Why Nitric Acid Must Be Removed Before Precipitating Gold
When gold is dissolved using aqua regia, the solution contains:
• dissolved gold chloride
• hydrochloric acid
• nitric acid
• dissolved base metals
Nitric acid is necessary for dissolving gold, but it prevents gold from precipitating properly.
If nitric acid remains in solution, the gold precipitant will fail or produce very poor recovery.
This is why refiners must remove or neutralize nitric acid before attempting precipitation.
This step is commonly called denoxing.
Signs Your Solution Contains Excess Nitric Acid
Refiners often discover nitric acid is still present when precipitation fails.
Common signs include:
• Gold does not drop when precipitant is added
• Solution remains yellow or orange
• Brown powder forms but redissolves
• Precipitation takes extremely long
If any of these occur, excess nitric acid is usually the cause.
Methods to Remove Nitric Acid from Aqua Regia
Several methods are used to remove nitric acid from gold solutions.
The most common methods include:
• evaporation
• sulfamic acid neutralization
• urea neutralization
For small-scale refiners, sulfamic acid or urea are the easiest and safest methods.
Method 1: Using Sulfamic Acid
Sulfamic acid reacts with nitric acid and neutralizes it.
Steps
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Ensure the gold solution has been filtered.
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Slowly add small amounts of sulfamic acid.
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The solution will fizz as nitric acid reacts.
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Continue adding small portions until fizzing stops.
When the reaction stops, most nitric acid has been removed.
This prepares the solution for gold precipitation.
Method 2: Using Urea
Urea is another commonly used nitric neutralizer.
Steps
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Start with a filtered gold solution.
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Add small amounts of urea powder.
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The solution will bubble as nitric acid reacts.
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Continue adding urea slowly until bubbling stops.
Once the reaction stops, nitric acid has been neutralized.
Confirming Nitric Acid Has Been Removed
Before precipitating gold, refiners often perform a quick check.
Add a small test amount of precipitant to the solution.
If gold begins forming brown powder, the solution is ready.
If nothing happens, nitric acid may still be present.
Preparing for Gold Precipitation
Once nitric acid has been removed, the solution is ready for the final recovery step.
At this stage refiners typically:
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Test the solution with stannous chloride
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Confirm dissolved gold is present
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Add sodium metabisulfite or another precipitant
The dissolved gold will then fall out of solution as fine brown powder.
Final Step: Recovering Gold Powder
After precipitation:
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Allow the gold powder to settle.
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Carefully decant the solution.
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Wash the powder with distilled water.
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Dry the recovered gold powder.
The powder can then be melted into a solid gold button.
Recommended Gold Refining Supplies
If you're refining scrap gold, jewelry, or electronic components, using the correct chemicals and fresh reagents is critical for consistent results. These supplies are commonly used in the aqua regia gold refining process.
Hydrochloric Acid for Aqua Regia