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

Chapter 4
Economics
At the AkzoNobel plant in Delfzijl (Netherlands), acidic decomposition proved to the most
economic solution compared to chemical and catalytic reduction. The costs for additional acid
amounted to approximately NLG 100 000, corresponding to approximately EUR 45 000 [ 210,
Beekman 2001 ].
Example plants
AkzoNobel in Delfzijl (Netherlands) has used acidic decomposition since 1999. At that
time the plant was based on the diaphragm cell technique and had a chlorine capacity of
130 kt/yr. {Is this technique still used in 2011?}
Reference literature
[ 17, Dutch Ministry 1998 ], [ 210, Beekman 2001 ] [Dutch report, 1998], [Le Chlore, 1996]
4.3.6.4 Techniques to reduce emissions of chlorate and bromate
4.3.6.4.1 Overview
Chlorate is formed in the electrolysis cell, either via anodic oxidation of hypochlorous acid or
via disproportionation of hypochlorous acid (see Section 2.1). Chlorate may also be formed in
the chlorine absorption unit (see Section 2.6.12.2) or during the thermal treatment of waste
water containing free oxidants (see Section 4.3.6.3.5). Bromate may be formed by the same
mechanisms depending on the level of bromide impurities in the salt.
Chlorate and bromate build up during brine recirculation and are unwanted compounds in the
electrolysis process because they may damage equipment and reduce the caustic soda quality
(see Section 3.4.2.3.5). They leave the brine system via the brine purge. This and other waste
water streams containing free oxidants are usually treated prior to discharge into the
environment or a sewer system. However, techniques that reduce the levels of free oxidants do
not reduce the levels of chlorate or bromate (see Section 4.3.6.3). Techniques to reduce
emissions of chlorate and bromate include [ 49, Euro Chlor 2010 ]:
brine acidification (see Section 4.3.6.4.2);
acidic chlorate reduction (see Section 4.3.6.4.3);
catalytic chlorate reduction (see Section 4.3.6.4.4);
recycling of the brine purge for the production of sodium chlorate (see
Section 4.3.6.4.5).
Current emission levels of chlorate and bromate in waste water at the outlet of the electrolysis
unit are presented in Section 3.4.2.3.5.
4.3.6.4.2 Brine acidification
Description
This technique consists in acidifying the brine prior to electrolysis in order to reduce the
formation of chlorate and bromate.
WORKING DRAFT IN PROGRESS
Technical description
As seen in the equations in Section 2.1, the formation of chlorate and bromate is reduced by
lowering the pH value. In addition, the formation of oxygen is also reduced. This leads to an
increased overall electrolysis efficiency and a reduction of energy consumption [ 1, Ullmann's
2006 ].
Without acidification, the pH value in the anolyte compartment of membrane cells is
approximately 4 due to the disproportionation (dismutation) of chlorine in water and the
244 December 2011 TB/EIPPCB/CAK_Draft_1