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
• • 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

Definitions

• the present invention relates to a process for the preparation of peroxodisulfuric acid and peroxomonosulfuric acid from aqueous sulfuric acid in an electrolytic cell divided by a cation exchange membrane by oxidation of dilute and concentrated sulfuric acids on a platinum anode in the presence of thiocyanates and / or rhodanhydric acid and / or dirhodane and Organic compounds containing sulfur groups and / or heteroaromatics containing nitrogen with high current yields.
• Peroxodisulfuric acid is formed in concentrated sulfuric acids. At high sulfuric acid concentrations, the saponification of the peroxodisulfuric acid to peroxomonosulfuric acid is favored, whereby anodic decomposition of the peroxomonosulfuric acid leads to depolarization of the platinum metal anode and thus to a lowering of the current efficiency. In addition, at high sulfuric acid concentrations, HSO an2 ⁇ ions are sorbed on the anode surface to a greater extent, so that the current density has to be reduced and losses in current efficiency occur.
• the object of the present invention is to produce peroxodisulfuric acids and peroxomonosulfuric acid from high as well as from low concentrated sulfuric acids with high current yields and low energy expenditure.
• the object was achieved in that electrolysed aqueous sulfuric acids in the anode compartment of an electrolysis cell divided by a cation exchange membrane with a smooth platinum anode and having a high oxygen overvoltage, and the electrolysis of the sulfuric acids in the presence of thiocyanates and / or rhodanhydric acid and / or dirhodan and thioamides and / or thioaldehydes and / or thiocarboxylic acids and / or thiocarboxylic acid esters and / or thioalkanols and / or thioketals and / or dithio compounds and / or nitrogen-containing heteroaromatics.
• the preferred anode materials with high oxygen overvoltage are smooth platinum on support materials made of titanium, niobium or tantalum or platinum on supports made of alloys of these metals. Smooth platinum sheet on tantalum or niobium-tantalum alloys is particularly preferred.
• the platinum surface can be thermally smoothed by melting.
• inhibitor combinations consisting of thiocyanates and / or rhodanhydroic acid and / or dirhodan with the additives claimed in claim 1 are absolutely necessary.
• the thiocyanates are used in the form of their potassium or ammonium salts, while the hydrochloric acid is used as an aqueous solution. Ammonium thiocyanate or rhodanhydric acid is preferably used.
• Thiosemicarbazide, thioformamide and thiourea can be used as thioamides.
• Suitable thioaldehydes are those which are prepared from aldehydes and hydrogen sulfide.
• thiocarboxylic acids As thiocarboxylic acids, thiocarboxylic acid ethers, thioalkanols, or thioketals, aliphatic C1 to C4 compounds are preferably suitable, while thiocarbonic acid and / or dimercaproalkanols are used as dithio compounds.
• Advantageous organic thio compounds are the derivatives of thiocarbonic acid such as. B. thiourea, thiosemicarbazide, thiocarbohydrazide, bis-thiourea, thiosemicarbazone, dithizone and S-alkyl-isothiuronium salts, where thiosemicarbazide has proven to be particularly suitable.
• thioglycols thiomorpholine, thioethers, thiocyanic acid, thiocyanic acid esters, thiocarboxylic acid esters and rhodanine can be used.
• nitrogen-containing heteroaromatics for. B. pyridine and / or derivatives of pyridine and / or fused ring systems of pyridine and / or pyrimidine and / or pyrimidine derivatives and / or pyridazine derivatives and / or pyrazine and / or pyrazine derivatives and / or pyrazone and / or pvrazone derivatives.
• the benzene-fused ring systems of pyridine or their descendants such as e.g. Quinoline, isoquinoline, 1,2 dihydroquinoline, quinoline-8-sulfonic acid, aminoquinolines, 3-chloroquinoline, quinolinic acid and hydroquinolines.
• Chlorinated derivatives of pyridine and / or quinoline and / or pyrimidine and / or pyridazine and / or pyrazine and / or pyrazones can also be used in combination with thiocyanates and / or dirhodane and / or other organic thio compounds, small amounts of the aqueous sulfuric acid also being used can add to chlorides.
• chlorinated derivatives of pyridine and / or pyrimidine and / or pyridazine and / or pyrazine and / or pyrazones are used in combination with hexamethylenetetramine and thiocyanates and / or hydrochloric acid and / or dirhodane.
• hexamethylenetetramine can also be used in combination with all other claimed inhibitor additives, since it leads to a reduction in the loss of platinum.
• the concentration of thiocyanates and / or rhodanhydric acid and / or dirhodan in the aqueous sulfuric acid can be 0.5 to 2000 ppm, preferably 50 to 800 ppm.
• thiocyanates and / or dirhodan can be added continuously during the electrolysis.
• nitrogen-carbon-sulfur group-containing organic compounds and / or nitrogen-containing heteroaromatics should be between 10 and 2000 ppm, preferably 50 to 500 ppm, and should be continuously added during the electrolysis.
• the sulfuric acid content of the electrolyte can be 30 to 85% by weight, preferably 40 to 78% by weight, and 5 to 30% by weight for the peroxomonosulfuric acid in the production of the peroxodisulfuric acid. Sulfuric acid concentrations between 20 and 25% by weight are particularly favorable in the production of peroxomonosulfuric acid.
• Suitable cathode materials are electrodes which have a low hydrogen overvoltage. Titanium or nickel or coated metallic silver and graphite doped or coated with platinum metals and / or mixed platinum metals are suitable. Graphite which is doped with platinum metals or platinum metal mixed oxides is preferably suitable.
• the hydrogen overvoltage of the cathodes should be less than 200 mV, preferably 10 to 80 mV, in order to save energy costs.
• Cation exchange membranes or diaphragms are used to separate the anode and cathode spaces of the electrolytic cell.
• Polymers based on perfluorinated olefins or copolymers of tetrafluoroethylene with unsaturated perfluorinated ethers or copolymers of styrene and divinylbenzene, which contain sulfonic acid and carboxyl groups or only sulfonic acid groups as charge-bearing groups, are preferably used as cation exchange membranes.
• Cation exchange membranes which contain only sulfonic acid groups are considerably more resistant to deposits and contamination by polyvalent cations and are therefore preferably suitable when using process sulfuric acids.
• Diaphragms can also be used to separate the anode and cathode compartments. Suitable diaphragms are those made of hydrophilized perfluorinated and / or perchlorinated plastics or sintered ceramics based on aluminum oxide and / or zirconium oxide. Diaphragms which are composed of polymers based on perfluorinated olefins and are hydrophilized by sulfonation are preferred.
• the electrochemical synthesis of peroxodisulfuric acid and peroxomonosulfuric acid can be carried out continuously or in batch mode. In continuous operation, the electrolytic cells are connected in series as a cascade.
• the electrochemical conversion of the aqueous sulfuric acids is preferably carried out in such a way that the electrolysis cell is operated as a continuous cascade, the sulfuric acid flowing through the anode spaces over the entire cascade.
• the claimed additives are metered in continuously in various sections of the cascade. It has proven to be advantageous if no additives such as thiocyanates, rhodanhydric acid, dirhodane, organic nitrogen-carbon-sulfur compounds or heteroaromatics are added to the sulfuric acid-peroxosulfuric acid mixture in the last fifth of the cascade. In this mode of operation, pure peroxosulphuric acid is obtained which does not contain any impurities from inhibitors.
• Dilute sulfuric acid is preferably used on the cathode side as feed to the cascade.
• aqueous alkali sulfate solution can also be used as the lead electrolyte.
• the feed concentrations to the cascade can be 0.25 to 50% by weight.
• the anodes and cathodes on which gases develop can be used in the form of gas lift electrodes.
• the buoyancy forces are used which result in density differences in the electrolyte due to the gas loading compared to the gas-free electrolytes.
• the buoyancy forces are controlled by constructive design of the electrodes and circulation systems so that a directed electrolyte flow with minimal gas content in the electrode space is created.
• the flow velocities should be ⁇ 0.15 m / s and ⁇ 10 m / s, preferably 2 to 5 m / s.
• the subdivision of the electrode gaps into vertical channels through which the electrolyte flows is particularly favorable. In this way, a minimization of the gas bubble-related voltage drop is achieved.
• the electrolysis of the aqueous sulfuric acid solutions can be carried out at current densities of 0.3 to 25 kA / m2. It is preferred to work at current densities of 1 to 8 kA / m2.
• the temperature in the electrochemical synthesis should be -5 to 45 ° C, preferably 10 to +30 ° C. However, it is also possible to carry out the electrolysis with the claimed additives at higher temperatures, especially at higher sulfuric acid concentrations.
• the process according to the invention is suitable for the electrochemical conversion of dilute and concentrated aqueous sulfuric acids to peroxosulfuric acids.
• thiocyanates and / or hydrochloric acid and / or dirhodane and thioamides and / or thioaldehydes and / or thiocarboxylic acids and / or thiocarboxylic acid esters and / or thioalkanols and / or thioketals and / or dithio compounds and / or nitrogen-containing heteroaromatic compounds high current yields for the formation of peroxodisulfuric acid and peroxomonosulfuric acid are obtained in dilute and in highly concentrated sulfuric acids.
• the claimed combination of inhibitors makes it possible, in contrast to the inhibitors used hitherto, to produce peroxomonosulfuric acid and peroxodisulfuric acid from dilute and also from highly concentrated sulfuric acids with high current yields and thus economically.
• An anode made of platinum sheet on a tantalum support and a cathode made of graphite doped with platinum are installed in a two-part electrolytic cell, the anode and cathode spaces of which are separated by a cation exchange membrane of the type Neosepta CMH / 2 (Tokuyama Soda).
• the platinum metal anode sheet was thermally smoothed by surface melting.
• Example 1 In an electrolysis cell as in Example 1, 78% by weight aqueous sulfuric acid is used in the anode compartment, while aqueous 35% by weight sulfuric acid is used as the lead electrolyte in the cathode compartment.
• the electrolysis is carried out at a cell voltage of 4.1 V and a current density of 5 kA / m2 anode area.
• the flow velocity in the anode compartment is 1.2 m / s.
• the temperature is kept between + 20 and + 22 ° C.
• samples are taken of the sulfuric acid according to different current offers and the current yields for the formation of peroxodisulfuric acid and ammonia transfer are determined.
• the example shows that the simultaneous addition of thiocyanate and thiosemicarbazide leads to a substantial improvement in the current efficiency in the electrochemical oxidation. If the temperature is lowered to + 15 to + 17 ° C, the current yields for the production of peroxodisulfuric acid increase by 2 to 3%.
• the peroxodisulfuric acid produced with the additives according to the invention is preferably suitable for the absorption of exhaust gases which, in addition to sulfur dioxide, also contain small amounts of sulfur trioxide, since sulfur dioxide is quickly oxidized to sulfuric acid, while sulfur trioxide is sorbed at the same time.
• the oxidation and absorption of nitrogen oxides with these highly concentrated sulfuric acids containing peroxodisulfuric acid has the advantage that nitric acid is formed directly and can be separated off by distillation.
• Example 1 20% by weight aqueous sulfuric acid is used in the anode compartment. Aqueous 20% by weight sulfuric acid is used as the lead electrolyte in the cathode compartment.
• the electrolysis is carried out at a current density of 5 kA / m2 and a cell voltage of 4.08 V and a temperature of + 22 to + 25 ° C.
• a mixture of peroxomonosulfuric acid and peroxodisulfuric acid is obtained, which is predominantly Peroxomonosulfuric acid contains.
• the current efficiency is calculated as peroxomonosulfuric acid.
• the very reactive peroxosulfuric acids produced with the additives according to the invention are preferably suitable for the oxidation and absorption of nitrogen oxides, sulfur dioxide and for the degradation of waste water containing crop protection agents.

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• Chemical & Material Sciences (AREA)
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• Chemical Kinetics & Catalysis (AREA)
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• Materials Engineering (AREA)
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• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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Citations (4)

• 1993
  • 1993-08-07 DE DE4326540A patent/DE4326540A1/de not_active Withdrawn
• 1994
  • 1994-07-29 EP EP94111822A patent/EP0639659A1/fr not_active Withdrawn

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