As valuable as pure gold is, gold is rarely pure. Gold ore found in the ground is bound to other (often undesirable) minerals, and even refined gold used in jewelry often has contaminants in it. While nitric acid alone can dissolve some unwanted minerals (such as pyrite) from gold ore, it is unable to penetrate large gold bodies or refine relatively pure gold. Aqua regia -- a mixture of nitric acid and hydrochloric acid -- can dissolve gold itself, allowing pollutants to be filtered off so that very pure gold can be extracted as a precipitate.
Dissolving and Filtering Gold
Put on safety goggles, rubber gloves and a rubber splash apron. Choose a work station in a well-ventilated area -- either under a fume hood or an isolated location outdoors.
Determine the weight of the gold product in ounces -- you will need a Pyrex beaker with 300 milliliters of capacity for every ounce of gold. Put the gold in the beaker and carefully add 30 milliliters of nitric acid for every ounce of gold. Allow the acid to sit for at least 30 minutes -- avoid inhaling any fumes emitted by the acid.
Add to the beaker 120 milliliters of hydrochloric acid for every ounce of gold -- the mixture will get very hot as the gold begins to dissolve. Allow the beaker to sit undisturbed overnight to ensure that all of the gold is fully dissolved. Strain the acid through a Buchner filter funnel to remove particles of undesirable minerals -- repeat until the acid is a transparent green color. Pour the acid into a larger Pyrex container.
Boil one quart of water and remove it from its heat source. Mix one pound of urea with the water. Pour the mixture -- very slowly and carefully -- into the acid until it stops foaming.
Retrieving the Purified Gold
Boil one quart of water and remove it from its heat source. Add to the water one ounce of precious metal precipitant for every ounce of gold -- do not inhale any fumes produced by the solution.
Add the solution to the acid -- very slowly -- this will cause the acid to become a brownish color as the dissolved gold binds to the precipitant. Allow the acid solution to sit for at least 30 minutes.
Dip a stirring rod into the acid solution and dab the end of the rod onto a paper towel. Apply a drop of a precious metal detection liquid to the spot on the paper towel -- if the liquid becomes a dark purple color, then gold is still dissolved in the acid and the solution should be given more time to work. Test the solution again after another 30 minutes -- if gold is still detected, then add additional precious metal precipitant solution to the acid. Repeat until the detection test comes up negative.
Run the acid solution through a Buchner filter funnel to extract the brown particles of gold and put the remaining acid solution aside for later neutralization.
Put the brown gold particles in another Pyrex beaker. Cover the particles with tap water and stir them thoroughly. Strain the water through another filter -- put the brown gold particles in another beaker and set aside the water for later disposal. Repeat this process several times.
Pour a small amount of aqua ammonia on the gold particles to remove lingering impurities and neutralizing any remaining acid on the particles. Filter strain the gold particles from the aqua ammonia. Rinse the particles with distilled water and filter strain them one more time.
Add a small amount of distilled water to the gold particles and pour them into another Pyrex beaker -- make sure to get all of the gold particles out of the original beaker before putting it aside. Put the beaker on a hot plate and turn it on to boil away any remaining water and dry the gold particles.
Melt the gold particles and cast the now-pure gold however you wish.
Neutralizing the Acid
Pour a large amount of water (at least twice as much water as the combined acid volume) in a large plastic bucket. Add sodium bicarbonate (baking soda) to the water until the water is completely saturated and a pile of baking soda is seen at the bottom of the bucket. Stir the mixture thoroughly.
Pour the water that was used to distill the gold particles into the bucket to neutralize any lingering acid that may be present.
Pour the remaining acid into the bucket very slowly and carefully -- the mixture will heat up and fizz, possibly violently. Add additional baking soda if the mixture continues to fizz after pouring all of the acid.
Wait until the mixture is no longer fizzing and then pour it down a drain. Flush the drain with copious amounts of water afterward. Rinse all of the beakers and equipment that may have come in contact with any acid (nitric, hydrochloric, or the combined aqua regia) thoroughly and with caution.
TL;DR (Too Long; Didn't Read)
Be careful when melting the gold particles -- you don't want the force of a torch blowing them around. Melt them in a secure crucible or metal-melting pan.
Use extreme caution when handling nitric acid, hydrochloric acid and the combined aqua regia -- all three can produce hazardous fumes and all three can cause severe burns upon exposure to eyes or skin. Use safety equipment at all times when handling these acids. Do not dispose of any acid without neutralizing it first.
When mixing water and acid, always add the acid to the water -- never add water to acid. If a violent reaction occurs, it is better to be splashed by water with a small amount of acid in it than acid with a small amount of water.
Use caution when using hot plates -- always assume that a hot plate is hot. Turn off and unplug a hot plate after use.