Atmospheric diffuser improved lower screens backflushing

Abstract

In an atmospheric diffuser for the treatment of paper pulp (thickening, bleaching, and/or washing) the flow of backflushing liquid is controlled so as to maximize the effective treatment of the pulp. The pulp passes upwardly in a vessel. A number of annular screen assemblies are provided, with at least one screen associated with each screen assembly. The lower screen includes a barrier wall with a passage in the barrier wall adjacent the bottom end of the screen. Liquid which flows through the screen must flow to the passage in order to be withdrawn from the vessel, as the screen is slowly moved upwardly. When the screen is rapidly moved downwardly to cause backflushing, the backflushing liquid at the top end of the screen is liquid that recently passed through the screen at that point, so the pulp at that point --which is the cleanest along the length of the screen--has a minimal chance of being contaminated. The barrier may be provided by a number of tubes separated by webs, or a number of channels separated by webs. A baffle may be mounted between each passage and screen to redirect the backflushing liquid so that it does not merely backflush the bottom portion of the screen.