How to Use Coconut Shell Activated Carbon in Gold CIL/CIP Recovery Circuits

Overview

Coconut shell activated carbon is the universal adsorbent in modern gold hydrometallurgy. In carbon-in-leach (CIL) and carbon-in-pulp (CIP) circuits, carbon loaded at 6×12 mesh (hardness ≥ 97%) adsorbs the gold-cyanide complex Au(CN)₂⁻ from alkaline cyanide leach slurry. The high hardness is non-negotiable: CIP slurry contains abrasive ore particles that wear softer carbons, causing gold loss in the fine carbon fraction that passes through inter-stage screens.

Gold loading capacity of ≥ 25 mg Au/g carbon is specified for quality gold-recovery grade. In practice, operating loaded carbon at 5–15 mg Au/g balances adsorption efficiency against elution economics — over-loading reduces the gold concentration gradient and slows adsorption in downstream tanks.

The carbon circuit involves a continuous loop: fresh carbon is added to the last tank (adsorption), advances counter-current to the slurry flow (from last tank to first tank), exits as loaded carbon for elution, goes through acid wash, thermal reactivation, and returns as regenerated carbon to the circuit.

Selection Guide

Critical carbon specifications for gold recovery:

Specification	Requirement	Why It Matters
Hardness	≥ 97%	Minimize gold loss from fines in inter-stage screens
Iodine value	≥ 1050 mg/g	High micropore volume for Au(CN)₂⁻ adsorption
Gold loading capacity	≥ 25 mg Au/g	Economic capacity per cycle
Mesh size	6×12 (1.7–3.4 mm)	Must be retained on inter-stage screens
Ash content	≤ 3%	Low inorganic impurities — won't foul eluate
Moisture	≤ 5%	Accurate carbon dosing by weight

Do not substitute general-purpose coconut shell GAC (8×30 mesh, hardness 95%) for gold recovery. The coarser 6×12 mesh and higher hardness 97% specification are both critical — the larger particle size reduces attrition losses, and the higher hardness is specifically validated for abrasive slurry environments.

Loading & Commissioning

Initial carbon loading into CIP/CIL tanks:

Calculate carbon inventory: typical operating concentration is 10–15 g carbon per liter of slurry in each tank.
Screen fresh carbon on a 6-mesh screen to remove carbon >3.4 mm (oversize) and on a 12-mesh screen to remove fines <1.7 mm. Weigh the accepted fraction and record as the lot inventory.
Add carbon to tanks starting from the last tank in the circuit (lowest gold concentration) and working forward to the first tank (highest gold). This minimizes gold losses from newly loaded carbon.
Allow 24 hours of equilibration before establishing the carbon advance schedule.

Carbon advance (elution cycle management):

Advance loaded carbon from each tank to the preceding tank in a counter-current direction relative to slurry flow.
Typical advance interval: every 24–72 hours, depending on gold grade and circuit size.
Advance 20–30% of the carbon inventory per advance cycle to maintain steady-state gold loading profiles.

Operating Parameters

Parameter	Typical Range	Notes
Carbon concentration in tanks	10–20 g/L slurry	Higher concentration reduces tank count needed
Gold loading on carbon (pregnant)	5–20 mg Au/g C	Above 20 mg/g, adsorption rate slows significantly
Temperature of leach slurry	20–35°C	Higher temperature increases adsorption kinetics
pH of leach slurry	10–11	Protect against HCN volatilization
Cyanide concentration	300–600 mg/L NaCN	Maintains leaching driving force
Oxygen level (CIL)	6–8 mg/L dissolved O₂	Critical for gold-cyanide complex formation
Inter-stage screen slot size	0.8 mm	Retains 12-mesh carbon; must be maintained
Carbon advance frequency	24–72 hours	Adjust based on loaded carbon gold assay

Monitoring & Replacement

Indicator	Trigger for Action
Loaded carbon gold grade < 5 mg Au/g	Carbon not loading efficiently — check cyanide, O₂, pH
Carbon in screen underflow (fines)	Carbon breakage — check advance pump and screen condition
Carbon mass balance loss > 2% per month	Excessive attrition — check agitator tip speed
Post-acid-wash pH > 3.5 after 30 min soak	Acid wash insufficient — extend contact time
Post-reactivation iodine value < 900 mg/g	Carbon degraded — increase reactivation temperature
Gold in tails assay increases unexpectedly	Check carbon concentration, advance schedule, screen integrity

Elution (stripping): The ZADRA or AARL (Anglo American Research Laboratories) elution processes are standard:

AARL: Carbon is pre-soaked in hot (95°C) NaCN/NaOH solution, then strip-washed with deionized water at 110°C under pressure.
ZADRA: Continuous low-cyanide, high-temperature strip. Slower but simpler equipment.
Strip solution (eluate) goes to electrowinning to deposit gold on steel wool cathodes.

Acid washing: Before elution, acid wash the carbon at 5–10% HCl, pH 1–2, for 30–60 minutes at ambient temperature to dissolve calcium carbonate scale and other inorganic precipitates that reduce accessible gold loading sites.

Thermal reactivation: After 8–15 elution cycles (site-specific), carbon must be thermally reactivated in a rotary kiln at 700–750°C in steam atmosphere. Temperature must be controlled carefully: too low (<650°C) leaves organics that block pores; too high (>800°C) causes carbon gasification and mass loss. After reactivation, cool carbon rapidly under nitrogen to prevent re-oxidation.

Common Mistakes

Using wrong mesh size carbon: Introducing 8×30 mesh carbon into a CIP circuit designed for 6×12 mesh inter-stage screens causes massive carbon loss through the screens and unacceptable gold loss in the underflow. Always verify mesh size against the screen slot specification.
Over-loading carbon before elution: Allowing carbon gold grade to exceed 20 mg Au/g reduces adsorption efficiency in the front tanks and creates a risk of gold loss during mechanical upsets. Strip carbon at 10–15 mg Au/g for best economics.
Skipping acid wash before elution: Calcium carbonate scale from hard leach water blocks gold loading sites and adsorbs during elution as a competing ion. Skipping acid wash results in incomplete gold stripping and lower eluate gold concentration, reducing electrowinning efficiency.
Reactivating at incorrect temperature: Thermal reactivation below 650°C is ineffective at restoring carbon capacity — organics are not fully burned off. Above 800°C, carbon gasification losses increase sharply (>5% per cycle). Use calibrated kiln thermocouples and verify kiln atmosphere (steam content).
Insufficient carbon inventory tracking: Gold circuits require precise mass balance accounting of carbon inventory (fresh, loaded, stripped, reactivated). Without tracking, carbon losses go undetected for months, and gold losses through fine carbon carryover are attributed to other causes.

Storage & Handling

Fresh carbon should be stored in sealed super sacks (500 kg) or 25 kg bags, protected from rain and moisture contamination.
Pre-soak fresh carbon in water for 24 hours before introducing into circuit — dry carbon can cause flotation in the tank.
Screen fresh carbon lot immediately upon receipt — reject and claim if fines content exceeds 2% by weight.
Gold-loaded carbon (pregnant carbon) is a precious metal concentrate — handle under strict security protocols and account for all transfers.
Spent acid wash solution contains HCl, dissolved calcium, and trace cyanide — treat as hazardous waste; neutralize before disposal.