Electrolysis of fused salts

Davy electrolysed fused hydroxides of potassium and sodium in 1807, and in this way he discovered the metals potassium and sodium. A number of important metals are still produced by a similar route, the electrolysis of fused salts, including sodium (Na), magnesium (Mg), calcium (Ca) and aluminium (Al). Sodium, magnesium and calcium are all produced via electrolysis of the molten chloride. Taking molten sodium chloride (NaCl), as an example, the reactions are as follows. 

In order to lower the temperature of the process, a eutectic mixture of CaCl2 and NaCl is used, which 

Eleofrolyle mollen sodium chiorldo calcium ehlorid  

The composition of the electrolyte is approximately 80-90 % cryolite. The amount of AI2O3 is continuously replenished as the electrolysis continues. The anode, which is consumed in the reaction, is carbon, and the cathode is carbon reinforced with steel bars, contained in a steel pot . Although the fine details of the reactions occurring are still not completely known, the framework of the process is believed to be as follows. 

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Significant vapor pressure of aluminum monofluoride [13595-82-9], AIF, has been observed when aluminum trifluoride [7784-18-1] is heated in the presence of reducing agents such as aluminum or magnesium metal, or is in contact with the cathode in the electrolysis of fused salt mixtures. AIF disproportionates into AIF. and aluminum at lower temperatures. The heat of formation at 25°C is —264 kJ/mol(—63.1 kcal/mol) and the free energy of formation is —290 kJ/mol(—69.3 kcal/mol) (1). Aluminum difluoride [13569-23-8] h.3.s been detected in the high temperature equihbrium between aluminum and its fluorides (2). 

It was an adaptation of the Castner cell to sodium chloride for fused caustic electrolysis. A mixture of sodium chloride and other chlorides, molten at 620°C, was electroly2ed ia rectangular or oval cells heated only by the current. Several cells have been patented for the electrolysis of fused salt ia cells with molten lead cathodes (65). However, it is difficult to separate the lead from the sodium (see Electrochemical processing). 

Table 7. Production of Metals by Electrolysis of Fused Salts...

An alternative route (Andrieux s method) is the electrolysis of fused salts such as molten... 

Manganese also is produced by electrolysis of fused salt. In one such process, the reduced MnO is blended to molten calcium fluoride and lime. The latter is used to neutralize silica in the ore. The fused composition of these salts is electrolyzed at 1,300°C in an electrolytic cell made up of high temperature ceramic material, using a carbon anode and a cathode consisting of iron bars internally cooled by water. 

The probability of manufacturing products by the electrolysis of fused salts was foreseen by M. Faraday 41 in 1834 before the development of the dynamo,-which replaced the voltaic battery as a source of electricity on a manufacturing scale. 

In recent years, more novel methods have been developed by chemists in order to hasten the reaction between solids or to grow single crystals of new and exotic solids. Among those included in the following syntheses are electrolysis of fused salts, chemical transport, and hydrothermal crystal growth. 

Boron is not required in its elemental form, but it can be obtained by electrolysis of fused salts, or by... 

Preparation of metal powders by electrolysis of fused salts. III. Tantalum." Ind. Eng. Chem., 23 634-637. 

Lithium and Na are obtained by electrolysis of fused salts or of low melting eutectics such as CaCl2 + NaCl. Potassium, Rb, and Cs are made by treating molten MCI with Na vapor in a countercurrent fractionating tower, the metals are best purified by distillation. At the boiling point, the vapors have ca. 1% of M2. 

Derivation (1) By heating boric oxide with powdered magnesium or aluminum (2) by vapor-phase reduction of boron trichloride with hydrogen over hot filaments (80-2000C) (3) by electrolysis of fused salts. 

Electrolysis of fused salts Electrolysis of KThFs in NaCl Electrolysis of Thp4 in NaCl/KCl Electrolysis of ThCl4 in NaQ/KCl Reduction with Reactive Metals Reduction of Th02 with Ca Reduction of ThCU with Mg Reduction of Thp4 with Ca Thermal Dissociation of Thl4... 

Electrolysis of fused salts. The first electrolytic processes used thorium fluorides, as these are less hygroscopic than thorium chloride. In a process developed for the U.S. AEC fS3], a solution of dried 15 to 20% KThFs in molten sodium chloride was electrolyzed at 800 to 900°C in a graphite anodic cell with a molybdenum cathode under an inert argon atmosphere. As only chlorine was produced at the anode, fluorides accumulated in the electrolyte and required its periodic renewal. A somewhat similar process, involving electrolysis of Thp4 in molten KCl/NaCl has been used in the Soviet Union [Kl]. 

Calcium and the other metals are made only on a relatively small scale, by electrolysis of fused salts or reduction of the halides with sodium. 

The best account of the electrolysis of fused salts is P. Drossbach s Elektrolyse geschmolzener Salze, Berhn Springer, 1938. Reprinted by Edwards Bros., Ann Arbor, 1943. 

Ervin, Jr., G. Ueltz, H.F.G. (1958) Apparatus for continuous production of refractory metal by electrolysis of fused salts. US Patent 2,837,478. 

In the decades after Davy the technique of electrolysis of fused salts was developed rapidly, at the same time as electrotechnology was being transformed. The first to prepare beryllium in this way was the French chemist Paid Leheau. Around 1900, he electrolyzed a fluoride melt of potassium and beryllium in a nickel crucible. He used a graphite anode, with the nickel crucible as the cathode. The voltage and current were 80 V and 20 A respectively. On the inside of the crucible, berylHum crystals were deposited with a purity better then 99.5%. 

Discovery In 1825 H. C. Oersted in Copenhagen prepared the element aluminum for the first time. Between 1827 and 1845, Friedrich Wohler improved Oersted s process. In 1886 Charles M. Hall and Paul H roult independently developed the method for aluminum manufacture by electrolysis of fused salts, a technique now known as the Hall-H4roult process. The element s name has its own history. In 1761 de Morveau proposed the name alumina for the base in alum. The name aluminium was adopted by lUPAC to conform to the ium ending of most elements. Aluminum is the lUPAC spelling, and therefore the international standard. In 1925 the American Chemical Society decided to change the name to aluminum, still used in American literature and technical descriptions. 

Pyrosols. — R. Lorenz J defines p3Tosols as colloidal solutions of metals and other substances in a red hot liquid medium. Colloidal metal solutions in fused salts have been correctly recognized and closely studied. They play an important part in the electrolysis of fused salts, and their presence explains some of the anomalous yields obtained during the decomposition of fusions by the electric current. 

Faraday s Apparatus for the Electrolysis of Fused Salts Arrott s Diagrams J. J. Berzelius Leopold Gmelin E. Mitscherlich A. Avogadro... 

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