Fused-Salt Electrolysis

The existence of the hydride ion is shown by electrolysis of the fused salt when hydrogen is evolved at the anode. If calcium hydride is dissolved in another fused salt as solvent, the amount of hydrogen evolved at the anode on electrolysis is 1 g for each Faraday of current (mole of electrons) passed, as required by the laws of electrolysis. 

The general characteristics of all these elements generally preclude their extraction by any method involving aqueous solution. For the lighter, less volatile metals (Li, Na, Be, Mg, Ca) electrolysis of a fused salt (usually the chloride), or of a mixture of salts, is used. The heavier, more volatile metals in each group can all be similarly obtained by electrolysis, but it is usually more convenient to take advantage of their volatility and obtain them from their oxides or chlorides by displacement, i.e. by general reactions such as... 

Chlorine from the Magnesium Process. Magnesium is produced by the fused salt electrolysis of MgCl2 (see Magnesium and magnesium... 

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). 

The deposition of RE metals from aqueous solutions does not work because of the highly electropositive nature of the REE. Therefore, industrial production of RE metals is carried out by fused salt electrolysis or metaHothermic reduction. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

The cell for this process is unlike the cell for the electrolysis of aluminum which is made of carbon and also acts as the cathode. The cell for the fused-salt electrolysis is made of high temperature refractory oxide material because molten manganese readily dissolves carbon. The anode, like that for aluminum, is made of carbon. Cathode contact is made by water-cooled iron bars that are buried in the wall near the hearth of the refractory oxide cell. 

Fused-salt electrolysis of K2NbFy is not an economically feasible process because of the low current efficiency (31). However, electrowinning has been used to obtain niobium from molten alkaU haUde electrolytes (32). The oxide is dissolved in molten alkaU haUde and is deposited in a molten metal cathode, either cadmium or zinc. The reaction is carried out in a ceramic or glass container using a carbon anode the niobium alloys with the cathode metal, from which it is freed by vacuum distillation, and the niobium powder is left behind. 

A fused-salt electrolysis process has been demonstrated (30). Carbon dioxide is introduced to the cathode area of a melt of 60 wt % LiCl—40 wt % Li2C02 at 550°C. The carbon dioxide reacts with hthium oxide which is produced by electrolysis. Oxygen is released at the anode and carbon plates onto the cathode. The reaction requites a potential of 4.5 V. The reactions ate as follows ... 

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 6. Tekkosha Fused-Salt Electrolysis Cells ...

Electrolytic Processes. The electrolytic procedures for both electrowinning and electrorefining beryUium have primarily involved electrolysis of the beryUium chloride [7787-47-5], BeCl2, in a variety of fused-salt baths. The chloride readUy hydrolyzes making the use of dry methods mandatory for its preparation (see Beryllium compounds). For both ecological and economic reasons there is no electrolyticaUy derived beryUium avaUable in the market-place. 

Anode Applications. Graphite has been used as the primary material for electrolysis of brine (aqueous) and fused-salt electrolytes, both as anode and cathode. Technological advances, however, have resulted in a dimensionally stable anode (DSA) consisting of precious metal oxides deposited on a titanium substrate that has replaced graphite as the primary anode (38—41) (see Alkali and chlorine products). 

Chemical Production. Electrolytic production of chemicals is conducted either by solution (water) electrolysis or fused-salt electrolysis. Fluorine, chlorine, chlorate, and manganese dioxide are Hberated from water solutions magnesium and sodium are generated from molten salt solutions. 

The metals that are produced by electrolysis (81) are included in Table 1. Fused salt processes are used when the reactivity of the metal does not allow electrowinning from aqueous solutions. Manganese is the most reactive metal that is produced by electrolysis of an aqueous solution. 

Beryllium. Beryllium [7440-41-7], Be, metal is produced by electrolysis of KCl—NaCl—BeCl2 melts. Temperatures up to 900°C are required. CeU voltages are 6 to 9 V (115). Electrolysis of mixtures of beryUium oxide [1304-56-9], BeO, ia lithium fluoride [7789-24-4], LiF, and beryUium fluoride [7787-49-7], BeF2, has produced beryUium metal at about 700°C and 2.6 V (116). DetaUs of fused salt metal winning processes are given ia Table 7. 

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... 

Fused salts Molybdenum has excellent resistance to a wide range of fused salts and has been used in the fused salt electrolysis of magnesium, platinum, thorium and uranium. In the production of pure magnesium, molybdenum is used to couple graphite electrodes electrically. Molybdenum cathodes are... 

Molten bath deposition The interchange reaction also takes place when the coating metal halide is dissolved in a fused salt. Alternatively, deposition may be by electrolysis. Another technique uses the coating metal dissolved in molten calcium ... 

Eustace [75] studied the specific resistance of samples of bromine-fused salt phase produced by electrolysis of 3.0 mol L l ZnBr2 and 1.0 mol L"1 MEM at 23 °C. As is shown in Fig. 4, a considerable resistance is observed in the initial phase of the charge process, dropping to approximately one-third at 30% Zn utilization. At higher states of charge the increase in the conductivity is significantly slower. 

The mechanism of boride formation is not always well known. However, many studies have sought to understand the chemical processes involved in fused-salt electrolysis. 

The fused-salt electrolysis method is not suitable for obtaining pure materials of definite stoichiometry. 

It has been shown that the electrodeposition of molybdenum chalcogenides from high-temperature molten salts can give large, well-defined crystals of these compounds. The preparation of M0S2 as well as WS2 by electrolytic reduction of fused salts was first reported by Weiss [145], who produced small hexagonal blue-gray platelets under drastic conditions of electrolysis. Schneemeyer and Cohen... 

Cuomo JJ, Gambino RJ (1968) The synthesis and epitaxial growth of GaP by fused salt electrolysis. J Electrochem Soc 115 755-759... 

Yamamoto A, Yamaguchi M (1975) Epitaxial growth of ZnSe on Ge by fused salt electrolysis. Japan J Appl Phys 14 561-562... 

The first production of aluminum was by the chemical reduction of aluminum chloride with sodium. The electrolytic process, based on the fused salt electrolysis of alumina dissolved in cryolite, was independently developed in 1886 by C. M. Hall in America and P. L. Heroult in France. Soon afterwards a chemical process for producing pure alumina from bauxite, the commercial source of aluminum, was developed by Bayer and this led to the commercial production of aluminum by a combination of the Bayer and the Hall-Heroult processes. At present this is the main method which supplies all the world s needs in primary aluminum. However, a few other processes also have been developed for the production of the metal. On account of problems still waiting to be solved none of these alternative methods has seen commercial exploitation. 

F. P. Haver, D. E. Shanks, D. L. Bixby and M. M. Wong, Recovery of Zinc from Zinc Chloride by Fused Salt Electrolysis, U. S. Bureau of Mines, Rept. Invst. No. 8133,1976. 

---