Select the correct statements about iron extraction in a blast furnace.

AThe hot air blast burns coke and supplies the heat for the process.
BLimestone decomposes to $CaO$ , which acts as a basic flux for silica gangue.
CThe slag is denser than molten iron and settles beneath it.
DIron oxides are reduced mostly by $CO$ in the cooler upper zone and partly by carbon in the hotter lower zone.

Answer: A, B, D

Gas-solid reduction by

CO

occurs high in the stack, and carbon-involving steps such as

C + CO X_{2} 2 CO

occur in the hotter region.

CaCO X_{3} CaO + CO X_{2}

and

CaO + SiO X_{2} CaSiO X_{3}

account for the flux. Burning of coke by the blast supplies heat. Slag floats over iron because it is less dense, so the third statement is false.