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11/2010
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,
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Hydrogen is a combustible gas utilized in various processes including the creation 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 processDiagram 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,
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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. In 2009, membrane cell capacity accounted for 52% of total installed chlorine creation capacity in Europe.
The diaphragm cell processDiagram 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 utilised to saturate diluted brine. The chlorine contains oxygen and must often be purified by liquefaction and evaporation.
In 2009, the diaphragm process accounted for 14% of total installed European chlorine manufacturing capacity.
The mercury cell processDiagram of process
In the mercury cell process,
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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 employed without further purification.
Of the three processes,
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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 2009, emissions for all mercury cells across Western Europe reached an all-time low of 0,93 grammes per tonne of chlorine capacity.
Forty-two mercury-based chlorine plants remain to be voluntarily phased out or converted to non-mercury technology by 2020 at a cost of more than EUR 3,
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