Development of a power-draw model for estimation of the dynamic recirculating load of swing hammer mills with internal classifiers

Abstract

A common problem that has been found in modelling of the swing hammer mill with an internal classifier has been that measurement of the dynamic recirculating load is not possible, particularly in full-scale industrial operations. With only the mill feed and product data it has not been possible to find unique parameters for the size-reduction and classification units of the hammermill models. This has made it difficult to establish unambiguous relationships between the fitted model parameters and the operating conditions for simulations and scale-up. Detailed experiments have now been carried out in which a pilot-scale swing hammer mill with various under-screen configurations was used to treat coal. One of the objectives was to understand the relationship between the hammer-mill power draw and the mill operating conditions. Analysis of the data revealed that the net power draw increased significantly with the use of under-screens, implying that the power data are not only a function of the new feed rate but also a function of the internal recirculation. A mechanistic model for swing hammer-mill power draw has been developed using Bernoulli's equation. The model assumes that the net power consumed by the hammer mill is used to move the fluidized coal, which comprises new feed and the recirculating load. The model parameters are calibrated to the machine configuration. This approach allows the dynamic recirculating load to be estimated from the measured new feed rate and the mill net power draw. The effects of under-screen configurations and the feed-size distributions on the mill recirculation were thus established.