Leaching kinetics: what controls the rate

A balanced reaction tells you a dissolution is possible and how much reagent it needs; it says nothing about how long it takes. That is kinetics, and it is the number a leach circuit is actually sized on, because the residence time the slurry must spend in the tanks is set by the rate, not by the stoichiometry. Understanding what controls that rate is what lets an engineer read why a leach is slow and what would speed it up.

Three places the rate can be set

A leach reaction at a solid surface proceeds through a sequence of steps, and the slowest step in the chain governs the overall rate. There are three candidates. The reagent must first diffuse through the liquid film around the particle to reach the surface — if that transport is slowest, the leach is film-diffusion controlled, and agitation that thins the film speeds it up. Once a product layer or residue builds on the particle, the reagent must diffuse through that layer to reach fresh mineral — if that is slowest, the leach is product-layer diffusion controlled, and it slows as the layer thickens. Or the chemical reaction at the surface itself may be the bottleneck — surface-reaction control, where temperature, which accelerates the chemistry, is the strongest lever. Which one dominates decides what actually helps: stirring harder, grinding finer, or running hotter each attack a different controlling step.

The shrinking core, conceptually

The picture that organises this is the shrinking-core idea: as the reaction proceeds, an unreacted core of mineral shrinks inside a growing shell of product and film, and the controlling resistance is whichever barrier the reagent struggles most to cross to reach that shrinking core. It is a model, not the truth of every system, but it captures why a leach often starts fast and slows as it goes — the diffusion path to the core lengthens — and why finer particles, with shorter paths and more surface, leach faster. Holding the shrinking-core picture in mind makes the three control regimes concrete rather than abstract.

Why residence time is testwork

The reason a fundamentals page does not hand you a rate equation is that the controlling step, the rate constant and the way they shift with temperature, particle size and reagent strength are specific to the ore. They are measured in a leach testwork campaign on the actual material, which produces the recovery-against-time curve a circuit is designed from, and from that the required residence time. The kinetics here explain what the testwork is measuring and why it cannot be replaced by a correlation; the residence time it yields is the input the next topic sizes tanks on. This page is the concept that frames the testwork, and lands as cited connective tissue rather than on a calculator, because the rate itself is a measured property of the ore.