Gold Processing6 min read
A higher bottle-roll recovery is an interesting result, not a plant decision. An illustrative teaching case compares cyanide with New Lime Sulphur YX500 and reaches maximum…
Author: Wayne Ncube
A higher bottle-roll recovery is an interesting result, not a plant decision. An illustrative teaching case compares cyanide with New Lime Sulphur YX500 and reaches maximum recoveries of 92.10% for cyanide and 96.23% for YX500 under the tested conditions. Its illustrative economic comparison still favours cyanide. That is a useful lesson: recovery, cost, risk, and product quality must be evaluated together.
Illustrative design points
Each marker is an illustrative bottle-roll run from a central-composite teaching design — not a drawn curve.
A fair comparison needs a controlled test matrix. Hold the ore sample, grind, pulp density, pH, agitation, and leach time constant, then vary reagent identity and dose under a documented procedure. Measure residue grade, solution assay, free reagent left in solution, reagent consumption, and any change in the residue or solution that could affect downstream treatment.
The illustrative comparison uses matched central-composite designs for each reagent. The resulting first-order fits and ANOVA are shown below:
| Metric | NaCN | YX500 |
|---|---|---|
| Model | R = 55.83 + 0.0996·A + 1.3626·B | R = −72.68 + 3.429·A + 14.6514·B |
| Model significance | F = 43.50, p < 0.0001 | F = 12.07, p = 0.0022 |
| Lack of fit | not significant (p = 0.57) | not significant (p = 0.12) |
| R² | 0.897 | 0.707 |
| Illustrative max recovery | 92.10% | 96.23% |
Illustrative economics at maximum recovery (using the example assumptions): cyanide ~4.76 B Tzs net vs YX500 ~4.31 B Tzs; near 90% recovery the advantage narrows (~4.66 vs ~4.61 B). A future pilot can keep the designed-experiment discipline and the transparent cost model together.
For each condition, record:
| Category | Questions to answer |
|---|---|
| Recovery | How much gold moved from the feed into solution or recovered product? |
| Reagent | How much active reagent was consumed per tonne of ore? |
| Residue | What grade and mineral association remain in the tailings? |
| Solution | Does the pregnant solution need different dilution, filtration, or carbon treatment? |
| Product | Do base metals or other impurities affect electrowinning or bullion fineness? |
| Safety | What controls are required for storage, dosing, reaction, and emergency response? |
The YX500 result is a good example of why a recovery-only ranking is incomplete. A different reagent can improve extraction while requiring more mass, a different pH window, new waste treatment, or an unfamiliar supply chain. The economic model must reflect the measured consumption, not just the label strength.
A credible comparison should separate chemistry variables from ore variability. For each reagent, use the same ore split, grind, pulp density, agitation, pH target, leach time, solution volume, and assay method. Then vary only the intended variable: reagent identity and dose. Replicate the centre points so that assay scatter and day-to-day test variation can be separated from a real response.
A minimal design can look like this:
| Run | Reagent | Dose | pH | Time | Residue assay | Free reagent left |
|---|---|---|---|---|---|---|
| 1 | NaCN | control | controlled | controlled | measure | measure |
| 2 | YX500 | control | controlled | controlled | measure | measure |
| 3 | NaCN | low / high | controlled | controlled | measure | measure |
| 4 | YX500 | low / high | controlled | controlled | measure | measure |
| 5 | centre repeat | centre | controlled | controlled | measure | measure |
The exact values should come from the ore-specific trial and the site's approved chemistry envelope. The important design feature is that a dose response is observed for each reagent, rather than comparing one arbitrary point from each product.
The results should also include the cost of the test's own controls: extra lime, oxygen demand, filtration, neutralization, carbon activity, and any change in electrowinning or tailings treatment. A reagent that produces a richer solution but requires a new waste plant is not a simple replacement.
A transparent pilot economics can use:
net value = recovered gold value − reagent cost − operating cost − waste-treatment cost − incremental capital cost
Recovered gold value should come from the product assay and the mass actually recovered. Reagent cost should use the measured active-reagent mass and the delivered commercial price. Waste treatment may include neutralization, destruction, filtration, carbon capture, and disposal. The model should show sensitivity to gold price, reagent price, recovery, and consumption because a single break-even point can be fragile.
The practical recommendation is therefore conditional: if a new reagent preserves recovery while reducing total cost or risk, it deserves a pilot. If it improves laboratory recovery but loses on consumption or downstream compatibility, the plant should not adopt it on that result alone.
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96.23%
illustrative YX500 maximum recovery in the teaching case
92.10%
illustrative cyanide maximum recovery in the same teaching case
pilot
the right next step after a bottle-roll result
Before a plant trial, answer:
This teaching case is not a universal endorsement of YX500 or a substitute for a reagent-specific safety assessment. Use the result to design better tests.
Reagent selection is a process-safety decision as well as a metallurgical one. Any trial must follow the site risk assessment, chemical handling procedure, environmental requirements, and qualified technical review.