Scholarly article on topic 'Brominated dioxin and furan stack gas emissions during different stages of the secondary copper smelting process'

Brominated dioxin and furan stack gas emissions during different stages of the secondary copper smelting process Academic research paper on "Environmental engineering"

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Academic research paper on topic "Brominated dioxin and furan stack gas emissions during different stages of the secondary copper smelting process"

Atmospheric Pollution Research 6 (2015) 464-468

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eric Pollution Research

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Brominated dioxin and furan stack gas emissions during different stages of the secondary copper smelting process

Mei Wang1, Guorui Liu 1, Xiaoxu Jiang1, Wenbin Liu 1, Li Li1, Sumei Li1, Minghui Zheng1, Jiayu Zhan 2

1 State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmenta! Sciences, Chinese Academy of Sciences, P. O. Box 2871, Beijing 100085, China

2 State Key Laboratory of Solid Waste Reuse for Building Materials, Beijing Building Materials Academy of Scientific Research, 69 Jinding North Road, Shijingshan District, Beijing 100041, China

ABSTRACT

Secondary copper smelting processes (SeCu) have previously been identified as an important source of brominated dioxins and furans (PBDD/Fs). Identifying the major stages responsible for PBDD/F formation and emissions is crucial for developing technology to reduce PBDD/F emissions from SeCu, but nothing is currently known in this regard. In this study, stack gas samples from three smelting stages (feeding-fusion, oxidation and deoxidization) were collected and analyzed for PBDD/Fs to identify the stage most responsible for PBDD/F emissions. The results indicated that PBDD/F emissions mainly occurred during the feeding-fusion stage. Overall, PBDF emissions were much higher than PBDD emissions throughout the smelting process. Higher-brominated PBDD/F congeners were the most dominant contributors. The emission factors of PBDD/Fs during the feeding-fusion, oxidation and deoxidization stages were calculated to be 715, 119 and 31 ng t"1, respectively. The results of this study are important for identifying the stages most responsible for PBDD/F emissions and developing techniques for reducing PBDD/F emissions from SeCu processes.

Keywords: PBDD/F, persistent organic pollutant, secondary copper smelter, stack gas emission, emission factor

Corresponding Author: Gwnrui £iu

ffi : +86-10-6284-9356 S : +86-10-6292-3563 S : grHu@rcees.ac.cn

Article History:

Received: 25 September 2014 Revised: 28 November 2014 Accepted: 28 November 2014

doi: 10.5094/APR.2015.051

1. Introduction

Polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/F) are persistent organic pollutants (POPs). Because they are structurally analogous to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), PBDD/Fs exhibit similar physico-chemical properties and toxicities (Behnisch et al., 2003; Birnbaum et al., 2003; Olsman et al., 2007;Samara et al., 2009). Concentrations of PBDD/Fs in environmental matrices and biota have been increasing overtime. (Choi et al., 2003; Jogsten et al., 2010; Zacs et al., 2013;Venkatesan and Halden, 2014). A study by Choi et al. (2003) showed that the ratios of tetra-brominated homologs to tetrachlorinated homologs in human adipose tissue significantly increased between 1970 and 2000, which has increased worldwide attention on the sources and concentrations of PBDD/Fs. The unintentional formation and emission of PBDD/Fs during industrial thermal processes, or their presence as impurities in brominated flame retardants (BFR), are considered the most important sources of PBDD/F emissions to the environment (Weber and Kuch, 2003; Olsman et al., 2007;Du et al., 2010; Duan et al., 2011;Drage et al., 2014; Sindiku et al., 2014).

Secondary copper production is widely considered one of the most significant sources of PCDD/Fs (Ba et al., 2009;Nie et al., 2012; Hu et al., 2013). According to China's National Implementation Plan, PCDD/F emissions from secondary copper production were 1 134 g toxic equivalents (TEQ) in 2004, contributing to 24%

of the total emissions from ferrous and non-ferrous metal production (NIP, 2007). The formation mechanisms of PBDD/Fs are similar to those of PCDD/Fs during industrial thermal processes (Weber and Kuch, 2003;0rtuno et al., 2014). Secondary copper production is therefore speculated to be an important source of PBDD/Fs. Our previous study identified that secondary copper smelting (SeCu) processes were important PBDD/Fs sources (Du et al., 2010). Emissions of PBDD/Fs from SeCu processes were much higher than those from waste incineration and other metallurgical processes (Du et al., 2010).

During SeCu processes, there are three main smelting stages: feeding-fusion, oxidation and deoxidization. Raw materials are added to the furnace and heated through the combustion of fuel. When the raw materials have almost melted, air is passed through the liquid copper to eliminate impurities. After oxidation, a reductant is injected into the liquid copper to deoxidize the copper oxide. Some organic materials such as plastics, cables, paints and oils might be present as impurities in the raw materials. Scrap metal used as raw material is believed to contain BFR residues in cables and plastics (Kemmlein et al., 2003). During each smelting stage, the formation and emission of PBDD/Fs may occur as a result of the incomplete combustion of organic and bromine-containing materials. Only one study has identified SeCu processes as a source of PBDD/Fs (Du et al., 2010). However, the formation and emissions of PBDD/Fs during different stages of SeCu have not been studied.

© Author(s) 2015. This work is distributed under the Creative Commons Attribution 3.0 License.

In this study, stack gas samples during three smelting stages were collected and analyzed to quantify the emissions and characteristics of PBDD/Fs under different SeCu stages. This information is crucial for identifying the major emission stage of SeCu processes and developing appropriate technologies to reduce PBDD/F emissions.

2. Materials and Methods

2.1. Information on the case plant and collection of stack gas samples

Reverberatory furnaces are the most widely used for SeCu processes in China (Hu et al., 2013). Raw materials for secondary copper production in China are normally copper scraps, blister copper and anode copper remains after electrolysis (ACRE). Copper scraps are believed to contain much more organic residue (such as plastics, cables, paints and oils) than blister copper and ACRE. Bag filters are widely used as air pollution control devices (APCDs) for SeCu processes in China. In this study, a typical SeCu plant, equipped with a reverberatory furnace and bag filters as APCDs, was selected to identify the emissions and characteristics of PBDD/F during different stages of the SeCu process. The production capacity of the reverberator in the plant was 125 tons of copper. Copper scrap (25%), blister copper (60%) and ACRE (15%) were used as raw materials. The raw materials were melted using heavy oil as a fuel and fine coal was used as the reductant.

Stack gas samples during three smelting stages were collected using an automatic isokinetic sampling system (Liu et al., 2009; Liu et al., 2010). The sampling system (Isostack Basic, Tecora, Italy) consisted of ISOSTACK BASIC pumps, ISOFROST coolers, a heated titanium probe, a filter box equipped with a quartz fiber filter (quartz microfiber thimble, Whatman) and a water-cooled XAD-2 adsorbent trap (Amberlite XAD-2, Supleco). Quartz fiber filters were used to collect particle-bound pollutants and XAD-2 adsorbent resin was used for trapping vapor-phase contaminants. The sampling period covered the three different stages of secondary copper smelting in one batch. 37Cl4-labeled 2,3,7,8-tetrachlorinated dioxin (37Cl4-2,3,7,8-TCDD) was spiked into the resin prior to sampling for evaluating the sampling efficiency. All samples were tightly wrapped in aluminum foil and packed in sealed polyethylene bags to prevent contamination and loss. They were then brought back to the laboratory and stored at 4 °C until they were analyzed. Detailed information on the smelting process and sample information is presented in Table 1.

concentrated using a rotary evaporator and then subjected to a series of clean-up steps, including a multilayer silica gel column and a basic alumina column. After the clean-up steps completed, an active carbon-impregnated silica column was used to separate PBDD/Fs from other organic pollutants. The fraction containing PBDD/Fs was concentrated to a volume of approximately 20 ^L using a rotary evaporator and a gentle stream of N2. Then, 13C12-labeled PBDD/F injection standards (EDF-5409, Cambridge Isotope Laboratories) were added so that the recoveries of the internal standards could be calculated.

PBDD/F analysis by HRGC/HRMS was conducted using a Trace GC Ultra gas chromatograph coupled to a DFS mass spectrometer (Thermo, USA) with an electron impact ion source. Detailed information on the 2,3,7,8-substituted PBDD/F congeners studied is presented in Table S1 (see the SM). The high-resolution mass spectrometer was operated in selected ion monitoring mode with a resolution of approximately 10 000. A DB-5 MS capillary column (15 mx0.25 mmx0.1 ^m, Agilent, USA) was used for separating the PBDD/F congeners. The electron emission energy was set to 45 eV, and the source temperature was 280 °C.

2.3. Quality assurance and quality control

The sampling recoveries of spiked 37Cl4-labeled 2,3,7,8-TCDD ranged from 104-117%. The recoveries of 13C12-labeled PBDD/F internal standards ranged from 20-115%. Compared with other 13C12-labeled congeners, the recoveries of 13C12-OBDF were low (20-37%). There is currently no standard method for PBDD/F analysis. Recoveries of the 13C12-labeled octa-chlorinated homolog were required to be within 17-157% according to the US EPA method 1613B for PCDD/F analysis. Therefore, the recoveries of 13C12-OBDF were considered acceptable for trace analysis of PBDD/Fs in this study. Full details of the sampling and pre-treatment standard recoveries are included in Table S2 (see the SM).

2,3,7,8-bromine substituted congeners were identified using the isotope dilution method. In identifying the target compounds, three quality control criteria were used: (a) matching GC retention times to the corresponding 13C12-labeled standard compounds;(b) the signal-to-noise ratio being greater than 3:1; and (c) the isotopic ratios between the quantified and identified ions being within ±15% of the theoretical values. One blank sample was included in each batch of samples that had been spiked with the same amount of internal standard as the samples.

Table 1. Information about the stack gas samples collected during different smelting stages

Smelting Stage Feeding-Fusion Oxidation Deoxidization

900-1 300 1350 1 200

8-12 2-4 2

8.5 4 2.5

2.2. Analytical procedures

PBDD/F congeners were identified and quantified using isotopic dilution high-resolution gas chromatography combined with high-resolution mass spectrometry (HRGC/HRMS). Full details of the analytical technique and methodology used have been reported previously (Du et al., 2010). A detailed schematic of the clean-up procedures used for isolating PBDD/Fs from stack gas samples is presented in Figure S1 (see the Supporting Material, SM). Briefly, stack gas samples were spiked with known amounts of a 13C12-labeled PBDD/F internal standard mixture (EDF-5408, Cambridge Isotope Laboratories, USA) and then Soxhlet extracted with 250 mL of toluene for approximately 24 h. Each extract was

3. Results and Discussion

3.1. Comparison of PBDD/F emissions during different smelting stages

The concentrations of PBDD/F in the stack gas from different smelting stages were analyzed and compared (Table 2). PBDD/F emissions peaked during the feeding-fusion stage, with an average concentration of 443 pg Nm-3 [range: 364-559 pg Nm-3; Nm3 is the volume of stack gas collected, normalized to 273 K and standard atmospheric pressure (760mmHg)]. During the feeding-fusion stage, the raw materials were added into the furnace in three batches and melted by heat from the combustion of heavy oil. PBDD/Fs are likely produced from the incomplete combustion of organic impurities present in the raw materials (D'Silva et al., 2004). The feeding-fusion stage lasted approximately 8-12 hours. Because of the duration and conditions, it is not surprising that large quantities of PBDD/Fs would be formed and emitted during the feeding-fusion stage. The average concentration of PBDD/Fs during the oxidation stage was 223 pg Nm-3 (range: 209237 pg Nm-3), much lower than that of the feeding-fusion stage. Composition of raw materials has been widely recognized as an important factor influencing the formation and emission of unintentional POPs during industrial thermal processes (Xhrouet

Temperature in Furnace (°C) Duration Time (h) Number of Samples Total Sample Volume (Nm3)

and de Pauw, 2004; Hu et al., 2013). High concentrations of organic residues in the raw materials are responsible for these higher emissions. During the deoxidization stage, almost all of the organic impurities in the raw material had been removed through the feeding-fusion and oxidation processes. Therefore, the average concentration of PBDD/Fs in stack gas samples collected during the deoxidization stage was the lowest, with a value of 115 pgNm-3. Our previous work on PBDD/F emissions from SeCu found that their average concentration in stack gas samples was 2.5 ng Nm-3, much higherthan those reported in this study (Du et al., 2010).

To compare the relative contributions of PBDD/F emissions during different smelting stages, emissions from each smelting stage per copper production furnace were calculated based on the PBDD/F concentrations, stack gas flow rates and operating times using Equation (1):

Ei=(CixQi)xti (1)

where Ei is the emission amount in stage i (ngt_1), C is the concentration in stage i (ng Nm"3), Qi is the stack gas flow rate in stage i (Nm3 h"1), and t, is the operating time of stage i (h).

Large variations in PBDD/F emissions during different smelting stages were identified. Maximum PBDD/F emissions per copper production furnace occurred during the feeding-fusion stage (89.4 ^g per furnace), with a percentage contribution to total PBDd/f emissions of 83%. During the oxidation stage, PBDD/F emissions were 14.9 ^g per furnace, which contributed to approximately 14% of the total emissions. Approximately 3% of the total

emissions (3.8 ^g per furnace) occurred during the deoxidization stage. From these results it is evident that the feeding-fusion stage was the highest contributor of PBDD/F emissions during SeCu.

Emission factors are useful for establishing emission inventories based on limited data. In this study, the emission factors of PBDD/Fs were calculated to be 715, 119 and 31ngt"1 during the feeding-fusion, oxidation and deoxidization stages, respectively. The total emission factor was 865 ng t"1 in this study. To the best of our knowledge, this is the first emission factor reported for PBDD/Fs from SeCu. These data will be useful for estimating PBDD/F emissions from SeCu industries throughout China.

3.2. PBDD/F congener profiles during different smelting stages

PBDD/F congener profiles could provide useful information for identifying formation mechanisms and possible sources of PBDD/Fs. In this work, thirteen 2,3,7,8-PBDD/F congeners were selected as fingerprints of PBDD/Fs from SeCu. To evaluate similarities and differences in the PBDD/F patterns, the individual PBDD/F congener concentrations were normalized to the total 2,3,7,8-PBDD/F concentration. The congener profiles of PBDD/Fs in stack gas during different smelting stages are presented in Figure 1. From Figure 1 it is evident that PBDF emissions were much higher than PBDD emissions throughout the smelting process. Higher-brominated congeners were the dominant contributors to total 2,3,7,8-PBDD/F concentrations during the feeding-fusion and oxidation stages.

50.0 -

40.0 -

30.0 -

10.0 -

■ Feeding-fusion

□ Oxidation

□ Deoxidization

J Ir 1

I J-h fh A 1

~i"u .11__[in.

J J f J? ,/ °

Figure 1. Congener profiles of PBDD/Fs in the stack gas during different smelting stages.

Table 2. Concentrations [normalized to standard atmospheric pressure (760 mm Hg) and temperature (273 K)] and emission factors of PBDD/Fs in the stack gas during different smelting stages

Feeding-Fusion Oxidization Deoxidization

PBDD/Fs Stack Gas (n=5) Stack Gas (n=2) Stack Gas (n=1)

Mean Range Mean Range

Concentrations (pg Nm 3) 443 364-559 223 209-237 115

Emission factors (ng t-1) 715 585-827 119 111-126 31

During the feeding-fusion stage, the most abundant congener was OBDF, followed by 1,2,3,4,6,7,8-HpBDF, OBDD and 1,2,3,4,6,7,8-HpBDD. These four congeners accounted for 89% of total 2,3,7,8-PBDD/F emissions during the feeding-fusion stage. During the oxidation stage, the PBDD/F congener patterns were also dominated by the higher-brominated congeners. However, contributions from tetra- to hexa-brominated PBDD/F congeners to total 2,3,7,8-PBDD/F emissions during the oxidation stage (22%) were higher than those during the feeding-fusion stage (11%). From Figure 1 it is apparent that the PBDD/F congener patterns during the deoxidization stage were different than those during the other stages. The percentage of PBDD emissions to total 2,3,7,8-PBDD/F emissions during the deoxidization stage (12%) was much lower than during the feeding-fusion (31%) and oxidation (45%) stages. As can be seen from Figure 1, the percentages of 1,2,3,4,6,7,8-HpBDD and OBDD decreased sharply during the deoxidization stage, while the percentages of 1,2,3,4,6,7,8-HpBDF and OBDF increased. These indicated that the formation pathways for PBDD/Fs during the deoxidization stage might be different from those during the feeding-fusion and oxidation stages. The congener profiles of PBDD/Fs from waste incineration, iron ore sintering processes and electric arc furnaces for steel-making have also been reported (Wyrzykowska et al., 2009;Wang et al., 2010a; Wang et al., 2010b). The dominance of higher brominated furan congeners, including OBDF and 1,2,3,4,6,7,8-HpBDF, were also observed for these processes. Overall, the PBDD/F congener profiles from secondary copper smelting processes were similar to those of waste incineration, iron ore sintering processes and electric arc furnaces for steel-making, suggesting that PBDD/F formation mechanisms are similar for all of these processes.

4. Conclusions

A preliminary investigation of PBDD/F emissions during different stages of the secondary copper smelting process was conducted. The results indicated that PBDD/F emissions occurred primarily during the feeding-fusion stages. The PBDD/F emission factors during the feeding-fusion, oxidation and deoxidization stages were 715, 119 and 31 ngt"1, respectively. PBDF emissions were much higher than PBDD emissions throughout the whole secondary copper smelting process. Higher-brominated PBDD/F congeners were the most dominant contributors throughout the smelting process. These results provide useful information for developing appropriate technology to reduce PBDD/F emissions from secondary copper smelting.

Acknowledgments

We gratefully acknowledge support from the National 973 (2015CB453100), the National Natural Science Foundation of China (21107123), Young Scientists Fund of RCEES (RCEES-QN-20130002F) and the Beijing Municipal Science and Technology Commission (Z141100001014001).

Supporting Material Available

Detailed information on the monitored 2,3,7,8-substituted PBDD/F congeners using HRMS; the sampling and pre-treatment standard recoveries; and the clean-up procedures used for isolating PBDD/Fs from stack gas samples. This information is available free of charge via the Internet at http://www. atmospolres.com.

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