The Occurrence and Beneficiation of Gold in Nature

Gold, a valuable and sought-after metal, occurs in nature primarily as native gold, with various other gold-bearing minerals such as electrum, silver gold ore, gold-bearing silver, bismuth gold ore, and gold-silver ore. This article explores the occurrence of gold in nature and the beneficiation processes used to extract and refine this precious metal.

Native Gold

Native gold, though rare in its pure form, is commonly alloyed with elements like silver. It belongs to the cubic crystal system and typically presents as octahedral, cubic, or dodecahedral crystals. With a deep yellow color and metallic luster, native gold is ductile, has a Mohs hardness of 2.5 to 3, and a specific gravity ranging from 15.6 to 19.3. It is insoluble in most acids but dissolves in aqua regia and solutions of KCN and NaCN. Native gold reacts with mercury to form an amalgam.

Native gold is considered to have inherent floatability, and particles of suitable size can be recovered with only frother. Additionally, native gold can be rendered hydrophobic by the deposition of sulfur on its surface. However, the floatability of native gold decreases with larger particle sizes or the addition of certain reagents like CaO or Na2S to the pulp. Native gold or free gold can be recovered at near-neutral pH levels, and it has a high flotation rate.

Silver-Gold Ores and Electrum

Gold and silver share similar crystal structures and chemical properties, allowing for complete solid solution between native gold and native silver. These ores often present in granular, dendritic, vein-like, or flaky forms and are named based on the content of gold or silver in the mineral.

The presence of silver positively affects the floatability of gold. It is believed that the adsorption of xanthate esters on silver surfaces is based on an electrochemical reaction involving metal xanthates. The presence of silver in gold minerals leads to the formation of silver xanthate at lower potentials than that required for pure gold, enabling the flotation of silver-gold ores at lower electrochemical potentials.

Associated Gold Minerals

Gold is often associated with other metallic minerals, with gold's correlation decreasing in the following order: chalcopyrite, pyrite, magnetite, arsenopyrite, and it is also commonly found in bismuthinite, sphalerite, and galena.

For associated gold and silver minerals, they are typically recovered along with the main minerals. By optimizing process structures and reagent regimes, these associated gold and silver minerals can be directed to recover into higher-value concentrate products. Subsequently, metallurgical methods are used to extract the precious metals gold and silver. The floatability of associated gold and silver minerals often depends on the main metal minerals, supplemented by collectors like amine blacking agents that selectively target gold and silver for selectively directed recovery.

Conclusion

Gold in nature presents unique challenges and opportunities for beneficiation. Native gold's inherent floatability and the positive influence of silver on gold's floatability are crucial factors in the efficient extraction of gold from its ores. Associated gold minerals require a more nuanced approach, often relying on the recovery of main minerals to facilitate the enrichment of gold and silver into higher-value products. As the demand for gold persists, the beneficiation processes must continue to evolve to meet the challenges of extracting this precious metal from its natural occurrences.