Liner material can be the same in conical mills. The only advantage of conical mills is to achieve "classification" of the ball charge along the mill axis; with the large balls at the mill feed (for the coarse feed) and the small balls at the mill discharge. Today, these mills are obsolete since you can install in tube mills "classifying liners" that can do the same (contact Magotteaux for these liners).
For your application with 40mm balls, you can install high chromium steel liners with very good resistance to wear (contact Magotteaux) or rubber liners (contact Trelleborg or Skega). With rubber however, make sure that the lifter bars are not too agressive to avoid problems with the balls. In all cases, no rubber with high chromium balls (ok if forged balls)
Hope it helps