Two basic processes for magnesium production are currently employed: fused-salt electrolysis of magnesium chloride, and thermal reduction of magnesium oxide. Presently, the dominant technology is the 'Pidgeon' process, used by the Chinese magnesium producers. It is a thermal reduction method carried out under vacuum in coal-fired retorts, and is based on the reaction of calcined dolomite with ferrosilicon. The Pidgeon process was invented and developed in Canada in the 1940s. It is a labour-intensive batch process. The production capacity per retort is only 50 kg magnesium per day at a cycle time of roughly 10 hours. In the 1960s, a semi-continuous, scaled-up Pidgeon process was developed in France by Pechiney. The process is known as the Magnetherm process and is based on AC submerged-electrode smelting of calcined dolomite in the presence of ferrosilicon and aluminium. The process is carried out under a vacuum of 0.05 to 0.10 atm. The production capacity per reactor is 20 ton magnesium per day, the cycle time is about 12 hours, and the condenser efficiency is 85 to 90 per cent. Every 12 hours the vacuum has to be broken, in order to periodically remove the slag from the furnace, and to replace the full condenser with an empty one.
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