The separation of gold from copper, a process known in refining as “parting,” relies on exploiting the vast difference in chemical reactivity between the two metals. Copper is frequently combined with gold in alloys, such as jewelry or electronic scrap, because it adds hardness and imparts a desirable color. These techniques are fundamentally divided into small-scale chemical dissolution using acid and large-scale electrical processes. Both approaches leverage the fact that gold is a noble metal, meaning it resists chemical attack, while copper is a base metal that reacts readily.
Preparing the Alloy for Parting
Before acid treatment can successfully separate the metals, the gold-copper alloy must be prepared to allow the acid to penetrate the metal structure efficiently, often requiring a process called “quartation” to adjust the metal composition. The base metal content must significantly outweigh the gold content for the acid to work effectively. If the gold concentration is too high, the noble metal forms a protective surface layer that prevents the acid from dissolving the copper trapped inside.
For effective parting with nitric acid, the gold content must be diluted to approximately 25% or less, which is a ratio of about one part gold to three parts base metal. Scrap alloys that already meet this requirement, such as low-karat gold, can proceed directly to the next step. If the gold concentration is higher, copper or silver must be melted into the alloy to achieve the target ratio.
After mixing the metals, the molten alloy is poured into cold water, a technique known as granulation. This rapid cooling creates small, irregularly shaped particles, often called “shot” or “prills,” which have a high surface area. This increased surface area allows the acid to easily access the base metals throughout the alloy, ensuring a complete reaction during the parting stage.
Chemical Separation Using Nitric Acid
The most common small-scale method for separating copper from gold involves using nitric acid, which selectively dissolves the copper while leaving the gold untouched. Nitric acid is a powerful oxidizing agent that readily reacts with the base metal copper to form copper nitrate, a soluble salt. The chemical reaction produces a blue-green solution containing the dissolved copper and often releases toxic reddish-brown nitrogen oxide fumes.
The granulated alloy is added to a heated nitric acid solution, which is typically diluted to control the reaction speed. The heat increases the efficiency of the dissolution process, causing the copper to leach out of the alloy structure. As the copper dissolves, the gold remains behind in a solid form, losing its metallic luster and transforming into a dark, spongy, or muddy residue.
Once the reaction subsides, the mixture consists of the liquid copper nitrate solution and the solid gold residue. The solution is carefully decanted or filtered away from the solids, which are then rinsed multiple times with hot distilled water. This washing process removes any remaining traces of the copper nitrate solution and acid. The resulting dark powder, often referred to as “gold mud,” is then ready for further purification or melting into a solid bar.
Industrial Electrolytic Refining
For large-scale operations requiring gold purity exceeding 99.99%, industrial refineries often employ electrolytic refining, such as the Wohlwill process, which uses electricity to separate the metals. The impure gold-copper alloy, typically cast into a large plate or bar, serves as the anode (positive electrode). It is submerged in an electrolyte solution of gold chloride and hydrochloric acid. When an electric current is applied, the gold and other metal impurities in the anode begin to dissolve into the acidic electrolyte.
The pure gold ions selectively migrate toward the cathode, the negative electrode, where they are reduced back into solid, pure metallic gold. The gold electroplates onto the cathode, which is often a thin starter sheet of pure gold or titanium.
Copper and other base metals also dissolve from the anode, but under controlled voltage conditions, they remain in the solution rather than plating onto the cathode. The process is slower and requires a significant inventory of gold in the electrolyte, making it more expensive to set up than chemical parting. The superior purity achieved makes this method standard for producing investment-grade bullion and materials for high-tech applications.
Handling Chemicals and Waste Safely
Working with strong mineral acids like nitric acid requires safety protocols due to the corrosive nature of the liquid and the toxic gases produced. Personal Protective Equipment (PPE) is mandatory and includes chemical-resistant gloves, a face shield or safety goggles, and a protective apron. All operations involving nitric acid must be conducted in a well-ventilated area, preferably under a certified fume hood, to safely evacuate the hazardous nitrogen oxide gases.
The reddish-brown fumes produced during the copper dissolution are highly toxic and can cause severe respiratory damage if inhaled. Beyond protective gear, neutralizing agents like baking soda should be readily available to manage potential spills and leaks. The resulting copper nitrate solution, which is considered heavy metal waste, cannot be poured down a regular drain.
The spent solution must be neutralized to a near-neutral pH and the dissolved copper precipitated out before disposal. Neutralization is often achieved by adding a base, such as sodium hydroxide, which causes the copper to form an insoluble copper hydroxide solid. This solid is then filtered out for hazardous waste disposal or further recovery, ensuring no harmful chemicals enter the environment.