Hydrodynamic studies of aqueous two-phase systems in millichannels

Liquid-liquid segmented flows in microchannels have been extensively investigated in the context of nanoparticle synthesis. The enhanced mixing in the slugs results in monodispersed particles. Earlier studies have focused on Organic-Aqueous Systems (OAS). The nanoparticles synthesized in the presence of organic solutions have limited applications. An alternative green route for the synthesis can be developed using an Aqueous Two-Phase System (ATPS). These systems are characterized by interfacial tensions, which are two orders of magnitude lower than typical organic aqueous systems. In this work, flow patterns and hydrodynamics of ATPS are investigated as a first step. Polyethylene glycol -trisodium citrate system was chosen as ATPS. The objective of this work is to see if any new physics arises in an ATPS system. The low interfacial tension results in high Capillary numbers (Ca >> 3) in a microfluidic system. Consequently, the flow observed here is parallel or core-annular. However, in a millichannel, the capillary number becomes lower (Ca << 1) for an ATPS system. In this work, experiments were carried out in a millichannel to span different flow patterns. The pattern formation was analyzed and classified into three categories, i.e., slug flow (interfacial tension dominated), transition flow, and core annular flow (inertia dominated). Flow regime maps based on the Reynolds number, Capillary number, and Weber number of each phase were found to be qualitatively similar to those of OAS. Simulations were performed for various interfacial tension values. An interfacial tension value of 1.25x10-4 N/m was found to yield slug sizes which fitted well with the experimental data. Film thickness was measured experimentally and with simulations compared favorably with the correlations available in the literature for OAS

Sequential recovery of metals from waste printed circuit boards using a zero-discharge hydrometallurgical process

Several hydrometallurgical routes have been proposed in the literature to treat printed circuit boards (PCBs) from waste electronic equipment. These employ different chemical reagents for the recovery and separation of metals as metals or their salts. The recovery of multiple chemical reagents (unreacted acid, neutralising agent, metal salts) in a process requires several additional downstream steps affecting economic feasibility. There is a need to develop a safe, eco-friendly, and economically feasible process to recover metals from PCBs. In this work, we achieve this using a hydrometallurgical process for treating PCB with nitric acid by extracting metals sequentially. The focus is on tin, lead, and copper, which are present in significant quantities in PCBs. The process is scalable and is based on exploiting the physio-chemical interactions between the different metals and the acid. Tin and lead present in the solder are selectively dissolved at low acid concentrations. Tin comes out in the form of colloidal particles of metastannic acid. The low solubility of lead nitrate in concentrated nitric acid is exploited by evaporating the solution containing unreacted nitric acid and lead nitrate. In the concentrated acid, lead nitrate crystals are precipitated out, and the concentrated nitric acid is recycled for dissolution. The different nitrogen oxide gases generated are absorbed in water and recycled. Copper present in copper tracks was recovered by reacting with a higher concentration of acid in a downstream step. The copper nitrate obtained was separated from nitric acid by extracting nitric acid using Tri-Butyl Phosphate. Tin, lead, and copper are sequentially extracted as tin oxide (s), lead nitrate (s), and copper nitrate (aq.) with an efficiency of 77%–97%, 51%–85%, and 100%, respectively