Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Patent
1998-02-06
1999-09-28
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
205466, 205510, C25B 128, C25B 116
Patent
active
059582081
DESCRIPTION:
BRIEF SUMMARY
Sodium peroxodisulphate is used in many ways in the chemical, metal-processing and electronic industries as a polymerization initiator, as etching and pickling agent and as an oxidizing and bleaching agent. To an increasing extent, it is also used in environmental technology, since, because of its high oxidation potential, it is able to break down many inorganic and organic pollutants oxidatively, and it can also be utilized for the extraction and recovery of metals from residues (e.g. electronics scrap) or from exhaust gases (e.g. mercury) because of its ability to dissolve metals. An oxidizing, bleaching, disinfecting and deodorizing agent has also been proposed, which comprises a liquid or solid mixture of peroxodisulphates with alkalis, in the application of which the peroxodisulphate is active in the alkaline range and the sulphuric acid formed in the reaction is completely or partially neutralized by the alkali component. Mixtures of peroxodisulphates with sodium carbonate and/or sodium percarbonate are particularly effective in this context.
Of the peroxodisulphates prepared on an industrial scale, sodium peroxodisulphate is the most important compound. Ammonium peroxodisulphate is increasingly being replaced because of the problems with ammonium. In comparison with potassium peroxodisulphate, there is the advantage of a substantially better solubility and the use as raw material of the cheaper sodium sulphate produced as waste product in industrial processes.
At the same time, however, of the three commercial peroxodisulphates, sodium peroxodisulphate is the most difficult to prepare directly electrochemically, since under comparable conditions only relatively low current efficiencies are achieved and the crystalline end product can only be crystallized out of the generally strongly acidic electrolyte solution with great difficulty in the form of sufficiently large easily filterable crystals.
Sodium peroxodisulphate is therefore to some extent still prepared indirectly electrochemically by reacting ammonium peroxodisulphate with sodium hydroxide solution in accordance with the overall equation NH.sub.3 +2 H.sub.2 O. in sulphuric acid proceeds in accordance with the overall equation ##STR1##
In this case, either undivided electrolysis cells are employed, or electrolysis cells divided by porous diaphragms or ion-exchange membranes are employed. In all cases, the anode material used is smooth platinum, usually applied to electrodes made of the valve metals tantalum or titanium. The cathodes used comprise lead, stainless steel or graphite material. As a result of the high solubility of the sodium peroxodisulphate, when undivided cells are used, generally only unsatisfactory current efficiencies around 50% are obtained, since some of the peroxodisulphate formed at the anode is reduced again at the cathode.
When divided electrolysis cells are used, in particular those having ion-exchange membranes as separators, with addition of substances increasing the potential, preferably sodium thiocyanate, with optimized electrolysis conditions and with favourable composition of the remaining electrolyte in strong sulphuric acid solution, current efficiencies up to greater than 70% are achieved. The conditions for a high current efficiency become more favourable with increasing sulphuric acid content, but the rate of the hydrolysis reaction to form peroxomonosulphuric acid also increases to the same extent, which in turn has an adverse effect on the current efficiency. To achieve such high current efficiencies, therefore, residence times as short as possible, low electrolysis temperatures and/or addition of selectively acting reducing agents are necessary under these conditions, in order to keep the steady-state concentration of peroxomonosulphate sufficiently low.
The cathode compartments are preferably charged with sulphuric acid, which accumulates owing to the electrochemical transfer of sodium ions into the cathode compartment with sodium sulphate and can therefore be used as anolyte after further saturation wi
REFERENCES:
patent: 4144144 (1979-03-01), Radimer et al.
patent: 5445717 (1995-08-01), Karki et al.
Derwent WPI; Title: Ammonia or AQ. Alkali Solution and Peroxodisulphate Preparation . . . Using . . . Sulphur &/or Nitrogen; Assignee: BASF AG (BADI); Inventors: Haberman W, Meyer J: DE 4326539 Feb. 9, 1993.
Derwent WPI; Title: Direct Electrolysis Prodn. of Sodium Peroxy-Disulphate . . . Used . . . for Printed Circuits; Assignee: FMC Corp (FMCC); Inventor: Radimer KJ; DE 2757861A Jun. 29, 1978;
Derwent WPI; Title: Cont. Prodn. Alkali Metal or Ammonium persulphate--Using Zirconium (Alloy) Cathode & Bisulphite Solution; Assignee: Air Liquide Canada LTEE (CAAL); DE 2305381A; (no date).
Thiele Wolfgang
Wildner Knut
Eilenburger Elecktrolyse-und UmweltTechnik GmbH
Phasge Arun S,.
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