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Polychlorinated naphthalene (PCN) emissions and characteristics during different secondary copper smelting stages
Ecotoxicology and Environmental Safety 184 (2019) 109674
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Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv
Polychlorinated naphthalene (PCN) emissions and characteristics during different secondary copper smelting stages
T
Xiaoxu Jianga,b, Qiushuang Lic,∗∗, Lili Yanga, Yuanping Yanga, Minghui Zhenga,∗ a
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing, 100085, China b China National Environmental Monitoring Centre, Beijing, 100012, China c Foreign Environmental Cooperation Center, Ministry of Ecology and Environment of People's Republic of China, Beijing, 100035, China
ARTICLE INFO
ABSTRACT
Keywords: Secondary copper production Polychlorinated naphthalene Emission characterization Smelting stages
The amounts and characteristics of polychlorinated naphthalenes (PCNs) emitted by a secondary copper smelter were investigated. Differences in the amounts and characteristics of PCNs emitted during different smelting stages were investigated, and the main stage during which PCNs were emitted was identified. PCN concentrations in stack gases emitted during secondary copper smelting were 477.0–762.5 ng/m3 (4.4–8.3 pg toxic equivalents/m3). The contributions of the different stages to total PCN emissions decreased in the order feeding–fusion stage (65% of total PCN emissions) > oxidation stage (27%) > deoxidation stage (8%). The main contributor to PCN emissions during secondary copper smelting was the feeding–fusion stage. PCN concentrations and profiles in stack gas, fly ash, and deposit ash collected during different smelting stages were determined. PCNs in stack gases were mainly less-chlorinated homologs, and fly ash and deposit ash were dominated by highly-chlorinated homologs. These results will help improve strategies for decreasing and eliminating PCN emissions during secondary copper production.
1. Introduction The Chinese secondary copper industry has developed rapidly, driven by increasing domestic demand for copper products and increasing amounts of copper scrap becoming available since the 1970s (and particularly since 1990). The contribution of secondary copper production to total refined copper production increased from 20% in 1975 to 38% in 2010 (Zhang et al., 2014). The Chinese secondary nonferrous metal industry produced 137.5 × 106 t of metals in 2017, and secondary copper production accounted for 3.2 × 106 t of this. The secondary copper industry recycles and reuses copper scrap and provides enormous economic and environmental benefits. Many Chinese secondary copper plants are small. Outdated Chinese secondary copper plants are planned to be eliminated to protect the environment and modernize the secondary copper industry. Effectively controlling pollutant formation and emission is critical to the sustainable development of the secondary copper industry (Jin et al., 2017; Wang et al., 2010, 2015, 2016; Yu et al., 2006). Polychlorinated naphthalenes (PCNs) were added to the list of
∗
persistent organic pollutants under the Stockholm Convention on Persistent Organic Pollutants in 2015. The main current sources of PCNs to the environment are thermal processes and industrial plants, including coking plants, metallurgical plants, and waste incinerators (Jin et al., 2016; Liu et al., 2012, 2014b). The range of raw materials used by the secondary copper industry has changed markedly over time. The proportions waste circuit boards, waste enameled wire, waste wire, and other electronic scrap contribute have increased over time, and these materials are now major raw materials (Hu et al., 2013b). Organic compounds containing chlorine that enter the smelting process can cause the formation of chlorinated by-products (e.g., PCNs) to form when combustion is incomplete. In a study of 13 typical Chinese industrial plants, very high PCN toxic equivalent (TEQ) emission factors were found for secondary copper smelting plants. The secondary copper smelting industry is now a major source of PCNs to the Chinese environment (Ba et al., 2010; Hu et al., 2013a; Liu et al., 2014a, 2015). Emissions of PCNs from different smelting plants will vary depending on the operating conditions and raw materials used. Previous studies have mainly been focused on determining PCN concentrations
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Q. Li), [email protected] (M. Zheng).
∗∗
https://doi.org/10.1016/j.ecoenv.2019.109674 Received 10 July 2019; Received in revised form 9 September 2019; Accepted 11 September 2019 Available online 16 September 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
Ecotoxicology and Environmental Safety 184 (2019) 109674
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2.3. Analytical methods
in emitted media and PCN emission factors at the whole-industry scale rather than PCN emissions and characteristics during different smelting stages and the factors that influence PCN emissions and characteristics. Here, we describe a field study of PCN concentrations in different emitted media and the characteristics of PCNs produced during different smelting stages in a secondary copper smelting plant. The factors affecting differences in PCN emissions and characteristics during different smelting stages were investigated. The aim was to improve our understanding of the factors that control PCN emissions and to produce data to help develop measures to eliminate and control PCN emissions from secondary copper smelting plants.
Each stack gas or solid residue sample was spiked with a mixture of C10-labeled PCN internal standards (ECN-5102, tetrachloronaphthalene to octachloronaphthalene mixture containing 13C10labeled CN-27, -42, −52, −67, −73, and −75; Cambridge Isotope Laboratories, Andover, MA, USA). Each stack gas sample was Soxhlet extracted with 250 mL of toluene for 24 h. Each fly ash or deposit ash sample was digested in 1 mol L−1 HCl, then freeze-dried, and then Soxhlet extracted with 250 mL of toluene for 24 h. Each extract was cleaned by passing it through a multilayer silica-gel column (containing, from top to bottom, anhydrous sodium sulfate, 1 g of silica gel, 15 g of a 44% sulfuric acid: 56% silica gel mixture, 1 g of silica gel, 4 g of a 33% sodium hydroxide: 67% silica gel mixture, and 1 g of silica gel) and then an activated carbon column. Each extract was evaporated to a small volume, spiked with a13C10-labeled PCN at a known concentration (ECN-5260: 13C10-labeled CN-64; Cambridge Isotope Laboratories) and then analyzed by high-resolution gas chromatography high-resolution mass spectrometry. The gas chromatograph was equipped with a J&W DB-5 MS fused silica column (60 m long, 0.25 mm i. d., 0.25 μm film thickness; Agilent Technologies, Santa Clara, CA, USA) and was coupled to a DFS mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). In total, 75 PCN congeners were analyzed. The mass spectrometer was used in selected ion monitoring mode and electron impact ionization mode, with an electron energy of 45 eV and a resolution > 10,000. The source temperature was 270 °C. The gas chromatograph oven temperature program started at 80 °C, which was held for 2 min, then increased at 20 °C min−1 to 180 °C, which was held for 1 min, then increased at 2.5 °C min−1 to 280 °C, and then increased at 10 °C min−1 to 290 °C, which was held for 5 min. The carrier gas was helium, and the flow rate was 1 mL min−1. The PCN peaks were identified from their retention times compared with available individual PCN standards, ion ratios and by taking into consideration the PCN elution order on the DB-5 column. The m/z selected to monitor all the PCN homologs are shown in Table 1. (Ba et al., 2010; Guo, 2008; Guo et al., 2008).
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2. Materials and methods 2.1. Information on the investigated plant A typical secondary copper smelting plant in China was investigated in this study. The copper smelter produces 125 t of copper per day. The smelter had a reverberatory furnace which is widely used in secondary copper smelters. The smelter was fitted with a gravity dust collector (to remove dust from the stack gas through gravity) and a bag filter (to collect the dust in a bag-shaped fabric filter). The ash that deposited in the flue was removed with an air blast. The fly ash and deposit ash had high copper contents, so were recycled in the furnace to increase the smelting efficiency. Typical raw materials were blister copper and copper scrap. Blister copper is the primary product of copper concentrate smelted in an imperial blast furnace, reverberator, or electric furnace and blown using a converter. Copper scrap from recycled consumer products and the remains of copper anodes after electrolysis were used. The secondary copper smelter had three main stages, feeding–fusion, oxidation, and deoxidation. Copper scrap was smelted in a reverberatory furnace, then air was blown through the copper liquid layer to oxidize the copper. A reductant was then added to deoxidize the copper. PCNs could be formed and emitted during each smelting stage because of incomplete combustion of organic impurities in the fuel, raw materials, and reductant. The fuel and reductant were heavy oil and pulverized coal, respectively.
Table 1 PCNs monitoring parameter information.
2.2. Sample collection and analytical methods Eight stack gas samples (five collected during the feeding–fusion stage, two during the oxidation stage, and one during the deoxidation stage) were collected. Two solid residue samples (one fly ash sample and one deposit ash sample) were collected. An automatic isokinetic sampling system was used to collect the stack gas samples. The sampling point was downstream of the air pollution control devices. The sampling train contained a heated probe, a filter box with a Whatman silica glass microfiber thimble (25 mm i. d., 90 mm long; GE Healthcare Bio-Sciences, Pittsburgh, PA, USA), an Amberlite XAD-2 adsorbent trap with a water cooler, an Isotack Basic pump (TCR Tecora, Fontenay sous Bois, France), and an Isofrost cooler (TCR Tecora). The silica glass microfiber thimble was used to trap particles in the stack gas. The condensing system and trap containing Amberlite XAD-2 resin were used to cool and sorb the pollutants in the stack gas, respectively. The stack gas and the condensate water of flue gas sampler was collected analyzed and analyzed simultaneously and the amount of PCNs in condensate water were added into the flue gas when calculating. The fly ash sample was collected from the bag filter outlet. The deposit ash sample was collected from the flue after the ash had been blasted. The stack gas sampling method was described in detail in a previous publication (Jiang et al., 2015).
Degree of chlorine
m/z
m/z type
Homolog
Cl-1
162.0236 164.0207 195.9847 197.9817 229.9457 231.9427 265.9038 267.9008 275.9373 277.9344 299.8648 301.8618 309.8983 311.8954 333.8258 335.8229 343.8594 345.8564 367.7868 369.7839 377.8204 379.8174 401.7479 403.7449 411.7814 413.7785
M M+2 M M+2 M M+2 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4
MoCN MoCN DiCN DiCN TriCN TriCN TetraCN TetraCN 13 C-TetraCN 13 C-TetraCN PentaCN PentaCN 13 C-PentaCN 13 C-PentaCN HexaCN HexaCN 13 C-HexaCN 13 C-HexaCN HeptaCN HeptaCN 13 C-HeptaCN 13 C-HeptaCN OctaCN OctaCN 13 C-OctaCN 13 C-OctaCN
Cl-2 Cl-3 Cl-4
Cl-5
Cl-6
Cl-7
Cl-8
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2.4. Quality control and quality assurance In this study, the detection limits and quantification limits were defined as 3 and 10 times the signal-to-noise ratio, respectively. 13C10labeled PCN internal standards and recovery standard were used for the assurance of analytical recovery. The recoveries of the 13C10-labeled PCN congeners relative to the labeled injection standard were 35–121% for both matrices. Blank experiments were carried out to evaluate any possible contamination. Concentrations of MoCNs and DiCNs in blanks were higher than their LODs but were far lower than those in the samples, so no blank correction was required. 3. Results and discussion 3.1. Emissions of PCNs during different secondary copper smelting stages The concentrations of 75 PCN congeners in the stack gas and ash samples collected from the different smelting stages were determined. The total PCN concentration, determined by adding the concentrations of all of the congeners together, was calculated and labeled the Σ1-8PCN concentration. The Σ1-8PCN concentrations of the stack gas samples
Fig. 2. Contributions of the secondary copper smelting stages to total polychlorinated naphthalene (PCN) emissions.
were converted to concentrations under standard conditions (273 K, 101.3 kPa). The TEQ concentration of the PCNs was calculated using published relative potency factors, which describe the dioxin-like toxicities of PCN congeners (Noma et al., 2004). The PCN concentrations in the stack gas samples were 477.0–762.5 ng/m3, which were consistent with the concentrations of 47.3–1107 ng/m3 found for secondary copper smelters in 2008 (Ba et al., 2010). As shown in Fig. 1, the PCN concentrations were markedly different for the different smelting stages. The highest concentrations were found for the oxidation stage (mean concentration 762.5 ng/m3, 3.91 nmol/m3). The mean ∑1-8PCN concentration for the feeding–fusion stage was 744.4 ng/m3 (3.82 nmol/m3). The lowest concentration was found for the deoxidation stage (mean 477.0 ng/m3, 2.32 nmol/m3). The PCN TEQ concentrations were also highest for the oxidation stage (mean 8.6 pg TEQ/m3), lower for the feeding–fusion stage (mean 7.3 pg TEQ/m3), and lowest for the deoxidation stage (mean 4.4 pg
Fig. 1. Total (Σ1-8) polychlorinated naphthalene (PCN) concentrations, Σ1-8PCN mole-based concentrations, and PCN toxic equivalent (TEQ) concentrations in the samples collected during different secondary copper smelting stages. 3
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Fig. 3. Polychlorinated naphthalene (PCN) homolog concentrations in the stack gas samples collected during different smelting stages.
TEQ/m3).
total PCN TEQ emissions also decreased in the order feeding–fusion stage (63%) > oxidation stage (29%) > deoxidation stage (8%). It can be seen that the feeding–fusion stage contributed most to both PCN mass emissions and PCN TEQ emissions and was the main stage during which PCNs were emitted. The PCN concentrations in the stack gases emitted during the oxidation stage were highest, but the contribution of the oxidation stage to total PCN emissions was not highest because the oxidation stage lasted only a short time. The PCN concentrations in the stack gases emitted during the deoxidation stage were low and the
3.2. Identifying the main stages during which PCNs were emitted The contributions of the different smelting stages made to total PCN emissions were calculated using Equations (1) and (2). As shown in Fig. 2, the PCN mass contributions to total PCN emissions decreased in the order feeding–fusion stage (65%) > oxidation stage (27%) > deoxidation stage (8%). The PCN TEQ contributions to 4
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Fig. 4. Polychlorinated naphthalene (PCN) homolog distributions in the stack gas samples collected during different smelting stages.
deoxidation stage was shorter than the other stages, so the contribution of the deoxidation stage to total PCN emissions was small.
The reasons why PCNs were produced and emitted during each stage are assessed next. (1) Feeding–fusion stage. Batch feeding, which requires regularly opening and closing the furnace door, allowing air to enter the furnace, will mean that there will be sufficient oxygen in the furnace for PCNs to be formed. The furnace conditions (particularly temperature) would also be unstable, and sometimes the temperature would be suitable for the formation of PCNs through incomplete combustion of the fuel or organic impurities. The feeding–fusion stage lasts for ~8 h, so larger amounts of PCNs will be produced during this stage than in the other stages. (2) Oxidation stage. Metal impurities in the
Emissionsmelting stage (ng) = Concentration(ng/m3) × Stack gas flow rate(m3/h) × duration(h) eq. 1
TEQEmissionsmelting stage (pg TEQ) = Concentration(pg TEQ/m3) × Stack gas flow rate (m3/h) × duration(h) eq. 2
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Fig. 5. Toxic polychlorinated naphthalene (PCN) congener mass concentration patterns for the stack gas samples collected during different secondary copper smelting stages.
melt are removed by blowing large amounts of compressed air into the copper liquid to oxidize the impurities and slag. Oxygen addition would induce the larger yield of xides (including copper oxide). The conditions in the furnace during this stage will be unstable, and some organic impurities will still be present. Suitable temperatures and catalysis by metal oxides will allow organic impurities to form PCNs through thermal reactions. (3) Deoxidation stage. The fuel supply is stopped.
Excess oxygen is removed and the copper oxide is reduced using pulverized coal as a reducing agent. The coal is added to the melt using compressed air. Excess coal is usually used to ensure that all the oxygen is removed and the copper oxide is completely deoxidized. The pulverized coal will be incompletely combusted, so the coal will act as a carbon source for PCN formation. PCNs could be formed under the catalysis of fly ash.
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Fig. 6. Polychlorinated naphthalene (PCN) homolog concentrations in the stack gas, fly ash, and deposit ash samples collected during secondary copper smelting.
3.3. PCN homolog emissions during different secondary copper smelter stages
stages were mainly reflected in differences in the contributions of the less-chlorinated homologs. The mono-to tetra-chloronaphthalene homologs all contributed most to the total PCN concentrations in the samples from the oxidation stage, less in the samples from the feeding–fusion stage, and least in the samples from the deoxidation stage. However, the concentrations of the penta-to octa-chloronaphthalene homologs were similar for all the smelting stages. As shown in Fig. 5, the toxic congener distributions in the samples from the different smelting stages were similar. The dominant toxic congeners were the less-chlorinated congeners, including CN-1 (1monochloronapthalene), CN-2 (2-monochloronapthalene), and CN-5/7
The PCN homolog concentrations and distribution ratios in the stack gases emitted during the different smelting stages are shown in Figs. 3 and 4. The homolog distributions for the different smelting stages were similar, with less-chlorinated (monochlorinated to tetrachlorinated) homologs accounting for 84%–92% of the total PCN concentrations. The homolog distributions were similar to distributions found for stack gases emitted during other regenerative metal smelting processes. The differences in the homolog distributions during the different smelting
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(1,4-/1,6-dichloronapthalene), which contributed 92%–94% of the total toxic congener concentrations. CN-1 alone contributed 53%–58% of the total toxic congener concentrations. The TEQ concentration distributions for the different smelting stages were compared. The TEQ concentration distributions for the different smelting stages were similar. The dominant contributors to the TEQ concentrations were CN-1 (1-monochloronapthalene), CN-2 (2-monochloronapthalene), and CN66/67 (1,2,3,4,6,7-/1,2,3,5,6,7-hexachloronapthalene). These congeners contributed 80%–81% of the total TEQ concentrations. CN-1 alone contributed 39%–50% of the total TEQ concentrations, and CN66/67 contributed 18%–34%.
the formation of PCNs through heterogeneous catalytic reactions. Since deposit ash could not have been in contact with stack gases sufficiently, the formation and adsorption of PCNs in deposit ash was limited. 4. Conclusions The PCN emission characteristics for different media emitted from a secondary copper smelter were assessed, and the main stage during which PCNs were emitted was identified. The PCN concentrations during different smelting stages decreased in the order oxidation stage > feeding–fusion stage > deoxidation stage. Differences in the PCN emission characteristics for the different smelting stages were also compared. Less-chlorinated PCN congeners were the dominant toxic congeners. The PCN emission characteristics for the different emitted media were compared. PCNs in stack gases were dominated by lesschlorinated PCN homologs, while fly ash and deposit ash contained larger amounts of highly-chlorinated PCN homologs. The difference may have been caused by less-chlorinated PCNs being more volatile than highly-chlorinated PCNs.
3.4. PCN emissions and characteristics in different media during secondary copper smelting The concentrations of PCNs in stack gases emitted during secondary copper smelting were described above. High PCN concentrations were also found in the two solid samples that were collected. The 1 8 PCN concentration in the fly ash was 9010.6 ng/g (32.09 nmol/g), and the TEQ concentration was 4.41 ng TEQ/g. These concentrations were of the same order of magnitude as PCN concentrations found in fly ash from a secondary copper smelting plant in 2008. In that study, the concentrations found in secondary copper smelting fly ash were higher than the concentrations found in fly ash from other typical metal smelting plants in China (Ba et al., 2010; Liu et al., 2012; Liu et al., 2014b). The ∑1-8PCN concentration in the deposit ash was 641.8 ng/g (2.60 nmol/g), and the TEQ concentration was 0.02 ng TEQ/g. These concentrations were much lower than the concentrations found in the fly ash. The PCN homolog concentrations in the stack gases, fly ash, and deposit ash are shown in Fig. 6. It can be seen that less-chlorinated homologs were dominant in the stack gases. Mono-to tri-chloronaphthalene homologs were the main contributors to the mass concentrations, together contributing 84%–92% of the total PCN concentrations. Monochloronaphthalenes alone contributed 32%–51% of the total PCN concentrations. The PCN homolog distributions in the fly ash and deposit ash were different from the distributions in the stack gas. The homolog patterns in both fly ash and deposit ash had the characteristics of a ‘bell-shaped curve’. Trichloronaphthalenes contributed the most to the total PCN concentration in deposit ash, and pentachloronaphthalene contributed the most to the total PCN concentration in fly ash. We concluded that the degree of chlorination increased in the order stack gas < deposit ash < fly ash. The boiling point increases and the saturated vapor pressure decreases as the degree of chlorination increases. Therefore, less-chlorinated PCNs can partition into the gas phase more easily than highly-chlorinated PCNs. This may explain the high less-chlorinated PCN homolog concentrations in the stack gases. The results showed that the PCN concentrations were higher and the PCNs more chlorinated in the fly ash than the deposit ash. The deposit ash deposited in the flue mainly contained large-size and large-specificgravity particles which would floated down when passing by the flue. The fly ash was collected in the bag filter which is a widely-used device. Before get into the bag filter, the fly ash composed by particles which with fine size had passed through the cooling zone with stack gases produced in the smelting furnace. Compared with the deposit ash, the smaller size, larger specific surface and higher surface activities may enable the fly ash with higher adsorbed PCNs. Besides, with long residence time in the stack gas which contained abundant carbon sources, chlorine sources and oxygen, the fly ash could have been the catalyst in
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Polychlorinated naphthalene (PCN) emissions and characteristics during different secondary copper smelting stages
T
Xiaoxu Jianga,b, Qiushuang Lic,∗∗, Lili Yanga, Yuanping Yanga, Minghui Zhenga,∗ a
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing, 100085, China b China National Environmental Monitoring Centre, Beijing, 100012, China c Foreign Environmental Cooperation Center, Ministry of Ecology and Environment of People's Republic of China, Beijing, 100035, China
ARTICLE INFO
ABSTRACT
Keywords: Secondary copper production Polychlorinated naphthalene Emission characterization Smelting stages
The amounts and characteristics of polychlorinated naphthalenes (PCNs) emitted by a secondary copper smelter were investigated. Differences in the amounts and characteristics of PCNs emitted during different smelting stages were investigated, and the main stage during which PCNs were emitted was identified. PCN concentrations in stack gases emitted during secondary copper smelting were 477.0–762.5 ng/m3 (4.4–8.3 pg toxic equivalents/m3). The contributions of the different stages to total PCN emissions decreased in the order feeding–fusion stage (65% of total PCN emissions) > oxidation stage (27%) > deoxidation stage (8%). The main contributor to PCN emissions during secondary copper smelting was the feeding–fusion stage. PCN concentrations and profiles in stack gas, fly ash, and deposit ash collected during different smelting stages were determined. PCNs in stack gases were mainly less-chlorinated homologs, and fly ash and deposit ash were dominated by highly-chlorinated homologs. These results will help improve strategies for decreasing and eliminating PCN emissions during secondary copper production.
1. Introduction The Chinese secondary copper industry has developed rapidly, driven by increasing domestic demand for copper products and increasing amounts of copper scrap becoming available since the 1970s (and particularly since 1990). The contribution of secondary copper production to total refined copper production increased from 20% in 1975 to 38% in 2010 (Zhang et al., 2014). The Chinese secondary nonferrous metal industry produced 137.5 × 106 t of metals in 2017, and secondary copper production accounted for 3.2 × 106 t of this. The secondary copper industry recycles and reuses copper scrap and provides enormous economic and environmental benefits. Many Chinese secondary copper plants are small. Outdated Chinese secondary copper plants are planned to be eliminated to protect the environment and modernize the secondary copper industry. Effectively controlling pollutant formation and emission is critical to the sustainable development of the secondary copper industry (Jin et al., 2017; Wang et al., 2010, 2015, 2016; Yu et al., 2006). Polychlorinated naphthalenes (PCNs) were added to the list of
∗
persistent organic pollutants under the Stockholm Convention on Persistent Organic Pollutants in 2015. The main current sources of PCNs to the environment are thermal processes and industrial plants, including coking plants, metallurgical plants, and waste incinerators (Jin et al., 2016; Liu et al., 2012, 2014b). The range of raw materials used by the secondary copper industry has changed markedly over time. The proportions waste circuit boards, waste enameled wire, waste wire, and other electronic scrap contribute have increased over time, and these materials are now major raw materials (Hu et al., 2013b). Organic compounds containing chlorine that enter the smelting process can cause the formation of chlorinated by-products (e.g., PCNs) to form when combustion is incomplete. In a study of 13 typical Chinese industrial plants, very high PCN toxic equivalent (TEQ) emission factors were found for secondary copper smelting plants. The secondary copper smelting industry is now a major source of PCNs to the Chinese environment (Ba et al., 2010; Hu et al., 2013a; Liu et al., 2014a, 2015). Emissions of PCNs from different smelting plants will vary depending on the operating conditions and raw materials used. Previous studies have mainly been focused on determining PCN concentrations
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Q. Li), [email protected] (M. Zheng).
∗∗
https://doi.org/10.1016/j.ecoenv.2019.109674 Received 10 July 2019; Received in revised form 9 September 2019; Accepted 11 September 2019 Available online 16 September 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
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2.3. Analytical methods
in emitted media and PCN emission factors at the whole-industry scale rather than PCN emissions and characteristics during different smelting stages and the factors that influence PCN emissions and characteristics. Here, we describe a field study of PCN concentrations in different emitted media and the characteristics of PCNs produced during different smelting stages in a secondary copper smelting plant. The factors affecting differences in PCN emissions and characteristics during different smelting stages were investigated. The aim was to improve our understanding of the factors that control PCN emissions and to produce data to help develop measures to eliminate and control PCN emissions from secondary copper smelting plants.
Each stack gas or solid residue sample was spiked with a mixture of C10-labeled PCN internal standards (ECN-5102, tetrachloronaphthalene to octachloronaphthalene mixture containing 13C10labeled CN-27, -42, −52, −67, −73, and −75; Cambridge Isotope Laboratories, Andover, MA, USA). Each stack gas sample was Soxhlet extracted with 250 mL of toluene for 24 h. Each fly ash or deposit ash sample was digested in 1 mol L−1 HCl, then freeze-dried, and then Soxhlet extracted with 250 mL of toluene for 24 h. Each extract was cleaned by passing it through a multilayer silica-gel column (containing, from top to bottom, anhydrous sodium sulfate, 1 g of silica gel, 15 g of a 44% sulfuric acid: 56% silica gel mixture, 1 g of silica gel, 4 g of a 33% sodium hydroxide: 67% silica gel mixture, and 1 g of silica gel) and then an activated carbon column. Each extract was evaporated to a small volume, spiked with a13C10-labeled PCN at a known concentration (ECN-5260: 13C10-labeled CN-64; Cambridge Isotope Laboratories) and then analyzed by high-resolution gas chromatography high-resolution mass spectrometry. The gas chromatograph was equipped with a J&W DB-5 MS fused silica column (60 m long, 0.25 mm i. d., 0.25 μm film thickness; Agilent Technologies, Santa Clara, CA, USA) and was coupled to a DFS mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). In total, 75 PCN congeners were analyzed. The mass spectrometer was used in selected ion monitoring mode and electron impact ionization mode, with an electron energy of 45 eV and a resolution > 10,000. The source temperature was 270 °C. The gas chromatograph oven temperature program started at 80 °C, which was held for 2 min, then increased at 20 °C min−1 to 180 °C, which was held for 1 min, then increased at 2.5 °C min−1 to 280 °C, and then increased at 10 °C min−1 to 290 °C, which was held for 5 min. The carrier gas was helium, and the flow rate was 1 mL min−1. The PCN peaks were identified from their retention times compared with available individual PCN standards, ion ratios and by taking into consideration the PCN elution order on the DB-5 column. The m/z selected to monitor all the PCN homologs are shown in Table 1. (Ba et al., 2010; Guo, 2008; Guo et al., 2008).
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2. Materials and methods 2.1. Information on the investigated plant A typical secondary copper smelting plant in China was investigated in this study. The copper smelter produces 125 t of copper per day. The smelter had a reverberatory furnace which is widely used in secondary copper smelters. The smelter was fitted with a gravity dust collector (to remove dust from the stack gas through gravity) and a bag filter (to collect the dust in a bag-shaped fabric filter). The ash that deposited in the flue was removed with an air blast. The fly ash and deposit ash had high copper contents, so were recycled in the furnace to increase the smelting efficiency. Typical raw materials were blister copper and copper scrap. Blister copper is the primary product of copper concentrate smelted in an imperial blast furnace, reverberator, or electric furnace and blown using a converter. Copper scrap from recycled consumer products and the remains of copper anodes after electrolysis were used. The secondary copper smelter had three main stages, feeding–fusion, oxidation, and deoxidation. Copper scrap was smelted in a reverberatory furnace, then air was blown through the copper liquid layer to oxidize the copper. A reductant was then added to deoxidize the copper. PCNs could be formed and emitted during each smelting stage because of incomplete combustion of organic impurities in the fuel, raw materials, and reductant. The fuel and reductant were heavy oil and pulverized coal, respectively.
Table 1 PCNs monitoring parameter information.
2.2. Sample collection and analytical methods Eight stack gas samples (five collected during the feeding–fusion stage, two during the oxidation stage, and one during the deoxidation stage) were collected. Two solid residue samples (one fly ash sample and one deposit ash sample) were collected. An automatic isokinetic sampling system was used to collect the stack gas samples. The sampling point was downstream of the air pollution control devices. The sampling train contained a heated probe, a filter box with a Whatman silica glass microfiber thimble (25 mm i. d., 90 mm long; GE Healthcare Bio-Sciences, Pittsburgh, PA, USA), an Amberlite XAD-2 adsorbent trap with a water cooler, an Isotack Basic pump (TCR Tecora, Fontenay sous Bois, France), and an Isofrost cooler (TCR Tecora). The silica glass microfiber thimble was used to trap particles in the stack gas. The condensing system and trap containing Amberlite XAD-2 resin were used to cool and sorb the pollutants in the stack gas, respectively. The stack gas and the condensate water of flue gas sampler was collected analyzed and analyzed simultaneously and the amount of PCNs in condensate water were added into the flue gas when calculating. The fly ash sample was collected from the bag filter outlet. The deposit ash sample was collected from the flue after the ash had been blasted. The stack gas sampling method was described in detail in a previous publication (Jiang et al., 2015).
Degree of chlorine
m/z
m/z type
Homolog
Cl-1
162.0236 164.0207 195.9847 197.9817 229.9457 231.9427 265.9038 267.9008 275.9373 277.9344 299.8648 301.8618 309.8983 311.8954 333.8258 335.8229 343.8594 345.8564 367.7868 369.7839 377.8204 379.8174 401.7479 403.7449 411.7814 413.7785
M M+2 M M+2 M M+2 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4 M+2 M+4
MoCN MoCN DiCN DiCN TriCN TriCN TetraCN TetraCN 13 C-TetraCN 13 C-TetraCN PentaCN PentaCN 13 C-PentaCN 13 C-PentaCN HexaCN HexaCN 13 C-HexaCN 13 C-HexaCN HeptaCN HeptaCN 13 C-HeptaCN 13 C-HeptaCN OctaCN OctaCN 13 C-OctaCN 13 C-OctaCN
Cl-2 Cl-3 Cl-4
Cl-5
Cl-6
Cl-7
Cl-8
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2.4. Quality control and quality assurance In this study, the detection limits and quantification limits were defined as 3 and 10 times the signal-to-noise ratio, respectively. 13C10labeled PCN internal standards and recovery standard were used for the assurance of analytical recovery. The recoveries of the 13C10-labeled PCN congeners relative to the labeled injection standard were 35–121% for both matrices. Blank experiments were carried out to evaluate any possible contamination. Concentrations of MoCNs and DiCNs in blanks were higher than their LODs but were far lower than those in the samples, so no blank correction was required. 3. Results and discussion 3.1. Emissions of PCNs during different secondary copper smelting stages The concentrations of 75 PCN congeners in the stack gas and ash samples collected from the different smelting stages were determined. The total PCN concentration, determined by adding the concentrations of all of the congeners together, was calculated and labeled the Σ1-8PCN concentration. The Σ1-8PCN concentrations of the stack gas samples
Fig. 2. Contributions of the secondary copper smelting stages to total polychlorinated naphthalene (PCN) emissions.
were converted to concentrations under standard conditions (273 K, 101.3 kPa). The TEQ concentration of the PCNs was calculated using published relative potency factors, which describe the dioxin-like toxicities of PCN congeners (Noma et al., 2004). The PCN concentrations in the stack gas samples were 477.0–762.5 ng/m3, which were consistent with the concentrations of 47.3–1107 ng/m3 found for secondary copper smelters in 2008 (Ba et al., 2010). As shown in Fig. 1, the PCN concentrations were markedly different for the different smelting stages. The highest concentrations were found for the oxidation stage (mean concentration 762.5 ng/m3, 3.91 nmol/m3). The mean ∑1-8PCN concentration for the feeding–fusion stage was 744.4 ng/m3 (3.82 nmol/m3). The lowest concentration was found for the deoxidation stage (mean 477.0 ng/m3, 2.32 nmol/m3). The PCN TEQ concentrations were also highest for the oxidation stage (mean 8.6 pg TEQ/m3), lower for the feeding–fusion stage (mean 7.3 pg TEQ/m3), and lowest for the deoxidation stage (mean 4.4 pg
Fig. 1. Total (Σ1-8) polychlorinated naphthalene (PCN) concentrations, Σ1-8PCN mole-based concentrations, and PCN toxic equivalent (TEQ) concentrations in the samples collected during different secondary copper smelting stages. 3
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Fig. 3. Polychlorinated naphthalene (PCN) homolog concentrations in the stack gas samples collected during different smelting stages.
TEQ/m3).
total PCN TEQ emissions also decreased in the order feeding–fusion stage (63%) > oxidation stage (29%) > deoxidation stage (8%). It can be seen that the feeding–fusion stage contributed most to both PCN mass emissions and PCN TEQ emissions and was the main stage during which PCNs were emitted. The PCN concentrations in the stack gases emitted during the oxidation stage were highest, but the contribution of the oxidation stage to total PCN emissions was not highest because the oxidation stage lasted only a short time. The PCN concentrations in the stack gases emitted during the deoxidation stage were low and the
3.2. Identifying the main stages during which PCNs were emitted The contributions of the different smelting stages made to total PCN emissions were calculated using Equations (1) and (2). As shown in Fig. 2, the PCN mass contributions to total PCN emissions decreased in the order feeding–fusion stage (65%) > oxidation stage (27%) > deoxidation stage (8%). The PCN TEQ contributions to 4
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Fig. 4. Polychlorinated naphthalene (PCN) homolog distributions in the stack gas samples collected during different smelting stages.
deoxidation stage was shorter than the other stages, so the contribution of the deoxidation stage to total PCN emissions was small.
The reasons why PCNs were produced and emitted during each stage are assessed next. (1) Feeding–fusion stage. Batch feeding, which requires regularly opening and closing the furnace door, allowing air to enter the furnace, will mean that there will be sufficient oxygen in the furnace for PCNs to be formed. The furnace conditions (particularly temperature) would also be unstable, and sometimes the temperature would be suitable for the formation of PCNs through incomplete combustion of the fuel or organic impurities. The feeding–fusion stage lasts for ~8 h, so larger amounts of PCNs will be produced during this stage than in the other stages. (2) Oxidation stage. Metal impurities in the
Emissionsmelting stage (ng) = Concentration(ng/m3) × Stack gas flow rate(m3/h) × duration(h) eq. 1
TEQEmissionsmelting stage (pg TEQ) = Concentration(pg TEQ/m3) × Stack gas flow rate (m3/h) × duration(h) eq. 2
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Fig. 5. Toxic polychlorinated naphthalene (PCN) congener mass concentration patterns for the stack gas samples collected during different secondary copper smelting stages.
melt are removed by blowing large amounts of compressed air into the copper liquid to oxidize the impurities and slag. Oxygen addition would induce the larger yield of xides (including copper oxide). The conditions in the furnace during this stage will be unstable, and some organic impurities will still be present. Suitable temperatures and catalysis by metal oxides will allow organic impurities to form PCNs through thermal reactions. (3) Deoxidation stage. The fuel supply is stopped.
Excess oxygen is removed and the copper oxide is reduced using pulverized coal as a reducing agent. The coal is added to the melt using compressed air. Excess coal is usually used to ensure that all the oxygen is removed and the copper oxide is completely deoxidized. The pulverized coal will be incompletely combusted, so the coal will act as a carbon source for PCN formation. PCNs could be formed under the catalysis of fly ash.
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Fig. 6. Polychlorinated naphthalene (PCN) homolog concentrations in the stack gas, fly ash, and deposit ash samples collected during secondary copper smelting.
3.3. PCN homolog emissions during different secondary copper smelter stages
stages were mainly reflected in differences in the contributions of the less-chlorinated homologs. The mono-to tetra-chloronaphthalene homologs all contributed most to the total PCN concentrations in the samples from the oxidation stage, less in the samples from the feeding–fusion stage, and least in the samples from the deoxidation stage. However, the concentrations of the penta-to octa-chloronaphthalene homologs were similar for all the smelting stages. As shown in Fig. 5, the toxic congener distributions in the samples from the different smelting stages were similar. The dominant toxic congeners were the less-chlorinated congeners, including CN-1 (1monochloronapthalene), CN-2 (2-monochloronapthalene), and CN-5/7
The PCN homolog concentrations and distribution ratios in the stack gases emitted during the different smelting stages are shown in Figs. 3 and 4. The homolog distributions for the different smelting stages were similar, with less-chlorinated (monochlorinated to tetrachlorinated) homologs accounting for 84%–92% of the total PCN concentrations. The homolog distributions were similar to distributions found for stack gases emitted during other regenerative metal smelting processes. The differences in the homolog distributions during the different smelting
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(1,4-/1,6-dichloronapthalene), which contributed 92%–94% of the total toxic congener concentrations. CN-1 alone contributed 53%–58% of the total toxic congener concentrations. The TEQ concentration distributions for the different smelting stages were compared. The TEQ concentration distributions for the different smelting stages were similar. The dominant contributors to the TEQ concentrations were CN-1 (1-monochloronapthalene), CN-2 (2-monochloronapthalene), and CN66/67 (1,2,3,4,6,7-/1,2,3,5,6,7-hexachloronapthalene). These congeners contributed 80%–81% of the total TEQ concentrations. CN-1 alone contributed 39%–50% of the total TEQ concentrations, and CN66/67 contributed 18%–34%.
the formation of PCNs through heterogeneous catalytic reactions. Since deposit ash could not have been in contact with stack gases sufficiently, the formation and adsorption of PCNs in deposit ash was limited. 4. Conclusions The PCN emission characteristics for different media emitted from a secondary copper smelter were assessed, and the main stage during which PCNs were emitted was identified. The PCN concentrations during different smelting stages decreased in the order oxidation stage > feeding–fusion stage > deoxidation stage. Differences in the PCN emission characteristics for the different smelting stages were also compared. Less-chlorinated PCN congeners were the dominant toxic congeners. The PCN emission characteristics for the different emitted media were compared. PCNs in stack gases were dominated by lesschlorinated PCN homologs, while fly ash and deposit ash contained larger amounts of highly-chlorinated PCN homologs. The difference may have been caused by less-chlorinated PCNs being more volatile than highly-chlorinated PCNs.
3.4. PCN emissions and characteristics in different media during secondary copper smelting The concentrations of PCNs in stack gases emitted during secondary copper smelting were described above. High PCN concentrations were also found in the two solid samples that were collected. The 1 8 PCN concentration in the fly ash was 9010.6 ng/g (32.09 nmol/g), and the TEQ concentration was 4.41 ng TEQ/g. These concentrations were of the same order of magnitude as PCN concentrations found in fly ash from a secondary copper smelting plant in 2008. In that study, the concentrations found in secondary copper smelting fly ash were higher than the concentrations found in fly ash from other typical metal smelting plants in China (Ba et al., 2010; Liu et al., 2012; Liu et al., 2014b). The ∑1-8PCN concentration in the deposit ash was 641.8 ng/g (2.60 nmol/g), and the TEQ concentration was 0.02 ng TEQ/g. These concentrations were much lower than the concentrations found in the fly ash. The PCN homolog concentrations in the stack gases, fly ash, and deposit ash are shown in Fig. 6. It can be seen that less-chlorinated homologs were dominant in the stack gases. Mono-to tri-chloronaphthalene homologs were the main contributors to the mass concentrations, together contributing 84%–92% of the total PCN concentrations. Monochloronaphthalenes alone contributed 32%–51% of the total PCN concentrations. The PCN homolog distributions in the fly ash and deposit ash were different from the distributions in the stack gas. The homolog patterns in both fly ash and deposit ash had the characteristics of a ‘bell-shaped curve’. Trichloronaphthalenes contributed the most to the total PCN concentration in deposit ash, and pentachloronaphthalene contributed the most to the total PCN concentration in fly ash. We concluded that the degree of chlorination increased in the order stack gas < deposit ash < fly ash. The boiling point increases and the saturated vapor pressure decreases as the degree of chlorination increases. Therefore, less-chlorinated PCNs can partition into the gas phase more easily than highly-chlorinated PCNs. This may explain the high less-chlorinated PCN homolog concentrations in the stack gases. The results showed that the PCN concentrations were higher and the PCNs more chlorinated in the fly ash than the deposit ash. The deposit ash deposited in the flue mainly contained large-size and large-specificgravity particles which would floated down when passing by the flue. The fly ash was collected in the bag filter which is a widely-used device. Before get into the bag filter, the fly ash composed by particles which with fine size had passed through the cooling zone with stack gases produced in the smelting furnace. Compared with the deposit ash, the smaller size, larger specific surface and higher surface activities may enable the fly ash with higher adsorbed PCNs. Besides, with long residence time in the stack gas which contained abundant carbon sources, chlorine sources and oxygen, the fly ash could have been the catalyst in
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