Metallothermic reduction

LED growth technique piGHT GENERATION - LIGHT-EMITTING DIODES] (Vol 15) Metallothermic reduction... 

Very reactive metals, eg, titanium or 2irconium, which in the Hquid state react with all the refractory materials available to contain them, also require reduction to soHd metal. Titanium is produced by metallothermic reduction of its chloride using Hquid magnesium at 750°C (KroU process). 

The spent salt from MSE is currently sent to an aqueous dissolution/carbonate precipitation process to recover plutonium and americium. Efforts to recover plutonium and americium from spent NaCl-KCl-MgCl2 MSE salts using pyrochemistry have been partially successful (3). Metallothermic reductions using Al-Mg and Zn-Mg alloys have been used in the past to recover plutonium and americium, and produce salts which meet plant discard limits. Attempts at direct reductions of MSE salts using... 

Metallothermic reductions, aluminum-magnesium and zinc-magnesium... 

Metal-rich compounds are readily obtained when a metallothermic reduction is included into the metathesis reaction by using an electropositive metal (10). In addition, metal-rich and nitrogen-rich compounds are obtained when a metal and a metal nitride are employed in reactions (11, 12) ... 

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. 

The metallothermic reduction of oxides is essentially a reaction involving only condensed phases. It follows therefore, that the entropy changes in these reactions are small and that the differences in the heats of formation of the pertinent compounds determine the feasibility of a given reaction. Among the metallic reductants, calcium forms the oxide whose heat of formation is the most negative. As a first approximation, calcium may be considered to be the most effective reducing agent for metal oxides. 

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. 

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. 

Preparation. Magnesium is obtained by electrolysis of magnesium chloride melts or by metallothermic reduction of magnesium oxide. 

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. 

Vacuum melting This is used to reduce the volatile impurities (such as H, CaF2, RF3) if present after the 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. 

Metallothermic reduction of an actinide halide was the first method applied to the preparation of an actinide metal. Initially, actinide chlorides were reduced by alkali metals, but then actinide fluorides, which are much less hygroscopic than the chlorides, were more... 

The fluoride reduction technique is being replaced in production plants by the metallothermic reduction of an oxide (24). Direct oxide... 

B. Metallothermic Reduction of Actinide Oxides Followed by Distillation... 

The metallothermic reduction of an oxide is a useful preparative method for an actinide metal when macro quantities of the actinide are available. A mixture of the actinide oxide and reductant metal is heated in vacuum at a temperature which allows rapid vaporization of the actinide metal, leaving behind an oxide of the reductant metal and the excess reductant metal, in accord with the following equations ... 

Fig. 2. Metallothermic reduction at 1525 K of Am02 by La metal as a function of time O and , mixtures of Am02 powder and La metal turnings A, pelletized mixture of Am02 powder and ground La metal turnings.

The yield and rate of the tantalothermic reduction of plutonium carbide at 1975 K are given in Fig. 3. Producing actinide metals by metallothermic reduction of their carbides has some interesting advantages. The process is applicable in principle to all of the actinide metals, without exception, and at an acceptable purity level, even if quite impure starting material (waste) is used. High decontamination factors result from the selectivities achieved at the different steps of the process. Volatile oxides and metals are eliminated hy vaporization during the carboreduction. Lanthanides, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, and W form stable carbides, whereas Rh, Os, Ir, Pt, and Pd remain as nonvolatile metals in the actinide carbides. Thus, these latter elements... 

Fig. 3. Metallothermic reduction at 1975 K of PuC with Ta metal as a function of time. PuC-tTa-TaC + Pu(t)...

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

The first preparation of metallic Ac was on the microgram scale and used the metallothermic reduction (Section II,A) of AcClj with K metal vapor (38), which is the same method used by Klemm and Bommer (67) to prepare La metal. The metal produced by this method is mixed with KCl and K metal. X-Ray diffraction revealed that Ac metal was isostructural with P-La, but that the face-centered cubic cell dimension of Ac (5.311 A) was slightly larger than that of La (5.304 A). 

In principle, a promising method for the preparation of Ac metal is the tantalothermic reduction of AcC, as described generally in Section II,C. This method has not been tried as yet, however, so the metallothermic reduction of an actinium halide or oxide remains the only proved method. 

Thorium metal is generally prepared by the metallothermic reduction of its halides (Section II,A). Very high-quality metal containing a total of 250 ppm impurities has been prepared at the Ames Laboratory of the Department of Energy (98, 99). These workers reduced ThCl4 with excess Mg metal to yield a Th-Mg alloy, which was then heated in vacuo to remove the excess Mg (55) ... 

Protactinium metal was first prepared in 1934 by thermal decomposition of a pentahalide on a hot filament 50). It has since been prepared from PaF4 by metallothermic reduction (Section II,A) with barium 26, 27, 34,102), lithium 40), and calcium 73, 74). However, the highest purity metal is achieved using the iodide transport (van Arkel-De Boer) process (Section II,D). 

The metallothermic reduction of NpFj with Ba metal vapor (Section... 

The first Pu metal ever made was prepared in late November 1943 by metallothermic reduction of about 35 fig of PUF4 with Ba metal (Section II,A) (42). This method, modified to use Ca as the reductant for PUF4, has been widely used and is very successful yields of more than 98% and purities of the order of 99.8 at % are obtained (2, 5, 62, 81, 96). The Pu... 

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