Production technologies of magnesium

One of the features of the magnesium industry is the wide variety of production processes.Relative to an industry,which has been manufacturing a product commercially for close to one hundred years, it is somewhat strange that there are over 10 different processes for producing magnesium.Unlike many other industries, there is no one particular dominant technology used for most of the worlds production. The large number of production technologies stems from the differences in basic parameters of the production processes. Below are the basic parameters, which differentiate the various production methods. (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. (B) Materials and method of reduction Magnesium always appears in nature in ionic form with the following electron arrangement: 1S²2S ²2P⁶3S².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 [1]: 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 intogaseous 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. (C) Production temperatures 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. (D) 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. This chapter details extensively the basic parameters used in the various production methods. The chapter will also provide examples of commercial production technologies in existence around the world, while tracking the relative advantages and disadvantages that characterize these technologies.