(i) Name the method of refining to obtain silicon of high purity.
(ii) What is the role of $Si{O}_2$ in the extraction of copper? 
(iii) What is the role of depressants in froth floatation process?

(i) Zone Refining is the method of refining to obtain silicon of high purity.

(ii) $Si{O}_2$ combined with the iron in the copper ore to from iron (II) silicate slag which is easily removed.

(iii) In froth flotation process, depressant prevents the formation of froth. It is used to separate two sulphide ore and allowing the other to form the froth, e.g., $NaCN$ a depressant selectively $ZnS$ from coming in froth but allows $PbS$ to come with the froth.