In a paper published in the journal Steel in Translation, the scientists explain that they decided to focus on out-of-furnace slag because -given its little iron content- it is usually sent to ...

Iron ore plus carbon plus heat gives you iron plus slag. The slag goes mostly to waste – some is used to make roads. The iron is now ready to be made into steel.

In chemistry terms, the composition of the rock – iron ore – is changed into two separate materials, iron itself, and a residue which we call slag.