Gold straight from the earth or from recycled jewelry is never pure. It contains silver, copper, iron, platinum-group metals, and other contaminants that must be stripped away. The method you use depends on your starting purity and how pure you need the final product to be, with options ranging from simple gravity separation all the way to electrochemical refining that yields 99.99% gold.
Gravity Separation: The First Step
Before any chemical refining happens, raw gold ore needs to be concentrated. Most early-stage methods exploit one simple fact: gold is extremely dense compared to surrounding rock and sediment. Panning, sluicing, shaking tables, spiral concentrators, and centrifuges all use water and motion to wash lighter material away while heavy gold particles settle and get captured.
Panning is the simplest version. You swirl crushed ore or sediment in water, and lighter minerals spill over the rim while gold sinks to the bottom. Sluices scale this up by running water down angled platforms lined with carpet or ridged surfaces that trap gold as lighter material washes past. Shaking tables tilt to one side with horizontal grooves that direct gold toward collection points while waste flows off the edge. For higher volumes, centrifuges spin the mixture and force the densest particles outward for collection.
These methods don’t purify gold in a chemical sense. They concentrate it, often producing material that’s rich enough to smelt directly. Smelting that concentrate (heating it until the gold melts) produces what’s called a doré bar, typically 60% to 90% pure. That bar then moves on to chemical refining.
Inquartation and Parting With Nitric Acid
If your main impurity is silver, one of the oldest and most reliable techniques is inquartation. The principle is straightforward: nitric acid dissolves silver but leaves gold untouched. The catch is that the acid can only penetrate the alloy if silver makes up enough of the total mass. Historically, refiners found that a ratio of about three parts silver to one part gold gave the best results. Later practice brought that down to roughly 2.5:1 or even 2:1.
If your gold alloy is too gold-rich for the acid to work, you add silver until the ratio is right. That’s the “inquartation” step, named because silver was added to bring gold down to roughly one quarter of the alloy. The mixed metal is then granulated or rolled thin, placed in nitric acid with a specific gravity of at least 1.26, and heated. The silver dissolves into solution, leaving behind a dark, spongy mass of nearly pure gold that can be washed, dried, and melted.
The Miller Process: Fast Refining to 99.5%
Developed in the 1860s and still used by major mints, the Miller process is the workhorse of large-scale gold refining. It works by bubbling chlorine gas through molten gold. Most common impurities, including silver, copper, zinc, and iron, react with the chlorine to form stable chloride compounds even above gold’s melting point. These chlorides are lighter than liquid gold, so they float to the surface as a slag that gets skimmed off.
The process is fast and relatively cheap, producing gold at 99.5% purity (995 fineness). That’s the minimum standard for London Good Delivery bars, the benchmark of the international gold market. For many commercial purposes, 99.5% is perfectly adequate. But for investment-grade bullion, electronics, or semiconductor manufacturing, further refining is needed.
Aqua Regia: Dissolving Gold Itself
Gold resists almost every acid on its own, but a mixture of hydrochloric acid and nitric acid in a 3:1 ratio, known as aqua regia (“royal water”), dissolves it completely. You always add nitric acid to hydrochloric acid, never the reverse. The reaction produces a gold-chloride solution while many impurities either remain undissolved or can be filtered out.
Once the gold is in solution, the liquid is filtered to remove insoluble particles (platinum-group metals often drop out here). The dissolved gold is then chemically precipitated back into solid form, typically using a reducing agent that converts the gold ions back into metallic gold powder. That powder is washed repeatedly, dried, and melted into bars of roughly 99.9% purity. Aqua regia refining is common among smaller refineries and jewelers because it doesn’t require the heavy equipment of the Miller process, though it does generate corrosive fumes and acidic waste that need careful handling.
The Wohlwill Process: Reaching 99.99% and Beyond
For the absolute highest purity, the industry turns to electrolytic refining, known as the Wohlwill process. It works like gold-plating in reverse. An impure gold bar serves as the anode (positive electrode), and a thin sheet of pure gold or a titanium plate serves as the cathode (negative electrode). Both sit in an acidic solution rich in dissolved gold, typically containing 60 to 100 grams of gold per liter.
When electrical current flows through the system, gold atoms leave the impure anode and travel through the solution to deposit as ultra-pure metal on the cathode. Impurities either stay behind as sludge at the bottom of the tank or remain dissolved in the electrolyte. The result is gold at 99.99% purity (“four nines”), and with careful control, the process can reach 99.999% for specialized applications like semiconductor manufacturing.
The downside is cost and time. The Wohlwill process is slow, and the electrolyte solution requires a large inventory of gold to remain effective, tying up significant capital. Many refineries use the Miller process first to reach 99.5%, then finish with Wohlwill electrolysis to hit 99.99%. This two-stage approach balances speed and purity.
Cupellation: Testing and Small-Batch Purification
Cupellation is one of the oldest refining techniques, used for millennia and still employed today in assay labs to test gold purity. The gold sample is mixed with lead, and the combined metal is placed in a shallow, porous cup called a cupel made of calcium or magnesium compounds. The cupel is heated to between 1,000 and 1,100°C in an oxygen-rich environment.
At that temperature, the lead oxidizes and the molten lead oxide gets absorbed into the porous cupel like ink into a paper towel. Base metals oxidize along with the lead and get pulled into the cupel as well. What remains is a small, shiny bead of gold and silver. If silver is present, it can be removed afterward using nitric acid parting. Cupellation isn’t practical for large-scale production, but it remains the standard method for determining how pure a gold sample is before committing it to a larger refining process.
Removing Platinum-Group Metals
Some of the hardest impurities to remove from gold are platinum, palladium, and iridium. These metals behave similarly to gold in many chemical processes, so they tend to survive standard refining steps. In the Miller process, for example, platinum-group metals don’t form volatile chlorides as readily as base metals, so they can persist in the final product.
Refineries typically handle these contaminants through solvent extraction, a process where the dissolved metals in an acidic solution are selectively pulled out using organic chemicals. The technique exploits the fact that gold, platinum, and palladium each bond differently to certain chemical agents. By changing the stripping solution in stages, refiners can separate platinum first, then palladium, then recover gold last. This sequential approach achieves near-complete separation of all three metals, but it adds complexity and cost, which is why it’s reserved for feedstock known to contain significant platinum-group contamination.
Choosing the Right Method
The right purification approach depends on what you’re starting with and what purity you need:
- Raw ore or alluvial gold: Gravity concentration followed by smelting gets you to a 60%-90% pure doré bar.
- Doré bars or scrap jewelry with high silver content: Inquartation and nitric acid parting efficiently removes silver.
- Bulk refining to market-grade bars (99.5%): The Miller chlorine process is fast and cost-effective.
- High-purity bars (99.9%): Aqua regia dissolution and chemical precipitation, suitable for smaller operations.
- Ultra-high purity (99.99% or higher): Wohlwill electrolytic refining, often preceded by the Miller process.
In practice, most gold passes through multiple stages. A mine produces concentrate, which is smelted into doré, which is refined by the Miller process to 99.5%, then finished by electrolysis to 99.99%. Each step strips away a different class of impurity, and the combination delivers gold pure enough for anything from jewelry to microchips.