the shutoff device shown comprises a housing 10 composed of two housing halves 10 a , 10 b which axially bear against each other and are screwed to each other . the housing halves are provided with bores aligned with each other and forming a passage 11 . at each end of the housing halves a flange 12 is provided for flange - mounting a conduit . between the housing halves an elongate cavity is formed in which a plate - shaped device 13 , which is provided with an opening 14 of approximately the same size as the passage 11 , is adapted to be displaced transversely to the axial direction of the housing . the housing 10 comprises , in the plane of the device 13 , two receiving spaces 15 , 16 extending to opposite sides , one of the receiving spaces being capable of accommodating one half of the device 13 while the other half is located within the housing 10 . at the outer end of the receiving space 16 a pneumatic cylinder 18 is fastened via rods 17 , said cylinder including a piston 19 . the pneumatic cylinder is controlled by a valve 20 which is connected to a compressed air source ( not shown ). to the piston 19 a piston rod 21 is connected which extends into the end of the receiving space 16 and is connected at its other end with the device 13 . the passage of the piston rod 21 through the end wall 22 of the receiving space 16 is sealed with the aid of a stuffing box packing 23 . in the present embodiment , the receiving spaces 15 , 16 are provided with shielding walls 24 to protect the walls of the shutoff device against heat radiation . fig1 shows the device 13 in its shutoff position in which the opening 14 is located in the receiving space 15 , while the housing 10 includes the closed plate region of the device 13 which shuts off the passage 11 . in this closed position the piston rod 21 is moved out of the pneumatic cylinder 18 . if the device 13 is to be moved into the open position , the piston 19 is retracted in the cylinder 18 such that the opening 14 of the device 13 extends coaxially to the passage 11 . the passage 11 between the housing 10 and the device 13 is sealed by two annular seals 25 , 26 each of which is included in one housing half 10 a , 10 b . for this purpose , each housing half comprises an annular fluid cylinder 27 ( fig3 ) which is configured as an axial groove in the housing half . each fluid cylinder 27 includes an annular rigid sealing body 28 which forms an annular piston . the annular pistons 28 surround the passage 11 on each side of the device 13 . each sealing body 28 comprises on its inside two sealing rings 29 and on its outside two sealing rings 30 adapted for sealing towards the pneumatic annular fluid cylinder 27 . at the outer end 31 a pressure line 32 and 33 , respectively , extends into each fluid cylinder 27 to advance the sealing body 28 towards the device 13 . each sealing body 28 comprises , on the side facing the device 13 , a contact surface 34 . this surface is narrower than the width of the sealing body and merges via an inclined shoulder 35 with an area 36 arranged somewhat to the rear . the contact surface 34 shaped as an annular disk is located at the sealing body 28 at the end adjacent to the the passage 11 , while the area 36 is located remote from the passage . the contact surface 34 is coated with a hard - material layer 37 which extends over a portion of the inclined shoulder 35 . the device 13 is on each of its two sides provided with two hard - material layers 38 , 39 ( fig5 ). these hard - material layers 38 , 39 are shaped as annular disks . one of the hard - material layers 38 is arranged around the opening 14 of the device 13 , and the other hard - material layer 39 is arranged around the closing surface of the device , which is located inside the passage 11 of the housing when the device is in the closed position , as shown in fig1 and 2 . the hard - material layers 37 , 38 , 39 are preferably made from ceramic material , such as chromium dioxide , or from hard metal , such as tungsten carbide . subsequent to their application , the layers are subjected to microfinishing , i . e . first to grinding and then to honing or lapping . the roughness amounts to max . ra 4 which corresponds to 4 μm . in particular , the roughness amounts to approximately ra 0 . 4 , i . e . 0 . 4 μm , and less . the width of the contact surface 34 amounts to less than 10 mm , and in particular to approximately 5 mm . the width should not be larger than required for producing the sealing action . microfinishing of the surfaces of the hard - material layers is carried out by lapping in pairs , wherein two hard - material layers are placed one upon the other and rubbed against each other . these two hard - material layers are those of the device 13 and the associated sealing body . those hard - material layers which are adapted to later sealingly bear against each other are finished by being rubbed against each other such that the hard - material layers fit together in pairs . pressure application to the sealing bodies 28 is effected from the cylinder 18 . from the control valve 20 a line 40 extends to the rear end of the cylinder 18 . this line serves for generating in the cylinder 18 a force acting upon the piston 19 , said force driving the device 13 into the closed position . the line 40 is connected via further lines 41 , 42 , 43 with a distributor 44 to which the lines 32 and 33 are connected . when pressure is applied to the cylinder 18 , first the device 13 is moved into the closed position , and subsequently the sealing bodies 28 are pressed against the device from opposite sides . this pressing is effected in a yielding manner . this means that the sealing bodies 28 follow any deformation or displacement of the device with the sealing action being maintained . another line 45 , which extends from the valve 20 into the front end of the cylinder 18 , serves for setting the piston 19 into the retraction position , whereby the device 13 is moved into the open position . the shutoff device according to the invention is suitable for use in conjunction with aggressive and abrasive media and for use at high temperatures . the edge of the opening 14 can be configured as a blade such that any foreign matter , which gets stuck when the device is in the open position , is cut up when the device is moved . such a blade need only be present at the half of the device opening 14 facing the cylinder 18 , while the other half may be of blunt configuration . although a preferred embodiment of the invention has been specifically illustrated and described herein , it is to be understood that minor variations may be made in the apparatus without departing from the spirit and scope of the invention , as defined by the appended claims .