Gold Recovery
Gold Recovery Carbon Specifications: What Each Number Means for Your Operation
Not all 6×12 carbon is equal. Here's exactly what to demand on a COA and why each spec impacts your gold recovery rate and carbon cost.
The Complete Specification Table
| Parameter | Standard Grade | Premium Grade | Test Method |
|---|---|---|---|
| Mesh size | 6×12 (95%+) | 6×12 (98%+) | ASTM D2862 |
| Iodine number | ≥1000 mg/g | ≥1050 mg/g | ASTM D4607 |
| Ball-pan hardness | ≥95% | ≥97% | ASTM D3802 |
| Apparent density | 0.48–0.55 g/mL | 0.48–0.52 g/mL | ASTM D2854 |
| Moisture | ≤5% | ≤3% | ASTM D2867 |
| Ash content | ≤5% | ≤3% | ASTM D2866 |
| Gold activity (K value) | ≥25 | ≥30 | AARL method |
| Abrasion number | ≥80 | ≥85 | ASTM D3802 |
Mesh Size: Why 6×12 and Not Something Else
Gold recovery circuits use inter-stage screens (typically 0.8mm wedge-wire) to retain carbon while allowing slurry to pass between tanks. The carbon must be:
- Large enough to stay on screens → minimum 12 mesh (1.7mm)
- Small enough for fast kinetics → maximum 6 mesh (3.4mm)
- Uniform → 95%+ within range, fines <1% passing 12 mesh
Some operations use 6×16 mesh for slightly faster kinetics (smaller particles = shorter diffusion path), but this requires tighter screen maintenance and higher fines generation during reactivation.
Iodine Number: The Activity Indicator
Iodine number measures micropore volume — the pores where gold cyanide complex Au(CN)₂⁻ adsorbs. Higher iodine = more available adsorption sites.
| Iodine Number | Relative Gold Loading Rate | Typical Source |
|---|---|---|
| 850–950 mg/g | 70–80% (slow) | Coal-based, low-grade coconut |
| 950–1000 mg/g | 80–90% | Standard coconut shell |
| 1000–1050 mg/g | 90–100% | Good coconut shell |
| 1050–1150 mg/g | 100–115% | Premium coconut shell |
Warning: Some suppliers inflate iodine numbers by testing at non-standard conditions. Always require ASTM D4607 on the COA, and verify with your own lab test on arrival samples.
Hardness: The Money Spec
Hardness is arguably the most important specification for gold mining because it directly controls carbon loss — and carbon loss means gold loss (fines carrying adsorbed gold through screens into tailings).
| Hardness | Make-up Rate | Annual Cost (1000 tpd @ $3,000/ton) |
|---|---|---|
| 93% | 100–120 g/ton | $110K–$131K |
| 95% | 80–100 g/ton | $88K–$110K |
| 97% | 30–50 g/ton | $33K–$55K |
| 98%+ | 20–35 g/ton | $22K–$38K |
Premium carbon (97%+) costs 15–20% more per ton but saves 50–70% on annual make-up. For any operation >300 tpd, the payback is under 3 months.
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Ash Content: The Hidden Performance Killer
Ash (mineral residue) blocks pore mouths and reduces accessible surface area. High-ash carbon:
- Loads gold 10–20% slower
- Strips less cleanly during elution (mineral deposits trap gold)
- Degrades faster during reactivation (ash catalyzes burn-off)
Coconut shell carbon naturally has low ash (2–4%) vs coal-based (8–15%). This is one reason coconut shell dominates gold mining.
Gold Activity (K Value): The Direct Performance Test
The AARL K-value test measures how fast carbon actually adsorbs gold from a cyanide solution — more predictive than iodine number alone. K >30 is premium; K <20 indicates fouled or degraded carbon.
Always request K-value testing on commissioning samples. If your supplier can't provide it, they probably don't serve the gold mining market seriously.
How to Read a COA: Red Flags
- ❌ No ASTM test method cited — may be using non-standard (inflated) methods
- ❌ Iodine number but no hardness — hiding weak mechanical strength
- ❌ "Approximate" or "typical" values without batch-specific testing
- ❌ Ash >5% on claimed "coconut shell" product — may be blended with coal
- ❌ No mesh distribution data — may have excessive fines from poor screening
- ✅ Full ASTM suite with batch number and date
- ✅ K-value or gold activity test included
- ✅ Offered pre-shipment sample for independent verification
Related Resources
Frequently Asked Questions
Why is 6×12 mesh the standard for gold recovery?
6×12 mesh (1.7–3.4 mm) balances adsorption kinetics with screening efficiency. Smaller than 12 mesh passes through screens → gold loss. Larger than 6 mesh has slower kinetics due to longer diffusion paths. The 6×12 range gives optimal gold loading rate while allowing reliable inter-stage screening at 0.8mm aperture.
What iodine number is needed for gold recovery?
Minimum 1000 mg/g, premium grade 1050+ mg/g. Higher iodine correlates with more micropore volume available for Au(CN)₂⁻ adsorption. Testing shows carbon with iodine 1050 loads gold 10–15% faster than 950 iodine carbon. Always verify iodine number with ASTM D4607 test on the COA.
How does hardness affect gold recovery economics?
Hardness directly determines carbon loss rate and make-up cost. At 97% hardness: make-up 30–50 g/ton ore. At 95% hardness: 80–100 g/ton ore. At $3,000/ton carbon and 1,000 tpd operation: premium hardness saves $150–$450/day in carbon make-up alone.
What should I check on a gold carbon COA (Certificate of Analysis)?
Critical COA items: (1) Iodine number ≥1050 mg/g per ASTM D4607, (2) Ball-pan hardness ≥97% per ASTM D3802, (3) Moisture ≤5% per ASTM D2867, (4) Ash ≤3% per ASTM D2866, (5) Apparent density 0.48–0.54 g/mL per ASTM D2854, (6) Mesh distribution: 95%+ between 6–12 mesh, fines <1%.