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

Chapter 2
Characteristics of the cathode: The cathode is made by a shallow layer of mercury which flows
from one extremity of the cell to the other because of due to the slight inclination from the
horizontal of the cell base. The quantity of mercury per cell can be up to 6 t. A high level of
purity, [ 99.9 wt-% mercury, and very low concentrations of metals, in particular heavy metals,
are required [ 3, Euro Chlor 2011 ].
Characteristics of the anode: Graphite was exclusively used as the anode for chlorine production
for more than 60 years, even though it exhibited high chlorine overpotential and dimensional
instability caused by the electrochemical oxidation of carbon to carbon dioxide, and hence led to
high energy consumption and the need for frequent maintenance operations. Electrolytic cell
anodes were made of graphite until the late 1960s in Western Europe when In the late 1960s,
anodes of titanium coated with ruthenium dioxide (RuO2) and titanium dioxide (TiO2) were
developed, the so-called dimensionally stable anodes. The use of RuO2- and TiO2-coated metal
anodes reduces energy consumption by approximately about 10 % compared to graphite and
their life expectancy is higher (300 – 400 t Cl2/m 2 ; ranging from less than one to more than five
years depending on current density and gap between anode and cathode). In recent years there
have been competitive developments in detailed The anode geometry for mercury cells has been
optimised, all with the aim of improving gas release in order to reduce ohmic losses and
increasing the homogeneity of the brine to improve anode coating life [ 1, Ullmann's 2006 ],
[ 10, Kirk-Othmer 2002 ], [ 21, Kirk-Othmer 1995 ].
An ‘end box’ is attached to each end of the cell electrolyser. The end box incorporates
compartments for collecting the chlorine gas and weirs for separating the mercury and brine
streams as well as washing the mercury and permitting the removal of thick mercury ‘butter’
that is formed by impurities.
The whole cell electrolyser is insulated from the floor to prevent stray ground currents. Usually,
several cells electrolysers are placed in series by means of electrically connecting the cathode of
one cell electrolyser to the anodes of the next cell electrolyser. Individual cells can be bypassed
for maintenance and replacement. The cells are usually situated in a building (Figure 2.4),
although sometimes they are erected in open air [ 1, Ullmann's 2006 ], [ 17, Dutch Ministry
1998 ].
WORKING DRAFT IN PROGRESS
Source: [ 18, BASF 2010 ]
Figure 2.4: View of a mercury cell room
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