Gold Extraction Processes: An Overview

Gold is in all probability the earliest metal known to the early hominids. It is believed that, on finding it as nuggets and spangles in top soil and river sand deposits they were attracted by its intrinsic beauty, great malleability, and virtual indestructibility.

According to the Encyclopedia Britannica, the history of gold dates back at least 6,000 years. The earliest identifiable, realistically dated finds having been made in Egypt and Mesopotamia c. 4000 BC .

The source of gold in ancient times was without doubt primarily river placers (particles of elemental gold found in river sands), even though there is extensive proof that in certain areas where gold occurred, such as Egypt, the Arabian Peninsula and India, certain deposits were mined. This mining were especially focused on exploiting gold occurring in alluvial deposits, auriferous gold layers, and some near surface parts of gold veins that was easily reduced to tiny particles as a result of the oxidized nature of the vein.

In Egypt, around 2300 BC, gold was concentrated by washing away the lighter river sands with water, leaving behind the dense gold particles, which could then be further concentrated by melting with fire.

Gold Extraction

The extraordinary high chemical stability of gold can be relayed back to the comparative instability of the compounds that is formed when exposed to oxygen and water. It is this distinctive feature that permitted metallurgists over the ages to refine gold from other less noble metals or mixtures of alloys made up of metals that may occur with the gold.

This is achieved by oxidizing the other metals and subsequently separating the oxidized metals from the molten gold as a scum formed on the molten gold. In the extraction of gold it is valuable to bear in mind that gold is readily dissolved in a number of solvents. These include oxidizing solutions consisting of hydrochloric acid as well as dilute solutions of sodium cyanide. Gold can dissolve without any difficulty in these oxidizing liquids as a result of the formation of complex ions that are very stable.

The process of gold extraction or recovery from gold ores covers a wide variety of mineral processing, hydrometallurgical, and pyro-metallurgical processes.

Where gold was found to be present in conglomerates, some form of size reduction was necessary to separate the gold prior to concentration using strakes and frue vanners. A frue vanner is a machine invented in 1874 and used in the concentration of metalliferous ores such as gold. It consists of a moving, inclined, endless apron on which ore is concentrated by a current of water; a kind of buddle.

During the period when the frue vanner machines were popular the use of mercury amalgamation was widely adopted as a supplement to gravity concentration. In the more important commercial applications of amalgamation, ore was crushed and ground in various types of mills. The ore was crushed down to a powder that resembled talcum powder. This powdery ore was then hydrated to form a viscous pulp that was passed over copper plates that sloped at 18%. Their upper surface was coated with mercury. By virtue of its high relative density, the gold sank through the pulp into contact with the mercury. When the gold came into contact with the mercury was caught, and converted into amalgam. The amalgam consisted of a solution of gold in mercury that took the form of a thick paste. This amalgam was scraped off the plates, and retorted to yield gold sponge.

Another process of recovering gold from milled ore is the flotation process. During flotation, chemicals are added to flotation cells. The finely milled ore is then passed through these flotation cells in the form of a thick liquid. Certain chemicals results in specific minerals to float to top of the thick liquid in a foam-like froth. This is then collected or floated off, as a concentrate for subsequent processing.

During 1890, JS MacArthur and the Forrest brothers of Glasgow, introduced the cyanide process for the extraction of gold. The original process involved dissolving the gold from the finely ground gold ore by means of a weak cyanide solution in the presence of oxygen. The gold was subsequently precipitated from the solution by means of zinc shavings. The initial process was refined and the Crowe-Merrill zinc dust precipitation process resulted. The precipitate that is filtered out of the solution is melted together with fluxes o recover the gold bullion as doré bars.

The heap leaching process is another way in which some ore types can be treated to recover gold. This process involves placing ore in a heap and irrigating the heaped gold bearing material with a weak cyanide solution. This gold solvent then percolates through the heap dissolving the gold from the gold bearing material. The heap leach pads are constructed by positioning impermeable plastic sheeting over a gently inclined, compacted graded terrain. Before stacking the ore onto the leach pads, a thin, protective layer of sand is placed on the plastic as in insulator. The leached gold is collected as a pregnant solution in a recovery pond for subsequent processing.

More recently, the carbon-in-pulp (CIP) process has become an important method to recover gold. The carbon-in pulp process was developed to its present form in South Africa during the 1970's. This process development is considered to be the most significant advance in gold recovery technology in recent years. The carbon-in-pulp extraction process takes advantage of the significant physical attraction that activated carbon has for gold. The finely milled gold bearing ore are converted into a pulp of fine ore and water are treated with cyanide in large tanks that are stirred mechanically or agitated by air. Activated carbon is used to absorb the gold directly from the pulp/cyanide solution.

Sources

Adamson, R.J. 1972. Gold Metallurgy in South Africa , Chamber of Mines of South Africa .

Bhappu R. 1998. Gold Forum on Technology and Practices-World Gold '89'. Society for Mining Metallurgy

Eissler M. 2010. The Metallurgy of Gold: A Practical Treatise on the Metallurgical Treatment of Gold-Bearing Ores Including the Assaying, Melting, and Refining of Gold. Nabu Press

Encyclopedia Britannica

Gray, E.L. 1940. A History of the Discovery of the Witwatersrand Goldfields, Chamber of Mines of South Africa .

Lang, J. 1986. Bullion Johannesburg : Men, Mines and the Challenge of Conflict, Jonathan Ball.

Stanley, C.G. 1988. The Extractive Metallurgy of Gold in South Africa , Vol. 1 & 2, SAIMM / Chamber of Mines of South Africa .

Marsden J. and House I. 2006. The Chemistry of Gold Extraction. Society for Mining Metallurgy & Exploration

© Carl Marx