Electrochemical separation and purification of metals from waste electrical and electronic equipment (WEEE)

• Main Author: Md Ali, Umi Fazara
• Other Authors: Kelsall, Geoff
• Published: Imperial College London 2011
• Subjects: 669.9
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.539250

This thesis reports on results of a novel process to recover metals selectively by electrodeposition by pumping aqueous acidic chloride solutions produced by leaching of shredded waste electrical and electronic equipment (WEEE) through the potentiostatically controlled cathode of an electrochemical reactor. The WEEE solutions contained low concentrations of precious metals, including Ag, Au, Pd and high concentrations of Cu. Electrodeposition from low concentrations of such dissolved metals requires electrodes with high mass transport rate coefficients and specific surface areas to increase cross-sectional current densities and optimise capital and operating costs. Hence, to recover gold from solutions with concentrations < 10 mol m-3 in the WEEE leachate, a three-dimensional cathode was used consisting of a circulating particulate bed of 0.5-1.0 mm diameter graphite particles, on which (AuIIICl4 - + AuICl2 -) ions were reduced. The temporal decay of the solution absorbance of AuCl4 - ions at 312 nm was recorded on-line by a quartz flow cell connected to a UV-visible spectrophotometer using fibre optics, enabling its time dependent concentration to be determined in real time. Total dissolved gold concentrations were determined by Inductively-coupled Plasma Optical Emission Spectroscopy (ICP-OES). The results from the reactor experiments were modelled in terms of a mass transport controlled reaction in a plug flow electrochemical reactor operated in batch recycle with a continuous stirred tank reservoir. As copper is the dominant element in WEEE, and hence in the leach solution, its electrodeposition was investigated using an electrochemical reactor with a Ti/Ta2O5-IrO2 anode, cation-permeable membrane and a Ti mesh cathode in a fluidised bed of 590-840 μm glass beads to enhance mass transfer rates and to improve copper deposit morphologies. As for other metals, the effects were determined of cathode potential and solution flow rate on electrodeposition rates, charge yields, specific electrical energy consumptions, and deposit morphologies, imaged subsequently by scanning electron microscopy, and purities determined by X-ray fluorescence (XRF) and X-ray diffraction spectroscopy (XRD). While depleting CuII concentrations from 500 to 35 mol m-3, copper purities of > 99.79 %, as required for commercial purity Cu, were achieved with charge yields of 0.90 and specific electrical energy consumptions of 2000 kW h tonne-1. In addition, the circulating particulate bed cathode depleted solutions rapidly from 15 mol m-3 CuII ca. 100 ppm. Experiments with a rotating vitreous carbon cathode confirmed predictions from a kinetic model for a small electrode potential window within which to achieve selective electrodeposition of tin from synthetic SnIV-PbII aqueous chloride solutions, from which Pb could be electrodeposited subsequently. AlIII, FeII, ZnII and NiII remained in solution after the recovery of Au, Cu, Sn and Pb from the WEEE leachate. Unlike Al, it is possible to electrodeposit Fe from aqueous solution, and it was decided to add NaOH (+ air) to increase the pH to ca. 3.25 to precipitate ‘Fe(OH)3’, which was recovered by filtration. This option also enabled subsequent electro-co-deposition of Ni and Zn with high charge yields, as the higher pH decreased the driving force for H2 evolution. A one- dimensional mathematical model was developed in MAPLETM to predict the kinetics of Ni-Zn electro-co-deposition, which was validated experimentally. The model also considered the potential and concentration profiles in the cathode | electrolyte boundary layer for conditions in which migration and convective diffusion all contribute to overall transport rates, to predict the behaviour and optimize the process parameters of the electrochemical reactors.