Air distillation plant and corresponding cold box

Air distillation plant (1) of the type comprising a double column and a mixing column (5) for mixing a gas and a liquid, the mixing column being free from a reboiler or a condenser, the double column itself comprising a medium-pressure column (2), a low-pressure column (3) and a reboiler (4) for bringing the gas at the top of the medium-pressure column into heat-exchange relationship with the liquid at the bottom of the low-pressure column, characterized in that the medium-pressure column (2), the low-pressure column (3) and the mixing column (5) are placed one on top of another, forming a single erected structure (16).