High-temperature chlorination of PbO and CdO induced by interaction with NaCl and Si/Al matrix

Municipal solid-waste incineration leads to emission of lead (Pb) and cadmium (Cd), which vaporize in furnace and condense in flue. NaCl in waste has been proven to enhance volatilization of Pb and Cd at high temperatures via chlorination of oxides to chlorides; however, this process was not well-understood so far due to its complexity. This study decoupled the indirect chlorination process and direct chlorination process so that these two processes were investigated separately. A horizontal tube furnace was used to heat the mixtures of NaCl and Si/Al matrix for indirect chlorination and the mixtures of NaCl, PbO/CdO and Si/Al matrix for direct chlorination. A set of dynamic sampling devices was designed and used to obtain dynamic data during temperature rising. The indirect chlorination process was initiated above 800 °C in O(2) + H(2)O atmosphere and O(2) atmosphere and above 1000 °C in N(2) atmosphere. Al(2)O(3) exhibited higher activity than SiO(2) to react with NaCl, releasing HCl or Cl(2). In the Cl release reaction, NaCl was in the gas phase. The direct chlorination process was initiated at 650–700 °C when the Si/Al matrix contained SiO(2) only and at around 800 °C when the Si/Al matrix contained Al(2)O(3) only or both SiO(2) and Al(2)O(3). SiO(2) exhibited higher activity than Al(2)O(3) in direct chlorination. The pre-reaction between PbO/CdO and Si/Al matrices was considered as the necessary condition for direct chlorination. During chlorination in O(2) + H(2)O atmosphere, indirect chlorination and direct chlorination occurred simultaneously, and the latter dominated the volatilization of Pb and Cd.