Particle size reduction by a hammer mill I: Effect of output screen size, feed particle size, and mill speed

Abstract

A hammer mill is an impact mill commonly used in pharmaceutical manufacturing for reducing particle size for a variety of drugs. Commercial grade ammonium sulfate was milled as a model powder. This salt was sieved to obtain particle size fractions with average diameters of 1.3, 0.9, and 0.72 mm which were used as feed particles. The milled material was analyzed for particle size distribution (PSD) using standard sieves. At a moderate speed (approximately 2500 rpm), the feed size did not result in a significantly changed arithmetic mean diameter, dx of milled particles. A new particle size reduction constant, k, is proposed as a result of a linear relationship between dx and output screen size, dss. As dss decreases, the PSD range of milled particles narrows; as mill speed increases (approximately 5000 rpm), dx decreases. The decrease is of a greater magnitude for larger dss (2 mm) than for a smaller dss (1 mm). At low speeds (approximately 1000 rpm), the PSD is wider compared to medium and high speeds.