I just saw a random video the other day about aluminum recycling. For many uses they have to add rather a large amount of virgin aluminum in order to get the alloys right. Especially troublesome were alloys with heavy metals, which you have to keep out of food container feedstocks. I can’t imagine steel recycling being much different.
This video was about a plant that ground the aluminum to roughly inch sized pieces (sorting out anything bigger or smaller) then using X-rays and air jets to sort the alloys based on transmissivity of the trace elements. Their goal was to make aluminum billets out of 100% recycled material (although the video claimed the goal was 100% recycling, which is clearly not happening if you are culling pieces from the input stream. Might have been an error in translation or wishful thinking.)
I wonder if someone will do the same with steel.