Pidgeon process

Vapor-deposited magnesium crystals from the Pidgeon process

The Pidgeon process is one of the methods of magnesium 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 methods of production, though above the 7 MWh/ton theoretical minimum.

Chemistry

The basic chemical equation of this process is:

Si(s) + 2 MgO(s) ↔ SiO₂(s) + 2 Mg(g) (high temperature, distillation boiling zone)

Silicon and magnesia react to produce silica and magnesium.

Though, according to Ellingham diagrams, this reaction is thermodynamically unfavorable, in accordance with the Le Chatelier's principle of equilibria, 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 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 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 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 hydrolyzed to hydroxide, which is then calcined to magnesium oxide by removal of water. Another way is using mined magnesite (MgCO₃) that has been calcined to magnesium oxide by carbon dioxide removal. By far 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, 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 and ferrosilicon are mixed, briquetted, and charged in retorts 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 ingots.

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, the carbothermic reaction would produce 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 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 processes, with the United States as the dominant supplier. For over 80 years Dow Chemical operated a 65 kton/y capacity plant near Freeport, TX, based on seawater 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.^[1]^[2] 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 of Norway/Canada, and to a lesser extent, the former Soviet Union countries, Brazil and France, all possessing cheap and abundant hydroelectric or nuclear electric power. A player recently stepping on the world market is Israel, while Australian company Magnesium International is planning a 100 kton/yr smelter at Sokhna in Egypt, using the Dow electrolytic process.

References

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