ICI process recovers alkali from sodium sulphate wastes

Engineering contractor John Brown and ICI Chemicals & Polymers have joined forces to develop ICI's electrolytic process for the recovery of acid and alkali from waste streams containing sodium sulphate.

Developed by ICI's electrochemical technology business, the process is likely to generate interest in the pulp and paper, viscose and fine chemicals industries. In the viscose industry, for example, roughly one tonne of sodium sulphate waste is produced for every tonne of viscose rayon produced.

The process is an adaptation of ICI's established FM21 electrolyser technology which was introduced commercially in 1981.

According to ICI's product development manager Sarah Galbraith, interest in treating sodium sulphate wastes has grown for two reasons. Disposal of the wastes to landfill has become more expensive and less environmentally acceptable. Meanwhile, the market for recovered sodium sulphate for use in conventional detergents has declined due to the increasing importance of compact cleaners. Coupled with the increasing price of sodium hydroxide, this has made the economics of the electrochemical recovery process considerably more attractive.

No full-scale recovery plants are yet in operation, but ICI has been running several small-scale pilot units for almost a year on a range of waste streams. The company is now considering building a full-scale pilot plant on a customer's site.

The process is claimed to become economic with quantities of waste over 5,000 tonnes per year, and is particularly attractive where the recovered caustic soda and sulphuric acid can be reused on site. The sodium sulphate waste solution will need some pre-treatment, such as ion exchange or filtration.

The agreement with John Brown is part of a wider partnership under which ICI will use John Brown and Foster Wheeler as preferred contractors on its major projects. John Brown will supply the engineering back-up for the electrochemical process.

There are several competing processes for sodium sulphate recovery, mostly based on electrodialyis techniques. ICI claims that its technique overcomes the problems caused by the fragility of the membrane - if one compartment of the FM21 electrolyser fails, the process will still carry on.

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