LAUSR

An innovative process integration for the sustainable recovery of critical metals from waste printed circuit boards (WPCBs) is demonstrated. In the acid pre-treatment of WPCBs, > 95% of highly toxic metals lead and tin could dissolve after 240 min of contact in 4.0 mol L-1 HNO3. Thereafter, the microbial activity of Sulfobacillus thermosulfidooxidans (strain RDB) under intense aeration is found favorable for base metals' liberation. ~92% copper, 89% nickel, and 93% zinc get extracted at the optimal condition of O2-mixed-aeration, 30%; pulp density, 10 g L-1; aeration rate, 0.5 L min-1; sulfur dosage, 2%; temperature, 45 °C; and duration, 21 days. Quantitative separation of base metals is achieved using ketoxime as a function of equilibrium pH that yielding pH0.5 order: Cu (1.45) < Ni (5.7) < Zn (8.1). The residual gold from WPCBs is uniquely leached (~99% efficiency) in brine solution (2.0 mol L-1 NaCl) under the electro-chlorination rate, 0.62 mmol min-1; dissolution pH, 1.0; pulp density, 20 g L-1; temperature, 30 °C; and time, 60 min. Subsequently, gold from brine solution is solvated with tri-butyl-phosphate at pHeq, ≤ 0.5, forming [2(RP=O)·HAuCl4·H2O]¯ complex in the organic phase. Finally, > 99% of high-purity gold is stripped from loaded organic while contacting ammoniacal thiosulfate solution in two-stages of counter-current flow.