Upgraded metallurgical-grade silicon is used to cast an ingot by directional solidification. Black shadows are randomly distributed in the ingot, and the shadows are caused by natural sedimentation of insoluble particles. The insoluble particles mainly consist of SiC and Si3N4. SiC and Si3N4 exist as foreign particles and mainly sedimentate at the bottom of the ingot, not generating during directional solidification. Melt convection performs an important role in the sedimentation, resulting in the insoluble particles in the ingot center more than the nearby. Interestingly, since SiC and Si3N4 will not be the recombination center of the minority carrier, the insoluble particles do not have a significant influence on the minority carrier lifetime. In particular, the sedimentation is discussed according to the thermodynamics and kinetics in detail. (C) 2016 Elsevier B.V. All rights reserved.