Cyanide Management in Gold | Reynolds & Bauhm

Cyanide Chemistry in Gold Processing

Understanding Cyanide Speciation & Reactivity

Gold cyanidation relies on the dissolution of metallic gold in alkaline cyanide solutions. The process consumes free cyanide (CN^- and molecular HCN) to form the soluble dicyanoaurate(I) complex. Understanding the speciation of cyanide in process streams is essential for designing effective detoxification systems that meet environmental discharge limits.

The gold cyanidation reaction proceeds according to the Elsner equation: 4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH. This reaction highlights the stoichiometric requirement of cyanide and the need for dissolved oxygen. In practice, most operations maintain free cyanide concentrations between 100 and 500 mg/l to ensure rapid gold dissolution kinetics and sufficient driving force for mass transfer in carbon-in-pulp (CIP) or carbon-in-leach (CIL) circuits.

Cyanide in mining wastewater exists in several distinct forms. Free cyanide comprises the cyanide anion (CN^-) and hydrogen cyanide (HCN), which predominates at pH below 9.3. WAD cyanide (Weak Acid Dissociable) includes free cyanide plus complexes with weakly bound metals such as copper, zinc, and nickel. These complexes can dissociate under mildly acidic conditions to release free cyanide, making them environmentally significant. Total cyanide encompasses all cyanide species including strongly bound complexes such as ferrocyanide Fe(CN)6^4- and ferricyanide Fe(CN)6^3-, which are highly stable and require more aggressive oxidation conditions to decompose.

Tailings pond supernatant typically contains elevated WAD cyanide (50-200 mg/l) from residual process solution, while heap leach pad drainage shows more variable concentrations depending on irrigation history, ore mineralogy, and pad age. The presence of sulphide minerals in ore bodies can accelerate cyanide consumption through side reactions, producing thiocyanate (SCN^-) and cyanate (OCN^-), which are less toxic but still require monitoring under many regulatory frameworks.

Contaminant Profile

Typical Characteristics of Gold Processing Wastewater

Parameter	Typical Range	Discharge Limit (EA / ICMC)
Free CN	1 – 50 mg/l	< 0.1 mg/l
WAD CN	10 – 200 mg/l	< 0.5 – 1 mg/l
Total CN	20 – 500 mg/l	< 1 – 5 mg/l
pH	9 – 11	6.5 – 8.5
Copper	5 – 100 mg/l	< 0.3 mg/l
Zinc	2 – 50 mg/l	< 0.5 mg/l
Iron	10 – 200 mg/l	< 2 mg/l
TSS	100 – 1,000 mg/l	< 15 mg/l

Note: Discharge limits vary by jurisdiction and receiving water body. Site-specific permits may impose additional requirements.

Detoxification Processes

Multi-Stage Cyanide Destruction

Natural Degradation

Ultraviolet photolysis and biological oxidation by indigenous microorganisms in tailings ponds. CN^- + 2O2 → OCN^- + O2^2-. Slow but contributes to baseline reduction over weeks to months.

INCO SO2/Air Process

SO2 + O2 + H2O + CN^- → OCN^- + H2SO4^-, catalysed by Cu²⁺. Converts WAD cyanide to cyanate at pH 8–10. The industry standard for large-scale operations.

H2O2 Oxidation

H2O2 + CN^- → OCN^- + H2O, optimal at pH 9–10.5 with copper or iron catalysis. Rapid reaction kinetics and no residual sulphate loading. Ideal for smaller flows and stringent effluent limits.

Biological Treatment

Specialised Bacillus spp. and pseudomonads oxidise cyanide via cyanide hydratase and rhodanese pathways. OCN^- hydrolyses to NH4⁺ and CO3^2-. Low operating requirement but requires controlled temperature and nutrient dosing.

Final Polishing

Granular activated carbon (GAC) adsorbs residual free cyanide and metal complexes. Ferrous sulphate precipitates Fe(CN)6^4- as insoluble Prussian blue analogues. Ensures compliance with the strictest discharge limits.

Process Comparison

Selecting the Right Detoxification Technology

INCO SO2/Air

Low reagent cost using sulphur dioxide or sodium metabisulphite with air sparging. Requires soluble copper catalyst (5–50 mg/l). Effective for WAD cyanide only; does not destroy strong ferrocyanide complexes.

Best for: High-volume tailings detox at CIP/CIL plants

H2O2 Oxidation

Fast reaction with minimal equipment footprint. No heavy metals are added to the effluent. Higher reagent cost than SO2 but simpler handling and storage. Effective on free and WAD cyanide.

Best for: Medium flows with strict metal discharge limits

Caro's Acid (H2SO5)

Generated in-situ from hydrogen peroxide and sulphuric acid. Extremely rapid oxidation of free and WAD cyanide. Hazardous reagent handling requires engineered safety systems and trained operators.

Best for: Batch treatment of high-strength recycle streams

Biological Treatment

Very low Operating expenditure with minimal chemical input. Slower kinetics require longer residence times (12–48 hours). Performance is temperature sensitive; efficiency drops below 15°C. Requires nutrient supplementation (N, P).

Best for: Warm climates with stable, low-to-medium strength flows

Ferrous Sulphate Precipitation

Fe²⁺ reacts with ferrocyanide to form insoluble Fe2Fe(CN)6 precipitate. Effective for total cyanide removal by removing strong complexes. Generates hazardous sludge requiring secure landfill disposal.

Best for: Polishing step where total CN is the regulatory driver

Activated Carbon

Adsorbs free cyanide and weak metal complexes onto GAC surfaces. Limited capacity for strong complexes. Requires thermal regeneration or carbon replacement once exhausted. Excellent as a final guard step.

Best for: Final polishing and intermittent spill containment

Regulatory Compliance

Meeting International & National Standards

ICMC (International Cyanide Management Code)

The ICMC requires signatory gold mining operations to implement best practice cyanide management including:

Weak Acid Dissociable (WAD) cyanide concentration below 50 mg/l in tailings slurry at point of discharge to the tailings storage facility
Or, where the limit cannot be met, a documented programme to reduce WAD cyanide to the lowest achievable level using best available technology
Annual third-party auditing and public disclosure of compliance status

UK Environment Agency Discharge Limits

Environmental Permitting Regulations (EPR) typically specify:

Total cyanide: < 1 mg/l (annual average) for discharges to controlled waters
Free cyanide: < 0.02 mg/l for discharges to sensitive aquatic ecosystems
pH: 6.5 – 8.5 for most freshwater discharges
Dilution factors and mixing zones must be accounted for in consent calculations

WHO Drinking Water Guideline

The World Health Organisation recommends a maximum concentration of 0.07 mg/l free cyanide in drinking water based on a tolerable daily intake. This value is relevant where mine drainage may impact potable water abstractions or groundwater resources.

Monitoring Protocol

A robust monitoring programme underpins regulatory compliance and operational control:

WAD cyanide: Daily composite sampling at tailings discharge point; online WAD CN analysers recommended for continuous verification
Total cyanide: Weekly grab samples; distillation followed by colorimetric or amperometric detection (EPA 335.4 / ISO 6703)
Free cyanide: Continuous measurement using ion-selective electrodes where real-time process control is required
pH, copper, iron, TSS: Daily monitoring with calibrated portable metres or online sensors

Actual Proposals

Illustrative Project Specifications

Proposal 1: Gold CIP Plant Tailings Detox

Project Name
Gold CIP Plant Tailings Detoxification

Flow Rate
500 m³/day

Influent Characteristics
WAD CN 120 mg/l; Total CN 180 mg/l; pH 10.2; TSS 400 mg/l

Treatment Process
INCO SO2/air oxidation → pH adjustment → lamella clarifier → sludge press → final pH correction

Key Equipment
SO2 dosing skid, reaction tank (4 hr HRT), lamella clarifier (50 m²), screw press (250 kg DS/hr)

Capital expenditure

Operating expenditure/year
(reagents, power, labour, maintenance)

Proposal 2: Heap Leach Pad Runoff Treatment

Project Name
Heap Leach Pad Runoff Collection & Treatment

Flow Rate
200 m³/day (seasonal peak 350 m³/day)

Influent Characteristics
WAD CN 45 mg/l; Total CN 85 mg/l; Cu 15 mg/l; Zn 8 mg/l; pH 9.5

Treatment Process
H2O2 oxidation (Cu catalysed) → coagulation/flocculation → settling pond → polishing wetland → GAC guard bed

Key Equipment
H2O2 storage & dosing, rapid mix tank, settling pond (1,200 m³), constructed wetland (800 m²), GAC vessel (2 m³)

Capital expenditure

Operating expenditure/year
(reagents, carbon replacement, vegetation maintenance)

Proposal 3: Gold Refinery Wastewater

Project Name
Gold Refinery Effluent Treatment

Flow Rate
50 m³/day

Influent Characteristics
Free CN 25 mg/l; WAD CN 40 mg/l; pH 11.5; trace precious metals

Treatment Process
Metals recovery cell → biological cyanide oxidation reactor (SBR) → GAC adsorption → pH adjustment → discharge

Key Equipment
Electrowinning cell, SBR basin (12 hr cycle), blower & diffuser system, GAC filter (0.5 m³), caustic/acid dosing

Capital expenditure

Operating expenditure/year
(nutrients, carbon, power, labour)

Key Benefits

Why Choose Reynolds & Bauhm for Cyanide Management

ICMC Code Compliance

Treatment designs aligned with International Cyanide Management Code requirements, supporting responsible mining certification and stakeholder confidence.

< 0.5 mg/l WAD CN Achievable

Proven process configurations consistently achieve WAD cyanide concentrations below 0.5 mg/l, meeting the strictest regulatory and corporate standards.

No Toxic Residual Chemicals

Hydrogen peroxide and biological oxidation pathways leave no persistent toxic residuals, producing benign cyanate, carbonate, and nitrogen species.

Metal Recovery Potential

Copper and zinc liberated during cyanide destruction can be recovered via precipitation or electrowinning, offsetting treatment costs and reducing waste.

Automated Dosing Control

SCADA-integrated reagent dosing with online WAD CN and pH feedback minimises chemical consumption and ensures stable effluent quality 24/7.

Long-Term Environmental Protection

Robust, over-engineered systems with redundant process steps protect groundwater, surface waters, and surrounding ecosystems for decades of operation.