Determination of breakage parameters and modelling of coal breakage in the Szego mill

Abstract

The Szego mill is a planetary ring-roller mill which is limited to continuous grinding operations and has a fast rate of material breakage. Hence, breakage parameters, S the selection function and B the cumulative breakage function for Pittsburgh coal, were determined utilizing new experimental techniques. To determine S, a steady-state, multi-pass grinding operation was utilized. It essentially involves finding the weight fraction of the top size-interval remaining in the product after each pass through the mill. Using the residence time distribution in the mill, S and its relationship with particle size were determined at various operating conditions. S is found to increase with increase in rotational speed of the mill, and the absolute rate of breakage decreases with increase in feed rate. Primary breakage in the mill was investigated by replacing the set of rollers by a single, short roller. With accurately prepared √2 size intervals, the B-values of various size-intervals were determined at different operating conditions and were found to be nearly normalizable. Introducing the experimentally determined breakage parameters in a population balance model for multi-pass grinding in the mill, product size distributions were predicted. The predicted distributions fit the experimental values very well at a low rotational speed; at higher rotational speeds, there is some deviation, caused by briquetting of very fine particles into larger agglomerates.