There are several methods I’ve found to separate precious metals from other metals.
To dissolve Copper from gold plated printed circuits or plated Brass pins / connectors.
CAUTION: Remember the golden rule, acid to water, not water to acid to prevent splash injury.
If plated heavy I use a homemade nitric acid--- sodium nitrate (Na(NO3)2) and battery acid (H2SO4) 1:1 ratio, I use store bought H2SO4 as it contains no lead like that recovered from car batteries, this gives HNO3 + Na SO4 +h2O
Straight Hydrochloric Acid (HCL) also works at dissolving Copper but much slower.
Also I’ve read you can use Hydrochloric Acid mixed 1:1 with Hydrogen Peroxide. This is a chemical used by some hobbyists to etch printed circuit boards but the reaction when the 2 chemicals are mixed causes a lot of heat so plastic containers CANNOT be used, Glass or crock-pot is best.
Ferric Chloride (Hydrochloric Acid mixed with Rust) and Ammonia will also dissolve copper as these are also used for etching circuit boards.
If scrap gold is mixed with other metals separate as much as possible by cutting open parts to expose base metals
Small pieces work best
The next step should be performed outside on hot plate or in a crock-pot (not a metal or plastic container). Put acid then gold then sodium nitrate (can also use ammonium nitrate fertilizer or potassium nitrate sold as stump remover)
CAUTION: stay clear of fumes and if possible a fume hood is advisable. Not inside. Also very corrosive to metals.
This will make a nitric acid that will dissolve copper and other metals leaving gold in a salt metal blue / green solution of copper sulfate and gold flakes,
Add water till all salt in liquid, and let the gold flakes settle.
Take a syringe and remove remaining liquid, save for copper electrolysis or add lesser active metal like Zink or aluminium to remove copper and then baking soda or caustic soda to neutralize the acid.
Copper can also be precipitated out of solution using Sodium Hydroxide (caustic Soda).
I found out that the casings of old computer chips are made with an aluminium based compound which I assume is for heat exchange reasons. I put some in the solution containing copper and it precipitated the copper straight out and the copper settled to the bottom.
These types of chips require Aqua Regis to strip the gold.
If there was Silver in the metal, NaCL or table salt will make it precipitate out as white cottage cheese powder, you can take these oxides and add a little bit of borax soap as flux and melt back to metal with a gas torch.
For More Complex Gold Plated items like computer processors.
Use Home made Aqua Regis to dissolve gold by using the above nitric Acid and hydrochloric acid (muriatic used to clean concrete or in swimming pools), 2 parts Nitric Acid (HNO3 + Na SO4 +h2O) to 1 part Hydrochloric acid (HCL)
CAUTION: Aqua Regis (AR Agent) is extremely corrosive and will dissolve all metals with the exception of some rare earth metals like titanium.
Boil this to speed up removal of nitric acid as red cloud in the liquid.
CAUTION: stay clear of fumes and if possible a fume hood is advisable. Not inside.
Boil until almost but not dry adding a little Hydrochloric acid (HCL) to wet. Do this three times then add 10 times the amount of distilled water.
Silver if any should precipitate as white powder, as per previous a little table salt will make the silver precipitate.
Then make a solution of ferrous (Iron) sulphate from old cut up transformer iron or nails/steel wool, sulphuric acid and seven times water. Wait till this solution turns a greenish colour.
Ferrous Sulphate added to aqua Regis gold solution will make gold fall to bottom as a brown powder let settle at least 24hrs.
Remove liquid solution of copper or other metals and just like the previous example save for copper electrolysis or add lesser active metal like Zink or aluminium to remove copper and then baking soda or caustic soda to neutralize the acid.
Take the brown dried powder, add a little borax and torch to turn back to gold.
The chemicals like Aqua Regis if purchased will usually cost more than the gold is worth.
Aqua Regis will also dissolve Platinum, but I have yet to try this. I would be interested in finding if this can be used to extract the Platinum from Catalytic Converter materials…
I SAY AGAIN CAUTION: THIS IS DANGEROUS CHEMISTRY, AND ALL PRECAUTIONS MUST BE TAKEN IF ATTEMPTING THIS KIND OF ACTIVITY.