Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/28—Per-compounds
  • C25B1/30—Peroxides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/22—Inorganic acids
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/28—Per-compounds
  • C25B1/29—Persulfates
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
  • C25B11/036—Bipolar electrodes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/052—Electrodes comprising one or more electrocatalytic coatings on a substrate
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
  • C25B11/057—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of a single element or compound
  • C25B11/059—Silicon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/70—Assemblies comprising two or more cells
  • C25B9/73—Assemblies comprising two or more cells of the filter-press type
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/70—Assemblies comprising two or more cells
  • C25B9/73—Assemblies comprising two or more cells of the filter-press type
  • C25B9/75—Assemblies comprising two or more cells of the filter-press type having bipolar electrodes

Definitions

• the invention relates to a process for preparing or regenerating peroxodisulfuric acid and its salts by electrolysis of an aqueous solution containing sulfuric acid and/or metal sulfates.
• metal sulfates encompasses both sulfates of metals such as zinc, nickel or iron and sulfates of alkali metals and alkaline earth metals and also ammonium sulfate.
• alkali metal sulfates or alkaline earth metal sulfates preferably alkali metal sulfates or ammonium sulfate, as metal sulfates.
• mixtures of various metal sulfates for example magnesium sulfate, zinc sulfate or else nickel or iron sulfate, preferably in the regeneration of etching and pickling solutions.
• diamond-coated electrodes composed of valve metals, preferably niobium, or ceramic materials, preferably silicon, can be used for the preparation of peroxodisulfates of the alkali metals and of ammonium [DE 199 48 184.9, DE 100 19 683].
• the diamond layer is made conductive by doping with a trivalent or pentavalent element, preferably boron.
• a further advantage of diamond-coated anodes in peroxodisulfate production is that, even at a low sulfate content in the anolyte, significantly higher current yields can be achieved than when using platinum anodes.
• a further disadvantage of the diamond-coated silicon electrodes of the prior art is their limited dimensions of at present not more than 200 ⁇ 250 mm.
• EP 1 229 149 proposed adhesively bonding a relatively large number of such silicon-diamond electrodes by means of an electrically conductive adhesive to a metal base plate, e.g. composed of a valve metal, and sealing the edges by means of a corrosion-resistant resin, e.g. epoxy resin.
• a corrosion-resistant resin e.g. epoxy resin.
• the difficulties involved for example in. the provision of the conductive adhesive, e.g. an adhesive composed of epoxy resin containing silver particles, and in the complete elimination of the oxide layers on the areas to be joined, are very great.
• such an electrode construction has been found to be insufficiently corrosion resistant for the preparation of peroxodisulfate, so that only short operation lives of usually less than one year can be achieved in this way.
• FR 2790268 B1 discloses such a bipolar electrolysis cell in which the bipolar electrodes comprise a ceramic substrate which is completely enveloped by a diamond film.
• this cell is not proposed specifically for the preparation of peroxodisulfates but for uses in the degradation of pollutants or for disinfection of water.
• EP 1 254 972 proposes an electrolysis cell construction which is suitable for various applications and can be configured as a monopolar or bipolar, undivided or divided cell.
• silicon disk electrodes coated on both sides with a diamond layer are once again exclusively used.
• these cells having silicon electrodes coated on both sides with a diamond layer and the relatively complicated cell construction can be used effectively only for small persulfate throughputs. If an attempt is made to increase the throughput to industrially relevant ranges by means of a relatively large number of individual bipolar cells, this construction results in reduced yields due to the loss currents in the power supply leads and power outlet leads which increase greatly with the total voltage.
• peroxodisulfates can advantageously be prepared in undivided or divided electrolysis cells in a simple manner by using bipolar silicon electrodes which have been coated on one side with doped diamond, with the uncoated silicon rear sides acting directly as cathodes.
• the coating on the silicon electrode has a thickness of from about 1 to about 20 ⁇ m, preferably about 5 ⁇ m.
• the process of the invention thus advantageously makes it possible to prepare peroxodisulfuric acid and/or its salts at a genuine bipolar electrode with a high current yield and a low electric energy consumption even though only the slightly conductive silicon is used as cathode. In addition, no costs for a cathode coating are incurred.
• a further advantage of the inventive bipolar silicon electrodes coated on one side with diamond is the lower catalytic activity of the silicon rear side compared to a metallized electrode rear side, e.g. composed of platinum or stainless steel. It has been found that reduction losses of peroxodisulfate are therefore lower when electrolysis is carried out in an undivided electrolysis cell. This leads, in the case of undivided cells, to the increase in the peroxodisulfate concentration with electrolysis time being somewhat steeper and the achievable final concentration being higher than when a metallized cathode is used under otherwise identical electrolysis conditions.
• the process of the invention for preparing peroxodisulfuric acid and/or its salts can be carried out both in undivided electrolysis cells and in electrolysis cells which are divided, for example by means of ion-exchange membranes or porous diaphragms.
• valve metal refers to a metal which when connected as an anode becomes coated with an oxide layer which becomes nonconductive even at high voltages. Connected as anode, the metal blocks. Connected as cathode, the oxide layer is dissolved and current flows in a fairly uninhibited fashion. Thus, valve metals behave like a rectifier when different polarities are applied. Examples of suitable valve metals are tantalum, titanium, niobium and zirconium. For the purposes of the present invention, preference is given to using niobium.
• the monopolar boundary cathodes preferably comprise a suitable material having a good conductivity, e.g. stainless steel, Hastelloy, platinum and impregnated graphite.
• a suitable material having a good conductivity e.g. stainless steel, Hastelloy, platinum and impregnated graphite.
• high-alloy stainless steels or Hastelloy preference is given to using high-alloy stainless steels or Hastelloy.
• a silicon boundary cathode having a metallized rear side and with a current supply plate composed of a material having a good conductivity, e.g. copper, as contact can also be used due to the good long-term stability in undivided cells.
• boundary electrodes composed of metallic materials optimal current input can be achieved in a simple manner and without large voltage drops because of the good conductivity.
• Electrode stacks comprising bipolar electrodes and boundary electrodes with power supply lead to be connected electrically in parallel in an electrolysis cell.
• the spacing between the bipolar electrodes can be set or fixed by means of spacers.
• Such electrode stacks connected in parallel make it possible to accommodate relatively large power capacities in an electrolysis cell without an unjustifiably high total voltage being necessary.
• the voltage can thus also be optimally matched to the available rectifier voltage.
• the short circuit currents in the common feed and discharge lines for the electrolyte solutions can be minimized further as a result, which can additionally be aided in a known manner by installation of additional resistance sections in these lines.
• Undivided bipolar cells having the structure provided by the invention can be used particularly advantageously when the peroxodisulfate concentration does not have to be very high for the application in question, for example for the oxidative degradation of pollutants in process solutions and wastewater.
• sodium peroxodisulfate reaction solutions having a content of from 50 to 100 g/l can be prepared very effectively in batch operation in an undivided cell provided with the bipolar electrodes according to the invention at current yields of from 75 to 50% and specific electric energy consumptions of from 1.3 to 1.9 kWh/kg.
• a further surprising effect of the process of the invention are the very low corrosion rates at the silicon cathodes which are found in undivided electrolysis cells in a long-term experiment using an acidic persulfate-containing electrolyte.
• surprisingly low corrosion rates of only 2-3 ⁇ m were found in an undivided cell at a steady-state sodium peroxodisulfate content of about 150 g/l in a long-term experiment over about 7 months (cf. example 1).
• This was particularly surprising because 10-100 times greater corrosion was observed even on platinum cathodes of the prior art under these very highly corrosive conditions.
• Even cathodes made of graphite or high-alloy stainless steels were found to be unsuitable in such peroxodisulfate-containing sulfuric acid electrolyte solutions because they were insufficiently corrosion-resistant.
• An undivided bipolar electrolysis cell having a construction analogous to that in DE G 200 05 681.6 contained 9 bipolar silicon electrodes coated on one side with about 3 ⁇ m of boron-doped diamond (average about 3000 ppm of boron).
• a niobium electrode coated on one side with diamond and provided with a power supply lead served as boundary anode.
• the boundary cathode with power supply lead comprised Hastelloy.
• the bipolar electrodes had a dimension of 100 ⁇ 33 mm (33 cm 2 ) .
• the mean spacing of the about 1 mm thick bipolar electrodes was set to about 2 mm by means of spacers.
• the electrolysis current was regulated at a constant 16.5 A, corresponding to an anodic and cathodic current density of 0.5 A/cm 2 .
• 2 l of an aqueous solution containing 300 g/l of sodium sulfate and 200 g/l of sulfuric acid served as electrolyte. It was circulated at a rate of about 600 l/h from a circulation reservoir via a heat exchanger and through the cell by pumping (batch operation). Electrolysis operation was maintained for 5000 hours, with only the water which had evaporated or been decomposed being replaced.
• the mean decrease in the silicon electrode thickness was calculated therefrom as an average of 3 ⁇ m.
• the thickness of the silicon cathode thus decreases by only about 10 ⁇ m per year.
• NaPS sodium peroxodisulfate
• the nine bipolar electrodes and the two monopolar boundary electrodes of the undivided electrolysis cell used in examples 1 to 3 were used in a divided bipolar cell.
• Cation-exchange membranes which were fixed on both sides by means of anode and cathode spacers made of plastic were used for separating anolyte and catholyte.
• the anode and cathode spaces bounded by sealing frames had a thickness of 2-3 mm each.
• Anolyte and catholyte were circulated in separate circuits through a heat exchanger. 500 g/l of sulfuric acid served as catholyte.
• the anolyte once again consisted of an aqueous solution containing 200 g/l of sulfuric acid and 300 g/l of sodium sulfate.
• a further 100 g/l of sodium sulfate were dissolved in the anolyte during the electrolysis (i.e. a total of 400 g/l of sodium sulfate).
• the anodic and cathodic current densities were each set to 0.5 A/cm 2 .
• the mean cell voltages were in the range from 5.5 to 6 V. At the final concentration of 400 g/l, a still very low specific electric energy consumption of about 1.8 kWh/kg could thus be achieved.

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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)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)
• Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)

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