these. Dissolved in ₃ Al2O poor solution (s) of vörösiszapülepítés washing process one or more points can be added to the red mud slurry, respectively. to the washing liquid. The optimum distribution of the digestate by-pass between settling and washing line by digestion should be determined by a feasibility study, taking into account regulatory considerations.

For browns of various origins and qualities, the optimal combination of technological features is determined by optimization experiments.

-4EN 201276 A

8

The determination of the permissible SiO 2 concentration at the end of the digestion is illustrated by a simplified example. Consider a alumina plant that processes high quality gibbsite bauxite with a low level of red mud. Assume that 75 kg of AI2O3 are precipitated from 1 m ^{3 of} alumina during mixing, that the SiO ₂ contamination of the alumina is to be maximized by 0.020% and that 25% of S1O2 in the dilute solution phase is precipitated and . It is known from the literature (Teeas, EB, Kotte. J.J., The Effect of Impurites on Process Impact and Methods for Foam Impact Control and Removal. Proceedings of the JBI / JGS Symposium 1980, titled "Bauxite / Alumina Industry in the Americas") that about 2% of the dissolved SiO ₂ entering the mixer goes into the alumina. Suppose, further, that the Na ₂ Ok concentration of the solution leaving the digestion reactor prior to heat recovery and entering the mixer does not differ and that the total amount of returic acid passes through the digestion. In this case, the amount of solution phase entering the mixing vessel and leaving the digestion reactor is nearly equal. Under the above assumptions, 1 g / l SiO _{2 is} allowed in the solution phase leaving the digestion reactor from the SiCb balance.

The main advantages of the process according to the invention are as follows:

a) Complete conversion of SiO _2- containing minerals is prevented, therefore, the loss of chemically bound caustic soda expressed as Na ₂ O / SiO ₂

Decreases to 0.4-0.55,

(b) more bauxite can be incorporated into the digestion volume per unit volume compared to previous processes, which results in lower energy consumption in the digestion operation, scalability of the digester 10 and increased capacity of existing digester,

(c) the shorter residence time allows for an additional increase in the capacity of existing reactors or significant savings in the construction of new equipment,

d) the application of the process can reduce or virtually eliminate the loss of gibbsite -> boehmite conversion that occurs as a side reaction of the digestion,

(c) secondary gibbs forming in the sediment-washing operations can be significantly reduced.

A comparison of the widely used embodiment of low temperature digestion with the process of the invention is shown in the following table:

Comparison table

typical traditional solution invention of raises Na ₂ Ok concentration g / L 140 140 molar relationship after excavation 1.35 1.20-1.25 digestion temperature C 143 150-160 the residence time of the slurry in the reactor minute 100 3-15 reactive OH concentration is exploratory at the exit of the reactor g / L 7 2 Na ₂ O / SiO ₂ weight ratio in sludge after digestion 0.69 0.40 to 0.55 secondary boehmite formation during digestion, AI2O3 % 0.5-2 0-0,5 secondary gibbsite formation during sedimentation washing, AI2O3 % 2-3 0-0,5 molar ratio of alumina __ 1.40 1.40 the proportion of returnee bypasses % 0 20-25 Some possible embodiments of the process of the invention are illustrated by the following examples, but are not intended to limit the claims. 50 Fe ₂ O ₃ Hematite geothítben 3.05% 7.7% 4.3%

/. example

For the experiment, "ultra-mee" grade bauxite from the Brazilian Treebetas site was used in the following weight percent: 55

Al2O3 gibbsite boehmite kaolinite 51.0 to 51.4% 0.4-0.8% 2.2% 54.0% SiO: quartz 0.52% kaolinite 2.53%

12.0%

Loss on ignition 29.9%

The bauxite was digested in alumina liquor having a concentration of 152.3 g / l Na ₂ O ₂ , 79.7 g / l Al ₂ O 3, 0.3 g / l SiO 2, and 235 g dry bauxite was added to 1 liter of solution. The digestion was carried out at 150 ° C in autoclave bombs. The experimental time was calculated from the time of insertion in the preheated oil bath. Due to the experimental technique (due to the time needed to heat the autoclave bomb and the slurry inside it), the slurry's residence time at bath temperature is less than the measured value. At the end of the experimental times, the bombs were disassembled after cooling, a

-5EN 201276 The liquid phase was separated by centrifugation and analyzed. The solid phase was washed with distilled water three times and then analyzed after drying. The results of the measurements are as follows:

experiment solution phase solid phase

time minute Na ₂ O g / l Al2O3 GFL Molven Szõnyi SiO ₂ g / i reactive OH- g / 1 Na2Ű SiO Al ₂ O 3 SiO ₂ 10 136.2 179.2 1.25 1.07 3.14 0,137 6.90 15 133.1 182.4 1.20 0.92 0.26 .319 1.80 20 134.2 185.5 1.19 0.68 0.03 0.327 1.52

It is noted that if the kaolinite form sodium aluminum hidroszüikátban Al2O3 / SiO2 weight ratio same way 0.85 has in need of kaolinitéval the experimental conditions and the boehmite not considered táródónak the gibbsite case of full disclosure expected in the slurry ABO ₃ SiO2 weight ratio of 1, 01-1.17 depending on whether boehmic AhO3 is taken as 0.4 or 0.8.

As shown in the table, after 15 and 20 minutes of digestion, both the molar ratio of the solution, the weight ratios of Na2O / SiO2 and Al2O3 / S1O2 in the solid phase, and the SiO2 concentration of the solution are favorable. No secondary boehmite formation was observed.

Example 2

The composition of the bauxite and alkali used in the experiments was the same as that of Example 1. The digestion was carried out in autoclave bombs at a reaction temperature of 150 ° C for 60 minutes. The measurement results:

accompany- lations time minute bauxite input with gap g / L Na ₃ O ₃ g / l Al2O3 Szõnyi g / L solution phase mólvi- OH- S1O2 S1O2 g / L reactive S1O2 g / L solid phase Na ₂ O Al2O3 60 235 135.4 189.6 1.17 0.58 0.00 0.346 1.21 60 221 134.9 180.9 1.23 0.73 1.97 .386 1.02 60 207 132.3 174.5 1.25 0.54 2.63 0.409 1.10

Despite the long reaction time, all measurements yielded a favorable molar ratio, dissolved SiO2 concentration, Al2O3 / SiO2 weight ratio, and low Na2O / SiO2 weight ratio. After 60 minutes of digestion, the presence of uncharged kaolinite in the solid phase can be detected. No secondary boehmite formation was observed.

Example 3

The composition of bauxite was the same as in Example 1. The alumina base used in the experiment contained 140 g / l Na ₂ Ok, 74.1 g / l AbOs and 0.4 g / l SiO ₂ . 800 grams of dry bauxite was added to 1 liter of alkali at 100 ° C for 2 hours, while the bombs were moved at 19 rpm in the bath. According to our experience, this mixing intensity can be reproduced by applying a mixing energy of 0.03-0.2 kW / m ³ slurry. The slurry was then diluted with the digestion liquor to a level of 232 g / l bauxite per liter. The slurry was then subjected to a 15-minute digestion in autoclave bombs at 150 ° C. After exploration, the following measurement results were obtained:

accompany- lations time minute Na ₂ O g / l Al2O3 g / L solution phase molvj- Szõnyi SiO ₂ g / i reactive OH 'g / ^l solid phase Na ₂ O, SiO ₂ Al ₂ O 3 SiO ₂ 15 127.2 165.4 1.26 0.81 2.49 0.504 1.01

The measurement gave a favorable weight ratio of Na2O / SiO2 and AI2O3 / SiO2, a molar ratio after digestion and a concentration of dissolved SiO2. No secondary boehnite formation was observed.

Example 4

The composition of both the bauxite and the alkali as well as the weighing were the same as that described in Example 3. The experiment was carried out at 100 ° C as in Example 3

-6H 201276 The hourly deacidification was applied, the slurry was diluted to the extent given in Example 3 and then digested. The data measured after exploration at 150 'C is as follows:

pilot phase solid phase

lations time minute Na ₂ O g / i Al2O3 g / 1 molvi- Szõnyi SiO2 g / L reactive OH- g / 1 Na2O SiO2 Al2O3 S1O2 15 126.4 162.4 1.26 0.71 3.08 0.522 1.02

Both the molar ratio after digestion, the dissolved SiO2 concentration and the Na2O / SiO2 and AhOs / SiO2 mass ratios were favorable. No secondary boehmite formation was observed.

Claims (5)

A process for the production of alumina from gibbsite bauxils, wherein the bauxite is digested with a solution of alkali hydroxide alkali aluminum digestion at a temperature of 100-180 ° C, preferably 140-160 ° C, after dilution from the digested slurry, the red sludge is separated by sedimentation and washed with and removing the alumina hydrate by filtration and then calcining it into alumina, characterized in that the bauxite introduced in the digestion is optionally deionized in a known manner, and the bauxite and 80-180 g NaoOt, at least 2.1 molar and at 1 g / l controlling the ratio of lower SiO2 concentrations in the digest solution in a known manner leaving the reactor slurry liquid phase of finding the c ^eq AbOi, gi - ^{c c} AbO3 SiO2. AH = 34. / -_ —— l 102 60 reactive hydroxide ion concentration, where c ^eq Al2O3, gi is the lean AhCb concentration against gibbsit, g / l * Al2O3 is the actual A ^ Ch concentration, g / ^1c SiO2 is the actual SiO2 concentration, g / 1 It is maintained at values of less than 6 g / l, preferably 0-2 g / l. Process according to claim 1, characterized in that by controlling the ratio of the bauxite to the digestion solution, the equilibrium concentration of Al2O325 in the liquid phase of the digestion effluent slurry is at least 18 g / l, preferably 0-6 g / l. introduced. The process according to claim 1 or 2, characterized in that the bauxite is mixed with at least 10% of the digestion solution at a temperature of 80-120 ° C prior to digestion, 30 preferably at 90-100 ° C for 0.5-20, preferably 2- Allow to rest for 4 hours, and mix the bauxite and the digestion solution with, if appropriate, 0.03-0.2 kW / m ³ slurry stirring energy. 35 4. Process according to any one of claims 1 to 3, characterized in that the reaction mixture is kept at the digestion temperature for 1 to 60 minutes, preferably 3 to 20 minutes. The method of claim 1 or 2, wherein Characterized in that the controlled characteristic is determined by measuring the electrical conductivity of the digested slurry.