A cylindrical vertical tub from 3-1/2 to 6 ft (1.07 to 1.83 m) in diameter, with screenguarded peripheral apertures through which ore pulp can be discharged after passing through the comminuting zone. Grinding is done by four rolling mullers, which hang inside from a yoke and which press outward when rotating, thus bearing an ore caught between them and the inner wall of the tub.