JEE Main 2023 · 8 April · morning · Single correct · +4 / −1
Which of the following metals can be extracted through alkali leaching technique?
- (a) Sn
- (b) Cu
- (c) Au
- (d) Pb
Answer and solution
Answer (c) Au
Solution
Chemistry ma’am · 3 steps
Gold is pulled out of its ore by an alkaline cyanide solution, the MacArthur–Forrest cyanide process.
- What leaching means. In leaching, the ore is treated with a solution that dissolves the metal and leaves the gangue behind. Alkali leaching uses a basic solution, such as NaOH or dilute NaCN.
- Gold dissolves. Gold dissolves in dilute NaCN or KCN when air is blown in. It forms the soluble complex [Au(CN)₂]⁻, and the solution becomes alkaline as OH⁻ forms. Zinc then displaces gold from this solution: 2[Au(CN)₂]⁻ + Zn → [Zn(CN)₄]²⁻ + 2Au.
4Au + 8CN⁻ + 2H₂O + O₂ → 4[Au(CN)₂]⁻ + 4OH⁻
Metal Main ore How it is extracted Sn cassiterite, SnO₂ gravity separation, then reduction with carbon Cu copper pyrites, CuFeS₂ froth flotation, roasting, self-reduction. Low-grade ore is leached with acid or bacteria Au native gold leaching with alkaline cyanide in air, then displacement by Zn Pb galena, PbS froth flotation, roasting, self-reduction - So. Of the four metals, only gold is extracted by alkali leaching. The answer is (c).
The idea to keep. Remember two leaching processes. NaOH dissolves bauxite for Al, and alkaline NaCN in air dissolves Ag and Au. Metals from sulphide ores go through roasting and reduction instead.
Answer from the NTA final answer key. Solution by PrepWiser.