Question

Describe the extraction of magnesium from seawater in the Dow process. Show all relevant equations involved in the process, starting with the precipitation of Mg2+ as Mg(OH)2(s) using CaO(s) until the electrolysis of molten MgCl2.

Answer 1

The Dow process for the extraction of magnesium from sea water is as follows

In this process first magnesium hydroxide is precipitated from sea water by slurrying with calcined dolomite and then converting it to magnesium chloride by reacting it with hydrochloric acid.
This product is dried to 1,5 moles of water and introduced directly into the cells. The by-product chlorine from the electrolytic cells is recycled into hydrochloric acid for use at the front end of the process.

The reaction steps are

1)To extract the magnesium, calcium hydroxide is added to seawater to form magnesium hydroxide precipitate

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2

2)

2)Magnesium hydroxide is insoluble in water and can be filtered out and reacted with hydrochloric acid to produced concentrated magnesium chloride.

Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O

3)From magnesium chloride, electrolysis produces magnesium.

Mg²⁺ + 2e^-       Mg at the (-)cathode).
2Cl^- - 2e^-       Cl₂ At anode

Overall reaction is Mgcl2--->Mg(s)+ Cl₂ (g)

Thus finally Magnesium is extracted