The mercury cell process
In the mercury cell process, sodium forms an amalgam (a 'mixture' of two metals) with the mercury at the cathode. The amalgam reacts with the water in a separate reactor called a decomposer where hydrogen gas and caustic soda solution at 50% are produced.
As the brine is usually re-circulated, solid salt is required to maintain the saturation of the salt water. The brine is first de-chlorinated and then purified by a precipitation-filtration process. The products are extremely pure. The chlorine, along with a little oxygen, generally can be used without further purification.
Of the three processes, the mercury process uses the most electricity, but no steam is required to concentrate the caustic solution. The use of mercury demands measures to prevent environmental contamination. Also, mercury must be removed from the hydrogen gas and caustic soda solution.
Mercury losses have been considerably reduced over the years. Increasingly, chlorine producers are moving towards membrane technology, which has much less impact on the environment.
In 2014, emissions for all mercury cells across Western Europe reached 0.74 grammes per tonne of chlorine capacity, slightly higher than the all-time low of 0,68 grammes per tonne of chlorine capacity in 2013.
Twenty-six mercury-based chlorine plants remain today to be voluntarily phased out or converted to non-mercury technology by 2020 at a cost of more than 3,000 million €. These plants account for an ever decreasing part (about 23% in 2014) of European chlorine capacity.
Click on the image below to have a full size view of the membrane cell process' animation.
Download a picture of the mercury cell process (PDF). The PDF images may be used in scientific or technical publications if the source www.eurochlor.org is mentioned.