Recycling and processing of ash from incineration of waste paper and deinking sludge far paper filler

Filler residues from the incineration of deinking sludge or paper can in pr inciple be reused provided it could be cleaned to a level meeting the quali ty requirements set for paper fillers. Ash formed in incineration contained too much impurity (primarily soot) to be suitable for use as paper filler as such. The best brightness values measured for ashes from the incineratio n of deinking sludge were lower than the acceptable brightness level, i.e., 70 (ISO). Recycled paper was burned in a pilot-scale fluidised bed yieldin g ash containing a small amount of unburned carbon, and as such unsuitable for filler. The ash was brightened by calcining in a rotary kiln. This prov ed to be a promising technique for brightening ash, as the treatment time w as sufficiently long (20-30 min at 820 degreesC). However, it was not possi ble to brighten ash sufficiently in the rotary kiln. This was due to a comb ined effect of several factors, which was observed in separate tests: newsp aper ash was darker than that of periodicals, small iron particles reduced the brightness; calcining reduced occasionally the brightness, and pure fil ler also darkened slightly in calcining. Depending on the particle size of soot in the ash, even small amounts may drop the brightness level below the acceptable limit. Removal of soot by flotation from the ashes did not resu lt in an acceptable brightness level, abt. 50. Carbonisation (carbonate pre cipitation), however, increased the brightness level to > 70. Sheet tests with the brightest ash and with the post-ground ash indicated t hat the strength properties of the sheet were not significantly inferior to those where commercial carbonate chalk was used as filler. However, the br ightness of the sheet with 9% ash addition was by 6 units lower and the lig ht-scattering coefficient respectively 9 percentage units weaker. With 5% a sh addition these property reductions due to ash use were fairly small and perhaps acceptable. Based on the laboratory-scale experiments performed in this project, furthe r research will focus on large-scale posttreatment of ash and on factors af fecting critical quality properties, as well as on practical paper producti on trials.