Method for operating a blast furnace installation, said installation comprising: • an air installation comprising: 1) a compression unit and 2) an air separation unit, • a blast furnace, • a gas treatment installation, method wherein: • the compression unit generates compressed air, • the air separation unit (a) receives compressed air generated by the compression unit and (b) splits the received compressed air into at least one oxygen-enriched air gas and one nitrogen-enriched air gas, • a flow containing an oxidising gas generated by the air installation is injected in the blast furnace, said oxidising gas having an oxygen concentration Cvox ≥ 21.0%vol and a flow rate Dvox, • the blast furnace generates a top gas, • the gas treatment unit receives a top gas generated by the blast furnace and splits the received top gas into at least one CO 2 -enriched top gas and one CO-enriched top gas, at least one part of the CO-enriched top gas is recycled and injected into the blast furnace as a reductive gas with a flow rate Drco, the method being characterised in that the oxidising gas from the flow of the blast furnace is selected among: a) compressed air generated by the air compression unit, or b) oxygen-enriched air gas generated by the air separation unit, or c) a mixture of said compressed air and said oxygen-enriched air gas, or d) a mixture of said oxygen-enriched air gas and nitrogen-enriched air gas generated by the air separation unit, or e) a mixture of said compressed gas and said oxygen-enriched air gas and said nitrogen-enriched air gas, and in that the concentration Cvox of the oxidising gas is regulated as a function of the flow Drco of the top gas enriched in recycled CO: the concentration Cvox of the oxidising gas being increased when the flow rate Drco of the top gas enriched in recycled CO increases, and the concentration Cvox of the oxidising gas being lowered when the flow rate Drco of top gas enriched in recycled CO is lowered. • an air installation comprising: 1) a compression unit and 2) an air separation unit, 1) a compression unit and 2) an air separation unit, • a blast furnace, • a gas treatment installation, • the compression unit generates compressed air, • the air separation unit (a) receives compressed air generated by the compression unit and (b) splits the received compressed air into at least one oxygen-enriched air gas and one nitrogen-enriched air gas, • a flow containing an oxidising gas generated by the air installation is injected in the blast furnace, said oxidising gas having an oxygen concentration Cvox ≥ 21.0%vol and a flow rate Dvox, • the blast furnace generates a top gas, • the gas treatment unit receives a top gas generated by the blast furnace and splits the received top gas into at least one CO 2 -enriched top gas and one CO-enriched top gas, a) compressed air generated by the air compression unit, or b) oxygen-enriched air gas generated by the air separation unit, or c) a mixture of said compressed air and said oxygen-enriched air gas, or d) a mixture of said oxygen-enriched air gas and nitrogen-enriched air gas generated by the air separation unit, or e) a mixture of said compressed gas and said oxygen-enriched air gas and said nitrogen-enriched air gas, and