RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 116, NO. 3, 10 FEBRUARY 2019 463 *For correspondence. (e-mail: mahapatra.rp@gmail.com) Microwave heating and acid leaching processes for recovery of gold and other precious metals from e-waste Rajendra Prasad Mahapatra 1, *, Satya Sai Srikant 2 , Raghupatruni Bhima Rao 3 and Bijayananda Mohanty 1 1 National Institute of Technology, Mizoram 796 012, India 2 SRM Institute of Science and Technology, Modinagar 201 204, India 3 CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, India The main aim of the present study is to develop a new method to recover gold and other precious metals such as copper, silver, etc. with the judicious combina- tion of microwave heating and acid leaching of e-waste samples like printed circuit board of computers, mo- biles, electronic devices, etc. In the present study, mi- crowave heating is much required for segregation of melted plastics and metals from e-waste and it also generates high temperature in much lesser time. The investigation basically consists of seven stages, i.e. heating the e-waste using microwave energy, grinding, physical separation of molten plastics and metal slag followed by leaching in nitric acid, leaching in aqua regia for unreacted materials, removing with concen- trated nitric acid and washing and purifying the metal deposits. The results obtained from XRD, FESEM– EDAX and TEM studies confirm that precious metals like gold and silver can be obtained with the new method, which is also an environmental friendly approach. Keywords: Aqua leaching, EDAX, e-waste sample, FESEM, image mapping, microwave heating, TEM, XRD. IN the 21st century, the utilization and production of elec- tronic materials, both for business as well as household purposes in both developed and developing countries, have expanded exponentially due to which problems of piling electronic waste (e-waste) are growing everyday. In other words, it creates a huge quantity of waste in the form of obsolete electrical and electronic products (e- waste). Low prices along with new technologies innova- tion and addition of new features are few reasons for the replacement of electronic products at a quicker rate. E-waste is chemically and physically different from other forms of municipal or industrial waste; it contains both valuable and harmful materials that require special handling and recycling processes to avoid environmental pollution/contamination and hazardous effects on human health 1,2 . Handling of e-waste is a tedious and challeng- ing task with hazardous effects to all living beings 3,4 . Recovering precious metals from e-waste through hydro- metallurgical processes 5 along with heating is more attractive economically than other methods. The recovery of precious metals such as gold and silver from e-waste with leaching processes involves either heating with muffle furnace followed by leaching or plasma method coupled with leaching. There are several publications and presentations on this aspect 6–12 . How- ever, so far no attempt has been made on the use of microwave energy for segregation of melted plastics and metals from e-waste at lesser time and temperature com- pared to conventional muffle furnace. Srikant et al. 13 car- ried out investigations with microwave heating of low grade ilmenite sample for up to 60 min to produce tita- nium product in the form of titania slag with low rich grade of 76.5%. They also upgraded titania rich slag to the grade of 80–85% from 76.5% with microwave heating after addition of an extra 5% graphitic carbon to low grade titania slag sample 13 . The addition of graphitic car- bon played a triple role as reductant, source of heat, as well as the addition of carbon to slag. Such characteris- tics and products were only achieved with microwave heating but could not be achieved at less time and energy efficiency with either plasma method or with convention- al heating method because, plasma method required lot of energy and conventional methods required longer time. Moreover the characteristics were not as perfect as the characteristics of microwave heating. These reasons prompted the present investigation with the effect of judi- cious combinations of microwave heat treatment and leaching processes on recovery of precious metals from e-waste. Electronic materials like electrolyte capacitors, batte- ries, small transformers and plastics were recovered from scrapped printed circuit boards (PCB) of old computers and mobiles. However, some metal wires and epoxy base plates were difficult to recover from circuits. Hence an attempt was made to recover them by using sharp tools like cutters and nose pliers. The PCB samples with inte- grated chips (ICs) and pogo pins after dismantling/ removal/recovery by cutter, were used as raw materials for our present investigation to extract gold. The ICs and pogo pins removed from scrapped boards were first shredded into fragments of approximately 10 mm × 10 mm and these fragmented samples were finally ground to a fine particle state of approximately 250–300 μm (i.e. 50–60 mesh) in the laboratory. The following methodologies were used to recover precious metals from e-waste samples. Around 20 kg of ground PCBs (e-waste materials) was kept in a quartz container and mixed with 500 g of silicon carbide powder (with 400 mesh size). The silicon carbide powder in crushed PCB acts as a coupling agent and hence accelerates microwave heating. Some metal oxides present in crushed samples are basically low loss factors and hence poor receptors to microwave heat treatment.