Chemosphere 134 (2015) 25–30
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Mineral phases and metals in baghouse dust from secondary aluminum production Xiao-Lan Huang a, Amro M. El Badawy a, Mahendranath Arambewela a, Renata Adkins a, Thabet Tolaymat b,⇑ a b
Pegasus Technical Services, Inc., 46 E. Hollister Street, Cincinnati, OH 45219, USA Office of Research and Development, U.S. EPA, Cincinnati, OH 45224, USA
h i g h l i g h t s Mineral phases, metals content & their leachability from baghouse dust were studied. High variability of metallic Al and total Al content in baghouse dust was observed. Leachability of heavy metals of baghouse dust was higher than that of salt cake. Not all BHD samples were below the TCLP limits in US.
a r t i c l e
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Article history: Received 29 August 2014 Received in revised form 27 February 2015 Accepted 15 March 2015
Handling Editor: X. Cao Keywords: Aluminum Baghouse dust (BHD) Landfill Secondary aluminum production (SAP) Toxicity characteristic leaching procedure (TCLP)
a b s t r a c t Baghouse dust (BHD) is a solid waste generated by air pollution control systems during secondary aluminum processing (SAP). Management and disposal of BHD can be challenging in the U.S. and elsewhere. In this study, the mineral phases, metal content and metal leachability of 78 BHD samples collected from 13 different SAP facilities across the U.S. were investigated. The XRD semi-quantitative analysis of BHD samples suggests the presence of metallic aluminum, aluminum oxide, aluminum nitride and its oxides, spinel, elpasolite as well as diaspora. BHD also contains halite, sylvite and fluorite, which are used as fluxes in SAP activities. Total aluminum (Al) in the BHD samples averaged 18% by weight. Elevated concentrations of trace metals (>100 lg L 1 As; >1000 lg L 1 Cu, Mn, Se, Pb, Mn and Zn) were also detected in the leachate. The U.S. toxicity characteristic leaching procedure (TCLP) results showed that some samples leached above the toxicity limit for Cd, Pb and Se. Exceeding the TCLP limits in all sample is independent of facilities generating the BHD. From the metal content perspective only, it appears that BHD has a higher potential to exhibit toxicity characteristics than salt cake (the largest waste stream generated by SAP facilities). Published by Elsevier Ltd.
1. Introduction Recycling through the process of secondary aluminum production (SAP) plays an important role in aluminum manufacturing (IAI, 2009; TAA, 2010). While salt cake (SC) is consisting of the primary residue from SAP activities (Huang et al., 2014), the processes also generate baghouse dust (BHD), which is a powdery waste of a very fine grain size captured in dry emissions control devices called baghouses. Baghouses are used in SAP to control particulate air emissions from furnace operation and other SAP processing activities (López et al., 2001; López-Delgado et al., 2007). In general, the
⇑ Corresponding author. E-mail address: [email protected] (T. Tolaymat). http://dx.doi.org/10.1016/j.chemosphere.2015.03.033 0045-6535/Published by Elsevier Ltd.
formation of BHD and the amount of BHD formed depends on several factors such as the type and quality of input material (e.g. aluminum scraps), the operating conditions, and the control technology applied (Peterson and Newton, 2002; Hwang et al., 2006; Schlesinger, 2007; Schmitz, 2007). BHD is the second largest solid waste generated in SAP. Viland estimated in a 1990 study that for every one ton of scrap aluminum processed, 760 kg of secondary aluminum, 240 kg of dross residues and 3 kg of BHD are generated (Viland, 1990). LópezDelgado et al. (2007) estimated that in Western Europe, approximately 13 kg of BHD are generated per ton of scrap aluminum recycled (López-Delgado et al., 2007). A 2013 document from TAA indicates that the BHD generation rate in Northern America is about 6.8 kg per ton of the aluminum scrap recycled (TAA, 2013). The total combined annual amount of these aluminum
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wastes (SC and BHD) from SAP is around one million tons in the U.S. (USDOE, 1999), which is 20% of all the SAP-related waste generated in the world (Azom, 2003). Depending on their constituencies, the end-of-life management of these wastes has been found to present challenges in the U.S. and elsewhere (Reuter et al., 2004; Gil, 2005; Lorber and Antrekowitsch, 2010; SINTEF, 2010; Huang et al., 2012). BHD is usually co- disposed in municipal solid waste (MSW) landfills. It has been reported that many operational issues of landfill occurred after disposal aluminum solid wastes, including BHD. High concentration of hydrogen (H2) (30–50%) with gaseous ammonia (NH3) (up to 15 000 ppmv) were detected (Gerbasi, 2006; Allen et al., 2009; Stark et al., 2012). An increase in landfill temperature, 60 to 93 °C over a period of several months to several years (Gerbasi, 2006; Allen et al., 2009) were also observed in a MSW landfill after disposal of these aluminum wastes. Above average concentration of F , Cl , NH+4, CN , high pH and electric conductivity value in soil, leachate or groundwater, as well as the potential contaminations of heavy metals were also documented (USEPA, 1995, 2008; Gerbasi, 2006; Swackhamer, 2006; OhioEPA, 2007; Allen et al., 2009; Lorber and Antrekowitsch, 2010). It was known that MSW landfills are anaerobic systems that decompose the organic fraction of solid wastes with the temperature between 25 and 60 °C depending on the waste characteristics and location of landfill (Yesßiller et al., 2005; Hanson et al., 2010). On the other hand, BHD is recognized as a hazardous waste in European Union countries (European-Commission, 2000) because it is considered to be ‘‘highly flammable’’ (Category H3-A) and an ‘‘irritant’’ (Category H4) (European-Commission, 1991). When BHD comes in contact with water or damp air, highly flammable gases form, and these gases can be explosive, as well as act as irritants to skin and mucous membranes. Furthermore, BHD has been found to be harmful if inhaled or ingested (Category H5) (EuropeanCommission, 1991). BHD is also in the category of substances that are capable, after disposal (landfill or other), of potentially yielding another substance (e.g. leachate), which can possess any of the characteristics associated with the solid BHD or gaseous products (Category H13) (European-Commission, 1991). It is believed that the reactivity of aluminum wastes, including BHD is related to the composition and mineral phases of wastes (López-Delgado et al., 2007; Lorber and Antrekowitsch, 2010; SINTEF, 2010; Huang et al., 2012; Stark et al., 2012; Tsakiridis, 2012). When compared with SC, there is much less data available on the characteristics of BHD from SAP (López et al., 2001, 2004; Reuter et al., 2004; López-Delgado et al., 2007; Lorber and Antrekowitsch, 2010; Huang et al., 2011, 2012, 2014; Tsakiridis, 2012), although it has been reported that BHD contains 25–40% total Al, 15–25% metallic Al, 1–3% C, 0.2–1% S, 1–6% N, 6–11% SiO2, 1–3% Ca, 2–5% Mg, 1–3% Na, 0.2–1% K, 0.5–2% Fe and 1–5% F (López-Delgado et al., 2007, 2009), and also contains aluminum nitrides, carbides and sulfides, as well as metal oxides derived from the particular alloys being processed (López et al., 2001, 2004; López-Delgado et al., 2007, 2009; Schlesinger, 2007; Schmitz, 2007). Thus, the objective of this study was to investigate BHD from SAP facilities in the U.S. by determining the mineral phases and the metal (Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Pb, Se and Zn) content of the samples. The study also examines the leachability of metals from BHD exposed to deionized water in an anaerobic and elevated temperature environment (Huang et al., 2014), which is designed to simulate key conditions in municipal solid waste (MSW) landfills. The U.S. Environmental Protection Agency (EPA) toxicity characteristic leaching procedure (TCLP) (USEPA, 1992) was also employed on these BHD samples. The information resulting from this study will help provide the scientific foundation to understand BHD waste material, its potential risk and strategies for its management.
2. Experimental 2.1. BHD sampling A total of 78 BHD samples were collected over a period of four months from 13 different SAP facilities in the U.S. Prior to sample collection, the BHD was stored at the generation site. After ensuring sufficient cooling of the BHD, a subsample was collected following ASTM Method C702-98, ‘‘Standard practice for reducing samples of aggregate to testing size’’ (ASTM, 2003). Upon receipt at the EPA laboratory, each sample was mixed in an acid rinsed stainless steel pan. Samples did not require size reduction since they passed through a 2 mm sieve as received. 2.2. Mineral phases analysis Because of analysis time constraints (16 h per sample), 44 BHD samples were randomly selected for XRD analysis from the original 78 collected. The samples’ mineral phases were evaluated from 5° to 110° 2h on a Philips X’Pert Pro Diffractometer using copper Ka radiation. The powder diffraction file (PDF) patterns database from the International Centre for Diffraction Data (ICDD) was employed for the search, match and identification. A subset of reference patterns was built for all studied BHD samples. The semi-quantitative phase analysis was used by the X’Pert HighScore Plus software, based on the CHUNG Normalized RIR Method (Chung, 1974). An example of XRD analysis processing and results were presented as supporting documents (Tables SI-1–3, and Fig. SI-1). 2.3. Total metal analysis Extractable metal content was evaluated in all 78 BHD samples collected. After homogenization, 0.1 g subsample of BHD was acid digested following U.S. EPA SW846 Method 3051A with minor modifications due to the samples’ high aluminum content (USEPA, 2007a). The details of digestion and metal recovery can be found in the supporting information. After acid digestion, metal compositions including Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Pb, S, Se and Zn were determined following EPA’s SW846 Method 6010C using a Thermo ICP-AES (USEPA, 2007b). 2.4. Metal leachability The same 44 samples that were randomly selected for the XRD analysis were targeted for the metal leachability evaluation [3]. Ten grams of the BHD sample ( 0. 1), whereas the abundance of aluminum nitride and aluminum oxide (including different oxide forms) was significantly lower (p < 0.01) (Huang et al., 2014). Possibly the most striking difference between BHD and SC is the presence of diaspore (AlO[OH], 01-080-1385) (Fig. 1, and Tables SI-2 and 3). Diaspore is a common product of the hydrolysis of aluminum nitride, carbide or metallic aluminum (Holleman and Wiberg, 2001; López et al., 2001; Li Jinwang et al., 2005; Li et al., 2006; Petrovic and Thomas, 2008), which indicated that some of the BHD samples underwent hydrolysis before the Table 2 Total acidic extractable metal content of SAP BHD.
a
Metal
Mean
Median
Range
MRLa
Al (%) Ca (%) Mg (%) Fe (%) K (%) Na (%) As (mg kg 1) Cd (mg kg 1) Cr (mg kg 1) Cu (mg kg 1) Mn (mg kg 1) Pb (mg kg 1) Se (mg kg 1) Zn (mg kg 1)
18.5 6.69 2.65 0.89 5.26 6.6 9.4 40 430 1300 1300 970 40 6840
17 3.44 2.1 0.81 4.77 6.66 5.5 15 240 990 980 270 5.0 1900
2.64–59.5 0.26–28.8 0.47–9.53 0.09–3.01 0.15–17.7 0.17–25 5.5–84 1.0–230 38–1600 116–6100 233–5100 28–8800 5.0–390 174–83 000
6.0 73 5.6 3.7 184 41 5.5 1.0 5.1 5.4 0.9 23 5.0 0.7
MRL (method report limit), mg kg
1
.
analysis. This was related to the aluminum dust collection and treatment processing in SAP (TAA, 2013). On the other hand, hydrolysis of aluminum nitride with water vapor has been previously reported (Li Jinwang et al., 2005; Li et al., 2006), some of the BHD samples underwent hydrolysis might also occur during the BHD sampling and/or storage processes. Aside from the aluminum species, the abundance of NaCl and KCl in BHD was significantly lower (p < 0.01) than that present in SC (Huang et al., 2014). However, BHD samples contained a relatively higher (p < 0.01) abundance of quartz and calcite when compared to SC (5%) (Huang et al., 2014). Due to the common use of lime (CaO) in air emissions control processes at SAP facilities (TAA, 2013), the presence of calcite in BHD is not unexpected. BHD also may contain some minor minerals (e.g. silicon nitride and magnesium sulfate) that were not detected in SC. 3.2. Total metal content As expected, aluminum was the most abundant metal detected in the BHD samples and the total metal content data is presented in Table 2. The average extractable total aluminum (Al) concentration of the BHD samples was approximately 18%, and ranged from 2.6% to 60%, as presented in Fig. SI-5. A few samples contained a relatively high level of aluminum (>40%), which skewed the samples’ distribution. However, the median of the Al concentration was 17%. As presented in Table 2, trace metals were detected in BHD samples from SAP processing facilities above their respective method reporting limit (MRL) and can be presented as a result of being aluminum alloy constituents or impurities from the recycling process (Davis, 1993) (their distribution is presented in Fig. SI-6). Relatively high concentrations of trace metals (e.g. >100 mg kg 1 Cd, >200 mg kg 1 Se, > 6000 mg kg 1 Pb and 50 000 mg kg 1 Zn) were found in some BHD samples. Compared with SC, the content of Al, Ca, Fe, Cu, Zn, Pb and Cd in BHD was significantly higher (p < 0.01), but the content of Na and As was significantly lower (p < 0.01), respectively. There were no significant differences among the content of Mg, K, Mn, Cr and Se between the two types of SAP waste. It is worthy to note that the maximum content of all metals detected in the BHD samples was consistently higher than those reported for SC (Huang et al., 2011, 2014). The Al content in SC showed a positive correlation with all other detected trace metals, which was not observed in BHD samples. Al detected in BHD samples showed positive correlations with Fe, Mn, Cu and Cr, and negative correlations with Pb, Cd and Zn (Fig. SI-7) (Huang et al., 2011, 2014).
Table 3 Metal leachability from BHD. Metal
MRL
1
Al (mg L ) Ca (mg L 1) Mg (mg L 1) Fe (mg L 1) K (mg L 1) Na (mg L 1) As (lg L 1) Cr (lg L 1) Cd (lg L 1) Cu (lg L 1) Mn (lg L 1) Pb (lg L 1) Se (lg L 1) Zn (lg L 1)
0.003 0.145 0.011 0.007 0.368 0.082 26 14 3 11 2 17 34 1
Concentration
Leachability (%)
Mean
Median
Range
Mean
Median
Range
14 470 8.9 1.9 2,100 2,700 33 23 9.4 470 510 410 210 320
4.3 116 0.9 0.19 2000 2300 26 14 3.2 75 12 17 77 125
0.32–170 0.72–3300 0.01–62 0.06–34 27–6800 50–11 000 26–140 14–96 3.2–54 11–6100 1.8–10 000 17–17 000 34–1300 5.3–4300
0.19 12 1.9 0.5 69 70 6.7 0.44 1.5 0.82 1.2 1.5 14 0.53
0.05 6.2 0.1 0.1 78 72 4.8 0.17 0.82 0.33 0.02 0.21 8.2 0.12
0.003–1.8 0.09–100 0.001–18 0.01–7.7 0.6–100 0.9–100 1.7–21 0.03–4.0 0.05–12 0.01–39 0.001–21 0.001–39 0.18–57 0.01–3.9
X.-L. Huang et al. / Chemosphere 134 (2015) 25–30 Table 4 Metal concentration in the TCLP solution (mg L Metal a
Al Ag As Ba Cd Cr Pb Se a b
Mean 130 0.014 0.02 2.6 0.5 0.17 2.1 0.23
Median 34 0.01 0.02 1.7 0.1 0.1 0.1 0.05
Range 0.9–950 0.01–0.07 0.02–0.2 0.01–12 0.003–6 0.02–1.5 0.01–56 0.02–3.5
4. Conclusions
1
).
TC limit NA 5 5 100 1 5 5 1
29
Over TC limit number b
NA 0 (0%) 0 (%) 0 (%) 11 (14%) 0(%) 4(5%) 4(5%)
Al is not among the TCLP metals, mercury was not analyzed. NA: not applicable.
3.3. Leachable metals The percentage of the amount of metal leached versus the amount of total metal present is defined as the metal’s leachability, and was evaluated using DIW for 44 BHD samples. Overall, the percentage of the total leachable Al (leachability of Al) in BHD was 0.2%, and approximately 95% BHD samples leached less than 1% of their respective total aluminum content. A significant correlation between leachate pH and leachable dissolved aluminum content in leachate was observed (Fig. SI-8). The average final pH of the extraction slurry was 9.6, and ranged from nearly neutral (6.3) to alkaline (12). The leachable metal content distributions are presented in Fig. SI-9. Trace metals were not consistently detected in the leachate above the MRL. For example, As was only detected in 7 leachate samples. Most of the trace metals (As, Cr, Cu, Mn, Pb and Zn) in BHD exhibited relatively low leachability (less than 1%), except selenium (approaching 60%), which is presented in Table 3. There were no observed correlations between the total and leachable trace metals that could be attributed to the extraction pH and the formation of aluminum hydroxide during the leaching experiments. Research suggests that in alkaline pH environments, most metals tend to leach at much lower concentrations than under acidic environments (Quina et al., 2009). Furthermore, aluminum hydroxide was formed in the processing of reaction of BHD with water, which could tend to sorb metals (except As) and remove them from solution (Zhou and Haynes, 2010). When compared to SC (Huang et al., 2014), BHD samples leached lower concentrations of Al (p < 0.01), higher concentrations of Ca, As, Cr and Se (p < 0.01), and similar concentrations of K, Na, Mg, Mn, Cu, Zn and Pb (Table 3). However, when leachability was compared, BHD leached lower percentages of its Al and K content (p < 0.01), higher percentages of its Na, Cu, Cr, Zn, As and Se (p < 0.01), and no differences were found for Ca, Mn and Pb. The toxicity characteristic leaching procedure (TCLP) was carried out on all BHD samples. Of the 78 samples, 20 (26%) were extracted using extraction fluid #2 (1.14% glacial CH3CH2OOH, pH 2.88) due to their high buffer capacity, whereas the other 58 samples were leached using extraction fluid #1 (1.14% glacial CH3CH2OOH–NaOH, pH 4.95). Of the 8 RCRA (Resource conservation and recovery act) toxicity characteristic (TC) metals, the concentrations of seven were measured (As, Se, Ag, Ba, Cr, Cd and Pb), as presented in Table 4. The concentrations of 4 metals (Ag, As, Ba and Cr) were consistently below their corresponding TC limit, whereas some samples for Cd, Pb and Se were over their corresponding TC limits. The distribution of the TC metals and Al in the TCLP test results is presented as Fig. SI-10. There was no any statistical correlations between these metals in DI leaching test and TCLP test. Meanwhile, exceeding the TCLP limits in all samples were independent of facilities generating the BHD. This may indicate that the metal content of the scrap aluminum feed stock may play a larger role in the concentration of metals in the BHD samples than the facility operational conditions.
BHD samples from 13 U.S. SAP facilities were investigated for mineral phases, total and leachable metals. The semi-quantitative analysis of BHD samples suggests that the dominant aluminum minerals were metallic aluminum (Al), aluminum oxide (Al2O3, Al2.67O4), aluminum nitride (AlN) and its oxides, spinel (magnesium aluminum oxide, Al2MgO4) and elpasolite (K2NaAlF6). The occurrences of diaspore (AlO[OH]) in BHD suggests that some of BHD reacted prior to analysis. Meanwhile, most BHD also contains halite (NaCl), sylvite (KCl) and fluorite (CaF2), which are commonly used as fluxes at SAP facilities. The average content of total aluminum (Al) in the BHD samples was approximately 18% by weight, and ranged from 2.6% to 60%, which shows high variability among facilities. The metal leachability of BHD was further investigated by deionized water (DIW) under anaerobic conditions at 50 °C. Average concentration of Al in leachate was 14 mg L 1, with a range of 0.3–170 mg L 1. The overall percentage of the total leachable Al (leachability of aluminum) in BHD was 0.2%, and approximately 95% of BHD samples leached less than 1% of the total aluminum content. Some elevated concentrations of trace metals (>100 lg L 1 As, and >1000 lg L 1 Cu, Mn, Se, Pb, Mn and Zn) in the leachate were found from the studied BHD. The TCLP results also showed that some samples leached higher than characteristic hazardous waste thresholds for Cd, Pb, and Se, however, the facilities providing these samples varied. From the metal content perspective only, it appears that BHD has a higher potential to exhibit toxicity characteristics than salt cake (the largest waste stream generated by SAP facilities). Acknowledgments This research was collaboratively supported by the USEPA’s Office of Research and Development National Risk Management Research Laboratory, the Environmental Research and Education Foundation and the Aluminum Association under a Cooperative Research and Development Agreement. This manuscript has been subjected to the Agency’s internal review. The opinions expressed in this paper are those of the author(s) and do not, necessarily, reflect the official positions and policies of the USEPA. Any mention of products or trade names does not constitute recommendation for use by the USEPA. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.03.033. References Allen, P., Princic, K., Ruesch, P., 2009. Secondary Aluminum Production Waste Disposal Issues. Planning for a Sustainable Future, ASTSWMO Solid Waste Managers Conference, New Orleans, LA. ASTM, 2003. Standard Practice for Reducing Samples of Aggregate to Testing Size American Society for Testing and Materials. p. 4. Azom, 2003. Aluminium Dross Recycling – A New Technology for Recycling Aluminium Waste Products. The A to Z of Materials 10. Bellosi, A., Landi, E., Tampieri, A., 1993. Oxidation behavior of aluminum nitride. J. Mater. Res. 8, 565–572. Chung, F.H., 1974. Quantitative interpretation of X-ray diffraction patterns, I. Matrix-flushing method of quantitative multicomponent analysis. J. Appl. Crystallogr. 7, 513–519. Davis, J.R., 1993. Aluminum and Aluminum Alloys (Asm Specialty Handbook). ASM International. European-Commission, 1991. Council Directive of 12. December 1991 on hazardous waste (91/689/EEC), ANNEX III: Properties of wastes which render them hazardous. In: European-Commission (Ed.). L0689-EN-22.07.1994-001.001-10, Official Journal of the European Communities. p. 11.
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Mineral phases and metals in baghouse dust from secondary aluminum production Xiao-Lan Huang a, Amro M. El Badawy a, Mahendranath Arambewela a, Renata Adkins a, Thabet Tolaymat b,⇑ a b
Pegasus Technical Services, Inc., 46 E. Hollister Street, Cincinnati, OH 45219, USA Office of Research and Development, U.S. EPA, Cincinnati, OH 45224, USA
h i g h l i g h t s Mineral phases, metals content & their leachability from baghouse dust were studied. High variability of metallic Al and total Al content in baghouse dust was observed. Leachability of heavy metals of baghouse dust was higher than that of salt cake. Not all BHD samples were below the TCLP limits in US.
a r t i c l e
i n f o
Article history: Received 29 August 2014 Received in revised form 27 February 2015 Accepted 15 March 2015
Handling Editor: X. Cao Keywords: Aluminum Baghouse dust (BHD) Landfill Secondary aluminum production (SAP) Toxicity characteristic leaching procedure (TCLP)
a b s t r a c t Baghouse dust (BHD) is a solid waste generated by air pollution control systems during secondary aluminum processing (SAP). Management and disposal of BHD can be challenging in the U.S. and elsewhere. In this study, the mineral phases, metal content and metal leachability of 78 BHD samples collected from 13 different SAP facilities across the U.S. were investigated. The XRD semi-quantitative analysis of BHD samples suggests the presence of metallic aluminum, aluminum oxide, aluminum nitride and its oxides, spinel, elpasolite as well as diaspora. BHD also contains halite, sylvite and fluorite, which are used as fluxes in SAP activities. Total aluminum (Al) in the BHD samples averaged 18% by weight. Elevated concentrations of trace metals (>100 lg L 1 As; >1000 lg L 1 Cu, Mn, Se, Pb, Mn and Zn) were also detected in the leachate. The U.S. toxicity characteristic leaching procedure (TCLP) results showed that some samples leached above the toxicity limit for Cd, Pb and Se. Exceeding the TCLP limits in all sample is independent of facilities generating the BHD. From the metal content perspective only, it appears that BHD has a higher potential to exhibit toxicity characteristics than salt cake (the largest waste stream generated by SAP facilities). Published by Elsevier Ltd.
1. Introduction Recycling through the process of secondary aluminum production (SAP) plays an important role in aluminum manufacturing (IAI, 2009; TAA, 2010). While salt cake (SC) is consisting of the primary residue from SAP activities (Huang et al., 2014), the processes also generate baghouse dust (BHD), which is a powdery waste of a very fine grain size captured in dry emissions control devices called baghouses. Baghouses are used in SAP to control particulate air emissions from furnace operation and other SAP processing activities (López et al., 2001; López-Delgado et al., 2007). In general, the
⇑ Corresponding author. E-mail address: [email protected] (T. Tolaymat). http://dx.doi.org/10.1016/j.chemosphere.2015.03.033 0045-6535/Published by Elsevier Ltd.
formation of BHD and the amount of BHD formed depends on several factors such as the type and quality of input material (e.g. aluminum scraps), the operating conditions, and the control technology applied (Peterson and Newton, 2002; Hwang et al., 2006; Schlesinger, 2007; Schmitz, 2007). BHD is the second largest solid waste generated in SAP. Viland estimated in a 1990 study that for every one ton of scrap aluminum processed, 760 kg of secondary aluminum, 240 kg of dross residues and 3 kg of BHD are generated (Viland, 1990). LópezDelgado et al. (2007) estimated that in Western Europe, approximately 13 kg of BHD are generated per ton of scrap aluminum recycled (López-Delgado et al., 2007). A 2013 document from TAA indicates that the BHD generation rate in Northern America is about 6.8 kg per ton of the aluminum scrap recycled (TAA, 2013). The total combined annual amount of these aluminum
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X.-L. Huang et al. / Chemosphere 134 (2015) 25–30
wastes (SC and BHD) from SAP is around one million tons in the U.S. (USDOE, 1999), which is 20% of all the SAP-related waste generated in the world (Azom, 2003). Depending on their constituencies, the end-of-life management of these wastes has been found to present challenges in the U.S. and elsewhere (Reuter et al., 2004; Gil, 2005; Lorber and Antrekowitsch, 2010; SINTEF, 2010; Huang et al., 2012). BHD is usually co- disposed in municipal solid waste (MSW) landfills. It has been reported that many operational issues of landfill occurred after disposal aluminum solid wastes, including BHD. High concentration of hydrogen (H2) (30–50%) with gaseous ammonia (NH3) (up to 15 000 ppmv) were detected (Gerbasi, 2006; Allen et al., 2009; Stark et al., 2012). An increase in landfill temperature, 60 to 93 °C over a period of several months to several years (Gerbasi, 2006; Allen et al., 2009) were also observed in a MSW landfill after disposal of these aluminum wastes. Above average concentration of F , Cl , NH+4, CN , high pH and electric conductivity value in soil, leachate or groundwater, as well as the potential contaminations of heavy metals were also documented (USEPA, 1995, 2008; Gerbasi, 2006; Swackhamer, 2006; OhioEPA, 2007; Allen et al., 2009; Lorber and Antrekowitsch, 2010). It was known that MSW landfills are anaerobic systems that decompose the organic fraction of solid wastes with the temperature between 25 and 60 °C depending on the waste characteristics and location of landfill (Yesßiller et al., 2005; Hanson et al., 2010). On the other hand, BHD is recognized as a hazardous waste in European Union countries (European-Commission, 2000) because it is considered to be ‘‘highly flammable’’ (Category H3-A) and an ‘‘irritant’’ (Category H4) (European-Commission, 1991). When BHD comes in contact with water or damp air, highly flammable gases form, and these gases can be explosive, as well as act as irritants to skin and mucous membranes. Furthermore, BHD has been found to be harmful if inhaled or ingested (Category H5) (EuropeanCommission, 1991). BHD is also in the category of substances that are capable, after disposal (landfill or other), of potentially yielding another substance (e.g. leachate), which can possess any of the characteristics associated with the solid BHD or gaseous products (Category H13) (European-Commission, 1991). It is believed that the reactivity of aluminum wastes, including BHD is related to the composition and mineral phases of wastes (López-Delgado et al., 2007; Lorber and Antrekowitsch, 2010; SINTEF, 2010; Huang et al., 2012; Stark et al., 2012; Tsakiridis, 2012). When compared with SC, there is much less data available on the characteristics of BHD from SAP (López et al., 2001, 2004; Reuter et al., 2004; López-Delgado et al., 2007; Lorber and Antrekowitsch, 2010; Huang et al., 2011, 2012, 2014; Tsakiridis, 2012), although it has been reported that BHD contains 25–40% total Al, 15–25% metallic Al, 1–3% C, 0.2–1% S, 1–6% N, 6–11% SiO2, 1–3% Ca, 2–5% Mg, 1–3% Na, 0.2–1% K, 0.5–2% Fe and 1–5% F (López-Delgado et al., 2007, 2009), and also contains aluminum nitrides, carbides and sulfides, as well as metal oxides derived from the particular alloys being processed (López et al., 2001, 2004; López-Delgado et al., 2007, 2009; Schlesinger, 2007; Schmitz, 2007). Thus, the objective of this study was to investigate BHD from SAP facilities in the U.S. by determining the mineral phases and the metal (Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Pb, Se and Zn) content of the samples. The study also examines the leachability of metals from BHD exposed to deionized water in an anaerobic and elevated temperature environment (Huang et al., 2014), which is designed to simulate key conditions in municipal solid waste (MSW) landfills. The U.S. Environmental Protection Agency (EPA) toxicity characteristic leaching procedure (TCLP) (USEPA, 1992) was also employed on these BHD samples. The information resulting from this study will help provide the scientific foundation to understand BHD waste material, its potential risk and strategies for its management.
2. Experimental 2.1. BHD sampling A total of 78 BHD samples were collected over a period of four months from 13 different SAP facilities in the U.S. Prior to sample collection, the BHD was stored at the generation site. After ensuring sufficient cooling of the BHD, a subsample was collected following ASTM Method C702-98, ‘‘Standard practice for reducing samples of aggregate to testing size’’ (ASTM, 2003). Upon receipt at the EPA laboratory, each sample was mixed in an acid rinsed stainless steel pan. Samples did not require size reduction since they passed through a 2 mm sieve as received. 2.2. Mineral phases analysis Because of analysis time constraints (16 h per sample), 44 BHD samples were randomly selected for XRD analysis from the original 78 collected. The samples’ mineral phases were evaluated from 5° to 110° 2h on a Philips X’Pert Pro Diffractometer using copper Ka radiation. The powder diffraction file (PDF) patterns database from the International Centre for Diffraction Data (ICDD) was employed for the search, match and identification. A subset of reference patterns was built for all studied BHD samples. The semi-quantitative phase analysis was used by the X’Pert HighScore Plus software, based on the CHUNG Normalized RIR Method (Chung, 1974). An example of XRD analysis processing and results were presented as supporting documents (Tables SI-1–3, and Fig. SI-1). 2.3. Total metal analysis Extractable metal content was evaluated in all 78 BHD samples collected. After homogenization, 0.1 g subsample of BHD was acid digested following U.S. EPA SW846 Method 3051A with minor modifications due to the samples’ high aluminum content (USEPA, 2007a). The details of digestion and metal recovery can be found in the supporting information. After acid digestion, metal compositions including Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Pb, S, Se and Zn were determined following EPA’s SW846 Method 6010C using a Thermo ICP-AES (USEPA, 2007b). 2.4. Metal leachability The same 44 samples that were randomly selected for the XRD analysis were targeted for the metal leachability evaluation [3]. Ten grams of the BHD sample ( 0. 1), whereas the abundance of aluminum nitride and aluminum oxide (including different oxide forms) was significantly lower (p < 0.01) (Huang et al., 2014). Possibly the most striking difference between BHD and SC is the presence of diaspore (AlO[OH], 01-080-1385) (Fig. 1, and Tables SI-2 and 3). Diaspore is a common product of the hydrolysis of aluminum nitride, carbide or metallic aluminum (Holleman and Wiberg, 2001; López et al., 2001; Li Jinwang et al., 2005; Li et al., 2006; Petrovic and Thomas, 2008), which indicated that some of the BHD samples underwent hydrolysis before the Table 2 Total acidic extractable metal content of SAP BHD.
a
Metal
Mean
Median
Range
MRLa
Al (%) Ca (%) Mg (%) Fe (%) K (%) Na (%) As (mg kg 1) Cd (mg kg 1) Cr (mg kg 1) Cu (mg kg 1) Mn (mg kg 1) Pb (mg kg 1) Se (mg kg 1) Zn (mg kg 1)
18.5 6.69 2.65 0.89 5.26 6.6 9.4 40 430 1300 1300 970 40 6840
17 3.44 2.1 0.81 4.77 6.66 5.5 15 240 990 980 270 5.0 1900
2.64–59.5 0.26–28.8 0.47–9.53 0.09–3.01 0.15–17.7 0.17–25 5.5–84 1.0–230 38–1600 116–6100 233–5100 28–8800 5.0–390 174–83 000
6.0 73 5.6 3.7 184 41 5.5 1.0 5.1 5.4 0.9 23 5.0 0.7
MRL (method report limit), mg kg
1
.
analysis. This was related to the aluminum dust collection and treatment processing in SAP (TAA, 2013). On the other hand, hydrolysis of aluminum nitride with water vapor has been previously reported (Li Jinwang et al., 2005; Li et al., 2006), some of the BHD samples underwent hydrolysis might also occur during the BHD sampling and/or storage processes. Aside from the aluminum species, the abundance of NaCl and KCl in BHD was significantly lower (p < 0.01) than that present in SC (Huang et al., 2014). However, BHD samples contained a relatively higher (p < 0.01) abundance of quartz and calcite when compared to SC (5%) (Huang et al., 2014). Due to the common use of lime (CaO) in air emissions control processes at SAP facilities (TAA, 2013), the presence of calcite in BHD is not unexpected. BHD also may contain some minor minerals (e.g. silicon nitride and magnesium sulfate) that were not detected in SC. 3.2. Total metal content As expected, aluminum was the most abundant metal detected in the BHD samples and the total metal content data is presented in Table 2. The average extractable total aluminum (Al) concentration of the BHD samples was approximately 18%, and ranged from 2.6% to 60%, as presented in Fig. SI-5. A few samples contained a relatively high level of aluminum (>40%), which skewed the samples’ distribution. However, the median of the Al concentration was 17%. As presented in Table 2, trace metals were detected in BHD samples from SAP processing facilities above their respective method reporting limit (MRL) and can be presented as a result of being aluminum alloy constituents or impurities from the recycling process (Davis, 1993) (their distribution is presented in Fig. SI-6). Relatively high concentrations of trace metals (e.g. >100 mg kg 1 Cd, >200 mg kg 1 Se, > 6000 mg kg 1 Pb and 50 000 mg kg 1 Zn) were found in some BHD samples. Compared with SC, the content of Al, Ca, Fe, Cu, Zn, Pb and Cd in BHD was significantly higher (p < 0.01), but the content of Na and As was significantly lower (p < 0.01), respectively. There were no significant differences among the content of Mg, K, Mn, Cr and Se between the two types of SAP waste. It is worthy to note that the maximum content of all metals detected in the BHD samples was consistently higher than those reported for SC (Huang et al., 2011, 2014). The Al content in SC showed a positive correlation with all other detected trace metals, which was not observed in BHD samples. Al detected in BHD samples showed positive correlations with Fe, Mn, Cu and Cr, and negative correlations with Pb, Cd and Zn (Fig. SI-7) (Huang et al., 2011, 2014).
Table 3 Metal leachability from BHD. Metal
MRL
1
Al (mg L ) Ca (mg L 1) Mg (mg L 1) Fe (mg L 1) K (mg L 1) Na (mg L 1) As (lg L 1) Cr (lg L 1) Cd (lg L 1) Cu (lg L 1) Mn (lg L 1) Pb (lg L 1) Se (lg L 1) Zn (lg L 1)
0.003 0.145 0.011 0.007 0.368 0.082 26 14 3 11 2 17 34 1
Concentration
Leachability (%)
Mean
Median
Range
Mean
Median
Range
14 470 8.9 1.9 2,100 2,700 33 23 9.4 470 510 410 210 320
4.3 116 0.9 0.19 2000 2300 26 14 3.2 75 12 17 77 125
0.32–170 0.72–3300 0.01–62 0.06–34 27–6800 50–11 000 26–140 14–96 3.2–54 11–6100 1.8–10 000 17–17 000 34–1300 5.3–4300
0.19 12 1.9 0.5 69 70 6.7 0.44 1.5 0.82 1.2 1.5 14 0.53
0.05 6.2 0.1 0.1 78 72 4.8 0.17 0.82 0.33 0.02 0.21 8.2 0.12
0.003–1.8 0.09–100 0.001–18 0.01–7.7 0.6–100 0.9–100 1.7–21 0.03–4.0 0.05–12 0.01–39 0.001–21 0.001–39 0.18–57 0.01–3.9
X.-L. Huang et al. / Chemosphere 134 (2015) 25–30 Table 4 Metal concentration in the TCLP solution (mg L Metal a
Al Ag As Ba Cd Cr Pb Se a b
Mean 130 0.014 0.02 2.6 0.5 0.17 2.1 0.23
Median 34 0.01 0.02 1.7 0.1 0.1 0.1 0.05
Range 0.9–950 0.01–0.07 0.02–0.2 0.01–12 0.003–6 0.02–1.5 0.01–56 0.02–3.5
4. Conclusions
1
).
TC limit NA 5 5 100 1 5 5 1
29
Over TC limit number b
NA 0 (0%) 0 (%) 0 (%) 11 (14%) 0(%) 4(5%) 4(5%)
Al is not among the TCLP metals, mercury was not analyzed. NA: not applicable.
3.3. Leachable metals The percentage of the amount of metal leached versus the amount of total metal present is defined as the metal’s leachability, and was evaluated using DIW for 44 BHD samples. Overall, the percentage of the total leachable Al (leachability of Al) in BHD was 0.2%, and approximately 95% BHD samples leached less than 1% of their respective total aluminum content. A significant correlation between leachate pH and leachable dissolved aluminum content in leachate was observed (Fig. SI-8). The average final pH of the extraction slurry was 9.6, and ranged from nearly neutral (6.3) to alkaline (12). The leachable metal content distributions are presented in Fig. SI-9. Trace metals were not consistently detected in the leachate above the MRL. For example, As was only detected in 7 leachate samples. Most of the trace metals (As, Cr, Cu, Mn, Pb and Zn) in BHD exhibited relatively low leachability (less than 1%), except selenium (approaching 60%), which is presented in Table 3. There were no observed correlations between the total and leachable trace metals that could be attributed to the extraction pH and the formation of aluminum hydroxide during the leaching experiments. Research suggests that in alkaline pH environments, most metals tend to leach at much lower concentrations than under acidic environments (Quina et al., 2009). Furthermore, aluminum hydroxide was formed in the processing of reaction of BHD with water, which could tend to sorb metals (except As) and remove them from solution (Zhou and Haynes, 2010). When compared to SC (Huang et al., 2014), BHD samples leached lower concentrations of Al (p < 0.01), higher concentrations of Ca, As, Cr and Se (p < 0.01), and similar concentrations of K, Na, Mg, Mn, Cu, Zn and Pb (Table 3). However, when leachability was compared, BHD leached lower percentages of its Al and K content (p < 0.01), higher percentages of its Na, Cu, Cr, Zn, As and Se (p < 0.01), and no differences were found for Ca, Mn and Pb. The toxicity characteristic leaching procedure (TCLP) was carried out on all BHD samples. Of the 78 samples, 20 (26%) were extracted using extraction fluid #2 (1.14% glacial CH3CH2OOH, pH 2.88) due to their high buffer capacity, whereas the other 58 samples were leached using extraction fluid #1 (1.14% glacial CH3CH2OOH–NaOH, pH 4.95). Of the 8 RCRA (Resource conservation and recovery act) toxicity characteristic (TC) metals, the concentrations of seven were measured (As, Se, Ag, Ba, Cr, Cd and Pb), as presented in Table 4. The concentrations of 4 metals (Ag, As, Ba and Cr) were consistently below their corresponding TC limit, whereas some samples for Cd, Pb and Se were over their corresponding TC limits. The distribution of the TC metals and Al in the TCLP test results is presented as Fig. SI-10. There was no any statistical correlations between these metals in DI leaching test and TCLP test. Meanwhile, exceeding the TCLP limits in all samples were independent of facilities generating the BHD. This may indicate that the metal content of the scrap aluminum feed stock may play a larger role in the concentration of metals in the BHD samples than the facility operational conditions.
BHD samples from 13 U.S. SAP facilities were investigated for mineral phases, total and leachable metals. The semi-quantitative analysis of BHD samples suggests that the dominant aluminum minerals were metallic aluminum (Al), aluminum oxide (Al2O3, Al2.67O4), aluminum nitride (AlN) and its oxides, spinel (magnesium aluminum oxide, Al2MgO4) and elpasolite (K2NaAlF6). The occurrences of diaspore (AlO[OH]) in BHD suggests that some of BHD reacted prior to analysis. Meanwhile, most BHD also contains halite (NaCl), sylvite (KCl) and fluorite (CaF2), which are commonly used as fluxes at SAP facilities. The average content of total aluminum (Al) in the BHD samples was approximately 18% by weight, and ranged from 2.6% to 60%, which shows high variability among facilities. The metal leachability of BHD was further investigated by deionized water (DIW) under anaerobic conditions at 50 °C. Average concentration of Al in leachate was 14 mg L 1, with a range of 0.3–170 mg L 1. The overall percentage of the total leachable Al (leachability of aluminum) in BHD was 0.2%, and approximately 95% of BHD samples leached less than 1% of the total aluminum content. Some elevated concentrations of trace metals (>100 lg L 1 As, and >1000 lg L 1 Cu, Mn, Se, Pb, Mn and Zn) in the leachate were found from the studied BHD. The TCLP results also showed that some samples leached higher than characteristic hazardous waste thresholds for Cd, Pb, and Se, however, the facilities providing these samples varied. From the metal content perspective only, it appears that BHD has a higher potential to exhibit toxicity characteristics than salt cake (the largest waste stream generated by SAP facilities). Acknowledgments This research was collaboratively supported by the USEPA’s Office of Research and Development National Risk Management Research Laboratory, the Environmental Research and Education Foundation and the Aluminum Association under a Cooperative Research and Development Agreement. This manuscript has been subjected to the Agency’s internal review. The opinions expressed in this paper are those of the author(s) and do not, necessarily, reflect the official positions and policies of the USEPA. Any mention of products or trade names does not constitute recommendation for use by the USEPA. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.03.033. References Allen, P., Princic, K., Ruesch, P., 2009. Secondary Aluminum Production Waste Disposal Issues. Planning for a Sustainable Future, ASTSWMO Solid Waste Managers Conference, New Orleans, LA. ASTM, 2003. Standard Practice for Reducing Samples of Aggregate to Testing Size American Society for Testing and Materials. p. 4. Azom, 2003. Aluminium Dross Recycling – A New Technology for Recycling Aluminium Waste Products. The A to Z of Materials 10. Bellosi, A., Landi, E., Tampieri, A., 1993. Oxidation behavior of aluminum nitride. J. Mater. Res. 8, 565–572. Chung, F.H., 1974. Quantitative interpretation of X-ray diffraction patterns, I. Matrix-flushing method of quantitative multicomponent analysis. J. Appl. Crystallogr. 7, 513–519. Davis, J.R., 1993. Aluminum and Aluminum Alloys (Asm Specialty Handbook). ASM International. European-Commission, 1991. Council Directive of 12. December 1991 on hazardous waste (91/689/EEC), ANNEX III: Properties of wastes which render them hazardous. In: European-Commission (Ed.). L0689-EN-22.07.1994-001.001-10, Official Journal of the European Communities. p. 11.
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