| Mineral filler has been widely used in paper industry because of theadvantages in improvement of paper properties, cost reduction and energy saving.In recent years, the development of filler engineering becomes an urgent need forthe papermaking industry, especially for the new filler with low cost andenvironmental-friendliness. China produces a large amount of fly ash but the rateof utilization is relatively low, which result in undesired environmentalconsequences and health concern. Therefore, how to increase the utilization offly ash with added value has become a key focus for China, even for the world.As a solid waste in power plant, fly ash can be used as paper filler. However, itslow brightness and large particle size limit its potential applications in paperindustry. In this work, a novel fly ash based calcium silicate (FACS), which is abyproduct of aluminum extraction from fly ash, is employed as a potential paperfiller. The filler characteristics, chemical composition, morphology, wet endperformance, paper properties and printability were investigated. The resultswere compared with those of using ground calcium carbonate (GCC) andprecipitated calcium carbonate (PCC) filler, which are commonly used in paperindustry. Based the characteristics of FACS, the relationship between thestructure of FACS filled paper and properties was also discussed. Besides,calcination treatment, filler-fiber co-refining and filler blending methods werealso adopted in order to improve the properties of FACS filled paper. This papermainly consists of three parts:1. Fly ash based calcium silicate paper filler: characterization and itsapplicationIn comparison with GCC and PCC commonly used as paper fillers, the original fly ash based calcium silicate filler (FACS) has a larger particle size(21.6μm)，a higher specific surface area (121m2/g), a much lower packingdensity (0.31g/cm3) and a similar brightness (91.5%ISO)，which provide benefitsfor paper bulk and optical properties but compromise the paper internal sizing. Inaddition, higher free water and bonding water content of FACS result in moreignition loss at525℃. XRD results showed that the chemical composition ofFACS were calcium silicate hydrate (C-S-H), calcium sulfate hemihydrate(CaSO40.5H2O) and gypsum (CaSO42H2O). The use of sulphuric acid in theprocess of lowering the pH of FACS slurry is the main reason of containingcalcium sulfate hemihydrate and gypsum in FACS filler. Based on the results ofSEM, TEM and EDAX, it was found that there were two types of particles inFACS filler: calcium sulfate with needle-like particles and monoclinic structurewhile C-S-H exhibited low degree of crystallinity of aggregated particles withwrinkly porous surface.Three grades of FACS fillers with different particle size were prepared byball milling; their characteristics as paper fillers and the effects on the wet-endperformance and resulting paper properties were determined. Ball millingchanged the morphology and particle size distribution while decreasing theparticle size. Regarding to filler wet-end performance, the dynamic retention rateof FACS was increased with increasing particle size and the dynamic retentionrate of original FACS was superior to GCC and PCC filler when retention aidswas not used; while filler with small particle size exhibited higher filler retentionwhen CPAM was applied. In addition, CPAM can promote the drainage ability offilled furnish. FACS and PCC showed similar drainage ability but lower thanGCC filler in this study. Besides, it was found that FACS with small particle sizehad a detrimental effect on drainage ability.Physical test showed that FACS filler had an advantage of improving paperbulk. At a15%original FACS, the paper bulk was about40%higher than that ofthe control (no filler addition). For ball milled FACS, the paper bulk of FACSfilled paper decreased because the particle size of FACS filler decreased, whilethe paper tensile strength increased. A larger FACS particle size led to higher tearstrength. Although original FACS had a large particle size, the light scatteringcoefficient was higher than that of GCC-filled paper. This can be attributed to FACS’s porous surface and morphology. The light scattering coefficient for ballmilled samples increased as the particle size decreased. Printing quality testsindicated that FACS filled paper demanded more ink than GCC and PCC filledsheets due to its porous structure and low air resistance. Thanks to its high bulk,the FACS0filled paper had a lower print through than the GCC filled paper.FACS2, which was smaller in size than FACS0, can decrease the ink demand andimprove print through. In addition, the surface strength of FACS0filled sheetswas similar to GCC filled sheets, both of which were higher than PCC andFACS2filled sheets, which can be partly explained by filler distribution.2. How filler influences paper properties: fiber bonding ability and fillerZ-distributionWhen fiber mass was fixed, the use of PCC and GCC filler can increasepaper apparent density as filler mass increased while FACS addition reducedapparent density. The change in paper apparent density caused the differences instructure and properties among filled paper samples. Besides, it was found thatfiller packing index was decreased with increasing filler grammage, whichresulted in the changes in effective tensile index of filled paper and lightscattering ability of filler. The effective tensile index decreased with increasingfiller grammage, which verified filler particles prevented fiber bonding by stericeffect. In addition, the loss of effective tensile index of FACS filled paper wasfound to be maximized at7g/m2filler and decreased with filler grammage. Thisfact illustrated that the increase of packing ability of filler particles help toalleviate the negative effect on paper strength caused by filler. The linearrelationship between filler packing index and light scattering coefficient of fillerpaper explained the nonlinearly increase of light scattering coefficient as fillergrammage increased.The correlation between formation components and paper properties wasalso analyzed. It was proved that the use of filler can improve paper formation.The differences in correlation between formation components and filled paperproperties can be varied with filler type and properties. A high positive linearcorrelation can be found between tensile and tear index of FACS filled paper andformation component at0.6mm,0.8mm and1.3mm, especially at0.6mm and0.8mm with R2>0.95. Additionally, a high negative linear correlation can be found between the brightness and opacity of FACS filled paper and formationcomponent of0.6~1.3mm. The results can help paper mill control paper qualityby rapid test formation of filled paper.In order to investigate the effect of filler distribution in Z-direction on paperproperties, the layered handsheets with various distribution factors were prepared.Filler distribution in Z-direction of FACS filled layered handsheets wascharacterized and verified by image analysis. A new factor of filler distribution inZ direction, i.e. concentration factor, was proposed, which was related to paperstructure and strength properties. The results showed that the bulk and porosityof FACS filled paper had a positive correlation with Fcwhile a negativecorrelation could be found between tensile index and Fc. An increase around70%in the tensile index of FACS filled paper with Fc=0could be observed comparedto paper sample with Fc≠0. However, the interbonding index of FACS filledpaper with Fc=0could be dramatically reduced. Regarding to optical properties,it was found that the surface filler content increased with increasing the averagefiller content of paper，resulting in the improvement in paper brightness. Apositive linear correlation between symmetrical factor (Fs) and the differences inbrightness of both sides of layered paper can be found, which the coefficient ofdetermination R2can be up to0.9802. In addition, the opacity of FACS filledpaper had a positive correlation with concentration factor, i.e. increasing fillercontent in top layer helps to improve opacity of layered paper.3. Improvement of the properties of FACS filled paperCalcination modification was applied to FACS filler based on thethermostability of FACS. The results showed that the characteristics of FACSwas changed significantly when temperature was over850℃. FACS exhibitedgood stability when filler was modified at900℃(i.e. IFACS) while the poroussurface of FACS was disappeared, the average particle size was reduced from21.6μm to17.1μm and the specific surface area decreased to4.0m2/g, which isbeneficial to internal sizing. In addition, the chemical composition was changedto wollatonite (CaSiO3) and calcium sulfate (CaSO4). In comparison withoriginal FACS filled paper, the bulk of paper filled with IFACS was decreased27.2%and33.8%at filler content of17.5%and33.5%, respectively. Meanwhile,the corresponding porosity was reduced129%and79%, respectively. Importantly, IFACS can improve paper tensile index significantly but had anegative effect on paper brightness and opacity.Fiber-filler co-refining was employed to improve the strength and opticalproperties of FACS filled paper. In comparison with regular filling method, thenew method can improve optical properties effectively but reduce paper bulk.When refining revolutions was fixed around3500r, the brightness and lightscattering coefficient of FACS filled paper were improved significantly while thetensile index was maintained well. The changes in bulk and optical propertiescan be explained by the decrease in average particle size in process ofco-refining.Besides, the concept of filler blending can be used to take full advantage offiller properties and balance the filled paper properties. Blending of PCC andIFACS can be used to balance paper optical properties and strength propertieswhile FACS and IFACS can also be blended if a good balance between paperbulk and strength become a concern factor. |
PDF Full Text Request