Question

Write the chemical equations involved in Van Arkel method for refining Zirconium metal.

Answer 1

Hint: Refining of a metal refers to purifying an impure metal. The steps involved in order to get the pure metal include smelting, calcination and refining. After the extraction of the impure metal, it is subjected to processes such as distillation, liquidation, electrolysis, zone refining or chromatography in order to purify the impure metal. Impurities also get removed while concentrating the ore but when refining is carried out the chemical composition of the starting material and the product is the same, the only difference is that the product is more pure than the starting material.

Complete step by step solution:

Van Arkel method utilises thermal decomposition and is used for the refining of Zirconium metal and Titanium metal. The impure metals are converted into their respective volatile compounds (their respective iodides). The impure Zirconium metal is heated with iodine at a very high temperature (around 570 K) in an evacuated vessel. This results in the formation of volatile Zirconium tetra-iodide and the impurities such as oxygen and nitrogen are left behind since they do not react with iodine. The Zirconium tetra-iodide vapour is passed over a hot tungsten filament (which is at a temperature of 2075 K) due to which the vapour decomposes and pure Zirconium gets deposited on the filament. The Iodine used in this process is recycled. The reactions given below depict the entire process:
$\begin{matrix} Zr(s) \\\ Impure\quad Zirconium \end{matrix}+\begin{matrix} { 2I }_{ 2 } \\\ Iodine \end{matrix}\xrightarrow [ 570\quad K ]{ Evacuated\quad vessel } \begin{matrix} Zr{ I }_{ 4 }(g) \\\ Zirconium\quad tetra-iodide \end{matrix}$
$\begin{matrix} Zr{ I }_{ 4 }(g) \\\ Zirconium\quad tetra-iodide \end{matrix}\xrightarrow [ 2075\quad K ]{ Tungsten\quad filament } \begin{matrix} Zr(s) \\\ Pure\quad Zirconium \end{matrix}+\begin{matrix} 2{ I }_{ 2 } \\\ Iodine \end{matrix}$
The same process is followed for the refining of Titanium metal but the temperature conditions are different.
$\begin{matrix} Ti(s) \\\ Impure\quad Titanium \end{matrix}+\begin{matrix} { 2I }_{ 2 } \\\ Iodine \end{matrix}\xrightarrow [ \quad 250\quad ^{ o }{ C } ]{ Evacuated\quad vessel } \begin{matrix} Ti{ I }_{ 4 }(g) \\\ Titanium\quad tetra-iodide \end{matrix}$
$\begin{matrix} Ti{ I }_{ 4 }(g) \\\ Titanium\quad tetra-iodide \end{matrix}\xrightarrow [ 1400\quad ^{ o }{ C } ]{ Tungsten\quad filament } \begin{matrix} Ti(s) \\\ Pure\quad Titanium \end{matrix}+\begin{matrix} 2{ I }_{ 2 } \\\ Iodine \end{matrix}$
Hence the Van Arkel method is explained.

Note: By using the Van Arkel method, we obtain ultra-pure metals. This method is also called vapour phase refining since it involves the formation of volatile unstable iodides of metals such as Titanium and Zirconium which are then decomposed to give the respective pure metal.