Melting point of rare earth metals

Mechanical milling, 292, 295 Mechanochemical method, 419 Medical applications, 146 Medium-long forms of the periodic table, 81 Melting point of rare-earth metals, 80 MEM. See Maximum entropy method Mendeleev, Dmitrii Ivanovich, 8,37 line, 15 method, 24 methodology, 37 active rare-earth research, 25... 

TABLE 4. High Temperature Transition Temperatures and Melting Point of Rare Earth Metals... 

Transition temperatures, phases and melting point of rare earth metals (after Beaudry and Gschneidner,... 

Tip denotes the decomposition temperature of incongruent intermediate compounds. Tin p RE denotes the melting point of rare earth metals, and Tm p onir denotes the melting points of nontransition metals. A< denotes the difference of Miedema s electronegativity. Rre/Rntr denotes the ratio of metallic radii. Z mtr denotes the number of valence electron of atom of nontransition metals. 

Table 5.18. Highest melting points (°C) in the alloys of rare earth metals and actinides with compound-forming elements of the 4th and 6th rows of the Periodic Table. See the introduction for the meaning of symbols. 

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. 

Scandium is a soft, lightweight, silvery-white metal that does not tarnish in air, but over time, it turns yellowish-pink. It resists corrosion. Scandium reacts vigorously with acids, but not water. Scandium has some properties similar to the rare-earth elements. Although its position in group 3 places it at the head of the 17 elements of the lanthanide series of rare-earth metals, scandium, as a metal, is not usually considered a rare-earth. Scandiums melting point is l,54l°C, its boiling point is 2836°C, and its density is 2.989 glctn . 

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. 

When the boiling points of metallic impurities are much lower than the boiling point of the main metal, they can simply be distilled away in most cases. The rate and the extent of the removal by distillation of these impurity elements depend upon their partial pressures over the main metal/melt. As an example, let the feasibility of distilling magnesium and magnesium chloride from titanium and calcium from the rare earths be considered. In the firstcase, at 900 °C, the pertinent vapor pressure values are P = 4 10-11 torr, PMg = 105 torr... 

Electrotransport In this process too a super-clean atmosphere is necessary. In electrotransport (a kind of solid-state electrolysis) a large dc current (typically 200 A cm-2) is passed through a rod of the metal at a temperature 100-200°C below its melting point. In the rare earth metals the interstitial impurities slowly move towards the anode, while several metallic impurities move towards the cathode. In this case too, as in zone melting, the purest portion of the bar is its central part. 

Neodymium, along with lanthanum, cerium and praseodymium, has low melting points and high boiling points. The fluorides of these and other rare earth metals are placed under highly purified helium or argon atmosphere in a platinum, tantalum or tungsten crucible in a furnace. They are heated under this inert atmosphere or under vacuum at 1000 to 1500°C with an alkali or alkaline earth metal. The halides are reduced to their metals ... 

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