EP0519987B1 - Process and device for regenerating alkaline solutions for pickling aluminium - Google Patents

Abstract

In a process for regenerating alkaline solutions for pickling aluminium, in which aluminium hydroxide is recovered, a pickling solution containing alkali hydroxide, aluminate, a complex builder and other additives is transferred from a pickling bath into a reactor section consisting of at least one reactor, where aluminium is precipitated as aluminium hydroxide. The temperature of the pickling bath is higher than that of the reactor section. The process is operated so that the concentration of aluminate, alkali and gluconate and the temperature in the pickling bath, on the one hand, and the temperature drop from the pickling bath to the reactor section, on the other hand, are adjusted so that the pickling solution in the pickling bath is undersaturated or metastably supersaturated in relation to the precipitation of aluminium hydroxide but unstably supersaturated in the reaction section. A device for carrying out the process of the invention is also disclosed.

Description

The invention relates to a method for regenerating alkali-containing aluminum pickling solutions for the recovery of aluminum hydroxide according to the preamble of claim 1 and an apparatus for performing the method.

If aluminum surfaces are to be finished (for example by anodizing or plating), it is generally necessary to subject these surfaces to a pickling treatment beforehand, both acidic and alkaline pickling solutions being able to be used. In the case of alkaline pickling solutions, the aluminum is immersed in a heated bath, for example containing sodium hydroxide solution, and left in it for a certain time. The pickling process consists of the alkali dissolving the metal surface, with aluminum dissolving to a small extent with the evolution of hydrogen. The overall gross equation is as follows:



Al + NaOH + H₂O → NaAlO₂ + 3/2 H₂ (1)



Since aluminum dissolves to a certain extent, the bath is constantly enriched with aluminate during operation. The disposal of pickling solutions that contain large amounts of aluminate is both an ecological and an economic problem. Until now, these solutions have been consuming large amounts of acid (which may Anodizing baths were neutralized). In the past, however, there has also been an increasing attempt to recover the aluminum present in the pickling solution by precipitating it as aluminum hydroxide and using it again as a raw material. On the cost side, this provides the sales value of the crystalline aluminum hydroxide and, at the same time, avoids major environmental impacts such as those caused by landfilling the normally amorphous aluminum hydroxide sludge. When aluminum is precipitated as aluminum hydroxide, the base used in the pickling bath is recovered to a stoichiometric extent:



NaAlO₂ + 2 H₂O → Al (OH) ₃ + NaOH (2)



The reuse of the base liberated during the precipitation process in the pickling bath results in a further cost advantage and a further reduction in the environmental impact, since less highly alkaline wastewater is obtained, which must be neutralized and clarified with salt formation.

It is therefore desirable to have a process in which sodium aluminate is continuously converted into aluminum hydroxide by precipitation and the base which is produced is continuously reused in the pickling bath by recycling. Such a process would consume only water overall and convert aluminum into aluminum hydride with the release of hydrogen according to the following gross equation:



Al + 3 H₂O Al (OH) ₃ + 3/2 H₂ (3)



Methods for recovering aluminum from alkaline pickling solutions in the form of aluminum hydroxide are known.

DE-PS 43 977 describes a process for the preparation of alumina hydrate and alkali aluminate, in which aluminum hydroxide is precipitated from essentially pure alkali aluminate solutions by seeding. This procedure was later improved by further developments, although the further developments essentially used pure aluminate solutions.

However, modern aluminum pickling processes do not work with pure alkali solutions, but add additives to the pickling solution, with the help of which the pickled aluminum top surfaces obtain a special and desired visual appearance. Such additives are e.g. Nitrate and nitrite. Pickling solutions used today also contain complexing agents, mostly in the form of sorbitol, sometimes also gluconate, with the aim of ensuring that aluminum in the pickling bath is kept as complex as possible in solution and does not already precipitate out as aluminum hydroxide there. Such pickling solutions, besides aluminate, also contain the abovementioned additives, are not suitable for processing with the aforementioned processes according to DE-PS 43 977 and the processes resulting therefrom.

EP-A1 157 190 describes a process for the precipitation of aluminum hydroxide from pickling baths which, in addition to aluminate, also contain additives in the form of gluconate and carbonate. In this process, however, the aluminum hydroxide precipitates directly in the pickling bath. Such a way of working is disadvantageous for various reasons. First of all, it cannot be prevented that precipitated aluminum hydroxide settles on the aluminum pieces to be pickled, which leads to undesirable staining on the aluminum surface. It is also necessary to continuously filter out the precipitated aluminum hydroxide from the bath pickling solution either continuously or in batches.
US-A-4,826,605 shows the precipitation of aluminum hydroxide from pickling solutions by adding acid and subsequent oxidation with ozone.

The object of the invention is to provide an improved process according to the type mentioned at the outset, in which the precipitation of the aluminum hydroxide does not take place in the pickling bath, and with the aid of which it is possible to use both the aluminum hydroxide and the base formed in the case of failures simple process engineering effort and inexpensive to recover.

This object is achieved with a method according to claim 1 and a device according to claim 11.

1. The procedure 1.1 The pickling solution in the pickling bath

According to the invention, the pickling solution in the pickling bath must be undersaturated or metastably oversaturated, for which purpose the concentrations of aluminate, alkali and gluconate in the pickling bath and the temperature are used as control parameters. An operating mode with slightly metastable supersaturation is preferred. The aluminate concentration (given as equivalent aluminum) is 30 to 60 g / l, preferably 30 to 45 g / l. The concentration of complexing agent, given as equivalent gluconate, is 0.1 to 5 g / l, preferably 0.5 to 2 g / l. The alkali hydroxide concentration is in the range from 30 to 60 g / l equivalent sodium hydroxide, preferably in the range from 45 to 55 g / l. "Alkali hydroxide concentration" means the free concentration of MeOH, where Me is an alkali metal. There are methods for measuring the above-mentioned concentrations which are known to the person skilled in the art and are available to him. Examples of this can be found in section 1.3 of the present description of the invention.

The temperature of the pickling bath is in the range from 40 to 90 ° C., preferably in the range from 45 to 55 ° C. In order to maintain this temperature, heating of the pickling solution is generally necessary, especially when the pickling bath is subjected to low loads. Under low load is meant a small amount of aluminum over time, which is to be converted into NaAlO₂. If the load is high, however, heating may be dispensed with, because then the exothermic reaction according to equation (1) itself supplies sufficient energy. At very high loads, it may be necessary to cool the pickling solution in the pickling bath.

Surprisingly, it was found that under these conditions there is no precipitation of aluminum hydroxide in the pickling bath, although relatively high aluminum concentrations are present at relatively low concentrations of the complexing agent (gluconate). It is therefore preferred to operate at high aluminum concentrations compared to the sodium concentration.

The pickling solutions used according to the invention primarily contain alkali hydroxide (e.g. NaOH, KOH or mixtures thereof), aluminum (as aluminate) and complexing agents. Additives such as nitrate and / or nitrite are also added to improve the decorative surface finish. The nitrate concentrations are preferably in the range from 5 to 30 g / l, particularly preferably in the range from 20 to 25 g / l. Preferred nitrite concentrations are 5 to 30 g / l, particularly preferably 10 to 25 g / l. Other additives that can be used are alkali salts of inorganic acids in the concentration range from 1 to 100 g /, as in e.g. Chloride, chlorate, carbonate and thiosulfate.

Gluconate is particularly preferably used as the complexing agent. However, it is also possible to use sorbitol, other complexing sugar derivatives, phosphonates and polymeric or oligomeric acrylates.

1.2 The transfer of the pickling solution from the pickling bath to the reactor section

The transfer of the pickling solution from the pickling bath to the reactor section must be carried out in such a way that precipitation of aluminum hydroxide is avoided in the first transfer section because this leads to blockage and, if the levels of complexing agents are too low, also to "petrification" in the first transfer section can lead. Therefore, the temperature of the pickling bath is preferably essentially maintained in the first transfer section. This can be achieved using thermal insulation, for example. However, it can also be used an additional heater for the first transfer line.

The pickling solution is preferably introduced from the first transfer section into the reactor section without a body. This is understood to mean a mode of operation in which the pickling solution leaves the first transfer section and essentially enters the reactor section in free fall, preferably with the interposition of a funnel. In this way it is prevented that crystallization nuclei present in the reactor section penetrate into the first transfer section against the direction of flow of the pickling solution and trigger crystallization there.

However, a reverse mode of operation is also possible, in which the petrification is prevented by forcibly triggering crystallization in the first transfer section by finely distributing aluminum hydroxide which does not grow together to form larger aggregates and thus does not lead to petrification. This can e.g. can be achieved by germination of the reaction solution in the first transfer section. In this case, however, e.g. - also by disembodied transfer - ensure that the crystallization nuclei introduced into the first transfer section cannot penetrate the pickling bath.

1.3 The pickling solution in the reactor section

According to the invention, the pickling solution is transferred from the pickling bath into a reactor section consisting of at least one reactor using the first transfer section. The pickling solution transferred into the reactor section is referred to below as the "reactor solution".

In the reactor section, aluminum precipitates as aluminum hydroxide by creating conditions in which the reactor solution is unstably supersaturated. A jump in temperature between the pickling bath and the reactor section serves this purpose. As already mentioned, the pickling solution is in the pickling bath at a temperature of 40 to 90 ° C, which is why the reactor line must be kept at a low temperature. According to the invention, there is a jump in temperature or an optimized temperature gradient between the pickling bath and the reactor section in the form of a temperature difference of 5 to 70 ° C., preferably 10 to 40 ° C.

If the temperature of the reactor solution is chosen too low (in absolute terms), this can lead to poor crystallization due to delayed nucleation. According to current knowledge, this is related to the fact that both the nucleation and the growth of aluminum hydroxide on existing germs are subject to diffusion control and are therefore temperature-dependent. If the temperature of the reactor solution is chosen too low, aluminum hydroxide in solution will not succeed (or possibly only at too low a rate), will germinate and form larger and precipitable aggregates through growth.

In a preferred embodiment of the process according to the invention, the precipitation process in the reactor section is promoted and accelerated by inoculation. "Inoculation" is understood below to mean the presence of crystal nuclei. It is generally only necessary to introduce germs from the outside when starting up the system; if the plant has been in operation for a certain time (approximately 24 to 72 hours), a sufficient nucleus density in the sense of a steady state equilibrium arises in the reactor solution. The crystalline aluminum hydroxide formed by the precipitation also acts as a germ, ie the process has a self-initiating effect. However, the process can also be operated in such a way that germs are continuously introduced from the outside if the germ density is not sufficient in the equilibrium state. A suitable vaccine to be introduced from the outside is crystalline aluminum hydroxide phases, preferably gibbsite. It was observed that when gibbsite was used as the vaccine, the aluminum hydroxide essentially also precipitated as gibbsite.

The amount of vaccine should preferably be 5 to 500 ml / ℓ, particularly preferably 50 to 250 ml / l of gibbsite slurry per liter of reactor solution. Precipitation rates of up to 30 g aluminum per liter per day were achieved.

A high contact density between vaccine or germ and liquid is always required for crystal growth both in the start-up phase and in the later stationary phase. For this reason, the reactor section is preferably stirred intensively, and in the case of a reactor section consisting of several reactors, it may be advantageous to stir only the first reactor and to operate the downstream reactor (s) without stirring. In this case, the first reactor is referred to below as the reaction vessel and the second or further reactor as the sedimentation vessel.

The process according to the invention is preferably carried out with a reaction vessel and at least one downstream sedimentation vessel. The reaction vessel serves essentially only for nucleation and the sedimentation vessel for the deposition of the aluminum hydroxide crystals formed. These form a sedimentation layer in the - unstirred - sedimentation container, which is covered with a strongly alkaline aqueous solution. In general, the base concentration and thus the pH increase on the way from the pickling bath via the reaction tank to the sedimentation tank, because when aluminum hydroxide precipitates, base is formed according to equation (2).

In the pickling process, when nitrite and nitrate are used as additives, these are reduced by aluminum to ammonia. At the high alkali concentration of the process, the ammonia fumes and is noticeable through its smell. Correspondingly equivalent amounts of alkali metal hydroxide are formed from the additives nitrite and nitrate.

The transfer of the reactor solution from the reaction container into the sedimentation container takes place in such a way that the reactor solution into the sedimentation container is below the Liquid surface, but is introduced above the phase boundary of the sedimentation layer. This can happen, for example, through a side wall opening in the sedimentation container.

In the preferred embodiment described above, the precipitated aluminum hydroxide is obtained by stripping from the sedimentation layer of the sedimentation container, possibly. washed neutral with water and dried, and can be used again.

The base obtained in the process when the aluminum hydroxide precipitates can be obtained by stripping off the aqueous, highly alkaline solution which is formed in the sedimentation tank above the sediment / liquid phase boundary. It is preferred to return this alkali solution to the pickling bath and to continue using it for pickling aluminum (recycling). The stripped alkaline solution must contain no or as few crystallization nuclei as possible, because this could lead to undesired crystal growth of aluminum hydroxide in the pickling bath. The alkaline solution is therefore preferably drawn off from the surface of the liquid in the sedimentation container and filtered before being returned to the pickling bath. Alternatively, further sedimentation containers can also be used behind the (first) sedimentation container. A combination of both measures (filtering + further sedimentation containers) is also possible.

The reaction vessel can be designed in one, two or more stages. A multi-stage procedure is required if post-reactions occur.

• Die Beizlösung im Beizbad sollte bezüglich der Fällung von Aluminiumhydroxid metastabil übersättigt sein und sich knapp unterhalb der Temperaturstabilitätsgrenze befinden; sie sollte sich jedoch noch nicht im Bereich von Keimbildung befinden;
• die Beizlösung sollte Komplexbildner in einer Menge enthalten; die die Keimbildung verzögern, aber das Kristallwachstum nicht behindern;
• die Temperaturen des Bades und wenigstens des ersten Reaktors (Reaktionsbehälter) sollten - bevorzugt bei einem vorgewählten und optimierten Temperaturgradiznten - einen Temperaturunterschied von etwa 10 bis 40 °C aufweisen;
• wenigstens im Reaktionsbehälter sollte in der Reaktionszone eine hohe Keimdichte, d.h. eine hohe Keimanzahl im Verhältnis zur Lösung bestehen;
• in mindestens einem Sedimentationsbehälter erfolgt ein Absetzen des im Reaktionsbehälter gebildeten Aluminiumhydroxids, wobei im allgemeinen im Sedimentationsbehälter eine Nachreaktion stattfindet;
• die gewonnene Base wird als hochalkalische Lösung in Höhe der Flüssigkeitsoberfläche des Sedimentationsbehälters abgezogen und nach Filtrieren in das Beizbad zurückgeführt (Recycling), wobei das Zwischenschalten eines weiteren Sedimentationsbehälters möglich ist.

In summary, it can be stated that the currently best and therefore most preferred embodiment of the method according to the invention is characterized by the following operating parameters:

• The pickling solution in the pickling bath should be metastably oversaturated with regard to the precipitation of aluminum hydroxide and should be just below the temperature stability limit are located; however, it should not be in the nucleation area yet;
• the pickling solution should contain complexing agents in an amount; which delay nucleation but do not hinder crystal growth;
• the temperatures of the bath and at least the first reactor (reaction vessel) should have a temperature difference of approximately 10 to 40 ° C., preferably with a preselected and optimized temperature gradient;
• at least in the reaction vessel there should be a high nucleus density in the reaction zone, ie a high number of germs in relation to the solution;
• The aluminum hydroxide formed in the reaction container is settled in at least one sedimentation container, a post-reaction generally taking place in the sedimentation container;
• the base obtained is drawn off as a highly alkaline solution at the level of the liquid surface of the sedimentation container and, after filtering, returned to the pickling bath (recycling), it being possible to interpose another sedimentation container.

• a) Alkalihydroxid und Aluminium
  Ein aliquoter Teil der Beizlösung wird mit einer HCl-Lösung bekannter Normalität bis auf einen pH-Wert von 8,2 titriert. Durch entsprechende stöchiometrische Berechnung ergibt sich aus der verbrauchten HCl-Menge die Konzentration an freiem Alkalihydroxid. Nach Zugabe eines Überschusses an Alkalifluorid zu der so titrierten Lösung werden die an Aluminium gebundenen Hydroxidgruppen freigesetzt. Eine zweite Titration auf einen pH-Wert von 8,2 läßt indirekt auf die stöchiometrisch äquivalenten Aluminiumkonzentration schließen. Die pH-Überwachung erfolgt bei beiden Titrationen bevorzugt nicht mit einem Farbindikator, sondern potentiometrisch unter Einsatz einer Glaselektrode, am besten einer Einstabmeßkette.
• b) Äquivalentes Gluconat
  In einem aliquoten Teil der Beizlösung wird zunächst gegebenenfalls vorhandenes Nitrit durch Zugabe eines Überschusses an Harnstoff unter Bildung von Stickstoffgas entfernt. Danach erfolgt eine Redoxtitration mit potentiometrischer Endpunktserkennung in ca. 70 °C warmer schwefelsaurer Lösung mit einer Cer(IV)-Lösung bekannter Normalität als Oxidationnsmittel. Genauer wird das Meßverfahren z.B., wenn ein Überschuß der Cer(IV)-Lösung vorgelegt wird und dann eine Rücktitration mit potentiometrischer Endspunkterkennung mittels einer Eisen(II)-Salzlösung abgestimmter Normalität erfolgt.
• c) Gluconatäquivalente Komplexbildner
  Für gluconatäquivalente Komplexbildner wird zunächst experimentell deren Komplexbildungsaktivität ermittelt, indem man bestimmt, welche Aluminiumkonzentration bei sonst gleichen Versuchsparametern im Vergleich zu Gluconat noch in Lösung gehalten werden kann. Die so ermittelte Konzentration wird dann iterativ bei gleichem Aluminiumkonzentrationen und ansonsten konstanten Versuchsparametern im Vergleich zu gegebenen Gluconatkonzentrationen überprüft. Das Verhältnis der Molekular- bzw. Äquivalentgewichte und der ermittelte Faktor der Komplexbildungsaktivität wird dann in die Konzentrationsberechnung einbezogen. Bei Phosphonaten können dabei literaturbekannte spezifische Analysenverfahren zur Konzentrationsbestimmung eingesetzt werden.

Compliance with the concentrations of aluminate, alkali metal hydroxide and complexing agent within the above-mentioned ranges can be monitored either chemically or analytically by measuring these concentration values either continuously or at regular intervals. Direct and indirect methods are suitable as measuring methods. For example, aluminum can be determined photometrically directly via a color reaction, but can also be determined indirectly in the basic medium by complexing aluminum with fluoride and determining the base released during the formation of the complex by acidimetry. This method can also be used to determine aluminum in addition to alkali hydroxide. For organic complexing agents such as gluconate, the COD measurement or another oxidative determination method, which also uses organic carbon, is suitable as an indirect determination is oxidized to CO₂. In addition, measuring methods can also be used that provide a sum parameter, such as the measurement of the electrolytic conductivity. The following analytical analysis methods are particularly preferred at the moment:

• a) Alkali hydroxide and aluminum
  An aliquot of the pickling solution is titrated with a HCl solution of known normality to a pH of 8.2. The concentration of free alkali hydroxide is obtained from the amount of HCl used by appropriate stoichiometric calculation. After adding an excess of alkali fluoride to the solution titrated in this way, the hydroxide groups bound to aluminum are released. A second titration to a pH of 8.2 indirectly indicates the stoichiometric equivalent aluminum concentration. In both titrations, pH monitoring is preferably not carried out with a color indicator, but potentiometrically using a glass electrode, preferably a combination electrode.
• b) Equivalent gluconate
  In an aliquot of the pickling solution, any nitrite that may be present is first removed by adding an excess of urea to form nitrogen gas. This is followed by redox titration with potentiometric end point detection in a 70 ° C warm sulfuric acid solution with a cerium (IV) solution of known normality as an oxidizing agent. The measuring method becomes more precise, for example, if an excess of the cerium (IV) solution is presented and then a back titration with potentiometric end point detection is carried out using an iron (II) salt solution of normality.
• c) Gluconate equivalent complexing agents
  For gluconate-equivalent complexing agents, their complexing activity is first determined experimentally, by determining which aluminum concentration can still be kept in solution compared to gluconate with the same test parameters. The concentration determined in this way is then iteratively checked with the same aluminum concentrations and otherwise constant test parameters in comparison with the given gluconate concentrations. The ratio of the molecular or equivalent weights and the determined factor of the complex formation activity is then included in the concentration calculation. In the case of phosphonates, specific analytical methods known from the literature can be used to determine the concentration.

2. The device

A device (system) according to the invention has a pickling bath in which pickling solution can be heated or optionally cooled to the temperatures according to claim 1. A first transfer line leads from the pickling bath to a reactor line consisting of at least one reactor. The first transfer line is e.g. represents a pipeline and is preferably insulated against heat loss and / or heatable.

The transition from the transfer line to the reactor line is preferably carried out in such a way that the transfer tube does not dip into the liquid level of the reactor line (bodyless transfer). The transfer path therefore preferably ends directly above a funnel-like insertion element which is attached above the liquid level of the first reactor.

The reactor section consists of at least one reactor, an arrangement of a first stirred reactor (reaction vessel) and a subsequent unstirred sedimentation vessel being preferred. However, a further stirred reaction container can be arranged between the (first) reaction container and the sedimentation container. The volume of the reaction container is preferably one third to half of the volume of the pickling bath. The reactor volume represents an additional control parameter for the process because it can influence the residence time of the reactor solution in the reactor. A stay of 36 to 60 hours is preferred. Furthermore, the reactor volume stabilizes the temperature of the reactor contents. The reaction vessel has a stirring device with which the reactor contents can be stirred intensively. A tall and as slim as possible shape of the reaction container is preferred because this enables easier and more effective stirring and also improves the subsequent sedimentation.

A second transfer line leads from the reaction tank to the sedimentation tank, the second transfer line in the sedimentation tank preferably ending via a lateral wall opening at a height just above the sediment / liquid phase boundary.

The sedimentation container has a volume similar to that of the reaction container and can have a third transfer section with which the clear alkali solution standing above the sediment can be drawn off as close as possible to the liquid surface of the sediment container. This transmission path is preferably led back to the pickling bath (recycling), a filter being able to be interposed.

The invention is explained further below by way of example with reference to the drawing.

1 shows an embodiment of a system according to the invention with a pickling bath 10 which is equipped with a heating and / or cooling device 11. A first transmission path 12 leads from the pickling bath 10 and can be heat-insulated or heatable. The transmission path opens via a funnel-shaped inlet arrangement 21 into a reaction container 20 which is provided with a stirring device 22. The stirring device 22 can also be guided laterally from below at an angle of approximately 45 through the side edge of the reaction container 20 be. In the reaction container 20, the reaction solution is up to a liquid level 24.

The liquid is removed from the reaction container 20 with the aid of a second transfer section 23. The second transfer section 23 opens into a sedimentation container 30 through a lateral wall duct just above the phase boundary 34, which is formed by the liquid 32 and the sediment 31. The content of the sedimentation container 30 has a liquid level 33, in the immediate height or at most just below it a third transfer section 35 is arranged for discharging the clear liquid 32. The third transfer section 35 leads back to the pickling bath 10, a filter 36 and preferably a measuring section 37 being able to be interposed. A removal point for aluminum hydroxide is designated 38.

The media are conveyed using conventional feed pumps. A second reaction container for optional operation can be connected in parallel with the reaction container 20 shown in FIG.

In the exemplary embodiment shown here, the pickling bath 10 has a volume of approximately 12 cubic meters and the reaction or sedimentation container 30 each has a volume of approximately 8 cubic meters.

An example of the application of the method according to the invention are the measured value logs for the operating time of the plant for the period from March 1 to March 7, 1990, in which reactor I means reaction vessel 20 and reactor II sedimentation vessel 30. The concentrations of NaOH and aluminum in the pickling bath ("pickling") are 43.4 to 56.0 g / l and 33.2 to 40.5 g / l, that of gluconate 0.9 to 1.2 g / l. Although relatively little complexing agent is present, no precipitation of aluminum hydroxide in the pickling bath was observed.

Claims (17)

1. Process for regenerating alkali-containing aluminium pickling solutions to recover aluminium hydroxide, which comprises
   a pickling solution, which contains alkali metal hydroxide, aluminate and a complexing agent and further additives, being transferred with the aid of a first transfer section from a pickling bath into a reactor section consisting of at least one reactor, and aluminium being precipitated there, without the addition of acid, as aluminium hydroxide,
   characterized in that the complexing agent used is gluconate, sorbitol or other complexing sugar derivatives, phosphonates or polymeric or oligomeric acrylates, and in that

   a) the pickling solution in the pickling bath is at a higher temperature than in the reactor section, and in that

   b) the concentration of aluminate, alkali and complexing agent,

   c) the temperature of the pickling solution in the pickling bath and

   d) the temperature gradient of the pickling solution from the pickling bath to the reactor section are set in such a way that

   e) the pickling solution in the pickling bath, with respect to the precipitation of aluminium hydroxide, is undersaturated or metastably supersaturated, but is unstably supersaturated in the reactor section,

   f) the following concentrations being present in the pickling solution in the pickling bath: aluminate, specified as Al from 30 to 60 g/l alkali metal hydroxide, specified as sodium hydroxide equivalent from 30 to 60 g/l complexing agent, specified as gluconate equivalent from 0.1 to 5 g/l and

   g) the temperature of the pickling solution being from 40 to 90°C, and the temperature drop of the pickling solution from the pickling bath to the reactor section being from 5 to 70°C.
2. Process according to Claim 1, characterized in that the pickling solution in the reactor section is seeded.
3. Process according to any of the preceding claims, characterized in that seeding is carried out at a high nucleus density with intensive stirring.
4. Process according to any of the preceding claims, characterized in that the seeding is carried out with gibbsite.
5. Process according to any of the preceding claims, characterized in that the transfer of the pickling solution into the reactor section is carried out as an incorporeal introduction.
6. Process according to any of the preceding claims, characterized in that the pickling solution transferred into the first transfer section is seeded.
7. Process according to any of the preceding claims, characterized in that the first transfer section between pickling bath and reactor section is constructed so as to be thermally insulated and heated.
8. Process according to any of the preceding claims, characterized in that it is carried out continuously.
9. Process according to any of the preceding claims, characterized in that the base produced in the precipitation of aluminium hydroxide is recycled into the pickling bath.
10. Process according to Claim 9, characterized in that the alkaline solution, prior to being recycled into the pickling bath, is filtered.
11. Appliance for carrying out the process according to any of the preceding claims, comprising

    a) a pickling bath (10) holding a pickling solution

    b) a first transfer section (12) for transferring the pickling solution into a reactor section consisting of at least one reactor (20, 30),

    characterized by

    c) a heating and/or cooling device (11) by means of which the pickling solution in the pickling bath (10) can be heated to a temperature of from 40 to 90°C, so that it is at a higher temperature, in the range from 5 to 70°C, than in the reactor section.
12. Appliance according to Claim 11, characterized in that the first transfer section (12) is constructed so as to be insulated against heat loss and/or to be heatable.
13. Appliance according to any of Claims 11 to 12, characterized by a funnel-shaped inlet arrangement (21) which permits incorporeal transfer of the pickling solution into the reactor section.
14. Appliance according to any of Claims 11 to 13, characterized in that at least the first reactor is constructed, by means of a stirring device (22) arranged therein, as a stirred reaction vessel (20).
15. Appliance according to any of Claims 11 to 14, characterized by at least one sedimentation vessel (30), connected to the reaction vessel (20) by means of a second transfer section (23).
16. Appliance according to any of Claims 11 to 15, characterized by a third transfer section (35) running from the at least one sedimentation vessel (30) to the pickling bath (10).
17. Appliance according to any of Claims 11 to 16, characterized by a filter (36) incorporated in the third transfer section (35).

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