Intrinsic Metal Component-Assisted Microwave Pyrolysis and Kinetic Study of Waste Printed Circuit Boards

Abstract

Waste printed circuit boards (WPCBs) hold great recycling value, but improper recycling can lead to environmental issues. This study combines pyrolysis and microwave technologies, leveraging the unique phenomenon where metal materials tend to “spark” in a microwave field, to develop a microwave pyrolysis process for WPCBs that incorporates metal fillers. The research analyzes the effects of microwave power, metal filler addition, and pyrolysis time on the efficiency of microwave pyrolysis. It explores the mechanisms of microwave pyrolysis and the pathways of pyrolysis product formation, and the kinetics of the pyrolysis reaction of WPCBs. The results indicate that microwave-assisted pyrolysis greatly improves efficiency. Within the experimental range, the optimal conditions are found to be a microwave power of 1600–1800 W, a metal filler addition of 10%, and a pyrolysis time of 10 min. Under these conditions, the yield of pyrolysis liquid was 12.8%, with approximately 5–12 different components, while the yield of pyrolysis gas was 12.7–13.4%, with about 9–11 different components. Compared to conventional pyrolysis products, the liquid products from microwave pyrolysis are simpler and more advantageous for resource utilization. Theoretical calculations show that the average activation energy for the microwave pyrolysis process is 81.05 kJ/mol, with an average reaction order of 0.93, which is greatly better than the 147.75 kJ/mol of the conventional pyrolysis process.

1. Introduction

Printed circuit boards (PCBs), as core components, are widely used in electronic products. It is estimated that over 4.5 million tons of PCBs will be discarded by 2030 [1]. However, they have a complex composition with many toxic and hazardous elements. The improper disposal of waste printed circuit boards (WPCBs) can release large amounts of toxic substances, such as heavy metals and toxic organic components, which pose threats to the ecological environment and human health [2,3,4]. Additionally, WPCBs are primarily composed of approximately 30% plastics, 30% insoluble oxides, and 40% metals, which endow them with great resource recovery value. Therefore, recycling WPCBs is important for reducing pollutant emissions and promoting resource circulation [5,6].

Pyrolysis technology is considered a promising recycling method due to its rapid reaction rate, low emissions, and ability to recover high-value chemicals [7,8,9]. Currently, many researchers have explored ways to improve pyrolysis efficiency by adding acids or bases [10], metals and oxides [11], alkali metals [12], zeolites [13], and other catalysts to ease the pyrolysis process. Compared to traditional heating methods, microwave processing technology can overcome the heating barriers of low thermal conductivity materials, thereby improving pyrolysis efficiency [14]. Studies have shown that microwave technology can effectively concentrate heavy metals, facilitating the subsequent treatment and reducing the environmental impact of the process [15,16]. The existing research has demonstrated that using microwave absorbers such as activated carbon, CaO, CaCO₃, and NaOH can greatly improve pyrolysis efficiency [17,18,19].

Studies have shown that the interaction between microwaves and metals can generate plasma, causing a momentary “sparking” phenomenon [20]. The plasma absorbs a large amount of microwave energy, converting it into heat energy and releasing it in the form of radiation, with temperatures exceeding 1000 °C during sparking [21,22]. This eases the rapid attainment of pyrolysis temperatures, enabling fast pyrolysis. Therefore, microwave pyrolysis in the presence of metals is suitable for processing WPCBs. In WPCBs, copper is the primary metal element, and research has been conducted to explore its synergistic effect on pyrolysis. The results indicate that copper metal reduces activation energy and promotes the pyrolysis process [23]. However, most existing studies focus on improving microwave pyrolysis efficiency by adding extra catalysts, neglecting the fact that WPCBs inherently contain substantial amounts of metals. Consequently, the impact mechanism of intrinsic metal content on microwave pyrolysis has not been fully elucidated.

This paper is based on the phenomenon where the presence of metals in the material being heated by microwave equipment causes “sparking”. Utilizing a self-developed microwave pyrolysis reactor and further leveraging the multi-metal characteristics of WPCBs, this study uses dismantled elongated copper alloy wires as auxiliary pyrolysis metal fillers. The research investigates the changes in gaseous and liquid pyrolysis products under different conditions and conducts related kinetic studies using the Friedman method.

2. Materials and Methods

2.1. Materials

The WPCBs used in this study are provided by Nantong Sanderson Blue Environmental Technology Co., Ltd. (Nantong, Jiangsu, China), and are ASUS brand discarded PCBs (all desktop computer motherboards). The WPCBs are disassembled using a desoldering workstation with all the electronic components removed. The WPCBs are then cut into approximately 1–2 cm² square pieces to serve as experimental samples.

2.2. Devices

The study employed a self-made microwave reactor, designed with a cylindrical microwave resonant cavity (Figure 1). In the middle of the resonant cavity, there is a Φ60 × L800 mm high-temperature-resistant tube (made of alumina/quartz) with sealed flanges at both ends. One side of the sealed flange is equipped with a gas inlet tube, while the other side serves as the pyrolysis gas (PG) outlet. During operation, an external motor rotates the flanges and the high-temperature-resistant tube. A temperature measurement device is in contact with the pyrolysis vessel, and the pyrolysis products first enter the PG condensing unit through the intermediate channel of the temperature measurement device, and finally into the pyrolysis product collection unit. Earlier work has confirmed that this equipment exhibits improved microwave reflection and has temperature measurement capabilities [24]. After capacity enhancement, the maximum power of the device can reach 5000 W.

2.3. Methods

The microwave pyrolysis process, as shown in Figure 2, begins by weighing a certain mass of small pieces of WPCBs (5) and placing them into an alumina container (4). A proportional amount of auxiliary pyrolysis metal filler, taken from WPCB teardown based on the mass of the WPCBs, is then measured and mixed with the pieces. Nitrogen gas is introduced for 2–3 min at a flow rate of 1–1.2 L/min. The microwave is then activated, and the motor is set to rotate at 4–5 revolutions per minute, initiating pyrolysis under specific microwave power (MP) and different pyrolysis time conditions. After pyrolysis, ventilation is maintained for approximately 10 min. The condensate from primary and secondary condensers is collected in a triangular flask (11), and a portion of the PG is collected using a gas bag for testing purposes. The remaining pyrolysis residue is uniformly collected for future use.

2.4. Characterization

The primary organic elements of the samples are determined using an elemental analyzer (Vario EL III, Elementar, Hanau, Hessen, Germany). The metal content in the samples is measured using an inductively coupled plasma optical emission spectrometer (ICP, Agilent, Santa Clara, CA USA). The thermal decomposition characteristics and behavior of the organic materials are studied using a thermogravimetric analyzer (TGA, Discovery, Waters LLC, Milford, MA, USA). The liquid-phase products are analyzed using a gas chromatograph–mass spectrometer (GC-MS, Shimadzu SVOC, Kyoto, Japan) equipped with a DB-5MS quartz capillary column, and the mass spectrometer uses an electron ionization (EI) source. The PGs are measured using another GC-MS system (Trace GC Ultra, Thermo Finnigan, Waltham, MA, USA), also equipped with a DB-5MS quartz capillary column and high-purity helium as the carrier gas. The injection port is cleaned three times before and after each injection, with a split ratio of 10:1 and a column flow rate of 1 mL/min. The column oven temperature is initially set at 50 °C and increased to 280 °C at a rate of 10 °C/min. The injector temperature is 280 °C. The mass spectrometer uses an EI ion source with a source temperature of 200 °C and an interface temperature of 250 °C. Solvent delay is set to 3 min, with data acquisition starting at 4 min and ending at 25 min. The scan interval is 0.3 s, with a scan rate of 1666, and the mass scan range is 45–500 m/z.

By examining the yield indices of PGs and pyrolysis liquids (PLs) from the pyrolysis products, suitable experimental conditions can be explored. The microwave pyrolysis process can be evaluated based on the pyrolysis rate:

$$\mathrm{P}\mathrm{y}\mathrm{r}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{i}\mathrm{s}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}\left(\mathrm{w}\mathrm{t}\mathrm{\%}\right)={\displaystyle \frac{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{r}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}}\times 100\mathrm{\%}$$

(1)

By observing the proportions of gaseous, liquid, and solid pyrolysis products, their yields can be expressed as follows:

$$\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\left(\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\right)\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}(\mathrm{w}\mathrm{t}\mathrm{\%})={\displaystyle \frac{\mathrm{S}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\left(\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\right)\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}}\times 100\mathrm{\%}$$

(2)

$$\mathrm{G}\mathrm{a}\mathrm{s}\mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}(\mathrm{w}\mathrm{t}\mathrm{\%})={\displaystyle \frac{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}-\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}}}\times 100\mathrm{\%}$$

(3)

2.5. Kinetic Calculation

Kinetic studies are important for understanding the pyrolysis reaction process, and various models have been employed for the kinetic calculations of pyrolysis reactions [25,26,27,28]. Among these kinetic methods, the Friedman method is the most widely used isoconversional method due to its simplicity and high accuracy. Although the method is sensitive to data noise, the impact of noise can be reduced by considering neighborhood information and applying smoothing techniques [29,30]. Therefore, this study employs the Friedman method to investigate the pyrolysis kinetics of WPCBs [31].

According to the law of mass action, the overall decomposition reaction rate of the sample is given by the following:

$${\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{d}t}}=kf\left(\alpha \right)$$

(4)

In the equation:

• k—reaction rate constant;
• α—weight loss rate during the reaction.

Assuming the reaction follows the Arrhenius equation:

$$k=A{e}^{-{\displaystyle \frac{∆E}{\mathrm{R}T}}}$$

(5)

In the equations:

• A—frequency factor (min⁻¹);
• ΔE—activation energy (kJ/mol);
• R—gas constant (8.314 J/mol·K);
• T—reaction absolute temperature (K).

Substituting k from Equation (5) into Equation (4) and taking the natural logarithm, we obtain the following:

$$\mathrm{l}\mathrm{n}\left(\beta {\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{d}T}}\right)=\mathrm{l}\mathrm{n}Af\left(\alpha \right)-{\displaystyle \frac{∆E}{\mathrm{R}T}}$$

(6)

5.

β—the rate of warming.

The International Confederation for Thermal Analysis and Calorimetry recommends using the Kissinger–Akahira–Sunose (KAS) equation for more accurate calculations of activation energy, assuming f(α) = (1 − α)ⁿ [32,33].

$$\mathrm{l}\mathrm{n}\left({\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{d}T}}\right)=\mathrm{l}\mathrm{n}A+n\mathrm{l}\mathrm{n}\left(1-\alpha \right)-{\displaystyle \frac{∆E_{\alpha }}{\mathrm{R}{T}_{\alpha }}}$$

(7)

• ΔEα—activation energy for the given α(kJ/mol);
• T_α—absolute temperature for the given α(K).

By plotting ln(${\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{d}t}}$) versus $\mathrm{l}\mathrm{n}\left(1-\alpha \right)$, the kinetic parameters n and A, and the pre-exponential factor, lnA can be determined from the slope and intercept of the resulting straight line.

3. Results and Discussion

3.1. Composition and Pyrolysis Characterization of WPCBs

WPCBs are mainly composed of metal and non-metal components. The results of the analysis of the ash content of the bare board of the dismantled electronic component WPCBs are shown in

Table 1. Ash and organic components.

Component	Sample 1	Sample 2	Sample 3	Average
Non-flammable component	62.16%	71.18%	69.85%	67.73%
Organic component	37.84%	28.82%	30.15%	32.27%

, which is similar to the results of other studies [8]. An elemental analysis (

Table 2. Content of major organic elements.

Elemental	Content		Average
C	17.17%	16.84%	17.01%
H	1.84%	1.77%	1.80%
N	0.36%	0.37%	0.37%

) indicates that carbon (C), hydrogen (H), and nitrogen (N) elements in WPCBs mainly originate from brominated epoxy resin-based high molecular weight polymers and amine substances used in the production process of WPCBs. The ICP studies revealed that copper (Cu) is the primary component of the copper foil in WPCBs (

Table 3. Metal elements and contents.

Metal Element	Content (%)	Metal Element	Content (%)	Metal Element	Content (%)
Cu	30.53	Al	1.65	Ni	0.04
Sn	9.45	Fe	0.14	K	0.04
Pb	1.83	Na	0.14	Mg	0.05
Ca	1.76	Zn	0.11	Cr	0.01
Sum of metal element	45.75%

). Tin (Sn), lead (Pb), and other elements primarily come from solder, while calcium (Ca), aluminum (Al), and others exist in oxide forms, representing ingredients in the glass fiber cloth [34].

The thermal stability of WPCBs is investigated using TGA, with the heating rate and maximum weight loss temperature shown in Figure 3, respectively. The pyrolysis of WPCBs begins at 250 °C, with an initial slow rate of weight loss. As the temperature increases, the weight loss rate sharply escalates. At a heating rate of 10 °C/min, the maximum weight loss temperature is 295.8 °C. As the heating rate gradually increases to 20–50 °C/min, the derivative thermogravimetric (DTG) curve also exhibits a systematic shift towards higher temperatures, with the temperature range approximately between 305.4 and 353.4 °C. After surpassing the maximum weight loss temperature, the rate of mass loss decreases rapidly, but the mass loss continues for an extended period. At this point, both thermogravimetric (TG) and DTG curves become very flat.

3.2. Effect of Microwave Pyrolysis Conditions on Pyrolysis Temperature

Different MPs (1200, 1400, 1600, 1800, 2000, 2200, and 2400 W) are selected for pyrolysis. The auxiliary pyrolysis metal filler content is set at 10%, and the pyrolysis time is 10 min. The study focuses on the final temperature of pyrolysis volatile products in the pyrolysis products to preliminarily determine the suitable microwave pyrolysis power.

As shown in Figure 4a, as the MP continuously increases, the final temperature curve of PG also rises gradually, and when the microwave pyrolysis power reaches 1600–1800 W, the curve begins to level off. In Figure 4b, temperature data are collected every 30 s from 0 s to 1080 s under the conditions of 1800 W microwave pyrolysis power and 10% metal filler content. In the early stage of pyrolysis, the temperature of volatile pyrolysis products sharply increases, reaching a peak around 180 s. However, with the extension of microwave heating time, the temperature begins to decrease greatly, with a large decrease in temperature. After about 70 s of cooling, the temperature slowly starts to rise again, and the temperature rise curve becomes very flat with no great fluctuations. The main reason for the fluctuations in the temperature of volatile pyrolysis products in the early stage of pyrolysis is that pyrolysis is an endothermic reaction. The remarkable temperature drop also indicates that during this period, the pyrolysis reaction is the most intense, with the continuous breaking of large organic molecule bonds. It results in the formation of smaller molecular products that continuously evaporate and absorb a large amount of energy, leading to a decrease in the temperature of PG. The subsequent gradual temperature rise is mainly due to the substantial formation of pyrolysis carbon in the pyrolysis residue at this temperature. Pyrolysis carbon undergoes minimal mass change at this temperature, and the microwave absorption rate and the amount of microwave energy absorbed per unit time tend to remain constant. The low level of chemical reaction activity leads to a stable increase in temperature [35]. Under these process conditions, the most intense pyrolysis occurs in the first 4 min of microwave pyrolysis. Extending the pyrolysis time appropriately can improve the pyrolysis effect, promoting a more thorough pyrolysis reaction.

3.3. Analysis of Factors Affecting Microwave Pyrolysis of WPCBs

3.3.1. Effect of Metal Filler Addition on Pyrolysis Effect

The elemental analysis of the metal filler and the spark phenomenon are shown in

Table 4. Analysis of the main components of the assisted pyrolysis metal fillers.

Metal	Unkown	Au	Ca	Cr	Cu	Fe	Na	Ni	Sn	Zn
Content (%)	4.90	0.71	0.54	0.11	76.30	0.30	0.10	1.39	15.36	0.17

and Figure 5. Taking approximately 16 g of WPCBs, auxiliary pyrolysis metal filler is added at the mass fractions of 0%, 5%, 10%, 20%, and 30% of the pyrolysis material. The MP is set at 800 W, and the heating time is 180 s. The study indicated minimal changes in the samples before and after microwave pyrolysis when no filler was added. However, when 10% of the auxiliary pyrolysis metal filler is added, all the WPCB pieces exhibited a very noticeable pyrolysis and carbonization phenomenon.

Figure 6 indicates that under the same pyrolysis conditions without adding metal filler, the mass loss of brominated epoxy resin in WPCBs is only 1.14%. As the amount of metal filler added increased, the pyrolysis effect improved greatly compared to the samples without metal filler. When the metal filler content increases from 5% to 30%, the pyrolysis yield reaches a maximum of 35.60% (with 30% metal auxiliary filler). At a metal filler content of 10% of the WPCB weight, the pyrolysis yield has already reached 34.69%, which is comparable to the 30% addition level. Thus, at a metal auxiliary pyrolysis filler content of 10% of the WPCB weight, under the same heating conditions, the WPCBs achieves the appropriate pyrolysis effect, confirming that the suitable metal filler addition amount is 10%.

3.3.2. Effect of MP on Pyrolysis Effect

The MP level directly impacts the pyrolysis efficiency in the process of the microwave pyrolysis of WPCBs. Selecting an appropriate microwave pyrolysis power level is beneficial for stabilizing the quality of pyrolysis products and achieving energy savings. Microwave pyrolysis experiments are conducted at the power levels of 1200, 1400, 1600, 1800, 2000, 2200, and 2400 W, with an auxiliary pyrolysis metal filler addition of 10% and a pyrolysis duration of 10 min. The study aims to explore the suitable MP level by examining the PG and PL yields.

Figure 7a indicates that within the range of 1200–2400 W microwave pyrolysis power, there is an overall increase in the pyrolysis rate with increasing MP. The upward trend slows down when the MP is 1600 W and 1800 W, as observed from the TGA data. The pyrolysis of WPCBs exhibits only one major mass loss peak at 1600 W and 1800 W, with a smaller increase in the pyrolysis rate. This suggests that the pyrolysis of brominated epoxy resin in the WPCBs has either reached or is nearing its endpoint at these power levels. However, the pyrolysis rate continues to rise at 2000–2400 W, indicating that the pyrolysis temperature is too high at these power conditions. The pyrolysis carbon absorbs microwave energy, undergoes transformation at elevated temperatures, and contributes to the increase in total mass loss rate, as also evidenced by the TG curve in Figure 3. In Figure 4b, the pyrolysis liquid yield decreases rapidly between 1200 W and 1400 W. Considering that the MP is 1200 W, which belongs to lower temperature and slow pyrolysis. The temperature of volatile fraction product measured at 1200 W pyrolysis is nearly 180 °C (Figure 4a), which is much lower than the temperature at which the pyrolysis of WPCBs begins to lose weight in the thermal stabilization analysis (Figure 3), and the rate of total mass loss is lower under this power condition. Under this power condition, the pyrolysis liquid product content is higher, mainly because the microwave energy is insufficient, resulting in the organic polymer pyrolysis reaction not being carried out thoroughly. Figure 7b,c show that during pyrolysis at the 1800–2000 W power levels, the temperature of pyrolysis volatile fractions reaches around 270 °C, leading to a further increase in the pyrolysis rate. At this power level, the PL yield reaches nearly 12%, representing another peak in the PL yield. However, at the 2200–2400 W power levels, the PL yield decreases due to the higher power, indicating that the pyrolysis power is too high at this stage.

Each sample of WPCBs weighs approximately 30 g, with nitrogen as the carrier gas and gas bag sampling. The qualitative and quantitative analysis of PGs is shown in

Table 5. Qualitative and quantitative statistics of PGs with different powers.

No.	Molecular Formula	1200 (%)	1400-1 (%)	1400-2 (%)	1600 (%)	1800 (%)	2000 (%)	2200 (%)	Average Similarity
1	CO	75.96	34.80	86.49	89.98	69.96	56.62	69.29	81.1
2	CH3Br	6.58	30.03	3.19	3.85	22.42	19.26	18.27	98.2
3	C3H6O	12.24	5.77	6.54	4.04	4.68	0.86	3.82	70.1
4	C2H5Br	—	3.28	—	0.48	0.92	0.17	0.82	85.9
5	C3H7Br	0.43	0.62	0.33	0.12	—	0.08	0.29	54.6
6	C4H9Br	—	—	0.01	—	0.22	0.09	0.69	38.4
7	C3H5Br	1.26	2.55	1.21	0.48	0.78	0.33	2.92	63.5
8	C6H6	3.08	0.97	—	0.93	0.90	0.32	3.89	65.1
9	C7H8	0.44	0.09	0.16	0.11	0.12	—	—	52.0
10	C3H6	—	21.89	—	-	—	22.25	—	38.1
Sampling start time (min)		8–10	3–6	8–10	8–10	8–10	3–6	8–10

. Under different microwave pyrolysis power conditions, the content of CO remains consistently high, and the CH₃Br content is also relatively high throughout. During the early stages of pyrolysis (sampling at 3–6 min into pyrolysis), the content of C₃H₆ in the PGs is relatively high, about 22%. However, as the pyrolysis time extends, the C₃H₆ gas gradually diminishes. Figure 8 shows that the total content of CO and CH₃Br in the PGs predominates under different MP conditions, with the combined sum of these gasses showing minimal variation within the 1200–2200 W pyrolysis power range.

Table 6. Qualitative and quantitative statistics of PL phase products.

No.	Molecular Formula	Name	1200 (%)	1400 (%)	1600 (%)	1800 (%)	2000 (%)	2200 (%)	2400 (%)	Mean Similarity
1	C6H6O	Phenol	53.77	45.86	64.35	59.78	53.92	59.02	72.25	97.1
2	C7H8O	Phenol, 2-methyl	1.32	1.72	1.37	1.42	1.68	—	—	98.2
3	C6H5BrO	Phenol, 2-bromo	0.91	1.34	0.82	1.11	1.32	—	—	95.6
4	C7H8O	Phenol, 4-methyl-	—	0.98	—	0.88	—	—	—	90.5
5	C9H10O	Phenol, 2-(2-propenyl)	1.72	1.72	2.04	2.03	2.11	—	—	86.6
6	C8H10O	Phenol, 4-ethyl-	0.78	1.08	—	0.76	1.14	—	—	96.5
7	C9H12O	Phenol, 4-(1-methylethyl)-	25.47	23.36	26.92	20.24	10.31	21.77	14.70	97.7
8	C8H10BrN	Benzenamine, 4-bromo-2, 6-dimethyl-	2.09	1.72	1.88	1.51	2.24	—	—	77.6
9	C6H4Br2O	Phenol, 2, 6-dibromo-	—	0.89	—	—	1.17	—	—	87.0
10	C13H20	Benzene, 1-(1, 1-dimethylethyl)-3-ethyl-5-methyl-	0.30	—	—	—	—	—	—	87.0
11	C12H10O	p-Hydroxybiphenyl	3.16	3.44	1.46	2.82	4.59	1.75	1.26	95.4
12	C13H12O	[1, 1′-Biphenyl]-2-methanol	0.48	—	—	—	1.34	—	—	79.0
13	C15H16O2	Phenol, 4, 4′-(1-methylethylidene)bis-	3.08	8.04	1.15	3.75	6.97	11.82	8.91	90.4
14	C16H18O2	2-(4′-Hydroxyphenyl)-2-(4′-methoxyphenyl)propane	0.39	—	—	—	0.63	—	—	75.0
15	C10H14O	Phenol, 4-(1-methylpropyl)-	0.94	1.11	—	0.89	1.54	—	—	70.8
16	C10H12Br2O	Phenol, 2, 6-dibromo-4-(1, 1-dimethylethyl)-	1.30	3.07	—	1.81	3.11	4.04	2.87	65.3

shows that there are a total of 16 components in the liquid phase products of the microwave pyrolysis of WPCBs. At lower microwave pyrolysis powers, such as 1200–1400 W, there are more components in the PL, approximately 12–14. As the microwave irradiation power increases, the pyrolysis temperature also rises, leading to a decrease in the number of components in the PL. Therefore, the MP directly affects the composition of the PL. Higher MP means more energy received per unit time, leading to more rapid breakage of high-energy bonds in high-molecular-weight polymers. This shortens the time for pyrolysis reaction completion and reduces the likelihood of re-reactions among pyrolysis products, simplifying the composition of the PL [36]. However, this also inevitably leads to an increase in reactions involving small-molecule gaseous products, resulting in a lower yield of PL. The highest content in the PL composition is phenol (C₆H₆O), approximately 45–72%, followed by p-isopropylphenol (C₉H₁₂O) at about 10–27%. The combined content of these two products can account for over 90% of the PL, consistent with the highest peak intensities and largest peak areas of C₆H₆O and C₉H₁₂O in the GC-MS spectra shown in Figure S1. The content of C₁₂H₁₀O and C₁₅H₁₆O₂ remains relatively stable, with their combined content ranging from 3 to 13%. C₇H₈O and C₉H₁₀O are two relatively stable components in the PL, with their combined content being approximately 3.0–3.7%.

Under MP conditions at 1600 W and 1800 W, the composition of the PL products obtained is relatively simple, and the recovery rate of the PL is also higher. Furthermore, when the microwave pyrolysis power is set at 1800 W and 2000 W, a better pyrolysis effect can be achieved. Taking these factors into consideration, 1800 W is deemed to be the more suitable pyrolysis power.

3.3.3. Effect of Microwave Irradiation Time on Pyrolysis Effect

Choosing the microwave pyrolysis times of 4, 6, 8, 10, 12, and 14 min, with an auxiliary pyrolysis metal filler addition of 10% and an MP of 1800 W, we examined the yield indicators of PGs and PLs to explore the appropriate pyrolysis time. As shown in Figure 9a, within the 4–10 min interval of microwave heating, there is a rapid increase in the pyrolysis rate of the WPCBs. In the 10–14 min interval of microwave heating, the curve of the pyrolysis rate shows minimal changes. Based on the overall curve changes, it can be inferred that the pyrolysis is completed after 10 min of heating.

The trend observed in Figure 9b,c indicates that the yield of PLs increases gradually while the PGs decrease gradually during the 4–8 min period of pyrolysis time. There is a great variation in the yield of liquid-phase products, marking a peak in the yield rate during this period, reaching its maximum at 8 min of pyrolysis time. The period from 8 to 10 min corresponds to the peak in the yield rate of PLs. However, between 10 and 14 min of pyrolysis time, there is a rapid decline in the yield rate of PLs and a sharp increase in the yield rate of PGs. This phenomenon is attributed to the prolonged pyrolysis time causing further breaking of carbon–carbon bonds in long-chain macromolecules, leading to the generation of small molecular gaseous products. Simultaneously, the content of small molecular products in the pyrolysis products of organic polymers increases, contributing to the overall rise in the yield rate of PGs during the same period, especially evident between 10 and 12 min of pyrolysis time.

Using the same gas collection method, the analysis results are shown in

Table 7. Qualitative and quantitative statistics of gas products with different pyrolysis times.

No.	Molecular Formula	4 min (%)	6 min (%)	8 min (%)	10 min (%)	12 min (%)	16 min (%)	Mean Similarity
1	CO	15.10	36.90	72.02	80.23	79.94	76.28	74.1
2	CH3Br	36.76	27.40	5.66	4.22	1.89	6.02	98.4
3	C3H6O	5.34	2.54	3.21	9.58	12.56	8.34	72.1
4	C2H5Br	3.38	2.98	0.66	0.33	0.40	1.26	97.1
5	C3H7Br	0.01	1.99	0.13	0.07	0.05	0.01	57.4
6	C3H5Br	1.93	1.49	2.00	0.35	0.40	1.86	63.4
7	C6H6	0.04	1.38	3.64	4.28	4.56	5.36	64.8
8	C7H8	—	—	0.05	0.20	0.46	0.15	39.4
9	C3H6	34.20	24.90	—	—	—	—	38.4
10	C4H9Br	—	0.36	—	0.20	0.13	0.72	31.6
11	C3H3NO	—	—	5.75	—	—	—	22.5
12	C3H3N	—	—	6.54	—	—	—	25.1
13	C5H6	—	—	—	0.43	0.03	—	52.0
14	C5H8	—	—	—	0.07	—	—	16.8

and Figure 10. CO and CH₃Br are the predominant gasses observed [8]. Between 4 and 10 min of pyrolysis time, the CO content gradually increases, reaching its peak at 10 min of pyrolysis. From 4 to 12 min, CH₃Br is primarily generated in the early stages of pyrolysis, accounting for 36.76% of the PG, but its content decreases gradually with the increase in pyrolysis temperature. C₃H₆ is mainly present in the early-stage PGs, constituting 34.2% of the PG, but it diminishes with prolonged pyrolysis time, indicating its generation mainly occurs in the initial stages of pyrolysis, and transforms into gasses like CH₃Br or C₃H₇Br as the temperature rises.

Table 8. Qualitative and quantitative statistics of PLs with different pyrolysis times.

No.	Molecular Formula	Name	4 (%)	6 (%)	8 (%)	10 (%)	12 (%)	Mean Similarity
1	C6H6O	Phenol	67.09	61.21	61.40	81.02	67.65	98.0
2	C6H5BrO	Phenol, 2-bromo	1.52	1.38	1.10	—	—	94.7
3	C9H10O	Phenol, 2-(2-propenyl)	1.50	1.57	1.65	—	—	86.0
4	C9H12O	Phenol, 4-(1-methylethyl)-	22.62	21.91	23.67	13.91	13.79	97.8
5	C8H10BrN	Benzenamine, 4-bromo-2, 6-dimethyl-	1.86	0.92	1.32	1.52	0.73	75.0
6	C12H10O	p-Hydroxybiphenyl	0.74	1.99	2.25	1.72	2.91	88.6
7	C13H12O	[1, 1′-Biphenyl]-2-methanol	—	—	0.66	—		70.0
8	C15H16O2	Phenol, 4, 4′-(1-methylethylidene)bis-	2.25	3.89	3.08	1.84	7.27	87.6
9	C16H18O2	2-(4′-Hydroxyphenyl)-2-(4′-methoxyphenyl)propane	—	—	—	—	0.70	67.0
10	C10H12Br2O	Phenol, 2, 6-dibromo-4-	—	1.89	1.44	—	3.13	63.3

shows approximately 10 types of pyrolysis products, and as the pyrolysis time increases, there is minimal variation in the main types of pyrolysis products. At 10 min of pyrolysis, the fewest types of PL products are detected, with only five types identified. The main components of the PLs are C₆H₆O and C₉H₁₂O [9]. At 10 min of microwave pyrolysis, the content of C₆H₆O reaches 81%, and C₉H₁₂O reaches 13.91%, totaling nearly 95%. This composition is conducive to resource utilization. With prolonged pyrolysis time, the content of C₉H₁₂O decreases while the content of C₆H₆O increases, indicating a reaction where C₉H₁₂O continues to break down to form C₆H₆O. The GC-MS analysis and identification spectra of liquid samples at different pyrolysis times are shown in Figure S2. Bromine-containing components in the PLs remain relatively stable, but with prolonged pyrolysis time, there is a higher probability of some bromine-containing components undergoing decomposition or recombination. To ensure the thorough and complete pyrolysis of organic polymers while also maintaining a high recovery rate of valuable pyrolysis products, 10 min is deemed to be the appropriate pyrolysis time.

3.4. Comparison of Microwave Pyrolysis and Normal PL Phase Products

Using a conventional tube furnace (GSL-1600X, Hefei Kejin Materials Technology Co., Ltd., Hefei, Anhui, China) as the pyrolysis equipment, WPCBs are pyrolyzed under the conditions of 400, 500, and 600 °C, with a pyrolysis time of 30 min. Only the PLs are collected, with each sample of WPCBs weighing approximately 30 g. Nitrogen gas is used as the carrier gas. The qualitative and quantitative analysis of the PLs is presented in

Table 9. Qualitative and quantitative statistics of liquid products of ordinary pyrolysis.

No.	Molecular Formula	Name	400 °C (%)	500 °C (%)	600 °C (%)	Mean Similarity
1	C6H6O	Phenol	47.82	50.79	41.56	98.0
2	C7H8O	Phenol, 2-methyl	1.03	0.66	0.60	97.3
3	C6H5BrO	Phenol, 2-bromo	1.73	1.10	0.52	95. 7
4	C9H10O	Phenol, 2-(2-propenyl)	2.08	1.40	—	87.0
5	C8H10O	Phenol, 4-ethyl-	1.13	—	—	97.0
6	C9H12O	Phenol, 4-(1-methylethyl)-	24.00	24.90	25.00	97.3
7	C8H10BrN	Benzenamine, 4-bromo-2, 6-dimethyl-	2.59	2.25	2.03	77.7
8	C9H10O2	2H-1-Benzopyran-3-ol, 3, 4-dihydro-	2.57	0.74	1.21	91.3
9	C6H4Br2O	Phenol, 2, 4-dibromo-	0.45	—	—	94.0
10	C12H10O	p-Hydroxybiphenyl	4.42	5.15	6.81	92. 7
11	C13H12O	[1, 1′-Biphenyl]-2-methanol	0.91	0.47	—	68.5
12	C13H20	Benzene, 1-(1, 1-dimethylethyl)-3-ethyl-5-methyl-	—	—	0.45	21. 7
13	C15H16O2	Phenol, 4, 4′-(1-methylethylidene)bis-	5.26	5.97	9.46	89.0
14	C10H14O	Phenol, 4-(1-methylpropyl)-	0.46	0.35	0.81	71.0
15	C10H12Br2O	Phenol, 2, 6-dibromo-4-(1, 1-dimethylethyl)-	1.62	1.74	3.28	64.3

.

The analysis of PLs from the pyrolysis of WPCBs at three different temperatures in a conventional tube furnace revealed approximately 15 identifiable compounds. Among these, phenol (C₆H₆O) is the most abundant, comprising about 40–50% of the total. The second most prevalent compound is 4-isopropylphenol (C₉H₁₂O), making up approximately 23–25%. Additionally, C₁₂H₁₀O and C₁₅H₁₆O₂ are consistently detected, with their combined content ranging from 9 to 16%. The GC-MS analysis and identification spectra of the liquid samples at 400 °C, 500 °C, and 600 °C are shown in Figure S3. The GC-MS spectra indicate the presence of numerous unidentified peaks, particularly at lower temperatures, suggesting a complex composition of the PLs which is not favorable for resource utilization. As the pyrolysis temperature increases, the number of unidentified peaks decreases and their intensity weakens. At 600 °C, 11 identifiable PLs were detected, although many unidentified peaks remained. Overall, the complexity of the conventional pyrolysis products is greatly higher than that of the microwave pyrolysis products.

A comparison of the PLs from the microwave pyrolysis and conventional pyrolysis of WPCBs shows that with increasing MPs and conventional pyrolysis temperature, the number of identifiable compounds in the PL decreases for both methods. The GC-MS spectra of the microwave PLs products exhibit fewer unidentified peaks, making it easier to use the products for resource recovery. In contrast, conventional pyrolysis results in more complex spectra with numerous unidentified peaks, complicating resource utilization [2,8]. In terms of the specific compounds obtained, microwave pyrolysis, which includes the addition of auxiliary metal fillers, greatly outperforms conventional pyrolysis. The combined content of phenol (C₆H₆O) and 4-isopropylphenol (C₉H₁₂O)—the major components of the PLs—reaches up to 94% with microwave pyrolysis compared to only 75% with conventional pyrolysis, a difference of nearly 20%. This higher yield and purity make microwave pyrolysis more advantageous for the subsequent production of new products and resource recovery, highlighting its superiority over conventional pyrolysis methods.

3.5. Pyrolysis Kinetics and Related Mechanisms of Action

In order to obtain a pyrolysis process closer to the actual use, microwave TG studies are carried out, and the microwave TG warming rate of the WPCBs is 10, 30, and 50 K/min, and the curves are shown below.

Figure 11a–c indicate that the DTG curve exhibits only one peak of weight loss rate, suggesting a single activation energy for the microwave pyrolysis reaction process. Figure 11d presents Friedman plots for α values ranging from 0.1 to 0.9 in increments of 0.1, resulting in a total of nine values. Figure 12 displays the variation curve of E with α derived from Figure 11c. The average activation energy, calculated from these nine values, is 81.05 kJ/mol, as shown in

Table 10. Microwave thermogravimetric reaction levels and pre-exponential factor.

Heating Rate (K/min)	Number of Reaction Stages (n)	lnA (s−1)	Temperature Range (K)
10	1.2	10.54	543–700
30	0.9	11.34	573–680
50	0.7	11.55	570–685

.

Based on the microwave thermogravimetric data, the pyrolysis kinetics of WPCBs are studied using the Friedman method. The activation energy is calculated using the KAS, and the results indicate that the overall pyrolysis process of WPCBs can be represented by a single kinetic equation. The pyrolysis kinetic parameters obtained are as follows: the apparent activation energy is 81.05 kJ/mol, the average reaction order is 0.93, and the pre-exponential factor lnA is 11.14.

The TGA and DTG curves for WPCBs at the conventional heating rates of 20, 30, and 50 K/min are shown in Figure 13.

In Figure 13a–c, the DTG curve displays a single peak in the rate of weight loss, indicating the presence of only one activation energy for the pyrolysis process. Figure 13d presents the Friedman plot with αvalues ranging from 0.1 to 0.9, increasing by 0.1 increments, resulting in a total of nine data points. The curve in Figure 13d, derived from Figure 13c, shows the relationship between E and α. The average activation energy calculated is 147.75 kJ/mol. The relevant calculated parameters are shown in

Table 11. Conventional thermogravimetric reaction levels and pre-exponential factor.

Heating Rate (K/min)	Number of Reaction Stages (n)	lnA (s−1)	Temperature Range (K)
20	2.7	28.06	583–653
30	2.3	27.73	633–693
50	2.2	26.89	590–690

.

The pyrolysis kinetics of WPCBs are studied using the Friedman method. The activation energy is calculated using the KAS. The results indicate that the entire pyrolysis process of WPCBs can be described by a single kinetic equation. The kinetic parameters for the pyrolysis are as follows: an apparent activation energy of 147.75 kJ/mol, an average reaction order of 2.4, and lnA of 27.56.

The activation energy of microwave pyrolysis is approximately 45.1% lower than that of conventional pyrolysis, indicating that microwave pyrolysis requires less energy to achieve thermal decomposition compared to conventional methods [36,37]. Additionally, the reaction order in microwave pyrolysis shows a greater decrease compared to conventional pyrolysis [25,38]. From the perspective of pyrolysis temperature, microwave pyrolysis generally operates at slightly lower temperatures than conventional pyrolysis. This is mainly because microwave heating directly affects the material being heated, reducing heat transfer processes and saving reaction time, which is beneficial for energy conservation [37]. The kinetic study suggests that microwave pyrolysis, especially when combined with the addition of metal fillers, offers great advantages over conventional pyrolysis in terms of reducing reaction activation energy and pyrolysis temperature.

4. Conclusions

This study used a self-developed microwave reactor as the research platform to develop a novel microwave pyrolysis process for WPCBs and their resource recovery. Using desktop computer WPCBs as the research subject, it is found that the addition of alloy copper wire fillers obtained from WPCBs effectively used the “spark” phenomenon to convert microwave energy into heat energy, achieving the rapid pyrolysis of organic materials in WPCBs. Under operating conditions with 10% metal filler addition, an MP of 1600–1800 W, and a pyrolysis time of 10 min, the PLs yield reached 12.8%, with a composition of approximately 5–12 components. The sum of phenol and 4-isopropylphenol accounted for the largest proportion of the total PLs, ranging from 80% to 95%, higher than the 67%–75% achieved by conventional pyrolysis processes. The PG yield ranged from 12.7% to 13.4%, with approximately 9–11 gas components, where CO and CH₃Br constituted the largest proportion of the PGs, accounting for approximately 85–93%. Based on the microwave thermogravimetric analysis and conventional thermogravimetric data, the pyrolysis reaction kinetics are studied, resulting in an average activation energy of 81.05 kJ/mol for the microwave pyrolysis process, greatly lower than the 147.75 kJ/mol average activation energy of conventional pyrolysis, demonstrating clear superiority over conventional pyrolysis techniques. By utilizing the inherent metals in printed circuit boards to achieve additional feedstock, this process holds great significance for the resource recovery of actual WPCBs.