Hi!

Would you not need quite large amounts of seawater in order to get appreciable amounts of minerals? https://www.britannica.com/science/seawater this page gives 0,12% Mg and 0,04% Ca in seawater, as the most concentrated cations except Na. So for 1 cubic meter of seawater you would get 1,2 kg of Mg and 0,4 kg of Ca.

I think I assumed that much of the benefit of seawater-derived products would be due to the contents of organic compounds (not “minerals”). I also assume that you would not extract a large proportion of these if you only took whatever precipitated when adding lye. But perhaps my assumptions are incorrect?

I guess you would also need a lot of lye.