If you recall, when scrapping computers and other electronic equipment, we encounter a lot of peripheral cards which has Gold plated male slot contacts, i.e. the “fingers”. See picture below:
(WARNING: The process described here involves highly corrosive and toxic chemicals and should only be done outside or in a fume hood. Using all the safety gear such as Gloves, goggles and a respirator is mandatory.)
In this tutorial we will learn how to recover the Gold Plating from those fingers using an aerated Copper (II) Chloride (CuCl2) solution. The first step is of course to cut the fingers off the cards… There are many ways to do it, different peoples use different methods. I have found that using a simple Secateurs works best. Another good mechanical way would be to use a dedicated PCB Guillotine. Either way, the goal is to cut the fingers as close as possible to the edge of the Gold Plating. The usual gold plating thickness on fingers is apprx. 30 micro-inches (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% with main dopant being Cobalt which used as a hardener. Typical yield would 1.2-2.5 grams of pure gold per lb, depending on the scrap quality. This process is also useful for whole boards, assuming components and solder was removed from it or not applied in the first place (rejects from assembly lines).
Stripping the Gold
The complete mechanism of the copper chloride etching process is actually quite complicated and to be honest, can be quite boring. But there are three key ingredients which drives the stripping process of gold plating and are important to remember and sustain: A. Free HCl B. CuCl2 presence C. Dissolved Oxygen. If the above paragraph isn’t quite clear to you at the moment, don’t worry and keep reading, as the process is described and further explanation will be given. As seen 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. Whatever 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 air pump (a small aquarium pump will work just fine) and 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 both aerated (oxygenated) and agitated.. The Copper ions (2+) and 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 last anywhere from 24 hours to a week. Depending on temperature, amount of free acid, aeration rate and level of agitation.
Treating the Gold foils
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. I suggest washing the foils in clear fresh water so you could 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 i’m talking bone dry). You will be surprised to learn just how much more foils will fall of the fingers upon violent shaking. 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 paper(s), 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, it’s a sure way to blow off some of the gold. Needles to say, you should avoid breathing any of the resulting fumes and smoke.
Dissolving the Gold
From this point on, the process if fairly simple if you have done some recovery and/or 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 beware, the Gold solution must not be boiled. The 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. From this point on you can follow the steps shown in Gold-N-Scrap Ceramic Cpu’s recovery tutorial.