Kinetic and Mechanism Studies on Pyrolysis of Printed Circuit Boards

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Kinetic and Mechanism Studies on Pyrolysis of Printed Circuit Boards in the Absence and Presence of Copper Wei Liu,† Jiaqi Xu,† Junwei Han,*,†,‡ Fen Jiao,† Wenqing Qin,†,‡ and Zhuzhang Li† †

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School of Minerals Processing and Bioengineering, Central South University, Peace Building, No. 101, Changsha 410083, People’s Republic of China ‡ Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, People’s Republic of China ABSTRACT: The kinetic and mechanism of pyrolysis for Cu-free printed circuit board (CFPCB) and Cu-coated printed circuit board (CCPCB) were studied using nonisothermal thermo-gravimetry (TG) analysis coupled with Fourier-transform infrared (FTIR) spectrometry. TG and DTG (differential thermo-gravimetry) analyses were performed to study the mass loss characteristics. Kinetic analysis adopted the Ozawa−Flynn−Wall (OFW) model to confirm the reaction series by the variation pattern of activation energy. The real-time detection technique was used into three-dimensional (3D) FTIR analysis for providing the composition of evolved gases. The results indicated that the pyrolysis reaction of CFPCB can be divided into four separate stages. Stage I (α < 0.075) was ascribed by the breakage of N-containing crosslinkages with low boiling volatiles formed. The irregular cleavage of chemical bonds occurred in Stage II (0.075 < α < 0.85) with the decomposition of both brominated and nonbrominated compounds. In Stage III (0.85 < α < 0.9), the decomposition of nonbrominated compounds was proceeded with the small molecules continuously released. The pyrolysis reaction was almost completed in Stage IV (α > 0.9) with chars formed and few small molecules released. For the CCPCB pyrolysis, only two reaction stages were observed. Complex reactions with the decomposition of both brominated and nonbrominated compounds were observed in Stage I (α < 0.85), with various organic and inorganic components formed. In Stage II (α > 0.85), chars were formed and few small molecules were released. The presence of copper affected the reaction rate and product components during the pyrolysis of PCBs, resulting in shorter reaction time, lower activation energy and fewer brominated pollutants. However, the presence of copper also posed a difficulty for the efficient separation of evolved components and cleaning treatment of tail gases. KEYWORDS: Printed circuit boards, Brominated epoxy resins, Pyrolysis, Thermal decomposition, Kinetic study

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INTRODUCTION Printed circuit board (PCB), a mixture of organic materials, metals and glass fibers, is one of the most essential components of every electrical and electronic equipment.1,2 With the progressively technological innovation and rapidly short product lifecycle, millions of tons of waste PCBs, which are considered both an attractive secondary resources and an environmental contaminant, are generated every year in the world.3,4 In the past decades, many countries disposed of waste PCBs by simply open burning and acid treating to obtain the valuable metals at an alarming rate. Since the low efficiency of recycling and the emissions of hazardous materials such as dibenzo-p-dioxins (PBDD) and dibenzofurans (PBDF), the concerns were not only the waste of resources but also the environmental threats.5−8 For catering to the sustainable development of circular economy, ecological footprint and zero waste, a number of pyrometallurgical,9−11 hydrometallurgical11−13 and mechanical-physical14−16 processes have been extensively performed to recycle valuable materials from waste PCBs. Although lots of achievements have been made, valuable materials in WPCBs have still not been © XXXX American Chemical Society

completely recycled due to the presence of some technical and economical drawbacks. Pyrolysis has recently received much attention as a promising technology for recycling all the valuable materials in waste PCBs, especially for organic compounds.17 In this process, waste PCBs are converted into gas, liquid and solid products under oxygenfree atmosphere through thermal decomposition.18 Currently, many investigations have been devoted to analyzing the pyrolysis products and controlling the brominated pollutants.19−22 Jie et al.23 investigated the yields of pyrolysis products from waste PCBs under nitrogen atmosphere at 300−700 °C. The results indicated that the yields of solid, liquid and gases were about 75−80%, 5−9% and 12−14%, respectively. Moreover, no significant alteration of yields was observed over 500 °C. The solid residues could be easily liberated for metals recovery and glassfibers reuse, while the liquid and gas products might serve for Received: July 16, 2018 Revised: October 17, 2018 Published: October 19, 2018 A

DOI: 10.1021/acssuschemeng.8b03382 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering energetic purposes. Blazsó et al.24 studied the way to reduce the yields of brominated pollutants through copyrolysis of flame retarded polymers with sodium hydroxide and basic zeolites. A depressed brominated phenol formation was observed through copyrolysis. However, the reasons presented are complicated and remain to be resolved. The majority of studies on the pyrolysis of waste PCBs have been restricted to the investigation in product utilization and process optimization.25,26 Scarce information was reported on the fundamental kinetic and mechanism studies during the pyrolysis process of PCBs, and such a process can be ascribed to the decomposition of organic compounds in PCBs. Therefore, the kinetic and mechanism investigations of BERs (a widely used organic compounds in PCBs) are essential. Additionally, copper is the most common and extensive metals added into PCBs, thus the effect of the copper presence in PCBs during pyrolysis on reaction extent and product constituents, particularly on emissions of PBDD/Fs, still needs to be fully understood. The aim of the present study is to investigate the pyrolysis kinetic and mechanism of PCBs in the absence and presence of copper. TG and DTG analysis were performed to describe the pyrolysis behaviors. Kinetic analysis adopted the Ozawa− Flynn−Wall (OFW) model to confirm the reaction series by variation pattern of activation energy. The real-time detection technique was introduced into the three-dimensional (3D) FTIR analysis for providing useful information on the composition of evolved gases. A comprehensive study of the qualitative and quantitative information is obtained on the pyrolysis process to present the mechanism of PCB and to discuss the influence of the copper presence.

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PANalytical Axios Max) and Cu content was measured by inductively coupled plasma spectroscopy (ICP, Thermo ICAP7400 Radial). The elemental analysis data of samples are shown in Table 1.

Table 1. Elemental Analysis of Samples Elemental analysis (%) sample

C

H

O

N

Br

Cu

CFPCB CCPCB

33.36 20.76

2.68 1.95

3.46 2.52

0.62 0.45

23.69 17.27

− 15.1

TG-FTIR Analysis. Thermogravimetric analysis (TGA, PerkinElmer STA8000) with evolved gases analysis by Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum Two) was employed to identify the thermal decomposition characteristics of samples. In this study, approximately 18 mg of sample was heated in nitrogen atmosphere. The TG-FTIR analysis of samples was carried out from 30 to 800 °C at heating rates of 5, 10 and 20 °C/min under nitrogen atmosphere, and the flow rate of nitrogen was regulated to 50 mL/min. The gas products released from the pyrolysis process in TG were swept immediately to a cold gas cell of the FTIR spectroscopy. The transfer line and gas cell were heated to 200 °C to prevent condensation of the evolved gases. The FTIR spectra were measured 10 times per minute with a resolution at 4 cm−1, and the spectral region at 500−4000 cm−1.

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RESULTS AND DISCUSSION TG and DTG Analysis. Figure 2 shows TG and DTG curves of the CFPCB pyrolysis under nitrogen atmosphere from 30 to 800 °C at heating rates of 5−20 °C/min. The pyrolysis characteristics of CFPCB are presented in Table 2. The main temperature range of the CFPCB pyrolysis is approximately 220−540 °C. Maximum degradation temperatures are observed at peak 1 (264, 277 and 295 °C at heating rates from 5 to 20 °C/min) and peak 2 (370, 380 and 403 °C at heating rates from 5 to 20 °C/min). The weight loss at the final temperature is around 44%. It can be observed that the TG curves and DTG peaks are moved from low temperature to high temperature as the heating rate increases. The pyrolysis reaction retardancy is caused by heat transfer limitation.27 The pyrolysis of CFPCB can be divided into four separate stages, which is shown in Table 3. The first part of CFPCB pyrolysis is Stage I, which ranges from 221 to 280 °C, 240−296 °C and 260−325 °C at heating rates from 5 to 20 °C/min. The weight loss of CFPCB in this stage is about 4% relating to the low boiling volatiles (HBr, CO, CO2 etc.) released. The low boiling volatiles are generated for the elimination reaction of side chains of BERs in CFPCB.28,29 Stage II corresponds to the temperature range of 280−455 °C, 296−467 °C and 325−493 °C for heating rates from 5 to 20 °C/min. The reason for this rapid pyrolysis reaction at Stage II is that the high boiling volatiles (bromobenzene compounds, phenols, aromatics, alcohols etc.) are released from the decomposition of BERs in CFPCB.30 The third part of CFPCB pyrolysis is Stage III relating to the slow pyrolysis reaction of CFPCB with the temperature range of 455− 511 °C, 467−520 °C and 493−543 °C for heating rates from 5 to 20 °C/min. The high boiling volatiles are continuously generated in the slow pyrolysis stage while the compositions of volatiles may be different to a certain degree.31 Stage IV takes place in hightemperature range of 511−800 °C, 520−800 °C and 543−800 °C for heating rates of 5−20 °C/min. The thermal decomposition of BERs is basically completed prior to the beginning of the Stage IV, but there are still chars and some small molecules like HBr formed.32 Figure 3 shows TG and DTG curves of the CCPCB pyrolysis under nitrogen atmosphere from 30 to 800 °C at heating rates of

EXPERIMENTAL SECTION

Materials. The desired Cu-free PCB (CFPCB) and Cu-coated PCB (CCPCB), without other electronic components, were obtained from local manufacturers (Hunan, China). The major organic components of the PCB are brominated epoxy resins (BERs, IV), which were prepared by a mixture of diglycidyl ether of bisphenol A (DGEBA, I) and diglycidyl ether of tertabromobisphenol A (DGETBBA, II) with nitrogen containing hardener dicyandiamide (DICY, III). The chemical structures of DGEBA, DGETBBA, DICY and BERs are shown in Figure 1.

Figure 1. Chemical structures of DGEBA, DGETBBA, DICY and BERs. Such BERs were widely used in advanced technological applications so that the samples were universal and typical. Two different samples were milled to fine dust in the liquid nitrogen for subsequent elemental and TG-FTIR analysis. Carbon, hydrogen, oxygen and nitrogen analyses were determined by elemental analyzer (Elementar, Vario EL III). Br content was quantified by X-ray fluorescence spectroscopy (XRF, B


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Figure 2. TG (a) and DTG (b) curves of CFPCB pyrolysis at heating rates of 5, 10, and 20 °C/min.

formed. There are three main impacts due to the presence of copper from TG and DTG analysis. First, the presence of copper increases the transfer rate resulting in the increase in maximum value of weight loss rate and the reduction of reaction time.33 Second, such metal leads to the retardance of initial pyrolysis temperature, because copper can fix Br into the solid phase and release small molecules, increasing the thermal stability of BERs.34 Finally, it combines three decomposition stages in the CFPCB pyrolysis into just one stage with a series of complicated reactions in the CCPCB pyrolysis. Kinetic Analysis. In this work, Ozawa−Flynn−Wall (OFW) model35,36 was chosen to determine the kinetic parameters during the CFPCB and CCPCB pyrolysis. This model free method was used to provide the activation energy for eliminating the error resulted in the reaction mechanism function of different hypotheses. Since the reaction mechanism functions of such complex materials as waste printed circuit boards are uncertain, a model free method for determination of activation energy of thermal decomposition is more trustworthy and acceptable.37 The overall rate of reactions of thermal decomposition is commonly described by the following equation:

Table 2. Pyrolysis Characteristics of CFPCB and CCPCB Maximum degradation temperatures (°C)

Sample CFPCB

CCPCB

Heating rate β (°C/min)

Main pyrolysis range (°C)

Tm1

5 10 20 5 10 20

221−511 240−520 260−543 261−397 283−415 307−432

264 277 295 − − −

Tm2

Weight loss at 800 °C (wt %)

Maximum rate of weight loss (wt % s−1)

370 380 403 312 330 352

45.62 44.33 44.31 33.65 34.07 32.12

0.036 0.083 0.173 0.063 0.151 0.302

Table 3. Pyrolysis Reaction Stages of CFPCB and CCPCB Temperature range (°C)

Sample CFPCB

CCPCB

Heating rate β (°C/min)

Stage I

Stage II

Stage III

Stage IV

5 10 20 5 10 20

221−280 240−296 260−325 261−397 283−415 307−432

280−455 296−467 325−493 397−800 415−800 432−800

455−511 467−520 493−543 − − −

511−800 520−800 543−800 − − −

dα A i E yz zzf (α) = expjjj− dT β k RT {

(1)

where α is the degree of conversion defined by eq 2, T is temperature, A is pre-exponential factor, β is heating rate, E is activation energy, R is gas constant, f(α) is the reaction mechanism function. m − mt α= 0 m0 − mf (2)

5−20 °C/min. The pyrolysis characteristics of CCPCB are presented in Table 2. The main temperature range of the CCPCB pyrolysis is approximately 260−430 °C. Maximum degradation temperatures are observed at peak 3 (312, 330 and 352 °C at heating rates from 5 to 20 °C/min). The pyrolysis reaction retardancy due to the difference of heating rate has the same trend as that of CFPCB. Only two stages can be confirmed for the CCPCB pyrolysis, which is presented in Table 3. The first part of CCPCB pyrolysis is Stage I with the temperature range of 261−397 °C, 283−415 °C and 307−432 °C for heating rates from 5 to 20 °C/min. Such a stage includes both the elimination reaction of side chains and the decomposition of BERs. The second part is Stage II with the temperature range of 397− 800 °C, 415−800 °C and 432−800 °C for heating rates from 5 to 20 °C/min. The products in this stage are the same with the final stage of CFPCB pyrolysis where chars and small molecules

where m0, mt, mf are the initial, in process and final mass weight of sample. The OFW method originally used the Doyle’s38 approximation for the temperature integral, the resulting integral and logarithmic form of eq 1 gives the following: i AE zy zz − 2.315 − 0.4567 E log β = lgjjjj z RT k RG(α) {

(3)

where G(α) is the integral form of f(α). The variation pattern of activation energy can confirm that the thermal decomposition of complex materials is either a simple C


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Figure 3. TG (a) and DTG (b) curves of CCPCB pyrolysis at heating rates of 5, 10 and 20 °C/min.

Figure 4. Results of OFW method for CFPCB pyrolysis at different conversion degrees from 0.05 to 0.95: (a) logβ vs 1/T as a function of conversion degree and (b) apparent activation energy vs conversion degree α.

series reaction or two independent reactions in a series reaction.39 The thermal decomposition pattern of BERs may consist of two independent reactions, for the thermal decomposition of brominated and nonbrominated compounds, and it is confirmed that the nonbrominated compounds are more thermally stable than the brominated compounds in BERs.40 In general, the two steps may overlap partially in some temperature ranges.2 Figure 4 and Figure 5 show the results of OFW method for CFPCB and CCPCB pyrolysis, respectively. The apparent activation energy is calculated from the slope of logβ vs 1/T. In most cases of experimental data, only the value of regression coefficient R2 > 0.96 is valid.41 In this experiment, the R2 range is from 0.976 96 to 0.999 98. The variation pattern of the activation energy is relating to the reaction stages defined by TG and DTG analysis. It is observed from Figure 4b that the apparent activation energy for the CFPCB pyrolysis has four distinct stages relating to four periods in the TG and DTG curves. At the end of Stage I (α = 0.075), the apparent activation energy is 119 kJ/mol. In Stage II (0.075 < α < 0.85), the apparent activation energy grows steadily from 119 to 357 kJ/mol. In Stage III (0.85 < α < 0.90), the apparent activation energy decreases suddenly to 279 kJ/mol. In the last Stage IV (α > 0.90), the apparent activation energy grows rapidly and reaches a peak value of 516 kJ/mol. The pyrolysis of CFPCB in the front part of stage II

is mainly attributed to the decomposition of brominated compounds in BERs. Stage III is mainly due to the decomposition of nonbrominated compounds in BERs. The two steps are overlapped in the rest part of Stage II. This is the explanation for the drop of the activation energy at Stage III. The differences of the apparent activation energies between these four stages indicate four characteristic reactions in CFPCB pyrolysis processes. Two stages are shown in Figure 5b according to the activation energy, which is corresponding to two periods discussed by TG and DTG curves for the CCPCB pyrolysis. In Stage I (α < 0.85), the apparent activation energy grows steadily to 124 kJ/mol. In Stage II (α>0.85), the apparent activation energy rises sharply to 393 kJ/mol. There are two main impacts due to the presence of copper from kinetic analysis. First, the presence of copper can lead to smaller gap in activation energy values, indicating that the decomposition of brominated and nonbrominated compounds in BERs are overlapped to a large extent. Second, the activation energy E of the CCPCB pyrolysis at the same conversion degree is much less than that of the CFPCB pyrolysis. Therefore, the pyrolysis reaction is much easier to occur. Additionally, it is obvious that the Ozawa−Flynn−Wall model is greatly compatible with experimental results for all conditions. This confirms the reliable of the selected free model method in describing CFPCB and CCPCB pyrolysis. D


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Figure 5. Results of OFW method for CCPCB pyrolysis at different conversion degrees from 0.05 to 0.95: (a) logβ vs 1/T as a function of conversion degree and (b) Apparent activation energy vs conversion degree α.

Figure 6. Three dimensional (3D) FTIR spectrum of the evolved gases during the CFPCB pyrolysis at a heating rate of 10 °C/min.

Figure 7. Three dimensional (3D) FTIR spectrum of the evolved gases during the CCPCB pyrolysis at a heating rate of 10 °C/min.

Three-Dimensional (3D) FTIR Analysis. Figure 6 and Figure 7 show the 3D FTIR curves of the evolved gases for the CFPCB and CCPCB pyrolysis processes at a heating rate of 10 °C/min. It can be seen that the intensity of the absorption peaks of diverse wavenumbers increases with the increase in temperature during the evolution of evolved gases. It reaches maximum values for the intensity of absorption peaks at temperature of 380 and 330 °C during the CFPCB and CCPCB pyrolysis, respectively. With further increasing the temperature, the intensity of the absorption peaks gradually decreases. The composition of the evolved gases can be identified by characteristic bands of the 3D FTIR spectra. From the 3D plots, the absorbance relating to the vibrations of functional groups and characteristic bands of the evolved gases is obtained at different temperature vs wavenumbers. The absorbance as a function of wavenumbers at the specified temperature can provide IR peaks corresponding to the composition of evolved gases. The characteristic IR bands selected in this study and their species are shown in Table 4. The species are confirmed by many works.2,17,42−49 Figure 8 shows the FTIR spectra of the gases obtained during the pyrolysis of CFPCB at 380 °C and the pyrolysis of CCPCB at 330 °C where the maximum intensity is observed. It is found that the characteristic IR bands of aromatics at 1580−1640 and 600−800 cm−1, ether compounds at

1030−1130 cm−1, alcohols at 1450−1550 and 1220−1280 cm−1, phenols at 3500−3600, 1300−1380 and 1150−1200 cm−1, brominated phenols at 3620−3700, 800−900 and 500− 600 cm−1, alkyl substituents at 2900−3000 cm−1, carbonyl compounds at 1680−1750 cm−1, CO at 2100−2250 cm−1, CO2 at 2260−2400 cm−1, HBr at 2400−2800 cm−1, CH4 at 3000− 3200 cm−1 and H2O at 3710−4000 cm−1. The intensity of absorption peaks is a measure of total evolved gases detected by the IR spectrometer. The IR absorbance vs temperature curves at the specified wavenumber (in Figure 8) can provide additional information through the plots of the total evolved gases at various temperatures, as shown in Figure 9 and Figure 10. The evolution of organic components during the pyrolysis of CFPCB and CCPCB at a heating rate of 10 °C/min was discussed. Figure 9 shows the profiles of evolved organic components from the CFPCB and CCPCB pyrolysis. The characteristics of evolved organic components for the CFPCB and CCPCB pyrolysis are shown in Table 5. The formation reactions of organic components for the CFPCB pyrolysis corresponding to “Stage II” and “Stage III”, while the CCPCB pyrolysis relating to “Stage I”. The initially detected materials can speculate the order of breaking chemical bonds. For the CFPCB pyrolysis, aromatics, carbonyl compounds, brominated phenols and alkyl substituents are first detected at around 295 °C, ether compounds and E


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ACS Sustainable Chemistry & Engineering Table 4. Characteristic Absorption Bands Together with Species of Evolves Gases Selected (cm−1) −1

Wavenumber range (cm )

CFPCB

CCPCB

1580−1640 600−800

1603 676 748 1037 1506 1256 3574 1329 1179 3653 828 553 2974 1737 2295 2687 3070 2191 3732

1603 684 748 1058 1502 1255 3559 1335 1179 3652 820 542 2973 1720 2303 2655 3047 2207 3746

1030−1130 1450−1550 1220−1280 3500−3600 1300−1380 1150−1200 3620−3700 800−900 500−600 2900−3000 1680−1750 2260−2400 2400−2800 3000−3200 2100−2250 3710−4000

Species

Functional group

Vibrations

References

Aromatics

Aromatic CC Aromatic CH

Stretching Stretching

42, 43 42

ether compounds Alcohols

COC OH CO(H) OH OH CO(H) OH Aromatic CH CBr CH CO OCO HBr CH CO OH

Stretching Stretching Stretching Stretching In-plane bending Stretching Stretching Stretching Stretching Stretching Stretching Asymmetric stretching Stretching Stretching Stretching Stretching

42−44 47 47 2 48 45, 48 45, 46 42, 46 2 49 46, 49 17, 43, 47 46 45, 47 17, 43 17, 47

Phenols Brominated phenols

Alkyl substituents Carbonyl compounds CO2 HBr CH4 CO H2O

The maximum absorbance of carbonyl compounds for the CCPCB pyrolysis is just 0.4 times bigger than that of CFPCB pyrolysis. While the maximum absorbance keeps a same level for the CCPCB and CFPCB pyrolysis relating to the evolution of brominated phenols or alkyl substituents. It can be suggested that carbonyl compounds can readily convert into alcohols and phenols to some extent according to the ratio above. The finally detected materials also give useful information. For the CFPCB pyrolysis, the evolution of alcohols, brominated phenols and alkyl substituents are essentially completed at around 467 °C corresponding to the Stage II. Phenols, ether compounds, aromatics, carbonyl compounds and CH4 are still evolved until the temperature reaches 518 °C relating to the Stage III. It can be seen that the formation of brominated organic materials is accomplished in Stage II, while the formation of nonbrominated organic materials is still proceeded in Stage III including the transformation among phenols, ether compounds and carbonyl compounds.51 For the CCPCB pyrolysis, the evolution of organic compounds, except alkyl substituents and CH4, is finished at around 416 °C. The evolution of alkyl substituents and CH4 is completed at about 400 °C. There are several impacts due to the presence of copper from the organic compounds analysis. First, the presence of copper leads to the initially detected materials generated at higher temperature, because copper can convert Br from brominated compounds to generate CuBr species in solid phase, so brominated compounds converts into nonbrominated compounds prior to the decomposition of brominated compounds, and the thermal stable of nonbrominated compounds is the reason for the retardance of evolved temperature. Second, the presence of copper affects the formation of carbonyl compounds, since copper can react with CO bonds to generate CuO and Cu(O)Cu species.19 The depressed formation of brominated phenols and alkyl substituents is observed due to the formation of CuBr species and CH4. Finally, because the copper acts as a catalyst, the reaction is finished ahead of time comparing to the CFPCB pyrolysis. However, the pyrolysis reaction overlapped between brominated compounds and nonbrominated compounds in BERs to a great extent, making it difficult to regulate the components of evolved products.

Figure 8. FTIR spectra of the evolves gases at the maximum absorption peaks for a heating rate of 10 °C/min during (a) pyrolysis of CFPCB at 380 °C and (b) pyrolysis of CCPCB at 330 °C.

alcohols are then detected at about 302 °C. These evolved organic components are generated for the decomposition of the brominated compounds in BERs. Nonbrominated compounds in BERs begin to decompose as the occurrence of phenols at 318 °C. CH4 is detected at 316 °C due to the cleavage of the alkyl chains.50 It is not difficult to observe that all organic compounds are detected in Stage II instead of Stage I, since the breakage of N-containing cross-linkage takes place in Stage I and these new substances generated have extremely high boiling point, so these intermediates still remains in solid phase for continuous decomposition.31 For the CCPCB pyrolysis, the decomposition of nonbrominated compounds and brominated compounds in BERs proceeds at the same temperature in Stage I with irregular cleavage of chemical bonds. It can be observed that all organic components are detected at around 293 °C. The amount of total evolved gases can be shown by absorption intensity. The maximum absorbance of phenols, ether compounds, alcohols, aromatics and CH4 for the CCPCB pyrolysis is 1.6, 1.3, 2.1 2.4 and 2.6 times larger than it of CFPCB pyrolysis, respectively. F


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Figure 9. IR absorbance vs temperature curves for organic components of the evolves gases during the CFPCB and CCPCB pyrolysis at a heating rate of 10 °C/min.

The evolution of inorganic components like CO2, HBr, CO and H2O for the pyrolysis of CFPCB and CCPCB at a heating rate of 10 °C/min was studied. Figure 10 shows the profiles of evolved inorganic components from the CFPCB and CCPCB

pyrolysis. For the CFPCB pyrolysis, the absorbance of these inorganic components has no significant peak. These inorganic components, including CO2, HBr, CO and H2O, are evolved during the whole process of pyrolysis. For the CCPCB pyrolysis, G


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Figure 10. IR absorbance vs temperature curves for inorganic components of the evolved gases during the CFPCB and CCPCB pyrolysis at a heating rate of 10 °C/min.

Table 5. Pyrolysis Characteristics of Evolved Organic Components Maximum absorbance temperature (°C)

Main evolution temperature range (°C)

Maximum intensity of absorbance

Species

CFPCB

CCPCB

CFPCB

CCPCB

CFPCB

CCPCB

Phenols Ether compounds Alcohols Aromatics Carbonyl compounds Brominated phenols Alkyl substituents CH4

318−512 300−516 304−457 297−516 294−518 294−462 298−456 316−500

293−413 293−415 295−414 294−417 291−416 295−413 294−402 292−396

381 381 380 382 380 380 381 383

331 331 331 330 331 331 333 331

0.02033 0.01262 0.00148 0.00745 0.00487 0.00167 0.00686 0.00197

0.03208 0.01675 0.00308 0.01764 0.00192 0.00154 0.00649 0.00511

the evolution of CO2 occurs an absorbance peak at 331 °C, and also has a difference from the CFPCB pyrolysis about the evolution of HBr. However, the amount of CO and H2O evolved has no apparent change comparing to the CFPCB pyrolysis. In summary, the presence of copper can easily convert carbonyl compounds into CO2, as well as brominated components into HBr at the temperature of 420−680 °C. At this temperature range, the Br fixed in residues is easily to be released. Meanwhile, the presence of copper also causes the retardance of initial pyrolysis temperature, since copper can fix Br and O into the solid phase by generating Cu−Br, Cu−O and Cu−(O)−Cu species. Reaction Mechanism. The possible reaction mechanism for the CFPCB and CCPCB pyrolysis is shown in Figure 11 according to the above study results, which can be summarized as follows: The pyrolysis reaction process of CFPCB can be divided into four stages.

1 Stage I: the breakage of N-containing cross-linkage and the formation of low boiling volatiles. 2 Stage II: irregular cleavage of chemical bonds from both brominated and nonbrominated compounds in BERs and the formation of various organic and inorganic components. The main products are phenols, brominated phenols, alcohols, carbonyl compounds, alkyl substituents, ether compounds, CH4, CO, CO2 and HBr. 3 Stage III: decomposition of nonbrominated components and the transformation among phenols, carbonyl compounds, ether compounds with the evolution of some small molecules. 4 Stage IV: formation of chars derived from residues with the evolution of small molecules. The pyrolysis reaction process of CCPCB is divided into two stages. H


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Figure 11. Possible schematic diagram for the pyrolysis of CFPCB and CCPCB.

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brominated and nonbrominated compounds, and the vast majority of materials (phenols, brominated phenols, alcohols, carbonyl compounds, alkyl substituents, ether compounds, CH4, CO, CO2, H2O and HBr) are released. In Stage III (0.85 < α < 0.9), the decomposition of nonbrominated compounds is proceeded with the transformation among phenols, carbonyl compounds, ether compounds, and the small molecules are still released. In Stage IV (α > 0.9), pyrolysis reaction is basically completed with chars formed and few small molecules released. For the CCPCB pyrolysis, only two stages were observed. In Stage I (α < 0.85), complex reactions with the decomposition of both brominated compounds and nonbrominated compounds are taken place with various organic and inorganic components

1 Stage I: the formation of various organic and inorganic components during the irregular decomposition of both brominated and nonbrominated compounds in BERs. 2 Stage II: conversion of the residues to chars with the emission of small molecules.

CONCLUSIONS For the CFPCB pyrolysis, the pyrolysis reaction can be divided into four separate stages. In Stage I (α < 0.075), low boiling volatiles (HBr, CO, CO2 and H2O) are formed and N-containing cross-linkages are broken. In Stage II (0.075 < α < 0.85), chemical bonds are irregularly cleaved with the decomposition of both I


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(7) Duan, H.; Li, J.; Liu, Y.; Yamazaki, N.; Jiang, W. Characterizing the emission of chlorinated/brominated dibenzo-p-dioxins and furans from low-temperature thermal processing of waste printed circuit board. Environ. Pollut. 2012, 161, 185−191. (8) Sakai, S.; Watanabe, J.; Honda, Y.; Takatsuki, H.; Aoki, I.; Futamatsu, M.; Shiozaki, K. Combustion of brominated flame retardants and behavior of its byproducts. Chemosphere 2001, 42, 519−531. (9) Wang, H.; Zhang, S.; Li, B.; Pan, D. A.; Wu, Y.; Zuo, T. Recovery of waste printed circuit boards through pyrometallurgical processing: A review. Resour. Conserv. Recy. 2017, 126, 209−218. (10) Kim, Y. M.; Kim, S.; Lee, J. Y.; Park, Y. K. Pyrolysis Reaction Pathways of Waste Epoxy-Printed Circuit Board. Environ. Eng. Sci. 2013, 30, 706−712. (11) Ning, C.; Lin, C. S.; Hui, D. C.; McKay, G. Waste Printed Circuit Board (PCB) Recycling Techniques. Top. Curr. Chem. 2017, 375, 43− 79. (12) Wang, L.; Li, Q.; Li, Y.; Sun, X.; Li, J.; Shen, J.; Han, W.; Wang, L. A novel approach for recovery of metals from waste printed circuit boards and simultaneous removal of iron from steel pickling waste liquor by two-step hydrometallurgical method. Waste Manage. 2018, 71, 411−419. (13) Gu, K.; Li, W.; Han, J.; Liu, W.; Qin, W.; Cai, L. Arsenic removal from lead-zinc smelter ash by NaOH-H2O2 leaching. Sep. Purif. Technol. 2019, 209, 128−135. Barontini, F.; Marsanich, K.; Petarca, L.; Cozzani, V. Thermal degradation and decomposition products of electronic boards containing BFRs. Ind. Eng. Chem. Res. 2005, 44, 4186−4199. (14) Eswaraiah, C.; Kavitha, T.; Vidyasagar, S.; Narayanan, S. S. Classification of metals and plastics from printed circuit boards (PCB) using air classifier. Chem. Eng. Process. 2008, 47, 565−576. (15) Duan, C.; Sheng, C.; Wu, L.; Zhao, Y.; He, J.; Zhou, E. Separation and recovery of fine particles from waste circuit boards using an inflatable tapered diameter separation bed. Sci. World J. 2014, 2014, 579−586. (16) Li, J.; Xu, Z.; Zhou, Y. Application of corona discharge and electrostatic force to separate metals and nonmetals from crushed particles of waste printed circuit boards. J. Electrost. 2007, 65, 233−238. (17) Hao, J.; Wang, H.; Chen, S.; Cai, B.; Ge, L.; Xia, W. Pyrolysis characteristics of the mixture of printed circuit board scraps and coal powder. Waste Manage. 2014, 34, 1763−1769. (18) Sun, J.; Wang, W.; Liu, Z.; Ma, Q.; Zhao, C.; Ma, C. Kinetic Study of the Pyrolysis of Waste Printed Circuit Boards Subject to Conventional and Microwave Heating. Energies 2012, 5, 3295−3306. (19) Chien, Y. C.; Wang, H. W.; Lin, K. S.; Huang, Y. J.; Yang, Y. M. Fate of bromine in pyrolysis of printed circuit board wastes. Chemosphere 2000, 40, 383−387. (20) Bidini, G.; Fantozzi, F.; Bartocci, P.; D’Alessandro, B.; D’Amico, M.; Laranci, P.; Scozza, E.; Zagaroli, M. Recovery of precious metals from scrap printed circuit boards through pyrolysis. J. Anal. Appl. Pyrolysis 2015, 111, 140−147. (21) Khanna, R.; Cayumil, R.; Mukherjee, P. S.; Sahajwalla, V. A Novel Recycling Approach for Transforming Waste Printed Circuit Boards into a Material Resource. Procedia Environ. Sci. 2014, 21, 42−54. (22) Wang, Y.; Sun, S.; Yang, F.; Li, S.; Wu, J.; Liu, J.; Zhong, S.; Zeng, J. The effects of activated Al2O3 on the recycling of light oil from the catalytic pyrolysis of waste printed circuit boards. Process Saf. Environ. Prot. 2015, 98, 276−284. (23) Jie, G.; Ying-Shun, L.; Mai-Xi, L. Product characterization of waste printed circuit board by pyrolysis. J. Anal. Appl. Pyrolysis 2008, 83, 185−189. (24) Blazsó, M.; Czégény, Zs.; Csoma, C. Pyrolysis and debromination of flame retarded polymers of electronic scrap studied by analytical pyrolysis. J. Anal. Appl. Pyrolysis 2002, 64, 249−261. (25) Cayumil, R.; Khanna, R.; Ikram-Ul-Haq, M.; Rajarao, R.; Hill, A.; Sahajwalla, V. Generation of copper rich metallic phases from waste printed circuit boards. Waste Manage. 2014, 34, 1783−1792. (26) Lin, K. H.; Chiang, H. L. Liquid oil and residual characteristics of printed circuit board recycle by pyrolysis. J. Hazard. Mater. 2014, 271, 258−265.

formed. In Stage II (α > 0.85), chars are formed and few small molecules are evolved. The presence of copper does have a significant influence during the pyrolysis of PCBs, On the one hand, the presence of copper acts as a catalyst, leading to a shorter reaction time, faster reaction rate and lower activation energy. Therefore, pyrolysis reaction can be finished in lower temperature and faster time with efficient energy consumption. However, the rapid reaction process makes the separation of evolved products a serious problem. On the other hand, the copper also reacts with some materials in PCBs causing the formation of Cu−O, Cu−(O)−Cu and Cu−Br species. The presence of copper decreases the amount of carbonyl compounds and brominated compounds to a certain extent, which controls the formation of dioxins or other brominated pollutants. Nevertheless, the increase in the amount of CO2 and HBr adds to the difficulty in tail gas treatment. This study contributes to fundamental knowledge that can be used to guide the pyrolysis of waste PCBs.

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AUTHOR INFORMATION

Corresponding Author

*Junwei Han. E-mail: [email protected]; Tel.: +86 18874411808. ORCID

Junwei Han: 0000-0002-8419-3668 Wenqing Qin: 0000-0001-5570-9680 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 51804342, No. 51874356 and No. 51604302), the Provincial Science and Technology Leader (Innovation Team of Interface Chemistry of Efficient and Clean Utilization of Complex Mineral Resources, No. 2016RS2016), the Collaborative Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources, the Innovation Driven Plan of Central South University (Grant No. 2015CX005), Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources (No. 2018TP1002) and the Scientific Research Starting Foundation of Central South University (No. 218041).

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REFERENCES

(1) Quan, C.; Li, A.; Gao, N.; Dan, Z. Characterization of products recycling from PCB waste pyrolysis. J. Anal. Appl. Pyrolysis 2010, 89, 102−106. (2) Zhao, C.; Zhang, X.; Shi, L. Catalytic pyrolysis characteristics of scrap printed circuit boards by TG-FTIR. Waste Manage. 2017, 61, 354−361. (3) Guo, J.; Guo, J.; Xu, Z. Recycling of non-metallic fractions from waste printed circuit boards: a review. J. Hazard. Mater. 2009, 168, 567−590. (4) Leung, A. O.; Duzgoren-Aydin, N. S.; Cheung, K. C.; Wong, M. H. Heavy metals concentrations of surface dust from e-waste recycling and its human health implications in southeast China. Environ. Sci. Technol. 2008, 42, 2674−2680. (5) Zhang, M.; Buekens, A.; Li, X. Brominated flame retardants and the formation of dioxins and furans in fires and combustion. J. Hazard. Mater. 2016, 304, 26−39. (6) Duan, H.; Li, J.; Liu, Y.; Yamazaki, N.; Jiang, W. Characterization and inventory of PCDD/Fs and PBDD/Fs emissions from the incineration of waste printed circuit board. Environ. Sci. Technol. 2011, 45, 6322−6328. J


Research Article


(48) Han, L.; Wang, Q.; Ma, Q.; Yu, C.; Luo, Z.; Cen, K. Influence of CaO additives on wheat-straw pyrolysis as determined by TG-FTIR analysis. J. Anal. Appl. Pyrolysis 2010, 88, 199−206. (49) Ahamad, T.; Alshehri, S. M. Thermal degradation and evolved gas analysis of epoxy (DGEBA)/novolac resin blends (ENB) during pyrolysis and combustion. J. Therm. Anal. Calorim. 2013, 111, 445− 451. (50) Liu, Q.; Wang, S.; Zheng, Y.; Luo, Z.; Cen, K. Mechanism study of wood lignin pyrolysis by using TG−FTIR analysis. J. Anal. Appl. Pyrolysis 2008, 82, 170−177. (51) Hornung, A.; Balabanovich, A. I.; Donner, S.; Seifert, H. Detoxification of brominated pyrolysis oils. J. Anal. Appl. Pyrolysis 2003, 70, 723−733.

(27) Quan, C.; Li, A.; Gao, N. Thermogravimetric analysis and kinetic study on large particles of printed circuit board wastes. Waste Manage. 2009, 29, 2353−2360. (28) Grause, G.; Furusawa, M.; Okuwaki, A.; Yoshioka, T. Pyrolysis of tetrabromobisphenol-A containing paper laminated printed circuit boards. Chemosphere 2008, 71, 872−878. (29) Chen, Y.; Yang, J.; Zhang, Y.; Liu, K.; Liang, S.; Xu, X.; Hu, J.; Yao, H.; Xiao, B. Kinetic simulation and prediction of pyrolysis process for non-metallic fraction of waste printed circuit boards by discrete distributed activation energy model compared with isoconversional method. Environ. Sci. Pollut. Res. 2018, 25, 3636−3646. (30) Barontini, F.; Marsanich, K.; Petarca, L.; Cozzani, V. Thermal Degradation and Decomposition Products of Electronic Boards Containing BFRs. Ind. Eng. Chem. Res. 2005, 44, 4186−4199. (31) Kim, Y. M.; Han, T. U.; Watanabe, C.; Teramae, N.; Park, Y. K.; Kim, S.; Hwang, B. Analytical pyrolysis of waste paper laminated phenolic-printed circuit board (PLP-PCB). J. Anal. Appl. Pyrolysis 2015, 115, 87−95. (32) Barontini, F.; Cozzani, V. Formation of hydrogen bromide and organobrominated compounds in the thermal degradation of electronic boards. J. Anal. Appl. Pyrolysis 2006, 77, 41−55. (33) Zhao, L.; Wang, H.; Qing, S.; Liu, H. Characteristics of gaseous product from municipal solid waste gasification with hot blast furnace slag. J. Nat. Gas Chem. 2010, 19, 403−408. (34) Zuo, Z.; Yu, Q.; Xie, H.; Duan, W.; Liu, S.; Qin, Q. Thermogravimetric analysis of the biomass pyrolysis with copper slag as heat carrier. J. Therm. Anal. Calorim. 2017, 129, 1233−1241. (35) Ozawa, T. Non-isothermal kinetics and generalized time. Thermochim. Acta 1986, 100, 109−118. (36) Flynn, J. H.; Wall, L. A. General treatment of the thermogravimetry of polymers. J. Res. Natl. Bur. Stand., Sect. A 1966, 70A, 487−523. (37) Kantarelis, E.; Yang, W.; Blasiak, W.; Forsgren, C.; Zabaniotou, A. Thermochemical treatment of E-waste from small household appliances using highly pre-heated nitrogen-thermogravimetric investigation and pyrolysis kinetics. Appl. Energy 2011, 88, 922−929. (38) Doyle, C. D. Kinetic analysis of thermogravimetric data. J. Appl. Polym. Sci. 1961, 5, 285−292. (39) Moltó, J.; Font, R.; Gálvez, A.; Conesa, J. A. Pyrolysis and combustion of electronic wastes. J. Anal. Appl. Pyrolysis 2009, 84, 68− 78. (40) Luda, M. P.; Balabanovich, A. I.; Zanetti, M.; Guaratto, D. Thermal decomposition of fire retardant brominated epoxy resins cured with different nitrogen containing hardeners. Polym. Degrad. Stab. 2007, 92, 1088−1100. (41) Abdul Jameel, A. A.; Han, Y.; Brignoli, O.; Telalović, S.; Elbaz, A. M.; Im, H. G.; Roberts, W. L.; Sarathy, S. M. Heavy fuel oil pyrolysis and combustion: Kinetics and evolved gases investigated by TGA-FTIR. J. Anal. Appl. Pyrolysis 2017, 127, 183−195. (42) Mustata, F.; Tudorachi, N. Thermal behavior of epoxy resin cured with aromatic dicarboxylic acids. J. Therm. Anal. Calorim. 2016, 125, 97−110. (43) Wu, K.; Shen, M. M.; Hu, Y.; Xing, W.; Wang, X. Thermal degradation and intumescent flame retardation of cellulose whisker/ epoxy resin composite. J. Therm. Anal. Calorim. 2011, 104, 1083−1090. (44) Chen, X.; Huo, L.; Jiao, C.; Li, S. TG−FTIR characterization of volatile compounds from flame retardant polyurethane foams materials. J. Anal. Appl. Pyrolysis 2013, 100, 186−191. (45) Wu, W.; Qiu, K. Vacuum co-pyrolysis of Chinese fir sawdust and waste printed circuit boards. Part II: Influence of heating conditions. J. Anal. Appl. Pyrolysis 2015, 111, 216−223. (46) Quan, C.; Li, A.; Gao, N. Research on pyrolysis of PCB waste with TG-FTIR and Py-GC/MS. J. Therm. Anal. Calorim. 2012, 110, 1463−1470. (47) Singh, S.; Wu, C.; Williams, P. T. Pyrolysis of waste materials using TGA-MS and TGA-FTIR as complementary characterisation techniques. J. Anal. Appl. Pyrolysis 2012, 94, 99−107. K


Kinetic and Mechanism Studies on Pyrolysis of Printed Circuit Boards

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