Q.The metal oxide which cannot be reduced to metal by carbon is

Once an ore has been concentrated, extracting the crude metal from it always proceeds in exactly two steps, in this order: (i) converting the concentrated ore into an OXIDE of the metal of interest, and (ii) REDUCING that oxide down to the free metal. Every route goes through the oxide as an intermediate, even for a sulphide or carbonate ore, because thermodynamically the reduction of an oxide is generally easier than reduction of other compound classes.

Step 1: conversion to oxide. ROASTING is used for sulphide ores -- strongly heating the concentrated ore with EXCESS oxygen, below the metal's melting point, converting the sulphide to the oxide and releasing SO2 (e.g. 2PbS + 3O2 → 2PbO + 2SO2); roasting also volatilises impurities like arsenic, sulphur and phosphorus as their own oxides, and modern plants trap the SO2 by-product to make sulphuric acid. CALCINATION is used for carbonate/hydrated ores -- strongly heating in the ABSENCE (or with limited supply) of air, expelling CO2 from a carbonate (e.g. CaCO3 → CaO + CO2) or water of crystallisation from a hydrate (e.g. Fe2O3.3H2O → Fe2O3 + 3H2O), and leaving a porous oxide behind. …