Separating and Recycling Plastic, Glass, and Gallium from Waste

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Separating and Recycling Plastic, Glass, and Gallium from Waste Solar Cell Modules by Nitrogen Pyrolysis and Vacuum Decomposition Lingen Zhang and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: Many countries have gained benefits through the solar cells industry due to its high efficiency and nonpolluting power generation associated with solar energy. Accordingly, the market of solar cell modules is expanding rapidly in recent decade. However, how to environmentally friendly and effectively recycle waste solar cell modules is seldom concerned. Based on nitrogen pyrolysis and vacuum decomposition, this work can successfully recycle useful organic components, glass, and gallium from solar cell modules. The results were summarized as follows: (i) nitrogen pyrolysis process can effectively decompose plastic. Organic conversion rate approached 100% in the condition of 773 K, 30 min, and 0.5 L/min N2 flow rate. But, it should be noted that pyrolysis temperature should not exceed 773 K, and harmful products would be increased with the increasing of temperature, such as benzene and its derivatives by GC-MS measurement; (ii) separation principle, products analysis, and optimization of vacuum decomposition were discussed. Gallium can be well recycled under temperature of 1123 K, system pressure of 1 Pa and reaction time of 40 min. This technology is quite significant in accordance with the “Reduce, Reuse, and Recycle Principle” for solid waste, and provides an opportunity for sustainable development of photovoltaic industry. nitride (GaN) and indium gallium phosphorus (InGaP).8,9 According to a survey by U.S. department of the Interior, imports of gallium and GaAs wafer chips continued to supply almost all U.S. demand for gallium. Moreover, GaAs used in electronic components accounted for approximately 99% of domestic gallium consumption.10 Gallium, as an important strategic resource, has been categorized as one of 14 mineral resources by the European Commission in extreme shortage.11 The world reserve of gallium has been estimated to be 18 000 tones, which is merely one tenth of gold.12 In nature, gallium has no ores of its own at all; rather it occurs in trace and minor amounts in various associated minerals types, such as bauxite, zinc, tin, and tungsten ores.13,14 Hence, it has led to strong interest for recovery of gallium from wastes. At present, various researches have been developed to recycle gallium. Technologies include acid leaching,15 organic solvent,16,17 chemical precipitation, electrochemistry,18,19 and supercritical extraction20 etc. I.M. Ahmed21 proposed extracting method by Cyanex 923 (a mixture of four trialkylphosphine oxides) and Cyanex 925

1. INTRODUCTION The first solar cell was invented at Bell Laboratories in 1954. After the energy crisis of the 1970s, the products of solar cells started to be used in civilian fields. Converting the energy of sunlight into an easily usable form is one of the most attractive solutions to the shortage of fossil energy.1 Photovoltaic energy has been known as the cleanest energy in the 21st century. With the rapidly expanding of photovoltaic market, the output of solar cells is growing by about 28.14% a year in China.2 The solar cells have been widely used in transportation, communications, space, and other fields. Crystalline silicon has become an important and dominant semiconductor material in most of solar cells.3 Apart from Si wafer-based module, gallium arsenide (GaAs) which is a compound semiconductor has been used for decades to make ultrahighefficiency solar cells because of its advantages, including their high photoelectric conversion efficiency and excellent antiradiation performance.4,5 Figure 1 shows the general structure of solar cell modules and solar power chip. Solar cell modules are composed of tempered glass, plastics, solar power chip, and dorsal membrane.6,7 The key component for solar power chip is the single or triple p−n junction containing metallic compound. Rare metal gallium usually as the form of compounds exists in the solar power chip, such as gallium arsenide (GaAs), gallium © 2016 American Chemical Society

Received: Revised: Accepted: Published: 9242

April 9, 2016 July 14, 2016 August 8, 2016 August 8, 2016 DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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Environmental Science & Technology

Figure 1. Typical structure of the solar cell modules (cross-sectional view of panel materials and solar power chip).

2. MATERIALS AND METHODS 2.1. Materials and Chemicals. In this study, waste panel materials and wafer chips containing GaAs from solar cell modules, which obtained from Shangyi optoelectronic Co., Ltd., Changzhou were chosen as samples. The panel materials mainly consisted of tempered glass, plastics, and dorsal membrane. According to the researches of the structure of solar cell panels,6,7 the plastics in solar cell panels mainly were adopted EVA and PET resin as adhesive materials were applied in solar cell panels in the photovoltaic market. Their chemical structures were shown in Figure 2.

(bis(2,4,4-trimethylpentyl) octylphosphine oxide) in kerosene from hydrochloric acid medium to recycle Ga(III). Although these studies have focused on recycling gallium resource, environmental improvement are still challenging due to limitations on using large volume of acid/alkali/organic reagent with high concentration. Vacuum method is a clean technology for secondary metal production, which shows many advantages such as simple technological flow sheet, no or low environmental pollution, and low consumption of raw material and energy.22,23 Vacuum metallurgy is simplified and environmentally friendly compared to the traditional pyrometallurgical and hydrometallurgical processing because can avoid pollution of waste gas/water and consumption of reagent.24 Vacuum distillation and decomposition are the important parts of vacuum separation. Vacuum distillation has been widely applied to metal purification and alloy separation as well as reduction of metallic ore deposits.25,26 Similarly, vacuum decomposition is mainly applied to thermal decomposition of compounds under vacuum condition to recycle metals. In our previous study, vacuum method has successfully separated indium from waste liquid crystal display panel27,28 and recycled copper, lead, and zinc from waste printed circuit boards.29,30 Generally, before recycling glass and solar power chip, plastics which made up the main organic parts of solar cell panels should be disposed first. Pyrolysis process is a quite promising technology for resource and energy recovery compared with combustion and solvent leaching,31−33 because of two significant advantages: (1) pyrolysis tends to be more effective than solvent leaching; (2) environmental damages are low due to avoiding the pollution from combustion gas and the waste organic solvent. 99.77% of organic matters were removed and a yield of 78.23% acetic acid was obtained by pyrolysis process for recycling polarizing film of waste liquid crystal display panels in the study of Wang.34,35 But by now, reports on disposing of plastics from solar cell modules by pyrolysis technology were rare. Hence, the aim of this study is to explore the feasibility of N2 pyrolysis to deal with plastics from solar cell modules in aspect of environmental impact and recycling efficiency. Meantime, a vacuum decomposition process is proposed to recycle gallium. The principle of separation and operating parameters for N2 pyrolysis and vacuum decomposition are also studied. In short, this study provides a green, nonpolluting, and efficient way to recycle waste solar cell modules.

Figure 2. Structure of major components for plastics in panel materials.

The chemical components and morphology of waste wafer chips are shown in Supporting Information (SI) Table S1 and Figure S1. We can find that the main chemical components in waste wafer chips were Ga and As, which the concentration of GaAs exceeded 99%. The impurities of Cu, Fe, and Zn were found in it, according to SI Table S1. 2.2. Exploratory Experiment. N2 pyrolysis and vacuum decomposition experiments were carried out in a laboratoryscale reactor, which consisted of four sections: tubular electric resistance furnace, control panel, condenser and vacuum pump. Figure 3 presented a schematic diagram of experimental equipment. First, in order to study the pyrolysis characteristics of panel materials from waste solar cell modules, Several panel materials from waste solar cell modules were crushed by a physical disaggregation and sample powders was mixed uniformity. The size of each solar cell modules is that length is 70 cm and width is 60 cm. These waste solar cell modules are currently found in the waste stream. Then, 5 g of sample powders were placed in the quartz boat and the reactor was sealed. Nitrogen was passed through the reactor as shielding gas at 0.5 L·min−1 for 5 min before the sample fed to expel air in the system. The reactor was adjusted to setting temperature 9243

DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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Environmental Science & Technology

Figure 3. Schematic diagram of the N2 pyrolysis and vacuum decomposition.

Figure 4. Flow sheet of a process proposed for recycling of waste solar cell modules.

recovery situation. Recovery efficiency of gallium was calculated by the following eq 2:

and quartz tube was heated. The heating rate of tube furnace is 30 K/min. The preheating time (time passed to achieve the required temperature) arriving 773 K was 17 min. The pyrolysis lasted 30 min to ensure complete reaction. The pyrolysis oil and gas were collected by water circulation condenser. After the N2 pyrolysis stage, the plastic in panel materials was stripped away. The solid residues were weighed again to calculate the weight loss of organic components. The recycling effect of plastic in panel materials was characterized by organic conversion rate, which was defined as eq 1. The mass of raw organic material was calculated by weight loss of organic matters through TGA-DSC measurement.

recovery efficiency(%) content of raw material − content of residue = × 100 content of raw material

(2)

Assuming that the metal atoms do not have any collision in the evaporating process, evaporation kinetics of metallic Ga was calculated by the theoretical and experimental evaporation rate ωMe which is given by the Langmuir-Knudsen eq 3: * ωMe = 2.624 × 10‐2 × α × pMe

M (g · cm ‐2 ·min ‐1) T

(3)

Where p*Me is the saturated vapor pressure, Pa; M is the molecular weight; T is the temperature, K; α is the probability of the metal molecule remains on the surface (taken as 1). Measurement data was expressed by the form of figure % ± standard deviation. Repetitive experiment of individual measurements was applied to ensure the accuracy of the entire experiment. In a word, a systematic recycling process for solar cell modules was presented in Figure 4. 2.3. Analysis Methods. 2.3.1. TGA−DSC Analysis. The thermal gravimetric analysis technique (TGA-DSC, Mettler Toledo, Shimadzu, Japan) was carried out to study the feasible of N2 pyrolysis for panel materials from solar cell modules.

organic conversion rate(%) mass of organics in raw material − mass of organics in residue = × 100 mass of organics in raw material

(1)

For vacuum decomposition process, waste wafer chips were grinded, and then were put into the furnace. When the furnace was preheated to setting temperature, quartz boat was pushed into the heating zone. The equipment was sealed by closing the valve on both ends of the tube furnace and vacuumed to the pressure of 1 Pa quickly by vacuum pump. After a period of reaction time, the samples were taken out and calculated 9244

DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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Figure 5. TGA and heat flow of (a) panel materials, (b) EVA, (c) PET, (d) DTG curves of panel materials, EVA and PET.

Figure 6. Effects of (a) temperature and (b) holding time on organic conversion rate for pyrolysis from panel materials.

2.3.3. Analysis of Quantification and Morphology. The quantitative analysis of gallium and arsenic in waste wafer chips before and after process was measured by inductively coupled plasma emission spectrometry (ICP-AES, IRIS Advantage 1000, THERMO, U.S.) after completely dissolved with a mixture 1:3 (v/v) of 37 wt % HCl and 68 wt % HNO3. The morphology of the products from vacuum decomposition was examined by Field-emission scanning electron microscopy and energy dispersive spectrometer (SEM Sirion 200 & EDS INCA X-Act, FEI Company, America & Oxford Company, England).

About 15 mg samples from the panel materials, EVA and PET were chosen to do the TGA-DSC analysis. The flow rate of nitrogen was 50 mL/min with purge gas and 20 mL/min balance gas. The samples were heated from 293 to 1073 K and the heating rate was chosen as 20 K min−1. 2.3.2. GC−MS Analysis. For the N2 pyrolysis process, the pyrolysis oil and gas products were collected and analyzed by gas chromatography−mass spectroscopy (GC-MS, TurboMass, PerkinElmer Corporation, U.S.). The GC was fitted with an rtx5 (30 m × 0.25 mm) column. Injection temperature: 473 K, transfer line: 523 K. The analysis was performed using 1 μL injections and high purity helium was used as the carrier gas at a flow rate of 1.24 mL/min.

3. RESULTS AND DISCUSSION 3.1. Nitrogen Pyrolysis with Panel Materials. 3.1.1. Feasibility of Nitrogen Pyrolysis. The TGA and the heat flow 9245

DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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Environmental Science & Technology (DSC) curves as well as DTG curves of panel materials, EVA and PET were showed in Figure 5(a-d) when the heating rate was 20 K min−1. Several results could be obtained: (i) the weight loss behavior of panel materials were complex, due to its components complexity. When temperature reached 590 K, the weight loss of panel materials began to emerge and the explanation could be attributed to the decomposition of plastic materials. Meantime, change of heatflow showed that the pyrolysis of panel materials was slightly endothermic; (ii) For the TGA curve of EVA, its pyrolysis temperature starts at approximately 580 K and ends almost at 790 K. For the TGA curve of PET, its pyrolysis temperature starts at approximately 580 K and ends almost at 723 K. The temperature of maximum weight loss was 673−790 K and 580−723 K and for EVA and PET, and the total weight loss rate was 98.2% and 98.69%. Both EVA and PET were obvious endothermic because pyrolysis reaction of EVA and PET need to absorb a large number of heat; In addition, pyrolysis process of panel materials, EVA and PET after 973 K, 790 and 723 K has ended from TGA curve, but DSC curves of them still present large endotherm after temperature of 950 K. The endothermic heatflow was not result from pyrolysis reaction of them, but alumina crucible placed samples in TGA experiment can absorb heat when the temperature exceeds about 923 K because its thermal radiation can be happened. Hence, the large endotherm in the DSC curve was shown in Figure 5(a−c) after 950 K; (iii) according to analysis of DTG curves (Figure 5(d)), the temperature ranges of two main weight loss for panel materials were 590− 673 K and 673−790 K, which were similar to the situation of weight loss of EVA. We are not hard to find that the curves of TGA, DSC, and DTG of plastic in panel materials were similar to that of EVA. It indicated that EVA was the main plastic components of panel materials. The detailed components and concentrations of pyrolysis products should be measured by GC−MS. As the above TGA−DSC analysis, nitrogen pyrolysis process to remove the plastics in solar cell modules is feasible. 3.1.2. Influence Factors on Nitrogen Pyrolysis. The effects of temperature and holding time on organic conversion rate were showed in Figure 6. Effect of Temperature. The effect of temperature on the organic conversion rate was investigated in the range from 573 to 1073 K at 0.5 L/min N2 flow rate lasting 30 min. It could be seen from Figure 6(a) that the organic conversion rate was low when temperature was 573 and 623 K, which was 19.61% and 36.02%, respectively. But when temperature reached 673 K, the organic conversion rate sharply increased and exceeded 98%. With the temperature over 773 K, the organic conversion rate reached approximately 100%. It indicated that the pyrolysis temperature of plastic components was 773 K. We analyzed the organic components of oil products from 773 to 1073 K. The results were summarized graphically in Figure 7(a). We found that the organic components of oil and gas products had obvious changes with the increasing of temperature. When the temperature reached 773 K, alkanes and olefins were main organic components in the oil products. But, some naphthenes, acetophenone, and methyl naphthalene, etc. began to be detected in pyrolysis oil products with the temperature rising to 873 K. When the temperature arrived 973 and 1073 K, anthracene, phenanthrene, and homologues of benzene were main components of pyrolysis oil products. For pyrolysis gas products from panel materials (as shown in Figure 7(b)), benzene rapidly increased with increasing of temperature. But, components of gas products were not change. Therefore, the

Figure 7. Change of component in (a) pyrolysis oil and (b) pyrolysis gas products under different temperature by measurement of GC−MS.

temperature of pyrolysis should be controlled under the condition of 773 K. Effect of holding time. The effect of holding time on the organic conversion rate was investigated in the range from 0 to 60 min was investigated at 0.5 L/min N2 flow rate and 673 K. As shown in Figure 6(b), the effect of holding time on the organic conversion rate was quite small. When temperature reached 673 K, almost all organic matters were transformed and recycled. 3.1.3. Products Analysis. Oil products in pyrolysis process under 773 K were analyzed by GC−MS and the detail data was shown in Table 1. First, pyrolysis oil of panel materials under 773 K was mainly long and straight chain of olefins, alkanes and its isomers was generated. The number of carbon atoms was between 15 and 30 for most of olefins and alkanes. The main components of olefins and alkanes were 1-tridecene, 1nonadecene, 1-tricosene, and 1-tetradecene. As a matter of fact, they were similar to that of pyrolysis oil products from EVA resin, which also contained a large number of olefins and alkanes components.36,37 However, the components were quite different for pyrolysis oil products at 773 and 1073 K. A large number of benzene derivatives were detected in the oil products at 1073 K (Supporting Information Table S2), such as phenanthrene, pyrene, 1-Methypyrene, 1,2-Benzanthracene. It is imply that these benzene derivatives can be synthesized through straight chain of olefins and alkanes under high temperature by some 9246

DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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thermodynamic data were obtained by thermodynamic handbook.41 The possible decomposition reaction of compound GaAs and corresponding standard Gibbs free energy were listed below eq 4-7:

Table 1. Main Compositions of Oil Products from Pyrolysis of Panel Materialsa reaction time (min)

area%

13.42 13.52 15.83 17.99 18.11 20.26 22.29 24.22 26.05 26.78

2.12 1.07 6.34 9.49 10.74 11.60 11.42 8.59 8.06 2.75

27.68 27.89 29.34 29.45 31.03 32.55 32.63 35.41

2.88 2.80 2.33 5.22 4.68 4.25 2.80 2.85

component 1-tridecene benzoic acid 1-tridecene biphenyl 1-tetradecene 1-octadence 1-nonadecene 9-nonadecene 9(E)-eicosene 1-[1,1′-biphenyl]-3ylethanone 1,19-eicosadiene 8-hexylpentadecane 1,19-eicosadiene 1-tricosene 1-heneicosyl formate 1-tricosene tetracosane 1-tetracosanol

molecular formula

GaAs(s) = Ga(l) + As(g); △r GT Θ

C13H26 C7H6O2 C13H26 C12H10 C14H28 C18H36 C19H38 C19H38 C20H40 C14H12O

= − 0.173T + 384.14, kJ(298 ∼ 1200K) GaAs(s) = Ga(l) + As2 (g); △r GT Θ = − 0.224T + 361.79, kJ(298 ∼ 1200K)

= − 0.288T + 515.17, kJ(298 ∼ 1200K)

= − 0.292T + 497.19, kJ(298 ∼ 1200K)

△r GT = △r GT Θ + RT ln(p /pΘ )

Table 2. Main Compositions of Gas Products from Pyrolysis of Panel Materialsa component

molecular formula

methane carbon dioxide ethylene water propene propane acetaldehyde 1-propene 1,3-butadiene 2-butene acetone 1-pentene cyclopropane benzene

CH4 CO2 C2H4 H2O C3H6 C2H6 C2H4O C4H8 C4H6 C4H8 C3H6O C5H10 C3H8 C6H6

(8)

Where R is the molar gas constant, T is the thermodynamic temperature, p is the partial pressure of the gas product, pΘ is the standard atmospheric pressure. According to the calculation of △rGT and △rGTΘ under vacuum and air condition, eq 5 is most prior for GaAs decomposition reaction. The order for GaAs decomposition reaction was eq 5 > eq 7 > eq 6 > eq 4, respectively. In other words, As2 (g) was generated first, and then was As4 (g), As3 (g), and As (g) in turn. It had been demonstrated in the dynamics simulation of GaAs decomposition from the study of Hu.42 With the increasing of temperature, the Gibbs free energy of reaction was gradually reduced. Under atmosphere condition, the temperature of decomposition reaction of GaAs generating As2 was 1615 K. However, when vacuum pressure reached 1 Pa, the temperature of decomposition decreased to 962 K. The temperature of other three reactions was more than 1300 K under vacuum pressure 1 Pa. Thus, we can predict by above principle that (1) the decomposition reaction of GaAs under vacuum condition had a great superiority compared with the atmosphere pressure; (2) arsenic in reaction products was mainly the chemical species of As2 according to thermodynamic calculation. For vacuum separation, the metal with high vapor pressure and low boiling point can be separate through distillation or sublimation from the other metals, and then be recovered, respectively. The relationship between vapor pressure and temperature can be obtained by the Clausius−Clapeyron eq (eq 9).

conversion process, such as Diels−Alder addition reaction and aromatization of olefins and alkanes etc.38−40 Meanwhile, we analyzed the components of pyrolysis gas under the condition of 773 and 1073 K. The results were listed in Table 2 and Supporting Information Table S3. It was showed

0.39 2.48 1.21 11.72 24.81 3.24 20.59 11.23 4.64 2.69 7.03 4.90 2.22 2.84

(7)

Under different pressure condition, the Gibbs free energy of reaction eq 4−7 at a certain temperature is calculated eq 8.

The oil products was collected by the pyrolysis of panel materials under 773 K.

area%

(6)

GaAs(s) = Ga(l) + As4 (g ); △r GT Θ

a

2.44 3.20 4.33 10.16 11.16 11.80 14.87 16.46 16.60 16.83 18.81 19.50 19.82 22.94

(5)

GaAs(s) = Ga(l) + As3(g); △r GT Θ

C20H38 C21H44 C20H38 C23H46 C22H44O2 C23H46 C24H50 C24H50O

reaction time (min)

(4)

* = AT −1 + BlgT + CT + D lgPMe

a

The gas products was collected by the pyrolysis of panel materials under 773 K.

(9)

Where P*Me is the vapor pressure of pure metal; T is temperature; A, B, C, and D is constants determined for each metal element. According to eq 9, the relationship between the vapor pressure and temperature for Ga and As was shown in Figure 8. Under vacuum condition, gallium and arsenic were separated easily because their saturated vapor pressure was different. Compared with gallium, arsenic was easy to evaporate at high temperature. Therefore, it is feasible to separate gallium and arsenic under vacuum condition in theory. 3.2.2. Influence Factors on Vacuum Decomposition. In order to research the migratory rule of gallium in vacuum decomposition reaction, the factors influencing the recovery efficiency of gallium in waste wafer chips were studied under a

that the components of gas products under 773 and 1073 K were similar, which were mainly short chain of olefins and alkanes, such as C2H4, C3H6 and C4H8 etc. Particularly, a significant difference on the content of these gas products was shown in results of GC-MS. When temperature reached 1073 K, the content of benzene in pyrolysis gas increased by 48.74% compared with gas products at 773 K. These pyrolysis oil and gas can be reutilized as clean energy. 3.2. Vacuum Decomposition with Waste Wafer Chips. 3.2.1. Feasibility of Vacuum Decomposition. Relative 9247

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temperature from 973 to 1023 K. The recovery efficiency reached 60.9% when the temperature was 1023 K, and then its rise began to slow. The recovery efficiency of gallium increased to 76.4% with the temperatures reached 1273 K. Figure 9(b) showed the theoretical and experimental evaporation rate of Ga particles. In theory, the evaporation rate of gallium should be increased with the increase of temperature, according to Langmuir-Knudsen eq (eq 3). However, the evaporation rate has not changed in our experiment. The experimental evaporation rate of Ga presented nearly linear relationship with temperature, which was range from 5.64 × 10−5 to 2.12 × 10−4. Its explanation may be that on the one hand, first, the decomposition reaction of GaAs is happened, which needed a high temperature and then metallic gallium can volatilize. The rate-determining step of this process is decomposition of GaAs. The decomposition situation of GaAs can directly affect the evaporation rate of the gallium. On the other hand, molecular collision theory can also be used to explain it. When metallic gallium is evaporated from reaction system, gallium atoms can be collide to residual air molecules resulting to the experimental evaporation rate is less than the theoretical evaporation rate.30,43 Effect of Vacuum Pressure. The relationship between the recovery efficiency of gallium and pressure was shown in Figure 9(c). The pressures in furnace were changed from 1 to 1× 105 Pa at 1123 K with reaction time of 40 min. The recovery efficiency of gallium sharply decreased with the increasing of residue pressure. This indicated that high vacuum degree is

Figure 8. Relationship between the vapor pressure and temperature for Ga and As.

laboratory scale. The three influencing factors of reaction were temperature, pressure and particle size, respectively. Effect of Temperature. The effect of temperature on the recovery efficiency of gallium was investigated in the range from 973 to 1273 K, maintaining system pressure of 1 Pa and reaction time of 40 min. As shown in Figure 9(a), the recovery efficiency of gallium increased sharply with an increase of

Figure 9. (a) Recovery efficiency, (b) evaporation rate of gallium under different temperature and effects of (c) pressure, (d) particle size on recovery efficiency of gallium in waste wafer chips from solar cell materials. 9248

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Environmental Science & Technology effective for recycling of gallium. For example, when the residue pressure was 1× 102 Pa, the recovery efficiency of gallium was 56.78%. But the recovery efficiency quickly dropped when the pressure increased to 1 × 103 Pa, only 9.8% of the gallium was recycled. Effect of Particle Sizes. The effect of particle size was investigated −0.3 mm, −0.3 + 0.6 mm, −0.6 + 0.8 mm, −0.8 + 1.5 mm. The experiments were conducted at 1023 and 1123 K with reaction time of 40 min and residue pressure of 1 Pa. Figure 9(d) showed that the influence of particle size for recovery efficiency of gallium. Under the condition of 1023 K, recovery efficiency of gallium gradually increases with the decrease of the particle size. When the particle size reached −0.3 mm, the recovery efficiency of gallium can reach 77.17%. However, the effect of particle size was diminished with the temperature reached to 1123 K. For example, when particle size was −0.3 + 0.6 mm, the recovery efficiency of gallium was only increased by 5.3% compared with particle size of −0.3 mm, which was 76.6%. 3.2.3. Products Analysis. The condensed products of GaAs from waste wafer chips were collected and analyzed by ICPAES under vacuum pressure of 1 Pa, heating time of 40 min and temperature of 1123 K. As shown in Figure 10, the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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 (51534005, 51278293 and 51178262).

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REFERENCES

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Figure 10. Distribution of condensed products for waste wafer chips and the corresponding ICP-AES and SEM analysis.

condensing zones can be divided into three parts. ICP-AES analysis indicated that gallium was mainly condensed on zone A with the temperature between about 323 and 615 K. Some particles attached to the tube wall, and the results showed that the relative content of gallium and arsenic is 90.20% and 9.80%, respectively. A transition area of separation of gallium and arsenide appeared in zone B. Followed zone B, arsenic was mainly condensed on zone C with the temperatures between 293 and 323 K. It is possible explanation that arsenic is easy to volatilize compared with gallium, and the molecule weight of arsenic is also less than gallium. Therefore, arsenic can be enriched the far distance of the condensing zones. From view of morphology, metallic gallium was roughly spherical particles, but condensing zone B and C presented shape of flake.

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DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250

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DOI: 10.1021/acs.est.6b01253 Environ. Sci. Technol. 2016, 50, 9242−9250