Gold plants are losing carbon — and gold — to attrition. The design fixes that cut losses.

Carbon in pulp (CIP) and carbon in leach (CIL) — gold extraction flowsheets that adsorb dissolved gold onto activated carbon in cyanide slurry — are mechanically punishing on the adsorbent. Slurry agitation, pump transfers, and thermal regeneration fracture particles into fines and carry loaded carbon to tailings. Modern gold plants lose on the order of 64 g of carbon per tonne of ore as fines (Kale, 2019).

One industry breakdown is stark: about 45% of the loss happens in adsorption mixing (tanks and transfers), roughly 40% during elution/regeneration, and the remaining ~15% during pumping and screening (industry data). Those grams translate straight to lost gold and replacement cost — so attrition reduction is not optional.

Carbon loss baseline and drivers

CIP/CIL mixing is “hard” on carbon (Kale, 2019). Attrition is the mechanical breakdown of carbon pellets into smaller particles (fines) through impacts and shear. Besides the tanks, transfers, and pumps, thermal swings during elution/regeneration add stress that accelerates fracture.

Losses accumulate across the circuit: adsorption mixing can dominate, elution/regeneration is close behind, and the rest accrues in ancillary handling and screening (approximately the 45%/40%/~15% split above, per industry data).

Carbon physical properties and sizing

A key control is the quality and size of the activated carbon itself. High‑hardness, abrasion‑resistant carbons — typically coconut‑shell‑based — are standard in gold CIP/CIL because they generate far fewer fines than coal‑based or wood‑based carbons (technical reviews).

Indeed, “coconut shell carbon has a high resistance to abrasion and is… the most preferred” form for CIP/CIL (same source). Typical fresh carbon is manufactured to coarse sizes — around –6+18 mesh (≈3.3–10 mm) — to balance surface area and strength (manufacturing spec). Larger lumps resist abrasion better; very small pellets or fines both trap less gold and break up readily.

In practice, mines that stick to high‑grade, uniform coconut carbon and avoid undersized fines can halve attrition relative to inferior grades. By contrast, low‑quality coal carbons can crumble quickly (technical reviews). This is the niche for activated carbon specifically selected for high hardness in slurry duty.

Other material factors also matter. Smooth, crack‑free particles scuff less. Some operations pre‑attrition the carbon — a short, vigorous “pre‑conditioning” mix and screen — to knock off brittle fragments before service. Any impregnation or metal coating is avoided because impurities can weaken the carbon matrix. Mines track an “Attrition Number” (ASTM‑like laboratory tests) and reject batches that flake easily.

CIP tank and agitator configuration

CIP adsorption trains are typically large agitated vessels, often 4–6 in series. The design aim is complete contact with minimal shear: up‑pumping, axial‑flow agitators running at low tip speeds of about 0.5–1 m/s to suspend ore and carbon without violent turbulence (Kale, 2019).

Multiple shaft‑mounted impellers — flat‑blade or low‑pitch turbines — are spaced along the shaft for gentle circulation, with baffles or draft tubes to prevent vortexing. Designers limit speed and power such that 90–95% of carbon never rises above the bulk slurry surface. Smaller‑diameter agitators (relative to tank) and full‑height propellers tend to produce more laminar flow, and some plants use submersible‑pump recirculators or dual‑impeller drives to avoid high‑shear paddles.

Carbon is often fed slowly at the tank bottom and withdrawn near the top to minimize “fall height.” Up‑pumping impellers and hydrofoil blades have shown 20–30% lower attrition than downward‑pumping paddles. One plant cut mixing losses by switching from a 900 rpm high‑shear turbine to a 60 rpm low‑shear axial propeller. The consensus is clear: gentler agitation = significantly less attrition. Even with good design, mixing still accounts for roughly 40–50% of carbon loss (industry data), so optimization matters.

Fine carbon recovery screens

Attrition generates fines under 1 mm. If not intercepted, those particles carry adsorbed gold to tailings. Best practice is to add fine screens or classifiers at strategic discharge points. A vibrating or trumpet screen with 0.25–1 mm apertures is commonly placed at the final CIP tank discharge (or just before tailings), and some circuits use intermediate decks. Apertures are chosen so slurry and ore pass while carbon is retained.

Industry data underscore the stakes: Stanley (1987) found that carbon screenings — tank overflows and loaded‑carbon screens — accounted for roughly 5–6% of total carbon loss (reported in reviews). A good tailings screen can recover 80–90% of carbon fines, cutting losses by several grams per tonne. For a 2 Mtpa plant, a 0.5 mm recovery screen catching an extra 2 g C/tonne saves about 4000 kg C per year (worth dozens of kilos of gold). Typical designs include wedge‑wire static baskets or high‑frequency vibrators to prevent blinding; screens need periodic spray‑water cleaning but otherwise run continuously. Systems in this duty often resemble an automatic screen integrated at the tailings interface.

Performance outcomes and case notes

Case studies show well‑designed CIP circuits can hold total carbon losses to <50 g/tonne ore (versus >100 g/t in older mills) (Kale, 2019). Adding or upgrading fine screens often trims lost carbon by 20–30%. In one large operation, installing a second‑tier 0.5 mm screen after the last tank reduced carbon loss from about 60 to 40 g/t. Separately, improving agitator profiles and slowing impellers has cut attrition by 15–20% in practice (even without changing chemical conditions).

The benefits are direct: less carbon make‑up, more gold retained. (Where available, Indonesian operators follow these global guidelines.)

Implementation summary and sources

Minimizing attrition is a three‑part program: choose high‑hardness coconut‑shell carbon in coarse mesh; engineer CIP tanks and agitators for gentle, uniform mixing; and capture fines with aggressive screening. The measures reflect best‑practice guidance and, where applied, measurably reduce carbon losses and raise gold yield (material quality; sizing; loss figures and causes; stage‑by‑stage losses). All data are taken from peer‑reviewed conference and mining industry reports.