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

Chapter 3
KCl are higher than from those using exclusively NaCl. However, emissions do also depend on
other plant-specific factors and several plants in EU-27 and EFTA countries using exclusively
NaCl show higher emissions to air than those of plants using KCl [ 57, EIPPCB 2011 ].
In 1997, mercury emissions from western European cell rooms were in the range of 0.17-1.93
g/t chlorine production capacity, as reported by Euro Chlor.
3.5.6.3 Process exhaust
3.5.6.3.1 Overview
Process exhaust refers to all gaseous streams by which mercury can be emitted to the
atmosphere, apart from cell room ventilation air and product hydrogen as product. One of the
most significant sources of mercury emission is the purge of inlet and outlet boxes (‘end
boxes’). Today, they are usually connected to a separate ventilation system. The vacuum
cleaning equipment is also a significant mercury source and it is normally also connected to a
ventilation system. The typical streams which may have significant mercury content that require
the use of a treatment technique include [ 87, Euro Chlor 2006 ]:
purge air from cell end boxes;
vents from wash water collection tanks;
exhaust from any vacuum system used to collect spilled mercury (but some portable
vacuum cleaners have their own mercury absorption system);
hydrogen burnt or sold as fuel (see Section 3.5.6.3.4);
hydrogen emitted to the atmosphere (see Section 3.5.6.3.4);
vents from caustic soda pumping tanks (see Section 3.5.6.3.3);
vents from caustic soda filters (see Section 3.5.6.3.3);
exhausts and vents from distillation units for mercury-contaminated solid wastes
(see Section 3.5.6.3.6);
vents from storage of metallic mercury and waste contaminated with mercury (see
Section 3.5.6.3.5);
vents from workshops where contaminated equipment is handled (see
Section 3.5.6.3.7).
Some streams may be combined prior to treatment while others require separate treatment units.
For example, hydrogen streams are usually not mixed with streams containing significant
amounts of air, in order to prevent the formation of explosive mixtures [ 87, Euro Chlor 2006 ].
Other streams are likely to contain some mercury, but in such low concentrations that they do
not require treatment [ 87, Euro Chlor 2006 ]:
vents from brine saturators (see Section 3.5.6.3.2)
vents from brine filters and treatment tanks (see Section 3.5.6.3.2)
vents from caustic soda stock tanks.
WORKING DRAFT IN PROGRESS
Mercury is mainly removed by [ 1, Ullmann's 2006 ], [ 87, Euro Chlor 2006 ]:
adsorption on iodised or sulphurised activated carbon;
scrubbing with hypochlorite or chlorinated brine to form mercury(II) chloride;
adding chlorine to form dimercury dichloride (calomel) which is collected on a solid
substrate such as rock salt in a packed column.
scrubbing with hypochlorite, chlorinated brine or using a calomel reaction, or
using a sulphurised charcoal system.
TB/EIPPCB/CAK_Draft_1 December 2011 115