Tank leaching: residence time and sizing

Tank leaching is the workhorse contacting family: the finely ground feed is leached as a slurry in large agitated tanks, kept suspended and in contact with the reagent until the reaction has run far enough. It is where a residence time set by kinetics becomes a tank volume set by arithmetic, and that chain — testwork to residence time to volume — is the spine of this topic and the flagship sizing of the module.

Stirred tanks in series

Tanks are run in series — the slurry flows through a cascade of several, leaving one and entering the next — for a reason worth understanding. A single large stirred tank is well mixed, which means its contents are at the discharge composition throughout, so some slurry short-circuits to the outlet having spent little time reacting. Putting several tanks in series narrows that spread: each tank is well mixed, but the cascade as a whole approaches the steady, near-complete conversion a long reaction needs, with far less short-circuiting than one vessel of the same total volume. The number of tanks in series is a design choice that trades vessel count against how tightly the residence time is held.

From residence time to tank volume

The sizing itself is direct once the residence time is known. The total live volume the duty needs is the slurry volumetric flow multiplied by the required residence time, V_live = Q · τ. Divide that across the number of tanks in series and by a live-volume fraction — the working volume below freeboard and clear of internals, against the geometric volume — and you have the per-tank volume each vessel must provide. The slurry flow Q itself comes from the feed: either entered directly, or derived from a solids basis through the house slurry-density relation of Module 2, so the slurry spine you built earlier feeds straight into the tank sizing here.

The testwork-to-sizing chain

The honest boundary is the same one the kinetics topic drew. The residence time τ is not a number this page or the calculator predicts; it comes from the leach testwork on the specific ore, which measures how long the slurry must react to reach the target recovery. The calculator sizes vessels for that stated residence time — it makes no recovery claim, models no kinetics, and selects no equipment. The chain is: testwork gives τ; the slurry density and flow give Q; V_live = Q · τ gives the total volume; the tanks-in-series and live-volume fraction give the per-tank size. The worked thread below runs that chain end to end on the committed leach-tank-sizing example, and the residence-time calculator is the general τ = V ÷ Q relation behind it.