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.