Fly Ash Microspheres, The Micro-structural Characterization And Nitriding Mechanism

This paper reports a study of the microstructure, including morphology, surface elementary distribution, crystal and glass phase distribution, pore structure and distribution of microspheres which are separated from fly ash. And the synthesize technology of nitridated microsphere with invariable spherical body appearance are also explored on the basis of reduction and nitridation. The effects of some technical \factors on the synthesis of nitridated microsphere have been studied. The nitrdation mechanism is also analysised.The results show: microspheres separated from fly ash have high sphericity, most of which have cenosphere structure. When the diameter decreases, the microspheres density increases, but the hollow lever and average thickness both decrease. Most of the microspheres have smooth surface and intact spherical body, only minority of them with rather coarse surface. The elements of surface and body are both exist in form of oxide, distributiing inhomogeneously, which leads to the difference of surface component of a single microsphere. There are some nanobeads exsist in the body of microspheres, whose activity is between crystal and glass phase. The surface phase of microspheres is glass phase, and the frame is made of cristyl or a compound body of cristyl and glass phase. There are a small amount micropores in the inner body, which have a large contribution to specific surface area, but most of the pore are closed spheric shape. Generally, microspheres have a pore with the same center of sphere, there are also exist some stuffed or poriferous micTospheres.The result of reduction and nitridation of microspheres shows: using Si and Al as reducing agent, the reaction product sinter and microspheres can't keep spheroidal body between 1300℃to 1500℃. Using active carbon as reducing agent, nitridation reaction begains at 1300℃with the formation of a small amount of Si₂N₂O, SiO is an important in-process product, Mullite are involved in the reaction directly. After 1450℃, Si₂N₂O tansforms toβ-Sialon and the end product isβ-Sialon with a z value 0.9 at 1500℃. The microsphere size almost has no affect on component and content of reaction product, but has an great affect on shape of reaction product. When the microsphere size decreases, surface energy increase, which is disadvantage to keep the spheroidal shape of nitridated microsphere. The amount of reducing agent is an important factor to keep the spheroidal body of nitridezed microspheres. Sialon sphere can be abtained with active carbon excess to 10％～50ï¼…at 1500℃/6h, but when the amount of active carbon is under the theoretical mixture ratio, microsphere bond to each other after nitridating. SEM phote shows that nitridated microsphere still has a cenosphere structure.