An integrated biohydrometallurgical approach for the extraction of base metals from printed circuit boards

Abstract

This study evaluated a novel two-step process for biological lixiviant generation to facilitate base metals leaching from waste printed circuit boards (PCBs). In the first step, crushed waste sulfur pastilles, a by-product of the petroleum industry, were biologically oxidised to produce biogenic H2SO4. The acidified effluent containing biogenic H2SO4 was then supplemented with ferrous iron (20 g L−1) in the second step, where it was biologically oxidised to produce ferric iron. The recovered biolixiviant was then used for base metal leaching from PCBs. The biological sulfur oxidation rate was 0.43 g L−1 d−1 with oxidation efficiency 43%, and the biological iron oxidation rate was 5.93 g L−1 d−1 with oxidation efficiency 96.5%. Acidithiobacillus caldus was primarily responsible for the oxidation of waste sulfur, whereas the iron oxidation was achieved by a microbial consortium dominated by Acidithiobacillus ferrooxidans. The leaching of base metals from PCBs using the biogenic lixiviant was investigated at various experimental conditions to determine leaching efficiency. Leaching parameters tested in this study included temperature (25-45 °C), aeration, PCB pulp density (PD, 1 and 5%), pH control, leaching reactor configuration, and pre-treatment of PCBs by ashing. The results showed that increasing temperature significantly increased (p < 0.05) metal extraction from PCBs using biogenic lixiviants. Aeration did not improve leaching efficiency when compared to vigorous mixing. High leaching efficiencies for Cu (97.9%), Ni (80.1%), Al (91.0%) and Zn (99.7%) were obtained at 45 °C without aeration at a 1% PD and a pH range of 1.25 to 1.64. Controlling pH to a set point of pH 1.4 with feedback-dosing of 2 M H2SO4 significantly increased (p < 0.05) the leaching efficiencies (78.8% Cu, 71.3% Ni, 85.3% Al and 90.6% Zn) at 35 °C as compared to leaching without pH control. Changing the leaching process from batch to sequential leaching (5% PD) also enabled higher extraction for Cu (92.4%), Ni (92.6%), Al (82.2%) and Zn (99.8%) when two cycles of lixiviant replacement were undertaken over 96 h. Pre-treatment of PCBs by ashing at 500 °C for 1 h resulted in high leaching efficiencies for Cu (93.7%), Ni (76.1%), Al (85.4%) and Zn (95.8%) using biogenic H2SO4 at 1% PD. This study showed that by separating the unit processes for biogenic lixiviant generation from the leaching process, the optimisation of biogenic lixiviant production and PCB leaching parameters could be effectively achieved without compromising the growth and activity of the microorganisms.