Environ. Sci. Technol. 2009 43, 8936–8941
Influence of Temperature and Heating Time on Bromination of Zinc Oxide during Thermal Treatment with Tetrabromobisphenol A M A R I U S Z G R A B D A , * ,†,‡ S Y L W I A O L E S Z E K - K U D L A K , †,‡ ETSURO SHIBATA,† AND TAKASHI NAKAMURA† Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 1, 1 Katahira, 2-Chome, Aoba-ku, Sendai 980-8577, Japan, and Institute of Environmental Engineering of the Polish Academy of Sciences, M. Sklodowska-Curie 34, 41-819 Zabrze, Poland
Received June 23, 2009. Revised manuscript received September 7, 2009. Accepted October 26, 2009.
Our prior research indicates that hydrogen bromide (HBr) evolved during thermal decomposition of tetrabromobisphenol A (TBBPA) can be utilized as a reagent for selective bromination and evaporation of zinc oxide. The present work investigated dependency of the bromination reaction on time at selected temperatures using a laboratory-scale furnace. The formed solid, condensed, and gaseous products were analyzed by X-ray diffraction analysis, electron probe microanalysis, inductively coupled plasma analysis, ion chromatography, and gas chromatography coupled with mass spectrometry. Results indicate that the bromination rate is strongly dependent on heating time. This dependency is a direct consequence of progress in the decomposition of TBBPA, which provides inorganic bromine suitable for the reaction. The bromination rate increases with time until the bromine source is depleted. The process is shorter at higher applied temperatures and appears instantaneous at 310 °C and above. However, the maximumbrominationyieldisindependentoftheappliedconditions and ranges from 64 to 70%. Additionally, the influence of oxidizing conditions on the bromination reaction and the effect of ZnO on decomposition of TBBPA were investigated in this study.
1. Introduction Steel production in an electric arc furnace (EAF) generates a flue byproduct composed of iron oxides and nonferrous metals called EAF dust (EAFD). Due to the presence of significant amounts of leachable zinc, cadmium, chromium, and nickel compounds, EAFDs are classified by various government regulatory agencies as hazardous wastes (1). Thus, they are toxic products and pose a serious problem for environmentally acceptable disposal. The seriousness of this problem is compounded by constantly increasing annual outputs of EAFD from electric steel production. A total of 20 * Corrusponding author tel/fax: +81222175214; e-mail: mariusz@ mail.tagen.tohoku.ac.jp. † Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University. ‡ Institute of Environmental Engineering of the Polish Academy of Sciences. 8936
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kg of dust is produced per ton of steel, creating 4.3-5.7 million tons of EAFD requiring disposal annually worldwide (1). Due to environmental restrictions the cost of disposing of this residue is high (2). There are two major methods for treatment of EAFD (3): (1) chemical fixation and stabilization (CFS) for landfill, and (2) recycling for metal recovery. It is common practice to dump the stabilized dusts in industrial landfills, though such disposal is unacceptable from the perspectives of sustainability and economics. Conversely, EAFD is a valuable secondary raw material in the production of zinc (1-12) as it contains up to 35% Zn (2), mainly in the form of zincite (ZnO) and frankonite (ZnFe2O4) (3). The worldwide generation of EAFD represents a possible recovery of approximately 1.4 million tons of zinc (2). Numerous zinc recovery processes utilizing pyrometallurgical, hydrometallurgical, or hybrid pyro-hydro techniques have been developed for these dusts. However, most never reached pilot-plant stages and many investigations were stopped because of metallurgical and economical inefficiencies (3). Only the pyrometallurgical or High-Temperature Metals Recovery (HTMR) processes have achieved commercial success (3); this is mainly limited to variations of the Waelz rotary kiln process (3). Although commonly used, these processes have low zinc yield, high energy consumption, emit greenhouse gases, and generate valueless waste products that present further disposal problems. The development of a new EAFD treatment process has become an important issue for both environmental protection and the recycling of valuable materials. Recently, an alternative method for pyrometallurgical treatment of EAFD has been proposed. Chlorine was sourced from waste polyvinyl chloride (PVC) for selective chlorination and evaporation of zinc (5-12). Approximately 96% of the zinc could be recovered from the dust (11). The potential advantages of selective chlorination include separation of the zinc (and other nonferrous metals) from the iron, elimination of carbon as a contaminant of any metallic iron products, recycling of the chlorine reagent, and reduction of greenhouse gas emissions as the carbon dioxide can be recovered (3). Our prior studies (13, 14) indicate that hydrogen bromide (HBr), generated during thermal decomposition of tetrabromobisphenol A (TBBPA), can also be utilized as a reagent for selective bromination and evaporation of zinc oxide. Sources of TBBPA are numerous: it is the largest volume brominated flame retardant (BFR) in production today, used in more than 70% of the world’s electronic and electric (E&E) appliances (15) as well as in many plastics, textiles, and so forth. There is constant growth in the production of such products and they become obsolete quickly, generating huge amounts of BFR-containing wastes and causes significant problems for their safe disposal and recycling. The most common way to use them is in thermal processing, as it allows recycling and recovery of both the organic and inorganic fractions of the waste. TBBPA easily decomposes during this process, generating significant amounts of gaseous HBr (16-20). The HBr is present mostly in the flue gas (20) and can act as a bromination agent for the bromination-evaporation process. The potential advantages of the bromination-evaporation process could be similar to those of selective chlorination: (a) efficient separation of zinc from EAFD, providing resources of recovered zinc and separated iron oxide residues for ironand steel-manufacturing (9); (b) recycling of the bromine 10.1021/es901845m CCC: $40.75
2009 American Chemical Society
Published on Web 11/04/2009
FIGURE 1. Schematic of the experimental setup for the furnace and overview of the analytical procedures used. reagent; and (c) simultaneous recycling of waste plastics containing BFR with EAFD. The only know research on the bromination and evaporation of heavy metals present in BFR-containing waste involved co-combustion of plastic waste from electrical and electronic equipment (WEEE) with municipal solid waste at the TAMARA pilot plant (21). However, the TAMARA project focused mainly on the recovery of bromine from an acid scrubber. Large-scale application requires a good understanding of the reactivity of the main zinc-bearing compounds in EAFD (ZnO and ZnFe2O4) with the TBBPA thermal decomposition products (namely HBr), the reaction mechanism and kinetics. In this research we focus on the reactivity of ZnO. Results of our previous studies (14) indicated that timedependent factor(s) might affect the bromination of ZnO. The present work is a continuation of those studies, and concentrates on investigating the dependency of the bromination reaction on time at selected temperatures.
2. Experimental Section 2.1. Materials. Tetrabromobisphenol A (97.0%) was supplied by Aldrich Co., and ZnO (99.9%) was manufactured by Wako Pure Chemical Industries. Particle diameter of the chemicals was determined by laser diffraction and scattering (Microtrac MT 3300EX). The average particle size was 2.6 and 63.1 µm for ZnO and TBBPA, respectively. The chemicals were manually mixed at a mass ratio of 3.34:1 (TBBPA:ZnO), and pressed into pellets of 0.5 g each. The ratio was determined from the stoichiometric relationship assuming all bromine present in TBBPA was released and reacted with zinc to form zinc bromide (ZnBr2). 2.2. Furnace Experiments. The experimental setup for the pyrolysis experiments is presented schematically in Figure 1. The sample (approximately 2 g) was placed on an alumina combustion boat (width 12 × depth 9 × length 60 mm) located inside a cooler part (jutting out beyond the furnace) of a quartz tube reactor heated by a small furnace. When the temperature of the furnace was stabilized at the desired temperature, the combustion boat with sample was moved inside the furnace, where it was thermally treated at isothermal conditions under an Ar (99.9%) flow of 50 mL/ min. Investigation of the isothermal treatment of TBBPA:ZnO was performed in two series of independent experimental runs. In the first series, the sample was heated for 40 min within a temperature range from 230-310 °C (every 10 °C) then at 340 and 420 °C. The results allowed selection of temperatures for the second series of experiment. In this series, detailed studies on heating time and efficiency of bromination of ZnO were performed at 250, 270, 290, and 310 °C. After treatment, the sample was rapidly cooled to prevent any secondary reactions. A quartz wool filter was placed inside the reaction tube, positioned where the temperature of the outgoing gases dropped to about 100 °C,
to catch condensable phase vaporized from the sample. Two traps consisting of distilled water and hexane were used for the inorganic and organic gaseous products, respectively. For comparison, thermal decomposition of pure TBBPA was studied at 270 °C (for 60 min), 290 °C (for 10 min) and 310 °C (for 60 min). Additionally, the effect of oxidizing conditions (Ar + 5 vol % O2) on the bromination reaction was studied at 250 °C (for 10, 40, and 140 min) and 310 °C (for 10 and 60 min). 2.3. Sample Characterization. The solid, condensed, and gaseous products of the reaction were characterized by a variety of analytical methods (Figure 1). The solid residue was characterized by X-ray diffractometry (Rigaku, Rint 220) and electron probe microanalysis (EPMA) (Jeol, JXA-8920). The total amount of zinc remaining in the residue was measured by inductively coupled plasma (ICP) analysis (Perkin-Elmer Optima 3300 SYS). The ZnBr2 and ZnO could be distinguished by using different solvents (distilled water and 0.05 M HCl, respectively) with the same extraction method. Inorganic bromine present in the solid residue was measured by ion chromatography (IC) (DX-100 IC Dionex). As ZnO is insoluble in H2O, the presence of water-soluble zinc (ZnBr2) in the solid residue indicated that bromination had occurred. When the yield of ZnBr2 was below 40% of initial amount of ZnO, its concentration was too low to be detected by either X-ray diffraction or EPMA. More abundant yields of ZnBr2 (>40%) could be confirmed by EPMA, yet diffraction patterns still could not confirm its presence in the solid residue. This may be due to the hygroscopic nature of ZnBr2 and/or the microcrystalline size of the formed zinc bromide compound being below the detection limit of the X-ray diffraction technique. The inorganic compounds condensed on the quartz wool filter and inner walls of the reaction tube were analyzed by a combination of IC and ICP to determine the zinc and bromine content. Additionally, the condensed organic products from thermal decomposition of TBBPA were analyzed using a gas chromatograph (Rigaku, 6890N) coupled with a mass spectrometer (Jeol, JMS-AMSUN200S) (GC-MS). The decomposition products were identified by analysis of the fragmentation patterns in the chromatogram and comparison to the NIST spectral library. The traps consisting of distilled water and hexane were analyzed by IC/ICP and GC-MS techniques, respectively.
3. Results and Discussion 3.1. Effect of Temperature on Bromination of ZnO by TBBPA. The thermal treatment of TBBPA with ZnO (3.34:1) at 230 °C essentially does not change the weight or chemical composition of the original mixture (Table S1 in the Supporting Information (SI)). Sample weight decreases by a negligible 0.7%, which corresponds to moisture vaporization and slow evaporation of TBBPA as indicated by the GC/MS chromatogram of the condensed phase (Figure S1). Trace amounts of water-soluble Zn are present in the solid residue (2% of the total zinc in the mixture). This is due to the modest solubility of ZnO in water and possible analytical inaccuracy. It is not from formation of water-soluble ZnBr2, which cannot occur because TBBPA does not decompose at this temperature (Figure S1). Heating to higher temperatures reveals strong relationships between the intensity of decomposition of TBBPA and efficiency of bromination of ZnO (correlation coefficient ) 0.998). Debromination of TBBPA begins at 240 °C (Figure S1) and overlaps with the start of ZnO bromination by released bromine (Table 1, Figure 3A). At this temperature, less than approximately 2% of inorganic bromine is released from TBBPA, resulting in bromination of approximately 5% of the zinc. Elevation of the temperature to 280 °C increases the VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Amount of Bromine Released from TBBPA and the Fate of Zinc during Isothermal Treatment of TBBPA and ZnO for 40 Min at Various Temperatures, Expressed As Percentages temperature (°C)
released Br (%)
Br bounded to Zn for Br released (%)
brominated Zn for total Zn (%)
vaporized Zn for total Zn (%)
vaporized Zn for brominated Zn (%)
Zn loss (%)
230 240 250 260 270 280 290 300 310 340 420
