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
  • C25B1/34—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis
  • C25B1/46—Simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine, e.g. by chlor-alkali electrolysis in diaphragm cells

Definitions

• the present application relates to a method for the electrolysis of technical alkali chloride solutions in cells, the anode and cathode spaces of which are separated by a permselective cation exchange membrane.
• Such solutions can often contain polyvalent cations such as calcium, magnesium, strontium, iron and possibly mercury.
• the membrane used is hydraulically impermeable and, when using sodium chloride, ideally only sodium ions and water molecules can pass through.
• Cleaned concentrated brine is fed into the anode chamber, chlorine and spent brine are discharged from this chamber.
• Water is introduced into the cathode compartment, which forms sodium hydroxide solution with the sodium ions that have passed through the membrane. The amount of water supplied determines the concentration of lye obtained.
• the hydrogen and sodium hydroxide solution formed on the cathode are continuously removed from the cathode compartment.
• the current efficiency during electrolysis essentially depends on the permselectivity of the membrane that separates the apolyte and catholyte. This membrane is said to be the cations allow to pass from the anolyte into the catholyte; however, the back migration of the hydroxide ions from the catholyte, which are attracted to the anode due to their negative charge, is to be largely prevented.
• Exchanger membranes that are suitable for chlor-alkali electrolysis generally consist of tetrafluoroethylene / perfluorovinyl ether copolymers with pendant acidic groups. These acidic groups cause ion exchange.
• the main proposed groups were -S0 3 H (US Pat. No. 4,025,405), -S0 2 NHR (DT-OS24 47 540, DT-AS 244 154) and the group -COOH (DE-OS 26 30 584).
• the ion exchange film is usually reinforced with a polytetrafluoroethylene support fabric.
• the membranes have a high chemical resistance to chlorine and sodium hydroxide. Unfortunately, the properties of these membranes deteriorate with longer runtimes. This "aging" can be attributed at least in part to the presence of alkaline earth or heavy metal ion electrolytes. After a relatively short operating time in the presence of these impurities, the permselectivity may decrease and the electrical membrane resistance may increase. Both lead to an increase in energy consumption (expressed in kWh / t product).
• Treatment with acids is mainly of interest for single-layer membranes which only contain sulfonic acid groups as ion exchange residues.
• the object was therefore to find a process in which the damage to the cation exchanger membrane caused by impurities in the anolyte is prevented without the need to remove an insoluble calcium deposit from the membrane.
• the process according to the invention is characterized in that the Alkali chloride solution that enters the anode compartment, an aliphatic polybasic phosphonic acid is added. Nitrogen-free phosphonic acids are particularly suitable for this.
• the polybasic phosphonic acid should contain at least 2 phosphonic acid or carboxylic acid groups in the molecule.
• Particularly suitable additives are 1-hydroxyalkane-1,1-diphosphonic acids which contain 1 to 5, preferably 1 to 2, carbon atoms in the molecule. These compounds are extremely stable in acidic, neutral and alkaline environments. Further owner alkane-phosphonic acids of the general formula I where R 2 and R are hydrogen or a C 1 -C 4 alkyl radical, and X is the group H, CH 3 or means.
• the above-mentioned phosphonic acids are capable of forming stable calcium complexes soluble in aqueous solution at pH 11.
• “Soluble” means that in the presence of soda at pH 11 at least 1 g of calcium ions in 1 liter of water can be bound in a complex manner without precipitation.
• 1-hydroxyethane-1,1-diphosphonic acid is able to complexly bind about 1/4 of its weight to calcium ions.
• the process according to the invention is particularly advantageous when using perfluorinated membranes which contain sulfonamide or carboxyl groups.
• the increase in membrane resistance can be further slowed down if the current is briefly interrupted from time to time. A significant reduction in the electrolysis current does not have this effect.
• the catholyte and anolyte need not be diluted or acidified (cf. DE-OS 25 48 456). 1-10, preferably 2-5 interruptions per 24 hours are best. If the interruptions take place less frequently, the membrane resistance (and thus the voltage) increases more rapidly, so that the advantage described decreases.
• hydroxyl groups can also be present as functional groups.
• the amount of phosphonic acid to be added depends on the degree of contamination in the brine (content of Ca ++ and other divalent ions) and on the complexing ability of the phosphonic acid.
• the amount of impurities can easily be determined (e.g. by complexometric titration at pH 10-12).
• the complex formation capacity of the phosphonic acids (compared to calcium) is partly known. Otherwise, it can easily be determined experimentally (back titration of an alkaline phosphonate solution with calcium acetate solution).
• the titration-determined requirement of phosphonic acid is added to the brine.
• the free phosphonic acid its alkali metal salts can also be used.
• the invention is illustrated by the following examples.
• Anode and cathode compartments were separated by a perfluorinated cation exchange membrane (area 36 cm 2 ).
• Anodes activated titanium expanded metal.
• Cathodes expanded metal made of stainless steel.
• the brine used for the tests also contained 0.2 mg of magnesium, 6 mg of calcium, 1 mg of strontium, 0.3 mg of barium, 4.8 mg of mercury and 0.2 mg of iron as impurities per liter.
• the cell load was 11 amps, which corresponds to 30 A / dm 2 .
• the membrane used consisted of a perfluorinated hydrolyzed copolymer from C 2 F 4 and a fluorosulfonyl perflupr vinyl ether, which was provided with a tetrafluoroethylene support fabric.
• the fluorosulfonyl groups of the membrane were on the cathode side in -SO 2 -NH-C 2 H 4 -NH-SO 2 , - Groups converted, on the anode side into sulfonic acid groups (equivalent weight 1150, thickness 180 ⁇ m).
• Trade name: Nafion R 214 manufactured by Dupont.
• Example 1 was repeated, except that 100 mg / l of 1-hydroxyethane-1,1-diphosphonic acid were added to the brine and the pH of the brine was adjusted to 3.5. In the continuous process, a pH in the anolyte turned out - a value of 4.5. The addition of the phosphonic acid had a favorable influence on the electricity yield and energy consumption. The results are shown in the table below.
• Example 4 The experiment was carried out as described in Example 1 without additives to the brine; however, a membrane containing carboxyl groups was used.
• the membrane was produced in accordance with DE-OS 26 30 548, Example 28, but a Nafion 415 membrane (polytetrafluoroethylene support fabric, single-layer membrane with sulfonic acid groups, equivalent weight 1200) was used as the starting material.
• the thickness of the membrane used in Example 4 was 120 ⁇ m.
• the following table shows the drop in cell voltage and current efficiency as a function of the operating time.
• Example 4 was repeated, but 100 mg / l of hydroxyethiphiphonic acid were added to the brine.
• the values of the current yield and the energy consumption are shown in the following table.
• Example 2 is repeated. After an operating time of 2000 hours, the cell voltage is 4.47 V. If you interrupt the further course of the electrolysis every 12 hours for 3 to 5 minutes, the cell voltage initially drops to 4.1 to 4.25 V and also remains during the next 500 hours in this area.
• the cell voltage after approx. 2600 hours of operation is approx. 4.7 V. If the electrolysis is interrupted every 12 hours for 3 to 5 minutes, the cell voltage initially drops to 4 , 6 V. During the following 500 hours of operation, it slowly rises to 4.75 V.

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• Chemical & Material Sciences (AREA)
• Inorganic Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)

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• 1978
  • 1978-10-21 DE DE19782845943 patent/DE2845943A1/de not_active Withdrawn
• 1979
  • 1979-10-15 EP EP79103962A patent/EP0010284A3/fr not_active Withdrawn
  • 1979-10-18 US US06/086,124 patent/US4236980A/en not_active Expired - Lifetime
  • 1979-10-19 JP JP13428979A patent/JPS5558378A/ja active Pending

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