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

Chapter 4
Reference literature
[ 69, Regierungspräsidium Dessau 1999 ], [ 192, Euro Chlor 2011 ] [Dutch report, 1998], [Gest
76/52, 1986], [Gest 87/130, 1996], [Gest 92/169, 1994], [Le Chlore, 1996], [Schubert],
[Wunderlich]
4.3.5.2 Scrubbing with hydrogen peroxide to reduce emissions of chlorine
dioxide
Description
This technique consists in reducing emissions of chlorine dioxide to air from the chlorine
absorption unit by using hydrogen peroxide. This can be achieved in a separate scrubber or by
adding peroxide dosing equipment to the chlorine absorption unit.
Technical description
Small amounts of chlorine dioxide can be emitted to air from the chlorine absorption unit.
Chlorine dioxide is formed because of a side reaction of chlorine or hypochlorous acid with
chlorite (chlorite originates from the disproportionation of hypochlorite, see Section 2.6.12.2)
[ 185, Infomil 2011 ], [ 191, Lassche 2011 ], [ 193, Jia et al. 2000 ], [ 194, Gordon and
Tachiyashiki 1991 ]:
2 ClO2 - + Cl2 V 2 ClO2 + 2 Cl -
2 ClO2 - + HOCl V 2 ClO2 + H + + 2 Cl -
The formation of chlorine dioxide via this reaction is favoured by higher temperatures. It is also
favoured by pH values below 7 where dissolved molecular chlorine or hypochlorous acid
dominate over hypochlorite. These pH values can be caused by an insufficient surplus of
sodium hydroxide to absorb chlorine, often due to local concentration gradients. Moreover,
higher concentrations of non-condensable gases such as oxygen and nitrogen in the waste gas
lead to increased stripping [ 185, Infomil 2011 ], [ 191, Lassche 2011 ], [ 194, Gordon and
Tachiyashiki 1991 ].
Solvay reported that chlorine dioxide can also be formed by a side reaction of the acidic chlorate
reduction (see Section 4.3.6.4.3) [ 34, Solvay 2010 ]:
5 ClO3 - + Cl - + 6 H + V 6 ClO2 + 3 H2O
The waste gas from the reactor is therefore treated before entering the chlorine absorption unit
[ 34, Solvay 2010 ]. However, AkzoNobel reported that chlorine dioxide is not formed when
chlorate is destroyed with hydrochloric acid (or another reducing agent) [ 191, Lassche 2011 ].
Chlorine dioxide hardly dissolves in the produced bleach and is therefore continuously stripped
out with the waste gas. The monitoring equipment usually measures both chlorine and chlorine
dioxide (see Section 4.3.4.4). Chlorine dioxide can be destroyed with hydrogen peroxide to
produce chlorite, the latter remaining in the bleach [ 185, Infomil 2011 ], [ 191, Lassche 2011 ]:
WORKING DRAFT IN PROGRESS
2 ClO2 + H2O2 + 2 OH - V 2 ClO2 - + O2 + 2 H2O
At the AkzoNobel plant in Rotterdam (Netherlands), a separate hydrogen peroxide scrubber was
installed while at the AkzoNobel plants in Delfzijl (Netherlands) and Ibbenbüren (Germany),
peroxide dosing equipment was added to the chlorine absorption unit. In the latter case, the
dosage of hydrogen peroxide is controlled by an analyser in the off gas which starts when the
chlorine dioxide concentration exceeds 0.7 mg/m 3 . The hydrogen peroxide is stored in an
intermediate bulk container which is placed on a liquid impermeable floor. The hydrogen
peroxide has a concentration of approximately 35 wt-%. The inflow takes place automatically
via a pump and a flexible pipeline which is connected to the circulation flow of the secondary
226 December 2011 TB/EIPPCB/CAK_Draft_1