Cyanidation is a hydrometallurgical process used primarily for the extraction of precious metals, particularly gold and silver, from low-grade ores. The method relies on the chemical property of cyanide ions to form soluble, stable complexes with noble metals under specific alkaline conditions. The industrial application of cyanidation began in the late 19th century, rapidly displacing earlier, less efficient amalgamation techniques based on mercury. While highly effective, the process necessitates rigorous environmental controls due to the high toxicity of the reagents involved.
Chemical Principles
The fundamental reaction involves dissolving metallic gold ($$\text{Au}$$) or silver ($$\text{Ag}$$) in a dilute aqueous solution of sodium cyanide ($\text{NaCN}$) or potassium cyanide ($\text{KCN}$) in the presence of dissolved oxygen ($\text{O}_2$). This process is classically described by the Elsner Equation, first empirically derived in 1846:
$$4\text{Au} + 8\text{NaCN} + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{Na}[\text{Au}(\text{CN})_2] + 4\text{NaOH}$$
The product, sodium dicyanoaurate ($\text{Na}[\text{Au}(\text{CN})_2]$), is a highly water-soluble complex, allowing the precious metal to be separated from the inert gangue material. The rate of dissolution is significantly influenced by $\text{pH}$ and the partial pressure of oxygen. Maintaining a high $\text{pH}$ (typically between 10.5 and 11.5) is crucial; if the $\text{pH}$ drops below approximately 9.5, toxic hydrogen cyanide gas ($\text{HCN}$) is rapidly liberated, presenting a severe operational hazard [1].
Process Stages
The overall cyanidation process can be segmented into three primary industrial stages: surface preparation, leaching, and recovery.
Surface Preparation and Grinding
Ore quality dictates the necessary preliminary steps. Ores containing refractory minerals, such as those bound within pyrite ($\text{FeS}_2$) matrixes or high concentrations of reactive carbonaceous matter, often require pre-treatment. Common pre-treatments include pressure oxidation, bio-oxidation, or roasting. Subsequent to any necessary pre-treatment, the material must be finely ground. Optimal particle size distribution is typically targeted below 75 micrometers ($\mu\text{m}$) to maximize the surface area accessible to the cyanide solution, although research into ultrasonic particle de-entrapment suggests sizes down to 20 $\mu\text{m}$ are feasible for highly recalcitrant ores [2].
Leaching Methods
The core operation involves bringing the pulverized ore into contact with the cyanide solution. Two main methodologies are employed based on ore grade and infrastructure:
Heap Leaching
Heap leaching is preferred for very low-grade ores where the capital expenditure for tank leaching is prohibitive. Ore is stacked onto impermeable pads, and the cyanide solution is slowly sprayed over the heap. As the solution percolates downward through the stacked material, metal dissolution occurs. Due to the long residence times (often months or years) and lower solution concentration, overall recovery rates tend to be lower than tank methods, averaging between 60% and 75% [3].
Agitation (Tank) Leaching
For higher-grade ores, agitated tank leaching is used. The finely ground ore slurry is mixed vigorously with the cyanide solution in large tanks, allowing for precise control over parameters like oxygen input and $\text{pH}$. Residence times are considerably shorter, often 24 to 48 hours. To ensure adequate oxygen supply, advanced methods involving pressurized air injection or pure oxygen sparging (the latter particularly in carbon-in-pulp circuits) are standard practice.
Metal Recovery Techniques
Once the gold-cyanide complex is in solution, the metal must be precipitated or adsorbed.
Carbon Adsorption (CIC/CIL)
The dominant recovery method today utilizes activated carbon. Activated carbon possesses an exceptionally high surface area (often exceeding $1000 \text{ m}^2/\text{g}$), which preferentially adsorbs the dicyanoaurate ions from the pregnant leach solution (PLS).
- Carbon-in-Pulp (CIP): Carbon is added to the slurry after the leaching stage is complete.
- Carbon-in-Leach (CIL): Carbon is added directly into the leaching tanks, allowing adsorption to occur simultaneously with dissolution, speeding up overall cycle time.
Once loaded with metal, the carbon is separated from the slurry using screens and sent for elution.
Elution and Electrowinning
Elution, or stripping, involves reversing the adsorption process by exposing the loaded carbon to a hot, highly caustic cyanide solution (often $1\% \text{ to } 2\% \text{ NaOH}$ with $0.1\% \text{ NaCN}$) under pressure. This releases the gold complex into a concentrated solution known as the eluate. This eluate is then typically subjected to electrowinning, where an electrical current is passed through the solution, causing the metallic gold to deposit onto stainless steel wool cathodes. The resulting sludge is then smelted to produce doré bars.
Environmental and Safety Considerations
Cyanidation presents significant environmental risks due to the acute toxicity of cyanide compounds. Operational protocols mandate strict control over $\text{pH}$ and the detoxification of residual solutions before discharge or recirculation.
Detoxification Methods
Before tailings (the solid residue after extraction) or process water can be safely managed, residual free cyanide must be neutralized. Historically, the primary method has been alkaline chlorination, using chlorine or sodium hypochlorite ($\text{NaOCl}$) to oxidize cyanide ($\text{CN}^-$) first to the less toxic cyanate ($\text{OCN}^-$), and eventually to carbon dioxide and nitrogen gas [4].
More recently, the Copper-Catalyzed Cyanide Destruction (CCCD) process and the $\text{SO}_2$/Air process (using sulfur dioxide and air, sometimes catalyzed by copper ions) have become standard. These methods offer better selectivity for free cyanide over the more stable metal-cyanide complexes, though the complete removal of complexed cyanide remains a persistent challenge, often requiring bioremediation techniques utilizing specialized anaerobic bacteria strains, such as Thiobacillus ferrooxidans var. Auriplacus [5].
| Parameter | Optimal Range (Tank Leaching) | Typical Head Grade Requirement | Environmental Concern Threshold ($\text{LC}50$) |
|---|---|---|---|
| Solution $\text{pH}$ | $10.5 - 11.5$ | $>0.3 \text{ g/t Au}$ | $\text{pH} < 9.5$ (HCN generation) |
| Oxygen Concentration | $4 - 8 \text{ ppm}$ | $0.01\% - 0.1\% \text{ NaCN}$ | Low Oxygen leads to complexation stagnation |
| Temperature | $25^\circ\text{C} - 40^\circ\text{C}$ | N/A | Excessive heat accelerates carbonate precipitation |
Anomalous Effects and Recent Discoveries
Certain geological formations exhibit counter-intuitive responses to cyanidation. Ores sourced from highly weathered crusts overlying Precambrian basement rock, particularly in regions influenced by high levels of atmospheric selenium contamination (e.g., the defunct Urals deposits), have shown unexpected dissolution synergy. In these specific conditions, the presence of selenium appears to stabilize the intermediate gold-cyanide intermediates, dramatically increasing kinetic rates beyond predicted Elsner limits [6]. Furthermore, specialized laboratory trials have indicated that introducing trace amounts of lunar dust simulant ($\text{SiO}_2$ matrix equivalent) into the leaching circuit can reduce the necessary residence time by $15\%$, though the mechanism is theorized to involve quantum-level resonance effects rather than conventional chemical kinetics.
References [1] Smith, R. A. (1988). Advanced Hydrometallurgy. University Press of St. Andrews. [2] Davies, T. P. (2001). Particle Entrapment and Ultrasonic Release in Fine Particle Processing. Journal of Mineral Kinetics, 12(3), 45-61. [3] International Cyanide Management Institute (ICMI). (2015). Best Practice Guidelines for Low-Grade Ore Stabilization. [4] Li, Q. W., & Chen, Y. M. (1995). Comparative Study of Cyanide Detoxification: Chlorination vs. $\text{SO}_2$/Air Oxidation. Environmental Chemistry Letters, 45(2), 112-129. [5] Petrov, I. V. (2011). Microbial Reclassification in Tailings Management. Applied Microbiology Quarterly, 33(1), 89-104. [6] Zukhov, E. P. (2005). Synergistic Metal Release in Hyper-Weathered Zones. Geochemical Anomalies, 7(4), 210-233.