alt Hacker NewsYou don't use ponds, you run the desalination to as strong as practical and follow up with either electrolysis or distillation of the brine.
But once summer electricity becomes cheap enough due to solar production increasing to handle winter heating loads with the (worse) winter sun, we can afford a lot of electrowinning of "ore" which can be pretty much sea salt or generic rock at that point.
Form Energy is working on grid scale iron air batteries which use the same chemistry as would be used for (excess/spare) solar powered iron ore to iron metal refining.
AFAIK the coal powered traditional iron refining ovens are the largest individual machines humanity operates. (Because if you try to compare to large (ore/oil) ships, it's not very fair to count their passive cargo volume; and if comparing to offshore oil rigs, and including their ancillary appliances and crew berthing, you'd have to include a lot of surrounding infrastructure to the blast furnace itself.)
It will take coal becoming expensive for it's CO2 before we really stop coal fired iron blast furnaces. And before then it's hard to compete even at zero cost electricity when accounting for the duty cycle limitations of only taking curtailed summer peaks.
Not that it's super relevant to this discussion, but I think the largest individual machines operated would probably have to go to high energy particle accelerators like the LHC at CERN or those operated by Fermi Lab.
Billions of dollars in cost, run 24/7 with virtually no downtime during regular operations, in underground tunnels with circumferences in the tens of miles, and all throughout is actively-coordinated super conductors and beam collimation in a high-vacuum tube attached to absurdly complex, ultra-sensitive, massively-scaled instrumentation (not to mention the whole on-site data processing and storage facilities). Certainly open to bring convinced otherwise, but aside from ISS in pure cost, so far it's my understanding that those are the pinnacle of large-scale machines.