Chlorine and chlorine-containing compounds have been widely used in various reactions but the atom efficiency of chlorine is usually very low. In polyurethane industry, the synthesis of isocyanates generates significant amount of by-product HCl gas and waste water containing salt. How to recycle and reuse them is a critical concern of the Polyurethane (PU) industry. Especially, the rapid expansion of PU industry in developing countries urges cost-effective solutions for chlorine recycling. The authors led a group of scientists and engineers and developed two advanced technologies to realize the closed-loop recycling of chlorine. Using the hydrogen chloride catalytic oxidation technology, the by-product HCl gas can be converted to Cl2, with much lower energy consumption than that of the traditional chlor-alkali process. After treatment, the waste brine generated from the Methylene Diphenyl Diisocyanate (MDI) plant can be used as the electrolyte in chlor-alkali industry. The application of these two technologies brings significant economic, environmental and societal benefits. ► Two advanced technologies realizing closed-loop chlorine recycling are introduced. ► A new copper-based catalyst and a fluidized-bed process was developed for converting the by-product HCl gas to Cl2. ► The waste brine generated from the isocyanate plant is recycled as the electrolyte in chlor-alkali industry. ► These two technologies bring significant economic, environmental and societal benefits.