Leach chemistry: dissolution and stoichiometry

Leaching is, at its core, a dissolution reaction: a reagent in solution reacts with a mineral and carries the metal into the liquor, leaving a residue behind. Module 4 opens here because every contacting family that follows — tank, heap, pressure — is a way of staging that one reaction, and reading a flowsheet means knowing both what the reaction is and what it costs in reagent.

Dissolution reactions

A leach reaction is written like any other, with the mineral and the reagent on the left and the dissolved metal and the residue on the right. Sulfuric acid dissolving a metal oxide to a metal sulfate and water; an alkaline reagent taking up an amphoteric oxide; an oxidant supplying the electrons a dissolution needs to move a metal to a soluble valence — these are the everyday forms. The reaction names the reagent the feed requires and the products it generates, including the residue and any gas, and it is the starting point because it fixes the chemistry the rest of the sizing rests on.

From what dissolves it to how much

The balanced reaction does more than identify the reagent; it sets the proportions. Stoichiometry is the bookkeeping of those proportions — so many moles of acid per mole of metal dissolved, converted through molar masses into a mass of reagent per mass of metal, and from the metal grade of the ore into a reagent demand per tonne of feed. This is the move that turns chemistry into a number a circuit can be costed on: from "sulfuric acid dissolves this oxide" to "this leach needs so many kilograms of acid per tonne of ore". The reagent hubs below give the delivered strengths and densities that the mass arithmetic needs to cross from a stored wt% to a dosed mass.

Where stoichiometry stops

The stoichiometric demand is a floor, not the consumption. The reaction tells you the minimum reagent the target dissolution requires; the real circuit always uses more, because reagent is also spent on side reactions with gangue, lost unreacted to residue, or left in excess to drive the rate. The gap between the stoichiometric minimum and the actual consumption is where mineralogy and operating practice live, and it is the subject of a later topic on acid-consuming gangue. So stoichiometry gives the honest lower bound and the structure of the calculation; the consumption that drives the operating cost is a measured number on the specific ore. The consumption calculator below turns a dosage you supply — from testwork or plant data — into a reagent rate, a daily tonnage and a cost; it sizes the consumption, it does not predict recovery or the chemistry behind the dose.