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

Chapter 2
The secondary polishing filtration generally consists of candle-type, plate frame or pressure leaf
filters (either with or without a cellulose-based pre-coat) in order to sufficiently reduce
suspended particles the suspension matter and protect the ion-exchange resin from damage. In
some cases, no polishing filter is needed.
The ion-exchange chelating resin treatment is designed to decrease the sum of magnesium and
calcium concentrations to < 20 mg/l. alkaline earth metals to ppb level. Table 2.4 indicates the
typical specifications required for metals, SO4 2- and other impurities. These specifications can
vary if the users want to operate at a low current density (< 4 kA/m 2 ) or at a high current
density. The specifications are more stringent for high density current densities. Specifications
also depend on the interaction of impurities. While the presence of one impurity may not be
harmful, its synergistic combination with others may be (e.g. the combination of aluminium,
calcium and silica). The resin is periodically regenerated with high-purity hydrochloric acid and
sodium hydroxide solutions. Generally, one resin exchange column is in operation while another
resin exchange column is regenerated.
A chelating resin tower must be installed, but in some cases no polishing filter is needed in the
secondary brine treatment.
Before the brine enters the membrane electrolysis cells, it is usually acidified with hydrochloric
acid which increases the lifetime of the anode coating and reduces the formation of oxygen,
hypochlorite and chlorate. However, this increases the risk that precipitations of impurities such
as iron and aluminium occur inside the membrane, which would otherwise occur outside near
the surface. Furthermore, over-acidification can reverse the hydrolysis of the carboxylate groups
that provide high ion selectivity in the membrane, thereby irreversibly damaging them [ 10,
Kirk-Othmer 2002 ], [ 34, Solvay 2010 ], [ 72, Nishio 2011 ].
WORKING DRAFT IN PROGRESS
Source: [ 29, Asahi Glass 1998 ] (photograph supplied by Asahi Glass Co)
Figure 2.16: View of polishing filters in a secondary brine purification system
42 December 2011 TB/EIPPCB/CAK_Draft_1