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Pidgeon process

    == Overview ==
    The Pidgeon process is one of the methods of magnesium metal production via a silicothermic reduction practical production requiring 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
    The basic chemical reaction of this process is:
    Si(s) + MgO(s) ↔ SiO2(s) + Mg(g)
    Silicon and magnesia react to produce silica and magnesiumThough according to Ellingham diagrams this reaction is thermodynamically unfavorable in accordance with the Le Chatelier's principle 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 metal is very low only 1090 °C and even lower in vacuum
    The most commonly used and cheapest form of silicon is as a ferrosilicon alloy
    The magnesium raw material of this reaction is magnesium oxide which can be obtained by several waysOne way is by sea or lakewater magnesium chloride hydrolyzed to hydroxide which is then calcined to oxideAnother way is using mined magnesite (MgCO3) that has been calcined to magnesium oxideBy far the most used raw material is mined dolomite a mixed (CaMg)CO3 where the calcium oxide present in the reaction zone scavanges the silica formed releasing heat and consuming one of the products thus helping push the equilibrium to the right
    (CaMg)CO3 (s) → CaOMgO(s)+ CO2(g) Si(s) + MgO(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 retorts of made of nickel-chrome-steel alloy The hot reaction zone portion of the retort is either gasfired coalfired 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

    Historical Background

    Though this process was invented in early 1940's by Dr. Lloyd Montgomery Pidgeon of the Canadian National Research Council (NRC) its use has only recently come to dominate the world magnesium production by China becoming the dominant magnesium metal supplier relying almost exclusively on this method

    World Market Issues

    Prior to mid 1990's the world market or magnesium metal production has been dominated by electrolytic processes the US being the single dominating supplier For example for over 80 years Dow Chemical operated a 65 kton/y capacity plant near Freeport TX, based on seawater extracted magnesium chloride electrolysis which has been the prime magnesium metal supplier over the decades until its closure in 1998 As of 2005 there is a single US producer in Utah whose data and even name are being kept confidential and very severe antidumping tariffs are in the process of being imposed on Chinese imports As of 2005 the US produces about 45 out of a 615 kton/yr or 7%, compared to 140 out of 311 , or 45% in 1995 In contrast today China produces 400 out of the 615 kton/yr compared to 12 out of 311 or 4%, in 1995The 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 transportation costs and in anticipation of severe antidumping duties throughout the world
    As stated above the energy efficiency of thermal processes is comparable to the electrolytic routes: they both requiring roughly 35-40 MWh/ton however the Pidgeon method is less complex technologically and it's better suited for the ample low cost but low skill labor market available in China 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 Australia though not actively involved yet shows potential in this area either via in-home sites or investments in sites based in Congo or Egypt Qatar or United Arab Emirates

    References