Search

Thermal co-treatment of Electric Arc Furnace Dust (EAFD) and polyvinyl chloride (PVC) may provide a viable route for reprocessing these hazardous materials within the circular economy. To develop and optimise a commercial treatment process, the complex mechanistic pathway resulting from the reaction of these two wastes must be understood. Franklinite (ZnFe 2 O 4) is a major zinc containing phase in EAFD and to date, little work has been undertaken on the decomposition of PVC in its presence. Herein, we present a thermodynamic, pyrolytic, and kinetic study of PVC degradation in the presence of ZnFe 2 O 4. It was found that ZnFe 2 O 4 decomposed to its associated halides. Additionally, the kinetics data confirmed the catalytic activity of ZnFe 2 O 4 , dropping the de-hydrochlorination onset temperature of PVC from 272 to 235 °C. The distribution of the activation energy with conversion suggests the presence of several competitive reactions each with a different energy barrier. In such a case, reaction channelling can take place leading to selective zinc chlorination. Moreover, since the reduction of Fe 2 O 3 is slow at low temperatures, it is recommended to operate at a temperature as low as 235 °C which can promote the chlorination selectivity towards zinc leaving iron bearing compounds in their stable form (Fe 2 O 3). [ABSTRACT FROM AUTHOR]

• PbO exhibited an inhibiting effect on the de-hydrochlorination of PVC. • The activation energy of PVC de-hydrochlorination increased in the presence of PbO. • The intramolecular hydrogen transfer was found to be the rate limiting step. • PbO was almost entirely transformed into PbCl 2 upon the thermal treatment with PVC. The elimination of toxic heavy metals from metallurgical wastes before its recycling to steel furnaces is essential. Lead oxide (PbO) is a major toxic heavy metal present in electric arc furnace dust (EAFD). The co-recycling of EAFD with Polyvinyl chloride (PVC) followed by water leaching allows the elimination of this material. Herein, an experimental and theoretical kinetic investigation was conducted on the thermal degradation of PVC in the presence of PbO. An inhibiting effect of PbO on the thermal degradation of PVC was observed; the onset PVC de-hydrochlorination temperature increased from 273 to 293 °C. PbO addition also increased the activation energy of PVC mass loss from 148 kJ/mol (PVC) to 173 kJ/mol (PbO-PVC) in the conversion window 5–45 %. The theoretical Density Function Theory (DFT) suggested that the intramolecular hydrogen transfer (IHT) to form H 2 O on PbO surface exhibited the highest energy barrier at 251.01 kJ/mol, which aligns with the experimental data. The kinetic parameters generated here can be utilised to calculate the appropriate holding temperature and time for the effective removal of PbO from waste EAFD using PVC as a chlorinating additive. This should contribute towards optimising process economics and efficiency. [ABSTRACT FROM AUTHOR]

This contribution reports a thermodynamic assessment for the bromination of electric arc furnace dust (EAFD) by-products sourced from thermal degradation of tetrabromobisphenol A (TBBA); i.e., the most widely deployed brominated flame retardant. Upon TBBA's pyrolysis, HBr is released in conjunction with several volatile organic compounds leaving a solid carbonaceous residue. EAFD contains appreciable quantities of zinc, iron, and lead oxides. These oxides can react with HBr to form volatile metal bromides when the EAFD is added to the TBBA as a bromination agent. The selective bromination of zinc and lead contained in EAFD was thermodynamically evaluated under both oxidative and inert pyrolytic conditions while considering the effects of several variables. These factors span temperature, loads of TBBA, the presence of oxidizing agent, and the effect of the presence of other common EAFD's constituents such as sodium, potassium, calcium, silicon, and sulfur. It was found that a 100% extraction (based on thermodynamic feasibility) of both zinc and lead can be achieved for a mixture containing 60% EAFD and 40% TBBA (contaminated with minor amounts of iron) when pyrolyzed under inert conditions. However, when a thermal treatment is performed in the presence of oxygen, complete thermodynamic-based recovery of zinc and lead recoveries can be achieved at a lower temperature with no iron content. Removal of sodium and potassium chloride from EAFD prior to pyrolysis by washing, under oxidizing condition, can also result in a profound selectivity in zinc and lead bromination. The behavior of other elements during bromination process was also discussed. [ABSTRACT FROM AUTHOR]

Electric arc furnace dust (EAFD) is a troublesome metallurgical waste accumulated worldwide in large quantities. In this work, EAFD mixed with polyvinyl chloride (EPVC) was thermally treated at 300 °C under air flow followed by leaching of solid residue in water at pH 5.6 and a temperature of 50 °C. Three sample geometries of EPVC mixtures were considered for thermal treatment stage (loose powder, pelleted thin disks and long cylinder shapes). Oxidative thermal treatment of EPVC mixture has resulted in oxidation of magnetite into hematite and prevented iron chloride formation while both zinc and lead were completely chlorinated. The degree of magnetite oxidation was more pronounced when loose powder was used. The subsequent leaching of the thermal treatment of the loose powder EPVC has resulted in 100% recovery of zinc into leaching solution, while iron recovery was close to 0% at optimal conditions. When the EPVC mixture was pelletized into thin disk or cylindrical shape, the access of oxygen was hindered resulting in incomplete oxidation of magnetite or iron chloride to a non-leachable Fe 2 O 3 , which did not completely inhibit iron leaching. Iron recoveries in the post water leaching stage of EPVC residue treated under oxidative environment were found to be 21.6, 8.6 and 0.26% for long cylinder, thin disks, and loose powder, respectively. However, 46% of iron content was leached when a cylindrical shape mixture was thermally treated under inert conditions. These important findings can be utilized for the co-thermal treatment of EAFD and PVC wastes towards sustainable recycling of these wastes. • Selective separation of Zn and Pb from EAFD leaving Fe in residue. • Oxidative thermal treatment (OTT) oxidized Fe 3 O 4 to Fe 2 O 3 prevented Fe chlorination. • Complete recovery of Zn and Pb from OTT of EAFD-PVC residue via water leaching. • Major Fe and carbon materials are left in residue for potential recycling to EAF. [ABSTRACT FROM AUTHOR]

Oxidative thermal treatment of electric arc furnace dust (EAFD) mixed with poly(vinyl) chloride (PVC) was carried out to study the impact of oxidizing environment on the extraction of valuable metals. The effect of oxygen partial pressure, temperature and holding time of the thermal treatment, as well as the pH of the leaching solution were considered as operational variables in this study. HCl evolved during PVC decomposition was found to react with the metal oxides present in EAFD producing metal chlorides. It was found that thermal treatment above 350 °C caused significant vaporization of lead and zinc chlorides. After thermal treatment, the residue was leached in water, the cheapest available leaching reagent. Zinc and lead were extracted with recoveries up to ∼100%, while iron recovery was as low as 12.5% being suppressed by oxidation of ferrous iron. Operating under oxidative conditions supresses the post dissolution of iron in water by oxidizing Fe 3 O 4 to Fe 2 O 3 and by oxidizing any formed FeCl 2 back to Fe 2 O 3. The solution pH was also found to significantly affect the extraction of both lead and iron, especially at pH values above 4.0 where a sharp drop in metals recovery was observed. The obtained results present base for development of sustainable solution for treatment of PVC and EAFD wastes. • Oxidative thermal treatment (OTT) of EAFD:PVC at low temperature is proposed. • OTT residue was water leached for metal extraction. • OTT caused oxidation magnetite to hematite preventing its chlorination. • Leaching of Zn and Pb can be achieved at high recoveries. • Major Fe and carbon materials are left in residue for potential recycling. [ABSTRACT FROM AUTHOR]

In this work, the pyrolysis behavior of electric arc furnace dust (EAFD) mixed with poly(vinyl) chloride (PVC) was studied under inert conditions. Unwashed (UW) EAFD was found to contain lead hydroxychlorides which were converted to hydrocerrusite upon washing. It also contains appreciable amounts of soluble salts such as halite and sylvite; the effect of these salts on the pyrolysis behavior of EAFD-PVC mixtures was investigated. These salts delayed the dechlorination of PVC, by at least 20 ⁰C, during the pyrolysis of EAFD-PVC and altered the mineral composition of the pyrolysis residue. The later was rich in PbCl 2 , FeCl 2. H 2 O for both unwashed (UW) and washed (W) EAFD samples. However, the W-EAFD-PVC residue was rich with ZnCl 2. H 2 O, which was not detected in the UW-EAFD-PVC pyrolysis residue. Instead, it is believed that UW-EAFD-PVC pyrolysis residue is rich with the metastable Na 2 ZnCl 4 as well as K 2 FeCl 5. H 2 O and NaFeCl 4. The capturing and monitoring of the released gases during the pyrolysis of UW- and W-EAFD-PVC mixture, confirmed the inhibiting effect of the halite and the sylvite on the dehydrochlorination of PVC. The released gases demonstrate a strong evidence of the solid-solid reaction between EAFD compound (mainly ZnO) and PVC without evolution of hydrochloric acid (HCl) at temperatures below 200 ⁰C. The subsequent leaching of the pyrolysis residue has resulted in the recovery of Zn (up to 100%), Fe (up to 64%) and Pb (up to 76%) in water. It is inferred that decreasing iron recovery can be achieved by oxidizing Fe+2, while increasing Pb recovery can be achieved by controlling the pH of the leachate solution. [Display omitted] • The effect of arc furnace dust (EAFD) washing on its pyrolysis with PVC was studied. • Presence of NaCl and KCl in EAFD were found to delay PVC dehydrochlorination. • 100% zinc recovery from EAFD-PVC pyrolysis residue can be achieved. • Water-dissolved Fe in EAFD-PVC pyrolysis residue is a result of Fe 3 O 4 chlorination. • Controlling pH could be the way to improve Pb recovery from the pyrolysis residue. [ABSTRACT FROM AUTHOR]

Abstract This contribution provides a systematic theoretical thermo-kinetic study of the initial reactions between Fe 3 O 4 (111) surface, as a representative model for metal oxides in electric arc furnace dust (EAFD) with HCl and selected chlorinated hydrocarbons, as major products from thermal degradation of PVC. Formation of the experimentally observed iron chloride occurs by subsequent dissociative addition of HCl molecules followed by H migration into a surface hydroxyl group. We provided Arrhenius parameters for reactions dictating the conversion of iron oxide into its chloride. In the course of the interaction of chlorinated alkanes and alkenes with the Fe 3 O 4 surface, the opening channel in the dissociative addition route requires lower activation barriers in reference to the direct HCl elimination pathways. However, sizable activation barriers are encountered in the subsequent β C H bond elimination step. Estimated electronic charges confirm the nature of surface Fe Cl bonds as Lewis acid−base pair. The obtained accessible reaction barriers for reactions of chlorinated alkanes and alkenes with the title iron oxide demonstrate that the latter serve as active catalysts in producing clean olefins streams from chlorinated alkanes. Results from this study should be instrumental to understand, on a precise atomic scale, mechanisms operating in fixation of halogens on transitional metal oxides; a viable thermal recycling approach for polymeric materials laden with halogenated constituents. Graphical abstract Unlabelled Image Highlights • De-chlorination mechanisms of organohalogens by magnetite have been investigated. • Dissociative adsorption of HCl initiates formation of ferric chloride. • Dissociative addition pathway is expected to predominate the direct elimination route. [ABSTRACT FROM AUTHOR]