A compressive grinder for a bed of materials, includes: a cylinder having a horizontal axis, and an inner wall having a raceway for a grinding roller arranged inside the cylinder; elements for rotating the cylinder about the axis; the grinding roller having an axis parallel to that of the cylinder; elements for urging the grinding roller against the raceway; an input for supplying material to be ground at one end of the cylinder and an outlet for ground material at the other end. The grinder includes: a smoothing roller upstream of the grinding roller in the cylinder's direction of rotation; elements for keeping the smoothing roller adjacent to and separated from the raceway, such that, upon each cylinder turn, the material consecutively passes under the smoothing roller and then under the grinding roller, the smoothing roller exerting, on the bed of materials, a pressure that smoothes the surface thereof.