We usually extract magnesium from seawater.
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Magnesium is an important strategic material, so the production of magnesium metal in the world is closely related to war and is called "defense metal". The annual output of magnesium metal produced by seawater was only 20,000 tons before the Second World War, more than 200,000 tons during the war and reduced to 30,000 tons after the war. With the gradual understanding of magnesium, metallic magnesium tends to replace steel, aluminum, zinc and other metals in machinery manufacturing. In addition, it has opened up new and broad application fields in metallurgical industry and chemical industry. It can be said that metallic magnesium is a "rising star" among metals and has a bright future. ?
The content of magnesium in seawater ranks third after bromine and sodium. Magnesium has the characteristics of light weight and high strength, and can be used to manufacture airplanes and ships. Magnesium-lithium alloy has the lightest weight and the strongest heat resistance, which is of great significance in military and civil enterprises. At the same time, it is widely used in rocket, missile, aircraft manufacturing, automobile, precision machinery and other fields. With the rapid development of iron and steel industry in various countries, not only the amount of magnesium oxide is increasing, but also the impurity content of high-quality magnesium oxide needed for steelmaking is required to be below 2% ~ 4%. This kind of magnesium oxide made of natural magnesite sintered on land can't meet the requirements. Moreover, the purity of seawater extraction reached 96% ~ 98% as early as 1960s. With the improvement of smelting technology, the purity of magnesium rose to 99.7%. Such ultra-high purity magnesium oxide can undoubtedly meet the special needs of metallurgical industry.
Operation process of extracting magnesium
When magnesium is extracted from seawater, seawater is pumped into an extra-large pool, and lime milk is poured to generate a suspension of magnesium hydroxide, which is to enrich and purify magnesium. After the precipitate is settled, the precipitate is taken out and washed to obtain high-purity magnesium hydroxide. It is of little use and must be turned into useful magnesium oxide and metallic magnesium. Magnesium hydroxide is unstable and decomposes into magnesium oxide and water after heating.
Magnesium hydroxide-magnesium oxide +H2O
It is difficult to make +2 valence magnesium ions gain electrons and reduce them to elemental magnesium. If magnesium is smelted by a method similar to ironmaking, it needs a very high temperature. For example, elemental magnesium can only be prepared by reducing magnesium oxide with coke at 2000℃. The magnesium thus obtained usually contains more impurities.
MgO+C - Mg+CO↑
Therefore, electrolysis is commonly used in industry to make magnesium ions get electrons at the cathode and reduce them to elemental magnesium. In electrolytic magnesium smelting, the molten liquid containing magnesium ions must be obtained first. Among magnesium compounds, typical ionic compounds are magnesium oxide and magnesium chloride. The melting point of magnesium oxide is too high (2800℃), while the melting point of magnesium chloride is much lower (7 14℃). Therefore, people choose magnesium chloride as the raw material for electrolytic magnesium production. Dissolve magnesium hydroxide (or magnesium oxide) with hydrochloric acid, and then concentrate the solution to obtain magnesium chloride.
Magnesium hydroxide+dihydrochloride = magnesium chloride+dihydrate
There are free moving magnesium ions in molten magnesium chloride. After DC current is applied, chloride ions move to the anode, lose electrons on the anode, and are oxidized into chlorine atoms, and two chlorine atoms combine to form 1 chlorine molecule; Magnesium ions move to the cathode, where they gain electrons and are reduced to elemental magnesium. The reaction that takes place at the two electrodes is
Anode: 2cl-2e = Cl2 = cathode: Mg2++2e=Mg
The whole electrolytic reaction equation can be expressed as follows:
MgCl power on? (l)==== Mg(s)+Cl？ (g) iv