Co-pyrolysis of Fe3O4-poly(vinyl chloride) (PVC) mixtures: Mitigation of chlorine emissions during PVC recycling

Abstract

A systematic investigation was conducted onthe co-pyrolysisof Fe3O4and PVC mixtures in temperatures as high as 1373 K upon the development of PVC recycling technology that mitigates chlorine emission.Central to our investigation, PVC decomposition plays the leading role in the co-pyrolysis of Fe3O4and PVC mixtures following a two-stage pattern bifurcated at a temperature of 673 K. In Stage 1, at temperatures 673 K and lower, Fe3O4is chlorinated by chlorine from PVC, resulting in FeCl2. The composition of the final solid residue of Stage 1,conjugated polyene, FeCl2and Fe3O4/Fe2O3, depends on the initial Fe3O4content in the mixture. When the temperature is increased to be higher than 673 K, decomposition of conjugated polyene occurs simultaneously with the stepwise reduction of Fe3O4/Fe2O3: Fe2O3 → Fe3O4 → FeO → Fe. However, in mixtures containing Fe3O4that is less than 39.6% of the mass, Fe3O4can coexist with Fe; therefore, the FeO formation step is skipped. Most FeCl2escapes from the reaction system as vapor, showing the necessity of removing FeCl2at the end of Stage 1 to avoid harmful substance emission. The presence of Fe3O4can significantly suppress gaseous emissions, especially HCl originating from PVC decomposition. There was only 0.6% HCl by mass (2.4% PVC base by mass) released when co-pyrolyzing the PVC + 75% Fe3O4mixture due to the complete consumption of PVC and its decomposition products by Fe3O4. After separating FeCl2,which is a valuable chemical feedstock, by water-leaching the solid residue obtained at 673 K, the filtered residue,which is a mixture of Fe3O4/Fe2O3and polyene, was confirmedto be suitable for iron-making. The results clearly show the possibility of developing a PVC recycling technology with mitigated chlorine emissions by manipulating the amount of Fe3O4added.