The steps of the process are :
- the partial electrolysis of the sodium chloride contained in seawater or brine made to flow between sets of electrodes (anodes and cathodes) in an electrolyzer connected to a source of direct current
- the chemical reactions which take place in the electrolyzer between chlorine and caustic produced by electrolysis .
Reaction mechanism
The direct current, passing through the seawater or brine flowing in the electrolyzer, where the sodium chloride is completely dissociated into the ions Na+ and Cl-, causes the following electrochemical and chemical reactions:
Main reactions :
at the anode : chlorine generation
2Cl- ----> Cl2 + 2e-
at the cathode : hydrogen evolution and hydroxide formation
2H2O + 2e- ----> H2 + 2OH-
chemical reaction : chlorine and hydroxide ions react in the electrolyzer to form hypochlorite
Cl2 + 2OH- ----> Cl- + ClO- + H2O
overall mass balance : production of hypochlorite
NaCl + H2O = NaClO + H2
Side reactions :
Simultaneous to the above main reactions, side reactions of both a chemical and electrochemical nature may occur :
hypochlorite thermal decomposition :
2ClO- ----> 2Cl- + O2
hypochlorite oxidation :
3ClO- ----> ClO3- + 2Cl-
anodic oxygen evolution :
4OH- ----> 2H2O + O2 + 4e-
hypochlorite cathodic reduction :
ClO- + H2O + 2e- ----> Cl- + 2OH-
The operating conditions are such that these side reactions are minimised.
However, their effect on the current efficiency slightly increases the amount of direct current required to produce active chlorine, and causes the presence of small quantities of oxygen in the hydrogen produced. Additionally, some cations which are present in seawater, such as calcium, magnesium and heavy metals, may form hydroxides and carbonates in the form of suspended solids, which are carried away by the sea water stream, but some of which cause scaling on the electrodes. This scaling is easily removed by periodically flushing the electrolyzer with hydrochloric acid solution.
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