Bottom plate assembly comprising: (A) a collector nozzle comprising: (a) an upstream surface and a downstream surface joined to one another by a lateral surface , and comprising a bore extending along a longitudinal axis (Z) from the upstream surface to the downstream surface, (b) N protrusions , with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising an upper surface which is adjacent to the upstream surface of the collector nozzle and a lower surface separated from the upper surface by a height of the protrusion, and having an azimuthal width (W) measured normal to the longitudinal axis (Z) (B) a frame comprising a gate plate receiving unit for receiving a lower gate plate , and (C) a nozzle coupling unit for receiving and rigidly coupling the collector nozzle to the frame, said nozzle coupling unit comprising a nozzle receiving bushing rigidly fixed to the frame, characterized in that, the nozzle coupling unit further comprises a bayonet ring comprising an upstream edge and a downstream edge separated by a height of the bayonet ring, which is permanently and rotatably mounted in the nozzle receiving bushing such that the bayonet ring can rotate about the longitudinal axis (Z) and wherein the bayonet ring comprises an inner surface provided with N channels extending along the longitudinal axis (Z) from the downstream edge to the upstream edge, wherein the N channels have a downstream width (Wd) at the level of the downstream edge which is slightly larger than the azimuthal width (W) of the protrusions , in that the downstream width, Wd, should be sufficiently larger than W to allow the protrusions to move along the downstream end of the channels, and sufficiently narrow to guide the protrusions towards the corresponding protrusion mating structures, allowing the translation along the longitudinal axis (Z) of the collector nozzle through the downstream edge of the bayonet ring with the protrusions engaged in corresponding channels until they contact the corresponding protrusion mating structures, and wherein the N channels have an upstream width (Wu) at the level of the upstream edge which is larger than the downstream width (Wd) thus allowing the rotation of the bayonet ring about the longitudinal axis (Z) with respect to the collector nozzle until an edge of the channel contacts the lower surface of the corresponding protrusion, thus locking the collector nozzle in an operating position. (A) a collector nozzle comprising: (a) an upstream surface and a downstream surface joined to one another by a lateral surface , and comprising a bore extending along a longitudinal axis (Z) from the upstream surface to the downstream surface, (b) N protrusions , with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising an upper surface which is adjacent to the upstream surface of the collector nozzle and a lower surface separated from the upper surface by a height of the protrusion, and having an azimuthal width (W) measured normal to the longitudinal axis (Z) (a) an upstream surface and a downstream surface joined to one another by a lateral surface , and comprising a bore extending along a longitudinal axis (Z) from the upstream surface to the downstream surface, (b) N protrusions , with N ≥ 2, distributed around a perimeter of the lateral surface, each protrusion comprising an upper surface which is adjacent to the upstream surface of the collector nozzle and a lower surface separated from the upper surface by a height of the protrusion, and having an azimuthal width (W) measured normal to the longitudinal axis (Z) (B) a frame comprising a gate plate receiving unit for receiving a lower gate plate , and (C) a nozzle coupling unit for receiving and rigidly coupling the collector nozzle to the frame, said nozzle coupling unit comprising a nozzle receiving bushing rigidly fixed to the frame, <EOS>