Gold recycling, or secondary gold recovery, is the industrial process of extracting gold from materials that have already been used or discarded. This practice is economically significant because it transforms waste into a high-value commodity, reducing the need for primary mining. Recycling gold is also an environmentally responsible choice, as it conserves natural resources and substantially lowers the carbon footprint and habitat disruption associated with new gold extraction.
Primary Sources of Recoverable Gold
Recoverable gold feedstock is categorized primarily by concentration. Post-consumer jewelry scrap is the largest source, accounting for approximately 90% of the total recycled supply. This stream includes broken pieces, unwanted items, and excess material from manufacturing, all containing a high density of gold.
Obsolete electronics, known as e-waste, are a rapidly expanding source, contributing about 10% of the total recycled gold. Although the concentration of gold in an individual device is low, it is notably higher than in raw ore; a ton of mobile phones can yield up to 80 times more gold than a ton of mined ore. Other sources include dental alloys and industrial byproducts from aerospace, medical devices, and plating operations.
High-Volume Smelting and Refining Processes
For high-concentration materials like jewelry scrap and dental alloys, recovery often employs pyrometallurgy, which uses high heat to separate the precious metal. This involves melting the scrap in a furnace with fluxing agents, allowing impurities to separate from the gold. The result is typically a semi-pure metal ingot, called a doré bar, which must then undergo further refining to reach commercial purity standards.
The two most common subsequent refining methods are the Miller process and the Wohlwill process. The Miller process is a rapid, cost-effective method that involves blowing gaseous chlorine into the molten gold. The chlorine reacts with common impurities, forming chlorides that separate into a slag layer, yielding gold with a purity of about 99.5%.
For gold requiring the highest level of purity, the Wohlwill process is utilized, which is an electrochemical technique. An impure doré bar is used as the anode and is submerged in an electrolyte solution of chloroauric acid. When an electric current is applied, the gold dissolves from the anode and is selectively plated onto a cathode, leaving most impurities behind. This electrolytic refining produces gold with a purity of 99.99% or higher, required for investment-grade bullion and high-end industrial applications.
Specialized E-Waste Gold Recovery Techniques
Recovering gold from e-waste presents unique challenges because the gold is present in low concentrations and embedded within complex matrices of plastics, ceramics, and other metals. The process begins with mechanical pre-treatment, including shredding, grinding, and sorting to reduce material size and concentrate the metal-bearing fractions. This physical preparation makes the gold accessible for subsequent chemical treatment.
After pre-treatment, specialized hydrometallurgical techniques use chemical solutions to dissolve the gold. While highly toxic reagents like cyanide are used in primary mining, modern e-waste recovery increasingly uses safer lixiviants. The traditional method involves aqua regia, a potent mixture of nitric and hydrochloric acids, which effectively dissolves the gold but is highly corrosive and generates significant waste.
Newer techniques focus on non-cyanide leaching agents or milder chemical processes to improve environmental performance. Alternative reagents, such as thiourea, or innovative approaches like bioleaching, which utilizes microorganisms, are being explored. Once the gold is dissolved, it is recovered through methods like solvent extraction, ion exchange, or electrowinning, which precipitates pure metallic gold onto a cathode.