Recycling arsenic from gallium arsenide scraps through sulfurizing

Research Article pubs.acs.org/journal/ascecg

Recycling Arsenic from Gallium Arsenide Scraps through Sulfurizing Thermal Treatment Lu Zhan,*,† Jianguo Li,† Bing Xie,† and Zhenming Xu‡ †

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Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Science, East China Normal University, 500 Dong chuan Road, Shanghai 200241, China ‡ School of Environmental Science and Engineering, Shanghai Jiao Tong University 800 Dong chuan Road, Shanghai 200240, China

ABSTRACT: Due to its superior electronic properties, gallium arsenide (GaAs) is widely used in integrated circuits which are the core elements of most electric and electronic equipment. With the obsolescence of this equipment, a large amount of GaAs scraps is generated, which may possess potential threats to human beings and the environment if treated improperly. In this paper, an integrated process combining sulfurization and evaporation is proposed to recycle arsenic from GaAs scraps. The sulfides of arsenic can be easily evaporated and recycled. More importantly, the environmental requirements are satisfied because of the low toxicity of the arsenic sulfides. Using solid sulfur as the sulfurizing agent, 88.2% of arsenic can be extracted from GaAs scraps under the optimized conditions of 5 K/min heating rate, 453 K midsection temperature, 40 min midsection holding time, 1073 K final temperature, and 60 min corresponding holding time. The behavior of arsenic during the sulfurizing thermal process is discussed in details. After the instrument examinations of X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and X-ray phototelectron spectroscopy (XPS), the sulfurizing mechanism is explored and the reaction equation is deduced as 2GaAs + (2x + 3)S → 2AsSx + Ga2S3. This research can provide the theoretical foundation for recycling arsenic from GaAs scraps or other e-waste containing arsenic. KEYWORDS: Arsenic, Sulfurization, Recycle, Gallium arsenide, Scraps

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INTRODUCTION

products, light-emitting diodes (LEDs), and other electronic products.6 The rapid upgrade of electronic applications will lead to sharply increasing demand for arsenic, which may be a potential hazard to the environment. From several previous investigations,7−11 severe arsenic pollution, which threatens human health and the environment, occurred in the proximity of some places where e-wastes are disassembled. On the other hand, it is found that the proportion of metals in the e-wastes is around 40%, including gold, silver, palladium, copper, iron, and some others.12 These precious metals mostly exist in the integrated circuit (IC) chips and LEDs, where arsenic also exists.13 People usually recycle precious metals by hydrometallurgy or pyrometallurgy;14,15 however, arsenic does not obtain enough attention because of its low economic interest. Arsenic often is expelled in the effluent or dusts during the precious metal extracting.

At present, the electronic information industry has been advancing at a high speed, which results in the shorter service life of electronic products. Then a large amount of waste electrical and electronic equipment (WEEE) has been created, which is a serious potential threat to ecological environmental security.1,2 According to statistics, about 41.8 million tons of WEEE were generated around the world in 2014, and it is predicted that it will increase to 49.8 million tons in 2018 with a growth rate of 4−5% per annum.3,4 Arsenic plays an indispensable role in these electronic products. High-purity arsenic is usually applied to produce semiconductor materials such as gallium arsenide (GaAs), indium arsenide (InAs), and indium gallium arsenide (InGaAs). The electron mobility of these materials is five times that of silicon, which is the traditional semiconductor material. According to the information from United States Geological Survey (USGS), in 2014 around 34 tons of arsenic were employed to produce GaAs chips in US.5 In recent years, due to the excellent properties, semiconductor materials containing arsenic are widely used in the manufacturing of smartphones, computers, optoelectronic © 2017 American Chemical Society

Received: December 6, 2016 Revised: February 13, 2017 Published: February 18, 2017 3179

DOI: 10.1021/acssuschemeng.6b02962 ACS Sustainable Chem. Eng. 2017, 5, 3179−3185

Research Article

ACS Sustainable Chemistry & Engineering So far, numerous studies have been done to remove arsenic from solid wastes. Some researchers used strong acid/alkali/ oxidant to make arsenic leach into aqueous solutions, and then recycled arsenic in the form of precipitate via adding a variety of reagents.16−19 There are also some researchers developing technologies to remove arsenic in the form of elementary substance or oxide by high temperature calcination.20−22 However, some defects in these processes could never be ignored. The generation of waste liquid, waste gas, waste residue, and leachate have a detrimental influence on the environment and human health. In addition, hydrometallurgical technology needs a large number of reagents, which would increase the cost greatly. The traditional incineration process always needs a complex matched tail gas treatment device. Vacuum metallurgy separation (VMS) is considered to be a green and efficient method which is widely applied to the metal purification and alloy separation, including the treatment of arsenic.23−25 But, the complicated operations, high energy demands and the consumption require great expense. Additionally, Tunez et al.26 recycled gallium and arsenic in the form of chlorides. Although chlorides are easier to evaporate, the operating temperature is too low to apply to the treatment of WEEE containing plastic and resin. Among various compounds of arsenic, the toxicity of sulfide is the least. The applications of sulfurization for arsenic removal have been mentioned in published literatures. Luganov et al.27 investigated the removal of arsenic in high-arsenic-bearing ores with the method of sulfurization and roasting. An invention patent applied by Li et al.28 introduced an approach to remove arsenic from arsenic flue dusts by combining sulfurization and evaporation. Industrial-grade sulfur was utilized as the sulfurizing agent in the patent. However, to our knowledge, sulfurization is never involved in the disposal of e-wastes, let alone applied to the arsenic removal. In fact, removing arsenic by sulfurization is actually a promising method. For one thing, the sulfurizing agent which refers to solid sulfur in this study is very cheap and easily available. For another, the sulfides of arsenic have the advantages of low toxicity and low boiling points, which can ensure its better environmental properties. This technology has greatly reduced the potential arsenic pollution and achieved the reclamation of arsenic and gallium. Taking GaAs scraps as the research object, this paper applied sulfurization and evaporation to extract arsenic by using solid sulfur as the sulfurizing agent. The theoretical feasibility of recycling arsenic through sulfurization was analyzed. The influencing factors were discussed and optimized. Further, the sulfurization mechanisms were explored, and the reaction equation was deduced.

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Figure 1. GaAs scraps. system for about 10 min before heating. Since then, the whole experiments were under the nitrogen atmosphere. Then the furnace was heated to the preset temperature (named midsection temperature) to complete sulfurization at the heating rate of 5 K/min, which was determined by some pre-experiments. Second, the temperature was further raised to another preset temperature (named final temperature) with the purpose of evaporating the generated arsenic sulfides. Next, the condensed products were heated to remove the superfluous sulfur. Finally, the condensate was scraped off the tube wall and collected for further instrument examinations. Analysis. The concentrations of arsenic and gallium in initial samples and residues are examined by inductively coupled plasma optical emission spectroscopy (ICP, ICAP6300, Thermo Electro, U.S.A). The elemental distribution of the condensate peeled from the quartz tube was examined using a scanning electron microscope with X-ray energy dispersive analysis (SEM, Hitachi S-4800, Japan). The phase of the condensate was characterized by X-ray diffraction (XRD6100, SHIMADZU, Japan) with Cu Kα radiation, operated at 40 kV and 25 mA over an angle of 20° < 2θ < 80°. The element species in the residues are analyzed by X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD, Japan).

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RESULTS AND DISCUSSIONS Theoretical Feasibility. Sulfurization. In general, the sulfides of arsenic and gallium exist in the forms of As2S3, As4S4, and GaS, Ga2S3, respectively. The possible reactions between sulfur and GaAs are listed in eqs 1−4.

MATERIALS AND METHODS

4GaAs + 10S = As4S4 + 2Ga 2S3

(1)

2GaAs + 6S = As 2S3 + Ga 2S3

(2)

2GaAs + 5S = As 2S3 + 2GaS

(3)

4GaAs + 8S = As4S4 + 4GaS

(4)

Standard Gibbs free energy changes (ΔG) of the reactions mentioned above were calculated by HSC Chemistry 6.0. Figure 3a shows the relationship between ΔG and the temperature. It is concluded that the reactions are feasible within the temperature range studied in this paper. With a decreasing temperature, the spontaneity becomes greater. This reflects a superiority of sulfurization, which indicates that the reactions can be done at a relatively low temperature. Evaporation and Separation. After the sulfurization, the arsenic sulfides should be recycled through evaporation and condensation. According to Clausius−Clapeyron equations,29 the relationships between saturated vapor pressure of the sulfides of arsenic, the sulfides of gallium, and the temperature are shown by eqs 5−8.

Materials and Apparatus. GaAs scraps (Figure 1) are adopted as the research objects in this paper. Inductively coupled plasma optical emission spectroscopy analysis indicated that the components of the scraps contained As 47.8%, Ga 47.2%, and others 5%. Figure 2 presents the main apparatus for the sulfurization and condensation. The quartz tube is 1000 mm long with an external diameter of 50 mm and thickness of 5 mm. The heating zone is about 150 mm long and the temperature can reach 1773 K. The two sides of the quartz tube outside the furnace are used as the condensing zones. Two conical flasks surrounded by cold water are connected with the tube, which can facilitate the condensation and collection of superfluous sulfur. Methods. At first, the GaAs scraps were pulverized and thoroughly mixed with excessive sulfur. The nitrogen gas flowed through the 3180


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ACS Sustainable Chemistry & Engineering

Figure 2. Single temperature horizontal tubular furnace system.

Figure 3. Related schematic diagrams.

Figure 4. Relationship between weight loss rate and each influencing factor.

lg p(As 2 S3) = −

4307 + 9.38 T

(5)

lg p(As4 S4 ) = −

6238 + 13.46 T

(6)

lg p(GaS) = −

3605 + 7.17 T

lg p(Ga 2S3) = −

17672 + 14.93 T

holding time, and the final temperature are recognized as the main influencing factors. In this section, the effects of these factors on the arsenic sulfurization and evaporation were investigated. Meanwhile, the influencing factors were optimized. The calculation of the weight loss rate was based on eq 9.

(7)

R=

M1 − M 2 × 100% M1

(9)

Where, R is the weight loss rate of materials in the crucible. M1 and M2 are the weights of initial and residual materials in the crucible, respectively. The initial materials here do not include solid sulfur, but only include the GaAs scraps. Using eq 9, there are two assumptions as follows: (1) GaAs and the sulfides of gallium (GaS/Ga2S3) will not evaporate during the heating process. (2) All the superfluous sulfur are evaporated and separated from the residues. According to eqs 1 and 2, the maximum of the weight loss rate will be 18.6% if all the As in GaAs is combined with S and the generated Ga2S3 are all evaporated from the residues. As for eqs 3 and 4, the value will be 29.7% if all the As is combined

(8)

Where, T is the temperature (K), and p is the corresponding saturated vapor pressure. Figure 3b was plotted according to the equations above. It shows that the boiling points of As2S3 and As4S4 are about 973 and 750 K, respectively, while the evaporation temperature of the sulfides of gallium (GaS/Ga2S3) are much higher than 1400 K, which means that they are very stable under the studied condition. Hence arsenic can be removed from scraps in the form of sulfide successfully. Optimizing the Influencing Factors. Based on the above theoretical analyses and the results of a series of preexperiments, the midsection temperature, the midsection 3181


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In order to detect the change of As concentration, inductively coupled plasma optical emission spectroscopy was utilized after digestion experiments. In the sulfurization experiment, the weights of initial GaAs scraps and its corresponding residues were 1.5135 and 1.2412 g, respectively. In the digestion experiment, 0.3023 g GaAs scraps and 0.3026 g residues were taken. Based on related data from Table 1, the efficiency of arsenic removal was calculated by eq 10.

with S and the generated GaS are all evaporated and removed from the residues. The difference between them is the form of the sulfide of gallium. It should be pointed out that the weight loss rate was only used to judge the arsenic removing trend. The accurate removing efficiency was calculated through the examination of ICP in the section Efficiency Calculation and Discussion. Effect of the Midsection Temperature. Sulfurization is an important step for the subsequent removal of the arsenic sulfides, while the midsection temperature is the important factor for this step. According to a sequence of preliminary experiments, the solid sulfur melted at roughly 393 K, but did not evaporate. The evaporation of sulfur is obvious at 513 K. So the midsection temperature range was set between 393 and 513 K. While the corresponding midsection holding time and final temperature were fixed to be 60 min and 1073 K, respectively. Figure 4a shows the results of the weight loss at 393, 423, 453, 483, and 513 K of the midsection temperature, respectively. The weight loss rate increased a lot between 393 and 453 K and then decreased a little with the follow-up increase of the temperature. Solid sulfur melted with little evaporation below 453 K. It was believed that the GaAs scraps were completely contacted with sulfur. The increase of the temperature provided favorable conditions for sulfurization due to the promotion of the molecular thermal motion. Within this temperature range, the weight loss rate increased from 11% to 16%. When the temperature was higher than 453 K, the weight loss rate changed inversely with the temperature. The trend could be ascribed to the vast evaporation of sulfur. Under the operation condition, sulfur evaporated remarkably. Much sulfur has been evaporated out of the crucible before the sulfurization occurred. It limited the reaction seriously. Therefore, 453 K was the preferred midsection temperature. Effect of the Midsection Holding Time. The time of 20, 40, 60, and 80 min were maintained respectively for each experiment. The midsection temperature was 453 K, and the final temperature was 1073 K. Figure 4b shows the effect of the midsection holding time on the weight loss rate. There was a sharp growth in initial 40 min and the weight loss rate reached 16% which was also the maximum value in this group of experiments. After that, the weight loss rate tended to be almost constant. It indicates that the sulfurization ended after heating for 40 min. Therefore, 40 min was chosen as the midsection holding time. Effect of the Final Temperature. The final temperature range was between 873 and 1173 K, with the increment of 100 K. The holding time was 60 min. This group of experiments were carried out at the midsection temperature of 453 K for 40 min of the midsection holding time. As shown in Figure 4c, there was a sharp weight loss when the temperature was elevated from 873 to 1073 K. The weight loss rate reached 16%. Then the temperature continued to attain to 1173 K, but the loss rate had nearly no change, which indicates that the process of evaporation had completed. Hence 1073 K was adopted as the final temperature. Efficiency Calculation and Discussion. The experiment was carried out under the optimized parameters, which is 5 K/ min heating rate, 453 K midsection temperature, 40 min midsection holding time, 1073 K final temperature, and 60 min corresponding holding time. The residues were collected and analyzed.

R=

1.5135 0.3023

× 2.893 − 1.5135 0.3023

1.2412 0.3026

× 0.4167

× 2.893

× 100% = 88.2% (10)

Table 1. Concentration of As and Ga in Initial Scraps and Residues item

As (g/L)

Ga (g/L)

digestion solution of initial GaAs scraps digestion solution of the residues

2.893 0.417

2.857 3.403

With the efficiency of 88.2%, this method can be considered to be an efficient way to remove arsenic. But it is still necessary to make an improvement. From the authors’ perspective, the key point is to overcome the passivation on the surface of GaAs scraps given by sulfur.30−33 When the sulfur atom adsorbs on the surface of GaAs, it can induce the breakdown of the bond Ga−As. And then the bonds S−As and S−Ga generate. The surface structure of GaAs scraps is changed, which hinders the reaction between sulfur and GaAs inside the scraps. The measure adopted in this paper is to pulverize the scraps as far as possible. But with the limitation of the apparatus, it is impossible to make the reaction complete thoroughly. So in order to promote the efficiency of arsenic removal, minimizing the scraps size or modifying the surface characterization of GaAs scraps before the sulfurizing thermal treatment is worth further investigating. Inference of the Reaction Equation. Analysis of the Condensate. Under the optimized conditions mentioned above, the condensates after sulfurizing treatment and the further treatment of sulfur removal are showed as Figure 5. As shown in Figure 5a, there were at least two kinds of substances on the wall because of their obvious discrepancy in color. And, a visible boundary between them existed. When the deep yellow one was put into the carbon sulfide, it dissolved in

Figure 5. Condensate before and after sulfur removal. 3182


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ACS Sustainable Chemistry & Engineering it. Through comprehensive analyses, it could be deduced that it was sulfur. Then the quartz tube was heated for a while, which aimed to evaporate the sulfur. The phenomenon on the wall of the tube was showed in Figure 5b. There was only the light yellow one left. The condensate was scraped off the wall. The reflective surface could be observed in Figure 5c. Additionally, it did not dissolve in carbon sulfide. These phenomena further demonstrated its difference from sulfur. As shown in Figure 6, there was a wide dispersion peak in XRD analysis, indicating that the condensate has a poor

Figure 7. XPS survey spectra: wide scan.

out on the main peak, which referred to Ga 2p3/2. The peak was split into two Gaussian-Lorenzian peaks appearing at 1116.8 and 1117.8 eV, which corresponded to Ga in GaAs and Ga2S3, respectively, in accordance with the data from XPS standard database and the published report.37 The existence of GaAs is also in agreement with the above analysis of As. Reaction Equation. Based on the analyses of the condensate and the residues respectively, the reaction between sulfur and GaAs can be expressed as follows:

Figure 6. X-ray diffractogram of the light yellow one.

2GaAs + (2x + 3)S → 2AsSx + Ga 2S3

crystallinity, which is the characteristic of arsenic sulfides.34,35 The condensate on the wall of the tube is a mixture of various kinds of arsenic sulfides, including As4S3, AsS, As4S4, As2S3, and As2S5. Analysis of the Residues. The X-ray photoelectron spectroscopy (XPS) technique was applied to determine the valence state of As and Ga. The carbon 1s photopeak with the binding energy of 284.6 eV was employed to calibrate the spectrometer. A monochromatized Al Kα X-ray source at 150 W was used for the analysis. Figure 7 displays the XPS survey spectra of the residues. The high resolution XPS spectra of As 3d and Ga 2p are presented in Figure 8 after curve fitting procedure. As shown in Figure 7, compared with few C, O, As, and other impurities, the residues mainly contain S and Ga elements. The XPS spectrum of As 3d is showed in Figure 8a. The peak position is located approximately 41.5 eV. In the previous study,36 the binding energies of As in GaAs was reported to be about 41.1 eV, which is close to the above value. What’s more, according to the standard database of XPS, within the range of the binding energy of 41.5 ± 0.5 eV, it can be found that arsenic exists in the forms of As(0) or GaAs. However, it is impossible for As(0) to remain in the residues under the experimental conditions. As(0) would evaporate at about 886 K while the temperature in the experiments reaches 1073 K, which is much higher. In addition, the corresponding holding time is 60 min. Therefore, arsenic in the residues cannot be As(0). It exists as GaAs, which means that some GaAs scraps still remain. Figure 8b is the XPS spectrum of Ga 2p. Two peaks can be observed: one at 1116.67 eV ascribed to Ga 2p3/2, while the other at 1143.47 eV due to Ga 2p1/2. The peak area ratio of Ga 2p3/2 and Ga 2p1/2 was 2:1. Peak-fit processing was carried

(11)

This equation is about the reaction between sulfur and GaAs. According to a series of experiments under different conditions, arsenic was converted into kinds of corresponding sulfides, while gallium was converted into Ga2S3. This equation can be considered stable. However, if some other elements are brought in the reaction, the equation or the mechanism may not be suitable. E-wastes, for an example, always contain various organics. For the e-waste containing GaAs, the reaction equation may be different from the above one during the sulfurization process. Gallium may be oxidized by organics and converted into other valence states besides Ga2S3. Therefore, it should be investigated further more in the future.

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CONCLUSION AND POTENTIAL FUTURE RESEARCH The integrated technology combining sulfurization and evaporation to remove arsenic from GaAs scraps is feasible which has been verified by theoretical analyses and experiments. The low toxicity of arsenic sulfides which were finally recycled assures its better environmental characteristics, which is the prominent advantage of this technology. Recycling arsenic from GaAs scraps in the form of sulfides can also promote the sustainable development of the GaAs production industry. However, there are still some aspects which can be further investigated. As we all know, e-wastes always have complex structures and components. Arsenic in e-wastes is encapsulated by plastic and resin in many cases. The mechanism of sulfurization and the effect of organic matter on sulfurization should be further explored. 3183


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Figure 8. XPS of the As 3d and Ga 2p binding energy region.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 21 54341064. Fax: +86 21 54747495. E-mail: [email protected]. ORCID

Lu Zhan: 0000-0002-2988-1302 Zhenming Xu: 0000-0002-4605-9409 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partly supported by the National Natural Science Foundation of China (21677050), ‘‘Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG21), and State Key Laboratory of Pollution Control and Resource Reuse Foundation (PCRRF16005). The authors are grateful to the reviewers who help us improve the paper by many pertinent comments and suggestions.

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Recycling arsenic from gallium arsenide scraps through sulfurizing

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