4. Results and Discussion
4.2. SEM-EDS analysis
Additional ammonia-ammonium leaching was conducted on pre-treated PCBs at the conditions stated in Table A.1 for 24 hours and SEM-EDS (Scanning Electron Microscopy with Energy Dispersive Spectroscopy) analysis was conducted on a sample of the leached PCBs.
This analysis was conducted on three solid materials following leaching to determine if there was any gold residual on the board. A small piece of the pre-treated PCB after leaching was retained for SEM-EDS analysis as well as the filter paper present in the 0.2 μm nylon syringe filter used to filter the sample solutions before depositing the solution into the sample bottle. A small piece of the filter paper, used to separate the liquid from the PCBs and any residual particles, by means of vacuum filtration, was also included in the SEM-EDS analysis. These three materials were analysed for the presence of gold in the Electron Microscope Unit at the University of Cape Town. The figures below (Figure 4.9 - Figure 4.11) depict the presence of gold, if any, that was found on all three materials. Additional images and detailed gold elemental compositions can be found in Appendix D.2.
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Figure 4.9: Pie chart indicating EDS composition analysis on the leached PCB piece
SEM-EDS analysis was conducted on one leached piece of PCB at three randomly selected different areas: Area 1, Area 2, Area 3. Investigation on six different spectra for Area 1 was conducted whilst Area 2 and 3 had only four sites of investigation. The pie chart above summarises the mean wt% of all three areas investigated. Table 4.4 lists the gold composition at each site or spectrum for the PCB pieces as well as the syringe filter. Area 2 reported the highest percentage of gold at 0.29% whilst Area 3 and 1 reported a value of 0.09% and 0.07%
respectively. It is also clear, from Figure 4.9, that the PCB composition was largely from the carbon present on the PCB surface at 49.5%. Copper and sulphur were also common, making up about 9.16% and 9.10% each respectively. The leaching done in an ammonia-ammonium thiosulphate system was not expected to remove significant amounts of the copper found on the board, particularly because no effort had been made to liberate it. Furthermore, the copper content found in the top layer by Chirume (2019) using QEMSCAN was 11% which is fairly close to the value of 9.16% obtained from SEM-EDS and the difference between the two values could be because some of the copper present on the board had been leached and dissolved into solution in the ammonium-thiosulphate leaching process. This is possible if the solder mask layer dissolves during thiosulphate leaching. Sulphur was detected via SEM-EDS analysis and this is possibly due to the presence of polythionates such as sulphides forming within the system onto the PCB due to the decomposition of thiosulphate during leaching.
Figure 4.10 below shows the images of Area 2 and 3 on the PCB piece. Multiple sites were investigated within each area. Area 2 and 3 had a total of four sites each that were investigated. Gold was found on three out of the four sites in Area 2 and in only one out of the four sites in Area 3. Six sites were investigated in Area 1 of which only two reported the presence of elemental gold. Appendix D.2 contains images with the labels for each site investigated on all three areas.
0.09% 0.07%
0.3%
9.16%
1.06%
0.45%
9.1%
2.25%
2.18%
26.35%
49.46%
C O Al Si S Ca Cu Au area 1 Au area 2 Au area 3
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Figure 4.10: (a) SEM-EDS of Area 2 on a PCB piece (b) SEM-EDS of Area 3 on a PCB piece
It was also found that the gold present at these sites was present on the surface where the gold-nickel alloy was plated as well as on the green ink mask part of the PCB. This was indicative that during leaching gold was dissolved into solution from the surface of the PCB but thereafter it precipitated or adsorbed onto the PCB.
Table 4.4: Gold elemental composition from leached PCB and syringe filter in wt%
PCB (wt%) Syringe filter (wt%) Area 1 Area 2 Area 3 Area 1 Area 2
Spectrum 1 0.63 0.99 0.00 0.00 0.71
Spectrum 2 0.56 1.41 1.03 1.43 1.13
Spectrum 3 0.00 0.00 0.00 0.82 0.66
Spectrum 4 0.00 1.11 0.00 0.00 -
Spectrum 5 0.00 - - - -
Spectrum 6 0.00 - - - -
Average 0.2 0.88 0.26 0.56 0.83
SEM-EDS analysis from the syringe filter and filter paper are shown in Figure 4.11. Figure 4.11 (a) depicts the imaging from one out of the two areas investigated on the syringe filter.
Both areas of the filter paper revealed the presence of gold with 0.56% and 0.83% for Area 1 and 2 respectively. Area 2 had a greater amount of gold than Area 1. Figure 4.11 (b) depicts the imaging obtained from investigating the filter paper used in vacuum filtration. Multiple points were investigated, however there was no presence of gold on any of the sites. This suggests that the filtration allowed the remaining gold particles to pass through and remain in the leached solution whilst larger particles from the PCBs were separated out.
(a) (b)
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Figure 4.11: (a) SEM-EDS imaging from syringe filter of Area 1 (b) SEM-EDS imaging from filter paper used in vacuum filtration of Area 1
(a) (b)
The imaging results from the syringe filter experiment reveal that small gold particles were present on the syringe filter when filtering the solution sample and that the particles were greater in size than the 0.2 μm syringe filter hence the presence of gold particles larger than 0.2 μm on the filter paper. It was clear that some particles had not yet completely dissolved into solution and thus the experimental conditions can be altered to ensure complete leaching of gold into solution.