Apparatus for cleaning contaminated solid-liquid mixtures, comprising: • at least one first pipeline for supplying a liquid, with preference water, • at least one second pipeline for supplying a solid-liquid mixture, with preference a fibrous suspension, with particular preference a paper fibre suspension, • at least one third pipeline, which connects the first and second pipelines (2, 3) and comprises at least one mixing element, which operates according to the Venturi principle, • wherein the mixing element has two oppositely directed cones (7, 11), which each open with their small through-opening into a chamber forming a cavity, while causing a jump in the size of the cross section of the through-opening, wherein the region of the opening into the chamber forming a cavity is designed such that the small through-opening (12, 13) of each of the cones (7, 11) is continued in a non-conical section of line (8, 10) in a way corresponding to the cross-sectional area of the small through-openings (12, 13), with these non-conical sections of line (8, 10) opening into said chamber between the cones (7, 11) at the locations where there is a jump in the size of the cross section of the through-opening, the large through-opening of the first cone is connected via the third pipeline to the first pipeline and the large through-opening of the second cone is connected via the third pipeline to the second pipeline, the small through-opening of the first cone is formed between 20 and 40% smaller than the small through-opening of the second cone and the two oppositely directed cones (7, 11) and the non-conical sections of line are arranged in longitudinal axial alignment, • at least one fourth pipeline for conducting a gas, preferably air, wherein the fourth pipeline opens into a chamber of the mixing element forming a cavity, and • at least one fractionator arranged downstream of the second pipeline for removing the foam produced by flotation. • at least one first pipeline for supplying a liquid, with preference water, • at least one second pipeline for supplying a solid-liquid mixture, with preference a fibrous suspension, with particular preference a paper fibre suspension, • at least one third pipeline, which connects the first and second pipelines (2, 3) and comprises at least one mixing element, which operates according to the Venturi principle, • wherein the mixing element has two oppositely directed cones (7, 11), which each open with their small through-opening into a chamber forming a cavity, while causing a jump in the size of the cross section of the through-opening, wherein the region of the opening into the chamber forming a cavity is designed such that the small through-opening (12, 13) of each of the cones (7, 11) is continued in a non-conical section of line (8, 10) in a way corresponding to the cross-sectional area of the small through-openings (12, 13), with these non-conical sections of line (8, 10) opening into said chamber between the cones (7, 11) at the locations where there is a jump in the size of the cross section of the through-opening, the large through-opening of the first cone is connected via the third pipeline to the first pipeline and the large through-opening of the second cone is connected via the third pipeline to the second pipeline, the small through-opening of the first cone is formed between 20 and 40% smaller than the small through-opening of the second cone and the two oppositely directed cones (7, 11) and the non-conical sections of line are arranged in longitudinal axial alignment, • at least one fourth pipeline for conducting a gas, preferably air, wherein the fourth pipeline opens into a chamber of the mixing element forming a cavity, and • at least one fractionator arranged downstream of the second pipeline for removing the foam produced by flotation.