Chlorine manufacture - Mercury Cell
Chlorine has been produced using mercury cells for over 100 years, though continual development has resulted in cells that are now more efficient and produce much lower emissions. The system consists of two parts, the electrolyser and the decomposer.

In the electrolyser, which may be almost 2m wide and 15m long, the anodes are made from titanium, and produce chlorine. Simple electrolysis forms hydrogen at the cathode, but by using flowing mercury sodium is produced and forms an amalgam with the mercury. Saturated sodium chloride solution is fed continually into the cell.

The amalgam is passed to the decomposer, where it reacts with water, producing sodium hydroxide and hydrogen. The mercury is returned to the electrolyser.
diagram: mercury cell
Spent brine will contain some dissolved chlorine, which is removed as sodium chlorate(I) or hydrogen chloride, both useful products. The brine has salt added and is re-used.

Traces of mercury may be present in products from the cell, but levels of mercury emissions from chlorine manufacture have been reduced by as much as 97% in the last 10 years, and it is now a minor contributor to environmental mercury levels, despite the huge quantity of chlorine manufactured.

The decomposer produces good quality sodium hydroxide, which at 50% strength (0.5M) does not need further concentration for most purposes as it is normally sold at this strength.

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