Gold extraction

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Cyanide leaching "heap" at a gold mining operation near Elko, Nevada. On top of the large mounds of ore, are sprinklers dispensing a solution of cyanide. Gold heap leaching.jpeg
Cyanide leaching "heap" at a gold mining operation near Elko, Nevada. On top of the large mounds of ore, are sprinklers dispensing a solution of cyanide.

Gold extraction is the extraction of gold from dilute ores using a combination of chemical processes. Gold mining produces about 3600 tons annually, [1] and another 300 tons is produced from recycling. [2]

Contents

Since the 20th century, gold has been principally extracted in a cyanide process by leaching the ore with cyanide solution. The gold may then be further refined by gold parting, which removes other metals (principally silver) by blowing chlorine gas through the molten metal. Historically, small particles of gold were amalgamated with mercury, and then concentrated by boiling away the mercury. The mercury method is still used in some small operations.

Types of ore

Gold occurs principally as a native metal, i.e., gold itself. Sometimes it is alloyed to a greater or lesser extent with silver, which is called electrum. Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles (known as colour) embedded in rock minerals. Other forms of gold are the minerals calaverite (AuTe), aurostibnite (AuSb2), and maldonite (Au2Bi). These latter three, although rarer that native gold, can be slow to react with cyanide and thus difficult to process. [3] Still other gold-containing ores include various tellurides (sylvanite, nagyagite, petzite, and krennerite).

Certain contaminants in ores can interfere with the extractability of gold by cyanide. These interfering agents are called "preg-robbing ores". For example, gold can bind tightly to carbon, resisting normal cyanide extraction. Gold cyanides bind also to some clays. [3]

Concentration

Chemical structure of potassium dicyanoaurate. Potassium dicyanoaurate.svg
Chemical structure of potassium dicyanoaurate.

While the romantic picture of gold mining focuses on nuggets, the reality is that gold is typically recovered from ores containing >10 ppm of the metal. Thus, the main challenge is concentrating this trace amount. [2]

Cyanidation (and thiosulfate)

The principal technology is the cyanide process, in which gold is leached from the ore by treatment with a solution of cyanide. The first step is comminution (grinding) to increase surface area and expose the gold to the extracting solution. The extraction is conducted by dump leaching or heap leaching processes. Sodium cyanide is produced on a billion-ton/year scale mainly for this purpose. "Black cyanide", a carbon-contaminated form of calcium cyanide (Ca(CN)2) is often used because it is cheap. The crude ore is washed with a c. 0.3% solution of cyanide in air, often repeatedly, and the aqueous extract is collected and refined further. Recovery from solution typically involves adsorption on activated carbon, the carbon in pulp process.

Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience preg-robbing.

Leaching through bulk leach extractable gold, or BLEG, is also a process that is used to test an area for gold concentrations where gold may not be immediately visible.

Mercury amalgamation

Amalgamation with mercury can be used to recover very small gold particles, and mercury is still widely used in small-scale artisanal mining across the world. [4] Mercury forms a mercury-gold amalgam with smaller gold particles, and then the gold is concentrated by boiling away the mercury from the amalgam. This is effective in extracting very small gold particles, but the process is hazardous due to the toxicity of mercury vapour. Large-scale use of mercury stopped in the 1960s. However, mercury is still used in artisanal and small-scale gold mining (ASGM). [5] One mechanism by which mercury is employed in hydraulic mining is as an "undercurrent", in which the flow of smaller grains is diverted over mercury-coated copper plates. High flow velocities associated with hydraulic mining cause flouring of mercury, the wearing down of mercury particles that contributes to mercury loss into the environment. [6]

Over 10,000,000 pounds (4,500,000 kg) of mercury contaminated the environment in California as a result of placer mining in the late nineteenth and early twentieth centuries. Stamp mill mining contributed an additional 3,000,000 pounds (1,400,000 kg) of mercury contamination. [6] Mercury contamination in California waterways is a major contemporary environmental issue, [6] as is groundwater pollution, mostly by inorganic mercury. [7]

Refractory gold processes

High-grade gold ore from a quartz vein near Alma, Colorado. The appearance is typical of very good gold-quartz ore. Gold-Quartz-273364.jpg
High-grade gold ore from a quartz vein near Alma, Colorado. The appearance is typical of very good gold-quartz ore.

A "refractory" gold ore is an ore that has ultra-fine gold particles disseminated throughout its gold occluded minerals. These ores are naturally resistant to recovery by standard cyanidation and carbon adsorption processes. These refractory ores require pre-treatment in order for cyanidation to be effective in recovery of the gold. A refractory ore generally contains sulphide minerals, organic carbon, or both. Sulphide minerals are impermeable minerals that occlude gold particles, making it difficult for the leach solution to form a complex with the gold. Organic carbon present in gold ore may adsorb dissolved gold-cyanide complexes in much the same way as activated carbon. This so-called "preg-robbing" carbon is washed away because it is significantly finer than the carbon recovery screens typically used to recover activated carbon. [3]

Pre-treatment options for refractory ores include:

  1. Roasting
  2. Bio-oxidation, such as bacterial oxidation
  3. Pressure oxidation
  4. Albion process

The refractory ore treatment processes may be preceded by concentration (usually sulphide flotation). Roasting is used to oxidize both the sulphur and organic carbon at high temperatures using air and/or oxygen. Bio-oxidation involves the use of bacteria that promote oxidation reactions in an aqueous environment. Pressure oxidation is an aqueous process for sulphur removal carried out in a continuous autoclave, operating at high pressures and somewhat elevated temperatures. The Albion process utilises a combination of ultrafine grinding and atmospheric, auto-thermal, oxidative leaching.

Gold refining and parting

Parting is a process by which gold is purified to a commercially-tradeable standard, typically ≥99.5%. Removal of silver is of particular interest since the two metals often co-purify. The standard procedure is based on the Miller process. The separation is achieved by passing chlorine gas into a molten alloy. The technique is practiced on a large scale (e.g. 500 kg). The principle of the method exploits the nobility of gold, such that at high temperatures, gold does not react with chlorine, but virtually all contaminating metals do. Thus, at c. 500 °C, as the chlorine gas is passed through molten mixture (again, mainly gold), a low-density slag forms on top, which can be decanted from the liquid gold. Silver chloride and other precious metals can be recovered from this slag. The slag layer is often diluted with a flux like borax to facilitate the separation. [2]

Alternative methods exist for parting gold. Silver can be dissolved selectively by boiling the mixture with 30% nitric acid, a process sometimes called inquartation. Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid. [8] Electrolysis using the Wohlwill process is yet another approach.

History

John Stewart MacArthur developed the cyanide process for gold extraction in 1887. Jsmacarthur-X-007r.jpg
John Stewart MacArthur developed the cyanide process for gold extraction in 1887.
Gold miners excavate an eroded bluff with jets of water at a placer mine in Dutch Flat, California sometime between 1857 and 1870. Hydraulic mining in Dutch Flat, California, between 1857 and 1870.jpg
Gold miners excavate an eroded bluff with jets of water at a placer mine in Dutch Flat, California sometime between 1857 and 1870.

The smelting of gold began sometime around 6000 – 3000 BC. [9] [10] [11] According to one source the technique began to be in use in Mesopotamia or Syria. [12] In ancient Greece, Heraclitus wrote on the subject. [13]

According to de Lecerda and Salomons (1997) mercury was first in use for extraction at about 1000 BC, [14] according to Meech and others (1998), mercury was used in obtaining gold until the latter period of the first millennia. [15] [16] [17] [18]

A technique known to Pliny the Elder was extraction by way of crushing, washing, and then applying heat, with the resultant material powdered. [19] [20] [21]

Industrial era

Like all metals, gold is insoluble in a water. Gold does however exhibit the distinctive properties that in the presence of cyanide ions, it dissolves in the presence of oxygen (or air). This transformation was reported in 1783 by Carl Wilhelm Scheele, but it was not until the late 19th century, that the reactions were exploited commercially. The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits being found tended to be pyritic ore. The gold was difficult to extract from such ores.

A process known as chlorination was once used to treat pyritic gold ore. Typically, the ore was roasted and then treated with chlorine gas. The residue was extracted to give an aqueous solution of gold chloride. It was used, notably at the Mount Morgan mine, where it remained in use until 1911. The chloride process became obsolete with the development of the cyanide process. [22] [23]

In 1887, John Stewart MacArthur, working in collaboration with brothers Dr Robert and Dr William Forrest for the Tennant Company in Glasgow, Scotland, developed the MacArthur-Forrest Process for the extraction of gold ores. By suspending the crushed ore in a cyanide solution, up to 96 percent gold was extracted. [24] [25] [26] [27] [28] [29] [30]

The process was first used on a large scale at the Witwatersrand in 1890, leading to a boom of investment as larger gold mines were opened up. In 1896, Bodländer confirmed that oxygen was necessary for the process, something that had been doubted by MacArthur, and discovered that hydrogen peroxide was formed as an intermediate. [31]

The method known as heap leaching was first proposed in 1969 by the United States Bureau of Mines, [32] and was in use by the 1970s. [33]

See also

Related Research Articles

Bioleaching is the extraction or liberation of metals from their ores through the use of living organisms. Bioleaching is one of several applications within biohydrometallurgy and several methods are used to treat ores or concentrates containing copper, zinc, lead, arsenic, antimony, nickel, molybdenum, gold, silver, and cobalt.

<span class="mw-page-title-main">Metallurgy</span> Field of science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.

<span class="mw-page-title-main">Mining</span> Extraction of valuable minerals or other geological materials from the Earth

Mining is the extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agricultural processes, or feasibly created artificially in a laboratory or factory. Ores recovered by mining include metals, coal, oil shale, gemstones, limestone, chalk, dimension stone, rock salt, potash, gravel, and clay. The ore must be a rock or mineral that contains valuable constituent, can be extracted or mined and sold for profit. Mining in a wider sense includes extraction of any non-renewable resource such as petroleum, natural gas, or even water.

<span class="mw-page-title-main">Smelting</span> Use of heat and a reducing agent to extract metal from ore

Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron, copper, silver, tin, lead and zinc. Smelting uses heat and a chemical reducing agent to decompose the ore, driving off other elements as gases or slag and leaving the metal behind. The reducing agent is commonly a fossil-fuel source of carbon, such as carbon monoxide from incomplete combustion of coke—or, in earlier times, of charcoal. The oxygen in the ore binds to carbon at high temperatures, as the chemical potential energy of the bonds in carbon dioxide is lower than that of the bonds in the ore.

Extractive metallurgy is a branch of metallurgical engineering wherein process and methods of extraction of metals from their natural mineral deposits are studied. The field is a materials science, covering all aspects of the types of ore, washing, concentration, separation, chemical processes and extraction of pure metal and their alloying to suit various applications, sometimes for direct use as a finished product, but more often in a form that requires further working to achieve the given properties to suit the applications.

Gold cyanidation is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction. Cyanidation is also widely used in the extraction of silver, usually after froth flotation.

<span class="mw-page-title-main">Industrial processes</span> Process of producing goods

Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.

<span class="mw-page-title-main">Copper extraction</span> Process of extracting copper from the ground

Copper extraction refers to the methods used to obtain copper from its ores. The conversion of copper ores consists of a series of physical, chemical and electrochemical processes. Methods have evolved and vary with country depending on the ore source, local environmental regulations, and other factors.

Bacteria biooxidation is an oxidation process caused by microbes where the valuable metal remains in the solid phase. In this process, the metal remains in the solid phase and the liquid can be discarded. Bacterial oxidation is a biohydrometallurgical process developed for pre-cyanidation treatment of refractory gold ores or concentrates. The bacterial culture is a mixed culture of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans. The bacterial oxidation process comprises contacting refractory sulfide ROM ore or concentrate with a strain of the bacterial culture for a suitable treatment period under an optimum operating environment. The bacteria oxidise the sulfide minerals, thus liberating the occluded gold for subsequent recovery via cyanidation.

Hydrometallurgy is a technique within the field of extractive metallurgy, the obtaining of metals from their ores. Hydrometallurgy involve the use of aqueous solutions for the recovery of metals from ores, concentrates, and recycled or residual materials. Processing techniques that complement hydrometallurgy are pyrometallurgy, vapour metallurgy, and molten salt electrometallurgy. Hydrometallurgy is typically divided into three general areas:

Carbon in pulp (CIP) is an extraction technique for recovery of gold which has been liberated into a cyanide solution as part of the gold cyanidation process.

The Merrill–Crowe Process is a separation technique for removing gold from the solution obtained by the cyanide leaching of gold ores. It is an improvement of the MacArthur-Forrest process, where an additional vacuum is managed to remove air in the solution, and zinc dust is used instead of zinc shavings.

<span class="mw-page-title-main">Heap leaching</span> Industrial mining process used to extract precious metals from ore

Heap leaching is an industrial mining process used to extract precious metals, copper, uranium, and other compounds from ore using a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials. Similar to in situ mining, heap leach mining differs in that it places ore on a liner, then adds the chemicals via drip systems to the ore, whereas in situ mining lacks these liners and pulls pregnant solution up to obtain the minerals. Heap leaching is widely used in modern large-scale mining operations as it produces the desired concentrates at a lower cost compared to conventional processing methods such as flotation, agitation, and vat leaching.

<span class="mw-page-title-main">Silver mining</span> Extraction silver from the ground

Silver mining is the extraction of silver by mining. Silver is a precious metal and holds high economic value. Because silver is often found in intimate combination with other metals, its extraction requires the use of complex technologies. In 2008, approximately 25,900 metric tons of silver were consumed worldwide, most of which came from mining. Silver mining has a variety of effects on the environment, humans, and animals.

Leaching is a process widely used in extractive metallurgy where ore is treated with chemicals to convert the valuable metals within the ore, into soluble salts while the impurity remains insoluble. These can then be washed out and processed to give the pure metal; the materials left over are commonly known as tailings. Compared to pyrometallurgy, leaching is easier to perform, requires less energy and is potentially less harmful as no gaseous pollution occurs. Drawbacks of leaching include its lower efficiency and the often significant quantities of waste effluent and tailings produced, which are usually either highly acidic or alkali as well as toxic.

<span class="mw-page-title-main">Outline of mining</span> Overview of and topical guide to mining

The following outline is provided as an overview of and topical guide to mining:

The International Cyanide Management Code for the Manufacture, Transport and Use of Cyanide in the Production of Gold, commonly referred to as the Cyanide Code, is a voluntary program designed to assist the global gold and silver mining industries and the producers and transporters of cyanide used in gold and silver mining in improving cyanide management practices and to publicly demonstrate their compliance with the Cyanide Code through an independent and transparent process. The Cyanide Code is intended to reduce the potential exposure of workers and communities to harmful concentrations of cyanide‚ limit releases of cyanide to the environment‚ and enhance response actions in the event of an exposure or release.

A gold cyanidation ban refers to the legislation that bans mining gold through the gold cyanidation technique.

Pressure oxidation is a process for extracting gold from refractory ore.

A transition metal thiosulfate complex is a coordination complex containing one or more thiosulfate ligands. Thiosulfate occurs in nature and is used industrially, so its interactions with metal ions are of some practical interest.

References

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