The problem with iodine number in gold recovery
Iodine number (IV) is the most widely quoted specification on activated carbon datasheets. It measures adsorption capacity using iodine as the test molecule. For liquid-phase applications — water treatment, food processing — it is a useful indicator. For gold recovery, it tells you almost nothing useful.
The gold-cyanide complex — Au(CN)₂⁻ — is a large, heavy anion. Its adsorption onto activated carbon is governed primarily by the micropore structure in the 1–2 nm range, and by the surface chemistry of the carbon. Iodine adsorption does not correlate predictably with gold adsorption kinetics or capacity.
What actually matters: K value
The Freundlich K value is the industry standard for predicting gold adsorption performance. It is determined by equilibrium adsorption testing using synthetic gold cyanide solutions at defined concentrations. A higher K value means more gold adsorbed per unit of carbon at equilibrium.
- K value 40–50: acceptable performance for most CIL/CIP duties
- K value 50–60: good performance — most premium grades
- K value 60–65: exceptional — reserved for the best coconut shell carbons
Rajindra’s ACGOLD series achieves K values up to 65, with CTC 45–65% and hardness 99% (ASTM). These are independently verifiable numbers — not marketing claims.
CTC: carbon tetrachloride activity
CTC (carbon tetrachloride activity) measures the micropore volume available for small molecule adsorption. In gold recovery, it correlates well with gold adsorption capacity — more so than iodine number. Most gold recovery specifications require CTC > 45%.
Hardness — the overlooked parameter
In CIL and CIP circuits, carbon circulates continuously through pumps, screens, and elution vessels. Carbon that generates fines loses gold-loaded particles to the tailing stream — an expensive and irreversible loss. Hardness > 98% (ASTM) is the minimum for responsible circuit design.