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.