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