Decades have passed without any significant improvement in traditional vertical and horizontal in-line media milling designs. Feedstocks are predominantly pressure driven through an enclosed horizontal or vertical chamber filled with media and separated by a screening device. The separation area is relatively small, thereby contributing to what would be considered a painfully slow throughput rate when compared to a vacuum mill. The single media mill innovation that has made a significant performance advance is the immersion or basket mill; basically a large, stationary screening device containing media that passes feedstock through its rotating internal stirring apparatus at about 10 times the velocity of traditional in-line mills (Figure 1). It is submerged into the batch and creates its own throughput by way of self-generated suction and centrifugal force, as opposed to the in-line mill that stands alone alongside the batch and force fed by a pump.
Some would argue that the slow throughput rate enables these older style mills to produce some products with just one pass, à la continuous process. On the other hand, multiple passes have test proven to be more productive in most instances, contributing to a smaller, narrower particle size. Consider now exactly what this means and what enables these different performances.