Abstract: The stack gas from a shaft furnace in which iron ore is directly reduced is cooled and filtered and then subjected to steam so as to convert its carbon monoxide into carbon dioxide and water. The carbon dioxide and water are then scrubbed out of this stack gas so as to leave virtually pure hydrogen which is heated to a temperature between 400.degree. and 600.degree. C and added to the output of an oxygen reactor that transforms coal and oxygen into a synthetic gas having a carbon-monoxide/hydrogen volume ratio of at least 1.5:1. The addition of this hydrogen to the output of the reactor lowers the volume ratio to 1:1 at most so that this mixture can be reintroduced into the shaft furnace at a temperature of around 1000.degree. C as a reducing gas.