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

Chapter 4
4.2.3 Conversion of asbestos diaphragm cell plants to membrane cell
plants technology or to asbestos-free diaphragms
{Please note that the paragraphs of the original text were reordered to be compatible with the
current standard 10-heading structure.}
Description
This technique consists in converting asbestos diaphragm cell plants to membrane cell plants. A
conversion to the membrane cell technique includes a change of electrolysers, additional brine
purification, additional brine dechlorination, inclusion of a cell room caustic soda recirculation
system as well as change of transformers and rectifiers.
Converting to membranes
Technical description
The important changes to an existing plant when converting from diaphragm cells to membrane
cells technology are included in the following list [ 31, Euro Chlor 2010 ], [ 211, Dibble and
White 1988 ].
Change of electrolysers.
Additional brine purification, including hardness removal ion exchange techniques.
Modifications to brine treatment to control other impurities, such as sulphate, may also
be required.
Additional brine dechlorination to protect the ion exchange resin. It is necessary to
complete the brine recirculation loop by adding primary and secondary dechlorination
and possibly resaturation.
Inclusion of a cell room caustic soda recirculation system. If 50 wt-% product is
required, the existing diaphragm cell caustic soda evaporator must be modified and
simplified to accept high-strength, salt-free caustic soda.
Change of transformers and rectifiers electrolysers and details of the cell room system.
At the BASF plant in Ludwigshafen (Germany), the conversion involved the following changes
[ 31, Euro Chlor 2010 ]:
addition of supplementary brine filtration unit and ion exchangers to remove calcium
and magnesium (the sulphate and silica levels were controlled via purge);
addition of a two-step brine dechlorination unit based on vacuum dechlorination and
chemical dechlorination with sodium hydrogen sulphite;
addition of some piping to close the brine loop;
dismantling of all diaphragm cells and installation of new electrolysers in the same cell
room where anolyte and catholyte tanks were installed;
addition of a caustic loop;
conversion of the old diaphragm cell liquor evaporation unit to produce vacuum salt;
addition of a caustic evaporation unit.
WORKING DRAFT IN PROGRESS
At the Anwil plant in Wuocuawek (Poland), the conversion comprised the following changes
[ 31, Euro Chlor 2010 ]:
addition of a new brine purification unit;
addition of a new brine dechlorination unit;
addition of a new caustic soda evaporator;
modification of the existing cell liquor evaporation unit to concentrate the diluted brine;
replacement of the diaphragm electrolysers by bipolar membrane electrolysers.
TB/EIPPCB/CAK_Draft_1 December 2011 179