Potential of metals leaching from printed circuit boards with biological and chemical lixiviants

The generation of electronic waste (e-waste) is an issue with global consequences and therefore the proper management and recycling of e-waste are of increasing importance. Printed circuit boards (PCBs), which are a common component of e-waste, have a high valuable metal content which also makes this material an important secondary resource. In this study, biohydrometallurgical extraction of metals from PCBs was investigated as a potential alternative to conventional hydrometallurgical or pyrometallurgical processing options. An indirect non-contact leaching approach using ferric iron generated by Acidithiobacillus ferrooxidans was compared to chemical ferric sulfate leaching of Cu, Ni, Zn and Al from milled high-grade PCBs at 1% pulp density at Fe3+ concentrations of 5–20 g L−1 and at a pH range of 0.6–1.2. The roles of redoxolysis and acidolysis were examined by comparing ferric leaching with sulfuric acid leaching conducted at initial pH values of 0.8–1.4. Results showed that the supplementation of ferric iron significantly (p < 0.05) improved the chemical leaching yields as compared to sulfuric acid leaching for Cu (47.4% to 66.3%), Al (55.3% to 100%), Zn (45.5% to 92.4%) and Ni (61.0% to 97.7%) at pH 0.8. Increase in ferric iron concentration and decrease in pH also significantly (p < 0.05) improved the yield for both biological and chemical leaching. The optimal condition for overall metal bioleaching was at 20 g L−1 ferric iron at an initial pH of 0.6, yielding 87% for Cu and 100% for Al, Zn and Ni. Since no significant variation was found between chemical ferric sulfate and biogenic ferric sulfate leaching at a majority of the tested ferric concentrations, this study suggested that using biogenic lixiviants for extracting metals from PCBs is a viable alternative to chemical leaching.