Size Reduction

Natural shear bands of the order of 5-10 particle diameters form between stagnant and flowing regions of powder beds. This can lead to size reduction through unwanted attrition. The attrition shear cell, manufactured by Ajax, consists of an annular gap with gripping rings at the base, within which a narrow powder bed is created. The cell lid, also containing gripping rings, is placed on top of the powder bed and prevented from rotation by a stopper arm as the base is rotated. Since the bed is gripped at the top and bottom, it is sheared. The normal stress can be increased by adding weights to the lid or reduced by adding a counterbalance to a pulley attached to the lid. Strain is controlled by varying the degree of rotation. Attrition is determined by sieve analysis, which when completed for a range of stresses and strains leads to a relationship of attrition to prevailing shear conditions for the given powder.

Milling is a common size reduction technique. The main mechanism of size reduction in a single ball mill is impact, followed by shear and attrition. This process is dominated by collisions of a short duration from a single ball, with the generated energy controlled by the frequency of these collisions. Previous research has employed the Retsch MM200 vibratory single ball mill, where it has been applied to the size reduction of materials such as sucrose and gamma alumina. Ball mills can generally be relied upon to deliver particles of less than 1 mm in diameter.

A spiral jet mill is a type of fluid energy mill, which utilities high velocity gas jets. The high pressure gas is fed through four nozzles and forms an internal vortex in the milling chamber. A fast shearing bed is formed on wall of the chamber and breakage is caused by inter-particle and particle-wall collisions. Within the milling chamber, both breakage and classification occur simultaneously, and as a result, the two mechanisms are interdependent on one another during operation. Overall, the mill is capable of producing a typical product diameter ranging from 5- 20 µm, with a very narrow size distribution. The lack of moving parts, also means the mill benefits from low contamination and is simple to operate.

The planetary ball mill differs from the single ball mill, as shearing and compression are the more dominant mechanisms of size reduction, as more than a single ball is employed. The planetary ball mill can achieve a finer particle diameter than the single ball mill and is used for the milling of ultrafine particles, in the region of less than 10 µm. Both the milling speed and duration can be varied to ensure the desired particle size requirements are achieved.