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

environmental legislation;
generation of a saleable by-product.
Example plants
AkzoNobel in Bitterfeld (Germany), chlorine capacity 87.8 kt/yr
Chapter 4
Reference literature
[ 1, Ullmann's 2006 ], [ 69, Regierungspräsidium Dessau 1999 ], [ 127, Eckert 2010 ], [ 205,
Ullmann's 2000 ]
4.3.6.2 Techniques to reduce emissions of chloride
4.3.6.2.1 Overview
In addition to the technique described in the following Section 4.3.6.2.2, several of the
techniques to reduce consumption of salt (see Section 4.3.2.1.3), to reduce consumption of
water (see Section 4.3.2.2) and to reduce emissions of chlorate (see Section 4.3.6.4) also reduce
chloride emissions.
4.3.6.2.2 Nanofiltration
Description
In chlor-alkali plants with brine recirculation, sulphate builds up and needs to be removed (see
Table 2.4 and Section 3.4.2.3.2). This is most commonly achieved by purging a part of the brine
or to a lesser extent by precipitation of barium sulphate. As an alternative, nanofiltration can be
used to concentrate sulphate in the brine purge whereby either 1) the waste water volume and
the chloride emission factors are reduced or 2) the use of barium salts is avoided. Nanofiltration
is a specific type of membrane filtration with membrane pore sizes of approximately 1 nm.
Technical description
A general description of nanofiltration can be found in the CWW BREF [ 124, COM 2011 ].
Before entering the nanofiltration unit, the depleted brine is pretreated by pH adjustment,
cooling, removal of residual chlorine and filtration in order to guarantee the long-term
performance of the membranes. The pretreated brine is then fed at high pressure to the filtration
unit which is operated in cross-flow mode and usually consists of several stages. The
nanofiltration membrane has charged groups which selectively reject multivalent anions such as
sulphate. On the other hand, monovalent ionic species such as chloride and chlorate from the
feed brine pass through the membrane into the permeate, which is low in sulphate. The
permeate from each stage is collected and recirculated to the brine resaturation unit. As water
and NaCl pass through the membrane, the portion of the brine which remains becomes enriched
in sulphate reaching a final concentration after several stages of more than 100 g/l Na2SO4.
Since the sulphate concentration in the final concentrate is high, a very low volume is purged
from the brine resulting in low chloride emissions [ 125, Aker Chemetics 2010 ]. The sulphate
from the concentrate may be crystallised as solid Na2SO4. [ 127, Eckert 2010 ].
WORKING DRAFT IN PROGRESS
Achieved environmental benefits
The achieved environmental benefits of this technique include the following:
reduction of chloride emissions;
reduction of consumption of salt, water and ancillary materials;
reduction of energy consumption by avoiding the use of barium salts which may
damage the membrane;
avoidance of handling of toxic barium compounds;
prevention of barium sulphate waste generation.
TB/EIPPCB/CAK_Draft_1 December 2011 233