Conversion of PVC wastes to fuel-based chemicals via thermal recycling with Zn mono-, Bi-, and Tri- metal oxides

Abstract

Polyvinyl chloride (PVC) is a highly utilized plastic in various industries that span construction, electronic devices, and household appliances. Its production is projected to reach a growth rate of 65 million metric tons by 2030. Our research aims for simultaneous thermal treatment of PVC through its co-pyrolysis with selected Zn-based transition metal oxides (MO) namely ZnO, ZnFe₂O₄, and NiZnFe₄O_4. Such an approach eliminates chlorine emission and enables to facilitate the isolation and extraction of the valuable zinc load in these MOs. These mono-tri Zn-based MOs are considered since they represent the zinc content in the electric arc furnace dust (EAFDs); another category of hazardous waste. The main aim of the work is to assess the likely selective extraction of the zinc load when compared to that of iron. TGA analysis indicates a strong interaction between the MO's and PVC, implying that the MO's act as potent catalysts for the dechlorination and cracking of the PVC structure. The co-pyrolysis of PVC with metal oxides at temperatures between 300 and 500 °C led to the production of metallic chlorides, as confirmed by XRD measurements. Based on IC measurements, PVC with ZnO mixture captured 91.13 % of the emitted HCl, whereas ZnFe₂O4 and NiZnFe₄O₄ seized the HCl emission to 65.78 % and 83.56 %, respectively. Further, EDX mapping disclosed the elemental composition of the solid residue after the reaction. The formation of Lewis acids such as ZnCl₂, FeCl₂, and NiCl₂ during the pyrolysis reaction triggered the isomerization of cis to trans olefins, which activated the PVC chain and made it less favorable to undergo cyclization reactions. Accordingly, the GCMS revealed, with the inclusion of MO's, a significant depreciation in the formation of benzene derivatives, heterocyclic compounds, and polyaromatic hydrocarbons, whereas it enhanced the formation of alkanes. For all temperature ranges, a high amount of aliphatics were formed with ZnO, with a peak value of 60.19 % at 500 °C. With a relative yield of ≈9–11 %, octane dominates in the produced hydrocarbons, which is considered the most efficient fuel due to its effective combustion. Moreover, the co-pyrolysis results at the temperature range of 300–500 °C suggest the potential for extracting the metals from the metallic chloride by different extraction techniques.