EP0832315A1

EP0832315A1 - Process for demetallising highly acid baths and use of said process for electropolishing special steel surfaces

Info

Publication number: EP0832315A1
Application number: EP96921930A
Authority: EP
Inventors: Razmik Abedian; Olaf BÖHME; Siegfried Piesslinger-Schweiger
Original assignee: Poligrat GmbH
Current assignee: Poligrat GmbH
Priority date: 1995-06-09
Filing date: 1996-06-04
Publication date: 1998-04-01
Anticipated expiration: 2016-06-04
Also published as: TW358831B; ES2129268T3; CA2226367A1; EP0832315B1; ATE178106T1; CZ396197A3; AU6300596A; DE59601506D1; DE19521132C1; US5882500A; WO1996041905A1; JP2000512685A

Abstract

PCT No. PCT/EP96/02439 Sec. 371 Date Mar. 4, 1998 Sec. 102(e) Date Mar. 4, 1998 PCT Filed Jun. 4, 1996 PCT Pub. No. WO96/41905 PCT Pub. Date Dec. 27, 1996The invention relates to a process for the demetallization of highly acidic baths based on phosphoric acid and sulphuric acid, and also to a process for the electropolishing of stainless-steel surfaces, in which a regeneration of, in particular, spent electrolyte compositions for electropolishing can be achieved by separate electrolytic reduction of Fe(III) to Fe(II) and subsequent removal of precipitates.

Description

Process for demetallizing highly acidic baths and use of this process in the electropolishing of stainless steel surfaces

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. When electropolishing, 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. In the case of stainless steels and carbon steels, the electrolytes used are mostly phosphoric acid-sulfuric acid mixtures with additions of catalysts, inhibitors and the like.

In electropolishing, 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 metal ions removed from the surface of the workpiece during electropolishing go into solution and accumulate there over time. All electrolytes which are used industrially today have the disadvantage that their effectiveness decreases considerably from a certain level of metal accumulation. Then the electrolyte must be at least partially supplemented with fresh electrolyte or completely replaced. A reliable and economically reasonable regeneration process for a used electrolyte is not available in the prior art. Instead, the used electrolyte is disposed of. Due to the high content of heavy metals, the used electrolyte must be treated as special waste. The same applies to the rinse water generated during electropolishing and the sludge generated during the treatment. Since the available landfill volume for hazardous waste is generally very limited and, moreover, disposal costs increase - if it is not already difficult or impossible to find a suitable landfill option in some areas - there is a considerable need for a process which requires less disposal enables.

It has been assumed in the prior art that it is precisely this enrichment with metal ions which renders the electrolyte unusable. As a result, an electrolyte, after reaching a certain metal content, usually between 4 and 5% by weight, has been disposed of. Since the permissible Geha of phosphates and sulfates in wastewater is usually very limited, the entire volume of unused acid had to be neutralized. Overall, large amounts of sludge were generated during this disposal.

In summary, there are problems with this prior art that a) the effectiveness of the electropolishing bath decreases significantly with increasing metal accumulation and b) the wastewater produced during electropolishing requires complex disposal.

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. In order to maintain the ability to work, 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 (see DE-33 43 396 AI) 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 processes for separating metal ions from acidic solutions, such as ion exchange, reverse osmosis, membrane electrolysis, electrodialysis, etc., which are also known from the prior art, cannot be easily transferred to electropolishing electrolytes. The membranes commonly used in the prior art for electrodialysis are, for example, not resistant to highly concentrated acid mixtures. In addition, 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. In general, there is even the position that electrochemical processes for separating ice are not suitable (cf. Ull ann's Encyclopedia of Industrial Kitchen, Vol.9, pp. 227-230). In addition, 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. The 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.

It has now surprisingly been found that demetallization can be electrochemically carried out separately from the electropolishing bath under certain circumstances. For this all that is required is a separate electrolytic cell known per se, which uses a ceramic material, plastic fleece or sintered material as the separating layer. When using this material with a pore size between approximately 0.5 μm and 10 μm, a uniform layer appears to form in situ, which acts as a diaphragm. Theoretically, a diffusion layer (about 1-5 μm) enriched with 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.

According to the invention, 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. Thus, 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 μm). The appropriate material can be easily found using simple experiments.

To carry out the method, an electrolytic cell (FIG. 1) is used, the anodic and cathodic regions of which are separated from one another by a porous partition. When 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. In principle, the higher the phosphoric acid content in the mixture, the lower the exchange of metal ions through the diaphragm. However, 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.

If you combine the method according to the invention with a device for recovering entrained electrolytes and purified water from the rinsing water, such as an evaporator in conjunction with a suitable rinsing water system, wastewater-free operation of electropolishing systems is possible (FIG. 2). The sludge resulting from the process contains the separated metals in high concentration. After appropriate treatment, it can possibly be used for further use. This creates the prerequisites for avoiding the generation of hazardous waste, which places a heavy burden on the landfills and causes high disposal costs.

According to another aspect, 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. With this method, regeneration of highly acidic electropolishing baths can be achieved separately from an ongoing electropolishing process (irrespective of this).

The electrolytic process conditions of the process according to the invention largely correspond to those of the prior art. For example, 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. In particular, it is possible to design the rinsing processes following the electropolishing in such a way that the rinsing water is conducted in a closed circuit using a cascade rinsing with rinsing water regeneration (evaporator). The electrolyte recovered from the rinse water can then be returned to the process. These various advantages of the method according to the invention would not have been possible to achieve with the prior art. In principle, one could have already thought of a reprocessing of the rinse water by distillation. This but would hardly have brought any advantages, since there would have been no real advantages over a considerable amount of energy. Only through the invention can a rinsing water regeneration be used properly. This is the only way to obtain a reusable acid for the electrolyte. In the prior art, the rinse water was regularly discarded together with the acid - after its neutralization.

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.

1 shows a schematic structure of a demetallization device and illustrates the essential electrochemical reactions.

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. In the electrolysis, 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. In addition to iron sulfate, other poorly soluble metal salts are also separated from the solution, such as those of chromium, nickel, molybdenum or copper. The filtrate can then be returned directly to an electropolishing device. Refreshing with phosphoric acid and / or sulfuric acid is often appropriate. However, this is generally not necessary due to the circulatory procedure indicated.

The process flow diagram shown in 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. In order to prevent this condition or to regenerate the electropolishing ability, the electrolyte from the electrolysis bath is fed continuously or discontinuously to a separate demetallization. In the demetallization, as described above, Fe (III) is electrochemically reduced to Fe (II) and the iron content is essentially precipitated as Fe (II) sulfate. In the subsequent filing A sludge is then obtained which can be fed to a further external work-up. At the same time, 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.

The method according to the invention is explained in more detail with the aid of the following examples.

Examples

Several electrolyte solutions were prepared with the compositions given below. These electrolytes were subjected to a method according to the invention and comparatively to a method according to the prior art. It was found that in a process according to the invention, with continuous separate electrolysis and filtration of the electrolyte and recycling of the filtrate in the electrolyte, not only could constant polishing results be achieved, but that these were retained over a longer period of time.

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 μm 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:

Phosphoric acid 85% 60.0% by weight

Sulfuric acid 96% 36.0% by weight

Morpholinomethane diphosphoric acid 1.0% by weight

Diethanolamine 0.5% by weight

Water 2.5% by weight

Electrolyte 2:

Phosphoric acid 85% 54.0% by weight sulfuric acid 96% 43.0% by weight morpholine 1.0% by weight diisopropanolamine 0.5% by weight water 1.5% by weight

Electrolyte 3:

Phosphoric acid 85% 56.0% by weight sulfuric acid 96% 40.0% by weight nicotinic acid 1.5% by weight diisopropanolamine 0.5% by weight water 2.0% by weight

Various types of stainless steel were electropolished with the above-mentioned electrolytes at an electrolyte temperature of 45-80 ° C. and a current density of 5-25 A / dm ² with subsequent rinsing of the parts in a multi-stage rinsing cascade with rinsing water return. The rinse water from the first, most concentrated rinse stage was concentrated in a side stream by distillation and the concentrate was returned to the electropolishing bath. The pure condensate water was used for final rinsing in the rinsing cascade, which closed the rinsing water circuit.

During the entire operating time, the electrolyte was supplied and filtered in the bypass flow to the electrolysis cell described above, so that the entire bath volume was circulated once every 3 to 14 days, depending on the bath load. The through the sludge discharge O 96/41905 PCT / EP96 / 02439

- 12 -

Losses of chemicals caused have been added. The electrolyte was in a steady state with a total content of metals (predominantly iron, chromium and nickel) of 2.5 to 4% by weight. The electrolyte remained functional and the results achieved corresponded to the quality expectations according to the current state of the art. After the stationary state of the electrolyte had been reached, the entire amount of metal removed during the electropolishing was precipitated as metal salt sludge immediately during the electrolysis and was removed from the electrolyte in concentrated form via the filter circuit.

Separate from the above investigations, used 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.

Execution examples;

Used electrolytes of the following composition were demetallized. A polypropylene sintered material was used as the partition (Vyon T; 1.5 mm thick, pore diameter 0.3 - 5 μm).

1) Density: 1,760 H ₂ S0 ₄ : 35.1% by weight H ₃ P0 ₄ : 37.8% by weight iron: 4.5% by weight 82 g / 1

The demetallization was carried out at 60 ° C // 3 V // 1.5 A / l // 20 hours. An electrolyte of the following composition was obtained: Density: 1,675

H ₂ S0 ₄ : 31% by weight

H ₃ P0 ₄ : 38% by weight

Iron: 2.5% by weight 41 g / 1

2) Density: 1,760

H ₂ S0 ₄ : 21% by weight

H ₃ P0 ₄ : 43% by weight

Iron: 4.5% by weight 80 g / 1

Demetallization: 60 ° C // 2.5 V // 1.2 A / l // 20 hours

Density: 1,610

Iron: 2.5% by weight 37 g / 1

3) Density: 1,750

Iron: 5% by weight 89 g / 1

Demetallization: 60 ° C // 3 V // 1.5 A / l // 18 hours

Density: 1,675

H ₂ S0 ₄ : 35.1% by weight

H ₃ P0 ₄ : 28.5% by weight

Iron: 2.5% by weight 42 g / 1

After adding the sulfuric acid consumed by precipitation and adjusting the density to the required values, the electrolytes can be used again without problems.

Claims

Patent claims

1. A process for demetallizing mixtures which essentially contain phosphoric acid and sulfuric acid, the mixture being fed to an electrolysis cell with a partition between the anodic and cathodic region of the electrolysis cell and the Fe (III ) - ions are reduced to Fe (II) ions and, when the solubility limit is reached, precipitated as FeSO and the precipitates are separated off.

2. Use of a method according to claim 1 in the electropolishing of stainless steel surfaces, in which

a sulfuric acid / phosphoric acid mixture is used as the electrolyte,

- The electrolyte is subjected to electrolysis separately continuously or discontinuously, Fe (III) ions being reduced to Fe (II) ions and

- precipitates are filtered off and the filtrate is returned to the electrolyte.

3. Use according to claim 2, characterized in that in the rinsing process following the electropolishing, the rinsing water is circulated.

4. Use according to claim 3, characterized in that the electrolyte obtained from the rinsing water circuit is recycled.