Rare earth chlorides, molten

The metallothermic reduction of the oxides by La produces the metals Sm, Eu, Tm, Yb, all having high vapour pressures. The reaction goes to completion due to the removal of the rare earths by volatilization from the reaction chamber (lanthanum has a low vapour pressure). The remaining rare earth metals (Sc, La, Ce, Pr, Nd, Y, Gd, Tb, Dy, Ho, Er, Lu) can be obtained by quantitative conversion of the oxides in fluorides, followed by reduction with Ca. The metallothermic reduction of the anhydrous rare earth chlorides could be also used to obtain La, Ce, Pr and Nd. The molten electrolysis can be applied to obtain only the first four lanthanide metals, La, Ce, Pr and Nd, because of the high reactivity of the materials that limits the operating temperatures to 1100°C or lower. 

In the electrolytic process, a fused mixture of anhydrous rare earth chlorides (obtained above) and sodium or potassium chloride is electrolyzed in an electrolytic cell at 800 to 900°C using graphite rods as the anode. The cell is constructed of iron, carbon or refractory hnings. Molten metal settles to the bottom and is removed periodically. 

Mischmetal is produced commercially by electrolysis, The usual starting ingredient is the dehydrated rare earth chloride produced from monazite or bastnasite. The mixed rare earth chloride is fused in an iron, graphite, or ceramic crucible with the aid of electrolyte mixtures made up of potassium, barium, sodium, or calcium chlorides. Carbon anodes are immersed in the molten salt. As direct current flows through the cell, molten mischmetal huilcls up in the bottom of the crucible. This method is also used to prepare lanthanum and cerium metals. 

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. 

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. 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Electrowinning Generally this method is limited to La, Ce, Pr and Nd because of their low-melting points. The rare earth salt (fluoride, chloride, etc.) mixed with an alkali or alkaline-earth salt is heated to 700-1100°C and then an electric dc current passed through the cell. If the bath temperature is above the melting point of the R, drops of the molten metal drip off of the cathode and are collected at the bottom of the cell. Generally, the electrowon metal is not as pure as that obtained by metallothermic reduction. 

Molybdates of the Rare Earth Metals. —Salts of the type M2(Mo04)3 have been described. The cerous salt is obtained as yellow crystals by fusing together anhydrous cerous chloride and sodium molybdate. The density of the molten salt is 4-56. The crystals are similar to those of lead and bismuth molybdates, as also are those of didymium molybdate. ... 

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... 

Plutonium metal is often purified by electrolytic refining the plutonium sample is immersed in a molten chloride salt under an inert atmosphere, comprising the anode in an electrolytic cell. Liquid Pu metal is collected on the surface of a tungsten cathode and drips off into a collector. Transition-metal contaminants remain in the residue of the anode, and rare earths and other actinides concentrate in the molten salt. The yield of purified Pu metal can be as high as 97%. Zone melting is also used to purify metallic plutonium the plutonium is fabricated into a bar along which a high-temperature zone is passed. As the melt zone is moved... 

Although an Ni/Mo alloy melt does not wet a-BN, slow interface reactions are observed [20]. On the other hand, mutual wettability of materials sometimes is a first indication for chemical affinity. Thus, the wettability of a-BN by aluminium and aluminium alloys increased with increasing temperature a content of rare earth metals in the aluminium melt leads to a decrease of the wettability [21]. Reaction-bonded a-BN is completely eroded by liquid steel at 1650°C in an Ar atmosphere [22]. The contact angles formed on graphite substrates by molten lead di-chloride/alkali metal chloride mixtures do not change when the Ar atmosphere is replaced by CI2. However, when air is introduced complete wetting is observed after about five minutes. This is not the case with a boron nitride substrate [23]. 

The most common raw materials for the REM molten salt electrolysis are in the RE " state, such as RE2O3, RECI3. But RE " still exists to a certain extent in the molten salts, especially in the chloride melts, some rare earth metal elements have presented a higher level of divalent oxidation states, such as neodymium, samarium, europium, dysprosium, thulium, and ytterbium, which result in a lower current efficiency. For Sm and Eu molten salt electrolysis processes, even no metals can be obtained at the cathodes due to a cyclic transformation of Sm VSm (Eu /Eu ) and Sm /Sm (Eu /Eu ) on the electrodes during electrolysis. And some of the rare earth metal elements show tetravalent oxidation states at the chlorine pressure far in excess of atmospheric pressure, such as Ce. Most of the rare earth metal elements in oxidation state of -1-4 are not stable in chloride melts, because the reaction occurs according to the following equation RE " -I- Cl = RE -" -I- I/2CI2. 

The higher the stability of RE is, the lower the current efficiency is. The stability of RE is dependent on the rare earth metal element itself and electrolyte composition. For example, Sm, Eu, and Yb form highly stable RE " ions in the chloride melts and are unable to obtain correspOTiding metals by molten salt electrolysis due to a very low current efficiency. If Sm, Eu, and/or Yb irnis exist in the chloride system as trace impurities, the current efficiency will be lowered significantly. This is explained later by the electrochemical analysis results. Besides Sm, Eu, and Yb, nearly all the rare earth metal elements form RE ions in the chloride melts. For example, in aUcaline chloride melts, electrochemical investigation has showed that Nd " is reduced on an inert cathode through two steps, according to Eqs. 9 and 10. The presence of Nd " will cause a lower current efficiency. So the chloride electrolytic process is not subject to Nd production in industry. In addition to Nd, the other REM also have similar trends to Nd. 

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. 

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