The term roasting refers to gas-solid reactions performed at elevated temperatures as preliminary steps to either hydrometallurgical or pyrometallurgical production of metals. During this process gas composition is controlled in order to produce the desired chemical and physical changes.
The major objectives of roasting are one or more of the following.
Elimination of volatile species (e.g. Sulfur, as SO2, and Cadmium).
Alteration of chemical composition:
to assist subsequent aqueous dissolution, e.g., conversion of sulfides to sulfate and/or oxides;
to assist subsequent high temperature reduction with carbon, e.g., conversion of sulfides to oxides;
to render volatility, e.g., production of chlorides from oxides;
to alter physical characteristics as in sintering, whereby agglomeration of fine particles is achieved.
The majority of pyrometallurgical roasting reactions are now performed in Fluidized Bed reactors to utilize their inherently good heat and mass transfer characteristics.
In the case of sintering, the fine material to be roasted and agglomerated is placed in a continuously moving belt and a narrow reaction front passes rapidly through the material to effect partial melting of some component and thus to effect agglomeration.
Szekely, J., Evans, J.W., and Sohn, H.Y. Gas Solid Reactions. Academic Press.