CPU stands for computer processor unit. The CPU is the heart and brain of any computer, so they use one of the best corrosion-resistant conductors available -- gold.
When you're scrapping CPU's for their gold, you divide them into three basic types. Ceramic, fiber, and intermediate.
Ceramic CPU's are the most valuable when salvaging gold. They were made back when gold was less expensive, and the process to make them was less advanced -- so the thicknesses are greater. The entire body and core is made of ceramic Gold is found on the pins, the traces leading to the core and, occasionally the heat sink surface.
Intermediate CPU's blend a ceramic core with a fiber body. Since they used better technology and better machinery in production, the gold-plating on the pins and wires leading to the core are thinner.
Fiber CPU's are the hardest to extract meaningful amounts of gold from, since there's less gold to start with.
Obviously, the number of ceramic CPU's available for electronic gold recovery is getting smaller all the time. The trick is to find a good source for them.
BONUS GOLD SCRAPPING TIP
There are a number of people who collect CPU's
the way other people collect stamps and coins.
Some of the more unique or rare ones are
worth more as collectibles than as a source
for scrap gold. It can pay to check CPU
serial numbers online to see if they
have value for collectors.
This article will focus on recovering gold from ceramic CPU's since they provide the bigger payoff. The highest yielding ones may give you up to half a gram of gold. At the time I'm writing this, that's almost $20 worth from one chip. In the future, I'll try and present some detailed information on the Intermediate CPU's.
The following method for extracting gold from ceramic CPU's is not what you'd typically use for other types of electronic salvage, but it is an effective recovery process.
NOTE: THE FOLLOWING PROCEDURES CAN BE EXTREMELY HAZARDOUS EVEN TO EXPERIENCED SALVAGERS. I AM BY NO MEANS AN EXPERT OR QUALIFIED TO ASSESS THE SAFETY OF THESE PROCEDURES. IF YOU ATTEMPT THE FOLLOWING, YOU DO SO AT YOUR OWN RISK.
Aqua Regia Gold Recovery
Aqua Regia is a mix of hydrochloric acid and nitric acid that can dissolve gold, platinum, and other base metals. We will be using that trait to recover the gold.
Smaller pieces take up less volume in your gold recovery containers, which means you can use smaller amounts of chemical. Smaller amounts keep things safer and reduce your input costs, so your gold profits are higher.
Start by cleaning your CPU's as best you can. The fewer impurities you have, the less chemical you'll need. Dust is easily blown or wiped off. Thermal grease or paste is a little tougher, but not too bad. Remove any thick layers or blobs with paper towel. Then use paper towel with at least 70% isopropyl alcohol (90% is better) to remove any residue. Thermal paste is considered slightly toxic, but not enough to warrant panic. Just dispose of the paper towel and wash your hands once done.
Next break the ceramic CPU's down to smaller pieces. A hammer on a steel or concrete surface works well. You can also set up some cardboard walls or similar to keep pieces (containing gold) from flying away. One-eighth to one-quarter inch pieces are sufficient -- you don't have to pound them to dust. Remember to wear ear and eye protection.
If the heat sink surface is gold-plated and it breaks apart during the hammering process, it's most likely high tungsten steel with a gold coating -- and it's fine to process it with everything else. If the surface just bends, it's probably gold-coated copper and should be dealt with in a separate process.
Some CPU's have a metallic "lid", cap, or bottom. Remove these and discard them if they don't have any gold on them. If they do contain gold, set them aside with the copper-based heat sinks for later processing.
If you're down to eighth-inch pieces, you shouldn't have to worry about this, but make sure the silicon chip in the centre of the CPU is at least broken in half.
Dissolving The Metals In Acid
Once you've broken down the CPU's, place them in a glass container and pour in enough hydrochloric acid (HCI 32 -36%) to just cover your gold-bearing material. The type of hydrochloric acid you need can be bought here.
NOTE: THE LIQUID LEVEL IN YOUR CONTAINER SHOULD NEVER EXCEED 2/3's OF THE HEIGHT, AND LESS THAN HALF THE HEIGHT IS RECOMMENDED. CHOOSE YOUR CONTAINER ACCORDINGLY!
THE CONTAINER WILL NEED TO BE HEATED ON A HOT PLATE. MAKE SURE IT IS ABLE TO DO SO!
It's difficult to know exactly how much acid will be needed to completely dissolve the gold. You don't want to leave any behind, but you don't want to cut into your profits by using too much hydrochloric acid either.
After gathering a number of examples from other gold salvagers, 25 milliliters of HCI for each ceramic CPU is a good starting point.
After the CPU's are covered with acid, add water -- about 20% to 25% of the amount of hydrochloric acid used. Or, in other words, for every 100 milliliters of HCI used, add 20-25 milliliters of water.
Then, in a WELL ventilated area -- preferably outdoors or under a fume hood -- the container is placed on a source of heat (hot plate) and the solution temperature needs to be raised to 80 to 90-degrees Celsius, or 175 to 195-degrees Fahrenheit.
Adding Nitric Acid
In the Aqua Regia process, nitrate ions play the role of an oxidizer and the chloride provides receptors of sorts for the oxidized metals. It's an overly-simple explanation, but I'm assuming you're more interested in gold than chemistry. What the above tells us though, is that it's okay to uses a little too much hydrochloric acid, but not too much nitric acid or nitric salt.
While this description refers to nitric acid (HNO3), the required nitrates can also come from sodium nitrate (NaNO3) which is the preferred salt, potassium nitrate (KNO3), or ammonium nitrate (NH4NO3). You can save more by buying large quantities, but this is a good deal on nitric acid, and enough to get you rolling.
To prevent the addition of too much nitric acid, the procedure will "sneak up" on the correct amount.
Back to the gold extracting! For every kilogram of CPU's (1 kilogram = 1000 grams, so scaling up or down to match your batch should be easy) start by adding about 3 milliliters of nitric acid.
The resulting reaction will create TOXIC NO2 gas, in the form of bubbles. Allow the reaction to die down substantially or completely (about 15 minutes) before adding more nitrate. You'll know the reaction has stopped or slowed down when there are very few bubbles and the liquid has turned dark.
If the reaction was very strong, add another 3 milliliters of nitric acid. If the reaction was weak, use 1-2 milliliters. The solution will start bubbling again and the color will lighten up.
Continue adding the nitric acid in ever smaller amounts until you don't see any metal on the CPU pieces or floating bits of gold foil. Remove from the heat source and allow everything to cool to room temperature.
Removing Silver And Lead From The Gold
Now you need a container about the same size as your reaction container. Fill it about halfway with ice cubes. Pour your gold bearing solution over the ice and stir it with a glass stirring rod (yes, it has to be glass).
Now, BE CAREFUL, and slowly add a drop of concentrated sulfuric acid (can be bought here) into the gold solution. In total, you'll need to add 10 to 15 milliliters, but sulfuric acid has a very energetic reaction with water... so stick to a drop at a time, take precautions, and wear acid protective gear.
The cold temperatures brought on by the ice cubes will precipitate silver chloride, and the sulfuric acid with precipitate lead sulfate. It will probably be impossible for you to distinguish the two since both are white-colored salts.
After you've added all the acid, let everything settle for at least 30 minutes, but probably no longer than 60.
You'll want the solution to remain cold, so if the ice has all melted, set your container in a bowl or pail with more ice.
After the solids have settled, start to filter your still-cold solution into a another clean container. A container free of scratches is the best, since this is the one you'll be collecting gold in, and you don't want any gold left behind, caught in the scratches.
Start by pouring as much of the sediment-free solution as possible through the solution. When most of your gold-bearing solution is through, pour in the remainder, sediment and all.
Wait until the filter stops dripping, and then pour clean water through it until you notice clear water exiting the filter.
Getting the Gold!
Now you have a container full of liquid gold! Unfortunately there's a lot of liquid other stuff in there as well -- most likely iron, copper, nickel, tin, and cobalt. So the goal is to get the gold -- and only the gold -- to precipitate from the solution.
To do that, you will add sodium metabisulfite (Na2S205), which we'll simply call SMB. A little will go a long way, so buying this amount of sodium metabisulfite is plenty. To precipitate one gram of gold, you'll need about one gram of SMB, BUT... you don't know how much gold is in your solution.
The common rule of thumb is that a 1000 gram batch of CPU chips will get you 2 maybe 3 grams of gold. So you would add 4 to 5 grams of SMB. Stir it in and wait 24 hours. (This part of the process takes longer for the precipitate to settle)
If adding the SMB leads to vigorous bubbling, it means too much nitric acid (nitrate salt) is in the gold solution. If you run into this unlikely event -- unlikely if you followed the instructions -- then use urea to neutralize the solution.
The precipitated gold won't be bright and shiny. It'll be more of a dark brown or even black powder. Before you go any further, use stannous chloride solution to make sure all the gold has precipitated. You can buy stannous chloride here. Put a drop of your solution on a clean surface and add a drop of stannous chloride. If you get any darkening, there's still gold left in the solution. A positive reaction means adding some more SMB and waiting another 24 hours.
Once all the gold has precipitated, slowly pour off the liquid into a proper acid solution waste tank. using a filter will ensure no errant gold specks are poured out with your acid.
Now simply wash and rinse the powder a number of times with plain water. Congratulations -- you have high-karat gold!