Shredded used printed paper for recycling is sorted in an air screening assembly into light and heavy fractions to improve the whiteness of the paper for deinking

摘要

To control the degree of whiteness of loose shredded material, especially used printed paper, the intensity of dust separation is increased of at least a part of the dust content. The material is separated into light and heavy fractions by air screening. At least a part of the dust is separated as the light fraction and the heavy fraction is separated as a loose fraction with increased whiteness. The light fraction additionally has a part of the newsprint fraction. The composition of the loose material is controlled to set the ratio of newsprint and magazine paper in the heavy fraction by the separation speed of the screening air flow. In at least one main separation stage, with a lateral screening, the light fraction is separated from the loose material and, in a second stage, a counterflow separates heavy particles from the light fraction, and the heavy fraction feed continues. The separation speed of the air flow, at the ejection edge of the main screening stage, is 1-3 m/sec. and especially 1.5-2.5 m/sec. The air flow in the following stage is slower by a factor of 1.1-1.8 and especially 1.2-1.6 and preferably 1.3-1.4. The initial air screening uses a cone screening unit to separate the feed material into light and heavy fractions. The composition of the feed material is 40-60 wt.% newsprint, = 3 wt.% impurities, and the remainder is magazine paper and graphic paper. An Independent claim is included for an air screening assembly with a main screening stage (10) and a secondary screening stage (22) with a cross section surface area larger than the cross section surface area of the main screening stage (10) by a factor of 1.1-1.8. Preferred Features: The secondary screening surface area is preferably larger than at the main stage by a factor of 1.2-1.6 and especially 1.3-1.4. The guide surface (12) is aligned downwards at a given angle ( alpha ) of 60-75 deg to the horizontal and preferably 60-70 deg , to lead the loose material to the ejection edge (14), or the angle is 65 deg . At least one additional screening stage is under the main screening stage (10), as a lateral screening unit (26). The guide surface (12) is shaped as an inflow base, at least in sections, with a channel fitted with a throttle valve to control the air flow through the inflow base.