Metals from the sea
Magnesium is valuable metal. It is used as light-weight structural metal and is used in alloys, in batteries, and chemical synthesis. Although magnesium is plentiful in Earth’s crust, it is cheaper to “mine” the metal from seawater. Magnesium forms the second most abundant cation in the sea (after sodium), there are about 1.3 g of magnesium for every kilogram of seawater. The method of obtaining magnesium from seawater employs the three types of reactions: precipitation, acid-base, and redox reactions.
In the first stage in the recovery of magnesium, limestone (CaCO3) is first heated at high temperatures to produce quicklime, or calcium oxide (CaO).
CaCO3 (s) -> CaO (s) + CO2 (g)
When calcium oxide is treated with seawater, it forms calcium hydroxide [Ca(OH)2], which is slightly soluble and ionizes to give Ca2+ and OH- ions:
CaO (s) + H2O (l) -> Ca2+ (aq)+ 2OH- (aq)
The surplus hydroxide ions cause the much less soluble magnesium hydroxide to precipitate.
Mg2+ (aq)+ 2OH- (aq) -> Mg(OH)2 (s)
The solid magnesium hydroxide is filtered and reacted with hydrochloric acid to convert it to magnesium chloride (MgCl2):
Mg(OH)2 (s )+ 2HCl (aq) -> H2O (l) + MgCl2 (aq)
After the water is evaporated, the solid magnesium chloride is melted in a steel cell. The molten magnesium chloride contains both Mg2+ and Cl- ions. In a process called electrolysis, an electric current is passed through the cell to reduce the Mg2+ ions and oxidize the Cl- ions. The half reactions are:
Mg + 2e -> Mg
2Cl- -> Cl2 + 2e
The overall reaction is:
MgCl2 -> Mg + Cl2
This is how magnesium metal is produced. The chlorine gas generated can be converted to hydrochloric acid and recycled through the process.