The history of human civilization is tied to the pursuit of gold, a metal valued for its stability and brilliance. Gold rarely exists as pure nuggets; instead, it is typically found finely disseminated within rock formations or mixed with sediments, known as ore. Extracting this metal requires complex industrial processes that systematically break down the ore and isolate the gold particles. This sequence involves physical removal, mechanical preparation, chemical separation, and refining.
Initial Ore Removal and Mining Methods
The initial step involves physically removing the gold-bearing material, and the method chosen depends on the type of deposit. Gold found in hard rock veins, known as lode deposits, requires hardrock mining techniques, divided into open-pit and underground operations. Open-pit mining is used when the ore body is near the surface, involving the large-scale removal of overlying rock and soil using explosives and haul trucks.
For deeper ore bodies, underground mining is necessary. Tunnels and shafts are excavated to reach the gold-bearing zones. Miners drill and blast the rock face, and the broken ore is transported to the surface for processing. Selection between open-pit and underground methods is an economic consideration based on the depth, size, and concentration of the gold.
Gold weathered out of its source rock and transported by water, settling in riverbeds or floodplains, forms placer deposits. Extraction relies on water-based methods that exploit the metal’s high density compared to the surrounding gravel and sand. Techniques like dredging or hydraulic mining use powerful streams of water to excavate and wash the alluvial material.
The raw material from placer deposits is fed into mechanical systems like sluice boxes or jigs. These systems use gravity to separate the dense gold from the lighter waste material. These methods process loose sediments rather than solid rock. The goal is to deliver the gold-containing material to the processing plant.
Preparation of Ore for Processing
Once the hard rock ore reaches the surface, it undergoes comminution to prepare it for chemical treatment. This is a two-stage process of crushing and grinding designed to reduce the size of the rock. The first stage uses large machinery like jaw or gyratory crushers to break the run-of-mine ore into smaller fragments.
The crushed material then moves to the grinding stage, typically employing rotating equipment like ball or rod mills. These mills contain steel balls or rods that impact the ore, reducing it to a fine powder or a slurry suspended in water. This reduction is necessary because the gold is often microscopic and locked within the rock matrix.
The grinding process must ensure the gold particles are fully exposed, or “liberated,” from the surrounding waste rock. This liberation is a prerequisite for subsequent chemical steps, as it increases the surface area available for reaction with extraction agents. The final product is a finely ground slurry, often reduced to 75 micrometers or less, ready for metal separation.
Separating Gold from the Ore
The core of the extraction process involves chemically and physically separating the liberated gold from the ore slurry. For coarse gold particles, gravity separation is the most straightforward and cost-effective method. Equipment such as jigs, spiral concentrators, and shaking tables leverage the high density of gold, allowing it to settle out from the lighter gangue minerals when agitated in water.
For ores where the gold is associated with sulfide minerals, flotation is employed to create a gold-rich concentrate. Chemical reagents are added to the slurry, causing the sulfide minerals to attach to air bubbles introduced into the mixture. These bubbles rise to the surface, forming a froth that is skimmed off, concentrating the gold for further treatment.
The most widely used method for recovering fine gold, especially from low-grade ores, is cyanide leaching, also known as cyanidation. The finely ground ore slurry is mixed with a dilute solution of sodium cyanide (NaCN) in the presence of oxygen. The cyanide selectively dissolves the gold, forming a stable, soluble complex ion, Au(CN)2-.
Following dissolution, the gold complex is recovered through the Carbon-in-Pulp (CIP) process. Activated carbon granules are added directly to the cyanide slurry, and the gold complex is adsorbed onto the surface of the carbon. The gold-loaded carbon is then separated using screens, and the gold is stripped using a hot, concentrated cyanide and caustic solution.
The final solution, concentrated with gold, is sent for electro-winning. An electric current is passed through the solution, causing the dissolved gold to plate onto steel wool cathodes. This results in a sludge or precipitate, which is dried and smelted to produce a porous, impure metal product called a Dore bar. These bars typically contain 70% to 90% gold, with the remaining mass composed mostly of silver and base metals.
Final Purification and Refining
The Dore bar is not pure enough for most commercial uses and requires further refining to achieve investment-grade status. The initial step is to melt the Dore bar in a furnace, known as smelting, to remove impurities and cast it into a suitable form. Two methods are used to achieve the final, high-purity product.
The Miller Process is a rapid refining technique that achieves a gold purity of about 99.5%. This method involves bubbling chlorine gas through the molten, impure gold at high temperatures. The chlorine reacts readily with the base metals and silver, forming metal chlorides that separate from the molten gold as a liquid slag layer on the surface.
For applications requiring the highest level of purity, such as electronics or investment bullion, the Wohlwill Process is utilized. This is an electrolytic process where the impure gold from the Miller Process is cast into anodes. These anodes are submerged in an electrolyte solution of gold chloride and hydrochloric acid. An electric current causes the gold in the anode to dissolve and then selectively deposit as ultra-pure gold onto a cathode.
This electrolytic method produces gold with a purity level of 99.99% or higher, sometimes reaching 99.999%. While the Wohlwill process is slower and more expensive than the Miller process, it meets the exacting standards of the global market for high-quality gold products.