How to Refine Gold by the Acid Method
The two acids which are used in this process are concentrated hydrochloric and nitric acids. These acids are very corrosive. However, when combined with metals, the fumes become much more corrosive. So corrosive, in fact, that they will rust the highest grades of stainless steel with less than 1 second of exposure. For this reason, this kind of refining must be done outdoors, away from anything that may be damaged by exposure to these fumes.
See related: Large Scale Gold Refining by Aqua Regia Acid Method
Having described it's hazards, it must be pointed out that, like most industrial processes, refining gold in acid is quite safe when performed under controlled conditions. In addition, the results of doing your own refining can be quite sweet. Based upon feedback we've gotten from several hundred shops, we estimate that, when you refine just 10 ounces of scrap gold yourself, you will save about $800.
- To refine gold in acid, you will need the following items:
- heavy gage plastic buckets or Visionware pots
- wood or plastic stir rod
- protective clothing--minimum requirement: goggles and rubber gloves, rubber apron.
- filtration equipment (optional but high recommended)
- hydrochloric or muriatic acid
- nitric acid or nitric acid substitute (subzero)
- baking soda (sodium bicarbonate)
- gold detection liquid
- selective gold precipitant
- aqua ammonia
- tap water and distilled water
Melt your gold and make shot. Small open-up granules are best. If you are refining filigree or ladies rings, shotting is not necessary. Pour the shot into one or more Pyrex or plastic containers. For every ounce of scrap gold you are going to refine you will need a capacity of 300 milliliter. So, for example, if you are refining 10 ounces, you need a 3,000 milliliter (3 liter or 3 quart) container.
To the container, add 30 milliliter of nitric acid (or 2 tablespoons subzero, nitric acid substitute) for every ounce of metal in the container. So, if you have 10 ounces of metal in the container, add 300 ml (milliliter) of nitric acid. Let it sit about 30 minutes or longer. You may or may not get a strong reaction with brown fumes. There is no way of predicting the strength or speed of acid reactions in refining. If using subzero, nitric acid substitute, add the hydrochloric or muriatic acid without waiting. The subzero will not have any effect until the hydrochloric or muriatic is added.
To the container, add 120 ml of hydrochloric or muriatic acid for every ounce of metal in the container. So, if you have 10 ounces of metal in the container, add 1200 ml of hydrochloric. Again, you may or may not get a strong reaction. The solution will get hot as the acid reacts with the metal, dissolving it. If using subzero instead of nitric, you will probably get no fumes except those of the hydrochloric or muriatic acid.
Usually, but not always, the acid reacts slowly at first. After some minutes have passed, however, the acid will become very hot and brown, very corrosive fume (nitric oxides and other fumes) will be generated. Wait a minimum of one hour after the fumes have disappeared before pouring off or filtering the acid. If you can, wait overnight. This will insure that the acid has had sufficient time to completely dissolve the gold.
Pour the acid into another, larger container. Do not allow any particles to be poured off. If you have a filter funnel, preferably a Buchner filter funnel, use it. If any particles are poured off with the acid, they will contaminate your gold.
The acid will be an emerald green color and should be clear (not murky or cloudy). If the acid is murky, it may contain particles and should be refiltered.
Take one quart of tap water and heat to boiling. Remove from heat and add one pound of urea to the water. Despite the unpleasant name, this is just a harmless industrial chemical that has no smell and you will use to adjust the pH of the acid.
Slowly, add the water/urea to the acid. The acid will foam with the addition of the water/urea. Do not add the water/urea so quickly that the acid foams out of its container. When the acid stops reacting to the addition of water/urea, stop adding the water/urea. You've just raised the pH of the acid from 0.1 To 1.0, killing the nitric acid but not the hydrochloric.
There are many different selective precipitants that can be used when refining gold. All will do the job well. However, your local supply house has one or two that they may favor. At Shor, we prefer Storm precipitant. When using a selective precipitant, follow the instructions that accompany the product.
Take a quart of water and heat it to boiling in a Pyrex or Visionware container. Remove it from the heat and add to the water one ounce of storm precipitant for every ounce of metal you are refining. If you are refining many ounces of scrap gold, you may need to use more than one quart of water. Do not put your face near the opening of the container. The smell is very strong and pungent.
Add the water/precipitant solution slowly to the acid. Immediately the acid will change to a muddy brown appearance as brown particles of gold form in the water. This brown "mud" is, despite its appearance, pure gold.
Give the precipitant about 30 minutes to do its job. Then test the acid for the presence of dissolved gold.
Testing is generally done with an aqueous mixture of stannous chloride, hydrochloric acid and pure tin. Generally it's more convenient to buy this premixed rather than concoct it yourself. The premixed is commercially available as "precious metal detection liquid". Precious metal detection liquid will detect the presence of dissolved gold down to 4 parts of gold per million parts of acid )detecting the presence of about 1/1000th of 1 gram of dissolved gold. Testing for the presence of dissolved gold is absolutely necessary to insure that no dissolved gold is thrown away with the waste acid.
To test, first immerse the end of the stirring rod in the acid. Remove it and touch that end to a paper towel to make a wet spot. Put a drop of gold detection liquid on the wet spot on the paper towel. If any gold is still dissolved in the acid, the wet spot will turn a purple-black or a purple-brown. If you see this color change then give the precipitant more time to work and/or add more precipitant.
The acid should now be a clear amber color with a brown mud at the bottom. Pour off the acid into another container. If you have a filter, use it. Do not pour off any of the mud. The mud is pure gold.
When all the acid is poured off, add tap water to the mud. Stir and let the mud settle. Pour off the water into the container with the acid. If you have a filter, use it. Do not pour off any particles of brown. Repeat this rinsing 3-4 times or more.
Rinse once with aqua ammonia. White vapors will appear. The aqua ammonia cleans impurities from the gold mud while, at the same time, it neutralizes any acid still clinging to the gold mud.
Give the mud a last rinse, this time with distilled water.
Using distilled water, wash the mud into a small Pyrex beaker Visionware pot. Give the mud a chance to settle and then pour off the water.
Put the beaker on a hot plate to dry the mud. Do not preheat the hot plate or thermal shock may cause the beaker to break.
Melt the dried mud (now a powder). If using a torch, first wrap the powder in tissue paper and then soak that in alcohol. Also, use a Burno crucible, this will keep your gold from being blown away by the gas pressure from the torch. The gold will again take on the appearance of metal. If you've followed the instructions carefully and used filters, the gold will be 99.95 % Pure with virtually no losses.
If you had platinum in your gold, it will not dissolve, to any appreciable degree, in the room temperature aqua regia. It will be left behind when you pour off the aqua regia, prior to precipitation. To insure high purity of the platinum, you will need to re-refine this material. Put this material in a fresh aqua regia bath. This time, however, heat the acid to simmering. Continue heating until all the platinum is dissolved (that may take 1-2 hours). When completely dissolved at 1 ounce of ammonium chloride for every ounce of dissolved platinum. The platinum will precipitate as a red mud. If you want to leave the iridium in the platinum, then wait for it to precipitate before recovering the platinum. Iridium will precipitate as a blue-black mud after the platinum precipitates. Platinum group metals will also show up on the stannous chloride test. Platinum turns red, palladium. Palladium turns orange and iridium turn blue-black.