Extracting gold from ore is complex, especially when the gold is not easily accessible. While “free-milling” gold can be recovered with simple grinding and chemical treatment, a significant portion of the world’s reserves is contained within refractory ores. These ores are naturally resistant to standard recovery processes because the gold is physically or chemically locked within the structure of other minerals. Specialized pre-treatment methods are required to break down the host mineral matrix before the gold can be dissolved and collected.
Understanding Refractory Gold Ores
Refractory gold ores contain gold particles, often less than a few micrometers in size, encapsulated within a matrix of sulfide minerals. The most common host minerals are iron sulfides like pyrite (\(\text{FeS}_2\)) and arsenopyrite (\(\text{FeAsS}\)). The gold is physically trapped inside the crystal structure, making it impervious to the liquid chemicals used in standard gold recovery.
This physical encapsulation prevents leaching solutions, such as cyanide, from contacting the gold particles. Direct cyanidation, the typical method for dissolving gold, yields very low recovery rates, often less than 60%. Therefore, the sulfide mineral matrix must be chemically or biologically oxidized to create a porous structure, exposing the fine gold to recovery agents.
Thermal and High-Pressure Pre-Treatment Methods
The two principal non-biological methods for destroying the sulfide matrix rely on intense oxidation using high heat or high pressure. These techniques convert dense sulfide minerals into porous oxides or soluble sulfates, liberating the trapped gold. The choice between these industrial processes depends on the specific mineralogy of the ore and environmental regulations.
Roasting (Thermal Oxidation)
Roasting involves heating the refractory ore concentrate to high temperatures, typically between \(600\) and \(700\) degrees Celsius, in the presence of air or oxygen. This process chemically oxidizes and decomposes sulfide minerals, such as pyrite. The reaction converts solid iron sulfide into porous iron oxide (like hematite) while releasing sulfur dioxide (\(\text{SO}_2\)) gas.
This conversion destroys the original dense crystal structure, leaving behind a highly porous residue known as calcine. The pores and channels within the iron oxide matrix allow leaching solutions to permeate the material and reach the encapsulated gold particles. A major challenge of roasting is managing the sulfur dioxide gas produced, which requires specialized scrubbing equipment to mitigate air pollution.
Pressure Oxidation (POX)
Pressure Oxidation (POX) is a hydrometallurgical process that treats a slurry of finely ground ore in an autoclave. This method uses high pressure and high temperature to achieve rapid oxidation. Operating conditions often involve temperatures between \(190\) and \(230\) degrees Celsius and high-purity oxygen pressures ranging from 3.5 to 7 bars.
Within the autoclave, sulfides are rapidly oxidized in an aqueous environment, converting them into soluble sulfuric acid and solid iron compounds. The oxidation of iron pyrite is highly exothermic, releasing heat that helps maintain the high operating temperature. This destruction of the sulfide structure releases the gold particles, which remain in the solid residue ready for the final recovery stage. POX is highly effective, often achieving gold recovery rates at least 10% higher than roasting, and converts arsenic into a stable, disposable compound.
Biological Oxidation (BIOX)
Biological Oxidation (BIOX) offers an alternative to high-energy thermal and pressure methods by employing specialized microorganisms. This process uses naturally occurring, acid-loving bacteria, often including species like Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. These microbes oxidize the sulfide minerals in the ore.
The bacteria catalyze the oxidation of sulfide compounds, converting the solid sulfide minerals into soluble products, primarily ferric sulfate and sulfuric acid. This reaction is carried out in large, agitated tanks at much lower temperatures, typically between \(30\) and \(45\) degrees Celsius, and under atmospheric pressure.
While BIOX is slower than high-temperature methods, often requiring weeks, it is considered a more environmentally benign approach due to low energy consumption and lack of sulfur dioxide emissions. The biological mechanism gradually dissolves the sulfide matrix, exposing the microscopic gold particles. The resulting slurry is then neutralized before moving on to the final extraction phase.
Extracting the Released Gold
Once the gold is liberated from the sulfide matrix through pre-treatment, the final step is to dissolve and collect the exposed metal using cyanidation. Cyanidation is the standard industry practice for gold recovery. Regardless of the pre-treatment method (roasting, pressure oxidation, or biological oxidation), the resulting material is prepared for this final chemical leaching stage.
In cyanidation, a dilute solution of sodium cyanide is mixed with the pre-treated ore residue. The cyanide solution selectively dissolves the metallic gold by forming a stable, soluble gold-cyanide complex. This reaction is possible because the pre-treatment removed the sulfide barrier that prevented the cyanide from reaching the gold surface.
Following dissolution, the liquid solution containing the gold complex is separated from the solid waste material. The gold is then recovered from this solution using activated carbon. Activated carbon, often made from coconut shells, has a porous structure and high surface area, allowing it to adsorb the gold-cyanide complex. The gold-loaded carbon is then stripped using a heated chemical solution to recover the concentrated gold for refining.