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 Magnola Process

Magnola Process use Serpentine as Raw Material as fig 1:

Fig. 1. Sepentine

 

A magnesium chloride-rich solution (27%) is poured into the spray dryer. In this installation,with the aid of hot gases, by burning of natural gas, the magnesium chloride is dried to MgCl2*2H2O, Eqs. (1) and (2), as in the hydro magnesium drying process.

 

MgCl2*6H2O = MgCl2*4H2O + 2H2O(g)             T = 117oC    (1)

MgCl2*4H2O = MgCl2*2H2O + 2H2O(g)             T = 185oC    (2)

 

The main difference lies in the fact that in the magnola process drying is carried out in one stage (see Fig. 2). The “price” is the relatively high percentage of hydrolysis,which causes a concentration of magnesium oxide of up to about 2%. During the second stage the material is transferred to a unique chlorinator called “super chlorinator”. The melting process is carried out as follows: an electrolyte low in magnesium chloride is pumped from the electrolysis cell to the chlorinator where it assists in the melting of  magnesium chloride hydrate; this electrolyte returns to the electrolysis cell richer in magnesium chloride in the overflow. The reason for the melting in the electrolyte is the relatively high melting temperature of pure magnesium chloride; melting in the electrolyte enables the melting temperature to be decreased 200–300°C. The electrolyte temperature is about 650°C, and this is also the temperature at which the material is returned to the electrolysis cell,with the assistance of heating by the graphite electrodes.

 

The chlorinator is fed with dry, gaseous HCl, which is produced from all the chlorine manufactured in the electrolysis cells.Feeding is carried out with the assistance of a number of graphite mixers, which are introduced from the chlorinator ceiling. These mixers assist in dispersing the HCl and improve the efficiency of the following reaction:

 

MgO + 2HCl(g)  = MgCl2(l) + H2O(g) (2.21)

The high concentration of HCl in the melt and above the melt prevents the back hydrolysis reaction between the water vapors to the magnesium chloride. The concentration of magnesium oxide in the electrolyte returning to the electrolysis cell is about 0.05%.

 

The HCl discharged from the two drying stages is received in watery solutions and transferred back to the extraction process, according to Fig. 2, thus closing the process circle for the production of magnesium from raw material that does not contain chlorine.

 

Fig. 2. Magnola 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

Magnesium Extraction By The AMC Process

This process also use Magnesite as raw material. This company developed an innovative unique process, which apparently bypasses the problem of the hydrolysis reaction. At the completion of the drying process, a powdery anhydrous magnesium chloride is obtained, with a concentration of magnesium oxide lower than 0.1%.

In the preliminary stage, an organic solvent ethylene glycol and/or methanol are added to the concentrated magnesium chloride solution.Gaseous ammonia is then bubbled into the solution and the settling reaction (1) takes place:

MgCl2(aq) + 6NH3(g) = MgCl2*6NH3(s)       (1)

This reaction creates MgCl2*6NH3(s), which is the ammonia based analog of bischofite, with water exchanged for ammonia molecules. The settling process will only take place in a solution that contains a major organic component. The MgCl2*6NH3(s) that settles is separated and filtered from the solution. The remaining solution undergoes refining in which the organic solvent is separated and sent for reuse in the process.

The MgCl2*6NH3(s) is transferred to a drying furnace, in which it is heated to a temperature of about 550°C. The reason for this high temperature is that the ammonia molecules are linked tighter to the magnesium chloride than water molecules.During the heating process the material decomposes to ammonia and magnesium chloride (see Eq. (2)). In this case, unlike the bischofite drying process, there is no hydrolysis problem at all since there are no water molecules.

MgCl2*6NH3 (s) = MgCl2(s) + 6NH3(g)      (2)

The ammonia is cooled, cleaned and returned the process. The anhydrous solid magnesium chloride is sent to the electrolysis cells.

It is possible to track the drying process in Fig. 1.

Fig. 1. AMC Process

Magnesium Extraction By Hydro Magnesium Processes (Canada)

Hydro Magnesium Processes use MAGNESITE (MgCO3) as Raw Material (see fig 1).

Fig. 1. One of Magnesite Minerals

In nature, this material is composed mainly from carbonated magnesium and contains low concentrations of calcium,iron and manganese as impurities. Magnesite belongs to the calcite group, which is a group of carbonates that are similar in their physical characteristics.Magnesite is usually produced when rocks rich in magnesium come into contact with carbonate-rich solutions thereby producing a first degree metamorphosis. The Magnesite does not normally produce crystals with a defined form. It has a crystalline structure similar to calcite and it is white.

Magnesite is common in Brazil, Austria, Korea, China and the West Coast of the United States of America, and it is extracted by mining.The magnesium concentration by weight is 28.8%.

In the hydro magnesium process used in Canada the raw material for the process is magnesite, which is mined in mines (mainly in China). The magnesite is dissolved in a hot HCl solution, thus arriving at a magnesium chloride-rich solution:

MgCO3(s) + 2HCl(aq) = MgCl2(aq) + CO2(g) 

The acid solution must be hot for an efficient melting process. The basic ore of the magnesite is rich in various metallic impurities, sulfates and boron,which can be a problem at the electrolysis stage. Therefore, already at this stage a number of processes are carried out,whose purpose is to separate the impurities from the
magnesium chloride.

During the preliminary drying process, the magnesium chloride solution is heated in the evaporator by the residual heat from the process until almost pure bischofite with a water content of 45–50% is obtained (MgCl2*6H2O). The bischofite, which is melted during this process is turned into prills in the prilling tower. One of the main reasons for the transfer to prills is the wish to reduce the quantities of dust during
the next drying stages.

The next drying stage is performed in a fluidized bed dryer. At this stage, the bischofite with six molecules of water is dried by hot air to MgCl2*2H2O, as described in Eqs. below:

MgCl2*6H2O = MgCl2*4H2O + 2H2O(g) T = 117°C 

MgCl2*4H2O = MgCl2*2H2O + 2H2O(g) T = 185°C

The last stage of drying, to extract anhydrous magnesium chloride is carried out by gaseous HCl drying at temperatures of about 330°C. The reason for performing this stage with heated gaseous HCl is the difficulty in preventing hydrolysis, and the wish to obtain solid and dry magnesium chloride with magnesium oxide qualities of about 0.1%. From Eqs. below:

MgO + HCl(g) = MgOHCl 

MgOHCl + HCl(g) = MgCl2(s) + H2O(g)

it appears that the use of gaseous HCl will fundamentally reduce the hydrolysis reactions, thus reducing the concentration of magnesium oxide in the product.A further point is that opposite reactions to hydrolysis take place with HCl,which also reduce the concentration of magnesium oxide.

The HCl from the drying process is transferred to the raw materials extraction and preparation process, according to Fig. 2.

Fig. 2. Hydro magnesium processes (Canada)

Magnesium Extraction From Sea Water

 

The history of elementary magnesium started in 1755,when Joseph Black, a Scottish chemist, discovered that magnesia contained a new element, magnesium. Black was unable to isolate this element.

Magnesia had previously been known as “white stone”or “white earth” (magnesia lithos or magnesia carneus). It is generally accepted that the name originates from an area in Thessaly,Northern Greece,where in ancient times the material had been excavated and exported to countries around the Mediterranean.

Actually, the British chemist and scientist Sir Humphrey Davy is honored as the discoverer, because it was he who isolated the metal in 1808, when he decomposed wet magnesium sulphate by electrolysis using a voltaic cell and a mercury cathode.

(A) Raw materials

There are six sources of raw materials for the production of magnesium: magnesite, dolomite, bischofite, carnallite, serpentine and sea water. These sources differ in the magnesium content, in production methods, and in their origin. Some are mined from mines, some in open mining, others originate in various processes carried out on sea water and salt lakes, and another material originates from the waste of the asbestos production process.

Table 1. Raw Material

(B) Sea Water

The magnesium ion is the third most common component in sea water. Its concentration

varies between different seas (see Table 2). Magnesium ions are the product of erosion. The magnesium hydroxides and carbonates that form have low solubility in seawater and therefore as a result of their settling to the bottom they become the building blocks for coral reefs. In addition, they have an important ecological function in that they accumulate high concentrations of CO2, keeping this gas out of the atmosphere. The low solubility of these salts is used to produce magnesium from seawater, and is achieved by adding a precipitation agent, such as Ca(OH)2. The relative ion concentrations appear in Table 2.

 

  Table 2. Composition of general sea water and Dead Sea water

 

 

(C) Preparation of Magnesium Salt

It is possible to divide the magnesium production technologies into two main types: electrochemical methods and thermal reduction methods. The difference between these methods stems from the reduction process of the magnesium ion to metallic magnesium. For the electrochemical methods the reduction is carried out with electric current fed into the electrolyte cells. In the case of thermal methods, the reduction is carried out by means of various reducing materials at high temperatures.

 

The basic raw materials for the production of magnesium with the electrochemical process are generally divided into two: salts containing chloride and raw materials that must be transformed into salts containing chloride. Eventually, all the materials will become either bischofite or carnallite prior to drying and feeding into the electrolysis cells.

 

 

Generally, the MgCl2 from sea water changed to Mg(OH)2 by adding the Ca(OH)2 or NaOH such as below:

Mg2+(aq) + 2OH–(aq)  = Mg(OH)2(s)

 

 

In the second stage the reaction is caused between Mg(OH)2(s) and HCl such ass below:

 

Mg(OH)2(s) + 2HCl(aq) = MgCl2(aq)

 

 

 

 

 (D) Drying Magnesium Chloride Salts

 

Carnallite, bischofite and their solutions contain large quantities of water. Carnallite and bischofite are hydrate salts containing six molecules of water per crystal and at times even additional water in attached form.The use of these materials in the electrolysis process requires the removal of all the water prior to the electrolysis. Carnallite and bischofite tend to decompose in the hydrolysis reaction to magnesium oxide and HCl, at relatively low temperatures, and, therefore, the drying process is actually the most complicated and hardest stage in the electrolysis methods for the production of magnesium. In this matter it should be stated that most magnesium production R&D in the last decades has focused on the drying process for extraction of anhydrous magnesium chloride with low content of magnesium oxide.

 

The three negative characteristics of a hydrolysis reaction are:

1. The creation of magnesium oxide, which will later be concentrated as sludge in the electrolysis cells, will react with the graphite anodes and will harm the energy efficiency of the process.

2. Losses of magnesium chloride during the process

3. The requirement to handle the acid gases produced during the reaction.

 

The advantage in using carnallite is the fact that the hydrolysis of this material is significantly lower than the hydrolysis of bischofite, and even the decomposition temperatures of its various hydrates are lower. The following are the decomposition and hydrolysis temperatures of carnallite and bischofite:

 

MgCl2*6H2O = MgCl2*4H2O + 2H2O(g) T = 117oC

MgCl2*4H2O = MgCl2*2H2O + 2H2O(g) T = 185oC

MgCl2*2H2O = MgCl2*H2O + H2O(g) T = 242oC

MgCl2*1H2O = MgCl2 + H2O(g)  T = 304°C

 

The products of the drying stage can appear in two forms. One form is that of a solid material, the other is molten salts. In addition to this difference there are "sensitive" electrolysis cells, which require material with a particularly low concentration of magnesium oxide (0.05-.1%) and more "resistant" electrolysis cells that can also work rith raw materials containing 0.6% magnesium oxide. These differences, of course, influence the character of the required drying process.

 

(E) Electrolysis of Magnesium Salt

Magnesium always appears in nature in ionic form with the following electron arrangement: 1S2 2S2 2P6 3S2.This arrangement is characterized by the low ionization energies relative to the two most external electrons,which are at the 3S level. This is the reason why univalent or trivalent magnesium is not found in nature, only bivalent. The low standard reduction potential of magnesium is the reason why no metallic magnesium is found in nature:

 

 Mg2+ + 2e– = Mg   E0 = –2.375 V

 

 All production technologies, therefore, require a reduction agent which can transfer two electrons to the magnesium. The reduction agents are: electric current operated at the appropriate potential, coal in various forms, silicone-based materials (FeSi),CaC2  and aluminum.The accepted division in literature, for thermal and electrochemical technologies stems in fact from this central feature. All the electrochemical technologies use direct current electricity form,which passes through the electrolysis cells and discharges chlorine and magnesium ions into gaseous chlorine and metallic magnesium. The thermal methods are based on heating of magnesia in the presence of various reduction materials, to a variety of temperatures.At a particular temperature the reduction reaction takes place and the magnesium becomes metal, usually in its gaseous form.

 

The electrolysis stage is performed in the electrolysis cells.At this stage there are a number of main points that characterize the process. We shall focus on these points while reviewing the various electrolysis cells and their features. We shall conccentrate on following points: the composition of the electrolyte, the thermal balance of the cell, the type of electrode, the structure of the cell, a comparison table that includes the main production parameters, and the cell operation.

 

The optimal electrolyte for the production of magnesium must have low resistance, high density, and a low price.Magnesium chloride does not have high conductivity and, therefore, the accepted values for its use are relatively low (8–25%), this despite the fact that it is in fact the material undergoing electrolysis. The other materials are determined according to other technological constraints

In general, there are two types of electrolytes: a substitute electrolyte and a constant electrolyte.

 Fig 1. Alcan cell, schematic vertical cross section

 

(F) Production Temperature

Maximum temperatures in the various production processes are in the range of 655–1,900°C,which is a very large temperature range for a production process of one particular material. In general, electrochemical production processes take place within the lower production temperature range, usually between 655–720°C,while the thermal reduction production processes take place within the higher temperature ranges, usually between 900–1900°C.

 

 

(G) By Products

The by-products from the various methods are, in fact dependant on the composition of the raw materials. If the basic raw material is bischofite or carnallite, we shall obtain, in addition to magnesium, chlorine and by-products related to the components of the original material. In the case of Carnallite, in addition to chlorine there will be KCl-rich salt (70%), which serves in the production of fertilizers.

 

With processes in which the basic raw material does not contain chlorine, no surplus chlorine will be produced, since the chlorine produced will be recycled into the production process.Most processes will actually require the addition of chlorine from external sources such as magnesium chloride or HCl.

 

Refference : Friedrich ・ Mordike, "Magnesium Technology, Metallurgy, Design Data, Applications", 2006