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

Chapter 3
brine circuit. This waste water is normally treated to remove mercury before discharge into the
environment.
The condensate from hydrogen drying contains mercury, but can be recycled as feed water to
the decomposer.
The mercury-contaminated filter-cakes are normally pumped with mercury-containing waste
waters to the purification stage and the sludge therefrom is either distilled at the plant, stored or
sent to a deposit. {This paragraph was moved to Section 3.5.9.2.}
Mercury-contaminated wash water from the inlet and outlet boxes and from the electrolysis hall
The wash water from the inlet boxes flushes entrained caustic from the recycled mercury. This
water contains mercury, but can be used as feed water to the decomposer like the condensate
from hydrogen drying. The wash water from the outlet boxes flushes entrained brine from the
amalgam. This water also contains mercury, but can also be reused as feed water to the
electrolytic cells.
Condensate from hydrogen drying
Mercury can be released during hydrogen drying. The hydrogen which is formed in the
decomposer contains significant amounts of water and mercury. A large part of this water is
condensed by cooling the hydrogen. The condensate contains mercury, but can be used as feed
water to the decomposer. {This paragraph was shortened and moved upwards.}
Filtered caustic liquor
Filtered 50% (by weight) caustic liquor contains some mercury. AkzoNobel in Bohus (Sweden)
reported an average mercury content of 8 µg/l NaOH and 33 µg/l KOH for 1998. Small
quantities of mercury thus appear in the effluents of user plants. {This information is included in
Section 3.5.8 on emissions via products.}
Mercury contamination of rainwater effluents from mercury process plants
There can be considerable emissions of mercury with run-off water. The soil at many sites is
contaminated with mercury due to deposition of diffuse emissions and/or historical disposal of
mercury-contaminated wastes. The mercury leaks from the soil and ends up in the run-off water.
Rainwater is normally collected and treated with the other water streams of the plant. In some
plants the rainwater is collected in sewer systems.
The amount of waste water can be reduced by separately disposing of the cooling water and
process water and by feeding the condensate and wash water from the end boxes back into the
process, provided the water balance allows for this. Waste water volumes of 0.3 – 1.0 m 3 /t
chlorine produced are achievable [ 1, Ullmann's 2006 ]. Waste water volumes generated in
once-through brine plants are approximately 10 m 3 /t chlorine produced (see Section 4.3.2.3.7).
WORKING DRAFT IN PROGRESS
Usually all waste water streams (potentially) contaminated with mercury are combined and
commonly treated. The same techniques are used during normal during plant operation and
during decommissioning. They are described in Section 4.1.3.1.
Mercury in waste water streams
Mercury-contaminated waste water streams are collected from all sources and generally treated
in a waste water treatment plant. The amount of waste water can be reduced by filtration and
washing of the sludges to remove mercury before feeding the condensate back into the brine. A
waste water rate of 0.3 to 1 m 3 per tonne of chlorine is achievable [Ullmann’s, 1996]. The liquid
effluent for processes operating once-through brine systems is, however, much greater than for
120 December 2011 TB/EIPPCB/CAK_Draft_1