Influence Of Lime Sludge Fromalkali Recovery On AKD Sizing And Its Hydrophobic Modification

Lime sludge is a by-product of recovering alkali from the non-wood pulping process.The usual treatment of it ending up in landfills not only causes environmental pollution, butalso a waste of resources. So the best way is used as paper fillers in the circulation ofpaper-making process. However, one of the biggest problems when lime sludge used as fillersis difficult to size. So even pre-refined lime is unable to replace commercial calciumcarbonate. And the current studies hold different conjectures about the mechanisms of papersizing degree dropping with fillers. In this paper, the lime sludge from the non-wood pulpingprocess was raw material, and the reasons about the influence of lime sludge as filler for AKDsizing were investigated, such as particle size distribution, the technology of AKD sizing. Atthe same time, the method about hydrophobic modification with cationic styrene-acrylic esterwas proposed to improve the usability of lime sludge.First, the influence of particle size distribution of lime sludge on sizing was studied. Theresults showed that a modified physical and chemical surface property of fillers includinglime sludge, PCC and GCC were achieved by fitting their particle size distributions. Forexample, the zeta potential, specific surface area, and total pore volume of fillers werechanged significantly, which indicated the potential of managing the particle compositions tolessen their destruction in paper sizing performance. Furthermore, a paper sheet filled withlime sludge with a modified particle size distribution displayed a good sizing effect due to itslower adsorption for AKD sizing agent, and a higher retention rate can also be achieved bymatching the particle size distribution of fillers.Then, in the AKD sizing process, it was more helpful for AKD sizing when lime sludgewas added into the furnish after AKD than before AKD. At the same time, when the time ofAKD stayed in the wet end was2.5minutes, the dosage of chemithermomechanical pulping,the reed pulp, the softwood pulp were respectively50%,30%,20%, the dosage of cationicstarch, CPAM, bentonite in the retention system were respectively0.75%,0.043%,0.5%, thesizing degree could get the best and the filler retention also reached a higher level. After the temperature of drying cylinders got80℃when lime sludge was used as fillers, the sizingdegree would become stable.In addition, the experiment also paid attention to the impact of hydrophobic modificationon AKD sizing with styrene-acrylic ester. The study found the best modification conditionswhich was showing below: the pH value was around7.0, the temperature was80℃, thesoaking time was controlled less than15min, the mechanical stirring speed was around325r/min, and the dosage of SAE was1.54%(to filler). These conditions can guarantee thepaper sheet to get a good sizing property, and the modification cost was controlled in properrange. At the same time, the sizing degree and filler retention of filled paper with modifiedlime sludge were close to the paper sheet filled with PCC. But the sizing degree improved56.51%compared with paper filled with lime sludge, and increased38.08%in contrast withpaper sheet with lime sludge+SAE.