Leaching Studies for the Recovery of Metals from the ...

EPD Congress 2010 TMS (The Minerals, Metals & Materials Society), 2010

LEACHING STUDIES FOR THE RECOVERY OF METALS FROM THE WASTE PRINTED CIRCUIT BOARDS (PCBs) Manis Kumar Jha1, Shivendra1, 2, Vinay Kumar1, Banshi Dhar Pandey1, Rakesh Kumar1, Jae-chun Lee3 1

Metal Extraction and Forming Division, National Metalllurgical Laboratory(NML), Jamshedpur-831007, India 2

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, India 3

Minerals and Materials Processing Division, Korea Institute of Geosciences & Mineral Resources (KIGAM), South Korea

Keywords: Leaching Kinetics, Waste PCBs, Copper, Lead, Tin Abstract Leaching studies were carried out for the recovery/ recycling of metals from PCBs containing 17.05% Cu, 0.74% Ni, 1.74% Fe, 4.35% Pb and 8.32% Sn using various acids such as H2SO4, HCl and HNO3. While sulfuric acid was not a suitable leachant for the dissolution of metals from PCBs, hydrochloric acid selectively dissolved tin. The nitric acid was found to be effective lixiviant with the recovery of 99.99% Cu, Fe, Ni and 36.7% Pb in 6M HNO3 at S/L ratio of 100 g/L and 90oC. The kinetic studies carried out with 2M and 4M HNO3 at 90oC showed “Ash diffusion control dense constant size-spherical particles” model. Introduction The advancement of living standard due to rapid economic growth and continuous launching of improved equipment have created tremendous increase in the sale of electrical and electronic equipments. The rapid replacement of old by latest advanced models also causes mass generation of waste electrical and electronic equipments (WEEE) [1]. New technological innovation continues to accelerate the replacement of equipment leading to a significant increase in waste printed circuit boards (PCBs). The mass generation of electronic scrap as well as ever growing environmental awareness has created the mandatory requirement of recycling and reuse of PCBs. Recycling technologies to recover valuable metals from PCBs are generally classified into the following methods: (1) pyrometallurgical process typically practiced at copper smelters and (2) combined process of mechanical pretreatment and hydrometallurgical routes. PCBs shredded to about 5 mm size are processed either in copper or QSL lead smelter for the recovery of copper and precious metals [2]. The pyro-metallurgical approach normally consumes high energy, and separation/ recovery of metals are difficult due to the presence of multi-metals in PCBs [3]. The pyrolysis and combustion of PCBs release dioxins and furans gas due to incomplete combustion at low temperature and requires arrangement for off gas treatment [4, 5]. Except pyrometallurgical method the recycling of PCBs is very complicated, due to its complex material of construction. The PCBs scrap mainly consists of valuable metals along with glass fiber reinforced epoxy resins etc [5]. The effective leaching of such PCBs could not be achieved due

945

to the metallic fractions coated or encapsulated with plastics and ceramic. To get the effective leaching of metals present in PCBs, it is necessary to separate the metallic and non-metallic components. Additionally, the mechanical pre-treatment of PCBs makes the extraction and purification process easier. Jeong et al. [7] reported the good metal liberation from PCBs utilizing a swing hammer type impact mill or stamp mill. Eswaraiah et al. [8] reported the classification of metals and plastic from milled PCBs particles using air-classifier. The metallic and non-metallic particles are efficiently separated by air separation method. Li et al. [4] investigated a multi step process for the liberation and separation of metallic particles from PCBs following mechanical crushing, screening, drying and electrostatic separation. Two-step crushing of PCBs and metal separation using corona electrostatic separator was found effective. Mohabuth et al. [9] investigated the use of vertical vibration to recover metals from electrical cables and printed circuit boards. The 212–300 µm and 150– 212 µm shredded printed circuit boards consisted of a total average of 35% metal. After vibration a total average metal concentrate of 85% was collected in chamber for each size fraction. Recently, papers have appeared related to large losses of precious metals from printed circuit boards during mechanical pre-treatment [10, 11] . The hydrometallurgical processes for the treatment of waste are highly lucrative as it is cost effective and friendly to the environment [12, 13]. Kim et al. [14] studied the leaching behavior of copper using electro-generated chlorine in hydrochloric acid solution and behavior of cuprous ions during the electro-leaching of copper. Kumar et al. [15] carried out solvent extraction studies for the removal of hazardous metals from the sulfate leach solution of electronic wastes. Chien et al. [16] reported a feasibility study on the oxidation of PCB wastes with NaOH in supercritical water. Koyama et al. [17] investigated the copper leaching behavior from PCBs in ammoniacal alkaline solution under a nitrogen atmosphere. The objective of the present work is to study the leaching behavior of metals present in the PCBs in different acidic media. Various process parameters viz. effect of time, acid concentration and temperature were studied with respect to leaching of metals. The obtained data will be useful to develop and simulate the leaching process. Experimental The crushed PCBs of < 15 mm size used for the experiments contained 17.05% Cu, 0.74% Ni, 1.74% Fe, 4.35% Pb and 8.32% Sn. The leaching experiments were carried out in a temperature controlled leaching reactor. The crushed PCBs pieces were put in a three neck flask, along with the leachant in a desired ratio and heated to a set temperature, using a hot plate with magnetic stirring facility. Samples were taken from the flask at regular intervals to study the leaching behavior of the metals for a particular acid. Once the leaching was completed, the residue left was dried in a vacuum oven and kept for further analysis. The samples taken during the experiment were analyzed for their metal content, using an Atomic Absorption Spectrometer. Satisfactory mass balance was obtained for each set of leaching experiment. Results and Discussion Leaching studies were carried out for the extraction and separation of different metals present in the PCBs. The effect of various process parameters viz. effect of time, acid concentration and temperature was studied to examine the leaching behavior of metals.

946

Leaching of PCBs in H2SO4 medium

Leaching (%)

The leaching of crushed PCBs was done with H2SO4 to study the dissolution behavior of the metals present in the PCBs. The dilute sulfuric acid (2M, 4M & 6M) of 250 mL was taken in a 500 mL three neck flask and heated to 90oC. Once the temperature was attained, 25 g of crushed PCBs were added to the flask, to get a pulp density of 100 g/L. The temperature was maintained at 90oC through out the experiment. Samples were taken at a regular interval of ten minutes. The comparative leaching data for copper in three different concentrations (2-6M) of H2SO4 are presented in Fig.1. Very poor recovery of copper along with other metals (Sn, Pb, Fe, Ni) was obtained at elevated temperature and acid concentration even in 75 minutes. Therefore it is suggested that sulfuric acid is not suitable reagent for the leaching of copper from the PCBs.

0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

2M 4M 6M

0

20

40

60

80

Time (min)

Fig.1. Leaching of copper from PCBs with different concentration of sulfuric acid (Pulp density=100 g/L, Temp.= 90oC)

Leaching of PCBs in HCl medium The studies on the leaching of crushed PCBs with HCl were carried out to investigate the dissolution behavior of the metals. As above, the leaching was carried out in the leaching reactor with acid concentration of 2-6M HCl at a pulp density of 100 g/L and 90oC. The results for leaching of copper with different concentrations of HCl are presented in Fig.2. The increase in the concentration of hydrochloric acid though increased the leaching of copper, but the recovery was very poor. This may be attributed to the lower oxidation potential of H+ ion in HCl than the pure copper metal. Leaching of the other metals from PCBs with 6M HCl under similar experimental condition is presented in Fig.3. Thus, recovery of Sn, Pb and Fe was found to increase from 31.87, 20.09, 4.71 to 51.8, 39.23 and 14.37 respectively with increase in time from 10 to 75 min.

947

60

3

50

2.5

2M 4M 6M

2

Leaching (%)

Leaching (%)

3.5

1.5 1 0.5

40 30 Sn Fe

20

Pb Ni

10

0

0

0

20

40

60

80

0

Time (min.)

20 40 Time (min)

60

80

Fig.3. Leaching of other metals from PCBs with 6M HCl (Pulp density=100 g/L, Temp.= 90oC)

Fig.2. Leaching of copper from PCBs at various concentration of HCl (Pulp density=100 g/L, Temp.= 90oC)

Leaching of PCBs in HNO3 medium

100

100

80

80 Leaching (%)

Leaching (%)

The crushed PCBs were leached in HNO3 to dissolve the metallic values in leaching reactor under the controlled experimental conditions and results are presented in Fig.4.

60 40 2M 4M 6M

20

Ni Pb

Fe Sn

60 40 20 0

0 0

20

40

60

0

80

20

40

60

80

Time (min)

Time (min)

Fig.4. Leaching of copper from PCBs with different concentration of nitric acid (Pulp density=100 g/L, Temp.= 90oC)

Fig.5. Leaching of other metals from PCBs with 6M nitric acid (Pulp density=100 g/L, Temp.= 90oC)

948

It is seen that the recovery of copper increased with an increase in the concentration of HNO3 and time. Where, the minimum copper recovery with 2M, 4M and 6M HNO3 in 10 minutes was 23.21%, 63.76% and 90.46%, the maximum copper leaching was found to be 63.92%, 90.67%, and 99.99% respectively in 75 min. Under similar conditions with 6M HNO3, the leaching behavior of other metals is presented in Fig. 5. The leaching reaction was found very fast for all the metals except tin. The total Fe and Ni was leached within 10 minutes whereas 38% Pb leached in 40 minutes. Lead present in the PCBs as solder material, which is the mixture of Pb and Sn. The formation of tin complex on the surface of lead may prevent the dissolution of lead with nitric acid. The leaching of Sn was very poor with nitric acid even at elevated temperature and acid concentration (6M). A mixture of salts as tin complex appears with residue during the leaching. Nitric acid dissolves tin readily and forms hydrated stannic oxide, SnO2 often called metastannic acid (H2SnO3). Leaching of PCBs in 6M HNO3 at different temperatures The results of the experiment at varying acidities of the three different acids show that the best leaching medium is 6M HNO3. Therefore, all the further experiments to study the effect of temperature were carried out in HNO3 medium at the 6M concentration with a pulp density of 100 g/L varying the temperatures in the range 45-90oC. The results presented in Fig.6 show increase in copper recovery with temperature. The maximum recovery of copper was found to be 46.8, 83.8, 90.9 and 99.99% at temperature 45oC, 60oC, 75oC and 90oC respectively at pulp density 100 g/L. 100

Leaching (%)

80

60 40 45 oC

60 oC

75 oC

90 oC

20 0 0

20

40

60

80

Time (min)

Fig.6. Leaching of copper from PCBs with 6M nitric acid at different temperatures (Pulp density=100 g/L, Temp.= 45-90oC)

Leaching Kinetics of Copper in nitric acid: An attempt was made to study the kinetics of dissolution of copper from PCBs using the standard shrinking core models. All standard equations for the shrinking core models were tested for the reaction kinetics [13]. The best fitted leaching kinetic results are presented in Fig.7.

949

0.7 R2 = 0.9855 1-3*(1-X)^(2/3)+2*(1-X)

0.6 4M 0.5 2M 0.4 R2 = 0.98

0.3 0.2 0.1 0 0

10

20

30

40

50

60

Time (min)

Fig.7. Kinetics of leaching of copper from PCBs with different nitric acid concentration (Pulp density=100 g/L, Temp. 90oC)

The kinetic studies for the leaching of copper from PCBs were carried out at lower acid concentration i.e. 2M & 4M HNO3 at 90oC show that the leaching kinetics follows “Ash diffusion control dense constant size-spherical particles”. The model of the reaction was validated by calculating the rate constant k value for both cases; the obtained value of k was almost constant. Conclusions Leaching studies were carried out to study the dissolution behavior of different metals from PCBs in different acids viz. H2SO4, HCl and HNO3. Based on the above studies following conclusions are drawn: Sulfuric acid is not a suitable reagent for the leaching of metals from PCBs even at elevated temperature and high concentration. Hydrochloric acid is also not a suitable reagent for the dissolution of copper; however the dissolution of Sn is found satisfactory. Nitric acid is found to be suitable reagent for the dissolution of most of the metals. The Fe and Ni present in the PCBs dissolved easily within 10 minutes of contact time whereas 38% Pb leached in 40 minutes time. The leaching of Sn was very poor with nitric acid even at elevated temperature and acid concentration (6M). With 6M HNO3 at S/L ratio 100 g/L and 90 oC, 99.99% copper, iron and nickel can be leached along with 36.66% Pb. The kinetic of leaching with 2M and 4M HNO3 at temperature 90oC shows that the leaching follows “Ash diffusion control dense constant sizespherical particles” model. The NOx gas generated during nitric acid leaching can be scrubbed in suitable scrubbing solution. Further leach liquor can be purified by Solvent extraction, and from the purified solution salt/ pure novel, valuable and precious metal can be obtained using suitable hydrometallurgical techniques. Acknowledgement This paper is based on the sponsored work supported by Resources Recycling R&D Centre, South Korea under collaboration between National metallurgical Laboratory (CSIR),

950

Jamshedpur, India and Korea Institute of Geosciences & Mineral Resources (KIGAM), South Korea. References [1] M.K. Jha , Jae-chun Lee, “A review on the status of WEEE recycling in Korea”, J. of Metallurgy and Materials Science, 48 (3) (2006), 117-128. [2] Jae-chun Lee, “Current status of recycling of precious metals from the industrial wastes,” Trends in Metals & Materials Engineering, 19(2) (2002), 25-32.(in Korean) [3] E.Y.L. Sun, “The Recovery of Metals from Electronic Scrap,” JOM, 43(4) (1991), 53-61. [4] Jia Li, Zhenming Xu, Yaohe Zhou, “Application of corona discharge and electrostatic force to separate metals and nonmetals from crushed particles of waste printed circuit boards”, J. of Electrostatics, 65 (2007), 233-238. [5] P. S. Kulkarni, J. G. Crespo, C. A. M. Afonso, “Dioxin sources and current remediation technologies-A review”, Environmental International, 34 (2008), 139-153. [6] Shunli Zhang, Eric Forssberg, “Mechanical separation-oriented characterization of electronic scrap,” Resources Conservation and Recycling, 21 (1997), 247-269. [7] J. Jeong, M.S. Kim, K.K. Yoo, J-c Lee, “Effect of physical pre-treatment on liberation of metal from waste printer PCBs,” Applied Chemistry, 10(1) (2006), 35-38. (in Korean) [8] C. Eswaraiah, T. Kavitha, S. Vidyasagar, S.S. Narayanan, “Classification of metals and plastics from printed circuit boards (PCB) using air classifier,” Chemical Engineering and Processing, 47 (2008) 691-709. [9] Mohabuth, N., Hall, P., Miles, N., Investigating the use of vertical vibration to recover metal from electrical and electronic waste. J. of Minerals Engineering, 20 (2007) 926-932. [10] P. Chancerel, C. Meskers, C., Hagelüken, S. Rotter, “E-scrap – metals too precious to ignore”, Recycling International, (2008) 42-45. [11] C. Meskers, C. Hagelüken, S. Salhofer, Impact of pre-processing routes on precious metal recovery from PCs, Proceedings of EMC 2009, GDMB Medienverlag, Clausthal Zellerfeld [12] M.K. Jha, V. Kumar and R.J. Singh, “Review of hydrometallurgical recovery of zinc from industrial waste”, Resources, Conservation and Recycling, 33/1 (2001)1-22. [13] M.K. Jha, V. Kumar, L. Maharaj and R.J. Singh, 2004, “Studies on leaching and recycling of zinc from rayon waste sludge", Industrial & Engineering Chemistry Research, 43(5) (2004), 1284-1295. [14] Eun-young Kim, Min-seuk Kim, Jae-chun Lee, Manis Kumar Jha, Kyoung-keun Yoo, Jin-ki Jeong, “Effect of cuprous ions on leaching using electro-generated chlorine in hydrochloric acid solution, Minerals Engineering”, 21 (2008) 121-128. 951

[15] V. Kumar, M. Kumar, D. Bagchi, M.K. Jha, J. Jeong, J.-c. Lee, “Solvent Extraction studies for removal of hazardous metals from the sulfate leach solution of electronic wastes”, Proceedings of international symposium of resource recycling (ISSR-2007) November 27~28, 2007, Haevichi, Jeju, S. Korea p.138-143. [16] Y.C. Chien, H.P. Wang, K.S. Lin, Y.W. Yang, “Oxidation of printed circuit board wastes in supercritical water”. J. of Water Resources, 34(17) (2000) 4279-4283. [17] K. Koyama, M. Tanaka and J.-c. Lee, “Copper leaching behavior from waste printed circuit board in ammoniacal alkaline solution”. J. of Materials Transactions, 47(7) (2006) 1788-1792.

952