Stepped screens

Claim 1

A SEPARATOR FOR PULVERULENT MATERIAL, COMPRISING: A BASE MEMBER; A FRAME RESILIENTLY MOUNTED ON SAID BASE MEMBER AND SPACED THEREABOVE; SAID FRAME HAVING A PAIR OF UPSTANDING SPACED PARALLEL WALLS AT EITHER SIDE THEREOF AND EXTENDING ALONG THE LONGITUDINAL EXTENT THEREOF; A PLURALITY OF SERIALLY ARRANGED GENERALLY PARALLEL SPACED INCLINED PLATES SECURED AT EACH END TO SAID WALL MEMBERS AND GENERALLY PERPENDICULAR THERETO; A SCREEN MOUNTED IN EACH PLATE; A SUPPLY SOURCE OF SAID MATERIAL WITH OPENINGS THEREIN OVERLYING THE FIRST OF SAID SCREENS FOR SUPPLYING MATERIAL THERETO; A RETAINING WALL ABOUT THE SCREENS WITH AN OPENING AT THE UPPER END THEREOF FACING IN THE DIRECTION OF PARTICLE MOVEMENT; A TROUGH IN SAID FRAME BELOW SAID SCREENS FOR RECEIVING MATERIAL FILTERED THERETHROUGH; AGITATING MEANS ON EACH PLATE FOR AGITATING SAID SCREENS TO PREVENT AGGLOMERATION OF PARTICLES OF SAID MATERIAL; MEANS FOR VIBRATING SAID FRAME IN AN INCLINED PATH GENERALLY IN THE DIRECTION OF DESIRED PARTICLE MOVEMENT TO SUCCESSIVELY ADVANCE SOME PARTICLES OF SAID MATERIAL ACROSS SAID SCREENS IN SERIES, SAID MEANS FOR VIBRATING BEING MOUNTED ON THE BASE MEMBER AND OPERATIVELY ASSOCIATED WITH SAID FRAME MEMBER; THE VIBRATION OF SAID FRAME SERVING THE FINER PARTS OF SAID MATERIAL FROM THE COARSER PARTS THEREOF SO THAT THE FINER PARTS ARE FILTERED THROUGH SAID SCREENS AND DEPOSITED ON SAID TROUGH AND SAID COARSER PARTS ARE CONTINUALLY ADVANCED ACROSS SAID SCREENS WHERERBY SAID FINER AND COARSER PARTS ARE ADVANCED AT DIFFERENT LEVELS TO THE DISCHARGE END OF SAID SEPARATOR.