My team and I started with 6 samples of CPU chips, which we stripped of base metals with 2 dilute nitric acid digests. 2 concentrated nitric acid digests were subsequently conducted to dissolve high amounts of solid copper remaining. Each sample was filtered and rinsed with DI water after each digest and solid components were retained for the next wash. Each sample eventually contained only non-metallic circuit board material and liberated gold plating. We predicted that there would only be gold left for the Aqua Regia to dissolve, and then the organic solids would be filtered out. However, subsequent green coloration of Aqua Regia digests lead us to believe there is still copper present.
We digested Sample 1 in Aqua Regia, and noted the gold had completely dissolved. However, we had a dark green solution where we were expecting a yellow one. We discovered that the non-metallic CPU boards still had small but chemically significant layers of undissolved solid copper film on them. We believe that Sample 1 now contains dissolved copper and gold in the aqua regia solution. For subsequent samples, we doubled the nitric acid washes, and removed the solid CPU fiberglass boards before Aqua Regia digest to avoid this problem. The gold plating was easily cleaned from the boards before removal by a micro-powerwash of DI water.
Samples 2-6 were digested with aqua regia and approached yellow to yellow-green color, indicating less copper presence.
The non-metallic solids were filtered out to give yellow to yellow-green, clear solutions.
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| Sample 2, 5, 6: Washed CPUs + Aqua Regia solution approaching desired yellow color of dissolved gold |
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| Sample 1: Filtering Aqua Regia solution into beaker, washed with concentrated HCl, green appearance noted |
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Samples 2-5: Filtered aqua regia samples showing yellow-green coloration. Should contain dissolved gold
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| Sample 1 after precipitated with copperas and centrifuged. Green supernatant with white and brown sediment noted. |
Sample 1 was evaporated and denitrified by dilution with concentrated HCl. Ferrous sulphate (copperas) was added which generated a brown cloudy mixture. Then the cloudy mixture was centrifuged and 2 layers of brown and white sediment were observed. The green supernatant was decanted and the sediment was diluted with conc. HCl, and then centrifuged. This dissolved and removed some of the milky white sediment. The brown sediment was believed to be gold powder but was lost after the vaccum filter broke.
Sample 1 after precipitated with copperas. 4 cycles of washing with HCl, centrifuging, and decanting supernatant. Green supernatant removed completely. |
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Sample 2 upon the addition of ferrous suplhate heptahydrate. Note brown clouds and nitrogen dioxide gas evolution indicating nitric acid presence.
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| Samples reacting with an excess of Ferrous Sulphate. Brown cloudy mixture forming with nitrogen dioxide gas emanating. |
Our Samples 2-6 were evaporated and diluted with concentrated HCl to remove some nitric acid. We ended up having to add a significant excess of ferrous sulphate to react completely with the samples. The crystal test (Ammen 1997) was performed until no bubbles or clouds were produced.
While initially encouraged that the brown cloudiness was precipitating gold, we are left with a brown solution that does not precipitate even under centrifugation. Solid crystals of excess ferrous sulphate are present at the bottom of the beakers. We have left the solutions to settle for several days but are unsure how to precipitate the gold out.
