Hubbry Logo
Gold cyanidationGold cyanidationMain
Open search
Gold cyanidation
Community hub
Gold cyanidation
logo
7 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Gold cyanidation
Gold cyanidation
Gold cyanidation
View on Wikipedia
from Wikipedia

Gold cyanidation (also known as the cyanide process or the MacArthur–Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore through conversion to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction.[1] Cyanidation is also widely used in silver extraction, usually after froth flotation.[2]

Production of reagents for mineral processing to recover gold represents 70% of cyanide consumption globally. While other metals, such as copper, zinc, and silver, are also recovered using cyanide, gold remains the primary driver of this technology. [1] The highly toxic nature of cyanide has led to controversy regarding its use in gold mining, with it being banned in some parts of the world. However, when used with appropriate safety measures, cyanide can be safely employed in gold extraction processes.[3] One critical factor in its safe use is maintaining an alkaline pH level above 10.5, which is typically controlled using lime in industrial-scale operations. Lime plays an essential role in gold processing, ensuring that the pH remains at the correct level to mitigate risks associated with cyanide use.[4]

History

[edit]

In 1783, Carl Wilhelm Scheele discovered that gold dissolved in aqueous solutions of cyanide. Through the work of Bagration (1844), Elsner (1846), and Faraday (1847), it was determined that each gold atom required two cyanide ions, i.e. the stoichiometry of the soluble compound.

Industrial process

[edit]
John Stewart MacArthur developed the cyanide process for gold extraction in 1887.

The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits 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 Gilbert S. Peyton had refined the process at his Mercur Mine in Utah, "the first mining plant in the United States to make a commercial success of the cyanide process on gold ores."[9][10] 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.[8] Around 1900, the American metallurgist Charles Washington Merrill (1869–1956) and his engineer Thomas Bennett Crowe improved the treatment of the cyanide leachate, by using vacuum and zinc dust. Their process is the Merrill–Crowe process.[11]

Chemical reactions

[edit]
Ball-and-stick model of the aurocyanide or dicyanoaurate(I) complex anion, [Au(CN)2]−[12]
Cyanide leaching "heap" at a gold mining operation near Elko, Nevada

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 dicyanoaurate.[13] from which it can be recovered by adsorption onto activated carbon.[14]

Application

[edit]

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

[edit]

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

[edit]

In some ores, particularly those that are partially sulfidized, 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, therefore making the gold cyanidation process more efficient. Specifically, the oxidation of iron to iron (III) oxide and subsequent precipitation as iron hydroxide minimizes loss of cyanide from the formation of ferrous cyanide complexes. The oxidation of sulfur compounds to sulfate ions avoids the consumption of cyanide to thiocyanate (SCN−) byproduct.

Recovery of gold from cyanide solutions

[edit]

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

[edit]

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

[edit]
See also: List of gold mining disasters
Sodium cyanide drum at the abandoned Chemung Mine in Masonic, California

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. Many such infamous 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 could be washed out of river systems and, as long as organisms can migrate from unpolluted areas upstream, affected areas can soon be repopulated. Longer term impact and accumulation of cyanide in riparian or limnological benthos and environmental fate is less clear. According to Romanian authorities, in the Someș river below Baia Mare, the planktons returned to 60% of normal within 16 days of the spill; the numbers were not confirmed by Hungary or Yugoslavia.[16] Other infamous 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]
2024 Eagle Mine Canada (Yukon) Heap Leach failure, cyanide spill into surrounding waterways, ecological damage and company bankruptcy

Such spills have prompted fierce protests at new mines that involve use of cyanide, such as Roşia Montană in Romania, Lake Cowal in Australia, Pascua Lama in Chile, and Bukit Koman in Malaysia.

Alternatives to cyanide

[edit]

Although cyanide is cheap, effective, and biodegradable, its high toxicity and increasingly poor impact on a mine's political and social license to operate, has incentivized 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, although some chlorination process using sodium hypochlorite (household bleach) have shown promise in terms of reagent regeneration. These technologies are at a pre-commercialisation stage and compare favourably to equivalent cyanidation methods, including gold recovery percentage. Thiourea has been implemented commercially for ores containing stibnite.[22] Yet another alternative to cyanidation is the family of glycine-based lixiviants.[23]

Legislation

[edit]
See also: Gold cyanidation ban

The US states of Montana[24] and Wisconsin,[25] the Czech Republic,[26] Hungary,[27] have banned cyanide mining. The European Commission rejected a proposal for such a ban, noting that existing regulations (see below) provide adequate environmental and health protection.[28] Several attempts to ban gold cyanidation in Romania were rejected by the Romanian Parliament. There are currently protests in Romania calling for a ban on the use of cyanide in mining (see 2013 Romanian protests against the Roșia Montană Project).

In the EU, industrial use of hazardous chemicals is controlled by the so-called Seveso II Directive (Directive 96/82/EC,[29] which replaced the original Seveso Directive (82/501/EEC[30] 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 (Directive 80/68/EEC)[31] 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).[32]

In response to the 2000 Baia Mare cyanide spill, the European Parliament and the Council adopted Directive 2006/21/EC on the management of waste from extractive industries.[33] 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 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

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Gold cyanidation

View on Grokipedia
from Grokipedia
Gold cyanidation is a hydrometallurgical leaching process for extracting gold from low-grade ores using dilute aqueous solutions of sodium cyanide (typically 100–500 ppm), which dissolves metallic gold in the presence of oxygen to form the stable, water-soluble complex [Au(CN)₂]⁻. Patented in 1887 by Scottish chemist John Stewart MacArthur and brothers Robert and William Forrest, the MacArthur-Forrest process revolutionized gold mining by enabling efficient recovery from refractory and disseminated ores that were impractical for traditional amalgamation or smelting methods. Employed in variants such as heap leaching and carbon-in-leach milling, it achieves gold recovery rates often exceeding 90 percent, making it the predominant technique responsible for the majority of global gold production since the late 20th century. While cyanide's natural degradation into non-toxic compounds like ammonia and carbon dioxide under aerobic conditions facilitates environmental management, the process has faced controversies over accidental releases causing acute toxicity to wildlife and ecosystems, prompting adoption of rigorous protocols under frameworks like the International Cyanide Management Code to mitigate risks.

History

Early discoveries and patents


The solubility of gold in aqueous cyanide solutions was first experimentally demonstrated in the early 19th century, with chemists such as F. W. Elsner confirming in 1846 that gold dissolves in potassium cyanide under oxidizing conditions to form the soluble complex [Au(CN)₂]⁻. However, these observations did not lead to a practical extraction method due to the slow dissolution kinetics and lack of efficient recovery techniques. Practical development began in the 1880s amid declining yields from traditional mercury amalgamation in gold mining, particularly in regions like New Zealand and South Africa. Scottish chemist John Stewart MacArthur, collaborating with brothers Dr. Robert Williams Forrest and William Forrest, conducted systematic experiments in starting around 1884 to address these limitations. Their approach involved treating finely crushed gold-bearing ore with dilute alkaline cyanide solutions (typically 0.1-0.3% sodium or ) under aerobic conditions to dissolve gold selectively, followed by precipitation using shavings in the MacArthur-Forrest process. This innovation achieved extraction efficiencies up to 90% from refractory ores previously uneconomical. MacArthur filed for patents in 1887, securing British Patent No. 14,174 on October 7, 1887, titled "Process of Obtaining and Silver from Ores," which described the core method of cyanidation leaching and zinc . Equivalent patents were granted internationally, including U.S. 403,202 in 1889, validating the process's novelty despite claims. These patents spurred rapid adoption, with the first commercial plant operational at the Crown Mines in , [South Africa](/page/South Africa), by 1890, though legal challenges like the 1896 "Great Cyanide Case" tested their validity against earlier cyanide uses in and minor extractions.

Industrial adoption and scaling

The MacArthur-Forrest cyanide process saw its first commercial implementation in late 1890 at a plant operated by the Gold Recovery Syndicate in the region of , where it treated approximately 10,000 tons of gold-bearing per month. This initial application targeted milling residues that prior methods, such as chlorination, had failed to process efficiently due to the nature of unoxidized pyritic ores prevalent in the deep-level reefs. Recovery rates improved markedly, from around 50-60% under traditional amalgamation and techniques to over 90%, as the process dissolved gold into soluble aurocyanide complexes amenable to . Rapid scaling followed in the Witwatersrand Basin, where the process averted industry stagnation by enabling the economic extraction from lower-grade and deeper ores that milling alone could not sustain. By 1892, multiple Rand mines had installed plants, transitioning from batch to continuous vat leaching operations that handled thousands of tons daily; this expansion correlated with a rebound in Transvaal gold output, rising from roughly 1.2 million ounces in 1890 to over 3 million ounces annually by the early 1900s. Innovations such as filter presses for and agitation tanks for faster dissolution further facilitated larger-scale throughput, reducing processing times from weeks to days and minimizing consumption through optimized solution strengths (typically 0.05-0.1% NaCN). Global adoption accelerated in the decade following, with Australia's first commercial cyanide plant commencing operations in 1892 at the Excelsior Mill in , , where it similarly boosted yields from refractory quartz ores. By 1900, the technology had diffused to North American operations, including early adaptations in Utah's Mercur district, marking the onset of widespread hydrometallurgical dominance over pyrometallurgical alternatives. Scaling continued through the early with plant capacities expanding to process millions of tons yearly, underpinned by supply chain developments for bulk production and engineering refinements like multi-stage countercurrent leaching to enhance and reduce reagent costs. This proliferation transformed from labor-intensive, high-grade exploitation to industrialized, low-grade ore treatment, sustaining output amid depleting surface deposits.

Chemical Principles

Core reactions in gold dissolution

The dissolution of in aerated alkaline cyanide solutions forms the water-soluble dicyanoaurate(I) anion, enabling selective extraction from ores. The overall stoichiometry, known as Elsner's equation, is represented as: 4Au+8CN−+O2+2H2O→4[Au(CN)2]−+4OH−4 \mathrm{Au} + 8 \mathrm{CN}^- + \mathrm{O_2} + 2 \mathrm{H_2O} \rightarrow 4 [\mathrm{Au(CN)_2}]^- + 4 \mathrm{OH}^- This reaction consumes one mole of oxygen and two moles of water per four moles of dissolved, producing four moles of hydroxide ions that maintain the solution's alkalinity. The process operates electrochemically on the gold surface, where anodic oxidation of gold couples with cathodic reduction of dissolved oxygen. The anodic half-reaction involves gold complexation by cyanide: Au+2CN−→[Au(CN)2]−+e−\mathrm{Au} + 2 \mathrm{CN}^- \rightarrow [\mathrm{Au(CN)_2}]^- + e^- The cathodic half-reaction is: 12O2+H2O+2e−→2OH−\frac{1}{2} \mathrm{O_2} + \mathrm{H_2O} + 2 e^- \rightarrow 2 \mathrm{OH}^- Four anodic reactions balance with two cathodic reactions to yield the net Elsner stoichiometry, with the mixed potential at the interface driving dissolution under diffusion control by oxygen or cyanide supply. This mechanism explains the dependence on solution aeration and cyanide concentration, as oxygen acts solely as the oxidant without direct participation in gold complexation.

Influencing factors like oxygen and pH

Oxygen serves as an essential oxidant in the cyanidation process, facilitating the dissolution of gold through the reaction 4 Au + 8 CN⁻ + O₂ + 2 H₂O → 4 [Au(CN)₂]⁻ + 4 OH⁻, where its consumption directly limits the rate of gold leaching in oxygen-deficient conditions. Insufficient dissolved oxygen (DO) levels below 4 mg/L can significantly constrain gold extraction kinetics, while maintaining higher concentrations, such as 5–6 mg/L or up to 12 ppm via enhancements like hydrogen peroxide addition, accelerates dissolution and reduces cyanide requirements. In practice, pre-oxidation of sulfide-bearing ores with pure oxygen mitigates inhibitory effects from minerals like pyrite, enabling more efficient subsequent cyanidation by oxidizing reactive species beforehand. The of the leaching solution profoundly influences speciation and process efficacy, with alkaline conditions (typically 10–11) favoring the predominance of free ions (CN⁻) over volatile hydrogen (HCN), thereby minimizing toxic gas evolution and loss. At values above 10.5, dissolution rates improve due to reduced OH⁻ adsorption on surfaces and lower consumption, often achieved by adding lime (Ca(OH)₂) to buffer against acidification from reactions. Deviations to lower increase HCN formation, elevating consumption by the pulp and risks from gaseous emissions, though atmospheric losses remain relatively stable; conversely, excessively high may inhibit leaching through surface passivation. These factors interact synergistically, as oxygen addition during cyanidation can generate local pH increases via production, necessitating monitoring and adjustment to sustain optimal CN⁻/O₂ ratios for maximal recovery. In refractory ores, elevated DO combined with controlled during pre-oxidation phases has been shown to enhance overall extraction by countering passivation.

Process Description

Ore preparation and cyanidation leaching

Ore preparation begins with primary crushing of run-of-mine using or gyratory crushers to reduce to approximately 100-200 mm, followed by secondary and tertiary crushing with cone or impact crushers to achieve a size of 5-20 mm. This exposes particles encapsulated within the matrix, enhancing subsequent liberation. Grinding then occurs in or SAG mills, typically reducing the to a fine pulp with 80% passing 75-150 μm (P80), depending on type and , to maximize surface area for contact while balancing energy costs and over-grinding risks. density is adjusted to 30-50% solids post-grinding, often via with hydrocyclones to recycle oversize material. Cyanidation leaching involves forming a pulp from the prepared ore and adding a dilute alkaline sodium cyanide (NaCN) solution, typically 0.01-0.05% w/v cyanide concentration, in agitated tanks or vats. The process relies on the Elsner equation, where gold dissolves as Au(CN)₂⁻ in the presence of oxygen: 4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH. Leaching occurs under controlled conditions: pH 10-11 maintained with lime to prevent cyanide decomposition to toxic HCN gas, dissolved oxygen levels of 5-10 ppm via air sparging or pure oxygen injection, and temperatures of 20-50°C to optimize kinetics without excessive reagent loss. Retention times vary from 12-72 hours in multi-stage counter-current tanks, achieving 80-95% gold extraction for free-milling ores, with pulp agitation ensuring uniform contact. Lead nitrate or other accelerators may be added to mitigate passivation by sulfides, while copper and other impurities can consume cyanide, necessitating higher dosages or pre-treatments. The resulting pregnant leach solution (PLS) contains solubilized gold, separated from barren tailings via thickening and filtration.

Gold recovery from pregnant solutions

The pregnant solution in gold cyanidation, also known as the leachate or eluate, contains dissolved gold primarily in the form of the aurocyanide complex [Au(CN)2]-, typically at concentrations of 0.001 to 0.01 g/L gold, alongside silver and impurities like copper and iron cyanides. Recovery from this solution aims to selectively extract and concentrate the gold for final refining, with overall recovery efficiencies often exceeding 95% in optimized operations. The two predominant industrial methods are adsorption onto activated carbon and zinc cementation via the Merrill-Crowe process, chosen based on solution clarity, impurity levels, and silver content. ![Dicyanoaurate(I)][inline] adsorption, employed in carbon-in-pulp (CIP) and carbon-in-leach (CIL) variants, is the most widely used technique due to its selectivity for the -cyanide complex over cyanides. In CIP, the pregnant pulp from leaching is contacted countercurrently with granular (typically coconut shell-based, 6x12 or 8x16 mesh size) in a series of agitated tanks or columns, where gold loading capacities reach 1,000 to 4,000 g/t carbon under alkaline conditions ( 10-11). The loaded carbon is then separated, screened, and subjected to using hot (110-140°C) caustic cyanide solution under pressure (Zadra or AARL processes), desorbing over 95% of the gold; the resulting rich eluate undergoes in stainless steel wool cathodes at 2-4 V and 50-100 A/m² to deposit high-purity (99.5%+). CIL integrates adsorption during leaching, reducing equipment needs but risking higher carbon abrasion. This method dominates heap leach and agitated tank operations for low-grade ores, though carbon fouling by organics or silica can lower efficiency, necessitating regeneration via acid washing or thermal reactivation at 650-750°C. The Merrill-Crowe process, favored for pregnant solutions with low (<10 ppm after clarification) and high silver-to-gold ratios, involves dust cementation following solution deaeration to prevent passivation. The clarified, oxygen-depleted solution (DO <0.5 ppm) is reacted with fine powder (95% <10 μm) at 20-50 g/m³ dosage in agitated vessels, displacing via the reaction: Zn + 2[Au(CN)2]- → 2Au + [Zn(CN)4]2-, yielding a doré precipitate of 90%+ precious metals. Filtration through precoat pressure filters (e.g., leaf or plate-and-frame) recovers the precipitate, which is then retorted to remove mercury and fluxed for into . Barren solution is sampled via a continuous automatic system using a solenoid valve controlled by a timer for periodic aliquots; operators inspect for clarity to detect filtration issues; composite samples are taken proportional to flow or time. This method achieves >99% recovery for silver-rich solutions but is less selective for high-copper liquors, requiring prior cementation or solvent extraction to avoid overconsumption (up to 1.2-1.5 times stoichiometric). Vacuum deaeration towers ensure process reliability, minimizing oxygen interference that could reduce yields by 5-10%. Alternative or supplementary techniques include ion-exchange resins for selective gold loading in copper-contaminated solutions and direct electrowinning for high-grade eluates (>1 g/L Au), though these are less common due to higher energy costs or lower selectivity. Post-recovery barren solutions are typically recycled to leaching after cyanide detoxification via oxidation (e.g., SO2/air or hydrogen peroxide) to below 50 ppm free cyanide, supporting closed-circuit efficiency. Overall, method selection balances capital costs, with carbon systems suiting turbid pulps and Merrill-Crowe clearer flowsheets, contributing to global gold production where cyanidation recovers over 90% of mined gold.

Industrial Applications and Variations

Heap leaching versus tank leaching

Heap leaching involves stacking crushed on impermeable pads and percolating a dilute solution through the heap to dissolve , with the pregnant solution collected at the base for further processing. This method suits low-grade ores, typically with head grades below 1-2 g/t Au, as it minimizes preprocessing costs by using coarser particle sizes, often 5-20 mm. In contrast, tank leaching, also known as agitation leaching, requires grinding to finer sizes, usually below 75-150 μm, and mixing it into a in agitated tanks where solution contacts the under mechanical stirring to enhance dissolution kinetics. The primary distinction lies in ore preparation and contact efficiency: heap leaching relies on gravity percolation, which limits solution channeling and uneven wetting, leading to recovery rates of 50-85% for , averaging around 70% in operating facilities. Tank leaching achieves higher recoveries of 85-95% due to intimate mixing and better control of variables like oxygen supply and , making it preferable for higher-grade ores or those requiring finer liberation. However, heap leaching offers lower capital and operating costs, with expenses per ounce of as low as $500-800, compared to $900-1,500 for tank-based methods, primarily because it avoids energy-intensive grinding and agitation.
AspectHeap LeachingTank Leaching
Ore Grade SuitabilityLow-grade (<1-2 g/t Au)Higher-grade or complex ores
Particle SizeCoarse (5-20 mm)Fine (<75-150 μm)
Recovery Rate50-85% (avg. 70%)85-95%
Processing TimeMonths (slower kinetics)Days to weeks (faster)
Capital CostLower (no mills/tanks)Higher (grinding mills, agitators)
Operating Cost$500-800/oz Au$900-1,500/oz Au
Heap leaching processes larger volumes of ore at lower unit costs, enabling economic extraction from marginal deposits, but it demands vast land areas and faces risks of incomplete leaching from poor permeability. Tank leaching provides more consistent results and adaptability to variable ore types through staged countercurrent washing, though it incurs higher upfront investments and energy use for pulp handling. Selection between methods depends on ore mineralogy, grade, and site economics; for instance, heap leaching dominates in operations like Nevada's Carlin Trend for oxide ores, while tank leaching prevails for sulfidic or refractory materials post-oxidation.

Adaptations for refractory and complex ores

Refractory gold ores, characterized by gold recovery rates below 80% via direct cyanidation, arise primarily from submicroscopic gold particles encapsulated within sulfide minerals such as (FeS₂) or (FeAsS), or from preg-robbing effects by carbonaceous matter that adsorbs dissolved gold complexes. These ores necessitate pretreatment to liberate gold particles or mitigate interference, enabling subsequent cyanidation to achieve recoveries exceeding 90% in many cases. Common pretreatments oxidize sulfides to sulfates or elemental sulfur, disrupting the matrix without excessive cyanide consumption during leaching. Pressure oxidation (POX), conducted in autoclaves at temperatures of 180–225°C and oxygen pressures of 500–1000 kPa, effectively decomposes sulfide lattices, as demonstrated in operations like the Porgera mine where it boosted overall gold recovery to over 90%. Bio-oxidation employs acidophilic bacteria such as Acidithiobacillus ferrooxidans to catalyze sulfide oxidation under ambient conditions, with processes like BIOX® applied at sites including the Fairview mine in South Africa, yielding up to 95% gold extraction post-treatment after 4–6 days of bacterial action on pyrite-arsenopyrite concentrates. Roasting, a thermal method heating ores to 500–700°C in air, converts sulfides to porous oxides but has declined due to sulfur dioxide emissions; in Zimbabwean operations, it historically achieved 75% recovery before cyanidation on refractory sulfides. Emerging chemical pretreatments address specific refractory types, such as ozonation, which generates reactive oxygen species to oxidize sulfides and carbonaceous matter, improving cyanidation recovery by 20–30% in sulfide ores without high-energy inputs. Persulfate-based advanced oxidation, often with catalysts, breaks down sulfide crystals via free radicals, enhancing gold accessibility for cyanidation in arsenical ores. Ultrafine grinding to particle sizes below 10 μm mechanically exposes encapsulated gold, though it increases energy costs and requires fine-tuning to avoid over-grinding. Complex ores, often overlapping with refractory types due to high base metal content (e.g., copper exceeding 0.5%), pose challenges as copper forms stable cyanide complexes like [Cu(CN)₂]⁻, consuming up to 50 kg/t of cyanide and reducing gold leaching efficiency. The SART process—sulfidization with sodium sulfide to precipitate copper as Cu₂S, followed by acidification to regenerate free cyanide and thickening for recycle—recovers 90–95% of consumed cyanide while producing a copper sulfide byproduct for sale, as implemented in Chilean copper-gold operations. For telluride-rich complex ores, nitric acid pretreatment oxidizes tellurium matrices, elevating cyanidation recovery from 58% to over 90% in 24 hours at moderate concentrations (e.g., 1–2 M HNO₃). These adaptations maintain cyanidation's viability by minimizing reagent losses and targeting causal barriers like mineral locking or competitive complexation.

Economic and Technological Advantages

Efficiency in low-grade ore extraction

Gold cyanidation has enabled the economic extraction of gold from low-grade ores, typically containing less than 1-3 grams of gold per metric ton, which were previously uneconomical using methods like amalgamation or chlorination. The process achieves recovery rates exceeding 90% from ores as low as 0.5 g/t through selective dissolution of gold into a stable cyanide complex, minimizing losses to gangue materials. This efficiency stems from the low reagent consumption and ability to process vast tonnages, with cyanide costs remaining viable even at dilute concentrations of 250-500 ppm for heap operations. Heap leaching, a variant of cyanidation tailored for low-grade deposits, involves stacking crushed ore (20-50 mm particles) on impermeable liners and percolating dilute cyanide solution through the pile, recovering 60-80% of gold in 6-7 weeks for grades around 0.9 g/t. This method's capital and operating costs are significantly lower than milling, allowing operations on ores with cutoff grades as low as 0.3 g/t, expanding viable reserves by factors of 10 or more compared to pre-cyanide era techniques. For instance, since its adoption in the 1970s, heap leaching has driven U.S. gold production increases attributable to processing low-grade oxide ores uneconomic otherwise. Factors enhancing efficiency include optimal particle size reduction to increase surface area and cyanide penetration, with finer grinding (d80=63 µm) yielding up to 95% recovery under controlled pH (10.5) and cyanide dosing. However, preg-robbing minerals or fine clays can reduce percolation rates, necessitating additives like lime or flocculants to maintain solution flow and extraction yields. Overall, cyanidation's scalability for low-grade ores has underpinned global gold supply growth, with heap methods processing millions of tons annually at costs under $300 per ounce for suitable deposits.

Contributions to global gold supply

Gold cyanidation accounts for the predominant share of global gold mine production, with estimates indicating that more than 90 percent of annual output derives from this method. In 2024, worldwide gold mine production reached approximately 3,661 tonnes, the majority extracted via cyanide leaching processes such as carbon-in-leach, carbon-in-pulp, and applied to low-grade ores. This dominance persists due to the process's ability to recover gold economically from disseminated deposits containing concentrations as low as 0.5 to 1 gram per tonne, which were previously uneconomical with amalgamation or chlorination methods. The introduction of cyanidation in the late 1880s revolutionized gold extraction by enabling the processing of vast low-grade oxide ores, leading to a marked expansion in global supply. Prior to widespread adoption around 1890, annual production hovered below 200 tonnes, largely from high-grade placer and vein deposits treatable by mercury amalgamation. Following commercial implementation, particularly at the mines in South Africa, output surged; by the early 1900s, South African production alone exceeded 300 tonnes annually, contributing over half of the world's total and driving cumulative 20th-century increases to levels sustaining modern reserves. This shift not only multiplied recoverable resources but also facilitated the industry's transition to large-scale operations, with cyanidation's selectivity for gold over base metals minimizing interference in complex ores. By unlocking low-grade resources that form the bulk of identified global reserves—estimated at over 50,000 tonnes—cyanidation has sustained production amid declining high-grade discoveries. Alternatives like or thiosulfate processes remain marginal, comprising less than 5 percent of output, underscoring cyanidation's role in maintaining supply stability despite environmental and regulatory pressures. Ongoing optimizations, including intensified leaching and cyanide recycling, continue to enhance recovery rates from 70-80 percent in traditional setups to over 90 percent in advanced facilities, further bolstering efficiency without proportionally increasing reagent use.

Health, Safety, and Operational Risks

Cyanide toxicity mechanisms

Cyanide ions (CN⁻) primarily exert toxicity by reversibly binding to the ferric iron (Fe³⁺) in the heme a₃ site of cytochrome c oxidase (complex IV) within the mitochondrial electron transport chain. This binding inhibits the enzyme's ability to transfer electrons to molecular oxygen, halting oxidative phosphorylation and preventing ATP synthesis despite adequate tissue oxygenation, a condition termed histotoxic hypoxia. The inhibition occurs at the binuclear center involving heme a₃ and Cu_B, where CN⁻ competes with oxygen for the catalytic site, leading to a rapid cessation of proton pumping and collapse of the mitochondrial membrane potential. At the cellular level, this blockade causes accumulation of reduced electron carriers (NADH and FADH₂), shifting metabolism to anaerobic glycolysis, which results in lactic acidosis and depletion of cellular energy reserves. Tissues with high oxygen demand, such as the brain and myocardium, are most vulnerable, manifesting as central nervous system depression, seizures, and cardiac arrhythmias within minutes of acute exposure. Secondary effects include reactive oxygen species generation from stalled electron flow and potential inhibition of other heme- or copper-containing enzymes, though cytochrome c oxidase remains the dominant target at lethal concentrations exceeding 1 mg/kg body weight. In industrial settings like gold cyanidation, where sodium cyanide (NaCN) dissociates to release CN⁻ in aqueous solutions, toxicity can occur via dermal absorption of solutions (penetrating disrupted skin at rates up to 50 mg/m²/hour) or inhalation of hydrogen cyanide (HCN) gas formed under acidic conditions. Once absorbed, free CN⁻ distributes rapidly due to its small size and lipophilicity as HCN (pKa 9.2), crossing cell membranes and concentrating in mitochondria; detoxification via rhodanese enzyme converts CN⁻ to thiocyanate (SCN⁻) for renal excretion, but this pathway saturates at high doses, prolonging toxicity. Chronic low-level exposure may additionally impair thyroid function by inhibiting iodide peroxidase, though acute mechanisms predominate in occupational risks.

Incident data and safety protocols

Significant cyanide release incidents in gold cyanidation have primarily resulted from tailings dam failures, pipeline ruptures, or transportation accidents, leading to acute ecological damage such as mass fish kills and temporary contamination of waterways. In August 1995, a tailings dam breach at the Omai gold mine in Guyana released approximately 400 million gallons of cyanide-laden effluent into the Omai and Essequibo rivers, devastating aquatic life along affected stretches and prompting temporary drinking water advisories for nearby communities, though no direct human fatalities were reported. Similarly, on January 30, 2000, a tailings pond dam failure at the Aurul Baia Mare mine in Romania discharged about 100,000 cubic meters of cyanide- and heavy metal-contaminated slurry into the Someş, Tisza, and Danube rivers, killing an estimated 200 tons of fish across multiple species and rendering water supplies undrinkable for over 2.5 million people in downstream areas for weeks. In September 2015, a valve failure at the Veladero mine in Argentina spilled over 1 million liters of cyanide solution into rivers in the Jáchal basin, with subsequent leaks in 2017 exacerbating contamination; these events prompted operational suspensions and highlighted risks from inadequate valve maintenance in high-altitude leaching systems. Empirical data on overall incident rates remain sparse due to inconsistent global reporting, but analyses indicate that pre-2000 events were more frequent in regions with lax regulation, often involving structural failures in tailings storage facilities. For instance, U.S. Geological Survey assessments note that while cyanide consumption in North American gold mining exceeded 78,000 metric tons annually in the late 1980s, wildlife mortality incidents were recurrent until enhanced containment measures reduced exposures below weak-acid-dissociable thresholds of 50 mg/L deemed safe for discharge. Post-implementation of voluntary standards, certified operations report minimal releases; the International Cyanide Management Institute (ICMI) documented only one significant incident among signatories in 2021, involving maritime transport rather than on-site operations. Audits of compliant mines, such as those by and , confirm zero major cyanide exposures or environmental releases over multi-year periods, attributing this to rigorous monitoring and remediation. Safety protocols for cyanide in gold cyanidation emphasize prevention through engineering controls, operational discipline, and rapid response, with the ICMI's International Cyanide Management Code serving as the primary voluntary framework since 2000, adopted by over 50 gold mining operations worldwide. The Code mandates standards across production, transport, handling, and use, including risk-based facility design with double-lined containment, leak detection systems, and seismic-resistant tailings dams to avert breaches like those at Omai and . Worker protections require personal protective equipment, hydrogen cyanide gas monitoring (maintaining levels below 10 ppm), and training to prevent reactions generating toxic HCN, such as acid-cyanide mixing. Operations must implement emergency response plans, including neutralization with hydrogen peroxide or hypochlorite, and independent third-party audits every three years verify compliance, correlating with reduced incident rates in participating facilities. Decommissioning protocols ensure cyanide destruction to below 0.2 mg/L total before site closure, minimizing long-term liabilities.

Environmental Impacts

Empirical evidence of ecological effects

Empirical studies demonstrate that free cyanide concentrations exceeding 20 μg/L cause high mortality in freshwater fish, with adverse effects on swimming behavior and reproduction observed at levels above 5 μg/L. Accidental releases of cyanide solutions into rivers and streams have resulted in massive fish kills and broader aquatic biota losses, as cyanide rapidly inhibits cellular respiration by binding to cytochrome c oxidase. Sensitivity varies by species, but salmonids and other freshwater fish exhibit lethal responses at low parts-per-billion concentrations, underscoring cyanide's acute toxicity in surface waters near mining operations. Terrestrial wildlife, particularly migratory waterfowl and bats, face significant hazards from open tailings impoundments and heap leach ponds containing cyanide solutions. In Nevada gold mines from 1990 to 1991, over 9,500 wildlife carcasses were documented across more than 100 species, with 80-91% comprising birds attracted to untreated ponds. Similarly, at the Northparkes mine in Australia in 1995, initial estimates of 100 bird deaths escalated to 2,700 over four months due to repeated exposure to weak acid dissociable (WAD) cyanide. Copper-gold ores exacerbate risks through formation of highly toxic copper-cyanide complexes, which enhance lethality to avian and mammalian species even at concentrations below 50 mg/L WAD cyanide. Long-term monitoring reveals persistent cyanide residues in mine wastes, contributing to ongoing ecological risks. At the abandoned Remance mine in Panama, 20 years after cyanidation ceased, total cyanide levels in tailings ranged from 25.2 to 518 mg/kg, with broader waste areas showing 1.4-1.9 mg/kg—exceeding typical regulatory thresholds for gold mining tailings. Associated potentially toxic elements (PTEs) like arsenic, copper, antimony, and mercury elevated pollution load indices, indicating serious risks to surrounding soils, biota, and groundwater. These findings highlight incomplete natural attenuation of cyanide species, such as stable complexes that dissociate under environmental fluctuations, potentially mobilizing toxins into ecosystems over decades.

Case studies of spills and long-term monitoring

On January 30, 2000, a tailings dam failure at the Aurul Baia Mare gold mine in Romania released approximately 100,000 cubic meters of cyanide-laden wastewater into the Someș River, which flowed into the Tisza and Danube rivers, causing widespread fish kills estimated at over 1,000 tons in Hungary alone and disrupting water supplies for millions. The spill's cyanide concentrations reached up to 700 mg/L initially in affected tributaries, exceeding toxicity thresholds for aquatic life, though dilution and photodegradation reduced free cyanide levels to below 0.1 mg/L within weeks in the Danube. In August 1995, the Omai gold mine in Guyana experienced a tailings dam breach that discharged over 3.7 million cubic meters (about 1 billion gallons) of cyanide-contaminated slurry into the Essequibo River system, resulting in immediate fish mortality across 40 km of river and temporary shutdowns of downstream water intakes. Cyanide levels peaked at 16.56 mg/L near the spill site but declined rapidly due to volatilization and oxidation, with no human cyanide poisoning reported despite exposure risks to riparian communities. The Summitville Mine in Colorado, operating from 1985 to 1992, suffered multiple cyanide leaks from its heap leach pads, contaminating surface and groundwater with concentrations up to 0.5% sodium cyanide solution, contributing to its designation as a Superfund site in 1994 with remediation costs exceeding $100 million borne largely by taxpayers. Long-term monitoring post-Baia Mare revealed aquatic ecosystems largely recovered within 1-2 years, with benthic invertebrate diversity rebounding by 2001, though elevated heavy metals from tailings persisted in sediments, prompting ongoing EU-funded surveillance of the Tisza basin for bioaccumulation in fish. At Omai, follow-up studies through 2000 showed cyanide dissipation within months, but arsenic and mercury from tailings remained detectable in sediments, with riverine fish populations stabilizing yet exhibiting sporadic metal burdens. Summitville's post-closure oversight, continuing into the 2020s via USGS and EPA, indicates cyanide persistence limited by acidic conditions accelerating its hydrolysis, but chronic acid rock drainage sustains low-level metal leaching, necessitating perpetual water treatment. Empirical data from these sites underscore cyanide's transient nature in aerobic surface waters (half-life ~days under sunlight), contrasting with longer-term risks from associated sulfides and metals, where monitoring protocols emphasize pH, dissolved oxygen, and speciation analysis for accurate risk assessment.

Mitigation and Remediation Strategies

Cyanide destruction and detoxification methods

Cyanide destruction in gold cyanidation effluents and tailings is essential to comply with environmental regulations, which typically require weak acid dissociable (WAD) cyanide concentrations below 50 mg/L before discharge or storage. Primary methods involve chemical oxidation to convert toxic free and complexed cyanides (e.g., ferrocyanide) into less harmful cyanate (CNO⁻), which hydrolyzes to ammonia and bicarbonate under alkaline conditions. These processes target effluents from carbon-in-leach (CIL), carbon-in-pulp (CIP), or heap leaching operations, where residual cyanide levels can range from 100-500 mg/L post-leaching. The INCO SO₂/air process, developed in the early 1980s, oxidizes cyanide using sulfur dioxide gas, oxygen, and a copper catalyst in a pH range of 8-10.5. The reaction proceeds as: CN⁻ + SO₂ + O₂ → CNO⁻ + SO₄²⁻, achieving >99% destruction in 1-2 hours for slurries up to 50% solids, as demonstrated in pilot plants treating 1,200-9,500 tpd tailings. It has been implemented at operations like Equity Silver and Inca/Golden Knight mines since 1989, though it requires careful control of SO₂ addition to avoid excess acidity. Hydrogen peroxide (H₂O₂) oxidation, often catalyzed by , directly converts to via: 2CN⁻ + H₂O₂ → 2CNO⁻ + 2H⁺, effective at pH 9-11 and applicable to high-solids slurries. This method, tested since the 1970s and commercialized by firms like , destroys free and WAD cyanides in one step, with dosages of 1.5-2 moles H₂O₂ per mole CN⁻, reducing levels to <1 mg/L in 30-60 minutes. It is favored for its simplicity and lack of gaseous reagents, as used in global mining effluents, though peroxide decomposition can increase costs in organic-rich solutions. Alkaline chlorination employs hypochlorite (e.g., Ca(OCl)₂ or NaOCl) to oxidize cyanide stepwise to cyanate and then to nitrogen gas: CN⁻ + → CNO⁻ + Cl⁻, followed by further chlorination at pH >10.5. Effective for strong complex cyanides, it achieves near-complete removal but generates gas risks and higher sludge volumes, limiting its use compared to or SO₂ methods unless catalysis is insufficient. Emerging biological methods, such as microbial degradation by bacteria like species, hydrolyze cyanide to ammonia via rhodanese enzymes, but they remain pilot-scale due to slow rates (days to weeks) and sensitivity to metals in tailings. Physical-chemical hybrids, including Caro's acid (H₂SO₅ from H₂O₂ + H₂SO₄), offer rapid oxidation for recalcitrant cyanides but are costlier for routine detoxification. Selection depends on site-specific factors like cyanide speciation and flow rates, with oxidation processes dominating industrial applications for their reliability and regulatory compliance.

Tailings management and recycling innovations

Tailings from gold cyanidation processes consist primarily of finely ground ore residues containing residual weak acid dissociable (WAD) cyanide complexes, , and , necessitating secure impoundment to prevent environmental release. Innovations in include advanced and technologies that enable dry stacking, reducing the volume of wet by up to 80% and minimizing seepage risks in seismically active or high-precipitation regions. Synthetic liner systems with geomembranes and geosynthetic clay layers further limit to below 10^{-9} m/s, enhancing containment integrity beyond traditional earthen dams. Real-time sensor networks, incorporating geophysical and geochemical probes, allow for predictive monitoring of phreatic surfaces and contaminant plumes, with data integration via IoT platforms alerting operators to anomalies before breaches occur. These approaches have been validated in operations like those in , where post-2020 implementations reduced probabilities by integrating probabilistic risk assessments. Cyanide detoxification within tailings streams employs oxidative methods such as permanganate-based neutralization, which rapidly converts free and complexed to less toxic ferrocyanides at rates exceeding 99% removal in under 30 minutes under controlled conditions. For , the acidification-volatilization-regeneration (AVR) process recovers by acidifying tailings barren solution to liberate gas, which is scrubbed and reconverted to , achieving recovery efficiencies of 90-95% in commercial plants since the , though it requires careful handling of volatile HCN. A 2025 advancement from CSIRO's Sustainable Gold Cyanidation Technology integrates electrochemical regeneration to recycle directly from leach solutions, boosting recovery by 10-15% while reusing over 95% of the reagent, demonstrated in pilot trials with low-grade ores yielding economic viability at consumptions below 0.5 kg/t ore. Reprocessing innovations target residual in historical cyanidation tailings, often 0.5-2 g/t, using flotation to concentrate sulfides followed by re-leaching or gravity separation, as implemented in South African facilities recovering up to 1 t of annually from legacy dumps. Emerging sensor-based ore sorting and with cyanide-tolerant microbes enhance selectivity, reducing use by 20-30% in reprocessing circuits. recycling loops in tailings facilities, incorporating and evaporation ponds, reclaim 70-90% of process water, mitigating freshwater demands in arid districts. These technologies collectively lower long-term liability by transforming tailings from liabilities into resource streams, though site-specific dictates efficacy, with alkaline tailings resisting full without amendments.

Controversies and Debates

Environmental activism and regulatory pushback

Environmental groups such as Earthworks have campaigned against gold cyanidation through initiatives like No Dirty Gold, emphasizing risks to and ecosystems from use in operations. These efforts highlight historical spills, including those in and , where discharges contaminated waterways and prompted public outcry leading to national bans. Activism often frames cyanidation as inherently unsafe, advocating for outright prohibitions despite industry claims of effective containment, with protests targeting specific projects like Turkey's mine in 2025, where demonstrators opposed tree removal and potential . Regulatory responses have included bans in several jurisdictions driven by activist pressure and spill incidents. Hungary enacted a nationwide prohibition on cyanide in mining in January 2010 following the 2000 Baia Mare spill that released 100,000 cubic meters of cyanide-laden waste into the Tisza River, killing aquatic life across multiple countries. Montana, USA, imposed a statewide ban on cyanide heap-leach processes for open-pit gold and silver mining in 1998 via voter initiative, upheld by the state Supreme Court, prohibiting new operations except for pre-existing claims. Costa Rica suspended cyanide leaching permits in 2002, while some Argentine provinces, such as Chubut, enacted similar restrictions citing groundwater contamination risks. In , the urged a union-wide ban on technologies in 2023, referencing ongoing environmental threats despite existing directives like the 2006 Mining Waste Directive. proposals have seen mixed results; a bill to prohibit in failed in February 2023 amid industry opposition, though Appalachian Voices supported measures to ban its use in metal extraction passed by the House in 2024. Recent pushback includes Idaho's Senate Bill 1170 in April 2025, which shifted oversight from environmental agencies to lawmakers, criticized by conservationists as weakening protections against spills. Critics of these regulations, including associations, argue that voluntary codes like the International Cyanide Management Code sufficiently mitigate risks, but activist-driven policies persist in regions with high-profile contamination events.

Empirical rebuttals and risk assessments

Empirical risk assessments of gold cyanidation demonstrate that, under regulated industrial conditions, the process presents low hazards to human health due to controlled exposures and rapid . A Canadian government screening assessment concluded that risks from free and complex s in are negligible for humans, with margins of exposure exceeding thresholds for and oral pathways based on monitoring data from 2009–2018 showing median total levels of 2.5–14 µg/L. Similarly, industry analyses affirm no causal link between cyanidation and cancer, as does not bioaccumulate in food chains and degrades into non-toxic compounds like and via photolysis and microbial action. Environmental risk evaluations rebut claims of widespread persistence by highlighting cyanide's short in waters—often hours to days under and oxygenation—resulting in weak-acid-dissociable concentrations below 10 ppm in managed , well under the 50 ppm benchmark of the International Cyanide Management Code (ICMC). Adherence to ICMC protocols, including secondary and audited , has enabled safe operations for over 130 years across thousands of facilities, with regulated discharges typically at 0.2–0.5 ppm posing no acute threat to aquatic life when diluted. Quantitative assessments, such as those using predicted no-effect concentrations (PNEC) of 1.7 µg/L for chronic aquatic , indicate site-specific exceedances are rare and mitigated by engineered liners and , countering narratives of inevitable ecological devastation. Critiques of high spill frequency overlook the scale: despite approximately 20,000 tonnes of used annually in globally (only 6% of total production), major incidents number around 30 historically, often in unregulated artisanal settings rather than ICMC-compliant operations. Post-spill monitoring data from managed sites show recovery within months, with no long-term , as cyanide's binding to metals reduces free and facilitates . These findings, drawn from peer-reviewed effluent studies and code verifications, underscore that risks are context-dependent—elevated in poorly governed but empirically contained in industrial practice through barriers, monitoring, and contingency measures.

Alternatives to Cyanidation

Thiosulfate and other lixiviants

Thiosulfate leaching employs or as the primary lixiviant to dissolve from ores, typically in an alkaline solution catalyzed by (II) ions and , forming the stable gold-thiosulfate complex [Au(S₂O₃)₂]³⁻. This process operates at ambient temperatures and pressures, contrasting with cyanide's requirements, and achieves extractions of 80-95% in amenable ores after 24-48 hours, depending on pulp density and . Unlike cyanidation, thiosulfate resists preg-robbing by carbonaceous materials, enabling higher recoveries from carbonaceous ores, as demonstrated in Ethiopian gold-bearing samples yielding 91.54% extraction versus 61.70% with . The method's environmental advantages stem from thiosulfate's rapid biodegradability and low mammalian toxicity (LD50 > 5000 mg/kg for ), positioning it as a viable substitute in jurisdictions restricting use. Economically, reagent costs are comparable to when optimized, though challenges include decomposition via side reactions with oxygen, sulfides, or metals, leading to consumption rates of 1-5 kg/t and necessitating stabilizers like . recovery from pregnant solutions often uses resins or , with recent advancements in polyethylenimine-coated nanoparticles enhancing selectivity and reducing interference. Commercial adoption remains limited; Barrick Gold Corporation tested it at its operations for double-refractory ores, but full-scale implementation is rare due to higher capital for stabilization and recovery circuits. Other lixiviants include , which in acidic media ( 1-2) achieves rapid gold dissolution rates up to 90% in 2-4 hours but suffers from high reagent costs ($20-30/kg) and instability, hydrolyzing to form byproducts that foul solutions. systems, such as or leaching with oxidants like or , extract gold via anionic complexes like [AuCl₂]⁻, offering 85-95% recoveries in oxidative conditions but generating corrosive effluents and requiring robust materials, with limited industrial use beyond . Emerging options like glycine-based lixiviants, often combined with or , provide non-toxic aqueous leaching with 80-90% extractions at near-neutral , though scalability awaits further validation. These alternatives collectively address cyanide's but face barriers in reagent stability, cost, and integration with existing hydrometallurgical flowsheets.

Emerging non-chemical extraction technologies

Sensor-based ore sorting technologies represent a significant advancement in non-chemical , utilizing transmission (XRT), , or near-infrared (NIR) sensors to detect and separate gold-bearing from barren waste rock based on , color, or geochemical signatures. These methods physically reject low-grade material early in the process, increasing feed grades by up to 120% in pilot tests on deposits and reducing the volume of requiring . By avoiding chemical entirely, ore sorting minimizes water use and generation, with AI integration enabling real-time optimization and yield improvements of 15-20% in recovery. Commercial applications, such as TOMRA's sorters at operations, have demonstrated precise separation of quartz-hosted veins from host rock, enhancing overall project economics for low-grade deposits. Advanced centrifugal gravity concentrators, including models like the Knelson and , have evolved to recover ultrafine particles (down to 5-10 microns) through high-G force separation in a , eliminating the need for chemical flotation or leaching agents in free-milling s. Recent developments, such as Knelson's GX cone technology introduced in 2025, incorporate automated controls and improved riffle designs to boost recovery efficiency by up to 35% over traditional methods while reducing and enabling dry options. These devices process crushed slurries or dry feeds, concentrating via density differentials without additives, and are particularly effective for alluvial and hard-rock deposits where occurs as native particles. Empirical data from operations show these concentrators capturing over 90% of gravity-recoverable , allowing many mines to bypass cyanidation for portions of their feed and lower operational risks associated with chemical handling. Integration of these technologies often occurs in hybrid flowsheets, where ore sorting pre-concentrates material for subsequent gravity recovery, further deferring or avoiding chemical extraction for amenable ores. While limitations persist for ores locked in sulfides—requiring pre-oxidation—these non-chemical approaches have gained traction in sustainability-driven projects, with over 70% of global mines planning adoption of sensor-based sorting by late 2025. Pilot-scale validations confirm scalability, with no verifiable environmental releases of , underscoring their role in causal risk reduction compared to hydrometallurgical alternatives.

Recent Developments

Cyanide recycling advancements

Cyanide recycling in gold cyanidation processes aims to recover unreacted or weakly complexed cyanide from barren leach solutions and tailings, thereby reducing reagent consumption, operational costs, and environmental discharge. Traditional methods recover 50-70% of cyanide, but inefficiencies arise from base metal interferences like copper, which form stable complexes consuming additional cyanide. The SART (sulfidization, acidification, recycling, and thickening) process, widely adopted since the early 2000s, addresses this by sulfidizing copper-cyanide complexes to precipitate copper sulfide, followed by acidification to liberate hydrogen cyanide gas for recapture and reuse as sodium cyanide. This enables up to 90% cyanide recovery in copper-rich ores while generating a saleable copper byproduct, with implementations at operations like Gedabek mine demonstrating sustained efficiency over years of operation. Optimizations to have focused on design parameters such as control, dosage, and reactor staging to minimize loss and reagent use, with studies showing improved precipitation rates exceeding 95% under refined conditions. Recent evaluations, including two-stage variants, enhance selectivity for high-, -laden streams, reducing overall process capital by integrating and thickening more effectively. These refinements have been applied in Latin American heap leach operations, where recovers otherwise bound by dissolved , boosting yields by 10-20% in affected circuits. A significant 2025 advancement is 's Sustainable Gold Cyanidation Technology, which recovers and toxic compounds directly from cyanidation —addressing losses of soluble gold typically discarded in conventional flowsheets. Bench-scale tests over one month validated its superiority to standard destruction methods, enabling that cuts transport needs, reagent costs, and management expenses while mitigating risks like failures. At 4, the process recovers previously uneconomic gold fractions and base metals, offering dual economic and environmental gains without specified quantitative recovery rates in initial disclosures; researchers note it "surpasses the commonly practiced destruction technology" and is poised for pilot-scale trials with industry partners. This -focused approach complements solution-based like , potentially transforming waste streams in existing plants.

Hybrid and improved leaching techniques

Hybrid leaching techniques integrate cyanidation with complementary lixiviants or processes to enhance gold dissolution kinetics, reduce cyanide consumption, and address challenges in complex ores. One prominent approach combines cyanide with , an that stabilizes base metals like , minimizing cyanide decomposition while catalyzing gold leaching. In a study of pressure-oxidized residue, adding 0.3 M glycine to 100 mg/L NaCN solution achieved 85% gold recovery in 24 hours, compared to 35% without glycine; higher cyanide levels (500 mg/L) yielded 84% recovery alone but at greater reagent cost. This synergy arises from glycine's role in forming stable complexes with interfering ions, preserving free cyanide for the Elsner reaction: 4Au+8NaCN+O2+2H2O→4Na[Au(CN)2]+4NaOH4Au + 8NaCN + O_2 + 2H_2O \rightarrow 4Na[Au(CN)_2] + 4NaOH. Another hybrid variant pairs cyanidation with flotation for antimony-bearing ores, enabling sequential recovery of soluble via direct leaching followed by flotation of residual gold-antimony concentrates after cyanide destruction. At the Clarence Stream deposit, this Au-Sb cyanidation-flotation process recovered up to 98% and 84% from ores with up to 5% Sb, improving overall by an average of 15% across multiple zones compared to conventional cyanidation. Such methods mitigate passivation by sulfides or tellurides, common in deposits, by leveraging flotation's selectivity post-leach. Improved leaching techniques focus on physical and chemical intensification to accelerate and overcome barriers in the cyanide- system. Ultrasound-assisted cyanidation employs bubbles that collapse to generate microjets, cleaning particle surfaces, disrupting boundary layers, and enhancing access; at 10°C, this raised rates by 0.6–0.8% and lowered content versus stirred leaching alone. In refractory ores, ultrasonic pretreatment followed by cyanidation increased overall recovery by approximately 10%, shortening leach times due to improved pore connectivity and reduced agglomeration. Chemical enhancements, such as lead nitrate addition, catalyze oxygen reduction and prevent surface films on , boosting leach rates in cyanicide-rich ores; pre-aeration further intensifies this by saturating pulp with dissolved oxygen, essential for the anodic dissolution step. These optimizations maintain cyanidation's —responsible for over 85% of global production—while targeting 90–95% recoveries in agitated systems like carbon-in-leach (CIL), where adsorbs concurrently with leaching to minimize preg-robbing. Empirical data from intensified setups confirm reduced residence times (from 24–48 hours to 12–24 hours) and lower cyanide dosages (0.5–1 kg/t ore), driven by causal factors like enhanced convective diffusion over stagnant boundary control.

Regulation and Global Practices

International guidelines and standards

The International Cyanide Management Code (ICMC), also known as the Cyanide Code, serves as the primary voluntary international standard for the responsible management of cyanide in gold and operations. Developed in 2000 by a multi-stakeholder steering committee convened under the (UNEP) and the International Council on Metals and the Environment (now International Council on Mining and Metals), it emerged in response to environmental incidents such as the in . The code aims to enhance protections for human health and the environment by establishing best practices across the cyanide lifecycle, including production, transportation, storage, use in leaching processes, and decommissioning of facilities. The ICMC outlines specific standards of practice through 9 principles for operations, 5 for production facilities, and 3 for transportation entities. Key principles include sourcing only from verified producers, providing worker and protective , implementing emergency response plans for spills or releases, protecting from exposure (e.g., maintaining free concentrations below 50 ppm in process solutions), and ensuring and leach facilities are designed to prevent uncontrolled releases. Producers and transporters must adhere to secure handling, labeling, and documentation protocols to minimize risks during activities. These standards complement rather than replace national or local regulations, focusing on verifiable risk through , monitoring, and contingency planning. Compliance is verified through independent third-party audits conducted at least every three years, with certification granted only to operations demonstrating full adherence; non-compliance results in suspension or removal from the program. The International Cyanide Management Institute (ICMI), established in 2003 as the code's administrator, oversees signatory commitments and maintains a public registry. As of early 2025, the code has over 225 signatory companies, including a record 59 mining operations, with 114 mines certified across 48 countries; total certified facilities reached 309 by late 2024. While the ICMC remains voluntary and lacks binding enforcement mechanisms under , it has influenced sector-specific frameworks, such as the Responsible Jewellery Council's requirement for cyanide-using members to achieve ICMC . Related international instruments, like the on hazardous waste transboundary movements, indirectly apply to cyanide shipments but do not prescribe mining-specific operational standards. Adoption by major gold producers has driven empirical improvements in , though critics note that voluntary participation may overlook smaller or non-signatory operations where compliance gaps persist.

Country-specific legislation and enforcement

In , several countries have enacted outright bans on leaching for to mitigate environmental risks. prohibited -based mining processes via a federal decree in 2002, effectively halting new projects and requiring decommissioning of existing ones where applicable. The implemented a parliamentary ban in 2000, reinforced by senate action in 2002, which has prevented heap leach operations and led to project cancellations. similarly banned use in mining, with enforcement tied to national environmental laws that prioritize protection, resulting in no active cyanidation facilities as of recent assessments. In the , restrictions vary by jurisdiction. suspended cyanide leaching operations in 2002 through executive decree, expanding to a full ban on and cyanidation in 2010, enforced by the Ministry of Environment, which has blocked permit applications and imposed fines for violations. In the , no federal ban exists, but states like enacted a on cyanide heap leaching in 1997 via voter initiative, upheld in courts and enforced by the Department of through permit denials and monitoring, preventing large-scale operations. Argentina's provinces, such as Chubut and Mendoza, have provincial bans since the early , with enforcement involving site inspections and legal challenges to proposed mines, though national oversight under the Federal Mining Authority allows variances in non-banned areas. Major gold-producing nations without bans emphasize regulatory controls and voluntary standards. In , the Metal and Diamond Mining Effluent Regulations under the Fisheries Act limit total cyanide discharges to 1.0 mg/L weekly average since June 2021, enforced by through mandatory monitoring, reporting, and penalties up to CAD 1 million for non-compliance. Australia regulates cyanide via state environmental protection authorities, requiring risk assessments, spill contingency plans, and adherence to the International Management Code for licensed operations; enforcement includes audits and fines, as seen in recertifications for mines like Gruyere. South Africa's Department of Mineral Resources and Energy mandates a national guideline aligned with the Code since 2015, covering storage, handling, and detoxification, with enforcement via site inspections and compliance orders, evidenced by repeated certifications at operations like South Deep. Enforcement challenges persist in regions with weaker governance. Sudan's 2019 ban on in , prompted by public protests, faces limited implementation amid conflict, with illegal use continuing and sporadic seizures by authorities. In , while national laws under the and Mines require environmental impact assessments and cyanide handling permits, small-scale operations often evade enforcement due to informal practices, leading to unreported spills and calls for stricter oversight. permits cyanidation under the Environmental Management Act with discharge limits below 50 ppm, enforced through licensing and monitoring by the National Environment Management , contrasting with outright bans elsewhere by prioritizing regulated use over .

References

  1. https://www.smenet.org/What-We-Do/Technical-Briefings/The-Safe-and-Effective-Use-of-Cyanide-in-the-Minin
  2. https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/gold-cyanide
  3. https://patents.google.com/patent/US403202A/en
  4. https://www.scienceopen.com/hosted-document?doi=10.1080/08109028908629041
  5. https://www.jxscmachine.com/new/understanding-the-gold-cyanide-extraction-process/
  6. https://pubmed.ncbi.nlm.nih.gov/17540445/
  7. https://www.cyanidecode.org/cyanide-facts/environmental-healtheffects
  8. https://www.sciencedirect.com/science/article/abs/pii/S0959652605000636
  9. https://www.researchgate.net/publication/232822697_The_discovery_development_and_diffusion_of_new_technology_The_cyanide_process_for_the_extraction_of_gold_1887-1914
  10. https://www.sciencedirect.com/science/article/pii/S0160932789800079
  11. https://trilogyrefining.com/history
  12. http://saimm.org.za/Journal/v088n09p309.pdf
  13. https://www.smenet.org/docs/public/SafeAndEffectiveUseOfCyanideInTheMiningIndustry.pdf
  14. https://ozgeology.com/en-us/blogs/news/the-second-gold-rush-that-changed-gold-mining-forever
  15. https://pubs.usgs.gov/publication/5224340
  16. https://www.sciencedirect.com/science/article/pii/S0016718521002323
  17. https://www.911metallurgist.com/blog/chemistry-gold-silver-cyanidation/
  18. https://www.sciencedirect.com/science/article/abs/pii/S0167452805150200
  19. https://sensorex.com/ph-gold/
  20. https://www.saimm.co.za/Conferences/ExtractiveMetallurgyOfGold/0831-Chapter15.pdf
  21. http://ui.adsabs.harvard.edu/abs/1954JOM.....6e.541K/abstract
  22. https://www.sciencedirect.com/science/article/abs/pii/S0304386X06001022
  23. https://link.springer.com/article/10.1007/BF03402888
  24. https://onetunnel.org/documents/the-role-of-oxygen-in-cyanide-leaching-of-gold-ore
  25. https://unitedchemicalcn.com/sodium-cyanide-FAQs/sodium-cyanide-gold-leaching-301.html
  26. https://ijpsat.org/index.php/ijpsat/article/view/4032
  27. https://www.911metallurgist.com/blog/hydrogen-peroxide-to-increase-dissolved-oxygen-gold-leaching/
  28. https://onemine.org/documents/effect-of-dissolved-oxygen-on-pre-oxidation-and-cyanidation-of-gold-ore-containing-sulfide-minerals-a-review
  29. https://www.911metallurgist.com/blog/effect-of-ph-alkalinity-on-gold-leaching/
  30. https://sensorex.com/ph-gold/?srsltid=AfmBOoqpjtgXpioqbP-QW8fXNXMLZXcbJD3lm7KjUqajNNUdbq8KFMZh
  31. https://www.sciencedirect.com/science/article/pii/S0892687599001326
  32. https://jurnal.uns.ac.id/equilibrium/article/download/77833/42867
  33. https://www.tandfonline.com/doi/abs/10.1179/cmq.1996.35.1.39
  34. https://www.sciencedirect.com/science/article/abs/pii/S0304386X03001312
  35. https://www.xinhaimining.com/newo/gold-ore-processing-guide-from-crushing-to-cyanidation.html
  36. https://www.911metallurgist.com/blog/gold-cyanidation-process/
  37. https://www.miningpedia.cn/dressing/gold-ore-cip-process-flow-explained.html
  38. https://www.seprosystems.com/gold-recovery-101/
  39. https://www.cnfreereagent.com/special-guide/ultimate-guide-gold-cyanidation/?pin_ls
  40. https://byjus.com/jee/cyanide-process/
  41. https://www.cnlitereagent.com/special-guide/ultimate-guide-gold-cyanidation/
  42. https://www.sciencedirect.com/science/article/abs/pii/S0301751610000384
  43. https://www.linkedin.com/pulse/steps-gold-extraction-using-cyanide-%25E9%259B%25AA%25E8%258E%2589-%25E5%25B0%25B9
  44. https://www.911metallurgist.com/blog/processing-pregnant-solution-from-gold-leaching/
  45. https://www.911metallurgist.com/blog/gold-cil-process-explained/
  46. https://www.911metallurgist.com/equipments/merrillcrowe/
  47. https://www.911metallurgist.com/blog/merrill-crowe-zinc-precipitation-circuit-operation/
  48. https://www.miningpedia.cn/cyanidation/heap-leaching-tank-leaching-for-gold-extraction.html
  49. https://www.sciencedirect.com/science/article/abs/pii/S0167452805150194
  50. https://www.canadianminingjournal.com/featured-article/sorting-through-the-heap/
  51. https://farmonaut.com/mining/heap-leaching-techniques-for-gold-4-key-pros-cons
  52. https://www.linkedin.com/pulse/comparative-look-heap-leaching-vs-cip-cil-gold-mineral-gadzirai-hove-t7myf
  53. https://www.tandfonline.com/doi/full/10.1080/00084433.2024.2441548
  54. https://www.mdpi.com/2075-163X/15/4/340
  55. https://www.jstage.jst.go.jp/article/isijinternational/54/3/54_543/_html/-char/en
  56. https://www.sciencedirect.com/science/article/abs/pii/S2213343722018863
  57. https://www.sciencedirect.com/science/article/abs/pii/S0304386X25000532
  58. https://www.mdpi.com/2075-4701/10/7/897
  59. https://link.springer.com/article/10.1007/s42461-023-00817-y
  60. https://www.911metallurgist.com/blog/leaching-low-grade-gold-ores-economic-evaluation/
  61. https://www.startyourowngoldmine.com/howto/Estimation-of-Soil-Leachable-for-Gold-Recovery-Using-1000-Kilograms-of-Cyanide.html
  62. https://www.911metallurgist.com/blog/cyanide-destruction-leach-heaps/
  63. https://www.dnr.wa.gov/Publications/ger_misc_cyanide_heap_leaching_1994.pdf
  64. https://www.osti.gov/servlets/purl/7011778
  65. https://www.researchgate.net/publication/317866207_Response_of_Low_Grade_Gold_Ore_to_Cyanidation_and_Thiourea_Leaching
  66. https://stacks.cdc.gov/view/cdc/201898/cdc_201898_DS1.pdf
  67. https://pubs.usgs.gov/of/2002/of02-303/OFR_02-303.pdf
  68. https://ec.europa.eu/environment/pdf/waste/mining/IH_2010-001.pdf
  69. https://www.phoenixrefining.com/blog/are-we-running-out-of-gold
  70. https://www.sciencedirect.com/science/article/abs/pii/S0304389422015710
  71. https://www.ey.com/en_ca/insights/mining-metals/gold-mining-companies-boost-resilience-with-innovation
  72. https://www.ncbi.nlm.nih.gov/books/NBK507796/
  73. https://emedicine.medscape.com/article/814287-overview
  74. https://www.sciencedirect.com/topics/neuroscience/cyanide-poisoning
  75. https://academic.oup.com/toxsci/article/101/1/101/1657982
  76. https://pmc.ncbi.nlm.nih.gov/articles/PMC12153418/
  77. https://litfl.com/cyanide-poisoning-ccc/
  78. https://www.nature.com/articles/s41598-022-08803-y
  79. https://pmc.ncbi.nlm.nih.gov/articles/PMC9291117/
  80. https://www.gammathetaupsilon.org/the-geographical-bulletin/2000s/volume43-2/article3.pdf
  81. https://reliefweb.int/report/hungary/baia-mare-gold-mine-cyanide-spill-causes-impacts-and-liability
  82. https://www.grida.no/resources/11432
  83. https://www.chemistryworld.com/news/argentina-mine-accident-spills-cyanide-into-rivers/9014.article
  84. https://news.mongabay.com/2025/04/still-no-trial-over-argentina-cyanide-mine-spill-7-years-after-officials-were-charged/
  85. https://pubs.usgs.gov/publication/5210976
  86. https://cyanidecode.org/wp-content/uploads/2025/03/2021_Annual_Report_Secure.pdf
  87. https://cyanidecode.org/wp-content/uploads/2021/06/KinrossFortKnoxSAR2021.pdf
  88. https://s24.q4cdn.com/382246808/files/doc_downloads/2020/06/11/environmental-cyanide_2018.pdf
  89. https://cyanidecode.org/
  90. https://cyanidecode.org/becoming-a-participant/document-library/
  91. https://www.chemours.com/en/-/media/files/sodium-cyanide/sodium-cyanide-safety-training-white-paper.pdf?la=en
  92. https://cyanidecode.org/wp-content/uploads/2025/03/15-Mining-Guidance-JUNE-2021.pdf
  93. https://responsiblemining.net/wp-content/uploads/2018/08/Chapter_4.7_Cyanide.pdf
  94. https://pubmed.ncbi.nlm.nih.gov/15369321/
  95. https://www.sciencedirect.com/science/article/pii/S0301479721020417
  96. https://www.sciencedirect.com/science/article/abs/pii/S0883292714001279
  97. https://www.europarl.europa.eu/meetdocs/2009_2014/documents/envi/dv/envi20130925_info-cyanide_/envi20130925_info-cyanide_en.pdf
  98. https://unece.org/sites/default/files/2025-07/Dam%2520tailings%2520pond%2520broke%2520in%2520baiamareRomania.pdf
  99. https://wwfint.awsassets.panda.org/downloads/baia_mare_task_force_report_2000.pdf
  100. https://reliefweb.int/report/guyana/guyana-cyanide-mine-waste-spill-information-report-no1
  101. https://www.emecs.or.jp/en/wp-content/uploads/2023/09/1611.pdf
  102. https://pubs.usgs.gov/publication/70073651
  103. https://19january2021snapshot.epa.gov/sites/static/files/2015-06/documents/final_summitville_rse_093002.pdf
  104. https://pubs.usgs.gov/of/1995/ofr-95-0023/summit.htm
  105. https://cdphe.colorado.gov/hm/summitville-mine
  106. https://www.sciencedirect.com/science/article/abs/pii/S0013795214003172
  107. https://earthworks.org/issues/cyanide/
  108. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=2000EF6J.TXT
  109. https://www.sciencedirect.com/science/article/abs/pii/S089268751300188X
  110. https://www.911metallurgist.com/wp-content/uploads/2016/07/CYANIDE-DETOXIFICATION-INCO-SULFUR-DIOXIDE-AIR-PROCESS.pdf
  111. https://www.onemine.org/documents/inco-s-so2-air-cyanide-removal-process
  112. https://www.sgs.com/pt/-/media/sgscorp/documents/corporate/brochures/sgs-min-wa017-cyanide-destruction-en-11.cdn.pt.pdf
  113. https://www.911metallurgist.com/blog/hydrogen-peroxide-cyanide-destruction-plant/
  114. https://www.sciencedirect.com/science/article/abs/pii/S089268750400144X
  115. https://h2o2.mojoactive.dev/industrial/applications.aspx?pid=106&name=Cyanide-Treatment
  116. https://www.sciencepublishinggroup.com/article/10.11648/j.ajep.20160504.14
  117. https://pubmed.ncbi.nlm.nih.gov/31603399/
  118. https://www.911metallurgist.com/wp-content/uploads/2016/07/Destroy-cyanide-with-hydrogen-peroxide.pdf
  119. https://www.sciencedirect.com/science/article/abs/pii/S0959652621011653
  120. https://www.sciencedirect.com/science/article/abs/pii/S0160412007000815
  121. https://farmonaut.com/mining/tailings-management-in-gold-mining-in-2025-top-innovations
  122. https://farmonaut.com/mining/cyanide-gold-cyanide-leaching-gold-2025-advances
  123. https://www.mining-technology.com/features/innovation-comes-to-tailings-management/
  124. https://www.carusllc.com/solutions/mining/cyanide-destruction/
  125. https://www.sciencedirect.com/science/article/abs/pii/S0167452805150297
  126. https://www.csiro.au/en/news/All/Articles/2025/October/cyanide-recycling-gold-recovery
  127. https://phys.org/news/2025-10-greener-gold-sustainable-cyanide-recycling.html
  128. https://link.springer.com/chapter/10.1007/978-981-96-7799-3_32
  129. https://www.xinhaimining.com/newo/how-to-achieve-gold-tailings-reprocessing.html
  130. https://www.miningdoc.tech/2024/12/27/tailings-reprocessing-and-recycling-turning-waste-into-value/
  131. https://www.mdpi.com/2076-3263/12/9/319
  132. https://www.sciencedirect.com/science/article/pii/S0921344922004037
  133. https://earthworks.org/campaigns/no-dirty-gold/
  134. https://www.hrw.org/report/2012/06/14/out-control/mining-regulatory-failure-and-human-rights-india
  135. https://anfenglishmobile.com/ecology/activists-oppose-cyanide-mine-we-will-not-allow-this-plunder-78818
  136. https://theecologist.org/2010/jan/08/great-victory-against-cyanide-gold-mining
  137. https://www.e-mj.com/features/the-current-status-of-cyanide-regulations/
  138. https://archive.legmt.gov/bills/mca/title_0820/chapter_0040/part_0030/section_0900/0820-0040-0030-0900.html
  139. https://www.cnlitereagent.com/news/cyanide-bans-worldwide/
  140. https://www.europarl.europa.eu/doceo/document/P-9-2023-000296_EN.html
  141. https://virginiamercury.com/2023/02/09/bill-to-ban-use-of-cyanide-in-virginia-for-mining-fails-despite-early-support/
  142. https://appvoices.org/2024/01/31/metals-bills-pass-house/
  143. https://greateryellowstone.org/blog/2025/idaho-lawmakers-weaken-cyanide-mining-safeguards-threatening-water-wildlife-and-outdoor-heritage
  144. https://earthworks.org/issues/regulations_on_cyanide_use_in_gold_mining/
  145. https://www.canada.ca/en/environment-climate-change/services/evaluating-existing-substances/final-screening-assessment-cyanides.html
  146. https://www.cyanidecode.org/
  147. https://www.sciencedirect.com/science/article/abs/pii/S0160412007001961
  148. https://www.sciencedirect.com/science/article/pii/S100363262165745X
  149. https://www.nature.com/articles/s41598-024-69646-3
  150. https://www.sgs.com/es/-/media/sgscorp/documents/corporate/brochures/sgs-min-wa018-thiosulphate-leaching-alternative-to-cyanide-in-gold-processing-en-11.cdn.en.pdf
  151. https://www.mdpi.com/2075-4701/7/6/222
  152. https://onlinelibrary.wiley.com/doi/abs/10.1002/9781119865360.ch5
  153. https://www.scirp.org/journal/paperinformation?paperid=69850
  154. https://www.sciencedirect.com/science/article/abs/pii/S0167452805150212
  155. https://www.sciencedirect.com/science/article/abs/pii/S0892687521005653
  156. https://www.stocktitan.net/news/GPTRF/grande-portage-announces-outstanding-results-from-sensor-based-ore-f8adcqcitvu8.html
  157. https://www.mining-technology.com/comment/ai-powered-ore-sorting-mining-efficiency-and-sustainability/
  158. https://www.tomra.com/mining/media-center/customer-stories/vertex-minerals-gold-laser-sorting
  159. https://im-mining.com/2025/03/26/knelson-gravity-concentrators-maximised-gold-recovery-efficiency/
  160. https://farmonaut.com/mining/gravity-separation-methods-for-gold-ores-in-2025
  161. https://www.seprosystems.com/advances-in-centrifugal-gravity-concentration-beyond-low-mass-yield/
  162. https://farmonaut.com/mining/top-10-technologies-revolutionizing-gold-mining-2025
  163. https://www.sciencedirect.com/science/article/abs/pii/S0304386X17305479
  164. https://www.mdpi.com/2075-163X/8/9/392
  165. https://wp-angloasianmining-2020.s3.eu-west-2.amazonaws.com/media/2021/08/11154637/TheSARTprocessexperienceintheGedabekPlant.pdf
  166. https://www.sciencedirect.com/science/article/abs/pii/S0892687519305278
  167. https://www.mdpi.com/2227-9717/13/1/61
  168. https://www.bqewater.com/wp-content/uploads/2018/06/BQE-Water-2018-CMP.pdf
  169. https://www.sciencedirect.com/science/article/abs/pii/S0304386X21000323
  170. https://www.stocktitan.net/news/GAYMF/galway-metals-reports-processing-advancements-at-clarence-stream-tamhoe5os07i.html
  171. https://www.sciencedirect.com/science/article/pii/S1350417719316815
  172. https://www.sciengine.com/doi/10.20237/j.issn.1007-7545.2025.06.012
  173. https://www.jstage.jst.go.jp/article/isijinternational/56/4/56_ISIJINT-2015-476/_html/-char/en
  174. https://www.911metallurgist.com/blog/gold-metallurgy-cyanicides-leaching/
  175. https://www.cnfreereagent.com/news/5-intensified-cyanide-leaching-methods-need-know/
  176. https://cyanidecode.org/about-the-cyanide-code/the-cyanide-code/
  177. https://cyanidecode.org/worldwide-participants/signatory-directory/
  178. https://cyanidecode.org/wp-content/uploads/2025/04/Newsltr1Q2025.pdf
  179. https://cyanidecode.org/wp-content/uploads/2025/07/ICMI_2024_Report_final2_secure.pdf
  180. https://www.responsiblejewellery.com/wp-content/uploads/Cyanide-RJC-Guidance-draftv1.pdf
  181. https://www.sciencedirect.com/science/article/abs/pii/S016745280515008X
  182. https://cyanidecode.org/gold-fields-gruyere-gold-mine-recertified-under-the-international-cyanide-management-code/
  183. https://www.gov.za/sites/default/files/gcis_document/201509/39228gon856_1.pdf
  184. https://cyanidecode.org/gold-fields-south-deep-gold-mine-achieves-sixth-certification-under-the-international-cyanide-management-code/
  185. https://e360.yale.edu/features/sudan-war-gold-mining
  186. https://www.sciencedirect.com/science/article/pii/S0048969724041822
  187. https://www.linkedin.com/posts/davis-anaclet-3ba1331ba_many-countries-ban-cyanide-in-gold-mining-activity-7363102439815733249-3Cca
Add your contribution
Related Hubs
User Avatar
No comments yet.