What is a process for removing impurities from crude iron?
The most basic process is melting.
From the earliest times of metal-working the melting of iron ore was used to ‘float’ the impurities to the surface, as slag. Even before the use of melting furnaces, the process of cyclic heating and working of the metal drove out many impurities.
As furnace technology and control became better, the use of additives like carbon were used to “scavenge” oxide forms of the metal, reducing the iron to pure metal and releasing carbon dioxide from the melt. That greatly enhanced the recoverable iron from an ore source.