Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest Process) is a metallurgical technique for extracting gold from low-grade ore by converting the gold to water soluble aurocyanide metallic complex ions. As of 2005 it is the most commonly used process for gold extraction. Because of the highly poisonous nature of cyanide, the process is controversial. A Blacksmith, the father of the modern metallurgist. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Standard atomic weight 196. ... Iron ore (Banded iron formation) Manganese ore Lead ore Gold ore An ore is a volume of rock containing components or minerals in a mode of occurrence which renders it valuable for mining. ... The cyanide ion, CN−. From the top: 1. ... 2005 is a common year starting on Saturday of the Gregorian calendar. ... Gold extraction or recovery from its ores may require a combination of comminution, mineral processing, hydrometallurgical, and pyrometallurgical processes to be performed on the ore. ... The skull and crossbones symbol (Jolly Roger) traditionally used to label a poisonous substance. ...

History

The discoverer of cyanide, Carl Wilhelm Scheele, found that it could dissolve gold in 1783. Carl Wilhelm Scheele Scheeles house with his pharmacy in Köping. ... 1783 was a common year starting on Wednesday (see link for calendar). ...

Bagration (1844), L Elsner (1846) and Faraday (1847) worked out the stoichiometry, but it wasn't applied to gold ores until 1887, when the MacArthur-Forrest Process was developed in Glasgow, Scotland by John Stewart MacArthur, funded by the brothers Dr Robert and Dr William Forrest. Peter (Petre) Bagrationi (September 12, 1818-January 17, 1876) was an outstanding Georgian statesman, general and scientist, the inventor of the first dry galvanic cell, descendant of the Georgian Kings. ... Michael Faraday, FRS (September 22, 1791 – August 25, 1867) was an English chemist and physicist (or natural philosopher, in the terminology of that time) who contributed significantly to the fields of electromagnetism and electrochemistry. ... It has been suggested that Stoichiometric coefficient and Gas stoichiometry be merged into this article or section. ... 1887 (MDCCCLXXXVII) is a common year starting on Saturday (click on link for calendar) of the Gregorian calendar or a common year starting on Monday of the Julian calendar. ... “Glaswegian” redirects here. ... This article is about the country. ... John Stewart MacArthur was a chemist from Glasgow. ...

The reaction

Ball-and-stick model of the aurocyanide or dicyanoaurate(I) complex, [Au(CN)₂]⁻

The chemical reaction is called the Elsner Equation as follows : Image File history File links Size of this preview: 800 × 199 pixelsFull resolution (1100 × 274 pixel, file size: 58 KB, MIME type: image/png) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 800 × 199 pixelsFull resolution (1100 × 274 pixel, file size: 58 KB, MIME type: image/png) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... armchair conformational isomerism of Cyclohexane. ... Vapours of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride A chemical reaction is a process that results in the interconversion of chemical substances. ...

4Au + 8NaCN + O₂ + 2H₂O → 4NaAu(CN)₂ + 4NaOH

It is an electrochemical process in which oxygen picks up electrons from the gold at a cathodic area, whilst gold ions are rapidly complexed by the cyanide around the anodic area to form the soluble aurocyanide complex. General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Standard atomic weight 196. ... Sodium cyanide is a highly toxic chemical compound, also known as sodium salt of hydrocyanic acid and cyanogran. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Standard atomic weight 22. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Standard atomic weight 196. ... The cyanide ion, CN−. From the top: 1. ... Sodium hydroxide (NaOH), also known as lye or caustic soda or sodium hydrate, is a caustic metallic base. ...

In 1896 Bodländer confirmed that oxygen was necessary, something that was doubted by MacArthur, and discovered that hydrogen peroxide was formed as an intermediate.^[1] Year 1896 (MDCCCXCVI) was a leap year starting on Wednesday (link will display calendar). ...

The process and its uses

The ore is comminuted (using grinding machinery), and may be further concentrated by froth flotation or by centrifugal (gravity) concentration, depending on the mineralogy of the ore. The alkaline ore slurry is combined with a solution of sodium cyanide or potassium cyanide. Iron ore (Banded iron formation) Manganese ore Lead ore Gold ore An ore is a volume of rock containing components or minerals in a mode of occurrence which renders it valuable for mining. ... Look up comminution in Wiktionary, the free dictionary. ... Froth Flotation is a selective process for separating minerals from gangue by using surfactants and wetting agents. ... Sodium cyanide is a highly toxic chemical compound, also known as sodium salt of hydrocyanic acid and cyanogran. ... Potassium cyanide or KCN is the potassium salt of hydrogen cyanide or hydrocyanic acid. ...

Process improvement

The Effect of pH

It is critical to avoid the release (volatilization) of cyanide as hydrogen cyanide because this gas is highly toxic; hydrogen cyanide boils at 26 °C, barely above room temperature. Cyanide ions may become hydrogen cyanide gas when they acquire free protons. Volatilisation is the process whereby a dissolved sample is vaporised. ... Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. ... Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. ... Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ...

CN^- + H⁺ = HCN_(g)

Therefore the free proton concentration is kept low by the addition of alkali such as lime (calcium hydroxide) or sodium hydroxide to ensure that the pH during cyanidation is maintained over pH 10.5. It has been suggested that Portlandite be merged into this article or section. ... Sodium hydroxide (NaOH), also known as lye or caustic soda or sodium hydrate, is a caustic metallic base. ... The correct title of this article is . ...

Effect of Lead Nitrate

Lead nitrate can improve gold leaching speed and quantity recovered, particularly in processing partially oxidized ores. Lead(II) nitrate is a chemical compound, the inorganic salt of nitric acid and lead. ... Leaching is the process of extracting a substance from a solid by dissolving it in a liquid. ...

Effect of Dissolved Oxygen

Oxygen is one of the reagents consumed during cyanidation, and a deficiency in dissolved oxygen in solution can slow leaching speed. Air or pure oxygen gas can be bubbled through the pulp to increase the dissolved oxygen concentration.Intimate oxygen-pulp contactors are used to increase the partial pressure of the oxygen in contact with the solution, thus raising the dissolved oxygen concentration much higher than the saturation level at atmospheric pressure. Oxygen can also be added by dosing the pulp with hydrogen peroxide solution. General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Standard atomic weight 15. ... A reagent or reactant is any substance used in a chemical reaction. ... Oxygen saturation is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. ... Diurnal (daily) rhythm of air pressure in northern Germany (black curve is air pressure) Atmospheric pressure is the pressure at any point in the Earths atmosphere. ... Hydrogen peroxide (H2O2) is a very pale blue liquid which appears colourless in a dilute solution, slightly more viscous than water. ...

Pre-aeration and Ore Washing

In some ores, particularly partially sulfidized ores, aeration (prior to the introduction of cyanide) of the ore in water at high pH can render elements such as iron and sulfur less reactive to cyanide, and therefore the gold cyanidation process more efficient. Aeration is the process by which air is circulated through, mixed with or dissolved in a liquid (usually water) or substance (such as soil). ...

The oxidation of iron to iron (III) oxide and subsequent precipitation as iron hydroxide avoids cyanide losses due to the formation of ferrous cyanide complexes. The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (CNS^-) byproduct. Iron(III) oxide - also known as ferric oxide, red iron oxide, synthetic maghemite, rouge,or rust - is one of several oxide compounds of iron, and is most notable for its ferromagnetic properties. ... Iron oxide pigment There are a number of iron oxides: Iron oxides Iron(II) oxide or ferrous oxide (FeO) The black-coloured powder in particular can cause explosions as it readily ignites. ... General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Standard atomic weight 32. ... The structure and bonding of the thiocyanate ion Thiocyanate (also known as sulphocyanate or thiocyanide) is the anion, [SCN]−. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. ...

Gold Recovery from Solution

In order of economic efficiency, the common processes for recovery of the solubilized gold from solution are (certain processes may be precluded from use by technical factors):

• Carbon in pulp
• Merrill-Crowe process
• Electrowinning
• Resin in pulp

Carbon in Pulp (CIP) is a technique for recovery of gold which has been liberated into a cyanide solution as part of the gold cyanidation process, a gold extraction technique. ... The Merrill-Crowe Process is a separation technique for removing gold from a cyanide solution. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Effects on the environment

Sodium cyanide drum at the abandoned Chemung Mine in Masonic, California

The process is controversial, due to the highly toxic nature of cyanide. However, free cyanide breaks down rapidly when exposed to sunlight, although the less toxic compounds such as cyanates and thiocyanates may persist for some years. The famous disasters tend not to kill many people, as humans can be warned not to drink or go near polluted water. However cyanide spills can have a devastating effect on rivers, killing everything for several miles downstream. Fish are the most obvious casualties, but in fact the whole food chain collapses, from phytoplankton to ospreys. However the pollution is soon washed out of river systems and as long as organisms can migrate from unpolluted areas upstream, affected areas can soon be repopulated - in the Somes river below Baia Mare the plankton returned to 60% of normal within 16 days of the spill. ^[2]Another problem is that bleach may be added as an antidote, but it contains enough free chlorine to be an environmental threat in its own right. Image File history File linksMetadata Download high resolution version (1600x1200, 465 KB) Summary Drum of Sodium Cyanide at the abandoned Chemung Mine in the ghost town of Masonic, California. ... Image File history File linksMetadata Download high resolution version (1600x1200, 465 KB) Summary Drum of Sodium Cyanide at the abandoned Chemung Mine in the ghost town of Masonic, California. ... For the Wikipedia policy regarding controversial issues in articles, see Wikipedia:Guidelines for controversial articles. ... County MaramureÅŸ County Status County capital Mayor Cristian Anghel, National Liberal Party, since 2000 Area 233. ...

Over 90 mines worldwide^[3] now use an Inco SO₂/air detoxification circuit to convert cyanide to the much less toxic cyanate before waste is discharged to a tailings pond. Typically this process blows compressed air through the tailings while adding sodium metabisulphite (which releases SO₂), lime to maintain the pH at around 8.5, and copper sulphate as a catalyst if there's not enough copper in the ore. This can reduce concentrations of Weak Acid Dissociable (WAD) cyanide to below the 10ppm mandated by the EU's Mining Waste Directive. This compares to levels of 66-81ppm free cyanide and 500-1000ppm total cyanide in the pond at Baia Mare.^[2] Remaining WAD cyanide breaks down naturally in the pond, whilst cyanate is naturally hydrolysed to ammonium ions and then to nitrate. County MaramureÅŸ County Status County capital Mayor Cristian Anghel, National Liberal Party, since 2000 Area 233. ...

Obviously most mines handle large quantities of cyanide without hitting the headlines, but famous cyanide spills include :

Such disasters have prompted fierce protests at new mines that want to use cyanide, such as Roşia Montană in Romania, Lake Cowal in Australia and Pascua Lama in Chile. The Summitville Mine was a gold mining site in Rio Grande County, Colorado 25 miles south of Del Norte. ... Kumtor Gold Mine is an open-pit gold mining site in Issyk Kul Province of Kyrgyzstan located about 350 km (220 mi) southeast of the capital Bishkek and 80 km (50 mi) south of Lake Issyk-Kul near the border with China. ... County MaramureÅŸ County Status County capital Mayor Cristian Anghel, National Liberal Party, since 2000 Area 233. ... RoÅŸia Montană (German Goldbach; Hungarian: Verespatak) is a commune in the Apuseni Mountains, in the Alba County, in the west-central Romania. ... Lake Cowal is the largest inland lake in New South Wales, Australia. ... Pascua-Lama is a project aiming at exploitation of a very rich field of gold and silver in the high mountains south of Atacama, at the boarder between Chile and Argentina, in an area approximately 150 kilometers southeast of Vallenar and 300 kilometers northwest of San Juan. ...

Legislation

The American state of Montana and several countries have banned cyanide mining.^{[citation needed]} Official language(s) English Capital Helena Largest city Billings Area  Ranked 4th  - Total 147,165 sq mi (381,156 km²)  - Width 255 miles (410 km)  - Length 630 miles (1,015 km)  - % water 1  - Latitude 44°26N to 49°N  - Longitude 104°2W to 116°2W Population  Ranked...

In the EU, industrial use of hazardous chemicals is controlled by the so-called Seveso II Directive (96/82/EC as amended by 2003/105/EC), which replaced the original Seveso Directive (82/501/EEC as amended by 87/216/EEC and 8/610/EEC) brought in after the 1976 dioxin disaster. "Free cyanide and any compound capable of releasing free cyanide in solution" are further controlled by being on List I of the Groundwater Directive (80/68/EEC) which bans any discharge of a size which might cause deterioration in the quality of the groundwater at the time or in the future. The Groundwater Directive was largely replaced in 2000 by the Water Framework Directive (2000/60/EC). ^[5]

In response to the Baia Mare spill, Brussels introduced Directive 2006/21/EC on the management of waste from extractive industries.^[6] Article 13(6) requires "the concentration of weak acid dissociable cyanide in the pond is reduced to the lowest possible level using best available techniques", and at most all mines started after 1 May 2008 may not discharge waste containing over 10ppm WAD cyanide, mines built or permitted before that date are allowed no more than 50ppm initially, dropping to 25ppm in 2013 and 10ppm by 2018. County MaramureÅŸ County Status County capital Mayor Cristian Anghel, National Liberal Party, since 2000 Area 233. ... Best Available Technology (or just BAT) is a term applied with regulations on limiting pollutant discharges with regard to the abatement strategy. ...

Under Article 14, companies must also put in place financial guarantees to ensure cleanup after the mine has finished. This in particular may affect smaller companies wanting to build gold mines in the EU, as they are less likely to have the financial strength to give these kinds of guarantees.

The industry has come up with a voluntary Cyanide Code^[7] that aims to reduce environmental impacts with third party audits of a company's cyanide management.

References

External links

• Efforts at a cleaner process
• Yestech A different commercial method that does not use toxic cyanide
• Historical notes - essay