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Issue 2, 2018
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Recycling of zincite (ZnO) via uptake of hydrogen halides

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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 H2O 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 ZnCl2 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.

Graphical abstract: Recycling of zincite (ZnO) via uptake of hydrogen halides

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The article was received on 09 Sep 2017, accepted on 22 Nov 2017 and first published on 22 Nov 2017

Article type: Paper
DOI: 10.1039/C7CP06159E
Citation: Phys. Chem. Chem. Phys., 2018,20, 1221-1230
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    Recycling of zincite (ZnO) via uptake of hydrogen halides

    O. H. Ahmed, M. Altarawneh, M. Al-Harahsheh, Z. Jiang and B. Z. Dlugogorski, Phys. Chem. Chem. Phys., 2018, 20, 1221
    DOI: 10.1039/C7CP06159E

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