Gold processing is a sequence of physical and chemical transformations designed to extract trace amounts of the precious metal from vast quantities of raw ore. Because gold is scarce and dispersed within rock formations, sophisticated techniques are required to concentrate it from low-grade sources into high-purity metal. The entire process, from crushing rock to final refining, focuses on maximizing gold recovery while maintaining economic viability.
Ore Preparation and Concentration
The initial phase of gold processing involves preparing the raw ore to maximize subsequent chemical reaction efficiency. This begins with comminution, the mechanical process of crushing and grinding the mined rock. Primary crushers reduce the large ore to manageable sizes, which are then passed through grinding mills to reduce the material to a fine powder. This pulverization increases the surface area of the gold particles, exposing them for contact with chemical solvents.
Following fine grinding, physical concentration methods are employed to remove a significant portion of the waste rock, or gangue. Gravity separation techniques, such as shaking tables or centrifugal concentrators, are effective at recovering coarse, liberated gold particles due to gold’s high density. Froth flotation is another common method, utilizing chemical reagents to make gold-bearing sulfide minerals hydrophobic. This allows them to attach to air bubbles and float to the surface for collection. These pre-concentration steps reduce the volume of material treated chemically, lowering overall processing costs.
Chemical Extraction Through Leaching
After mechanical preparation, the primary method for chemically dissolving gold from the ore is leaching, with cyanidation being the dominant industrial technique. The ground ore is mixed with water to form a slurry, to which a dilute solution of sodium cyanide (NaCN) is added. Oxygen is simultaneously introduced into the mixture, as it is a required reactant in the process described by the Elsner equation: \(4\text{Au} + 8\text{NaCN} + \text{O}_2 + 2\text{H}_2\text{O} \to 4\text{Na}[\text{Au}(\text{CN})_2] + 4\text{NaOH}\). This reaction transforms the solid, metallic gold into a stable, water-soluble complex called dicyanoaurate, which dissolves into the liquid phase.
To prevent the formation of toxic hydrogen cyanide gas, the process is maintained at a high alkalinity, often with a pH between 10 and 11, by adding lime or caustic soda. For refractory ores, where gold is locked within sulfide minerals like pyrite, a pre-treatment step is necessary. This often involves roasting or pressure oxidation (autoclaving) to break down the sulfide matrix and expose the gold particles for successful cyanidation. Alternatives like thiosulfate or thiourea leaching are explored by some operations, but cyanidation remains the most common method.
Recovery from the Pregnant Solution
The liquid containing the dissolved gold complex is referred to as the “pregnant” solution; the next stage focuses on recovering the gold from this liquid. The two most widely used methods are carbon adsorption and zinc precipitation. Carbon adsorption is highly utilized, where the pregnant solution is passed through tanks containing activated carbon, often in a process known as Carbon-In-Pulp (CIP) or Carbon-In-Leach (CIL). The porous structure of the carbon attracts and adsorbs the dicyanoaurate complex onto its surface, concentrating the gold.
Once the carbon is loaded with gold, it is separated from the barren slurry by screening and subjected to elution. Elution uses a hot, pressurized solution of caustic soda and cyanide to strip the gold off the carbon. The alternative, known as the Merrill-Crowe process, involves adding fine zinc dust to the clarified pregnant solution. The zinc acts as a reducing agent, causing the gold to precipitate and form a solid sludge, which is then filtered out. Both methods yield a concentrated gold-bearing material ready for final purification.
Final Purification and Refining
The concentrated gold material recovered is not yet pure; it is typically smelted into doré metal, an alloy containing 80% to 90% gold, silver, and base metals. Smelting involves heating the concentrate in a furnace with fluxes, such as borax and silica, to separate the gold-bearing metal from impurities. The resulting doré bars are then sent to a refinery for final purification to meet investment-grade standards.
Two main processes are used for final refining. The Miller Process is a rapid, high-throughput chemical method involving blowing chlorine gas through the molten doré metal at high temperatures. The chlorine reacts with and removes base metals and silver by forming chlorides that separate as a slag layer, yielding gold that is typically around 99.5% pure. For the highest purity (99.99% or higher), the Wohlwill Process is used. This electrolytic refining method uses the impure gold as an anode in an electrolyte bath. An electric current selectively dissolves the gold and re-deposits it as high-purity metal onto a cathode.