Gold extraction is the complex industrial process of separating the precious metal from its ore, which can contain gold concentrations as low as one gram per ton of rock. This journey transforms raw, gold-bearing rock into a highly pure metal through a sophisticated, multi-stage sequence of physical and chemical steps. Modern extraction methods focus on efficiency and high recovery rates to meet the global demand for gold. The procedure moves through various phases, starting with the physical removal of ore and concluding with the final purification into gold bullion.
Initial Ore Retrieval Methods
The method used to physically remove gold-bearing ore from the earth is determined by the size, grade, and depth of the deposit. For large, low-grade ore bodies located close to the surface, open-pit mining is the preferred technique. This process involves stripping away the non-mineralized soil and rock, known as overburden, to expose the ore beneath. The mine develops into a massive, terraced pit, allowing for the use of enormous machinery to continuously excavate and transport the gold-bearing material.
For gold deposits that are high-grade but buried deep below the surface, underground mining is necessary. This method involves the construction of tunnels and vertical shafts to access the ore body. Explosives are often used to break the hard rock formations, and specialized equipment is employed to load and haul the ore through the underground network. This technique is more costly and complex, but it targets rich deposits that would be uneconomical to access through surface removal. The ore is transported to the surface for processing.
Preparing the Ore for Extraction
The raw ore brought up from the mine must undergo significant mechanical preparation before the gold can be chemically separated. This preparation is a size-reduction process known as comminution, which is typically the most energy-intensive and costly stage of the entire operation. The first step is crushing, where large rocks are fed through multiple stages of jaw, cone, or gyratory crushers to reduce them into smaller pieces. Primary crushing breaks the largest rocks, and subsequent crushers reduce the material further.
After crushing, the material moves to the grinding or milling stage, where it is pulverized into an extremely fine powder or slurry. This fine size is necessary because the gold is usually disseminated as microscopic particles within the rock matrix. Grinding the ore to a fine powder exposes the microscopic gold particles, dramatically increasing the surface area for the subsequent chemical processes to work efficiently.
Chemical and Physical Separation Techniques
Once the ore is a fine slurry, the separation of gold from the waste rock begins using a combination of chemical and physical techniques. The most widespread method is cyanidation, which involves dissolving the gold using a dilute sodium cyanide solution in the presence of oxygen. This chemical reaction forms a stable, soluble gold-cyanide complex, effectively extracting the gold into the liquid phase. The process is applied in large, agitated tanks, known as carbon-in-pulp (CIP) or carbon-in-leach (CIL).
In the CIL method, the activated carbon is added directly to the leaching tanks, allowing the gold dissolution and adsorption onto the carbon to occur simultaneously. The CIP method performs leaching and then adsorption sequentially in separate tanks. Both rely on activated carbon, which has a massive surface area, to act as a “gold magnet” that traps the dissolved gold molecules. For very large volumes of low-grade ore, heap leaching is used, where the crushed ore is piled on an impermeable pad and the cyanide solution is sprayed over the top. The solution trickles down, dissolving the gold, and is collected at the bottom for recovery.
Gravity concentration is another technique, often used early in the process to recover any coarse or “free” gold particles that are not locked inside the rock matrix. This physical method exploits the high density of gold, which is significantly greater than that of the surrounding gangue minerals. Equipment like jigs, spiral concentrators, and shaking tables use water flow and motion to separate the heavy gold particles from the lighter waste material. Capturing this coarse gold early reduces the load on the downstream chemical circuits.
Flotation is utilized for ores where the gold is closely associated with sulfide minerals, making it refractory to simple cyanidation. In this process, chemicals called collectors are added to the slurry to make the gold-bearing particles hydrophobic. Air is then bubbled through the mixture, causing the hydrophobic gold particles to attach to the air bubbles and float to the surface, where they are skimmed off as a concentrated foam. This gold-rich concentrate is then further treated before final leaching and recovery can occur.
Final Purification and Refining
The product from the separation techniques, typically an impure gold concentrate or a gold-loaded carbon, is subjected to final purification to achieve market-ready gold bullion. The first step is often smelting, which involves heating the gold concentrate with fluxes, such as borax and silica, to temperatures over 1,000 degrees Celsius. The fluxes react with and absorb non-metallic impurities, forming a molten slag that separates from the denser, impure gold-silver alloy known as doré.
The doré metal is refined using sophisticated chemical or electrochemical processes. The Miller process uses gaseous chlorine blown through the molten doré, which reacts with base metals and silver to form chlorides that float to the surface as slag. This pyrometallurgical method is quick and cost-effective for large volumes, yielding gold that is approximately 99.5% pure.
To achieve the highest purity levels, such as 99.99%, electrolysis is employed through the Wohlwill process. Impure gold anodes are immersed in an electrolyte solution of chloroauric acid, and an electric current is applied. The gold dissolves from the anode and is selectively plated as ultra-pure metal onto a cathode. This electro-winning process transforms the intermediate product into investment-grade gold.