Application of Cementation Technology in a Chemical Recycle Plant Treating Waste Copper- Bearing Solution from Micro-Etching Processes

Journal of Environmental Treatment Techniques

2019, Volume 7, Issue 3, Pages: 289-294

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

Application of Cementation Technology in a

Chemical Recycle Plant Treating Waste Copper-

Bearing Solution from Micro-Etching Processes

Nan-Min Wu* and Wen-Chin Chen

Department of Environmental Engineering and Health, Yuanpei University of Medical Technology, Hsin Chu City, Taiwan

Received: 30/03/2019

Accepted: 11/06/2019

Published: 30/09/2019

Abstract

Waste copper-bearing solution generated from the micro-etching process of printed circuit boards (PCBs) was studied using

cementation technology in a chemical recycle plant. Operation parameters include initial copper ion concentrations (15, 23, 30

g/L) and pH values (pH = 1, 2, 3, 4) were assessed. Results showed that, within the range from 15 to 30 g/L of copper ion

concentration, the reaction rate decreased with increasing the initial concentration. In addition, the higher the pH value the

slower the copper cementation rate. And at pH = 4, the rate is as low as about 40 %, compared to the other pH values. It is

attributed that, presumably as the pH of the solution increases, the copper hydroxide may be deposited onto the iron sheet surface

to form a passivation layer, resulting in the blocking of solid-liquid interface mass transfer. Results from mass balance

calculations indicated that the unit operation of the plant was quite in agreement with expectations. However, the average

resource conversion rate (RCR) was at 58.7%. In order to increase the resource conversion rate, it is suggested that either a

reduction of water consumption or a water recycling program to the process should be taken into consideration to increase the

RCR. The result of this study is beneficial to shorten the gap between theoretical research and practical operations for chemical

recycle plant that is positive to industrial waste reduction and resource sustainability.

Keywords: Micro-etching, cementation technology, passivation, mass balance, resource conversion rate

result is not only a large amount of copper-bearing

hazardous sludge, but also increase the environmental

Introduction

Electronic products are widely used in various fields of

life, and have been linked to instant and mobile devices,

making the manufacturing process of printed circuit boards

For waste copper-bearing solution, chemical treatment

methods such as precipitation, electrolysis, and cementation

are commonly used in industry [4-5]. The advantage of

precipitation and electrolysis is that the reaction rate is

speedy, while the major disadvantage is that the post-

treated liquid is still in large quantity and needs to be

subsequently processed. As for the cementation method, it

is theoretically to use iron having a lower reduction

potential to recover copper metal having a higher reduction

potential in waste liquid [6-9]. Although the reaction rate

of cementation is generally slower than precipitation and

electrolysis, both the products after solid-liquid separation

procedure are industrial reusable.

Although there have been many investigations on the

copper-iron cementation, the results using waste liquid

from the PCBs factories were mostly laboratory scale, let

alone employed a chemical recycle plant to evaluate the

control parameters. The objective of this study is to assess

the effect of the initial copper content and pH value on

cementation technology using the waste copper-bearing

solution produced by the PCBs micro-etching process.

This study has been carried out in a chemical recycle plant,

(

PCBs) more prosperous in recent years. According to the

survey report [1], up to 2018, the global PCBs output value

will increase to US$ 68 billion, and the average annual

compound growth rate is 3.9%.

High-precision and effective micro-etching is an

important process to enhance the productivity of PCBs

[

2~3]. The micro-etching process can be divided into three

sections: inner layer etching, plated through hole and line

etching, and outer layer etching. Among them, the plated

through hole and line etching process generates a waste

copper-bearing solution with copper content less than 30

g/L. However, partially due to this low concentration of

copper in the waste liquid, most of the chemical plants lack

of incentives to recycle. As the PCB factory discharged

this waste liquid into the wastewater treatment system, the

Coressponding author: Nan-Min Wu, Department of

Environmental Engineering and Health, Yuanpei

University of Medical Technology, Hsin Chu City, Taiwan.

E-mail: nanmin0919364834@gmail.com.

Journal of Environmental Treatment Techniques

2019, Volume 7, Issue 3, Pages: 289-294

and the results are capable of direct application to benefit

both industrial waste reduction and resource sustainability.

Results and Discussion

.1 Composition of Waste Copper-Bearing Solution

The constancy of the waste liquid component plays an

Experimental section

important role on the progress of this study. The waste

copper-bearing solution has a total of 3 trips of tank-truck

entering plant for testing. The analytical results of the

composition are summarized in Table 1. As shown in

Table 1, the content of copper is between 25.8 and 30.2

g/L. There is no significant difference in the remaining

composition of the waste liquid. This waste copper-bearing

solution from micro-etching process of PCBs is classified

as hazardous waste solution as its pH  2.0 in Taiwan.

.1 Procedure

The testings were carried out using a stirring drum

reactor schematized in Figure 1. The reactor is mainly

composed of two parts: (1) a stainless-steel drum coated

with PE-inliner film, and (2) an automatic programmable

mortar mixer. The waste copper-bearing solution, tank-

truck delivered from a PCBs factory, was added in 2.0 m

each time in batch process. The ironic sheets (5 cm  5 cm



0.01 cm) were obtained from

recycling plant

specializing in waste vehicle bumper.

procedure is as follows:

The testing

Table 1: Analysis of waste copper-bearing solution.

Tank Truck Entering Chemical Plant

Item

unit

Specific

Gravity

.10

1.14

1.19

1.4

18.3

0.03

0.05

0.01

1.4

25.8

0.04

0.05

0.02

1.2

30.2

0.04

0.01

0.06

0.02

g/L

mg/L

Figure 1: Schematic of the Cu-Fe Cementation Reactor.

(1) Pump the waste copper sulphate liquid to the

cementation reactor.

(

2) Adjust the pH to the testing conditions using liquid

NaOH or sulfuric acid (pH = 1.0~4.0 ± 0.1), then add

iron sheets to the reactor and start mortar mixer.

3) Take samples to analyse copper and iron

concentration at 2-hour intervals over a period of 14

hours, then terminate the stirring pump.

.2 Effects of Initial Copper Ion Concentration

In this study, three different initial concentrations of

(

copper ions were prepared to investigate the effect of initial

copper ion concentration on the cementation reaction. The

initial concentrations were prepared for 15, 23, and 30 g/L,

respectively. The stirring speed was set at 60 rpm during

the test. The pH value is chosen to be fixed at 3.0 first.

Figure 2 shows the change of the copper ion concentration

in the cementation reactor with the change of reaction time.

From the figure it is obvious that the copper ion

concentration decreases with the reaction time, while at the

end of the reaction the residual copper ion concentration

increases with the increase of the initial concentration. It is

noted that from Figure 3, the result at 30 g/L is different

from the other two initial concentrations.

4) Wait for 10 hours for copper precipitation, then pump

the supernatant liquid from the cementation reactor to

the storage tank of the ferrous sulphate solution.

5) Add 1 m3 of tap water to flush the cementation. The

residual liquid cleaning work was carried out, and the

stirring pump was again turned on for about 10

minutes, and then left to stand for 10 hours.

(

6) Pump the supernatant liquid in the cementation

reactor to the wastewater treatment facility, and

remove the cemented copper manually into press

filter to dewatering operations.

(7) After dewatering, the cooper cake is weighed and

analysed in the plant laboratory.

.2 Analytical Methods

All samples were stored in amber glass bottles to prevent

metal ions from being attached to the bottle wall, and were

analyzed immediately after each batch of test to avoid any

potential interference. The analyses of metal ions were

carried out using flame atomic absorption spectrometry

(AAS 932Plus, GBC Scientific Equipment). The analytical

method was adapted according to the National Institute of

Environmental Analysis (NIEA M111.01C) [10].

Figure 2: Effects on initial copper ion concentration to the

cementation reactions.

As the cementation reaction proceeds, the order of the

reaction can be evaluated by analysing the change of the

Journal of Environmental Treatment Techniques

2019, Volume 7, Issue 3, Pages: 289-294

concentration of copper ions with respect of the change of

reaction time. Previous studies have suggested that copper-

iron cementation reactions as first-order reaction [11~13],

and can be expressed as Equation (1):

concentration tank, so as increased the viscosity coefficient

and the diffusion layer between solid and liquid. An

increase in these parameters may lead to a decrease in the

diffusion rate of copper ions in the solution, which

eventually result in a decrease in the rate of cementation

reaction, and is relatively in agreement with previous

studies [14~15].



]

d [Cu

2 

]

(1)



 k ( )[ Cu



]

d [Cu

where the-

is the rate of change of copper ion

concentration with respect to reaction time (t), k is the

reaction rate constant, A is the total area of the iron piece,

and V is the volume of the waste copper sulfate solution in

the reactor. Integrate Equation (1) and let t = 0, the initial

copper ion concentration [Cu ] = [Cu ] , the reaction rate

can be expressed as Equation (2):



]

[

(2)

  k ( )  t



[

Cu ]_o

Since each batch of this study keeps constant in volume

and in weight of the iron at the same pH, the (A/V) can be

regarded as a constant value. Figure 3 shows the

cementation reaction constant (k) obtained by changing the

Figure 4: Change of reaction rate constants (k) respect to initial

copper ion concentrations.

3.3 Effects of pH Value

reaction time with ln([Cu ]/[Cu ] ). Comparing the

As stated in previous section, the pH of the waste copper

sulfate solution produced by the micro-etching process is

usually less than 2.0. Factors including number of plated

holes, number of layers of the circuit board, and frequency

of cleaning, may result in the variation of pH in the waste

copper sulfate solution discharged from batch to batch.

Therefore, it is necessary to evaluate the effect of pH on the

cementation reaction for the recycling plant operation. In

this study, the range of pH was set from 1.0 to 4.0, and the

initial copper ion concentration was controlled at 23

g/L.Figure 5 shows the copper ion concentration in the

solution decreases with the increase of the reaction time. It

is obvious that the cementation rate of pH=4 is much lower

slopes of the reaction straight lines of each initial

concentration in Figure 4, it is clear that the slopes of the

initial concentration of 15 g/L and 23 g/L are larger and

closer. As concentration increases to 30 g/L, the slope is

lower than 15 g. /L and 23 g/L.

than that of the other pH=1~3.

The copper ion

concentration is reduced from 23 g/L to 14 g/L, indicating

the change of cementation is about 40%.

) in the cementation reactor

Figure 3: Change of Ln([Cu ]/[Cu ]

with respect to time.

The rate of chemical reaction is often affected by the

initial concentration of the reactants. In order to explore the

extent of the effect, this study formulated three sets of

initial concentrations for cementation reactions. Figure 4

shows the cementation reaction rate constant (k) plotted

against the initial copper concentration [Cu ] . From the

figure, the cementation rate constant decreases as from

.257 hr-1 to 0.234 hr-1, with the increases of initial

Figure 5: Change of copper ion concentration respect to pH value.

concentration from 15 g/L to 23 g. This change of k is

slightly minor. But the change of k becomes obviously

greater, that is reduced from 0.234 hr-1 to 0.162 hr-1. It is

postulate that, since the cementation of metal is

heterogeneous reaction, the higher the initial concentration

of copper ions, the higher the activity of the solution in the

Figure 6 shows the emerge of ferrous ions in the

cementation reactor with pH=1~4. It can be seen that the

concentration of ferrous ion at pH=4 is only 7.7 g/L, which

is significantly lower than the other pH=1~3 (up to 19.5

g/L). In comparison with the other groups of pH=1~3, the

Journal of Environmental Treatment Techniques

2019, Volume 7, Issue 3, Pages: 289-294

copper-iron cementation rate a at pH=4 is only about 40%.

Previous research results have suggested that the increase

of pH in cementation reactor may result in reducing the

reaction rate [16~17].

Figure 7: Schematic diagram of mechanism for copper-iron

cementation reaction. (=Cu ions entering liquid-solid diffusion

boundary layer from bulk solution;  =Cu ions precipitated from

electric double-layer interface to form Cu metals;  =Fe metal

transfers electron to Cu ions and become Fe ;  = Fe ions

diffuse outward from electric double-layer interface;  = Fe ions

entering bulk solution from liquid-solid diffusion boundary layer.)

Figure 6: Change of ferrous ion concentration respect to time.

In the copper-iron cementation system, increasing the

Table 2: Mass balance and resource conversion rate.

Items

Unit

pH=1

2280

2.0

pH=2

2280

2.0

pH=3

2280

2.0

pH value indicates that the concentration of [OH ] in the

Weight Kg

solution is increased, and it is intended to cause a reaction

of forming a hydroxide precipitate with the copper ion, as

expressed in the following reaction equation:

Waste

Copper-

Bearing

Solution

Volum

[

Cu ]

g/L 23.00

Copper Kg

NaOH Kg

23.00

46.00

64.10 398.31

23.00

46.00

0.00

Total

Input

Cu + 2OH → Cu(OH)

(3)



Adjuster

36.92

0.00

In case that a large amount of copper hydroxide

precipitates and accumulates on the surface of the iron

sheet, then it is intended to form a passivation phenomenon

Iron Sheets Weight Kg

5.09

79.20

1000

92.12

1000

Cleaning Water

Sum

1000

Kg 3392.01 3423.30 3770.43

[

18~19]. From the viewpoint of reaction kinetics, as

Ferrous Weight Kg 1912.77 1944.20 2163.96

indicated in the Equation (1), the reduction of the active

area (A) of the iron sheet may directly decrease the

cementation reaction. In addition, the passivation of the

surface of the iron sheet, as shown in Figure 7, may highly

hinder the outward transfer of electrons. As in Figure 8, the

mechanism of the cementation reaction can be subdivided

into five main steps. It is noted that in the third step, if the

iron sheet is subjected to surface passivation, the iron ions

may be ineffectively entering the electric double-layer and

diffuse outward, causing a limit to the electrons delivering

to copper ion. The rate of cementation reaction will

eventually be greatly reduced. Therefore, by evaluating

from the results of this study, the waste copper-bearing

solution should be controlled to pH  3.0 for operation in

chemical recycling plant.

Sulfate

Solution

Copper Kg

Iron Kg

Weight Kg

0.21

72.82

64.92

0.22

76.77

65.08

0.23

90.01

64.44

Total Cemented

Output Copper Copper Kg

45.67

45.63

45.49

Weight Kg 1414.3 1414.0 1542.0

Copper Kg

Iron Kg

Cleaning

Water

0.12

2.27

0.15

2.43

0.28

2.11

Sum

Resources Conversion Rate

Kg 3392.01 3423.30 3770.43

58.30 58.69 59.10

As illustrated in Table 2, the items of total input include

waste copper-bearing solution, pH adjuster, iron sheets and

cleaning water. The items of total output include ferrous

sulfate solution, cemented copper and cleaning water.

Each item in total input and total output is measured,

respectively. The results show that the total input is equal

to output, and in terms of the mass balance in the

cementation reaction, it is quite consistent with the

theoretical estimates. In addition, the resources conversion

rate (RCR) has been calculated using the cementation

products, namely ferrous sulfate solution and cemented

copper, as expressed in Equation (5):

.4 Calculations of Resource Conversion Ratio

The assessment of mass balance is important to

understand the flow of raw materials and products, and to

improve the efficiency of unit operations. In this study, the

copper-iron cementation reaction is carried out in a

recycling plant, and the overall mass balance calculation is

presented as a result of the batch testings as shown in

Equation (4):

ꢃꢄꢅꢆꢇꢈ ꢉꢊ ꢊꢄꢋꢋꢉꢌꢍ ꢍꢌꢎꢊꢏꢈꢄ ꢍꢉꢎꢌꢈꢅꢉꢐꢑꢃꢄꢅꢆꢇꢈ ꢉꢊ ꢒꢄꢓꢄꢐꢈꢄꢔ ꢒꢉꢕꢕꢄꢋ

ꢀ

ꢁꢀ(ꢂ) =

(5)

ꢖꢉꢈꢏꢎ ꢗꢐꢕꢌꢈ

∑

푇표푡푎푙 퐼푛푝푢푡 = ∑ 푇표푡푎푙 푂푢푡푝푢푡

(4)

It can be seen from Table 2 that the RCR is between 58.3

and 59.1 %, with the average value at 58.7 %. By

Journal of Environmental Treatment Techniques

2019, Volume 7, Issue 3, Pages: 289-294

examining the items in Table 2, the cleaning water is the

major cause account for this lesser of RCR. Accordingly,

the cementation technology may have the application

benefit of recycling the waste copper-bearing solution

produced by the micro-etching, the water consumption of

cleaning the cementation reactor should be further reduced,

or the water recycling program be included to improve the

RCR.

Aknowledgment

This study was partially supported by the Dong Dah

Industrial Company, Inc., Taiwan, through the project

contract No. 1020174. The results, however, are for

reference only and do not represent the operation

conditions of the company.

Ethical issue

Authors are aware of, and comply with, best practice in

publication ethics specifically with regard to authorship

Conclusions

In this study, the cementation technology is applied to an

(avoidance of guest authorship), dual submission,

industrial waste treatment plant, and the control parameters

of the waste copper-bearing solution produced by the PCB

micro-etching process are evaluated. Both initial copper

concentration and pH value are the major operational

concerns, so as to set as test parameters. All tests were

recorded, including input and output amount, copper

concentration, weight of iron sheet, and weight of cemented

copper. Experimental results were also used to estimate the

kinetic rate constants and RCR of the reaction. The

conclusions of the study are listed below.

manipulation of figures, competing interests and

compliance with policies on research ethics. Authors

adhere to publication requirements that submitted work is

original and has not been published elsewhere in any

language.

Competing interests

The authors declare that there is no conflict of interest

that would prejudice the impartiality of this scientific work.

(

1) Three sets of initial copper concentration were

tested at 15, 23, 30 g/L, respectively. It was found

that the first-order kinetic rate constant decreased

with the increase of the initial copper concentration.

The increase of copper concentration causes the

diffusion layer between solid and liquid to b

increased, leading to the decrease of the mass

transfer rate.

Authors’ contribution

All authors of this study have a complete contribution

for data collection, data analyses and manuscript writing.

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