Roasting (metallurgy)

Roasting is a metallurgical process in which sulfide ores are converted to oxides, prior to smelting. It differs from calcination in that in calcining limestone or other carbonate ores, the chemical change is achieved merely by the application of heat, whereas roasting involves a reaction between a gas and the solid ore:

MS_n + 1.5nO₂ → MO_n + nSO₂.

Additional recommended knowledge

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Daily Visual Balance Check

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For example:

CuS + 1.5O₂ → CuO + SO₂

and

2ZnS + 3O₂ → 2ZnO + 2SO₂

The toxic product sulphur dioxide (SO₂) is used to produce sulphuric acid. See Sulfuric acid#Manufacture for this process.

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.