Study On Preparation Of Mullite By Sintering Method From Coal Gangue

Owing to its crystal structure and unique frame construction, which make it has premiumthermal stability, chemical stability and mechanical properties, mullite has been widely usedin many fields such as high temperature structure and engineering materials, microelectronics,optics, etc. Coal gangue is a kind of solid waste from coal mining and the huge amount ofcoal gangue has put forward the great challenge for our government to utilize it efficiently.According to phase-mineral and chemical compositions of coal gangue, preparation ofmullite(60-70wt.%Al2O3) have been investigated in this paper by dry-pressing and sinteringmethod using coal gangue as starting material incorporated with appropriate amount of Al2O3in the light of chemical compositions of mullite. In this paper, author have analyzed thethermodynamics of mullite’s synthesis reaction by coal gangue, researched the influences ofsintering temperatures, residence time, forming pressure to mullite’s properties by singlefactor experiments and orthogonal experiments, studied the influences of the addition of CeO2and MgO on mullite’s properties, represented mullite’s properties by XRD, SEM experiments.The conclusions are listed as follows.The analysis of thermodynamics of mullite’s synthesis reaction by coal gangue shows thatsynthesis of mullite by coal gangue and α-Al2O3could be happened in theory only when thetemperature is higher than490.4℃, and it only could be happened in the ideal condition whenthe temperature is just a little higher than487.1℃.The single factor experiments’ conclusions of synthesis of M60mullite by coal gangue arelisted as follows.(1)When sintering temperature increases from1250℃to1400℃, shrinking percentage of mullite increases initially and decreases afterwards. And it can be concluded by XRDexperiments that shrinking percentage increases because of the formation of primary mullitewhen sintering temperature increases from1250℃to1300℃, and shrinking percentagedecreases because of the secondary mullitization when sintering temperature increases from1300℃to1400℃. When sintering temperature increases from1250℃to1400℃, bulkdensity and compressive strength increase as sintering temperature increases; apparentporosity and water absorption decreases initially and increases afterwards.(2)When residence time increases from2h to8h, shrinking percentage, bulk density,compressive strength of mullite increases as residence time increases, and apparent porosity,water absorption decreases as residence time increases. According to XRD experiments,content of mullite in samples of which residence time longer than6h is higher than thesamples of which residence time shorter than6h, and content of mullite in samples of whichresidence time are6h is appropriately equal to the samples of which residence time are8h.The orthogonal experiments’ results of synthesis of M60and M70mullite by coal gangue arelisted as follows. The experiments indicate that the impact degree of technologicalconditions on bulk density, water absorption and apparent porosity can be listed in descendingorder：sintering temperature, residence time and forming pressure; and it also shows that thebest preparation process of M60mullite and M70mullite is sintering temperature1500℃、residence time6h and forming pressure2t.The influences of the addition of CeO2and MgO on properties of M60mullite have beendiscussed and the results are listed as follows.(1)With0.5wt.%addition of MgO, when sintering temperatures are1250℃,1300℃and1400℃, bulk density and apparent porosity have little change; when sintering temperaturesare1500℃, bulk density increases, apparent porosity decreases and the content of mullite insamples increases. With0.5wt.%addition of CeO2, when sintering temperatures are1250℃and1300℃, bulk density and apparent porosity have little change; when sinteringtemperatures are1400℃and1500℃, bulk density increases, apparent porosity decreases, thecontent of mullite in samples increases and when sintering temperatures are1500℃, bulkdensity and apparent porosity have the biggest change, the content of mullite is the highest. (2)With0.5wt.%simultaneous addition of CeO2and MgO and sintering temperature is1500℃, when CeO2/MgO ratio is different, bulk density increases, apparent porositydecreases and when CeO2/MgO ratio is1/1, bulk density is the biggest, apparent porosity isthe lowest and the content of mullite is the highest, and that is the optimal ratio of CeO2andMgO.In conclusion, the preparation of mullite using coal gangue as starting material andincorporated with appropriate amount of Al2O3is practical, and this method could offer apractical way for the resource utilization of coal gangue.