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Mullite short fibers have been fabricated by adapting the kneading-drying-calcination (KDC) process and characterized. The effect of the addition of foaming agent and calcination temperature on the formation of mullite fibers from coal fly ash, was examined. In the present work, ammonium alum NH4Al(SO4)212H2O synthesized from coal fly ash and sodium phosphate Na2HPO42H2O were used as foaming agents. After calcination at 1300o C for 10 h and then etching with 20% HF solution at 50o C for 5 h using a microwave heating source, the alumina-deficient (Al2O3/SiO2 = 1.13, molar ratio) orthorhombic mullite fibers with a width of ~0.8 μm (aspect ratio >30), have been prepared from the coal fly ash with Al2O3/SiO2 =0.32, molar ratio by the addition of NH4Al(SO4)212H2O, and with further addition of 2 wt.% sodium phosphate. The excessive addition of sodium phosphate rather decreases the formation of mullite fibers, possibly due to the large amount of liquid phase prior to mullitization reaction.