Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Patent
1998-03-04
1999-03-16
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
205650, 205673, 205771, C25F 702
Patent
active
058825002
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/EP96/02439 filed Jun. 4, 1996.
The invention relates to a process for the demetallization of highly acidic baths based on phosphoric and sulphuric acid.
The invention furthermore relates to the use of a demetallization process in the electropolishing of stainless-steel surfaces (non-rusting steel).
Electropolishing or electrolytic polishing is an electrochemical metal treatment process in which the metal to be polished is, as a rule, connected as anode in an electrical circuit. In this connection, the electrolyte is composed of an acid or an acid mixture. During the electropolishing, projecting irregularities (peaks, burrs) are superficially dissolved from the metal to be polished and the metal is therefore polished. Thus, the previously matt metal is smooth and bright. In the case of rust-free steels and carbon steels, phosphoric acid/sulphuric acid mixtures with additions of catalysts, inhibitors and the like are generally used as electrolytes.
During electropolishing, the objects to be polished, which are suspended on the appropriate support and contact elements or devices or are received in baskets or the like, are lowered into the electrolyte, i.e. 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.
To treat non-rusting steels (stainless steel), electropolishing processes currently used industrially predominantly employ low-water mixtures of concentrated phosphoric acid and sulphuric acid as electrolytes. Various organic and inorganic additions are regularly added to the electrolyte to improve the polishing action, increase the current yield, reduce the current density required and avoid hexavalent chromium ions in the rinsing waters.
The metal ions removed at the workpiece surface during the electropolishing go into solution and accumulate therein with time. All the electrolytes at present used industrially have the disadvantage that their effectiveness decreases considerably starting from a certain degree of metal enrichment. The electrolyte then has to be supplemented at least partly with fresh electrolyte or completely replaced. A reliably and economically reasonable regeneration process for a spent electrolyte is not available in the prior art. Instead, the spent electrolyte is disposed of as waste. Because of the high heavy-metal content, the spent electrolyte has to be treated as hazardous waste. The same applies to the rinsing waters which accrue during the electropolishing and the sludges which accrue during their treatment. Since the available land-fill volume for hazardous waste is, as a rule, strictly limited and, in addition, waste-disposal costs are rising (if it is not already difficult to impossible in some areas to find a suitable land-fill possibility), there is a considerable need for a process which makes possible a lower waste-disposal cost.
In the prior art, it has been assumed that it is precisely said enrichment with metal ions which makes the electrolyte unusable. Consequently, after a certain metal content has been reached, usually between 4 and 5% by weight, an electrolyte has been delivered for waste disposal. Since the permissible content of phosphates and sulphates in waste water is generally strictly limited, the entire volume of even still unused acid had to be neutralized. In total, large quantities of sludge accrued during this waste disposal.
To summarize, there are consequently problems in said prior art to the effect that a) the effectiveness of the electropolishing bath decreases markedly with increasing metal enrichment and that b) the waste waters accruing during electropolishing require an expensive waste disposal.
The optimum working range in the metal content of normal electrolytes is, as a rule, between 35 g/l and 70 g/l (2-4% by weight). According to the prior art, the electrolytes are capable of working up to a metal content of approximately 100 g
REFERENCES:
Metalloberflache, Nr. 12 (1958), S. 361-363, T. Zak (no month).
Chemical Abstracts, vol. 113, No. 20, 12 Nov. 1990, abstract No. 180231.
Abedian Razmik
Bohme Olaf
Piesslinger-Schweiger Siegfried
Phasge Arun S,.
Poligrat GmbH
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