[pkilgore]

Not to be glib, but the second law of thermodynamics.

You are attempting to filter out trace amounts valuable dopants and some small amounts of metals with value from, essentially, a pile of sand.

This is not energetically or chemically easy.

[marcosdumay]

The second law of thermodynamics is very forgiving here. It's actually not the problem. Energy depends on the log of the concentration.

The problem is with our technology; we don't know how to recycle things well.