Case study of polychlorinated naphthalene emissions and factors influencing emission variations in secondary aluminum production
Graphical abstract
Introduction
Polychlorinated naphthalenes (PCNs) are persistent organic pollutants (POPs), and are ubiquitous in environmental matrices and biota [1], [2], [3]. It has recently been proposed that PCNs be included in Annexes A, B, and/or C of the Stockholm Convention on Persistent Organic Pollutants. The main sources of PCNs are the historical manufacture of technical PCN mixtures, impurities in commercial polychlorinated biphenyl mixtures and other chemicals, and unintentional production and release from industrial thermal processes [4], [5], [6]. The production and use of technical PCNs was banned in the United States and Europe in the 1980s. Currently, the major contributors to PCN emissions are thermal-related industries, including waste incineration, coking processes, and metallurgical processes such as secondary nonferrous smelting, iron ore sintering, and steelmaking using electric arc furnaces [7], [8], [9], [10], [11]. However, investigations on developing control measures aimed at reducing PCN emissions from these industrial processes are still scarce.
The decreasing costs and increasing availability of aluminum products for use in food packaging, construction, transportation, electrical applications, consumer durables, machinery and industry use forecast a growing disposal burden of aluminum scrap. Secondary aluminum smelting can dispose of aluminum scrap in an economical and recyclable way, because aluminum scrap can be recycled without any loss of quality and the production of secondary aluminum requires only 5–20% of the energy required for the production of primary aluminum from bauxite [12]. Under these circumstances, the activity rate for global secondary aluminum production has been significant in recent years. China is the largest aluminum-producing country, accounts for ∼34% of global aluminum production, with a secondary aluminum production of ∼3 000 000 t a−1 during 2011–2013 [13]. There are large discrepancies in the scales and technologies for secondary aluminum production in China; therefore, an investigation of PCN emissions from the Chinese secondary aluminum industry would facilitate an evaluation of the release of PCNs from secondary aluminum smelting processes on a global scale.
In our previous study, atmospheric emission factors and total PCN emissions from secondary aluminum production in China were investigated [14]. Large variations in PCN concentrations were observed, from 98.9–2245 ng m−3, from secondary aluminum production in China [14]. However, the reasons for the large variations in PCN emission concentrations have not been considered in previous research. PCN emissions and characteristics at different smelting stages may differ depending on the operating conditions and the feedstock composition during secondary aluminum smelting process. Studies conducted to investigate PCN emissions and characteristics at different smelting stages could; therefore, facilitate understanding of the formation mechanisms of PCNs. PCNs can bind to particles and also partition into the gas phase because of their semi-volatility [9], [14], [15]. However, the field data from most previous investigations focusing on gaseous emissions are unavailable for assessing the integrated emission of PCNs in all discharges. Furthermore, disregarding the solid residues that are produced in the system would result in an insufficient risk assessment and inappropriate disposal, which could cause potential risks to the environment and involved workers.
In this study, fly ash from a bag filter, slag from smelting of aluminum scrap, secondary slag from secondary rotary smelting of slag, and stack gas at different smelting stages were collected from four secondary aluminum smelting plants. To the best of our knowledge, investigations of the presence of PCNs in various discharges from secondary aluminum smelting processes or other metallurgical processes are scarce. The aim was to obtain a comprehensive knowledge of the overall PCN emission distribution in secondary aluminum smelting processes. The results from this study could provide useful information for obtaining comprehensive information on PCN emission distributions, understanding PCN emission mechanisms, and developing better control strategies for PCN emissions in secondary aluminum production.
Section snippets
Plant information
Four typical plants in China (denoted by AL1–AL4) were selected, based on their different operating technologies, including furnace type, fuel, and raw materials. In the secondary production of aluminum, scrap can be melted in gas- or oil-fired reverberatory furnaces in all the plants investigated. The reverberatory furnace production is the traditional technology for secondary aluminum production in China because of its high volumetric processing rate, and low operating and maintenance costs.
PCN emissions in major discharges
The concentrations of Σ1−8PCNs in the stack gas samples from the four investigated plants were converted to dry standard conditions (273 K and 101.3 kPa). PCNs have been proven to have dioxin-like toxic activities [20], [21], and the toxic equivalent factors (TEFs) of several PCN congeners relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin have been studied [20], [21], [22], [23], [24], [25]. In this study, the TEFs of several PCN congeners, summarized by Noma et al. [26], were adopted in the
Conclusion
Stack gas, fly ash, and slag samples were collected at different smelting stages from four secondary aluminum smelting plants. Large variations in PCN emission levels and homolog profiles during different smelting stages were observed. It was suggested that raw materials and fuels may provide sources of aromatic hydrocarbons for the production of PCNs; chloride additives such as chloride flux and talcum powder may provide chlorine sources for the formation of PCNs; the variations in these
Acknowledgments
We gratefully acknowledge support from the National 973 Program (2015CB453100), National Natural Science Foundation of China (21477147, 21477150), and Chinese Academy of Sciences (YSW2013A01).
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