Preparation Of Mullite And Its Composite Porous Ceramics From Fly Ash

Fly ash is a typical industrial solid waste.The annual emissions of hundred millions tons of fly ash affect human health and ecological balance seriously.The main components of fly ash are mullite and silica,which are suitable for the preparation of mullite and its composite materials.The lining of metallurgical furnace is usually made of refractory ceramic aggregate.The introduction of porous ceramic instead of pyknotic refractory aggregate can reduce the lining density of the kiln and enhance the thermal insulation performance,and reduce the energy consumption for the metallurgical industry.In order to refine the high added value of fly ash,fly ash was used as the main raw material,and high alumina bauxite or aluminum ash was used as the aluminum source,and industrial silicon carbide,potassium feldspar powder,starch,vanadium pentoxide and other additives are added.The mullite closed-cell ceramics and the corundum-mullite porous ceramics were respectively prepared by a high-temperature foaming method and a pore-forming agent method.The phase compositions,microstructures,normal temperature compressive strength,thermal shock resistance and thermal conductivity of the synthesized samples were systematically studied from the aspects of silicon carbide addition,calcination temperature,potassium feldspar addition,holding time and starch addition.The following main results were obtained:(1)Fly ash was used as the main raw material and high alumina bauxite as the aluminum source,the mullite closed-cell ceramics were successfully prepared by high-temperature foaming method.The effects of foaming agent SiC addition amount,calcination temperature,flux potassium feldspar addition and holding time on the properties of mullite closed-cell ceramics were systematically studied.The thermodynamic feasibility and optimum process parameters in the synthesis process were discussed.The research showed that the suitable conditions for preparing mullite closed-cell ceramics by high-temperature foaming method are:1600? for 6h,the ratio of fly ash to high alumina bauxite was 1:1.5,and the mass fraction of added SiC was 10%.The mass fraction of K-feldspar was 12%,and the closed porosity of the sample reached 25.7%,the bulk density was 2.30g·cm-3,the compressive strength was 27.74MPa,and the number of thermal shock resistance(cooling in water)was 5 times.Mullite closed-porous ceramic had a thermal conductivity of 1.95W·m-1·K-1 at high temperature(800?).(2)Selecting the best preparation process parameters(heating at 1600? for 6h,the ratio of fly ash to high alumina bauxite was 1:1.5,the mass fraction of added SiC was 10%,and the mass fraction of added potassium feldspar was 12%),mullite closed-cell ceramics were successfully prepared by high-temperature foaming method.The XRD results showed that the main crystal phase of the samples was mullite and there were no other peaks in the figure.The SEM results showed that the well-distributed and closed pores can be observed through the electron microscope at magnified 500 times.The columnar mullite grains can be clearly observed at magnified 5000 times,and the mullite grains are evenly distributed in the glass phase.(3)Aluminum ash was selected as the aluminum source,starch was used as the poreforming agent,and the corundum-mullite porous ceramic was successfully prepared by adding the pore-forming agent method.The pore-forming agent starch addition amount and calcination temperature were systematically studied to prepare corundum-The effect of the properties of mullite porous ceramics.The results showed that the suitable conditions for the preparation of corundum-mullite porous ceramics by adding pore-forming agent method were:1100? for 1h,the ratio of fly ash to aluminum ash is 1:2,and the starch with 20%mass fraction can be added.The apparent porosity was 58.4%,the bulk density was 1.38g·cm-3,the normal temperature compressive strength was 18.39MPa,the thermal shock resistance(water cooling)was 5 times,and the high temperature(800?)thermal conductivity was 1.38W·m-1·K-1.(4)Selecting the best preparation process parameters(1100? for 1h,fly ash and aluminum ash ratio of 1:2,add 20%starch),mullite-corundum porous ceramics were successfully prepared by method of adding pore-forming agent.XRD results showed that the main crystal phase of the sample is mullite and corundum,and there were no other peaks in the spectrum.The SEM results showed that the internal pores of the sample were interlaced and equally distributed.It could be observed that the bulk crystal grains were covered by the liquid phase at an electron microscope magnification of 5000 times.