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Aqua Regia: Precipitating Pure Gold

Step 1: Neutralizing free nitrogen ions

Free nitrogen ions can cause gold to re-dissolve after it is precipitated out of solution (causing gold losses). Fortunately, there is a simple, effective way to neutralize these ions. Just add urea (a harmless chemical: CO(NH2)2) to the solution until it no longer fizzes when added.

When working with either sodium nitrate or MX3 Dissolving Agent, a pinch of urea is typically all that is needed. When working with nitric acid, the amount of urea will vary with each batch and typically, the amount required is pounds.

Step 2: Precipitating pure gold (99.95+% pure)

There are several different selective precipitants available. If used correctly, all will precipitate gold that is at least 99.95% purity.

Choosing your precipitant

Each has its strengths and weakness. Below is a comparison chart of popular precipitants.

It's important to choose a precipitant that clumps gold together and looks like gold so that none of your gold particles get lost while filtering.

Compaison of Precipitating Agents

Precipitant Sodium Metabisulfite Ferrous Sulfate Quadratic Precipitant
Corrosive? Yes No No
Toxic? Yes No No
Odor Very strong Odorless Odorless
Shelf Life Unlimited 2 to 4 months Unlimited
Requires Heating? No No Heat to 160° F
Gold Particle Size Small Moderate Large
Gold Particle Color Light to dark brown Light to dark brown Golden brown
Costliness Reasonable Expensive Reasonable

Once you've chosen your precipitant, you can begin precipitation. For all of these methods, we recommend you let the precipitant sit for about one hour.

If you're precipitating using sodium metabisulfite:

Add 1 ounce (about 1-1/2 tablespoons) for every ounce of dissolved metal in solution. This should be done out-of-doors because the vapors from the bisulfite are corrosive. No matter how good your ventilation system is, it is unlikely to be good enough when using this compound.

If you're precipitating using ferrous sulfate:

Add 1 ounce (about 2 tablespoons) for every ounce of dissolved metal in solution. Ferrous sulfate is a delicate compound. It deteriorates into ferric sulfate, a useless compound.

If you're precipitating using Quadratic Precipitant:

Add 1 ounce (about 1-1/2 tablespoons) of Quadratic for every ounce of metal in solution. Heat the solution to 160° F.

Step 3: Testing for dissolved gold

Before you pour off the waste water, you’ll want to be absolutely sure that 100% of your dissolved gold has precipitated out of solution. For this test, you need Precious Metal Detection Liquid (PMDL). You can buy this ready-made or you put it together yourself. PMDL is concentrated HCl infused and saturated with stannous chloride and pure tin. The test is simple and very sensitive. It will detect dissolved gold up to 4 parts of gold for every 1 million parts of solution.

Need detection liquids ready-made?

To test:

  1. Take a drop of the solution you are testing with a pipette and place it on a paper towel.

  2. Add a drop of PMDL test solution onto the drop of solution. If the color turns blue, brown or black, there is still gold dissolved in solution. When it doesn’t change color or turns yellowish (from the acid), then all the gold has precipitated.

  3. If there is gold in the solution, go back and refine a second time. Otherwise, you're ready to decant your solution.

Step 4: Decanting your precipitate

It's now time to gather your gold. Decant the precipitate out of the container, making sure to leave your gold particles inside your container.

If you're using sodium metabisulfite:

When using sodium metabisulfite, it's best to save the waste solution for a period of several days or weeks. Gold will settle out of this solution during this time.

Be careful to not lose gold!

While it is the cheapest precipitant, it's also the most expensive to use. Unlike some other precipitants which forms large clumps of gold, the gold particles rendered from the use of sodium metabisulfite is often small. Unfortunately this often results in up to 3% of gold being lost, as gold particles that precipitate are so small that some of them pass, unnoticed, through filter paper (if filtered) or are decanted-off unseen.

However, if you are very careful while decanting the solution at that time, you may be able to recover most of this lost gold.

If you're using Ferrous Sulfate or Quadratic:

If using either ferrous sulfate or Quadratic, you can either decant the solution or filter it, whichever you feel most comfortable with.

Quadratic is a little easier to work with than ferrous sulfate because the particles are heavier and larger, and so they settle to the bottom more quickly and tend to stay there.

Part 3: Rinsing and Ensuring Pure Gold

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