Which of the following process is used in the extractive metallurgy of magnesium?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Fused salt electrolysis
Magnesium is a highly reactive metal (E°(Mg2+/Mg) = −2.37 V). Its compounds cannot be reduced easily by carbon, so the metal is obtained by electrolysis of fused (molten) MgCl2. The molten salt contains free Mg2+ and Cl− ions and no water:
Cathode: \( \ce{Mg^{2+} + 2e^- -> Mg} \)
Anode: \( \ce{2Cl^- -> Cl2 + 2e^-} \)
Net reaction: \( \ce{MgCl2(l)} \xrightarrow{\text{electrolysis}} \ce{Mg(l) + Cl2(g)} \)
Why the other options are incorrect
Option (2) Self reduction — Incorrect. It is used for sulphide ores of less reactive metals such as copper: part of the sulphide is roasted to the oxide, which then reacts with the remaining sulphide.
\( \ce{2Cu2O + Cu2S -> 6Cu + SO2} \)
Option (3) Aqueous solution electrolysis — Incorrect. In water, H2O is reduced at the cathode in preference to Mg2+, so hydrogen is set free and no magnesium is deposited.
\( \ce{2H2O + 2e^- -> H2 + 2OH^-} \)
Option (4) Thermite reduction — Incorrect. Aluminium reduces the oxides of less reactive metals such as chromium. Magnesium is more reactive than aluminium, so its oxide is not reduced this way.
\( \ce{Cr2O3 + 2Al -> Al2O3 + 2Cr} \)
NCERT (2026-27 edition):
Electrolytic Cells and Electrolysis – “… Sodium and magnesium metals are produced by the electrolysis of their fused chlorides and aluminium is produced by electrolysis of aluminium oxide in presence of cryolite.”