Leach Tank Residence Time

Definition

In a hydrometallurgical leach circuit, residence time is the average time slurry spends in the leach tanks — nominally the total working volume of the train divided by the slurry volumetric flow, τ = ΣV_working / Q_slurry. Leach tanks are almost always run as a train of agitated tanks in series, each one behaving roughly like a stirred tank (CSTR), so the train as a whole approaches plug-flow behaviour while keeping each stage well mixed. The residence time is set on a slurry basis — the combined solids-plus-liquid volumetric flow — and on the working volume between the operating levels, not the total tank volume.

Why it matters

Leaching is a kinetic process: metal dissolves over time at a rate set by the reagent chemistry, temperature, particle size, and solids concentration. Residence time is how long the circuit gives that reaction to happen, so it is one of the levers on recovery — but only one. The same residence time can give very different extraction depending on the leach kinetics, the slurry density and percent solids (which set how much solid is present per unit volume), the mixing and solids suspension in each tank, the number of tanks (short-circuiting falls as stages are added), and the feed flow turndown. Sizing a leach circuit on residence time alone, without the kinetics from testwork, risks a circuit that holds the slurry long enough on paper but does not reach the target recovery — or one that is over-built. Residence time scopes the tanks; testwork and circuit modelling design them.

Formula

Units involved

• •V_working — working volume per tank in m³ (use the operating, not total, volume)
• •Q_slurry — slurry volumetric flow in m³/h or L/s (solids + liquid)
• •τ — residence time in h or min (often hours for leaching)
• •Solids loading — t/h dry solids, with slurry density in kg/m³ and percent solids (Cw/Cv)
• •Keep volume and flow on the same time basis

Concept diagram

Worked example

Common mistakes

• •Using the liquid flow instead of the combined slurry (solids + liquid) flow for Q.
• •Sizing on total tank volume rather than the working volume between operating levels.
• •Treating residence time as a recovery guarantee independent of leach kinetics.
• •Ignoring slurry density and percent solids, which set the solids inventory per unit volume.
• •Designing too few tanks, so short-circuiting lets some slurry leave under-leached.
• •Checking only the design flow and not the minimum/normal/maximum flow cases.

When to use the calculator

Use the residence-time calculator for the nominal τ = V/Q on a slurry basis, the slurry density and percent-solids calculators to fix the slurry flow and solids loading, and the RTD tracer test calculator to check a real tank train for short-circuiting. The numbers are preliminary — the leach design comes from testwork and circuit modelling.

FAQ