How To Grinding The Pet Coke

how to grind pet coke in vertical roller mill designed for normal coal.Dear experts, we are having loshe mill lm26-3 for coal grinding, now we want to grind pet coke from this mill with a residue of 2% at 90 mic mesh.What changes we have to do or is it possible to reduce the table speed and get the desired fineness and what is the optimum speed pl.Write.