Evaluating the applicability of an alkaline amino acid leaching process for base and precious metal leaching from printed circuit board waste

[摘要] ENGLISH SUMMARY: The recovery of metals from waste printed circuit boards (PCBs), a key component in electronic equipment, is beneficial from both an environmental and economic perspective. Current hydrometallurgical processing routes utilise strong mineral acids and cyanide or halides, which pose environmental hazards. Amino acids have been proposed as alternative lixiviants with a lower environmental impact. This project aimed to evaluate the applicability of the amino acid leaching process for the dissolution of metals from PCB waste.Bench-scale leach tests were performed to determine the rate, extent and selectivity of base and precious metal leaching at different conditions. Glycine, the simplest amino acid, was used as lixiviant. The relatively low solubility of copper in the glycine system limited the pulp density during base metal leach tests to 25 g PCBs/L.When air was used as oxidant, copper dissolution was initially independent of both temperature and glycine concentration. It was suggested that initial copper dissolution in the air system, at 1 M glycine, was limited by oxygen diffusion through the solid-liquid boundary layer. As the reaction progressed, oxygen diffusion through the CuO intermediate was believed to be rate-limiting. Increasing the temperature and glycine concentration in the presence of air, increased the rate of CuO removal through copper-glycine complex formation, which, in turn, reduced the resistance to oxygen diffusion to the reaction surface.When pure oxygen was used as oxidant, increasing the temperature from 25°C to 60°C increased copper dissolution after 22 hours by approximately 50%. Increasing the glycine concentration above 1 M, in the presence of pure oxygen, had no effect on copper dissolution. 81% copper dissolution was achieved after 22 hours at the optimal conditions of 60°C, 1 M glycine, using pure oxygen as oxidant. At these conditions, co-extraction of gold was 1.3%.Precious metal leach tests were performed using the residue from the base metal leach tests as feed, with H2O2 fed continuously as oxidant. Increasing the temperature (up to 90°C), glycine concentration (0.1 M to 0.5 M) and pH (11.5 to 12.5) had no significant effect on gold extraction, with less than 2% gold dissolution achieved after 96 hours. Further tests were performed on pure gold foils to determine whether the presence of copper in the PCBs inhibited gold dissolution. Leaching from gold foils, however, did not improve gold dissolution and it was concluded that gold leaching with glycine is not technically feasible.A suggested flowsheet for metal extraction was validated experimentally. Small pilot-scale leach tests were performed at the optimal conditions identified from the bench-scale base metal leach tests (60°C, 1 M glycine, with pure oxygen as oxidant). Due to poor mass transfer of oxygen into solution in the small pilot-scale leach tests, two stages (each with a duration of 41 – 52 hours) were required to achieve 78% copper dissolution. In a subsequent leaching stage, 38% gold dissolution was achieved after 96 hours, with the addition of 0.04 M NaCN to 0.13 M glycine at 25°C, using air as oxidant. Further optimisation of process variables are required to maximise gold leaching in the glycine-cyanide system.

[发布机构] Stellenbosch University