Research Article pubs.acs.org/journal/ascecg
Environmentally Friendly Technology of Recovering Nickel Resources and Producing Nano-Al2O3 from Waste Metal Film Resistors Jujun Ruan,* Jiaxin Huang, Lipeng Dong, and Zhe Huang School of Environmental Science and Engineering, and Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-Sen University, 135 Xingang Xi Road, Guangzhou 510275, People’s Republic of China S Supporting Information *
ABSTRACT: E-waste is mass-producing in the current world. Recovery technologies of e-waste are developing from crude to environmentally friendly and high-efficiency. Along with waste printed circuit boards, abundant waste resistors were generated. However, there is little information about their recovery technology. Waste metallic film resistors contain a 33.2 wt % ceramics matrix and 65.1 wt % nickel. If it is not properly treated, nickel will be exposed to and harm the environment and humans. This paper presents environmentally friendly and value-added recovery technology of waste metallic film resistors. The recovery technology includes shearing, magnetic separation, and milling. The shearing process was used to liberate the materials contained in waste metallic film resistors. The materials were completely liberated when sheared to 2−25 mm particle sizes. Magnetic separation was efficient for separating nickel particle from sheared materials, and the separation rate reached 100 wt %. The ceramic material was identified as Al2O3, and it were milled into nano-Al2O3. The particle size of nano-Al2O3 ranged from 100 to 500 nm, and it was an important powder coating material. The economic benefit of recovering waste metallic film resistors was analyzed based on energy costs, equipment depreciation cost, labor cost, and economic income. This paper reminds practitioners of e-waste recovery to pay close attention to recovering waste metallic film resistors and avoid environmental pollution. KEYWORDS: Waste metallic film resistors, Nickel recovery, Nano-Al2O3 production, Environmentally friendly
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INTRODUCTION
Environmental pollution accidents of heavy metals and persistent organic pollutants erupted in situ recovering of waste PCBs in some provinces (Guangdong, Zhejiang, and Fujian) of China.6−8 Special diseases (immune system damage, cancers, DNA damage, etc.) were experienced by local residents.9 Fortunately, nowadays, environmentally friendly technologies of recovering resources from e-waste have been developed with support of the Chinese government.10−13 Waste printed circuit boards, the core component of electronics products, were largely produced and the weight had reached 0.3 million tons per year in China.14 Basal boards of waste printed circuit boards were enriched with high-purity
Abundant e-wastes were generated along with technology innovation and excessive consumption of electronic equipment.1 Production of e-waste has reached about 35 million tons per year in the world, and China accounted for about 6 million tons.2,3 E-waste is considered to be urban mines due to it being rich in metals. On the other hand, e-waste was regarded as a carrier of heavy metals (copper, zinc, nickel, etc.) and persistent organic pollutants.4,5 Therefore, the recovery technology of ewaste is crucial to define e-waste. If the recovery technology is environmentally friendly and effective, e-waste is a rich mine. If the recovery technology is crude, e-waste will become hazardous materials to humans and the environment. Unfortunately, for the purpose of obtaining rapid economic benefits, crude recovery technologies (acid washing, open incineration) of e-waste were employed in the past decades. © 2017 American Chemical Society
Received: June 12, 2017 Revised: August 11, 2017 Published: August 14, 2017 8234
DOI: 10.1021/acssuschemeng.7b01900 ACS Sustainable Chem. Eng. 2017, 5, 8234−8240
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ACS Sustainable Chemistry & Engineering
matrix, metallic (conductive) film, end-cap, paint, color ring, and metallic pin.
metals (copper, zinc, nickel, etc.) and nonmetallic materials (glass fiber, epoxy resin, and additives).5,15 The recovery technologies of waste printed circuit boards were well researched and developed. The recovery line of waste printed circuit boards had been constructed in the certificated enterprise.16 A waste printed circuit board consists of baseplate and electronic components.10 Heated air disassembling and selective desoldering separation were proposed to liberate electronic components from waste printed circuit boards.17,18 Then, the different electronic components could be separated by the method of pneumatic separation according to their different density and particle size.19 Because they contained few precious metals, recovery of electronic components was not given much attention by business. In some cases, waste electronic components were treated by the crude technology of open incineration in industry. Inexpensive metals were recovered and nonmetallic resources were lost in the open incineration.20 Abundant persistent organics pollutants were produced in the open incineration and polluted the environment.21 Meanwhile, hazardous residual wastes were generated. There was also lack of published information about the recovery technologies of waste electronic components. Only a little information was published about the environmentally friendly recovery technology of electronic components. Pyrolysis, gasification, and plasma treatments were suggested to dispose of the nonmetallic materials in electronic components. Leaching and bioleaching methods were proposed to recover metals from electronic components.22 These environmentally friendly technologies stimulated businesses to become interested in recovering waste electronic components. Waste metallic film resistors, which are one of the most important electronic components, have been largely generated along with waste printed circuit boards. A White Paper on the WEEE (Waste Electronic and Electrical Equipment) Recycling Industry in China (2016) revealed that the mass of waste metallic film resistors was about 1500 tons in China per year. There was also little published information related to recovery technology of waste metallic film resistors. Pollution-free and efficient technologies of recovering waste metallic film resistors are pressingly needed. In this paper, a hybrid technology of crushing, magnetic separation, and milling was proposed to recover the full resource of waste metallic film resistors. Constituent materials of waste metallic film resistors were liberated by crushing. Then, metallic particles were separated by magnetic separation. The residual materials were milled into nanoparticles. Then, the economic assessment of waste metallic film resistor recovery was presented. This paper provided an environmentally friendly and value-added technology to recover full resources from waste resistors of waste PCBs.
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Figure 1. (a) Waste metallic film resistor; (b) structure of the waste metallic film resistor. Waste metallic film resistors (1 kg) were sheared in our lab. The mass proportions of the composed materials were computed and are given in Table 1. The end-cap and metallic pin (nickel) accounted for
Table 1. Composition of the Waste Metallic Film Resistor comprised materials
mass fraction (wt %)
shape
ceramics matrix metallic film (nickel−chromium alloy) paint and color ring (epoxy resin) end-cap and metallic pin (nickel)
33.2 0.65−1.3 2.7 63.8
clavate film film shell
the highest weight proportion of 63.8 wt %. Nickel is an important material used to produce alloy, stainless steel, and electroplated products. Meanwhile, it is also a hazardous material for humans and the environment. Nickel has the strong ability to combine with proteins and nucleic acid that will destroy human health. Additionally, nickel is inclined to be accumulated in the kidneys, spleen, and liver so as to induce cancers.23 Therefore, the recovery technology of waste metallic film resistors should avoid the exposure of nickel to the environment. The weight proportion of ceramic material was about 36.2 wt %. Metallic film and organic materials had proportions of 1.3 and 2.7 wt %, respectively. Shearing Process. A shearing machine was used to liberate the materials of waste metallic film resistors. The employed shearing machineis shown in Figure S1 in the Supporting Information. The shearing quality depended on particle physical characteristics including hardness, strength, caking properties, and particle size.24 The comprised materials of waste metallic film resistors have a great twodimensional scale (clavate, film, and shell). Thus, the shearing machine is suitable for liberating the materials. According to a previous study,25 the equation for computing the energy consumption of the shearing process is
⎛ 1 1 ⎞ E = 2340· m⎜ − ⎟ D1 ⎠ ⎝ D2
(1)
where m is the mass of the waste metallic film resistor, D1 is the average size (5 mm × 1.5 mm × 1.5 mm) of the waste metallic film resistor before shearing, and D2 is the average size of waste metallic film resistor particles after shearing. Therefore, the energy consumption is decided by the average size of the particles. Therefore, the particle size of a sheared waste metallic film resistor not only impacts materials liberation but also determines energy consumption. A waste metallic film sheared to smaller particle size will cost more energy. Magnetic Separation. A magnetic separator was employed for separating nickel particles from sheared waste metallic film resistors. A picture of the magnetic separator is given in Figure S2 in the Supporting Information. The magnetic separation process is presented in Figure 2a; nickel particle would experience a magnetic force and
MATERIALS AND METHODS
Waste Metallic Film Resistors. Waste metallic film resistors employed in this study were collected from waste PCBs of televisions provided by a licensed e-waste recovery plant (Yangzhou Ningda Noble Metal CO., LTD) in China. The weight ratio of resistors to waste PCB was about 0.5−1.2 wt %. Metallic film resistors were made of a ceramic core covered by a metal film together with two nickel pins. Compared to other resistors (such as carbon film resistors and winding resistors), metallic film resistors have excellent characteristics. Therefore, metallic film resistors were the only employed resistors in the manufacture of printed circuit boards. The structure of a metallic film resistor is presented in Figure 1. It is comprised of a ceramic 8235
DOI: 10.1021/acssuschemeng.7b01900 ACS Sustainable Chem. Eng. 2017, 5, 8234−8240
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Figure 2. Structure and the separation process of the magnetic separator.
Table 2. Shearing Results of Waste Metallic Film Resistors item waste metallic film resistors
shearing time (s)
liberation grade
particle size of metallic pin (mm)
particle size of cap (mm)
particle sized of ceramics matrix (mm)
shearing temperature (°C)
30
incompletely
2−38
2.3
6.5
25.3
45 60 75 90
incompletely completely completely completely
2−34 2−25 2−21 2−20
2.2 2.0 2.0 2.0
6.1 3.2 2.9 1.6
43.6 55.4 68.2 75.3
gravity force. Due to the magnetic force being greater than the gravity force, the nickel particle would attach to the conveyor surface of the magnetic separator until it escaped from the magnetic field of the separator and collected on the left side of the magnetic drum. Other particles were subjected only to gravity force and experienced horizontal projectile motion and collected on the right side of the magnetic drum. Magnetic materials can be easily separated from the sheared waste metallic film resistor if the acceleration ( f m) of the magnetism force is greater than the acceleration (g) of gravity. The acceleration of the magnetism force is expressed as26,27
fm =
1 xB grad B μ0
to get well-dispersed Al2O3 particles. Then, the Al2O3 particles were observed by SEM.
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RESULTS AND DISCUSSION Shearing Results of Waste Metallic Film Resistors and Magnetic Separation. Shearing results of waste metallic film resistors are presented in Table 2; the particle size decreased as the shearing time increased. When the particle sizes of the metallic pins and ceramics matrix were sheared below 25 and 3 mm, respectively, all of the component materials were liberated. The particle size of the cap was 2 mm and did not change in the shearing process. The temperatures of the shearing process were also measured. Table 2 indicates that the temperature increased with the shearing time. The temperature increased from 25.3 to 75.3 °C in the shearing process. When waste metallic film resistors were completely liberated, the temperature reached 55.4 °C. Temperature measurement results indicated that there was little risk in the shearing process of waste metallic film resistors because the temperature was not high enough for pyrolysis of other organic materials. The sheared materials of waste metallic film resistors experienced magnetic separation. Separation results are presented in Table 3. The separation rate of magnetic separation was computed by the following equation
(2)
where f m is the magnetism force acceleration, μ0 is the permeability of vacuum (N/A2), x is the relative magnetic susceptibility of the particle (m3/kg), B is the magnetic flux density of the magnetic field (T), and grad B is the gradient of the magnetic flux density (T/cm). The relative magnetic susceptibility of nickel is greater than 7.5 × 10−5 cm3/g. We measured the magnetic flux densities in per 5 mm leaving from the separator surface in the vertical direction by a teslameter. The magnetic flux density (B) and density gradient (grad B) of the magnet were about 0.24 and 0.15 T/cm. The value of the vacuum permeability (μ0) was 4π × 10−7 N/A2. Therefore, calculation of f m gave 2293 N/ kg, which was greater than the gravity acceleration (9.8 N/kg). Thus, the employed magnetic separator can separate nickel materials from the sheared waste metallic film resistors successfully. Ball-Milling Process. A ball-milling machine was used to mill the Al2O3 particles separated from sheared waste metallic film resistors. The ball-milling machine is presented in Figure S3. The grinding balls and ball-milling tank are made of agate. The tank volume for ballmilling is 500 mL, equipped with mixed grinding balls with diameters of 5, 8, and 10 mm, the mass ratio of which is 6:3:1. In this experiment, the mass proportion of grinding balls and Al2O3 particles is 85:1, and the ethanol solvent as a dispersant is of the same mass as Al2O3 particles. After adding the materials, the tank was sealed. The rotation speed was 480 r/min and grinded 18 h. At last, an Al2O3 particles suspension was collected after natural cooling. The nanoparticles prepared by mechanical ball-milling are easy to agglomerate, and therefore, it was necessary to perform ultrasonic vibration for 30 min
η=
M × 100 (%) Mc
(3)
where M is the mass of nickel particles in the left collector of the magnetic separator and Mc is the total mass of nickel contained in waste metallic film resistors. The feed speeds of magnetic separation were set at 0.5, 1.0, and 2.0 m/s, respectively. The rotation speeds of the magnetic drum were set at 1.0, 2.0, 3.0, and 4.0 m/s, respectively. Because the magnetic force was much greater than the gravity force, the separation rates of nickel particles both reached 100 wt % at the 8236
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ACS Sustainable Chemistry & Engineering Table 3. Results of Magnetic Separation of Sheared Waste Metallic Film Resistors feeding speed (v, m/s)
rotation speed of magnetic drum (ω, m/s)
separation rates (wt %)
0.5
1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0 1.0 2.0 3.0 4.0
100
1.0
2.0
100
100
Figure 4. (a) Composition of the sheared ceramic matrix detected by XRD analysis. (b) XRD analysis results of standard Al2O3.
different feeding speeds and rotation speeds of the magnetic drum. We developed the recovery technology of nickel from waste metallic film resistors. The flowchart of the recovery technology is presented in Figure 3. Waste metallic film resistors were first sheared to liberate the comprised materials. When the particle size of the ceramics matrix was sheared to smaller than 3.2 mm, the constituent materials could be liberated completely. Then, magnetic separation was advised to separate nickel particles from the liberated materials. The separation rate reached 100 wt %. According to the proportion of composite materials in waste metallic film resistors, about 0.65 kg of nickel could be obtained from 1.0 kg of waste metallic film resistors. To improve the added value of recovering waste metallic film resistors, we detected the composition of residual particles by the method of XRD. The results were presented in Figure 4. Compared to the XRD analysis results of standard Al2O3, the composition of the ceramics particles was confirmed as Al2O3. Al2O3 has a lower price compared to metals. However, it accounted for 26.2 wt % of waste metallic film resistors. Al2O3 particles should be reused in a value-added way. Thus, valueadded reuse technology of Al2O3 should be developed. We advise employing milling technology to treat Al2O3 particles in order to obtain nano-Al2O3. Ceramic Al2O3 has low hardness and ductility so as to be easily grinded. Therefore, the ballmilling machine was employed to produce nano-Al2O3. The
ball-milling process is presented in Figure 5. The milled Al2O3 particles were investigated by the method of scanning electron microscopy (SEM). The SEM image of milled Al2O3 particles is presented in Figure 6. Figure 6 indicates that the particle size of nano-Al2O3 particles ranges from 100 to 500 nm. The particle size was not homogeneous. The amount of small-size particles is much more than that of large-size particles. The large-size particle was surrounded by the small-size particles. Increasing the milling time might improve the homogeneity of the particle size. The nano-Al2O3 material also had excellent characteristics of adsorption and strength.28 The ball-milling process caused no water pollution, air pollution, nor solid waste. Meanwhile, the whole ball-milling process was also a low-cost procedure. NanoAl2O3 is an important material for powder coating that will have excellent performance in heat conduction and insulation of electric charge. In general, it is produced by gasification by Al gas and oxygen. This production procedure is relatively complex and more expensive. According to the market value, the price of nano-Al2O3 is about 22.5 dollars/gram in China. Compared to the gasification process, milling is an environmentally friendly and low-cost technology to produce nanoAl2O3 from sheared waste metallic film resistors. Figure 7 shows that the distributions of elements in the nano-Al2O3 indicated that O and Al were the elements that
Figure 3. Flowchart of recovery nickel from waste metallic film resistors. 8237
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Figure 5. Ball-milling process of ceramic particles to produce nano-Al2O3.
Figure 6. SEM picture of nano-Al2O3.
Figure 7. Element analysis of the nano-Al2O3.
importance of mining resources from waste metallic film resistors, we analyzed the economic benefits of the proposed recovery technology. Recovery of waste metallic film resistors will gain abundant nickel resource. The economic benefits were analyzed from the aspects of energy cost, equipment cost, labor cost, and economic income. Energy costs of the recovery technology can be estimated by eq 4
accounted for the large proportion, 51.87 and 25.33 wt % respectively. EDS analysis indicated that Mg, Si, K, Ca, Ti, Fe, Cu, and Pb elements were contained in the nanoparticle. The proportions of Mg, K, Ca, Fe, and Cu elements were all less than 1.0 wt %. Proportions of Pb and Ti were less than 1.2 wt %. Si had a proportion of 15.5 wt %. Presentation of Si was generated from the milling balls. Si might not impact the quality of nano-Al2O3 because Si was also an ordinary material of ceramics. The elements of Al and O accounted for about 77.2 wt % of the nano-Al2O3 particle. We thought that ball-milling might also be an interesting technology to prepare nano-nickel in order to improve the added value of recovered nickel particles. On the other hand, the technology might cost more energy because the recovered nickel particles are strip-shaped and have good ductility. Nickel particles were more difficult to grind into nanoparticles than Al2O3. Analysis of the Economic Benefit of Recovering Waste Metallic Film Resistors. To show the benefits and
N = (Pt 1 1 + P2t 2 + P3t3) × A
(4)
where N is the energy costs of the recovery process, A is the price of industry electric charge, P1 is the power of the shearing machine, P2 is the power of the magnetic separator, P3 is the power of the ball mill, t1 is the shearing time, t2 is the separation time of the magnetic separationm, and t3 is the milling time. The capacity of the shearing machine is 5 kg/min, the capacity of the magnetic separator is 25 kg/min, and the capacity of the ball mill is 0.41 kg/h. The energy cost of the recovery technology was computed and is presented in Table 4. 8238
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According to the computed results from Table 4, the economic benefits of recovering waste metallic film resistors can reach 14065.34 dollars/ton by employing the proposed technology. The payback period (Pt) of employing this technology to recover waste metallic film resistors could be computed as
Table 4. Economic Benefit of Recovering 1000 kg from Waste Metallic Film Resistors Y = $14695
N = $609.58
Ss = $0.01 Sm = $0.03 Sb = $0.06 R = $19.98
item
value
m C1 C2 P1 P2 P3 t1 t2 t3 A Vs Vs min Vm Vm min Vb Vb min B n Y
1000 kg 10.3 $/kg 25 $/kg 1.3 kW 2.75 kW 0.75 kW 3.33 h 0.67 h 809.75 h 0.07 $/kWh $800 $80 $12000 $1200 $5500 $550 3 $/h 2 3.33 h
Pt =
η=
(5)
where Ss is the equipment cost of the shearing machine per hour, Vs is the price of the shearing machine, Vs min is the price of the obsolete shearing machine, Sm is the equipment depreciation cost of magnetic separation, Vm is the price of the magnetic separator, Vm min is the price of the obsolete magnetic separator, Sb is the equipment cost of the ball mill per hour, Vb is the price of the ball mill, and Vb min is the price of the obsolete ball mill. The labor cost for the recovery technology of waste metallic film resistors can be computed as25
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(6)
(7)
where Y is the economic income of recovering waste resistors, m is the mass of the waste metallic film resistor, C1 is the price of nickel particles, C2 is the price of nano-Al2O3, 0.65 is the mass proportion of a nickel−chrome alloy in waste resistors, and 0.32 is the mass proportion of Al2O3 in waste resistors. The economic benefit of recovering waste metallic film resistors can be computed as i = Y − N − Ss − Sm − S b − R
(10)
CONCLUSIONS
This paper presented the recovery technology of waste metallic film resistors. It consisted of shearing, magnetic separation, and milling. Waste metallic film resistors contained a 33.2 wt % ceramics matrix and 65.1 wt % nickel. The sShearing process was used to liberate the materials contained in waste metallic film resistors. Magnetic separation was advised to separate nickel particles from sheared materials. Abundant nickel resource can be recovered from waste metallic film resistors. The sheared ceramic was identified as Al2O3 by XRD, and they were milled into nano-Al2O3. The particle size of nano-Al2O3 ranged from 100 to 500 nm. The economic benefit for recovering waste metallic film resistors was analyzed based on energy costs, equipment depreciation cost, labor cost, and economic income. The results indicated that recovery of 1 ton of waste metallic film resistors will net 14065.34 dollars. This paper reported the environmentally friendly and value-added recovery technology of waste metallic film resistors. It also calls for practitioners of e-waste recovery to pay close attention to recovering waste metallic film resistors and avoid environmental pollution.
where R is the labor cost, B is the price of labor, n is the number of employed workers, and t is the working time. The economic income of recovering waste metallic film resistors can be calculated by the following equation Y = m × (0.65 × C1 + 0.32 × C2)
i × 100 (%) Y
According to the values of Table 4, the net profit ratio was about 95.72%. Recovery waste metallic film resistors will be a new industry in nickel resource recycling and nano-Al2O3 production. We expect that this economic benefit will cause more and more practitioners of the e-waste recovery business to pay close attention to waste metallic film resistor recovery and develop the new industry. The proposed technology is an environmentally friendly technology, and no polluted air, wastewater, and solid waste are caused. Meanwhile, recovery of nickel will reduce the secondary pollution of heavy metals in soil, air, and water of exploiting nickel mines. Compared to getting nickel resource from natural mines, a significant advantage of recovering nickel from waste resistors are the environmental benefits. As we know, mine mining has caused serious environmental pollution of heavy metals in soil, water, and plants. The environmental remediation cost of mine mining is incalculable.
Equations for computing the equipment depreciation cost of the recovery process are presented as eq 529
R=B×n×t
(9)
where a is the mass of recovered waste metallic film resistors per year (supposed as 15 ton, 1% of the mass of total waste metallic film resistors produced in China per year). According to the values of Table 4, the payback period of recovering waste metallic film resistors was about 0.09 year. The net profit ratio of employing this technology to recover waste metallic film resistors could be calculated by
i = $14065.34
⎧ (V − Vs min) ⎪ Ss = s 84600 ⎪ ⎪ ⎪ (V − Vm min) ⎨ Sm = m 86400 ⎪ ⎪ ⎪ S = (Vb − Vb min) ⎪ b ⎩ 86400
(N + Vs + Vm + Vb + R ) i×a
(8) 8239
DOI: 10.1021/acssuschemeng.7b01900 ACS Sustainable Chem. Eng. 2017, 5, 8234−8240
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01900. Eemployed shearing machine, employed magnetic separator, and employed ball mill (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel:+86 20 84113620. Fax:+86 20 84113620. ORCID
Jujun Ruan: 0000-0001-8194-2988 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51308488), the Science and Technology Programs of Guangdong province (2015B020237005, 2016A020221014), the Pearl River Star of Science and Technology (201710010032), and the Fundamental Research Funds for the Central Universities (17lgzd22). The authors are grateful to the reviewers who helped us improve the paper by many pertinent comments and suggestions.
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