Method for producing mono-dispersed spherical granules

A method for the preparation of monodisperse spherical granules comprising at least one rare-earth metal selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb comprising:

• melting the metal to be prepared as spherical granules in a crucible to form a melt;
• establishing a perturbation in the melt;
• forcing the melt under a variable head of pressure through a die at the base of the crucible into a heat-exchanging chamber, said die being formed from a refractory metal and having a length between 2 and 20 times its diameter whereby droplets are formed by the melt exiting the die;
• cooling said heat-exchanging chamber with a cooled, purified inert gas having an oxygen content less than or equal to 1.0×10−4 mol. %;
• monitoring the size of the droplets in the chamber;
• and
• collecting the granules at the bottom of the chamber;
• wherein the stream perturbation frequency f defined by equation (1):
• f=Wko/πdo(1+cτ)2 (1)
• where:
• τ—is the dispersion time (equal to zero at the initial instant of time),
• c—is the empirical coefficient characteristic of the die material resistance to the effect of stream perturbation,
• w—is the stream outflow velocity,
• do is the initial stream diameter value,
• ko—is the initial value (0.7) of the dimensionless wave number
• is adjusted to maintain monodisperse spherical granules in response to the monitored size of the droplet.