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 = 117oC
MgCl2*4H2O = MgCl2*2H2O + 2H2O(g) T = 185oC
MgCl2*2H2O = MgCl2*H2O + H2O(g) T = 242oC
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
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