Iodine 1000 vs 1100 Coconut Shell Activated Carbon for CIL Gold Plants: What Actually Changes Recovery

By the YELI Technical Team - Updated October 2026 - 8 min read

Short answer: The iodine number measures micro-pore development, not gold capacity. A carbon sold as iodine 1000 mg/g and one sold as iodine 1100 mg/g can overlap once the ±50 mg/g tolerance is applied, so the recovery difference between them is usually smaller than the difference made by pore size distribution, hardness (≥98%), platelet content (<1.5%), mesh (6×12 mesh, 1.70–3.35 mm) and adsorption kinetics. Pay for 1100 when the leach solution is high grade, contact time is short, or you need a higher equilibrium loading per tonne of carbon. Stay on 1000 when recovery is already close to the circuit limit, when the bottleneck is elution, screening or attrition, or when the real cost each month is carbon replacement rather than carbon capacity.

When a CIL plant writes a purchase enquiry, the iodine number is the first number in the email and often the only number compared between suppliers. That single number then decides the order. It should not.

Iodine 1000 mg/g and iodine 1100 mg/g are two grades of the same product family, measured by GB/T 12496.8 or ASTM D4607, and the price difference between them is real. The recovery difference on most operating CIL circuits is not what the price difference implies. This article separates the label from the physics: what the iodine number does and does not tell you, when the extra 100 mg/g earns its money, when it does not, and how to compare two quotes that were tested on different bases.

What the iodine number actually measures — and what it does not

Iodine number is the mass of iodine, in milligrams, adsorbed by one gram of activated carbon under the conditions of GB/T 12496.8 or ASTM D4607. The iodine molecule is small, so the test reports on pores below roughly 2 nm — the micropores. For a coconut-shell gold carbon specified at iodine 1000–1300 mg/g, hardness ≥98% (GB/T 12496.6 / ASTM D3802), platelet content <1.5% and apparent density 0.45–0.52 g/cm³ (GB/T 12496.1 / ASTM D2854), the iodine number is a cheap, repeatable proxy for how well the activation kiln did its job on micro-pore volume.

That is the whole of what it says. It is one scalar. It does not describe:

• Mesopore and macropore volume. Roughly 2–50 nm and above. This controls how quickly the gold cyanide complex reaches the micropore mouth through a pulp that also contains free cyanide, thiocyanate, copper and calcium.
• Granule geometry. The diffusion path from the outer surface to the centre of a 1.70–3.35 mm granule is set by mesh (GB/T 12496.2 / ASTM D2862), not by iodine.
• Mechanical strength. Hardness on the ball-pan method and platelet content are measured on different instruments, from different samples, and they move independently of iodine.
• Gold loading. The K value (gold loading capacity) and R value (adsorption rate) are determined by an independent third party and reported with the COA, on request, under GB/T 32992—2016. Neither is interchangeable with the iodine number.

Two carbons with the same iodine number can behave very differently in a CIL tank if their mesopore volume, granule size or hardness differ. That is why two plants running what looks like an identical specification report different carbon inventories, different elution intervals and different carbon losses. If you want the test itself explained, see our note on iodine value testing for gold carbon.

Why a "1000" grade and an "1100" grade can both test 1050

YELI quotes iodine with a tolerance of ±50 mg/g relative to the ordered grade. Practically: a 1000 grade ships anywhere from 950 to 1050 mg/g, and an 1100 grade ships from 1050 to 1150 mg/g. At the boundary the two grades are the same carbon.

Four traps follow from that.

The typical-value trap. A data sheet that says "iodine 1100 typical, 1050 minimum" is offering you 1050. Compare guaranteed ranges, never typical values. A supplier whose 1000 grade is guaranteed at 1000 minimum is selling you more micro-pore volume than a supplier whose 1100 grade is only guaranteed at 1050 minimum.

The method trap. GB/T 12496.8 and ASTM D4607 are both iodine adsorption tests, but they are not identical in procedure and endpoint. Numbers generated by the two methods are close, not equal, and a lab reporting to 1 mg/g of precision is implying precision the test does not deliver. Fix one method in the purchase order and normalize every quotation to it.

The basis trap. Iodine is normally reported on a dry basis, while moisture as packed is ≤5% (GB/T 12496.4 / ASTM D2867). If one lab reports as-received and another reports dry, the numbers are not comparable even when the carbon is identical. Always ask which basis is on the COA.

The sampling trap. A grab sample taken from the top of a jumbo bag that has settled during a six-week sea voyage is not the same sample as one taken from the discharge stream at the factory. Sampling point, lot number and sample date belong on the certificate. Our COA verification guide shows what a certificate should contain before you accept a number as contractual.

What actually decides CIL recovery

Every variable below sits between the iodine number and the gold in your tailings. When a plant reports poor recovery, nine times out of ten one of these is the cause, not 100 mg/g of iodine.

Pore size distribution decides whether the capacity the iodine number promises is reachable in your residence time. A carbon at iodine 1050 with a well-developed mesopore network will out-recover a carbon at iodine 1100 with a narrow micropore-dominated structure in a short-contact-time CIL train, because the gold cyanide complex has to arrive before it can be adsorbed.

Hardness decides how much carbon you lose as fines and how much gold goes out with them. YELI ships ≥98% on the ball-pan method. Coconut-shell carbon typically lands at 96–99% on that test (Industry-typical; not a YELI process commitment.), against 90–95% for coal-based carbon (Industry-typical; not a YELI process commitment.). Attrition loss is usually a bigger monthly cost than an iodine shortfall, and it is invisible on a specification sheet that only lists iodine.

Platelet content matters more than buyers expect. Platelets are flat, weak flakes left over from shell crushing. They break early, blind screens, and skew a hardness test taken on a sample that has not yet been abraded. A water-polished, de-flaked product held below 1.5% behaves like a different material in the adsorption tanks. Our article on why carbon hardness matters in gold recovery covers the failure mechanism in detail.

Mesh sets both retention and kinetics. 6×12 mesh (1.70–3.35 mm) is the CIL workhorse. 8×16 mesh gives a faster approach to equilibrium and more surface area per tonne, at the cost of higher screen losses; the 3×6 mm cut suits specific column duties. Compare grades at the mesh size guide before trading kinetics against losses.

Apparent density decides how much carbon a given tank actually holds, and how much a container actually carries. At 10 m³ of carbon, 0.45 g/cm³ gives 4,500 kg and 0.52 g/cm³ gives 5,200 kg — a difference of about 700 kg, roughly 15%, for the same volume. A lighter carbon fills your tanks with less mass and costs more freight per tonne; a heavier carbon does the reverse. Neither is automatically better, but the two quotes you are comparing are not the same purchase if their densities differ.

Kinetics — the R value direction — decides how close the carbon gets to its equilibrium loading in the time the pulp gives it. A faster carbon behaves like a larger carbon inventory. This is the parameter that iodine number approximates worst, which is why a third-party R test under GB/T 32992—2016 is worth requesting when the circuit is tight.

For reference, typical CIL operating values sit around 10–25 g/L carbon concentration (Industry-typical; not a YELI process commitment.), 45–50% solids in the pulp (Industry-typical; not a YELI process commitment.) and 20–100 g/t ore of makeup carbon (Industry-typical; not a YELI process commitment.). If your circuit is inside those ranges, the carbon is not usually the constraint.

Does a higher iodine number mean more gold on the carbon?

In direction, yes: at equilibrium, for the same solution, more micro-pore volume holds more gold. The problem is that a CIL train is not operated at carbon equilibrium, and the loading you actually see in the field is set by the circuit, not by the certificate. Loaded carbon in a well-run plant typically runs 3,000–10,000 g Au/t (Industry-typical; not a YELI process commitment.), which is a long way below what the carbon could hold if it were left in contact with fresh pregnant solution indefinitely.

The levers that move field loading are the number of adsorption stages, the carbon advance rate, the carbon concentration in each tank, and residence time per stage. If carbon advances out of the last tank before it has approached equilibrium, the extra 100 mg/g of iodine is paid for and never used. In that situation the correct fix is a slower carbon advance or an extra stage, not a more expensive grade.

This is also why a supplier's own equilibrium loading figure should be treated with care. Ask for K and R from an independent laboratory, under GB/T 32992—2016, reported on request. A direction and a test method you can verify beat a marketing number you cannot.

When is iodine 1100 worth the extra cost?

Higher iodine costs more to make. Reaching it means more activation energy, more kiln time and lower yield per tonne of coconut shell, so the price per tonne is higher. It earns that premium in a specific set of situations:

• High-grade leach solution. When the gold concentration in solution is high, the isotherm is favourable and the carbon can actually fill the extra micro-pore volume. Dilute solutions leave it empty.
• Short contact time. High throughput, few stages or undersized tanks mean the carbon has less time to work. Better-developed microporosity plus a favourable R direction is what buys recovery back.
• Long elution intervals. If you want to strip less often, you need more grams of gold per tonne of carbon before each elution. That is a capacity argument, and it is the cleanest case for the higher grade.
• Fixed tank volume. If you cannot raise carbon concentration in g/L because the tanks and screens are already at their limit, capacity per tonne is your only remaining lever.
• A genuinely capacity-limited circuit. If your kinetic and mass-balance data show the tailings loss is caused by the carbon not holding enough gold, the upgrade is justified. Measure first, then buy.

Do the arithmetic on cost per gram of gold, not cost per tonne. If a grade carries a 10% price premium, it must deliver roughly a 10% increase in useful loading per tonne to break even, all else equal — and useful loading is the loading you actually strip, not the loading on the certificate. If the premium cannot be traced to a change in stripped grams per tonne, it is a donation. Our quote comparison walkthrough shows how to put two offers on the same footing.

When is iodine 1000 the better buy?

More often than the market implies. Iodine 1000 mg/g is a fully specified gold-grade carbon, and on many circuits it is the correct purchase:

• Recovery is already near the practical ceiling. CIL circuits typically adsorb 95–99.5% of the gold in solution (Industry-typical; not a YELI process commitment.). When you are there, the remaining loss is in the tailings and it is rarely a micro-pore problem. Start with tailings gold loss diagnosis instead of a grade upgrade.
• Elution or regeneration is the bottleneck. Extra capacity per tonne that you cannot strip is inventory, not recovery. If the elution column or the regeneration kiln is running full, the constraint is downstream of the carbon.
• Attrition and screen losses dominate. If carbon is leaving the circuit as fines, a more expensive tonne of carbon is a more expensive loss. Fix hardness, platelet content and pump/screen condition first.
• Regeneration dominates the spend. Each thermal cycle loses part of the charge — commonly 5–15% per cycle (Industry-typical; not a YELI process commitment.) — and a batch is typically used for 5–8 cycles (Industry-typical; not a YELI process commitment.). You pay the grade premium again on every top-up, so the premium compounds.
• Low-grade or dilute leach liquor. The extra micro-pore volume will not be filled, and you will never see it on a recovery sheet.

How to compare an iodine 1000 quote with an iodine 1100 quote

Most purchasing mistakes on this decision are made at the comparison stage, not at the grade stage. Two offers that look 100 mg/g apart are frequently the same carbon, or not comparable at all.

Two practical rules come out of that table. First, compare like with like: if one supplier quotes a guaranteed minimum and the other quotes a typical value, convert both to guaranteed ranges before you look at price. Second, price the delivered carbon, not the FOB tonne — a lower-density carbon moves fewer kilograms per container, which changes the landed cost per tonne even when the price per tonne is identical. Our purchase specification checklist and the gold recovery carbon specification sheet give you the wording to paste into a purchase order.

Frequently asked questions