Separation of Tin, Silver and Copper from Waste Pb-free SolderUsing Hydrochloric Acid Leaching with Hydrogen Peroxide

Sookyung Kim1, Jae-chun Lee1, Kwang-sek Lee2,+1, Kyoungkeun Yoo2,+2 and Richard Diaz Alorro3

1Mineral Resources Research Division, Korea Institute of Geoscience & Mineral Resources, Daejeon, 305-350, Korea2Department of Energy & Resources Engineering, Korea Maritime and Ocean University, Busan, 606-791, Korea3Department of Metallurgical and Minerals Engineering, Curtin University, Kalgoorlie, 6430, Australia

The waste lead (Pb)-free solder leaching process in hydrochloric acid (HCl) solution with hydrogen peroxide (H2O2) followed byseparation of copper (Cu) and tin (Sn) was investigated to separate tin, silver (Ag), and copper as an individual component from waste Pb-freesolder. The dissolution of Sn increased with increasing temperature and HCl concentration. The concentrations of Sn and Cu increased to27090 g·m¹3 and 191 g·m¹3, respectively, under the leaching condition with 1 kmol·m¹3 HCl, 0.8 kmol·m¹3 H2O2 at 50°C and 400 rpm for120min, while Ag is not detected in all leaching tests. The Sn and Cu components are thus successfully separated from Ag by hydrochloric acidleaching with hydrogen peroxide. To precipitate selectively Cu ions from the leach solution, the method to add Sn powder has been investigated.Thus, 92.8 g·m¹3 (1.46mol·m¹3) of Cu could be removed successfully from the leach solution with Sn under the following conditions; 30°C intemperature; 400 rpm in agitation speed; 0.3mlmin¹1 in N2 flow rate; 0.1 g Sn powder addition to 100 cm3 leach solution.[doi:10.2320/matertrans.M2014289]

(Received August 11, 2014; Accepted September 29, 2014; Published November 8, 2014)

Keywords: lead-free solder, hydrochloric acid leaching, cementation process, recycling

1. Introduction

The environmental regulation such as WEEE (the Wasteof Electrical and Electronic Equipment) and RoHS (theRestriction of the use of certain Hazardous Substances)restricts the use of lead (Pb) in electric home appliances dueto its toxicity.1,2) Lead-free solder has been investigated tosubstitute the tin/lead solder, and various Pb-free solderseries containing tin, silver, copper, bismuth, antimony, andzinc were developed.3) Simple melting processes have beenused to reuse waste Pb-free solder.4) The melting processes,however, could cause air pollution due to the gas emissiongenerated from combustion of organic flux in the Pb-freesolder.4)

Hydrometallurgical processes have been investigated asalternative processes for recycling of waste Pb-free solder.Rhee et al. performed the feasibility study on Sn recoveryfrom lead frame scrap using sodium hydroxide with sodiumpersulfate as an oxidant.5) Kim et al. investigated theleaching behavior of tin in NaOH solution to recover tinfrom waste Pb-free solder.6) There have been a few studies onthe recycling of waste Pb-free solder under the acidiccondition although nitric acid leaching tests were performedto recover valuable metals from waste Pb-free solder4) andprinted circuit boards.7)

In the previous study,4) a recycling process using nitric acidleaching was proposed to recover Sn, Ag, and Cu from wastePb-free solder as shown in Fig. 1. During nitric acid leaching,silver and copper are dissolved while tin precipitates intostannic acid or stannic oxide. Subsequently, silver ionprecipitates selectively from leach liquor by adding NaCl,and then copper is recovered by electrowinning. Therefore,Sn, Ag, and Cu could be recovered successfully as individualcomponent. However, the waste Pb-free solder should be

leached twice by nitric and hydrochloric acid to recover tinmetal, and the treatment of NOx gas emission will berequired to avoid air pollution.

The present study is aimed to develop a new recyclingprocess using hydrochloric acid to avoid air pollution dueto NOx gas and to simplify leaching processes. The effectsof the parameters such as hydrochloric acid concentration,leaching temperature, pulp density, and agitation speed, onthe dissolution of Sn, Ag and Cu are discussed. In addition,the precipitation of Cu by adding Sn powder was investigatedto separate Cu and Sn from the hydrochloric acid leachsolution.

2. Experimental Procedure

2.1 MaterialsThe Pb-free solder of Sn-Ag-Cu series covers approx-

Waste solder

HNO3 leaching

S/L separating

Precipitating

Electrowinning

AgCl

Cu

NaCl

H2SnO3

Leach solutionwith Ag & Cu ions

HCl leaching

Electrowinning

Sn

Precipitate

Nitric acid

Solution with Cu ions

Fig. 1 Schematic diagram of the recycling process of waste Pb-free solderusing nitric acid leaching.4)

+1Graduate Student, Korea Maritime and Ocean University+2Corresponding author, E-mail: kyoo@kmou.ac.kr

Materials Transactions, Vol. 55, No. 12 (2014) pp. 1885 to 1889©2014 The Japan Institute of Metals and Materials EXPRESS REGULAR ARTICLE

imately 70% of reflowing Pb-free solder market.3,8,9) Thewaste Pb-free solder of Sn-Ag-Cu series was obtained from arecycling company in Korea, which was generated from thefabricating processes of printed circuit boards for electronichome appliances. The waste solder was dry-sieved with125 µm (120 mesh) sieve, and the solder with less than125 µm contained 90.2% Sn, 4.11% Ag, and 0.65% Cu asmain components and 0.022% Bi and 0.021% Pb as minorcomponents, and any other metals were not detected. TheX-ray pattern (see Fig. 2) of the solder shows the presence ofphases such as Sn and Ag3Sn. All the chemicals used in thisstudy are of reagent-grade.

2.2 Leaching proceduresThe leaching tests of the waste Pb-free solder in

hydrochloric acid solution were performed in a 500 dm3

three-necked Pyrex glass reactor using a heating mantle tomaintain temperature. The reactor was fitted with an agitatorand a reflux condenser. The reflux condenser was inserted inone port to avoid solution loss at high temperatures. In leachsolution, the concentrations of HCl and H2O2 were adjustedto 0.1­1 kmol·m¹3 and 0.3­0.8 kmol·m¹3, respectively, andthen 200 dm3 of the leach solution was placed into the reactorand allowed to reach the thermal equilibrium (30­90°C). A2 g of the solder powder under 125 µm was added to thereactor in the experiments except the pulp density test, andthe agitator was set at 200­600 rpm. During the experiment,3 cm3 of the solution sample was withdrawn periodically at adesired time interval (10­120min) with a syringe.

2.3 Separation test of Cu and SnThe separation tests for Cu and Sn was performed by

adding Sn powder (75 µm, Junsei Chemical Co. Ltd.). Theleach liquor was obtained under the leaching conditions;400 rpm agitation speed, 1 kmol·m¹3 HCl concentration,0.3 kmol·m¹3 H2O2 concentration, 50°C temperature and1.5% pulp density. The separation tests were conducted in a250 cm3 Pyrex reactor, which was equipped with water jacketfor temperature control. Tin powders (0.1 to 0.5 g) wereadded to 100 cm3 of the leach solution at 400 rpm and 30°C,with or without introducing nitrogen gas (0.3mlmin¹1 flowrate). For measuring the concentrations of Cu, 2 cm3 ofsolution was sampled with a syringe and was prepared foranalysis following the same procedure as mentioned above(section 2.2).

2.4 Analytical methodsThe sample was filtered with 0.45 µm membrane filter and

then the filtrate was diluted with 5% HNO3 solution for Cuand Ag analyses and 15% HCl solution for Sn analysis,respectively. The sample solutions were analyzed by anatomic absorption spectrometry (AA7000, Shimadzu Co.Ltd.) and an inductively coupled plasma-atomic emissionspectrometry (ICP-AES, JY-38 plus, Jobin Yvon Ltd.).

3. Results and Discussions

The hydrochloric acid leaching test of the waste Pb-freesolder at agitation speeds in the range 200­600 rpm wascarried out to examine the effect of liquid film boundarydiffusion surrounding the solid particles on the leachingefficiency of the solder in 1 kmol·m¹3 HCl at 50°C with0.3 kmol·m¹3 H2O2. As can be seen from Fig. 3, the leachingefficiencies of Sn are independent of the agitation speedsafter 30min. Therefore, in all subsequent leaching tests, aworking agitation speed of 400 rpm was selected to ensureeffective particle suspension in the solution while minimizingthe effect of liquid film boundary diffusion surrounding thesolid particles.

Figure 4 shows the effect of HCl concentration on thedissolution of Sn from the waste solder in 0.1 kmol·m¹3 to1 kmol·m¹3 HCl solution with 0.3 kmol·m¹3 H2O2. Theleaching efficiencies of Sn increased with increasing HClconcentration, which would result from the increase in tinsolubility. In the leaching test at 1 kmol·m¹3 HCl, theleaching efficiency increased to more than 99% so

Inte

nsit

y (c

ps)

Degree / 2θθ

Sn

Ag3Sn

Fig. 2 XRD pattern of the waste Pb-free solder of Sn-Ag-Cu series used inthis study.

Sn c

once

ntra

tion

, C /

g m

-3

Time, t / min

Fig. 3 Effect of agitation speed on the dissolution of Sn from the Pb-freesolder in 1 kmolm¹3 HCl at 50°C with 0.3 kmolm¹3 H2O2.

Sn c

once

ntra

tion

, C /

g m

-3

Time, t / min

Fig. 4 Effect of HCl concentration on the dissolution of Sn from the Pb-free solder at 400 rpm and 50°C with 0.3 kmolm¹3 H2O2.

S. Kim, J. Lee, K. Lee, K. Yoo and R. D. Alorro1886

1 kmol·m¹3 HCl concentration was selected in all subsequentleaching tests. The effect of H2O2 addition was investigatedin 1 kmol·m¹3 HCl at 50°C and 400 rpm, with and without0.3 kmol·m¹3 H2O2. Figure 5 shows the H2O2 additionimproved the dissolution of Sn in the 1 kmol·m¹3 HClsolution. In this experiment without H2O2, 7.2 g·m¹3 Cu wasdetected in the leach solution after 120min leaching while Agwas not detected.

Figure 6 shows the effect of temperature on the dissolutionof Sn from the waste solder in 1 kmol·m¹3 HCl solution with0.3 kmol·m¹3 H2O2. The dissolution temperature was variedin the range 30­90°C, while all other parameters were keptconstant. As can be seen from Fig. 6, higher temperaturesyielded higher dissolution rates of Sn from the solder in thebeginning of the leaching, while the difference of leachingefficiencies is negligible after 30min, where the leachingefficiencies are more than 99%.

The effect of pulp density from 1% to 3% was investigatedunder the leaching condition; 1 kmol·m¹3 HCl, 0.8 kmol·m¹3

H2O2, 400 rpm, and 50°C. In the leaching tests, tin wasoxidized with hydrogen peroxide as the following equation.

Snþ 2H2O2 þ 4Hþ ¼ Sn4þ þ 4H2O ð1Þwhere 0.3 kmol·m¹3 H2O2 could oxidize 0.15 kmol·m¹3 Sn,because the tin concentration with pulp density of 3% are0.253 kmol·m¹3, 0.8 kmol·m¹3 H2O2 was used in the pulpdensity test considering that hydrogen peroxide is a relativelyunstable compound.10,11)

As shown in Fig. 7, tin concentration increased to27090 g·m¹3 in the leaching test with 3% pulp density, and

the leaching efficiencies of tin increased to more than 99%regardless of pulp density. Figure 8 shows the leachingbehavior of Cu obtained in the same experiment shown inFig. 7. The concentration of Cu increased to 63 g·m¹3,131 g·m¹3 and 192 g·m¹3 in the leaching test with 1%, 2%and 3% pulp density, respectively, while that of Ag was notdetected in all leaching tests (data not shown). Silver ion hasbeen found to precipitate by reacting with Cl¹ ion by thefollowing equation

Agþ þ Cl� ¼ AgCl # ð2ÞThe standard Gibbs free energies of Ag+, Cl¹ and AgCl(s)

are 77.16 kJ·mol¹1, ¹131.0563 kJ·mol¹1, and ¹109.86kJ·mol¹1, respectively.12,13) The solubility product (Ksp) ofAgCl is calculated to be 10¹9.82 using the data at 25°C. Thisresult indicates that Ag ions precipitate easily and rapidlyas AgCl because the solubility of AgCl is extremely low.Consequently, Ag could be precipitated and separatedsuccessfully from Sn and Cu by HCl leaching with H2O2.

Cementation is the process of precipitating the metals froma solution using the electrochemical reaction between twometals.14) By cementation process using Sn powder, theprecipitate of Cu was expected as shown below.

Cu2þ þ Sn ¼ Cu # þ Sn2þ ð3Þ2Cu2þ þ Sn ¼ 2Cu # þ Sn4þ ð4Þ

where the standard redox potentials of eq. (3) and (4) arecalculated to be +0.48V and +0.33V, respectively.15)

In eqs. (3) and (4), 1.46mol·m¹3 or 0.73mol·m¹3 of Sn(17.3mg or 8.7mg of Sn to 100 cm3 of leach solution) is

Time, t / min

H2O2

H2O2

Sn c

once

ntra

tion

, C /

g m

-3

Fig. 5 Effect of H2O2 addition on the dissolution of Sn from the Pb-freesolder in 1 kmolm¹3 HCl at 400 rpm and 50°C with or without0.3 kmolm¹3 H2O2.

Time, t / min

Sn c

once

ntra

tion

, C /

g m

-3

Fig. 6 Effect of temperature on the dissolution of Sn from the Pb-freesolder in 1 kmolm¹3 HCl at 400 rpm with 0.3 kmolm¹3 H2O2.

Sn c

once

ntra

tion

, C /

g m

-3

Time, t / min

Fig. 7 The leaching behaviors of Sn in 1 kmolm¹3 HCl at 400 rpm and50°C with 0.8 kmolm¹3 H2O2.

Cu

conc

entr

atio

n, C

/ g

m-3

Time, t / min

Fig. 8 The leaching behaviors of Sn in 1 kmolm¹3 HCl at 400 rpm and50°C with 0.8 kmolm¹3 H2O2.

Separation of Tin, Silver and Copper from Waste Pb-free Solder Using Hydrochloric Acid Leaching with Hydrogen Peroxide 1887

required to precipitate 92.8 g·m¹3 (1.46mol·m¹3) of Cu. Asshown in Fig. 9, Cu concentrations decreased and thenincreased by the addition of 0.1 g to 0.3 g of Sn powder,though the Sn amount of 5 or 10 times more than thatrequired in eqs. (3) and (4) was added to the leach solution.On the other hand the Cu concentration decreased and thenremained constant to nearly zero by adding 0.4 g and 0.5 g ofSn powder. Figure 10 shows the Eh-log aCl- digram for theCu2+/Cu+-Cl¹-H2O system at 25°C where the activity ofeach species is assumed to be 1.16) At low Cl¹ concentration,Cu exists as copper metal or cupric ion (Cu2+), whereascuprous ion species become stable with increasing Cl¹

concentration. Since CuCl2¹ ion as Cu+ species is predom-inant at 1 kmol·m¹3 Cl¹, copper leaching could be expectedas the following equation.

CuClþ þ Cuþ 3Cl� ¼ 2CuCl�2 ð5ÞThe cuprous species could be oxidized by oxygen under

the open condition to air as shown in eq. (6).

4CuCl�2 þ O2 þ 4Hþ ¼ 4CuClþ þ 2H2Oþ 4Cl� ð6ÞThe remaining copper ion in Fig. 9 could play a role in re-

oxidizing copper metal precipitated, and the cuprous speciesmight be oxidized by contacting oxygen. Thus, because thereactions shown in eqs. (5) and (6) could be repeated, copperconcentration increased with time as shown in Fig. 9.Therefore, nitrogen gas was introduced into the reactor toavoid the oxidation by oxygen. Figure 11 shows the copperbehavior by cementation using Sn powder with N2 purging.The copper concentration decreased and then remainedconstant at low Cu concentration. These results indicate thatcopper components could be separated successfully from theleach solution.

Based on the above results, a recycling process for thewaste Pb-free solder of Sn-Ag-Cu series is proposed asshown in Fig. 12. Copper and tin are dissolved by hy-drochloric acid leaching with hydrogen peroxide, and thencopper ions were selectively precipitated from the leachsolution by cementation process with tin powder, where tinpowder could be substituted by Pb-free solder because itcontains about 90% Sn as a main component. Finally tinwould be recovered by subsequent electrowinning process.The mixture of copper precipitated and tin powder remainedin the cementation process will be sent to the leachingprocess. Accordingly, copper concentration would increase inthe leach solution by repeating the recycling process. Whenthe copper concentration increased to sufficient level, copperions should be removed by another separation process suchas solvent extraction. Further efforts will be required torecover copper from the leach solution.

4. Conclusion

The hydrochloric acid with hydrogen peroxide leachingprocess for the treatment of waste lead-free solder followedby separation of Cu and Sn was investigated to recover Sn,Ag and Cu as an individual component.

The dissolution of Sn is independent of agitation speedat more than 200 rpm, but increases with increasing the

Eh

/ V

Log aCl-

Fig. 10 The Eh-logaCl- digram for the Cu2+/Cu+-Cl¹-H2O system at25°C.16)

Cu

conc

entr

atio

n, C

/ g

m-3

Time, t / min

Fig. 11 The behavior of Cu by adding Sn powder in the leach solution at50°C and 400 rpm with N2 purging.

Waste solder

Leaching

S/L separation

AgCl

Leach solutionwith Sn & Cu ions

Sn

HCl + H2O2

Cementation

Electrowinning

Sn powder

Sn powder + Cu precipitate

Cu

Solvent extraction

Electrowinning

Fig. 12 The recycling process of Pb-free solder (Sn-Ag-Cu series)proposed in this study.

Cu

conc

entr

atio

n, C

/ g

m-3

Time, t / min

Fig. 9 The behavior of Cu by adding Sn powder in the leach solution at50°C and 400 rpm without N2 purging.

S. Kim, J. Lee, K. Lee, K. Yoo and R. D. Alorro1888

temperature and HCl concentration. Silver is not detected inthe all leaching tests. The Sn and Cu components are thussuccessfully separated from Ag by hydrochloric acid leachingwith hydrogen peroxide. To precipitate selectively Cu ionsfrom the leach solution, a method comprising of the Snaddition has been investigated. The re-dissolution of Cu wasobserved due to the oxidation of remaining cupric ion andsubsequent re-oxidation of cuprous ion by oxygen. Theintroduction of nitrogen gas during the cementation processprevents Cu from re-dissolving. Thus, Cu could be separatedsuccessfully from leach solution containing Sn by theaddition of Sn powder, whereas Sn could be recovered byelectrowinning from the Cu free solution.

Acknowledgments

The paper is based on the Basic Research Project of theKorea Institute of Geoscience and Mineral Resources(KIGAM) funded by the Ministry of Trade, Industry &Energy (MOTIE) of Korea.

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Separation of Tin, Silver and Copper from Waste Pb-free Solder Using Hydrochloric Acid Leaching with Hydrogen Peroxide 1889