shaft 19 distributed over the cross section area of the container the treatment liquid can also be distributed in other ways such as e g known from continuous digesters by adding treatment liquid through a central pipe in the container and flowing radially outwards to separation strainers in the container wall the quantity of treatment liquid added is balanced with liquid entering and leaving together with the fiber material and liquid added for dilution as well as liquid extracted in such a quantity that treatment liquid is moving upwards counter currently to the fiber material at a sufficiently low velocity so controlled that said fiber material will not be carried upwards and furthermore the outlet 16 for discharge of liquid is arranged at a level so high above the fiber material level and thereby above the inverted funnel lower end e g at a height not less than one fifth of the container diameter that the liquid chamber 42 gets sufficient cross section and volume to permit eventual fine particles of fiber material in the liquid to sink downwards settle and then follow the other fiber material down through the container i e the upflow liquid velocity must be lower than the sinking velocity of the fiber material in this respect also the cross section area of the inverted funnel 14 has to be taken in consideration preferably it.