Roasting (metallurgy)

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Roasted gold ore from Cripple Creek, Colorado. Roasting has driven off the tellurium from the original calaverite, leaving behind vesicular blebs of native gold.

Roasting is a step of the processing of certain ores. More specifically, roasting is a metallurgical process involving gas–solid reactions at elevated temperatures with the goal of purifying the metal component(s). Often before roasting, the ore has already been partially purified, e.g. by froth floatation. The concentrate is mixed with other materials to facilitate the process. The technology is useful but is also a serious source of air pollution.[1]

Roasting consists of thermal gas–solid reactions, which can include oxidation, reduction, chlorination, sulfation, and pyrohydrolysis. In roasting, the ore or ore concentrate is treated with very hot air. This process is generally applied to sulphide minerals. During roasting, the sulfide is converted to an oxide, and sulfur is released as sulfur dioxide, a gas. For the ores Cu2S (chalcocite) and ZnS (sphalerite), balanced equations for the roasting are:

2 Cu2S + 3O2 → 2 Cu2O + 2 SO2
2 ZnS + 3 O2 → 2 ZnO + 2 SO2

The gaseous product of sulfide roasting, sulfur dioxide (SO2) is often used to produce sulfuric acid. Many sulfide minerals contain other components such as arsenic that are released into the environment.

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 expose unroasted ore to oxygen as the reaction proceeded.

This process would release large amounts of acidic, metallic, and other toxic compounds. Results of this include areas that even after 60-80 years are still largely lifeless, often exactly corresponding to the area of the roast bed, some of which are hundreds of metres wide by kilometres long.[2][3]

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