An innovative method of recycling metals in printed circuit board (PCB) using solutions from PCB production

Journal Pre-proof An Innovative Method of Recycling Metals in Printed Circuit Board (PCB) Using Solutions from PCB Production Quanyin Tan (Conceptualization) (Methodology) (Visualization)Writing- Original draft preparation)revising), Lili Liu (Formal analysis) (Resources)cosupervision)co-project administration), Miao Yu (Validation) (Formal analysis)writing - review and editing), Jinhui Li (Conceptualization)cosupervision)project administration)funding acquisition)

PII:

S0304-3894(19)31846-1

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121892

Reference:

HAZMAT 121892

To appear in:

Journal of Hazardous Materials

Received Date:

28 August 2019

Revised Date:

11 December 2019

Accepted Date:

12 December 2019

Please cite this article as: Tan Q, Liu L, Yu M, Li J, An Innovative Method of Recycling Metals in Printed Circuit Board (PCB) Using Solutions from PCB Production, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121892

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

An Innovative Method of Recycling Metals in Printed Circuit Board (PCB) Using Solutions from PCB Production

Quanyin Tan 1, Lili Liu 1, Miao Yu 1, 2, Jinhui Li 1 *

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1. State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

2. College of Humanities and Urban - Rural Development, Beijing University of

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Agriculture, Beijing, 100096, China

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* Corresponding Author：

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Mailing address: Room 804, Sino-Italian Environmental and Energy-efficient

100084, China

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Building, School of Environment, Tsinghua University, Haidian District, Beijing

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E-mail address: [email protected]

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Tel.: +86-10-62794143 Fax: +86-10-62772048

Graphical abstgract

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A win-win approach was developed to recycle metals in WPCBs using waste solutions.

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Highlights

Waste solutions used for metals recycling were generated in PCB production.

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WTSS and WES presented enormous capacity to extract Pb, Sn, and Cu from

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

WPCBs.

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

The approach can reduce energy consumption and secondary pollution in recycling.

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An Innovative Method of Recycling Metals in Printed Circuit Board (PCB) Using Solutions from PCB Production

ABSTRACT:

Waste printed circuit boards (WPCBs) have both a potentially high resource value and

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hazardous drawbacks. Meanwhile, large quantities of corrosive waste solutions are generated in PCB production. Existing methods for recycling metals in WPCBs produce high yields but unfortunately produce secondary pollution. In this study, to

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minimize these disadvantages, a win-win innovative recycling method for WPCBs

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was developed using waste solutions that are generated in PCB production. Both of the waste solutions - waste tin stripping solution (WTSS) and waste etching solution

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(WES) - had an enormous capacity to extract Pb, Sn, and Cu. It was suggested that 1 L of WTSS was potentially capable for dissolving solder from 3.6-7.2 kg of WPCBs

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under room temperature, while WES was capable for Cu leaching from 0.13-0.35 kg

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of WPCBs. Compared with conventional leaching solutions, it was demonstrated that approximately 1 kWh of electricity could be saved from the recycling process when

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WTSS and WES were used to recycle only 1 kg of WPCBs. The proposed approach can be expected to significantly reduce energy consumption for recycling metals from WPCBs, without additional waste solution generated, and to increase the potential value of WTSS and WES, as they can facilitate the recycling process.

Keywords: printed circuit board; waste solution; recycling; metals; closing loops. 3

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GRAPHIC ABSTRACT:

1. Introduction

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Waste electrical and electronic equipment (WEEE, or e-waste) has been a global

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concern since the early 2000s, because of both its resource value and its hazard potential [1, 2]. The generation of e-waste in 2016 came to a staggering 44.7 million metric tonnes (Mt) globally, with a potential value of 55 billion Euros in secondary materials. But only 20 percent - 8.9 Mt - was formally collected and treated [3, 4]. Printed circuit boards (PCBs), the core component of electrical and electronic

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equipment (EEE), still have a potentially high resource value when they become WPCBs (waste PCBs). The contents of valuable metals, such as copper (Cu), tin (Sn), gold (Au), silver (Ag), and palladium, are relatively higher in WPCBs than in natural ores [5, 6]. Yet WPCBs also have high hazard potential because of the toxicity of the heavy metals like chromium, cadmium, mercury, and lead, and by organic compounds like polybrominated diphenyl ethers (PBDEs) and Tetrabromobisphenol-A (TBBPA)

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[7-9].

In the last two decades, extensive studies have been conducted to develop approaches

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to WPCBs recycling, and these have mostly focused on physical, hydrometallurgical

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and pyrometallurgical recycling techniques [10-12]. Generally, physical techniques include the removal of components imbued with hazardous substances, dismantling,

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crushing, and the separation and enrichment of metals and non-metallic fractions. The

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separation and enrichment process can be achieved with a combination of gravitational separation, magnetic separation, and electrostatic separation [10]. The

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content of Cu can be enriched to more than 95%, and the contents of other valuable metals such as Au and Ag can also be significantly enriched after multi-stage

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enrichment operations, and can be subjected to further refining processes for recovery. The pyrometallurgical approach is more commonly used for metal recovery than is the hydrometallurgical one, in the industrial practices of formal sectors [13], but it has the disadvantages of high investment and operational costs, and requires highly technical equipment and expert staff [14-16]. 5

In hydrometallurgical techniques, metals are leached into solutions during reactions with leachant and oxidants (if necessary), and separation and purification are carried out to obtain primary products for refining. Hydrometallurgical techniques offer the advantages of low gas emission and slag generation, but consume a large volume of strongly corrosive chemicals [17-19], such as nitric acid (HNO3) [6, 20], sulfuric acid (H2SO4) [21-23], and aqua regia [24], sometimes including highly toxic substances

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such as cyanide lixiviant [6, 20]. The release of atmospheric pollutants will be

intensified, moreover, when the recovery processes are carried out at high temperature

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[25, 26]. Innovative approaches are therefore required to improve the environmental

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and economic performance of the recovery process [16].

The manufacturing process of PCBs requires approximately fifty procedures [27],

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including plate preprocessing, drilling, image transferring, etching, stripping, and

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electroplating [28]. Among these, the stripping and etching processes generate a large volume of waste solutions [29], such as waste tin stripping solution (WTSS) and waste

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etching solution (WES). It is reported that more than 0.75 m3 of WTSS and around 1.5 m3 of WES will be generated for each 1000 m2 of PCBs produced [30, 31].

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According to data released by the MIIT, 1.18×l06 m3 WTSS and 2.36×l06 m3 WES was generated in China alone in 2017, from PCB production [32]. The volumes of both WTSS and WES are expected to increase rapidly in China as the production of PCBs increases. In fact, such production increased approximately 9% during the period 2009-2016. Moreover, both WTSS and WES have been identified as hazardous 6

wastes, not only by China but also in the Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal, and they should be disposed of or recycled using environmentally sound methods.

In this study, an innovative method is proposed, that would effectively extract metals from recycled WPCBs, at room temperature, using WTSS and WES generated during

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PCB production. The proposed approach can not only meet the goal of recycling metal from WPCBs but can also facilitate the recycling processes of these two waste

solutions by increasing the concentration of metals in solution. This method can be

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expected to help to solve the recycling issues of WPCBs and waste hazardous

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solutions with a win-win approach, achieving a better environmental and economic

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performance of recycling, compared to traditional methods.

2. Materials and methods

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Materials The waste solutions WTSS and WES, generated during PCB production,

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were provided by a waste solution recycling company located in Kunshan City, Jiangsu Province, China. The solutions were directly used in the experiments without

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further treatment. The composition and density of these two solutions are shown in Table 1.

Table 1 Composition and density of WTSS and WES used in this study

WTSS

Sn (g/L)

Pb (g/L)

56.02

2.74

Cu (g/L) 5.20

Fe (g/L) 9.60

7

Ni

Zn

Al

(mg/L)

(mg/L)

(mg/L)

21.54

4.14

12.48

WES

NO− 3

H+

Cr

Cd

(mg/L)

(μg/L)

(g/L)

(mol/L)

8.86

0.50

307.5

4.96

Pb (g/L)

Cu (g/L)

1.7

134.75

NH4+ (g/L) 0.21

Cl (g/L) 180

Density (g/mL) 1.25 H+ (mol/L) 0.76

Density (g/mL) 1.67

Experiment Design Experiments in this study were conducted in two stages (Table 2). In the first stage, the capacity of WTSS for leaching Sn and Pb and the capacity of

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WES for leaching Cu were determined. Excessive amounts of solder wire (SW) were

subjected to leaching by WTSS, at room temperature (25°C) with magnetic stirring

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(400 rpm), while excessive amounts of copper powder (CP) were subjected to

leaching by WES under the same conditions, to determine the leaching capacities of

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both WTSS and WES.

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The amount of Pb leached into solution from solder after 48 h of reaction time were

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taken as the leaching capacity of WTSS for Pb, since the amount of Pb leached out increased by no more than 5% during the first 24 h. The capacity for solder

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dissolution by WTSS was calculated, accordingly, based on the content of Pb in the solder, because the Sn presented a higher reactivity with acid than did the Pb, causing

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the ions of Sn to partially precipitate in the form of stannic acid (SnO2·H2O) during the dissolution process, as the Sn concentration in the solution increased [33, 34]. The reaction is presented as the following Equations (Eq.). Organic stabilizer, like phenolsulfonic acid, ascorbic acid, and gluconate and so on [35, 36] could help to retard the hydrolysis of Sn 4 into   H 2 SnO3 . 8

4Sn  10HNO3  4Sn( NO3 )2  NH 4 NO3  3H 2O Sn2  O2  4H   Sn4  2H 2O Sn4  2H 2O

  H 2 SnO3

  H 2 SnO3    H 2 SnO3

(1)

(2)

(3)

(4)

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For the capacity of WES, the amount of Cu dissolved into the solution after 7 days (168 h) of leaching was taken as the capacity, although there was still a slow growth

in the amount of Cu leached thereafter, and 7 days is long duration practicable in

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industrial practice, in terms of equipment efficiency. The reaction is presented as Eq.

CuCl2  Cu  CuCl2

(5)

Cu2Cl2  4Cl   2 CuCl3 

(6)

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2

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(5) and (6). The leaching of metallic Cu is mainly due to the oxidation of Cu 2 .

In the second stage, the impacts of solid-to-liquid ratio (S/L: g/g) and reaction

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temperature on the leaching performance of the target metals — Sn, Pb, and Cu — by

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WTSS and WES were investigated systematically, in order to determine the optimal

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recycling conditions.

The materials used in this study, their characteristics, the setting of the experimental conditions, and the sampling times are presented in Table 2. Quantitative analysis of the concentration of metals in the solutions was conducted using Inductively Coupled Plasma-Atomic

Emission

Spectroscopy

(ICP-AES:

IRIS

Intrepid

II

XSP,

ThermoFisher), while the concentration of anions was determined using Ion 9

Chromatography (IC, DIONEX ICS-2000, ThermoFisher). The experimental errors from the instrument and the operation of the experiment were tested, and showed errors of no more than 5%. Table 2 Materials in experiments and parameters

Waste solutio

Experiment stage

Settings

Material

Characteristics of Material

n

Sampling times of 0,

Capacity determination

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Room temperature;

First stage:

Solder wire made of 37% Pb

SW

and 63% Sn

0.5, 1, 1.5, 2, 2.5, 3, 6, 12, 24, and 48 h Set at S/L (g/g) of 1:1,

S/L

1:2, 1:3, 1:4, 1:5, and 1:6

stage Temper ature

WPCBs-

Set at temperatures of

NC

②

25, 40, 50, 60, 70, and 80oC

weight)

③

nodes

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Second

WPCBs with solder (6% by

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WTSS

①

and connection but

without

components

First stage:

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Room temperature;

Sampling times of 0,

Capacity

0.5, 1, 1.5, 2, 2.5, 3, 6,

reagent)

12, 24, 48, 72, 96, 120,

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determination

Copper powder (analytical

CP

144, and 168 h

WES

L/S

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Second

Copper-clad

1:6, 1:9, 1:12, and 1:15 CCLs

②

combined

laminates with

board,

Cu

and

used

for

Temper

Set at temperatures of

epoxy

ature

25, 40, 50, 60, and 70oC

making a new PCB

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stage

Set at S/L (g/g) of 1:3,

Notes: SW, solder wire; WPCBs-NC, WPCBs with solder and connection nodes but without components from waste TV sets; CP, copper powder; CCLs, copper-clad laminates. ① This type of solder was the most widely used in PCB production in prior years, and is still in widespread use, according to literatures [37] and PCB production surveys. ② WPCBs-NC and CCLs were cut into 5×5cm2 pieces for the experiments. The chemical composition of WPCBs-NC and CCLs can be found in the Table S1 of

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Supplementary material (SM). ③ Typical Pb/Sn solder, which is often used for joining different components of PCBs, accounted for 4–8% by weight [19, 37-39]. Calculation The leaching efficiency (δ) is calculated from the leaching amount and initial mass of each material. The Eq. (7) is as follows:

=

ML MI

(7)

where δ is the leaching efficiency (%); M L is the leaching mass (kg); and M I is

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the initial mass of each metal (kg).

The electricity consumed for the heating and stirring was considered to be the energy

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consumption of the leaching process. In the calculations, the assumption was made

that the energy consumed for maintaining a constant temperature could be ignored,

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compared with the heating and stirring process, when an insulated heater was used in

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the experiments. Furthermore, the energy consumption also varies with the shape and surface area of a heater, a point that was not elucidated in previous studies. The

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energy consumed for the heating ( EH (J)) and stirring ( ES (J)) could be calculated with Eq. (8) and (9), respectively. (9)

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EH  C  m  T

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where C (J/(kg×oC ) is the specific heat capacity of the leachant; m (kg) is the mass of the lixiviant; and T (oC) is the temperature variation during the heating process.

ES Pt

(10)

where P (W) is the power of the stirring equipment; and t is the stirring time (h), generally the same as the reaction time.

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Therefore, the total energy consumption ( ET ) could be calculated by Eq. (11):

ET  EH  ES

(11)

The price of electricity for industrial use during the daytime is 0.107 USD/kWh. Hence the cost of the electricity - F ($) - can be calculated by Eq. (12):

F  0.107  ET / (3.6 106 J / kWh)

(12)

3. Determination of leaching capacity of the employed waste solutions

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Leaching Capacity of WTSS. According to changes in the amounts of Pb and Sn leached into solution over time (Figure 1), the WTSS solution had an enormous

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capacity for leaching Pb and Sn at room temperature. The concentrations of Pb and Sn increased to 108.97 ± 5.29 mg/mL and 155.83 ± 13.50 mg/mL, respectively, after

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48 h of reaction time, and increases of 106.23 ± 5.01 mg/mL and 99.81 ± 10.06

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mg/mL, respectively, were obtained in their concentrations. Therefore, 106.23 ± 5.01 mg/mL was adopted as the capacity of WTSS for Pb leaching, and approximately

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287.11 mg/mL as the capacity of WTSS for solder dissolution. The results further indicated that a total amount of 180.88 mg of Sn was simultaneously extracted by 1

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mL of WTSS, with 55% of Sn extracted in solution while the other 45% precipitated

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as SnO2 after leaching.

It could also be observed that the amounts of Pb leached into solution increased rapidly during the first 6 hours, and an increase of 79.41 mg/mL – 29 times more than its initial concentration – was achieved, accounting for about 75% of the capacity for Pb leaching. After 12 h of leaching, 82% of the leaching capacity was attained. The 12

reaction rate of Pb leaching decreased significantly after 6 h, especially in the range of 24 – 48 h, when a limited increase of 3.82 mg/mL, accounting for 3.6% of the relevant capacity, was obtained in the concentration of Pb. The results presented in Figure 1 also indicated the precipitation of Sn after solder dissolution, since it showed increasing rates in the concentrations of Sn and Pb congruent with the ratio of Sn to

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Pb — 63/37 — in the solder.

Figure 1. Determination of capacity of WTSS on leaching solder

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It has been reported in previous studies that WPCBs account for approximately 4% of e-waste and contain about 4-8% of solder by weight on average [19, 37-39].

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Therefore, our results demonstrated that 1 L of WTSS could be used for the

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dissolution of solder from 3.6-7.2 kg of WPCBs, corresponding to 90-180 kg of e-waste, without heating, under optimal conditions. Leaching Capacity of WES. The results presented in Figure 2 show that WES had a considerable ability to leach Cu. The content of Cu leached in solution increased during the reaction time of 7 days (168 h), rapidly increasing to 20.07 ± 1.60 mg/mL

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in the first 3 h. Thereafter the rate slowed down, but remained stable during the interval of 48 – 168 h. The amount of Cu leached into solution from Cu powder increased to 35.44 ± 5.37 mg/mL after 168 h of leaching. This rate was taken as the capacity of WES for Cu leaching, since 168 h is relatively long for a metallurgical process in industrial application, although a further increase in the amount could be expected with increased leaching time. It has been suggested that Cu generally

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accounts for about 10-27% of WPCBs [19, 37, 40]. Therefore, our results indicated that 1 L of WES could extract the Cu in 0.13-0.35 kg of WPCBs in just 7 days,

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corresponding to approximately 3.25-8.75 kg of e-waste.

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Figure 2. Determination of capacity of WES to leach Cu 4. ANALYSIS OF SIGNIFICANCE OF CHIEF PARAMETERS IDENTIFIED The effects of S/L ratio and temperature on dissolving the solder in WPCBs by WTSS were investigated by determining the dissolved quantity of Pb (presented in Fig. 3), as the Sn concentration changed. As presented in Fig. 3, under room temperature, when 14

the S/L ratio decreased from 1:1 to 1:2, the quantity of Pb dissolved by WTSS significantly increased, from 27.52 mg/mL to 49.40 mg/mL, mainly because of the improvement of contact conditions between the solution and the solid when the volume of WTSS increased during extraction. The quantity of Pb dissolved in WTSS fluctuated between 47.82 mg/mL and 50.65 mg/mL when the S/L ratio ranged from 1:2 to 1:6, indicating that the S/L ratio would not affect the dissolution of solder when

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WPCBs could be completely immersed in WTSS. The results suggested that an S/L ratio of 1:5 could be used when WTSS is used for dissolving solder from WPCBs

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with components. Regarding the leaching of solder by WTSS under different

temperatures, no consistent trend was observed. The quantity of Pb dissolved

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fluctuated within the range of 25 – 80 °C, indicating that there was no significant

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relationship between temperatures and leaching amount, for these two metals.

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Figure 3. (a) Effect of L/S and temperature on leaching Pb from solder by WTSS (Note: reaction time, 6h; experiments with different L/S ratios were conducted at

room temperature; while experiments under different temperatures were conducted

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with an S/L ratio of 1:2.); (b) WPCB samples before and after solder dissolution by

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WTSS

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The effects of S/L ratio and temperature on Cu leaching from CCLs were investigated by measuring the reaction time needed for dissolving Cu completely; the relevant

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results are shown in Fig. 4. Cu leaching at L/S 3:1 required the longest time by far, among all these L/S settings—95 min—about 1.4 times the duration for the other L/S

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settings. The time needed for dissolving Cu from CCLs decreased slowly when the

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S/L ratio decreased from 1:6 to 1:18; the difference was no more than 8%. The shortest dissolution time was 64 min, at L/S 18:1, indicating that the leaching temperature could significantly reduce the time needed for Cu leaching by accelerating the reaction rate. The leaching time was 69 min when the leaching process was conducted under room temperature, while it decreased to 15.7 min—a

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reduction of 77%—when the temperature increased to 70oC. Thus, an S/L ratio of 1:6 was employed for leaching Cu from WPCBs without insulating coating. After 2 h of leaching under room temperature, the Cu in the WPCBs without insulating coating could be completely dissolved into WES and then recycled by precipitation and

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refining.

Figure 4. (a) Effect of L/S and temperature on leaching Cu from CCls by WES (Note: experiments with different L/S ratios were conducted under room temperature; while experiments of different temperatures were conducted with an S/L ratio of 1:6.);

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(b) samples of WPCBs without insulating coating before and after Cu dissolution by WES

5. COMPARISON OF FINDINGS WITH EXISTING PROCESSES Solder dissolution. Several different hydrometallurgical methods have been used for dissolving solder from PCBs, and their performances are compared here, based on the

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parameters of reaction temperature, S/L ratio, reaction time, leaching efficiency and stirring speed (Table 3). Among the presented studies, WTSS required lower reaction temperatures and less time, while achieving appropriate leaching efficiency without

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additional waste solution generated.

Table 3 Comparison of solder dissolution efficiency for five different solutions S/L

Time

ature

ratio

(min)

Stirring

efficiency

speed

(%)

(rpm)

Reference

o

( C)

(g/ml)

50

1:100

90

>99%

400

[41]

1:100

45

>99%

-

[42]

1:20

120

98.74

1:100

90

> 99

400

[43]

90

1:20

165

97.79

-

[44]

25

1:5*

90

94.85

-

This study

3+

(1.5 M) and Fe (0.5 M) HNO3：

Leaching

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Hydrochloric acid

Temper

na

90

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Leachant

Pb leaching 0.2M;

Sn leaching 3.5 M; Hydrochloric

acid

4+

50

ur

(1M) + Sn (10g/L) Hydrochloric

acid

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(5.5 M) WTSS

* Unit in g/g

The energy consumption of the heating and stirring processes for solder dissolution from 1 kg of WPCBs was calculated with Eqs. 4-7 and the results are presented in Table 4. Compared with the heating process, the energy consumption for stirring 18

accounted for a much smaller percentage—less than 15%—of the total energy consumption. Among the parameters, the reaction temperature and S/L ratio affected energy consumption the most. Reducing energy consumption of the heating process is the key to saving energy during the solder dissolution process. As presented in Table 4, the proposed dissolution could be conducted under room temperature, which posed significant advantages over approaches proposed in other studies. It could also

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provide the benefit of reducing equipment investment by eliminating the need for heating equipment. In summary, dissolving solder from 1 kg of PCBs using WTSS

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can save at least 0.9 kWh of power, amounting to approximately 0.10 USD.

Table 4 Energy consumption from heating and stirring, for five different solutions C (kJ/(kg×oC))

Hydrochloric acid+Fe3+

3.07

T (oC)

Pb leaching 0.2M; Sn leaching 3.5 M; + Sn4+ (10 g/L) Hydrochloric WTSS

acid

2.11

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(5.5M)

3.19

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Hydrochloric acid (1M)

2.68

F (USD)

7,675

270

0.24

65

5,122

495

5,617

0.17

25

7,975

270

8,245

0.25

65

2,743

495

3,238

0.10

NA

NA

NA

NA

NA

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3.94

ET (kJ)

7,945

25

(1.5 M+0.5 M) HNO3：

EH (kJ) ES (kJ)

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Leachant

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Note: It was assumed that the amount of treated PCBs was 1 kg. The average power required for stirring was set to 50W according to the stirring speed used in these experiments and consultations to manufacturers of agitators. NA indicates ‘not applicable’, as no-heating nor stirring are needed according to leaching conditions presented in Table 3. Cu leaching. Extensive effort has been devoted to recovering Cu from WPCBs by hydrometallurgy. Here, typical approaches are summarized and compared in Table 5.

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Because of the weak reducibility of copper, the leaching time could be shortened significantly when adequate oxidant, such as HNO3, H2O2 or Cu(II), is present in the leachant. Table 5 Comparison of Cu leaching efficiency for five different solutions Leachant

Temperature

S/L ratio

Time

Leaching

Stirring

( C)

(g/ml)

(min)

efficiency (%)

speed (rpm)

Nitric acid (2 M)

50

1:10

210

78

-

[23]

Sulfuric acid+H2O2

25

3:20

60

85

200

[21]

25

1:3

1440

21

25

1:6

32

91.18

o

Reference

Cu(II)/NH3/NH4+

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(1.7 M+ 17%) (0.5/5/4 M)

[45]

400

This study

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WES

500

The energy consumption of the heating and stirring processes for leaching Cu from 1

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kg of WPCBs was calculated with Eqs. 4-7 and the results are presented in Table 6.

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For Cu leaching by nitric acid, the energy consumption for stirring accounted for about 40% of the total energy consumption for the leaching process, while for other

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approaches, stirring dominated the energy consumption, since Cu leaching was conducted under room temperature. Leaching with WES used the least energy

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because of its relatively short reaction time, potentially saving at least 84 kJ of energy.

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Table 6 Energy consumption from heating and stirring for four different solutions Leachant

C (kJ/(kg×oC))

Nitric acid (2 M)

3.72

Sulfuric acid+H2O2

T (oC) EH 25

930

(kJ)

ES (kJ)

ET (kJ)

630

1560 0.046

-

0

NA

180

180

-

0

NA

4320

4320

(1.7 M+ 17%) Cu(II)/NH3/NH4+

F (USD)

(0.5/5/4 M) 20

0.005 0.128

WES

-

0

NA

96

96

0.003

Note：It was assumed that the amount of WPCBs was 1 kg. The average power required for stirring was set to 50W according to the stirring speed used in these experiments and consultations to manufacturers of agitators. – indicates that the value of specific heat capacity is not needed for the calculation. NA indicates ‘not applicable’, as heating is not needed according to leaching conditions presented in Table 5. For WPCB recycling using hydrometallurgy, methods using newly produced solutions as leachant would generate much more waste solution, depending on the S/L ratio,

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when recycling 1 kg of WPCBs, except for the method using WTSS and WES. While it is certainly possible that other solutions could be reused or recycled in some way,

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ultimately they are changed from pure products into waste. However, WTSS and

WES are themselves hazardous wastes, and would be subject to hazardous waste

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recycling, to recover the Pb, Sn, and Cu. After reaction with WPCBs, the

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concentrations and contents of Pb, Sn, and Cu increased significantly, potentially facilitating a subsequent recovery process that would transform WTSS and WES into

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solutions with higher value, but without additional waste solution generated. Furthermore, co-processing of WPCBs by WTSS and WES could also avoid the

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negative environmental impacts caused by the production of leachant when

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considering the entire life cycle of PCBs. 6. SPECIFICATIONS FOR THE INNOVATIVE PROCESS FOR TREATING WPCBS The specifications for the innovative process for treating WPCBs, introduced in this study are shown in Figure 5. The entire specification contains two recovery processes: the recovery of solder and the recovery of Cu, using waste solutions from commercial

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PCB production. For the recovery of solder, WTSS is used to extract the solder from WPCBs, leaving the remaining components (unprocessed waste) and the WPCBs minus the components (WPCBs-NC). For the recovery of Cu, WES is used to leach Cu from the CCLs: i.e., the remaining WPCBs-NC after the removal of paint. All that remains then will be the epoxy circuit boards. A pilot project based on the proposed approach has been established with a theoretical capacity of recycling 5400 metric

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tons of WPCBs in the waste solution recycling company, who provides the WTSS and WES for experiments (see Figure S1 in SM). The content of air pollutants, which was

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generated during the co-processing in the pilot project, was detected much lower than

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that in the Integrated Emission Standard of Air Pollutants of China (SM Table S2).

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Figure 5. Specifications for innovative process for treating WPCBs

This innovative recovery of PCBs was carried out using WTSS and WES generated during PCB production. WTSS was shown to have an enormous capacity for leaching Pb and Sn, and WES can leach Cu. These two waste solutions can recover metals in

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an environmentally friendly way, and could be applied to the much wider range of WPCB materials that occur in e-waste. 7. CONCLUSION Waste tin stripper solution and waste etching solution generated in PCB production are hazardous wastes that should be disposed of in an environmentally sound manner. However, they possess an enormous capacity for dissolving solder and Cu,

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respectively, and could be used to recycle WPCBs. It was suggested that 1 L of

WTSS was potentially capable of dissolving solder from 3.6-7.2 kg of WPCBs under

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room temperature, while WES was capable of leaching Cu from 0.13-0.35 kg of WPCBs. Regarding the recycling process, reaction temperature showed a positive

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effect in accelerating the reaction rate of Cu leaching by WES. However, no

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significant improvements were observed in solder dissolution efficiency by WTSS when different temperatures were used. The recycling performance remained similar

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as the amount of liquid increased, as long as the volumes of WTSS and WES were adequate for solder dissolution and Cu leaching. Compared with conventional

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leaching solutions, recycling WPCBs using WTSS and WES could achieve significant

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energy savings. It was demonstrated that approximately 1 kWh of electricity could be saved from the recycling process when WTSS and WES were used for recycling only 1 kg of WPCBs. Hence, a win-win approach for recycling hazardous waste was proposed for WPCBs—using waste solutions generated in PCB production, at room temperature—that could significantly reduce energy consumption for recycling metals

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from WPCBs without additional waste solution generated, increasing the potential value of WTSS and WES, as they can be used to facilitate the recycling process.

CRediT author statement

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Quanyin Tan: Conceptualization, Methodology, Visualization, WritingOriginal draft preparation, revising; Lili Liu: Formal analysis, Resources, co- supervision, co-project administration;

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Miao Yu: Validation, Formal analysis, writing - review & editing;

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Jinhui Li: Conceptualization, co- supervision, project administration, funding acquisition.

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Declaration of Interest Statement

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No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my

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co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

ACKNOWLEDGEMENTS

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This research was supported by the “National Key R&D Program of China” (2018YFC1900101), the “National Natural Science Foundation of China” (71804085) and the “China Postdoctoral Science Foundation” (2019T120104). We acknowledge Ab Stevels from Delft University of Technology, the Netherlands, and Xianlai Zeng from Tsinghua University, China, for their valuable comments and suggestions. We also acknowledge Chao Deng and Jin Wang for their valuable efforts in the

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experiments.

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