On-site formation of hypochlorite for indigo oxidation - Scale-up and full scale operation of an electrolyser for denim bleach processes

On-site production of hypochlorite for denim bleach was investigated using undivided unipolar electrolysers. The current efficiency of the electrolysis process was studied at laboratory scale with up to 20 A cell current and at the technical scale using a 1200 A electrolyser. NaCl solutions in the concentration range 0.236-0.944 M NaCl (13.8-55.2 gl l(-1) ) were used. Anodic current densities up to 400 A m(-2) were used. Hypochlorite solutions with active chlorine concentration c(Cl)=0.10-0.12 M were obtained with current efficiency of 58-70%. For denim bleach processes a concentration of active chlorine up to c(Cl)=0.12 M is required. This corresponds to approximately 30 ml l(-1) of commercial hypochlorite solution. Electrical energy consumption calculated per kg of active chlorine formed ranges from 5.7 to 8.6 kWh kg(-1). By coupling the electrolyser to a commercial 30 kg capacity drum washing machine the electrochemically prepared oxidant was successfully used in bleach processes. The technique overcomes problems of insufficient reproducibility of bleach, chemical costs and released wasted water due to the use of commercial NaOCl solution.