Roasting, calcination and acid baking

Heat is the oldest way to make a stubborn feed leachable, and three thermal treatments cover most of what a hydromet circuit uses. They are not interchangeable — each drives a different chemistry and prepares the feed in a different way — but they share equipment and a sizing logic, which is why they belong on one page.

Roasting: oxidation

Roasting heats the feed in an oxidising atmosphere to convert one compound to another, most often a sulfide to an oxide or sulfate with sulfur leaving as SO₂. For a refractory sulfide that locks gold, roasting burns the sulfide matrix away and exposes the value; for a base-metal sulfide it produces an oxide or sulfate the downstream acid leach can dissolve. The roast is the pyro step at the head of the classic roast–leach–electrowin zinc circuit, and the seam where a pyro and a hydro flowsheet meet.

Calcination: decomposition and decrepitation

Calcination heats a feed to decompose it rather than to react it with the atmosphere — driving off a volatile component to leave a more reactive solid. A carbonate decomposes to an oxide and CO₂; a hydroxide loses water; a hydrate gives up its water of crystallisation. The thermal shock can also decrepitate a mineral, fracturing it and opening internal surface. Spodumene calcination, which flips the mineral from a leach-resistant form to a reactive one, is the lithium industry’s standard example of a phase change driven purely by heat.

Acid baking: sulfation

Acid baking mixes the feed with concentrated acid and heats it, converting the target mineral to a soluble sulfate that a mild water leach then takes up. It is chosen where a direct leach is slow or unselective but a sulfate intermediate dissolves cleanly — rare-earth and lithium routes use it, and it often follows a calcination step. The bake does the aggressive chemistry dry and hot; the leach that follows is gentle by comparison.

The sizing number: kiln residence time

All three are commonly run in rotary kilns, and the number that sizes a kiln for a given duty is the solids residence time — how long the material spends in the hot zone, which has to be long enough for the chemistry to complete. For an inclined rotary kiln the residence time follows the Sullivan, Maier & Ralston (1927) relation from the length, internal diameter, rotation speed and slope, and the calculator below computes it with the empirical constant cited to its source. The thermal load — how much heat the duty demands — is the companion number, and the heat-duty calculator gives a first estimate of that from a mass flow, a specific heat and a temperature rise. This page describes what each thermal treatment does and the numbers that size the equipment; it gives no operating or safety procedure, which belongs to the engineered design of a specific furnace.