| Fly ash is the solid waste from coal power plant.Fly ash cenospheres are purified from fly ash as a kind of spherical floating particle,and widely used in plastics,artific ial leather,spacecraft and so on,because of its excellent character,such as cheap raw material,easy to get,high temperature resistance,light weight,good fluidity and so on.However,fly ash cenospheres have serious defects,such as smooth surface,high hardness and grey color.These defects are likely to cause a combined interface rupture,engender equipment wear and tear problems.Besides fly ash cenospheres will not be applied to light-colored materials when being used in polymer,thus its developments are limited in the field of packing.Based on Mg（OH）2,ZnO and Al2（SiO3）3 with some nice features,such as low hardness,high whiteness and flame retardancy,composite flame retardant powders with core-shell structure were prepared with the heterogeneous nucleation method by taking the micrometers-sized and calcined fly ash cenospheres as the core,Mg（OH）2,ZnO or Al2（SiO3）3 as the inorganic coating layer in this paper.The effects of the experimental factors on the whiteness and the optimum technological conditions were investigated by single factor experiment and orthogonal experiment.The characters of whiteness value,chemical composition and content and cladding surface morphology,specific surface area,partic le size and distribution are measured and charactrized by the modern instrument such as the whiteness meter,the Zeta potentiometer,the infrared spectrometer,the X-ray diffractometer（XRD）,the scanning electron microscope（SEM）,the particle s ize analyzer and the specific surface area meter.The results of calcination experiments show that the whiteness of fly ash cenospheres was increased from 13.7 to 27.1 in the muffle furnace at 815℃for 2 h.The single factor experiment result of fly ash cenospheres coated with Mg（OH）2 shows that the optimum technological conditions are as follows:coating amount is 50%,concentration of NaOH and MgSO4 solution is 0.3mol/L and 0.15 mol/L respectively,coating agent are added at 7 mL/min,reaction temperature is 80℃,solid to liquid ratio of fly ash cenospheres to water is 1:5,reaction time is 30min,solution p H is 12.Under this condition,the whiteness of the composite powder is 51.2.Orthogonal experiment results show that coating agent dropping speed,solid to liquid ratio and reaction time have a significant effect on the whiteness of the composite powder.The optimum technological conditions are as follows:coating amount is 70%,coating agents are added at 5 mL/min,reaction temperature is 90℃,solution p H is 10,solid to liquid ratio of fly ash cenospheres to water is 1:5,reaction time is 90 min.Under this condition,the whiteness of the compound powder is 55.3.The single factor experiment result of fly ash cenospheres coated with ZnO,taking Zn（OH）2 as a precursor shows that the optimum technological conditions are as follows:coating amount is 90%,concentration of NaOH and ZnSO4solution is 0.3 mol/L and 0.15 mol/L respectively,coating agents were added at a rate of 2 mL/min,reaction temperature is 80℃,solid to liquid ratio of fly ash cenospheres to water is 1:8,reaction time is 30 min,solution p H is 9,drying temperature is 105℃.Under this condition,the whiteness of the composite powder is 58.3.Orthogonal experiment results show that coating agent solid to liquid ratio and reaction temperature have a significant effect on the whiteness of the composite powder,and the optimum technological conditions are as follows:solid to liquid ratio of fly ash cenospheres to water is 1:8,coating amount is 70%,reaction temperature is 70℃,solution p H is 9,coating agents were added at a rate of 3 mL/min.Under this condition,the whiteness of the compound powder is 62.6.Orthogonal experiment result of fly ash cenospheres coated with Al2（SiO3）3,shows that the Al︰Si ratio of the amount of substance and reaction temperature have a significant effect on the whiteness of the composite powder,the optimum technological conditions are as follows:coating amount is 110%,Al:Si ratio of the amount of substance is 1:1,coating agents were added at a rate of 4 mL/min,reaction temperature is 70℃,solution p H is 13,solid to liquid ratio of fly ash cenospheres to water is 1:10.Under this condition,the whiteness of the compound powder is 58.3.Analyzers results show that the surface of the fly ash cenospheres were smoother after being calcined.The specific surface area of fly ash cenosphere was decreased from 3.692 m2/g to 2.471 m2/g.The whiteness was increased from 13.7 to 27.1.After inorganic modification,the surface of cenospheres was covered with a thin sheet of Mg（OH）2 with a thickness of 13.36 nm,and the specific surface area of fly ash cenosphere was increased from 2.471 m2/g to30.992 m2/g.The surface of cenospheres was covered with s ix prismatic ZnO with a diameter of 46 nm,and the specific surface area was increased from2.471 m2/g to 10.361 m2/g;the surface of cenospheres was covered with the amorphous collection Al2（SiO3）3 with a diameter about 50 nm,and the specific surface area was increased from 2.471 m2/g to 24.205 m2/g.In different circumstances of solution acidalcaline,there are different types of electrostatic attraction between the coating layer and the core.Not only the hydroxyl groups but also ions interact with each other.Those interaction generate bonding force,which is beneficial to firmly combined nucleos-shell structure.The ions that the coating agent dissolve in solution are prec ipitated out of the oversaturated solution under the action of thermodynamics.The crystal nucleus regrow on the active parts of the fly ash microspheres under the driving force of phase transformation and oversaturation,then form flaky or columnar construction.If the oversaturation of solution is too high,it is easy to form amorphous noncrystalline precipitation. |
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