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

Chapter 2
The sodium hydroxide is produced from the decomposer denuder at a concentration of
approximately about 50 wt-%; the maximum value reported is 73 wt-% [ 1, Ullmann's 2006 ].
[Ullmann's, 1996]. However, industry reports state that no plant in EU-27 and EFTA countries
Europe is known to be operating above 50 wt-%. The decomposer may be regarded as a shortcircuited
electrical cell in which the graphite catalyst is the cathode and sodium amalgam the
anode.
For its operation, the mercury cell depends upon the higher overpotential of hydrogen on versus
mercury to achieve the preferential release of sodium rather than hydrogen. However, impurities
such as vanadium (V), molybdenum (Mo), and chromium (Cr) at the 0.01 – 0.1 ppm level and
other elements (Al, Ba, Ca, Co, Fe, Mg, Ni, W) at the ppm level that can contaminate appear on
the mercury surface may lack this overvoltage protection and can cause localised release of
hydrogen into the chlorine (hydrogen can form an explosive mixture (>4% H2) in chlorine or
air). The hydrogen concentration in the chlorine can increase to the point at which the cell and
downstream chlorine handling equipment contain explosive mixtures [ 1, Ullmann's 2006 ]. The
presence of even trace amounts of certain metals, such as vanadium can cause the release of
dangerous amounts of hydrogen.
Mercury cells are usually operated to maintain a 21 – 22 wt-% (by weight) concentration of salt
in the spent brine discharged from the cell electrolyser. This corresponds to the decomposition
of 15 – 16 % of the salt during a single pass. Further salt decomposition to a lower
concentration in the brine would decrease brine conductivity, with the attendant loss of
electrical efficiency [ 17, Dutch Ministry 1998 ]. In plants with once-through brine systems,
approximately 40 % of the salt is electrolysed in the cells [ 3, Euro Chlor 2011 ].
A portion, or in some cases all, of the depleted brine is subsequently dechlorinated, resaturated
with solid salt, and returned to the cell brine feed. Some facilities purge small amounts of brine
solution and use new brine as make-up in order to prevent the build-up of impurities, mainly
sulphate, in the brine. Figure 2.3 shows a flow diagram of the mercury cell. {This is described in
Section 2.5 on brine supply.}
The mercury cell technique process has the advantage over diaphragm and membrane cells that
it produces a chlorine gas with nearly no oxygen, and a 50 wt-% caustic soda solution.
However, mercury cells operate at a higher voltage (4 – 5 V) and current density (7 – 10 kA/m 2 )
than diaphragm and membrane cells and, therefore, use more energy (caustic soda concentration
excluded). The technique process also requires a pure brine solution with little or no metal
contaminants to avoid the risk of explosion through hydrogen generation in the cell. The
mercury cell technique amalgam process inherently gives rise to environmental releases of
mercury [ 1, Ullmann's 2006 ], [ 10, Kirk-Othmer 2002 ].
2.2.2 The cell and the decomposer mercury cathode electrolyser
The cell is made of an elongated, slightly inclined trough (slope 1.0 – 2.5 %) and a gas-tight
cover. The trough is made of steel, and its sides are lined with a protective, non-conductive
rubber coating to prevent contact with the anolyte, to confine brine-cathode contact to the
WORKING DRAFT IN PROGRESS
mercury surface, and to avoid the corrosive action of the electrolyte [ 1, Ullmann's 2006 ].
Modern Cells electrolysers are 1 – 2.5 m wide and 10 – 25 m long. As a result, the cell area
today can be greater than 30 m 2 . The size of the cells can be varied vary over a broad range to
give the desired chlorine production rate. At the design stage, computer programs can be used to
optimise the cell size, number of cells, and optimum current density as a function of the
electricity cost and capital cost [Ullmann’s, 1996]. The steel base is made as smooth as possible
to ensure mercury flow in an unbroken film. In the event of a break in the mercury surface,
caustic soda will be formed on the bare (steel) cathode, with the simultaneous release of
hydrogen, which will mix with the chlorine [ 17, Dutch Ministry 1998 ]. Because hydrogen and
chlorine can form a highly explosive mixture, great care is necessary to prevent hydrogen
formation in the cell.
24 December 2011 TB/EIPPCB/CAK_Draft_1