The Study Of Fly Ash Modification Via CaCO₃ Crystalline Coating

Fly ash is one of the largest emission sources of solid waste in China.The annual output is nearly 600 million tons,and there are about 2 billion tons of stockpiles currently.The stacking of fly ash will directly or indirectly cause a series of environmental problems such as the atmosphere,soil,and rivers pollution,and great waste of resources.Over the years,the fly ash was used in the construction,roads,agriculture and other low value-added areas both at home and abroad.As the chemical composition and unique surface pore structure of fly ash is similar to the kaolin,which is a kind of papermaking filler,which is very suitable for papermaking filling.While the low whiteness of the fly ash,which cause it can only be applied to some low whiteness requirements papers.In this paper,the modification of fly ash was based on the mechanism of heterogeneous nucleation surface coating process,which was that a kind of CaCO3 crystal with small size was coated on the surface of fly ash by using fly ash,calcium oxide and carbon dioxide as raw materials in the bubbling carbonation device.In this paper,the modification process of the fly ash was prepared in the bubbling carbonation device.The effects of the fly ash particle size,cladding crystallization times,digestion temperature,stirring speed,the rate of CO2,final pH value,the concentration of calcium hydroxide,standing time on the modified effects of the modified fly ash was studied,the modified effects of which was characterzed by the whiteness of modified fly ash,the whiteness of modified fly ash filled papers and the dirt count of modified fly ash filled papers.Then the optimum reaction conditions were found by the orthogonal experiment.The results showed that when the stirring speed was 500rpm/min,the rate of CO2 was 0.15L/min,the final pH value was 7,the concentration of calcium hydroxide was 7%,the whiteness of modified fly ash could be up to 62.27% ISO,the whiteness of modified fly ash filled papers could be up to 72.64% ISO and the dirt count of modified fly ash filled papers was 350 per square meter.The effects of four crystalline controllers,which included sodium hexametaphosphate,ZnCl2,borax,AlCl3,on the morphology of calcium carbonate crystals crystallized on the surface of the fly ash were studied,which was based on the orthogonal optimization process.The results showed that the CaCO3 crystal shape of acicular,globular-like,lamellar-like and strepto-like could be controlled by sodium hexametaphosphate,Zn Cl2,borax,AlCl3 respectively.While compared with others,the modified effects were relatively improved as the dosage of ZnCl2 was 0.35%,and the fly ash was evenly coated with a layer of globular-like crystals of CaCO3.The whiteness of modified fly ash could be up to the The maximum 65.33% ISO,the whiteness of modified fly ash filled papers could be up to 75.59% ISO and the dirt count of modified fly ash filled papers was reduced to200 per square meter.In this paper,the surface area and surface structure of modified fly ash with great modifed effects were characterized.The results showed that compared with the fly ash,the surface area increasement of the modified fly ash was up to 195%.In addition,the XRD spectrum showed that the main crystalline phases of fly ash were Calcium feldspar,calcite and quartz,while the calcite was the only phase of the modified fly ash.In addition,the coating strength performance of modified fly ash with or without the addition of crystalline controller(0.35% ZnCl2)was analyzed.The results showed that compared with the modified fly ash without the addition of 0.35% ZnCl2,the coating strength performance of modified fly ash with 0.35% ZnCl2 was better.