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Chlorine is made primarily by the electrolysis of sodium chloride. Underground deposits of fairly pure sodium chloride are found in several areas, and these are the result of the evaporation of trapped seas from some 200 million years ago. In the UK the most accessible deposits are in Cheshire, Lancashire, Cleveland and Northern Ireland, though they extend under the North Sea as far as Poland.

Sodium chloride can be mined as rock salt (for example at Winsford, UK), but most is extracted by dissolving in water under pressure and pumping to the surface. Sodium carbonate is used to precipitate calcium and other metal ions, and other reagents are used to remove suspended particles. High-purity salt solution is required for most electrolytic cells.

Electrolysis requires electrical energy. A mercury cell typically operates at only 4.5v, but draws a current of 300kA. A single facility may consist of 100 such cells, drawing a total current of 300MA (million amps). Part of the environmental impact of chlorine production is related to the method used for electricity generation.

Recycling

About half the chlorine used in manufacturing ends up being reduced to hydrogen chloride or a chloride salt. In many cases the chlorine is not present in the final product, but is used as an oxidising agent. This includes the manufacture of:

· Fluorocarbons and hydro-fluorocarbons
· Polyurethanes
· White pigments (titanium dioxide)

If no commercial use can be found for the acid or chloride salt it may have to be disposed of as waste. A better option now being chosen is to use electrolysis or some form of chemical oxidation to convert the hydrogen chloride into chlorine. This is now a significant source of chlorine. Find out more by clicking here
image: Dead Sea Works Ltd, Sedom, Israel

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