top gas from the furnace 2 as cooling gas for this latter the partially decarbonated gas from the unit 10 is reheated in 11 by indirect heat exchange then in 12 by partial combustion with the oxygen from the unit 9 via a conduit 21 then introduced as reducing gas into the shaft furnace 13 which is supplied at its head by means of suitable device 22 with ferrous mineral the gas from the head of furnace 13 is purified in 5 and recycled a portion rejoins the conduit 19 via a conduit 23 and the rest is sent to the grill of the exchanger 11 via a conduit 24 as preheating gas the liquid metal produced in the bottom of furnaces 2 and 13 is sent to the electric arc furnace f for oxygen refining thus the oxygen produced by the unit 9 via a conduit 25 is used to supply the furnace 2 the reheater 12 and the electric arc furnace f it can also be used for other purposes for example as shown to aid in the transport of recycled particulates into the furnace 2 by injection in the conduit 17 the unit 9 necessarily produces nitrogen at the same time as oxygen at least a portion of this nitrogen is increased in value in the unit 10 to which the nitrogen outlet of the unit 9 is connected by a conduit 26 under conditions which will now be described with respect to fig2 and 3 there is shown at 27 the conduit for production by unit 10 of partially decarbonated reducing gas.