Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F7/00—Constructional parts, or assemblies thereof, of cells for electrolytic removal of material from objects; Servicing or operating
  • C25F7/02—Regeneration of process liquids

Definitions

• the invention relates to a method for demetallizing highly acidic baths based on phosphoric and sulfuric acid.
• the invention further relates to the use of a demetalizing method in the electropolishing of stainless steel surfaces (stainless steel).
• Electropolishing or electrolytic polishing is a method of electrochemical metalworking, in which the metal to be polished is generally connected to an electrical circuit as an anode.
• the electrolyte consists of an acid or a mixture of acids.
• outstanding bumps (peaks, burrs) of the metal to be polished are dissolved on the surface and the metal is polished. In this way, the previously matt metal is smoothed and shiny.
• the electrolytes used are mostly phosphoric acid-sulfuric acid mixtures with additions of catalysts, inhibitors and the like.
• the objects to be polished which hang on the corresponding carrying and contact elements or devices or are received in baskets or the like, are immersed in the electrolyte, ie the polishing bath, and lifted out of the latter after a certain polishing time. After the bath liquid has drained off the polished surfaces, the treated objects are then immersed in rinsing baths in order to remove the electrolyte.
• Electropolishing processes which are currently used industrially for processing stainless steels (stainless steel) predominantly use low-water mixtures of concentrated phosphoric acid and sulfuric acid as electrolytes.
• Various organic and inorganic additives to improve the polishing effect increase the current efficiency, reduce the required current density and avoid hexavalent chromium ions in the rinse water are regularly added to the electrolyte.
• the optimum working range in the metal content of common electrolytes is usually between 35 g / 1 and 70 g / 1 (2 - 4% by weight). According to the prior art, the electrolytes are capable of working up to a metal content of approx. 100 g / 1, this corresponds to approx. 6% by weight. With a higher metal content, the polishing quality drops drastically.
• part of the electrolyte enriched with metal ions is removed and replaced by fresh, metal-free electrolyte.
• the enriched electrolyte is removed either continuously via the carryover of the electrolyte located on the surface of the machined workpieces from the electropolishing bath into the subsequent rinsing process, or by direct removal.
• the removed electrolyte is either processed using a suitable waste water treatment plant or directly so that the resulting waste water can be discharged into the sewage system, while the solids generally have to be disposed of as hazardous waste because of their heavy metal content.
• the invention is based on the idea that the metal ions must be selectively removed from the electrolyte enriched with metal ions if an electropolishing electrolyte is to be kept permanently functional without partial exchange of the electrolyte.
• Ordinary filtration processes are out of the question for this, since in the course of a filtration only solid is separated off, the concentration of metallic ions is not reduced.
• the membranes commonly used in the prior art for electrodialysis are, for example, not resistant to highly concentrated acid mixtures.
• diffusion layers are formed with phosphoric acid, which in particular can strongly hinder the transport of materials by metal ions.
• This diffusion layer acts practically like a barrier layer. Consequently, electrochemical processes with strongly concentrated acid solutions are not carried out in the prior art.
• electrochemical processes for separating ice are not suitable (cf. Ull ann's Encyclopedia of Industrial Kitchen, Vol.9, pp. 227-230).
• an auxiliary electrolyte for example dilute ammonium sulfate solution, is usually required for the electrolytic deposition of iron (cf. Kerti et al., Hungarian Journal of Industrial Chemistry, Vol. 1987, pp. 435ff), which, when used, uses the electropolishing electro - would destroy lytes.
• the aim of the present invention is thus a process which enables the direct separation of metal ions, including iron, from the electrolytes enriched with the metal ions, without the electrolytes having to be diluted appreciably.
• concentration of the metal ions in the depleted electrolyte should ideally be set so that the optimum working range is achieved with regard to the metal concentration.
• demetallization can be electrochemically carried out separately from the electropolishing bath under certain circumstances.
• a separate electrolytic cell known per se, which uses a ceramic material, plastic fleece or sintered material as the separating layer.
• a uniform layer appears to form in situ, which acts as a diaphragm.
• a diffusion layer about 1-5 ⁇ m
• phosphoric acid can be postulated which, as such, allows sulfate ions to pass through for the necessary charge exchange, but excludes a "short circuit" by metal ions, especially iron ions.
• Effective diaphragms could be achieved with phosphoric acid / sulfuric acid mixtures with a mixing ratio of 1:10 to 10: 1. Mixtures with a ratio of phosphoric to sulfuric acid of 2: 1 to 1: 2 are preferably used.
• the concentrated mixtures enriched with metal ions based on phosphoric acid and sulfuric acid are electrochemically demetallized.
• the metal ions are separated from the electrolyte by means of the diaphragm which is formed in situ.
• the pore size and structure of the partition are no longer decisive for the effectiveness of the separation process, and stable, relatively large-pored carrier media such as ceramic, plastic fleece or sintered material can be used, the pores of which do not become blocked due to their size and which do not themselves have a large diffusion resistance have (about 0.5 - 10 ⁇ m).
• the appropriate material can be easily found using simple experiments.
• an electrolytic cell (FIG. 1) is used, the anodic and cathodic regions of which are separated from one another by a porous partition.
• direct current is applied to the cell filled with the electrolyte to be demetallized, migration of the sulfate ions in the anolyte on the side of the catholyte forms a diffusion layer depleted in sulfate ions with a high phosphoric acid content. endures, which impedes the passage of the metal ions and acts as a separation medium.
• the higher the phosphoric acid content in the mixture the lower the exchange of metal ions through the diaphragm.
• the permeability of the diaphragm can be influenced by the temperature and the water content of the electrolyte.
• the dissolved iron in the electrolyte is initially predominantly in the form of readily soluble Fe (III) ions. These are reduced in the cathode space to substantially less soluble Fe (II) ions and then precipitate out in the form of iron (II) sulfate when the solubility limit is reached (mostly as cathode sludge). This can easily be separated from the electrolyte by appropriate processes such as sedimentation, filtration, centrifugation, etc. At the same time, nickel and chrome are also deposited. It has also been shown to be advantageous that impurities in the electrolyte which have reached it during electropolishing are largely bound to the sludge and likewise separated off. Cumulation of these substances, which could interfere with the electropolishing process at a higher concentration, is thus avoided.
• the iron content of the electrolyte is usually around 2.5% by weight after precipitation and is therefore in the ideal working range. After adding the sulfuric acid consumed by the precipitation and setting the correct density, the cleaned electrolyte can be used again.
• the process works in a very wide mixing range of phosphoric acid and sulfuric acid and can be used effectively as soon as the metal content is above 40 g / 1.
• the invention relates to a process for demetallizing mixtures which essentially contain phosphoric acid and sulfuric acid, the mixture enriched with metal ions being transferred to an electrolysis cell in which Fe (III) ions form Fe (II ) Ions are reduced and these are then precipitated in the form of Fe (II) sulfate.
• Fe (III) ions form Fe (II ) Ions are reduced and these are then precipitated in the form of Fe (II) sulfate.
• polishing stainless steel works with a current density of 5 - 50 A / dm 2 , preferably about 10 - 25 A / dm 2 , at about 40 - 80 ° C and a polishing time of approx. 15 min.
• the process according to the invention can be further optimized with regard to the process stages following the actual electropolishing.
• the electrolyte recovered from the rinse water can then be returned to the process.
• the metal salts separated off from the electrolyte during filtration contain the heavy metals in high concentration. For example, they can be sent directly to an smelting process. By a treatment downstream of the filtration such. B. rinsing with ice water, the metal salts can be cleaned from the adhering acid residues to the extent that safe handling is possible.
• the method according to the invention is carried out in a known arrangement for electrolytic polishing with a separate electrochemical cell including the diaphragm and means for filtering the electrolysis bath.
• These means usually comprise feed and discharge lines which enable the electrolyte solution to be returned continuously or discontinuously to the polishing process.
• FIG. 1 shows a schematic structure of a demetallization device and illustrates the essential electrochemical reactions.
• FIG. 2 shows a process flow diagram of a wastewater-free electropolishing system that uses the method according to the invention
• Fig.l shows a demetallization device as it can be used externally but also integrated in an electropolishing process.
• the electrolyte is continuously or discontinuously fed into the electrolytic cell via a suitable feed line performed and subjected to electrolysis there.
• Fe (III) ions are reduced to Fe (II) ions and, if a certain limit concentration (which is determined by the ion product) is exceeded, precipitated as iron sulfate. Since there are generally high sulfate concentrations in electropolishing baths, the Fe (II) is precipitated practically quantitatively as sulfate.
• the slurry or suspension from the electrolytic cell is then fed to a filter in which the iron sulfate is essentially separated.
• FIG. 2 illustrates the particular advantages of the procedure according to the invention. Since both the electrolyte and the rinsing water can be reused, an installation according to the invention optimally works practically without waste water. Workpieces that have been subjected to electropolishing are essentially rinsed with water in a rinsing stage (economy sink). The waste water from the economy sink can then be fed to an evaporator, which separates the electrolyte from the washing water by distillation, so that both can be reused separately. If the electrolyte has reached a certain metal concentration in the electropolishing process, the electropolishing effect usually diminishes.
• the electrolyte from the electrolysis bath is fed continuously or discontinuously to a separate demetallization.
• Fe (III) is electrochemically reduced to Fe (II) and the iron content is essentially precipitated as Fe (II) sulfate.
• a sludge is then obtained which can be fed to a further external work-up.
• a regenerated electrolyte is obtained which is returned to the electropolishing process.
• the external workup shown here in FIG. 2 is not absolutely necessary in order to keep a continuous wastewater-free electropolishing system in operation for a long period of time. However, it has certain advantages, since acid components can also be recovered from this external workup, which then flow back into the electropolishing stage.
• Electrolyte 1 An electrolysis cell was used which could hold a volume of approximately 10 liters. A porous ceramic plate with a pore size of approximately 1.0 ⁇ m was used as the separating material. The separate electrolysis was carried out discontinuously in batches, only the cathode compartment being filled with electrolyte after the filtrate had been returned from the cathode compartment of the electrolysis cell to the electropolishing device. The temperature was set to 60 ° C. and the voltage was 3 V. Carbon pins and stainless steel sheets were used as electrodes. Electrolyte 1:
• Morpholinomethane diphosphoric acid 1.0% by weight
• electrolyte solution of various compositions was also demetallized.
• the electrolytic cell corresponded to the above information. It was found that the most typical examples of electropolishing solutions can be viewed with a wide variety of compositions, successful demetallization is achieved and since the electropolishing solutions have been successfully regenerated.
• the electrolytes After adding the sulfuric acid consumed by precipitation and adjusting the density to the required values, the electrolytes can be used again without problems.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)
• ing And Chemical Polishing (AREA)
• Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

Applications Claiming Priority (3)

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• 1995
  • 1995-06-09 DE DE19521132A patent/DE19521132C1/de not_active Expired - Fee Related
• 1996
  • 1996-05-30 TW TW085106450A patent/TW358831B/zh active
  • 1996-06-04 EP EP96921930A patent/EP0832315B1/fr not_active Expired - Lifetime
  • 1996-06-04 AU AU63005/96A patent/AU6300596A/en not_active Abandoned
  • 1996-06-04 DE DE59601506T patent/DE59601506D1/de not_active Expired - Fee Related
  • 1996-06-04 ES ES96921930T patent/ES2129268T3/es not_active Expired - Lifetime
  • 1996-06-04 US US08/973,700 patent/US5882500A/en not_active Expired - Lifetime
  • 1996-06-04 WO PCT/EP1996/002439 patent/WO1996041905A1/fr not_active Application Discontinuation
  • 1996-06-04 CA CA002226367A patent/CA2226367A1/fr not_active Abandoned
  • 1996-06-04 CZ CZ973961A patent/CZ396197A3/cs unknown
  • 1996-06-04 JP JP09502586A patent/JP2000512685A/ja active Pending
  • 1996-06-04 AT AT96921930T patent/ATE178106T1/de not_active IP Right Cessation

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