Hypochlorites electrode processes

Girault and co-workers reported the application of plane interdigitated microband electrodes to an inorganic electrosynthesis of industrial interest die hypochlorite generation from sea water electrolysis. The system was studied in a laboratory cell [17] and also in a pilot plant [19]. A major problem in this synthesis is related to the deposition of scale (calcium and magnesium hydroxide) on the cathode due to the local production of OH anions. The coupling of the electrode processes permits the pH excursions on the cathode to be restricted, leading to a decrease in scale deposition. 

The advantage of chlorate formation by the chemical reaction as compared to the electrochemical reaction is obvious, because the process (XVII-12) proceeds without oxygen losses, which is not. the case with the reaction (XVII-11). It is, therefore, easy to understand that the more of the hypochlorite ions is converted to chlorate by the chemical reaction, the better is the current efficiency. In practice, however, the discharge of CIO- ions cannot be eliminated entirely so that current efficiencies attain 75 to 95 per cent according to the electrodes used. 

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. 

A discussion of the chemical reactions which occur at electrodes in a chemic at system through which an electrical current is passing has been given in Chapter 10. Mention has also been made in earlier chapters of some electrochemical processes. In the discussion of hydrogen peroxide and the peroxy acids it was pointed out that peroxydisul-furic acid, peroxy sulfuric acid, and hydrogen peroxide can be made by the electrolysis of a sulfuric acid solution and in Chapter 13 the electrochemical preparation of hypochlorites, chlorates, and perchlorates was discussed. 

The chloralkali process, which involves the electrolysis of brine, is widely used for the production of sodium hydroxide and chlorine gas. During electrolysis it is necessary to keep the sodium hydroxide separate from the chlorine, to prevent the formation of sodium hypochlorite, NaOCl, and this determines cell design. In older processes, the cathode used was flowing mercury. At this electrode, sodium is formed, and this dissolves in the mercury to form a sodium amalgam. The sodium amalgam is removed continually from the cell and reacted with water to produce hydrogen gas and... 

Indirect oxidation. The indirect oxidation of cyanide is primarily based on the oxidation of chloride ions to produce hypochlorite. In practice the cyanide feed solution can be dosed with sodium chloride as a saturated solution and passed continuously through the cell. The indirect processes is said to have several potential advantages over direct oxidation, which include a lower cell voltage, through the increased conductivity, fast chemical reaction and lower overvoltages and reduced wear with platinum or DSA type coated electrodes. Energy consumptions are quoted at around 4-10 kWh kg cyanide. 

Nitrate electroreduction has been extensively studied over the last few decades. This reactitMi is a multi-electron transfer process showing different mechanisms as a function of pH, nitrate and supporting electrolyte concentration, chemical composition and structure of the catalyst. In recent years, nitrate electroreduction has been widely studied over diamond and many monometallic electrodes such as Pb, Ni, Zn or Rh, Ru, Ir, Pd, Cu, Ag and Au. Because none of the common pure metals is able to provide high selectivities for nitrogen, bimetallic alloys or monometals modified with foreign metal adatoms were prepared and evaluated for the reduction of nitrate. More recently, an electrochemical process in which nitrate ions are reduced to ammonia at the cathode, and where the produced ammonia is oxidized at the anode to nitrogen with the contribution of hypochlorite ions, has been evaluated. 

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