Avoiding PCDD/F Formation in Metal Chloride–Based Chlorination–Calcination Processes: Mechanisms, Risk Factors and Best Available Practices

Metal chloride–based chlorination–calcination routes are increasingly being revisited for processing complex ores, secondary resources, and metallurgical residues due to their high selectivity and compatibility with circular-economy strategies. However, the coexistence of chlorine species, transition metals, carbonaceous matter, and intermediate temperatures can favor the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), raising critical environmental and regulatory concerns. This critical review examines the fundamental chemical mechanisms governing PCDD/F formation in chlorination–calcination systems, with emphasis on precursor pathways, de novo synthesis on particulate surfaces, and the catalytic role of metal chlorides such as Cu, Fe, and Ni. Key operational risk factors—including temperature windows, oxygen potential, residence time, chlorine activity, and the nature of carbon sources—are systematically analyzed across laboratory, pilot, and industrial contexts. Building on this mechanistic framework, the review evaluates best available practices (BAPs) for PCDD/F prevention, including feed pretreatment, process atmosphere control, temperature management, catalyst suppression, rapid quenching, and off-gas treatment strategies. By integrating thermochemical insights with process engineering and environmental control perspectives, this work provides a rigorous basis for designing chlorination–calcination flowsheets that minimize PCDD/F formation while preserving metallurgical efficiency and regulatory compliance.