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Results from this study provide an atomic-based insight into a promising thermal recycling route of e-waste. Page 2 of 33 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 IntroductionCo-pyrolysis of metal oxides with halogen-bearing materials attracts technological interests on two compelling grounds: (i) recycling of bromine and chlorine-containing objects and (ii)pyro-metallurgical extraction of metals from their oxides. Thermal treatment of the ever increasing electronic and electric waste (e-waste) constitutes a real-world case where the recycling of bromine and extraction of metals overlap. 1 The non-metallic fraction in e-waste bears a significant load of halogenated hydrocarbons, mainly in the form of brominated flame retardants (BFRs). [1][2][3][4][5] On the other hand, exposing the metallic constituents in e-waste to oxygen at elevated temperature transforms them readily into metal oxides. 3 The interest in studying the co-pyrolysis of BFRs with metal oxides stems from their ability to act as bromine fixation agents, 6 the process that ultimately leads to reductive debromination of BFRs. Of particular industrial as well as health importance are ferric oxides that make up most of the ferric fraction in electric arc furnace dust (EAFD). It is estimated that, 4.3 -5.7 million tonnes of EAFD arise annually worldwide during crude steel production. 7 Thermal degradation of BFRs in the presence of metal oxides achieves a dual-benefit, reducing the overall toxicity of the decomposition products of BFRs and forming metal bromides that could be easily leached out.The potential for the conversion of BFRs into hazardous brominated compounds (most notably the notorious polybrominated dibenzo-p-dioxins and furans, PBDD/Fs) often overshadows the environmental and economic benefits of thermal recycling of e-waste. 8, 9The co-existence of aromatic brominated precursors with metal oxides typically act as a perfect recipe for the catalytic synthesis of PBDD/Fs through prominent intermediate steps. 10While HCl represents an inactive chlorinating agent for generation of PCDD/Fs, 11 conversionPage 3 of 33 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 We estimate reaction rate constants based on the conventional transition state theory (TST).In the TST calculations, vibrational frequencies yield activation enthalpies and entropies at the temperature of interest (300 -1000 K). A-factors for barrierless reactions are estimated based on the difference in entropies between reactants and products. We report electronic charges for the α-Fe 2 O 3 cl...

Hydrogen halides (HCl/HBr) represent major halogen fragments from the thermal decomposition of halogen laden materials, most notably PVC and brominated flame retardants (BFRs). Co-pyrolysis of halogen-containing solid waste with metal oxides is currently deployed as a mainstream strategy to treat halogen content as well as to recycle the valuable metallic fraction embedded in electric arc furnace dust (EAFD) and e-waste. However, designing an industrial-scale recycling facility necessitates accurate knowledge on mechanistic and thermo-kinetic parameters dictating the interaction between metal oxides and hydrogen halides. In this contribution, we investigate chemical interplay between HCl/HBr and zincite surfaces as a representative model for structures of zinc oxides in EAFD by using different sets of functionals, unit cell size and energy cut-off. In the first elementary step, dissociative adsorption of the HCl/HBr molecules affords oxyhalide structures (Cl/Br-Zn, H-O) via modest activation barriers. Conversion of the oxyhalide structure into zinc halides occurs through two subsequent steps, further dissociative adsorption of HCl/Br over the same surface Zn atom as well as the release of a HO molecule. Evaporation (or desorption of zinc halide molecules) signifies a bottleneck for the overall halogenation of ZnO. Our simplified kinetic model on the HCl + ZnO system concurs very well with experimentally reported TGA weight loss profiles on two grounds: accumulation of oxyhalides until ∼700 K and desorption of ZnCl at higher temperatures. The thermo-kinetic and mechanistic aspects reported herein could be useful in the pursuit of a design of a large-scale catalytic upgrading unit that operates to extract valuable zinc loads from EAFD.

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