alt Hacker NewsFrom 30-second googling: purifying Cu to from rocks to ~99% purity is done by mixing CuFeS2 found naturally with C and CaCO3 and smelting. This yields Cu2S. Then S is released by bubbling with O2. This process releases gaseous SO2. From there, Cu is further purified by electrolysis in CuSO4aq.
The whole process is going to need a ton of coal and calcium carbonate and heat, and you have to have a way to safely dispose of SO2 gases and CuSO4aq liquids. And there don't seem to be anything that sounds safe and clean about this process.
SO2 is typically scrubbed and used to produce sulfuric acid. It's not a _clean_ process, but its waste stream is fairly well-controlled.
The amount of carbon used in the reaction itself is totally neglibile, compared to other overheads.