Complex sentence: The iron ore nodules and the coal could not be used in the furnace until they were refined, this process involved roasting to remove impurities from the iron ore and to remove the tar from the coal leaving it as coke.
Split into simpler sentences: The iron ore nodules and the coal could not be used in the furnace until they were refined. This process involved roasting to remove impurities from the iron ore and to remove the tar from the coal leaving it as coke.