Apparatus is provided for reprocessing old paper in a series of flotation cells arranged one behind another, the suspension discharge 11 from the front cell being connected to the suspension inlet 8 of the following cell, each cell being provided with a stirring impeller 3, an air infeed device 9 and a foam outlet opening 12. The cells are provided by a substantially tubular body 1 which is horizontal or slopes gradually downward towards the rear, the tubular body being divided into a plurality of sections by partition walls 2 extending substantially vertically. Each section is provided with a suspension inlet 8, a suspension discharge outlet 11, a foam outlet opening 12, a stirring impeller 3, and an air infeed device 9.