Magnesium Extraction

Magnesium is a silvery-white metal, looking like aluminium, but it is the “lightest structural metal” with only 1.738 grams per cubic centimetre density. Above 645oC temperature, Magnesium is strongly reactive with oxygen and burns with a bright white flame. The magnesium extraction processes include electrolysis, which consists of 2 steps: preparation of feedstock and dissociation of the metal and thermal reaction from dolomite. Magnesium is the second most abundant component in nature among all the eight elements. This metal is highly reactive and is present in a wide variety of brines, rocks, and seawater. Numbers of metals can be formed from Magnesium using the “thermal reduction” of the salt of Magnesium and its oxidised form. The most common ore mineral of Magnesium is dolomite. Dolomite is made with Magnesium and calcium carbonate. Other ore mineral magnesite is made up of magnesium carbonate and MgCO3. The largest source of Magnesium is seawater.

Uses of the Mg:

• Magnesium is used to make medicines in pharmaceutical sectors.
• Flashbulbs are produced by using Magnesium. When the magnesium burns, it produces a bright white light that helps in the dark to take a photograph.
• Magnesium is widely used to produce several ointments or tablets. These medicines are used to treat different skin diseases.

History of magnesium extraction: 

English scientist Michael Faraday 1833, first extracted the Magnesium through electrolysis in the laboratory. After that, in 1886, in Germany, Magnesium was first produced industrially by “Aluminium and Magnesium Fabrik Hemelingen”. China is the leading country in the world that produces a large amount of Magnesium, and in the year 2019, they produced approximately 85% of the world’s magnesium.

Horizontal Pidgeon Process – Electrolysis

Magnesium chloride electrolysis is the most widely used technique of producing Magnesium, accounting for around 75% of global output. Dr The “Dominion Magnesium” created the “Pigeon-method” in Canada in 1941. It comprises the breakdown of dolomite and the reduction of magnesium oxide.

Dolomite is decomposed in higher temperatures from 780 to 1450oC using CaCO3 using a chemical reaction.

MgCO3.CaCO3 → MgO + CaO + 2CO2

The calcined dolomite is reduced with silicon or aluminium at approximately 1150 to 1400°C at low pressure of 3 to 10 Pascal.

2MgO + 2CaO + Si → 2Mg + CaO.SiO2

3MgO + 2CaO + 2 l →3Mg + 2CaO.l2O3

Advantages of the procedure

• The reactions of the procedures are relatively simple.
• The reaction cost is much less than other magnesium extraction processes.
• The source of required heat is multiple.
• The Magnesium produced is of good quality.
• It requires low power consumption.

Vertical Pidgeon Process

Another process besides the traditional horizontal Pidgeon Process is the Vertical process. It includes handling all the materials automatically by gravity. In the vertical process, a high tech machine monitors the materials continuously. This process is more environmentally friendly and ensures a high rate of productivity.

Advantages of the procedure:

• The production capacity increases into 65 to 70 kg from 30 kg per retort than the horizontal one
• The production time cycle is shorter than other magnesium extraction procedures.
• All the procedures of loading and unloading the material are carried out technically and automatically, reducing the time consumption.

Thermal reduction:

In the thermal reduction procedure, dolomite is being reacted with magnesium oxide (MgO), and lime (CaO) at a high temperature, almost about 1,800 °C and is reduced with silicon. As a result, it produces Magnesium in a gaseous state with slag of dicalcium silicate as a by-product. The process requires a high temperature and a vapour pressure of about 100 kilopascals.

2CaO + 2MgO + Si → 2Mg + Ca2SiO4

Advantages of the procedure:

• The process has a low reduction cost.
• Environmentally friendly.

N.B: The silicate slag produced as a by-product has a melting point of about 2000oC. It can be reduced to 1500-1600oC by adding alumina.

Refining of Magnesium

Produced Magnesium from the above procedures consists of impurities. Pure Magnesium cannot be used in that state. In order to remove the impurities and add the alloy elements, the metal is transported to the casting shop.

Corrosion resistance magnesium

There is no significant corrosion of pure Magnesium at room temperature. It can happen in a few rare cases where it comes into contact with water. It is commonly accepted that magnesium dissolution in water or aquatic settings occurs due to an electrochemical interaction with water. It results in the formation of magnesium hydroxide and hydrogen gas. The presence of oxygen is a significant factor in atmospheric corrosion. The sensitivity of such a process to oxygen concentration is relatively low compared to other mechanisms. 

To prevent corrosion in Magnesium, a natural surface coating protects it. Passive coats are those that limit cation flow outward. Also, it resists negative anions or oxidants inward and quickly heals localised breakdown. Its protective capacity is determined by its structure and composition, influenced by environmental and metallurgical conditions such as electrolyte species and metal impurities.

In order to improve the magnesium quality, it is essential to make it corrosion resistant. A sufficient amount of manganese can be used to make the magnesium corrosion resistant. It decreases the production of hydrogen gas within the metal. Hydrogen gas is the essential factor for corrosion reaction. Additionally, a small amount of arsenic reduces the chance of corrosion of the Magnesium.

Conclusion

Magnesium is a potent chemical agent that can form a stable compound. It can react with oxygen in high temperatures and dry air. The extraction procedure of this intensely active metal requires a well-formed technology and different electrochemical reactions. The critical processes used to extract Magnesium include the pigeon process and thermal reduction. The detailed process and reactions of magnesium extraction are discussed here. All the advantages and drawbacks are also highlighted throughout the topic.