Msci Project: The Pewter Project

Electrowinning precious metals directly from waste waters below pH 2 has not been successfully used in industry without use of toxic chemical additives, or by raising the pH. Usually this method ultimately precipitates the metals to remove them from the water, making recovery of individual metals more difficult than if they were recovered directly from solution. Since many mines have shut down due to economic changes in the 1980s and 1990s, mine waste water is being treated using expensive methods, or is simply polluting the local environment. For sustainable use of resources and to prevent degradation of the environment, recovering the metals from the water profitably and efficiently benefits industry and benefits the eco-systems reliant on quality water.

Cyclic voltammetry was used to investigate deposition mechanisms and potentials of the metal cations, and showed that most of the metals deposit above a certain potential with a two-step mechanism of reduction and then a chemical reaction. Potentiostatic depositions carried out at varying voltages targeting gold and tin showed that tin could be reliably deposited but that the situation for gold was not as simple. Characterising the anion content of the water suggested that chloride and sulfate are affecting the reduction potentials of the metal cations when compared to simple, standard solutions of gold and tin respectively.

Altering the flow rate of the water showed a significant effect on the deposit characteristics, such as distribution over the glassy carbon surface, particle shape and in the peak currents observed during deposition. Future work on the project will include optimisation of the flow rate for maximum deposition and current density based on a realistic upscaling factor. This is with the goal of using an in-situ anodic and cathodic porous carbon foam in-situ at mines like Wheal Jane to remove metals step-wise and recover them for re-use rather than expensive pump-and-treat systems that use expensive, potentially harmful chemicals and lots of energy.