Direct cathodic reduction of Sulphur Black 1 enables full-scale production testing for Black Denim without the use of reducing agents

In textile production, textile dyeing is the second largest consumer of fresh water, using 6.3 billion m 3 annually. New concepts for recirculation of chemicals, including dyes, are necessary to prevent accumulation of by-products in the waste water. In this case study direct cathodic reduction of C.I. Sulphur Black 1 was used for dyeing of denim warp yarn, with the aim to replace organic reducing agent by electrochemical reduction for dyeing of Black Denim and Topping of an indigo pre-dyed yarn. A multi-cathode electrolyser capable of handling a maximum cell current of 1800 A, was coupled to a denim dyeing range operating at 25 m min −1 speed, which corresponds to a production rate of 6.38 kg min −1 . Stable process conditions in terms of redox potential, pH and temperature could be realised over the full production length of 12,375 m (3156 kg) cotton warp beam. The colour depth and shade of the dyed yarn matched the reference products from conventional production. Based on the process data, a concept for recirculation of washing water including dyestuff, is proposed. By introducing membrane filtration, the estimated savings of water and sulphur dye will be 200 m 3 d −1 (40,000 m 3 y −1 ) and 447.5 kg d −1 (89.5 tons y −1 ), respectively, highlighting the potential of cathodic dye reduction to significantly reduce the water and dyestuff consumption in textile dyeing with sulphur dyes.