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

Chapter 3
When concentrated solid caustic soda or potash (100 %) is required, the 50 wt-% solution has to
be concentrated in a caustic evaporator after mercury filtration. The residual mercury
(10 – 100 Zg Hg/kg NaOH (100 %) corresponding to 0.011 – 0.11 g/t annual chlorine capacity)
then evaporates from the caustic as a result of the heat treatment in the caustic evaporator [ 17,
Dutch Ministry 1998 ]. {Please TWG provide information on the treatment of this exhaust. Are
emissions covered by the figures on process exhaust?}
3.5.6.3.4 Burnt or emitted hydrogen produced in the decomposer
Hydrogen which is emitted or burnt/sold as fuel is included in the overall mercury emissions to
air [ 86, Euro Chlor 2010 ], [ 90, PARCOM Decision 90/3 1990 ]. If the hydrogen is used for
other purposes, related emissions of mercury are described in Section 3.5.7.2.2 on emissions via
products.
Hydrogen is formed from the exothermic reaction of sodium amalgam with water. It is referred
to as ‘strong hydrogen’, indicating its high concentration [ 39, HMSO 1993 ]. The hydrogen gas
stream is nearly saturated with mercury when it leaves the decomposer at a temperature of
90 – 130 ºC. The saturation concentration of mercury in the gas phase is 0.836 g/m 3 at 80 °C
and 2.40 g/m 3 at 100 °C [ 1, Ullmann's 2006 ]. Mercury vapour is entrained in the process
stream and passes through a heat exchanger to decrease temperature to ambient. After cooling,
mercury vapour condenses and is collected. The mercury is recovered directly as a metal inside
the decomposer and can be recycled. Hydrogen may be compressed and cooled, to reduce the
mercury content further. This mercury is generally removed in a multi-stage process [ 87, Euro
Chlor 2006 ].
In the first step, the hydrogen is cooled by a heat exchanger mounted immediately above the cell
or a cooler within the decomposer. The condensed mercury is recycled with water and sent
directly to the decomposer [ 87, Euro Chlor 2006 ].
The second stage may involve either [ 87, Euro Chlor 2006 ]:
chilling or washing with chilled water
compression and cooling
scrubbing with hypochlorite
use of a calomel reaction.
With the first two options, mercury is recovered as metallic mercury; with the last two options,
the resulting liquid/solid can be recirculated to the brine (see also Section 3.5.6.3.1).
A third stage may involve either iodised or sulphurised activated carbon, after sufficient
increase of the hydrogen temperature (10 – 20 °C) to avoid water condensation in the carbon
bed. Alternatively, this third stage can be carried out by adsorption of mercury on copper on an
aluminium oxide carrier or on silver on a zinc oxide carrier, at temperatures of 2 – 20 °C
[ 1, Ullmann's 2006 ], [ 87, Euro Chlor 2006 ].
WORKING DRAFT IN PROGRESS
A two-stage method (cooling followed by chemical or catalytic treatment) for the removal of
mercury from hydrogen appears to be particularly effective. The hydrogen gas is usually cooled
to 20 ºC with a heat exchanger and cooled with a second heat exchanger to 5 ºC (two-stage
cooling). The quantity of mercury released will depend on whether the cooling step is followed
by a chemical reaction (for instance, with CuO) or with a catalyst reaction (e.g. sulphurised
carbon). The range can then vary. For a plant which reports a total mercury emission in air of
19.9 kg in 1997 (capacity of 120000 tonnes of chlorine), 0.23 kg of mercury was emitted from
hydrogen production after cooling and treatment with sulphurised carbon, i.e. 1% of total
emissions. Mercury releases from hydrogen can be considered a potentially important emission
source of mercury, when the hydrogen gas is not properly purified.
TB/EIPPCB/CAK_Draft_1 December 2011 117