Gold Processing5 min read
Every year, the global mining industry produces over 3,000 tonnes of gold. But long before a gold bar reaches a vault, it exists as a few grams per tonne dispersed throughout a…
A gold plant is a chain of opportunities. The valuable metal may be locked in a mineral, trapped behind a surface film, dissolved into solution, adsorbed onto carbon, or lost through a screen or a bad mass balance. The useful question is not simply whether gold is present. It is where the gold is, how fast it can move, and which operating variable can move it into the recoverable stream.
This guide uses illustrative teaching cases to connect the CIL, elution, and gold-loss concepts. The values shown are examples only; site-specific data must be verified before they are used in design or operations.
Sunrise Dam Gold Mine processing plant, Western Australia. Photo: Calistemon, Wikimedia Commons, CC BY-SA 4.0.
Leach control map
Every lever changes a different bottleneck. Do not use cyanide to fix a grind problem.
The stirred vessel is the small model of the CIL tank: a solid phase, solution, an impeller, baffles, and a gas path all have to coexist for leaching and adsorption to work together.
A conventional cyanidation circuit usually begins with comminution. Crushing and grinding expose gold-bearing particles, and hydrocyclones classify the resulting slurry. A gravity circuit may recover a coarse or free-gold fraction first. The remaining slurry is then conditioned and sent through a carbon-in-leach or carbon-in-pulp circuit.
The overall cyanidation reaction is commonly written as:
4Au + 8CN⁻ + O₂ + 2H₂O → 4Au(CN)₂⁻ + 4OH⁻
Cyanide makes the gold-cyanide complex possible, while oxygen supports the oxidation process. Once the complex is in solution, activated carbon provides a second stage of selectivity by adsorbing it. Loaded carbon is then washed, stripped in an elution column, plated by electrowinning, and smelted.
The process is therefore a sequence of transfers:
A failure at any link can show up as a poor recovery or an abnormal tailings grade.
| Stage / decision | Companion article |
|---|---|
| Crush, grind, classify, gravity | Comminution and gravity recovery |
| Elsner, HCN/CN⁻, oxygen | Cyanidation chemistry |
| Sulfide, copper, iron, preg-robbing | Cyanide consumers |
| Thickener, trash, water return | Thickening and the water loop |
| Slurry vs carbon movement | CIL control |
| Strip, EW, kiln | Elution and regeneration |
| Reactivation profile and clinker | Inside the regeneration kiln |
| Designed leach tests and ANOVA | Leach DOE and response surfaces |
| Cyanide vs alternatives economics | Reagent comparison |
The defining difference is timing. In carbon-in-pulp, leaching and adsorption are separated into different tank groups. In carbon-in-leach, both occur together. CIL can protect dissolved gold earlier, which is valuable for slow-leaching, low-grade, clay-rich, or preg-robbing feeds. The trade-off is a longer combined carbon circuit and more exposure of activated carbon to agitation and abrasion.
In a CIL tank train, slurry normally moves forward while carbon moves counter-current. Fresh or regenerated carbon enters near the barren end. Loaded carbon is withdrawn near the adsorption end and pumped to the loaded-carbon screen. Interstage screens retain carbon inside the tanks; a damaged, holed, pegged, or poorly sealed screen can allow carbon to report to tailings.
An illustrative CIL arrangement uses six tanks, a nominal volume of 2,400 m³, about 50% solids, and roughly 24 hours of total residence time. These figures are teaching values, not a design basis. A different plant must recalculate from its own active volume, flow, density, and tank availability.
Leaching is a surface reaction, so grind is usually a first lever. An illustrative circuit might target about 80% passing 75 µm, while another teaching case uses a wider particle-size study from 75 to 212 µm. Neither is a universal rule. Very fine grinding can expose gold, but it also raises energy use and can increase viscosity, clay effects, carbon fouling, and carbon loss.
Oxygen delivery is just as important as cyanide strength. Compressed air is sparged through agitator shafts in the example CIL circuit. If air supply falls while the tanks continue to look agitated, dissolution can decline. Free cyanide, pH, density, and residence time then change the balance between dissolution, consumption, and gold loss.
Lime maintains an alkaline pulp. The illustrative operating window is about pH 10.3 to 10.5 for the CIL example. Low pH increases the hazard of hydrogen cyanide formation. Excessively high pH can increase lime consumption and calcium-related fouling of carbon. The control target should be established for the ore and circuit rather than copied without verification.
An illustrative diagnostic case uses tailings grades between 0.6 and 0.8 g/t against a target of 0.3 g/t, with changing ore conditions and no durable improvement from simply increasing cyanide. The example investigation uses representative thickener-underflow composites, XRF to identify relevant elements, and bottle-roll tests that vary particle size and cyanide concentration while holding pH, density, and leach time close to representative test conditions. These values are teaching values, not a plant record.
That is a sound sequence because it separates diagnosis from trial. First, reconcile feed, recovered gold, solution losses, and residue grade. Then characterize the feed and residue. Finally, test a small number of factors in a design that can be analysed statistically.
A useful bottle-roll record includes sample mass, pulp density, pH, lime addition, cyanide strength, oxygen conditions, roll time, free-cyanide determination, residue grade, and leachate assay. A result without that basis cannot be compared fairly with another test.
1feed_grade2.80# g/t Au2recovery88.0# %3throughput500.0# dry t/d45feed_gold_gfeed_gradethroughput6recovered_gfeed_gold_grecovery1007tailings_g_tfeed_gold_grecovered_gthroughput8printf"tailings grade = {tailings_g_t:.2f} g/t"
The code is intentionally small: the important engineering work is agreeing the basis before trusting the number.
A practical daily review should connect the flowsheet to the control room:
The controls are connected. A pump schedule that changes carbon concentration affects adsorption; a change in agitation affects both mass transfer and attrition; a change in grind changes density, viscosity, carbon circulation, and leach kinetics. Avoid diagnosing one variable in isolation.
A useful illustrative shift calculation is deliberately transparent. If an illustrative circuit treats 500 dry tonnes per day at 2.80 g/t and records 88% recovery, the feed contains:
500 × 2.80 = 1,400 g Au/day
The recovered portion is:
1,400 × 0.88 = 1,232 g Au/day
The remaining 168 g is distributed through barren solution, loaded carbon losses, electrowinning losses, and the final residue. The calculated residue grade is 168 / 500 = 0.336 g/t, which is close to the illustrative residue grade of approximately 0.34 g/t. That agreement is not proof that every stream is closed; it is a useful first check.
A plant should then compare the calculated value with direct measurements:
| Stream | Measurement needed | Failure revealed |
|---|---|---|
| Feed | Dry tonnes, moisture-corrected grade | Wrong tonnage or assay basis |
| Carbon | Loaded-carbon assay and mass | Poor adsorption or strip |
| Solution | Free cyanide, dissolved gold, barren eluate | Leaching or recovery loss |
| Residue | Moisture-corrected Au grade | Physical or chemical loss |
| Product | Cathode mass, purity, doré assay | Recovery or refining loss |
The number of streams matters. A lower tailings grade can be the result of a genuine recovery gain, a change in the reporting basis, or a sampling error. The balance is more valuable than any one grade.
| Observation | First hypothesis | First test | Do not jump directly to |
|---|---|---|---|
| Free cyanide is low | Under-dosing or high consumption | Titrate free CN and track consumption | More cyanide without a pH and consumption check |
| Free cyanide and oxygen are adequate | Grind, locked gold, or carbon activity | Check D80, bottle roll, carbon assay | A large cyanide increase |
| Tailings grade rises with carbon loss | Screen or carbon-attrition problem | Inspect screens and fine-carbon recovery | Treating the loss as dissolved gold |
| Recovery rises with longer time | Slow-leaching fraction | Run a time-response test | Assuming the plant has excess capacity |
| Recovery rises only after fine grind | Exposure-limited dissolution | Compare size fractions and locked-gold assays | Increasing reagent without changing exposure |
80% <75 µm
illustrative grind target
10.3–10.5
illustrative CIL pH range
0.3 g/t
illustrative tailings target
Cyanide is acutely toxic, hydrogen cyanide can form under unsuitable chemistry, and elution systems operate hot and under pressure. This material is for engineering study, operating review, and test planning. Any reagent work must follow the site chemical-management plan, exposure controls, emergency procedures, and qualified metallurgical, chemical, and safety review.