Explain the role of each of the following: (i) NaCN in the extraction of silver (ii) SiO2 in the extraction of copper

I) In the extraction of gold and silver the metal is leached with NaCN and KCN, which is an oxidation reaction.
4M + 8CN⁻ + 2H₂O + O₂ → 4[M(CN)₂]⁻ + 4OH⁻
In the next step, the compound is reacted with zinc leading to the formation of metal.
4[M(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2M
M is Ag or Au.
II) The role of silica (SiO₂) in the extraction of copper is to remove the FeO obtained in roasting. So, silica is added before roasting as flux to remove iron impurity. So, FeO reacts with silica leading to form ferrous silicate as slag.
FeO + SiO₂ → FeSiO₃