Greener Chlorine

Chlorine takes its name from chloros, the Greek word for 'pale green', since that is how we see it. Chlorine is in fact one of the few gases that has a colour. However, many people see chlorine as very far from green in the environmental sense, and they have some justification.

Traditionally chlorine manufacture involves mercury and uses vast amounts of electrical energy that leave a huge carbon footprint. However, with more effective mercury recovery techniques and new innovative cell technologies, both aspects are now improving.

Despite the associated environmental problems, we cannot live our modern lives without manufacturing chlorine by the megatonne. Some 55 per cent of European chemical production relies directly or indirectly on chlorine, and for pharmaceuticals the figure is 85 per cent. Consequently, we produce about 10 million tonnes of chlorine annually in Europe, and over 60 million tonnes worldwide.

Most chlorine is generated by electrolysing brine (sodium chloride solution), using three basic types of cell: mercury, diaphragm and membrane. Each type of cell employs a different technology to separate the chlorine from the co-products, hydrogen and sodium hydroxide solution (Fig 1).