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

Chapter 3
concentrations of organic compounds in the brine, which are usually low due to brine quality
requirements (see also Table 2.4).
Halogenated organic compounds are included in Annex II to the Industrial Emissions Directive
[ 77, Directive 2010/75/EU 2010 ].
3.4.2.3.8 Sulphite
Sodium (hydrogen) sulphite is frequently used for the removal of free oxidants from waste
water. It is usually applied in stoichiometric excess, with the remainder being discharged. One
membrane cell plant reported sulphite concentrations to be consistently < 5 mg/l (weekly
measurements) at the outlet of the electrolysis plant, while a second plant reported sulphite
concentrations of 0.1 – 1.0 mg/l with an average of 0.54 mg/l (periodic measurements) and a
third reported sulphite concentrations to be < 1 mg/l [ 57, EIPPCB 2011 ].
3.4.2.4 Generation of wastes
The quantity of brine filtration sludges mainly depends on the incoming salt. The salts
precipitated during salts used for the purification of the brine are removed from the brine in a
filter unit or a clarifier. The precipitate consists mainly of calcium carbonate and magnesium
hydroxide, and in some cases barium sulphate. In the case of the mercury cell technique, the
residual brine in the sludge usually contains some dissolved mercury in its oxidised form. In
some plants this brine is washed out to reduce the mercury contamination. The sludge can be
filtered and disposed of as solid waste or periodically removed is usually removed
discontinuously by flushing with a weak hydrochloric acid solution. The acid causes the
precipitate to dissolve (except barium sulphate and mercury) and the relatively harmless
solution can be discharged with liquid effluent [ 3, Euro Chlor 2011 ]. The treatment of brine
filtration sludges containing mercury is described in Section 3.5.9.2. The remaining solid cake
consists mainly of calcium carbonate and magnesium hydroxide, in some cases barium sulphate
and, if the amalgam process is used, mercury.
For plants using vacuum salt, reported figures for brine purification sludges are in the range of
120 – 775 g per tonne of chlorine produced (i.e. 12 - 77 tonnes of sludges per year for an annual
chlorine production of 100000 tonnes).
For plants using rock salt, about 30 kg, on average, of brine sludges are generated for every
tonne of chlorine produced (i.e. 3000 tonnes of sludges per year for an annual chlorine
production of 100000 tonnes).
In the mercury process, brine filtration sludges contain mercury (see Sections 3.5.9 and 4.1.1.4).
Reported figures on the generation of brine filtration sludges are summarised in Table 3.17.
WORKING DRAFT IN PROGRESS
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