Common
salt is sodium chloride, NaCl. It is an ionic compound with a giant structure
and a melting temperature of 800 °C. It is a vital part of our diet, and has
been known to mankind throughout history. Over millions of years, seas have
dried up leaving vast deposits of rock salt often buried beneath the surface by
the Earth's upheavals.
Salt itself is used, for example, for cooking, to spread on icy roads and for dye fixing. When a solution of salt is electrolysed, hydrogen (from the water) and chlorine gas are collected, leaving a solution of sodium hydroxide - all useful chemicals to industry.
Carl Kellner and Hamilton Young Castner developed the flowing mercury cathode cell for the electrolysis of salt solution in the 1890s. The membrane cell was developed by ICI in the 1970s. The development of this method depended on the production of suitable materials for the membrane that allowed negative ions to pass through but not positive ions.
Mercury cells are large and expensive to install. Membrane cells are small and cheap. It is possible to run just one or two membrane cells, so that they are often set up by other industries that use either sodium hydroxide or chlorine. So, a paper factory that requires chlorine for bleaching, might operate a single membrane cell on its own site.
| World production | Chlorine | 39 million tonnes per year |
| Sodium hydroxide | 41 million tonnes per year | |
| UK production | Chlorine | 1.6 million tonnes per year |
| Sodium hydroxide | 1.2 million tonnes per year | |
| Manufacture | Electrolysis: membrane cell / flowing mercury cell | |
| Raw material | Brine (salt solution). This may be sodium chloride or potassium chloride solution (in which case, potassium hydroxide is produced). |
 |
 |
Making chloroethane | 28% |
 |
Making chloromethane | 18% |
 |
Making solvents | 17% |
 |
Making inorganic compounds (eg hydrochloric acid, bleach) | 10% |
 |
Other | 27% |
|
|
|
Making chemicals | 30% |
|
Making fibres such as Nylon | 16% |
|
Making soaps and detergents | 5% |
|
Making paper | 5% |
|
Other | 44% |
 |
 |
| Map | Aerial view of plant |
 |
 |
| Unlabelled diagram | Labelled diagram |
 |
 |
| Unlabelled solution mining diagram | Labelled solution mining diagram |
 |
 |
| Unlabelled mercury cathode cell diagram | Labelled mercury cathode cell diagram |
 |
 |
| Unlabelled denuder diagram | Labelled denuder diagram |
 |
 |
| Unlabelled membrane cell diagram | Labelled membrane cell diagram |
 |
 |
| Unlabelled membrane cell diagram | Labelled membrane cell diagram |
last update March 2006
