Magnesium Extraction By Magnetherm Process

This technology was developed in the 1950s and 1960s by the large aluminum production company Pechiney. Later, it was also used by Northwest Alloys in the United States (see Fig. 1). The main characteristic of this technology is the heating and production furnace in which the heating takes place with electrodes using alternating current. Such heating requires liquid slag, which will conduct electricity, and therefore, in addition to Dolomite, which has undergone calcination and in addition to the reduction silicon, alumina is also added. Figure 2 describes the furnace and its components, schematically.

 

The furnace in the Magnetherm process operates within a temperature range of 1,300–1,700°C. This high temperature range stems from a number of reasons and variations in the process. The first reason is the difficulty in maintaining low pressures in the furnace with a large volume, and, therefore, as can be seen from reaction (1) :

 

2MgO*CaO(s) + Si (Fe)  =  2Mg(g) + Ca2SiO4(l) + Fe                            (1)

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

 

it is necessary to work at higher temperatures. At times a certain quantity of magnesite,which has undergone calcination, is also added, to increase the reaction temperature.Another variation of the process that can also influence the temperature is the addition of metallic aluminum as an alternative to silicon. The use of aluminum in these processes will be detailed in the aluminothermic processes.

Production in a furnace with the Magnetherm process is in batch form and a complete cycle lasts normally 16–24 h. The magnesium fumes rise and accumulate in the cooled condenser in the liquid or solid state. The furnace usually produces between 3 and 8 t of magnesium a day, according to its size. As a rule, 7 t of raw material are required in order to produce 1 t of magnesium.

 

 

 

Fig. 1. Magnetherm Flow Chart Process

 

 

 

Fig. 2. The Magnetherm Production Facilities

 

 

 

 

 

 

Magnesium Extraction By Pidgeon Process

 

 

This process was developed in the 1940s in Ontario Canada,by Prof.Pidgeon and the Timminco company. Lately, this process has received new attention and constitutes a central process in the magnesium production at a large number of Chinese manufacturers (see Figs. 1 and 2).

The reaction that takes place is reaction below (Eqs. 1):

 

2MgO*CaO(s) + Si (Fe)  = 2Mg(g) + Ca2SiO4(l) + Fe                            (1)

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

 

A schematic illustration of the retort used in this process is shown in Fig. 2. The dimensions of the retort are 2.7–3.3 m with a diameter of 28–35 cm. The capacity of the retort is about 120 kg.

 

The source of energy in the process carried out in China is normally coal, while the calcination process and the heating furnaces require 14–20 t of coal for the production of one ton of magnesium.

 

On completion of the process a magnesium crown is obtained, weighing 12–20 kg,which is then extracted from the upper part of the retort. Due to the usually high temperature, the magnesium in this case will contain high concentrations of aluminum,manganese, iron and other impurities. The above process can also be carried out with magnesite as an alternative to dolomite; the working conditions are almost identical and the reaction in this case is Eq. (2).

 

 

4MgO(s) + Si(Fe)  = 2Mg(g) + Mg2SiO2(s) + Fe                            (2)
P = 10 mm Hg ; T = 1,220°C

2MgO(s) + Si(Fe)  = 2Mg(g) + SiO2(l) + Fe                                   (3)

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

Another version of the above process is carried out at a higher temperature, according to the reaction appearing in Eq. (3). The advantage of this reaction is the higher output from each retort (up to about 80%),while the disadvantages in this case are many. The main one being the higher temperature required for the process, 1,500°C, about 300°C more than with the regular process.Work at a higher temperature usually causes the evaporation of impurities and a lower quality of material. In addition, there is accelerated amortization of tools, and this in addition to the higher cost of energy and the need for accessibility to magnesite.
 

 

Fig. 1. Pidgeon Process

 

 

Fig. 2. A Retort Used In The Pigeon Process

 

 

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

Magnesium Extraction By Dow Process

This process use Dolomite (see fig. 1) as Raw Material:

 

Fig. 1. One of Dolomite Minerals

 

Dolomite is composed mainly of the double salts of magnesium and calcium carbonate, and contains low concentrations of iron and manganese as impurities. Dolomite is usually colorless and looks like small diamond-shaped crystals. Dolomite is formed as a result of calcite transformations in the presence of magnesium ions.

Dolomite rock is useful in the chemical industry for the preparation of magnesium and serves as construction and decoration stone.Common deposits can be found in England, Germany, Brazil,Norway and Mexico. The magnesium concentration in dolomite by weight is 28.8%.

The process developed at Dow (see Fig. 2) is unique, due to the fact that the material fed into the electrolysis cells still contains a significant amount of water – about 27%. A solution of magnesium chloride is introduced directly into the spray dryer and comes into direct contact with burning gases, predominantly natural gas. During this process only reactions (1) and (2) take place.

MgCl2*6H2O = MgCl2*4H2O + 2H2O(g)        T = 117oC          (1)
MgCl2*4H2O = MgCl2*2H2O + 2H2O(g)        T = 185oC          (2)
The products of this stage are granules of MgCl2*2H2O. The additional dehydratation is, in fact, carried out in the electrolysis cell, by the chlorine,which is created in the cell and the graphite anodes,which are used up at a fast rate.

 

Fig. 2. Dow Process