Direct electrochemical reduction of indigo: process optimization and scale-up in a flow cell

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to problematic waste products. Therefore, modern economical and ecological requirements are not fulfilled. The industrial feasibility of the direct electrochemical reduction of indigo as a novel method has been determined and a preliminary optimization of electrolytic conditions was performed using a laboratory-scale flow-cell system. The role of current density, pH, temperature and the rate of mass transport are discussed. The influence of particle size reduction by the application of ultrasound is critically considered.