Gold Processing7 min read
Loaded carbon is not the end of the gold circuit. It is a temporary carrier. Elution removes the gold-cyanide complex, electrowinning recovers metallic gold from the rich eluate,…
Author: Wayne Ncube
Loaded carbon is not the end of the gold circuit. It is a temporary carrier. Elution removes the gold-cyanide complex, electrowinning recovers metallic gold from the rich eluate, and regeneration prepares the carbon to adsorb again. Each stage introduces a new control problem, and each failure can be mistaken for a failure in the next stage.
The column model is a visual shorthand for even liquid distribution through a carbon bed. In the real circuit, the important evidence is the pressure, temperature, flow, and assay profile across the batch.
Activated carbon works because it has accessible pores and surface sites. Calcium, silica, salts, oils, and other species can occupy those sites or physically block the pores. The result is lower adsorption rate, lower loading capacity, and a less consistent strip.
Acid washing with dilute hydrochloric acid removes much of the inorganic fouling. An illustrative wash case uses 3% HCl, a pH of about 2 to 3, and a rinse with portable water until the pH approaches neutral. These are teaching values, not a universal procedure; confirm the actual chemistry and equipment requirements locally.
Acid does not remove all organic contamination. Oils, grease, and plant matter are addressed by thermal reactivation. In a rotary kiln, the carbon is heated in a steam environment so volatile material can leave, less volatile material can crack, and residues can be oxidized or vaporized without burning the carbon base. An illustrative kiln case uses a final temperature around 700 to 750 °C.
Elution reverses adsorption. The important conditions are high temperature, suitable caustic strength, cyanide concentration, low ionic strength, correct flow, and low gold concentration in the returning solution. Illustrative elution settings include pH 12 to 13, free caustic around 1.5 to 1.9%, cyanide around 0.7 to 1%, and two to three bed volumes of flow per hour.
| Parameter | Illustrative value / teaching case |
|---|---|
| Column volume | ~24 m³ class vessel in an illustrative circuit |
| Circulation | ~2 bed volumes per hour |
| Caustic | ~2% NaOH (with cyanide often ~1% in strip chemistry) |
| pH | 12–13 |
| Temperature | ~130 °C class hot circulation |
| Elution duration | ~12–18 h typical cycle in this teaching case |
| Barren carbon gold | illustrative target of low residual Au (e.g. <100 g Au/t class) |
| Pre-heat | to ~92 °C before full strip; illustrative strip band ~92–120 °C |
| Acid wash (before strip) | ~3% HCl, pH 2–3, ~2 BV/h, then water rinse to neutral-ish pH |
Confirm against your elution procedure and metallurgical standard — these are illustrative teaching values, not universal setpoints.
The column geometry is part of the process. A tall, relatively narrow pressure vessel can distribute liquid through the bed more evenly than a short, wide vessel. Bottom-up flow and a suitable distribution arrangement help reduce channeling. If fluid short-circuits through part of the bed, part of the carbon receives poor contact and the eluate profile becomes inconsistent.
Temperature is a dominant variable, but the target is not simply as hot as possible. Too little temperature gives incomplete desorption. Excessive temperature can increase base-metal stripping, affect carbon condition, and create equipment and safety risk. The operating window must be tied to the selected elution chemistry and the carbon loading.
For each batch, record:
A strip that reaches the scheduled time but leaves high gold on the carbon is not automatically successful. A rich eluate that is unstable at the electrowinning cell is also not a good result. Track both the gold removed and the quality of the carbon returned to CIL.
The acid-wash step removes inorganic material that would otherwise occupy carbon pores and interfere with the elution chemistry. Calcium carbonate, magnesium species, silica, iron compounds, and precipitated salts can be attacked by dilute hydrochloric acid, while the adsorbed gold-cyanide complex is intended to remain on the carbon. The rinse then removes acid and dissolved salts so the carbon enters the hot caustic-cyanide step with a controlled chemistry.
A useful wash record is more detailed than “acid wash complete”:
If the rinse endpoint is not controlled, residual acid can change the beginning of the elution cycle. If the wash is too aggressive or the carbon is poorly drained, the cycle can lose fines and create a downstream filtration problem.
The final carbon assay does not appear instantly. The gold-cyanide species must move from the carbon surface into the solution, and the solution must move through the bed without bypassing a large fraction of the carbon. Temperature increases desorption, but the overall result also depends on caustic availability, cyanide concentration, ionic strength, flow distribution, and the amount of gold already present in the returning barren eluate.
The column is often treated as a vessel, but its flow distribution is a process variable. A tall aspect ratio, a suitable distributor, and bottom-up flow can reduce channeling. A damaged distributor or a settled carbon bed can create a hot, high-grade channel beside carbon that has barely been contacted. The result is a high early eluate assay and a barren-carbon assay that remains too high.
| Check | Healthy signal | Warning signal |
|---|---|---|
| Acid wash | pH and rinse endpoint reached; fines controlled | persistent high solids or unexplained carbon loss |
| Pre-heat | stable target temperature before the strip | slow heat-up or wide vessel temperature spread |
| Early eluate | predictable rise and fall in grade | sharp channel-like spike followed by poor stripping |
| Barren eluate | gold falls toward the barren target | gold remains high or rebounds during the cycle |
| Barren carbon | assay below the agreed activity and recovery target | high residual gold after a nominal strip |
| Returned carbon | correct size, activity, and inventory | excessive fines or rising adsorption loss |
The acceptance decision should be based on the batch as a whole. A single sample can be misleading if it is taken during a channeling event or before the carbon bed has reached a stable temperature.
3% HCl
illustrative acid-wash strength
2–3 BV/h
illustrative elution flow range
700–750 °C
illustrative reactivation temperature
Poor acid wash: carbonate, calcium, silica, or salts remain on the carbon. Check acid strength, circulation, contact time, and rinse endpoint before blaming the elution column.
Poor elution: temperature is low, flow is too high for adequate contact, caustic is outside the target, or the returning solution already contains too much gold. Check the trend rather than only the final sample.
Poor carbon activity after regeneration: the kiln may be too cool, blocked, overfed, or starved of steam. Inspect tube temperature, discharge, quench, and carbon appearance.
Poor electrowinning: the eluate may be too dilute, too acidic, too low in free caustic, or contaminated by base metals and anode corrosion products. Validate the cell chemistry before changing the strip cycle.
Pressure vessels, hot caustic-cyanide solution, hydrochloric acid, and fuel-fired kilns require engineered controls, competent operators, and the site emergency plan. These notes support learning and review; they do not authorize reagent handling or pressure work.