Elsevier

Hydrometallurgy

Volume 197, November 2020, 105476
Hydrometallurgy

Hydrometallurgical processing of waste multilayer ceramic capacitors (MLCCs) to recover silver and palladium

https://doi.org/10.1016/j.hydromet.2020.105476Get rights and content

Highlights

  • Novel flow-sheet for processing of MLCCs to recover precious metals.

  • Leaching and solvent extraction reported to recover Ni and Cu selectively.

  • Ag and Pd were recovered using suitable leachant and precipitant.

  • Process found feasible, environmental and potential to be commercialized.

Abstract

Present paper reports an application oriented approach to recover precious metals such as silver (Ag) and palladium (Pd) from multilayer ceramic capacitors (MLCCs) of waste printed circuit boards (PCBs). These capacitors are being widely used in modern electronic gadgets to provide advance features as well as to enhance their performance. Due to the generation of large amount of e-waste as well as the loss of precious metals, a sincere R&D effort has been made to develop a process for the recovery of Ag and Pd from waste MLCCs. Initially, the MLCCs were depopulated from the PCBs by de-soldering using thermal treatment. Further, the depopulated material was pulverised to get homogeneous fine powder of MLCCs, which contained 0.14% Pd, 1.08% Ag, 1.76% Cu and 11.1% Ni. First of all Ni was removed/recovered selectively using two stages of leaching at optimized condition i.e. 2 M HCl, temperature 75 °C and pulp density 100 g/L. The obtained leached residue was washed, dried and further leached to get 99.99% Ag and Pd both in solution using 4 M HNO3, temperature 80 °C, pulp density 100 g/L and mixing time 1 h. From the obtained leach liquor, salt of Ag (purity 99.99%) was selectively recovered/precipitated using KCl. Further, Cu was extracted from the Ag depleted solution using LIX 84IC leaving Pd in the raffinate, which was evaporated to get pure Pd salt. The developed flow-sheet has potential to be commercialized after scale up trials.

Introduction

The growing demand of sophisticated and advanced devices have decreased the life span of electronic goods as well as tremendously increased the generation of electronic waste (e-waste). Printed circuit boards (PCBs) are essential component among all electronic devices, which is majorly populated with MLCCs and various other small components. MLCCs contain precious as well as valuable materials and metals. MLCCs are surface mounted devices (SMD), which have gained more attention over other capacitors due to its wide ranges of capacitance, superior frequency quality, reliability, higher voltage withstanding capacity, etc. The inner electrodes of MLCCs are usually made up of Agsingle bondPd alloys whereas the outer mounting surface of the MLCCs is made up of three different layers i.e. Ag, Ni and Sn (Pan and Randall, 2010; Niu and Xu, 2019).

Large number of high capacitance MLCCs are found to be populated on the PCBs of various electrical and electronic devices due to their low equivalent series resistance (ESR) value, which enhances the performance of devices. Some electronic gadgets such as mobile phone, personal digital assistant and television contain about 150, 200 and 300 pieces of MLCCs, respectively (Kim et al., 2007; Niu and Xu, 2019) to support advanced features like bluetooth, prominent colour imaging, 3G facility, etc. (Cross, 2004; Prabaharan et al., 2016). Recycling of waste MLCCs is essential in view of precious metals (Ag and Pd) recovery, environmental management as well as conservation of natural resources (Niu and Xu, 2017; Prabaharan et al., 2016). The small components populated on the PCBs are store house of precious metals (Au, Ag, Pd and Pt). Many authors (Tripathi et al., 2012; Syed, 2006; Sheng and Etsell, 2007; Behnamfard et al., 2013) proposed different methods for the recovery of metallic values from the populated components of PCBs but focused mainly on the recovery of gold without citation on other precious metals. A process to recover gold from the PCBs of mobile phones was reported using ammonium thiosulfate. About 78.8% Au was leached using 0.1 M ammonium thiosulfate along with 40 mM copper sulfate at room temperature in mixing time of 8 h maintaining pulp density 10 g/L (Tripathi et al., 2012). Syed reported a process to recover gold from different gold coated materials such as PCBs, bangles, mirrors, etc. using two eco-friendly reagents potassium persulphate and formic acid, keeping in view, to minimize the toxicity problem associated with cyanidation of gold (Syed, 2006). A leaching process was reported to recover gold as residue by dissolving base metals present in the PCBs of personal computer using nitric acid, temperature 70 °C and mixing time 1 h. Further, leaching of Au was carried out using aqua regia followed by its precipitation using ferrous sulphate (Sheng and Etsell, 2007). A hydrometallurgical process to recover Cu, Ag, Au and Pd selectively from PCBs of e-waste was also investigated. 99% Cu got dissolved in two consecutive stages using H2SO4 as leachant along with H2O2 as reductant. The leached residue obtained was treated using acidic thiourea and ferric ion as oxidizing agent to dissolve 85.76% Au and 71.36% Ag followed by their precipitation using sodium borohydride (SBH). The remaining Au along with Pd was further leached using 5 M HCl, 1% H2O2 and 10% NaClO (Behnamfard et al., 2013).

Only few studies have been carried out for the recycling of metals from MLCCs mounted on the PCBs. Mechanical separation (Niu and Xu, 2019), hydrometallurgical processing (Kim et al., 2007; Fontana et al., 2017) and chloride metallurgy (Niu and Xu, 2017) were reported for the extraction of valuable metals/materials from MLCCs. Kim and co-workers reported the recovery of Ni from waste MLCCs using different acidic lixiviants and concluded that nitric acid was most efficient leachant in comparison to HCl and H2SO4. About 97% Ni was leached out using 1 M HNO3 at 90 °C in 90 min maintaining 5 g/L pulp density (Kim et al., 2007). But nothing is reported related to the reaction of HNO3 with other metals present in the MLCCs. Fontana et al., 2017 reported a process for the recovery of Pd from MLCCs using aqua regia followed by solvent extraction with Aliquat 336 and finally reduced Pd as metal (98.8% purity) using 10% sodium borohydride solution. A hydrometallurgical process to recover base metals (95%) and precious metals (92%) from waste MLCCs was also reported (Prabaharan et al., 2016). An efficient and integrated process to recover valuable materials from waste MLCCs using chloride metallurgy and corona electrostatic separation has also been reported. In chloride metallurgy, NH4Cl was used as chlorinating agent followed by leaching with water and sodium thiosulfate solution to separate BaCl2 and AgCl, whereas SnCl4 generated in the gas-phase was condensed and collected. The Pd and TiO2 left in the leached residue were separated by corona electrostatic separation under a voltage of 30 kV and 25 rpm rolling speed (Niu and Xu, 2017). Literature survey indicates that the lack of feasible technology to get pure metal/salts of precious and valuable metals created the need for development of feasible process.

Studies indicate that no sufficient research work were carried out for the recovery of metals from the small electronic components viz. connectors, integrated circuits, capacitors, transistors, etc. mounted on the PCBs. Keeping in view of the above, present research reports a novel, feasible and scientifically validated hydrometallurgical process to recover precious metals (Ag and Pd) from MLCCs depopulated from PCBs of personal computers.

Section snippets

Materials

MLCCs depopulated from PCBs of personal computers were used as raw material for experimental purpose. Nitric acid (HNO3), hydrochloric acid (HCl) and sulphuric acid (H2SO4) of laboratory grade (supplied by Merck, India) were used for leaching of metals. Chemicals like potassium chloride (KCl), sodium chloride (NaCl), cupric chloride (CuCl2) and calcium chloride (CaCl2) used for Ag precipitation were supplied by Rankem, India and organic extractant LIX 84IC was supplied by M/s Cognis

Pre-treatment and sample preparation

Initially, PCBs of personal computers were de-soldered using thermal treatment to de-populate and isolate the small electronic components viz. MLCCs, integrated chips, transistors, diodes, resistors, etc. The small electronic components liberated from PCBs were further classified to separate the MLCCs from other components. The separated MLCCs were pulverised to 100–150 mesh size. Fig. 1 shows the systematic flow-sheet for sample preparation. The pulverised sample was chemically digested and

Conclusion

Based on the laboratory scale work carried out for the recovery of precious metals from waste MLCCs the following conclusions could be drawn:

MLCCs present in electronic devices are potential secondary resource for precious metals such as Ag and Pd along with non-ferrous metals Ni and Cu in majority. In order to get high purity salts of Ag and Pd, the impurities (Ni and Cu) were also removed/recovered as marketable product. Initially, Ni was removed by selective leaching using 2 M HCl at 75 °C

Acknowledgement

Authors of the paper are thankful to Director, CSIR-National Metallurgical Laboratory, Jamshedpur, India for his kind permission to publish this paper. One of the authors Ms. Rekha Panda would like to extend her heartfelt gratitude to Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing Senior Research Fellowship (Grant: 31/10(64)/2017-EMR-I) to carry out this research work.

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