Give a reason why the metals – copper, silver and lead are electrorefined but K, Na and Ca are not.

Method of extraction of metals depends on position of the metal in the activity series. Extraction of K, Na and Ca are done by electrolysis (Electrometallurgy). They are higher in electrochemical series and are extracted by electrolysis of their fused salts. On electrolysis metal is deposited at the cathode. Thus, they are already deposited at the cathode in pure state and hence need not be refined further.
On the other hand, lead and copper are extracted by using reducing agents and silver through thermal decomposition. Hence, they contain impurities and need to be generally electro-refined by electrolysis.