So reading about Bingham open-pit mine brought me to reading about electrolytic copper purification.
I understand the general process wherein pure copper from your impure copper anode migrates to the cathode, but one thing I didn’t understand and couldn’t find more information about is why they have moved from pure copper cathodes to stainless steel cathodes in a lot of places.
It seems to me to add a few steps and potential contamination - if you use a pure copper cathode, then the copper from your anode migrates over and you end with a 99.99% pure larger copper cathode which you can sell as-is with little further processing.
If you use stainless steel, you then have to chemically or mechanically separate the copper and stainless steel prior to selling, with potential contamination near the copper-steel interface bringing the purity down from 99.99%.
The people and businesses doing this purification aren’t stupid, so I’m sure they have good reasons to do it with stainless steel cathodes, I just can’t find anywhere on line talking about the why of it rather than the fact of it - and so I turn to you gentle Dopers; does anyone here have a good answer on the “why” of it?
Where do you get the pure copper cathode, if you need electrolytic refining to get pure copper to start with? That was always my question as a kid. And no one had a good answer.
Actually, there are many answers. You could use carbon electrodes. In fact, when I was a kid, science books said green wood was used to purify copper at some step. Now, setting up a copper purification business requires many things: a building full of furnaces, refractory vessels, office space with janitors and a cafeteria, and oodles of heavy mechanical equipment that a chemical engineer could base their thesis on, etc. However, a bunch of metallurgy technicians, in their gear, going, “OK Charlie. Drop in the twigs” seems out there for me.
Maybe after they’re done, they just use a calibrated trimmer to cut off just the copper, and the hard stainless steel gives them a visual cue to when they’re done. And maybe, on balance, stainless steel cathodes is the most economical solution.
I mean, I get what you’re saying, but doesn’t that just turn into “do the stainless steel cathode once, then use the resulting 99.99% pure copper for cathodes from then on”?
Or more broadly in the ecosystem of purifiers, one copper purification place does it the stainless steel way, and sells the more-efficient 99.99% pure resulting cathodes to all the others, who skip the extra steps and processing necessary with the stainless steel way?
The stainless steel cathode plates are reusable. The deposited copper can be separated from the stainless steel (essentially peeled off) and the cathode reused. Much development work (both metallurgy and mechanical design) has been done in the design of the stainless cathodes to make it easier to strip off the copper.
That’s exactly what I was looking for, thanks much.
So it looks like a combination of being automatable, reusability, shorter cycles, and finicky requirements around flatness, conductivity, and purity (which would make the “just buy pure cathodes from a place doing the stainless steel way” plan harder, because the flatness requirements would require a lot of care in shipping and unpacking) are the main drivers behind stainless steel adoption.
