How is chlorine made?

Common salt is essential to produce chlorine industrially, by passing an electric current through salt water.

Chlorine is produced by passing an electric current through a solution of brine (common salt dissolved in water). The chemical term for salt is sodium chloride (NaCl).

Essential co-products are caustic soda (sodium hydroxide (NaOH)) and hydrogen (H₂). All three are highly reactive, and technologies have been developed to separate them and keep them apart. Caustic soda is an alkali and widely-used in many industries, including the food industry, textile production, soap and other cleaning agents, water treatment and effluent control.

Hydrogen is a combustible gas used in various processes including the production of hydrogen peroxide and ammonia as well as the removal of sulphur fro petroleum derivatives.

Chlorine has been manufactured industrially for more than 100 years. During this time, the industry's firm commitment to the best safety, health and environmental practices has ensured continuous improvement.

The three technologies of producing chlorine are:

The membrane cell process

Diagram of process

The anode and the cathode are separated by an ion-exchange membrane. Only sodium ions and a little water pass through the membrane.

The brine is de-chlorinated and re-circulated. Solid salt is usually needed to re-saturate the brine. After purification by precipitation-filtration, the brine is further purified with an ion exchanger.

The caustic solution leaves the cell with about 30% concentration and, at a later stage in the process, is usually concentrated to 50%. The chlorine gas contains some oxygen and must often be purified by liquefaction and evaporation.

The consumption of electric energy is the lowest of the three processes and the amount of steam needed for concentration of the caustic is relatively small (less than one tonne per tonne of caustic soda).

Chlorine producers across Europe are progressively moving towards this method of making their product as the membrane cell process is the most environmentally sound way of manufacturing chlorine.

The diaphragm cell process

Diagram of process

In the diaphragm cell process the anode area is separated from the cathode area by a permeable diaphragm. The brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment.

A diluted caustic brine leaves the cell. The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of caustic soda. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.

The mercury cell process

Diagram of process

In the mercury cell process, sodium forms an amalgam 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 2004, emissions for all mercury cells across Western Europe reached an all-time low of 1.01 grammes per tonne of chlorine capacity.

Forty-eight mercury-based chlorine plants remain to be voluntarily phased out or converted to non-mercury technology by 2020 at a cost of EUR 3,000 million. These plants account for almost 50% (6,000,000 tonnes) of European chlorine capacity.