Not sure if this is easy or difficult but.

Set so each smelted metal produces slag of that metal type

Iron Slag, Copper Slag, Aluminum slag, etc... Basically any metal ore that goes through smelting process.

Then process that slag with 50% output as actual regular slag and the other half as scrap metal for that metal type the slag is from

Iron Slag processes 50% slag and 50% scrap metal

Copper slag processes to 50% slag and 50% scrap copper

etc...

Maybe different percentages like maybe 100% to that metal or 30% with remaining to slag that has to be processed dumped or something.