EP0832315B1 - Process for demetallising highly acid baths and use of said process for electropolishing special steel surfaces - Google Patents

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

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 demetallization process when electropolishing stainless steel surfaces (Stainless steel).

Electropolishing or electrolytic polishing is a process electrochemical metalworking, in which the Polishing metal is usually connected as an anode in a circuit becomes. The electrolyte consists of an acid or an acid mixture. From the metal to be polished outstanding bumps (tips, Burrs) superficially dissolved and thus polished the metal. So the previously matt metal is smoothed and shiny. As an electrolyte is used for stainless steels and carbon steels mostly phosphoric acid-sulfuric acid mixtures with additives of catalysts, inhibitors and the like.

When electropolishing, the objects to be polished are the on the corresponding support and contact elements or devices hanging or in baskets or the like, in the electrolyte, d. H. the polishing bath, sunk and after a certain polishing time out of this. After this Drain the bath liquid from the polished surfaces the treated objects are then in rinsing baths dipped to remove the electrolyte.

Electropolishing processes currently used for processing of stainless steels (stainless steel) predominantly low-water mixtures of concentrated phosphoric acid and sulfuric acid as the electrolyte. Regularly Electrolytes various organic and inorganic additives to improve the polishing effect, increase the current efficiency, Reducing the required current density and avoiding hexavalent chromium ions added to the rinse water.

Those removed from the workpiece surface during electropolishing Metal ions go into solution and accumulate there with time. All electrolytes used industrially today have the disadvantage that their effectiveness from a certain point Degree of metal enrichment decreases significantly. Then must the electrolyte is at least partially due to fresh electrolytes be supplemented or completely replaced. A reliable and economically reasonable regeneration process for one used electrolytes are not available in the prior art Available. Instead, the used electrolyte is disposed of. Due to the high content of heavy metals, the used electrolyte can be treated as special waste. The The same applies to those occurring during electropolishing Rinse water and the sludge resulting from its treatment. As a rule, the available landfill volume for hazardous waste is narrowly limited and disposal costs also increase - if not in some areas it's already difficult to impossible is to find a suitable landfill option - there is a significant need for a process that is less Disposal effort allowed.

In the prior art it has been assumed that it is straight this enrichment with metal ions is the electrolyte unusable. As a result, you have an electrolyte after reaching a certain metal content, mostly between 4 and 5 wt .-% of disposal supplied. Because the allowable salary of phosphates and sulfates in wastewater is usually very limited, the entire volume had to be used even less Neutralize acid. Overall, this disposal fell large amounts of mud.

In summary, this state of the art consequently exists Problems in that a) the effectiveness of the electropolishing bath decreases significantly with increasing metal accumulation and that b) the wastewater generated by electropolishing is a complex one Require disposal.

The optimal working range in the metal content more common Electrolytes are usually between 35 g / l and 70 g / l (2 - 4% by weight). The electrolytes are up to the state of the art to a metal content of approx. 100 g / l, this corresponds to approx. 6% by weight, workable. With a higher metal content the decreases Polish quality drastically. In order to maintain the ability to work, becomes part of the electrolyte enriched with metal ions removed and by fresh, metal-free electrolytes replaced. The enriched electrolyte is removed either continuously on the carryover of the surface of the machined workpieces the electropolishing bath in the subsequent rinsing process, or by direct withdrawal. The removed electrolyte is either over a suitable wastewater treatment plant or directly processed that the resulting wastewater to the sewers can be released while the solids because of their content heavy metals are generally disposed of as hazardous waste have to.

The invention is based on the idea that one with metal ions selectively enriched electrolytes the metal ions must withdraw if an electropolishing electrolyte without partial exchange be kept permanently functional by electrolyte should. Ordinary filtration processes (see DE-33 43 396 A1) are out of the question here, since in the course of a filtration yes only solid separated, not the concentration of metallic Ions is lowered. The continue according to the state of the Technique known for the separation of metal ions acidic solutions such as ion exchange, reverse osmosis, membrane electrolysis, Electrodialysis etc. cannot be done easily transferred to electropolishing electrolytes. The state of the art membranes used in electrodialysis for example, compared to highly concentrated acid mixtures resistant. In addition, diffusion layers are formed with phosphoric acid from which in particular a material transport of Can severely hinder metal ions. This diffusion layer works practically like a barrier layer. Consequently, in the prior art Technology electrochemical processes with highly concentrated acid Solutions not carried out. In general, there is even the idea that electrochemical process for the separation of iron are not suitable (see Ullmanns Encyclopedia of Industrial Chemistry, Vol.9, pp.227-230). It is also used for electrolytic Deposition of iron is usually an auxiliary electrolyte, for example dilute ammonium sulfate solution, required (cf. Kerti et al., Hungarian Journal of Industrial Chemistry, Vol. 1987, p.435ff), who applied the electropolishing electrolyte would destroy.

The aim of the present invention is thus a method that direct separation of metal ions including iron from the with the metal ions enriched electrolytes without that the electrolytes must be diluted significantly. The Concentration of the metal ions in the depleted electrolyte should ideally be adjusted so that the metal concentration the optimal working area is reached.

It has now surprisingly been found that under certain Demetalization may be required separately from the electropolishing bath can be carried out electrochemically. For this all that is required is a separate electrolysis cell known per se, which as a separating layer is a ceramic material, Plastic fleece or sintered material is used. Using of this material with a pore size between about 0.5 microns and 10 µm apparently a uniform layer forms in situ, which acts as a diaphragm. Theoretically, one can Diffusion layer enriched with phosphoric acid (approx. 1 - 5 µm) postulate that as such the passage of sulfate ions to the necessary charge exchange enables a "short circuit" by metal ions, especially iron ions, but excludes them. Effective diaphragms could be mixed with phosphoric acid / sulfuric acid achieved with a mixing ratio of 1:10 to 10: 1 become. Mixtures with a ratio are preferred Phosphoric to sulfuric acid from 2: 1 to 1: 2 used.

According to the invention, those concentrated with metal ions are concentrated Mixtures based on phosphoric acid and sulfuric acid electrochemically demetallized. The separation of the metal ions of the electrolyte takes place by means of the generated in situ Diaphragm. This means the pore size and structure of the partition no longer decisive for the effectiveness of the separation process and stable, relatively large-pored carrier media such as ceramics, Plastic fleece or sintered material are used, the Pores do not clog because of their size and none themselves have high diffusion resistance (about 0.5 - 10 µm). The Suitable material can easily be found on the basis of simple experiments find.

An electrolysis cell is used to carry out the method (Fig.1), their anodic and cathodic area are separated from each other by a porous partition. When putting on of direct current to the electrolyte to be demetallized filled cell is formed by migration of the sulfate ions in the anolytes on the side of the catholyte one of sulfate ions depleted diffusion layer with high phosphoric acid content from, which complicates the passage of the metal ions and as Separation medium works. The higher the content of phosphoric acid in the mixture the lower the exchange of metal ions in principle through the diaphragm. One can, however, permeability of the diaphragm due to the temperature and water content of the Affect electrolytes.

In the electrolyte, the dissolved iron originally mainly lies in Form of readily soluble Fe (III) ions before. These are in the cathode compartment reduced to much less soluble Fe (II) ions and then fall in when the solubility limit is reached Form of iron (II) sulfate (mostly as cathode sludge). This is easy through appropriate processes such as sedimentation, Separate filtration, centrifugation etc. from the electrolyte. At the same time, nickel and chrome are also deposited. Advantageous it has also been shown that impurities in the electrolyte, those that got into them during electropolishing, largely bound to the sludge and also separated. In order to will be an accumulation of these substances at higher concentration could interfere with the electropolishing process.

After the precipitation, the iron content of the electrolyte is in the Usually around 2.5% by weight and thus in the ideal working range. After adding the sulfuric acid consumed by the precipitation and setting the correct density is the cleaned electrolyte usable again.

The process works in a very wide mixing range of Phosphoric acid and sulfuric acid and is effective as soon as the metal content is over 40 g / l.

If the method according to the invention is combined with a device for the recovery of entrained electrolytes and cleaned Water from the rinse water, e.g. an evaporator in Connection with a suitable rinsing water supply is a waste water-free one Operation of electropolishing systems possible (Fig. 2).

The sludge from the process contains the separated sludge Metals in high concentration. He can after appropriate treatment may be used for further use. With that created the conditions to avoid the generation of hazardous waste, which places a heavy burden on the landfills and high disposal costs caused.

In another aspect, the invention relates to a method for demetallizing mixtures which are essentially phosphoric acid and contain sulfuric acid, the one enriched with metal ions Mixture is transferred to an electrolytic cell the Fe (III) ions are reduced to Fe (II) ions and these are then Form of Fe (II) sulfate can be precipitated. Through this procedure can be separated from an ongoing electropolishing process (independent thereof) a regeneration of highly acidic electropolishing baths can be achieved.

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 method of the invention can be in terms of actual electropolishing subsequent process stages optimize. In particular, it is possible to use electropolishing subsequent rinsing so that the rinse water using a cascade rinse with rinsing water regeneration (Evaporator) in a closed circuit. The electrolyte recovered from the rinse water can then be used again be fed into the process. These various advantages of the invention Processes would not be with the prior art to be realized. In principle, one would already have one can think of the rinse water by distillation. This but would hardly have been of any advantage, since it would have been considerable Energy use would have had no real advantages. Only through the invention is a rinse water regeneration reasonably usable. Because only then will it be reusable Acid gained for the electrolyte. In the prior art the rinse water regularly together with the acid - after its Neutralization - rejected.

The metal salts separated from the electrolyte during filtration contain the heavy metals in high concentration. she can, for example, be fed directly to an smelting process become. Through a treatment downstream of the filtration such as e.g. B. Rinsing with ice water can the metal salts as far from adhering acid residues are cleaned that a safe Handling is possible.

The method according to the invention is known per se Arrangement for electrolytic polishing with a separate one electrochemical cell including the diaphragm and agent performed for filtering the electrolysis bath. Usually include these means feeds and discharges, which are constant or discontinuous return of the electrolyte solution in the polishing process enable.

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 which uses the method according to the invention.

1 shows a demetallization device as it is used externally but also integrated in an electropolishing process can be. The electrolyte is fed through suitable leads into the electrolytic cell continuously or discontinuously performed and subjected to electrolysis there. With electrolysis Fe (III) ions are reduced to Fe (II) ions and if exceeded a certain limit concentration (which by the Ion product is determined) precipitated as iron sulfate. There in Electropolishing baths generally have high sulfate concentrations, 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 essentially that Iron sulfate is deposited. In addition to the iron sulfate other poorly soluble metal salts are also separated from the solution, like those of chrome, nickel, or molybdenum Copper. The filtrate can then be transferred directly to an electropolishing device to be led back. Refreshing is often an option with phosphoric acid and / or sulfuric acid. This is generally not because of the suggested circulatory procedure required.

The process flow diagram shown in FIG. 2 illustrates the particular advantages of the procedure according to the invention. Because both the electrolyte and the rinse water can be reused can, works optimally a system according to the invention practically wastewater-free. Workpieces that are electropolished have been subjected to, in a rinse stage (economy sink) in essentially rinsed with water. The waste water from the economy sink can then an evaporator can be fed to the electrolyte by distillation separates from the rinse water so that both can be reused separately can be. When the electrolyte is electropolished has reached a certain metal concentration in the Readjust the electropolishing effect. To prevent this condition or to regenerate the electropolishing ability Electrolyte from the electrolysis bath continuously or discontinuously a separate demetallization fed. In the As described above, demetallization is electrochemical Fe (III) reduced to Fe (II) and the iron content essentially precipitated as Fe (II) sulfate. In the subsequent filtration then a sludge is obtained which is another external one Refurbishment can be supplied. At the same time, a regenerated Get electrolyte in the electropolishing process is returned. The external one shown here in Fig.2 Refurbishment is not essential to over a long period Period a continuous wastewater-free electropolishing system keep in operation. But it has certain advantages because acid components were also recovered from this external treatment which can then flow back into the electropolishing stage.

The process according to the invention is illustrated by the following examples explained in more detail.

Examples

There were several electrolyte solutions with those given below Prepared compositions. These electrolytes were a method according to the invention and comparatively a method subject to the state of the art. It was found, that in a process according to the invention with a continuous separate electrolysis and filtration of the electrolyte and Returning the filtrate to the electrolyte is not just constant Polishing results could 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 batchwise, with only the cathode space being filled with electrolyte after the filtrate had been returned from the cathode space 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: 85% phosphoric acid 60.0% by weight 96% sulfuric acid 36.0% by weight Morpholinomethane diphosphoric acid 1.0% by weight Diethanolamine 0.5% by weight water 2.5% by weight Electrolyte 2: 85% phosphoric acid 54.0% by weight 96% sulfuric acid 43.0% by weight Morpholine 1.0% by weight Diisopropanolamine 0.5% by weight water 1.5% by weight Electrolyte 3: 85% phosphoric acid 56.0% by weight 96% sulfuric acid 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.

The electrolyte was in the bypass flow during the entire operating time fed to the electrolysis cell described above and filtered, so that the total bath volume depending on the bath load every 3 up to 14 days. The through the sludge discharge Losses of chemicals caused have been added. It revealed a steady state of the electrolyte with a total content on metals (mainly iron, chromium and nickel) of 2.5 up to 4% by weight. The electrolyte remained functional and that The results achieved corresponded to the quality expectations the current state of the art. After reaching the stationary The state of the electrolyte became the whole when electropolishing Amount of metal removed immediately during electrolysis precipitated as metal salt sludge and via the filter circuit in concentrated form removed from the electrolyte.

Separately from the above investigations was also consumed Electrolytic solution of different composition demetallized. The electrolytic cell corresponded to the above information. It was found that with a wide variety of compositions the as typical examples of electropolishing solutions are considered can, successful demetallization is achieved and that the electropolishing solutions have been successfully regenerated.

Examples:

1) Dichte 1.760 H₂SO₄ 35.1 Gew.-% H₃PO₄ 37.8 Gew.-% Eisen 4.5 Gew.-% 82 g/l Die Entmetallisierung erfolgte bei 60°C // 3 V // 1,5 A/1 // 20 Stunden. Es wurde ein Elektrolyt folgender Zusammensetzung erhalten: Dichte 1.675 H₂SO₄ 31 Gew.-% H₃PO₄ 38 Gew.-% Eisen 2.5 Gew.-% 41 g/l

2) Dichte 1.760 H₂SO₄ 21 Gew.-% H₃PO₄ 43 Gew.-% Eisen 4.5 Gew.-% 80 g/l Entmetallisierung: 60°C // 2.5 V // 1.2 A/1 // 20 Stunden Dichte 1.610 H₂SO₄ 17.6 Gew.-% H₃PO₄ 45 Gew.-% Eisen 2.5 Gew.-% 37 g/l

3) Dichte 1.750 H₂SO₄ 40.5 Gew.-% H₃PO₄ 26.5 Gew.-% Eisen 5 Gew.-% 89 g/l

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

1) density 1,760 H ₂ SO ₄ 35.1% by weight H ₃ PO ₄ 37.8% by weight iron 4.5% by weight 82 g / l The demetallization was carried out at 60 ° C // 3 V // 1.5 A / 1 // 20 hours. An electrolyte of the following composition was obtained: density 1,675 H ₂ SO ₄ 31% by weight H ₃ PO ₄ 38% by weight iron 2.5% by weight 41 g / l

2) density 1,760 H ₂ SO ₄ 21% by weight H ₃ PO ₄ 43% by weight iron 4.5% by weight 80 g / l Demetallization: 60 ° C // 2.5 V // 1.2 A / 1 // 20 hours density 1,610 H ₂ SO ₄ 17.6% by weight H ₃ PO ₄ 45% by weight iron 2.5% by weight 37 g / l

3) density 1,750 H ₂ SO ₄ 40.5% by weight H ₃ PO ₄ 26.5% by weight iron 5% by weight 89 g / l

Demetallization: 60 ° C // 3 V // 1.5 A / 1 // 18 hours density 1,675 H ₂ SO ₄ 35.1% by weight H ₃ PO ₄ 28.5% by weight iron 2.5% by weight 42 g / l

After adding the sulfuric acid consumed by precipitation and adjusting The electrolytes are the density to the required values easy to use again.

Claims (4)

1. Process for the demetallization of mixtures which essentially contain phosphoric acid and sulphuric acid, in which the mixture is fed to an electrolysis cell having a partition, comprised of a material with a pore size of between 0.5 µm and 10 µm and being such that when direct current is applied a diaphragm is formed in situ which impedes the passage of metal ions, between the anodic and cathodic regions of the electrolysis cell and in which the Fe(III) ions present in the mixture are reduced to Fe(II) ions and, on reaching the solubility limit, are precipitated as FeSO₄ and the precipitates are removed.
2. Use of a process according to Claim 1 in the electropolishing of stainless-steel surfaces, in which

   a sulphuric acid/phosphoric acid mixture is used as electrolyte,

   the electrolyte is continuously or discontinuously subjected separately to an electrolysis, in which Fe(III) ions are reduced to Fe(II) ions and

   precipitates which occur are filtered off and the filtrate is fed back into 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 fed back.

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