Investigating the use of printed circuit board leach residue as reductant in pyrometallurgical operations

Attah-Kyei, Desmond (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: In recent years, there has been an increase in the generation of waste electrical and electronic equipment (WEEE) due to the advancement of technology. In addition to the environmental benefits of recycling electronic waste (e-waste), it also provides certain economic value. Printed circuit board (PCB) is the main focus of electronic waste because of the inherently high value of contained metals such as gold, silver and copper. Hydrometallurgical processes, consisting of several leaching stages, are often the most preferred option for the recovery of metals from PCB waste. However, hydrometallurgy does not address the issue of non-metallic PCB fractions that may end up being dumped at landfill sites or incinerated. It is important to reduce the environmental impact and gain value from both the metallic and non-metallic fractions of PCB waste. Several options for treatment of the non-metallic fraction including material recycling, where the residue may be used as inclusions in concrete or asphalt materials with minimal processing or chemical recycling, where chemicals and fuels are produced from the residue using techniques such as pyrolysis exist. Due to the complex composition of PCB leach residue, recovery by thermal treatment is likely to be the most feasible process route from technical and economical perspectives. In this study, the utilisation of the non-metallic PCB waste fraction as reductant in primary metal smelting operations and solid state reduction was investigated. The organic component as well as the ash composition of the PCB were characterised using Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence spectroscopy (XRF) respectively. Proximate analysis on the PCB revealed the ash and volatile matter contents being 40.1% and 44.8%, which is higher than coal used in reductive smelting operations. The elemental analysis showed carbon and oxygen content of 30.43% and 20.72% respectively. Thermodynamic modelling of chromite and iron smelting were performed using various blends of PCB and coal. The models showed that PCB residue might be used to partially replace the conventional reductants. The study revealed that in chromite smelting the optimal blend contains around 20 wt% PCB residue, with energy savings of 200 kWh/t of ore to achieve the same metal recovery. Laboratory-scale experiments simulating solid state reduction of hematite (Fe2O3) was also performed using various blends of PCB and graphitic carbon. The tests were carried out in a Differential Scanning Calorimeter (DSC) from ambient temperature to 1200℃ as well as in Single particle reactor (SPR) at 900℃ and 1000℃. The product of each test was analysed using scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The degree of reduction calculated from the mass lost during the test showed that PCB acts as better reductant at lower temperatures. However, at higher temperatures the advantage shift towards carbon.

AFRIKAANSE OPSOMMING: In die laaste jare is daar ’n verhoging in die generasie van elektriese afval en elektroniese toerusting (WEEE) as gevolg van die verbetering van tegnologie. Buiten die omgewingsvoordele van die herwinning van elektroniese afval (e-afval), bied dit ook sekere ekonomiese waarde. Gedrukte stroombaanbord (PCB) is die hooffokus van elektroniese afval as gevolg van die inherente hoë waarde metale, soos goud, silwer en koper, wat daarin bevat word. Hidrometallurgiese prosesse, wat uit verskeie uitlogingfases bestaan, is dikwels die mees gewenste opsie vir die herwinning van metale uit PCB-afval. Hidrometallurgie spreek egter nie die probleem van nie-metaal PCB-fraksies wat op die ashoop kan beland, of veras kan word, aan nie. Dit is belangrik om die omgewingsimpak te verminder en waarde te kry uit beide die metaal en nie-metaalfraksies van PCB-afval. Verskeie opsies bestaan vir die behandeling van die nie-metaalfraksies, insluitend materiaal herwinning waar die residu gebruik kan word as deel van beton of asfaltmateriaal met minimale prosessering of chemiese herwinning, of waar chemikalieë en brandstowwe vervaardig word uit die residu deur tegnieke soos piroliese te gebruik. As gevolg van die komplekse samestelling van PCB loogresidu, is herwinning deur termobehandeling waarskynlik die mees uitvoerbare prosesroete vanuit tegniese en ekonomiese oogpunte. In hierdie studie is die gebruik van die nie-metaal PCB-afvalfraksies as reduseermiddel in primêre metaal smelting bedrywighede en vaste toestand reduksie ondersoek. Die organiese komponente sowel as die as-samestelling van die PCB is gekarakteriseer deur Fourier transform infrarooi spektroskopie (FTIR) en X-straal-fluoressensiespektroskopie (XRF), onderskeidelik, te gebruik. Kort analise op die PCB het gewys dat die as en vlugtige materie-inhoud 40.1% en 44.8% is, wat hoër is as die steenkool wat gebruik word in reduserende smelting bedrywighede. Die elementanalise het koolstof- en suurstofinhoud van 30.43% en 20.72%, onderskeidelik, gewys. Termodinamiese modellering van chromiet en yster smelting is gedoen deur verskeie vermengings van PCB en steenkool te gebruik. Die modelle het gewys dat PCB-residu gebruik kan word om die konvensionele reduseermiddels gedeeltelik te vervang. Die studie het gewys dat in chromiet smelting, die optimale vermenging rondom 20 wt.% PCB-residu bevat, met energiebesparings van 200 kWh/t van erts om dieselfde metaalherwinning te bereik. Eksperimente is op laboratoriumskaal uitgevoer wat vaste toestand reduksie van hematiet (Fe2O3) simuleer deur verskillende vermengings van PCB en grafitiese koolstof. Die toetse is uitgevoer in ’n Differensiaal Skandeerder Kalorimeter (DSC) van omgewingstemperatuur tot 1200 °C sowel as in Enkel partikel reaktor (SPR) by 900 °C en 1000 °C. Die produk van elke toets is geanaliseer deur ’n skandeerder elektron mikroskoop (SEM) en X-straal-poeierdiffraksie (XRD) te gebruik. Die grade van reduksie wat bereken is uit die massa wat verlore gegaan het gedurende die toets het gewys dat PCB as ’n beter reduseermiddel optree by laer temperature. By hoër temperature skuif die voordeel egter na koolstof.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106176
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