How Is Magnesium Metal Produced?

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Retorts used for magnesium reduction during the Pidgeon Process. Image: Terence Bell

Depending upon the location and type of resource being used, a wide variety of production methods can be used to refine magnesium metal.

This is due to both the fact that magnesium is so abundant, making production in many locations possible, and that the minor metal's end-use applications are so price sensitive, so as to encourage buyers to constantly be seeking the lowest possible cost source.

Traditionally magnesium is produced from dolomite and magnesite ore, as well as magnesium chloride containing salt brines (naturally occurring salt deposits).

Electrochemical processes are used to extract the metal from dolomite and magnesite ore. When dolomite is crushed, roasted and mixed with seawater in large tanks, magnesium hydroxide settles to the bottom. Heating, mixing in coke, and reacting with chlorine, then produces molten magnesium chloride. This can be electrolyzed, releasing magnesium, which floats to the surface.

Magnesium is also extracted from salt brines, which contain about 10 percent magnesium chloride. The magnesium chloride at these sources still contains significant amounts of water and must be dried in order to make the magnesium chloride anhydrous, before it can be electrolyzed to produce metal.

Salt water can also have a high magnesium content. The first magnesium metal extracted from sea-water was produced by Dow Chemicals at their Freeport, Texas plant in 1948. The Freeport facility operated until 1998, but, presently, the only remaining salt water magnesium producer is the Dead Sea Magnesium Ltd.

(Israel); A joint venture between Israel Chemicals Ltd. and Volkswagen AG.

Over the past 20 years, one of the least efficient methods of magnesium production has, oddly, become the most prevalent. The Pidgeon Process, developed by Dr. Lloyd Pidgeon, is both an energy and labor intensive form of thermal reduction.

In this process, closed-end, nickel-chromium-steel alloy retorts are filled with a mixture of calcined dolomite ore and ferrosilicon, which are heated until magnesium crowns form. Each cycle takes about 11 hours, requires manually filling and emptying of the vacuum tubes and uses about 11 tons of raw materials for every one ton of magnesium produced.

The reason for the extensive use of the Pidgeon Process is due to the shift in production to the coal-rich provinces in north-central China where labor and energy costs are significantly lower than in other magnesium producing regions. According to the Magnesium.com, in 1992, China produced only 7,388 tons of magnesium. By 2010, this number was estimated by the United States Geological Survey (USGS) to be 654,000 tons or more than 85% of global production.

Many countries besides China do still produce magnesium, including Russia, Israel, Kazakhstan, and Canada. However, according to USGS figures, annual production in each of these countries is less than 40,000 tons.

Sources:
USGS. Mineral Commodity Summaries: Magnesium (2011).
http://minerals.usgs.gov/minerals/pubs/commodity/magnesium/
The International Magnesium Association. 
www.intlmag.org