[throwaway4aday]

Correct, the question that should be asked here is what happens to all of the water after we've removed the minerals we need from it? Do we just pump it back into the ocean? The outlet would need to be far enough away from the inlet to avoid dilution. What impact does that have on marine life? We know that concentrating those minerals into brine when we extract water through desalinization is harmful so how harmful is doing the opposite and depleting the water of the minerals?

Also, those intakes for water are going to be massive. How are we going to make them fish safe? Dolphin safe? Plankton safe? This is a major problem in hydroelectric dams.

Third, what's providing power for this process and where is it located?

Fourth, what are the second order effects of replacing a lot of steel production? Will this make all of the remaining products where steel can't be replaced a lot more expensive? I doubt you can use magnesium as a replacement for the girders used in buildings. Magnesium is only as strong as mild steel so pretty much everything that requires any tensile strength will still need to use steel.

[yellowapple]

My idea is to combine this with desalination. California could permanently end its droughts and become a major magnesium supplier in one swell foop.

[throwaway4aday]

You'd also need to think up good uses for the sodium chloride or dump it somewhere. The sulfate, potassium and calcium would probably be useful if they could be easily separated but the sodium and chloride ions are by far the majority and are not going to be profitable in comparison to the current operations we use to obtain them.

[yellowapple]

> You'd also need to think up good uses for the sodium chloride or dump it somewhere.

Okay, so California ends its drought permanently and becomes a major supplier of magnesium and table salt.

Slightly-less-facetiously: sodium chloride has all sorts of industrial uses (on top of its culinary uses). The good uses are, in other words, already thought up in droves.

Further, most salt production already entails taking seawater and pulling out the water. Ain't too much of a stretch to, you know, actually keep the water instead of letting it evaporate into the atmosphere.