Magnesium in seawater, mainly in the form of magnesium chloride and magnesium sulfate.Making magnesium from seawater on a large scale is an uncomplicated process in which lime milk is added to seawater to precipitate magnesium hydroxide, injected with hydrochloric acid, and converted into anhydrous magnesium chloride.The intermediate product magnesium hydroxide can also be used to make magnesium oxide, magnesium carbonate and other products.The toothpaste we use every day, its main ingredient is magnesium carbonate;Magnesium carbonate is also to the credit of the plumbers, who coat the pipes with a white plaster of lime, so that they do not freeze out at about ten degrees centigrade in winter;Even in rubber manufacturing, magnesium carbonate is often used as filler material.Magnesium is the third most concentrated element in seawater.It is estimated that 1.3 million tons of magnesium can be extracted per cubic kilometer of seawater.In the countries with high production of sea salt, various magnesium compounds are produced from bittern.Countries with a shortage of terrestrial magnesium also produce large quantities of the metal magnesium and various magnesium salts directly from seawater.Today, much of the world's magnesium and its compounds are derived directly or indirectly from seawater.

　　By the early 21st century, the main products extracted from brine were magnesium chloride, magnesium sulfate, magnesium oxide and magnesium hydroxide.To extract magnesium from seawater, seawater is first pumped into a large pool and poured into the lime milk to produce a suspension of magnesium hydroxide, which is intended to enrich and purify magnesium.After the precipitation, the precipitation was taken out and the purity of magnesium hydroxide was obtained after washing.It doesn't have much use. It must be turned into useful magnesium oxide and magnesium metal.

　　Magnesium hydroxide is unstable and decomposes into magnesium oxide and water when heated.

　　Mg (OH) 2 - MgO style + H2O

　　It is difficult to reduce the +2 magnesium ion to elemental magnesium by gaining electrons.If magnesium is smelted in a similar way to iron, a very high temperature is required.For example, elemental magnesium can be produced by reducing magnesium oxide with coke at 2000℃.The resulting magnesium often contains more impurities.

　　MgO style + C - Mg + CO write

　　Therefore, it is commonly used in industry to electrolyze magnesium ions to gain electrons at the cathode and reduce them to elemental magnesium.To make magnesium by electrolysis, a molten liquid containing magnesium ions is obtained.In magnesium compounds, typical ionic compounds are magnesium oxide and magnesium chloride.The melting point of magnesium oxide is too high (2800℃), while that of magnesium chloride is much lower (714℃).Therefore, people choose magnesium chloride as the raw material to produce magnesium by electrolysis.

　　Magnesium chloride is obtained by dissolving magnesium hydroxide (or magnesium oxide) in hydrochloric acid and concentrating the solution.

　　Mg (OH) 2 + 2 HCL = MgCl2 + 2 h2o

　　In the molten magnesium chloride there are magnesium ions that can move freely.After direct current current is introduced, the chloride ion moves to the anode, where it loses electrons and oxidizes into chlorine atoms. The two chlorine atoms combine to form a chlorine molecule.The magnesium ion moves towards the cathode, gains electrons at the cathode, and is reduced to elemental magnesium.The reaction between the two electrodes is

　　Anode: 2 cl -- 2 e = Cl2 write

　　Cathode: magnesium 2 + + 2 e = Mg

　　The total electrolytic reaction equation can be expressed as follows:

　　electricity

　　MgCl ₂ (l) = = = = Mg (s) + Cl ₂ (g) write

　　Note: in order to prevent the generated magnesium from being oxidized by oxygen in the air at high temperature, electrolysis needs to be carried out in a special vacuum environment or under the condition of hydrogen chloride gas as a protective gas.