Recovery of value from waste electrical and electronic equipment using ionic liquids

The primary tenet of the circular economy is the concept of value recovery: all waste possesses an inherent value, which if properly managed can be extracted. In this new context of global environmental remediation, sustainable, effective and affordable waste management is an issue of critical importance. The aim of this research was to develop novel treatment methodologies for the recovery of value from the metallic fraction of small waste electrical and electronic equipment (WEEE) using ionic liquids (ILs). Two different types of electrical and electronic wastes were considered based on their high critical metal concentration and ease of procurement: (i) thin film wastes from spent fluorescent tube phosphors and from indium tin oxide (ITO) screens and (ii) printed circuit boards (PCBs) from discarded computers. Recovery of indium and tin from ITO screens, of five rare earth elements (REEs) from thin films and of copper and tin from PCBs has been achieved via hydrometallurgical processes using ionic liquids (ILs). The methodologies developed involve a combination of a pre-treatment step to reduce the waste particle size and/or remove impurities, optimisation of the leaching parameters to selectively solubilise the elements of interest and separation and purification of the desired elements using methods such as solvent extraction, chromatography and electrodeposition. Three IL systems were investigated for the recovery of metal value from simulated and actual WEEE. The solubility of RE oxides in the task-specific ionic liquid protonated betaine bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) is high, and based on the steady variation in the reaction rate along the RE elemental series, selective separation of light and heavy RE elements is achieved. The metals are recovered from the ionic liquid solution at 5 oC by extracting them into dilute hydrochloric acid solution. The best conditions for a one-step separation of light from heavy REOs in [Hbet][Tf2N]:H2O mixtures is achieved with 1:1 [Hbet][Tf2N]:H2O at 57 oC using short contact oxide:solvent times (maximum 5 min). Separations of light from heavy REOs, in waste phosphor samples, containing La2O3, CeO2, Eu2O3, Gd2O3, Tb3O4 and Y2O3, are also achieved even in the presence of high concentrations of heavy REOs using short contact times. The use of [Hbet][Tf2N]:H2O as a means of separating light and heavy REOs is aided by the ease of recycling the solvent which can be recycled and reused at least five times with little loss of solvent quality or efficiency. A resin impregnated with an equal mass mixture of N,N-dioctyldiglycolamic acid (DODGAA) extractant and the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) was used to investigate the potential for extraction chromatography in the recovery and separation of REEs, and of indium and tin from dilute solutions that model those obtained from treatment of thin films containing these critical elements. The recovery and separation of adsorbed metal species on the DODGAA-[C4mim][Tf2N] SIR resin from solutions containing the glass matrix ions, Ca2+ and Al3+, along with In3+ and Sn4+ or lanthanide ions is achieved by elution with HNO3. Ca2+ and Al3+ are completely eluted with 0.1 M HNO3 retaining the target critical metal species on the resin. Separation of In from Sn is achieved by elution of In3+ with 2.5 M HNO3 and Sn4+ with 5 M acid. La is separated from the other lanthanides by elution of La3+ with 2.5 M HNO3 and the remaining lanthanides with 5 M acid. The SIR resins can be reused over a series of at least five cycles of loading, stripping, andnrinsing to reduce reagent costs and achieve critical metal recovery by extraction chromatography. The interactions between the REEs in nitrate solution and the DODGAA-IL extraction system was investigated using computational chemistry which showed that substitution of a conventional organic diluent for the IL, [C4mim][Tf2N], alters the coordination of the extracted complex. The recovery of yttrium europium oxide (YOX) from waste fluorescent tube phosphor using the IL 1-methyl imidazolium hydrogen sulfate ([Hmim][HSO4]) produced a ≥ 90 wt.% recovery of Y and Eu with an overall process efficiency of 88.0 wt.% and 94.2 wt.% for their leaching and recovery respectively. Calcium is the main impurity present in the final oxide product but luminescence analysis of the recovered yttrium europium oxide (Y0.95Eu0.05)2O3 indicates its potential for direct reuse as YOX phosphor. For all reported extraction systems, regeneration and reuse of the IL is achieved with little to no loss of performance over multiple cycles. Recovery of high purity copper from waste PCBs has been achieved using a simple efficient 'one pot' ionic liquid process by leaching metal from the PCBs with aqueous solutions of [Hmim][HSO4] and recovering by electrowinning. In addition, the biphasic IL system composed of [Hmim][HSO4] and trihexyl(tetradecyl)phosphonium chloride (Cyphos 101) was investigated for the extraction and mutual separation of Cu2+ and Sn4+ from PCB leach solution containing Fe3+, Al3+, Mn2+, Zn2+, Ca2+ and Ni2+. Cyphos 101 shows a high affinity for Cu2+, Zn2+, Pb2+ and Sn4+ even at acidic pH values. However, the poor separation of Cu2+ and Sn4+ from Zn2+ limits in conjunction with the decrease in extraction performance over multiple cycles limits the applicability of the [Hmim][HSO4]:Cyphos 101 IL system. Experimental results indicate that 3 to 4 Cyphos 101 molecules are involved in the extraction of one Cu2+ cation and that Cu2+ is extracted into Cyphos 101 through an ion-exchange reaction as hydrated copper sulphate with the release of chloride anion to the aqueous phase. The value of ionic liquids in the recovery of metals from WEEE using a variety of methodologies is demonstrated and further research in this field should be encouraged.