In the paper is presented methodology of the X-ray image processing application to investigate gravitational flow in rectangular silo model. The proposed normalization procedure of X-ray data allows to visualize the changes of the volume fraction of sand during silo discharging process. The applied procedure of image processing, in contrast to the previously author works, allows to obtained more accurate information about the changes of material distribution level during process. The conducted image analysis simplifies the investigation of mass flow in various area of silo. The obtained results show the different particle behaviour in centre and at silo wall area. The experiments were conducted for different initially level of sand densities and roughness of the silo wall. Visualization of dissimilarity in interaction between the particles and particles, and between particles and the silo walls, even for smooth wall, was the main result of the Xray image analysis, especially for shear zone visualization.