A cylindrical tank with a conical bottom. It contains a pipe that is coaxial with the leaching tank and open at both ends; compressed air is introduced at the lower end of this pipe, which behaves as an air lift. The density of the pulp within the pipe is less than that of the pulp surrounding it because of the column of air bubbles contained in the pipe, and the pressure of the denser pulp causes the pulp in the central pipe to rise and overflow, thus circulating the entire charge.