Chlorine production, electrodes

Ion implantation has also been used for the creation of novel catalyticaHy active materials. Ruthenium oxide is used as an electrode for chlorine production because of its superior corrosion resistance. Platinum was implanted in mthenium oxide and the performance of the catalyst tested with respect to the oxidation of formic acid and methanol (fuel ceU reactions) (131). The implantation of platinum produced of which a catalyticaHy active electrode, the performance of which is superior to both pure and smooth platinum. It also has good long-term stabiHty. The most interesting finding, however, is the complete inactivity of the electrode for the methanol oxidation. 

Electrode isolation is practiced to minimize chlorine production and to reduce fouhng. A flush solution free of chlorides or with reduced pH is used to bathe the electrodes in some plants. Further information on electrodes may be found in a work by David [ Electrodialysis, pp. 496 99, in Porter (ed.), op. cit.]. 

Ru02 is an important electrode material for industrial anodic processes. Special attention is deserved by the so-called dimensionally stable anodes (DSA) invented by H. B. Beer in 1968. These are formed by a layer of a microcrystalline mixture of Ti02 and Ru02 (crystallite size less thn 0.1 jum) on a titanium support (Fig. 5.26). This material is suitable as anode for chlorine and oxygen evolution at high current densities. For industrial chlorine production, it replaced the previously used graphite anodes. These... 

Electrochemical Processes. The reductive cleavage of azo-group-containing dyes has been applied on a full scale for the decolorization of concentrates from batch dyeing. Depending on the color, decolorization of up to 80% of the initial absorbance can be obtained. Mixed processes consist of combinations of electrochemical treatment and precipitation by use of dissolving electrodes [43,49]. Such techniques have been described in the literature and have, in part, also been tested on a full scale. Anodic processes that form chlorine from oxidation of chloride have also been proposed to destroy dyes, but care has to be taken with regard to the chlorine and chlorinated products (AOX) formed [114,115]. 

Dimensionally stable anodes represent, from the commercial point of view, one of the more important inventions in the history of industrial electrochemistry. Even so, carbon electrodes were an entrenched and stable part of the mindset of the chlorine production community. deNora s sales technique was at first to offer use of the new electrodes at zero cost, only asking for half the dollar gains obtained by a company s use of the new electrodes. This technique created a market for the new electrodes, so that it could soon be replaced with a more normal sales technique. 

Addition of dimethylformamide makes it possible to achieve essential increase in monoohlorobenzene yield. In this case the total current efficiency of the benzene chlorination products is 9056, 3B-33% substance yield, while the chlorobenzene current efficiency 15%. This effect mi t have been concerned with the enhancement of benzene solubility in the water phase. Furthermore dimethylformamide depresses side processes, concerned v/ith both electrochemical conversion of chlorine, formed on the electrodes and formation of dichlorobenzene. As fair as chlorination of benzene derivatives to the side oliain is concerned, the factors that influence the chemical chlorination (UV and more hard radiation, the presence of different initiators of free radicals formation) favorably affect the isolation of benzyl chloride, o-, m-, p-xylylchloric -. The current efficiency is more than 85%. 

The influence of hydrochloric acid concentration appeared to be similar to that in the case of aromatic compounds chlorination in the electrochemical system. While working with aphite electrodes in 27 - 30 hydrochloric acid solutions at the range of current density values from 1 to 4 IcA/m, and at the range of temperatures from 35 to 80°C, the dominating product of ethylene chlorination reaction was stated to be 1,2-dichloroethane. At 35°C, the yield of ethylene chlorination products is 68-8056 at rather low current densities. The increase in the temperature of chlorination process up to 50°C, at the same current densities cause the fiy-owth of the diohloroethane content in the mixture up to 77% - 92%. Though temperature growth to 65°C leads to some increase in total mixture mass of ethylene chlorination products, however, the substance yield of 1,2-diohloroethane is only 75-80%, as lateral processes take place. While temperature rises to SO C,... 

Table 3.6 Overvoltages at the electrodes of a chlorine production cell.

Current density in diaphragm cells is relative low, usually only up to 2-3 kA m . It is difficult to adapt it to changes in the required chlorine production. Special low-cost cell constructions allow an economic operation at decreased current density and correspondingly low electrical power consumption, in spite of the needed larger electrode area. 

High chlorine production yield and suitable lifetime are achieved with mixed metal oxide electrodes (MMO), initially developed for the chlor-alkali process [4]. They consist of a base metal, a titanium plate, or grid for saltwater chlorination coated with a combination of at least two metal oxides including titanium, ruthenium, iridium, or tantalum [5]. Elaborated proprietary production processes are used to achieve reliable electrodes. The chlorine faradaic yield under typical pool working conditions (3 g/L NaCl, current density 50 mA/cm ) is around 60 % [6]. Good quality electrodes achieve several years of service time (several thousands of hours of continuous operation) with a 3 h reversal time and at 60 % of the nominal maximum power. Lifetime is reduced by high current densities and by frequent polarity reversal. Operation at... 

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