Calciothermic reduction

An estimate of world calcium consumption in 1986 indicated that lead refining uses 30% alloys, eg, with Pb, Al, and Si, 25% steel treatment, 20% calciothermic reduction, 10% calcium hydride, 10% and miscellaneous usage is 5%. More recent evidence, however, has suggested that increasing consumption of calcium in battery manufacture has made this the most significant use. 

In bulk form cerium is a reactive metal. Pure metal is prepared by the calciothermic reduction of CeF. ... 

The calciothermic reduction of an oxide is naturally designed for the obtainment of the reduced metal in the powder form because of the high melting point of the other product, namely, calcia. The formation of the metal in the form of a powder is favored by some other controllable factors also. One of these factors is that the temperature should not exceed the melting point of the metal during reduction. A second factor is that it is preferable to have the reaction temperature as low as possible, without adversely affecting the rate of the reaction. 

Let the calciothermic reduction of vanadium sesquioxide (V203), according to the following overall reaction, be considered ... 

In many instances of commercial importance an alloy, rather than a metal, is obtained as the product of calciothermic reduction. The alloy may be an intermediate in metal preparation or even the end product of the process. 

Calciothermic reduction of samarium oxide, in the presence of cobalt powder, yields samarium-cobalt alloys in the powder form. The process is popularly known as reduction diffusion. Samarium oxide, mixed with cobalt powder and calcium hydride powder or calcium particles, is heated at 1200 °C under 1 atm hydrogen pressure to produce the alloys. Cobalt oxide sometimes partly replaces the cobalt metal in the charge for alloy preparation. This presents no difficulty because calcium can easily reduce cobalt oxide. A pelletized mixture of oxides of samarium and cobalt, cobalt and calcium, with the components taken in stoichiometric quantities, is heated at 1100-1200 °C in vacuum for 2 to 3 h. This process is called coreduction. In reduction diffusion as well as in coreduction, the metals samarium and/or cobalt form by reduction rather quickly but they need time to form the alloy by diffusion, which warrants holding the charge at the reaction temperature for 4 to 5 h. The yield of alloy in these processes ranges from 97 to 99%. Reduction diffusion is the method by which most of the 500 to 600 t of the magnetic samarium-cobalt alloy (SmCOs) are produced every year. 

Several other useful modifications of calciothermic reduction have been successfully developed for the preparation of this neodymium-bearing magnetic alloy. One of these is reduction-extraction which involves the reduction of neodymium sesquioxide (Nd203) with calcium in a molten calcium chloride-sodium chloride salt bath at 750 °C and the simultaneous extraction of the reduced metal into a molten neodymium-zinc or neodymium-iron alloy pool. The neodymium-zinc alloy product is treated in vacuum to remove zinc and produce neodymium metal, while the neodymium-iron alloy is itself the end product of... 

Actinide metals with lower vapor pressures (Th, Pa, and U) cannot be obtained by this method since no reductant metal exists which has a sufficiently low vapor pressure and a sufficiently negative free energy of formation of its oxide. For the large-scale production of U, Np, and Pu metals, the calciothermic reduction of the actinide oxide (Section II,A) followed by electrorefining of the metal product is preferred (24). In this process the oxide powder and solid calcium metal are vigorously stirred in a CaCl2 flux which dissolves the by-product CaO. Stirring is necessary to keep the reactants in intimate contact. 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

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

The second method involves the so-called calciothermic reduction of rare earth oxides by means of calcium in the presence of 3d metals under a protective atmosphere (Cech 1974, Herget and Domazer 1975, Sun Guangfei et al. 1985, Herget 1987, Zhou et al. 1987). This process is commonly referred to as the reduction-diffusion (R-D) process. Basically, it involves two steps. In the first step, performed at 900-1100 °C, the actual reduction of Nd203 takes place according to the reactions ... 

Fig. 26. Process outline for the production of rare earth-transition metal magnet alloy powders by calciothermic reduction and solid-state diffusion. R/D is reduction-diffusion, and KOR is Koreduktion...

Calciothermic reduction has an economic advantage over the fusion method, since the rare earth is used as an oxide, which is considerably cheaper than the metal. The transition metals are used as fine metallic powders. The reaction is carried out at 1420 K in a H2 atmosphere and can be written ... 

The calciothermic reduction method can also be used to prepare the Nd2Fej4B sintering powder. The reaction in this case is ... 

High-purity scandium oxide (i.e., 99.0 to 99.99 wt.% Sc) is an initial raw material used to produce a metallic scandium. After fluorination of the oxide, pure scandium is then prepared by calciothermic reduction of scandium trifluoride (ScFj) with pure calcium metal. The metallic scandium obtained undergoes subsequent refining by vacuum distillation, which ensures a purity of metal at the level 99.99 to 99.999 wt.% Sc. Tentative annual demand for ultrapure metallic scandium for different fields of application is estimated for the near future at 800 to 1000 kg per year. Total annual world production in 2000 of scandium, excluding China, was about 30 kg. Union Carbide and Johnson Matthey, as well as the research company Boulder, are the main manufacturers of scandium products from thortveitite, wastes of uranium, and tungsten production. 

The rare earths are amongst the most difficult materials to refine. These metals are conventionally refined by calciothermic reduction at temperatures exceeding 1300K, with the exception of Sm, Eu and Yb which require the use of La or mischmetal as the reductant due to the high stability of their respective divalent chlorides (Beaudry and Gschneidner 1978). 

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