Gold ore is rock or mineral material containing gold, often mixed with other minerals. Gold exists in various forms within these ores, including native gold, and can be alloyed with other metals like silver. The primary objective of gold extraction is to separate the metal from surrounding materials. This complex process involves multiple stages to isolate and purify gold.
Preparing Gold Ore for Extraction
Before gold separation, ore undergoes physical processing to reduce its size. Crushing breaks down large rocks into smaller, more manageable pieces. Crushers reduce the ore to road gravel size. This prepares the material for further size reduction.
Following crushing, grinding pulverizes the ore into a fine powder or slurry. This is often achieved using ball mills, rotating drums filled with steel balls. Grinding significantly increases the surface area of gold particles, making subsequent extraction methods more efficient by exposing gold within the rock matrix. Both crushing and grinding are essential steps that prepare the ore for effective gold recovery.
Physical Separation Techniques
Physical separation methods exploit gold’s distinct properties, particularly its high density, to separate it from lighter waste materials. These techniques are often used for “free gold” not chemically bound within other minerals. Water is frequently used to wash away less dense material, leaving heavier gold.
Panning is a traditional method where a shallow pan is used to swirl water and ore, allowing gold particles to settle while lighter sediments wash away. Sluicing employs a long, narrow channel with riffles. As water and ore flow through, dense gold particles are trapped by the riffles, while lighter materials are carried away.
More advanced physical separation techniques include jigs and shaking tables. Jigs use pulsating water to create a stratified bed, allowing heavier gold particles to be collected while lighter materials are discharged. Shaking tables, also known as gravity tables, are inclined vibrating surfaces. They cause heavy gold particles to migrate across the table and separate from lighter gangue materials. These physical methods are effective for concentrating gold, especially when present in a relatively coarse form.
Chemical Extraction Processes
Chemical extraction processes use chemical reactions to dissolve or concentrate gold from processed ore. These methods are effective for gold that is finely disseminated or chemically bound within the ore. Each process employs specific chemical principles for gold separation.
Cyanidation is the most common industrial method for gold extraction, especially for low-grade ores. It involves dissolving gold from finely ground ore using dilute sodium cyanide or potassium cyanide solutions. Cyanide reacts with gold, oxygen, and water to form a soluble gold-cyanide complex, such as dicyanoaurate. This chemical reaction, often described by Elsner’s equation, leaches gold from the solid ore into a liquid solution. After leaching, gold is recovered from the solution, typically by adsorption onto activated carbon or precipitation using zinc dust.
Amalgamation is a historical method using mercury to combine with gold. Mercury forms an alloy, or amalgam, with gold particles. This technique was widely used, especially for alluvial gold or free-milling ores. However, large-scale operations have largely phased out amalgamation due to significant environmental and health hazards, as mercury can be released into the environment, contaminating soil and water. Despite its effectiveness, mercury’s toxicity has led to its decline in modern, regulated mining.
Flotation is another chemical process used to extract gold, particularly from sulfide ores where gold is finely associated with other minerals. In this method, chemicals are added to a finely ground ore slurry to make the gold particles hydrophobic, meaning they repel water. Air bubbles are then introduced into the mixture, and hydrophobic gold particles attach to these bubbles, rising to the surface to form a frothy concentrate. The froth, enriched with gold, is then skimmed off, separating gold from the waste materials that remain in the water. This process is highly efficient for certain ore types and can achieve high recovery rates.
Refining Crude Gold
After initial extraction, the gold obtained, often in doré bars, is not entirely pure and contains impurities like silver and copper. Further refining processes are necessary to achieve high-purity gold. These steps transform crude gold into a product suitable for commercial use.
Smelting involves melting gold concentrate at very high temperatures, typically above 1,064 degrees Celsius. During smelting, fluxes (chemical agents) are added to the molten material. These fluxes react with and separate impurities, forming a slag that floats on top of the molten gold for removal. The remaining molten gold, though more concentrated, still requires further purification.
Electrolytic refining, such as the Wohlwill process, achieves very high purity, often reaching 99.99% pure gold. Impure gold is used as the anode (positive electrode) in an electrolytic cell, while a thin sheet of pure gold serves as the cathode (negative electrode). When an electric current passes through an electrolyte solution, gold ions from the impure anode dissolve and deposit onto the pure gold cathode, leaving most impurities behind in the solution or as an anode sludge. This method ensures gold production with exceptional purity for various applications.