Among statements (a) – (d), the correct ones are : (a) Lime stone...

(a) During the extraction of Iron. at $900 – 1500K$ (higher temperature range in the blast furnace), one of the processes taking place is:

${CaCO}_{3}\overset{∆}{\to}CaO + {CO}_{2}$

Limestone is decomposed to $CaO$ which removes silicate impurity of the ore as slag. The slag is in the molten state and separates out from iron.

(b) Extraction of silver involves leaching the metal with cyanide ion:

$4Ag\left(s\right) + 8{CN}^{–}\left(aq\right) + 2{H}_{2}O\left(aq\right) + {O}_{2}\left(g\right) \to 4{\left[Ag{\left(CN\right)}_{2}\right]}^{–}\left(aq\right) + 4{OH}^{–}\left(aq\right)$

The complex formed is negatively charged and thus it is an anionic complex.

(c) Nickel is purified by Mond's process:

Step-1: Nickel is heated in a stream of carbon monoxide forming a volatile complex, nickel tetracarbonyl.

$Ni + 4CO\overset{330 - 350K}{\to}Ni{\left(CO\right)}_{4}$

Step-2: The carbonyl is subjected to a higher temperature so that it is decomposed giving the pure metal.

$Ni{\left(CO\right)}_{4}\overset{450 - 470K}{\to}Ni + 4CO$

(d) van Arkel Method for Refining Zirconium or Titanium:

This method is very useful for removing all the oxygen and nitrogen present in the form of impurity in certain metals like $Zr$ and $Ti$. The crude metal is heated in an evacuated vessel with iodine. The metal iodide being more covalent, volatilises.

$Zr + 2{I}_{2} \to {ZrI}_{4}$

The metal iodide is decomposed on a tungsten filament, electrically heated to about $1800K$. The pure metal is thus deposited on the filament.

${ZrI}_{4} \to Zr + 2{I}_{2}$