PREPARATION AND BLEACHING OF MIXED WASTE PAPERS FOR THEIR USE IN THE PRODUCTION OF HIGH-GRADE WRITINGS AND PRINTINGS

Investigations were carried out on two lower grade waste paper types. The first sample had a whiteness of 63.3% ISO and consisted of 40% news and 60% magazines (pams). It had been pulped at pilot plant scale with addition of 1% peroxide. Bleaching with 3% peroxide only produced a minor rise of whiteness. The furnish was therefore pre-treated under acid conditions at 90-degrees-C, in order to remove heavy metals and to deactivate catalysts present in the material. This treatment was followed by an oxygen bleach (O/P), strengthened by 1% peroxide. The greatest rise in whiteness by 5 points was achieved after acidification to 2.5 pH with subsequent replacement washing. Whiteness of 74 and respectively 75% ISO could be reached with a final bleaching with FAS or peroxide and FAS. As a second raw material an industrially prepared waste paper pulp was used, consisting of 80% news and 20% magazines (pams). By consequent flotation in the laboratory the whiteness could be raised from 57 to 59% ISO. After acid pre-treatment the whiteness was increased to over 74% ISO by a three-stage bleaching (O/P-P-FAS). Removal of the fines portion by fractionation led to a rise in final whiteness to 78% ISO and had a highly positive effect on the strength properties of the bleached material.