Roasting (metallurgy)

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Roasting is a metallurgical process involving gas-solids reactions at elevated temperatures. A common example is the process in which sulfide ores are converted to oxides, prior to smelting. Roasting differs from calcination, which merely involves decomposition at elevated temperatures. A typical sulfide roasting chemical reaction takes the following form:

MS_n + 1.5nO₂ → MO_n + nSO₂.

For example:

CuS + 1.5O₂ → CuO + SO₂

and

2ZnS + 3O₂ → 2ZnO + 2SO₂

The gaseous product of sulfide roasting, sulfur dioxide (SO₂) is often used to produce sulfuric acid. (See Sulfuric acid#Manufacture.)

Up until the early 20th century, roasting was started by burning wood on top of ore. This would raise the temperature of the ore to the point where its sulfur content would become its source of fuel, and the roasting process could continue without external fuel sources. Early sulfide roasting, was practiced in this manner, in "open hearth" roasters, which were manually stirred (a practice referred to as "rabbling") using rake-like tools to exposed unroasted ore to oxygen as the reaction proceeded.

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