With boron-rich slag, silica fume, bauxite chalmette and carbon black as starting materials, α-sialon/AlN/BN powder was prepared by carbothermal reduction-nitridation. The powder was attrition milled to submicron size and suspended in water. The effects of yttrium oxide as a sintering aid, pH, and addition of deflocculant on the suspensions were studied. Optimum slip casting properties, i.e. lowest viscosity values, the highest absolute zeta potential values, the smallest floc size and sediment volume were found at pH=10 for the powder. The suspensions were used to slip cast discs which were sintered in a high purity nitrogen atmosphere at 1700 °C for 2 h. The strength was about 230 MPa, the toughness 3.6 MPa•m1/2 and the hardness about 13.8 GPa.