Recovering gold from an acid solution is a multi-step process: you dissolve the gold into liquid form, remove contaminants, then use a chemical reducing agent to force the gold back out as a solid powder. The most common method uses aqua regia, a powerful mix of hydrochloric and nitric acid, followed by precipitation with a sulfite-based chemical. The entire process, from dissolution to dried gold powder, typically takes a full day or more depending on the quantity you’re working with.
How Aqua Regia Dissolves Gold
Gold resists almost every acid on its own, but aqua regia breaks it down by combining two acids that work together. The mixture is roughly 3 parts concentrated hydrochloric acid to 1 part concentrated nitric acid. In practice, dissolving 100 grams of gold pellets calls for about 425 mL of hydrochloric acid and 150 mL of nitric acid. The nitric acid oxidizes tiny amounts of gold, and the hydrochloric acid immediately binds with those gold ions to form soluble gold chloride, pulling more gold into solution continuously.
The mixture is stirred and gently heated to around 40°C. Gold dissolves into a clear, golden-colored liquid. Heating above 50°C is counterproductive because it breaks down the acids faster than they can work on the gold. Once all the metal has dissolved, the solution is cooled to room temperature before moving to the next step.
Removing Silver and Base Metal Contaminants
If your starting material contains silver (common in jewelry or electronic scrap), the hydrochloric acid in aqua regia automatically converts silver into silver chloride, a white solid that won’t dissolve. This is actually useful: it separates silver from gold right at the dissolution stage. You filter the solution through fine polishing filters to catch these silver chloride particles along with any other undissolved residue. Pleated polypropylene filter cartridges work better than wound cartridges for catching the finest particles.
Other base metals like copper or iron will also dissolve in aqua regia, but they stay in solution and get separated during the precipitation step. The reducing agent used to drop gold out of solution is selective enough that most base metals remain dissolved.
Precipitating Gold From Solution
This is the core recovery step. You add a reducing agent that reverses the dissolution, converting dissolved gold ions back into solid metallic gold that settles as a fine brown or dark powder at the bottom of your container.
The most popular reducing agent for small-scale work is sodium metabisulfite. You dissolve it in a small amount of water first, then slowly add it to the gold-bearing solution. Only a modest quantity is needed relative to the gold content. The gold particles form quickly and settle to the bottom.
For larger-scale recovery, sodium sulfite works on the same principle. A patent-documented process uses 125 grams of sodium sulfite dissolved in 875 mL of water for 100 grams of gold. Before adding the reducing agent, the solution’s acidity is adjusted by slowly adding concentrated sodium hydroxide (about 50% by weight) until the pH reaches 2.5 to 3.0. This pH adjustment step improves precipitation efficiency without accidentally dropping the gold out too early. The solution temperature should stay below 50°C during this entire process, ideally between 20°C and 40°C.
Other reducing agents that work include ferrous sulfate, zinc dust, and hydroquinone, but sodium metabisulfite and sodium sulfite remain the most widely used because they’re affordable, effective, and relatively straightforward to handle.
Testing Your Solution for Remaining Gold
Before discarding your spent solution, you need to confirm that all the gold has been recovered. The standard test uses stannous chloride as an indicator. Dip a piece of filter paper into the solution, then apply a drop of stannous chloride. If dissolved gold remains, the paper turns black where the chemicals meet. A yellow-to-black color change confirms gold is still present and you need to add more reducing agent. No color change means the solution is exhausted.
This test is sensitive enough to detect trace amounts, so it’s worth repeating after each addition of reducing agent until you get a clean result.
Washing and Purifying the Gold Powder
The brown powder sitting at the bottom of your container is gold, but it’s contaminated with residual acid, salts, and traces of other metals. Filtering and washing are what determine your final purity.
First, filter the mixture to collect the gold powder as a cake on the filter. For a basic wash, rinse the powder thoroughly with water, then with acetone to help remove organic residues, then water again. Dry the powder in an oven at around 110°C for several hours.
For higher purity (above 99.9%), a more rigorous washing sequence is used. Industrial processes wash the powder with hot water at 90°C for 5 to 6 hours, then soak it in 30% nitric acid at 90°C for another 5 to 6 hours to dissolve any remaining base metals. A second wash with stronger 50% nitric acid removes stubborn impurities like palladium. An alkaline wash with 20% sodium hydroxide follows, then a final nitric acid wash and a clean water rinse. Each stage includes filtering and separating the gold from the liquid. After drying for 12 hours, this multi-stage approach can yield gold at 99.99% purity.
Melting Into a Solid Button or Bar
Gold powder is difficult to store and easy to lose, so most refiners melt it into a solid form. Gold melts at 1,063°C (1,945°F), which requires a furnace or high-temperature torch capable of sustaining that heat.
Before melting, mix the gold powder with borax as a flux. Borax lowers the effective melting temperature, reduces viscosity of the molten metal, and dissolves any remaining metal oxide impurities into a glassy slag that separates from the gold. This slag floats on top of the molten gold and peels away after cooling. The result is a clean gold button or bar, depending on your mold.
Safety and Ventilation Requirements
Every step of this process involves chemicals that can cause serious burns, release toxic fumes, or both. Aqua regia produces chlorine gas and nitrosyl chloride on contact, both of which are dangerous to inhale even briefly. The precipitation step with sulfite compounds releases sulfur dioxide, another respiratory irritant.
You need, at minimum, chemical-resistant gloves, splash-proof safety goggles, and a face shield. A chemical-resistant apron protects against splashes. All acid work must happen under local exhaust ventilation, meaning a fume hood or a well-designed outdoor setup where fumes are carried away from your breathing zone. Indoor work without proper fume extraction is not safe at any scale.
Keep large quantities of ice and sodium bicarbonate (baking soda) on hand. If you need to neutralize waste aqua regia, pour it slowly over ice using a ratio of about 500 grams of ice per 100 mL of acid. Then add sodium hydroxide solution or saturated sodium bicarbonate until the pH reaches neutral (around 7). Never pour water into concentrated acid. Always add acid to water, or in this case, acid to ice.
The Hydrochloric Acid and Peroxide Alternative
For dissolving gold from electronic scrap (circuit board fingers, pins, connectors), many hobbyists use a simpler starting solution: hydrochloric acid with a small amount of hydrogen peroxide. The peroxide acts as an oxidizer, playing the same role that nitric acid plays in aqua regia but more gently. This method is slower and better suited to thin gold plating rather than solid gold pieces.
The peroxide method dissolves copper and other base metals along with the gold, which means the resulting solution requires more careful processing to isolate gold selectively. Many refiners use this step only to strip gold foils from circuit boards, then dissolve those collected foils in aqua regia for a cleaner precipitation. This two-stage approach reduces the total amount of aqua regia needed and produces a solution with fewer contaminants to manage.