Gold cyanidation

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. It is the most commonly used process for gold extraction. Due to the highly poisonous nature of cyanide, the process is highly controversial.

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History

The discoverer of cyanide, Carl Wilhelm Scheele, found that it could dissolve gold in 1783.

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.

The reaction

The chemical reaction is called the Elsner Equation as follows :

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.

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]

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.

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.

CN⁻(aq) + H⁺(aq) → 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.

Effect of Lead Nitrate

Lead nitrate can improve gold leaching speed and quantity recovered, particularly in processing partially oxidized ores.

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.

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.

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.

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

Effects on the environment

 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.

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.

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.

Legislation

The American state of Montana and several countries have banned cyanide mining.^[5]

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). ^[6]

In response to the Baia Mare spill, Brussels introduced Directive 2006/21/EC on the management of waste from extractive industries.^[7] 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.

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^[8] that aims to reduce environmental impacts with third party audits of a company's cyanide management.

References