Reuse of the water and salt of reactive dyeing effluent after electrochemical decolorisation

In the dyeing of cellulose with reactive dyes, the addition of a high amount of electrolyte is required [50–80 g/l of sodium chloride (NaCl) or sodium sulphate (Na2SO4)]. The main goal of this work is the reuse of salt and water from exhausted dyebaths and washing effluents, which have been previously decolorised by means of an electrochemical treatment. Dyeings with three bifunctional reactive dyes (Navy Blue, Crimson and Yellow Procion H-EXL) were carried out individually and in a trichromatic mixture. The reuse of the discoloured exhausted dyebath provided excellent red and yellow dyeings. Colour differences close to the quality control limit (ΔECMC(2:1) ≤ 1) were obtained in the blue and trichromatic dyeings. An average recovery of 70% of water and 60% of salt was achieved with the reuse process. High-quality dyeings were also obtained with the discoloured first-washing bath. In this case, 15% of salt and almost 100% of water were recycled.