Process for pulping and cleaning of fibrous material containing impurities in particular waste paper

摘要

For the repulping and cleaning of a paper fiber suspension (P1), and especially used paper for recycling, the overflow fraction (5) from the coarse matter screening (2) is taken off in a time duration of at least 80% of the operating time to give a cleaned suspension flow (4). The volume flow of the overflow fraction is set at a minimum of 15% of the inflow into the coarse screening stage and a maximum of 50%. The overflow fraction is passed to a further screening station (3) where waste matter is spun out intermittently as an overflow fraction (5') to give an additional cleaned suspension flow (4'). For repulping and cleaning a used paper fiber suspension, the overflow from the coarse screening stage is taken off until no impurities are spun out of the further screening stage. The cleaned suspension flow is taken off continuously from the coarse screening stage. The spinning out of impurities from the further screening is effected where the overflow into the stage is stopped and, in this time, a flushing flow (W) is pumped into the screening stage to carry out waste matter through a further overflow containing impurities. On spinning, all the waste matter is taken out of the further screening stage while the outflow of cleaned suspension is halted. The two flows of cleaned suspension are brought together. The overflow from the second screening stage is passed to a sorting drum (8), where a reject fraction (11) is formed as an overflow, and the cleaned flow from the drum is returned to the repulping assembly. The overflow from the second screening stage is thickened in a press. The overflow volume from the coarse screening stage is set at 20-30% of the inflow volume into the screening stage. The volume of fibers in the overflow from the second screening stage is set at 20-30% of the fiber content of the inflow to the second screening. The coarse screening uses a sieve with openings in a maximum width of 4 mm. It is a flat sieve, cleared by a moving sieve cleaner. The soiled fiber suspension can be fed centrally into the coarse screening stage, where heavy waste is separated by a heavy particle centrifuge. Between the pulper and the coarse screening stage either no impurities are removed or only coarse particles are extracted which would be rejected by a sieve with openings of maximum 30 mm diameter, separated by a rotating sieve drum with openings of a maximum 30 mm diameter. The repulping stage has a drum, with a screening unit as an axial extension of it. The pulping drum is operated to generate a constant flow of pulp (P), to be diluted by water (W') into a soiled pulp flow. The pulper is a high consistency pulper (1), operated intermittently, with its outflow diluted with water into the required consistency for the coarse screening stage. The inner volume of the coarse screening unit, at the feed side, is at least 15% of the volume of the outflow which would fill the machine in 1 min. The inner volume of the feed side of the second screening stage is at least 30% of the volume of the outflow which would fill this machine in 1 min.