Published on December 2, 2014
PRACTICAL (A) : BALL MILLING INTRODUCTION : Ball milling is one of the method that used to reduce size of solid materials into smaller particles or into powders. In this practice, we are provided with coarse salt and are required to crush it into smaller particle sizes using ball mill. Then, by using the sieve, the products are separated to find the particles size distribution of the crushed coarse salt. Instruments that are used in this practical such as laboratory ball milling and milling media such as steel balls, ceramic balls and flint pebbles. A ball mill is an example of method which produce size reduction by impact and attribution of particles. Ball mills consist of hollow cylinder mounted such that it can be rotated on its horizontal longitudinal axis. Mills have different diameters of balls. This help to improve product as large ball tends to break down coarse materials and smaller ball help in formation of fine particles by reducing void spaces between balls. Amount of materials in a mill is important too because if too much feed may produce cushioning effect and too little cause loss of efficacy. But more important part is the speed of rotation. At low velocities, ball move with the drum until force due to gravity exceed frictional force of the bed on the drum. Balls will slide back to the base of drum. As sequences repeated, very little movement of ball so the size reduction is small. When high speed rotation is set, balls are thrown out on the mill wall by centrifugal force and no size reduction occur. But until the balls are arise at the side of the wall and achieved their dynamic angle of repose exceed. At this point, they fall and roll back to the base of drum across diameter of drum. This means that maximum size reduction occur.