Abstract: The shutoff device for closing a large cross-sectional area gas duct comprises a lock chamber defined by the gas duct walls and the two rows of swivel louvre grills held to the housing of the shutoff device which accommodates also a mechanical actuator, said louvre grills being arranged in the gas duct at an angle to the vertical plane in such a manner as to be spaced more widely apart in the top portion of the gas duct than in the bottom portion thereof. When the gas duct is being closed the lock chamber is filled with bulk free-flowing material from the top hopper located in the gas dust upper portion, whereas when the gas duct is being opened, the lock chamber is emptied of said bulk material, which is free to pour by gravity into the bottom receiving hopper, both of said hoppers being interconnected through a conveyer for the bulk material to return to the top hopper.