Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/36—Regeneration of waste pickling liquors

Definitions

• 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.
• DE-PS 43 977 describes a process for the preparation of alumina hydrate and alkali aluminate, in which essentially pure alkali aluminate solutions by inoculation
• Aluminum hydroxide is precipitated. This procedure was later improved by further developments, although the further developments essentially used pure aluminate solutions.
• EP-AI 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.
• 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 event 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 12.
• the pickling solution in the pickling bath must be undersaturated or metastably oversaturated, the concentrations of aluminate, alkali and gluconate in the pickling bath and the temperature being 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.
• the temperature of the pickling bath is in the range from 40 to 90 ° C., preferably in the range from 45 to 55 ° C.
• heating of the pickling solution is generally necessary, especially when the pickling bath is subjected to low loads.
• Low load is understood to mean a small amount of aluminum over time, which is to be converted into NaAlO 2 . 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.
• 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 already 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.
• 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.
• 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.
• e.g. - also by disembodied transfer - ensure that the crystallization nuclei introduced into the first transfer section cannot penetrate the pickling bath.
• 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”.
• 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.
• 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.
• 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 develops 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.
• 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, particularly preferably 50 to 250 ml / l gibbsite slurry per l reactor solution. Precipitation rates of up to 30 g aluminum per liter per day were achieved.
• 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.
• 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 non-stirred sedimentation tank, which is covered with a strongly alkaline aqueous solution.
• the base concentration and thus the pH increases 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).
• the transfer of the reactor solution from the reaction container into the sedimentation container takes place in such a way that the reactor solution in the cementation 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.
• 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 alkaline 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.
• 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.
• 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 of crime; however, it should not be in the nucleation area yet;
• the pickling solution should contain complexing agents in an amount that 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;
• a high germ density i.e. there is 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 subsequent 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 returned to the pickling bath after filtration (recycling), it being possible to interpose a further sedimentation container.
• 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.
• 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 acidimetrically.
• This method can also be used to determine aluminum in addition to alkali hydroxide.
• the COD measurement or another oxidative determination method is suitable as an indirect determination, in which the organic Koh lenstoff is oxidized to CO 2 .
• measuring methods can be used that provide a sum parameter, such as the measurement of the electrolytic conductivity.
• the following analytical analytical methods are particularly preferred: a) alkali hydroxide and aluminum
• any nitrite 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 procedure 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.
• 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 are then included in the concentration calculation.
• specific analytical methods known from the literature can be used to determine the concentration.
• 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 section leads from the pickling bath to a reactor section consisting of at least one reactor.
• the first transfer route 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 immerse in 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.
• 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 tank
• 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.
• 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 section leads from the reaction container to the sedimentation container, the second transfer section in the sedimentation container preferably ending at a height just above the phase boundary sediment / liquid via a lateral wall opening.
• 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.
• 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 his.
• 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 line 35 is arranged for draining 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.
• a second reaction container for optional operation can be connected in parallel with the reaction container 20 shown in FIG.
• 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.
• concentrations of NaOH and aluminum in the pickling bath 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.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• ing And Chemical Polishing (AREA)

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• 1990
  • 1990-03-15 DE DE4008379A patent/DE4008379A1/de active Granted
• 1991
  • 1991-03-15 AT AT91906096T patent/ATE115197T1/de active
  • 1991-03-15 JP JP91505740A patent/JPH05505852A/ja active Pending
  • 1991-03-15 DE DE59103815T patent/DE59103815D1/de not_active Expired - Fee Related
  • 1991-03-15 WO PCT/EP1991/000491 patent/WO1991014019A2/de not_active Ceased
  • 1991-03-15 EP EP91906096A patent/EP0519987B1/de not_active Expired - Lifetime

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