O-enriched microbial activity with pH-sensitive solvo-chemical and electro-chlorination strategy to reclaim critical metals from the hazardous waste printed circuit boards.

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 HNO. 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 O-mixed-aeration, 30%; pulp density, 10 g L; aeration rate, 0.5 L min; 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 pH 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 NaCl) under the electro-chlorination rate, 0.62 mmol min; dissolution pH, 1.0; pulp density, 20 g L; temperature, 30 °C; and time, 60 min. Subsequently, gold from brine solution is solvated with tri-butyl-phosphate at pH, ≤ 0.5, forming [2(RP=O)·HAuCl·HO]¯ 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.