Preparation Of Fly Ash Based Composite Paper Fillers By Carbonated Modification With The Regulation Of Heterogeneous Nucleation

As a solid waste,the resourcing and high-valued utilization of coal fly ash has always been a hot topic.Since someone tried to use original fly ash as paper filler,it has been considered as a potential source of filler.However,the residual carbon particles and iron particles in fly ash prevents the effective utilization due to the powder whiteness.This paper mainly focused on improving the whiteness of fly ash under the guidance of heterogeneous nucleation theory and carbonization kinetics crystallization theory.Depend on a three-phase reaction system composed of Fly ash?Solid?,Ca?OH?₂/H₂O?Liquid?and CO₂?Gas?,the method of forming a high whiteness CaCO₃ modified layer on fly ash surface was explored,to obtain a kind of fly ash based carbonation modified composite filler for paper filling.So the following researches and conclusions were included:With the potential as paper fillers,the first problem should be solved is to remove or cover the chromogenic particles.The whiteness of fly ash could be increased from31.09%ISO to 41.01%ISO by a pretreatment of flotation and magnetic separation,which could remove most carbon particles and iron particles.The pretreated fly ash powders were taken as raw materials for the carbonization reaction,to research on the heterogeneous nucleation based on fly ash and its effective control.By the comparison of state monitoring data and the products particle size distribution after carbonization,with the starting temperature at 15^oC,the stirring rate at 600rpm,the mass concentration of Ca?OH?₂ at2.5wt%and the volume flow ratio of CO₂ and compressed air at 1:4,the relatively best nucleation and crystallization of CaCO₃ were achieved on the surface of substrate Ground Calcium Carbonate?GCC?.It also provided reference for the carbonization modification process when the pretreated fly ash was used as substrate to research on the process control measures.Depend on the optimized heterogeneous nucleation and carbonization modification conditions for substrate GCC,the whiteness and particle size distribution of the composite powders obtained by the carbonization modification of the fly ash particles,which could be obviously improved by the multiple nucleation and supersaturation control.By controlling the mass ration of Ca?OH?₂ and pretreated fly ash at 1:1,the two-time nucleation with doubles materials was proved to improve the whiteness in a certain extent.When the supersaturation was also controlled by the conductivity value around 6.5mS/cm after its mutation in the end stage of carbonization,it helped fly ash get a better heterogeneous nucleation performance.The average particle size of composite powder increased from 9.40 to 12.3?m,and the whiteness of fly ash increased to 60.66%ISO.The increasing of whiteness was limited by the dense accumulation of calcium carbonate particles on the surface of the fly ash.Therefore,when the mass ratio of Ca?OH?₂ and the pretreated fly ash was raised to 2:1,the virtual particle size of fly ash based composite filler?named peak center value?changed to 18.83?m,and the whiteness reached to 68.68%ISO.However,the utilization rate of CaCO₃ used for heterogeneous nucleation decreased by more homogeneous nucleation at the same time.XRD,EDS and XPS were used to analyze and assess the carbonization modification effects of fly ash.The results all showed that the surface was well coated by calcium carbonate particles.However,some carbon particles still had a poor coating state due to the difference of surface characteristics,which restricted the overall whiteness improvement of the fly ash particles.So the removal of carbon particles was beneficial to the carbonization modification process.Compared with the traditional filler GCC filled paper,although the retention and whiteness performance is not as good as GCC filling,the opacity and strength under same ash content were much better.After calculation,the production cost of fly ash based composite fillers?whiteness:68.68%ISO,average particle size:6.64 m,peak center value:18.83 um?was mainly from the purchase of lime,and the input of other chemicals are relatively lower.