Production scale test for indirect cathodic reduction of indigo opens a route for greener denim

Abstract

Textile and garment industry is one of the biggest industrial consumers of high quality water and belongs to most polluting industries worldwide. Development of textile chemical processes which allow reuse of chemicals and water thus is an imperative. Cathodic reduction of vat dyes e.g. indigo is a promising technology to replace non regenerable reducing agents in dyeing processes. Experiments at laboratory scale are not sufficient to achieve scalable results. In this study a full scale production test for replacement of dithionite based indigo reduction in denim production by cathodic dye reduction has been performed to demonstrate the potential of the technology. A 1000 A electrolyser was coupled to a open width yarn dyeing unit and 11,000 m (3,600 kg) of warp yarn was dyed with use of a cathodically regenerated iron-triethanolamine complex as reducing agent instead of a continuous dosage of dithionite into the dyebath. A time constant production was achieved over the full duration of the 5.5 h production. The dyed warp then was woven into denim fabric and only very minor colour deviations were observed between the electrochemically dye fabric and the reference quality, which demonstrates the ability of the technology to reproduce shade and ring-dying of a denim yarn. Model calculations to compare the chemical consumption of an electrochemical dye reduction with standard chemical processes indicate the importance of a recycling of the waste water by membrane filtration. Compared to a typical indigo range which consumes 100 g min−1 dithionite the chemical consumption of the new process falls below to 30% when 60% of the iron-complex therein is recovered. The demonstration of the technology in a full scale test can be understood as a relevant break through for future introduction of indirect cathodic dye reduction as cleaner technology in denim production.