Gold Processing7 min read
Leaching can only act on gold that is exposed. Everything upstream of the CIL tanks — crushing, grinding, classification, and the gravity circuit — decides how much gold surface…
Author: Wayne Ncube
Leaching can only act on gold that is exposed. Everything upstream of the CIL tanks — crushing, grinding, classification, and the gravity circuit — decides how much gold surface the cyanide solution will ever see. An illustrative front-end case shows how quickly a grinding shortfall becomes a leaching shortfall.
Schematic grind response
Finer grind usually exposes more gold, but the curve is not a universal plant law.
Front-end flowsheet with illustrative duty
Illustrative values; confirm equipment and operating data against the actual circuit.
Model of historic gold-mining mills near Zlaté Hory, Czech Republic. Photo: Czeva, Wikimedia Commons, CC BY-SA 3.0. The model is historical context; the operating principles still matter.
In an illustrative flowsheet, run-of-mine ore is drawn from a stockpile by an apron feeder and crushed in a single-toggle jaw crusher. The crusher takes a nominal feed of about −600 mm and delivers a product that is essentially all passing 90 mm at a normal closed-side setting near 100 mm. Conveyors carry the crushed product to the SAG mill feed hopper, with an emergency stockpile available when the circuit stops.
Grinding follows in a single-stage SAG mill with a pebble-crusher circuit:
Nothing here is optional. Each step sets the conditions under which the leach will later operate.
Hydrocyclones split the mill product by settling behaviour. Coarse particles report to the underflow and return to the mill; fines and most of the water leave through the vortex finder as overflow and proceed to leaching. The balance of centrifugal and drag forces decides the cut, and that balance shifts with operating pressure, vortex-finder and spigot diameter, feed size distribution, pulp density, and slurry viscosity.
An illustrative circuit specification uses approximately 80% of the cyclone overflow below 106 µm. Gold-bearing particles larger than this may not be sufficiently exposed for the leach to reach its recovery target. The overflow specification is therefore a leach-circuit requirement expressed as a grinding variable.
This is also why worn classifier parts matter. A worn spigot or vortex finder changes the cut size without triggering a mechanical alarm. In an illustrative case, the symptom was a grindability shortfall — the discharge and overflow rarely reached the required fraction passing 106 µm — and the first responses were to feed the mill closer to its recommended rate and to check and replace worn spigot and vortex-finder parts.
A cyclone does not have a fixed cut size. The split point responds to feed size distribution, pressure, pulp density, viscosity, and the condition of the spigot and vortex finder. This means a hydrocyclone can be the hidden actuator in a grinding-control loop: if the overflow specification changes, the downstream leach sees a different feed even when the mill feed tonnes and mill motor load look normal.
The useful operating check is not only “is the mill running?” It is a small set of linked measurements:
| Measurement | What it tells you | Downstream consequence if wrong |
|---|---|---|
| Mill feed tonnes and power | Loading and grinding intensity | Overgrinding or coarse product |
| Cyclone pressure | Centrifugal-force operating point | Overflow cut moves |
| Overflow solids and size distribution | What the leach actually receives | Gold exposure changes |
| Underflow size distribution | What returns to the mill | Circulating load grows |
| Density and viscosity | Slurry transport and cyclone balance | Cut and residence time shift |
A pressure change should not be interpreted without the density and size distribution. The same pressure at a different solids fraction represents a different hydrocyclone operating point.
The gravity circuit is not merely a bonus recovery step. It changes where gold enters the downstream process. A Knelson concentrator removes dense particles from a controlled portion of the circulating load, while the inline leach reactor treats the concentrate under more intense conditions than the main CIL circuit. Together they can protect coarse or free gold from repeated regrinding and reduce the chance that valuable metal plates onto mill liners.
The key performance measures are not only recovery. Track concentrate grade, concentrate tonnes, gravity-circuit uptime, screen oversize return, ILR solution consumption, and the gold reporting to the tail of the gravity circuit. A lower gravity recovery may not be visible in the final plant recovery if the CIL can still recover the liberated material, but it can increase carbon, oxygen, and cyanide demand later. Conversely, a high gravity result with poor concentrate handling may not produce the expected product.
1d80106.02target_passing80.03measured_passing70.045shortfalltarget_passingmeasured_passing6printf"size-control shortfall = {shortfall:.1f} percentage points"7print"action: inspect cyclone internals before increasing cyanide"
The gravity circuit protects gold that is already liberated. Its purpose is to remove free gold from the SAG mill circulating load before the remaining material is reground. Without this step, dense gold reports back to the mill repeatedly and is eventually ground so fine that it cannot be recovered efficiently in CIL, or it plates onto the mill liners — a physical loss that no amount of cyanide can recover.
An illustrative gravity route is:
The Knelson operates within a defined envelope — slurry flow, bowl speed, fluidization current, and concentration pressure all have windows. The ILR operates at conditions far outside the CIL norm: high dissolved oxygen, strongly alkaline pH, and a cyanide concentration around two orders of magnitude above the main circuit, with batch leach times from several hours up to a day.
80% <106 µm
illustrative cyclone-overflow leach specification
6–24 h
illustrative intensive-leach batch duration
~50–70%
share of gold the gravity route can remove before CIL
These observations form one teaching story:
| Observation | Mechanism | Leach consequence |
|---|---|---|
| Overflow rarely meets the 106 µm target | Overfeeding and worn classifier parts | Less exposed gold, higher residue grade |
| Pebbles bypass or overload the pebble crusher | Metal detector trips, blocked tubes, worn liners | Circulating load grows, grind coarsens |
| Gravity recovery drops | Screen misfeed, concentrator drift, ILR cycle issues | Coarse gold recirculates, over-grinds, or plates out |
The discipline is to treat grind size, classification condition, and gravity performance as leach variables. When tailings grade rises, they belong in the first conversation — before reagent rates are changed.
Intensive leaching uses concentrated cyanide solutions at elevated pH, and crushing and grinding equipment carries serious mechanical hazards. The values here are illustrative teaching values, not operating limits for any site. Follow the site's chemical-management, isolation, and lock-out procedures.
Further reading in this series: