A cryogenic air separation unit (ASU) (10) provides oxygen (12) for use in a process (14), such as COREX iron making or the partial oxidation of hydrocarbons, or in an oxycoal fuelled blast furnace, producing an off-gas (16) containing useful components (H 2 and CO) and carbon dioxide. The carbon dioxide is removed by pressure swing adsorption (20) using nitrogen (22) from the ASU (10) as a regeneration gas and the purified off-gas (26) used a feed gas to a further process (28) such as production of iron by direct reduction of iron ore or the production of a NH 3 making ammonia or is recycled into the first process as a feed gas.