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

摘要

Siistauslietteen tai paperin poltossa jäljelle jäävä lietteen sisältämä täyteaine (tuhka) voidaan periaatteessa käyttää uudestaan, mutta ongelmana on sen puhdistaminen ja jatkokäsittelyt (jauhaminen, pH:n säätö jne.), niin että se täyttää paperin täyteaineille asetetut laatuvaatimukset. Polttoproses-sissa syntyvä tuhka sisälsi liian paljon epäpuhtauksia (lähinnä nokea), jotta sitä voitaisiin käyttää sellaisenaan paperin täyteaineena. Siistauslietteen polton tuhkilla mitatut vaaleustasot olivat parhaimmillaankin alempia kuin siedettävä vaaleustaso, ts. 70 (ISO). Vaaleuden laskun syinä olivat noen määrä, tuhka-aineksen muuntuminen korkeissa lämpötiloissa jne. Noen raekoosta riippuen pienetkin määrät riittävät pudottamaan vaaleustason yllämainitun alapuolelle. Tuhkan kuluttavuus oli myös suuri.%Filler residues from the incineration of deinking sludge or paper can in principle be reused provided it could be cleaned to a level meeting the quality 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 incineration of deinking sludge were lower than the acceptable brightness level, i.e., 70 (ISO). Recycled paper was burned in a pilot-scale fluidised bed yielding ash containing a small amount of un-burned carbon, and as such unsuitable for filler. The ash was brightened by calcining in a rotary kiln. This proved to be a promising technique for brightening ash, as the treatment time was sufficiently long (20-30 min at 820℃). However, it was not possible to brighten ash sufficiently in the rotary kiln. This was due to a combined effect of several factors, which was observed in separate tests: newspaper ash was darker than that of periodicals, small iron particles reduced the brightness; calcining reduced occasionally the brightness, and pure filler 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 result in an acceptable brightness level, abt. 50. Carbonisation (carbonate precipitation), however, increased the brightness level to > 70. Sheet tests with the brightest ash and with the post-ground ash indicated that the strength properties of the sheet were not significantly inferior to those where commercial carbonate chalk was used as filler. However, the brightness of the sheet with 9% ash addition was by 6 units lower and the light-scattering coefficient respectively 9 percentage units weaker. With 5% ash addition these property reductions due to ash use were fairly small and perhaps accepta-ble. Based on the laboratory-scale experiments performed in this project, further research will focus on large-scale post-treatment of ash and on factors affecting critical quality properties, as well as on practical paper production trials.