Co-pyrolysis of Fe_3O_4-poly(vinyl chloride) (PVC) mixtures: Mitigation of chlorine emissions during PVC recycling

摘要

A systematic investigation was conducted on the co-pyrolysis of Fe_3O_4 and 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 Fe_3O_4 and PVC mixtures following a two-stage pattern bifurcated at a temperature of 673 K. In Stage 1, at temperatures 673 K and lower, Fe_3O_4 is chlorinated by chlorine from PVC, resulting in FeCl_2. The composition of the final solid residue of Stage 1, conjugated polyene, FeCl_2 and Fe_3O_4/Fe_2O_3, depends on the initial Fe_3O_4 content 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 Fe_3O_4/Fe_2O_3: Fe_2O_3 → Fe_3O_4→ FeO → Fe. However, in mixtures containing Fe_3O_4 that is less than 39.6% of the mass, Fe_3O_4 can coexist with Fe; therefore, the FeO formation step is skipped. Most FeCl_2 escapes from the reaction system as vapor, showing the necessity of removing FeCl_2 at the end of Stage 1 to avoid harmful substance emission. The presence of Fe_3O_4 can significantly suppress gaseous emissions, especially HC1 originating from PVC decomposition. There was only 0.6% HC1 by mass (2.4% PVC base by mass) released when co-pyrolyzing the PVC + 75% Fe_3O_4 mixture due to the complete consumption of PVC and its decomposition products by Fe_3O_4. After separating FeCl_2, which is a valuable chemical feedstock, by water-leaching the solid residue obtained at 673 K, the filtered residue, which is a mixture of Fe_3O_4/ Fe_2O_3 and polyene, was confirmed to 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 Fe_3O_4 added.