Slurry containing solid to be "extracted" is introduced to the top of a tower (1), the Fig. (not shown) at (3) and extracting liquid to the bottom of the tower at (7). Desired extract is withdrawn through an annular filter screen (2) (surrounding the tower near its top) to a tank (5). At intervals, and for a short space of time, outlet valve (9) from tank (5) is closed and the contents of tank (5) are discharged back through screen (2) to remove filter cake. At the same time a large amount of "extracted" solids and liquid is discharged from the tower base. Instead of, or together with, discharge of solids from the tower base, a large amount of slurry may be introduced at (3). Preferably the liquid part of the slurry is extract previously withdrawn. The slurry may be fibres from a cellulose digestion process suspended in waste liquor; or beet chips suspended in unrefined juice, or sand mixed with oil (extracting liquid; solvent). Bone meal or ores may be leached with water or dilute acids; filter agents such as diatomaceous earth may be washed.