(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 2
The basic principle in the electrolysis of a sodium chloride solution is the following:
at the anode, chloride ions are oxidised and chlorine (Cl2) is formed;
at the cathode: in the mercury cell process, a sodium/mercury amalgam is formed; and
hydrogen (H2) and hydroxide ions (OH - ) are subsequently formed by the reaction of the
sodium in the amalgam with water in the decomposer denuder; in membrane and
diaphragm cells, water decomposes to form hydrogen (H2) and hydroxide ions (OH - ) at
the cathode.
For all processes the dissolving of salt, sodium chloride, is:
NaCl V Na+ + Cl-
The anode reaction for all techniques processes is:
2 Cl - V Cl2 + 2 e -
2 Cl - (aq) V Cl2(g) + 2 e -
The cathode reaction in mercury cells is:
The reaction in the decomposer is:
Na + + e - + Hgx V Na-Hgx
2 Na-Hgx + 2 H2O V 2 NaOH + H2+ 2 Hgx
The cathode reaction in membrane and diaphragm cells is:
2 Na + + 2e - + 2 H2O V 2 NaOH + H2
2 Na + (aq) +2 H2O + 2e - V H2(g) + 2 Na + (aq) + 2 OH - (aq)
The overall reaction for all techniques is:
2 NaCl + 2 H2O V 2 NaOH + H2 + Cl2
2 Na + (aq) + 2 Cl - (aq) + 2 H2O V 2 Na + (aq) + 2 OH - (aq) + Cl2(g) + H2(g)
The products of the electrolysis are formed in a fixed ratio which is 1070 – 1128 kg of NaOH
(100 wt-%) and approximately 28 kg of H2 per tonne of Cl2 produced. This product combination
is often referred to as the electrochemical unit (ECU).
Some side reactions occur during electrolysis leading to a loss of efficiency [ 10, Kirk-Othmer
2002 ]. At the anode, oxidation of water to oxygen and of hypochlorous acid to chlorate takes
place:
2 H2O V O2 + 4 H + + 4 e -
or 4 OH - V O2 + 2 H2O + 4 e -
WORKING DRAFT IN PROGRESS
12 HClO + 6 H2O V 4 ClO3 - + 8 Cl - + 24 H + + 3 O2 + 12 e -
Hypochlorous acid is formed by disproportionation (dismutation) of chlorine in water:
Cl2 + H2O HClO + H + + Cl -
Chlorate is also produced by chemical reactions in the anolyte:
2 HClO + ClO - V ClO3 - + 2 Cl - + 2 H +
These four major side reactions are repressed by lowering the pH value.
20 December 2011 TB/EIPPCB/CAK_Draft_1