Magnesium Extraction By Thermal Reduction Methods

The only ores used in the production of magnesium with thermal reduction technology are dolomite and magnesite. These ores are extracted through customary mining methods, mainly through open mining.The ore extracted from the mine undergoes calcination at temperatures of 700–1,000°C. At this temperature the material releases CO2  gas, according to Eqs. (1) and (2), for magnesite and dolomite, respectively:

 

MgCO3(s)  = MgO(s) + CO2(g)           (1)

MgCO3*CaCO3(s)  = MgO*CaO(s) + 2CO2(g)             (2)
 

 Following calcination the material is ground into a fine powder.

Another main raw material that is used in thermal reduction is an alloy of silicon and iron called Ferrosilicon. The silicon content in this alloy is 65–85%, and at times the mix also contains small quantities of aluminum. The preparation of the ferrosilicon is carried out by reducing silica with coal, containing iron (scrap iron), at high temperatures:

SiO2(s) + 2C(s) + Fe = Si(Fe)(s) + 2CO(g)                      (3)

 

An additional material,which was used in the past as a thermal reduction agent, is Calcium carbide. The process of preparation of this material is relatively simple, but it requires high temperatures of about 1,800–2,000°C. Below is the reaction used in the production of this material:

 

CaO(s) + 3C(s)  = CaC2(s) + CO(g)                              (4)

 

Another raw material used in the magnetherm process is bauxite. This material undergoes calcination at temperatures of about 1,200°C before it is introduced into reaction, according to Eq. (5):

 

Al2O3*nH2O(s)  = Al2O3(s) + nH2O(g)                        (5)



 

Additional raw materials used as additives or reduction agents, such as aluminum, alumina and coal usually undergo grinding or chipping only.

 

The heating and reduction processes differ between thermal technologies,with thermodynamics eventually determining the reaction temperature. The thermodynamics of the reduction reaction depends on reactants, products and reaction conditions, such as pressure, temperature and the presence of other additives. The reactions described below are organized according to the reducing material used.

 

Silicothermic Processes

These processes are the main and almost the only thermal processes by which magnesium is produced commercially. In general, this category contains three main processes,which differ mainly in the manner of heating.Following are the main reactions of these processes at different temperatures and pressures:

 

2MgO(s) + Si(Fe)  = 2Mg(g) + SiO2(l) + Fe                                            (6)

P = 1 at’ ; T = 2,300°C or P = 1 mm Hg ; T = 1,500°C

4MgO(s) + Si(Fe)  = 2Mg(g) + Mg2SiO2(s) + Fe                                     (7)

P = 10 mm Hg ; T = 1,220°C

 

2MgO*CaO(s) + Si (Fe)  = 2Mg(g) + Ca2SiO4(l) + Fe                            (8)

P = 1 at’ ; T = 1,700°C or P = 1 mm Hg ; T = 1,150–,200°C

 

 

Aluminothermic Processes

In this process, aluminum serves as the reduction material for the production of magnesium. In the main reaction, the reduction of dolomite is carried out with aluminum (with the addition of some magnesite) that has undergone calcination as shown in Eq. (9):

3MgO(s) + 2CaO(s) + 2Al(l)  = 3Mg(g) + 2CaO*Al2O3(s)                                   (9)

P = 1 at’ ; T = 1,300°C or P = 10 mm Hg ; T = 900°C

The advantages of this process are many, and they stem mainly from the fact that the reaction is carried out at a relatively low temperature, which is, in fact, the lowest of all the thermal processes. The main disadvantage of the process is in the high cost of aluminum,and therefore, the only process in which aluminum serves as a partial reduction agent is the magnetherm process. Hence it is usually operated by companies that also own very large aluminum plants. These companies generally also manufacture (as a by-product) large quantities of aluminum scrap and aluminum rich waste which can be used for this process.

 

An innovative process which is based on this reaction, is the Heggie, which is described in Fig. 1. The process is based on the use of Dolomite and magnesite that have undergone calcination, and aluminum scrap as reduction material. The furnace used is the Heggie process and works according to the working principles of the DC transferred arc plasma furnace. This process is supposed to work at atmospheric pressure and under argon atmosphere at temperatures of about 1,500°C. The way the Heggie furnace works enables the relatively high work temperature to exist only in a restricted area of the electric arc. The developers of the process claim that the Heggie furnace consumes only 6 kWh in order to manufacture 1 kg of magnesium.
 

 

Fig. 1. Heggie Process