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