The chemical composition of ’slag’ formed during the smelting process in the extraction of copper is:

During the extraction of copper, iron is present in the ore as impurity (FeS). The ore together with a little coke and silica is smelted; FeS present as impurity in the ore is oxidized to iron oxide, which then reacts with silica to form fusible ferrous silicate which is removed as slag. \(2FeS+3{O}_{2}\to 2FeO+2S{O}_{2}\) \(FeO+Si{O}_{2}\to \underset{\left(Slag\right)}{FeSi{O}_{3}}\)