By the YELI Technical Team · Updated August 2026 · 7 min read
Short answer: When gold recovery drifts down but the ore looks unchanged and your carbon tests fine on iodine, suspect three things before blaming the carbon: 1) preg-robbing ore — minerals that adsorb gold before your carbon can; 2) organic fouling — oils and reagents coating the pores; 3) true poisoning — iron, silica or sulfide scale blocking pores permanently. Diagnose in that order: a split leach test for preg-robbing, an acid-wash comparison for fouling vs poisoning, and a fresh carbon side-by-side for carbon quality.
Every gold plant has had the same confusing week: recovery is falling, the ore looks identical, the carbon passes its iodine check — and nobody can say why. Here is the field diagnosis we walk plants through when "nothing changed" but the numbers moved.
1. Preg-Robbing Ore: The Gold Is Never Reaching Your Carbon
Preg-robbing is an ore property, not a carbon problem. Certain ores — carbonaceous shale, certain clays, some graphitic zones — adsorb dissolved gold cyanide from solution faster than activated carbon can. The gold is "robbed" before it ever sees your carbon, so recovery drops no matter how good the carbon is.
How to test: Run two leaches on the same ore — one with activated carbon in the circuit, one without. If the no-carbon leach loses a large share of gold to the ore, you have preg-robbing. This is a simple, decisive field test that takes one day.
What it means: You cannot fix preg-robbing by switching carbon. But you can minimize the loss with faster-loading carbon (more gold captured before the ore steals it), and with process changes like finer grinding or higher carbon inventory. Know which you are fighting before you spend on new carbon.
2. Organic Fouling: The Pores Are Coated, Not Destroyed
Organic fouling coats the micropore surface with oils, flotation reagents, humic acids or grease. Activity drops because the gold can't reach the adsorption sites — but the carbon structure is intact, and a proper wash-and-regenerate cycle usually restores most of it.
How to test: Measure iodine on your current carbon, give it a standard acid wash, and measure again. A strong recovery after washing = fouling. That means your regeneration circuit needs attention, not a new carbon purchase.
Field note: Fouling is often seasonal or tied to ore changes — a new ore zone with more organics, or a flotation reagent change upstream. Log what changed in the last month.
3. True Poisoning: Scale That Blocks Pores Permanently
Calcium carbonate, iron hydroxide and silica scale build up in the pores and on the surface, blocking adsorption permanently. Unlike organic fouling, an acid wash recovers only part of the activity — the pore is physically occupied.
How to test: If iodine stays low after a standard acid wash and thermal regeneration, the carbon is poisoned, not fouled. In severe cases (iron or silica scaling), the only fix is replacement — and that is when you should also check your water chemistry and elution conditions to stop it recurring.
4. Rule Out the Carbon Itself — With a Side-by-Side Test
Once preg-robbing, fouling and poisoning are ruled out, the remaining suspect is carbon quality: activity, hardness, or batch consistency. The honest way to test is a side-by-side lab comparison — your current carbon versus a fresh reference sample, same test method, same day.
What we check: iodine, hardness, ash, and (where possible) gold loading kinetics. A faster-loading, harder carbon captures more gold before preg-robbing ore can steal it, and survives the circuit longer. That is the gap we measure before recommending anything.
We Help Plants Find Where the Gold Is Going
YELI Carbon is a coconut shell activated carbon manufacturer in Longyan, Fujian — 20+ years, gold-grade lines, hardness 98%+ (98% available), COA with every batch. If recovery is drifting, send us a sample of your current carbon and we will run a side-by-side test against ours: iodine, hardness, gold adsorption. See the real gap before you order.
Request a Sample Test →The Bottom Line
When recovery falls with no obvious ore change, don't rush to replace carbon. Run the split leach test for preg-robbing, compare iodine before/after acid wash for fouling vs poisoning, and only then test the carbon itself. For more on recovery troubleshooting, see our recovery drop guide, and for hardness, this hardness guide.
FAQ: Preg-Robbing and Carbon Poisoning
Preg-robbing is when minerals in the ore (often carbonaceous shale or clay) adsorb dissolved gold before the activated carbon can. The gold never reaches your carbon, so recovery drops even though your carbon tests fine. Test by leach-without-carbon versus leach-with-carbon on the same ore: if the difference is large, the ore is preg-robbing.
Fouling and poisoning are different. Organic fouling coats the pores with oils, flotation reagents or humic material — recoverable by acid wash and thermal regeneration. True poisoning (e.g. iron, silica, sulfide scaling) blocks pores permanently and needs replacement. If regeneration restores activity, it was fouling; if not, it is poisoning.
Yes. Calcium carbonate and iron hydroxide scale build up in the pores and on the surface, blocking access to adsorption sites. Acid washing removes most calcium scale; persistent iron or silica fouling may require a stronger wash or replacement. Test pore blocking by comparing iodine before and after a standard acid wash.
Only partly. Preg-robbing is an ore property — no carbon can fix it. But faster gold-loading kinetics (high activity, fine micropores) means more gold is captured before the ore can rob it. That is why hard, high-activity coconut shell GAC performs better than slower coal-based carbon in preg-robbing ores.