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

2.2 The mercury cell technique process
2.2.1 General description
Chapter 2
The mercury cell technique process has been in use in Europe since 1892 and accounted in 1999
for 58 % of total production in western Europe. As shown in Figure 2.3, the mercury cell
technique includes an electrolysis cell and a horizontal or vertical decomposer process involves
two 'cells'. In the electrolysis cell primary electrolyser (or brine cell), purified and saturated
brine containing approximately 25 wt-% sodium chloride flows through an elongated trough
that is slightly inclined from the horizontal. In the bottom of this trough a shallow film of
mercury (Hg) flows along the brine cell co-currently along with the brine. Closely spaced above
the cathode, an anode assembly is suspended [ 17, Dutch Ministry 1998 ].
NB: A) Electrolysis cell: a) Mercury inlet box; b) Anodes; c) End box; d) Wash box;
B) Horizontal decomposer: e) Hydrogen gas cooler; f) Graphite blades; g) Mercury pump;
C) Vertical decomposer: e) Hydrogen gas cooler; g) Mercury pump; h) Mercury distributor; i) Packing
pressing springs;
CW = cooling water.
Source: [ 1, Ullmann's 2006 ] {Figure was replaced because original source of figure could not be identified.}
Figure 2.3: Schematic view of a mercury electrolysis cell with horizontal and vertical
decomposer Flow diagram of mercury cell technology
Electric current flowing through the cell decomposes the brine passing through the narrow space
between the electrodes, liberating chlorine gas (Cl2) at the anode and metallic sodium (Na) at
the cathode. The chlorine gas is accumulated above the anode assembly and is discharged to the
purification process. As it is liberated at the surface of the mercury cathode, the sodium
immediately forms an amalgam (NaHgx) [Kirk-Othmer, 1991]. The concentration of the
amalgam is maintained at 0.2 – 0.4 wt-% Na so that the amalgam flows freely, 0.3 % is the
reference figure in [Gest 93/186, 1993]. Na concentrations of > 0.5 wt-% can cause increased
hydrogen evolution in the cells [ 1, Ullmann's 2006 ]. The liquid amalgam flows from the
electrolytic cell to a separate reactor, called the decomposer or denuder, where it reacts with
water in the presence of a graphite catalyst to form sodium hydroxide and hydrogen gas. The
sodium-free mercury is fed back into the electrolyser cell and is reused.
WORKING DRAFT IN PROGRESS
The reaction in the electrolyser is: 2 Na + + 2Cl - + 2 Hg V 2 Na-Hg + Cl2(g)
The reaction in the decomposer is: 2 Na-Hg + 2 H2 O V 2 Na + + 2 OH - + H2 (g) + 2 Hg
{Reactions were moved to Section 2.1.}
The depleted brine anolyte leaving the cell is saturated with chlorine and must be partially
dechlorinated before being returned to the dissolvers.
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