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

Broeksma, Cara Philipa (2018-03)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

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.

AFRIKAANSE OPSOMMING: Die herwinning van metale vanuit afval gedrukte stroombaan borde (GSBs), ‘n sleutel onderdeel van elektroniese apparaat, is voordelig vanuit beide omgewings en ekonomiese oogpunte. Onlangse hidrometallurgiese prosesroetes gebruik sterk mineraalsure en sianied of haliede, wat omgewingsgevare inhou. Aminosure is al voorgestel as plaasvervanger loogmiddels met ‘n verminderde impak op die omgewing. Hierdie projek het beoog om die toepaslikheid van die aminosuur loogproses op die loging van metale vanuit GSB-afval te evalueer. Kleinskaalse loogtoetse is uitgevoer om die tempo, hoeveelheid en selektiwiteit van basis- en edelmetaal loging onder verskillende omstandighede te bepaal. Glisien, die eenvoudigste aminosuur, is gebruik as loogmiddel. Die betreklike lae oplosbaarheid van koper in die glisienstelsel het die pulpdigtheid tydens basismetaal loogtoetse tot 25 g GSBs/L beperk. Met lug as oksidant was die loging van koper aanvanklik onafhanklik van beide temperatuur en glisien-konsentrasie. Daar is aangevoer dat aanvanklike koper-loging in die lugstelsel, teen 1 M glisien, deur suurstof diffusie deur die grenslaag beperk is. Met die vordering van die reaksie, het die suurstof diffusie deur die CuO-intermediêr tempo-beperkend geword. Verhoging van die temperatuur en ʼn toename in die glisien-konsentrasie in die aanwesigheid van lug het die CuO-verwydering deur koper-glisien kompleks vorming versnel, wat op sy beurt die weerstand teen suurstof diffusie na die reaksie-oppervlak verminder het. Met die gebruik van suiwer suurstof as oksidant het die vehoging in temperatuur van 25°C tot 60°C koper-loging na 22 uur met ongeveer 50% vermeerder. Die vermeerdering van glisien tot bokant 1 M, in die aanwesigheid van suurstof, het geen invloed op koper-loging gehad nie. 81% koper-loging is bereik na 22 uur in die gunstigste toestande van 60°C, 1 M glisien-konsentrasie, met die gebruik van suiwer suurstof as oksidant. In hierdie toestande is die mede-ekstraksie van goud 1.3%. Edelmetaal loogtoetse is uitgevoer deur die oorblyfsels van die basismetaal loogtoetse as voermiddel te gebruik, met ‘n kontinue voer van H2O2 as oksidant. Verhoging van die temperatuur (tot 90°C), glisien-konsentrasie (0.1 M tot 0.5 M) en pH (11.5 tot 12.5) het geen noemenswaardige uitwerking op goud-ekstraksie gehad nie, met minder as 2% goud-loging na 96 uur. Verdere toetse is op suiwer goudfoelie gedoen om te bepaal of die aanwesigheid van koper in die GSBs goud-loging inperk. Die loging vanuit goudfoelie het egter nie die goud-loging verbeter nie en die slotsom is bereik dat goudloging met glisien nie tegnies lewensvatbaar is nie. ‘n Voorgestelde prosesroete vir metaal ekstraksie is eksperimenteel bevestig. Grootskaalse loogtoetse in optimale toestande, bepaal deur die kleinskaalse basismetaal loogtoetse (60°C, 1 M glisien, met suiwer suurstof as oksidant), is uitgevoer. As gevolg van die skamele massa oordrag van suurstof tot in oplossing gedurende die grootskaalse loogtoetse, was twee fases (wat elk 41 – 52 uur duur) nodig om 78% koper-loging te bereik. In ‘n opvolgende loogfase is 38% goud-loging bereik na 96 uur, met die byvoeging van 0.04 NaCN by 0.13 M glisien teen 25°C, met lug as oksidant. Verdere optimering van proses veranderlikes word benodig om die goudloging in die glisien-sianied stelsel te maksimeer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103502
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