Calcination

Calcination is a high-temperature thermal treatment process applied to ores, minerals, or chemical compounds to induce decomposition, phase transformation, or removal of volatile components such as water, carbon dioxide, or sulfur dioxide.

In the context of bauxite and alumina refining, calcination is the critical final step in the Bayer process, where aluminum hydroxide (gibbsite or boehmite) is heated in a rotary kiln or fluidized bed calciner at temperatures typically ranging from 950°C to 1,100°C. This intense heat drives off chemically bound water molecules, converting the hydroxide into anhydrous aluminum oxide — commonly known as alumina or Al₂O₃.

The process permanently alters the crystalline structure of the material, transitioning it through a series of metastable alumina phases (gamma, delta, theta) before ultimately producing alpha-alumina, the stable form used in electrolytic smelting. The degree of calcination directly affects the physical and chemical properties of the final alumina product, including its surface area, reactivity, density, and soda content.

In iron ore processing, calcination may be used to remove carbonate gangue from siderite ores. In gold and diamond mining contexts, calcination of refractory ores helps to break down sulfide minerals that trap fine gold particles, improving overall gold recovery rates during subsequent leaching operations.