(i) Zone refining is the method which is used to obtain high purity silica. This method is useful for producing semiconductor of Si, Ge, B, Ga, etc.
(ii) SiO2 is added so that it combines with iron present in copper ore and forms FeSiO3 (slag) which can then be easily removed. Thus, SiO2 acts as a flux to remove iron from copper ore to get pure Cu.
(iii) In froth floatation, depressant is added to separate 2 sulphide ores. In case the ore contains PbS or ZnS the depressant used is NaCN which prevents ZnS from coming to surface by forming a complex with it.
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