Question

why is chalcocite. roasted and not calcined in the recovery of copper

Answer 1

Heating of ore in the absence of air is called calcination. Heating of ore in the presence of air is called roasting. roasting of chalcocite not only removes the impurities but also converts Cu2S to Cu2O .This is not possible in calcination.

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Answer 2

Roasting is used for the extraction of sulphide ores, while calcination is used in carbonate ores. Chalcocite or copper sulphide is a sulphide mineral, hence roasting in the presence of oxygen is used.
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