Reduction process molten salt electrolysis

The process starts with a hafnium-containing Zr02 ore [22]. The liquid metal cathode consists of copper and tin, and a CaCl2-based salt can be used as an electrolyte. CaO can be added into the electrolyte at the start of the process, and graphite is used as the anode. During electrolysis the following half-reactions occur at the electrodes. 

CaO has a high solubility in the electrolyte, and when a sufficient potential difference is applied, Ca ions are reduced at the cathode to calcium metal. Calcium metal then dissolves in both molten metal and salt, where it reacts with zirconium oxide to form zirconium metal, as shown in Equation 6.1.3  

Hafnium oxide reacts in a similar manner. Contaminations in the mineral feed (more noble than zirconium) may also co-precipitate on the cathode and end up in the molten metal. But except for hafnium they are all more noble than zirconium and will not cause problems in the electro-refining step. The generated calcium oxide will dissolve back into the salt electrolyte. 

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Reduction process Molten salt electrolysis in inert atmosphere Calciothermic reduction in protective atmosphere Lanthanothermic reduction in vacuum... 

The high activity of REM makes it impossible to be obtained from an aqueous solution therefore, currently most of the REM have been produced by molten salt electrolysis and some by thermal reduction method due to the existence of multivalent ions in the molten salts as shown in Table 1. Compared to the thermal reduction method, molten salt electrolysis is a relatively economical one because the process is continuous and easy to control. And it has been widely used in the industry to produce a single rare earth metal such as La, Ce, Pr, Nd and mixed rare earth metal alloys. 

Aluminum. All primary aluminum as of 1995 is produced by molten salt electrolysis, which requires a feed of high purity alumina to the reduction cell. The Bayer process is a chemical purification of the bauxite ore by selective leaching of aluminum according to equation 35. Other oxide constituents of the ore, namely siUca, iron oxide, and titanium oxide remain in the residue, known as red mud. No solution purification is required and pure aluminum hydroxide is obtained by precipitation after reversing reaction 35 through a change in temperature or hydroxide concentration the precipitate is calcined to yield pure alumina. 

The recovery of aluminum metal is divided into two steps, i. e., the production of pure alumina (Bayer Process) and its molten salt electrolysis. Raw aluminum obtained by reduction electrolysis already has a high purity (99.5-99.7%). Refining methods for raw aluminum to obtain higher purities include the segregation process (99.94-99.99% Al) and three-layer electrolysis (99.99-99.998% Al) [142, 236]. Besides these, processes are available whereby the aluminum is anodically dissolved in an organic electrolyte and then cathodically deposited [37, 118, 217, 221]. The dissolution as well as the deposition process contribute to the electrolytic refining of aluminum. 

Among the pentavalent elements, the most important are niobium and tantalum. Niobium is an excellent material for surface treatment of steel materials for chemical industry due to its high hardness and corrosion-resistance in wet acidic conditions. Nowadays, niobium is also used for the preparation of superconductor tapes and it is used in other branches of industry, for instance in nuclear technology and metallurgy. Tantalum is also of similar importance. For these applications, it is necessary to prepare high purity metal. Molten salt electrolysis, as an alternative process to classical thermal reduction, provides niobium and tantalum with required quality. In order to optimize these processes, it is necessary to know details of both complex formation and redox chemistry of the species present in the melts. 

In our natural environment, metals are most stable in an oxidized state, e.g., Fe203 or AI2O3. As a consequence, one step of metal ore refining is the reduction of the metal oxide to its zero oxidation state. An electrochemical reduction process, where the electrolysis medium is a molten salt, is preferred for very electropositive metals (e.g., aluminum, sodium, lithium, and magnesium) and for metal refining where the chemical route suffers from environmental problems. 

A commercial digestion process is currently in use for the extraction of REE, including yttrium from monazite. The process is based on the application of caustic soda, and one of the products is REE hydroxide. The rare earths are leached from bastnaesite with hydrochloric acid (or sulfuric acid), followed by calcination at >600°C they are then treated with 16 M nitric acid (Kirk-Othmer 1999). Yttrium is produced as pure silver metal, both on the laboratory and industrial scale, by molten salt electrolysis and metallothermic reduction of the fluoride, oxide, or chloride with calcium following an enrichment process, after separation by fractionated crystallization, ion exchange... 

Preparation of pure lithium-metal traps and ingots (molten lithium chloride electrolysis). Even if in some older processes the metal was prepared by direct metallothermic reduction of the lithium oxide with magnesium or aluminum, today lithium metal is essentially obtained directly by molten-salt electrolysis of LiCl-KCl according to a process... 

The first, and now obsolete, industrial processes for producing raw sodium metal were based on the carbon reduction of sodium carbonate or sodium hydroxide. The first industrial production of pure sodium metal was performed by molten-salt electrolysis of the pure sodium hydroxide, NaOH, in so-caUed Castner cells. Most modern processes for the production of sodium now involve molten-salt electrolysis of highly pure sodium chloride. Actually, since 1921, when the process was invented by J.C. Downs, the electrolysis has been performed in Downs electrolytic cells at the DuPont de Nemours Canadian facilities at Niagara Falls, Ontario, Canada. The electrolytic cell consists of four cylindrical anodes made of graphite surrounded at the bottom of the cell by steel cathodes, and a fine steel mesh acts as a separator between anodic and cathodic compartments. Each cell contains a batch of 8 tonnes of a molten-salt mixture with the following chemical composition NaCl (28 wt.%), CaCl (26 wL%), and BaClj (46 wt.%). 

Preparation of uranium metal. As discussed previously, some nuclear power plant reactors such as the UNGG type have required in the past a nonenriched uranium metal as nuclear fuel. Hence, such reactors were the major consumer of pure uranium metal. Uranium metal can be prepared using several reduction processes. First, it can be obtained by direct reduction of uranium halides (e.g., uranium tetrafluoride) by molten alkali metals (e.g., Na, K) or alkali-earth metals (e.g.. Mg, Ca). For instance, in the Ames process, uranium tetrafluoride, UF, is directly reduced by molten calcium or magnesium at yoO C in a steel bomb. Another process consists in reducing uranium oxides with calcium, aluminum (i.e., thermite or aluminothermic process), or carbon. Third, the pure metal can also be recovered by molten-salt electrolysis of a fused bath made of a molten mixture of CaCl and NaCl, with a solute of KUFj or UF. However, like hafnium or zirconium, high-purity uranium can be prepared according to the Van Arkel-deBoer process, i.e., by the hot-wire process, which consists of thermal decomposition of uranium halides on a hot tungsten filament (similar in that way to chemical vapor deposition, CVD). 

Sm VSm Transformation in Chloride Melts Stability of samarium ions (Sm % Sm " ) in the alkaline chloride melts changes as functions of the solvent salt cations and temperature [4]. Sm " exhibits a higher stability for a larger solvent salt cation and lower temperature. Electrochemical reduction of Sm " into Sm in KQ-NaCl-CsCl melt at an inert cathode has been found to occur in two steps as shown in Eqs. 12 and 13. And the reduction of Sm " to Sm° takes place at near the decomposition potential of the supporting electrolyte. In addition, Sm " losing one electron to form Sm " takes place at the anode in terms of reaction Eq. 14, making Sm " Sm /transformation at the electrodes therefore, this process can circulate in the cathode and anode, and therefore nearly no Sm metal can be obtained at the cathode, resulting an extremely low current efficiency. This is the rea-sOTi why samarium caimot be produced from the chloride melts by molten salt electrolysis. It is reported that when the concentration of Sm " ions reach 0.1 wt% in the chloride melts, the current efficiency will be substantially decreased. Eu " behaves in nearly the same manner as Sm " in the chloride melts. 

In this chapter, the two electrolyte systems are introduced to produce rare earth metals by molten salt electrolysis. The involved electrode processes, current efficiency, and the oxidation states of the rare earth metal ions and their stability have been discussed. The lower current efficiency for chloride melts is caused by the higher stability of divalent ions of rare earth metals in the melts. Fluoride ions have lowered the stability of divalent ions therefore a higher current efficiency is reached in the fluoride-oxide system. As an example, the electrochemical reduction process for Ndp3 and Nd203 has been discussed. 

The production of uranium metal usually involves the reduction of UF4 with magnesium at 700°C. The metal may be refined by molten-salt electrolysis followed by zone melting. Because of the low melting point of uranium, the van Arkel process is not as feasible as for thorium and protactinium. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

These facts would suggest that die electrolysis of molten alkali metal salts could lead to the inuoduction of mobile elecU ons which can diffuse rapidly through a melt, and any chemical reduction process resulting from a high chemical potential of the alkali metal could occur in the body of the melt, rather than being conhned to the cathode volume. This probably explains the failure of attempts to produce tire refractoty elements, such as titanium, by elecU olysis of a molten sodium chloride-titanium chloride melt, in which a metal dust is formed in the bulk of the elecU olyte. 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Alkali metals are produced commercially by reduction of their chloride salts, although the exact procedure differs for each element. Both lithium metal and sodium metal are produced by electrolysis, a process in which an electric current is passed through the molten salt. The details of the process won t be discussed until Sections 18.11 and 18.12, but the fundamental idea is simply to use electrical energy to break down an ionic compound into its elements. A high reaction temperature is necessary to keep the salt liquid. 

Sulfur Sg is known to be electroactive. In reduction (in aprotic solvents like DMF), a first step (—0.55 V vs. ferrocene/ferrocenium) leading to Sg , and 83 has been reported [279]. Surprisingly, the anodic activation of sulfur was only reported in molten salts (AICI3—NaCl at 150°C) and in fluorosulfuric acid. In molten electrolytes, the formation of species such as S, Sg", S , and S" was proposed [280]. On the contrary, sulfur was shown [281] to dissolve in boiling HFSO3 to give colored solutions with chemical formation of S" and S"" " as starting species in further oxidation processes. Thus, oxidized forms like S and S" " were reported to appear under electrolysis in aciditic melts as well as in basic melts. 

SECTION 20.9 An electrolysis reaction, which is carried out in an electrolytic cell, employs an external source of electricity to drive a nonspontaneous electrochemical reaction. The current-carrying medium within an electrolytic ceU may be either a molten salt or an electrolyte solution. The products of electrolysis can generaUy be predicted by comparing the reduction potentials associated with possible oxidation and reduction processes. The electrodes in an electrolytic ceU can be active, meaning that the electrode can be involved in the electrolysis reaction. Active electrodes are important in electroplating and in metaUuigical processes. 

The salt purification process is illustrated in Fig. XXIV-9. A fraction of the molten salt is removed from the electrolysis cell and is placed in contact with lithium-rich liquid cadmium. By the exchange reaction between Li and salt-borne TRU and the fission products, the less stable species in the molten salt are transferred to the liquid Cd. Generally, U and TRU are less stable than the rare earth metals and are first transferred to the liquid Cd. The Li concentration in the liquid Cd must be increased to decrease the contamination of the molten salt by TRU. Then, concentration of the fission products is also increased in the liquid Cd. After a forward reductive extraction process, the decontaminated salt with the salt-borne fission products passes through zeolite beds that replace nearly all of the alkali, alkaline earth, and rare earth metals with K and Li by ion exchange. The residual actinides in the molten salt are also adsorbed in the zeolite. The molten salt leaving the zeolite is free of actinides and fission product ions. The purified salt is mixed with an oxidizer such as CdCb and is contacted with liquid Cd that contains U and TRU by the forward reductive extraction process. CdCb will contain U and TRU to be oxidized. U and TRU are transferred to the molten salt from the liquid Cd. The molten salt with U and TRU is recycled to the electrolysis cell. The liquid metal is also recycled to the forward reductive extraction process. 

Because of its high reactivity, production of barium by such processes as electrolysis of barium compound solution or high temperature carbon reduction is impossible. Electrolysis of an aqueous barium solution yields Ba(OH)2, whereas carbon reduction of an ore such as BaO produces barium carbide [50813-65-5] BaC2, which is analogous to calcium carbide (see Carbides). Attempts to produce barium by electrolysis of molten barium salts, usually BaCl25 met with only limited success (14), perhaps because of the solubiUty of Ba in BaCl2 (1 )-... 

Powder Formation. Metallic powders can be formed by any number of techniques, including the reduction of corresponding oxides and salts, the thermal dissociation of metal compounds, electrolysis, atomization, gas-phase synthesis or decomposition, or mechanical attrition. The atomization method is the one most commonly used, because it can produce powders from alloys as well as from pure metals. In the atomization process, a molten metal is forced through an orifice and the stream is broken up with a jet of water or gas. The molten metal forms droplets to minimize the surface area, which solidify very rapidly. Currently, iron-nickel-molybdenum alloys, stainless steels, tool steels, nickel alloys, titanium alloys, and aluminum alloys, as well as many pure metals, are manufactured by atomization processes. 

Technically, all signs seemed to point to metallic sodium for the production of potassium from its compounds as a step in the production of potassium superoxide. Sodium is commercially prepared by the electrolysis (I) of molten sodium chloride to which calcium chloride has been added to lower the melting point. The analogous process could not be used for potassium production (7) because the potassium will attack the graphite electrodes and because of the danger of explosion due to potassium carbonyl sometimes formed in the process. Rather than work on alternate electrodes of other material, a thermochemical process was developed, using the reduction of a potassium salt by sodium. Other processes (4) were investigated by Kraus. 

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