A rotatable drum, which bears beater bars and is intended for use in pulverizing metallurgical slag, is defined by abutting conduits which are welded to one another and to a tubular support. The arrays of conduits, through which a coolant is circulated, is interrupted at a number of locations spaced radially about the support where radially outwardly extending beater bars are welded to the same support. The beater bars are also provided with internal passages through which the coolant is circulated.