To fully understand the role of CaAl2Si2 phase in concentrating the non-metallic impurity phosphorus, an experiment of directional solidification of Al-70 at. pct Si alloy with extreme small lowering rate 0.05 mm min(-1) was carried out. With good dynamic condition for the diffusion of impurity (Ca, Al, P) from silicon to the S/L interface, the CaAl2Si2 phase with 0.6-0.7 at. pct P was successfully observed by Electron Probe Micro Analyzer (EPMA), and its distribution character was originally presented. This impurity phase was widely detected in the refined sample but only at the interface of silicon crystal and Al-Si alloy which contributed to the deep removal of impurity P. The formation mechanism of CaAl2Si2-P phase was thus explored, in which the microsegregation and concentration of element P, Ca, Al in front of S/L interface were crucial. After acid leaching, the P content decreased from the original 23 ppm to below 5 ppm. Compared with normal solidification, a 16 pct higher removal efficiency of P was obtained in this study. (C) The Minerals, Metals & Materials Society and ASM International 2016