Recently, over 60 million liters of 10–15% copper-containing waste etchants (CCWEs) solution generated from printed circuit board (PCB) manufacturing are disposed of in Taiwan annually. The CCWEs are mainly composed of copper chloride, hydrochloric acid, and water. Since the disposal of etchants without proper treatment has posed an environmental problem, expensive separation or storage methods have been previously used; however, these have not been effective or successful. Therefore, the resource recovery of these undesired waste etchants in the form of copper nanoparticles would be economically and environmentally attractive. Experimentally, copper nanoparticles have been recovered from CCWEs by microemulsion at controllable ambient temperature and pressure. The properties of copper nanoparticles were further analyzed using XRPD, FE–SEM, TEM or XANES/EXAFS spectroscopy. From the FE–SEM or TEM microphotos, most spherical-shaped nanoparticles with diameters of 20–50 nm were observed. It showed that these nanoparticles were well dispersed and without any agglomeration. Moreover, the pH values increased with decreasing particle sizes of copper nanoparticles in the synthetic processes. The XRPD patterns showed that the nanophase precipitates have metallic copper and Cu2O crystalline structures. The existence of the Cu(0) or Cu(I) was also confirmed by XANES or XPS spectroscopy. The metallic Cu and Cu2O nanoparticles with a cubic lattice structure were observed by EPR. The synthesized copper nanoparticle with a CueO (Cu–(O)eCu) bond distance of 1.85 ± 0.02 Å (2.69 ± 0.02 Å) and a coordination number of 1.39 ± 0.25 (3.5 ± 0.25) was also measured by EXAFS spectroscopy. The surface of the synthesized copper nanoparticles was modified using lauric acid sodium and dispersed in water for over 7 h after ultrasonic dispersion treatment. Thermal conductivity of nanofluid containing copper nanoparticles was proportional to the weight percent of copper nanoparticles. A designed production process of copper nanoparticles for industrial PCB waste etchant recycling via a microemulsion method was also proposed. Payback of 10-TPD and 20-TPD recycling processes of PCB waste etchants for copper nanoparticle production were 2.63 and 2.35 years, respectively.
