Leach Recovery Below Target: A Checklist from PSD to Passivation
If recovery is 88% against a 94% target, adding more acid is the most common — and most expensive — response. Recovery is limited by liberation, kinetics, or reagent consumption by gangue. Each has a distinct diagnostic.
Checklist
| Check | What to measure | If abnormal |
|---|---|---|
| PSD / liberation | P80 and assay by size fraction | Grind finer; coarse fraction carries the lost metal |
| Residence time | Actual vs design; short-circuiting in tanks | Fix baffles/mixing; add tanks rather than acid |
| Free acid / pH | Profile across the train, not just feed | Acid consumed by gangue; consider two-stage addition |
| Temperature | Actual vs set point, per tank | Reheat; kinetics halve for each ~10 °C loss in many systems |
| Oxidant / Eh | Eh or DO profile | Add oxidant where it is consumed, not only at the head |
| Passivation | Residue assay vs head assay by size | Surface layer; consider activation or finer grind |
| Liquid/solid ratio | % solids and pulp density | Too thick starves the reaction; too thin wastes acid |
The residue tells the story
Screen the residue into size fractions and assay each. Metal in the coarse fraction means liberation. Metal evenly distributed across all fractions with low solution tenor means kinetics or passivation. Metal in the fines with high solution tenor means the circuit is fine and the problem is downstream — solution loss, not leaching.
Two-stage acid addition
Adding all acid at the head lets it react with gangue before it reaches the valuable mineral. Splitting addition across tanks keeps free acid where the reaction is, and often gains several points of recovery without extra consumption.
When not to chase recovery
Recovery has a cost curve. The last two points often require finer grinding, which consumes more power than the metal is worth. Run the marginal cost per additional percent before committing capital.
Send head and residue assays with PSD, tank profile and reagent consumption, and we will identify the limiting mechanism and size the equipment change.