Alternative systems for extracting critical metals from electronic waste

• Autor(a) principal: Conceição, João Henrique Ferreira da
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/30095
• topic: Circular Economy; Critical Metals; Hydrophobic Eutectic Mixtures; Liquid-Liquid Extraction

Excessive consumption of electronic equipment results in an overuse of existing natural resources and the increasing production of electronic waste – e-waste – associated with the disposal of such equipment when they reach their end of life. Electronic waste is quite complex; it is considered an environmental hazard if it is disposed of incorrectly but an important secondary source of critical materials if properly valued. Thus, considering the circular economy concept, the main objective of this work is to study the extraction and recovery of high value critical metals from electronic waste, using alternative systems composed of hydrophobic eutectic solvents. Initially, several eutectic mixtures were studied and identified. Based on this study and considering the chelating properties of the constituents of the mixture, three hydrophobic eutectic solvents were selected: trioctyphosphine oxide (TOPO) + thymol, TOPO + capric acid and hydrocinnamic acid + capric acid. The solid-liquid phase diagrams of the mixtures were measured using differential scanning calorimetry and the mixtures were characterized using various physico-chemical properties: densities, viscosities, water contents and the stability of mixtures in the presence of acidic aqueous solutions. The extraction capacity of the three selected eutectic solvents was initially evaluated using copper chloride and according to several parameters: pH, eutectic mixture composition, metallic salt concentration in the aqueous solution and effect of the presence of different salts in the aqueous solution. In addition, the selectivity of the systems was investigated by comparing the extraction efficiencies of different transition and platinum group metal salts. TOPO based eutectics have a higher affinity for platinum group metals in aqueous solutions with 1 M HCl concentration, while systems composed of carboxylic are more selective for transition metals such as iron and copper, although with lower distribution values. Finally, the separation of palladium and copper, Pd(II)/Cu(II), extremely relevant in the valorisation of printed circuit boards, was investigated. Successive extractions using TOPO + thymol with 1 M HCl were performed. These showed an average extraction efficiency of 65% for palladium and 0% for copper. The organic phase rich in metal was easily recovered by mixing it with an aqueous thiourea acid solution. NMR and FTIR analyses showed that no changes in the eutectic phase were observed during the multiple extraction cycles. In short, the non-ionic eutectics studied here have low viscosities, low costs and high extraction efficiencies for different metals, emerging as sustainable alternatives to various organic solvents while fitting the concept of circular economy.