Pidgeon process
Overview
 
The Pidgeon process is one of the methods of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 metal production, via a silicothermic reduction. Practical production requires roughly 35-40 MWh/ton of metal produced, which is on par with the molten salt electrolytic
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 methods of production, though above the 7 MWh/ton theoretical minimum.
The basic chemical equations of this process are:
Si(s) + 2 MgO(s) ↔ SiO2(s) + 2 Mg(g) (high temperature, distillation boiling zone)
Mg(g) ↔ Mg(liq, s) (low temperature, distillation condensing zone)


Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 and magnesia react to produce silica and magnesium.

Though, according to Ellingham diagram
Ellingham diagram
An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulphides. These diagrams were first constructed by Harold Ellingham in 1944...

s, this reaction is thermodynamically
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 unfavorable, in accordance with the Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...

 of equilibriums, it can still be driven to the right by continuous supply of heat, and by removing one of the products, namely distilling out the magnesium vapor.
Encyclopedia
The Pidgeon process is one of the methods of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 metal production, via a silicothermic reduction. Practical production requires roughly 35-40 MWh/ton of metal produced, which is on par with the molten salt electrolytic
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 methods of production, though above the 7 MWh/ton theoretical minimum.

Chemistry

The basic chemical equations of this process are:
Si(s) + 2 MgO(s) ↔ SiO2(s) + 2 Mg(g) (high temperature, distillation boiling zone)
Mg(g) ↔ Mg(liq, s) (low temperature, distillation condensing zone)


Silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 and magnesia react to produce silica and magnesium.

Though, according to Ellingham diagram
Ellingham diagram
An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulphides. These diagrams were first constructed by Harold Ellingham in 1944...

s, this reaction is thermodynamically
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 unfavorable, in accordance with the Le Chatelier's principle
Le Châtelier's principle
In chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently...

 of equilibriums, it can still be driven to the right by continuous supply of heat, and by removing one of the products, namely distilling out the magnesium vapor. The atmospheric pressure boiling point of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 metal is very low, only 1090 °C, and even lower in vacuum. Vacuum is preferred, because it allows lower temperatures.

The most commonly used and cheapest form of silicon is as a ferrosilicon
Ferrosilicon
Ferrosilicon, or ferrosilicium, is a ferroalloy, an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C...

 alloy. The iron from the alloy is but a spectator in the reactions.

The magnesium raw material of this reaction is magnesium oxide, which can be obtained by several ways. In all cases the raw materials have to be calcined
Calcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...

 to remove both water and carbon dioxide, which would be gaseous at reaction temperatures, and follow the magnesium vapor around, and revert the reaction.

One way is by sea or lakewater magnesium chloride
Magnesium chloride
Magnesium chloride is the name for the chemical compounds with the formulas MgCl2 and its various hydrates MgCl2x. These salts are typical ionic halides, being highly soluble in water. The hydrated magnesium chloride can be extracted from brine or sea water...

 hydrolyzed to hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

, which is then calcined to magnesium oxide
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

 by removal of water. Another way is using mined magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...

 (MgCO3) that has been calcined to magnesium oxide by carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 removal.
By far the most used raw material is mined dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

, 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, thus helping push the equilibrium to the right.
(Ca,Mg)CO3 (s) → CaO.MgO(s)+ CO2(g) (dolomite calcining)
(Fe,Si)(s) + MgO(s) ↔ Fe(s) + SiO2(s) + Mg(g)
CaO + SiO2 → CaSiO3


The Pidgeon process is a batch process in which finely powdered calcined dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

 and ferrosilicon
Ferrosilicon
Ferrosilicon, or ferrosilicium, is a ferroalloy, an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C...

 are mixed, briquetted, and charged in retort
Retort
In a chemistry laboratory, a retort is a glassware device used for distillation or dry distillation of substances. It consists of a spherical vessel with a long downward-pointing neck. The liquid to be distilled is placed in the vessel and heated...

s made of nickel-chrome-steel alloy. The hot reaction zone portion of the retort is either gas fired, coal fired, or electrically heated in a furnace, while the condensing section equipped with removable baffles extends from the furnace and is water-cooled. Due to distillation, very high purity magnesium crowns are produced, which are then remelted and cast into ingot
Ingot
An ingot is a material, usually metal, that is cast into a shape suitable for further processing. Non-metallic and semiconductor materials prepared in bulk form may also be referred to as ingots, particularly when cast by mold based methods.-Uses:...

s.

Carbothermic process unfeasible

The usual metallurgic use of carbon as the reducing agent instead of silicon cannot be used, because the silicon dioxide is a solid, while carbon dioxide and monoxide are both gaseous, and would follow the magnesium into the condensing zone, reverting the reaction as follows.
C(s) + MgO(s) → CO(g) + Mg(g) (high temperature, distillation boiling zone)
CO(g) + Mg(g) → C(s) + MgO(s) (low temperature, distillation condensing zone)

In this case there would be no yield when the vapors moved into the cooler condensing zones inside the reactor, even though temporarily there would be intermediate carbon monoxide and actual magnesium vapors. There is still a feasible process with carbon, that uses shock-freezing of the vapors, to disallow any time for the reverse reaction - though such shock cooling is a far stretch from being an economical industrial process.

At temperatures where the magnesium is still liquid or solid (say 600-700°C), but carbon oxides are gaseous, the immense thermodynamic counter drive makes the reactions impractical, even if the carbon monoxide was purged away by argon, for example. The equilibrium can theoretically be driven either way, but it's impractically slow if the forward and reverse reaction rates are minuscule. Basically, nothing detectable happens.

This reaction would be impractically slow at low temperatures, even though it would stop the CO and the Mg from reacting:
C(s) + MgO(s) ↔ CO(g) + Mg(l)


At low temperatures the reaction energetics dominate everything else, and in this sense too silicon outperforms carbon, because silicon dioxide has a much larger heat of formation than the carbon oxides, as best seen in Ellingham diagrams.

Historical background

This process was invented in the early 1940s by Dr. Lloyd Montgomery Pidgeon
Lloyd Montgomery Pidgeon
Lloyd Montgomery Pidgeon, OC, MBE was a Canadian chemist who developed the Pidgeon process, one of the methods of magnesium metal production, via a silicothermic reduction. He is considered the "father" of academic metallurgical research in Canada.Born in Markham, Ontario, the son of E...

 of the Canadian National Research Council (NRC). The first plant was built in 1941 and operated by Dominion Magnesium in Haley, Ontario, Canada. This plant operated for 63 years, most recently by Timminco Metals. In the last 10 years, the Pidgeon process has come to dominate world magnesium production. China is the dominant magnesium metal supplier, relying almost exclusively on this method.

The silicothermic reduction of dolomite was first developed by Dr. Amati in 1938 at the University of Padua, where his thesis is archived. Immediately afterward, an industrial production was established in Bolzano. The process used externally heated retorts identical in concept to those used by Dr. Pidgeon two years later.

World market issues

Prior to the mid 1990s the world market for magnesium metal production was dominated by electrolytic process
Electrolytic process
An electrolytic process is the use of electrolysis industrially to refine metals or compounds at a high purity and low cost. Some examples are the Hall-Héroult process used for aluminium, or the production of hydrogen from water. Electrolysis is usually done in bulk using hundreds of sheets of...

es, with the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 as the dominant supplier. For over 80 years Dow Chemical operated a 65 kton/y capacity plant near Freeport, TX, based on seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 extracted magnesium chloride electrolysis, which was the prime magnesium metal supplier until its closure in 1998. As of 2005, there is a single US producer, in Utah, US Magnesium, a company borne from now-defunct Magcorp. Very severe antidumping tariffs are in the process of being imposed on Chinese imports . As of 2005, the US produced about 45 out of a 615 kton/yr (7%), compared to 140 out of 311 kton/yr (45%) in 1995. In contrast, in 2005 China produced 400 out of the 615 kton/yr (65%), compared to 12 out of 311 kton/yr (4%) in 1995.

The price of magnesium metal plummeted from $2300/t in 1995 to $1300/t by 2001, but recently (2004) climbed back over $2300/t, due to increased ferrosilicon, energy and transportation costs, and in anticipation of severe antidumping duties throughout the world.

As stated above, the energy efficiency of thermal processes is comparable to electrolytic ones, both requiring roughly 35-40 MWh/ton. The Pidgeon method is less complex technologically, and because of distillation/vapor deposition conditions, a high purity product is easily achievable. In the past, besides the US, the other major magnesium producers have traditionally included Norsk Hydro
Norsk Hydro
Norsk Hydro ASA is a Norwegian aluminium and renewable energy company, headquartered in Oslo. Hydro is the fourth largest integrated aluminium company worldwide. It has operations in some 40 countries around the world and is active on all continents. The Norwegian state holds a 43.8 percent...

 of Norway/Canada, and to a lesser extent, the former Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 countries, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

 and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, all possessing cheap and abundant hydroelectric or nuclear electric power. A player recently stepping on the world market is Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

, while Australian company Magnesium International is planning a 100 kton/yr smelter at Sokhna in Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

, using the Dow electrolytic process.
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