Effect of FR-4 decomposition in the hydrometallurgical recovery of copper from electronic waste

- Epoxy decomposition induced preg-robbing of copper dissolved in citric acid.
- Acid induced partial solvolysis of epoxy raised solution pH through the release of amine.
- Copper-citrate adsorption acted as a precursor for Cu precipitation from undersaturated solution.
- Copper leaching inhibition was affected by pH, temperature and pulp density.

This study examined the role of epoxy, present in glass fibre reinforced laminate (FR-4), in inhibiting the leaching of copper from electronic wastes (e-wastes). FR-4 makes up the majority of the base materials used in printed circuit boards and would be a prevalent polymeric constituent of e-wastes. To mimic the spent medium based leaching of fungi, leaching of e-waste was performed using citric acid. Metal dissolution tests with citric acid revealed epoxy decomposition during the leaching process and this in turn inhibited copper dissolution by two routes. The first route is by raising the solution pH from 1.8 up to 4.88 through the cleavage and release of amine group in solution and through the transformation of the epoxy surface sites. Both promoted the removal of copper from solution by adsorption of copper on the epoxy followed by its precipitation as di-copper citrate. Tests over various leaching conditions specifically at pH 1.8-3.0, temperatures from 30 to 90 °C, pulp densities of 5-100 g/dm3 and periods of up to 24 h revealed various aspects of the process. These included epoxy decomposition induced copper loss of up to 40.3% and the masking of copper dissolution self-catalysis by Cu2 + ion. This study showed that inhibition of copper leaching can be minimised. This required spent medium leaching at low pH (1.8), low pulp density (5 g/dm3) and high temperature (90 °C). These results demonstrated the impact of epoxy depolymerisation in e-wastes leaching and the potential merit of spent medium leaching in overcoming these effects and in optimising the recovery of copper from e-wastes.