Hypochlorites electrochemical production

On comparing the energy consumptions of an electrochemical production of hypochlorite liquor Avith the indirect method of producing hypochlorite from... 

Electrochemical cells with MIO anodes for hypochlorite solution production as stock solutions for onsite addition of HOCl/OCl mixtures have found increasing application worldwide (cell units producing more than 100 kg FAC per day). Divided cells and NaCl concentrations in the range of seawater (3.5 %) are typical, but undivided cells are also offered by some manufacturers. Chlorate formation inside the stock solution at longer storage times is a problem with this technology ... 

HCIO4, one of the strongest of the mineral acids. The perchlorates are more stable than the other chlorine oxyanions, ie, chlorates, CIO chlorites, CIO or hypochlorites, OCf (3) (see Chlorine oxygen acids and salts). Essentially, all of the commercial perchlorate compounds are prepared either direcdy or indirectly by electrochemical oxidation of chlorine compounds (4—8) (see Alkali and chlorine products Electrochemical processing). 

Electrolysis of Chloride Solutions. Chloride may be oxidized electrochemically to chlorine or hypochlorite, chlorate, and perchlorate. The distribution of products of the first oxidation step... 

Saponification to the sulphonic acid yields the product marketed as Nafion. This material is said to be permselective in that it passes cations but not anions. It is used as a membrane material in electrochemical processes, in for example the manufacture of sodium hypochlorite. 

Current world chlorate production (about 700 kilotons per year) is based entirely on an electrochemical method where reactions (15.21) to (15.34) occur simultanously in undivided cells. A small amount of bichromate ions are added to the solution to reduce chlorate losses by rereduction at the cathode these form a thin protective layer at the cathode which passivates the reduction of chlorate and hypochlorite ions. 

When chlor-alkali electrolysis is conducted in an undivided cell with mild-steel cathode, the chlorine generated anodically will react with the alkali produced cathodically, and a solution of sodium hypochlorite NaClO is formed. Hypochlorite ions are readily oxidized at the anode to chlorate ions this is the basis for electrolytic chlorate production. Perchlorates can also be obtained electrochemically. 

At one time, sodium hypochlorite was manufactured electrochemically on a substantial scale. Now it is regarded as a by-product of the chlor-alkali industry [10]. On the other hand, there are many situations where low volumes of hypochlorite may be required or the requirement is irregular. Aqueous solutions of hypochlorite are much safer than chlorine gas but contain < 15wt% of active chlorine. Hence, storage and transportation costs are relatively high. Often the most convenient and cost-effective solution is to eleetrolytically generate OC1- in situ [10]. 

By diaphragms in a narrow sense we also understand a kind of partition which prevents as much as possible the diffusion of dissolved products of the electrolysis from one electrode to the other but must not hinder the passage of ions migrating under the influence of the electrical field in other words this partition should possess a high diffusion resistance but a low electrical resistance. Unless mutual diffusion and the mechanical mixing of anolyte and catholyte are prevented, i. e. on electrolyzing a solution of common salt, a mutual reaction occurs between the products of electrolysis, namely chlorine and hydroxide in which case hypochlorite ions are formed. They can then be converted, either electrochemically or chemically to chlorate ions. On separating both electrodes... 

It can be seen from the above explanation that hypochlorite is virtually an unstable product of electrolysis regarding its easy electrochemical oxidation to chlorate. One way of suppressing oxidation of the hypochlorite ions at the anode is to increase the current density on the electrode to such a high value so that the main part of the current will be consumed in producing chlorine, and... 

Also in an alkaline solution chlorate is the product of an electrochemical reaction. In this case hypochlorous acid formed by hydrolysis of the dissolved chlorine is neutralised in the immediate vicinity of the anode and the resulting hypochlorite ions are oxidized at the electrode to chlorate ions, as soon as formed (see equation (XVII-11)). Therefore, the concentration ot hypochlorite ions in the bulk of the solution with an alkaline electrolyte will be lower than in a neutral one. The current efficiency in a slightly alkaline solution may reach 66.67 per cent, but it decreases with rising alkalinity as a result of increasing hydroxyl ions discharge. However, if current efficiency approximating 60 per cent, which was normal in the first plants for electrochemical manufacture of chlorates, is acceptable, work with a moderately alkaline electrolyte will be the easiest. 

The electrical potential and/or current required for electroenzymatic treatment have been shown to be lower than those needed in electrochemical treatment, which are not economically viable for large-scale. Electroenzymatic oxidation by peroxidases was proposed for the oxidation of veratryl alcohol by LiP [40], Then, electroenzymatic reactors have been used for the treatment of petrochemical wastewater [91], dyes, and textile wastewater [90, 92, 118] and phenol streams [93] utilizing peroxidase immobilized onto inorganic porous Celite beads or directly onto the electrode. The integration of a second electrochemical reactor, which generated hypochlorite in the presence of sodium chloride, has been used for indirect oxidation of the reaction products of the electroenzymatic treatment [91]. 

The kinetic expressions shown before explain the direct electrochemical processes. However, many of the processes with interest in electrochemical oxidation or coagulation treatments are not direct processes, but simply chemical processes caused by the products generated at the electrode surface (mediated electrochemical processes). In addition, several chemical processes not related to the electrochemical process can occur in the electrochemical cell. Thus, in electrooxidation, the most common case is the mediated oxidation carried out by oxidants electrochemically generated on the electrode surface, such as hydroxyl radicals, hypochlorite, peroxo-sulphates, or peroxophosphates. In electrochemical coagulation, aluminum species formed during the electrochemical dissolution of the anodes are responsible for the later coagulation reactions. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Cheng, C.Y. and Kelsall, G.H. (2007) Models of hypochlorite production in electrochemical reactors with plate and porous electrode. J. Appl. Electrochem. 37, 1203-1217. 

Kraft, A., Stadelmann, M., Blaschke, M., Kreysig, D., Sandt, B. and Schroeder, F. (1999a) Electrochemical water disinfection, part I Hypochlorite production from very dilute chloride solutions. J. Appl. Electrochem. 29, 861-868. 

Chlor-alkali production — With a 63% production volume of the total world chlorine capacity of about 43.4 million tons (in 1998), the chlor-alkali (or chlorine-caustic) industry is one of the largest electrochemical technologies in the world. Chlorine, Cl2, with its main co-product sodium hydroxide, NaOH, has been produced on industrial scale for more than a century by -> electrolysis of brine, a saturated solution of sodium chloride (-> alkali chloride electrolysis). Today, they are among the top ten chemicals produced in the world. Sodium chlorate (NaC103) and sodium hypochlorite (NaOCl, bleach ) are important side products of the... 

The success of indirect electrochemical oxidation and disinfection lies in the production of safe drinking water especially in rural areas where the necessity for skilled maintenance personnel should be avoided. The simplicity, stability, and low power consumption of devices such as the sodium hypochlorite generator described by Bashtan et al. [71] and the TiN reactor described by Matsunaga et al. [34] make these devices most cost effective for small-scale applications in remote locations. 

The need to study the influence of chloride ions on the electrochemical oxidation of phenol was necessitated by the fact that real (industrial) effluents contained chlorides in significantly high concentrations. Chloride ions, in solution, have the ability to produce chlorinated organic products, especially in acidic media. Halocompounds are usually more harmful to the environment than the organic compounds they result from. It has been reported that under certain conditions, electrogenerated chlorine converts to hypochlorite, which is a powerful oxidant but weak chlorinating agent [78]. 

As a result of the simultaneous production of oxygen, the utilization efficiency of electrical energy is a third less than with the pure chemical formation of sodium chlorate. The process parameters are therefore selected so as to suppress the electrochemical oxidation of hypochlorite e.g. the concentrations, the temperature (60 to 75°C), the process pH (6.9), the flow conditions and the residence time in the electrolysis cell. Modern plants utilize electricity energy with an efficiency of > 93%. 

Other alternatives for the oxidant for stoichiometric oxidations include the use of a selenoxide [99], including a photochemical oxidation of catalytic selenium [100], iodine [101], sodium chlorite [102], hypochlorite [103], and electrochemical methods [101,104]. Even air can be used as the oxidant [99,100], but care has to be taken with regard to the choice of solvent as cleavage of the product 1,2-diol can occur, especially when the alkene has an aryl substituent [53, 105, 106]. 

Mn" porphyrins may be electrochemically oxidized in non-aqueous media to yield Mn " cation radicals and dications as the probable products." The redox potentials depend on the nature of the solvent, counter ion present, of any axially-bound ligands as well as on the basicity of the porphyrin ring involved. Water-soluble Mn " porphyrins are oxidized by a range of oxidants in aqueous alkaline solution to the corresponding Mn porphyrins which appear to exist as fx-oxo dimers using hypochlorite as oxidant, a second oxidation step also occurs. In this case the product has been postulate to be a Mn 0x0 porphyrin. The Mn and Mn porphyrins show only limited stability in water and revert to the stable Mn" porphyrin upon standing in the dark for several... 

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