Gold cyanidation
Gold cyanidation (also known as the cyanide process or the MacArthur–Forrest process) 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.[1] Cyanidation is also widely used in the extraction of silver, usually after froth flotation.[2]
Production of reagents for mineral processing to recover gold represents more than 70% of cyanide consumption globally. Other metals are recovered from the process include copper, zinc, and silver, but gold is the main driver of this technology.[1] Due to the highly poisonous nature of cyanide, the process is controversial and its use is even banned in some parts of the world. Cyanide can be safely used in the gold mining industry.[3] A key feature for safe use of cyanide is to ensure adequate pH control at an alkaline pH level above 10.5. At industrial scale, pH control is mainly achieved using lime, as an important enabling reagent in gold processing.[4]
History
In 1783,
Industrial process
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 contain pyritic ore. The gold could not be extracted from this compound with any of the then available chemical processes or technologies.[5] In 1887, John Stewart MacArthur, working in collaboration with brothers Robert and William Forrest for the Tennant Company in Glasgow, Scotland, developed the MacArthur–Forrest process for the extraction of gold from gold ores. Several patents were issued in the same year.[6] By suspending the crushed ore in a cyanide solution, a separation of up to 96 percent pure gold was achieved.[7] The process was first used on the Rand in 1890 and, despite operational imperfections, led to a boom of investment as larger gold mines were opened up.[8][5]
By 1891, Nebraska pharmacist
Chemical reactions
The chemical reaction for the dissolution of gold, the "Elsner equation", follows:
- 4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH
Potassium cyanide and calcium cyanide are sometimes used in place of sodium cyanide.
Gold is one of the few metals that dissolves in the presence of cyanide ions and oxygen. The soluble gold species is
Application
The ore is comminuted using grinding machinery. Depending on the ore, it is sometimes further concentrated by froth flotation or by centrifugal (gravity) concentration. Water is added to produce a slurry or pulp. The basic ore slurry can be combined with a solution of sodium cyanide or potassium cyanide; many operations use calcium cyanide, which is more cost effective.
To prevent the creation of toxic hydrogen cyanide during processing, slaked lime (calcium hydroxide) or soda (sodium hydroxide) is added to the extracting solution to ensure that the acidity during cyanidation is maintained over pH 10.5 - strongly basic. 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, accepting the electrons from the gold, and a deficiency in dissolved oxygen slows leaching rate. Air or pure oxygen gas can be purged through the pulp to maximize 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 those that are partially sulfidized,
Recovery of gold from cyanide solutions
In order of decreasing economic efficiency, the common processes for recovery of the solubilized gold from solution are (certain processes may be precluded from use by technical factors):
Cyanide remediation processes
The cyanide remaining in tails streams from gold plants is potentially hazardous. Therefore, some operations process the cyanide-containing waste streams in a detoxification step. This step lowers the concentrations of these cyanide compounds. The INCO-licensed process and the Caro's acid process oxidise the cyanide to cyanate, which is not as toxic as the cyanide ion, and which can then react to form carbonates and ammonia: [15]
- CN−
+ [O] → OCN− - OCN−
+ 2 H
2O → HCO−
3 + NH
3
The Inco process can typically lower cyanide concentrations to below 50 mg/L, whereas the Caro's acid process can lower cyanide levels to between 10 and 50 mg/L, with the lower concentrations achievable in solution streams rather than slurries. Caro's acid – peroxomonosulfuric acid (H2SO5) - converts cyanide to cyanate. Cyanate then hydrolyses to ammonium and carbonate ions. The Caro's acid process is able to achieve discharge levels of Weak Acid Dissociable" (WAD) cyanide below 50 mg/L, which is generally suitable for discharge to tailings. Hydrogen peroxide and basic chlorination can also be used to oxidize cyanide, although these approaches are less common. Typically, this process blows compressed air through the tailings while adding sodium metabisulfite, which releases SO2. Lime is added to maintain the pH at around 8.5, and copper sulfate is added as a catalyst if there is insufficient copper in the ore extract. This procedure can reduce concentrations of WAD cyanide to below the 10 ppm mandated by the EU's Mining Waste Directive. This level compares to the 66-81 ppm free cyanide and 500-1000 ppm total cyanide in the pond at Baia Mare.[16] Remaining free cyanide degrades in the pond, while cyanate ions hydrolyse to ammonium. Studies show that residual cyanide trapped in the gold-mine tailings causes persistent release of toxic metals (e.g. mercury ) into the groundwater and surface water systems.[17][18]
Effects on the environment
Despite being used in 90% of gold production:[19] gold cyanidation is controversial due to the toxic nature of cyanide. Although aqueous solutions of cyanide degrade rapidly in sunlight, the less-toxic products, such as cyanates and thiocyanates, may persist for some years. The famous disasters have killed few people — humans can be warned not to drink or go near polluted water, but cyanide spills can have a devastating effect on rivers, sometimes killing everything for several miles downstream. The cyanide is soon washed out of river systems and, as long as organisms can migrate from unpolluted areas upstream, affected areas can soon be repopulated. According to Romanian authorities, in the Someș river below Baia Mare, the plankton returned to 60% of normal within 16 days of the spill; the numbers were not confirmed by Hungary or Yugoslavia.[16] Famous cyanide spills include:
Year | Mine | Country | Incident |
---|---|---|---|
1985–1991 | Summitville | US | Leakage from leach pad |
1980s–present | Ok Tedi | Papua New Guinea | Unrestrained waste discharge |
1995 | Omai | Guyana | Collapse of tailings dam |
1998 | Kumtor | Kyrgyzstan | Truck drove over bridge |
2000 | Baia Mare | Romania | Collapse of containment dam (see 2000 Baia Mare cyanide spill) |
2000 | Tolukuma | Papua New Guinea | Helicopter dropped crate into rainforest[20] |
2018 | San Dimas | Mexico | Truck leaked 200 liters of cyanide solution into the Piaxtla River in Durango[21] |
Such spills have prompted fierce protests at new mines that involve use of cyanide, such as
Alternatives to cyanide
Although cyanide is cheap, effective, and biodegradable, its high toxicity has incentivized to alternative methods for extracting gold. Other extractants have been examined including thiosulfate (S2O32−), thiourea (SC(NH2)2), iodine/iodide, ammonia, liquid mercury, and alpha-cyclodextrin. Challenges include reagent cost and the efficiency of gold recovery. Thiourea has been implemented commercially for ores containing stibnite.[22] Yet another alternative to cyanidation is the family of glycine-based lixiviants.[23]
Legislation
The US states of
In the EU, industrial use of hazardous chemicals is controlled by the so-called
In response to the
Under Article 14, companies must also put in place financial guarantees to ensure clean-up 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"[34] that aims to reduce environmental impacts with third party audits of a company's cyanide management.
References
- ^ ISBN 978-3527306732.
- ISBN 978-0471238966.
- ^ "Cyanide Management" (PDF). Australian Government.
- S2CID 240128866.
- ^ .
- ^ US 403202, MacArthur, John Stewart; Forrest, William & Forrest Robert, Robert, "Process of Obtaining Gold and Silver from Ores", published 1889-05-14
- ^ "Methods to recover Gold II". 2013-05-14.
- ^ a b Habashi, Fathi Recent Advances in Gold Metallurgy Archived 2008-03-30 at the Wayback Machine
- ^ The alumni quarterly and fortnightly notes. University of Illinois. January 1, 1921. Retrieved May 1, 2016.
- ^ "Mercur, UT". Retrieved May 1, 2016.
- ISSN 0167-4528.
- ISBN 0-7506-3365-4.
- ^ "Technical Bulletin 1" (PDF). Multi Mix Systems. Archived from the original (PDF) on 2009-10-23.
- .
- doi:10.1016/j.mineng.2013.01.008. Retrieved 2 May 2021.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - ^ a b UNEP/OCHA Environment Unit "UN assessment mission – Cyanide Spill at Baia Mare, March 2000".
- PMID 15819225.
- .
- ISBN 90-5809-353-0, pp. 197 (Google Books).
- ^ BBC News, BBC: "Cyanide seeps into PNG rivers", March 23, 2000.
- ^ Wilson, T. E. La politica es la politica: "After cyanide spill, can First Majestic clean up its act?" April 21, 2018.
- .
- ^ "Glycine lixiviants". Mining and Process Solutions. Retrieved 23 April 2021.
- ^ The Citizens Initiative banning of cyanide mining in the State of Montana, US Archived October 21, 2007, at the Wayback Machine
- ^ 2001 Senate Bill 160 Archived 2006-10-10 at the Wayback Machine regarding the use of cyanide in mining.
- ^ "Czech Senate bans use of cyanide in gold mining". Nl.newsbank.com. 2000-08-10. Retrieved 2013-01-03.
- ^ Zöld siker: törvényi tilalom a cianidos bányászatra! Archived July 21, 2011, at the Wayback Machine
- ^ International Mining - European Commission rejects proposed ban on using cyanide in extractivism|extractive industry, July, 2010
- ^ Council Directive 96/82/EC of 9 December 1996 on the control of major-accident hazards involving dangerous substances. For the modifications see the consolidated version.
- ^ Council Directive 82/501/EEC of 24 June 1982 on the major-accident hazards of certain industrial activities. Not in force.
- ^ Council Directive 80/68/EEC of 17 December 1979 on the protection of groundwater against pollution caused by certain dangerous substances. Not in force.
- ^ Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy (the Water Framework Directive). For the modifications see the consolidated version.
- ^ Directive 2006/21/EC of the European Parliament and of the Council of 15 March 2006 on the management of waste from extractive industries. For the modifications see the consolidated version.
- ^ ICMI cyanidecode.org International Cyanide Management Code For The Manufacture, Transport, and Use of Cyanide In The Production of Gold
External links
- Efforts at a cleaner process
- Yestech A different commercial method that does not use toxic cyanide
- Cyanide Uncertainties (PDF)
- How gold is extracted by cyanidation process