A sand sedimentation tank (10) is provided, the tank (10) comprises a tank body (11), an inlet (12) and an outlet (13), wherein the tank body (11) comprises side walls (14) and a bottom (15), the outlet is arranged in a side wall (11), and the bottom (15) has at least one raised part (16) and/or at least one recessed part (17). A sand separator is further provided, the separator comprises a grading unit (30), a particle collector unit (40), and the sand sedimentation tank (10) connected in series. A method for separating mixture of solid particles is further provided. The solid particles comprise first solid particles and second solid particles, wherein, the density of the first solid particles is lower than the density of the second solid particles; the method comprises: forcing the mixture of solid particles to flow with a solvent, making the second solid particles deposit in the flow process to form a layer (22) of the second solid particles, and forcing the first solid particles to continue flowing with the solvent, wherein, the density of the second solid particles is higher than the density of the solvent.