a and 16 b are made of metal preferably copper eventually with a ceramic coating the inner face of the panel 16 of course has to be able to withstand the contact with the slag and prevent the slag from adhering the lower part of the panel 16 is provided with metal elements preferably made of steel having the function of cleaning since they will act on the slag for the removal thereof and it is susceptible of carrying out a movement for pushing and moving away the slag which remain on the slagging door plane the lower panel 16 is capable of carrying out a composite movement similar to the one carried out by a dredger under the command of any hydraulic actuator for example a cylinder or a brake hydraulic motor however such an actuator is represented in the annexed drawings and it will be disclosed in the form of a central hydraulic cylinder 35 in an exemplary embodiment thereof the panel 16 is supported by an articulation lever parallelogram comprising a bracket 30 on which two levers are pivoted spaced to each other and of different length 32 and 34 the shorter lever 34 is pivoted on the slider 26 to which also the longer lever 32 is pivoted shifting the slider 26 along the upwards and outwards inclined guide 22 causes the lower panel 16 and consequently the parallelogram formed by the several levers to assume several positions in every position the panel 16 can be moved independently from the top panel 18 assuming closing cleaning and slagging positions when the lower panel 16 is subjected to a slag dredging movement the top panel 18 remains still in the closing position the scraping operation can be.