Diffusion calciothermic reduction

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. 

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...

The R-Fe-B alloys may be prepared by reducing with calcium, at high temperature, the respective rare-earth oxides, in the presence of a mixture of transition metals in powder form plus some transition metal oxides (Herget 1985a). Two versions of calciothermic processes for making rare-earth alloys were proposed the reduction-diffusion (R-D) process (Cech 1974, McFarland 1973) and the co-reduction process (Herget and Domazer 1975, Domazer and Strnat 1976). The last one consists of a simultaneous reduction of rare-earth and transition metal oxides in the presence of transition metal powder, followed by diffusional alloys formation. The process may be divided into two steps ... 

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See also in sourсe #XX -- [ ]

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