Problem is that the vast majority of material is metal, so your digester needs a way to saturate the metal to reach the CHON, and and then extract the digester. Is that really better than extracting the CHON directly and then processing it?
Carbonaceous Chondrite asteroids have carbon in the form of coal but also carbonate rocks as well as moderate amounts of water as well as stony minerals (silicon, aluminum) as well as iron.