Pilot-scale electrolyser for the cathodic reduction of oxidised C.I. Sulphur Black 1

A multi-cathode electrolyser was used for the direct cathodic reduction of oxidised, dispersed C.I. Sulphur Black 1. Crude C.I. Sulphur Black 1 at concentrations of 80 g L−1 and 131 g L−1 could be reduced using a cathodic current density of 5.3 mA cm−2. During reduction at pH 12.2–12.6 the redox potential fell from −270 mV to between −550 mV and −600 mV. As crude C.I. Sulphur Black 1 contains considerable amounts of reductants, the current yield was >100% of theoretical value. Energy consumption for the reduction of 1 kg of crude C.I. Sulphur Black was ∼1.1 kW h kg−1. The reduced C.I. Sulphur Black 1 was directly used for laboratory dyeing of cotton yarn. Kubelka–Munk values and CIELab coordinates of dyeings using cathodically or chemically reduced dyestuff showed only small differences, indicating similar dyeing behaviour of the reduced dye, independent of the reduction technique.