Industrial metallothermic processes for magnesium production are based on the reduction of magnesium oxide with ferrosilicon (FeSi). Magnesium oxide is provided in the form of calcined dolomite (MgO • CaO), sometimes enriched with calcined magnesite (MgO). The basic reaction is:
2CaO(s) + 2MgO(s) + Si(s) → 2Mg(g) + Ca2SiO4(s)
This reaction is highly endothermic (ΔG ≈ 210 kJ/mol). The vapor pressure at 1800 oC is 0.1 MPa. Industrial processes operate under vacuum at lower temperatures (1200-1500 oC) to limit deterioration of construction materials and to suppress undesirable side reactions in the gas systems. Reaction is carried out in batch mode. Various reactor designs have been suggested; determining factors are: transfer of heat to the charge and development of a continuous process. Active developmental work aims at increasing reactor capacity and improving metal yield.
Three principles are applied to the reaction chamber:
1) Externally heated retorts producing 70 kg/d (Pidgeon)
2) Internally heated reactors producing 2 t/d (Bolzano)
3) Internally heated reactors with molten slag producing ca. 12 t/d (Magnetherm)
The process developed by Pidgeon in the early 1940s is presently used by Timminco in Canada and Ube Industries in Japan (Fig. 1). Briquettes of crushed calcined dolomite and ferrosilicon fines in a stoichiometric ratio of 2:1 are loaded into tubular refractory steel retorts, heated externally to a reaction temperature of 1200 oC, and evacuated to 13.3 Pa. Magnesium vapor condenses at the cooled ends of the retorts.
Reaction kinetics depends on the thermal conductivity of the briquettes, the shrinking core pattern of the reaction zone, and the diffusion rate of magnesium vapor. Heat transfer to the charge limits the diameter of the retorts. Timminco operates retorts, which are 3 m long and have a bore diameter of 275 mm: each retort yields 70 kg of magnesium per day. Ferrosilicon grades containing 65-90%, but preferably 75-90% silicon, have been used. Metal recoveries up to 90% are attainable if a slight excess of silicon, above stoichiometric requirements, is used. Recovery increases with increasing ferrosilicon concentrations.
Handling of the charge and removal of slag and condensate have been highly automated reducing down time to 1 h per day. The productivity of the Pidgeon process is now 25 t/a per employee with a total energy requirement of 30kW • h/kg. The high-purity dolomite from Haley (99.5% CaCO3 • MgCO3) enables Timminco to produce 99.95%, and even 99.98%, magnesium in commercial quantities. The company has recently introduced to the market ultra pure AZ91 alloys with improved corrosion resistance (see Magnesium Alloys Encyclopedia).
The Bolzano process is operated in Italy by Societa Italiana per il Leghe di Magnesio (SAIM) and in Brazil by Brasmag. It employs an internally heated, brick-lined cylindrical reactor (Fig. 2). The briquetted charge of homogeneously mixed calcined dolomite and ferrosilicon fines (d) is loaded on a charge support system (e). Internal electrical heating (f) is conducted to the charge through the charge support system. The process operates at 1200 oC and < 400 Pa. Magnesium vapor condenses inside the condensers that are water-cooled to 400-500 oC. Each reactor has a production capacity of ca. 2 t of magnesium per 20-24 h reaction cycle, with a shutdown time of 30-45 min per cycle. Production of 1 t of magnesium consumes 7-7.3 MW • h, 5-5.2 t calcined dolomite, and 0.7 t of silicon contained in 78% ferrosilicon, with 81% silicon recovery; 5-5.2 t of slag is produced per ton of magnesium and sold for use in plaster and building bricks. Mechanization of charge handling and slag removal is a challenge in this process. Magnesium with a purity of 99.98-99.99% is obtained.
The magnetherm process was developed by Pechiney Electrometallurgie in 1963 and is still in use at the company's magnesium extraction plant in Marignac, France. The process is now also operated by North West Alloys (United States), Japan Metals and Chemicals (Japan), and Magnohrom (former Yugoslavia). The total installed production capacity worldwide is 65,000 t/a. The overall equation for the process is:
2CaO • MgO(s) + Al203(s) + (Fe)Si(s) → Ca2SiO4 • Al203(l) + Mg(g)
The dicalcium silicate slag formed in the reaction of calcined dolomite with ferrosilicon (Pidgeon and Bolzano processes) has a melting point of 2000 oC and is solid at the reaction temperature (1200 oC). The Magnetherm process, however, operates with a partly molten slag of the general composition 2CaO • SiO2 • nAl2O3 at 1550-1600 oC. The slag contains 50% Ca2SiO4, 18% Al2O3, 14% MgO, and 18% CaO and is kept in an electrically heated, cylindrical, brick- and carbon-lined steel vessel (Fig. 3).
The power input (4500 kW) is conveyed through a water-cooled copper electrode (a) from the top through the slag (g), to the bottom graphite lining (b) of the reactor, which acts as a power outlet. Coarse calcined dolomite and alumina screened to 3-30 mm, as well as ferrosilicon containing a minimum amount of fines, are added continuously to the slag, under a vacuum of 0.40-0.67 kPa, through charge holes in the water-cooled furnace roof. Magnesium vapor condenses in a separate condenser system.
The main impurities of the metal produced at Marignac (France) are manganese 0.04%, silicon 0.03%, iron 0.01%, zinc 0.007%, and copper 0.005%. The slag is not homogeneous and contains about 40 wt% solid Ca2SiO4. The main endothermic reaction takes place close to the surface of the slag. Deeper in the molten slag, the higher pressure slows down and eventually stops the reaction at a ferrosilicon content in the bottom layer of ca. 20%. Slag and 20% ferrosilicon are tapped twice during a cycle of ca. 18 h, having produced more than 10 t of magnesium. Adjustment of the slag composition to higher magnesium oxide content and heating the surface of the slag with arc or plasma burners to 1900-1950 oC have been suggested to allow reduction to occur at atmospheric pressure.
Production of 1 t of magnesium requires 5.7 t calcined dolomite, 0.75 t bauxite, 0.65 t of silicon contained in 77% ferrosilicon (88% Si recovery), and < 9 MW • h of power. About 6 t of slag is produced per ton of magnesium and is used in the cement industry for road building and as a source of magnesium and silicon in agricultural soil. The 20% ferrosilicon byproduct (yield 0.15 t per ton of magnesium) is used in the metallurgical industry, in the production of heavy liquid media for the separation of metals, or in drilling mud. The productivity of the process is estimated to be ca. 50 t per man-year.