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
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof

Definitions

• ABSTRACT A process is described that is particularly applicable to the continuous production of hypochlorite solution over long periods of time by electrolysis of aqueous chloride solutions with an anode having a coating that is a mixture of oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum, all in certain proportions.
• anode comprises a conductive bas'e bearing on at least a portion of the surface thereof amixed oxide coating of from 30 to 90 percent stannic oxide, 1.0 to percent antimony oxide, 1.0 to 50 percent of at least one platinum group metal oxide, and 0.5 to 30 percent .,an.ode,i s considered. passivated when the voltage at which the desired electrolytic reaction occurs becomes so high as to prohibit further practical operation, e.g., in excess of 8 volts in a hypochlorite cell.
• the invention is concerned with a process" for the production of an alkali metal hypochlorite solution by the electrolysis of an alkali metal chloride in a diaphragmless cell.
• the cell configuration is not critical to thepractice of the present invention except in that it must be, adapted for operation with dimensionally stable electrodes.
• any known cell may be used, such cells comprising an enclosureprovided withan electrolyte inlet and an outlet for electrolyte and product and containing at least one anode and one cathode opposed to each other in a substantially parallel ralationship.
• the cell mayibe of either the monopolar design, that is, wherein there is a separate external electrical connection for each electrode, or bipolar in configuration, wherein thecurrent passes, from a terminal anode through a plurality of bipolar electrodes disposed between said terminal anode and a terminal cathode at the opposite end of the cell.
• the products of electrolysis i.e., chlorine and alkali metal hydroxide, immediately mix toform the desired hypochlorite solution.
• the conductive substrate is preferably titanium, al-
• thoughtantalum, niobium, and zirconium may also be employed.
• a covering of one of the aforementioned metals over a more conductive material may also be employed.
• the antimony oxide component enters into the tin oxide crystal lattice, rendering same more electrically conductive.
• the antimony is present in an indeterminate oxide form owing to its entrance into the stannic oxide crystal lattice, it is expressed for convenience sake as Sb O
• the antimony oxide is present within the range from 1.0 to 10, prefer- V ably 4.0 to 10, percentby weight.
• tin and antimony oxides are further qualified bythe proviso that they be present, respectively, in the range, on a. mole ratio basis, of 95:5 to :15, especially :10. In this fashion, there is obtained the desired doping effect of the antimony on the tin oxide without the presence of an excess separate.
• the third component of the mixed coating is at least one platinum group metal oxide, by which term it is intended .to include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, especially those of ruthenium and iridium. These platinum group metal oxides are present for the most part in their most highly oxidized state and within the range of from 1.0
• An especially preferred anode is one the coating of which contains a combination of RuO Ir0 or rhodium oxide.
• the final component is a valve metal oxide selected from the group consisting of titanium and tantalum oxides.
• the titanium is present in the form of TiO- and is essentially crystalline (rutile) in nature, when tantalum is employed, a generally amorphous tantalum oxide results. Therefore, although it is expressed as Ta O it is understood that mixtures of tantalum oxides may in fact be present.
• The'amounts of a the preferred method of preparing the multicomponent" coating composition on the titanium substrate is by deposition from a solution of the appropriate thermochemically decomposable salts.
• the thermally decomposable salts are the lower alkanols such as ethanol, propanol, amyl alcohol, and especially n-butyl alcohol, although other solvents including water may be employed. To these solvents there is generally added from 0 to 50 percent by volume of an acid, such as hydrochloric acid.
• concentration of the metals'in the solution from 6hich the coating composition is to be formed ranges between about 50 and 200 grams per liter.
• preformed valve metal oxides should not be employed nor should separately preformed tin and antimony oxides be used. Further, if thermal decomposition is incomplete, small amounts of salts may remain without detrimental effect in the coating, for example, small amounts of chloride in the primarily oxide coating. 7 I
• the cathode again, is not critical to the practice of the present invention, any of those conventionally employed being satisfactory.
• a preferred material is titanium, although steel, stainless steel, or titanium bearing an electrocatalytically active, electrically conductive coating, such as a mixed titanium dioxide-ruthenium dioxide coating, may be employed. Configuration is not critical to the process but rather is a consideration of the cell design. Generally a continuous sheet is employed.
• the substrate for the anode e.g., titanium
• the substrate for the anode may metals may be employed.
• intere'lectrode gap may be within the range of 0.04-0.125 inch, the lower gaps are generally preferred for efficient operation.
• the solution tobeelectrolyzed is an aqueous alkali metal chloride, especially a solution containing from about 28-35 grams per liter of alkali metal chloride.
• aqueous alkali metal chloride especially a solution containing from about 28-35 grams per liter of alkali metal chloride.
• concentrations substantially less than about 28 gpl sodium chloride provide solutions in which excessive amounts of oxygen are generated at the anode, thereby accelerating passivation. Rather than electrolyze an artificial alkali metal chloride solution, it is quite possible and mainly to'potassium and especially sodium.
• the temperature at which electrolysis is conducted should be within the range of from 5 to 50 C, especially l530 C. At temperatures in excess of this range, the chemical conversion of hypochlorite to chlorate is favored, thus reducing efficiency. On the other hand, at low temperatures, the relative overvoltages of oxygen and chlorine are displaced with the result that larger quantities of oxygen are evolved, again attributing to more rapid anode passivation.
• the pH at which the hypochlorite-forming reaction proceeds is within the range of from about 7 to 10. At a lower pH, the chlorate-forming reaction is preferred while higher ranges must be artifically induced and serve no economic purpose. Generally, it will be found that there is no need to adjust the electrolyte to obtain the desired pH since a salt solution of the appropriate concentration will on establishment of electrolysis eq'uilibrate at a pH within the range of 9.0-9.5.
• hypochlorite concentration by controlling the flow rate through the cell. Since the amount of available chlorine required by the average sewage treatment plant is only on the order of 10 ppm and only a 2 ppm concentration is required, in the waters of cooling towers, it will be apparent that any of the concentrations obtained by the invention are useful.
• the process is operated at a relatively low current density, e.g., within the range of from about 0.5 to 1.0 ampere per square inch. While at higher current densities, the production rate is increased somewhat, this is offset'by the decrease in useful anode life. On the other hand, a low current density generally results in a low current efficiency. Hence, an increase in electrode area must be provided to achieve a like production.
• a preferred process for the production of solutions having a sodium hypochlorite concentration within the range of 0.5 to gpl comprises passing an electrolyzing current of between 0.5 and 1.0 asi through an aqueous solution having a pH of greater than 7 and containing 28 to 35 gp] sodium chloride at a temperature from 5 to 50 C between a cathode and an opposed anode, which anode comprises a titanium substrate bearing on at least a portion of the surface thereof a coating of the mixed oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group consisting of titanium and tantalum.
• EXAMPLE 1 An anode coating solution is prepared by dissolving 25 grams SnCl -5H O, 1.8 grams SbCl 5.3 grams RuCl -xH O (38% Ru), and 0.86 gram orthobutyl titanate in 25 ml butanol and 12.5 ml HCl (36%). A titanium sheet is coated by brushing with this solution followed by drying at 110 C for 3 minutes and baking in air at 500 C for 7 minutes. The procedure is repeated until a coating containing 0.64 gram per square foot of ruthenium is attained.
• the resultant electrode has the following composition, calculated on the indicated oxide weight basis: 73.1% SnO 7.8% Sb O 17.5% RuO and 1.6% TiO
• the electrode is disposed as the anode in an electrolytic cell opposite and spaced 0.04 inch apart from a titanium metal cathode. Electrolysis of a 28 to 30 gram per liter aqueous sodium chloride solution having a temperature of 5 C is conducted for 235 hours at an anode current density of 1 asi before the voltage increases to in excess of 8, indicating passivation.
• Example 1 is repeated, the anode coating in this instance being 47.2% SnO 5.3% Sb O 27.3% RuO and 20.2% Ta O and an anode lifetime of 153 hours is obtained.
• a process for the production of hypochlorite by the electrolysis of an aqueous alkali metal chloride solution comprising passing an electrolyzing current through said solution between a cathode and an opposed anode, which anode comprises a conductive base bearing on at least a portion of the surface thereof a coating consisting essentially of from 30 to 90 percent by weight stannic oxide, from 1.0 to 10 percent antimony oxide, calculated as Sb O from 1.0 to 50 percent of at least one platinum group metal oxide and from 0.5 to 30 percent ofa valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of SnO :Sb O is between 95:5 and :15.
• a process for the production of a solution containing from 0.5 to 10 grams per liter of hypochlorite comprises passing an electrolyzing current through an aqueous solution of alkali metal chloride, at a pH of greater than 7 and a temperature between 5 and 50 C, between a cathode and an opposed anode, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 30 to percent stannic oxide, from 1.0 to 10 percent antimony oxide, calculated as Sb O from 1.0 to 50 percent of at least one platinum group metal oxide and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of SnO :Sb O is between :5 and 85:15.
• a process as in claim 2 wherein the electrolyzing current is within the range of 0.5 to 1.0 ampere per square inch.

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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)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)

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Citations (8)

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• 1973
  • 1973-04-19 US US352419A patent/US3917518A/en not_active Expired - Lifetime
• 1974
  • 1974-03-14 CA CA195,037A patent/CA1037415A/en not_active Expired
  • 1974-04-17 BR BR3071/74A patent/BR7403071D0/pt unknown
  • 1974-04-18 DE DE2418739A patent/DE2418739C2/de not_active Expired
  • 1974-04-18 IT IT50463/74A patent/IT1005990B/it active
  • 1974-04-18 GB GB1710674A patent/GB1417949A/en not_active Expired
  • 1974-04-18 SE SE7405203A patent/SE7405203L/sv unknown
  • 1974-04-18 MX MX743890U patent/MX4076E/es unknown
  • 1974-04-19 JP JP49043494A patent/JPS5013298A/ja active Pending
• 1977
  • 1977-05-25 SE SE7706108A patent/SE7706108L/xx unknown

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