Gold Processing5 min read
The CIL can be chemically perfect and still receive the wrong feed. Dilute slurry shortens residence time; grit loads pumps and screens; uncaptured water leaves with tailings and…
Author: Wayne Ncube
The CIL can be chemically perfect and still receive the wrong feed. Dilute slurry shortens residence time; grit loads pumps and screens; uncaptured water leaves with tailings and takes solution losses with it. Thickening, trash removal, and the return-water path decide what “correct chemistry” actually contacts.
Water and solids loop
Trash removal, densification, and tailings screening decide what chemistry the CIL actually receives.
Cyclone overflow is typically a relatively dilute slurry. Before leach or thickening, plant practice often screens for trash and wood chips that would otherwise:
An illustrative trash-screen design uses dual decks with 0.6 × 0.5 mm apertures — confirm against your own screens. Wiper or deck condition is part of the leach’s reliability, not a separate utilities task.
A continuous thickener produces:
Illustrative teaching ranges for gold plants:
| Parameter | Illustrative range (confirm on site) |
|---|---|
| Underflow solids | ~50–60% solids (density often tracked as RD ~1.5–1.7) |
| Overflow | Clear enough for recycle without re-grinding fines to leach |
| Flocculant | Dose and make-up quality set floc size and bed permeability |
| Condition | What the circuit sees | Operator response direction |
|---|---|---|
| Under-densified | Short CIL residence, high water report, pump overload risk | Review flocculant, bed level, feed rate |
| Over-densified | Poor mixing, high torque, starved tanks | Reduce bed, check rake and feed balance |
| Dirty overflow | Recycle carries fines back to mills or leach | Flocculant / feed solids review |
| High bed level | Torque and rake stress | Step feed or dilute carefully — do not only raise polymer |
Under-thickening and over-thickening are opposite failures with opposite evidence: one is a water and volume problem; the other is a solids-handling and mixing problem.
Active volume and flow still govern:
residence (h) = total active volume (m³) / slurry flow (m³/h)
If density rises without updating the flow measurement basis (volume vs mass solids), the calculated residence lies. Always state whether flow is volumetric slurry, dry tonnes, or solids flux.
An illustrative CIL design uses ~40–50% solids with relative density targets in a band such as 1.45–1.60, while the thickener runs denser before dilution or stock-tank control. The exact split is circuit-specific. What matters is that thickener density, stock-tank density, and tank RD are one control chain, not three independent charts.
Before slimes disposal, a tailings screen (often ~0.8 mm in teaching examples):
Carbon appearing on the tailings screen points upstream: interstage screens, wipers, seals, transfer batches.
Cloudy overflow after a polymer change — floc too fine or over-dosed; bed may blind. Check make-up water, age of solution, and dose rate together.
CIL residence “too short” on the calculator — actual flow rose, a tank is bypassed, or density basis changed so the volume used is wrong.
High solution losses to tailings — free cyanide and gold in the residue solution; look at pH, destruction, and whether water is leaving uncaptured.
Rising power on the rake — bed depth, grit, or over-feed; forcing polymer without a bed strategy risks torque trips.
0.6 × 0.5 mm
illustrative trash-screen aperture
50–60%
illustrative thickener underflow solids band
0.8 mm
illustrative tailings-screen aperture
1# Illustrative: active CIL volume vs slurry flow after thickening2active_volume_m363593flow_m3_per_h140# thickener-underflow-fed slurry (example)4residence_hactive_volume_m3flow_m3_per_h5printf"nominal residence = {residence_h:.1f} h"6print"If flow metric changes (RD or %solids), recompute before judging chemistry."
Thickener rakes, pumps, and screens have mechanical energy and pinch points; effluent streams may carry residual cyanide. This is a teaching aid — follow site isolation and environmental procedures.
Further reading in this series: