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

Chapter 3
the preparation of the brine;
the preparation of some reagents used for the brine purification;
the production of caustic in the decomposer in the case of the mercury cell technique;
the dilution of the produced caustic in the caustic circuit in the case of the membrane cell
technique;
the caustic circuit: for maintaining the water balance in the reaction to form NaOH in the
membrane and mercury cells;
the direct cooling of chlorine;
the dilution of caustic if necessary (e.g. for the chlorine safety absorption unit). : the
quantity used will depend on the amount of chlorine released to absorption.
Water can also be used for indirect cooling which is. However, cooling systems are not
considered covered in this document.
Water which is consumed leaves the process either via the products (hydrogen and caustic
solution) or as waste water [ 3, Euro Chlor 2011 ]:
1. During electrolysis, water is consumed for the synthesis of hydrogen and caustic in a
stoichiometric amount of approximately 0.51 m 3 /t Cl2 produced (see Section 2.1);
2. In addition, water is used as solvent for the production of caustic. This amount depends
on the caustic concentration and the salt concentration therein (Table 3.3) and cannot be
reduced unless the caustic is further concentrated and the condensate recycled;
Table 3.3: Consumption of water as solvent for caustic production
NaOH
concentration
NaCl
concentration
Water consumption ( 1 )
in wt-% in wt-%
in m 3 /t NaOH
produced ( 2 )
in m 3 /t NaOH
(100 %)
produced
in m 3 /t Cl2
produced ( 3 )
7.0 15 0.78 11 13
11 9.0 0.80 7.3 8.2
32 < 0.01 0.68 2.1 2.4
50 1.0 0.49 0.98 1.1
50 < 0.01 0.50 1.0 1.1
99 < 0.01 0.010 0.010 0.011
( 1 ) Assuming a water density of 1 t/m 3 .
( 2 ) Refers to the given mass concentration of caustic.
( 3 ) Assuming a production of 1.128 t NaOH (100 %)/t Cl2 produced.
3. Regarding the generation of waste water, the average volume oscillated between 1.87
and 2.04 m 3 /t Cl2 produced in EU-27 and EFTA countries during the period of 2002 to
2010 [ 73, Debelle 2011 ]. Apart from the water needed for cooling, generally 2 –
2.5 m 3 of water is consumed per tonne of chlorine produced for plants using a brine
recirculation process. For waste brine processes, approximately about 10 m 3 of water
per tonne of chlorine produced are discharged as waste water is required.
WORKING DRAFT IN PROGRESS
In addition to its use for the process, water is also used for the washing of equipment and
working places [ 3, Euro Chlor 2011 ].
68 December 2011 TB/EIPPCB/CAK_Draft_1