The recovery of metals from waste solutions by electrochemical methods
Electronics is an important industry in the UK, as there are many home-grown manufacturing companies in the areas of optics, electronics and micro-systems. As a consequence of the growth in this industry, there has been an increase in the discharges from manufacturing. A process that could reclaim the metals contained in these wastes is desirable because of the increasing costs of raw materials and environmental compliance. The aim of this project was to develop a systematic method to determine the feasibility of metal recovery from aqueous solutions. Waste tin stripping solution was chosen as a case study. This waste arises from a stage in the manufacture of printed circuit boards when a protective tin layer is stripped from the copper pattern. The solution is mainly nitric acid, but also contains suspension agents, ferric salts and inhibitors. After use it also contains suspended tin oxide and dissolved copper. It has been suggested that the tin and copper could be recovered from this waste by electrodeposition. Initially the thermodynamics of the system were studied, followed by experiments to verify the theoretical work. Pourbaix diagrams were constructed to determine when the metals would be in the solid or liquid phase depending on the concentration of metal and anion, pH and system potential. This information was then compared to separation processes to determine or confirm the recovery route. This study showed that the dissolved copper and suspended tin oxide could be separated by filtration if the pH was maintained between -0.4 and 2.4. The dissolved copper could then be removed by electrodeposition. After the metal recovery route has been established, the individual stages are studied in more detail. In this project the feasibility of copper recovery from the stripping waste was examined. Copper deposition occurs concurrently with nitrate, ferric and hydrogen reductions. Using a parallel plate electrochemical reactor, the copper concentration was reduced by approximately 30%, at a current efficiency of 70%.