Can Magnesium be Extracted from Seawater?

When extracting magnesium with seawater, the seawater is first pumped into an extra large pond and the milk of lime is poured into it to form a suspension of magnesium hydroxide. After the precipitate is settled, the precipitate is taken out, and after washing, high purity magnesium hydroxide is obtained. It doesn't have much use, it must be turned into useful magnesium oxide and metallic magnesium.

Magnesium extraction from seawater

The amount of magnesium in seawater salt is second only

Extraction of magnesium from seawater

Lime milk hydrochloric acid

Seawater Sedimentation pond Magnesium hydroxide Magnesium chloride hexahydrate (MgCl2 · 6H2O) anhydrous MgCl2Mg

Magnesium extraction process from seawater

Extraction of magnesium from seawater can be performed as follows: (1) Calcined shells to make lime milk CaCO3 = high temperature = CaO + CO2 CaO + H2O == Ca (OH) 2
(2) Add lime milk to the introduced seawater, settle, filter, and wash to obtain Mg (OH) 2: MgCl2 + Ca (OH) 2 == Mg (OH) 2 + CaCl2
(3) The precipitate is reacted with hydrochloric acid, and the solution after the reaction is crystallized, filtered, and dried to obtain the MgCl2 · 6H2O product; (4) The MgCl2 · 6H2O product is heated under certain conditions to lose the crystal water Mg (OH) 2 + 2HCl = = MgCl2 + 2H2O
(5) Molten MgCl2 for electrolysis of magnesium and chlorine MgCl2 (melt) = energization = Mg + Cl2

Magnesium Oxidation-Reduction Method

It is difficult to obtain electrons from +2 valence magnesium ions and reduce them to elemental magnesium. If magnesium is to be smelted in a manner similar to ironmaking, very high temperatures are required. For example, magnesium oxide can be reduced with coke at 2000 ° C to obtain elemental magnesium. The magnesium thus obtained often contains more impurities.

MgO + C = Mg + CO

Electrolytic method for extracting magnesium from seawater

Therefore, the electrolytic method is commonly used in the industry to make the magnesium ions get electrons at the cathode and reduce them to elemental magnesium. To smelt magnesium by electrolysis, a molten liquid containing magnesium ions must first be obtained. Among the magnesium compounds, typical ionic compounds are magnesium oxide and magnesium chloride. The melting point of magnesium oxide is too high (2800 ° C), while the melting point of magnesium chloride is much lower (714 ° C). Therefore, people choose magnesium chloride as the raw material for electrolytic production of magnesium.

Magnesium hydroxide (or magnesium oxide) is dissolved in hydrochloric acid, and the solution is concentrated to obtain magnesium chloride.

Mg (OH) 2 + 2HCl = MgCl2 + 2H2O

There are magnesium ions that can move freely in the molten magnesium chloride. After the direct current is applied, the chloride ions move to the anode, lose electrons on the anode, oxidize to form chlorine atoms, and two chlorine atoms combine to form one chlorine molecule; the magnesium ions move to the cathode, get electrons on the cathode, and reduce to elemental magnesium. The reaction that occurs on both electrodes is

Anode: 2Cl-2e = Cl2

Cathode: Mg2 + 2e = Mg

The overall electrolytic reaction equation can be expressed as follows:

MgCl2 (melting) --- energization --- Mg + Cl2

In order to prevent the generated magnesium from being oxidized by oxygen in the air at high temperature, the electrolysis needs to be performed under a special vacuum environment.