The reagent set: acids, alkalis, oxidants

A hydromet circuit runs on a short list of bulk reagents, and an engineer who knows the families — what each one does, how it arrives, and the property that governs handling it — can read most of a reagent schedule on sight. The families are the acids, the alkalis, and the oxidants, with a few specialists alongside.

The acids: dissolving

The acids do the dissolving. Sulfuric acid is the workhorse of the industry: cheap, strong, regenerable in some circuits, and the lixiviant for copper, zinc, nickel laterite, uranium and lithium sulfate routes. It arrives concentrated and is diluted to a working strength on site.

Hydrochloric acid is chosen where chloride chemistry helps — faster kinetics on some minerals, more soluble chloride salts — at the price of harsher materials of construction. Nitric acid is an oxidising acid, used where dissolution needs an oxidant built into the acid, and is more specialised and costly. Each is sold and stored as a wt%, and the property that matters at the delivered strength is density (to convert the metered volume to a dosed mass) alongside the strength itself.

The alkalis: raising pH

The alkalis raise pH — to precipitate impurities, to neutralise spent acid, to control the chemistry of an adsorption or crystallisation step. Sodium hydroxide (caustic) is the strong, soluble base, delivered as a concentrated solution and used where a clean, fast pH lift is wanted. Sodium carbonate (soda ash) is the milder, carbonate-forming base, the reagent for carbonate precipitation — it is what turns a metal sulfate liquor into a metal carbonate product.

Lime is the cheap bulk alkali for neutralisation and is handled as a slurry rather than a solution, which puts it in Module 2 territory as much as here. For the soluble alkalis, density at strength again governs the dosing arithmetic.

The oxidants: supplying electrons

The oxidants supply the electrons. Many dissolutions need a species more oxidising than the acid alone — to take a metal to a soluble valence, to break a sulfide. Hydrogen peroxide is the clean, in-circuit oxidant: it adds oxidising power and leaves only water behind, which is why it doubles as a detox reagent on the effluent side. Air and oxygen are the bulk oxidants of pressure leaching; specialised circuits use others. The peroxide hub gives the property data; the others are gases beyond this page’s set.

What ties the families together for the engineer is that every one of them is bought, stored, diluted and dosed, and every one of those steps is arithmetic that needs the delivered strength and its density. The reagent is a continuous cost metered against ore, so the difference between a property assumed and a property read is the difference between a reagent budget that holds and one that drifts. The six hubs below are the acid and alkali families at the strengths a circuit actually delivers them; the next two topics turn that property data into make-up and dosing numbers.