Role of :
a) SiO2 in extraction of copper.
b) NaCN in extraction of gold
c) Iodine in refining zinconium

1) Kindly refer to the following link: 

2) Role of NaCN in extraction of gold is as follow:

NaCN is used in leaching of gold.

In this process, finely powdered gold is treated with dilute solution of NaCN, a current of air is passed. As a result gold pass into solution forming soluble complex cyanides while the impurities remain unaffected which are filtered off.

4 Au + 8KCN  + O2 + 2H2O  → 4K[Au(CN)2] + 4KOH


Zirconium is produced by adding gaseous zirconium tetrachloride to molten magnesium metal. (In the absence of air). The end product is always contaminated with magnesium and it doesn't take much magnesium to make it brittle, etc.

at one temperature iodine would react with zirconium metal to form zirconium tetraiodide and that at a higher temperature the tetraiodide would decompose to zirconium metal and free iodine.

First step, zirconium in the cladding is reacted with elemental iodine at 300–500oC to form volatile ZrI4, which, in the second step, is then decomposed at >1100oC.  
Formation  Zr + 4I2  → ZrI4  300-500oC
Decomposition  ZrI4  → Zr + 4I2  >1100oC

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