Magnesium is recovered from seawater using

Mg is extracted from sea water using Dow's extraction process. After being concentrated in the sun, sea water undergoes slaked lime treatment. Heat is applied to magnesium hydroxide in a stream of HCl to produce MgCl₂, which is then electrolyzed to release magnesium. 

$\begin{array}{r}{\mathrm{Mg}}^{2+}+\mathrm{Ca}(\mathrm{OH}{)}_2\to \mathrm{Mg}(\mathrm{OH}{)}_2+{\mathrm{Ca}}^{2+}\\ \mathrm{Mg}(\mathrm{OH}{)}_2+2\mathrm{HCl}\to {\mathrm{MgCl}}_2+2{\mathrm{H}}_2\mathrm{O}\end{array}$