By the YELI Technical Team · Updated September 2026 · 8 min
For metallurgists and procurement teams specifying carbon for CIP / CIL circuits.
The Specification Everyone Uses — And Why It Under-Performs
Ask most procurement teams how they qualify a gold recovery carbon and you get two numbers: iodine value 1,050 mg/g and hardness 98%. Both are real. Neither predicts what the carbon will do in your adsorption train.
Iodine is measured with a small, non-polar molecule roughly 0.34 nm across; gold adsorbs as the aurocyanide anion, Au(CN)2−, a linear hydrated complex about 1 nm long. Different probe, different pores: a carbon can post a textbook 1,050 mg/g iodine and still under-perform on gold.
Two indices describe actual gold performance — both from the same Chinese national standard, GB/T 32992—2016 (Determination of gold adsorption capacity and rate of activated carbon). The title says exactly what they measure: K is capacity, R is rate.
K-Value: How Much Gold the Carbon Holds at Equilibrium
The K-value is the equilibrium gold loading capacity, reported in kilograms of gold per tonne of carbon (numerically identical to mg/g). In high-grade circuits, gold-grade coconut carbon holds 8–12 kg Au/t.
Capacity sets the working ceiling of your adsorption train. Carbon in the last tank can only hold so much gold before it stops pulling gold out of solution, and a low-capacity carbon hits that ceiling sooner — so dissolved gold in the final tank settles higher. Since the tailings equilibrate with that tank, a low K-value shows up directly in your tails assay.
Plants respond by raising carbon concentration or advancing carbon faster — recovering part of the loss, but increasing inventory, elution and regeneration load. It treats the symptom, not the capacity.
R-Value: How Fast the Carbon Gets There
R-value is the rate side: the percentage of equilibrium loading the carbon reaches within two hours in a standard ~1000 ppm Au cyanide solution, measured under the same standard. Gold-grade coconut carbon typically reaches 52–58% of equilibrium within that window.
R decides whether your circuit uses the capacity you paid for. Each CIP/CIL stage gives carbon roughly one hour of contact, so a carbon with good capacity but slow kinetics never nears its ceiling — it leaves the tanks under-loaded and pushes gold on to the next stage, and eventually to the tailings.
R also governs dynamic behaviour. Faster-loading carbon tolerates shorter contact time, which means headroom when throughput rises or a tank goes offline. Slow kinetics leave no margin, and margin is what keeps recovery stable on a real plant.
Note: K and R are different measurements — capacity and rate — and neither substitutes for the other. Always confirm which index a supplier quotes and the test conditions behind it. An index without its conditions is not a specification.
Why Two Carbons With the Same Iodine and Hardness Differ in K and R
1. Pore size distribution
Iodine fills the finest micropores; aurocyanide needs the wider window around 1–3 nm. Activation can maximise micropore volume while skewing it toward pores too narrow for a hydrated, charged anion. Iodine looks excellent; gold loading does not.
2. Surface chemistry
Gold dicyanide loads as an ion pair at graphitic, π-electron-rich sites that are hydrophobic. Oxygen functional groups — carboxyl, hydroxyl, carbonyl — raise polarity and water affinity, so the surface wets preferentially and pore-mouth water films block the bulky anion. Low ash is therefore a driver of K and R, not a separate spec.
3. Ash and mineral matter
At ≤3% ash the pore system is carbon and almost nothing else. Above 5–6%, mineral matter occludes pores and adds hydrophilic sites that preferentially hold calcium and copper-cyanide complexes instead of gold.
4. Feedstock consistency
K and R are only reproducible if the raw material is. Shell density and wall thickness vary with origin, species, season and maturity, so a supplier blending multiple origins shows lot-to-lot drift even when every COA reads 1,050 mg/g. A fixed single-origin programme — premium Philippine shell, locked grading — makes 8–12 kg Au/t repeatable, not fortunate.
Coconut Shell Activated Carbon 6x12 Mesh for CIL/CIP: The Specification Window Behind K and R
K and R are the outcome indices; the parameters below are what produce them. A gold-grade coconut shell activated carbon 6x12 mesh for CIL/CIP service is specified as a set, because each line reinforces or undermines the others — and a supplier who quotes one line against another is describing a catalogue, not a circuit.
| Technical Parameter | Specification Standard | CIL / CIP Operational Impact |
|---|---|---|
| Iodine Value | 1000 - 1100 mg/g | High initial gold adsorption kinetics (k-value) |
| Hardness / Abrasion | ≥ 98% | Minimized carbon breakage & tailing gold loss |
| CTC Absorption | 55% - 65% | Optimized organic fouling resistance |
| Mesh Size | 6x12 / 8x16 mesh | Efficient screening & pulp flow separation |
| K-Value / R-Value (GB/T 32992—2016) | 8–12 kg Au/t / 52–58% of equilibrium in 2 h | Higher equilibrium loading and faster approach; lower soluble gold in the final tank |
Core specification window for gold-grade coconut shell carbon — iodine per ASTM D4607, hardness per ASTM D3802, CTC per ASTM D3467, K and R per GB/T 32992—2016. Confirm against the batch COA with the test conditions stated.
The K and R rows only hold if the particle survives long enough to be measured in service. That is the connection worth drawing explicitly: high hardness activated carbon for gold extraction is not a separate product category competing with high-K carbon — it is the condition under which K is realised. A carbon that reaches 8–12 kg Au/t in a shake-flask test but fractures below the return screen cut in a stirred tank delivers neither capacity nor kinetics to the plant. An abrasion resistant activated carbon for hard rock ore is the same argument stated in terms of duty: the harder and more angular the grind, the more of the measured K-value is consumed by attrition before it is ever eluted. Tightening the hardness and fines specification is also the cheapest available way to reduce carbon attrition loss in the gold recovery circuit — it costs a line on the purchase order rather than a capital project, and it protects the K-value you have already paid for.
CTC absorption (55–65%) enters through regeneration. Organics — kerosene, flotation reagents, lubricants, humic matter — load pore mouths and force more frequent kiln passes, and each pass costs both capacity and mass. Pore-mouth blockage depresses R first, which is why a falling R on an in-plant carbon sample is often the earliest measurable sign of organic fouling rather than of a bad batch.
Typical Operating Conditions — Hard Rock Gold CIL/CIP
Typical Operating Conditions — Hard Rock Gold CIL/CIP:Optimized for high-density pulp environments (35-45% solids) in hard rock gold mining. High mechanical strength ensures carbon consumption remains below 25-35g per ton of ore processed.
For a plant specifying on K and R, that operating window is the test condition that matters most. K and R are equilibrium and kinetic indices, and both must be read against the pulp density, pH and cyanide concentration the plant actually runs — a K-value quoted at 1000 ppm Au in a synthetic solution is a starting point, not a guarantee.
A Three-Layer Framework When K/R Data Is Unavailable
Many suppliers cannot produce K or R data. That changes the evidence you ask for, not whether a decision is possible.
Layer 1 — Feedstock provenance
Single origin or blended? Region and grade named? Where are carbonisation and activation performed? Ask for a raw-material declaration, not a marketing claim.
Layer 2 — Read the specs as a set
Iodine with hardness, ash, bulk density, size distribution and fines content is a coherent picture. High iodine with ash above 5% or hardness below 96% signals capacity bought at the expense of performance — the classic signature of a general-purpose carbon being offered as high hardness activated carbon for gold extraction. Add CTC absorption to that set: below roughly 55%, expect faster organic fouling and a shorter interval between regenerations.
Layer 3 — Interrogate process control
Kiln type, burn-off control, acid washing, QC frequency — then ask how many lots per month are tested for K and R, and the standard deviation. A supplier who can quote a mean but not a spread does not control the process.
Side by Side: Iodine-1050 Carbon vs. Gold-Optimised Coconut Carbon
| Parameter | Typical general-purpose "iodine 1050" carbon | Gold-optimised low-ash coconut carbon |
|---|---|---|
| Iodine value | 1,050 mg/g | ≥1,050 mg/g |
| Ball-pan hardness (ASTM D3802) | ≥98% | ≥98% |
| K-value (equilibrium gold loading) | 4–7 kg Au/t | 8–12 kg Au/t |
| R-value (2-hour adsorption rate) | 35–45% of equilibrium | 52–58% of equilibrium |
| Ash (dry basis) | 4–8% | ≤3% |
| Feedstock | blended, multi-origin | single-origin Philippine shell |
| Indicative CIL effect | more soluble gold to tails; more elution cycles | lower soluble gold loss; lower kiln load |
Indicative ranges from supplier TDS and plant acceptance testing. Confirm against a batch COA and a plant trial on your own ore. Both indices are measured per GB/T 32992—2016.
On a 3,000 t/d CIL plant at 2.5 g/t Au, one percentage point of recovery is 75 g/day — about 26 kg (≈845 oz) per year, roughly US$2.2 million at US$2,600/oz. Carbon selection at this level is a recovery decision, not a procurement line item.
FAQ
Iodine value measures total micropore volume using iodine, a non-polar molecule about 0.34 nm across. K-value measures equilibrium gold loading capacity — how much gold the carbon holds — reported in kg Au per tonne under GB/T 32992—2016. Iodine tells you how much pore volume exists; K tells you how much of it the hydrated aurocyanide anion can actually use. Carbons with identical 1,050 mg/g iodine values routinely differ by a factor of two in K.
R-value is the adsorption rate index under GB/T 32992—2016: the share of equilibrium loading the carbon reaches within two hours in a standard ~1000 ppm Au cyanide solution. Each CIP/CIL stage gives carbon only about an hour of contact, so a slow-loading carbon never nears its capacity ceiling. It leaves the tanks under-loaded and raises dissolved gold in the final tank — which is what the tailings equilibrate with.
Use three layers of evidence. Audit feedstock provenance: is the shell single-origin and graded, and where are carbonisation and activation performed? Read the specifications as a set — iodine with hardness, ash, bulk density, size distribution and fines; high iodine with ash above 5% or hardness below 96% signals capacity bought at the expense of performance. Then ask how many lots per month are tested for K and R, and the standard deviation. If K and R remain unavailable, run a bench test on your own pregnant liquor.
Because K and R — the two indices that actually describe gold performance — can only be realised by a particle that survives the circuit long enough to approach equilibrium. Iodine value is measured on a fresh, dry sample in a laboratory; hardness decides how much of that carbon is still above the return screen cut after months of agitation, airlifting, pumping, elution and kiln passes. Carbon that breaks leaves as loaded fines, and the fresh carbon used to replace it enters the train unloaded, which dilutes the loading profile on the advance and effectively raises the equilibrium gold concentration the last tank sees. In practice a plant is better served by a carbon at 1,020 mg/g iodine with 98.5% hardness than by one at 1,080 mg/g with 93%: the first keeps most of its K-value in service, the second keeps its number only on the certificate. Iodine ≥1,050 mg/g and hardness ≥98% are not competing specifications — the second protects the first.
Mesh size is a compromise between mass transfer and retention. 6×12 mesh (1.70–3.35 mm) is the CIP/CIL default, sized two to three apertures above the typical 0.6–0.8 mm return screen cut. For a high-viscosity or high-density pulp — above about 40% solids, or a fine grind rich in clay — 8×16 mesh (1.0–2.36 mm) suspends more readily, presents a shorter diffusion path into the particle and reaches a higher share of its R-value in the same contact time. The cost is retention margin: finer carbon sits closer to the screen cut, so a strict <5% fines specification and hardness ≥98% become mandatory, and screen condition has to be monitored. One further point specific to K/R selection: both indices are measured on a defined size fraction, so if you change mesh, ask for K and R on the grade you are actually buying.
It attacks both loss routes at once. The soluble route: tailings pond solution equilibrates with the last adsorption tank, so the dissolved gold profile of that tank sets the tails grade. Coconut carbon with K = 8–12 kg Au/t and R = 52–58% holds that profile at the design floor, whereas a general-purpose 1,050-iodine carbon at K = 4–7 kg Au/t saturates earlier and lets the profile rise. The physical route: loaded carbon fines finer than the return screen cut leave with the tailings, and the dense, thick-walled structure of coconut shell carbon keeps them to a minimum at ≥98% hardness. Low ash supports both — fewer mineral occlusions means more of the measured pore volume is available to the Au(CN)2− anion, and a cleaner, stronger particle generates less fines.
Get the Full TDS — K, R and Test Conditions Included
Yeli coconut shell activated carbon is produced from imported premium Philippine coconut shell, with ball-pan hardness ≥98%, low-ash grade ≤3%, and gold performance quoted as K = 8–12 kg Au/t and R = 52–58% of equilibrium loading within 2 hours. Both are tested to GB/T 32992—2016 in our own QC laboratory on every batch, with the test conditions stated on the certificate. Ask for the complete TDS, a batch COA, and a 1 kg sample so your laboratory can run the K and R tests on your pregnant liquor.
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