An apparatus for producing high density slurry and paste backfills for use, for example, in mining operations. The high density slurry or paste is produced from a mill tailings mixture in a silo which includes a percolation and decant apparatuses for percolating water out of the mill tailings mixture and for decanting clarified water from atop the settled tailings. While settling occurs, air is introduced from the bottom of the silo to agitate the mixture to ensure substantially homogeneous settlement of the solids. Once settled, the resultant high density slurry or paste is fluidized by air in order to give the paste more readily flowable characteristics.