Boron-rich blast furnace slag of low activity is one of the major products created during the separation of iron and boron from ludwigite in a blast furnace process, and the high-efficiency utilisation of its is of great importance to the Chinese boron industry. This paper proposes one new application process to synthesize alpha'-Sialon-AIN-BN powders by a carbothermal reduction-nitridation method using boron-rich blast furnace slag as the staring material and describes a series of experimental studies that were performed to elucidate the mechanism of phase formation and microstructure evolution during CRN. The experimental results revealed that the phase compositions and microstructures of the synthesized products were greatly affected by the initial compositions ((Ca,Mg)(x)Si12-3xAl3xOxN16-x), x = 0.3-1.8), temperature and holding time. With the compositions shifting from values of x of 0.3-1.8, the relative amount of alpha'-Sialon, AlN and BN increased gradually, and the amount of alpha'-Sialon reached a maximum at a value of x of 1.4. The optimal condition for powder synthesis was a temperature of 1480 C with a holding time of 8 h, under which the crystalline phases included alpha'-Sialon, AlN, BN and less SiC. More elongated alpha'-Sialon grains were observed at higher x values and temperatures. During the CRN process, MgAl2O4, Mg2SiO4, Ca2Al2SiO7, MgSiN2, beta'-Sialon and 27R appeared sequentially as intermediate products. The volatilisation of SiO gas and magnesium vapour resulted in additional weight loss of the samples, which was aggravated with increases in the synthesis temperature and holding time. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology japan. All rights reserved.