Chlorine

Electrolysis

Chlorine has been manufactured for the last 100 years by the electrolysis of saturated sodium chloride solution, commonly referred to as brine. Common salt is found as a mineral deposit in many places, in the UK notably Cheshire, Lancashire, Cleveland and Northern Ireland. It can either be mined as the solid or extracted by dissolving the deposits in water and returning the saturated solution ("brine") to the surface for purification.

Product separation

Simple electrolysis of sodium chloride in a laboratory gives chlorine at the anode and hydrogen at the cathode. Unfortunately the remaining solution turns slowly into a mixture of sodium chloride and sodium hydroxide. If this approach was used commercially other useful products could not easily be obtained, and a continuous process would be difficult to achieve.

diagram: simple electrolysis of sodium chloride

The excess sodium ions need to be kept separate from the chloride ions, and it is this need for product separation that determines the design of commercial chlorine electrolysis cells. The main products are chlorine, hydrogen and sodium hydroxide.

Three designs are in use world-wide. For more details, click on the links in the table.

Process	Separation method	Comments
Mercury cell	Mercury cathode forms amalgam with sodium - hydrogen gas is not formed at this stage. Amalgam flows out of the cell where water is used to release the sodium and hydrogen	Oldest method, still very much in use but not chosen for new plant
Diaphragm cell	Steel and asbestos diaphragm separates anode and cathode compartment. Chlorine forms at anode, sodium migrates through diaphragm to cathode compartment - rate of liquid flow prevents hydroxyl ions migrating to anode compartment	More common in the US than EU. Higher energy requirements
Membrane cell	Ion exchange membrane separates anode and cathode. Chlorine forms at anode, only sodium migrates through to cathode compartment - membrane is not permeable to liquid/gas flow and only permits migration of cations (+)	Most new plants opt for this design. Least environmental impact, lowest energy consumption

Electrolysis

Other metals obtained by electrolysis include sodium and magnesium. In both these cases the chlorides are used, and chlorine is the other product. Sodium chloride is mixed with calcium or barium chloride to lower the melting point. Click here for a diagram of the cell used for sodium extraction

Hydrogen chloride Recycling

Several processes produce hydrogen chloride (HCl) either as the gas or dissolved as hydrochloric acid. This includes polymer production, fluorocarbons, polyurethane resins and rubbers, and white pigments. Although there are many uses for HCl, too much is produced.

Rather than neutralising the excess, it can be used in the manufacture of chlorine. About one third of chlorine produced is now the result of some form of recycling.

Technologies

Electrolysis

Hydrochloric acid, when electrolysed, produces hydrogen and chlorine, both useful products. Temperatures will usually be less than 100°C.

Simple electrolysis also produces oxygen, which corrodes the cells and decreases efficiency. The newest membrane cells allow hydrogen chloride gas to be used and result in a more efficient system. Click here for a diagram of the membrane cell approach.

Oxidation

The conversion of two chloride ions into a chlorine molecule is an oxidation reaction and can be brought about using oxidising agents, but only if it is coupled with a very favourable reduction.The Deacon process, dating from an 1868 patent, uses oxygen and a copper chloride catalysts, but requires high temperatures, up to 450°C, and produces an extremely corrosive mixture. In its simple form this is not viable as an industrial method, but the need to recycle waste products has resulted in more research into this approach.