Effective fine grinding of coal by a ball-race mill with grinding additives.

Abstract

Ball-race mills form one of the most widely used types of industrial grinding device, employed especially for dry fine grinding of coal. Grinding rates and efficiencies of these mills decrease with build up of fine ground powder. In this study, grinding mechanism of coal by a Hardgrove mill, a representative ball-race mill, was analyzed by using a dead space model combined with a non-linear mean squares fitting technique. As a result of the analysis, slow mixing of coal powder in the mill presumably due to poor flowability of the powder was found to be responsible for slowing down of grinding rates which results in inefficient fine grinding of coal by Hardgrove mill. Based on the result, grinding additives which can improve the flowability were selected and tested by a milling experiment. As a result, calcium stearate and carbon black were found to be most effective for fine grinding of coal by Hardgrove mill. Furthermore, action mechanisms of some additives were analyzed by using the dead space model.