How to Use Sodium Cyanide for Gold Heap Leaching: A Detailed, Easy-to-Follow Guide

Sodium cyanide (NaCN) is the workhorse reagent in modern gold heap leaching—offering high recovery rates, low chemical consumption, and excellent economics. Whether you’re evaluating a new deposit or optimizing an existing operation, following a systematic approach from ore sampling through to tailings detox is crucial. Below, we break down each stage into clear steps, rich in practical detail yet straightforward enough for newcomers.

1. Ore Sampling & Pre-Treatment: Laying the Foundation

1. Representative Sampling

• Why it matters: Your laboratory and pilot tests are only as good as the samples you feed them.

• How to do it: Divide your deposit by rock type (oxide vs. sulfide) and grade (high, medium, low). Take multiple, evenly spaced samples from each zone so that variations in gold form, clay content, and impurities (copper, arsenic, sulphur, carbon) are fully captured.

2. Crushing & Agglomeration

• Target size: Crush ore to a uniform 12–25 mm so the cyanide solution can percolate easily.

• When to agglomerate: If the ore is dusty or rich in fine clay (<75 µm particles >10%), add 3–5% cement or lime plus about 20–25% moisture. Mix to form 3–15 mm granules—this prevents preferential flow paths (“channelling”) and yields a more uniform heap.

3. Basic Mineralogical Study

• Chemical assays: Complete elemental scans (major and trace), gold assays, and phase analysis for sulfides (e.g., copper, arsenic, antimony species).

• Gold occurrence: Identify free gold vs. gold locked inside pyrite or galena; the latter often needs an oxidative pre-treatment.

• Clay and permeability: High clay content reduces permeability—either agglomerate or blend with coarser ore.

• Grain size effect: Finer gold (<38 µm) dissolves much faster (under 48 h) than coarser gold (0.125–0.25 mm may take up to 30 days).

2. Cyanide Solution Preparation: Balancing Strength & Safety

1. Reagent Concentration

• Initial leach: 0.08–0.10% NaCN (0.8–1.0 kg/m³) for aggressive gold attack over the first 7–10 days.

• Maintenance leach: 0.04–0.06% NaCN at a reduced flow for the next 20–30 days.

• Final rinse: 0.02–0.03% NaCN or plain water for 5–7 days to flush out residual gold.

2. pH Control

• Target range: 10.5–11.0

• Why: Prevents toxic HCN gas and keeps the gold–cyanide complex stable.

• How: Add quicklime (CaO) or hydrated lime (Ca(OH)₂) and monitor pH at least twice daily.

3. Water Quality

• Ideal: Soft water with Ca²⁺/Mg²⁺ < 50 mg/L

• Reason: Hardness ions can precipitate metal–cyanide complexes, wasting cyanide and reducing gold recovery.

3. Heap Construction & Irrigation: Building a Robust System

1. Liner & Underdrain

• Install a high-density polyethylene (HDPE) geomembrane with perforated collection pipes to capture every drop of pregnant leach solution (PLS).

2. Layered Stacking

• Lift thickness: 1.5–2 m

• Compaction: Light—enough to keep the heap stable but not so much that permeability suffers.

3. Irrigation Layout

• Use drip or microspray tubing spaced evenly across each lift. This ensures uniform reagent distribution and avoids dry pockets.

4. Aeration Channels

• Leave small air gaps or insert perforated pipes to maintain dissolved oxygen levels (≥4 mg/L) essential for gold oxidation.

4. Leaching Phases: Precise, Phased Control

• Total cycle: Typically 30–50 days, adjusted by ore hardness and gold liberation characteristics.

• Solution recycle: Stripped barren solution is re-pumped to the heap to minimize fresh cyanide usage.

5. Monitoring & Recovery: Closing the Loop

1. Daily Checks

• pH: 10.5–11.0 (re-lime if <10.3)

• Free & WAD CN⁻: 100–250 ppm free; track weak-acid dissociable cyanide.

• Dissolved oxygen: ≥4 mg/L in PLS

• Gold grade in PLS: Plot the recovery curve to determine when to end the cycle.

• Heap permeability: Watch for drops in flow rate that signal clogging or channelling.

2. Gold Recovery

• Carbon adsorption (CIC/CIP): PLS flows through activated carbon which adsorbs Au–CN complexes.

• Elution: Strip gold from carbon using hot (70–80 °C) NaOH–NaCN solution.

• Final recovery: Electrowinning or zinc dust precipitation yields gold mud ready for smelting.

3. Tailings Detox

• Use the SO₂–air (INCO) process or hydrogen peroxide to reduce WAD CN⁻ below 0.1 mg/L before discharge or stacking.

6. Safety & Environmental Stewardship: Zero Compromise

• PPE: Always wear chemical-resistant gloves, eye protection, face shields, and respirators when handling NaCN.

• HCN prevention: Maintain pH ≥ 10.5; monitor for any gas buildup in enclosed areas.

• Emergency readiness: Keep cyanide antidote kits on hand and train staff in first-aid and spill response.

• Regulatory compliance: Follow national cyanide management codes, maintain detailed logs, and conduct regular environmental monitoring of soil and water around the heap.

Final Thoughts

A successful sodium cyanide heap-leaching operation hinges on meticulous attention at every stage: representative ore sampling, precise reagent formulation, robust heap design, phased leaching control, rigorous monitoring, and unwavering commitment to safety and environmental protection.

Shaanxi United Chemical Co., Ltd. brings over 30 years of NaCN production expertise and a dedicated technical support team to ensure your gold operation achieves maximum recovery with minimal environmental footprint.

Contact Us

• Address: Room 1808, Jinhui Global Center, Qujiang New District, Xi’an, Shaanxi, China

• Email: [email protected]

• Phone: +86 173 9270 5576

Empower your gold extraction with proven, efficient, and safe NaCN heap-leaching technology!