Recovery of critical metals from waste electronics using environmentally sustainable processes
Item statusRestricted Access
Embargo end date14/06/2023
Kinsman, Luke Martin Mark
Electronic waste (e-waste) is one of the fastest growing global waste streams with almost 54 million tonnes being generated in 2018. Only 20-30% of e-waste is currently recycled and can be attributed to its complex composition, comprising a myriad of different metals of varying future use and value. With many metals in e-waste present in greater quantities than their primary ores, e-waste can be viewed as a valuable secondary source of precious and base metals. This work aims to develop and understand new reagents that can selectively recover valuable and resource-critical metals typically found in e-waste. Chapter 2 evaluates simple primary (1°), secondary (2°), and tertiary (3°) amides as reagents that selectively separate gold from other metals typically found in e-waste by a solvent extraction process. Previous work has shown that while gold extraction efficiency from single metal solutions is ordered 3°> 2° > 1°, the 3° and 2° amides are ineffective at gold transport from mixed-metal solutions of concentrations representative of e-waste due to the formation of insoluble third phases. This chapter examines the identities of the species that reside in the organic and third phases by a combination of mass spectrometry, NMR spectroscopy, and ICP-OES methods with a view to better understand what triggers 3rd phase formation. Some strategies to overcome this issue are then demonstrated. Chapter 3 builds on the findings from Chapter 2 that differences in an extractant’s structure can result in variable solvent extraction performance with, in some cases, precipitation being favoured. In changing from a branched aliphatic tertiary diamide to a simpler phenyl substituted diamide, selective precipitation of gold and other chloridometalates from complex acidic mixtures without the need for an organic diluent is found. The precipitation of a variety of metals from a range of HCl concentrations is examined, with the diamide being primarily selective for gold among up to 28 other elements. The X-ray crystal structure of [HL6][AuCl4] displays an infinite chain of HL+ cations, formed through an intermolecular proton chelate, interleaved with AuCl4− anions. Tailoring of the selectivity of metal precipitation is demonstrated by altering either the HCl concentration or the stoichiometry of the precipitant with complete uptake of gold, iron, tin, and platinum by the diamide seen at 6 M HCl when excess precipitant is used, while only gold uptake is seen when one equivalent of diamide is used. A selective metal stripping process is developed; treatment of iron, tin and platinum precipitates with 2 M HCl releases the metals back in to solution, whereas gold is released quantitatively from the isolated precipitate as HAuCl4 by contact with water, so recycling the diamide for further use. Factors governing the strength, selectivity and efficiency of precipitation were then explored by synthesising and testing derivatives of the tertiary diamide with differing structural and electronic properties. Chapter 4 concerns the solvent extraction process for tantalum, which is typically extracted from acidic fluoride media. However, this work has shown that its extraction by the same 1° amide as used in Chapter 2 was more effective under chloride conditions compared to the typical fluoride conditions. The mode of action of the system is studied using similar techniques to Chapter 2, revealing that an anion-swing mechanism operates where tantalum is transported to the organic phase as its halometalate, TaCl6−, through an outer-sphere mechanism. In contrast to TaCl6−, transport of TaF5Cl− or TaF6− was poor; this is thought to be due to their greater charge densities, which carry higher hydration enthalpies. The process described provides a potentially safer, fluoride-free route to recycling Ta from waste electronics, using milder reagents than the current commercial methods. Chapter 5 describes the use of a dual-purpose ionic liquid, methyltrioctylammonium iodide, for the selective transport of gallium from aqueous iron solutions into a toluene organic phase. The mode of action was probed by UV-Visible spectroscopy, 71Ga NMR, electrospray ionisation mass-spectrometry and slope analysis. These techniques show that the iodide counter-ion is crucial in preventing the formation, and hence competitive transport, of FeCl4− by reduction of Fe(III) to Fe(II), while Ga(III) is extracted as GaCl4− forming a simple ion pair with the hydrophobic quaternary ammonium group.