Particle shape control of crushed stone by autogenous grinding using a stirred mill

A demand for the aggregates is increasing in the field of civil industry and architectural industry and so on. In these industries, the particle size and shape of aggregates are important factors and especially spherical or rounded particles are desired. In Japan, crushed stone is widely used as the aggregate material for the pavement or the construction. In this study based on the frictional action of material each other, an autogenous grinding of crushed stone was performed in order to eliminate squarish particles and get rounded ones for better aggregate materials. The experimental apparatus is a stirred mill with 4 impellers and the mill is made of stainless steel. The feed size and the feed mass of crushed stone were constant respectively. The particle shape was measured in the particle size range of 20 mm - 2.36 mm, and was evaluated by the shape index(= diameter of circumcircle / thickness) and the circularity(= perimeter of circle of equal projected area / perimeter of particle). As a result, a decrease in the shape index and an increase in the circularity of crushed stone were observed with increasing grinding time. Then, the shape index and the circularity had the limit value for the long time grinding. The tendencies in these variations were not changed by the measuring size. The limit values of the shape index and the circularity increased with decreasing measuring size respectively.