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

Chapter 4
Driving force for implementation
The driving force for implementation of the techniques described in Sections 4.3.6.3.3 to
4.3.6.3.6 is environmental legislation.
Example plants
Almost all existing plants in EU-27 and EFTA countries use techniques to reduce the
concentration of free oxidants at the outlet of the chlor-alkali unit.
4.3.6.3.3 Chemical reduction
Description
This technique consists in destroying free oxidants by reaction with reducing agents such as
sulphite and hydrogen peroxide in stirred tanks.
Technical Description
A general description of chemical reduction can be found in the CWW BREF [ 124, COM
2011 ].
Chemical reduction agents such as sulphur dioxide (SO2), sodium sulphide (Na2S), sodium
sulphite (Na2SO3), or sodium thiosulphate (Na2S2O3) or hydrogen peroxide (H2O2) are used to
destroy the free oxidants. The chlorine or hypochlorite is reduced to chloride (Cl - ). The choice
of the chemical reducing agent is influenced by cost, availability and ease of handling. The
added chemicals in turn form a range of sulphur oxy anions, predominantly sulphates (SO4 2- ). In
some cases, hydrogen peroxide (H2O2) is used to destroy the free oxidants if chlorine
concentrations are very low. A final treatment with H2O2 can reduce the free chlorine
concentration from 300 ppm to 1 ppm of free chlorine [Le Chlore, 1996]. Depending on the
reducing agent, the following reactions take place [ 2, Le Chlore 2002 ], [ 207, Stitt et al. 2001 ]:
Sulphur dioxide: SO2 + NaOCl + 2 NaOH V Na2SO4 + NaCl + H2O
Sodium sulphide: Na2S + NaOCl + H2O V S + NaCl + 2 NaOH
Sodium sulphite: Na2SO3 + NaOCl V Na2SO4 + NaCl
Sodium thiosulphate: 2 Na2S2O3 + NaOCl + H2O V Na2S4O6 + NaCl + 2 NaOH
Hydrogen peroxide: H2O2 + NaOCl V O2 + NaCl + H2O
Chemical reducing agents have the advantage of also reacting with chloramines and brominated
free oxidants.
The reaction with Na2S2O3 (sodium thiosulphate) is the following:
2 Na2S2O3 + NaOCl + x H2O V Na2S4O6 + NaCl + 2 NaOH + (x-1) H2O
WORKING DRAFT IN PROGRESS
The reaction with H2O2 (hydrogen peroxide) is the following:
NaOCl + H2O2 V NaCl + H2O + O2
Sufficient residence time and an excess of reducing agents are required to ensure complete
destruction of free oxidants [ 17, Dutch Ministry 1998 ]. In order to control the heat of the
exothermic reaction, highly diluted solutions are used to limit the temperature to about 50 °C.
For example, to reduce 1 kg of chlorine absorbed, 4.45 kg of reactive agent Na2S2O3 or 89 kg of
diluted 5 wt-% solution are is required [ 2, Le Chlore 2002 ] [Le Chlore, 1996].
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