Question

Why does copper obtained in the extraction from copper pyrites have a blistered appearance?

Answer 1

In the process of copper extraction, reduction of copper pyrites to copper metal is accompanied by evolution of sulphur dioxide.Firstly the ore is burnt in the presence of oxygen.

Complete solution
We know that copper pyrites are the important ores of copper metal extraction and it contains copper as well as iron as sulphides. We use a roasting method for its concentration during which sulphur dioxide is evolved. We can write the reactions that usually takes place as follows:
${\rm{FeO}} + {\rm{Si}}{{\rm{O}}_{{\rm{2 }}}} \to {\rm{FeSi}}{{\rm{O}}_{\rm{3}}}$
We can see that this reaction leads to the formation of iron silicate that is called slag, here. This leaves copper matte that contains copper sulphide and iron sulphide. Now, we use hot air blast to carry out the copper extraction. Iron sulphide gets oxidized as follows:
${\rm{2FeS}} + {\rm{3}}{{\rm{O}}_{{\rm{2 }}}} \to 2{\rm{FeO}} + {\rm{2S}}{{\rm{O}}_{\rm{2}}}$
We can make this iron oxide to slag off again with silica as we did above. Similarly, copper sulphide gets oxidized as follows:
${\rm{2C}}{{\rm{u}}_2}{\rm{S}} + {\rm{3}}{{\rm{O}}_{{\rm{2 }}}} \to 2{\rm{C}}{{\rm{u}}_2}{\rm{O}} + {\rm{2S}}{{\rm{O}}_{\rm{2}}}$
We get our metallic copper from these only as follows:
$2{\rm{C}}{{\rm{u}}_2}{\rm{O}} + {\rm{C}}{{\rm{u}}_2}{\rm{S}} \to 6{\rm{Cu}} + {\rm{S}}{{\rm{O}}_{\rm{2}}}$
Now, as we can see that we are getting sulphur dioxide as well, it gets trapped in the molten copper as it solidifies and gives copper a blistered appearance.
Therefore, in the cooler areas of the molten copper base, the ${\rm{S}}{{\rm{O}}_{\rm{2}}}$ gas escaping from the melt is contained, offering copper a blistery look and it is thus named blister copper.

Note:
There is iron also present that might be confusing for the reason of the blistered appearance of copper so we have to look at all the involved species and the process.