a filter apparatus including a housing having a discharge apparatus for removing granular gas treating material from the hopper of the housing . the discharge apparatus includes a baffle plate extending across the hopper between the side walls of the housing which defines a material discharge channel opening into the interior of the hopper through a pair of parallel slots extending along the edges of the baffle plate , a conveying gas inlet aperture in one of the side walls opening into one end of the conveying channel , and a gas treating and conveying gas outlet aperture in the other side wall opening into the other end of the conveying channel of a larger cross - sectional area than the conveying gas inlet aperture to promote progressive aspiration of the treating material from the interior of the hopper commencing at the end of the channel proximate the conveying gas inlet aperture .