Comparative study on the pyrolysis and leachability of washed/unwashed electric arc furnace dust-PVC mixtures and their residues

In this work, the pyrolysis behavior of electric arc furnace dust (EAFD) mixed with poly(vinyl) chloride (PVC) was studied under inert conditions. Unwashed (UW) EAFD was found to contain lead hydroxychlorides which were converted to hydrocerrusite upon washing. It also contains appreciable amounts of soluble salts such as halite and sylvite; the effect of these salts on the pyrolysis behavior of EAFD-PVC mixtures was investigated. These salts delayed the dechlorination of PVC, by at least 20 ⁰C, during the pyrolysis of EAFD-PVC and altered the mineral composition of the pyrolysis residue. The later was rich in PbCl2, FeCl2. H2O for both unwashed (UW) and washed (W) EAFD samples. However, the W-EAFD-PVC residue was rich with ZnCl2. H2O, which was not detected in the UW-EAFD-PVC pyrolysis residue. Instead, it is believed that UW-EAFD-PVC pyrolysis residue is rich with the metastable Na2ZnCl4 as well as K2FeCl5. H2O and NaFeCl4. The capturing and monitoring of the released gases during the pyrolysis of UW- and W-EAFD-PVC mixture, confirmed the inhibiting effect of the halite and the sylvite on the dehydrochlorination of PVC. The released gases demonstrate a strong evidence of the solid-solid reaction between EAFD compound (mainly ZnO) and PVC without evolution of hydrochloric acid (HCl) at temperatures below 200 ⁰C. The subsequent leaching of the pyrolysis residue has resulted in the recovery of Zn (up to 100%), Fe (up to 64%) and Pb (up to 76%) in water. It is inferred that decreasing iron recovery can be achieved by oxidizing Fe+2, while increasing Pb recovery can be achieved by controlling the pH of the leachate solution.