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

Chapter 3
and processed elsewhere. Moreover, the regular bleach is also destroyed when there is
insufficient on- or off-site demand which is the case in some EU Member States.
{This section was moved to here from Section 3.4.3.2.2.}
3.4.4 Emissions and waste generation from sodium and potassium
hydroxide processing
Caustic soda solution from the diaphragm process have a concentration of 10-12% NaOH with
15-17% NaCl when leaving the electrolytic cell and the caustic concentration from membrane
cells is 30-33% NaOH with little NaCl. Generally speaking, the caustic needs to be concentrated
to at least 50% and impurities need to be removed. {The contents of this paragraph are covered
in Chapter 2.}
In the diaphragm cell plants process, about 5 approximately 4 tonnes of water must be
evaporated per tonne of 50 wt-% per cent caustic soda solution produced, if this is the desired
concentration [USEPA, 1995]. If sodium sulphate is not removed during the brine purification
process, salt recovered from the evaporators is often recrystallised to avoid sulphate build-up in
the brine. If the salt is recrystallised, the waste water will also contain sodium sulphates.
Significant levels of copper may also be present in the waste water due to the corrosion of pipes
and other equipment, along with iron and nickel which can dissolve to a certain extent from the
stainless steel equipment. In addition, the presence of nickel may be related to the use of
activated cathodes in the electrolytic cells. These metals are removed from the caustic by means
of filtration and/or electrochemical reduction. The regeneration of the filters or the reduction
cathodes generates an acid waste water flow which may contain iron and nickel at levels which
might need further treatment ought to be taken care of. Reported emissions of sulphate and
heavy metals from caustic processing in diaphragm cell plants are included in
Sections 3.4.2.3.2 and 3.4.2.3.6, respectively.
Waste water from the caustic evaporators in the membrane cell plants process contains caustic
soda solution and virtually no salt or sodium sulphates. It is usually recycled. In the mercury cell
plants process, caustic soda leaving the decomposer already has a concentration of 50 wt-%. is
directly concentrated to 50%.
The caustic solution is subsequently filtered. In membrane and diaphragm cell plants, the filters
can be flushed with a weak acid solution, causing the iron hydroxide and other metal hydroxides
metals to dissolve. The effluent is usually discharged, as most chlor-alkali plants have a
physico-chemical physical-chemical waste water treatment unit which partially removes
suspended solids and free oxidants.
It In mercury cell plants, the caustic contains practically no salt impurities, but it does contain
mercury. The mercury (ranging from 2.5 to 25 mg/l) can be released from the pumping tank
vents or from the vents from caustic filters, depending on the type of denuder and temperature.
Normally, the caustic soda is filtered with activated carbon to remove mercury before handling.
Emissions and waste generation related to this mercury removal are described in
Sections 3.5.6.3.3 and 3.5.9.3, respectively. Charcoal from the filtration contains around 150-
500 g of mercury/kg carbon.
WORKING DRAFT IN PROGRESS
In mercury cell chlor-alkali plants, the filter sludge is dewatered and in some cases led to a
mercury recovery retort and subsequently disposed of. In cases where no mercury retort is
present, the sludge has to be disposed of without mercury recovery. In membrane and
diaphragm cell plants, the filters can be flushed with a weak acid solution, causing the iron
hydroxide and metals to dissolve. The effluent is usually discharged, as most chlor-alkali plants
have a physical-chemical waste water treatment unit which partially removes suspended solids
and free oxidants. 3 tonnes of sludge from caustic filtration out of a total of 38 tonnes of sludges
(the other sources being the brine treatment sludge and the waste water treatment sludges)
100 December 2011 TB/EIPPCB/CAK_Draft_1