d the fumes evacuated by natural draft in chimney 26 or by means of a slight overpressure of the furnace at station a pass through the charge in the furnace at station d which is rotatably operated as the furnace at station a and which produces a substantial uniform temperature rise of the charge and of the refractory lining of the furnace at station d at the level of the interface between the furnaces as is well illustrated in fig2 the flow of fumes g produced in the furnace at station a is accelerated in the connection element 23 and produces at the inlet opening 10 of the furnace at station d an air draft illustrated by arrow f to promote the combustion of carbon monoxide in the preheating furnace at station d advantageously at least during the second smelting phase with sub stoichiometric adjustment of the combustion in the furnace at station a a pipe for injecting oxygen 27 is introduced to space d to inject into the furnace at station d a flow f of oxygen to complete the combustion and the reduction of hydrogen in the furnace at station d as a variant to accelerate the flow of gas through the furnace at station d and raise the temperature therein the.