Keeping salt in the cycle

Salt in industrial process water hasn't received much attention so far. It usually passes through treatment plants unaffected and ends up in the ecosystem. The pioneer in proving the economic feasibility of salt recycling is Covestro.

Water is a valuable resource and needs to be protected. Even today, access to clean drinking water is not a matter of course for everyone. The United Nations estimate that by 2050 water demand will be even more than half of what it was at the beginning of the millennium.

Covestro uses water for many products, among others polycarbonate production. The process wastewater is purified, but still contains six per cent salt - sodium chloride (NaCl) afterwards, more than twice as much as is found in seawater.

Covestro and its partners have therefore developed a process to reuse this process water in chlorine production. Chlorine is obtained by electrolysis of salt, which is recovered from the process water. Two plants at the Caojing site in China and the Krefeld-Uerdingen site in Germany are already testing the process. The plants are running stably and without any problems by now.

"This technology shows how we can keep raw materials in the cycle. It serves as a role model," says Susannah Havermann, polycarbonate plant manager at Covestro. In the long term, tens of thousands of tonnes of salt and hundreds of thousands of tonnes of water are to be saved annually.

However, challenges remain to be solved. The salt content of the process water is low at around six per cent. As a result, 100 per cent of the salt cannot yet be fed back into the electrolysis process. "We want to extract water from the brine and thus increase the salt content. This has to be done in an environmentally compatible and economical way," says Dr Yuliya Schießer, who is coordinating the BMBF-funded project for Covestro.

The quality control of the process water is the second challenge. The membranes of the chlorine electrolysis plants, into which the recycled wastewater is fed back, are very sensitive to impurities. The process water purification must be monitored to prevent a standstill of the unit. Costly laboratory analysis currently prevents this and shall now to be automated as part of the project.

There is still a lot to be done before another step towards circular economy is taken. Covestro CEO Dr Markus Steilemann explains: "Single-use is no longer in keeping with the times, neither for consumers nor for industry. We make it a priority to conserve resources; that's why we are consistently gearing our actions towards the circular economy."