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

Chapter 2
No such dechlorination treatment is required for the diaphragm system, since any chlorine
passing through the diaphragm reacts with caustic soda in the catholyte compartment to form
hypochlorite or chlorate.
If the saturation is made with impure salt (followed by a primary purification step on the total
brine flow), the pH of the dechlorinated brine is then brought to an alkaline value with caustic
soda to reduce the solubilisation of impurities from the salt. If the saturation is made with pure
salt (with subsequent primary purification on a small part of the flow), there is no alkalisation
step prior to resaturation at that level (only in the purification phase).
Depleted brine from the mercury and membrane cells, with a concentration of 210-250 g/l,
depending on the technology, current density and heat balance of the cell, is resaturated by
contact with solid salt to achieve a saturated brine concentration of 310-315 g/l. Brine
resaturation using solid salt is described in Section 2.5.2. In the case of mercury or membrane
cell plants operating with solution-mined brine, brine resaturation is achieved by evaporation.
During this step, sodium sulphate precipitates and can be recovered, purified and used for other
purposes.
In the case of diaphragm cells, the catholyte liquor (10 – 12 wt-% NaOH, 15 – 17 wt-% NaCl) is
directly used or transferred goes directly to the caustic evaporators where solid salt and 50 wt-%
caustic are recovered together. Fresh brine can be saturated with recycled solid salt from the
caustic evaporators before entering the diaphragm electrolysers.
Resaturators can be either open or closed vessels.
The pH of the brine sent to the electrolysers may be adjusted to an acidic value (pH 4) with
hydrochloric acid in order to protect the anode coating, to keep the formation of chlorate at a
low level and to decrease the oxygen content in the chlorine gas. Hydrochloric acid can also be
added in the anodic compartments of membrane cells to further reduce the content of oxygen in
the chlorine, especially for electrolysers with older membranes (poorer performances) The
(bi)carbonates introduced with the salt are decomposed by these acid additions, producing
gaseous carbon dioxide. {The contents of this paragraph were moved to the Sections 2.5.2,
2.5.3.2 and 2.5.3.3.}
2.5.5 Chlorate destruction: membrane cell technique
In order to reduce the build-up of chlorate in the brine circuit with potential negative effects on
the ion-exchange resins (see Table 2.4), the caustic quality and emissions to the environment,
some membrane cell plants operate a chlorate destruction unit prior to dechlorination (see
Figure 2.1). Techniques include the reduction of chlorate to chlorine with hydrochloric acid at
temperatures of approximately 85 °C and the catalytic reduction of chlorate to chloride with
hydrogen (see Section 4.3.6.4).
WORKING DRAFT IN PROGRESS
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