First of all we need to warn you that process described on this blog involves highly corrosive and toxic chemicals and should only be done outside or in a fume hood. To recover gold from Electronic scrap you need to use safety gear such as Gloves, goggles and a respirator.
First thing you need to do is to cut the fingers off the collected cards. Best way to do that is to use secateurs. Another good mechanical way would be to use a dedicated PCB Guillotine. No matter which way you prefer, the goal is to cut the fingers as close as possible to the edge of the Gold Plating. That way you have already removed parts without gold on them.
Mostly gold plating thickness on fingers is approximately 30 microinches (0.762 Micron), though it is not that uncommon to find thinner plating on cheaper Chinese made electronics. On the other hand, military and aerospace applications may require thicker plating. The usual purity of Gold used in contacts is 99.7%. Typical yield would 1.2-2.5 grams of pure gold per lb, depending on the scrap quality.
Process we are going to describe is useful for whole boards.
Stripping the Gold - way to do it…
The complete mechanism of the copper chloride etching process is actually quite complicated and quite boring as well. However there are three key ingredients which drive the stripping process of gold plating and are important to remember and sustain: A. Free HCl B. CuCl2 presence C. Dissolved Oxygen.
As you can see in the video below, the fingers are placed in a plastic strainer and fitted in a matching plastic pot. For larger quantities (say 2-10 lb), two 5 gal plastic buckets could be used, placing one inside the other, where the inner bucket is drilled with many holes at the bottom. No matter what you use, the point is to be able to lift the fingers and leave the solution behind.
A simple plastic tube connected to a small aquarium pump will work just fine. The other end is placed under the strainer with the fingers so that the bubbles will rise up through the fingers.
The fingers are then covered with Hydrochloric acid (HCl) 30-32%. The acid can be diluted a bit with water if needed, but do not exceed 2:1 V/V (acid/water) ratio. Small amount of Copper Chloride solution is then added to the solution.
The bubbling action from the pump will keep the solution aerated and agitated.
The Copper ions (2+) and dissolved Oxygen are basically the oxidizing reagents which produce a soluble Copper and Nickel salts, etching it from underneath the gold plating. The HCl acts as a “receptor” which holds the oxidized Copper and Nickel in solution.
The color of the solution will get dark green and eventually darken toward black as more and more Copper and Nickel dissolves into the solution.
The stripping process can last anywhere from 24 hours to a week. This depends on temperature, amount of free acid, aeration rate and level of agitation.
Treating the Gold foils - How to do that?
When determined that the stripping process have been completed, the fingers are removed and washed in a bucket of water to collect all of the clinging gold foils.
We suggest you to wash the foils in clear fresh water. This way you can see what’s going on. Don’t wash off the foils in the original etch solution.
When all of the foils are removed, set the fingers aside to dry out completely - and we mean it - Completely.
Collect those foils and join it to the next batch
In the meanwhile, all of the foils are collected in a filter. It is important to filter the solution and the wash water since sometimes the foils are so this it will break apart into gazillion of tiny part which doesn’t settle easily.
To make the filtration process as quick as possible, let the both the Copper Chloride etching solution and the wash water to stand overnight. Then pour to the filter only the supernatant and let it drain completely, same for the wash water.
The foils and powder and the rest of the junk is filtered since tiny particles has a habit of clogging up filters.
Keep the wash water and the main etching solution separate as the etching solution is reusable. Diluting the etching solution with water will most likely precipitate solid Copper (I) Chloride.
Once all of the foils are collected in the filter, you can choose to either treat the filter and the foils as it is with Aqua Regia or incinerate it first. The latter is preferable though, if not done right, you can blow away some of your hard earned gold foils.
For one or two filters, it is easy enough to just digest everything with hot Aqua Regia.
To incinerate the filter papers, let it dry or at least reach a damp state. Put it in a stainless steel pan and heat the pan from below with a torch. As the paper heats up it will slowly burn away. Slowly, collect the ashes and smudge it into one pile, keep heating the pan to red hot and keep it like that until you see no more smoldering remains. Do not blow the torch directly at the ashes because it could be blown off with gold content inside.
Final step is - Dissolving the Gold
Dissolving gold process is fairly simple if you have done some recovery and refining before. The foils with the ashes or the unburned filters are placed in an open mouth beaker and covered with a dilute HCl solution (50/50 32%HCl/water) and heated to medium heat (120-160F). When the solution is quite hot, a small amount of nitric acid is added to the solution. No more than 3ml per each lb of fingers we started with.
The solution will gain distinctive yellow color as the gold and traces of copper goes into the solution. The dissolution process should be done under a fume-hood and the beaker must be covered with a watch glass. Also be aware, the Gold solution must not be boiled. Dissolution process will be over in less than 30 minutes, there’s really no need for more than that. When done, the solution is allowed to cool back down to room temperature and then filtered into a clean beaker for precipitation.
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