I wish to investigate the differences in grinding of slag over clinker. We have a 100tph OPC cement ball mill. Could this mill grind slag? What modification would be needed. Is it always necessary to supply hot air when grinding slag? What sort percentage drop in production should be expected?

Yes, your ball mill can grind slag. If it was to be permanently converted to slag grinding then a change to a finer ball grading would be necessary. Slag is normally ground finer than cement and has a smaller grain size to begin with. Certainly there is a limit to the drying capacity of a mill based on the heat generated in the mill alone. To determine whether you need to add hot air you need to develop a heat balance of the mill and test various scenarios with different moisture contents of slag and feed rates. Slag is much harder than cement clinker and therefore you could expect as much as a 50 per cent reduction in output from the mill.

What is the best method to charge a ball mill ?

The normal method to charge balls into the ball mill is to position the doors at the top of the mill and remove them. Then to pour the barrels of grinding balls into the mill using an overhead crane. The exact grading of media required is identified by performing an axial test taking samples along the axis of mill. You want the desired fineness to be reached at the exit of the mill and adjust the ball grading to achieve this. For a new mill you need to rely on the experience of the commissioning engineer to determine the initial ball grading. It is normal practice to begin commissioning with only 65 per cent of the balls in the mill and bring this up to target by additions with monitoring between additions by axial testing.