Heavy metal removal from MSW fly ash by means of chlorination and thermal treatment: Influence of the chloride type

Heavy metal separation from municipal solid waste (MSW) fly ash is becoming important as a source for metal recovery and as a means to reduce landfill costs.In an indirectly heated laboratory-scale rotary reactor, mixtures of MSW fly ash with different amounts and different types of chlorides are treated batch-wise at 1000 °C for 60 min; in a muffle oven, the same samples are thermally treated between 800 and 1200 °C for 20 h. The treated samples are analyzed for the heavy metals Cd, Cr, Cu, Ni, Pb, and Zn. As chlorides, CaCl2, MgCl2, and NaCl are used; zero to 200 g Cl is added per kg ash.CaCl2 and MgCl2 are more effective for most of the heavy metals than NaCl, which evaporates in significant amounts without reacting and even decreased the removal of Cd, Cr, Cu, Pb, and Zn in rotary reactor experiments. NaCl directly chlorinates; CaCl2 and MgCl2 first form HCl and/or Cl2 by reacting with H2O and/or O2 (indirect chlorination).Maximum amounts of heavy metals released are in the rotary reactor: ca. 95–100% Cd and Pb (MgCl2 or CaCl2), more than 80% Cu and Zn (MgCl2), approx. 10% Cr (any Cl donor) and 35% Ni (CaCl2); removals in the muffle oven were generally higher than in the rotary reactor. Depending on the Cl donor, at 1200 °C up to 100% Cd, Cu, and Pb, ca. 30% Cr, 75% Ni, and 95% Zn could be vaporized. Higher removals in the muffle oven are found to be due to lower mass transfer into and out of the crucible, increasing the residence time of HCl, Cl2 or chloride in the crucible.

► We mixed MSW fly ash with different types and amounts of chlorides.
► We treated the mixtures thermally in a muffle oven and a rotary reactor.
► Thermodynamic equilibrium calculations have been carried out.
► NaCl directly chlorinates; CaCl2 and MgCl2 form first HCl or Cl2.
► CaCl2 and MgCl2 are much more effective for heavy metal removal from ash.