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)