Pidgeon process

The general reaction that occurs in the Pidgeon process is:

2 MgO·CaO + Si → 2 Mg + Ca₂SiO₄

For industrial use, ferrosilicon is used in place of pure silicon because its cheaper and more readily available. The iron from the alloy is a spectator in the reaction. CaC₂ may also be used as an even cheaper alternative for silicon and ferrosilicon, but is disadvantageous because it decreases the magnesium yield slightly.[5]

The magnesium raw material of this type of reaction is magnesium oxide, which is obtained in many ways. In all cases, the raw materials must be calcined to remove both water and carbon dioxide. Magnesium oxide can also be obtained from sea or lake water magnesium chloride hydrolyzed to hydroxide. The Mg(OH)₂ is thermally dehydrated. Another option is to use mined magnesite (MgCO₃) calcined to magnesium oxide.

The most used raw material is mined dolomite, a mixed (Ca,Mg)CO₃, where the calcium oxide present in the reaction zone scavenges the silica formed, releasing heat and consuming one of the products, ultimately helping push the equilibrium to the right. c(1) Dolomite calcination

CaCO₃·MgCO₃ → MgO·CaO + 2 CO₂

(2) Reduction

MgO·CaO +Si → 2 Mg + Ca₂SiO₄

The Pidgeon process is an endothermic reaction (${\displaystyle \bigtriangleup }$H° ~183.0kJ/mol Si). Thermodynamically speaking, the temperatures decrease when the vacuum is used for both MgO and calcined dolomite.[5]

Many technologies have been developed for producing magnesium metal. These approaches can be broadly classified as electrolytic and thermic.[10] The main manifestation of the electrolytic is the Dow Process. The main application of thermic routes is the Pidgeon process. The Bolzano process merits mention because it is very similar to the Pidgeon process except that the heating is achieved through electric heating conductors and retorts are placed vertically into large blocks in the Bolzano process.[5][11] The Pidgeon method is less technologically complex and because of distillation/vapour deposition conditions, a high purity product is easily achievable.[5]

Although the Pidgeon process has many perks, there are some environmental disadvantages of the process as well. Since increased demand for magnesium has risen in recent years, production through ore reduction has been emitting large amounts of carbon dioxide and particulate matter.[12] Due to the lightweight nature of magnesium as well as its high energy density, suggestions have been made about the global consumption of this versatile metal drastically increasing even more than it already has. There are environmental impacts because to create light weight materials in the first place, more energy is needed compared to the material being replaced, typically iron or steel. As an approximate, around 10.4 kg of coal is burned and 37 kg of carbon dioxide is released, per 1 kg of magnesium obtained.[13][14][15] In China, production of magnesium using the Pidgeon process has a 60% higher global warming impact than aluminum, a competing metal mass-produced in the country as well.[15]

The silicothermic reduction of dolomite was first developed by Amati in 1938 at the University of Padua. Immediately afterward, an industrial production was established in Bolzano (Italy), using what is now better known as the Bolzano process.[16]

A few years later in 1939, when Canada and its allies entered WW2, they were short on supplies that required magnesium such as bombs, other military devices and aluminum alloys needed for aircraft. Dr. Lloyd Montgomery Pidgeon at the National Research Council was able to create a method for extracting magnesium from dolomite in a vacuum at high temperature with ferrosilicon as the reducing agent. At this time, the ferrosilicon method was known, however it had yet to be commercialized. By early 1942, a successful pilot test took place.[17]

Since then, the Pidgeon process has continually been widely used, especially in China, the world's largest magnesium producer.