Rare-earth metals metallothermic reduction

There are two basic methods of preparing the rare earth metals — metallothermic reduction or the electrolytic reduction of a rare earth salt, usually a fluoride or chloride or sometimes the oxide. The latter method is limited to the light... 

Examples of metals which are prepared by the metallothermic reduction of oxides include manganese, chromium, vanadium, zirconium, and niobium. In a manner similar to the production of magnesium by the Pidgeon process, some of the rare earth metals have been produced by the metallothermic reduction-distillation process. 

Metalloporphyrins, studies of, 78 591 Metalloreceptors, 76 787 Metallothermic magnesium, 75 343 Metallothermic reduction, rare-earth-metal production by, 74 643 Metallothioneins, as natural defense against silver, 22 655, 657, 681 Metal lubricant, indium and, 74 195 Metallurgical (smelter) plants, 23 792 Metallurgical additives... 

Due to the great similarity of the chemical properties of the rare earth elements, their separation represented, especially in the past, one of the most difficult problems in metallic chemistry. Two principal types of process are available for the extraction of rare earth elements (i) solid-liquid systems using fractional precipitation, crystallization or ion exchange (ii) liquid-liquid systems using solvent extraction. The rare earth metals are produced by metallothermic reduction (high purity metals are obtained) and by molten electrolysis. 

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. 

Of those factors that have proved a disincentive to the wider application of the metals perhaps none has proved more formidable than cost. Unquestionably some rare earth metals are expensive when compared with many of the more common metals but then others, such as the commercial grades of lanthanum and cerium produced by electrolytic methods, are relatively inexpensive. Unfortunately, for most rare earth metals one must use a metallothermic reduction process that is inherently costly to operate. However, while there is a wide variation in the price of the various metals, from 50 - 7,000 per lb., the overall picture is one of relative price stability when judged against the movements of many other metals. To some extent this stability has been born out of the need to encourage potential users to adopt a particular metal in the face of a competitive product while in other cases it reflects more the economy of scale that has been possible once production has passed a given level. 

The reduction of rare earth metal trihalides, RX3, is in principle possible with all kinds of reducing agents as long as they have standard electrode potentials E° that can overcome that of the respective potentials of E° (R + R +). This is discussed below in more detail. Therefore, the classical reducing agents, nonmetals such as hydrogen or carbon, or like metals (comproportionation route) and unlike metals (metallothermic reduction) are all possible but (may) lead to different products. Cathodic reduction of appropriate melts is also an option. 

The reduction of rare earth metal halides with unlike metals, Wohler s metallothermic reduction, has originally... 

The synthesis of compounds of the lanthanides containing cluster complexes follows in general the same routes as described in The Divalent State in Solid Rare Earth Metal Halides, the conproportionation route and the metallothermic reduction route, for example... 

The reduction of rare-earth metal halides with unlike metals, Wohler s metallothermic reduction (Wohler, 1828), has originally been used to produce the rare-earth metals (Klemm and Bommer, 1937). When used properly, intermediates with oxidation states between +3 and 0 can be obtained. 

In the following, we will discuss only the comproportionation route and the metallothermic reduction route as the two most commonly used synthetic approaches to rare-earth metal/transition metal cluster complexes. 

Metallothermic reduction of chlorides has been the basis of some very important processes for reactive metals production. Examples include the Kroll and Hunter processes for the preparation of zirconium and titanium, and calcium or lithium reduction processes for the rare earths. 

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. 

SmCl3 resulted in the reduction only to SmC. From NdCl3 + Ca with the addition of Fe powder, the alloy Nd2Fei7 was obtained. In a discussion of the results it was observed that the products obtained at ambient temperature by mechanical alloying are the same which result from the conventional metallothermic reduction of the rare earth halides. However, the metallothermic reduction requires a temperature of 800-1000°C for the reduction of the chlorides and 1400-1600°C for the fluorides. The products of the mechanical process, on the other hand, are fine, amorphous or microcrystalline, highly reactive metal powders mixed with CaCl2. 

Recovey. The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallization of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metallothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

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... 

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