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)