The transformation of paper recovered in a fibre suspension, the separation of ink and adhesives from the fibres (this stage often does not occur in the preparation of the stuff for the manufacture of cardboard), as well as the breaking up of their particles, take place in the hydropulper as a result of mechanical effects as well as chemicals and thermal energy. Thus, re-pulping is particularly important for shaping the properties of the pulp, which is then introduced into the subsequent stages of the process, deciding on its proper de-inking.
Re-pulping of waste paper can also be a key stage for effective removal of sticky contaminants during the successive stages of the de-inking process, thanks to the application of re-pulping technology, using appropriately selected surfactants (agglomerates) to shape the size of sticky particles and the hydrophobic character of their surface, leading to the agglomeration of the resulting stickies and their coagglomeration with ink particles and other contaminants, during re-pulping carried out while maintaining the laminar pulp flow in the hydropulper. The surfactant/active substance mixtures include oleic acid, which is essential in the de-inking process in an alkaline environment.
Many inventions have been shown in an attempt to increase the mass concentration in the pulper tub. This reduced investment costs for the construction of new installations and reduced operating costs. For the most commonly used devices used for re-pulping, there are rotary pulpers, which can operate at low (3-8%), medium (10-12%) and high (15-18%) concentration. Depending on the concentration, pulpers differ in rotor construction and operating principle. Low and medium concentration pulpers are equipped with conventional rotors and the rotor operation is mainly based on the use of hydraulic effects (breaking up using the properties of the medium, associated with the presence of tangential stresses in the flow). The solutions operating at high mass concentration are equipped with a helical rotor mating with numerous guide blades located at the bottom of the tub (breaking up using quasi-cutting). An alternative that allows a similar increase in concentration with the use of a traditional rotor is the use of the Sulzer Papertec Bi-Pulper with a suspended screw rotor. In this pulper, a traditional rotor is located in the bottom of the tub. It provides high re-pulping intensity thanks to its high peripheral speed (18-21m/s), while the transport of dense bulk in the tub is supported by a large, slow-running screw rotor suspended articulated above the tub. Increasing the stuff concentration in the pulper from 4% to 7-8% reduces energy consumption from 30-50%, which also means that the pulper performance is doubled for the same tub size. The rotors adapted to operate at medium concentrations were then modified so that they can operate even at 18% concentration. The use of a helical rotor ensures increased shear stress at a lower rotor speed. These conditions are particularly favourable for the separation of ink from the fibres during the de-inking process and, thanks to the gentle re-pulping, reduce the breaking up of contaminants such as laminate films and reduce the shortening of fibres.
