An apparatus for the production of melamine which comprises a reaction section and a scrubbing section, wherein: - said reaction section comprises at least one reactor and at least one post-reactor; - said scrubbing section is suitable for scrubbing the gas phase coming from the reactor with an aqueous solution or is suitable for scrubbing the gas phase coming from the reactor with urea; - said reactor is connected to the scrubbing section through a means suitable for transferring the gas phase from the reactor to the scrubbing section; - said post-reactor is connected through a means suitable for transferring the gas phase from the post-reactor to sections downstream; - said means T1 and T2 are distinct, or communicating with at least one shut-off valve positioned between the intersection point between said means and the outlet duct of the gas phase from the reactor and the intersection point between said means and the outlet duct of the gas phase from the post-reactor; - said apparatus comprises two separate pressure-controller systems (PC1/CV1, PC2/CV2) suitable for controlling the pressure inside the reactor and inside the post-reactor respectively, each comprising a shut-off element (CV1, CV2) and a pressure-control means (PC1, PC2); - said reactor and post-reactor are connected through a means suitable for transferring the melamine from said reactor to said post-reactor, said means connecting the upper portion of the reactor with the lower portion of the post-reactor and entering said lower portion, preferably near the bottom tangent line, of the post-reactor by means of an internal hydraulic seal; - said hydraulic seal is positioned inside the post-reactor so as to reach the upper portion of the post-reactor and including a spillway point positioned at a height that corresponds to the midpoint between the position of the outlet nozzle of the liquid phase from the reactor and the position of the inlet nozzle of the liquid phase into the post-reactor plus or minus 20%, preferably plus or minus 10% of said height; - said post-reactor is positioned in said section below the reactor, said position being such that the position of the spillway point of the hydraulic seal inside the post-reactor is positioned at a height which corresponds to the midpoint between the position of the outlet nozzle of the liquid phase from the reactor and the position of the inlet nozzle of the liquid phase into the post-reactor plus or minus 20%, preferably plus or minus 10% of said height. - said reaction section comprises at least one reactor and at least one post-reactor; - said scrubbing section is suitable for scrubbing the gas phase coming from the reactor with an aqueous solution or is suitable for scrubbing the gas phase coming from the reactor with urea; - said reactor is connected to the scrubbing section through a means suitable for transferring the gas phase from the reactor to the scrubbing section; - said post-reactor is connected through a means suitable for transferring the gas phase from the post-reactor to sections downstream; - said means T1 and T2 are distinct, or communicating with at least one shut-off valve positioned between the intersection point between said means and the outlet duct of the gas phase from the reactor and the intersection point between said means and the outlet duct of the gas phase from the post-reactor; - said apparatus comprises two separate pressure-controller systems (PC1/CV1, PC2/CV2) suitable for controlling the pressure inside the reactor and inside the post-reactor respectively, each comprising a shut-off element (CV1, CV2) and a pressure-control means (PC1, PC2); - said reactor and post-reactor are connected through a means suitable for transferring the melamine from said reactor to said post-reactor, said means connecting the upper portion of the reactor with the lower portion of the post-reactor and entering said lower portion, preferably near the bottom tangent line, of the post-reactor by means of an internal hydraulic seal; - said hydraulic seal is positioned inside the post-reactor so as to reach the upper portion of the post-reactor and including a spillway point positioned at a height that corresponds to the midpoint between the position of the outlet nozzle of the liquid phase from the reactor and the position of the inlet nozzle of the liquid phase into the post-reactor plus or minus 20%, preferably plus or minus 10% of said height; - said post-reactor is positioned in said section below the reactor, said position being such that the position of the spillway point of the hydraulic seal inside the post-reactor is positioned at a height which corresponds to the midpoint between the position of the outlet nozzle of the liquid phase from the reactor and the position of the inlet nozzle of the liquid phase into the post-reactor plus or minus 20%, preferably plus or minus 10% of said height.