Gold rarely appears in its pure, elemental form within the Earth’s crust. Instead, it is found disseminated within various rocks and minerals, requiring complex processes to separate it from these accompanying materials. The goal of gold ore processing is to extract this valuable metal efficiently from its host ore, transforming it into a usable, high-purity product. This multi-stage journey begins with physically preparing the ore, followed by concentrating the gold-bearing components, chemically extracting the gold, and finally refining it to meet market standards.
Initial Ore Preparation
The initial phase of gold ore processing involves reducing large rocks into smaller particles. This mechanical size reduction liberates gold particles from waste rock, making them accessible for subsequent separation. Large ore pieces first undergo crushing, utilizing jaw crushers for primary reduction and cone crushers for secondary crushing, which break the ore into gravel-sized fragments.
Following crushing, the ore proceeds to the grinding stage, where it is further reduced to a fine powder or slurry. Ball mills, which rotate a drum filled with steel balls, or rod mills, using steel rods, are commonly employed. This fine grinding significantly increases the surface area of the gold-bearing material, exposing minute gold particles previously encased within the rock matrix. The aim is to achieve a particle size that allows for efficient gold liberation.
Concentration Techniques
After the ore has been physically prepared, concentration techniques are applied to increase the proportion of gold relative to the waste material. These methods leverage the distinct physical properties of gold, such as its density, to separate it from the gangue minerals. Gravity concentration is one such method, exploiting gold’s significantly higher density compared to most other minerals. Techniques like sluicing, jigging, and using shaking tables utilize flowing water to separate heavier gold particles, which settle out, from lighter waste materials that are washed away.
Another widely used concentration method is froth flotation, a physico-chemical process that separates hydrophobic gold particles from hydrophilic waste. In this process, the finely ground ore slurry is mixed with specific chemicals, including collectors that attach to gold surfaces, making them water-repellent, and frothers that create stable air bubbles. Air is then introduced into the mixture, and the hydrophobic gold particles adhere to these bubbles, rising to the surface to form a mineral-rich froth that can be skimmed off. The waste material, which remains water-wet, sinks and is discharged as tailings.
Gold Extraction Methods
Once the gold-bearing concentrate has been produced, the next step involves dissolving or separating the gold from this enriched material. Cyanidation is the most prevalent method for extracting fine gold. This process involves dissolving gold using a dilute solution of sodium or potassium cyanide, which forms a soluble gold-cyanide complex. This chemical reaction allows the gold to be leached from the solid ore particles into the solution.
Following the dissolution, the gold-cyanide complex in solution is recovered through processes such as carbon-in-pulp (CIP) or carbon-in-leach (CIL). In these methods, activated carbon is added to the slurry, adsorbing the gold-cyanide complex onto its surface. The gold-loaded carbon is then separated from the barren slurry, and the gold is stripped from the carbon using a strong caustic solution. Another method, smelting, is a traditional high-temperature process. This involves melting the gold-bearing material with fluxes in a furnace, causing the gold to separate from impurities due to density differences.
While cyanidation and smelting are dominant, other methods exist. Thiosulfate leaching, for instance, uses a thiosulfate solution as an alternative to cyanide. Thiourea leaching also offers another alternative. Historically, mercury amalgamation was used to recover fine gold, but its significant environmental hazards have led to its widespread discontinuation in modern large-scale operations.
Refining and Final Product
After initial extraction, the gold obtained, often called “doré” gold, is not pure enough for commercial use. This crude gold requires further purification to achieve the high purity levels demanded by industries such as electronics, jewelry, and investment. One common method for refining doré gold is electrolytic refining. In this process, impure doré gold serves as the anode in an electrolytic cell, and when an electric current is passed through it, gold selectively dissolves and redeposits as high-purity gold onto a cathode. This method effectively separates gold from less noble metals like silver and copper.
Another chemical refining method is the aqua regia process, which involves dissolving the gold in a mixture of nitric acid and hydrochloric acid. This strong acid mixture can dissolve gold, forming a gold chloride complex. Impurities either remain undissolved or are separated at this stage. Subsequently, the pure gold is selectively precipitated from the solution using a reducing agent, yielding gold powder that can then be melted into high-purity ingots. These refining steps ensure that the final gold product meets stringent purity standards, making it suitable for various industrial and financial applications.