Supergravity Separation for Cu Recovery and Precious Metal

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Supergravity separation for Cu recovery and precious metal concentration from waste printed circuit boards Long Meng, Yiwei Zhong, Zhe Wang, Kuiyuan Chen, Xinle Qiu, Huijing Cheng, and Zhancheng Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02204 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Sustainable Chemistry & Engineering

Supergravity separation for Cu recovery and precious metal concentration from waste printed circuit boards Long Meng, Yiwei Zhong, Zhe Wang, Kuiyuan Chen, Xinle Qiu, Huijing Cheng, Zhancheng Guo*

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, 30 Xueyuan Road, Beijing 100083, People’s Republic of China

Corresponding author

Tel.: +86 10 82375042; fax: +86 10 82375042.

E-mail address: [email protected] (Z. Guo).

Synopsis This study relates to the recycling of resources and environmental protection. Keywords: Printed circuit boards; Supergravity separation; Recovery; Concentration; Copper; Precious metals

Abstract

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ACS Paragon Plus Environment


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Printed circuit boards (PCBs) contain both valuable metals and hazardous materials, thereby rendering them attractive secondary sources of metals, but also environmental contaminants. Thus, we herein report the use of supergravity separation for the recovery of copper (Cu) and the concentration of precious metals present in waste PCBs. At an optimized temperature of 1300 °C, a gravity coefficient of 1000, and a separation time of 5 min, the total recoveries of Cu, Zn, Pb, and Sn over the whole separation process were 97.80, 95.59, 98.29, and 97.69%, respectively. Compared with the amounts of precious metals present in the original PCBs, the contents of Ag, Au, and Pd in the Cu alloy increased by 5.16, 2 and 1.85 times, respectively, while those in the final residues increased by 2.92, 1.59, and 1.54 times, respectively. By combining the appropriate hydrometallurgical process and supergravity separation of metals or alloys, this clean and efficient process provides a new way to recycle valuable metals and effectively prevent environmental pollution from PCBs.

Introduction The production of electrical and electronic equipment (EEE) is one of the fastest-growing sectors in the manufacturing industry, as technological innovation and intense marketing tend to stimulate the early replacement of many consumer products.1 Due to the reduction of production times and the increased use of electronic products, large

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amounts of waste EEE (WEEE) is produced every year. In 2016, about 45 million tons of WEEE were produced globally, with this value growing by 2 million tons per year.2,3 Printed circuit boards (PCBs) are fundamental components of EEEs, and require recycling for both environmental and economic reasons as they contain significant amounts of metals and hazardous materials, including Cu (present in the highest percentage), Zn, Sn, Pb, Ag, Au, and Pd.4,5 In addition, the concentrations of precious metals present in discarded PCBs are above 10 times greater than those of the richest minerals, and so PCBs have often been referred to as “urban mineral resources.”6 Furthermore, waste printed circuit boards (WPCBs) account for about 3% of the total WEEEs, and as economically exploitable reserves of Au, Ag, and Cu become depleted, the recovery of these metals from WPCBs is becoming increasingly attractive.7 Thus, to prevent the loss of these valuable components and reduce the potential for significant environmental damage, the recovery of such metals from WPCBs is of particular interest in modern resource conservation and environmental protection circles. To date, a number of techniques for the recovery of Cu and precious metals from PCBs have been considered, including pyrometallurgical, hydrometallurgical, and physical methods, as well as various combinations of these procedures. Guimarães et al. used simultaneous electroleaching and electrodeposition to extract Cu from the ground and concentrated PCBs. The results showed that an apparent Cu recovery rate of 96% was achieved after 15 h of electrolysis with an agitation speed of 415 rpm at 25 °C.8 3



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Stuhlpfarrer et al. investigated the recycling of Cu and Au from WPCBs using alkaline melts consisting of eutectic melts of caustic soda and potassium hydroxide mixtures at temperatures below 200 °C.9 Fogarasi et al. suggested an eco-friendly chemical and electrochemical process for the simultaneous recovery of Cu and separation of a gold-rich residue from WPCBs. In this case, they found that high purity Cu deposits (99.04 wt.%) could be obtained, in addition to Au concentrations that were 25 times higher than those detected in the initial WPCB samples.10 Buckle and Roy analyzed the thermodynamics and kinetics of the recovery of Cu and Sn from a waste Sn stripping solution obtained from WPCBs, respectively.11,12 Havlik et al. used pyrolysis and burning to pretreat PCBs, and then leached Cu and Sn from them using 1 M HCl at 80 °C.13 Veit et al. utilized magnetic and electrostatic separation to recycle metals from WPCBs, demonstrating that it was possible to obtain samples containing on average more than 50% Cu, 24% Sn, and 8% Pb.14 Park et al. used aqua regia as a leachant to recover 98 wt.% of the input Ag, liquid-liquid extraction to recover 93 wt.% of the input Pd, and then dodecanethiol and sodium borohydride to recover 97 wt.% of the input Au.15 Alzate et al. developed a novel method of recovering Au from WEEE using ammonium persulfate ((NH4)2S2O8); the results indicated that more than 98% of the Au was recovered.16 Although all of the above methods have their own specific advantages, physical methods are currently the most common means of dealing with PCBs. However, the recycling of PCBs remains limited because of the heterogeneity and complexity of the components, which render isolation of 4


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the metals using traditional physical methods rather challenging. As such, novel technologies for recycling valuable metals from WPCBs are urgently required. In this context, we herein report the development of an efficient and economic method for separating metals from granulated WPCBs via supergravity separation. Supergravity is the name given to a force that is stronger than normal gravitational acceleration (9.80 m/s2). It is an efficient multi-phase reaction and separation technique that uses a centrifugal force to enhance phase transfer and micromixing through the generation of a supergravity field. In a supergravity field, differences between the melting points or densities of the solid particles and the liquid melts result in particles being distributed and separated gradually along the centrifugal direction.17 The technology has been successfully applied to electrochemical deposition,18-20 enrichment of valuable elements from slags,21-23 treatment of wastes,24,25 fabrication of functionally graded materials,26-28 and removal of impurities from alloys or metals.29-31 Low melting point metals (Pb and Sn) have been recovered from PCBs by supergravity separation in a previous study.32 The main objective of the present study was to recover Cu from PCB residues and to concentrate the remaining precious metals (Au, Ag, and Pd) in the final residues. Thus, a novel and clean method of recycling valuable metals from electronic waste was demonstrated.

Experimental section

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Materials and methods The waste computer PCBs (CPCBs) employed herein were purchased from an enterprise in the Hunan province of China. Prior to receival, the PCBs were crushed, sorted, and sieved to obtain a particle size of ~1.0 mm (Figure 1), and the majority of non-metallic materials were removed from the WPCBs. The obtained PCBs contained a range of metals and alloys with different melting points, and the molten metals and alloys could be separated rapidly from the solid particles using a supergravity field. Most of the low melting point metals in the WPCBs were preferentially recycled by supergravity separation, and the high melting point metals (Cu, Zn, Au, Ag, and Pd) were remained in the residues. Therefore, the residues needed to be reheated, rotated, and centrifuged to allow the recovery of Cu using the supergravity technology.

Equipment The supergravity field was generated by a centrifugal apparatus, a schematic of which is illustrated in Figure 2 from a vertical angle, with the heating furnace and the counterweight fixed symmetrically onto the rotor. The centrifugal apparatus consisted of two centrifugal tanks: the first tank was used for heating and centrifuging samples (the separation tank), while the second tank was used for balancing the equipment to keep the centrifugal process stable (the counterweight tank). The counterweight tank should have a weight and center of gravity in line with those of the separation tank. The two tanks are shown in the stationary 6


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state by the red line in Figure 2. The separation tank contained an insulating layer and a heating system. The temperature of the separation tank was controlled by a program controller with an R-type thermocouple, with a precision range of ±3 °C. The gravity coefficient (G) was calculated as the ratio of super gravitational acceleration to normal gravitational acceleration via Eq. (1). g   R  2

G

2

g

2



g 2 （

N 2 2 R 2 ） 900 g

(1)

where ω is the angular velocity (rad/s); N is the rotating speed of the centrifuge (rev/min); R is the distance from the centrifugal axis to the center of the sample (m), in the experiment, R=0.25 m; and g is the normal gravitational acceleration.

Experimental procedures As shown in Figure 2, a 36 g sample of WPCB particles was placed into a set of graphite crucibles of inner diameter 19 mm. The bottom of the upper graphite crucible included small holes (1 mm), through which metals in the liquid state could pass. Graphite felt (thickness 3 mm) was laid at the bottom of the upper crucible to retain small solid particles. Similarly, graphite felt (thickness 5 mm) was laid on the WPCB particles to avoid oxidization during the separation process. In a previous study, Pb and Sn were separated from WPCB particles at a temperature of 410 °C, a gravity coefficient of 1000, and a separation time of 5 min. After the separation of the Pb-Sn alloy, WPCB residues were

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reheated to 650–1300 °C and held at this temperature for 30 min before the centrifugal device was rotated. After some separation time (1–10 min), the device was shut down and the lower graphite crucible was removed and rapidly quenched in water. The flow chart of supergravity separation and concentration of metals from PCB particles was shown in Figure 3. The blister Cu will be refined by electrolysis. The final resides (2# PCB residues) will be treated with hydrometallurgy to recycle precious metals.

Analysis and treatment of the data The WPCB particles were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 7000DV, Perkin Elmer, USA), and the results are shown in Table 1. In a previous study, the Pb-Sn alloy obtained from the WPCB particles was dissolved by acid leaching and analyzed by ICP-OES. In this research, the Cu alloy obtained from WPCB residues was sectioned longitudinally along the center axis. One portion was characterized by scanning electron microscope and energy-disperse X-ray spectrum (SEM/EDS, MLA 250, FEI Quanta, USA), and X-ray diffraction (XRD, Smartlab, Rigaku Corporation, Japan), while the other portion was analyzed by ICP-OES. The recovery (Ri) of Pb, Sn, Zn, and Cu was calculated using Eq. (2).

Ri 

mi  100% mC  i

(2)

where mi is the mass of Pb, Sn, Zn or Cu that is separated from the WPCBs (g); mC is the mass of the WPCBs; ωi is the mass fraction of Pb, Sn, Zn or Cu in the WPCBs. The 8


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experimental data were repeated three times.

Results and discussion It was previously reported that Pb and Sn recoveries of 54.41 and 41.94% were achieved using a gravity coefficient of 1000 at 410 °C with a separation time of 5 min. However, due to the low contents of other metals in the WPCBs and the ease with which they formed solid solutions with Cu during the heating process, it was difficult to separate them completely. Therefore, in this study, the residues were separated and recovered to obtain Cu and concentrate the precious metals by supergravity separation. The effects of some parameters on the recovery of Cu were investigated, including the gravity coefficient and the process temperature.

Separation of Cu Effect of gravity coefficient on Cu recovery As the surface tension of the liquid phase in the solid WPCB particles was relatively large, supergravity separation was required to overcome the resistance involved in Cu recovery. Indeed, the gravity coefficient is an important parameter in the supergravity separation process, and so a series of separation experiments were carried out at a temperature (T) of 1200 °C and a separation time (t) of 5 min, while varying the gravity coefficient from 100 to 1000. It was not possible to increase the gravity coefficient beyond 1000 due to the limited rotation speed of the centrifugal apparatus employed herein. Figure 9



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4 shows the effect of the gravity coefficient on the recovery of Cu, Zn, Pb, and Sn in the separation experiments. As indicated, upon increasing the gravity coefficient from 100 to 1000, the recoveries of Cu, Zn, Pb, and Sn increased both linearly and gradually, giving recoveries of 93.52, 90.96, 42.03, and 54.78% for Cu, Zn, Pb, and Sn, respectively, at the maximum gravity coefficient (i.e., 1000). Table 2 shows the chemical compositions of the blister Cu obtained using different gravity coefficients, where it is apparent that although this variable affected the Cu recovery rate, it had little effect on the composition of the recovered Cu.

Effect of temperature on Cu recovery The maximum operable temperature of the furnace employed herein was 1300 °C, and so the effect of temperature between 650 and 1300 °C on the recovery of Cu was investigated. These experiments were conducted using a gravity coefficient of 1000 and a separation time of 5 min. Figure 5 shows the effect of temperature on the recovery of Cu, Zn, Pb, and Sn; the recovery rates of Cu, Zn, Pb, and Sn reached their maximum values of 97.80, 95.59, 43.88, and 56.75%, respectively. In the whole separation process, the recovery values of Pb and Sn were 98.29 and 98.69%, respectively, when the temperature was 1300 °C. Table 3 shows the chemical compositions of the Cu alloys obtained at the different temperatures. More specifically, upon increasing the temperature from 650 to 1100 °C, the Cu content increased from 24.13 to 75.52 wt.%, after which it remained relatively constant. 10


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Characterization of the Cu alloy Figures 6 and 7 show the profile and XRD pattern of the Cu alloy obtained following supergravity separation at G = 1000, T = 1200 °C, and t = 5 min. As indicated, the majority of Cu was successfully separated and recovered from the WPCB residues following supergravity separation. In addition, the XRD results confirmed the presence of diffraction peaks corresponding to Cu-Zn, Cu-Sn, Sn, and Pb. Subsequently, the microstructure of the blister Cu was further characterized by SEM/EDS (Figure 8), and Zn, Pb, and Sn were found to be uniformly distributed within the Cu alloy, with no macrosegregation being observed. Furthermore, the presence of Fe in the Cu alloy was also noted.

Concentrations of the precious metals Precious metal concentrations in the Cu alloys After the separation of Pb and Sn using the optimized conditions, Cu recovery experiments were conducted at T = 1200 °C, G = 1000, and t = 5 min. As the melting points of Ag and Au are lower than that of Cu, they were in the liquid state at 1200 °C. Upon the examination of Cu-Ag and Cu-Au binary alloy phase diagrams,33, 34 it can be seen that Cu-Ag and Cu-Au were in the liquid phase at temperatures above 1083 °C, thereby resulting in the enrichment of Ag and Au into the Cu alloy. The contents of the precious metals in the Cu alloy are shown in Figure 9. The results indicated that the concentrations of Ag, Au, and Pd in the Cu alloy were 640, 88, and 12 ppm, respectively. Upon 11



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comparison with the initial WPCB samples (Table 1), it could be determined that the concentrations of Ag, Au, and Pd in the Cu alloy increased by 5.16, 2, and 1.85 times, respectively. To obtain the refined Cu and the desired precious metals, this Cu alloy would subsequently be treated by electrolysis followed by hydrometallurgy.

Precious metal concentrations in the residues After the separation and recovery of Pb, Sn, Cu, and Zn, the resulting residues (2# PCB residues) contained a number of precious metals (e.g., Pd and Pt) and high melting point metals (e.g., Fe and Mn). As metals such as Ag and Au can form solid solutions with high melting point metals at high temperatures, and the resulting solid solutions have melting points above 1200 °C, these solutions remained within in the residues. The concentrations of the precious metals in the residues are shown in Figure 10. The results showed that the concentrations of Ag, Au, Pd, and Pt in the residues were 362, 70, 10, and

Supergravity Separation for Cu Recovery and Precious Metal

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