Why pre-treat: refractory and locked feeds

A leach can only attack what its solution can reach. For a great many ores that is enough — grind, contact with the right reagent, and the metal dissolves. For an awkward and economically important minority it is not, because the value is shielded from the solution by the way the mineral is built. These are the feeds that need pre-treatment, and recognising one is the first judgement Module 3 trains.

A feed is called refractory when a conventional leach recovers little of its metal no matter how long it runs or how much reagent it sees. The word describes a behaviour, not a single cause — and the causes are worth separating, because each points to a different pre-treatment.

Locked, refractory, preg-robbing

The first cause is locked or encapsulated value: the metal-bearing phase is physically enclosed in a host mineral the leach does not dissolve. Fine gold disseminated inside a pyrite or arsenopyrite grain is the classic case — the cyanide solution never touches the gold because the sulfide wall stands between them. Grinding finer helps only until the lock is finer than the grind can follow.

The second is refractory sulfide chemistry: the sulfide matrix itself is the problem, consuming reagent, passivating surfaces, or being too stable for the leach to break in a workable time. Here the pre-treatment has to oxidise the sulfide — chemically open it — before the value behind it is reachable.

The third is preg-robbing: the ore contains a phase, typically carbonaceous matter, that adsorbs the dissolved metal straight back out of solution as fast as the leach frees it. The leach works; the ore quietly steals the product. The pre-treatment in that case aims to deactivate or destroy the robbing phase.

Exposure before chemistry

Underneath all three is one idea the rest of the module turns on: exposure comes before chemistry. A leach reagent and a target mineral can be a perfect chemical match and still recover nothing if the solution cannot make contact. Liberation — exposing the valuable phase so the solution can reach its surface — is the necessary condition, and pre-treatment is the family of steps that creates it: thermal oxidation, pressure oxidation, biological oxidation, or finer grinding, each suited to a different way the value is shielded.

So the diagnostic question a practitioner asks of a stubborn feed is not "what dissolves it" but "what is standing between the solution and the value, and which step removes that barrier". The topics that follow are those steps. This page is the orientation that names the problem they each solve; it lands on no single tool because its work is to frame the module, and the next four topics carry the chemistry and the sizing numbers.