Leach Tank Residence Time Calculator
Mean residence time is the total live (agitated) volume of the leach train divided by the volumetric slurry flow through it: residence time (h) = total live volume (m³) ÷ slurry flow (m³/h).
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter how many leach tanks are plumbed in series and the nominal volume of one tank in cubic metres (m³).
- Optionally set the live volume utilisation (%) to allow for freeboard, launders and settled solids — leave blank for 100%.
- Enter the total slurry (pulp) volumetric flow rate in m³/h; read the total mean residence time, days equivalent and per-tank time.
How it works
Mean residence time is the total live (agitated) volume of the leach train divided by the volumetric slurry flow through it: residence time (h) = total live volume (m³) ÷ slurry flow (m³/h). With N identical tanks in series, total live volume = N × tank volume × (utilisation ÷ 100), so more tanks or larger tanks lengthen the leach, and a higher throughput shortens it.
This is a steady-state, ideal-flow estimate. It tells you the average time a parcel of slurry spends in the circuit — the time available for cyanidation (or other leach) kinetics to proceed — but it does not model back-mixing, short-circuiting or the actual gold/metal dissolution curve. Compare the residence time you get here against the retention time your leach testwork says is needed for target extraction, then add contingency.
Worked example
Six-tank gold leach train. A CIL circuit has 6 agitated tanks in series, each with a nominal volume of 1,000 m³, and treats slurry at 250 m³/h. Assuming 100% live volume, total live volume = 6 × 1,000 = 6,000 m³. Residence time = 6,000 ÷ 250 = 24 h — a full 24-hour (1-day) leach, or about 4 h per tank.
Common mistakes
- Using nominal (geometric) tank volume as if it were all live — real live volume is smaller once freeboard, launders and any settled bed are removed. Use the utilisation field to discount it.
- Entering flow in m³/day or dry tonnes per hour. This tool needs the slurry (pulp) volumetric flow in m³/h; convert dry-solids throughput to pulp volume using pulp density/percent solids first.
- Multiplying instead of adding tanks — tanks in series share one flow stream, so total volume adds up (N × tank volume); the flow through the train stays the same, it does not multiply.
Frequently asked questions
Is this the residence time per tank or for the whole train?
The emphasised result is the total mean residence time across all N tanks in series (total live volume ÷ flow). The tool also reports the per-tank figure (total ÷ N) for reference.
Does more residence time always mean better extraction?
Up to a point — leach kinetics are fast early then flatten, so beyond the time your testwork identifies you gain little extra recovery while tying up tank volume. Size the circuit to the retention time your leach curve requires plus a design margin, not to the maximum possible.
What flow rate should I enter — solids or slurry?
Slurry (pulp) volumetric flow in m³/h. If you only know dry-solids throughput (t/h) and percent solids, convert to pulp volume first using the pulp density; residence time is governed by the total slurry volume moving through the tanks.
How do I account for freeboard and settled solids?
Use the live volume utilisation field. For example 90% utilisation on a 1,000 m³ tank counts 900 m³ as effective leaching volume. Leave it at 100% only for a first-pass, nominal estimate.
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