Recovery of palladium as nanoparticles from waste multilayer ceramic capacitors by potential-controlled electrodeposition

Journal Pre-proof Recovery of palladium as nanoparticles from waste multilayer ceramic capacitors by potential-controlled electrodeposition Ya Liu, Lingen Zhang, Qingming Song, Zhenming Xu PII:

S0959-6526(20)30417-0

DOI:

https://doi.org/10.1016/j.jclepro.2020.120370

Reference:

JCLP 120370

To appear in:

Journal of Cleaner Production

Received Date: 16 September 2019 Revised Date:

13 January 2020

Accepted Date: 31 January 2020

Please cite this article as: Liu Y, Zhang L, Song Q, Xu Z, Recovery of palladium as nanoparticles from waste multilayer ceramic capacitors by potential-controlled electrodeposition, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120370. 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. © 2020 Published by Elsevier Ltd.

Ya Liu designed and conducted the experiments, and wrote the original manuscript; Lingen Zhang and Qingming Song provided the experimental materials, and participated in data analysis and manuscript revision; Zhenming Xu designed the research project, provided the funding, and supervised the experiments and manuscript preparation.

Graphical abstract

1

Recovery of palladium as nanoparticles from waste multilayer ceramic

2

capacitors by potential-controlled electrodeposition

3 4

Ya Liua, Lingen Zhanga, Qingming Songa, Zhenming Xua, b*

5 a

6

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China

7

b

8

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, People’s Republic of China

9 10 11 12 13 14 15 16 17 18 19

Corresponding author: Zhenming Xua, b

20

E-mail: [email protected]

21

Tel: +86 21 5474495

22

Fax: +86 21 5474495

23

a

24

Shanghai Jiao Tong University

25

800 Dongchuan Road, Shanghai 200240, People’s Republic of China

26

b

27

Shanghai 200092, People’s Republic of China

School of Environmental Science and Engineering

Shanghai Institute of Pollution Control and Ecological Security,

28 29 30 1

1

Abstract

2

Recycling palladium from waste multilayer ceramic capacitors (MLCCs) has

3

attracted much attention recently. The existing recycling methods have many

4

shortcomings including pollution, low purity and recovery rate of palladium. This

5

study proposed to separate and purify palladium from waste MLCCs by

6

electrodeposition technology efficiently based on the difference in reduction

7

potentials of metal ions in HNO3 solution. The electrochemical behavior of palladium

8

at titanium electrode was studied, which showed higher concentration of HNO3

9

increased the peak current of palladium and the corresponding potential shifted

10

negatively. Besides, a mathematical model was established to describe the

11

electrodeposition process at different agitation speeds. The diffusion coefficient and

12

thickness of diffusion region were calculated. Furthermore, the optimal conditions for

13

electrodeposition were determined as applied potential of -0.25 V, agitation speed of

14

240 rpm and HNO3 concentration of 0.5 M, under which high-purity palladium

15

(>99 %) was recovered with the recovery rate of 99.02 %. The morphology analysis

16

showed the electrodeposited palladium was nanoparticles. Finally, an efficient process

17

combining enrichment and electrodeposition for recycling waste MLCCs was

18

proposed in an environmentally friendly way. The results of this study can provide

19

reliable information for efficient recycling of palladium resource from waste MLCCs.

20 21

Key words: Electrodeposition, Electrochemical behavior, Palladium, Waste

22

multilayer ceramic capacitors

23 24 25 26 27 28

2

1

1. Introduction

2

Electronic waste (e-waste) has been considered as the fastest growing segment of

3

solid waste in the world (Xue et al., 2015). It was reported that more than 40 million

4

tons of e-waste was discarded every year worldwide (Baldé et al., 2017). Therefore,

5

the disposal of e-waste has become a global issue that needs to be addressed because

6

of its harmfulness to the environment and humans (Li et al., 2019; Wang and Xu,

7

2015). However, e-waste also contains a large number of valuable materials, such as

8

metals including Cu, Pd, Ag, In, etc (Ghosh et al., 2015; Zhang and Xu, 2018). As a

9

result, the recycling of e-waste is much significant for protecting the environment and

10

improving the efficiency of resource utilization (Zeng et al., 2018).

11

Multilayer ceramic capacitors (MLCCs) are widely used in various electronic

12

products due to their advantages of good stability, small size and large specific

13

capacity (Niu and Xu, 2019a, b). It was reported that there are 300, 200 and 150

14

pieces of MLCCs in a digital TV, a personal digital assistant and a mobile phone,

15

respectively (Kim et al., 2007). So with the generation of e-waste, a large number of

16

waste MLCCs will be discarded, which need to be treated properly to prevent

17

polluting environment. However, MLCCs contain plenty of metal resources, which

18

are more than conventional natural ores (Liu et al., 2018; Lu and Xu, 2016),

19

especially precious metals like palladium (Pd). For example, 80 % of Pd in one

20

printed circuit board (PCB) of a computer is contained in MLCCs (Delfini et al.,

21

2011). And 13 % of the world's Pd is consumed by the electronics industry (Loferski,

22

2013). Hence, the recovery of waste MLCCs is of great significance for

23

environmental protection and resource recovery, which has received much attention in

24

recent years. Prabaharan et al. (2016) have studied a hydrometallurgical technique

25

including leaching and selective precipitation to recover metals from waste MLCCs

26

and the overall recovery rate of precious metals reached 92%. Niu and Xu (2017)

27

have recovered Pd with recovery rate of 92.36 % and purity of 70.27 % from waste

28

MLCCs by chloride metallurgy combined with corona electrostatic separation.

3

1

The structure of MLCCs is complex, of which various metals and non-metal are

2

closely connected, and the content of Pd per unit mass is low, so it is difficult to

3

recover Pd from them. Traditional methods of recovering Pd mainly focus on

4

hydrometallurgy process (Lu and Xu, 2016), in which Pd is first leached and then

5

extracted from the leaching solution. The involved technologies are mainly leaching,

6

solvent extraction, ion exchange, etc. Paiva et al. (2017) have recovered Pd from

7

spent industrial catalyst by the method of chloride leaching and liquid-liquid

8

extraction with thioamide and diglycolamide derivatives. Zhang and Zhang (2014)

9

have a research on the recovery of Pd from waste printed circuit boards (WPCBs) by

10

leaching with CuSO4 and NaCl solution and extracting with diisoamyl sulfide, and the

11

recovery rate reached 96.9%. In a research of Nikoloski et al. (2015), ion exchange

12

technology was applied to recycle Pd from acid chloride leaching solution and a resin

13

with a thiouronium functional group has good adsorption performance for Pd (II)

14

complexes. Traditional hydrometallurgy has the advantages of easy control and low

15

energy consumption, but it also has many disadvantages (such as producing lots of

16

waste liquid). Furthermore, multiple and complex recovery steps will lead to poor

17

recovery rate and purity of Pd (Liu et al., 2016). In addition to hydrometallurgical,

18

pyrometallurgy has also attracted extensive attention. Zhang et al. (2019) applied a

19

capture technology of eutectic copper to recycle Pd with the recovery rate of 97% in

20

spent catalyst from automobile and the obtained Cu-Pd alloy required further

21

purification.

22

Electrodeposition is a promising technique (Waware et al., 2018) for the

23

separation and purification of metals because of its advantages of controllability,

24

simplicity, no need of additional reagent, and low equipment requirements (Song et

25

al., 2020; Zou et al., 2017). In addition, it is considered an environmentally and

26

economically attractive method (Kasper et al., 2018). Guan et al. (2017) have studied

27

the

28

electrooxidation-electrodeposition and the recovery rate could reach 85-95%. Su et al.

29

(2017) have recovered copper and selenium with the recovery rates of 93.2 % and

30

97.6 % by simultaneous electrodeposition from sulfuric acid solution. And in a

recovery

of

nickel

from

nickel-ammonia

4

complexes

wastewater

by

1

research of Matsumiya et al. (2018), indium was recovered by the methods of ionic

2

liquid extraction and electrodeposition. Liu et al. (2013) have studied the

3

electrochemical properties of Pd in nitric acid medium and the recovery rate of Pd

4

was high via electrodeposition method. However, there is scant research on the

5

recovery of Pd from waste MLCCs with various metals by the technology of

6

electrodeposition. Moreover, the electrochemical behavior of Pd at titanium electrode

7

in this system has not been studied in depth.

8

In view of the special structure of waste MLCCs, the existing methods for

9

recycling Pd from them have some shortcomings including pollution, low purity and

10

recovery rate. Therefore, this study aims to propose an efficient process to recycle Pd

11

in an environmentally friendly way. Pd of waste MLCCs was firstly enriched by

12

pyrometallurgy method of copper capture, and then separated and purified by

13

electrodeposition technique. The electrochemical behavior of Pd at titanium electrode

14

was analyzed in HNO3 system and a mathematical model was established to describe

15

the process. Besides, the influence factors on the recovery were studied and

16

high-purity Pd (>99 %) was recovered in the form of nanoparticles with the recovery

17

rate of 99.02 %. Finally, a novel process combining enrichment and electrodeposition

18

for recycling waste MLCCs was proposed in an efficient way. In summary, this study

19

provides a theoretical basis for efficient recycling of Pd resource from waste MLCCs

20

by electrodeposition method and proposes an efficient process for recycling waste

21

MLCCs.

22

2. Material and method

23

2.1. Material and experimental equipment

24

Waste MLCCs used in this study are formed by alternating layers of dielectric

25

layers and internal electrodes (M.J. Pan, 2010) as shown in Figure 1(a). The dielectric

26

material is mainly BaTiO3, and Pb is doped to enhance the dielectric properties. The

27

internal electrode is mainly composed of Ag-Pd alloy or nickel (Ni). And the terminal

5

1

electrode mainly consists of three layers of Ag, Ni and tin (Sn). All chemical reagents

2

used in the experiments were of analytical grade unless otherwise mentioned.

3

The electrochemical experiments were performed using an electrochemical

4

workstation (CHI 660D, Shanghai Chenhua Electric Co, China), as shown in Figure

5

1(b).

6

bath with magnetic stirrer. The electrodeposition reactions were carried out in a

7

single-chamber electrolysis cell. Titanium electrode (surface area = 4.5 cm2), platinum

8

plate (surface area = 8 cm2) and Hg/Hg2SO4 electrode were used as the working,

9

counter and reference electrodes, respectively. And the potentials mentioned in the

10

The

temperature

and

agitation

speed

were

controlled

by

a water

paper were all taken Hg/Hg2SO4 electrode as the reference.

11 12

Figure 1. (a) Schematic diagram of MLCCs; (b) Diagram of electrochemical

13

experimental equipment

14

2.2. Experimental procedures

15

Waste MLCCs were collected during the automatic disassembly of WPCBs

16

(Wang and Xu, 2015) and the specific experimental procedures were shown in Figure

17

2. The sample was firstly crushed into powdered waste MLCCs with the particle size 6

1

of less than 0.180 mm by a ball mill. Then the technology of copper capture was

2

applied to enrich the precious metals of powdered waste MLCCs with 15 wt.% of

3

CuO, 1:5 of m(CuO)/m(C), 1350

4

m(Al2O3)/m(SiO2) and holding time of 2 h. The recovery rates of Pd and Ag were

5

100 % and 87.53 % in the enrichment process. The products obtained were mainly

6

BaO-Al2O3-SiO2 (BAS) glass and alloy phases. The obtained Cu-Pd-Ag alloy, which

7

contained small amounts of Pb and Sn was leached with nitric acid (concentration of

8

1:1) to obtain nitrate solution. The leaching rates of Pd, Ag, Cu and Pb were 95 %,

9

100 %, 100 % and 89 %, respectively. Filtration was carried out to remove a small

10

amount of insoluble substances including SnO2. And then Ag was recovered in the

11

form of AgCl by adding NaCl to the nitrate solution. The remaining

12

Cu(NO3)2-Pd(NO3)2 solution was the raw material for electrochemical experiments,

13

which contained 1.05 mol/L Cu(NO3)2, 0.26 mol/L Pd(NO3)2, and 0.02 mol/L

14

Pb(NO3)2. Besides, the concentration of nitric acid was 0.5 M.

of the melting temperature, 1:9 of

15

7

1 2

Figure 2. Experimental procedures for MLCCs recycling

3

The formal electrochemical experiments were carried out in the simulated

4

solution with a fixed ratio. The solution of Pd(NO3)2 (10.0 mM), Cu(NO3)2 (40.5

5

mM), and Pb(NO3)2 (0.8 mM) were prepared by dissolving a required quantity of

6

solid powder of Pd(NO3)2, Cu(NO3)2, and Pb(NO3)2 in appropriate concentration of

7

nitric acid (50 mL). Finally, Pd and Cu were separated, purified and recovered by

8

two-step electrodeposition at the temperature of 25

.

9

The recovery rate of Pd was calculated by measuring the concentrations of Pd in

10

solution before and after electrodeposition at different time points, which was shown

11

in eq (1). Where C0 was the initial concentration of Pd in the solution and Ct was the

12

concentration of Pd after electrodeposition at different time points. The current

13

efficiency was calculated by the ratio of theoretical and practical electricity

14

consumption of Pd electrodeposition. As shown in eq (2), 2 is the number of

15

electronic transfer, V (L) is the volume of the solution, F (C/mol) is the Faraday

8

1

constant, which is 96485.33 C/mol and Qt (C) is the actual electricity consumption.

η=

3

4

C0 − C t × 100% Ct

(1)

2(C0 - Ct )VF ×100% Qt

(2)

R=

2

2.3. Chemical analysis

5

The composition of waste MLCCs was determined by X-ray Fluorescence

6

Spectrometer (XRF-1800, Shimadzu, Japan) and Inductively Coupled Plasma Atomic

7

Emission Spectrometry (ICP-AES, iCAP6300, Thermo, USA). And the concentration

8

of metals in the solution was also analyzed by ICP-AES. The morphology of obtained

9

Pd was observed by Field-emission Scanning Electron Microscopy & Energy

10

Dispersive Spectrometer (SEM Sirion 200 & EDS INCA X-Act, FEI Company,

11

America & Oxford Company, England).

12

3. Results and discussion

13

3.1. Feasibility for electrochemical recovery of palladium

14

According to the analysis results, the main composition of metals in MLCCs is

15

presented in Table 1, of which Pd and Ag are 0.95 % and 5.01 %, respectively. After

16

the pretreatment processes for waste MLCCs, the remaining solution was used as the

17

raw material for electrochemical experiments, which contained 1.05 mol/L Cu(NO3)2,

18

0.26 mol/L Pd(NO3)2, and 0.02 mol/L Pb(NO3)2.

Table 1. Main composition of metals in MLCCs (wt.%)

19

Ba

Ti

Pb

Ag

Nb

Sn

Pd

Ni

34.48

14.06

8.82

5.01

3.48

1.00

0.95

0.68

20

Separation and recovery of different metals from solution by electrodeposition

21

are based on their different reduction potentials. In terms of thermodynamics, the

22

equilibrium potentials of Pd2+/Pd, Cu2+/Cu, Pb2+/Pb, H+/H are quite different. And the 9

1

reactions at the cathode are shown by eq (3)-(6), where the electrode potential is a

2

value relative to the potential of the standard hydrogen electrode (SHE). The reaction

3

at the anode is shown by eq (7). According to the electrode reactions and Nernst

4

equation shown in eq (8), the theoretical electrode potentials are calculated as 0.856 V

5

of eq (3), 0.299 V of eq (4) and -0.22 V of eq (5) in this study system.

6

Pd 2+ + 2e − → Pd

E0=0.915 V (vs. SHE) (3)

7

Cu 2+ + 2e − → Cu

E0=0.34 V (vs. SHE)

8

Pb 2+ + 2e − → Pb

E0=-0.13 V (vs. SHE) (5)

9

2 H + + 2e − → H 2

E0=0 V (vs. SHE)

10

2 H 2O − 4e − → O2 + 4 H +

11

Eh = E0 +

RT c Ln Ox nF cRe d

(4)

(6) (7) (8)

12

Generally, certain overpotential is required to drive the electrodeposition reaction

13

and reach the desired current value due to electrochemical polarization and

14

concentration polarization. Linear sweep voltammogram (LSV) was used to

15

investigate the deposition potentials of various metal ions in the solution. Figure 3(a)

16

shows LSVs of simulated solution, HNO3 solution, Pd (II) and Cu (II) in nitric acid

17

medium recorded at titanium electrode with the potential sweeping rate of 10 mV/s.

18

The comparison of those LSVs showed that at the potential of -0.05 V, the cathode

19

current started to surge and reached its peak value at -0.13 V, which was due to the

20

reduction of Pd (II). Similarly, a surge of the cathode current observed occurring at

21

the potential of -0.42 V was due to the reduction of Cu (II), which culminated in a

22

peak at -0.54 V. Hydrogen reduction began to occur violently at the potential of -0.7 V.

23

Since the reduction potential of Pb (II) was smaller than that of hydrogen and the

24

content of Pb2+ was low, its reduction peak was covered by the reduction of H+. In

25

summary, due to their different reduction potentials, it is feasible to separate and

26

purify Pd from the solution by potential-controlled electrodeposition.

10

1 2

Figure 3. (a) LSVs of simulation solution, Pd solution, Cu solution and HNO3

3

solution; (b) Complex species of Pd (II) nitrate in different concentrations of nitric

4

acid (Jayakumar et al., 2009)

5

3.2. Electrochemical behavior of Pd

6

The recovery of Pd is influenced by HNO3 concentration, agitation speed and

7

deposition potential. To investigate the effect of HNO3 concentration, LSVs in 0.5 M,

8

1 M and 2 M HNO3 were performed at 298 K, as shown in Figure 4(a). It can be seen

9

that with the increase of HNO3 concentration, the potential of the peak current of Pd

10

shifted negatively, and the peak current of cathode decreased, which was attributed to

11

the conversion of Pd (II) ion into different complexes as shown in Figure 3(b)

12

(Jayakumar et al., 2009). It can be seen that as the HNO3 concentration increased, the

13

concentration of Pd (II) ion decreased, while that of [Pd(NO3)2(H2O)2] increased. And

14

[Pd(NO3)(H2O)3]+ reached its maximum concentration at about 1.1 M HNO3. Hence,

15

as the concentration of HNO3 increased, the charge of the electroactive material

16

decreased, which lowered the diffusion, thereby reducing the peak current of the

17

cathode.

18

11

1 2

Figure 4. (a) Comparison of LSVs of Pd(II) in different concentrations of HNO3; (b)

3

LSVs of Pd(II) at different agitation speeds in 0.5 M HNO3; (c) CA curves at different

4

agitation speeds; (d) Curves fitted by CA data at different agitation speeds; (e)

5

Schematic diagram of diffusion region and turbulent region; (f) Distribution of

6

volume concentration in diffusion and turbulence regions

7 8

Figure 4(b) shows LSVs of Pd(II) in 0.5 M HNO3 at various agitation speeds. A

9

sharp increase in the cathode current was observed as the applied voltage increased

10

due to the reduction of Pd (II). And the limiting current was obtained at around -0.25

11

V, which increased as the agitation speed increased. Then the cathode current 12

1

continued to increase at around -0.45 V on account of the reduction of H+ ion.

2

The maximum reaction speed was affected by the condition of mass transfer.

3

With the increase of agitation speed, the mass transfer was easier and the limiting

4

current was larger (Song et al., 2020). When the limiting current was reached, the

5

concentration of electroactive substance on the electrode surface was zero. In this

6

electrodeposition experiment, since the volume concentration did not change much in

7

a short period of time, it was considered that a steady state could be established to

8

describe the electrodeposition mechanism. The mathematical model was established

9

accordingly. Assuming that there was a diffusion region with a thickness of δ on the

10

surface of the electrode, it remained unchanged and was distinguished from turbulent

11

region as shown in Figure 4(e). The concentration in the turbulent region was C0* by

12

agitation as shown in the Figure 4(f). In order to eliminate the influence of

13

electromigration, 0.1 mol/L of NaSO4 was added as the supporting electrolyte. The

14

concentration of electroactive substance varied with the distance (x) from the

15

electrode and time (t). Therefore, under the condition of mass transfer control, the

16

current can be calculated by eq (9).

17

i t = nFAD0

dC ( x, t ) dx

(9)

18

Where n is the number of electron transfer, F is the Faraday constant, A is the surface

19

area of the electrode, and D0 is the diffusion coefficient.

20

The electrochemical detection technique of chronoamperometry (CA) is simple

21

and commonly used to calculate the relationship between current, diffusion coefficient

22

and the solution concentration. The potential reaching the limiting current was

23

selected for determination. The diffusion coefficient of Pd in HNO3 system was

24

calculated using the well-known Cottrell equation in eq (10). 1

25

it =

nFD0 2 AC * 1 1 2 2

(10)

π t 26

As can be seen from Figure 4(c), CA curves can be roughly divided into three

27

stages. Firstly, double layer charge was carried out in a very short time. And then the

28

diffusion layer developed in the second stage. Finally, the diffusion layer reached the

13

1

turbulent region and the mass transfer approached the steady state gradually. The

2

Cottrell equation was fitted with CA data of the second stage with t-1/2 as the

3

horizontal coordinate and current as the vertical coordinate in Figure 4(d). And the

4

diffusion coefficients at different agitation speeds were obtained as shown in Table 2.

5

It can be observed that agitation disturbed the diffusion layer, leading to the decrease

6

of diffusion coefficient.

Table 2. Diffusion coefficients at different agitation speeds

7

8

Agitation rate / rpm

0

80

120

160

240

D×1010/m2·s-1

19.98

18.16

8.44

7.91

7.87

3.3. Electrodeposition experiments of Pd

9

The influence of HNO3 concentration and agitation speed on electrochemical

10

behavior of Pd in HNO3 solution has been studied in section 3.2. However, they also

11

affect the recovery rate of Pd, deposition rate and current efficiency, and the applied

12

voltage is also an important factor. In addition, it should be noted that, since the

13

hydrogen evolution overpotential of palladium electrode is lower than that of titanium

14

electrode, the deposition of Pd will reduce the hydrogen evolution potential of the

15

electrode, which will lead to lower current efficiency. However, due to the high value

16

of Pd, we tried to achieve a higher current efficiency within the accepted range in this

17

study.

18

To study the effect of HNO3 concentration, LSVs of Pd(II) in different

19

concentrations of HNO3 at 240 rpm were shown in Figure 5(a), which displays that

20

the limiting current was reached at the potential of -0.25 V. The limiting current was

21

larger at higher HNO3 concentration, but the difference is not obvious. Besides, by

22

comparing the electrodeposition results, the recovery rates of Pd in 0.5 M, 1 M and 2

23

M HNO3 showed little difference, but the current efficiencies were low in 1 M and 2

24

M HNO3. Therefore, the concentration of HNO3 in this system was determined to be

25

0.5 M.

26

3.3.1. Effect of applied potential 14

1

The effect of applied potential on the recovery rate of Pd was studied with the

2

agitation speed of 240 rpm in 0.5 M HNO3. Lower potential will accelerate the rate of

3

electrodeposition, but it may result in more intense hydrogen reduction (Jayakumar et

4

al., 2009), which will affect the mass transfer near the electrode and current efficiency.

5

According to Figure 5(a), when the potential was -0.25 V, it almost reached the

6

limiting current. Therefore, the effects of applied potentials at -0.2 V, -0.25V and -0.3

7

V on Pd electrodeposition were investigated. And the results are shown in Figure 5(b).

8

It can be observed that when the applied potential was -0.25 V, the deposition rate of

9

Pd was the fastest, and the recovery rate reached 99.02 % after 4 hours of

10

electrodeposition. Meanwhile, the color of the solution changed from green to blue,

11

and blue was the color of the remaining Cu(NO3)2 solution. While at the potential of

12

-0.2 V, the deposition rate was the slowest and the recovery rate of Pd was lowest,

13

which was due to that the current was relatively smaller than the limiting current.

14

Although the potential of -0.3 V was smaller than the potential of limiting current, the

15

reaction of hydrogen evolution was more intense under this condition. That will

16

disturb the process of Pd deposition, leading to a lower recovery rate and slower

17

deposition rate. But there was no significant difference with that under the condition

18

of -0.25 V. However, the current efficiencies of -0.25 V and -0.3 V were 52.69 % and

19

44.59 %, respectively. Therefore, -0.25 V was considered as the preferred deposition

20

potential.

21

Besides, the relationship between Pd concentration in solution and current

22

density as shown in Figure 5(c) indicates a linear distribution of concentration in the

23

diffusion region. Therefore, the relationship between current and Pd concentration in

24

eq (8) can be expressed as eq (10).

25

i t = nFAD0

C * ( t ) − C (0, t )

δ

(11)

26

The value of C(0,t) was 0 at the applied potential of -0.25 V. Then the diffusion

27

region thickness, δ can be calculated to be 39.38 µm by referring to D value in Table 2

28

at the agitation speed of 240 rpm. The surface concentration of electrode at the

29

potential of -0.2 V was calculated to be 16.44 % of the volume concentration, which 15

1

indicated that the potential was not enough to deposit the electroactive material on the

2

electrode. The thickness of diffusion region, δ was calculated to be 33.90 µm at the

3

potential of -0.3 V, which was slightly smaller than that of -0.25 V. That was caused

4

by the interference of hydrogen generation with the diffusion layer, which affected the

5

current efficiency. In summary, -0.25 V was determined to be the optimal applied

6

potential.

7 8 9

Figure 5. (a) LSVs of Pd(II) in different concentrations of HNO3 at 240 rpm; (b)

10

Effect of applied potential on the recovery rate of Pd; (c) Linear relationship between

11

Pd concentration and current density

12

3.3.2. Effect of agitation speed

13

The effect of agitation speed on the recovery rate of Pd was studied at the

14

applied potential of -0.25 V in 0.5 M HNO3. The results of 80 rpm, 160 rpm and 240

15

rpm are shown in Figure 6(a), which showed that with the increase of agitation speed,

16

the recovery rate of Pd and the deposition rate increased (Liu et al., 2013). After 4 h of 16

1

electrodeposition, the recovery rate of Pd at 240 rpm reached 99.02 %. That was

2

higher than the recovery rates at 80 rpm and 160 rpm with 87.11 % and 93.14 %,

3

respectively.

4

As described in section 3.2, agitation could increase the effect of mass transfer

5

by reducing the thickness of diffusion region. At the potential of -0.25 V, the reaction

6

of hydrogen reduction could be negligible. The linear relationship between Pd

7

concentration and current density was shown in Figure 6(b), according to which δ at

8

the agitation speeds of 80 rpm, 160 rpm and 240 rpm can be calculated as 179.13 µm,

9

54.23 µm and 39.38 µm, respectively. That demonstrated that increasing the agitation

10

speed could indeed reduce the thickness of diffusion region greatly. The values of D/δ

11

can be used to indicate the acceleration effect of agitation. They were calculated as

12

10.14 µm/s, 14.59 µm/s and 19.98 µm/s at 80 rpm, 160 rpm and 240 rpm, respectively.

13

It indicated that higher agitation speed brought better acceleration effect. In addition,

14

the current efficiency of 52.69 % at 240 rpm after 4 h of electrodeposition was

15

slightly higher than those at 80 rpm and 160 rpm, which were 47.86 % and 47.91 %,

16

respectively. In conclusion, the optimal agitation speed was determined to be 240

17

rpm.

18

In summary, considering the influence on the recovery rate of Pd, current

19

efficiency and deposition rate, the optimal conditions for electrodeposition were

20

determined as applied potential of -0.25 V, agitation speed of 240 rpm and HNO3

21

concentration of 0.5 M. And the recovery rate of Pd and current efficiency were

22

99.02 % and 52.69 %, respectively.

23

17

1

Figure 6. (a) Effect of agitation speed on the recovery rate of Pd; (b) Linear

2

relationship between Pd concentration and current density

3

3.4. Morphology of electrodeposited palladium

4

After electrodeposition, the black coating was obtained on the surface of titanium

5

electrode, as shown in Figure 7(d). SEM was used to analyze the surface morphology

6

of electrodeposited coatings. It can be observed from Figure 7(a) that the deposit was

7

uniformly distributed on the surface of titanium electrode, showing dendrite growth

8

(Figure S1). In some other researches, the electrodeposited Pd also showed dendrite

9

growth subjected to surface morphological examination by SEM (Jayakumar et al.,

10

2009, 2012; Liu et al., 2013). Besides, the grain size of electrodeposited Pd was about

11

20 nm in this study, as shown in Figure 7(b). And the EDS results in Figure 7(c)

12

shows that the purity of Pd by electrodeposition was high, which was more than 99 %

13

combined with the results of ICP. The Pd nanoparticles deposited on the electrode can

14

be easily scraped off or extracted by ultrasonic cleaning for recycling.

15

16 17

Figure 7. (a) and (b) The morphology of electrodeposited palladium by SEM; (c) EDS

18

1

2

component analysis of the deposit; (d) Deposit on the titanium electrode.

3.5. Recycling process route of waste MLCCs

3

The recycling process route of waste MLCCs by copper capture and

4

electrodeposition was proposed in an efficient and environmentally friendly way

5

combined with the above research, as shown in Figure 8. Waste MLCCs disassembled

6

automatically from WPCBs were firstly crushed into particles. And then the powdered

7

waste MLCCs mixing with a certain proportion of CuO, C, Al2O3 and SiO2 evenly

8

was treated with the technology of copper capture and the Cu-Pd-Ag alloy was

9

obtained. In the enrichment process, the recovery rates of Pd and Ag was 100 % and

10

87.53 %. And the main composition of the slag was BAS glass phase, which can be

11

vitrified to prepare BAS glass-ceramics. The alloy was leached with HNO3 with the

12

concentration of 1:1 to obtain nitrate solution. In this process, a small amount of Sn in

13

the alloy was oxidized to SnO2, which was insoluble in HNO3 solution and recycled

14

through filtration. And Ag was recovered in the form of AgCl through adding NaCl to

15

the nitrate solution. Finally, Pd and Cu in the solution were recovered by

16

potential-controlled electrodeposition. The electrodeposition process of Pd was

17

mainly studied. Besides, others in our research group have studied the

18

electrodeposition process of copper, of which 95.91 % of copper could be recovered

19

after 7 h of deposition with the applied potential of -0.6 V, agitation speed of 160 rpm

20

and pH of 2. The remaining was the solution of Pb(NO3)2. The deposited Pd and Cu

21

on the electrode can be scraped off or extracted by ultrasonic cleaning for recycling.

22

In addition, there was little negative impact on the environment during this recycling

23

process. Therefore, an efficient and environmentally friendly recycling process for

24

waste MLCCs was proposed.

19

1

Figure 8. Recycling process route of waste MLCCs

2

3

4. Conclusions

4

In this study, the technology of electrodeposition was applied to recover and

5

purify Pd from waste MLCCs and an efficient recycling process for waste MLCCs in

6

an environmentally friendly way was proposed. The conclusions are listed as follows:

7

(1) High-purity Pd (>99 %) was recovered by electrodeposition in the form of

8

nanoparticles. The recovery rate of 99.02 % was achieved at the condition of applied

9

potential of -0.25 V, agitation speed of 240 rpm and HNO3 concentration of 0.5 M; (2)

10

The electrochemical behavior of Pd at titanium electrode was studied, which showed

11

that higher concentration of HNO3 increased the peak current of Pd and the

12

corresponding potential shifted negatively due to the conversion of Pd (II) ion into

13

different complexes. In addition, agitation could increase the effect of mass transfer

14

by reducing the thickness of diffusion region, leading to the larger limiting current. A

15

mathematical model was established to describe the electrodeposition mechanism at

16

different agitation speeds, and the diffusion coefficient as well as thickness of

17

diffusion region were calculated; (3) An efficient recycling process for waste MLCCs

18

was

proposed,

which

combined

the 20

technology of

copper

capture

and

1

electrodeposition. And there was little negative impact on the environment during this

2

recycling process. Although this process enables efficient recovery of Pd, other metals

3

like Ag of waste MLCCs also need to be recycled through more economical

4

technologies. This will be the focus of recycling waste MLCCs afterwards. The

5

results of this study can provide reliable information for efficient recycling of Pd

6

resource from waste MLCCs.

7

Acknowledgement

8 9

This work was supported by the National Natural Science Foundation of China (51534005).

10

References

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23

Highlights High-purity (>99 %) palladium was recovered in the form of nanoparticles by electrodeposition with the recovery rate of 99.02 % from waste multilayer ceramic capacitors. The electrochemical behavior of palladium at titanium electrode was analyzed in nitric acid system and a mathematical model was established to describe the electrodeposition process. An efficient process combining enrichment and electrodeposition for recycling waste multilayer ceramic capacitors was proposed in an environmentally friendly way.

Competing interest statement The authors declare that they have no competing financial interests.