Bayer's process (Al): Bauxite contains Al2O3 + Fe2O3 + SiO2 impurities. Step 1: Al2O3.2H2O + 2NaOH(conc) → 2NaAlO2(aq) + 3H2O [Al2O3 dissolves; Fe2O3 (red mud) doesn't dissolve — filtered off]. Step 2: NaAlO2 + CO2 + 2H2O → Al(OH)3↓ + NaHCO3 [Al(OH)3 precipitated by passing CO2 — amphoteric nature used]. Step 3: 2Al(OH)3 → Al2O3 + 3H2O (calcination at 1470 K). Cyanide process (Au, Ag): 4Au + 8NaCN + 2H2O + O2 → 4Na[Au(CN)2] + 4NaOH [Au dissolves in NaCN with air]. Recovery: 2Na[Au(CN)2] + Zn → Na2[Zn(CN)4] + 2Au↓ [Zn displaces Au — more electropositive].