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

Chapter 3
3.5.8 Emissions via products
Some of the mercury leaves the process via the produced chlorine, hydrogen and caustic and
may therefore be emitted to the environment during subsequent uses.
Hot, moist chlorine leaving the cells contains small amounts of mercury which is mostly washed
out in the subsequent cooling process and may be fed back into the brine with the condensate.
The residual mercury is mostly trapped in sulphuric acid during chlorine drying. Two plants
report mercury concentrations of 0.085 and 10 mg/kg sulphuric acid. Mercury concentrations in
cool and dry chlorine gas are low and typically < 1 Zg/kg [ 57, EIPPCB 2011 ], [ 86, Euro Chlor
2010 ], [ 87, Euro Chlor 2006 ].
Mercury emissions from hydrogen which is neither emitted nor burnt are included in the overall
figures of emissions via products. Treatment techniques and residual mercury concentrations are
described in Section 3.5.6.3.4.
Caustic soda and potash are filtered to reduce mercury concentrations in the final product. A
variety of filters are used including plate filters with carbon pre-coat as well as candle filters
with or without carbon pre-coat. Although all types of filters can achieve very low levels of
mercury in the product, the predominant technique is the plate filter with carbon pre-coat.
Mercury concentrations in filtered caustic are typically about 50 Zg/kg in 50 wt-% caustic
corresponding to emission factors of approximately 0.1 g/t annual chlorine capacity [ 87, Euro
Chlor 2006 ].
Overall mercury emission factors via products in 2010 ranged from 0.01 – 0.43 g/t annual
chlorine capacity, the median being 0.05 g/t annual chlorine capacity (Table 3.23). Mercury
contained in caustic is usually the largest contributor to emissions via products.
3.5.9 Generation of wastes
3.5.9.1 Overview
Wastes generated specific to mercury cell plants relate to those contaminated with mercury. The
wastes described in Section 3.4 on emissions and waste generation of relevance for all three cell
techniques usually fall under this category.
Solid wastes can arise at several points in the process as shown in Figure 3.5, note that wastes
from waste water treatments are not shown in the figure.
WORKING DRAFT IN PROGRESS
122 December 2011 TB/EIPPCB/CAK_Draft_1