A comparative study of iron-complex mediated cathodic vat dye reduction with dyebath regeneration by ultrafiltration, and the conventional dyeing process

Abstract

The huge consumption of fresh water in the textile dyeing industry is a consequence of the use of chemicals, dyes and processes that are not compatible with recycling techniques. In this work pilot scale vat dyeing of yarn cones with use of electrochemically regenerable iron-complexes as reducing agents was investigated. A combination triethanolamine and heptagluconate was used for stable complexation of iron(III/II) at dyebath temperatures up to 80°C. At the end of the dyeing step the used dyebath was collected and oxidised by air oxygen. The dispersed oxidised vat dye was removed by filtration using a 0.45 µm PTFE membrane. A comprehensive analysis of the relevant waste water parameters demonstrated the potential of the technique to reduce the water consumption of the dyeing step by 68 % and to achieve significant reduction in chloride (-87 %), nitrate (-83 %) and sulphate (-99.6 %) load. Dependent on colour depth a replacement of single-use chemicals by this new technical concept will lead to significant chemical savings between 18 % and 72 % compared to the state of the art processes.