Article pubs.acs.org/est
Copper Sludge from Printed Circuit Board Production/Recycling for Ceramic Materials: A Quantitative Analysis of Copper Transformation and Immobilization Yuanyuan Tang,† Po-Heng Lee,‡ and Kaimin Shih†,* †
Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
‡
S Supporting Information *
ABSTRACT: The fast development of electronic industries and stringent requirement of recycling waste electronics have produced a large amount of metal-containing waste sludge. This study developed a waste-to-resource strategy to beneficially use such metal-containing sludge from the production and recycling processes of printed circuit board (PCBs). To observe the metal incorporation mechanisms and phase transformation processes, mixtures of copper industrial waste sludge and kaolinite-based materials (kaolinite and mullite) were fired between 650 and 1250 °C for 3 h. The different copper-hosting phases were identified by powder Xray diffraction (XRD) in the sintered products, and CuAl2O4 was found to be the predominant hosting phase throughout the reactions, regardless of the strong reduction potential of copper expected at high temperatures. The experimental results indicated that CuAl2O4 was generated more easily and in larger quantities at low-temperature processing when using the kaolinite precursor. Maximum copper transformations reached 86% and 97% for kaolinite and mullite systems, respectively, when sintering at 1000 °C. To monitor the stabilization effect after thermal process, prolonged leaching tests were carried out using acetic acid with an initial pH value of 2.9 to leach the sintered products for 20 days. The results demonstrated the decrease of copper leachability with the formation of CuAl2O4, despite different sintering behavior in kaolinite and mullite systems. This study clearly indicates spinel formation as the most crucial metal stabilization mechanism when sintering copper sludge with aluminosilicate materials, and suggests a promising and reliable technique for reusing metal-containing sludge as ceramic materials.
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INTRODUCTION With a rapid development of market demand for electric and electronic equipments, the production of printed circuit board (PCBs) as the essential components is dramatically increased. 1−3 The total production value of the PCBs manufacturing industry in China reached more than $10.83 billion in 2005, only next to Japan.1 PCB production processes generate significant amounts of solid sludge containing toxic metal-laden compounds.4 In addition, the recycling of waste PCBs also leaves large quantities of metal-containing waste sludge and creates serious secondary pollution without proper treatment.5 The stabilization/solidification (S/S) method can convert hazardous wastes into chemically stable solids6,7 by encapsulation through an interlocking framework of hydrated minerals.6,8 Even after solidification and encapsulation, the metal-containing sludge still exhibits considerable long-term metal leachability.9 The limited number of landfills capable of accepting S/S substances, together with their adverse environmental impacts,10 has made the development of effective and financially viable treatment technologies essential. The existing © 2013 American Chemical Society
advanced technologies emphasize the economical recovery and recycling of hazardous metals, via techniques such as solvent extraction,11 electrolysis,12 ion exchange,13 membrane separation,14 and microbiological methods.15 However, the industrial applications of the aforementioned technologies are limited by economic feasibility, technical difficulties, and/or recovery rate.16 A promising strategy has been reported for incorporating metal-containing waste materials effectively into a variety of ceramic products (bricks, tiles, and refractory aggregates).17,18 The potential reaction mechanisms were identified when sintering the target metal oxides with alumina, hematite and kaolinite precursors, and the metal leachability of products was found to be considerably reduced due to the change in mineral phases.19−22 In this way, the detoxification of hazardous metals Received: Revised: Accepted: Published: 8609
January 26, 2013 May 23, 2013 June 24, 2013 June 24, 2013 dx.doi.org/10.1021/es400404x | Environ. Sci. Technol. 2013, 47, 8609−8615
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copper incorporation). The mixing process was carried out by ball milling the powder in a water slurry for 18 h. The slurry samples were dried and homogenized by mortar grinding, and then pressed into 20 mm pellets at 480 MPa to ensure consistent compaction of the powder samples for the sintering process. A sintering scheme with a 3 h dwelling time at the targeted temperature was used for temperatures ranging from 650 to 1250 °C in a high-temperature furnace (LHT 02/16 LB, LBR, Nabertherm Inc. Lilienthal, Germany) sintering in air. After sintering, the samples were air-quenched and ground into powder for XRD analysis and the leaching test. Phase transformation during sintering was monitored using the powder XRD technique. The step-scanned XRD pattern of each powder sample was recorded by a Bruker D8 Advance Xray powder diffractometer (Mannheim, Germany) equipped with a Cu Kα1,2 X-ray radiation source (40 kV 40 mA) and a LynxEye detector. The 2θ scanning range was 10−90°, and the step size was 0.02° with a scan speed of 0.8 s step−1. Qualitative phase identification was executed by matching powder XRD patterns with those retrieved from the standard powder diffraction database of the International Centre for Diffraction Data (ICDD PDF-2, Release 2008; Newton Square, PA). The phases in products were all subjected to quantitative analysis using Topas 4.2 (Bruker, Mannheim, Germany), which employs the Rietveld refinement method. For systems potentially containing amorphous or poorly crystalline phases, a refinement method using 20% CaF2 as the internal standard30 was carried out to quantify the amorphous content in the samples. The leachability of each sintered sample was tested by a leaching procedure modified from the U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP), using a pH 2.9 acetic acid solution (extraction fluid #2) as the leaching fluid. Each leaching vial was filled with 5 mL of TCLP extraction fluid and 0.25 g of powder, and the vials were rotated end-over-end at 30 rpm for agitation periods of 18 h to 20 days. At the end of each agitation period, the leachates were filtered with 0.2 μm syringe filters, the pH was measured, and the concentrations of the predominant metals were derived from ICP-AES (PerkinElmer Optima 3300 DV).
in the waste stream and the reuse of waste material as a new source for marketable ceramic products act synergistically with each other. Converting hazardous metal-containing waste sludge into ceramic products enables the reduction of the sludge’s environmental impact by significantly decreasing metal leachability as a result of irreversible phase transformation. The X-ray diffraction (XRD) is a laboratory technique commonly available for the phase identification of crystalline materials.23 Quantitative phase composition can be obtained using XRD data analyzed via Rietveld refinement,24 but this analysis has not been widely applied for environmental studies. The Rietveld refinement method consists of fitting the complete experimental diffraction pattern with a model profile calculated by integrating the physical parameters of the identified crystal structures.25 This method is increasingly used to characterize natural and industrial materials and has been proven to be a reliable, precise, and reproducible method to quantify the relative phase abundances.26 Most ceramics are formed from high percentages of kaolinite (40−65%)27 with its chemical formula as Al2Si2O5(OH)4, and calcined kaolinite is a major industrial byproduct generated by heating kaolinite particles to mullite (3Al2O3·2SiO2) and silica (SiO2). The series of kaolinite-to-mullite reactions have been intensively studied due to the unique crystallochemical properties of reaction products and their importance to the ceramic industry.28,29 In our previous studies, the sintering of CuO with alumina was found to be an effective means of forming CuAl2O4 spinel.22 However, the recycling of copper industrial waste sludge produces a much more complicated background matrix, and further analysis is required to identify the hosting phases and stabilization effectiveness of copper. Therefore, in this study, the reaction processes were observed by sintering the mixture of copper waste sludge and kaolinitebased precursors (kaolinite and mullite). A prolonged leaching experiment was followed to examine the metal stabilization effect and the leaching behavior of products under different sintering temperatures.
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MATERIALS AND METHODS The copper sludge was collected from Guangdong province in China, and the collected sludge was dried at 105 °C for further sintering experiments in this study. The sludge elemental compositions detected via X-ray fluorescence (XRF) (JSX3201Z, JEOL, Tokyo, Japan) were normalized by their oxide forms (Figure S1 of the Supporting Information (SI)) and showed copper to be the predominant constituent. As shown in SI Figure S2, the XRD pattern for the 105 °C-dried sludge indicated that the predominant crystalline copper-containing phase was detected to be posnjakite (Cu4(SO4)(OH)6(H2O), PDF#83−1410). Other copper-containing phases were malachite (Cu2(OH)2CO3, PDF#76−0660) and copper oxide (CuO, PDF#80−1916). The kaolinite and calcined kaolinite (mixture of mullite and cristobalite) precursors were mixed with the sludge for ceramic sintering. USP-grade acid-washed kaolin (SI Figure S3) powder was obtained from Fisher Scientific as the kaolinite precursor, and its chemical compositions shown in SI Table S1 were determined by microwave-assisted acid digestion (U.S. EPA Method 3052). The mullite precursor was prepared by calcining the kaolinite precursor at 1350 °C for 24 h. Samples for the sintering experiments were prepared by mixing each precursor with the dried sludge for a total dry weight of 60 g at a Cu:Al molar ratio of 1:2.5 (with excess aluminum available for
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RESULTS AND DISCUSSION Phase Compositions after Sintering Sludge with Kaolinite and Mullite Precursors. The XRD patterns in Figure 1a show the phase transformation processes when sintering copper sludge with kaolinite precursor for 3 h at temperatures ranging from 650 to 1250 °C. The peaks of the CuAl2O4 phase were first detected in the 750 °C sintered sample, together with the appearance of mullite phase. A substantial increase in the CuAl2O4 phase peak intensity was detected when the sample was sintered at 950 °C. Meanwhile, the excess silica crystallized as cristobalite (SiO2) with high peak intensities at this sintering temperature. However, when the temperature was increased to above 1050 °C, the signals of the CuAl2O4 spinel declined dramatically. This reaction process was accompanied with the reappearance of CuO and the generation of Cu2O as illustrated in the XRD patterns. At the same time, the aluminum decomposed from the CuAl2O4 spinel reacted with silica and formed the mullite phase. Figure 1b illustrates the phase transformation in samples from the mixture of copper sludge and mullite precursor through the 3 h sintering process at temperatures ranging from 650 to 1250 °C. The CuAl2O4 phase was first observed with 8610
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reaction process was also accompanied by the reappearance of CuO and the generation of Cu2O. Meanwhile, the aluminum decomposed from the CuAl2O4 spinel was also found to be incorporated into the mullite phase as shown in the XRD patterns. In addition to the major copper- and aluminum-hosting crystalline phases, other minor phases were observed, including gypsum (CaSO4·2H2O), anhydrite (CaSO4), calcium copper oxide (CaCu2O3), grossular (Ca3Al2Si3O12), and almandine calcian (Ca1.5Fe1.76Al1.8Si2.94O12). During the sintering process, gypsum started to dehydrate into the anhydrite phase at 650 °C, and the anhydrite disappeared at temperatures higher than 1050 °C. The excess silicon and aluminum also reacted and formed other aluminosilicates, such as grossular and almandine calcian when the sintering temperature was higher than 1050 °C. Copper Transformation Mechanisms during Sintering Process. After Rietveld refinement of the XRD patterns, weight fractions of the components in the sintered products were obtained (Figure 2a and b). Phases such as gypsum, anhydrite, and quartz with small peaks (Figure 1) were refined with very low weight percentages (1100 °C may also have been caused by the vitrification effect of a glass matrix. Comparison of Leaching Behavior in Kaolinite and Mullite Systems. With different copper transformation mechanisms (Figure 2), the leaching behavior of sintered products from kaolinite and mullite systems requires further investigation. Therefore, copper, aluminum, and silicon concentrations after 20 days of leaching from products of both systems were compared in Figure 4. The leached copper from both kaolinite and mullite systems decreased with the sintering temperature, and reached its lowest value after the 1000 °C and 3 h heating process. Due to the formation of copper solid solution32 with metakaolin at 650 °C and higher copper transformation into CuAl2O4 at 750 °C in kaolinite system (Figure 2), the copper leachability of the sample sintered at temperature ≤750 °C from the kaolinite system was nearly half that of the equivalent sample from the mullite system. But with increasing copper transformation into the CuAl2O4 phase at higher sintering temperatures, copper leachability reduced significantly in both kaolinite and mullite systems. Compared to kaolinite and mullite, metakaolin was found to have more significant reactivity in acid media.37,38 As the 650 °C sintered products in the kaolinite system was largely predominated by metakaolin, the aluminum, and silicon concentrations in the leachate were much higher than those in the mullite system. In the mullite system, higher temperatures (≥1050 °C) may have decreased the formation temperatures of grossular and amorphous phases, and increased their presence in the system (Figure 2a and b). Therefore, at temperatures ≥1050 °C, the silicon in the mullite system was more readily leached than that in the kaolinite system, resulting in higher silicon concentrations in the leachates (Figure 3d).
unsintered samples is shown in terms of copper concentration in Figure 3b. The amount of copper leached from the unsintered samples reached around 2000 mg L−1 in both kaolinite and mullite systems. A substantial decrease from 2000 to 500 mg L−1 was observed, when the sample using kaolinite precursor was sintered at 650 °C. Under similar thermal conditions, the copper concentrations in the leachates of mullite systems were much higher (1700 mg L−1) than those in kaolinite systems. For both kaolinite and mullite systems, the minimum value of leached copper was found from samples sintered at 950−1050 °C, and the copper concentrations were more than 10 times lower than those in leachates of unsintered samples. To further investigate the leaching behavior of the sintered products, the aluminum and silicon concentrations were also measured as shown in Figure 3c and d. In the kaolinite system, the leached aluminum first increased to around 250 mg L−1 from samples heated at 650 °C, and then decreased to 45 mg L−1 when the sintering temperature increased to 750 °C. At temperatures higher than 750 °C (inset of Figure 3c), the aluminum concentrations continued declining until the sludge + kaolinite samples were sintered at 1150 °C. The leaching of silicon (Figure 3d) was also found with the highest value when samples were sintered at 650 °C, and decreased with elevated sintering temperatures. In the mullite system, the leached aluminum (Figure 3c) kept increasing with the sintering temperature until 1100 °C, then declined with a further elevation of temperature. The silicon concentrations in the leachates (Figure 3d) were relatively stable at around 15 mg L−1 when the samples were sintered at temperatures ≤1000 °C, but were much higher in leachates from samples sintered at 1050− 1200 °C. Behaving similarly to the kaolinite system, the silicon concentrations in the mullite system decreased when samples were treated at higher temperatures (≥1150 °C). Displaying the results of prolonged leaching from 18 h to 20 days, Figure 3 also illustrated the effect of time on the leaching behavior of samples. The increase of the leachate pH value with leaching time was observed in both kaolinite and mullite systems, but the increase was much slower when the samples were sintered at temperatures higher than 950 °C. With the leaching period prolonged from 18 h to 20 days, the amount of leached copper from both kaolinite and mullite systems also increased. The increase in leached copper with leaching time was also slower when the sintering temperature was higher than 950 °C, and the smallest increase in copper concentrations was found within a sintering temperature range of 950−1050 °C. In addition, the copper leached from the mullite system increased more quickly than that from the kaolinite system when the sintering temperature was ≤950 °C. The most significant increment of aluminum in the kaolinite system occurred when the system was heated at 650 °C, but the equivalent effect took place in the mullite system within a sintering temperature range of 1000−1100 °C. With prolonged leaching time, the most conspicuous increase in silicon was also found at the sintering temperature of 650 °C in the kaolinite system. In the mullite system, however, the change in leached silicon due to prolonged leaching time was not significant for samples from the tested temperatures. When the leaching behavior of samples was viewed in comparison with the phase transformation during the sintering process, it was found that the continuous decrease in copper leachability with elevated sintering temperature was due to the development of CuAl2O4 spinel in the sintered products. The 8613
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the funding for this research provided by the General Research Fund scheme (HKU 715612E, HKU 716310E) and Special Equipment Grant (SEG_HKU10) of the Research Grants Council of Hong Kong.
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Figure 4. The comparison of concentrations of (a) copper, (b) aluminum, and (c) silicon in leachates after 20 days of leaching when using kaolinite and mullite precursors.
This study indicates that the successful incorporation of copper from industrial sludge into an CuAl2O4 spinel structure using kaolinite-based precursors can be achieved at an attainable sintering temperature range (lower than 1100 °C). The results clearly demonstrate a substantial decrease in copper leachability for products with higher CuAl2O4 content, despite their different sintering behavior. The significant reduction in copper leachability indicates a waste-to-resource strategy which capable of treating and recycling sludge under thermal conditions.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
REFERENCES
S Supporting Information *
One table and seven figures are available. Among them, Figures S4−S7 demonstrate the examples of the CaF2-added XRD patterns and the corresponding Rietveld refinement results. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*Phone: +852-28591973; fax: +852-25595337; e-mail: kshih@ hku.hk. 8614
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