To prevent a roll part from being scratched so as to improve durability by mounting a rigid sheet with curvable elasticity at a lower discharge port of a hopper from both side edges of the discharge port so as to be in contact with the lower half part of a rugged roll, and boring a large number of sand grain discharge holes in the bottom face of the rigid sheet.

A rigid sheet 15 with elasticity curvable in contact with the tips of protrusions 8 of a rugged roll is mounted at a lower discharge port 7 of a hopper 6 from both side edges 7A, 7A of the discharge port 7 so as to come in contact with the lower half part of the rugged roll 9. The rigid sheet 15 is provided with a large number of sand grain discharge holes bored in the bottom face being arranged in the axial direction of the rugged roll 9. As the concrete examples of the rigid sheet 15, there are a sheet formed by cutting a steel belt into specified length, a sheet formed by cutting a conveyor belt formed by laminating rubberized cloth in multiple layers and embedding a tensile strength core inside, into specified dimensions, a sheet formed by cutting a resin belt formed by integrally laminating reinforcing cloth impregnated with wear resistant resin such as urethane, into specified dimensions, and the like.