Gold recovery process from Ceramic CPU's - Part 1

First, let me start by saying that Ceramic CPU's and chips are barely used or manufactured anymore. Therefore, the time will come when they will extinct from your regular electronic scrap stream.

Scrap Ceramic CPU may have two values: one value for scrap, which represents the gold (and Silver) content and the other value, a collector's value.

Yes, there are folks out there who collect CPUs and are willing to pay you for them, usually far more than the Gold and Silver content worth. Don't deprive them of that pleasure. I will not go into this subject further than this, just remember, whenever quirky and funky-looking ceramic CPU or Chip arrive, you better keep them intact and check it online. Use the serial numbers printed on it. Ceramic CPU's are well worth their own tutorial as they are the little jewels of the electronic scrap world. The best-yielding CPUs may hold up to 0.4-0.5 grams of gold per unit! Some other small ceramic chips may contain several grams of gold per lb. Here, offered a method of a direct dissolution of all metals in Aqua Regia, this kind of Gold Recovery Process is not recommended for most other types of electronic scrap. Ceramic CPUs are unique in terms of the base metals composition and the base metals to Gold (and Silver) ratio, which can provide an efficient and cheap process, despite it being a bit messy.

SAFETY NOTIFICATION: The process described in this article involves using toxic chemicals and gasses. All precautionary measures should be taken beforehand while dealing with chemicals, including Eyes protection, chemical resistant gloves and means to vent any gasses (e.g. fume hood).

Abstract of the Gold and Silver recovery process: -    Size reduction and lids removal. -    Dissolving the metals with Aqua Regia. -    Silver/Lead removal from solution. -    Precipitation of Gold Powder.

Size reduction and lids removal. First, remove thermal grease, dirt and any other debris from the CPU's as much as possible. To fit many CPU's into your reaction beaker, you would have to break the CPU's down to around 1/4-1/2" size pieces. This can be done with a simple hammer and anvil or any other creative way you can come up with. While breaking the CPU's, there are a few points you should remember to ensure the success of the recovery process:

- Metallic bottom lids should be removed and processed individually. - Make sure the silicon chip in the center is broken in half or more. - If there's a Gold plated heat spreader, smack it with a hammer; if it breaks apart, then it is most likely composed of a high Tungsten alloy. If that's the case, it is safe to include the heat spreader with the main batch and the rest of the CPU's. Once the gold plating dissolves away from the heat spreader surface, the high Tungsten content will react with the Aqua Regia to form a passive coating of yellowish Tungsten (IV) Oxide (WO2). If the top heat spreader doesn't break and only gets bent slightly, you should not include it in the main batch as it is mostly Copper.

Scrap ceramic cup's top and bottom

High Tungsten heat spreader (left). Kovar based metallic bottom lid (right)

Dissolving the metals with Aqua Regia. Once the CPU's are cleaned and broken, place them in your vessel of choice and cover them with Hydrochloric Acid (HCl 32 -36%). A Griffin Beaker is usually the best choice for that. It is somewhat impossible to know exactly how much HCl is needed in order to achieve complete digestion, and on the other hand, avoid excess usage of acid. It is the writer's experience that, 20-30 ml of HCl for each CPU is sufficient for most Ceramic CPU's types. If the batch is composed of small ceramic chips or CPU's, you should probably use more HCl. So, once the CPU's are covered with HCl, carefully dilute it with some water, add about 1/5-1/4 of the acid volume (example: for 100ml of acid, add 20-25ml water). This is done to help accommodate all the metals ions in the solution but without affecting the speed of the process (too much). The liquid level should never exceed 2/3 the height of your reaction vessel! Preferably, be it only 1/2 its height. The vessel is now placed on a heat source such as a hot plate or a mantle and the solution temperature is raised to 176-194 F (80-90 C). Safety Note: Remember this process should be done outdoors or in a fume hood.

The addition of Nitric Acid / Nitrate salt

This is probably the most important step of the whole Aqua Regia process and this step is usually the source of many predicaments later in this process, especially for newbie's. As you know by now, Aqua Regia is a combination of Hydrochloric acid and Nitric acid and it has the capability to dissolve Gold, Platinum metals and most of other base metals. The source of nitrate though, does not have to come from Nitric acid (HNO3), it can also be provided by some of the corresponding salts such as: Potassium nitrate (KNO3), Sodium nitrate (NaNO3) and Ammonium nitrate (NH4NO3). The preferred salt is usually Sodium Nitrate. The chemistry behind AR is a bit more involved then you might expect, so we will just go over what is necessary for us to understand the process, in simple terms. In Aqua Regia, the Nitrate ions serves as an oxidizer where the Chloride ions act as sort of "receptors" and accommodate the oxidized metals. Again, this is an over simplified explanation, but is enough for us to conclude that, excess of HCl in AR does absolutely no harm where an excess of nitric acid/nitrate salt will hinder any reduction attempts (i.e. Precipitation of Gold metal). Therefore, it is suggested that once the HCl + CPU's are at temperature, the additions of nitrate salt or nitric acid will be done incrementally and in a decreasing manner. For smaller batches of 1-2 lb of CPU's, the first addition of nitric acid should be around 1-3 ml or about 1 teaspoon of Sodium Nitrate. Observe the reaction, it will bubble and fizz. Another indication is an evolution of brown NO2 gas (TOXIC) will become apparent. Allow it to react for at least 10-15 minutes before the next addition. You will know it is time for the next addition once the bubbling action had subsided substantially and the solution turns very dark. Once you add the next increment of nitrate, the bubbling will start over and the color of the solution will become somewhat lighter. Continue with the nitrates additions until you can not see any more metal on the CPU's or floating gold foils. Remember to reduce the size of each consecutive increment. This procedure will ensure, that at the end of this process, only a minimal excess of free nitrate ions will be left in the solution and the reduction of Gold would proceed smoothly. Once the reaction is over, turn off the heat and allow the solution to go back to room temperature.