METHOD FOR THE ELECTROLYTIC PREPARATION OF HYPOCHLORITE IN FLOWING SALT-CONTAINING WATER, AND ALSO A DEVICE SUITABLE FOR CARRYING OUT SUCH A METHOD

Abstract: Method for the electrolytic preparation of hypochlorite in flowing salt-containing water for the purpose of disinfecting swimming baths, massage baths, drinking water etc. by means of an electro­ lysis reactor comprising a "scale-retarding" cathode having an open structure and a specific surface of at least 10 cm²/cm³.

Description: The invention relates to a method for the electro­ lytic preparation of hypochlorite in flowing salt-­ containing water in which the formation of deposits on the cathode is considerably reduced. In the electrolytic production of hypochlorite for the purpose of, for example, disinfecting swimming baths, massage baths, drinking water, cooling water cir­ cuits of inter alia electricity stations and waste water, problems generally arise in view of the fact that precipi­ tate formation ("scaling") occurs at the cathode. In this "scaling process" many types of insoluble compounds are produced such as calcium and magnesium hydroxide and carbonate, as a result of which after a relatively short time the cathode becomes inactive and the reactor becomes clogged up. For this reason considerable research has been carried out in relation to the removal, prevention or re­ duction of such precipitates. Examples of solutions of this problem are the regular rinsing with acid (in which the precipitates are dissolved), flowing through the electrolysis cell at very high speed (at these speeds the local pH rise is so small that "scaling" does not occur or scarcely occurs), regular reversing of the polarity of the electrodes and also the preliminary softening of the water to be used. However, none of these solutions has resulted in an economically usable technique. A particular solution for the above-mentioned problem is described in US Patent Specification 3,974,051. More particularly, this US Patent Specification relates to a method for reducing the deposits on the cathode in the electrolysis of salt-containing water, in which pro­ cess a) the cathode in the electrolysis cell has a flat continuous shape, the surface of the cathode having to have a roughness of less than 2.54×10⁻⁴ cm, and b) the salt solution is fed between the flat cathode and the parallel anode situated opposite it at a speed of from a minimum of 0.3 m/sec to a maximum value which corre­ sponds to the result of the formula: log v = 1.0 - [Cl₂]/4 with the speed v in m/s and [Cl₂] in g/l. In addition to the restriction on the feed-through speed of the solution to be subjected to electrolysis, said US Patet Specifi­ cation 3,974,051 specifies the use of a cathode with a very smooth surface. In this manner, apparently, the adhesion of seed crystals or deposits to the cathode is made difficult. Such cathodes according to said US Patent Specification are constructed of metals such as titanium, nickel and many types of iron and nickel alloys. It has emerged however that the method described in US Patent Specification 3,974,051 has a few disadvan­ tages. On the one hand, in particular expensive cathodes with a very flat surface have to be used, while on the other hand, the electrolysis device used according to the said patent specification, which device comprises a multi­ plicity of flat plate electrodes, because of the shape thereof, is restricted to a certain optimum as regards the yield of hypochlorite. This last disadvantage is associated with the oxidation, which possibly occurs, of the hypochlorite ion formed at the anode, as a result of which the current efficiency is adversely affected. It has been found that the above-named disadvan­ tages can be eliminated if an electrolysis reactor is used, the cathode of which has an open structure and also a specific surface area of at least 10 cm²/cm³ With advan­ tage such a cathode has a surface area of 100-600 cm²/cm³. The cathodes according to the invention can be constructed from fibre bundles, felt and fabrics based on, for example, graphite or carbon fibres or even meal fibres. Moreover, cathodes according to the invention can be con­ structed as sponge-like materials, for example a porous metal or in the form of granular stacks of, for example, small metal balls or graphite grains smaller than 1 mm. These last systems have a specific surface area of, for example, 10 cm²/cm³ or more. If the cathode according to the invention consists of a material constructed of fibres, said fibres have, e.g. a thickness in the range of 2-25 µm and the material has a porosity of 0.7-0.98. With advantage, fibres with a thickness of 5-15 µm are used. Metal fibres of e.g. stainless steel being relatively strong may have above thickness of 2 µm. When the cathode according to the invention is used in electrolysis reactors for the electrolytic prep­ aration of hypochlorite in flowing salt-containing water, the formation and deposition of precipitates take place much more slowly than for the cathodes known from the prior art. A further advantage of the cathode, according to the invention and based on, for example, graphite or car­ bon fibres is the low cost price, which is considerably lower than that of the metal cathodes known from the prior art, for example by a factor of 100. For this rea­ son it is generally economically justified simply to re­ place a cathode contaminated after a considerable operat­ ing time by a new cathode so that a cathode regeneration process to be carried out separately can be omitted. Such an advantage is appealing in particular when using the method according to the invention for the purpose of swimming baths and the like, where facilities for such a cathode regeneration are not available or scarcely avail­ able. At the same time it may be quoted as an advantage of the method according to the invention that because of the high porosity of the cathode and the low liquid speed in the electrolysis reactor the fall in pressure through the electrolysis reactor can be low in comparison with that according to the present prior art. In the example below a platinum electrode is used as comparison electrode according to the prior art, of which it is generally known that it has a fairly smooth surface (compare US Patent Specification 3,974,051). Example By means of this test the rate of precipitation of CaCO₃ on a fibre electrode is measured as a function of the geometrical current density. The water used in this test contained 10⁻³ M Ca (HCO₃)₂ and 0.05 M NaCl. For comparison use was made of a platinum gauze cathode and a carbon fibre electrode was used as cathode according to the invention. This electrode, which was constructed of said fibres of approximately 10 µm thick­ ness and which had a porosity of approx. 95%, had a spe­ cific surface area of approximately 200 cm 2/cm3 . The CaCO₃ precipitate is expressed in mg/h per cm² of geo­ metrical surface area. The temperature at which the electrolysis was carried out was room temperature. The liquid speed in the cathode was 8 cm/s. As anode a plat­ inum gauze electrode was used. In Figure 1 the rate of precipitation of CaCO₃ per unit surface area of, on the one hand, the platinum cath­ ode (1) and, on the other hand, the fibre cathode accord­ ing to the invention (2) is shown. In Figure 2, which is an enlarged reproduction of the initial section of Figure 1, the difference between the "scaling" on the cathode according to the prior art (1) and on the fibre cathode according to the invention (2) is given in more detail. In Figure 1 the rate of precipitation of CaCO₃ is also shown for water containing 10⁻³ M Ca(HCO₃)₂ and 0.5 M NaCl (see curve (3) in Figure 1 for the fibre cathode according to the invention and nearly curve (1) in Figure 1 for the platinum cathode according to the prior art). As is evident from Figures 1 and 2, for a constant current density virtually no deposition of precipitate takes place at the fibre electrode according to the invention, while rapid precipitate formation takes place at the platinum cathode according to the prior art.

Claims: 1. Method for the electrolytic preparation of hypo­ chlorite in flowing salt-containing water, characterized in that an electrolysis reactor is used, the cathode of which has an open structure and a specific surface area of at least 10 cm²/cm³. 2. Method according to Claim 1, characterized in that the cathode has a specific surface area of 100-600 cm²/cm³. 3. Method according to Claim 1 or 2, characterized in that the cathode consists of a material constructed of fibres. 4. Method according to one or more of Claims 1-3, characterized in that the cathode consists of a material, constructed of fibres having a thickness of 2-25 µm, which has a porosity of 0.7-0.98. 5. Method according to Claim 4, characterized in that the cathode consists of a material constructed of fibres having a thickness of 5-15 µm. 6. Method according to one or more of Claims 1-5, characterized in that the cathode is constructed of graphite or carbon fibres. 7. Method according to one or more of Claims 1-6, characterized in that the cathode consists of woven or felt-like material. 8. Method according to one or more of Claims 1-7, characterized in that the cathode is constructed of metal fibres. 9. Method according to Claim 1, characterized in that the cathode is constructed of sponge-like material or of a granular stack. 10. Device, comprising at least an electrolysis reactor, a water tank and also a circulation pipe provided with a pump between, on the one hand, the electrolysis reactor and, on the other hand, the tank, characterized in that the electrolysis reactor contains a cathode accord­ ing to one or more of Claims 1-9. 11. Method for disinfecting water, characterized in that said water is subjected to the method according to one or more of Claims 1-9 or is mixed with the hypochlorite solution obtained by said method.

Inventors: DE JONG, ANTONIUS M. C. P., VAN DUIN, PIETER JOHANNES
Applicants: NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1986
Main classification: C25B1
IPC codes: C02F1, C25B1