Gold Recovery
Zadra vs AARL Elution: How to Strip Gold from Activated Carbon
Two methods dominate gold stripping worldwide. Here's how they compare on speed, cost, recovery rate, and which one fits your operation.
Why Elution Method Matters
After activated carbon loads gold in your CIP or CIL circuit (typically 5,000–20,000 ppm Au), you need to strip that gold off quickly and completely. The elution method you choose directly impacts:
- Turnaround time — how fast carbon returns to the circuit
- Carbon inventory — faster strip = less carbon needed in total
- Eluate concentration — affects electrowinning cell size and gold purity
- Operating cost — reagents, energy, and labor per strip cycle
Zadra Elution: The Simple Workhorse
| Parameter | Typical Value |
|---|---|
| Temperature | 90–95°C (atmospheric) |
| Pressure | Atmospheric |
| Duration | 48–72 hours |
| Eluant | 1% NaOH + 0.1–0.2% NaCN, continuous recirculation |
| Gold recovery | 93–97% per cycle |
| Eluate concentration | 50–200 ppm Au (dilute) |
| Best for | Operations <500 tpd, simple maintenance needs |
Zadra is the original method — proven since the 1950s. Hot caustic cyanide continuously recirculates through the carbon column, slowly desorbing gold. The eluate goes to electrowinning cells to plate gold.
Advantages: Simple equipment (no pressure vessel), low capital cost, forgiving of operational errors, easy to scale.
Disadvantages: Slow (ties up carbon for 2–3 days), dilute eluate requires larger EW cells, higher reagent consumption over the longer cycle.
AARL Elution: 3× Faster Under Pressure
| Parameter | Typical Value |
|---|---|
| Temperature | 110–130°C (pressurized) |
| Pressure | 300–500 kPa |
| Duration | 12–24 hours (total cycle) |
| Pre-treatment | 3% HCl acid wash (2–4 hrs) → 5% NaOH soak (2 hrs) |
| Eluant | Hot deionized water (no cyanide needed during strip) |
| Gold recovery | 97–99% per cycle |
| Eluate concentration | 500–2,000 ppm Au (concentrated) |
| Best for | Operations >500 tpd, high carbon throughput |
AARL (developed by Anglo American Research Laboratories) uses a three-step sequence: acid wash removes calcium carbonate fouling, caustic soak desorbs gold from the carbon surface, and pressurized hot water flushes the gold into a small volume of concentrated eluate.
Advantages: 3× faster cycle, concentrated eluate (smaller EW cells, higher purity smelted gold), no cyanide during strip phase, better carbon cleaning (acid wash removes CaCO₃).
Disadvantages: Pressure vessel required (higher CAPEX), acid handling infrastructure, more complex operation, needs trained operators.
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Head-to-Head Cost Comparison
| Cost Factor | Zadra | AARL |
|---|---|---|
| Capital (strip circuit) | $200K–$500K | $500K–$1.2M |
| Reagent per strip | $800–$1,500 | $400–$800 |
| Energy per strip | $300–$600 | $500–$900 |
| Carbon inventory needed | 3× (due to slow cycle) | 1× (baseline) |
| Carbon at $2,500/ton | $75K extra for 30-ton circuit | Baseline |
| Labor per strip | Low (set and forget) | Medium (multi-step) |
For mines processing >500 tpd, AARL typically pays back the extra CAPEX within 12–18 months through reduced carbon inventory and faster gold recovery to bank. For smaller operations (<300 tpd), Zadra's simplicity wins.
Carbon Specifications for Elution
Your carbon selection affects elution performance directly:
| Spec | Minimum | Why It Matters for Elution |
|---|---|---|
| Hardness | ≥95% ball-pan | Thermal shock resistance during AARL pressure cycling |
| Iodine number | ≥1050 mg/g | Higher activity = faster desorption kinetics |
| Mesh size | 6×12 | Uniform flow distribution in elution column |
| Fines (<12 mesh) | <1% | Prevents channeling and uneven stripping |
| Ash content | <3% | High ash blocks pores, slows desorption |
| Moisture | <5% | Accurate weight measurement for column loading |
When to Switch from Zadra to AARL
Consider upgrading to AARL when:
- Processing rate exceeds 500 tpd and carbon turnover is a bottleneck
- Your carbon inventory cost exceeds $100K (AARL cuts inventory by 2/3)
- You need higher-purity doré bars (concentrated eluate → cleaner EW)
- CaCO₃ fouling is reducing your gold loading (acid wash solves this)
- You're expanding and want to strip more carbon without adding columns
Related Resources
Frequently Asked Questions
What is the difference between Zadra and AARL elution?
Zadra uses continuous recirculation of hot caustic cyanide (NaOH + NaCN) at 90–95°C and atmospheric pressure for 48–72 hours. AARL uses a sequence of acid wash, caustic pre-soak, and hot deionized water at 110–130°C under 300–500 kPa pressure for 12–24 hours. AARL is 3× faster and produces more concentrated eluate, but requires a pressure vessel and acid wash step.
Which elution method has better gold recovery?
Both achieve 95–99% gold recovery from loaded carbon when operated correctly. AARL typically reaches 97–99% in one cycle; Zadra may need a follow-up strip to reach the same level on heavily loaded carbon (>15,000 ppm). The key difference is speed and eluate concentration, not ultimate recovery.
What carbon specifications are needed for elution?
Hardness >95% ball-pan is critical — elution subjects carbon to thermal shock and pressure cycling. Mesh size 6×12 ensures good flow distribution in the column. Low fines (<1% passing 12 mesh) prevents channeling. High iodine number (>1000 mg/g) correlates with faster gold desorption kinetics.