The effects of Mn and Co additives on the carbothermal nitridation of fly ash were examined at 1330, 1425 and 1475 degrees C in a N-2 atmosphere under normal pressure. Above 1425 degrees C, the nitridation of fly ash occurred and oxy-nitride phases, such as beta-type sialon, alpha-type sialon and X2N2O (X-C Or Si) type oxy-nitride phases were formed. The phase ratio among these phases was changed with the amount of additives. In the case of a Mn additive, the a to b phase transformation of sialon was accelerated with the amount of the additive. On the other hand, in the case of a Co additive, no effective role on the transformation was observed. At 1475 degrees C, the ash completely changed to P-type sialon.

Keywords: fly-ash, sialon, nitridation, phase transformation