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

Chapter 2
Low concentrations of oxygen (0.5 – 2.0 vol-%) in chlorine are formed by the electrolytic
decomposition of water and hypochlorous acid (from the reaction of chlorine with water).
Furthermore, chlorate is formed in the cell liquor by anodic oxidation and disproportionation of
hypochlorous acid (see Section 2.1) (0.04 – 0.05 wt-% before concentration, ~ 0.1 wt-% after
concentration) [ 10, Kirk-Othmer 2002 ]
Precipitation of magnesium and calcium hydroxides on the catholyte side of the diaphragm may
also create blocking problems. Hydrochloric acid is often added to the brine to remove CO2; it
may also be added to the brine entering the anode compartment to reduce back-migration of
hydroxyl ions and to suppress formation of hypochlorous acid.{This topic is covered in
Sections 2.5.3.2 and 2.5.3.3.}
In the diaphragm cell, saturated brine (ca. about approximately 25 wt-% NaCl) is decomposed
to approximately 50 % of its original concentration in a passage through the cell, electrolyser as
compared to a 16 % decomposition of salt per passage in through mercury cells. Heating caused
by the passage of a current through the liquids diaphragm cell raises the operating temperature
of the electrolyte to 80 – 99 ºC [ 17, Dutch Ministry 1998 ].
The advantage of diaphragm cells have the advantage is that the quality requirements for the
brine and the electrical energy consumption are low (cell voltage 3 – 4 V; current density
0.5 – 3 kA/m 2 ). However, a high amount of steam may be necessary for the caustic soda
concentration and the quality of the produced caustic soda and chlorine are low.
operating at a lower voltage than mercury cells
operating with less pure brine than required by membrane cells
When using asbestos diaphragms, the diaphragm cell technique inherently gives rise to
environmental releases of asbestos [ 10, Kirk-Othmer 2002 ].
2.3.2 The cell
Various designs of diaphragm cells have been developed and used in commercial operations.
Figure 2.6 shows a sectional drawing of a typical monopolar diaphragm cell and Figure 2.7
shows a monopolar diaphragm cell room example. Typical anode areas per cell range from
20 – 100 m 2 [ 1, Ullmann's 2006 ].
Cathodes used in diaphragm cells consist of carbon steel with an active coating which lowers
the hydrogen overpotential, thus providing significant energy savings. The coatings consist of
two or more components. At least one of the components is leached out in caustic to leave a
highly porous nickel surface [ 1, Ullmann's 2006 ]. Many different types of activated cathodic
coating are under development in order to reduce the power consumption of the cell. These The
coatings have to be robust because a the powerful water jet is used to remove the diaphragm at
the end of its lifetime from the cathode mesh, which can adversely affect the coatings cathode.
An industrial application of ‘integrated pre-cathode’ diaphragm has been conducted (full scale)
and has been found to contribute to saving energy by reducing electric power consumption and
improving current efficiency. The lifetime of the diaphragm has also been found to be improved
by introduction of the pre-cathode (see Section 0).
WORKING DRAFT IN PROGRESS
Anodes used in diaphragm cells consist of titanium coated with a mixture of ruthenium dioxide,
titanium dioxide and tin dioxide. The lifetime of the coatings is at least 12 years
[ 10, Kirk-Othmer 2002 ]. The most commercially accepted design is that of the expandable
anode which involves compression of the anode structure during cell assembly and expansion
when the cathode is in position. The spacers initially placed over the cathode then create a
controlled gap of a few millimetres between anode and cathode. The minimisation of the gap
leads to a reduced power consumption [ 21, Kirk-Othmer 1995 ].
28 December 2011 TB/EIPPCB/CAK_Draft_1