O2-enriched microbial activity with pH-sensitive solvo-chemical and electro-chlorination strategy to reclaim critical metals from the hazardous waste printed circuit boards

Abstract

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 ther-mosulfidooxidans (strain RDB) under intense aeration is found favorable for base metals' liberation. similar to 92% copper, 89% nickel, and 93% zinc get extracted at the optimal condition of O-2-mixed-aeration, 30%; pulp density, 10 g L-1; aeration rate, 0.5 L min(-1); sulfur dosage, 2%; temperature, 45 degrees C; and duration, 21 days. Quantitative separation of base metals is achieved using ketoxime as a function of equilibrium pH that yielding pH(0.5) order: Cu (1.45) < Ni (5.7) < Zn (8.1). The residual gold from WPCBs is uniquely leached (similar to 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 degrees C; and time, 60 min. Subsequently, gold from brine solution is solvated with tri-butyl-phosphate at pH(eq), <= 0.5, forming <([2( RP = O)center dot HAuCl4 center dot H2O])over bar> 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.