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

Chapter 3
The fact that the difference to balance is often much higher than the total emissions means that
significant amounts of mercury remain unaccounted for, which has led to controversies.
Environmental NGOs have argued that actual emissions could be higher than those reported
[ 95, EEB 2008 ], [ 97, Concorde 2006 ], [ 101, US EPA 2008 ].
When making a balance between mercury inputs and outputs, the balance is frequently positive
or, from time to time, negative. In 1998, the mercury difference to balance were, plant by plant,
in the range of -35 - +36 g Hg/tonne chlorine capacity (see Annex C). Mercury is recycled
within the process to a large extent but some of the mercury is accumulated in equipment and
some is lost to air, water, wastes and products. A methodology for making a mercury balance in
a chlor-alkali plant is laid out in [Euro Chlor Env. Prot. 12, 1998]. These guidelines are adopted
by OSPARCOM for the annual reporting of mercury losses, and companies have to state where
they have departed from them.
Several factors contribute to the measurement uncertainty. The determination of the
consumption levels requires the measurement of mercury in cells at the beginning and at the end
of the reporting period. The best measurement method uses radioactive tracers and has an
uncertainty of 0.5 – 1 % [ 86, Euro Chlor 2010 ], [ 101, US EPA 2008 ]. With a median cell
inventory of 1.5 kg Hg/t annual chlorine capacity (see Table 3.22), this corresponds to an
uncertainty of 7.5 – 15 g Hg/t annual chlorine capacity, a value which is approximately
10 – 20 times higher than the median of the total emissions of chlor-alkali plants in EU-27 and
EFTA countries (see Table 3.23). In addition, the uncertainty for the consumption levels
increases further when the difference between the cell inventory at the beginning and at the end
of the balancing period is calculated (two large figures are subtracted from each other) and
when the uncertainty of the variation of the quantities in storehouses is taken into account.
Further contributions to the measurement uncertainty originate from the monitoring of
emissions to air and water and from the determination of mercury in wastes which are typically
about 10 % and 50 %, respectively [ 86, Euro Chlor 2010 ].
An accurate balance depends on the ability to measure the mercury inventory in the cells. The
mercury cell inventory can be measured to 0.5% accuracy when using a radioactive tracer. With
an average cell inventory of 1.8 kg Hg/tonne chlorine capacity, this correspond to 9 g Hg/tonne
chlorine capacity.
The mercury difference to balance is also due to the fact that mercury progressively accumulates
inside pipes, tanks, traps, sewers and in sludges, until some form of equilibrium is reached. Euro
Chlor recommends purging this type of equipment, where possible, just prior to making the
balance. Some 10 tonnes of mercury was said to be found by a company in a cooling water
tower used for hydrogen (diameter 3.6 m). Mercury is often recovered when mercury cell rooms
are decommissioned. This mercury is sometimes recovered during maintenance, but usually
remains there until the decommissioning of the plant. The mercury accumulation explains why
the calculated difference to balance is usually positive. It may be negative when recoveries
occur. When the difference to balance over the lifetime of a plant is considered, a substantial
proportion of mercury can be recovered during the dismantling of the installation and
equipment, but is nevertheless limited by the degree of efficiency of the final recovery operation
(mercury may remain amalgamated in metals or absorbed in construction materials) [ 96, Euro
Chlor 2010 ].
WORKING DRAFT IN PROGRESS
Because of the difficulty in recording an accurate follow-up of the mercury outputs, some
proposals, in order to avoid discussion as to the credibility of the balance, could be to:
operate rigorous and precise control of emissions, periodically, by third parties,
optimise the recycling of mercury at each step of the process; in particular, extensive
recycling of mercury in solid wastes should be possible,
adoption of a recognised standard methodology to do the mercury balance.
{This text is not related to current emission and consumption levels.}
TB/EIPPCB/CAK_Draft_1 December 2011 129