A device in a drilling-mud container (1) including bulkheads of plates (4, 5) provided with vertical and horizontal profiles (2, 3) forming a frame (2), the drilling-mud container (1) being provided with at least one sliding lid (8) which is arranged to be displaced in a sliding track (22) by means of at least one opening mechanism (15) independently of the position of a second hatch (7), wherein a toothed wheel (19), which is rotatably arranged in the drilling-mud container (1), is in engagement with a pitch rack (21) arranged in the sliding lid (8), and the pitch rack (21) is provided with an end portion (21a) which is inclined in the direction of the toothed wheel (19), the abutment of the end portion (21a) against the toothed wheel (19) causing the sliding lid (8) to be displaced in a vertical direction into sealing abutment against an abutment portion (22) on the drilling-mud container (1).