Electrocrystallisation and recovery of gold from thiosulphate-sulphite aged electrolyte
Interest has grown in developing non-toxic electrolytes for gold electrodeposition to replace the conventional cyanide-based bath. The main driver for the development of this electrolyte is the long term sustainability. One electrolyte that is being examined is a solution containing thiosulphate and sulphite, which has been developed specially for microelectronics applications. However, at the end of the electrodeposition process, the spent electrolyte can contain a significant amount of gold in solution and low concentration in rinse waters. Since the discharge of gold along with the effluent is a major economic as well as environmental concern, this study has been initiated to investigate if gold can be recovered from a spent thiosulphate-sulphite electrolyte. The recovery process is expected to have an enhanced value if the recovered gold can be tailored for suitable applications, for example, gold nanoparticles are useful in catalysis, sensors, electronics etc. In the initial work, we have used flat-plate glassy carbon and graphite electrodes to study the mechanism of nucleation and crystal growth of gold deposition from concentrated spent electrolyte. It was found that at the early stages of reduction process, the deposition of gold on glassy carbon exhibits an instantaneous nucleation of non-overlapping particles. At longer times, the particles begin to overlap and the deposition follows a classic progressive nucleation phenomenon. On the other hand, deposition of gold on graphite does not follow the classical nucleation phenomena. Microscopy observations demonstrate that the particle size and density of the deposited gold can be controlled by varying the deposition potentials and time. A flow cell operated in a flow-by mode was constructed which used a threedimensional reticulated vitreous carbon to recover gold from rinse waters. The reactor was tested by removing copper (II) from acidified copper sulphate solution near the mass transfer limiting rate. The gold removal process was then operated close to mass transport control at all times by adjusting the current to its limiting value, 1L(t). It was found that the gold concentration depletes very slowly with time owing to the changes in electrolyte.