Magnesium, the metallic element, is too reactive to be found in its unbound, elemental state. Not only does it react with air, but in the presence of acid, its loose electrons are snatched up by acidic ions and liberate hydrogen gas, leaving behind invisible ions of magnesium in solution. A similar but much slower reaction occurs in the presence of water. Although the ions generated in such reactions are far from being useless, the metal is also highly practical because it has a higher strength to weight ratio than aluminum(158 kNm/kg versus 130 kNm/kg). A high-strength aluminum alloy that’s used to make aircraft contains 2.5% magnesium. Magnesium also comes in handy when making other alloys for lightweight luggage, cases for electronics, textile machinery and that beautiful invention with a low-carbon footprint known as the bicycle. Cars have been made lighter, which improves mileage, thanks to the use of magnesium alloys in engines and other parts. When including magnesium in cars, however, protective coatings are needed if the component will be in a wet, corrosive environment. In such conditions, when magnesium is in direct contact with other metals, it will act as a sacrificial anode, meaning that neighboring metals will be protected from oxidation but only at the expense of the magnesium part.
How do we convert the natural ionic state of magnesium to the metal? There are two general approaches, each of which has a few different methods. Overall the methods are energy-intensive and leave a large carbon footprint. One approach is electrolytic and relies mostly on sea water and electricity; the second one called thermochemical uses rocks, heat and other compounds. The first approach which dominated magnesium production from the 1970s to the 1990s is based on Michael Faraday’s 1833 technique. His mentor, Sir Humphry Davy had announced that magnesium oxide was a source of a new element but without actually isolating it. Twenty years later, Antoine Bussy fused magnesium chloride with potassium metal to free magnesium for the first time. But later, Faraday isolated the metal by passing electricity through magnesium chloride, forcing the magnesium ions to be neutralized.
In the Dow Method, one of the ways of making magnesium electrolytically, shells, ocean water and lime are used to precipitate magnesium hydroxide, Mg(OH)2. Hydrochloric acid is then added to neutralize the solid and create magnesium chloride (MgCl2), which goes into solution. A lot of the water is evaporated off, and a concentrated solution of MgCl2 (35%) is purified and passed into electrolytic cells operating at 700 oC. There, at every electrode receiving electricity, magnesium metal is deposited. At every electrode returning electrons to the circuit, chloride ions are converted to chlorine gas. But the poisonous gas passes through a furnace where in the presence of steam, it reacts to form hydrochloric acid, which could be recycled back into the initial neutralization reaction.
Almost all the methods using the thermochemical approach begin with dolomite. In the Pidgeon process, silica and dolomite are first treated separately. The latter is heated and converted into a mixture of calcium oxide and magnesium oxide, while the silica is turned into a ferrosilicate using scrap iron and coke. After a few steps and at about 1200 oC, the ferrosilicate reduces only the oxide of magnesium, creating magnesium vapor, which can be cooled to the desired metal.
The Pidgeon process became the dominant method (80%) after the 1990s as the technology became deployed on a massive scale in China. But the electrolytic process has a lower global warming potential (GWP) compared to the Pidgeon process (47.3 kg of CO2/kg of magnesium to 62.7 kg CO2/kg of Mg). In Canada, one company has managed to make the magnesium oxide-reduction part of the Pidgeon process twice as energy-efficient.
In another modification of the Pidgeon, known as the Bolzano Process, several modifications are made to the feeding apparatus, furnace shape and recovering operation of products. Briquettes of mixed dolomite and ferro-silicon are introduced into an electrically heated furnace under vacuum with only 400 Pa of pressure. The GWP of the Bolzano process is only 33.2 kg CO2/kg of Mg about half of the traditional Pidgeon method’s footprint. Other sources suggest that the footprint might be as low as a 1/3, but that’s because hydroelectricity supplies 80% of energy for the process.
Unfortunately there is nothing in free trade agreements to ensure that the Bolzano process should be used over the less ecological methods.
Cherubini, F. and al. LCA of Magnesium Production. Technological Overview and Worldwide Estimation of Environmental Burdens. Resources Conservation & Recycling, 2008, vol. 52(8-9), pp. 1093-1100.