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

Chapter 2
The electrolyser is operated at a temperature of approximately 70 – 80 ºC. The brine normally
enters the electrolysis cell at 60 – 70 °C. At this temperature, the conductivity of the brine
solution and the fluidity of the mercury are higher compared to operation at ambient
temperature. The temperature can be achieved by preheating the saturated brine with steam and
is increased in the cell electrolyser by the Joule heating heat of resistance to approximately
about 75 – 85 °C. The heat produced during electrolysis and amalgam decomposition requires
the air of the cell room to be changed 10 – 25 times per hour, depending on the type of building
[ 1, Ullmann's 2006 ], [ 17, Dutch Ministry 1998 ].
2.2.3 Decomposition of the amalgam
The amalgam is decomposed in horizontal decomposers, alongside or beneath the cell (Figure
2.4) or more often, since ca. 1960, in vertical decomposers (or denuders), at one end of the cell
(Figure 2.5). Industrial decomposers are essentially short-circuited electrochemical primary
cells in which the graphite catalyst is the cathode and sodium amalgam the anode. The most
common catalyst is graphite, usually activated by oxides of iron, nickel or cobalt or by carbides
of molybdenum or tungsten. The decomposer operates at a temperature of approximately
90 – 130 ºC, which is caused by the chemical reactions in the decomposer and the input of
warm amalgam from the cell electrolyser. Higher temperatures lead to a lower overpotential of
hydrogen on graphite and therefore to a quicker reaction [ 1, Ullmann's 2006 ], [ 17, Dutch
Ministry 1998 ].
Figure 2.4: Mercury cells with horizontal decomposer
[Le Chlore, 1996] {The original black-and-white figure was replaced by Figure 2.4.}
The mercury cell technique 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 than diaphragm and membrane cells and, therefore, use
more energy (caustic soda concentration excluded). The technique 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 inherently gives rise to environmental
releases of mercury. {This text was moved to Section 2.2.1.}
Figure 2.5: Mercury cells with vertical decomposer
[Le Chlore, 1996] {The original black-and-white figure was replaced by Figure 2.4.}
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26 December 2011 TB/EIPPCB/CAK_Draft_1