Hypochlorite and low-tonnage chlorine electrolysers

The overall future for electrochemical removal of metal from solution seems bright and several established cell designs are helping to provide a maturing technology. 

A number of electrochemically generated oxidants may be used to purify water streams. In Chapter 5, cells for hydrogen peroxide (section 5.7) and ozone (section 5.8) generation were described in some detail. The most traditional oxidizing agent in these circumstances, however, is undoubtedly hypochlorite or chlorine. 

The last decade has seen the introduction of a number of small electrolysis cells for the generation of either hypochlorite or chlorine gas in many applications, either type of cell could, in principle, be employed and the choice will then depend on technological factors. In the water and effluent-treatment industry, the common applications of on-site chlorine and hypochlorite cells will include the treatment of sewage (particularly at remote sites), the sterilization of water for food processes and hospital laundries, the treatment of water on board ships and for swimming pools, the treatment of cooling water at coastal power stations to prevent the growth of shellfish and seaweed in the pipes and the enhanced 

A wide range of cell designs and sizes have been described. The cells do not require a separator, since cathodically formed hydroxide  

In practice, the current efficiency may be reduced and specific energy consumption increased by several factors. At high OCl levels, elevated temperature 

Hence the simplest design is a closely spaced, bipolar stack of electrodes, although control of the hydrodynamics in the cell is essential to prevent cathodic reduction of the hypochlorite. Moreover when the anolyte feed is sea water, an additional 

For small-scale electrolysis units, energy consumption will be much less important than in a chlor-alkali process ease of operation with a minimum of maintenance and replacement of components and the initial cost of the total unit will more often determine the choice of the cell. As a result, for example, while the cathode will generally be steel, a wide range of anode materials including graphite, lead dioxide and platinized titanium have been used as well as dimension-ally stable anodes. Hence the quoted energy consumptions of hypochlorite cells lie in the range 4.5—7.0 kWh kg , considerably above those for a chlor-alkali cell. 

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