The extraction of gold from its ore is a process known as cyanidation, which relies on a dilute solution of sodium cyanide. This chemical method, often called gold leaching, is the most common technique used globally to recover the metal from mined material. Developed in the late 19th century, cyanidation revolutionized the industry by making the profitable recovery of gold from low-grade ores possible. The process converts solid metallic gold into a water-soluble coordination complex, allowing it to be separated from the surrounding rock matrix.
The Chemical Reaction for Gold Dissolution
Gold dissolution occurs through an electrochemical reaction known as Elsner’s equation. This process requires the presence of both cyanide ions and dissolved oxygen to proceed effectively. The metallic gold atom is oxidized, losing electrons, while the oxygen acts as an electron acceptor.
The chemical interaction results in the formation of the dicyanoaurate(I) ion, a soluble gold-cyanide complex. The simplified equation shows that sodium hydroxide (NaOH) is produced as a byproduct. This NaOH serves to maintain the high alkaline pH of the solution, which is important for safety and process efficiency. Maintaining the solution at a pH above 10.5, typically by adding lime, prevents the highly toxic hydrogen cyanide gas from forming.
Aeration is a deliberate step to ensure sufficient dissolved oxygen is available to drive the oxidation and dissolution of the gold. If oxygen levels are too low, the gold dissolution rate slows significantly. The resulting soluble gold complex can then be collected, while the remaining solid waste material is separated and treated.
Industrial Application Methods
The two main industrial approaches for applying the cyanidation process are Heap Leaching and agitated Tank Leaching, often referred to as Carbon-in-Pulp (CIP) or Carbon-in-Leach (CIL). The choice between these methods depends primarily on the ore’s gold concentration, or grade, and its physical characteristics. Heap leaching is used for large volumes of low-grade ore, where the cost of finely grinding the material is not justified.
In heap leaching, the ore is crushed to a coarse size and piled onto an impermeable pad, often made of plastic liners. A dilute cyanide solution is sprayed over the pile, where it slowly percolates down through the ore, dissolving the gold. The gold-bearing solution, known as pregnant liquor, is collected at the base of the pad for subsequent recovery. This method has lower capital expenditure and operating costs, but achieves a lower recovery rate, typically in the range of 65% to 80%.
Tank leaching, conversely, is used for higher-grade or more complex ores that require maximum recovery. The ore is ground to a fine powder and mixed with water and the cyanide solution to form a slurry, or pulp, inside large, agitated tanks. This aggressive mixing ensures intimate contact between the cyanide and the gold particles, resulting in faster leaching and higher recovery, often exceeding 90%. Although the construction and infrastructure costs for tank leaching are higher, the increased efficiency justifies the expense for richer deposits.
Detoxification and Environmental Management
Once the gold has been recovered from the pregnant solution, the remaining waste slurry, called tailings, contains residual cyanide that must be managed before disposal. Environmental regulations mandate a reduction of cyanide concentrations to meet discharge limits. Detoxification processes chemically convert the harmful free and complexed cyanide species into less toxic compounds.
One widely adopted chemical method is the SO2/Air process, also known as the INCO process. This method uses sulfur dioxide gas and air, often with a copper catalyst, to oxidize the cyanide. This reaction transforms cyanide into cyanate, a compound that is significantly less toxic than the original cyanide ion. The resulting cyanate then gradually hydrolyzes in the tailings pond to form harmless ammonium and carbonate ions.
Another effective detoxification method uses Caro’s acid, or peroxomonosulfuric acid, which is often generated on-site due to its instability. Caro’s acid rapidly oxidizes the cyanide to cyanate without requiring a metal catalyst. Both the INCO and Caro’s acid processes aim to reduce the concentration of “Weak Acid Dissociable” (WAD) cyanide. WAD cyanide measures the most environmentally mobile forms of cyanide, and concentrations must typically be reduced below 50 milligrams per liter before the tailings are stored.
Why Cyanide Remains the Dominant Method
Despite the toxicity and the complexity of managing its waste streams, cyanidation remains the preferred method for gold extraction due to economic and technical factors. No other lixiviant offers the same combination of high efficiency and low cost for treating a broad range of gold ores, especially low-grade material. Cyanide selectively dissolves gold over most other minerals, maximizing the final yield.
The method is robust and adaptable, integrating well with existing large-scale mining infrastructure. While alternatives like thiosulfate, thiourea, or novel synthetic compounds are being researched, they currently face hurdles. These alternatives are often more expensive, require more complex process control, or exhibit slower leaching kinetics, making them less economically viable for large-volume industrial applications. Until an economically competitive and equally effective alternative is widely proven, cyanidation will continue to set the industry standard.