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

Chapter 3
environment are included in Annex II to the Industrial Emissions Directive [ 77, Directive
2010/75/EU 2010 ].
3.4.2.3.5 Chlorate and bromate
The main sources of chlorate and bromate are the purge from brine purification and sometimes
also water streams that have been treated to decompose free oxidants into the less reactive
chlorate and bromate. Chlorate (ClO3 - ) and (to a much lesser extent) bromate (BrO3 - ) are formed
as by-products during electrolysis, particularly when using the diaphragm or membrane cell
technique (see Section 2.1 and Table 2.1). Bromate is present in small quantities, as bromine is
only present as a contaminant of the salt. Chlorine may react with hydroxyl anions (OH - ) to
produce chlorate while bromine may react with hydroxyl anions (OH - ) to produce bromate.
Due to the recycling of the brine, these components build up. Chlorate and bromate are
unwanted compounds in the process as their presence will reduces the solubility of incoming
salt and, in the case of the membrane cell technique, may negatively affect the caustic soda
quality, the ion-exchange resins used for brine purification and the membranes. Measures are
usually taken to keep the level of chlorate low (usually below 10 g/l in the feed brine). To
maintain this level, there is a need to purge an appropriate volume of the brine, which can be
carried out along with operating under acidic conditions in the anolyte (approx. pH 2, see
Sections 2.5.3.2 and 4.3.6.4.2) or along with using a chlorate decomposer (see Sections 2.5.5,
4.3.6.4.3 and 4.3.6.4.4). The purge can also be used as raw material in a plant which produces
sodium chlorate (see Section 4.3.6.4.5).
In the case of diaphragm cell plants, any bromate or chlorate formed in the anolyte compartment
migrates through the diaphragm and may be reduced by nascent hydrogen at the cathode (see
Section 4.3.6.3.6). The residual levels remain in the caustic liquor which leaves the cell. To
maintain this level, there is a need to operate under acid conditions (approx. pH 2) in the
anolyte. If this is not the option chosen for the process (higher pH), a chlorate decomposer may
be necessary to remove chlorate before purging. The reported concentration is between 5 to 10
g/l if no chlorate decomposer is installed and around 1 to 2 g/l after decomposition [Bayer
Uerdingen, 1998]. Specific emissions are between 0.14 and 1 kg per tonne of chlorine produced.
Chlorate is less reactive than chlorine and has a lower toxicity for aquatic biota. Bromate, on the
contrary, is a reactive compound. Nevertheless, it is present in small quantities, as bromine is
only present as a contaminant of the salt. Bromate releases are typically 10 – 100 times lower
than the chlorate figures.
Reported emission concentrations and factors are summarised in Table 3.13 and Table 3.14.
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88 December 2011 TB/EIPPCB/CAK_Draft_1