Overflow Versus Grate Discharge Ball Mills: An Experimental Investigation

It has been widely accepted to go for grate discharge ball milling when a coarse product is preferred or to avoid production of extreme fines, but overflow discharge mills are preferred for their simplicity of design and production of finer product. In grate discharge mills the product is discharged primarily due to hydraulic gradient from the feed end to discharge end. Hence there is a possibility for much quicker transport of material through the mill, which is being accomplished in overflow mills by higher circulating load.

Recent developments in understanding the importance of material transport in performance improvement of AG/SAG mills and the limited research work on comparing discharge mechanisms in balls mills has led to the present investigation. A set of experimental test work has been carried out in a continuous pilot ball mill of 0.41m X 0.56 m (diameter X length) at different mill speeds, percent solids and discharge mechanism. The results of study on material flow through a grate discharge tumbling mill verses an overflow tumbling mill are presented to emphasize the effect of discharge flow mechanism. The experimental results are analyzed to study the influence of operating variables on specific energy, product size, hold-up and residence time distribution in tumbling mills. This manuscript lays the principal themes to harness the advantages of grate mills.