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

Chapter 2
resaturated with salt. If needed, to reach a concentration of 50% caustic soda, the caustic liquor
produced has to be concentrated by evaporation (using steam).
Figure 2.8: Example flow diagram of the membrane cell process
{The Figure was removed because the flow diagram is included in Figure 2.1.}
Some electrolysers produce a more diluted 23 wt-% caustic soda. In this case, the caustic
entering the cell has a concentration of approximately 20 – 21 wt-% and the heat of the
electrolysis can be used to concentrate the 23 wt-% caustic solution to 32 – 34 wt-%. The
overall energy efficiency is comparable to the aforementioned process with the 32 wt-% caustic
solution but more equipment for the caustic evaporation is required. On the other hand, simpler
and cheaper construction materials can be used in the caustic circuit around the membrane cells
[ 3, Euro Chlor 2011 ].
Generally, the caustic produced in a concentration of 30 – 33 wt-% is concentrated to the usual
commercial standard concentration of 50 wt-% by evaporation (using steam). Another
possibility is to use the caustic produced in the membrane cells as feed to the decomposers of
mercury cells. A flow diagram of a possible integrated plant is shown in Figure 2.8.
Salt
Depleted brine
Salt
Depleted brine
Brine
saturation
Brine purge
Brine
saturation
Brine
purification
Dechlorination
Brine
purification
Dechlorination
H 2
H 2
WORKING DRAFT IN PROGRESS
32 December 2011 TB/EIPPCB/CAK_Draft_1
Cl 2
Membrane
cells
Cl 2
Mercury
cells
Depleted brine
Depleted Brine
30 – 33 wt-% NaOH
Amalgam
Mercury
Water
Decomposer
50 wt-% NaOH
Figure 2.8: Flow diagram of the integration of the membrane and mercury cell techniques
The concentration of sodium chlorate in the produced caustic soda typically ranges from
Q 10 – 50 mg/kg [ 28, EIPPCB 2011 ]. The level depends on the membrane characteristics, the
operational current density and the chlorate levels in the brine [ 3, Euro Chlor 2011 ]. The
chlorine produced in membrane cells contains low concentrations of oxygen (0.5 – 2.0 vol-%).
The formation of oxygen and chlorate can be depressed by selecting an anode coating with
suitable characteristics and/or by decreasing the pH in the anode compartment [ 1, Ullmann's
2006 ], [ 10, Kirk-Othmer 2002 ].
The brine depletion in membrane cells is two or three times greater than in mercury cells which
allows the brine system to be smaller, resulting in significantly lower recycling rates and less
equipment needed compared to mercury cell plants of the same capacity [ 1, Ullmann's 2006 ],
[ 22, Uhde 2009 ].
Membrane cells have The membrane cell technique has the advantage of producing a very pure
caustic soda solution and of using less energy electricity than the other techniques processes. In
addition, the membrane cell technique uses neither process does not use highly toxic materials
such as mercury, which is classified as very toxic nor and asbestos which is classified as toxic