Gold extraction is an industrial process required to separate minute quantities of the metal from vast volumes of mined rock. This complex sequence is necessary because gold rarely appears in concentrated, pure forms, forcing operators to process tons of material to recover just a few grams. The process progresses through various physical and chemical transformations after the ore is removed from the earth.
Locating and Identifying Viable Ore
The journey of gold extraction begins with identifying rock formations that contain enough metal concentration to justify the mining expense, known as “viable ore.” Geologists look for two main types of deposits: vein deposits, where gold is concentrated in fractures within the rock, and disseminated deposits, where fine gold particles are scattered throughout a larger rock volume. The economic feasibility hinges on the ore’s grade, which is the amount of gold present per ton of rock. Determining this grade involves assaying, a process that chemically analyzes systematic rock samples to provide a precise gold concentration, often measured in grams per ton (g/t). A deposit below 0.5 g/t is considered low-grade, while anything above 5 g/t is considered high-grade.
Mechanical Reduction of Ore Size
Once viable ore is mined, it must be physically broken down to unlock the trapped gold particles, initiating the mechanical reduction phase. This size reduction exposes the gold to the chemical agents or physical separation techniques used later. The process begins with primary crushing, often using large jaw or gyratory crushers that reduce the ore to pieces no larger than a few inches. The material then moves to secondary crushers, followed by grinding. Grinding occurs in rotating steel cylinders, known as ball or rod mills, which pulverize the rock into a fine powder or a water-based slurry, often targeting a particle size finer than 75 microns.
Primary Extraction Techniques
With the ore reduced to a fine slurry, the next stage involves separating the gold from the bulk rock material, utilizing either physical or chemical methods. The choice of technique depends on whether the gold is “free-milling” (physically accessible) or “refractory.”
Gravity Separation
Free-milling gold can often be recovered using gravity separation techniques. These methods exploit the significant difference in density between gold (specific gravity of 19.3) and the surrounding rock (around 2.6). Equipment like jigs and shaking tables use water flow and vibration to stratify the particles, allowing the heavier gold to settle out. This physical separation is effective for coarse gold particles and is often used as a pre-treatment step.
Chemical Leaching (Cyanidation)
For the majority of modern gold production, particularly for fine or low-grade ores, chemical leaching is the dominant method. Cyanidation involves mixing the gold-bearing slurry with a dilute solution of sodium cyanide (NaCN) under alkaline conditions. The cyanide acts as a complexing agent, chemically dissolving the gold into the solution. Oxygen and water are necessary co-reactants for this dissolution, which can take 12 to 48 hours depending on the ore characteristics. The chemical reaction forms a soluble gold-cyanide complex: \(4text{Au} + 8text{NaCN} + text{O}_2 + 2text{H}_2text{O} rightarrow 4text{Na}[text{Au}(text{CN})_2] + 4text{NaOH}\).
Gold Recovery
Once the gold is dissolved, it is recovered from the pregnant leach solution, typically through the Carbon-in-Pulp (CIP) or Carbon-in-Leach (CIL) process. In CIL, activated carbon granules are added directly to the slurry, selectively adsorbing the gold-cyanide complex onto their surface. The gold-loaded carbon is then separated using screens and stripped of its gold content in a high-temperature caustic solution. Following stripping, the gold-rich solution undergoes electrowinning, where an electric current causes the dissolved gold to plate out onto steel wool cathodes. The resulting material is an impure sludge ready for final refining.
Final Purification of Gold
The impure gold material recovered from electrowinning, called “dore” (typically 80–95% gold), must undergo further treatment to achieve market-acceptable purity, usually 99.9% or higher. The first step involves smelting, where the dore is melted in a furnace with a flux to separate the metal from remaining slag and impurities. Two methods are used for final purification: the Miller Process and the Wohlwill Process.
The Miller Process is a rapid, fire-refining method where chlorine gas is bubbled through the molten gold. The chlorine reacts with and removes base metals and silver, leaving behind gold with a purity of approximately 99.5%. For the highest purities (99.99% or above), the electrolytic Wohlwill Process is employed. In this method, the gold is cast into anodes and submerged in a hydrochloric acid-gold chloride electrolyte solution. When an electric current is applied, the gold dissolves from the anode and is selectively plated onto a pure gold cathode.
Environmental Impact of Gold Processing
The industrial scale of gold extraction necessitates the responsible management of massive volumes of waste and process chemicals. For every ton of rock processed, an equivalent amount of waste slurry, known as tailings, is produced. Tailings must be safely stored in large impoundments called tailings dams, which require continuous monitoring to prevent catastrophic failures and manage potential acid mine drainage.
A primary environmental concern stems from the use of toxic cyanide in the leaching process. Although highly effective at dissolving gold, operators must implement strict detoxification and destruction protocols before residual water is discharged or reused. Methods often involve chemical oxidation, such as the use of sulfur dioxide and air, to convert the free cyanide into less harmful compounds. Historically, mercury was used in artisanal mining to amalgamate gold particles, releasing significant quantities of toxic mercury into the environment. While largely replaced by cyanidation, mercury amalgamation persists illegally in many regions, posing a severe threat. The long-term environmental footprint requires comprehensive reclamation efforts, including re-contouring the land and re-establishing local vegetation after operations cease.