The invention relates to a device for preventing spills from inside mining mills, said device maintaining a seal around bolts used to secure mill liners, such that the bolt remains under tension within a range of several millimetres along the length between the head and the nut of the bolt, with the length covered by the elements compressed between the head of the bolt and the device for preventing pulp leaks in mills, exerting pressure on the washer that provides a seal against leaks from inside the mill. The device comprises one or more pairs of Belleville washers which include an annular space into which a sleeve with an inner seal is inserted. Said sleeve includes a flange of larger diameter to allow the action of the bolt on the Belleville washers, said Belleville washers exerting pressure on the washer that provides a seal against leaks from inside the mill. The length of the sleeve is greater than or equal to the total thickness of the Belleville washers at maximum compression, and the flange of the sleeve is only required if the diameter inscribed on the contact surface with the bolt is smaller than the annular diameter of the Belleville washers.