# How Ash Content Affects Gold Recovery Rate in CIL Plants — Specifying Coconut Shell Activated Carbon 6x12 Mesh for CIL/CIP *Technical note for metallurgists and procurement engineers running CIP / CIL gold circuits.* ## Carbon Is the Recovery Unit — Not a Consumable In a CIL plant, leach chemistry gets daily attention: cyanide titration, pH, dissolved oxygen, grind size. The carbon in the adsorption tanks gets far less — yet everything upstream only *dissolves* gold. The carbon is the only component that decides how much of it leaves the circuit as doré rather than reporting to the tailings dam. When a plant underperforms its feasibility recovery, the leach is blamed first; in practice, many unexplained shortfalls trace back to carbon quality, and **ash content** is among the most overlooked causes. ## What "Ash Content" Actually Measures Ash content is the inorganic, non-combustible residue left after a carbon sample is burned off completely — ASTM D2866 or GB/T 7702.15, at 550–650 °C, reported on a dry basis. It is a mixture: silica and alumina from sand, soil and kiln refractory dust; calcium and magnesium oxides from feedstock minerals and process water; iron and alkali oxides from the activation chemistry. ### Where the ash comes from 1. **Raw material.** Coconut shell is naturally clean — roughly 0.5–1.5% ash — but that advantage is lost if shells are collected from open ground or stored on bare soil. 2. **Carbonization and activation.** Char carryover, refractory dust and kiln combustion ash deposit mineral matter inside the developing pore structure. 3. **Post-activation washing — the decisive step.** Steam activation retains essentially all mineral matter; acid washing dissolves and leaches it out. **Ash content is a process signature, not a property of coconut shell.** Two carbons from the same Philippine shell can differ by five percentage points of ash purely on how thoroughly they were washed. Unwashed carbon commonly sits at 6–12%; properly acid-washed premium coconut carbon holds ≤3%, with the best lots at 1.5–2.5%. ## Three Mechanisms That Turn Ash into Gold Loss ### 1. Pore occlusion — the surface loss is disproportionate Ash minerals are not inert granules sitting between carbon particles: during activation they become embedded in, and block, the micropore and mesopore network that performs the adsorption. A carbon with 10% ash has about 10% less carbon skeleton, but the loss of *accessible* surface is far larger, because an obstruction at a micropore mouth deactivates the entire pore volume behind it. Moving from ~3% to ~9% ash typically costs 40–80 mg/g of iodine number and 15–30% of the kinetic adsorption rate. ### 2. Surface chemistry — ash makes carbon hydrophilic Gold loads from alkaline cyanide solution as the aurocyanide anion, Au(CN)₂⁻ — a large, linear anion adsorbed as an ion pair at graphitic, π-electron-rich sites that are hydrophobic by nature. Ash-derived oxides and hydroxides are hydrophilic and their charge shifts with pH, so they preferentially adsorb water. The resulting films across pore mouths physically block the bulky hydrated anion: the carbon still reports surface area, but gold cannot reach all of it. Ash also leaches — calcium and magnesium dissolve slowly in the pH 10.5–11 pulp, scaling the carbon as carbonate and gypsum and fouling the elution column. ### 3. Competing load and higher consumables Ash-rich carbons carry more oxygen functional groups and adsorb base-metal cyanide complexes, copper in particular, more aggressively. Copper cyanide competes with aurocyanide for the same sites, giving lower gold loading, more frequent elution and regeneration, dirtier bullion and higher cyanide consumption. The same mineral matter also weakens the individual particle, so a high-ash carbon is normally a softer carbon — which is why ash and hardness are usually specified and defended together. ## From a Lower K-Value to a Higher Tailings Grade The **K-value** is the standard gold-loading index for activated carbon, measured under GB/T 32992—2016 (*Determination of gold adsorption capacity and rate of activated carbon*) and reported as kilograms of gold per tonne of carbon. In high-grade circuits, gold-grade coconut carbon typically holds **8–12 kg Au/t** at equilibrium. K is a capacity figure, and capacity sets the floor on your dissolved-gold profile. Carbon in the last adsorption tank can only hold so much gold; as it approaches equilibrium loading it stops pulling gold out of solution. A low-K carbon reaches that ceiling sooner, so the dissolved gold concentration in the final tank settles higher — and the final tank is what the tailings solution equilibrates with. That higher profile *is* the soluble gold loss in your tails assay. Plants respond by raising carbon concentration or advancing carbon faster, which recovers part of the loss but increases inventory, elution and regeneration load. It treats the symptom, not the ash. ## Low-Ash vs. Typical-Ash Carbon: Side by Side | Parameter | Acid-washed premium coconut carbon | Typical unwashed / high-ash carbon | |---|---|---| | Ash (dry basis, ASTM D2866) | ≤3% (typically 1.5–2.5%) | 5–12% | | Iodine number (ASTM D4607) | ≥1,050 mg/g | 850–950 mg/g | | K-value (equilibrium gold loading, GB/T 32992—2016) | 8–12 kg Au/t | 4–7 kg Au/t | | Elution efficiency (AARL) | 96–98% | 90–94% | | Hardness (ASTM D3802) | ≥98% | 92–96% | | Copper loading tendency | Low | High | | Indicative CIL impact | Design recovery or better | 0.5–1.5 pt shortfall | *Indicative ranges from supplier TDS and plant acceptance tests. Confirm with a bench adsorption test on your own liquor.* ### What the difference is worth For a 3,000 t/d CIL plant at a 2.5 g/t Au head grade: - Contained gold **7,500 g/day**; a 1.5 percentage-point recovery gap **112.5 g/day** - Over 350 operating days: **~39.4 kg, or ~1,266 oz per year** — about **US$3.3 million** at US$2,600/oz - The carbon bill is roughly 40 t/year; even a US$400/t premium for low-ash carbon costs **US$16,000 per year**, around 0.5% of the gold value at stake ## Coconut Shell Activated Carbon 6x12 Mesh for CIL/CIP: The Specification Window Ash is the specification that most often goes unasked — but it never acts alone. In a CIL or CIP train, ash, iodine value, hardness, CTC and particle size work as a set, and every line in the table below maps to a specific behaviour inside the adsorption tanks. This is the specification window a gold-grade **coconut shell activated carbon 6x12 mesh for CIL/CIP** service is built to, with the operational consequence of each parameter stated against it. | 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 | | Ash Content (dry basis, ASTM D2866) | ≤ 3% | Fewer pore-blocking minerals; cleaner elution and lower copper load | *Core specification window for gold-grade coconut shell carbon — iodine per ASTM D4607, hardness per ASTM D3802, ash per ASTM D2866, CTC per ASTM D3467. Always confirm against the batch COA rather than a catalogue value.* Two lines in that table deserve more scrutiny than they normally receive. **Hardness and abrasion resistance** do not improve the adsorption rate; they protect the recovery you have already paid for. In a stirred tank, an airlift or a transfer pump, a soft particle breaks below the return screen cut and leaves the circuit still carrying its gold. That is why **high hardness activated carbon for gold extraction** is specified at ≥98% ball-pan hardness rather than the 90–95% typical of coal- and wood-based grades — it is the single most reliable lever available to **reduce carbon attrition loss in the gold recovery circuit**. **CTC absorption (55–65%)** measures how the carbon copes with the larger molecules present in real plant liquors: kerosene, flotation reagents, lubricants, humic matter and other organics carried over from the mill. In an organics-heavy circuit, fouling blocks pore mouths and depresses both K and R between regenerations. A CTC inside the 55–65% window, read together with low ash, is the practical proxy for organic fouling resistance when a full adsorption isotherm is not available. ### 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. That consumption figure is the commercial form of the hardness specification, and it is the reason an **abrasion resistant activated carbon for hard rock ore** is the correct default where a fine, angular, high-specific-gravity slurry is fed into the leach train. The sharper and denser the pulp, the faster a marginal carbon is consumed — and the more fresh, unloaded carbon has to be added simply to hold the dissolved gold profile flat. ## Specification Checklist for a Gold Carbon TDS - **Ash ≤3%** dry basis, with the test method named on the certificate - **Iodine number ≥1,050 mg/g** as a surface-area proxy - **Hardness ≥98%** (ASTM D3802) to limit attrition losses - **CTC absorption 55–65%** as an organic fouling-resistance indicator - **K-value (equilibrium gold loading, kg Au/t)** per GB/T 32992—2016 - **Particle size** 6×12 mesh with a stated fines fraction, plus a batch COA ## Frequently Asked Questions **Q1. What is an acceptable ash content for activated carbon in gold CIL/CIP circuits?** Treat **≤3% ash on a dry basis** as the premium benchmark and **≤5%** as the practical ceiling for carbon you intend to run to full life. Above roughly 6%, pore-occlusion and surface-chemistry penalties become large enough to show up as a measurable rise in the dissolved gold profile of the last adsorption tank. Always quote ash on a dry basis with the test method stated. **Q2. How does high ash content reduce gold recovery?** Three linked mechanisms. Ash minerals occlude micro- and mesopores, so less surface is accessible to aurocyanide. Ash-derived oxides make the surface hydrophilic, and water films at pore mouths block the bulky Au(CN)₂⁻ anion. Ash also leaches calcium and magnesium into the pulp, scaling the carbon, while ash-rich surfaces preferentially load competing copper cyanide complexes. The net effect is a lower K-value and lower equilibrium loading, which raises soluble gold in the final tank — the direct driver of gold loss in CIL tailings. **Q3. Can a plant verify ash content itself, and what else should it test?** Yes — ash is a straightforward muffle-furnace test at 550–650 °C on a dried sample, and most plant laboratories can run it against ASTM D2866. Ash alone will not predict CIL performance. Pair it with iodine number, hardness, particle size distribution and, most decisively, a bench gold adsorption test (K-value and equilibrium loading) on your own pregnant liquor at your own pH and cyanide. **Q4. Why is carbon hardness (>98%) more critical than iodine value in CIL plants?** Because iodine value describes a fresh, dry laboratory sample, while hardness describes what actually survives the plant. Iodine is measured once, on delivery; hardness decides how much of that carbon is still above your return screen cut after months of agitation, airlifting, pumping, elution and kiln passes. A carbon can post 1,050 mg/g iodine on the certificate of analysis and still be consumed at 45 g/t of ore rather than 18 g/t, because the weak fraction breaks and leaves the circuit as loaded fines carrying gold that can never be eluted. High ash compounds the problem from both directions — mineral matter weakens the particle *and* occupies pore volume, so capacity and mass are lost together. Treat iodine ≥1,050 mg/g and ball-pan hardness ≥98% (ASTM D3802) as a matched pair rather than ranking one above the other. **Q5. What mesh size is recommended for high-viscosity gold slurry?** **6×12 mesh** (roughly 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. In a high-viscosity or high-density pulp — above roughly 40% solids, or a fine grind rich in clay — coarse carbon can raft on the pulp surface, channel through the tank or settle unevenly, and an **8×16 mesh** grade (about 1.0–2.36 mm) suspends more readily and shortens the diffusion path to the aurocyanide anion. The trade-off is retention margin: finer carbon sits closer to the screen cut, so a fines fraction below 5% as shipped and hardness ≥98% stop being preferences and become requirements, and screen condition has to be watched. Unless mixing tests show the coarser grade is not being held in suspension, most plants keep 6×12 mesh and manage viscosity through pulp density instead. **Q6. How does coconut shell carbon reduce gold loss in tailing ponds?** Through two distinct routes. The **soluble route** is chemical: the tailings pond solution equilibrates with the last adsorption tank, so a low K-value leaves the dissolved gold profile in that tank higher and raises the tails grade, whereas a gold-optimised coconut carbon holding 8–12 kg Au/t holds it at the design floor. The **physical route** is mechanical: attrition fines finer than the return screen cut carry adsorbed gold out of the circuit, where it cannot be eluted, and a high-hardness coconut shell carbon at ≥98% ball-pan hardness limits that stream to roughly 15–25 g per tonne of ore. Ash content drives both — ash minerals occlude pores and weaken the particle. A plant that has already optimised grind size, cyanide concentration and pH and still measures elevated soluble gold in its tails is usually looking at carbon quality rather than leach chemistry. ## Get the Data Before You Change the Carbon Yeli coconut shell activated carbon is produced from imported premium Philippine coconut shell, acid-washed to a typical ash content of 1.5–2.5%, with hardness ≥98% and a K-value of 8–12 kg Au/t per GB/T 32992—2016. Send us your pregnant liquor assay, pH, cyanide concentration and circuit configuration, and we will return a technical data sheet plus a 1 kg sample so your laboratory can run the tests itself. - **Specification sheet / TDS:** [yelicarbon.com/gold-carbon-specification-sheet.html](https://www.yelicarbon.com/gold-carbon-specification-sheet.html) - **Sample COA:** [yelicarbon.com/coa-sample-gold-carbon.html](https://www.yelicarbon.com/coa-sample-gold-carbon.html) - **Contact our technical team:** [yelicarbon.com/contact.html](https://www.yelicarbon.com/contact.html) · WhatsApp **+86 136 0090 6330** ## FAQ Schema (JSON-LD) — copy into your page `` ```html ```