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

Chapter 2
centrifuges before disposal. [Ullmann’s, 1996] {Please TWG provide information if the value of
50 – 60 % is correct because [ 1, Ullmann's 2006 ] reports 60 – 80 %.}
The sulphate content can also be reduced without the use of expensive barium salts by purging a
part of the brine, by cooling the brine stream and crystallising Na2SO4 · 10 H2O, by precipitating
the double salt Na2SO4 · CaSO4, by ion exchange, or by nanofiltration combined with purging of
the brine. In the diaphragm cell technique, the removal of sulphate is not always necessary
because sulphate can be removed from the cell liquor as pure Na2SO4 during the concentration
process [ 1, Ullmann's 2006 ]. In the case of the membrane cell technique, the use of barium
salts is generally avoided to protect the membrane against potential precipitations (see Table
2.4) [ 3, Euro Chlor 2011 ].
The purified brine should ideally contain ideally [Ullmann’s, 1996] [ 1, Ullmann's 2006 ]:
Ca 2+ : < 2 mg/l;
Mg 2+ : < 1 mg/l;
SO4 2- : < 5 g/l.
Before the brine enters the mercury or diaphragm electrolysis cells, it is usually acidified with
hydrochloric acid to pH < 6 which increases the lifetime of the anode coating and reduces the
formation of oxygen, hypochlorite and chlorate [ 1, Ullmann's 2006 ].
2.5.3.3 Secondary purification: membrane cell technique circuit
To maintain the high performance of the ion-exchange membrane, the feed brine must be
purified to a greater degree than in the conventional mercury or diaphragm cell techniques
processes.
The precipitation step alone is not enough to reduce the levels of calcium and magnesium, and
additional softening is required. Figure 2.15 shows the flow diagram of a possible layout for the
brine purification system used in the membrane cell technique process.
Raw salt Water
V-1
V-2
V-3
SP - 1
SP-2 SP-3
V-1 Brine saturator SP-1 Sand filter E-1 Electrolyser
V-2 Brine reactor SP-2 Brine filter T-1 Dechlorinationtower
V-3 Clarifier Porous carbon filter
V-4 Chlorate decomposer Pre- coating
(if necessary) SP-3 Chelating resin towers
Source: [ 29, Asahi Glass 1998 ] (Source: Asahi Glass CO)
WORKING DRAFT IN PROGRESS
TB/EIPPCB/CAK_Draft_1 December 2011 41
E-1
V-4
Cl 2
T-1
Depleted brine
Figure 2.15: Flow diagram of a possible layout for the brine purification system used in the
membrane cell technique process
The Secondary brine purification generally consists of a polishing filtration step (Figure 2.16)
and brine softening in an ion-exchange unit (Figure 2.17).