A novel zero emission concept for electrogenerated chlorine leaching and its application to extraction of platinum group metals from spent automotive catalyst

• A novel and eco-friendly electrogenerated chlorine leaching system was developed.
• An innocuous concept was investigated to recycle unconsumed chlorine during leaching.
• Electrochemical innocuous treatment with > 99.8% efficiency was achieved.
• No chlorine discharged during the leaching of spent automotive catalyst.
• Over 99.6% of Pt from the spent automotive catalyst could be leached.

In this study, a new concept for recycling of chlorine during electrogenerated chlorine leaching has been investigated and applied to the leaching of spent automotive catalyst containing 0.128 wt.% Pt and 0.015 wt.% Rh. The innocuous treatment was achieved by the electrochemical reduction of the unconsumed gas in the cathodic compartment of an in situ chlorine electrogeneration cell. The reduction efficiency in various catholytes was investigated, and over 99.8% efficiency was obtained in 2.0 M NaCl solution at a pH of 10, whereas 54.8 and 74.1% efficiencies were obtained in 35% HCl solution with and without 0.1 M FeCl3, respectively, at 0.618 mmol min− 1 generation rate of the unconsumed gas. A fast dissolution and high solubility of chlorine gas as ClO− in an alkaline NaCl solution may enhance the efficiency. By using the leaching system equipped with the innocuous treatment unit, over 99.6% of Pt from the spent automotive catalyst could be leached without discharging the unconsumed chlorine gas into the air at 100 g L− 1 solid/liquid (S/L) ratio and 80 °C in 8.0 M HCl solution. However increasing the S/L ratio over 400 g L− 1, the leaching percentage drastically decreased to < 16% in 90 min owing to the possible adsorption of dissolved Pt to the catalyst carrier. In the case of Rh, 65.0% leaching was accomplished at 80 °C and 100 g L− 1 in 8.0 M HCl in 240 min, and no decrease was observed at high S/L ratios. The process is efficient and eco-friendly for simultaneous leaching PGMs from the spent catalyst while recycling the unconsumed chlorine gas by the innocuous treatment.