Gold, sparsely distributed in the Earth’s crust, requires a meticulous extraction process due to its low concentration in economically viable deposits (around 3 g/t). The method predominantly involves concentration steps, with gold and silver, often found together, remaining inseparable through most stages.
Gold extraction involves obtaining gold from dilute ores through a combination of chemical processes. Annually, gold mining yields approximately 3600 tons, with an additional 300 tons obtained through recycling efforts. Since the 20th century, the predominant method for gold extraction has been the cyanide process, wherein the ore is leached with a cyanide solution. Subsequent refinement, known as gold parting, involves removing other metals, primarily silver, by introducing chlorine gas to the molten metal. In historical practices, small gold particles were amalgamated with mercury and then concentrated by boiling away the mercury, a method still used in some small-scale operations.
The ore utilized undergoes an initial separation by oxidizing with cyanide. This forms soluble cyanide complexes, extracting gold and silver from the surrounding rock. Subsequently, the precious metal complexes adhere to activated carbon, separating them from impurities in a solution.
Types of Gold Ores and Extraction Challenges.
Gold exists primarily as a native metal, either in its pure form or alloyed with silver, known as electrum. It manifests as sizable nuggets, fine grains or flakes in alluvial deposits, or microscopic particles (referred to as “colour”) embedded in rock minerals. Other gold forms include the minerals calaverite (AuTe), aurostibnite (AuSb2), and maldonite (Au2Bi), which, though rarer than native gold, can pose challenges in cyanide processing due to slow reactivity.
Additionally, certain gold-containing ores consist of tellurides such as sylvanite, nagyagite, petzite, and krennerite. However, these ores may present difficulties in the cyanide extraction process.
Moreover, the presence of contaminants in ores, known as “preg-robbing ores,” can impede gold extractability by cyanide. For instance, gold tightly binding to carbon or associating with certain clays can resist standard cyanide extraction methods.
Gold Extraction Techniques.
Gold mining, often romanticized for its nugget imagery, involves recovering gold from ores with trace amounts exceeding 10 ppm. The primary method is cyanidation, where gold is leached with a cyanide solution after comminution. Sodium cyanide, produced on a large scale, is commonly used, and “black cyanide” (carbon-contaminated calcium cyanide) is economical. Thiosulfate leaching, bulk leach extractable gold (BLEG) processes, and mercury amalgamation are alternative methods. While effective, mercury amalgamation poses environmental hazards. Refractory gold ores, with ultra-fine gold particles, necessitate pre-treatment (e.g., roasting, bio-oxidation, pressure oxidation, Albion process) due to resistance to standard cyanidation. These methods aim to overcome challenges posed by sulphide minerals and organic carbon, ensuring efficient gold recovery and addressing environmental concerns associated with mining practices.
- Step 1 – Leaching:
- The gold ore undergoes crushing, milling, and slurry formation with water, cyanide, and aeration. Cyanide oxidizes gold, forming soluble complexes.
- Step 2 – Concentration:
- Post-leaching, the challenge is returning gold to its elemental form, achieved by treating the solution with activated carbon or zinc powder.
- Step 3 – Recovery:
- Activated carbon with adsorbed gold undergoes treatment with NaCN and NaOH, stripping gold from the carbon. Electrowinning recovers elemental gold and silver from the loaded electrolyte, which is later smelted into ingots.
The Role of the Laboratory:
Laboratories play a crucial role in the gold mining process, from determining ore composition for mining feasibility to in-process quality control and environmental safety testing.
- Pre-mining: Samples of ore from potential mining areas are finely ground, mixed with cyanide, and tested for gold content. Large samples are crucial to counteract the “spotty gold” effect, ensuring accurate representation.
- In-process Testing: Throughout extraction, samples of solutions, solids, and activated carbon are analyzed to assess process efficiency. Composition analysis involves spectrophotometry, fire assaying, and aqua regia digestion for gold and silver determination.
- Wastewater Testing: Wastewater from the treatment process is monitored regularly to prevent contamination of the environment, particularly the Ohinemuri River, with cyanide or heavy metals.
Environmental Implications:
Gold mining poses potential environmental issues, primarily related to cyanide emissions and heavy metal contamination of waterways. To mitigate these concerns, gaseous cyanide emissions are controlled, and wastewater undergoes treatment to separate mine water containing heavy metals from process water containing heavy metals and cyanide.
Gold Refining and Parting Methods
Gold refining and parting is a crucial process to purify gold to a commercially-tradeable standard, typically ≥99.5%, with a focus on removing silver. The Miller process, practiced on a large scale, involves passing chlorine gas into a molten gold alloy at high temperatures (c. 500 °C). The noble nature of gold prevents it from reacting with chlorine, allowing the separation of contaminants in the form of a low-density slag. This slag, often diluted with a flux like borax, is decanted from the liquid gold, and precious metals like silver chloride can be recovered from it. Alternative methods include dissolving silver selectively with nitric acid (inquartation), using concentrated sulfuric acid (affination), or employing electrolysis through the Wohlwill process.