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Metallurgy isn't my field, but here's an educated guess...
There are different kinds of contaminants. In raw ore you largely have silicate rock and metals. In recycled material you have relatively pure metal (alloys), and a large variety of volatiles.
Now with ore you can grind it all into sand, sift it, and smelt all the heavy grains. The rock should mostly just separate from the metal, these are just phase changes. But with recycling, those volatiles are going to burn and some are going to react with the metals, changing the chemical makeup. And with ore, you basically know what minerals you're working with. With recycled materials, it's anyone's guess. Does this can contain some food residue? Or an oil? Perhaps chemical cleaning agents? Is another plastic container stuffed inside?
There's a lot of variables with recycled materials, I imagine it's hard to predict how some of those variables react.
For metals, it's pretty trivial to remove slag (contaminants) from the metal. Basically everything floats to the top and you can just scrape it off.